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WIRELESS OFDM SYSTEMS: CHANNEL PREDICTION AND SYSTEM CAPACITY

The general theme of this thesis is orthogonal frequency division multiplexing (OFDM) communications over time and frequency selective fading channels. We propose and study linear prediction techniques for acquiring channel state information (CSI) in OFDM receivers, and we perform an information-theoretic analysis of the performance of OFDM systems. After a review of the generic discrete-time pulse-shaping OFDM system (which comprises conventional cyclic-prefix OFDM systems as a special case), we consider the transmission over a time and frequency selective fading channel. We arrive at an approximate multiplicative system input-output relation in which intersymbol and interchannel interference is neglected. Based on this approximate input-output relation, we propose decision-directed channel predictors that are capable of yielding up-to-date CSI without regular transmission of pilot symbols. We derive the minimum mean-square error (MMSE) predictor and a reducedcomplexity version that allows for an efficient DFT implementation. We also develop adaptive predictors that do not need statistical prior knowledge and can track nonstationary channels. Several applications of channel prediction are discussed, and the excellent performance of the proposed techniques is demonstrated by computer simulations. The second major contribution of this thesis is an information-theoretic analysis of the performance of OFDM systems transmitting over time and frequency selective channels. We study the system capacity of wideband OFDM communications in the absence of CSI at the transmitter and the receiver. Using a codebook that is “peaky” in time and frequency, we show that OFDM can approach the infinite-bandwidth channel capacity. On the other hand, using a “nonpeaky” constant-modulus signaling scheme, we show that the information rate is reduced by a penalty term that is related to the predictability of the fading channel. We quantify the impact of the spread and shape of the scattering function on this penalty term. Finally, we formulate an upper and a lower bound on system capacity and demonstrate by simulations that both bounds are close to the AWGN channel capacity for large ranges of bandwidth and for practically relevant system parameters.