Presentation on theme: "OFDM Transmission over Wideband Channel"— Presentation transcript:
1 OFDM Transmission over Wideband Channel Gwo-Ruey Lee
2 Outlines OFDM Transmission over Wideband Channel The Time Dispersive Channel ModelEffects of Time Dispersive Channels on OFDMChannel EstimationTime Domain Channel EstimationFrequency Domain Channel EstimationEqualizationComparisons of Time- and Frequency- Domain Equalization
3 The Time Dispersive Channel Model 1/3A time-dispersive channel can be characterized by its channel impulse responsewhereis the number of multipath componentsis the path gain of path iis the path delay of path i
4 The Time Dispersive Channel Model 2/3Each path gain is a complex random variable with uniformly distributed phases and Rayleigh distributed magnitude.The average power of each path decays exponentially.
5 The Time Dispersive Channel Model 3/3The mathematical model for the time dispersive channel iswhere is the sampling period, and isthe root-mean-squared (RMS) delay spread.
6 Effects of Time Dispersive Channels on OFDM 1/1The 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 ofthe transmitted symbols and the discrete-timechannel impulse response.
7 Channel Estimation1/9Channel 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 estimationFrequency domain channel estimation
8 Time Domain Channel Estimation 2/9This 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.
9 Time Domain Channel Estimation 3/9The received time domain signals areis the channel impulse response.is the known symbol.is noise.
10 Time Domain Channel Estimation 4/9The 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.
11 Time Domain Channel Estimation 5/9Then receive signal is expressed as
12 Time Domain Channel Estimation 6/9The least square (LS) estimate for is
13 Frequency Domain Channel Estimation 7/9This channel estimation is performed using the frequency domain approach.In this, the channel frequency response is estimated by using two repeat training sequences.
14 Frequency Domain Channel Estimation 8/9We get (4.2) from (4.1) through FFT transform,If we have two repeat sequences,
15 Frequency Domain Channel Estimation 9/9The least square (LS) estimate for each is
16 Comparisons of Time- and Frequency-Domain Equalization 1/1Time-domain EqualizationFrequency-domain EqualizationConvolutionOne-tap multiplicationHigh complexitywhen L is lagerLow complexitywhen L is smallNo FFT RequiredFFT Required
17 References 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. Mark Engels, Wireless Ofdm Systems: How to Make Them Work? Kluwer Academic Publishers. John Terry, Juha Heiskala, OFDM Wireless LANs: A Theoretical and Practical Guide, Sams.