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# OFDM Transmission over Wideband Channel - PowerPoint PPT Presentation

OFDM Transmission over Wideband Channel. Gwo-Ruey Lee. Outlines. OFDM Transmission over Wideband Channel The Time Dispersive Channel Model Effects of Time Dispersive Channels on OFDM Channel Estimation Time Domain Channel Estimation Frequency Domain Channel Estimation Equalization

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### OFDM Transmission over Wideband Channel

Gwo-Ruey Lee

• OFDM Transmission over Wideband Channel

• The Time Dispersive Channel Model

• Effects of Time Dispersive Channels on OFDM

• Channel Estimation

• Time Domain Channel Estimation

• Frequency Domain Channel Estimation

• Equalization

• Comparisons of Time- and Frequency- Domain Equalization

1/3

• A time-dispersive channel can be characterized by its channel impulse response

where

is the number of multipath components

is the path gain of path i

is the path delay of path i

2/3

• Each path gain is a complex random variable with uniformly distributed phases and Rayleigh distributed magnitude.

• The average power of each path decays exponentially.

3/3

• The mathematical model for the time dispersive channel is

where is the sampling period, and is

1/1

• The phenomenon of multipath fading causes previous symbol to interfere with the latter symbol. This is known as the inter-symbol interference (ISI).

• The received signal is the convolution of

the transmitted symbols and the discrete-time

channel impulse response.

1/9

• Channel estimation is the task of estimating the frequency response of the radio channel the transmitted signal travels before reaching the receiver antenna.

• The channel estimation can be performed using the time domain approach and frequency domain approach.

• Time domain channel estimation

• Frequency domain channel estimation

2/9

• This channel estimation is performed using the time domain approach, before FFT processing of the training symbols.

• In this, the channel impulse response instead of the channel frequency response, is estimated.

3/9

• The received time domain signals are

is the channel impulse response.

is the known symbol.

is noise.

4/9

• The time domain convolution can be expressed as a matrix vector multiplication.

• The parameter L is defined as the maximum length of the impulse response, L<64.

5/9

• Then receive signal is expressed as

6/9

• The least square (LS) estimate for is

7/9

• This channel estimation is performed using the frequency domain approach.

• In this, the channel frequency response is estimated by using two repeat training sequences.

8/9

• We get (4.2) from (4.1) through FFT transform,

• If we have two repeat sequences,

9/9

• The least square (LS) estimate for each is

• [1] L. Hanzo, W. Webb and T. Keller, Single- and multi-carrier quadrature amplitude modulation – Principles and applications for personal communications, WLANs and broadcasting, John Wiley & Sons, Ltd, 2000.

• [2] Mark Engels, Wireless Ofdm Systems: How to Make Them Work? Kluwer Academic Publishers.

• [3] John Terry, Juha Heiskala, OFDM Wireless LANs: A Theoretical and Practical Guide, Sams.