Interference Cancellation for DS/CDMA Systems in Flat Fading Channels
Ann‐Louise Johansson · Chalmers Publication Library (Chalmers University of Technology) · 1996
This thesis investigates interference cancellation in direct-sequence codedivision multiple access (DS/CDMA) systems that support multiple data rates.Two approaches for implementing multiple data rates are considered. Oneis 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 toeach user. This enables the transmission of several parallel data streamsthat are mutually synchronous.We introduce and analyse a novel approach that combines the above describedmultiple data rate systems with interference cancellation (IC). The cancellationin the receiver is performed successively on each user, starting with theuser received with the highest power. This procedure can in turn be iterated,forming a multistage scheme, with the number of iterations set as a designparameter. We will evaluate the performance of these two multiple datarate schemes when used in combination with IC and compare them to the performanceof systems employing conventional detectors.We first analyse IC for M-ary rectangular QAM systems and then applythe results to mixed modulation systems and systems employing parallelchannels. Our analysis employs a Gaussian approximation for the distributionof the interference, and it includes both the AWGN and the flat Rayleighfading channel. The systems ate also evaluated via computer simulations.Our analysis and simulations indicate that the IC schemes used in mixedmodulation or parallel channel systems yield a performance close to thesingle BPSK user bound and, consequently, give a prospect of a considerableimprovement in performance compared to systems employing a conventionaldetector.