Performance of turbo equalizer employing MMSE filter and LDPC codes

The performance of the receiver combining a minimum mean square error (MMSE) filter and low-density parity-check (LDPC) codes is investigated. Spatial multiplexing techniques can improve spectral efficiency without channel information at the transmitter over rich scattering wireless channel environments. A multiuser scenario is assumed where different users transmit LDPC encoded information simultaneously at the same carrier frequency. The receiver separates the multiplexed signal into signals for each user by the MMSE filter. The log likelihood ratios (LLR) of these signals are updated by the LDPC decoder. From the LLR and channel information, soft replicas are generated. Soft interference cancellation is performed by subtracting interference components from the received signal. The MMSE filtering is applied to the resultant signal. The interference component is eliminated by an iterative process between the MMSE filter and LDPC decoders. The paper shows the performance of this scheme using feedback channel estimation assuming a 4×4 multiple-input multiple-output (MIMO) system.