High-performance 64-QAM OFDM via carrier interferometry spreading codes

OFDM (orthogonal frequency division multiplexing) is an excellent technology that will almost certainly play a large role in next generation wireless communication systems. However, in OFDM, no frequency diversity is exploited to improve BER performance. Today´s OFDM systems attempt to overcome this limitation by application of channel coding and interleaving, which requires a reduction in throughput. In our earlier work, we showed how carrier interferometry (CI) codes may be used to spread OFDM symbols over all N subcarriers to exploit frequency diversity without loss in throughput. However, only BPSK was considered in the earlier work. In this paper, we extend the proposed CI/OFDM (carrier interferometry with OFDM) system by updating it for application with QAM modulation schemes. (For example, minimized mean square error combining (MMSEC) is derived for CI/OFDM with QAM modulation schemes.) Simulation results over multi-path fading channels show that 64QAM CI/OFDM significantly outperforms 64QAM OFDM at the cost of a small increase in complexity.