utran long term evolution 1

8/7/2019 UTRAN Long Term Evolution 1

1/17

UTRAN Long Term Evolution (LTE) refers to the long term evolution of the

3GPP radio access technology and is considered the successor of the

current UMTS system with the rollout anticipated to begin with trials in

2009.

The LTE work in 3GPP is closely aligned to the 3GPP system architecture

evolution (SAE) framework which is concerned with the evolved core

network architecture. The LTE/SAE framework defines the flat, scalable,

IP-based architecture of the Evolved Packet System (EPS) consisting of a

radio access network part (Evolved UTRAN) and the Evolved Packet Core

(EPC).

Note that the Evolved Packet System is purely packet based - it does not

handle circuit-switched traffic at all. Circuit-switched applications (e.g.

voice) are carried over IP (e.g. Voice over IP, VoIP).

Move your mouse pointer over the items in the architecture figure for a

short introduction to each item.


2/17

LTE networks can be deployed both in existing and new frequency bands

such as:

the 900 and 1800 MHz bands widely used for GSM

the 850 and 1900 MHz bands used for GSM in North and South America

the new 700 MHz band previously used for analog television broadcasting

the 2100 MHz band and the combined 1700/2100 MHz band mainly used

for 3G (WCDMA and HSPA) systems outside and within America,

respectively

the new 2600 MHz band that is becoming available for mobile systems in

many parts of the world.

LTE will be initially deployed in the 2100 MHz and 1700/2100 MHz

frequency bands.

The LTE air interface supports both frequency division duplexing (FDD)

and time division duplexing (TDD). In FDD, the uplink and downlink signals

are carried in different parts of the paired frequency band, as shown in


3/17

the figure. In TDD, the uplink and downlink transmission takes place

during different time intervals within the same spectral bandwidth.

In practice, a frequency band is split up into several portions, depending

on the geographical location where each portion is allocated to a certain

network operator. LTE offers the possibility to further split up theallocated portion into a number of channels with a variety of bandwidths

between 1.4 and 20 MHz.


4/17

Whereas GSM is based on Time Division Multiple Access (TDMA), and

WCDMA and HSPA are based on Code Division Multiple Access, the LTE

physical layer is based on Orthogonal Frequency Division Multiple Access(OFDMA) in the downlink and Single-Carrier Frequency Division Multiple

Access (SC-FDMA) in the uplink direction.

The second part of this course is entirely devoted to explaining the basic

operation of these multiple access methods. For instance, the concept of

subcarriers in the frequency domain should be familiar at this point.

Obviously, the physical structure of the LTE interface contains more than

just the multiple access method. The third part of the course addresses

among others such issues as the frame structure, the basic idea of using

resource blocks, the physical channels in downlink and uplink, andadaptive resource allocation.

The course also briefly describes the protocol layers located above the

physical layer.


5/17

The LTE radio interface is standardised in the 36-series of 3GPP Release 8.

The detailed physical layer structure is described in five physical layer

specifications.

The LTE radio technology offers the following benefits:

LTE offers peak data rates of up to 173 Mbit/s in downlink (assuming 2 x 2

MIMO and 20 MHz channel bandwidth) and up to 58 Mbit/s in uplink.

LTE enables round trip times (RTT) of less than 20 ms. The round trip time

or user plane latency is the time it takes for information to travel from the

mobile terminal to the destination in the network and back to the

terminal.

Also the control plane latency - the time needed to allocate transport

resources - is important. The requirement for the control plane latency in

LTE is less than 100 ms.

Contrary to HSPA, LTE offers packet scheduling in the frequency domain in

addition to packet scheduling in the time domain. This feature greatly

increases the spectrum efficiency of LTE.


6/17

The LTE capacity or spectrum efficiency is two to four times higher than

that of a 3GPP Release 6 HSPA system.

A major advantage of LTE over WCDMA or HSPA is the possibility of

allocating spectrum bandwidths of varying size to the mobile users.

LTE offers several channel bandwidth values between 1.4 and 20 MHz. By

contrast, the channel bandwidth in WCDMA or HSPA is always fixed at 5

MHz.

A small channel bandwidth allows easier spectrum refarming and is

beneficial for mobile operators short on spectrum.

On the other hand, a large channel bandwidth is required if large peak

data rates are to be supported.


7/17

A basic concept in LTE is Orthogonal Frequency Division Multiplexing

(OFDM). OFDM forms the basis for OFDMA in the downlink, and is also

closely related to SC-FDMA in the uplink.

Let us briefly show how the OFDM symbols are generated in the

transmitter and processed in the receiver.

In the transmitter, a sequence of modulated symbols is mapped - together

with reference information - onto N subcarriers to which the Inverse Fast

Fourier Transform (IFFT) is applied. The output signal samples are then

converted into a serial sequence and a so-called cyclic prefix is added,

resulting in an OFDM symbol. After some additional processing

(windowing, D/A conversion, frequency up-conversion, RF processing, etc.)

the OFDM signal is transmitted over the radio channel.

In the receiver, again after some conventional processing not explicitly

shown, the cyclic prefix is removed from the OFDM symbol, the signal is

converted into a parallel set of samples, and the Fast Fourier Transform

(FFT) is applied to these samples. Using the reference signals, the

equaliser is able to remove the amplitude and phase distortion of the


8/17

signal-carrying subcarriers. Finally, these subcarriers are de-mapped and

converted into the original serial symbol sequence.

In the transmitter, before the actual OFDM signal processing, the user

data is encoded, using turbo coding or convolution coding, and modulated,

using QPSK, 16QAM, or 64QAM.

Similarly, in the receiver the serial symbol stream obtained after the

OFDM signal processing is demodulated and decoded, providing the

original user data.

The selected modulation scheme and coding rate depend among others on

the radio channel conditions, and can be changed once every millisecond

based on feedback received in the uplink direction. This is called adaptive

modulation and coding (AMC), a concept already employed in HSPA.

Note that the support of 64QAM is optional in the uplink direction.


9/17

In the transmitter, after the IFFT processing, the OFDM symbol is

extended with a cyclic prefix. The purpose of adding a cyclic prefix is to

avoid intersymbol interference, in other words interference between

successive OFDM symbols, in the receiver.

The cyclic prefix is constructed by taking the end part of the IFFT output

waveform and copying it in front of the waveform. Note that the cyclic

prefix part of the OFDM symbol will not be utilised for FFT processing in

the receiver.

In order to understand why it is necessary to use a cyclic prefix, let us

consider a typical multipath propagation environment. In our example,

there is the direct propagation path between the base station and the

mobile device, a second path with asmall delay, and a third path with a

large delay. The replicas of the transmitted signal are received with

different delays, causing the multipath delay spread of the radio channel.

The cyclic prefix should be designed such that it is always longer than the

multipath delay spread, in order to avoid inter-symbol interference


10/17

between successive OFDM symbols. Note that in our example, the cyclic

prefix is too short, so there will be inter-symbol interference!

Let us illustrate the importance of using a cyclic prefix with a small

example.

The figure shows a sequence of OFDM symbols with the n:th subcarrier

displayed. Each subcarrier has an integer number of cycles in the FFT

processing interval, as in our example three cycles. As a result, all

subcarriers are mutually orthogonal within the FFT processing interval.

Also notice that the amplitude and phase of each subcarrier changes from

one OFDM symbol to another.

In a multipath environment, delayed replicas of a subcarrier add up in the

receiver. If the multipath delay spread is less than the duration of the

cyclicWithin the FFT processing interval, the resultant subcarrier is a pure

sinusoid, since the sum of sinusoids with a certain frequency is still a

sinusoid with this same frequency.


11/17

However, if the multipath delay spread is larger than the duration of the

cyclic prefix, the resultant subcarrier within the FFT processing interval is

not a pure sinusoid any longer. This causes intersymbol interference

which cannot be removed in the equaliser. To put it another way, the

subcarriers are not mutually orthogonal any more within the FFT

processing interval.

In the previous animation, we observed how the multipath radio channel

caused amplitude and phase distortion in the received subcarriers. This

distortion has to be removed or "equalised" in the receiver.

For this purpose, so-called reference or pilot signals with known signal

values are added to the signal-carrying subcarriers before applying the

IFFT operation. In the receiver, these reference signals are utilised duringthe equalisation process.

Since the signal values of the reference signals are known, the amplitude

and phase distortion of these subcarriers can be calculated and corrected

in the receiver. Using interpolation techniques, the distortion of the

signal-carrying subcarrierslocated between the reference signal

subcarriers can also be estimated and corrected.


12/17

Equalisation in the frequency domain using reference signals is easier to

perform than equalisation in the time domain, especially when the

multipath delay spread causes severe frequency-selective fading. This

insight led to the development of the SC-FDMA scheme used in the uplink,

as will be explained later.

Up to this point, we have been looking at the OFDM technique. But the

downlink multiple access method is called OFDMA. So what is the

difference between OFDM and OFDMA?

The answer is simple. In the case of OFDMA, subcarriers for several users -

instead of a single user - are multiplexed or combined into a larger set of

subcarriers to which the IFFT operation is applied. Information on which

subcarriers belong to which user is also sent in the downlink. A certainuser - for instance user 3 - can then utilize the relevant subcarriers and

disregard the other subcarriers at the receiving end.


13/17

In the uplink direction, Single-Carrier Frequency Division Multiple Access

(SC-FDMA) was chosen instead of OFDMA for the LTE radio interface.

The advantage of SC-FDMA over OFDMA is that SC-FDMA has a lower peak-

to-average power ratio (PAPR), which means less power consumption and

less expensive RF amplifiers in the terminal.

SC-FDMA signal processing has some similarities with OFDMA signal

processing. As a result, parameterisation of downlink and uplink can be

harmonised.


14/17

n comparison with OFDMA, we see that SC-FDMA simply adds another FFT

block in the transmitter and another IFFT block in the receiver.

In the transmitter, the basic task of the FFT / IFFT pair is to multiplex

different uplink signals in the frequency domain - hence the name

Frequency Division Multiple Access (FDMA). Different users are allocated

different M-point subsets of the N-point sample space of the IFFT

processing block. In our example, there are four users, M = 4 for each

user, and N = 16.

On the receiver side, the FFT / IFFT pair performs the demultiplexing of

the desired uplink signal. In addition, equalisation in the frequencydomain

is possible, provided the uplink signal also contains the required reference

signals. As in the case of OFDMA, a cyclic prefix must be added to eachSC-FDMA symbol to avoid intersymbol interference due to the multipath

delay spread in the radio channel.

Obviously, it is essential that the signals from different users are perfectly

synchronised. This is achieved by using special timing correction signals.


15/17

The time domain waveforms of the SC-FDMA symbols depend on how the

M subcarriers are mapped to the N-point IFFT block.

In the case of localised subcarrier mapping, a block of M consecutive

subcarriers is reserved for a certain user. The other subcarriers are set to

zero, in other words, these subcarrier locations are available for other

multiple access users in the uplink. Localised subcarrier mapping is the

mapping method used in LTE.

At the other extreme, interleaved subcarrier mapping means that the M

subcarriers are evenly distributed over the N possible subcarrier locations.

This mapping method produces the lowest possible peak-to-average

power ratio (PAPR), but scheduling in the frequency domain is less

efficient. Consequently, this mapping method is not used in LTE.


16/17

LTE employs the Hybrid Automatic Repeat reQuest (HARQ) fast

retransmission scheme both in downlink and in uplink. Up to eight HARQ

processes can be active both in downlink and in uplink at the same time.

The HARQ scheme provides error correction by soft-combining the

information received in successive retransmissions until the packet is

received correctly. This process is known as incremental redundancy.

HARQ uses a stop-and-wait protocol. After transmitting a data block, the

transmitting entity waits until it receives an acknowledgment (ACK) or

negative acknowledgement (NACK) before transmitting the(NACK) before

transmitting the next data block or retransmitting the error-containing

data block.

Downlink HARQ processes are asynchronous in time. Retransmissions are

possible in any order without fixed timing. As a result, HARQ-related

information such as the HARQ process identifier must be sent over the

PDCCH in parallel with the data sent over the PDSCH.

In contrast, uplink HARQ processes are synchronous in time. If the data of

a certain HARQ process is sent in subframe n, the ACK or NACK is sent


17/17

back over the Physical Hybrid ARQ Indicator Channel in subframe n+4. The

data block is then retransmitted - or the next data block is sent - in

subframe n+8.

utran long term evolution 1

Documents