A Linear Complexity Turbo Equalizer Based on a Modified Soft Interference Canceller

Soft-input soft-output (SISO) equalizers based on linear filters have proven to be good, low complexity, alternatives to trellis-based SISO equalizers. In particular, the soft interference canceller (SIC) has recently received great interest, especially for receivers performing turbo equalization. In this paper, we modify the way in which the SIC incorporates soft information. In existing literature the input to the cancellation filter is the expectation of the symbols based solely on the a-priori probabilities coming from the decoder, whereas here we propose to use the conditional expectation of those symbols, given both the a-priori probabilities and the received sequence. This modification results in performance gains at the expense of increased computational complexity. However, by introducing an approximation to the aforementioned algorithm a linear complexity SISO equalizer can be derived. Simulation results for an 8-PSK constellation and hostile radio channels have shown the effectiveness of the proposed algorithms in mitigating the inter-symbol interference (ISI)