Comparing DS-CDMA and multicarrier CDMA with imperfect channel estimation

Probability of bit error expressions are derived for direct sequence CDMA (DS-CDMA) and multicarrier CDMA (MC-CDMA) with imperfect diversity combining. Pilot and data channels are transmitted through a Rayleigh fading channel with an exponential multipath intensity profile. Channel statistics are estimated using simple integrators. Then the multipath in the DS system and the multiple subcarriers in the MC system are weighted by the imperfect channel estimates and combined. Keeping the data rate, the transmit power, and the fading power constant, as the bandwidth increases, the number of multipaths increases in the DS system, and the number of subcarriers increases in the MC system. At the same time, the signal strength in each path/subcarrier decreases, and results in larger errors in the channel estimates. We show that there is a tradeoff between diversity order and SNR available for channel estimation in both DS-CDMA and MC-CDMA. Moreover, we also show that MC-CDMA performs better than DS-CDMA.