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Performance and receiver structures for OFDM on Rayleigh fading channels

University of British Columbia

This thesis is a study of performance and receiver structures for Orthogonal Frequency Division Multiplexing (OFDM) on land mobile radio channels characterized by flat or frequency-selective Rayleigh fading. The type of OFDM considered is a parallel modulation scheme in which 'N data symbols are assembled and used to simultaneously modulate N frequency-orthogonal tones, forming the OFDM symbol or block. Relative to a conventional serial modulation scheme of comparable bandwidth, OFDM has an extended signaling interval which allows for channel averaging in fading conditions. However, the channel variation over the duration of an OFDM block impairs the orthogonality of the modulated tones, causing intersymbol interference. A new matched filter bound is derived which does not require that the channel remain constant over the signaling interval, and has Doppler frequency as a parameter. Pulse shape, diversity order, and interray correlation may also be varied. The matched filter bound is used to establish analytic performance limits on the probability of bit error for any receiver of uncoded OFDM on flat or frequency-selective Rayleigh fading channels. In contrast to the AWGN channel, the optimal pulse shape used in the receive correlator is time-varying and the transmitter pulse shape affects the probability of error. Using the Maximum Likelihood Sequence Estimation (MLSE) criterion, an optimal receiver for OFDM on flat fading channels is derived. This turns out to require a constraint length L = N — 1, which is generally infeasible due to complexity constraints. However for BPSK OFDM a suboptimal truncated version of the MLSE receiver is able to approach the MFB to within 1 dB for a wide range of Doppler rates. For QPSK OFDM simple truncated MLSE is found to be impractical due to the required constraint length, which is greater than for BPSK OFDM. The Minimum Mean Square Error (MMSE) criterion is used to derive optimal linear and nonlinear decision feedback equalizing receivers for OFDM on flat fading channels. A continuous-time analysis is used to show that the optimal linear MMSE receiver requires a sampling rate in excess of N samples per OFDM block, and that the optimal weighting function applied to the received signal to mitigate channel fading is tone-dependent. Approximations are considered to remove the tone-dependency, yielding a result consistent with previous work. An MMSE-based criterion is proposed and used to derive a method for modifying the channel impulse response to an optimal desired impulse response having a specified constraint length. This limits error propagation with decision feedback and reduces the complexity of sequence estimation, making the latter feasible for QPSK OFDM as well as BPSK OFDM. The resulting nonlinear receiver structures have probability of error performances which improve on previously published results for the same modulation and channel. Finally optimal and suboptimal receivers for OFDM on frequency-selective Rayleigh fading channels based on MLSE and MMSE criteria are derived. The additional complexities of receiver design arising from the presence of delay spread are studied analytically and evaluated by simulation. It is shown that for the MLSE receiver, long blocklength OFDM is relatively insensitive to the distribution of signal strengths between the rays of the two-ray frequency-selective Rayleigh fading channel model.

Thesis/Dissertation

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2009-04-03

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http://hdl.handle.net/2429/6766

Electrical and Computer Engineering

University of British Columbia

UBCV

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