Turbo equalization in an LTE uplink MIMO receiver

Utilization of maximum likelihood and similar algorithms in the LTE uplink MIMO receiver is not practical due to high complexity. An alternative is the application of turbo equalization which has been proven to enhance the performance of MIMO receivers at a reasonable implementation cost. There has been a lot of research on turbo equalization in general and some work related to the LTE uplink in particular. The turbo equalization principle relates to the cooperative operation of the channel decoder and the channel equalizer in an iterative fashion. Some of the approaches are based on using the turbo decoder output to reconstruct QAM symbols for each of the transmitted data streams. These symbols are in turn used to reduce the interstream interference. After this “interference cancellation” phase the MMSE MIMO equalizer operates on a “cleaner” signal compared to the first pass when there was no feedback from the turbo decoder. This collaboration between the turbo decoder and the MMSE MIMO equalizer may be iterated a few times. This paper provides general foundation for this turbo equalization approach and includes simulation results demonstrating significant performance gain. Tradeoff discussions and guidelines for practical implementation of this scheme on a Texas Instruments TMS320C66x multicore digital signal processor are provided.