On some limiting performance issues of multiuser receivers in fading channels




Djonin, Dejan V.

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The problem of information-theoretic optimal resource allocation for the synchronous single-cell CDMA Gaussian multiple access channel is investigated. Several different cases are analyzed including: optimal sequence allocation without power control, optimal sequence allocation with optimal power control and optimal sequence allocation without power control with equal single user capacities. In order to simplify the mathematical description of the multiple access capacity region, a Cholesky decomposition characterization is introduced and utilized to find the optimal sequence allocation for equal single user capacities. The case of randomly chosen spreading sequences in a large system model, i.e. when number of users and processing gain increase without bounds while maintaining their ratio fixed, is also analyzed. Using this model, the performance of a conventional decision feedback receiver in flat fading channels is analyzed. A sequence allocation scheme that uses two sets of orthogonal users that can be decoded with a very simple decision feedback receiver is analyzed. It is shown that the spectral efficiency of this scheme is very close to the maximal possible. Finally, the issue of imperfect channel state information available at the receiver is discussed and the spectral efficiency loss compared to the perfect channel state information case is evaluated for the optimal multiuser receiver.



Wireless communication systems, Code division multiple access, Performance limitations, Multiuser receivers