Comparative study between Mobile WiMAX (IEEE802.16e based) and 3GPP LTE

Karim Ahmed Samy Banawan, Mohammed Salaheldin Abdullah,

and Mohamed Abdel Ghani Mohammed El-Gharabawy.

Abstract: In this paper we present a comparative study between Mobile WiMAX

(IEEE802.16e based) and 3GPP LTE, we present the key technologies that are

utilized in both systems, then PHY layers are presented ,besides Network

Architectures. Our conclusions and result are also introduced.

Key terms: Mobile WiMAX, IEEE802.16e, LTE, PHY layer, OFDMA,SC-

FDMA,MIMO, system architecture.

I.INTRODUCTION

The demand for high data rate

wireless multi-media applications has

increased significantly in the past few

years. The wireless user‘s pressure

towards faster communications, no

matter whether mobile, nomadic, or

fixed positioned, without extra cost is

nowadays a reality. Finding an optimal

solution for this dilemma is a

challenge, not only for manufacturers

but also for network operators. The

recent strategy followed within ETSI

3GPP LTE and the WiMAX Forum

was a new and evolutionary concept,

especially for mobile applications.

Both have adopted a new PHY layer

multi-carrier transmission with a

MIMO scheme, a promising

combination offering a high data rate

at low cost.

The 3GPP LTE is acronym for

long term evolution of UMTS.The

multiple access scheme in LTE

downlink uses Orthogonal Frequency

Division Multiple Access (OFDMA)

and uplink uses Single Carrier

Frequency Division Multiple Access

(SC-FDMA). These multiple access

solutions provide orthogonality

between the users, reducing the

interference and improving the

network capacity. The resource

allocation in the frequency domain

takes place with a resolution of 180

kHz resource blocks both in uplink and

in downlink. The frequency dimension

in the packet scheduling is one reason

for the high LTE capacity. The uplink

user specific allocation is continuous to

enable single carrier transmission

while the downlink can use resource

blocks freely from different parts of

the spectrum. The uplink single carrier

solution is also designed to allow

efficient terminal power amplifier

design, which is relevant for the

terminal battery life. LTE solution

enables spectrum flexibility where the

transmission bandwidth can be

selected between 1.4 MHz and 20

MHz depending on the available

spectrum. The 20 MHz bandwidth can

provide up to 150 Mbps downlink user

data rate with 2 × 2 MIMO, and 300

Mbps with 4 × 4 MIMO. The uplink

peak data rate is 75 Mbps. The high

network capacity also requires efficient

network architecture in addition to the

advanced radio features.

The target in 3GPP Release 8 is

to improve the network scalability for

traffic increase and to minimize the

end-to-end latency by reducing the

number of network elements. All radio

protocols, mobility management,

header compression and all packet

retransmissions are located in the base

stations called eNodeB. eNodeB

includes all those algorithms that are

located in Radio Network Controller

(RNC) in 3GPP Release 6 architecture.

Also the core network is streamlined

by separating the user and the control

planes. The Mobility Management

Entity (MME) is just the control plane

element while the user plane bypasses

MME directly to System Architecture

Evolution (SAE) Gateway (GW). The

architecture evolution is This Release 8

core network is also often referred to

as Evolved Packet Core (EPC) while

for the whole system the term Evolved

Packet System (EPS) can also be used.

WiMAX is the commonly

used name for broadband wireless

access based on the IEEE 802.16

family of standards.WiMAX stands for

worldwide interoperability for

microwave access. The WiMAX forum

is an industry-led, nonprofit

corporation formed to promote and

certify compatibility and inter-

operability of 802.16 broadband

wireless products. IEEE 802.16 is an

IEEE Standard for Wireless MANs

(WMANs). The most recent addition

to the WiMAX family of standards is

802.16e, which is also called ‗Mobile

WiMAX. ‘ The IEEE standards specify

the physical layer (PHY) and the

Medium Access Layer (MAC), with no

definition of higher layers. For IEEE

802.16, those are addressed in the

WiMAX Forum Network Working

Group. There is a range of options

specified in IEEE 802.16, making the

standards much more fragmented than

what is seen in 3GPP and 3GPP2

standards. The 802.16 standard defines

four different physical layers, of which

two are certified by the WiMAX

forum:

● OFDM-PHY: based on an FFT size

of 256 and aimed at fixed networks.

● OFDMA-PHY (scalable): based on

an FFT size from 128 to 2048 for

802.16e.

In addition to the multiple

physical layers, the 802.16 standards

support a range of options, including:

TDD, FDD, and half-duplex FDD (H-

FDD) operation, TDM access with

variable frame size (2–20 ms),OFDM

with a configurable cyclic prefix

length, A wide range of bandwidths

supported (1.25–28 MHz), Multiple

modulation and coding schemes:

QPSK, 16QAM, and 64QAM

combined with convolutional codes,

convolutional Turbo codes, block

Turbo codes, and LDPC (Low-Density

Parity Check) codes, Hybrid ARQ and

Adaptive antenna system (AAS) and

MIMO.

There is also a range of Radio

Resource Management (RRM) options,

MAC features and enhancements in the

standards. The WiMAX forum defines

system profiles that reduce all the

optional features to a smaller set to

allow interoperability among different

vendors. This is done through an

industry selection of features for MAC,

PHY, and RF from 802.16

specifications and forms the basis for

testing conformance and

interoperability. Products certified by

the WiMAX forum adhere to a

Certification Profile that is based on a

combination of band of operation,

duplexing option and bandwidth. The

intended applications with the original

802.16 standard were fixed access and

backhaul, mainly for line-of-sight

operation. The addition of a physical

layer for non-line-of-sight applications

in IEEE 802.16-2004 and support for

mobility in IEEE 802.16e opens up the

standard for nomadic and mobile use.

In addition, provisions for multicast

and broadcast services (MBS) are also

included. This makes the standard

more similar to the evolved 3G

standards, but coming from a

completely different direction. The

IEEE standards such as 802.16 are

driven by the datacom industry as

Layer 1 and 2 standards, starting with

line-of-sight use for limited mobility,

targeting best-effort data applications

and now moving to higher mobility

and encompassing also other

applications such as conversational

services. The evolved 3G standards are

driven by the telecom industry,

targeting non-line-of-sight use and

mobility from the beginning, optimized

end-to-end standards for voice and

later also data services, now moving to

broader data applications including

best-effort services.

II. KEY TECHNOLOGIES

1- Common key technologies:

(a) Multiple Antenna support

Multi-antenna techniques can

be seen as a joint name for a set of

techniques with the common theme

that they rely on the use of multiple

antennas at the receiver and/or the

transmitter, in combination with more

or less advanced signal processing.

Multi-antenna techniques can be used

to achieve improved system

performance, including improved

system capacity (more users per cell)

and improved coverage (possibility for

larger cells), as well as improved

service provisioning, for example,

higher per-user data rates .both

systems support Multiple antenna

systems. For the LTE Multiple antenna

systems are integral part of its

specifications.

Multiple antennas at the transmitter

and/or the receiver can be used to

provide additional diversity against

fading on the radio channel. In this

case, the channels experienced by

the different antennas should have

low mutual correlation, implying

the need for a sufficiently large

inter-antenna distance (spatial

diversity).

Multiple antennas at the transmitter

and/or the receiver can be used to ‗

shape ‘ the overall antenna beam in

a certain way, for example, to

maximize the overall antenna gain

in the direction of the target

receiver/transmitter or to suppress

specific dominant interfering

signals. Such beam-forming can be

based either on high or low fading

correlation between the antennas.

The simultaneous availability of

multiple antennas at the transmitter

and the receiver can be used to

create what can be seen as multiple

parallel communication‗ channels ‘

over the radio interface. This

provides the possibility for very

high bandwidth utilization without

a corresponding reduction in power

efficiency or, in other words, the

possibility for very high data rates

within a limited bandwidth without

an un-proportionally large

degradation in terms of coverage.

Herein we will refer to this as

spatial multiplexing. It is often also

referred to as MIMO (Multi-Input

Multi- output) antenna processing.

(b) OFDMA transmission scheme:

Both systems use OFDMA as

multiple access scheme, for Mobile

WiMAX it is the multiple access

scheme for both UL and DL and for

LTE it is the multiple access scheme in

the DL only.

The OFDM is used to mitigate

the multipath fading which will result

in delay spread and frequency selective

fading instead of using complex

equalizers or high complexity rake

receivers. OFDM systems break the

available bandwidth into many

narrower sub-carriers and transmit the

data in parallel streams; each OFDM

symbol is preceded by a cyclic prefix

(CP), which is used to effectively

eliminate ISI. In practice, the OFDM

signal can be generated using IFFT

with a CP of sufficient duration,

preceding symbols do not spill over

into the FFT period, Also, Once the

channel impulse response is

determined (by periodic transmission

of known reference signals), distortion

can be corrected by applying an

amplitude and phase shift on a

subcarrier-by-subcarrier basis.

Problems of OFDM are:

susceptibility to carrier frequency

errors and a large signal peak-to-

average power ratio (PAPR).

OFDMA is an excellent choice

of transmission scheme for the 3GPP

LTE downlink and Mobile WiMAX

which allows the access of multiple

users on the available bandwidth. Each

user is assigned a specific time-

frequency resource.

(C) Hybrid ARQ with soft

combining

Fast hybrid ARQ with soft

combining is used in LTE and

WiMAX to allow the terminal to

rapidly request re-transmissions of

erroneously received transport blocks

and to provide a tool for implicit rate

adaptation. The underlying protocol is

also similar to the one used for HSPA–

multiple parallel stop-and-wait hybrid

ARQ processes. Retransmissions can

be rapidly requested after each packet

transmission, thereby minimizing the

impact on end-user performance from

erroneously received packets.

Incremental redundancy is used as the

soft combining strategy and the

receiver buffers the soft bits to be able

to do soft combining between

transmission attempts.

2-Key technologies for 3GPP LTE

system

(a) Spectrum flexibility

A high degree of spectrum

flexibility is considered one of the

main characteristics of the LTE radio

access. The aim of this spectrum

flexibility is to allow for the

deployment of the LTE radio access in

diverse spectrum with different

characteristics, including different

duplex arrangements, different sizes of

the available spectrum and different

frequency-bands-of-operation.

(i) Flexibility in duplex arrangement

One important part of the LTE

requirements in terms of spectrum

flexibility is the possibility to deploy

LTE-based radio access in both paired

and unpaired spectrum. Therefore,

LTE supports both frequency-

division-based and time-division-based

duplex arrangements. Frequency

Division Duplex (FDD) as illustrated to

the left in Fig. 1 implies that downlink

and uplink transmission take place in

different, sufficiently separated,

frequency bands. Time Division

Duplex (TDD), as illustrated to the

right in Fig. 1, implies that downlink

and uplink transmission take place in

different, non-overlapping time slots.

Thus, TDD can operate in unpaired

spectrum, whereas FDD requires

paired spectrum.

Support for both paired and

unpaired spectrum is part of the 3GPP

specifications already from Release 99

through the use of FDD-based

WCDMA/HSPA radio access in paired

allocations and TDD-based TD-

CDMA/TD-SCDMA radio access in

unpaired allocations. However, this is

achieved by means of relatively

different radio-access technologies

and, as a consequence, terminals

capable of both FDD and TDD

operations are fairly uncommon. LTE,

on the other hand, supports both FDD

and TDD within a single radio-access

technology, leading to a minimum of

deviation between FDD and TDD for

LTE-based radio access.

LTE also supports half-duplex

FDD at the terminal (illustrated in the

middle of Fig. 1). In half-duplex FDD,

transmission and reception at a specific

terminal are separated in both

frequency and time. The base station

still uses full duplex as it

simultaneously may schedule different

terminals in uplink and downlink; this

is similar to, for example, GSM

operation. The main benefit with half-

duplex FDD is the reduced terminal

complexity as no duplex filter is

needed in the terminal, which is

especially beneficial in case of multi-

band terminals which otherwise would

need multiple sets of duplex filters.

Fig. 1. Frequency- and time-division duplex.

(ii) Flexibility in frequency-band-of-

operation

LTE is envisioned to be

deployed on a per-need basis when and

where spectrum can be made available,

either by the assignment of new

spectrum for mobile communication,

such as the 2.6 and 3.5 GHz band, or

by the migration to LTE of spectrum

currently used for other mobile-

communication technologies, such as

GSM or cdma2000 systems, or even

non-mobile radio technologies such as

in current broadcast spectrum. As a

consequence, it is required that the

LTE radio access should be able to

operate in a wide range of frequency

bands, from as low as 450 MHz band

up to, at least, 3.5 GHz.

(iii) Bandwidth flexibility

Related to the possibility to

deploy the LTE radio access in

different frequency bands is the

possibility of being able to operate

LTE with different transmission

bandwidths on both downlink and

uplink. The main reason for this is that

the amount of spectrum being available

for LTE may vary significantly

between different frequency bands and

also depending on the exact situation

of the operator. Furthermore, the

possibility to operate in different

spectrum allocations gives the

possibility for gradual migration of

spectrum from other radio access

technologies to LTE.

LTE supports operation in a

wide range of spectrum allocations,

achieved by a flexible transmission

bandwidth being part of the LTE

specifications. To efficiently support

very high data rates when spectrum is

available, a wide transmission

bandwidth is necessary. However, a

sufficiently large amount of spectrum

may not always be available, either due

to the band-of-operation or due to a

gradual migration from another radio-

access technology, in which case LTE

can be operated with a more narrow

transmission bandwidth. Obviously, in

such cases, the maximum achievable

data rates will be reduced

correspondingly.

The LTE physical-layer

specifications are bandwidth-agnostic

and do not make any particular

assumption on the supported

transmission bandwidths beyond a

minimum value. The basic radio-

access specification including the

physical-layer and protocol

specifications, allows for any

transmission bandwidth ranging from

roughly 1 MHz up to around 20 MHz.

At the same time, at an initially stage,

radio-frequency requirements are only

specified for a limited subset of

transmission bandwidth, corresponding

to what is predicted to be relevant

spectrum-allocation sizes and relevant

migration scenarios. Thus, in practice

LTE radio access supports a limited set

of transmission bandwidths, but

additional transmission bandwidths can

easily be supported by updating only

the RF specifications.

(b) Channel-dependent scheduling

and rate adaptation

At the core of the LTE

transmission scheme is the use of

shared-channel transmission, with the

overall time-frequency resource

dynamically shared between users.

This is similar to the approach taken in

HSDPA, although the realization of the

shared resource differs between the

two – time and frequency in case of

LTE vs. time and channelization codes

in case of HSDPA. The use of shared-

channel transmission is well matched

to the rapidly varying resource

requirements posed by packet data and

also enables several of the other key

technologies used by LTE.

The scheduler controls, for each time

instant, to which users the shared

resources should be assigned. The

scheduler also determines the data rate

to be used for each link, that is, rate

adaptation can be seen as a part of the

scheduler. The scheduler is thus a key

element and to a large extent

determines the overall downlink

system performance, especially in a

highly loaded network. Both downlink

and uplink transmissions are subject to

tight scheduling. A substantial gain in

system capacity can be achieved if the

channel conditions are taken into

account in the scheduling decision, so-

called channel-dependent scheduling.

This is exploited already in HSPA,

where the downlink scheduler

transmits to a user when its channel

conditions are advantageous to

maximize the data rate, and is, to some

extent, also possible for the Enhanced

Uplink. However, LTE has, in addition

to the time domain, also access to the

frequency domain, due to the use of

OFDM in the downlink and DFTS-

OFDM in the uplink. Therefore, the

scheduler can, for each frequency

region, select the user with the best

channel conditions. In other words,

scheduling in LTE can take channel

variations into account not only in the

time domain, as HSPA, but also in the

frequency domain. This is illustrated in

Fig. 2.

The possibility for channel-

dependent scheduling in the frequency

domain is particularly useful at low

terminal speeds, in other words when

the channel is varying slowly with

time. Channel-dependent scheduling

relies on channel-quality variations

between users to obtain a gain in

system capacity. For delay-sensitive

services, a time-domain only scheduler

may be forced to schedule a particular

user, despite the channel quality not

being at its peak. In such situations,

exploiting channel-quality variations

also in the frequency domain will help

improving the overall performance of

the system. For LTE, scheduling

decisions can be taken as often as once

every 1 ms and the granularity in the

frequency domain is 180 kHz. This

allows for relatively fast channel

variations to be tracked and utilized by

the scheduler.

Fig. 2. Downlink channel-dependent scheduling in time and frequency domains.

(i) Downlink scheduling

To support downlink

scheduling, a terminal may provide the

network with channel-status reports

indicating the instantaneous downlink

channel quality in both the time and

frequency domain. The channel status

can, for example, be obtained by

measuring on a reference signal

transmitted on the downlink and used

also for demodulation purposes. Based

on the channel-status report, the

downlink scheduler can assign

resources for downlink transmission to

different mobile terminals, taking the

channel quality into account in the

scheduling decision. In principle, a

scheduled terminal can be assigned an

arbitrary combination of 180 kHz wide

resource blocks in each 1 ms

scheduling interval.

(ii) Uplink scheduling

The LTE uplink is based on

orthogonal separation of different

uplink transmissions and it is the task

of the uplink scheduler to assign

resources in both time and frequency

domain (combined TDMA/FDMA) to

different mobile terminals. Scheduling

decisions, taken once per 1 ms, control

what set of mobile terminals are

allowed to transmit within a cell during

a given time interval and, for each

terminal, on what frequency resources

the transmission is to take place and

what uplink data rate (transport format)

to use.

Channel conditions can also be

taken into account in the uplink

scheduling process, similar to the

downlink scheduling. However,

obtaining information about the uplink

channel conditions is a non trivial task.

Therefore, different means to obtain

uplink diversity are important as a

complement in situations where uplink

channel-dependent scheduling is not

suitable.

(c) Inter-cell interference

coordination

LTE provides orthogonality

between users within a cell in both

uplink and downlink, that is, at least in

principle, there is no interference

between transmissions within one cell

(no intra-cell interference). Hence,

LTE performance in terms of spectrum

efficiency and available data rates is,

relatively speaking, more limited by

interference from other cells (inter-cell

interference) compared to

WCDMA/HSPA. Means to reduce or

control the inter-cell interference can

therefore, potentially, provide

substantial benefits to LTE

performance, especially in terms of the

service (data rates, etc.) that can be

provided to users at the cell edge.

Inter-cell interference

coordination is a scheduling strategy in

which the cell-edge data rates are

increased by taking inter-cell

interference into account. Basically,

inter-cell interference coordination

implies certain (frequency-domain)

restrictions to the uplink and downlink

schedulers in order to control the inter-

cell interference. By restricting the

transmission power of parts of the

spectrum in one cell, the interference

seen in the neighboring cells in this

part of the spectrum will be reduced.

This part of the spectrum can then be

used to provide higher data rates for

users in the neighboring cell. In

essence, the frequency reuse factor is

different in different parts of the cell

(Fig. 3).

Fig. 3 Example of inter-cell interference coordination.

Inter-cell interference

coordination is mainly a scheduling

strategy, taking the situation in

neighboring cells into account. Thus,

inter-cell interference coordination is

to a large extent an implementation

issue and hardly visible in the

specifications. This also implies that

interference coordination can be

applied to only a selected set of cells,

depending on the requirements set by a

particular deployment. To aid the

implementation of various inter-cell

interference coordination schemes,

LTE supports exchange of interference

indicators between base stations.

(d) Multicast and broadcast support

Multi-cell broadcast implies

transmission of the same information

from multiple cells as described. By

exploiting this at the terminal,

effectively using signal power from

multiple cell sites at the detection, a

substantial improvement in coverage

(or higher broadcast data rates) can be

achieved. LTE takes this one step

further to provide highly efficient

multi-cell broadcast. By transmitting

not only identical signals from multiple

cell sites (with identical coding and

modulation), but also synchronize the

transmission timing between the cells,

the signal at the mobile terminal will

appear exactly as a signal transmitted

from a single cell site and subject to

multi-path propagation. Due to the

OFDM robustness to multi-path

propagation, such multi-cell

transmission, also referred to as

Multicast–Broadcast Single-Frequency

Network (MBSFN) transmission, will

then not only improve the received

signal strength, but also eliminate the

inter-cell interference. Thus, with

OFDM, multi-cell broadcast/multicast

throughput may eventually be limited

by noise only and can then, in case of

small cells, reach extremely high

values.

(e) New transmission scheme for the

uplink: SC-FDMA(DFTS-OFDM)

While many of the requirements

for the design of the LTE uplink

physical layer and multiple-access

scheme are similar to those of the

downlink, the uplink also poses some

unique challenges. Some of the

desirable attributes for the LTE uplink

include:

Orthogonal uplink transmission by

different User Equipment (UEs), to

minimize intra-cell interference

and maximize capacity.

Flexibility to support a wide range

of data rates, and to enable data

rate to be adapted to the SINR

(Signal-to-Interference plus Noise

Ratio).

Sufficiently low Peak-to-Average

Power Ratio (PAPR) of the

transmitted waveform, to avoid

excessive cost, size and power

consumption of the UE Power

Amplifier (PA).

Ability to exploit the frequency

diversity afforded by the wideband

channel (up to 20 MHz), even

when transmitting at low data rates.

Support for frequency-selective

scheduling.

Support for advanced multiple-

antenna techniques, to exploit

spatial diversity and enhance

uplink capacity.

The multiple-access scheme selected

for the LTE uplink so as to fulfill these

principle characteristics is Single-

Carrier Frequency Division Multiple

Access (SC-FDMA).

A major advantage of SC-

FDMA over the Direct-Sequence Code

Division Multiple Access (DS-CDMA)

scheme used in UMTS is that it

achieves intra-cell orthogonality even

in frequency-selective channels. SC-

FDMA avoids the high level of intra-

cell interference associated with DS-

CDMA which significantly reduces

system capacity and limits the use of

adaptive modulation. A code-

multiplexed uplink also suffers the

drawback of an increased PAPR if

multi-code transmission is used from a

single UE.

The use of OFDMA for the

LTE uplink would have been attractive

due to the possibility for full uplink-

downlink commonality. In principle,

an OFDMA scheme similar to the LTE

downlink could satisfy all the uplink

design criteria listed above, except for

low PAPR. SC-FDMA combines the

desirable characteristics of OFDM

with the low CM/PAPR of single-

carrier transmission schemes.

Like OFDM, SC-FDMA

divides the transmission bandwidth

into multiple parallel subcarriers, with

the orthogonality between the

subcarriers being maintained in

frequency-selective channels by the

use of a Cyclic Prefix (CP) or guard

period. The use of a CP prevents Inter-

Symbol Interference (ISI) between SC-

FDMA information blocks. It

transforms the linear convolution of

the multipath channel into a circular

convolution, enabling the receiver to

equalize the channel simply by scaling

each subcarrier by a complex gain

factor.

However, unlike OFDM, where

the data symbols directly modulate

each subcarrier independently (such

that the amplitude of each subcarrier at

a given time instant is set by the

constellation points of the digital

modulation scheme), in SC-FDMA the

signal modulated onto a given

subcarrier is a linear combination of all

the data symbols transmitted at the

same time instant. Thus in each symbol

period, all the transmitted subcarriers

of an SC-FDMA signal carry a

component of each modulated data

symbol. This gives SC-FDMA its

crucial single-carrier property, which

results in the PAPR being significantly

lower than pure multicarrier

transmission schemes such as OFDM.

(f) SC-FDMA Principles

(i) SC-FDMA Transmission Structure

An SC-FDMA signal can, in

theory, be generated in either the time-

domain or the frequency domain.

Although the two techniques are duals

and ‗functionally‘ equivalent, in

practice, the time-domain generation is

less bandwidth-efficient due to time-

domain filtering and associated

requirements for filter ramp-up and

ramp-down times.

(ii) T-Domain Signal Generation

Time-domain generation of an

SC-FDMA signal is shown in Fig. 4.

It is similar to conventional single-

carrier transmission.

Fig. 4. SC-FDMA time-domain transmit processing.

The input bit stream is mapped

into a single-carrier stream of QPSK or

QAM symbols, which are grouped into

symbol-blocks of length M. This may

be followed by an optional repetition

stage, in which each block is repeated

L times, and a user-specific frequency

shift, by which each user‘s

transmission may be translated to a

particular part of the available

bandwidth. A CP is then inserted.

After filtering (e.g. with a root-raised

cosine pulse-shaping filter), the

resulting signal is transmitted.

The repetition of the symbol

blocks results in the spectrum of the

transmitted signal only being non-zero

at certain subcarrier frequencies

(namely every Lth

subcarrier in this

example) as shown in Fig. 5.

Fig. 5. Distributed transmission with equal-spacing between occupied subcarriers.

Thus, the transmitted signal

spectrum in this case is similar to what

would be obtained if data symbols

were only modulated on every Lth

subcarrier of an OFDM signal. Since

such a signal occupies only one in

every L subcarriers, the transmission is

said to be ‗distributed‘ and is one way

of providing a frequency-diversity

gain.

By varying the block length M

and the repetition factor L, under the

constraint that the total number of

possible occupied subcarriers in the

bandwidth is constant (ML = constant),

a wide range of data rates can be

supported.

When no symbol-block

repetition is performed (L = 1), the

signal occupies consecutive subcarriers

and the transmission is said to be

‗localized‘. Localized transmissions

are beneficial for supporting

frequency-selective scheduling, for

example when the eNodeB has

knowledge of the uplink channel

conditions (e.g. as a result of channel

sounding), or for inter-cell interference

coordination. Localized transmission

may also provide frequency diversity if

the set of consecutive subcarriers is

hopped in the frequency domain,

especially if the time interval between

hops is shorter than the duration of a

block of channel-coded data.

Different users‘ transmissions,

using different repetition factors or

bandwidths, remain orthogonal on the

uplink when the following conditions

are met:

The users occupy different sets of

subcarriers. This may in general be

accomplished either by introducing

a user-specific frequency shift

(typically for the case of localized

transmissions) or alternatively by

arranging for different users to

occupy interleaved sets of

subcarriers (typically for the case

of distributed transmissions). The

latter method as Interleaved

Frequency Division Multiple

Access (IFDMA).

The received signals are properly

synchronized in time and

frequency.

The CP is longer than the sum of

the delay spread of the channel and

any residual timing synchr-

onization error between the users.

The SC-FDMA time-domain generated

signal has a similar level of PAPR as

pulse-shaped single-carrier

modulation. ISI in multipath channels

is prevented by the CP, which enables

efficient equalization at the receiver by

means of a Frequency Domain

Equalizer (FDE).

(iii) F-Domain Signal Generation

(DFT-S-OFDM)

Generation of an SC-FDMA

signal in the frequency domain uses a

Discrete Fourier Transform-Spread

OFDM (DFT-S-OFDM) structure as

shown in Fig. 6.

Fig. 6. SC-FDMA frequency-domain transmission processing (DFT-S-OFDM) showing localized and

distributed subcarrier mappings.

The first step of DFT-S-OFDM

SC-FDMA signal generation is to

perform an M-point DFT operation on

each block of M QAM data symbols.

Zeros are then inserted among the

outputs of the DFT in order to match

the DFT size to an N-subcarrier OFDM

modulator (typically an Inverse Fast

Fourier Transform (IFFT)). The zero-

padded DFT output is mapped to the N

subcarriers, with the positions of the

zeros determining to which subcarriers

the DFT-precoded data is mapped.

Usually N is larger than the

maximum number of occupied

subcarriers, thus providing for efficient

oversampling and ‗sinc‘ (sin(x)/x)

pulse-shaping. The equivalence of

DFT-S-OFDM and a time-domain-

generated SC-FDMA transmission can

readily be seen by considering the case

of M = N, where the DFT operation

cancels the IFFT of the OFDM

modulator resulting in the data

symbols being transmitted serially in

the time domain. However, this

simplistic construction would not

provide any oversampling or pulse-

shape filtering.

As with the time-domain

approach, DFT-S-OFDM is capable of

generating both localized and

distributed transmissions:

• Localized transmission. The

subcarrier mapping allocates a group

of M adjacent subcarriers to a user.

M<N results in zero being appended to

the output of the DFT spreader

resulting in an upsampled/interpolated

version of the original M QAM data

symbols at the IFFT output of the

OFDM modulator. The transmitted

signal is thus similar to a narrowband

single carrier with a CP (equivalent to

time-domain generation with repetition

factor L = 1) and ‗sinc‘ pulse-shaping

filtering (circular filtering).

• Distributed transmission. The

subcarrier mapping allocates M

equally-spaced subcarriers (e.g. every

Lth subcarrier). (L − 1) zeros are

inserted between the M DFT outputs,

and additional zeros are appended to

either side of the DFT output prior to

the IFFT (ML<N). As with the

localized case, the zeros appended on

either side of the DFT output provide

upsampling or sinc interpolation, while

the zeros inserted between the DFT

outputs produce waveform repetition

in the time domain. This results in a

transmitted signal similar to time-

domain IFDMA with repetition factor

L and ‗sinc‘ pulse-shaping filtering.

As for the time-domain SC-

FDMA signal generation,

orthogonality between different users

with different data rate requirements

can be achieved by assigning each user

a unique set of subcarriers. The CP

structure is the same as for the time-

domain signal generation, and

therefore the same efficient FDE

techniques can be employed at the

receiver.

It is worth noting that, in

principle, any unitary matrix can be

used in the place of the DFT for the

spreading operation with similar

performance. However, the use of non-

DFT spreading would result in

increased PAPR since the transmitted

signal would no longer have the single

carrier characteristic.

3- Key technologies of Mobile

WiMAX:

(a) Spectrum, bandwidth options

and duplexing arrangement

The Release 1 WiMAX profiles

cover operation in licensed spectrum

allocations in the 2.3, 2.5, 3.3, and 3.5

GHz bands. The channel bandwidths

supported are 5, 7, 8.75, and 10 MHz.

While WiMAX supports TDD,

FDD, and half-duplex FDD, the first

release only supports TDD operation.

TDD enables adjustment of the

downlink/uplink ratio for asymmetric

traffic, does not require paired

spectrum, and has a less complex

transceiver design. To counter

interference issues, TDD does,

however, require system-wide

synchronization and use of the same

uplink/downlink ratio in neighboring

cells. The reason is the potential for

mobile-to-mobile and base station-to-

base station interference if uplink and

downlink allocations overlap, which

becomes an issue in multi-cell

deployments. Because of adjacent

channel interference, system-wide

synchronization may also be required

for TDD operators deployed on

adjacent or near-adjacent channels.

While the initial profiles and

deployment of WiMAX use TDD as

the preferred mode, FDD may be

introduced in the longer term. The

reason may be local regulatory

requirements or to address need for

more extended multi-cell coverage

where FDD may become more

suitable.

(b) Quality-of-service handling

A connection-oriented Quality-

of-Service (QoS) mechanism is

implemented, enabling end-to end QoS

control. The QoS parameters are set

per service flow, with multiple service

flows possible to/from a mobile

station. The parameters define

transmission ordering and scheduling

on the air interface and can be

negotiated statically or dynamically

through MAC messages.

Applications supported through the

WiMAX QoS mechanism are:

● Unsolicited Grant Service (UGS):

VoIP

● Real-Time Polling Service (rtPS):

Streaming Audio or Video

● Extended Real-Time Polling Service

(ErtPS): Voice with Activity

● Non-Real-Time Polling Service

(nrtPS): File Transfer Protocol (FTP)

● Best-Effort Service (BE): Data

Transfer, Web Browsing, etc.

(c) Mobility

Mobile WiMAX supports both

sleep mode and idle mode for more

efficient power management. In sleep

mode, there is a pre-negotiated period

of absence from the radio interface to

the serving base station, where the

mobile station may power down or

scan other neighboring base stations.

There are different ‗ power saving

classes ‘ suitable for applications with

different QoS types, each having

different sleep mode parameters. There

is also an idle mode, where the

terminal is not registered to any base

station and instead periodically scans

the network at discrete intervals. There

are three handover methods supported,

with Hard Handover (HHO) being

mandatory and Fast Base-Station

Switching (FBSS) and Diversity

handover (MDHO) being optional.

(d) Fractional frequency reuse

WiMAX can operate with a

frequency reuse of one, but co-channel

interference may in this case degrade

the quality for users at the cell edge.

However, a flexible subchannel reuse

is made possible by dividing the frame

into permutation zones as described

above. In this way, it is possible to

have a subchannel reuse by proper

configuration of the subchannel usage

for the users. For users at the cell

edges, the Base Station operates on a

zone with a fraction of the

subchannels, while users close to the

Base Station can operate on a zone

with all subchannels. As shown in the

example in Figure 7, there can be an

effective reuse of not supported

frequencies for users at the cell edge,

while still maintaining a reuse of one

for the OFDMA carrier as a whole.

Fig.7 Fractional frequency reuse

III.PHY LAYER

1- PHY layer for WiMAX (IEEE802.16e based)

(a) System block diagram

Fig.8 The block diagram of the transmitter of the downlink PHY layer of mobile WiMAX with 2-

antennas.

(b) Overview of the PHY layer

blocks

(1) padding one: is used if the data

size from the MAC layer is less

than the frame size according to the

selected modulation scheme and

code rate, so this block pads ones

to reach the frame size.

(2) slot concatenation is used if the

data size from the MAC layer is

larger than the number of data to be

transmitted in one slot, so it divides

the data into blocks, each of them

is with the suitable size that can be

transmitted in one frame.

(3) Randomizer:

Aim: The randomization process

ensures that there is no long runs of

ones or zeros in the input bits. This

will result in:

Decrease the Peak to average

power ratio (PAPR).

Ensure the clock synchronization at

the receiver as the transition

between bit values helps the

receiver in synchronization.

If we have long runs of ones the

power of the signal will be

decreases until the threshold and

hence error happened due to Gibbs

phenomena.

This can be achieved by:

The randomization process is

carried out using pseudo random

binary generator (PRBG), as the output

of PRBG is used as the input to an

XOR Gate and the second input is the

block of data to be transmitted. Only

source bits are randomized .Elements

that are not a part of the source data,

such as framing elements and pilot

symbols shall not be randomized.The

LFSR shall be preset at the beginning

of each frame to the value

100101010000000 and shall be

clocked once per processed bit.

The Derandomizer It has the

same construction of the Randomizer,

as the data has a XOR operation with

the output of PRPG that has a linear

feedback shift register (LFSR) has the

same seed value of the Randomizer

used at the Transmitter

(4) Channel Coding:

The OFDMA PHY supports

Mandatory tail-biting

Convolutional Coding, The

convolutional encoder uses a

constituent encoder with a

constraintlength 7 and a native

code rate 1/2 The 6 bits from

the end of the data block are

appended to the beginning, to

be used as flush bits. These

appended bits flush out the bits

left in the encoder by the

previous FEC block. The first

12 parity bits that are generated

by the convolutional encoder

which depend on the 6 bits left

in the encoder by the previous

FEC block are discarded

and four optional coding

schemes: Zero Tailing

Convolutional code,

Convolutional Turbo

code(CTC) along with H-ARQ,

and Block Turbo code(BTC)

and low density parity check

(LDPC) codes

The most popular optional

channel coding scheme is

(CTC) WiMAX uses duobinary

turbo codes with a constituent

recursive encoder of constraint

length 4. In duo binary turbo

codes two consecutive bits

from the uncoded bit sequence

are sent to the encoder

simultaneously, have been

defined in WIMAX as optional

channel coding schemes but are

unlikely to be implemented in

fixed or mobile WiMAX.

(5) Puncturing: In order to achieve

code rates higher than 1/2, the

output of the encoder is punctured,

using a specified puncturing

pattern

(6) Interleaving: The interleaver is

defined by a two step permutation:

The first step ensures that the

adjacent coded bits are mapped

onto nonadjacent subcarriers,

which provides frequency

diversity and improves the

performance of the decoder.

The second step ensures that

adjacent bits are alternately

mapped to less and more

significant bits of the

modulation constellation, thus

avoiding long runs of lowly

reliable bits., The interleaver

indices are determined using

following equations

(7) Symbol Mapping: Mobile

WiMAX supports QPSK, 16QAM

and 64QAM in DL, but In the UL,

64QAM is optional in gray coded

scheme. Each modulation

constellation is scaled such that the

average transmitted power is unity,

assuming that all symbols are

equally likely. The symbols are

further multiplied by a

pseudorandom unitary number to

provide additional layer 1

encryption. Preamble and

midamble symbols are further

scaled by 2 2 which allowsboost

in the power and allows for more

accurate synchronization and

various parameter estimations,

such as channel response and noise

variance.

(8) OFDMA:

(i) OFDM Symbol Structure

The flexibility of the WiMAX

PHY layer allows one to make an

optimum choice of various PHY

layer parameters, such as cyclic

prefix length, number of

subcarriers, subcarrier separation,

and preamble interval, such that the

performance degradation owing to

ICI and ISI (intersymbol

interference) is minimal without

compromising the performance.

The four primitive parameters that

describe an OFDM symbol, and

their respective values in IEEE

802.16e-2005, are shown in Table

2.

Table 1 Primitive Parameters for OFDM Symbol

The OFDMA symbol structure consists

of three types of sub-carriers:

1. Data subcarriers are used for

carrying data symbols.

2. Pilot subcarriers are used for

carrying pilot symbols. The pilot

symbols are known a priori and can be

used for channel estimation and

channel tracking.

3. Null subcarriers have no power

allocated to them, including the DC

subcarrier and the guard subcarriers

toward the edge. The DC subcarrier is

not modulated, to prevent any

saturation effects or excess power draw

at the amplifier. No power is allocated

to the guard subcarrier toward the edge

of the spectrum in order to fit the

spectrum, of the OFDM symbol within

the allocated bandwidth and thus

reduce the interference between

adjacent channels.

The power in the pilot subcarriers, as

shown here, is boosted by 2.5 dB,

allowing reliable channel tracking even

at low-SNR conditions.

(ii) Scalable OFDMA

The IEEE 802.16e Wireless

MAN OFDMA mode is based on the

concept of scalable OFDMA (S-

OFDMA). S-OFDMA supports a wide

range of bandwidths to flexibly address

the need for various spectrum

allocation and usage model

requirements. The scalability is

supported by adjusting the FFT size

while fixing the sub-carrier frequency

spacing at 10.94 kHz. Since the

resource unit sub-carrier bandwidth

and symbol duration is fixed, the

impact to higher layers is minimal

when scaling the bandwidth. The

system bandwidths for the initial

planned profiles being developed by

the WiMAX Forum Technical

Working Group for Release-1 are 5

and 10 MHz.

Table2 OFDMA Scalability Parameters

(9) Subchannelization & subcarrier

permutation: In order to create the

OFDM symbol in the frequency

domain, the modulated symbols are

mapped on to the subchannels that

have been allocated for the

transmission of the data block.

A subchannel is a logical

collection of subcarriers. The

number and exact distribution of

the subcarriers that constitute a

subchannel depend on the

subcarrier permutation mode. The

number of subchannels allocated

for transmitting a data block

depends on various parameters,

such as the size of the data block,

the modulation format, and the

coding rate. In the time and

frequency domains, the contiguous

set of subchannels allocated to a

single user—or a group of users, in

case of multicast—is referred to as

the data region of the user(s) and is

always transmitted using the same

burst profile. A burst profile refers

to the combination of the chosen

modulation format, code rate, and

type of FEC.

Four subcarrier permutation are

applied:

FUSC: Each slot is 48 subcarriers

by one OFDM symbol.

Downlink PUSC: Each slot is 24

subcarriers by two OFDM

symbols.

Uplink PUSC and TUSC: Each slot

is 16 subcarriers by three OFDM

symbols.

Band AMC: Each slot is 8, 16, or

24 subcarriers by 6, 3, or 2 OFDM

symbols.

(i) DL Full Usage of Subcarriers

All data subcarriers are used to

create various subchannels.

Each subchannel is made up of 48

data subcarriers.

The pilot subcarriers are allocated

first then the data subcarriers are

mapped using permutation scheme.

Set of pilot subcarriers is divided

into 2 constant sets and 2 variable

sets.

Variable set allows receiver to

estimate channel response across

the entire frequency band.

When transmit diversity of 2, for

example, is used, each antenna uses

half of number of pilots.

Table3 Parameters of DL FUSC Permutation

(ii) Downlink Partial Usage of

Subcarriers

All subcarriers are divided into 6

groups.

All subcarriers (except null

subcarriers) are arranged into

clusters.

Cluster = 14 adjacent subcarriers

×2 OFDM symbols.

Cluster = 24 data subcarriers + 4

pilot subcarriers.

The clusters are then renumbered.

The clusters are then divided into 6

groups.

A subchannel is formed using 2

clusters from the same group.

(iii) Uplink Partial Usage of

Subcarriers

Subcarriers are divided into tiles.

Tile = 4 subcarriers ×3 OFDM

symbols.

Subcarrier = 8 data subcarriers + 4

pilot subcarriers.

Tiles are renumbered and divided

into 6 groups.

Subchannel = 6 tiles from a single

group

A special case from the UL PUSC

is Uplink Optional Partial Usage of

Subcarriers (OPUSC) where:

Tile = 3 subcarriers ×3 OFDM

symbols.

Subcarrier = 8 data subcarriers + 1

pilot subcarrier.

(iv) Band Adaptive Modulation and

Coding

Unique to the band AMC permutation

mode, all subcarriers constituting a

subchannel are adjacent to each other.

Although frequency diversity is lost to

a large extent with this subcarrier

permutation scheme, exploitation of

multiuser diversity is easier. Multiuser

diversity provides significant

improvement in overall system

capacity and throughput, since a

subchannel at any given time is

allocated to the user with the highest

SNR/capacity in that subchannel. Nine

adjacent subcarriers with eight data

subcarriers and one pilot subcarrier are

used to form a bin.

Four adjacent bins in the frequency

domain constitute a band.

An AMC subchannel consists of

six contiguous bins from within the

same band.

An AMC subchannel can consist of

1 bin × 6 consecutive symbols, 2

bins ×3 symbols, or 3 bins ×2

consecutive symbols.

(10) Channel estimation and

equalization form an estimate of the

amplitude and phase shift caused by

the wireless channel from the available

pilot information. The equalization

removes the effect of the wireless

channel and allows subsequent symbol

demodulation.

In WiMAX the reference

design estimates the channel frequency

response using linear interpolation in

time and frequency on a tile-by-tile

basis for each subchannel.

The pilot structure is also

outlined by Fig (6.31). In the first and

third OFDMA symbol, the outer

carriers of each tile are pilot

subcarriers, and so it is possible to

make an estimate of the channel

response at these frequencies by

comparison with the known reference

pilot subcarrier. The frequency

response of the two inner subcarriers

may be estimated by linear

interpolation in the frequency domain.

When the data and pilot information

has been assembled as shown in Fig

(6.31), it is possible to calculate

ℎ11 , ℎ14 , ℎ31, ℎ34 using the equation

ℎ𝑝 𝑡, 𝑘 =𝑟𝑝(𝑡, 𝑘)

𝑠𝑝(𝑡, 𝑘)

for the tile t of OFDMA symbol k

where:

𝑟𝑝 𝑡, 𝑘 is the pth received pilot

subcarrier

𝑠𝑝 𝑡, 𝑘 is the pth transmitted pilot

subcarrier

Subsequently, frequency domain linear

interpolation is performed to calculate

channel estimates using the following

equations:

ℎ 12 =1

3 ℎ14 − ℎ11 + ℎ11

,ℎ 13 =2

3 ℎ14 − ℎ11 + ℎ11

ℎ 32 =1

3 ℎ34 − ℎ31 + ℎ31,

ℎ 32 =2

3 ℎ34 − ℎ31 + ℎ31

Finally, time domain linear

interpolation is achieved as follows:

ℎ 21 =1

3 ℎ11 + ℎ31

,ℎ 22 =1

3 ℎ12 + ℎ32

ℎ 21 =1

3 ℎ13 + ℎ33 ,

ℎ 22 =1

3 ℎ14 + ℎ34

When all of the channel estimates

have been formed,a single-tap zero

forcing equalizer removes the channel

distortion by dividing the received

signal by the estimated channel

frequency response. Only a single-tap

equalizer is required, as the time

dispersion of the channel has been

removed by the use of OFDM and the

addition of a cyclic prefix.

(11) Advanced and Multiple antenna

support: there several advanced

multiple antenna techniques supported

in the IEEE 802.16 standard including

adaptive antenna systems (AAS), space

time coding(STC), multiple input

multiple output (MIMO) to provide

significant improvement in the overall

system capacity and spectral efficiency

of the network

There are 2 modes:

(1) open-loop mode the transmitter

does not know the CSI

(2) closed-loop mode, the

transmitter knows the CSI,

either due to channel

reciprocity, in case of TDD, or

to explicit feedback from the

receiver, in the case of FDD.

(a) Open loop mode:

(1) Space time coding(Alamouti

2×2)

For 2x2 Alamouti case, we have two

transmit antennas, Ant0 and Ant1. At a

given time instant, t, the transmitted

sympols are S0, S1 respectively. At

instant t+T, where T is the sympol

duration, the transmitted signals are –

S1*, S0

* . The received signals are as

follow:

𝑟0 = ℎ0𝑆0 + ℎ1𝑆1 + 𝑛0

𝑟1 = −ℎ0𝑆1∗ + ℎ1𝑆0

∗ + 𝑛1

𝑟2 = ℎ2𝑆0 + ℎ3𝑆1 + 𝑛2

𝑟3 = −ℎ2𝑆1∗ + ℎ3𝑆0

∗ + 𝑛3

where r0 is the received signal at Ant0

at time t, r1 is the received signal at

Ant0 at time t+T, r2 is the received

signal at Ant1 at time t, r3 is the

received signal at Ant1 at time t+T.

To decode, the combiner builds the

signals for S0, S1

ŝ0 = (𝛼02 + 𝛼1

2 + 𝛼22 + 𝛼3

2 − 1)𝑠0

+ ℎ0∗𝑛0 + ℎ1𝑛1

∗ + ℎ2∗𝑛2

+ ℎ3𝑛3∗

ŝ1 = (𝛼02 + 𝛼1

2 + 𝛼22 + 𝛼3

2 − 1)𝑠1

− ℎ0∗𝑛1 + ℎ1𝑛0

∗ − ℎ2∗𝑛3

+ ℎ3𝑛2∗

Then a Maximum Likelihood detector

searches for Si that minimizes:

(𝛼02 + 𝛼1

2 + 𝛼22 + 𝛼3

2 − 1) 𝑠0,1 2

+

𝑑2(ŝ0,1, 𝑠𝑖) for both S0, S1.

(1) Spatial multiplexing (SM)

Multiplex a data stream into several

branches and transmit via several

independent channels overlapping in

time and frequency.

SM Transmission (using V-BLAST

algorithm)

We consider a V-BLAST system with

2 transmit antennas and 2 receive

antennas.

At the transmitter, bit stream is

modulated then demultiplexed into 2

substreams, and each substream is sent

to its respective transmit antennas.

At the receiver, after estimating the

channel parametrs, the received signal

and and channel parameters are sent to

V-BLAST signal processing decoder,

which performs ordered successive

cancellation, taking the following

steps:

Ordering: selects the data stream

with the highest signal to interference

ratio

Nulling: remove the effect of other

streams by multipling the received

signal by zeroing weights

Slicing: quantize the output to get

the received symbol.

(2) Frequency-Hopping Diversity

Code

WiMAX also defines an optional

transmit diversity mode, known as the

frequency-hopping diversity code

(FHDC), using two antennas in which

the encoding is done in the space and

frequency domain, as shown in Figure

8 rather than the space and time

domain. In FHDC, the first antenna

transmits the OFDM symbols without

any encoding, much like a single-

antenna transmission, and the second

antenna transmits the OFDM symbol

by encoding it over two consecutive

subchannels, using the 2 × 2 Alamouti

encoding matrix

(b) Closed loop mode:

The various transmit diversity and

spatial-multiplexing schemes of IEEE

802.16 described in the previous

section do not require the transmitter to

know the CSI for the receiver of

interest. MIMO and diversity schemes

can benefit significantly if the CSI is

known at the transmitter. CSI

information at the transmitter can be

used to select the appropriate MIMO

mode number of transmit antennas,

number of simultaneous streams, and

space/time encoding matrix as well as

to calculate an optimum precoding

matrix that maximizes system capacity.

The CSI can be known at the

transmitter due to channel reciprocity,

in the case of TDD, or by having a

feedback channel, in the case of FDD.

The uplink bandwidth required to

provide the full CSI to the transmitter

the MIMO channel matrix for each

subcarrier in a multiuser FDD MIMO-

OFDM system is too large and thus

impractical for a closed-loop FDD

MIMO system. For practical systems,

it is possible only to send some form of

quantized information in the uplink.

The framework for closed-loop MIMO

in IEEE 802.16, as shown in consists

of a space/time encoding stage

identical to an open-loop system and a

MIMO precoding stage. The MIMO

precoding matrix in general is a

complex matrix, with the number of

rows equal to the number of transmit

antennas and the number of columns

equal to the output of the space/time

encoding block. The linear precoding

matrix spatially mixes the various

parallel streams among the various

antennas, with appropriate amplitude

and phase adjustment

.

Closed-loop MIMO framework in

IEEE 802.16In order to determine the

appropriate amplitude and phases of

the various weights, the transmitter

requires some feedback from the MS.

In the case of closed-loop MIMO, the

feedback falls broadly into two

categories: long-term feedback and

short-term feedback. The long-term

feedback provides information related

to the maximum number of parallel

streams: the rank of the precoding

matrix to be used for DL

transmissions. The short-term feedback

provides information about the

precoding matrix weights to be used.

The IEEE 802.16 standard defines the

following five mechanisms so that the

BS can estimate the optimum

precoding matrix for closed-loop

MIMO operations:

1. Antenna selection. The MS

indicates to the BS which

transmit antenna(s) should be

used for transmission in order

to maximize the channel

capacity and/or improve the

link reliability.

2. Antenna grouping. The MS

indicates to the BS the

optimum permutation of the

order of the various antennas to

be used with the current

space/time encoding matrix.

3. Codebook based feedback.

The MS indicates to the BS the

optimum precoding matrix to

be used, based on the entries of

a predefined codebook.

4. Quantized channel feedback.

The MS quantizes the MIMO

channel and sends this

information to the BS, using

the MIMO_FEEDBACK

message. The BS can use the

quantized MIMO channel to

calculate an optimum

precoding matrix.

5. Channel sounding. The BS

obtains exact information about

the CSI of the MS by using a

dedicated and predetermined

signal intended for channel

sounding.

(c) AAS support in IEEE Std 802.16

Through the AAS options, the IEEE

802.16 standard supports the use of

smart antennas to perform beam

forming. Beam forming can effectively

create a narrower signal beam,

resulting in increased gain and,

therefore, higher range. This in turn

increases capacity by increasing the

range at which a particular PHY burst

profile can be received. AAS also

allows for the suppression of noise

sources, improving the SNR at the

receiver, and discrimination on the

AoD allows energy to be concentrated

in the direction of the intended

recipient, enabling large cell ranges. In

addition, nulls can be steered in

particular directions, enhancing the

interference resistance of the system.

Drawbacks of these approaches

include the increased system

complexity and the inability to

broadcast messages, reducing the

spectral efficiency due to repetition of

broadcast MAC messages to the

various recipients.

(12) Mobile WiMAX TDD Frame

Structure: The 802.16e PHY supports

TDD and Full and Half-Duplex FDD

operation, however the initial release

of Mobile WiMAX certification

profiles will only include TDD. With

ongoing releases, FDD profiles will be

considered by the WiMAX Forum to

address specific market opportunities

where local spectrum regulatory

requirements either prohibit TDD or

are more suitable for FDD

deployments.

To counter interference issues,

TDD does require system-wide

synchronization; and TDD is the

preferred duplexing mode for the

following reasons:

TDD enables adjustment of the

downlink/uplink ratio to efficiently

support asymmetric

downlink/uplink traffic, while with

FDD, downlink and uplink always

have fixed and generally, equal DL

and UL bandwidths.

TDD assures channel reciprocity

for better support of link

adaptation, MIMO and other closed

loop advanced antenna

technologies.

Unlike FDD, which requires a pair

of channels, TDD only requires a

single channel or both downlink

and uplink providing greater

flexibility for adaptation to varied

global spectrum allocations.

Transceiver designs for TDD

implementations are less complex

and therefore less expensive.

BASICS OF OFDMA FRAME

STRUCTURE:

There are three types of OFDMA

subcarriers:

1. Data subcarriers for data

transmission.

2. Pilot subcarriers for various

estimation and synchronization

purposes.

3. Null subcarriers for no transmission

at all, used for guard bands and DC

carriers.

Active subcarriers are divided into

subsets of subcarriers called

subchannels. The subcarriers forming

one subchannel may be, but need not

be, adjacent. The pilot allocation is

performed differently in different

subcarrier allocation modes.

Fig illustrates the OFDMA frame

structure for a Time Division Duplex

(TDD) implementation. Each frame is

divided into DL and UL sub-frames

separated by Transmit/Receive and

Receive/Transmit Transition Gaps

(TTG and RTG, respectively) to

prevent DL and UL transmission

collisions. The downlink-to-uplink-

subframe ratio may be varied from 3:1

to 1:1 to support different traffic

profiles.

The relevant information about the

starting position and the duration of the

various zones being used in a UL and

DL subframe is provided by control

messages in the beginning of each DL

subframe.

In a frame, the following control

information is used to ensure optimal

system operation:

Preamble: The preamble is the

first OFDM symbol of the frame.

The preamble can be used for a

variety of PHY layer procedures,

such as time and frequency

synchronization, initial channel

estimation, and noise and interference

estimation. To create the preamble in

frequency domain, BPSK modulation

is used.

Frame Control Header (FCH):

The FCH follows the preamble. It

provides the frame configuration

information such as MAP

message length and the

modulation and coding scheme

and usable sub-channels.

DL-MAP and UL-MAP: The

DL-MAP and UL-MAP provide

subchannel allocation and

Multiple users data regions within

the frame and other control

information for the DL and UL

sub-frames respectively.

Since MAP contains critical

information that needs to reach all

users, it is often sent over a very

reliable link, such as BPSK with rate

1/2 coding and repetition coding.

The BS also transmits the downlink

channel descriptor (DCD) and the

uplink channel descriptor (UCD)

following the UL-MAP message,

which contains additional control

information pertaining to the

description of channel structure and the

various burst profiles that are allowed

within the given BS.

In order to conserve resources, the

DCD and the UCD are not transmitted

every DL frame.

UL Ranging: The UL ranging

sub-channel is allocated for

mobile stations (MS) to perform

closed-loop time, frequency, and

power adjustment as well as

bandwidth requests.

UL CQICH: The UL CQICH

channel is allocated for the MS to

feedback channel state

information.

UL ACK: The UL ACK is

allocated for the MS to feedback

DL HARQ ( Hybrid Automatic

Repeat Request ) acknowledge.

Burst Regions is used as Data

regions from different users each

burst has the same modulation

and code rate for all users that are

included in this burst.

TTG & RTG : Transmit/Receive

and Receive/Transmit Transition

Gaps .

Frame duration is almost 5 ms (it

is variable from 2 ms to 20 ms).

Each frame has 47 OFDM

symbols each symbol duration is 102.9

µs

2- 3GPP LTE PHY layer: Downlink Physical layer:

(1) CRC insertion:

In the first step of the transport-

channel processing, a 24-bit CRC is

calculatedfor and appended to each

transport block. The CRC allows for

receiverside detection of errors in the

decoded transport block. The

corresponding error indication is then,

for example, used by the downlink

hybrid-ARQ protocol as a trigger for

requesting retransmissions.

(2) Code-block segmentation and

per-code-block CRC insertion: The

LTE Turbo-coder internal interleaver

is only defined for a limited number of

code-block sizes with a maximum

block size of 6144 bits. In case the

transport block, including the

transport-block CRC, exceeds this

maximum code-block size, code-block

segmentation as illustrated in fig. is

applied before Turbo coding. Code-

block segmentation implies that the

transport block is segmented into

smaller code blocks that match the set

of code-block sizes defined for the

Turbo coder. In order to ensure that the

size of each code block is matched to

the set of available code-block sizes,

filler bits may have to be inserted at

the head of the first code block. Note

that filler bits may be needed also if

there is no actual codeblock

segmentation, that is if the transport-

block size does not exceed the

maximum code-block size. 15 As can

be seen in Figure 16.27 code-block

segmentation also implies that an

additional (24 bits) CRC is calculated

for and appended to each code block.

16 Having a CRC per code block

allows for early detection of correctly

decoded code blocks and

corresponding early termination of the

iterative decoding of that code block.

This can be used to reduce the terminal

processing effort and power

consumption. It should be noted that,

in case of no code-block segmentation,

that is in case of a single code block,

no additional code-block CRC is

applied

(3) Turbo coding

The overall structure of the LTE Turbo

encoding is illustrated in Figure The

Turbo encoding reuses the two

WCDMA/HSPA rate-1/2, eight-state

constituent encoders, implying an

overall code rate of 1/3. However, the

WCDMA/ HSPA Turbo encoder

internal interleaver has, for LTE, been

replaced by QPPbased17 interleaving ,

the QPP interleaver provides a

mapping from the input (non-

interleaved) bits to the output

(interleaved) bits according to the

function:

where i is the index of the bit at the

output of the interleaver, c(i) is the

index of the same bit at the input of the

interleaver, and K is the code-

block/interleaver size. The values of

the parameters f1 and f2 depend on the

code-block size K . The LTE

specification lists all supported code-

block sizes, ranging from a minimum

of 40 bits to a maximum of 6144 bits,

together with the associated values for

the parameters f1 and f2 .

(4) Rate-matching and physical-

layer hybrid-ARQ functionality

The task of the rate-matching and

physical-layer hybrid-ARQ

functionality is to extract, from the

blocks of code bits delivered by the

channel encoder, the exact set of bits to

be transmitted within a given TTI. As

illustrated in Figure 16.30 , the outputs

of the Turbo encoder (systematic

bits,first parity bits, and second parity

bits) are first separately interleaved.

The interleaved bits are then inserted

into what can be described as a circular

buffer with the systematic bits inserted

first, followed by alternating insertion

of the first and second parity bits. The

bit selection then extracts consecutive

bits from the circular buffer to the

extent that fits into the assigned

resource. The set of bits to extract

depends on the redundancy version

corresponding to different starting

points for the extraction of coded bits

from the circular buffer. As can be

seen, there are four different

alternatives for the redundancy

version.

(5) Bit-level scrambling

LTE downlink scrambling implies that

the block of code bits delivered by the

hybrid-ARQ functionality is multiplied

( exclusive-or operation) by a bit-level

scrambling sequence . In general,

scrambling of the coded data helps to

ensure that the receiver-side decoding

can fully utilize the processing gain

provided by the channel code. Without

downlink scrambling, the channel

decoder at the mobile terminal could,

at least in principle, be equally

matched to an interfering signal as to

the target signal, thus not being able to

properly suppress the interference. By

applying different scrambling

sequences for neighbor cells, the

interfering signal(s) after descrambling

are randomized, ensuring full

utilization of the processing gain

provided by the channel code

(6) modulation

The downlink data modulation

transforms the block of scrambled bits

to a corresponding block of complex

modulation symbols. The set of

modulation schemes supported for the

LTE downlink includes QPSK,

16QAM, and 64QAM, corresponding

to two, four, and six bits per

modulation symbol, respectively. All

these modulation schemes are

applicable to the DL-SCH, PCH, and

MCH transport channels. As will be

described in Chapter 18, only QPSK

modulation can be applied to the BCH

transport channel.

(7) Antenna mapping

The Antenna Mapping jointly

processes the modulation symbols

corresponding to, in the general case,

two transport blocks, and maps the

result to the different antenna ports

(8) Resource-block mapping

The resource-block mapping maps the

symbols to be transmitted on each

antenna port to the resource elements

of the set of resource blocks assigned

by the MAC scheduler for

transmission of the transport block(s)

to the terminal. Each resource block

consists of 84 resource elements (12

subcarriers during 7 OFDM symbols)

when deciding what set of resource

blocks to use for transmission to a

specific terminal, the network may

take the downlink channel conditions

in both the time and frequency domain

into account. Such time/

frequency-domain channel-dependent

scheduling, taking channel variations

However, in some cases downlink

channel-dependent scheduling is not

suitable an alternative means to handle

radio-channel frequency selectivity is

to achieve frequency diversity by

distributing a downlink transmission in

the frequency domain. In order to

provide the possibility for distributed

resource-block allocation in case of

resource allocation type 2, as well as to

allow for distributing the transmission

of a single resource-block pair in the

frequency domain, the notion of a

Virtual Resource Block (VRB) has

been introduced for LTE. What is

being provided in the resource

allocation is the resource allocation in

terms of VRB pairs. The key to

distributed transmission then lies in the

mapping from VRB pairs to Physical

Resource Block (PRB) pairs, that is, to

the actual physical resource used for

transmission. The LTE specification

defines two types of VRBs: localized

VRBs and

distributed VRBs.

In case of localized VRBs, there is a

direct mapping from VRB pairs to

PRB pairs as illustrated in Figure.

However, in case of distributed VRBs,

the mapping from VRB pairs to PRB

pairs is more elaborate in the sense that

Consecutive VRBs are not mapped

to PRBs that are consecutive in the

frequency domain,

even a single VRB pair is

distributed in the frequency

domain.The basic principle of

distributed transmission consists of

two steps:

o A mapping from VRB pairs to

PRB pairs such that consecutive

VRB pairs are not mapped to

frequency-consecutive PRB

pairs This provides frequency

diversity between consecutive

VRB pairs. The spreading in the

frequency domain is done by

means of a block-based ‗

interleaver ‘ operating on

resource-block pairs.

o A split of each resource-block

pair such that the two resource

blocks of the resource-block pair

are transmitted with a certain

frequency gap in between

(second step of Figure 16.32 ).

This provides frequency

diversity also for a single VRB

pair. This step can be seen as the

introduction of frequency

hopping on a slot basis.

(9) Multi-antenna transmission

LTE supports the following multi-

antenna transmission schemes or

transmission modes , in addition to

single-antenna transmission:

● Transmit diversity

● Closed-loop spatial multiplexing

including codebook-based beam-

forming

● Open-loop spatial multiplexing

Transmit diversity

In case of two antenna ports, LTE

transmit diversity is based on Space

Frequency Block Coding (SFBC). As

can be seen from Figure 16.33 , SFBC

implies that consecutive modulation

symbols 1 are mapped directly on

adjacent subcarriers on the first

antenna port. On the second antenna

port, the swapped and transformed

symbols are transmitted

on the corresponding subcarriers. In

case of four antenna ports, LTE

transmit diversity is based on a

combination of SFBC and Frequency

Shift Transmit Diversity (FSTD). As

can be seen in combined SFBD/FSTD

implies that pairs of modulation

symbols are transmitted by means of

SFBC with transmission alternating

between pairs of antenna ports

(antenna ports 0 and 2 and antenna

ports 1 and 3, respectively).

Closed loop Spatial multiplexing

As described in Chapter 6, spatial

multiplexing implies that multiple

streams or ‗ layers ‘ are transmitted in

parallel, thereby allowing for higher

data rates within a given bandwidth.

LTE spatial multiplexing allows for the

transmission of a variable number of

layers, up to a maximum of NA layers,

where NA is the number of antenna

ports. The LTE spatial multiplexing

may operate in two different modes:

closed-loop spatial multiplexing and

open-loop spatial multiplexing where

closed-loop spatial multiplexing relies

on more extensive feedback from the

mobile terminal.

One or two codewords, corresponding

to one or two transport blocks, are

mapped to the NL layers. The number

of NL layers may range from a

minimum of one layer up to a

maximum number of layers equal to

the number of antenna ports After

layer mapping, a set of NL symbols

(one symbol from each layer) is

linearly combined and mapped to the

NA antenna ports. This

combining/mapping can be described

by means of a pre-coder matrix W of

size NA *NL

As LTE supports multi-antenna

transmission using two or four antenna

ports, pre-coding matrices are defined

for:

two antenna ports ( NA = 2) and one

and two layers, corresponding to

precoder matrices of size 2×1 and 2×2,

respectively; four antenna ports ( NA =

4) and one, two, three, and four layers,

corresponding to pre-coder matrices of

size 4×1, 4×2, 4×3, and 4×4,

respectively.

Open loop SM

LTE also supports open-loop spatial

multiplexing , also sometimes referred

to as large-delay CDD

The structure of large-delay CDD is

illustrated in Figure 16.37 . As can be

seen, the overall pre-coding

functionality can in this case be seen as

a combination of two pre-coder

matrices, a matrix P of size NL×NL

and a matrix W of size NA ×NL . The

matrix P in Figure 16.37 can be

expressed as a product of two matrices

P =U. D , where U is a constant matrix

of size NL×NL and D ( i ) is matrix of

size NL×NL that varies between

subcarriers. As an example, the

matrices U and D ( i ) for the case of

two layers ( NL ×2) are given by:

The basic idea with the matrix P , that

is the ‗ large-delay CDD ‘ part of the

open-loop spatial multiplexing, is to

average out any differences in the

channel conditions as seen by the

different layers.

General beam-forming

As described above, closed-loop

spatial multiplexing includes beam-

forming as a special case when the

number of layers NL equals one. This

kind of beamforming can be referred to

as codebook-based beam-forming.

SC-FDMA Design in LTE

Transmit Processing for LTE

Although the frequency-domain

generation of SC-FDMA (DFT-S-

OFDM) is functionally equivalent to

the time-domain SC-FDMA signal

generation, each technique requires a

slightly different parameterization for

efficient signal generation. The pulse-

shaping filter used in the time domain

SC-FDMA generation approach in

practice has a non-zero excess

bandwidth, resulting in bandwidth

efficiency which is smaller than that

achievable with the frequency domain

method with its inherent ‗sinc‘ (zero

excess bandwidth) pulse-shaping filter

which arises from the zero padding and

IFFT operation.

For example, for a 5 MHz operating

bandwidth, physical layer parameters

optimized for time-domain

implementation might have a sampling

rate of 4.096 Mps (256 subcarriers

with 16 kHz subcarrier spacing)

resulting in bandwidth efficiency of

82%. An equivalent set of parameters

optimized for the frequency-domain

generation can support a bandwidth

efficiency of 90% (with 300 occupied

subcarriers and 15 kHz subcarrier

spacing). Thus, with frequency-domain

processing, a 10% increase in

bandwidth efficiency can be achieved,

allowing higher data rates.

The non-zero excess bandwidth pulse-

shaping filter in the time-domain

generation also requires ramp-up and

ramp-down times of 3–4 samples

duration, while for DFT-S-OFDM

there is no explicit pulse-shaping filter,

resulting in a much shorter ramp time

similar to OFDM. However, the pulse-

shaping filter in the time-domain

generation does provide the benefit of

reduced CM by approximately 0.25–

0.5 dB compared to DFT-S-OFDM, as

shown in Fig. ?. Thus there is a trade-

off between bandwidth efficiency and

CM/PAPR reduction between the time-

and frequency-domain SC-FDMA

generation methods.

Frequency-domain signal generation

for the LTE uplink has a further benefit

in that it allows a very similar

parameterization to be adopted as for

the OFDM downlink, including the

same subcarrier spacing, number of

occupied subcarriers in a given

bandwidth, and CP lengths. This

provides maximal commonality

between uplink and downlink,

including for example the same clock

frequency.

For these reasons, the SC-FDMA

parameters chosen for the LTE uplink

have been optimized under the

assumption of frequency-domain DFT-

S-OFDM signal generation.

An important feature of the LTE SC-

FDMA parameterization is that the

numbers of subcarriers which can be

allocated to a UE for transmission are

restricted such that the DFT size in

LTE can be constructed from multiples

of 2, 3 and/or 5. This enables efficient,

low-complexity mixed-radix FFT

implementations.

SC-FDMA Parameters for LTE

The same basic transmission resource

structure is used for the uplink as for

the downlink: a 10 ms radio frame is

divided into ten 1 ms subframes each

consisting of two 0.5 ms slots. As LTE

SC-FDMA is based on the same

fundamental processing as OFDM, it

uses the same 15 kHz subcarrier

spacing as the downlink. The uplink

transmission resources are also defined

in the frequency domain (i.e. before

the IFFT), with the smallest unit of

resource being a Resource Element

(RE), consisting of one SC-FDMA

data block length on one subcarrier. As

in the downlink, a Resource Block

(RB) comprises 12 REs in the

frequency domain for a duration of 1

slot, as detailed in Section 6.2. The

LTE uplink SCFDMA physical layer

parameters for Frequency Division

Duplex (FDD) and Time Division

Duplex (TDD) deployments are

detailed in Table 6.

Table 6. LTE uplink SC-FDMA physical layer parameters

Two CP durations are supported – a

normal CP of duration 4.69 μs and an

extended CP of 16.67 μs, as in the

downlink. The extended CP is

beneficial for deployments with large

channel delay-spread characteristics,

and for large cells.

The 1 ms subframe allows a 1 ms

scheduling interval (or Transmission

Time Interval (TTI)), as for the

downlink, to enable low latency.

However, one difference from the

downlink is that the uplink coverage is

more likely to be limited by the

maximum transmission power of the

UE. In some situations, this may mean

that a single Voice-over-IP (VoIP)

packet, for example, cannot be

transmitted in a 1 ms subframe with an

acceptable error rate. One solution to

this is to segment the VoIP packet at

higher layers to allow it to be

transmitted over several subframes.

However, such segmentation results in

additional signaling overhead for each

segment (including resource allocation

signaling and Hybrid ARQ

acknowledgement signaling). A more

efficient technique for improving

uplink VoIP coverage at the cell edge

is to use so-called TTI bundling, where

a single transport block from the MAC

layer is transmitted repeatedly in

multiple consecutive subframes, with

only one set of signaling messages for

the whole transmission. The LTE

uplink allows groups of 4 TTIs to be

‗bundled‘ in this way, in addition to

the normal 1 ms TTI.

In practice in LTE, all the uplink data

transmissions are localized, using

contiguous blocks of subcarriers. This

simplifies the transmission scheme,

and enables the same RB structure to

be used as in the downlink. Frequency-

diversity can still be exploited by

means of frequency hopping, which

can occur both within one subframe (at

the boundary between the two slots)

and between subframes. In the case of

frequency hopping within a subframe,

the channel coding spans the two

transmission frequencies, and therefore

the frequency diversity gain is

maximized through the channel

decoding process. The only instance of

distributed transmission in the LTE

uplink (using an IFDMA-like

structure) is for the ‗Sounding

Reference Signals‘ (SRSs) which are

transmitted to enable the eNodeB to

perform uplink frequency-selective

scheduling.

Like the downlink, the LTE uplink

supports scalable system bandwidths

from approximately 1.4 MHz up to 20

MHz with the same subcarrier spacing

and symbol duration for all

bandwidths. The uplink scaling for the

bandwidths supported in the first

release of LTE is shown in Table 7.

Note that the sampling rates resulting

from the indicated FFT sizes are

designed to be small rational multiples

of the UMTS 3.84 MHz chip rate, for

ease of implementation in a multimode

UE.

Note that in the OFDM downlink

parameter specification, the d.c.

subcarrier is unused. In contrast, no

unused d.c. subcarrier is possible for

SC-FDMA as it can affect the low

CM/PAPR property of the transmit

signal.

Table 7. LTE Uplink SC-FDMA parametrization for selected carrier bandwidths.

d.c. Subcarrier in SC-FDMA

Direct conversion transmitters and

receivers can introduce distortion at the

carrier frequency (zero frequency or

d.c. in baseband), for example arising

from local oscillator leakage.

In this section we explore three

possible configurations of the d.c.

subcarrier which were considered in

the design of the LTE uplink in order

to minimize d.c. distortion effects on

the packet error rate and the

CM/PAPR.

• Option 1. The d.c. subcarrier

distortion region falls in the middle of

a RB, such that one of the RBs

includes information modulated at d.c.

(e.g. 600 subcarriers for 10MHz

operation bandwidth with the d.c.

subcarrier being one of the subcarriers

for RB 26).

The performance of the RB containing

the d.c. subcarrier would be reduced at

the receiver; this effect would be most

noticeable with a narrow bandwidth

transmission consisting of a single RB.

• Option 2. One more subcarrier is

configured than is required for the

number of RBs (e.g. 601 subcarriers

for the 10 MHz bandwidth case). This

option would be beneficial for the case

of a system bandwidth with an even

number of RBs where the additional

subcarrier would be unused and

correspond to the d.c. subcarrier

located between RBs allocated to

different UEs.

• Option 3. The subcarriers are

frequency-shifted by half a subcarrier

spacing (±7.5 kHz), resulting in an

offset of 7.5 kHz for subcarriers

relative to d.c. Thus two subcarriers

straddle the d.c. location. This is the

option used in LTE, and is illustrated

in Fig. 5 for deployments with even

and odd numbers of RBs across the

system bandwidth.

IV. System Architecture

Specifying the PHY and MAC of the

radio link alone is not sufficient to build an

interoperable broadband wireless network.

Rather, the network architecture framework

that deals with the end-to-end service

aspects is needed.

a- WiMAX

Fig 9 shows the WiMAX network

reference model (NRM), which is a logical

representation of the network architecture.

The NRM identifies the functional entities in

the architecture and the reference points

between the functional entities over which

interoperability is achieved. The NRM

divides the end-to-end system into three

logical parts: (1) mobile stations used by the

subscriber to access the work; (2) the access

service network (ASN) which is owned by a

NAP and comprises one or more base

stations and one or more ASN gateways that

form the radio access network; and (3) the

connectivity service network (CSN), which is

owned by an NSP, and provides IP

connectivity and all the IP core network

functions. The subscriber is served from the

CSN belonging to the visited NSP; the home

NSP is where the subscriber belongs.

Fig.9 Network reference model

1- ASN Functions, Decompositions, and

Profiles

The ASN performs the following functions:

• IEEE 802.16e–based layer 2 connectivity

with the MS

• Network discovery and selection of the

subscriber‘s preferred CSN/NSP

• AAA proxy: transfer of device, user, and

service credentials to selected NSP AAA

and temporary storage of user‘s profiles

• Relay functionality for establishing IP

connectivity between the MS and the CSN

• Radio resource management (RRM) and

allocation based on the QoS policy and/or

request from the NSP or the ASP

• Mobility-related functions, such as

handover, location management, and paging

within the ASN, including support for

mobile IP with foreign-agent functionality

The ASN may be decomposed into

one or more base stations (BSs) and one or

more ASN Gateways (ASN-GW) as shown

in Fig.9. The WiMAX NRM defines multiple

profiles for the ASN, each calling for a

different decomposition of functions within

the ASN.

The BS is defined as representing

one sector with one frequency assignment

implementing the IEEE 802.16e interface to

the MS. Additional functions handled by the

BS in both profiles include scheduling for

the uplink and the downlink, traffic

classification, and service flow management

(SFM) by acting as the QoS policy

enforcement point (PEP) for traffic via the

air interface, providing terminal activity

(active, idle) status, providing DHCP proxy

functionality, relaying authentication

messages between the MS and the ASN-

GW, reception and delivery of the traffic

encryption key (TEK) and the key

encryption key (KEK) to the MS, serving as

RSVP proxy for session management, and

managing multicast group association via

Internet Group Management Protocol

(IGMP) proxy. A BS may be connected to

more than one ASN-GW for load balancing

or redundancy purposes.

The ASN-GW provides ASN

location management and paging; acts as a

server for network session and mobility

management; acts as an authenticator and

AAA; provides mobility tunnel

establishment and management with BSs;

acts as a client for session/mobility

management; performs service flow

authorization (SFA), based on the user

profile and QoS policy; provides foreign

agent functionality; and performs routing

(IPv4 and IPv6) to selected CSNs. Table 2.1

lists the split of the various functional

entities within an ASN between the BS and

the ASN-GW, as per the ASN profiles

defined by the WiMAX Forum. It should be

noted that the ASN gateway may optionally

be decomposed into two groups of

functions: decision point (DP) functions and

enforcement point (EP) functions. When

decomposed in such a way, the DP functions

may be shared across multiple ASN

Gateways. Examples of DP functions

include intra-ASN location management and

paging, regional radio resource control and

admission control, network session/mobility

management (server), radio load balancing

for handover decisions, temporary caching

of subscriber profile and encryption keys,

and AAA client/proxy. Examples of EP

functions include mobility tunneling

establishment and management with BSs,

session/mobility management (client), QoS

and policy enforcement, foreign agent, and

routing to selected CSN.

Table 2.1 Functional Decomposition of the ASN in Various Release 1 Profiles

2- CSN Functions

The CSN provides the following functions:

• IP address allocation to the MS for user

sessions.

• AAA proxy or server for user, device and

services authentication, authorization, and

accounting (AAA).

• Policy and QoS management based on the

SLA/contract with the user. The CSN of the

home NSP distributes the subscriber profile

to the NAP directly or via the visited NSP.

• Subscriber billing and interoperator

settlement.

• Inter-CSN tunneling to support roaming

between NSPs.

• Inter-ASN mobility management and

mobile IP home agent functionality.

• Connectivity infrastructure and policy

control for such services as Internet access,

access to other IP networks, ASPs, location-

based services, peer-to-peer, VPN, IP

multimedia services, law enforcement, and

messaging.

3- Reference Points

The WiMAX NWG defines a

reference point (RP) as a conceptual link

that connects two groups of functions that

reside in different functional entities of the

ASN, CSN, or MS. Reference points are not

necessarily a physical interface, except when

the functional entities on either side of it are

implemented on different physical devices.

Fig.9 shows a number of reference

points defined by the WiMAX NWG. These

reference points are listed in Table 2.2.

Table 2.2 WiMAX Reference Points

4- Network Discovery and Selection

WiMAX networks are required to

support either manual or automatic selection

of the appropriate network, based on user

preference. It is assumed that an MS will

operate in an environment in which multiple

networks are available for it to connect to

and multiple service providers are offering

services over the available networks. To

facilitate such operation, the WiMAX

standard offers a solution for network

discovery and selection. The solution

consists of four procedures:

NAP discovery, NSP discovery, NSP

enumeration and selection, and ASN

attachment.

NAP discovery: This process enables the

MS to discover all available NAPs within a

coverage area. The MS scans and decodes

the DL MAP of ASNs on all detected

channels. The 24-bit value of the ―operator

ID‖ within the base station ID parameter in

DL MAP as defined in IEEE 802.16 serves

as the NAP identifier.

NSP discovery: This process enables the

MS to discover all NSPs that provide service

over a given ASN. The NSPs are identified

by a unique 24-bit NSP identifier, or 32-byte

NAI (network access identifier). The MS

can dynamically discover the NSPs during

initial scan or network entry by listening to

the NSP IDs broadcast by the ASN as part

of the system identity information

advertisement (SII-ADV) MAC

management message. NSP-IDs may also be

transmitted by the BS in response to a

specific request by MS, using an SBC-REQ

message.

NSP enumeration and selection: The MS

may make a selection from the list of

available NSPs by using an appropriate

algorithm. NSP selection may be automatic

or manual.

ASN attachment: Once an NSP is selected,

the MS indicates its selection by attaching to

an ASN associated with the selected NSP

and by providing its identity and home NSP

domain in the form of a network access

identifier.

5- IP Address Assignment

The Dynamic Host Control Protocol

(DHCP) is used as the primary mechanism

to allocate a dynamic point-of-attachment

(PoA) IP address to the MS. Alternatively,

the home CSN may allocate IP addresses to

an ASN via AAA, which in turn is delivered

to the MS via DHCP. For nomadic, portable

and mobile access, dynamic allocation from

either the home or the visited CSN is

allowed, depending on roaming agreements

and the user subscription profile and policy.

See table 2.3

Service type PoA IP address scheme (IPv4) PoA IP address scheme (IPv6)

Fixed access Static or dynamic Static or stateful autoconfiguration

Nomadic access Dynamic Stateful or stateless autoconfiguration

Mobile access DHCP for P-MIP terminals Stateful or stateless authconfiguration

MIP based for C-MIP terminals

P-MIP=Proxy-Mobile IP mode.

C-MIP=Client-Mobile IP mode.

Table 2.3 PoA IP address method according to the WiMAX access services and IP version

6- Mobility

The mobility procedures are divided into

two mobility levels:

ASN anchored mobility procedures.

This refers to MS mobility where no CoA

address update is needed, also known as

micromobility.

CSN anchored mobility procedures.

The macromobility between the ASN and

CSN is based on mobile IP protocols

running across the R3 interface.

CSN anchored mobility implies that, in the

case of IPv4, the MS changes to a new

anchor FA (Foreign Agent). WiMAX

systems must support at least one of the

following mobile IP schemes:

Proxy-MIP. In this case, the MS is

unaware of CSN mobility management

activities and there is no additional

signalling/overhead over the air to complete

the CSN mobility.

Client MIP (CMIPv4). In this case,

the MIP client in the MS participates in

inter-ASN mobility.

b- LTE

LTE has been designed to support

only packet switched services, It aims to

provide seamless Internet Protocol (IP)

connectivity between User Equipment (UE)

and the Packet Data Network (PDN),

without any disruption to the end users‘

applications during mobility. While the term

‗LTE‘ encompasses the evolution of the

radio access through the Evolved-UTRAN

(E-UTRAN), it is accompanied by an

evolution of the non-radio aspects under the

term ‗System Architecture Evolution‘ (SAE)

which includes the Evolved Packet Core

(EPC) network. Together LTE and SAE

comprise the Evolved Packet System (EPS).

EPS uses the concept of EPS bearers

to route IP traffic from a gateway in the

PDN to the UE. A bearer is an IP packet

flow with a defined Quality of Service

(QoS) between the gateway and the UE.

1- Overall Architectural Overview

EPS provides the user with IP

connectivity to a PDN for accessing the

Internet, as well as for running services such

as Voice over IP (VoIP). This is achieved by

means of several EPS network elements

which have different roles.

Fig. 10 shows the overall network

architecture including the network elements

and the standardized interfaces. At a high

level, the network is comprised of the CN

(EPC) and the access network (E-UTRAN).

While the CN consists of many logical

nodes, the access network is made up of

essentially just one node, the evolved

NodeB (eNodeB), which connects to the

UEs. The EPC and E-UTRAN network

elements are described in more detail below.

a- The Core Network

The CN (called EPC in SAE) is

responsible for the overall control of the UE

and establishment of the bearers. The main

logical nodes of the EPC are:

• PDN Gateway (P-GW);

• Serving Gateway (S-GW);

• Mobility Management Entity (MME).

Fig. 10 The EPS network elements.

In addition to these nodes, EPC also

includes other logical nodes and functions

such as the Home Subscriber Server (HSS)

and the Policy Control and Charging Rules

Function (PCRF). Since the EPS only

provides a bearer path of a certain QoS,

control of multimedia applications such as

VoIP is provided by the IP Multimedia

Subsystem (IMS) which is considered to be

outside the EPS itself. The logical CN nodes

are shown in Fig. 10 and discussed in more

detail in the following.

• PCRF. provides the QoS

authorization (QoS class identifier and

bitrates) that decides how a certain data flow

will be treated and ensures that this is in

accordance with the user‘s subscription

profile.

• Home Location Register (HLR).

The HLR contains users‘ subscription data

such as the EPS-subscribed QoS profile and

any access restrictions for roaming. It also

holds information about the PDNs to which

the user can connect. In addition the HLR

holds dynamic information such as the

identity of the MME to which the user is

currently attached or registered. The HLR

may also integrate the Authentication Centre

(AuC) which generates the vectors for

authentication and security keys.

• P-GW. The P-GW is responsible

for IP address allocation for the UE, as well

as QoS enforcement and flow-based

charging according to rules from the PCRF.

The P-GW is responsible for the filtering of

downlink user IP packets into the different

QoS based bearers. It also serves as the

mobility anchor for inter-working with non-

3GPP technologies such as CDMA2000 and

WiMAX networks.

• S-GW. All user IP packets are

transferred through the S-GW, which serves

as the local mobility anchor for the data

bearers when the UE moves between

eNodeBs. It also retains the information

about the bearers when the UE is in idle

state (known as ECM-IDLE) and

temporarily buffers downlink data while the

MME initiates paging of the UE to re-

establish the bearers. In addition, the S-GW

performs some administrative functions in

the visited network such as collecting

information for charging (e.g. the volume of

data sent to or received from the user. It also

serves as the mobility anchor for inter-

working with other 3GPP technologies such

as GPRS and UMTS.

•MME. is the control node which

processes the signaling between the UE and

the CN. The protocols running between the

UE and the CN are known as the Non-

Access Stratum (NAS) protocols. The main

functions supported by the MME are

classified as:

Bearer management Functions: This

includes the establishment, maintenance and

release of the bearers.

Connection management Functions: This

includes the establishment of the connection

and security between the network and UE.

NAS control procedures are discussed in

more detail in the following section.

b- Non-Access Stratum (NAS)

Procedures

The NAS procedures, especially the

connection management procedures, are

fundamentally similar to UMTS.

The MME creates a UE context

when a UE is turned on and attaches to the

network. It assigns a unique short temporary

identity termed the SAE-Temporary Mobile

Subscriber Identity (S-TMSI) to the UE

which identifies the UE context in the

MME. This UE context holds user

subscription information downloaded from

the HSS. The local storage of subscription

data in the MME allows faster execution of

procedures such as bearer establishment

since it removes the need to consult the HSS

every time.

To reduce the overhead in the E-

UTRAN and processing in the UE, all UE-

related information in the access network

can be released during long periods of data

inactivity. This state is called EPS

Connection Management IDLE (ECM-

IDLE). The MME retains the UE context

and the information about the established

bearers during these idle periods.

To allow the network to contact an

ECM-IDLE UE, the UE updates the network

as to its new location whenever it moves out

of its current Tracking Area (TA); this

procedure is called a ‗Tracking Area

Update‘. The MME is responsible for

keeping track of the user location while the

UE is in ECM-IDLE.

When there is a need to deliver

downlink data to an ECM-IDLE UE, the

MME sends a paging message to all the

eNodeBs in its current TA, and the eNodeBs

page the UE over the radio interface. On

receipt of a paging message, the UE

performs a service request procedure which

results in moving the UE to ECM-

CONNECTED state. UE-related information

is thereby created in the E-UTRAN, and the

bearers are re-established. The MME is

responsible for the re-establishment of the

radio bearers and updating the UE context in

the eNodeB. This transition between the UE

states is called an idle-to-active transition.

c- The Access Network

The Access Network of LTE, E-

UTRAN, simply consists of a network of

eNodeBs, as illustrated in Fig. 11. For

normal user traffic , there is no centralized

controller in E-UTRAN; hence the E-

UTRAN architecture is said to be flat.

The eNodeBs are normally inter-

connected with each other by means of an

interface known as X2, and to the EPC by

means of the S1 interface – more

specifically, to the MME by means of the

S1-MME interface and to the S-GW by

means of the S1-U interface. The protocols

which run between the eNodeBs and the UE

are known as the Access Stratum (AS)

protocols.

Fig. 11. E-UTRAN architecture.

The E-UTRAN is responsible for all

radio-related functions, which can be

summarized briefly as:

Radio Resource Management. This covers

all functions related to the radio bearers,

such as radio bearer control, radio admission

control, radio mobility control, scheduling

and dynamic allocation of resources to UEs

in both uplink and downlink

Header Compression. This helps to ensure

efficient use of the radio interface by

compressing the IP packet headers which

could otherwise represent a significant

overhead, especially for small packets such

as VoIP.

Security. All data sent over the radio

interface is encrypted

Connectivity to the EPC. This consists of

the signaling towards the MME and the

bearer path towards the S-GW.

On the network side, all of these

functions reside in the eNodeBs, each of

which can be responsible for managing

multiple cells. Unlike some of the previous

second- and third generation technologies,

LTE integrates the radio controller function

into the eNodeB, thus reducing latency and

improving efficiency. Such distributed

control eliminates the need for a high-

availability, processing-intensive controller,

which in turn has the potential to reduce

costs and avoid ‗single points of failure‘.

One consequence of the lack of a centralized

controller node is that, as the UE moves, the

network must transfer all information related

to a UE, i.e. the UE context, together with

any buffered data, from one eNodeB to

another, mechanisms are therefore needed to

avoid data loss during handover. An

eNodeB may be served by multiple MME/S-

GWs, as is the case for eNodeB#2 in Fig.

11. The set of MME/S-GW nodes which

serves a common area is called an MME/S-

GW pool, and the area covered by such a

pool of MME/S-GWs is called a pool area.

V. Conclusions Table 8 below presents the key

elements of a comparison between the

Mobile WiMAX and 3GPP-LTE standards

as they converge to 4G broadband wireless

access systems. The comparison focuses

mainly on the physical layer aspects of the

radio access technology of these two

standards.

The parameters presented in Table 8

show that the Mobile WiMAX and 3GPP-

LTE standards are technically similar.

However, in terms of market perspective the

two standards differ in terms of expected

time to market and legacy. Mobile WiMAX

was first to market, whereas LTE has been

only recently standardized.

Following this observation, we may

conclude that due to timeline benefits new

service providers as well as existing cable

and DSL providers wishing to offer mobile

services are likely to select Mobile WiMAX

as their technology for mobile broadband

access. We may also conclude that in the

developed world major UMTS/HSPA

service providers will naturally evolve to

3GPP-LTE, whereas most CDMA2000

providers, as well as GSM/EDGE providers

in the developing world, will select Mobile

WiMAX for mobile broadband wireless

access while providing service continuity

over their legacy networks.

Table 8. Comparison of 3GPP LTE and IEEE 802.16e

REFERENCES

[1] Jeffrey G. Andrews, Arunabha Ghosh and Rias Muhamed, "Fundamentals of WiMAX: Understanding

Broadband Wireless Networking", Prentice Hall Communications Engineering and Emerging Technologies

Series

[2] Stefania Sesia, Issam Toufik and Matthew Baker, "LTE: The UMTS LongTerm Evolution", 2009 John Wiley

& Sons, Ltd.

[3] Erik Dahlman, Stefan Parkvall, Johan Sköld and Per Beming, "3G Evolution: HSPA and LTE for

Mobile Broadband", Second edition 2008, Academic Press

[4] Jacob Scheim, "A Comparison of Two Fourth Generation Technologies: WiMAX and 3GPP-LTE",

December 2006 White Paper, Communication & Signal Processing Ltd.

[5] "Advanced Technologies in Wireless Communication Systems with Mobile WiMAX System Simulation and

Implementation", 2008, Dr. Ibrahim Ghaleb's gradution project's book