lte air interface

1 Nokia Siemens Networks RA41202EN10GLA0

LTE RPESSLTE Air Interface


Nokia Siemens Networks Academy

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Module Objectives

After completing this module, the participant should be able to:

Understand the basics of the OFDM transmission technology Explain how the OFDM technology avoids the Inter Symbol Interference Recognise the different between OFDM & OFDMA Identify the OFDM weaknesses Review the key OFDM parameters Analyze the reasons for SC-FDMA selection in UL Describe the LTE Air Interface Physical Layer Calculate the Physical Layer overhead Identify LTE Measurements List the frequency allocation alternatives for LTE Review the main LTE RRM features Identify the main voice solutions for LTE


Module Contents

OFDM Basics OFDM & Multipath Propagation: The Cyclic Prefix OFDM versus OFDMA OFDM Weaknesses OFDM Key Parameters SC-FDMA LTE Air Interface Physical Layer Physical Layer Overhead LTE Measurements Frequency Variants RRM Overview VoIP in LTE


Module Contents



The rectangular Pulse

Advantages:+ Simple to implement: there is no complex filter system required to detect such pulses and to generate them.+ The pulse has a clearly defined duration. This is a major advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference.

Disadvantage: - it allocates a quite huge spectrum. However the spectral power density has null points exactly at multiples of the frequency fs = 1/Ts. This will be important in OFDM.

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Time Domain

frequency f/fs

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Frequency Domain

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FourierTransform

Inverse FourierTransform


TDMA

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Time Division

FDMA

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Frequency Division

CDMA

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Code Division

OFDMA

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Frequency Division

Orthogonal subcarriers

Multiple Access Methods User 1 User 2 User 3 User ..

OFDM is the state-of-the-art and most efficient and robust air interface


OFDM Basics

Transmits hundreds or even thousands of separately modulated radio signals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak ( centre frequency) of one subcarrier

intercepts the nulls of the neighbouring subcarriers

15 kHz in LTE: fixed

Total transmission bandwidth


OFDM Basics

Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission

The throughput is the sum of the data rates of each individual (or used) subcarriers while the power is distributed to all used subcarriers

FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined by the FFT size ( by the bandwidth)

Power

frequency

bandwidth


OFDM versus coventional FDM

OFDM allows a tight packing of small carrier - called the subcarriers - into a given frequency band.

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Frequency (f/fs) Frequency (f/fs)

SavedBandwidth


Fast Fourier Transform (FFT)

time frequency

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1/T

FFT

time frequency

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FFT

FFT is a method for calculating the Discrete Fourier Transform (DFT) and it is and fundamental element in OFDM

IFFT = Inverse FFT. FFT/IFFT allows to move between time & frequency domain representations. FFT & IFFT are blocks included in an OFDMA system:

FFT in the Receiver IFFT in the Transmitter


LTE is standardised in the 36-series of 3GPP Release 8:TS 36.1xx Equipment requirements (terminals, eNodeB)TS 36.2xx Layer 1 (physical layer) specificationsTS 36.3xx Layer 2 and 3 specificationsTS 36.4xx Network signalling specificationsTS 36.5xx User equipment conformance testing

Physical layer specifications:TS 36.201 Physical layer; General descriptionTS 36.211 Physical channels and modulationTS 36.212 Multiplexing and channel codingTS 36.213 Physical layer proceduresTS 36.214 Physical layer; Measurements

Frequency

eNodeB

Subcarriers

OFDMASC-

FDMA

LTE Air Interface Specifications

The LTE radio interface is standardised in the 36-series of 3GPP Release 8. The detailed physical layer structure is described in 5 physical layer specifications.


Module Contents



Multi-Path Propagation & Inter-Symbol Interference (ISI)

BTSTime 0 Ts

+

Time 0 Tt Ts+Tt

Tt

ISIInter Symbol Interference


Multi-Path Propagation & the Guard Period

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time

TSYMBOLTime Domain

1

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time

TSYMBOL

time

TSYMBOL

Tg

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Guard Period (GP)

Guard Period (GP)

Guard Period (GP)


Tg: Guard period durationISI: Inter-Symbol Interference

Propagation delay exceeding the Guard Period

12

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TSYMBOL

Time Domain

time

time

Tg

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Delay spread > Tg ISI


The Cyclic Prefix OFDM symbol

OFDM symbol

OFDM symbol

OFDM symbol

Cyclic prefix

Part of symbol used for FFT processing in the receiver

In all major implementations of the OFDMA technology (LTE, WiMAX) the Guard Periodis equivalent to the Cyclic Prefix CP.

This technique consists in copying the last part of a symbol shape for a duration of guard-time and attaching it in front of the symbol (refer to picture sequence on the right).

CP needs to be longer than the channel multipath delay spread (refer to previous slide).

A receiver typically uses the high correlation between the CP and the last part of the following symbol to locate the start of the symbol and begin then with decoding.


The OFDM Signal


Module Contents



OFDM

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Plain OFDM

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1 2 3 common info(may be addressed viaHigher Layers)

UE 1 UE 2 UE 3

OFDM stands for Orthogonal Frequency Division Multicarrier

OFDM: Plain or Normal OFDM has no built-in multiple-access mechanism.

This is suitable for broadcast systems like DVB-T/H which transmit only broadcast and multicast signals and do not really need an uplink feedback channel (although such systems exist too).

Now we have to analyze how to handle access of multiple users simultaneously to the system, each one using OFDM.


OFDMA

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Orthogonal FrequencyMultiple Access

OFDMA

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Resource Block (RB)

1 2 3 common info(may be addressed viaHigher Layers)

UE 1 UE 2 UE 3

OFDMA stands for Orthogonal Frequency Division Multiple Access

registered trademark by Runcom Ltd. The basic idea is to assign subcarriers to users based on their

bit rate services. With this approach it is quite easy to handle high and low bit rate users simultaneously in a single system.

But still it is difficult to run highly variable traffic efficiently. The solution to this problem is to assign to a single users so

called resource blocks or scheduling blocks.

such block is simply a set of some subcarriers over some time.

A single user can then use 1 or more Resource Blocks.


Module Contents



Inter-Carrier Interference (ICI) in OFDM

The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors. If the receivers frequency slips some fractions from the subcarriers center frequencies,

then we encounter not only interference between adjacent carriers, but in principle between all carriers.

This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform.

One possible cause that introduces frequency errors is a fast moving Transmitter or Receiver (Doppler effect).


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Leakage effect due to Frequency Drift: ICI

Two effects begin to work:

1. -Subcarrier 2 has no longer its power density maximum here -so we loose some signal energy.

2.-The rest of subcarriers (0, 1, 3 and 4) have no longer a null point here. So we get some noise from the other subcarrier.


Doppler in OFDM and Loss of Orthogonality

Doppler effect (shift): Change in frequency of a wave due to the relative motion of source and receiver.

Symbols are distorted in the time domain Frequency shifts make symbol detection inaccurate MCS schemes with high number of bits per subcarrier are not suitable for MSs

moving at high speed More difficult to support high data rates Doppler only impacts SINRs at the higher range i.e. > 20dB

It reduces orthogonality The frequency domain subcarriers are shifted causing inter-carrier interference (ICI) Frequency shift in the subcarriers limits the SNR values The nulls of interferers and peaks of signals will not coincide

ICI in the absence of orthogonality


Module Contents



Subcarrier types

Data subcarriers: used for data transmission Reference Signals:

used for channel quality and signal strength estimates. They dont occupy a whole subcarrier but they are periodically embedded in the

stream of data being carried on a data subcarrier.

Null subcarriers (no transmission/power): DC (centre) subcarrier: 0 Hz offset from the channels centre frequency Guard subcarriers: Separate top and bottom subcarriers from any adjacent

channel interference and also limit the amount of interference caused by the channel. Guard band size has an impact on the data throughput of the channel.

Guard (no power)

DC (no power)

data

Guard (no power)


OFDMA Parameters in LTE

Channel bandwidth: DL bandwidths ranging from 1.4 MHz to 20 MHz Data subcarriers: the number of data subcarriers varies with the

bandwidth 72 for 1.4 MHz to 1200 for 20 MHz


OFDMA Parameters in LTE

Frame duration: 10ms created from slots and subframes. Subframe duration (TTI): 1 ms ( composed of 2 x 0.5 slots) Subcarrier spacing: Fixed to 15kHz ( 7.5 kHz defined for MBMS) Sampling Rate: Varies with the bandwidth but always factor or

multiple of 3.84 to ensure compatibility with WCDMA by using common clocking

Frame Duration

Subcarrier Spacing

Sampling Rate ( MHz)

Data Subcarriers

Symbols/slot

CP length

1.4MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

10 ms

15 kHz

Normal CP=7, extended CP=6

Normal CP=4.69/5.12 s, extended CP= 16.67s.

1.92 3.84 7.68 15.36 23.04 30.72

72 180 300 600 900 1200

10ms


Module Contents



Peak-to-Average Power Ratio in OFDMA

The transmitted power is the sum of the powers of all the subcarriers

Due to large number of subcarriers, the peak to average power ratio (PAPR) tends to have a large range

The higher the peaks, the greater the range of power levels over which the transmitter is required to work.

Not best suited for use with mobile (battery-powered) devices


SC-FDMA in UL

Single Carrier Frequency Division Multiple Access: Transmission technique used for Uplink

Variant of OFDM that reduces the PAPR: Combines the PAR of single-carrier system with the

multipath resistance and flexible subcarrier frequency allocation offered by OFDM.

It can reduce the PAPR between 69dB compared to OFDMA

TS36.201 and TS36.211 provide the mathematical description of the time domain representation of an SC-FDMA symbol.

Reduced PAPR means lower RF hardware requirements (power amplifier)

SC

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OFD

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SC-FDMA and OFDMA Comparison (1/2)

OFDMA transmits data in parallel across multiple subcarriers SC-FDMA transmits data in series employing multiple subcarriers In the example:

OFDMA: 6 modulation symbols ( 01,10,11,01,10 & 10) are transmitted per OFDMA symbol, one on each subcarrier

SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA symbol using all subcarriers per modulation symbol. The duration of each modulation symbol is 1/6th of the modulation symbol in OFDMA

OFDMA SC-FDMA


SC-FDMA and OFDMA Comparison (2/2)


Uplink Air Interface TechnologySC-FDMA

User multiplexing in frequency domain (in OFDMA the user multiplexing is in sub-carrier domain)

One user always continuous in frequency Smallest UL bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in DL) Largest UL bandwidth: 20 MHz (same for OFDMA in DL)

Terminals are required to be able to receive & transmit up to 20 MHz, depending on the frequency band though

User 2 f

User 1 f

f

Receiver


Module Contents



LTE Physical Layer - Introduction

FDD..

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Downlink Uplink

Frequency band 1

Frequency band 2

.. ..Single frequency band

TDD

It provides the basic bit transmission functionality over air LTE physical layer based on OFDMA DL & SC-FDMA in UL

This is the same for both FDD & TDD mode of operation There is no macro-diversity in use System is reuse 1, single frequency network operation is feasible

no frequency planning required There are no dedicated physical channels anymore, as all resource

mapping is dynamically driven by the scheduler


LTE Physical Layer Structure Frame Structure (FDD)

10 ms frame

0.5 ms slot

s0 s1 s2 s3 s4 s5 s6 s7 s18 s19..

1 ms sub-frame

SF0 SF1 SF2 SF9..

sy4sy0 sy1 sy2 sy3 sy5 sy6

0.5 ms slot

SF3

SF: SubFrame

s: slot

Sy: symbol

FDD Frame structure ( also called Type 1 Frame) is common to both UL & DL Divided into 20 x 0.5ms slots

Structure has been designed to facilitate short round trip time

- Frame length = 10 ms

- FDD: 10 sub-frames of 1 ms for UL & DL- 1 Frame = 20 slots of 0.5ms each

- 1 slot = 7 (normal CP) or 6 OFDM symbols (extended CP)


LTE Physical Layer Structure Frame Structure (TDD)

SF#0

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time

UL/DL carrier

radio frame 10 ms

subframe

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subframe

half frame

DwPTS: Downlink Pilot time Slot

UpPSS: Uplink Pilot Time Slot

GP: Guard Period to separate between UL/DL

Downlink Subframe

Uplink Subframe

Frame Type 2 (TS 36.211-900; 4.2) each radio frame consists of 2 half frames Half-frame = 5 ms = 5 Sub-frames of 1 ms UL-DL configurations with both 5 ms & 10 ms DL-to-UL switch-point periodicity are supported Special subframe with the 3 fields DwPTS, GP & UpPTS; length of DwPTS + UpPTS +GP = 1

subframe DL / UL ratio can vary from 1/3 to 8/1 according to service requirements of the carrier


Subframe structure & CP length

Short cyclic prefix:

Long cyclic prefix:

Copy= Cyclic prefix

= Data

5.21 s

16.67 s

Subframe length: 1 ms for all bandwidths Slot length is 0.5 ms

1 Subframe= 2 slots Slot carries 7 symbols with normal CP or 6 symbols with long CP

CP length depends on the symbol position within the slot: Normal CP: symbol 0 in each slot has CP = 160 x Ts = 5.21s;

remaining symbols CP= 144 x Ts = 4.7s Extended CP: CP length for all symbols in the slot is 512 x Ts = 16.67s

Ts: sampling time of the overall channel basic Time Unit = 32.5 nsec

Ts =1 sec

Subcarrier spacing X max FFT size


Resource Block and Resource Element

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 60 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 60 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

Subcarrier 1

Subcarrier 12

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1 slot 1 slot

1 ms subframeResource Element

Physical Resource Block PBR or Resource Block RB: 12 subcarriers in frequency domain x 1 slot period in time domain Capacity allocation based on Resource Blocks

Resource Element RE: 1 subcarrier x 1 symbol period theoretical min. capacity allocation unit 1 RE is the equivalent of 1 modulation

symbol on a subcarrier, i.e. 2 bits (QPSK), 4 bits (16QAM), 6 bits (64QAM).


Physical Resource Blocks

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12 subcarriers

Time

Frequency

0.5 ms slot

1 ms subframe or TTI

Resource block

During each TTI, resource blocks for different UEs are scheduled in the eNodeB

In both the DL & UL direction, data is allocated to users in terms of resource blocks (RBs).

a RB consists of 12 consecutive subcarriers in the frequency domain, reserved for the duration of 0.5 ms slot.

The smallest resource unit a scheduler can assign to a user is a scheduling block which consists of two consecutive resource blocks


LTE Channel Options

Bandwidth options: 1.4, 1.6, 3, 3.2, 5, 10, 15 and 20 MHz

Subcarriers in frequency domain (15 kHz or 7.5 kHz subcarrier spacing)

Channel bandwidth (MHz)

Number of subcarriers

Number of resource blocks

1.4

72

6

3

180

15

5

300

25

10

600

50

15

900

75

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1200

100


DL Physical Resource Block

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe

or TTI

DL reference signal

Reference signals position in time domain is fixed (symbol 0 & 4 / slot for Type 1 Frame) whereas in frequency domain it depends on the Cell ID

Reference signals are modulated to identify the cell to which they belong.

This signal, consisting of a known pseudorandom sequence, is required for channel estimation in the UEs. (like CPICH in WCDMA).

Note that in the case of MIMO transmission, additional reference signals must be embedded into the resource blocks.


DL Physical Channels

PDSCH: Physical Downlink Shared Channel carries user data, L3 Signalling, System Information Blocks & Paging

PBCH: Physical Broadcast Channel for Master Information Block only

PMCH: Physical Multicast Channel for multicast traffic as MBMS services

PCFICH: Physical Control Format Indicator Channel indicates number of OFDM symbols for Control Channels = 1..4

PDCCH: Physical Downlink Control Channel carries resource assignment messages for DL capacity allocations & scheduling

grants for UL allocations

PHICH: Physical Hybrid ARQ Indicator Channel carries ARQ Ack/Nack messages from eNB to UE in respond to UL transmission

There are no dedicated channels in LTE, neither UL nor DL.


UL Physical Channels

PUSCH: Physical Uplink Shared Channel Transmission of user data, L3 & L1 signalling (L1 signalling: CQI, ACK/NACKs, etc.)

PUCCH: Physical Uplink Control Channel Carries L1 control information in case that no user data are scheduled in this subframe

(e.g. H-ARQ ACK/NACK indications, UL scheduling request, CQIs & MIMO feedback). These control data are multiplexed together with user data on PUSCH, if user data are

scheduled in the subframe

PRACH: Physical Random Access Channel For Random Access attempts; SIBs indicates the PRACH configuration (duration;

frequency; repetition; number of preambles - max. 64)


UL Physical Resource Block: DRS & SRS

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe or TTI

Frequency

Sounding Reference Signal on last OFDM symbol of 1 subframe;Periodic or aperiodic

transmission

Demodulation Reference Signal in subframes that carry

PUSCH

Note: when the subframe contains the PUCCH, the Demodulation

Reference Signal is embedded in a different way

The Demodulation Reference Signal is transmitted in the thirdSC-FDMA symbol (counting from zero) in all resource blocks allocated to the PUSCH carrying the user data.

This signal is needed for channel estimation, which in turn is essential for coherent demodulation of the UL signal in the eNodeB.

The Sounding Reference Signal SRS provides UL channel quality information as a basis for scheduling decisions in the base station. This signal is distributed in the last SC-FDMA symbol of subframes that carry neither PUSCH nor PUCCH data.

PUCCH: Physical UL Control Channel


b0 b1

QPSK

Im

Re10

11

00

01

b0 b1b2b3

16QAM

Im

Re

0000

1111

Im

Re

64QAM

b0 b1b2b3 b4 b5

3GPP standard defines the following options: QPSK, 16QAM, 64QAM in both directions (UL & DL)

UL 64QAM not supported in RL10 Not every physical channel is allowed to use any

modulation scheme: Scheduler decides which form to use depending on carrier

quality feedback information from the UE

Modulation Schemes

QPSK:

2 bits/symbol

16QAM:

4 bits/symbol

64QAM:

6 bits/symbol

Physical channel

Modulation

PDSCH QPSK, 16QAM, 64QAM

PMCH QPSK, 16QAM, 64QAM

PBCH QPSK

PDCCH, PCFICH

QPSK

PHICH BPSK

PUSCH QPSK, 16QAM, 64QAM

PUCCH BPSK and/or QPSK


Module Contents



DL Reference Signal Overhead

Reference Signal (RS)- If 1 Tx antenna*: 4 RSs per PRB- If 2 Tx antenna*: there are 8 RSs per PRB- If 4 Tx antenna*: there are 12 RSs per PRB

Example below: Normal CP (84 RE) & 2 Tx antenna*, DL RS overhead = 8 / 84 = 9.52 %

* with 1/2/4 Antenna Ports PRB: Physical Resource Block


Synchronization Signals Overhead

Primary Synchronization Signal (PSS) - occupies 144 Resource Elements per frame (20 timeslots); i.e. (62 subcarriers + 10

empty Resource Elements) x 2 times/frameExample: Normal CP, 10 MHz bandwidth; PSS overhead = 144 / (84 20 50) = 0.17 %

Secondary Synchronization Signal (SSS) Identical calculation to PSS; same overhead as for PSS

2 3 4 5 7 8 9 10

1 2 3 4 5 6 7

1 2 3 4 5 6

10ms Radio frame

1ms Subframe SSSPSS

0.5ms = 1 slot

Normal CP

Extended CP

PSS & SSS frame + slot structure in time domain

(FDD case)

checking for SSSat 2 possible positions

CP length


The combination of PDCCH, PCFICH & PHICH occupies the first 1, 2 or 3 symbols per TTI*

Resource Elements

reserved for

Reference Symbols

(2 antenna port case)

Control Channel

Region (1-3 OFDM symbols*)

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TimeData Region

One subframe (1ms)

PDCCH, PCFICH & PHICH overhead (1/2)

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth


PDCCH, PCFICH & PHICH overhead (2/2)

The number of RE occupied per 1 ms TTI is given by (12 y x), where: y depends upon the number of OFDM symbols per TTI (1, 2 or 3*) occupied by

Control Channels

x depends upon the number of RE already occupied by the Reference Signal x = 2 for 1 Tx antenna (Antenna Port) x = 4 for 2 Tx antennas (Antenna Ports) x = 4 for 4 Tx antennas (Antenna Ports) when y = 1 x = 8 for 4 Tx antennas (Antenna Ports) when y = 2 or 3

Example: in the case of normal CP, 2 Antenna Ports & 3 OFDM symbols occupied by Control Channels:

Control Channel Overhead = (12 3 - 4) / (12 7 2) = 19.05%

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth


PBCH Overhead

Occupies (288* x) Resource Elements (REs) per 20 timeslots per transmit antenna

The value of x depends upon the number of REs already occupied by the Reference Signal:

x = 12 for 1 Tx antenna, x = 24 for 2 Tx antennas & x = 48 for 4 Tx antenna

- Example: normal CP, 2 Tx antennas, 10 MHz bandwidth; PBCH Overhead = (288 24) / (84 20 50) = 0.31%

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Repetition Pattern of PBCH = 40 ms

one radio frame = 10 ms

PBCH* PBCH uses central 72 Subcarrier over 4 OFDM symbols in Slot 1


UL Demodulation Reference Signal Overhead (1/2)

Demodulation Reference Signal (DRS)

The DRS is sent on the 4thOFDM symbol of each RB occupied by the PUSCH.

PUCCH

PUCCH

PUSCH


Example:For 1.4 MHz Channel Bandwidth, the PUCCH occupies 1 RB per Slot. The number of RE per RB is 84 when using the normal CP. This means the DRS overhead* is: ((6-1) 12)/(6 84) = 11.9 %

Channel BW PUCCH RB/slot DRS Overhead*

1.4 MHz 1 ((6-1) 12) / (6 84) = 11.9 %

3 MHz 2 ((15-2) 12) / (15 84) = 12.38 %

5 MHz 2 ((25-2) 12) / (25 84) = 13.14 %

10 MHz 4 ((50-4) 12) / (50 84) = 13.14 %

15 MHz 6 ((75-6) 12) / (75 84) = 13.14 %

20 MHz 8 ((100-8) 12) / (100 84) = 13.14 %

UL DRS Overhead (2/2)

* for normal CP


PRACH Overhead

PRACH PRACH uses 6 Resource Blocks in the frequency domain. The location of those resource blocks is dynamic. Two parameters from RRC layer define it:

PRACH Configuration Index: for Timing, selecting between 1 of 4 PRACH durations and defining if PRACH preambles can be send in any radio frame or only in even numbered ones

PRACH Frequency offset: Defines the location in frequency domain PRACH Overhead calculation: 6 RBs * RACH Density / (#RB per TTI) x 10 TTIs per frame

RACH density: how often are RACH resources reserved per 10 ms frame i.e. for RACH density: 1 (RACH resource reserved once per frame)

Channel BW PRACH Overhead

1.4 MHz (6 1) / (6 10) = 10 %

3 MHz (6 1) / (15 10) = 4 %

5 MHz (6 1) / (25 10) = 2.40 %

10 MHz (6 1) / (50 10) = 1.20 %

15 MHz (6 1) / (75 10) = 0.8 %

20 MHz (6 1) / (100 10) = 0.6 %


PUCCH Overhead PUCCH Ratio between the number of RBs used for PUCCH and the total number of RBs in frequency

domain per TTIChannel BW PUCCH RB/slot PUCCH Overhead

1.4 MHz 1 1 / 6 = 16.67 %

3 MHz 2 2 / 15 = 13.33 %

5 MHz 2 2 / 25 = 8 %

10 MHz 4 4 / 50 = 8 %

15 MHz 6 6 / 75 = 8 %

20 MHz 8 8 / 100 = 8%

Time

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PUCCH

PUCCH

PUSCH

1 subframe = 1ms

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Physical Layer Overhead Example

Example of overhead:

DL 2Tx 2RX

UL 1TX - 2RX

PRACH in every frame

3 OFDM symbols for PDCCH


Module Contents



LTE Measurements

Physical layer measurements have not been extensively discussed in the LTE standardization. They could change.

Intra LTE measurements ( from LTE to LTE) UE measurements

CQI measurements Reference Signal Received Power (RSRP) Reference Signal Received Quality ( RSRQ)

eNB measurements Non standardized (vendor specific): TA, Average RSSI, Average SINR, UL CSI,

detected PRACH preambles, transport channel BLER Standardized: DL RS Tx Power, Received Interference Power, Thermal Noise Power

Measurements from LTE to other systems UE measurements are mainly intended for Handover.

UTRA FDD: CPICH RSCP, CPICH Ec/No and carrier RSSI GSM: GSM carrier RSSI UTRA TDD: carrier RSSI, RSCP, P-CCPCH CDMA2000: 1xRTT Pilot Strength, HRPD Pilot Strength

CSI: Channel State Information (received power per PRB)TA: Timing Advance


UE Measurements: RSRP & RSRQ

RSRP (Reference Signal Received Power) Average of power levels (in [W]) received across all Reference Signal symbols

within the considered measurement frequency bandwidth. UE only takes measurements from the cell-specific Reference Signal elements of

the serving cell If receiver diversity is in use by the UE, the reported value shall be equivalent to

the linear average of the power values of all diversity branches

RSRQ ( Reference Signal Received Quality) Defined as the ratio NRSRP/(E-UTRA carrier RSSI), where N is the number of

RBs of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks

Note: 3GPP has open issues on these e.g. measurement bandwidth on RSSI


eNodeB Measurements

DL Reference Signal Transmitted Power Average of power levels (in [W]) transmitted across all Reference Signal symbols

within the considered measurement frequency bandwidth Reference point for the DL RS TX power measurement: TX antenna connector The DL RS TX power signaled to the UE is not measured, it is just an eNB internal

setting

Received Interference Power: Received interference power, including thermal noise, within one PRBs bandwidthThermal noise power: No x W Thermal noise power within the UL system bandwidth (consisting of variable # of

resource blocks) No: white noise power spectral density on the uplink carrier frequency and W: denotes

the UL system bandwidth.

Optionally reported with the Received Interference Power Reference point: RX antenna connector In case of receiver diversity, the reported value is the average of the power in the

diversity branches


Module Contents



LTE Frequency Variants in 3GPP FDD

1

2

3

4

5

7

8

9

6

2x25

2x75

2x60

2x60

2x70

2x45

2x35

2x35

2x10

824-849

1710-1785

1850-1910

1920-1980

2500-2570

1710-1755

880-915

1749.9-1784.9

830-840

Total[MHz] Uplink [MHz]

869-894

1805-1880

1930-1990

2110-2170

2620-2690

2110-2155

925-960

1844.9-1879.9

875-885

Downlink [MHz]

10 2x60 1710-1770 2110-2170

11 2x25 1427.9-1452.9 1475.9-1500.9

1800

2600

900

US AWS

UMTS core

US PCS

US 850

Japan 800

Japan 1700

Japan 1500

Extended AWS

Europe Japan Americas

788-798 758-768777-787 746-756

Japan 800

US7002x102x1013

12 2x18 698-716 728-746

14 US700

US700

Band 15 16: reserved

815 830 860 875 704 716 734 746

2x152x1217

18

* digital dividend

US700

E-UTRAband

UHF (TV)*832 862 791 821 830 845 875 890

2x302x1519

20Japan 800


LTE Frequency Variants - TDD

33

34

35

36

37

39

40

38

1x20

1x60

1x15

1x20

1x40

1x60

1x100

1x50

1910 - 1930

1850 - 1910

2010 - 2015

1900 - 1920

1880 - 1920

1930 - 1990

2300 - 2400

2570 - 2620

Total[MHz]

Frequency[MHz]

UMTS TDD 1

UMTS TDD 2

US PCS

US PCS

US PCSEuro midle gap 2600

China TDD

China TDD

E-UTRAband


Module Contents

OFDM Basics OFDM & Multipath Propagation: The Cyclic Prefix OFDM versus OFDMA OFDM Key Parameters OFDM Weaknesses SC-FDMA LTE Air Interface Physical Layer Physical Layer Overhead LTE Measurements Frequency Variants RRM Overview VoIP in LTE


RRM building blocks & functionsOverview

Scope of RRM:

Management & optimized utilization of the radio resources:

Increasing the overall radio network capacity& optimizing quality

Provision for each service/bearer/user an adequate QoS (if applicable)

RRM located in eNodeB


LTE RRM: Scheduling (1/4)

Motivation Bad channel condition avoidance

OFDMA

The part of total available channel experiencing bad channel condition (fading)

can be avoided during allocation procedure.

CDMA

Single Carrier transmission does not allow to allocate only particular frequency parts. Every fading gap

effects the data.


Scheduler (UL/DL) (2/4)

Cell-based scheduling (separate scheduler per cell) Scheduling on TTI basis (1ms) Resource assignment in time and frequency domain (UL/DL) Proportional Fair (PF) resource assignment among UEs Priority for SRB (Signalling Radio Bearers) over DRB (Data Radio Bearers) Priority handling (UL/DL) for

Random Access procedure Signalling HARQ re-transmission

Uplink: Scheduler controls UEs & assigns appropriate grants per TTI Channel unaware UL scheduling based on random frequency allocation

(Channel-aware UL scheduling foreseen for RL30 & it will be SW licensed)

Downlink: Channel aware DL scheduling - Frequency Domain Packet Scheduling (FDPS) -

based on CQI with resources assigned in a fair manner

RL09


Downlink Scheduler (3/4)Algorithm

Determine which PRBs are available (free) and can be allocated to UEs

Allocate PRBs needed for common channels like SIB, paging, and random access procedure (RAP)

Final allocation of UEs (bearers) onto PRB. Considering only the PRBs available after the previous steps

Pre-Scheduling: All UEs with data available for transmission based on the buffer fill levels

Time Domain Scheduling: Parameter MAX_#_UE_DL decides how many UEs are allocated in the TTI being scheduled

Frequency Domain Scheduling for Candidate Set 2 UEs: Resource allocation in Frequency Domain including number & location of allocated PRBs

Pre-Scheduling:Select UEs eligible for scheduling

-> Determination of Candidate Set 1

Time domain schedulingof UEs according to simple criteria

-> Determination of Candidate Set 2

Start

End

Frequency domain schedulingof UEs/bearers

-> PRB/RBG allocation to UEs/bearers

RL09

Feature ID(s): LTE45


Uplink Scheduler (4/4)Algorithm

Evaluation of the #PRBs that will be assigned to UEs Available number of PRBs per user: resources are assigned via PRB groups (group of

consecutive PRBs)Time domain: Max_#_UE_UL which can be scheduled per TTI time frame is restricted by an O&M

parameter and depends on the bandwidth: 7 UEs (5 MHz), 10 UEs (10MHz) and 20 UEs (20MHz)

Frequency Domain: Uses a random function to assure equal distribution of PRBs over the available frequency

range (random frequency hopping)

a) b)

RL09


Example of allocation in frequency domain:

Full Allocation: All available PRBs are assigned to the scheduled UEs per TTI

Fractional Allocation: Not all PRBs are assigned. Hopping function handles unassigned PRBs as if they were allocated to keep the equal distribution per TTI


LTE RRM: Link Adaptation by AMC (UL/DL) (1/6)

Motivation of link adaptation: Modify the signal transmitted to and by a particular user according to the signal quality variation to improve the system capacity & coverage reliability.

It modifies the MCS (Modulation & Coding Scheme) & the transport block size (DL) and ATB (UL)

If SINR is good then higher MCS can be used -> more bits per byte -> more throughput.

If SINR is bad then lower MCS should be used (more robust) Flexi Multiradio BTS performs the link adaptation for DL on a TTI basis The selection of the modulation & the channel coding rate is based:

DL data channel: CQI report from UE UL: BLER measurements in Flexi LTE BTS


RL09

Optimizing air interface efficiency

74 Nokia Siemens Networks

Link Adaptation / AMC for PDSCH (2/6)

Procedure: Initial MCS is provided by O&M

(parameter INI_MCS_DL) & is set as default MCS

If DL AMC is not activated (O&M parameter ENABLE_AMC_DL) the algorithm always uses this default MCS

If DL AMC is activated HARQ retransmissions are handled differently from initial transmissions (For HARQ retransmission the same MCS has to be used as for the initial transmission)

A MCS based on CQI reportingfrom UE , shall be determined for the PRBs assigned to UE as indicated by the DL scheduler

START

Retrieve Default MCS

Dynamic AMC active?

HARQ retransmission?

Determine avaraged CQI value for allocated PRBs

Use the same MCS as for initial transmission

Determine MCS

Use Default MCS

END

yes

no

no

RL09


Link Adaptation / AMC for PUSCH (3/6)

Functionality UL LA is active by default but can be deactivated by O&M parameters. If not

active, the initial MCS is used all the time UE scope Two parallel algorithms adjust the MCS to the radio channel conditions:

Inner Loop Link Adaptation (ILLA): Slow Periodic Link adaptation (20-500ms) based on BLER measurements

from eNodeB (based on SINR in future releases) Outer Loop Link Adaptation (OLLA): event based

In case of long Link Adaptation updates and to avoid low and high BLER situations, the link adaptation can act based on adjustable target BLER:

- Emergency Downgrade if BLER goes above a MAX BLER threshold (poor radio conditions)

- Fast Upgrade if BLER goes below of a MIN BLER threshold (excellent radio conditions)

RL09


Downlink fast

1 TTI

channel aware CQI based

MCS selection 1 out of 0-28

output MCS TBS

up to 64QAM support

Uplink slow periodical

~30ms

channel partly aware average BLER based

MCS adaptation +/- 1 MCS correction

output MCS ATB

up to 16 QAM support

Comparison: DL & UL Link adaptation for PSCH (4/6)

MCS: Modulation & Coding SchemeTBS: Transport Block SizeATB: Automatic Transmission Bandwidth


Outer Link Quality Control (OLQC) (5/6)

Feature: CQI Adaptation (DL) CQI information is used by the scheduler & link adaptation in such a way that a certain

BLER of the 1st HARQ transmission is achieved

CQI adaptation is the basic mean to control Link Adaptation behaviour and to remedy UE measurement errors

Only used in DL Used for CQI measurement error compensation

CQI estimation error of the UE CQI quantization error or CQI reporting error

It adds a CQI offset to the CQI reports provided by UE. The corrected CQI report is provided to the DL Link adaptation for further processing

CQI offset derived from ACK/NACK feedback

RL09

Optimize the DL performance



Support of aperiodic CQI reports (6/6)

Functionality Aperiodic CQI reports scheduled in addition to periodic reports

Periodic CQI reports on PUCCH Aperiodic CQI reports on PUSCH

Description Controlled by the UL scheduler

Triggered by UL grant indication (PDCCH) Basic feature


Benefits Not so many periodic CQIs on PUCCH

needed

Allow frequent submission of more detailed reports (e.g. MIMO, frequency selective parts)

RL10


LTE RRM: Power Control (1/4)

Downlink: There is no adaptive or dynamic power control in DL but semi-static power

setting

eNodeB gives flat power spectral density (dBm/PRB) for the scheduled resources:

The power for all the PRBs is the same If there are PRBs not scheduled that power is not used but the power of the

remaining scheduled PRBs doesnt change: Total Tx power is max. when all PRBs are scheduled. If only 1/2 of the PRBs are

scheduled the Tx power is 1/2 of the Tx power max ( i.e. Tx power max -3dB)

Semi-static: PDSCH power can be adjusted via O&M parameters Cell Power Reduction level CELL_PWR_RED [0...10] dB attenuation in 0.1 dB steps

Improve cell edge behaviour, reduce inter-cell interference & power consumption

RL09



Power Control (2/4)

Uplink: UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

Closed Loop PC component f(i): Makes use of feedback from the eNB. Feedback are TCP commands send via PDCCH to instruct the UE to increase or decrease its Tx power

Improve cell edge behaviour, reduce inter-cell interference and power consumption

RL09

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _010 dBmifiPLjjPiMPiP TFPUSCHPUSCHCMAXPUSCH

1) Initial TX power level

2) SINR measurment

3) Setting new power offset 4) TX power level adjustment with the new offset

UL Power control is Slow power control: No need for fast power control as in 3G:

if UE Tx power was high it incremented the co-channel for other UEs.

In LTE all UEs resources are orthogonal in frequency & time

TPC: Transmit Power Control


Power Control (3/4)

Uplink (cont.): UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

PCMAX: max. UE Tx power according to UE power class; e.g. 23dBm for class 3 MPUSCH: # allocated PRBs. The UE Tx Power is increased proportionally to the # of allocated

RBs. Remaining terms of the formula are per RB

P0_PUSCH: eNB received power per RB when assuming path loss 0 dB. Depends on : Path loss compensation factor. Three values:

= 0, no compensation of path loss = 1, full compensation of path loss (conventional compensation) { 0 ,1 } , fractional compensation

PL: DL Path loss calculated by the UE Delta_TF: increases the UE Tx power to achieve the required SINR when transmitting a

large number of bits per RE. It links the UE Tx power to the MCS.

RL09

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _010 dBmifiPLjjPiMPiP TFPUSCHPUSCHCMAXPUSCH


Conventional & Fractional Power Control (4/4)

Conventional PC schemes: Attempt to maintain a constant SINR at the receiver UE increases the Tx power to fully compensate for increases in the path loss

Fractional PC schemes: Allow the received SINR to decrease as the path loss increases. UE Tx power increases at a reduced rate as the path loss increases. Increases in

path loss are only partially compensated. [+]: Improve air interface efficiency & increase average cell throughputs by reducing

Intercell interference 3GPP specifies fractional power control for the PUSCH with the option to disable it &

revert to conventional based on

Conventional Power Control: =1

If Path Loss increases by 10 dB the UE Tx power increases by 10 dB

Fractional Power Control: { 0 ,1}

If Path Loss increases by 10 dB the UE Tx power increases by < 10 dB

UE Tx Power UE Tx

Power

UL SINR UL

SINR


LTE RRM: Radio Admission Control (RAC)

Objective: To admit or reject requests for establishment of Radio Bearers (RB) on a cell basis

Based on number of RRC connections and number of active users per cell Non QoS aware Both can be configured via parameters

RRC connection is established when the SRBs have been admitted & successfully configured

UE is considered as active when a Data Radio bearer (DRB) is established Upper bound for maximum number of supported connections depends on the

BB configuration of eNB : RL10: support for 200, 400 & 800 active users respectively in 5, 10 & 20 MHz RL20: up to 840 active users in 20MHz

Handover RAC cases have higher priority than normal access to the cell RL09: All RRC connection setup request are admitted by default to avoid RAC complexity

RL09


LTE RRM: MIMO / Antenna Control (1/5)Transmit diversity for 2 antennas

Benefit: Diversity gain, enhanced cell coverage Each Tx antenna transmits the same stream of data with Receiver gets replicas of

the same signal which increases the SINR.

Synchronization signals are transmitted only via the 1st antenna eNode B sends different cell-specific Reference Signals (RS) per antenna It can be enabled on cell basis by O&M configuration Processing is completed in 2 phases:

Layer Mapping: distributing a stream of data into two streams Pre-coding: generation of signals for each antenna port

RL09


S1

S2

Spatial multiplexing (MIMO) for 2 antennas (2/5)

Benefit: Doubles peak rate compared to 1Tx antenna

Spatial multiplexing with 2 code words Supported physical channel: PDSCH

Two code words (S1+S2) are

transmitted in parallel to 1 UE double peak

rate

Layer Mapping

L1

L2

PrecodingMap onto Resource Elements

Map onto Resource Elements

OFDMA

OFDMA

Modulation

Modulation

Code word 1

Code word 2

Scale

W2

W1

2 code words transferred when channel conditions are good

Signal generation is similar to Transmit Diversity: i.e. Layer Mapping & Precoding

Can be open loop or closed loop depending if the UE provides feedback


Precoding (3/5)

Precoding generates the signals for each antenna port Precoding is done multiplying the signal with a precoding matrix selected from a predefined

codebook known at the eNB and at the UE side Closed loop: UE estimates the radio channel, selects the best precoding matrix (the one that

offers maximum capacity) & sends it to the eNB Open loop: no need for UEs feedback as it uses predefined settings for Spatial Multiplexing

& precoding

Pre-coding codebook for 2 Tx antenna case


DL adaptive open loop MIMO for 2 antennas (4/5)

Benefit: High peak rates (2 code words) & good cell edge performance (single code word)

2 TX antennas Dynamic selection between

Transmit diversity Open loop spatial multiplexing with 2

code words

Supported physical channel: PDSCH Dynamic switch considers the UE specific

link quality

Enabled/disabled on cell level (O&M) If disabled case either static spatial

multiplexing or static Tx diversity can be selected for the whole cell (all UEs)

2 code words (A+B) are transmitted in parallel to 1 UE which doubles the peak rate

1 code word A is transmitted via 2 antennas to 1 UE; improves the LiBu

AB

A


RL09

Note: DL adaptive closed loop MIMO has been moved to RL20

LiBu: Link Budget


MIMO, DL channels & RRM Functionality (5/5)

Available MIMO options vs. channel type Options for Transmit Diversity (2 Tx):

Control Channels PDSCH

Options for Dual Stream (SM): Only DL PDSCH

MIMO is SW feature

Channel can be configured to use MIMO mode

Channel cannot be configured to use MIMO mode

In UL, Flexi eNodeB has 2Rx Div. :

Maximum Ratio CombiningBenefit: increase coverage by increasing the received signal strength and quality

RRM MIMO Mode Control Functionality Refers to switch between:

Tx Diversity (single stream) MIMO Spatial Multiplexing (double stream) 1x1 SISO / 1x2 SIMO

Provided by eNB only for DL direction


LTE RRM: Connection Mobility Control (1/3)Handover Types

Intra-RAT handover Intra eNodeB Inter eNodeB Data forwarding over X2

High performance for 15120 km/h Optimized performance for 015 km/h

HO in case of no X2 interface configured between Serving eNB & Target eNB: HO via S1 interface RL20

Inter-RAT Handover PS domain only RL20: LTE WCDMA RL30: LTE CDMA2000 RL40: WCDMA LTE Not assigned: LTE GSM; GSM LTE


Intra frequency handover via X2 (2/3)

Basic Mobility Feature Event triggered handover based

on DL measurements (ref. signals)

Network evaluated HO decision Operator configurable

thresholds for coverage based & best cell based handover

Data forwarding via X2 Radio Admission Control (RAC)

gives priority to HO related access over other scenarios S1

S1 X2

MMES-GW

P-GW

RL09


A reliable and lossless mobility


Inter RAT Handover to WCDMA (3/3)

Only for multimode devices supporting LTE & WCDMA

Event triggered Handover based on DL measurement Reference Signal Received Power (RSRP)

Operator configurable RSRP threshold Inter-RAT HO measurements only

activated if there is not Intra-frequency neighbour cell

Network evaluated HO decision eNB broadcasts IRAT cell selection

information best target WCDMA cell may be selected

when above the threshold eNB initiates Handover via EPC

MMES-GW

P-GW

SGSNRNC

S1Iub

LTE

WCDMA

RL20


Module Contents

OFDM Basics OFDM & Multipath Propagation: The Cyclic Prefix OFDM versus OFDMA OFDM Key Parameters OFDM Weaknesses SC-FDMA LTE Air Interface Physical Layer Physical Layer Overhead LTE Measurements Frequency Variants RRM Overview VoIP in LTE


VoIP in LTE

Paging in LTE

2G/3G RAN

MMEE-UTRANMSC-S MGW

CS call setup in 2G/3G

CS Fallback handover

Voice is still important in LTE CS voice call will not be possible in LTE since there is no CS core interface Voice with LTE terminals has a few different solutions The first voice solution in LTE can rely on CS fallback Handover where LTE

terminal will be moved to 2G/3G to make CS call The ultimate LTE voice solution will be VoIP + IMS (not RL10)

RL20


Single Radio Voice Call Continuity (SR-VCC)

LTE VoIP

3G CS voice

LTE VoIP

3G CS voice 3G CS voice 3G CS voice

Single Radio Voice Call Continuity (SR-VCC)

Options for voice call continuity when running out of LTE coverage

1) Handover from LTE VoIP to 3G CS voice Voice Handover from LTE VoIP to WCDMA CS voice is called SR-VCC No VoIP needed in 3G

2) Handover from LTE VoIP to 3G VoIP VoIP support implemented in 3G

RL30


LTE Voice Evolution

Broadband LTE introduction

MGW MSS

LTEHSPA & I-HSPA

2G/3G

CS/PS

Increased radio efficiency for voice

serviceLTE

HSPA & I-HSPA2G/3G

Full IMS centric multimedia service

architecture

LTEHSPA & I-HSPA

PS

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NVSIMSMGW MSS NVS

CS/PS

Data only LTE Fast track LTE VoIP IMS multimedia

CS fallback handover

VoIP SR-VCC

VoIP SR-VCC

lte air interface

Documents

ofdm ofdma

ofdm technology

key ofdm parameters

cyclic prefix ofdm

ofdm transmission technology

main lte rrm features

frequency fs

scfdma selection