Effect of carrier frequency offset on single-carrier CDMA with frequency-domain equalization

Single-carrier code-division multiple access (SC-CDMA) with frequency-domain equalization (FDE) has attracted renewed interest in CDMA due to its lower peak-to-average power ratio (PAPR) and higher frequency diversity than orthogonal frequency-division multiple access (OFDMA), increased robustness to asynchronous multipath and multiuser interference than RAKE-based CDMA, and a reduced need for timing and frequency coordination than time-division multiple access, frequency-division multiple access, and OFDMA. However, existing studies on SC-CDMA are mostly conducted based on an optimistic assumption of perfect carrier frequency estimation; hence, the carrier frequency offset (CFO) effect on the performance of SC-CDMA has not adequately been addressed and is not well understood. In this paper, the system performance of SC-CDMA with FDE is evaluated in terms of the average bit error rate (BER) and signal-to-interference-plus-noise ratio (SINR) obtained through an analytical approach for downlink transmission over a frequency-selective fading channel. The results show that CFO gives rise to three effects: 1) a net phase rotation; 2) fading-like signal power reduction; and 3) intercode interferences (ICIs) due to the loss of code orthogonality. How the CFO-induced BER degradation varies with the number of active users, the number of virtual subcarriers, and constellation size is also investigated. We further show that, in practice, to prevent the BER performance from dramatically deteriorating, the residual CFO after synchronization should be kept within 5% of the virtual subcarrier spacing, irrespective of the constellation size.