Comparison of different detection algorithms for OFDM-CDMA in broadband Rayleigh fading

The orthogonal frequency division multiplexing (OFDM) transmission technique applied in a multipath environment has a bit error rate (BER) comparable with a narrowband radio channel because the fading of each subcarrier is frequency-nonselective. To overcome this behavior and to reduce the BER a combination of OFDM and CDMA has been previously proposed. In this case the information bits are spread over many subcarriers. The present authors discuss the performance of OFDM-CDMA with BPSK or QPSK signalling over a frequency-selective Rayleigh fading channel. An important question is the detection strategy in an OFDM-CDMA system. The authors make a quantitative comparison of different detection algorithms: conventional correlation (equal gain, EGC), orthogonality restoring correlation (ORC), ORC with a threshold in order to suppress subcarriers with low signal strength (TORC) and an iterative improvement based on a maximum likelihood approach. With TORC and an optimal threshold a gain of more than 8 dB over conventional OFDM can be obtained at p_b=10^-3, if the fading of subcarriers is uncorrelated