Soft-in/soft-out Hadamard despreader for iterative decoding in the IS-95(A) system

In the uplink of the CDMA IS-95(A) system decoding of a serial concatenated coding scheme has to be performed, where the inner code comprises M-ary orthogonal modulation with Hadamard (Walsh) spreading codes. For a coherent and a noncoherent RAKE-receiver structure we give the optimum symbol-by-symbol maximum a posteriori (MAP) decoding rules considering the a priori information about the systematic bits of the codewords if available. These algorithms are necessary for iterative decoding of the whole system. We present simulation results which demonstrate the gain of the modified receivers.