A low complexity turbo detection for coded DS-CDMA systems in multipath channels at rake computational load

An efficient, low complexity, at close to Rake computational load, turbo detection receiver for joint detection and decoding for coded DS-CDMA signals in multipath channels is derived. The new scheme is based on an asymptotic approximation of the maximum likelihood (ML) detector as we believe that for a number of iterations more than three, the summation over 2^vK-1 possible sequences (where K is the number of users and v is the channel length) will drop to a single factor that can be effectively represented by the expected values of the multiple access interference (MAI) and intersymbol interference (ISI) using a priori information from the decoder. Following the SISO detector is a bank of a full codeword MAP decoder (SISO channel decoder). Both stages are separated by interleaves and deinterleavers so that they iteratively exchange soft information of "likelihood" nature. The detection stage can be viewed as SISO detector that estimates soft MAI and ISI contribution in the received signal and determine after words the likelihood of the code bit of interest of a given user and feeds it to the appropriate decoder which in turn delivers an update of these likelihoods and so on in an iterative manner. Simulation results for performance evaluation are conducted under most interesting scenarios including asynchronous multipath channels, near far problem, time varying channels, channel estimation miss match and especially multirate systems.