OFDM AND OFDMA TECHNOLOGIES

OUTLINE NEED FOR MULTI-CARRIER

OFDM ENTERS INTO THE PICTURE

FFT / IFFT

GUARD TIME INSERTION

OFDM DRAWBACKS

CHANNEL ESTIMATION

OFDM BLOCK DIAGRAM

SIMULATION RESULTS

NEED FOR MULTI-

CARRIER Time Domain Analysis

NEED FOR MULTI-

CARRIER cont.

Pulse completely distorted.

ISI is significant in this case.

Pulse extended but the extension

are much smaller than T the output

behaves like the transmitted

rectangular pulse.

NEED FOR MULTI-

CARRIER cont.

Frequency Domain Analysis

NEED FOR MULTI-

CARRIER cont.

Conclusion

Wide pulses is needed for simple

equalization,

But

Narrow pulses is needed for high data rate

Solution

Multiplexing

NEED FOR MULTI-

CARRIER cont.

NEED FOR MULTI-

CARRIER cont.

Problem

Solution Orthogonality

NEED FOR MULTI-CARRIER cont.

NEED FOR MULTI-CARRIER cont.

OFDM ENTERS INTO THE

PICTURE Interference Orthogonality

B.W efficiency Min Separation

OFDM ENTERS INTO THE

PICTURE cont. Min Separation

Problem ◦ Difficult Implementation with traditional

oscillators

Solution ◦ DFT

But

◦ DFT needs high processing

Solution

◦ Easy implementation using FFT/IFFT

FFT / IFFT

IFFT DAC

Channel

ADC FFT

FFT/IFFT

GUARD TIME

INSERTION

X1 X2 …. Xn ….

Y1 Y2 …. Yn ….

Channel Filtering

hv …. h1 h0

GUARD TIME INSERTION

cont.

X1 X2 …. Xn ….

Y’’1 Y’’2 Yv+1 Yn Yv+2 …. Y’’v

X’1 X’2 …. X’n ….

….

Problem

ISI occurs

hv …. h1 h0

GUARD TIME INSERTION

cont.

X1 X2 …. Xn Xn-v+1

Xn ….

Y1 Y2 Yv+1 Yn Yv+2 …. Yv

X’1 X’n ….

….

Solution Cyclic Prefix

No ISI

Circular Convolution achieved.

hv …. h1 h0

Cyclic prefix

The CP allows the receiver to absorb much more

efficiently the delay spread due to the multipath

and to maintain frequency Orthogonality.

The CP that occupies a duration called the Guard

Time (GT), often denoted TG, is a temporal

redundancy that must be taken into account in data

rate computations.

OFDM DRAWBACKS cont.

Peak to Average Power Ratio (PAPR)

OFDM DRAWBACKS cont.

Sensitivity to frequency offset

Pilot

Signal

Extraction

Lowpass

FIR

Filter

Pilot

Signal

Estimation

CHANNEL ESTIMATION

Pilot Based Channel Estimation

Received

Signal after

FFT

Estimated

Channel

Response

Known Pilots

CHANNEL ESTIMATION

cont.

Fre

qu

en

cy(

su

b c

arr

iers

)

Data symbols

Pilot

symbols

Time (OFDM Symbols) Time (OFDM Symbols) F

req

ue

ncy(

su

b c

arr

iers

)

Pilot Arrangement Types

Block Pilot Patterns Comb Pilot Patterns

High channel frequency

selectivity rapid changing channels

OFDMA OFDMA is a multiple access method based on

OFDM

signaling that allows simultaneous transmissions to and from several users along with the other advantages of OFDM.

OFDM versus OFDMA

IEEE802.16d IEEE802.16e

Fixed WiMAX,256-OFDM Mobile WiMAX

DIVERSITY AND MIMO PRINCIPLES

What is diversity? Is a technique that combats the fading by ensuring that there will be many copies of the transmitted signal effected with different fading over time, frequency or space.

Diversity types

Time diversity

Frequency diversity

Spatial diversity

1- Time diversity:

We averaging the fading of the

channel over time by using :

1-The channel coding and

interleaving.

2-Or sending the data at different

times.

to explain this we will see an

example:

1-time diversity:

No interleaving x1 x2 x3 x4 y1 y2 y3 y4 z1 z2 z3 z4 h1 h2 h3

h4

interleaving x1 y1 z1 h1 x2 y2 z2 h2 x3 y3 z3 h3 x4 y4 z4

h4

So we can see that only the 3rd symbol from each codeword lost and

we can recover them from the rest symbols in each codeword.

|H(t)|

t

2- frequency diversity:

This type of diversity used for the

frequency selective channels as we

will averaging the fading over the

frequency by using:

1-Multi-carrier technique like OFDM.

2-FHSS (frequency hope spread

spectrum).

3-DSSS (direct sequence spread

spectrum).

2- frequency diversity:

We can see that each sub-band will effecting with different fading over the frequency.

3-spatial diversity:

we will have many copies of the transmitted signal effects

with different fading over the space .

we use multi-antenna systems at the transmitter or the

receiver or at both of them.

Spatial diversity

MISO SIMO MIMO MIMO-

MU

Receive diversity:

1-The receiver will has many antennas .

2-Each one has signal effecting with different

fading.

3-number of different paths =Mr.

Diversity order=Mr

MIMO:

In this type we use multi antennas at both the transmitter and receiver as shown.

Diversity order=Mt x Mr

Notes: The higher diversity order we have the better we combat the fading

Notes:

1-The diversity

reduces the BER

of the

communication

system.

2-Diversity order

BER .

Notes:

The distance between the antennas must be larger than the coherent distance to ensure that data streams are not correlated .

Question?

How the receiver get the signal from

the many copies reached ?

Answer

Diversity combining techniques

Selective combining SC

Maximal ratio combining

MRC

Equal gain combining

EGC

Diversity combining technique

1-Combines the independent fading paths signals to obtain a signal that passed through a standard demodulator.

2-The techniques can be applied to any type of diversity.

3-combining techniques are linear as the output of is a weighted sum of the different fading signals of branches.

4-It needs co-phasing.

Diversity combining technique

The signal output from

the combiner is the

transmitted signal

s(t) multiplied by a

random complex

amplitude term

Random SNR

from the

combiner

Fading of the path

Type of

technique

Diversity order

Diversity combining technique

Types of combining techniques

Selection combining

Threshold combining

Maximal ratio combining

Equal gain combining

selection combining technique

1-the combiner

outputs the

signal on the

branch with the

highest SNR .

2-no need here for

the co-phasing. 0 0 0 1

Threshold combining technique

As in SC since only one branch output is used at a time and outputting the first signal with SNR above a given threshold so that co-phasing is not required.

Special case at diversity order

=2 (SSC)

Does not take the

largest SNR so that

its performance less

than the SC

technique.

Maximal ratio combining

In maximal ratio combining (MRC) the output is a weighted sum of all branches due to its SNR

h1* h2* h3* hi*

Equal gain combining

technique A simpler technique is equal-gain

combining, which co-phases the signals

on each branch and then combines them

with equal weighting

MIMO

Traditional diversity is based on multiple receiver

antennas

Multiple-In Multiple-Out (MIMO) is based on both

transmit and receive diversity

Also known as Space Time Coding (STC)

With Mt transmission antennas and Mr receiver

antennas we have Mt Mr branches

Tx and Rx processing is performed over space

(antennas) and time (successive symbols)

47

MIMO or STC

In Mobile communication systems it may be difficult

to put many antennas in the mobile unit

Diversity in the downlink (from base station to

mobile station) can be achieved by Multiple-In

Single-Out (MISO) (i.e., Mr=1)

In the uplink (from mobile station to base station)

diversity is achieved my conventional diversity

(SIMO)

Hence, all diversity cost is moved to the base

station

All 3G and 4G mobile communication system

employ MIMO in their standard

48

Type of MIMO

Two major types of space time coding

◦ Space time block coding (STBC)

◦ Space time trellis coding (STTC)

STBC is simpler by STTC can provide

better performance

STBC is used in mobile

communications. STTC is not used in

any systems yet

We will talk only about STBC

49

Space Time Block Codes

There are few major types ◦ Transmit diversity: main goal is diversity gain

◦ Spatial multiplexing: main goal is increase data rate

◦ Eigen steering: main goal is both. Requires knowledge of the channel at the transmitter side

◦ Mix of the above: Lots of research

Transmit diversity, spatial multiplexing and simplified version of Eigen steering are used in 3G and 4G standards

While in 3G standards MIMO was an enhancement, in 4G MIMO is a main part 50

Transmit Diversity

Take Mt=2 and Mr=1

Two symbols so and s1 are transmitted over

two transmission periods

No change in data rate (denoted as rate 1

STBC)

Channel is known at receiver only

51

Transmit Diversity

Transmission matrix:

Transmission matrix columns are orthogonal to guarantee simple linear processing at the receiver

Other transmission matrices are defined in literature

Received signal is:

Performance is same as MRC with M=2

However, if Tx Power is the same, then transmit diversity (2x1) is 3 dB worse than (1x2)

52

1

* *

11 1 1

oo o o

o

s sr g nR

s sr g n

1

1

* *

1 1

oAnt Ant

o o

o

s s TimeS

s s Time

Transmit Diversity

Take Mt=2 and Mr=2

Performance is the same as MRC with M=4

However, if Tx Power is the same, then transmit

diversity (2x2) is 3 dB worse than (1x4)

53

Performance

MRRC=Maximal Ratio Receiver Combining

Note 3 dB difference in favor of Rx MRC diversity

Reference: S. Alamouti, a simple transmit diversity

technique for wireless communications,

IEEE JSAC, October 98

54

Order 2

Order

4

No diversity

Spatial Multiplexing

Purpose is to increase data rate (2x2 gives twice

data rate)

The 4 gains must be known at receiver

Simplest way zero forcing algorithm:

55

1 1o o or s g s g

1 2 1 3or s g s g

1

2 31 1

oo o

G

g gr s

g gr s

1

1 1

ˆ

ˆ

o oH Hs r

G G Gs r

Spatial Multiplexing

Optimum method: Maximum Likelihood

◦ Try all combinations of s1 and s2

◦ Find the combination that minimizes the squared error:

◦ Complexity increases with high order modulation

56

2 22 2

1 1 1 1 2 1 3ˆ ˆ ˆ ˆ

o o o o oe e r s g s g r s g s g

1 1o o or s g s g

1 2 1 3or s g s g

Performance Equal rate

comparison

Reference: David

Gesbert, Mansoor

Shafi, Da-shan

Shiu, Peter J.

Smith, and Ayman

Naguib, From

theory to practice:

an overview of

MIMO space–time

coded wireless

systems, IEEE

JSAC, April 2003

57

Zero forcing

ML

Alamouti

Eigenvalue Steering

Assume a MIMO system

58

Eigenvalue Steering Example with Mt = 2 and Mr=4

Any matrix H can be represented using Singular Value Decomposition as

U is Mr by Mr and V is Mt by Mt unitary matrices

is Mr by Mt diagonal matrix, elements σi

59

y H x n 1 11 12 1

2 21 22 1 2

3 31 32 2 3

4 41 42 4

H

y h h n

y h h x n

y h h x n

y h h n

HH U V

Eigenvalue Steering

Using transmit pre-coding and receiver shaping

60

H

H H

H H

H H H

y U H x n

U U V x n

U U V V x n

U U V V x U n

x n

Eigenvalue Steering

This way we created r paths between the Tx and

specific Rx without any cross interference

The channel (i.e., Channel State Information) must

be known to both transmitter and receiver

The value of r = rank of matrix H, r min(Mt, Mr)

Not all r paths have good SNR

Data rate can increase by factor r

See Appendix C for Singular Value Decomposition

See Matlab function [U,S,V] = svd(X)

61

Example

Reference: Sanjiv Nanda, Rod Walton, John Ketchum, Mark

Wallace, and Steven Howard, A high-performance MIMO OFDM

wireless LAN, IEEE Communication Magazine, February 2005

62

INTRODUCTION TO LTE AND ITS

UNIQUE TECHNOLOGIES.

What is LTE??

The 3GPP LTE is acronym for “long term evolution of UMTS “.

In order to ensure the competitiveness of UMTS for the next 10 years and beyond, concepts for UMTS Long Term Evolution (LTE) have been introduced in 3GPP release 8.

LTE is also referred to as EUTRA (Evolved UMTS Terrestrial Radio Access) or E-UTRAN (Evolved UMTS Terrestrial Radio Access Network)

What is LTE(cont.)?

The architecture that will result from this work is called EPS (Evolved Packet System) and comprehends E-UTRAN (Evolved UTRAN) on the access side and EPC (Evolved Packet Core) on the core side.

Can be considered the real 3.9G & invited to join the 4G family.

Also considered a competitive system to mobile WiMAX as we will show

What is LTE (cont.)?

LTE DESIGN TARGETS

-(a) capabilities: Scalable BW: 1.25, 2.5, 5.0, 10.0 and 20.0 MHz. Peak data rate:

Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channel

Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz channel

Supported antenna configurations: Downlink: 4x4,4x2, 2x2, 1x2, 1x1

Uplink: 1x2, 1x1

Duplexing modes: FDD and TDD

Number of active mobile terminals: LTE should support at least 200 mobile terminals in the

active state when operating in 5 MHz.

In wider allocations than 5 MHz, at least 400 terminals should be supported

Spectrum efficiency Downlink: 3 to 4 x HSDPA Rel. 65bits/s/Hz

Uplink: 2 to 3 x HSUPA Rel. 62.5bits/s/hz

Latency C-plane: <50 – 100 msec to establish U-plane

U-plane: <10 msec from UE to server

Mobility Optimized for low speeds (<15 km/hr)

High performance at speeds up to 120 km/hr

Maintain link at speeds up to 350 km/hr

Coverage Full performance up to 5 km

Slight degradation 5 km – 30 km

Operation up to 100 km should not be precluded by standard

INTRODUCTION TO LTE KEY TECHNOLOGIES

-)OFDM and OFDMA:1( One of the key technologies used in LTE and WiMAX

systems.

The problem ???

Due to the multipath the signal is received from many paths with

different phases that will result in

DELAY SPREAD :symbol received along a delayed path to

“bleed” into a subsequent symbol (ISI)

FREQUENCY SELECTIVE FADING: : some frequencies

within the signal passband undergo constructive interference

while others encounter destructive interference.The composite

received signal is distorted

Old solutions of multipath fading include direct channel

equalization or spread spectrum techniques(complex receiver is

needed).

OFDM: 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 and also this satisfy that the output

convolution with channel is complex gain multiplication.

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

(due either to local oscillator offset or Doppler shifts) and a large

signal peak-to-average power ratio (PAPR).

OFDMA

OFDMA is a multiple access method based on OFDM

signaling that allows simultaneous transmissions to and from several users along with the other advantages of OFDM.

OFDM versus OFDMA

IEEE802.16d IEEE802.16e

Fixed WiMAX,256-OFDM Mobile WiMAX

) Multi antenna transmission2(

LTE and WiMAX targets extreme

performance in terms of

◦ Capacity

◦ Coverage

◦ Peak data rates

Advanced multi-antenna solutions is

the key tool to achieve this

Multi antenna systems are integral part of

those systems

Different antenna solutions needed for

different scenarios/targets

◦ High peak data rates spatial multiplexing

◦ Good coverage Beam-forming

◦ High performanceDiversity

)Hybrid ARQ with soft 3(

combining

used in LTE and WiMAX to allow the terminal to rapidly request retransmissions of erroneously received transport blocks.

The underlying protocol multiple parallel stop-and-wait hybrid ARQ processes

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.

)Spectrum flexibility:1(

A high degree of spectrum flexibility is

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.

The flexibility includes: ◦ Different duplex arrangements.

◦ Different frequency-bands-of-operation.

◦ Different sizes of the available spectrum.

Duplex arrangement –G LTE 3(a)

Bandwidth flexibility –G LTE 3(b)

LTE physical layer supports any bandwidth from 1.25

MHz to well beyond 20 MHz in steps of 200 kHz (one

”Resource Block”)

dependent scheduling and rate -) Channel2(

adaptation LTE use of shared-channel transmission,

in which the time-frequency resource is dynamically shared between users.

)Interference coordination(soft reuse)3(

Adaptive reuse ◦ Cell-center users: Reuse = 1

◦ Cell-edge users: Reuse > 1

Relies on access to frequency domain ◦ Applicable for both downlink OFDM and

uplink SC-FDMA

(4)SC-FDMA:- LTE uplink requirements differ from downlink

requirements.

power consumption is a key consideration for UE

terminals.

The high PAPR and related loss of efficiency associated

with OFDM signaling are major concerns.

As a result, an alternative to OFDM was sought for use

in the LTE uplink.

Single Carrier – Frequency Domain Multiple Access

(SC-FDMA) is well suited to the LTE uplink

requirements.

The basic transmitter and receiver architecture is very

similar (nearly identical) to OFDMA,

and it offers the same degree of multipath protection.

because the underlying waveform is essentially single-

carrier, the PAPR is lower.

Basic block diagram:

transmitter :a QAM modulator coupled with the addition

of the cyclic prefix. This will eliminate ISI as OFDMA

Reciever: by using FFT & CP simple equalizer are

used (as OFDM).

Multipath distortion is handled in the same manner as in

OFDM(removal of CP, conversion to the frequency

domain, then apply the channel correction on a subcarrier-

by subcarrier basis).

-FDMA :-LTE practical SC

The practical transmitter is likely to take advantage of FFT/IFFT blocks

as well to place the transmission in the correct position of the transmit spectrum in case of variable transmission bandwidth.

SC-FDMA receiver

Frequency domain equalization (FDE) using

DFT/IDFT is more practical for such channels.

The fact of transmitting only a single symbol at a time

ensures a low transmitter waveform, compared with the

OFDMA case.

The resulting PAR/CM impact on the amplifier is thus directly

dependent on the modulation, whereas with the OFDMA case

it is the amount of subcarriers.

SC-FDMA subcarriers can be mapped in one of two ways:

localized or distributed

However, the current working assumption is that LTE will use

localized subcarrier mapping.

This decision was motivated by the fact that with localized

mapping, it is possible to exploit frequency selective gain

via channel dependent scheduling (assigning uplink

frequencies to UE based on favorable propagation

conditions).

(5) LTE Multicast/Broadcast MBMS – Multimedia Broadcast/Multicast Service

OFDM allows for high-efficient MBSFN operation ◦ Multicast/Broadcast Single-Frequency Networking

◦ Identical transmissions from set of tightly synchronized cells

◦ Increased received power and reduced interference

Substantial boost of MBMS system throughput

LTE allows for multicast/broadcast and unicast on the same carrier as well as dedicated multicast/broadcast carrier

LTE RADIO INTERFACE ARCHITECTURE

Introduction

Similar to WCDMA/HSPA, as well as to most other modern communication systems, the processing specified for LTE is structured into different protocol layers.

note that the LTE radio-access architecture consists of a single node –the eNodeB. The eNodeB communicates with

one or several mobile terminals, also known as UEs

Packet Data Convergence

Protocol (PDCP) performs IP header compression

to reduce the number of bits to transmit over the radio interface.

The header compression mechanism is based on Robust Header Compression (ROHC)a standardized header-compression algorithm also used in WCDMA

PDCP is also responsible for ciphering and integrity protection of the transmitted data. At the receiver side, the PDCP protocol performs the corresponding deciphering and decompression operations.

There is one PDCP entity per SAE bearer configured for a

mobile terminal

Radio Link Control (RLC) is responsible for segmentation/concatenation, retransmission

handling, and in-sequence delivery to higher layers.

Unlike WCDMA, the RLC protocol is located in the eNodeB since there is only a single type of node in the LTE radio-access-network architecture.

The RLC offers services to the PDCP in the form of radio bearers .

There is one RLC entity per radio bearer configured for a terminal.

Medium Access Control

(MAC) handles hybrid-ARQ retransmissions and uplink

and downlink scheduling.

The scheduling functionality is located in the eNodeB, which has one MAC entity per cell, for both uplink and downlink.

The hybrid-ARQ protocol part is present in both the transmitting and receiving end of the MAC protocol.

The MAC offers services to the RLC in the form of

logical channels .

MAC scheduling

The basic operation of the scheduler is so-called dynamic scheduling, where the eNodeB in each 1 ms TTI makes a scheduling decision and sends scheduling

information to the selected set of terminal.

Downlink

scheduling UL scheduling

dynamically controlling

the terminal(s) to

transmit to

the set of resource

blocks upon which the

terminal’s DL-SCH

should be transmitted.

Transport-format

selection(selection of

transport-block size,

modulation scheme, and

antenna mapping)

And logical-channel

multiplexing for downlink

transmissions

dynamically control

which mobile terminals

are to transmit on their

UL-SCH

and on which uplink

time/frequency resources

uplink scheduling

decision is taken per

mobile terminal and not

per radio bearer.

Physical Layer (PHY)

handles coding/decoding, modulation/demodulation, multi-antenna mapping, and other typical physical layer functions.

The physical layer offers services to the MAC layer

in the form of transport channels

DOWNLINK PHY LAYER OF (LTE)

LTE Generic Frame Structure The generic frame structure is used with FDD.(TDD is also

supported but not the trend). LTE frames are 10 msec in duration. They are divided into 10 subframes, each subframe being 1.0 msec

long. Each subframe is further divided into two slots, each of 0.5 msec

duration. Slots consist of either 6 or 7 ODFM symbols, depending on whether

the normal or extended cyclic prefix is employed.

Different time intervals within the LTE radio-access specification are defined as multiples of a basic time unit Ts = 1/30 720 000.

The time intervals can thus also be expressed as

Tframe = 307 200 Ts and Tsubframe = 30 720 Ts

-OFDMA For LTE Downlink: OFDMA is an excellent choice of multiplexing scheme for the 3GPP LTE

downlink

allows the access of multiple users on the available bandwidth.

Each user is assigned a specific time-frequency resource.

Allocation of PRBs is handled by a scheduling function at the 3GPP base station (eNodeB).

The total number of available subcarriers depends on the overall transmission bandwidth of the system. The LTE specifications define parameters for system bandwidths from 1.25 MHz to 20 MHz as shown in Table.

A PRB is defined as consisting of 12 consecutive subcarriers for one slot (0.5 msec) in duration.

A PRB is the smallest element of resource allocation assigned by the base station scheduler.

LTE does not employ a PHY preamble to facilitate carrier offset estimate, channel estimation, timing synchronization etc. Instead, special reference signals are embedded in the PRBs

Downlink resource block the OFDM subcarrier spacing has been chosen to Δf = 15 kHz. Sampling rate fs =15 000NFFT , where NFFT is the FFT size

the sampling rate Δf NFFT will be a multiple or submultiple of the WCDMA/HSPA chip rate (3.84 Mcps)

in the frequency domain the downlink subcarriers are grouped into resource blocks

where each resource block consists of 12 consecutive subcarriers. In addition, there is an unused DC-subcarrier in the center of the downlink band. it may be subject to un-proportionally high interference, for example, due to local-oscillator leakage.

Downlink reference signal

To carry out coherent demodulation of different downlink physical channels,

a mobile terminal needs estimates of the downlink channel

◦ Cell-specific downlink reference signals.

◦ UE-specific reference signal.

◦ MBSFN reference signals

Cell-specific downlink reference

signals consists of known reference symbols inserted within the first and third last

OFDM symbol of each slot and with a frequency-domain spacing of six subcarriers

the mobile terminal should carry out interpolation/averaging over multiple reference symbols

There are 504 different reference-signal sequences defined for LTE, where each sequence corresponds to one out of 504 different physical-layer cell identities

In case of downlink multi-antenna transmission the mobile terminal should be able to estimate the downlink channel corresponding to each transmit antenna

reference-signal structure for each antenna port in case of multiple antenna ports within a cell: ◦ In case of two antenna the reference symbols of the second antenna

port are frequency multiplexed with the reference symbols of the first antenna port, with a frequency-domain offset of three subcarriers.

◦ In case of four antenna ports ,the reference symbols for the third and fourth antenna ports are frequency multiplexed within the second OFDM symbol of each slot. Note that the reference symbols for antenna port three and four are only transmitted within one OFDM symbol

UE-specific reference signals

LTE also allows for more general beam-forming. In order to allow for channel estimation also for such transmissions, additional reference signals are needed.

As such a reference signal can only be used by the specific terminal to which the beam-formed transmission is intended, it is referred to as a UE-specific reference signal .

LTE block diagram (DL transport

channel processing)

(1)CRC insertion:

In the first step of the transport-channel processing, a

24-bit CRC is calculated for and appended to each transport block.

The CRC allows for receiver side 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 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

An additional (24 bits) CRC is calculated for and appended to each code block.

Having a CRC per code block allows for early detection of correctly decoded code blocks. This can be used to reduce the terminal processing effort and power consumption.

(3) FEC(forward error

correction):-

The UL-SCH uses the same rate 1/3 turbo encoding scheme (two 8-state constituent encoders and one internal interleaver) as the DL-SCH.

•The older interleaver used in HSPA been replaced by QPP based interleaving . •the QPP interleaver provides a mapping from the input (non-interleaved) bits to the output (interleaved) bits according to the function:

(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.

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

(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 (usually a gold code).

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

(6) Modulation The set of modulation schemes supported for the LTE downlink

includes QPSK, 16QAM, and 64QAM. All these modulation schemes are applicable to the DL-SCH, PCH, and

MCH transport channels. only QPSK modulation can be applied to the BCH transport channel.

(7) 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

LTE transmit diversity is based on Space Frequency Block Coding (SFBC)

SFBC implies that consecutive modulation symbols Si and Si+1 are mapped directly on adjacent subcarriers on the first antenna port.

On the second antenna port, the swapped and transformed symbols - S*

i+1 and Si*are transmitted on the corresponding subcarriers

SFBC/FSTD(combined SFBC

and )Frequency Shift Transmit

Diversity

Closed loop Spatial multiplexing

spatial multiplexing implies that multiple streams or ‘ layers ’ are transmitted in parallel, thereby allowing for higher data rates

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.

General beam-forming closed-loop spatial multiplexing includes beam-forming as a special

case when the number of layers equals one. This kind of beamforming can be referred to as codebook-based

beam-forming , indicating that ◦ the network selects one pre-coding vector (the beam-forming vector)

from a set of pre-defined pre-coding vectors (the ‘ codebook ’ ) with the selection, for example, based on the terminal reporting a recommended pre-coding vector.

◦ if not following the terminal recommendation, the network must explicitly inform the terminal about what pre-coding vector, from the set of predefined vectors, is actually used for transmission to the terminal.

UPLINK PHY LAYER OF (LTE)

Uplink transmission scheme

LTE uplink transmission is based on so-called DFTS-OFDM transmission

Which is a‘ single-carrier ’ transmission scheme that allows for ◦ flexible bandwidth assignment

◦ orthogonal multiple access not only in the time domain but also in the frequency domain.

◦ the use of a cyclic prefix allows low-complexity frequency-domain equalization at the receiver side.

Transmission method

“M”

determines

the BW

According to

OFDM mod.

position of signal

is determined

Mapping is

applied to

consecutive

carriers

localized

DFT implementation The DFT size should preferably be constrained to a power of two. However, such a constraint is in direct conflict with a desire to

have a high degree of flexibility of the bandwidth that can be dynamically assigned to a mobile terminal for uplink transmission all possible DFT sizes should rather be allowed.

For LTE, a middle way has been adopted where the DFT size is limited to products of the integers two, three, and five.

For example, DFT sizes of 60, 72, and 96 are allowed but a DFT size of 84 is not allowed.

In this way, the DFT can be implemented as a combination of relatively low-complex radix-2, radix-3, and radix-5 FFT processing

Uplink physical resource

parameters Chosen to be aligned, as much as possible, with the

corresponding parameters of the OFDM-based LTE downlink

◦ spacing equals 15 kHz

◦ resource blocks, consisting of 12 subcarriers

◦ Any number of uplink resource blocks ranging from a minimum of 6-110 resource blocks.

◦ time-domain structure, the LTE uplink is very similar to the downlink

However, in contrast to the downlink, no unused DC-subcarrier is defined for the LTE uplink

Uplink reference signals

Demodulation reference signals (DRS ) ◦ reference signals for channel estimation are also needed

for the LTE uplink to enable coherent demodulation of different uplink physical channels

Sounding reference signals (SRS) ◦ are transmitted on the uplink to allow for the network to

estimate the uplink channel quality at different frequencies.

Basic principles of uplink DRS

transmission Due to the importance of low power variations for

uplink transmissions

The principles for uplink reference-signal transmission are different from those of the downlink

certain DFTS-OFDM symbols are exclusively used for reference-signal transmission,

a reference signal is transmitted within the fourth symbol of each uplink slot

Uplink sequences

Limited power variations in the frequency domain to allow for similar channel-estimation quality for all frequencies.

Limited power variations in the time domain to allow for high power-amplifier efficiency.

Furthermore, sufficiently many reference-signal sequences of the same length, should be available to easily assigning reference-signal sequences to cells

Zadoff–Chu sequences have the property of constant power in both the frequency

and the time domain.

Zadoff–Chu sequences are not suitable for direct usage as

uplink: ◦ to maximize the number of Zadoff–Chu sequences and to maximize

the number of available uplink reference signals, prime-length Zadoff–Chu sequences would be preferred. At the same time, the length of the uplink reference-signal sequences should be a multiple of 12

◦ For short sequence lengths, corresponding to narrow uplink transmission bandwidths, relatively few reference-signal sequences would be available

Phase-rotated reference-signal

sequences by cyclically extending different prime-length Zadoff – Chu

sequences .

Additional reference-signal sequences can be derived by applying different linear phase rotations to the same basic

reference-signal sequences

sounding reference signals

(SRS) estimate the uplink channel quality at different frequencies

A terminal can be configured to transmit SRS at regular intervals ranging from as often as once in every 2 ms (every second subframe) to as infrequently as once in every 160 ms (every 16th frame

the frequency-domain scheduling:

◦ entire frequency band of interest with a single SRS OR

◦ narrowband SRS that is hopping in the frequency domain in such a way that a sequence of SRS transmissions jointly covers the frequency band of

interest.

Uplink transport-channel

processing uplink transport-channel

processing are similar to the corresponding steps of the downlink transport-channel processing

no spatial multiplexing or transmit diversity currently defined for the LTE uplink

As a consequence, there is also only a single transport block, of dynamic size, transmitted for each TTI.

LTE ACCESS PROCEDURE

LTE cell search Aim ◦ Acquire frequency and symbol synchronization to a cell.

◦ Acquire frame timing of the cell, that is, determine the start of the downlink frame.

◦ Determine the physical-layer cell identity of the cell.

two special signals are transmitted on the LTE downlink, ◦ the Primary Synchronization Signal (PSS)

◦ Secondary Synchronization Signal (SSS)

a terminal synchronizes to a cell. Once it knew PSS 5ms delay

acquires the physical-layer identity

(but not the identity group) of the cell using PSS .

Acquires physical layer identity group using SSS signal

detects the cell frame timing using SSS signal

Once this has been achieved, the terminal has to acquire the cell system

information

System information

In LTE, system information is delivered by two different mechanisms relying on two different transport channels ◦ A limited amount of system information, corresponding to the

so-called Master Information Block (MIB), is transmitted using the BCH.

◦ The main part of the system information, corresponding to different so-called System Information Blocks (SIBs), is transmitted using the downlink shared channel (DL-SCH).

Random access A fundamental requirement for any cellular system is the possibility

for the terminal to request a connection setup, commonly referred to as random access .

In LTE, random access is used for several purposes, including: ◦ for initial access when establishing a radio link (moving from

RRC_IDLE to RRC_CONNECTED;

◦ to re-establish a radio link after radio link failure;

◦ for handover when uplink synchronization needs to be established to the new cell;

◦ to establish uplink synchronization if uplink or downlink data arrives when the terminal is in RRC_CONNECTED and the uplink is not synchronized;

◦ as a scheduling request if no dedicated scheduling-request resources have been configured on PUCCH.

The first step consists of transmission of a random-access preamble, allowing the eNodeB to estimate the transmission

timing of the terminal. Uplink synchronization is necessary as the terminal otherwise cannot transmit any uplink data.

The second step consists of the network transmitting a timing advance command to adjust the terminal transmit timing, based on

the timing estimate in the first step. In addition to establishing uplink synchronization, the second step also assigns uplink resources to the terminal to be used in the third step in the

random-access procedure.

The third step consists of transmission of the mobile-terminal identity to the network using the UL-SCH similar to normal

scheduled data. The exact content of this signaling depends on the state of the terminal, in particular whether it is previously

known to the network or not.

The fourth and final step consists of transmission of a contention-resolution message from the network to the terminal on the DL-SCH. This step also resolves any

contention due to multiple terminals trying to access the system using the same random-access resource.

paging

Paging is used for network-initiated connection setup.

An efficient paging procedure should allow the terminal to sleep with no receiver processing most of the time and to briefly wake up at predefined time intervals to monitor paging information from the network.

In LTE, no separate paging-indicator channel is used

LTE ARCHITECTURE AND SAE

LTE System Architecture

LTE System Architecture cont.

Evolved Radio Access Network (RAN)

UE: User Equipment

eNB: enhanced Node B

-Contains PHY, MAC, RLC (Radio Link Control)

, PDCP (Packet Data Control Protocol).

eNBs are connected together through the SGW.

cont.LTE System Architecture

Functions of eNodeB:

Radio Resources management.

Admission control.

Enforcement of negotiated UL QoS.

Cell information broadcast.

Ciphering/deciphering of user and control plane data

Compression/decompression of DL/UL user plane packet headers.

cont.LTE System Architecture

Serving Gateway (SGW) -Routes and forwards user Data Packets. -Mobility anchor for eNB handovers and LTE to other 3GPP systems.

(relaying the traffic between 2G/3G systems and PDN GW).

Packet Data Network Gateway (PDN GW) -Connects UE to external packet data networks (serve IP functions) -Anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and

EvDO).

- Performs policy enforcement , charging

support.

LTE System Architecture

cont. Mobility Management Entity (MME)

-Manage the UE’s mobility.

-Idle-mode UE tracking and reachability .

-Paging procedure.

-Authentication and authorization.

- choosing the SGW for a UE at

the initial attach

-Security negotiations.

OVERVIEW OF LTE ADVANCED

Fundamental requirements for

LTE-Advanced complete fulfillment of all the requirements for IMT-

Advanced defined by ITU LTE-Advanced has to fulfill a set of basic backward

compatibility requirements ◦ Spectrum coexistence, implying that it should be possible to

deploy LTE-Advanced in spectrum already occupied by LTE with no impact on existing LTE terminals

◦ infrastructure, in practice implying that it should be possible to upgrade already installed LTE infrastructure equipment to LTE-Advanced capability

◦ terminal implementation

Extended requirements beyond

ITU requirements Support for peak-data up to 1 Gbps in the downlink and 500

Mbps in the uplink. Substantial improvements in system performance such as

cell and user throughput with target values significantly exceeding those of IMT-Advanced.

Possibility for low-cost infrastructure deployment and terminals.

High power efficiency, that is, low power consumption for both terminals and infrastructure.

Efficient spectrum utilization, including efficient utilization of fragmented spectrum

Technical components of LTE-

Advanced Wider bandwidth and carrier aggregation

Extended multi-antenna solutions

Advanced repeaters and relaying functionality

Coordinated multi-point transmission

Wider bandwidth and carrier

aggregation LTE-Advanced will be an increase of the maximum transmission

bandwidth beyond 20 MHz, perhaps up to as high as 100 MHz or even beyond

In case of carrier aggregation, the extension to wider bandwidth is accomplished by the aggregation of basic component carriers of a

more narrow bandwidth

Extended multi-antenna solutions

support for spatial multiplexing on the uplink is anticipated to be part of LTE-Advanced

extension of downlink spatial multiplexing to more four layers

benefits of eight-layer spatial multiplexing are only present in special scenarios where high SINR can

be achieved

Coordinated multi-point

transmission Coordinating the transmission from the multiple antennas can be used to

increase the signal-to-noise ratio for users far from the antenna

for example by transmitting the same signal from multiple sites.

Such strategies can also improve the power-amplifier utilization in the network, especially in a lightly loaded network where otherwise some power

amplifiers would be idle

Advanced repeaters and relaying

functionality

Repeaters simply amplify and forward the received analog signals and are used already today for handling coverage holes.

“L1 relays”schemes where the network can control the transmission power of the repeater and, for example, activate the repeater only when users are present in the area handled by the repeater

intermediate node may also decode and re-encode any received data prior to forwarding it to the served users. This is often referred to as decode-and-

forward relaying

The proposals could roughly be

categorized into: Various concepts for Relay Nodes

UE Dual TX antenna solutions for SU-MIMO and diversity MIMO

Scalable system bandwidth exceeding 20 MHz, Potentially up to 100MHz

Local area optimization of air interface

Nomadic / Local Area network and mobility solutions

Flexible Spectrum Usage

Cognitive Radio

Automatic and autonomous network configuration and operation

Enhanced precoding and forward error correction

Interference management and suppression

Asymmetric bandwidth assignment for FDD

Hybrid OFDMA and SC-FDMA in uplink

UL/DL inter eNodeB coordinated MIMO

Timeframe

Standardization is expected to be included in 3GPP Release 10 timeframe.

The importance and timeframe of LTE Advanced will of course largely depend on the success of LTE itself.

If possible LTE-Advanced will be a software

upgrade for LTE networks.

Technology

Demonstrations In February 2007 NTT DoCoMo announced the

completion of a 4G trial

where they achieved a maximum packet transmission rate of approximately 5 Gbit/s in the downlink using 100MHz frequency bandwidth to a

mobile station moving at 10 km/h