Peak to Average Power Ratio Properties of MC-CDMA and SM-CDMA

This paper compares peak to average power ratio (PAPR) of multi carrier code division multiple access (MC-CDMA) and serial modulation-CDMA (SM-CDMA) with each other and with corresponding non-spread spectrum techniques which are orthogonal frequency division multiplexing (OFDM) and serial modulation (SM). In this comparison we consider the effect of two spreading types- binary and complex spreading, and equal-level and multi-level constellation on the amplitude distribution and out-of-band radiation. While MC-CDMA and OFDM have similar amplitude distribution, we show that the PAPR of SM-CDMA can be larger than that of SM. If the spreading is binary and the constellation is multi-level, this effect is so powerful that the PAPR of SM-CDMA becomes similar to that of MC-CDMA. Finally we use a modified selected mapping (SLM) algorithm to decrease the PAPR of SM-CDMA so that it would be equal to that of SM. Therefore SM-CDMA has lower PAPR if we use it together with modified SLM, and the system will need amplifiers with lower power back-off values