Thuyết trình Orthogonal Frequency Division Multiple Access in WiMAX and LTE A Comparison

 INTRODUCTION USE OF OFDMA IN WIMAX AND LTE  NETWORK ENTRY STEPS IN WIMAX AND LTE  PERFORMANCE BOUNDS FOR SYNCHRONIZATION AND CHANNEL ESTIMATION  EVOLUTION OF WIMAX AND LTE  CONCLU

S Srikanth and P A Murugesa Pandian, Anna University Xavier Fernando, Ryerson

University

Orthogonal Frequency Division

Multiple Access

in WiMAX and LTE: A

Comparison

Group 4 –

 INTRODUCTION

 USE OF OFDMA IN WIMAX AND LTE

 NETWORK ENTRY STEPS IN WIMAX AND LTE

 PERFORMANCE BOUNDS FOR SYNCHRONIZATION AND CHANNEL ESTIMATION

 EVOLUTION OF WIMAX AND LTE

 CONCLUSION

- IEEE 802.16e-based WiMAX and Third Generation Partnership Project (3GPP)-based Long Term Evolution (LTE) are the two standards that are likely to dominate the fourth generation (4G) wireless landscape.

- Both LTE and WiMAX standards use several common technologies with subtle differences.The main common technology

is orthogonal frequency-division multiple access (OFDMA).

- There are many good reasons for choosing OFDMA such as multipath handling capability, scalability of operation in different bandwidths, the ability to handle different data rates, and the ability

to easily combine with multiple antenna techniques.

- The focus of this article is on the comparative use of OFDMA in LTE and WiMAX systems.

FRAME STRUCTURE

USE OF OFDMA IN WIMAX AND LTE

RESOURCE MAPPING FROM SUBCARRIERS

USE OF OFDMA IN WIMAX AND LTE

Subcarriers are the smallest granular units in the frequency domain, OFDM symbol duration is the smallest granular unit in the time domain in OFDMA systems

In WiMAX, subchannels are formed

from a group of subcarriers in an

OFDM symbol.

A slot is formed by combining a

subchannel with different numbers

of OFDM symbols

In LTE, 12 adjacent subcarriers are grouped as a unit in the frequency axis, and 7 OFDM symbols (or 6 OFDM symbols in special cases) are considered as a unit in the time axis

In LTE, the uplink uses SCFDMA, which can be viewed asdiscrete Fourier transform (DFT)-spread OFDMA The data is passed through

a DFT block before being input to the OFDMA module.

USE OF FREQUENCY DIVERSITY

USE OF OFDMA IN WIMAX AND LTE

- In WiMAX, use PUSC (Partially Used

Sub-Carrier), subchannels are formed by

grouping 24 subcarriers that are present

in different parts of the spectrum

- This pseudorandom selection of the

positions of the subcarriers

- Control messages are also sent using

this diversity-based subchannelization

method

- In LTE, an RB (Resource Block)

contains the same 12 contiguous subcarriers for 7 OFDM symbols

- Another RB can be used in the second slot of the subframes

- Control messages use small bunches of contiguously placed subcarriers spread out over the entire bandwidth

- Within an OFDM symbol, no FD

USE OF MULTIUSER DIVERSITY

USE OF OFDMA IN WIMAX AND LTE

- In WiMAX, the BAMC organize groups

of 9 contiguous subcarriers

- Each bin has 8 data &1 pilot subcarriers,

and 4 such contiguous bins are grouped in

a band user feeds back the best 4 bands

- Based on this, BS chooses 2 bins in one

of these bands and allocates the same bin

over three consecutive OFDM symbols

- Frequency diversity and MUD-based

transmission cannot coexist in time

- In LTE, BTS uses the channel feedback from the mobile to schedule an RB for the user in a frame

- The feedback can be periodic

- Different modes of channel feedback

- Channel feedback can be sent as one value for the entire operating bandwidth or

as a sequence of values for a sequence of subbands covering the entire bandwidth

- In LTE both can be used

USE OF INTERFERENCE DIVERSITY

USE OF OFDMA IN WIMAX AND LTE

- The subcarriers used in a reference

subchannel in a reference cell are

distributed in different subchannels in the

reuse cells The subcarriers used in a

subchannel in a certain cell are unlikely to

be repeated in a particular

subchannel in another cell provided the

downlink perm base values are different

- In LTE, there can be a clash in the positions of the data subcarriers that can be used in neighboring cells

- There is no interference diversity

NETWORK ENTRY STEPS IN WIMAX AND LTE

- In WiMAX, a preamble is transmitted at

the start of every frame

- The preamble signal is generated by

using every third subcarrier in the allowed

bandwidth

- The pseudorandom sequence to be sent

on the subcarriers is specified in the IEEE

802.16e standard

- A mobile uses the time domain properties

of the preamble sequence, along with the

structure of the cyclic prefix (CP)

- Since the duration of the CP is not fixed,

a search procedure might have to be

performed to obtain the CP duration

- In LTE, the frame synchronization is obtained by detecting the primary synchronization sequence (PSS)

- Unlike the preamble in WiMAX, the number of subcarriers used for PSS is fixed

- The Zadoff-Chu sequence loaded onto the subcarriers is specified in the standard

- ID of the PSS in the received signal gives two potential starting points in the frame

as there are two PSS transmissions in the frame

NETWORK

ENTRY

NETWORK ENTRY STEPS IN WIMAX AND LTE

In WiMAX, a mobile has to search for

a valid preamble to acquire frame

synchronization Once synchronized,

the mobile reads the frame control

header (FCH) message, which points to

the length of the DL-MAP message that

contains the various allocations in the

frame

In LTE, irrespective of bandwidth and

the number of subcarriers, the first step

remains the same for all mobiles:

locating the primary synchronization

signal (PSS) and secondary

synchronization signal (SSS)

WiMAX LTE

Maximum

delay spread

PUSC mode: Considering both the pilots

carrying symbols: ~22us

Maximum

Doppler spread

WiMAX LTE

Maximum

delay spread

Maximum

Doppler spread

PERFORMANCE BOUNDS FOR

SYNCHRONIZATION AND CHANNEL

ESTIMATION

PHYSICAL LAYER OVERHEAD

In WiMAX

The overhead due to the use ofpreambles and pilots in every OFDM symbol isgiven by the ratio :

In LTE

The subcarriers used for physical layer processing purposes are the ones that carry reference symbols, plus primary and secondary synchronization

sequences

In LTE

The subcarriers used for physical layer processing purposes are the ones that carry reference symbols, plus primary and secondary synchronization

sequences

PERFORMANCE BOUNDS FOR

SYNCHRONIZATION AND CHANNEL ESTIMATION

EVOLUTION OF WIMAX AND LTE

The WiMAX and LTE camps have submitted candidate proposals to satisfy the official requirements of the ITU IMT-Advanced criterion for 4G wireless systems.

WiMAX to WiMAXv2

Base on the IEEE 802.16m standard

Forming fixed size physical resource units (PRUs)

Forming distributed and contiguous localized resourceunits (DRUs and LRUs)

LTE to LTE-A

The aggregation of multiple carriers to obtain wider

channel bandwidths.

the OFDMA related features are very similar in both

WiMAXv2 and LTE-A

This article has compared the use of OFDMA in WiMAX and LTE standards in detail

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