Recognition of CDMA signals with orthogonal codes using cyclostationarity

The concept of cyclostationarity can be used for recognition, extraction, and synchronization. A signal exhibits cyclostationarity if its cyclic autocorrelation function (CAF) is non-zero for a non-zero cyclic frequency. The CAF and cyclic frequencies differ from one signal to another depending on their characteristics such as carrier frequency and baud rate. Therefore, the property of cyclostationarity exhibited by modulated signals can be exploited for signal recognition, extraction, and synchronization. In this paper, we employ cyclostationarity for the recognition of CDMA signals that use orthogonal spreading codes. First, we derive an explicit formula for the CAF of such CDMA signals, and then demonstrate how the cyclostationarity in association with the chip rate decreases as the number of multiplexed codes increases. We also show that this cyclostationarity completely disappears if a scrambling code is used together with the spreading code. Furthermore, we consider two cases for enhancing the detection probability: i) the case of antenna diversity and ii) the case when multiple CDMA signals exist in different channels in the same frequency bands. Finally, computer simulations show that the detection probability of a CDMA signal can be improved in the above two cases.