Department of Electrical and Computer Engineering
M.S. Thesis Abstract
TWO DIMENSIONAL SPREADING FOR DOUBLY DISPERSIVE CHANNELS
Systems supporting broadband mobile services over wireless channels suffer from dispersion along time and frequency. Hence transmission by replicating information along both these domains leads to diversity gain in each domain. This work proposes a scheme to implement this replication principle, motivated by the Variable Spread Factor-Orthogonal Frequency Code Division Multiplexing (VSF-OFCDM) scheme introduced by NTT-DoCoMo. In this scheme a symbol is transmitted across several subcarriers with a total power constraint and along each subcarrier it is spread with CDMA codewords. The information theoretic bounds on capacity for this scheme are derived for three different scenarios each corresponding to a specific nature of channel state information (CSI) available at the transmitter. The cases are perfect CSI, partial CSI characterized by one bit of channel information per subcarrier and no CSI. The receiver is assumed to have perfect CSI. The optimal codeword and power allocation strategies to achieve these bounds are also derived for the single user and multi-user uplink channel. We show that for perfect CSI, the optimal strategy is to transmit along the best subchannel. For imperfect CSI at transmitter, diversity benefits are observed for a large number of subcarriers. It is also observed that, 1 bit feedback per subcarrier is a good scheme, both in terms of achievable rates and implementation complexity.
Thesis Director: Professor Roy Yates
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