1. ofdm history

7/30/2019 1. OFDM History

1/17

OFDM History High-data-rate communications systems are limited not by noise,

but often more significantly by the intersymbol interference (ISI)due to the memory of the dispersive communications channel.

If the symbol rate exceeds the duration of channel impulse response(CIR), mechanisms must be implemented in order to combat the

effects of ISI. Channel equalization techniques can be used to suppress the echoes causedby the channel.

Significant research efforts have been invested into the development of suchchannel equalizers

Another approach is to utilize an FDM system which employs a set of subcarriers in order to transmit information in parallel subchannels over the same channel.

The data throughput of each channel is only a fraction of the data rate of thesingle-carrier system having the same throughput.


2/17

OFDM is a considerable option when the channel introduces ISI

Applications: ADSL, DAB, DVB, Hiperlan/2, ...

Frequency Selective Channel

OFDM Orthogonal Frequency Division Multiplexing 2001-05-31

Uses the entire bandwidth

Splits bandwidth into subchannels Short symbol times

This causes ISI

Sends information in parallel

OFDM: orthogonal subcarriers

Single Carrier Multicarrier

Why OFDM?


3/17

OFDM History In 1971, Weinstein suggested using a digital implementation based on the DFT.

The DFT is by its nature cyclically redundant in the frequency domain. The associated harmonically related frequencies can be used as a set of subchannels carriers

required by the OFDM system.


4/17

Multipath can be described in two domains:time and frequency

time

time

Sinusoidal signal as input

time

time

Sinusoidal signal as output

f

Frequency response

Time domain: Impulse response

Frequency domain: Frequency response

time

Impulse response

timetime


5/17

Modulation techniques:monocarrier vs. multicarrier

To improve the spectral efficiency :

To use orthogonal carriers (allowing overlapping ) Eliminate band guards between carriers

Selective Fading

Very short pulses

ISI is compartively long EQs are then very long

Poor spectral efficiencybecause of band guards

Drawbacks

It is easy to exploitFrequency diversity

Flat Fading per carrier

N long pulses

ISI is comparatively short N short EQs needed

Poor spectral efficiencybecause of band guards

Advantages Furthermore

It allows to deploy2D coding techniques

Dynamic signalling

N carriers

BPulse length ~ N/B

Similar toFDM technique

Data are shared among several carriersand simultaneously transmitted

BPulse length ~1/B

Data are transmited over only one carrier

Channel

Guard bands

Channelization


6/17

Orthogonal Frequency Division Modulation

Data coded in frequency domain

N carriers

B

Transformation to time domain:each frequency is a sine wavein time, all added up.

f

Transmit

Symbol: 8 periods of f 0



+

Receivetime

B

Decode each frequencybin separately

Channel frequencyresponse

f

f

Time-domain signal Frequency-domain signal


7/17

Each subcarrier is modulated at a low enough rate that dispersion(ISI) is not a problem. Subcarriers must be spaced so that they donot interfere.

f 0 f 1 f N-1Bandwidth, B

x

x

x

cos(2 p f 0 t )

cos(2 p f 1t )

cos(2 p f N-1t )

LPF

LPF

LPF

P/S

Demodulator

r(t)

Detector

Detector

Detector

S(f)


8/17

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a special case of multicarrier transmission that permitssubchannels to overlap in frequency without mutual interference increased spectral efficiency.

OFDM exploits signal processing technology to obtain cost-effective meansof implementation.

Mulitple users can be supported by allocating each user a group of

subcarriers.

Bandwidth, ~ B/2


9/17

Spectrum of OFDM Signal

When N is large, the power spectral density (PSD) of the transmitted signal is

PSD of OFDM Signal


10/17

Textbooks and References

Wireless OFDM Systems: How to Make Them WorkMarc Engels, Editor

OFDM Wireless LANs: A Theoretical and Practical Guide Juha Heiskala and John Terry

OFDM for Wireless Multimedia Communications

Richard Van Nee and Ramjee Prasad Single and Multi -Carrier Quadrature Amplitude Modulation Lajos Hanzo, William Webb, and Thomas Keller

ADSL, VDSL, and Multicarrier Modulation John Bingham

Implementing ADSL David Ginsburg

DSL Advances Massimo Sorbara, John Cioffi, and Peter Silverman


11/17

OFDM OFDM also known as

Multi-Carrier or Multi-Tone Modulation DAB-OFDM

Digital Audio Broadcasting DVD-OFDM

Digital Video Broadcasting ADSL-OFDM

Asynchronous Digital Subscriber Line Wireless Local Area Network

IEEE-802.11a, IEEE-802.11gETSI BRAN (Hyperlan/2)


12/17

OFDM Systems

System TransformSize

NumberCarriers

ChannelSpacing

kHz

BandwidthMHz

SampleRateMHz

SymbolDuration

sec

Data

Rate

Mbits/s

HyperLAN/2 64 524 312.5 16.25 20 3.20.8 6-54

802.11a 64 524

312.5 16.56 20 3.20.8

6-54

DVB-T 20481024

1712842

4.464 7.643 9.174 224 0.68-14.92

DAB 20488192

1536 1.00 1.536 2.048 24/48/96msec

3.072

ADSL 256 (down)64 (up)

36-1277-28

4.3125 1.104 1.104 231.9 0.64-8.192


13/17

OFDM Advantages

Efficiently Deals With Multi-path Fading Efficiently Deals With Channel Delay Spread

Enhanced Channel Capacity Adaptively Modifies Modulation Density Robustness to Narrowband Interference


14/17

OFDM Disadvantages

OFDM Sensitive toSmall Carrier Frequency Offsets

OFDM ExhibitsHigh Peak to Average Power Ratio

OFDM Sensitive toHigh Frequency Phase Noise

OFDM Sensitive toSampling Clock Offsets


15/17

Adjacent Symbol Interference (ASI)

Symbol Smearing Due to Channel


16/17

Guard Interval Inserted Between Adjacent Symbolsto Suppress ASI


17/17

1. ofdm history

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