OFDMA Networks

By Mohamad Awad

OutlineOutline

Wi l h l i i t d th i ff t on Wireless channel impairments and their effect on wireless communicationChannel modelingg

Sounding techniqueOFDM as a solutionOFDMA as an improved solutionOFDMA as an improved solutionMIMO-OFDMAConclusion

Propagation Mechanisms in wireless channels

Reflection

Three propagation mechanisms in two types of propagation channels

Diffraction Scattering

Indoor wireless channel Outdoor wireless channel

T iTransmitter

Receiver

Large Scale FadingLarge Scale Fading

Topographic and elevation data were obtained from Naturalresources Canada.

Large-Scale Fading Vs. Small-Scale Fading

Large-Scale Fading

Small Scale Fading

Coherent Summation of

2 multipaths

Incoherent Summation

Partly- Coherent Summation of 2

multipaths Im

Im

ed P

ower

Small-Scale Fading of 2 multipaths

ReIm

Re

Rec

eive

Re

Figure taken from :”Wireless communication, principle and practice” by T. S. Rappaport

Transmitter – Receiver Separation

Channel-Sounding

?Propagation mechanisms:- Reflection.

?

- Diffraction.- Scattering.

Measurement setting:- frequency. e Po

wer

frequency.- line-of-sight vs no-line-of-sight.- environment.- transmitter - Receiver separation.- human presence.

fl A

Cha

nnel

Impu

lse

Res

pons

e

pow

er

time- floor Area.- extent of Clutters.- detection Threshold.

P Ch l

ReceiverTransmitter

Propagation Channel

Channel Impulse-Response

( ) ( )nex

N

nex ah ττδτ −=∑1

22

Channel Impulse Response

n=1

Total number of multipathsNB)

na n-th multipath amplitude

ved

Pow

er (

dB

ex

ττ excess delay

Receiver Detection Threshold

mal

ized

Rec

eiv

nτ n-th multipath excess delay

Excess Delay (ns)

Nor

m

maxτ

Review: Convolution Review: Convolution http://www.jhu.edu/signals/convolve/

Frequency-Flat Vs. Frequency-Selective Fading

ChannelTransmitted Signal

Received SignalSignal

No Time DispersionISITime Dispersion and ISI

Ts @ Tx

time

maxτ

Ts >> maxτ

@ RxTs @ Tx

time

maxτ

maxτTs <<

Frequency-Flat FadingFrequency-Selective Fading q y g

FrequencyBs @ Tx Bs @ RxBw

q y g

BwBs @ Tx Bs @ Rx Frequency

Bs << Bw

Frequencyw

Bs >> Bw

s @ s @

Multicarrier Transmission

History of Multicarrier transmission History of Multicarrier transmission A principle for multi-channel transmission over a bandlimited channel was proposed in 1966 !!bandlimited channel was proposed in 1966 !!Why did take that long to be implemented?

Combine ?

DFT !

What is OFDM?What is OFDM?Orthogonal Frequency Division MultiplexingMethod to multiplex many “narrowband” signals into Method to multiplex many narrowband signals into an aggregate “wideband” data streamNormally the subcarriers would need sufficient Normally, the subcarriers would need sufficient separation to avoid adjacent channel interferenceOFDM uses orthogonal (1/T ) subcarrier spacing to OFDM uses orthogonal (1/Tsym) subcarrier spacing to maintain subcarrier separation

OrthogonalityOrthogonality

Orthogonal Carrier SpacingOrthogonal Carrier Spacing

1/Tsym

1

1.2

0.6

0.8

0

0.2

0.4

0 5 10 15 20 25 30-0.4

-0.2

OFDM System ModelOFDM System Model

OFDM-TDMA MAC

Sub-1

Frequency

User 2

User 1

05

-0.4

-0.2 0 0.2

0.4

0.6

0.8 1 1.2

Sub-2

Sub 1

S b 4

Sub-3

User 2

U 4

User 3

510

1520

Sub-4

Sub-5

Time

User 42530

Disadvantages

High latency # users large

Advantages

Easy implementation High latency, # users large

In-effecient power usage

In-effecient in frequency utilization

Easy implementation

Simple resource allocation

Low processing requirement In-effecient in frequency utilizationp g q

Low signaling overhead (channel gain)

ApplicationsApplications

Multi-user diversityMulti user diversity

Time Varying Channel

•The channel is generally time varying due to:•User changes location.•Object gets between the transmitter and receiver (bus)

τex

Transmitter Time τex

τex

OFDMAOFDMAOrthogonal Frequency Division Multiple Access.A subcarrier is exclusively assigned to a userA subcarrier is exclusively assigned to a user.Exploits multiuser diversity.Dynamic subcarrier assignmenty g

Frequency

Disadvantages of OFDM

High latency, # users large

Sub-2

Sub-1

S b 3

User 2

User 1 In-effecient power usage

In-effecient in frequency utilization

Sub-4

Sub-3

Sub-5 User 4

User 3

Time

OFDMA Resource Allocation Internet

Channel gain

QoS (rate, dropping rate, etc..)

Base Station ( BS )

•Maximize ThroughputTime

Freq

•Maximize Throughput •Assign power < Max•Each subcarrier is exclusively assigned

•New•gainse c us ve y ss g ed

Guarantee QoSgains

Allocated subcarriers

Trans Power

User # 1 User # 2

Trans Power

OFDMA MAC

Sub-1

FrequencyUser 1

Sub-2

Sub-4

Sub-3

User 2

User 3Sub-4

Sub-5

Time

User 4

Disadvantages

Complicated Res Alocation

Advantages

Simple implementation Complicated Res. Alocation

Signaling overhead (adaptive scheme)

Simple implementation

Resource allocation flexibility

Adaptation to channel characteristics (adaptive scheme)

Better BER performance (adaptive scheme)

Space Diversity

Two antennas separated by D

Two antennas separated by several wavelengths will not generally experience fades at h the same time

“Space Diversity” can be obtained by using two obtained by using two receiving antennas and switching instant-by-instant

Signal received by Antenna 1

Si l i d b to whichever is bestSignal received by Antenna 2

C bi d Si lCombined Signal

MIMO systemsMIMO systems

MIMOMIMO

Basic idea of MIMO: Improve quality (BER) and/or datarate (bits/sec) by using multiple TX/RX antennas( ) y g p

Core scheme of MIMO: space-time coding (STC)Two main functions of STC: diversity & multiplexingy p g

MIMO - OFDMAMIMO OFDMA

ConclusionConclusionWireless channel suffers from large scale fading and small scale fadingscale fadingAfter modeling the channel, communication systems are matched to the channel.OFDMA is one of advanced technologies to overcome the wireless channel impairments.OFDMA can be combined with MIMO to improve performance .

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