Two dimensional spreading for dispersive channels

Systems supporting broadband mobile services over wireless channels suffer from dispersion along time and frequency. Hence transmission by spreading information along both these dimensions leads to diversity gain in each dimension. Motivated by this principle, we propose a generalized two dimensional spreading scheme, in which 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 under different assumptions about the channel state information (CSI) available at the transmitter. These 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 derived for the single user point to point transmission. The solutions also apply to the cases of synchronous multi-user uplink transmission, or when the receiver schedules transmissions of multiple users. For asynchronous uplink communications, we show that the problem of optimal resource allocation is analytically intractable and instead suggest a heuristic solution which can be implemented in a distributed manner.