Low complexity LMMSE turbo equalization for linearly precoded OFDM

We study the turbo equalization approach for OFDM system with combined error control coding and linear precoding. While previous literature: employed linear precoder of small size for complexity reasons, we propose use of a linear precoder of size larger than or equal to the maximum length of the equivalent discrete-time channel in order to achieve full frequency diversity and reduce complexities of the error control coder/decoder. A low complexity LMMSE turbo equalizer is also derived for the receiver. Through simulation and performance analysis, it is shown that the performance of our proposed scheme over frequency selective fading channel reaches the matched filter hound: compared with the same coded OFDM without linear preceding, the proposed scheme shows an SNR improvement of at least 6 dB at a bit error rate of 10^-6 over a multipath channel with exponential power delay profile. Convergence behavior of the proposed scheme with turbo equalization using various types of linear precoder/transformer, various interleaver sizes and error control coders with various constraint lengths is also investigated.