Comparison of direct sequence and multicarrier CDMA systems employing multiuser detectors

Abstract

In CDMA, users in a given cell transmit simultaneously in the same frequency band. Two ways to spread the spectra of users' signals have been considered in this work. In the first technique, called Direct Sequence (DS) CDMA, each user transmits a digital data bit by transmitting a sequence of digital symbols of much smaller duration, called chips, one after the other on a given carrier frequency. The sequence of chips (spreading sequence) is unique for each user and is used to distinguish between the users. The second technique, called multicarrier CDMA uses as many carrier frequencies as the number of chips per bit. Each bit is transmitted by transmitting corresponding chips simultaneously on different carrier frequencies. In this case, the chip duration is equal to the bit duration. In this work, computer simulations are used to observe the performance of these two modulation techniques for two multiuser detectors viz. a Parallel Interference Canceller (PIC) and a decorrelator. The channel considered is a frequency selective slow Rayleigh fading channel. Thus, there are multiple resolvable paths through which the signal is received at the receiver. It has been observed that both the receivers employed in a multicarrier system significantly outperform their counterparts in a DS system. It is shown that the multicarrier system is comparatively much more resistant to self interference due to multipath. Its superior performance is attributed to this resistance. It is suggested that for a DS system, choice of spreading codes that have good autocorrelation properties can alleviate this problem up to some extent. However, it still cannot outperform a multicarrier system. Hence, use of multicarrier modulation is advocated.