Orthogonal Frequency Division Multiple Access (OFDMA)

Zeeshan Asim

SSUET

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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 OFDMA

4. 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

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Contention-based Multiple 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 terminals

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-channelization Non-contention-based MA

Terminals transmit sequentially using the same channel

Example: Polling based medium access

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

Channelization Non-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)

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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

Objectives

What is OFDM

How OFDM works

Types of OFDM

Differences from other multiplexing techniques

Applications

What is OFDM

Definition

The first OFDM schemes presented in 1966

Development of OFDM over years

A

F

How OFDM works

It distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other

than their own.

Types of OFDM

C-OFDM

MIMO-OFDM

V-OFDM

W-OFDM

Flash-OFDM

C-OFDM

Coded OFDM

Digital Audio Broadcasting (DAB)

Digital Video Broadcasting (DVB-T)

COFDM offers real benefit in the presence of isolated narrow-band interfering signals

Multiple Input, Multiple Output OFDM (MIMO-OFDM)

Developed by Iospan Wireless

Uses multiple antennas to transmit and receive radio signals

Spatial multiplexing

V-OFDM

Vector OFDM

Developed by CISCO

Increases subscriber coverage

Lowers the cost of provisioning and deploying infrastructure

Employs both frequency and spatial diversity

Creates a robust processing technique for multi-path fading and narrow band interference

Subscriber

Base Station

Paths

W-OFDM

Wideband OFDM

Invented by Wi-LAN

Large spacing between carriers

Advantages: - Optimal performance against Multi-path

- Less sensitive to carrier offset

-Optimal power efficiency of the transmitter amplifier

- More immune against fading

Flash-OFDM

Fast-hopped OFDM

Wide-band spread-spectrum technology

Avoids the compromises inherent in other mobile data systems

Capability to work around interfering signals

Differences from other multiplexing techniques

OFDM versus WDM

-more flexible

-higher bandwidth efficiency

Differences from other multiplexing techniques

OFDM versus TDM

-number of carriers

-synchronization

-sensitivity

-capacity/efficiency advantages

-complexity and cost issues

Applications

Digital Audio and Video Broadcasting

Asymmetric Digital Subscriber Line (ADSL)

Wireless Networking

Power-line Technology

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)

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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.

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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

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