Interference Cancellation for DS/CDMA Systems in Flat Fading Channels

Licentiate thesis, 1996

This thesis investigates interference cancellation in direct-sequence code division multiple access (DS/CDMA) systems that support multiple data rates. Two approaches for implementing multiple data rates are considered. One is the use of mixed modulation. The second is the use of parallel channels. With mixed modulation we denote that each user chooses a modulation format, e.g., BPSK, QPSK (4-QAM) or 16-QAM, according to the desired data rate. Parallel channels are realized by assigning several spreading codes to each user. This enables the transmission of several parallel data streams that are mutually synchronous. We introduce and analyse a novel approach that combines the above described multiple data rate systems with interference cancellation (IC). The cancellation in the receiver is performed successively on each user, starting with the user received with the highest power. This procedure can in turn be iterated, forming a multistage scheme, with the number of iterations set as a design parameter. We will evaluate the performance of these two multiple data rate schemes when used in combination with IC and compare them to the performance of systems employing conventional detectors. We first analyse IC for M-ary rectangular QAM systems and then apply the results to mixed modulation systems and systems employing parallel channels. Our analysis employs a Gaussian approximation for the distribution of the interference, and it includes both the AWGN and the flat Rayleigh fading channel. The systems ate also evaluated via computer simulations. Our analysis and simulations indicate that the IC schemes used in mixed modulation or parallel channel systems yield a performance close to the single BPSK user bound and, consequently, give a prospect of a considerable improvement in performance compared to systems employing a conventional detector.