Iterative Channel Estimation and Decoding for the Multi-Path Channels with RAKE reception

We study an improved receiver for 3G Reverse Link (RL) with iterative channel estimation and decoding. A soft modulator processes the soft output of the Turbo decoder after a decoding iteration. If some symbols are deemed reliable at the output of the modulator, then these symbols act as extra pilot symbols for the channel estimator to refine the channel estimates that aid the demodulator, whose output is given to the decoder for another decoding iteration. This synergistic interaction between the channel estimator and the decoder results in an improved performance of the system. The conventional Turbo decoder needs to be modified to yield soft information for both the systematic as well as the parity bits and two methods are presented to accomplish this. To keep the complexity in terms of memory and processing low, the iterative channel estimation is done on the equivalent channel at the output of the RAKE combiner instead of per-finger iterative channel estimation before the RAKE combining. Numerical results are presented that show that significant improvement is possible over the current receiver. The transmission formats and the error correcting codes are kept the same and the new receiver does not need any change in the 3G standards.