Orthogonal Frequency Division Multiple Access

(OFDMA)

Khaja Mohammad Shazzad

2

Outline

1. Background Multiple Access (MA) Methods

2. Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA) Orthogonality Principle OFDM OFDM-FDMA

3. Advantages and Disadvantages of OFDMA4. Conclusion

Multiple Access (MA)

General wireless cellular systems are multi-users systems

Radio resource are limited– Limited Bandwidth– Limited number of channels

The radio resource must be shared among multiple users

Multiple Access Control (MAC) needed– Contention-based– Non-contention-based

4

Contention-basedMultiple Access(MA)

Contention-based– Each terminal transmits in a decentralized way– No central controller (Base stations or access points)– Example:

ALOHA Carrier Sensing Multiple Access (CSMA)

Standard:– GSM [l] uses the slotted ALOHA in the terminal’s initial

access process– IEEE 802.11 uses CSMA/CA based contention access

scheme

Non-contention-based Multiple Access (MA)

A logic controller (BS or AP) is needed to coordinate the transmissions of all the terminals

The controller informs each device when and on which channel it can transmit

Collisions can be avoided entirely Two Subdivisions

1. Non-channelization

2. Channelization

Non-channelizationNon-contention-based MA

Terminals transmit sequentially using the same channel

Example:– Polling based medium access

Standard:– IEEE 802.15(WPAN)– IEEE 802.11(WLAN)

ChannelizationNon-contention-based MA

Terminals transmit simultaneously using different channels

Most commonly used protocols in cellular systems Example:

– 1. Time Division Multiple Access (TDMA)– 2. Code Division Multiple Access( CDMA)– 1. Frequency Division Multiple Access (FDMA)

Standard– 1. GSM (TDMA)– 2. IS-95 (CDMA)– 3. American Mobile Phone System, AMPS (FDMA)

8

Time Division Multiple Access (TDMA)

GSM– Time slot 0.577 ms– Frame 4.6 ms– 8 time slots per frame– Frequency band 20 KHz

Code Division Multiple Access (CDMA)

IS-95– Orthogonal Walsh codes– 64 codes (channels)– One pilot channel– Seven paging channels– 55 traffic channels– Each carrier 1.25 MHz

Frequency Division Multiple Access (FDMA)

American Mobile Phone System (AMPS)

– Total Bandwidth 25 MHz– Each Channel 30 KHz

Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)

Orthogonality PrincipleOFDMOFDM-FDMA

12

Orthogonality Principle

Vector space– A, B and C vectors in

space are orthogonal to each other

– A.B=B.C=C.A=0– (A+B+C).A=(mod A)^2– (A+B+C).B=(mod B)^2– (A+B+C).C=(mod C)^2

A

B

C

Orthogonality Principle cont..

Real Function space

0)()(

)cos()(

)sin()(

0)()(

)cos()(

)sin()(

21

2

1

dttftf

nwtNtf

mwtMtf

dttftf

wtBtf

wtAtf

n

T

m

n

m

T

Orthogonality Principle cont..

)2sin()sin()( wtwttf

T

00s(nwt)dtsin(mwt)co

nmwhereT

00n(nwt)dtsin(mwt)si

Νm.n

Here mw and nw are called m-th and n-th harmonics of w respectively

15

Orthogonal Frequency Division Multiplexing(OFDM)

It is a special kind of FDM The spacing between carriers are such that

they are orthogonal to one another Therefore no need of guard band between

carriers. One example makes the thing clear

Example of OFDM

Lets we have following information bits– 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …

Just converts the serials bits to parallel bitsC1 C2 C3 C4

1 1 -1 -1

1 1 1 -1

1 -1 -1 -1

-1 1 -1 -1

-1 1 1 -1

-1 -1 1 1

Example of OFDM cont..

Modulated signal for C1 Modulated signal for C2

Modulated signal for C3 Modulated signal for C4

Modulate each column with corresponding sub-carrier using BPSK

Example of OFDM cont..

Final OFDM Signal = Sum of all signal

)2sin()()(1

0nttItV

N

nn

Generated OFDM signal, V(t)

V(t)

OFDM-FDMA (OFDMA)

Each terminal occupies a subset of sub-carriers

Subset is called an OFDMA traffic channel

Each traffic channel is assigned exclusively to one user at any time user1

user2

user3

user4

OFDM-FDMA (OFDMA)

The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique– Bandwidth options 1.25, 5, 10, or 20 MHz– Entire bandwidth divided into 128, 512, 1024 or

2048 sub carriers– 20 MHz bandwidth with 2048 sub carriers has 9.8

KHz spacing between sub carriers

OFDM-FDMA (System View)

22

Advantages of OFDMA

Multi-user Diversity– broadband signals experience frequency selective

fading– OFDMA allows different users to transmit over

different portions of the broadband spectrum (traffic channel)

– Different users perceive different channel qualities, a deep faded channel for one user may still be favorable to others

Advantages of OFDMA cont..

Multi-user Diversity

Advantages of OFDMA cont..

Efficient use of Spectrum

4/3 Hz per symbol

6/5 Hz per symbol

Advantages of OFDMA cont..

Receiver Simplicity– It eliminates the intra-cell interference avoiding

CDMA type of multi-user detection– Orthogonality of code destroyed by selective

fading – Only FFT processor is required

Bit Error Rate performance is better only in Fading environment

Disadvantages of OFDMA

Peak to average power ratio (PAPR)

avgP

txPAPR

2

)(

The large amplitude variation increases in-band noise and increases the BER when the signal has to go through amplifier nonlinearities.

27

Disadvantages of OFDMA cont..

Synchronization– Tight Synchronization between users are

required for FFT in receiver– Pilot signals are used for synchronizations

Co-channel interference – Dealing with this is more complex in OFDM than in CDMA

– Dynamic channel allocation with advanced coordination among adjacent base stations

28

Research issues and Conclusion

Future works– Peak-to-average power reduction in OFDM – Timing and Frequency Synchronization– Efficient digital signal processing Implementation of OFDM

– Multiple input/Multiple output (MIMO) OFDM

Conclusion– Different variations of OFDMA are proposed and

have different pros and cons

Thank You

Questions

or

Comments?