Burst-by-burst adaptive multiuser detection CDMA techniques

Kuan, Ee Lin (1999) Burst-by-burst adaptive multiuser detection CDMA techniques. University of Southampton, Doctoral Thesis.

Abstract

This thesis explores the application of burst-by-burst adaptive joint detection (JD) Direct Sequence Code Division Multiple Access (DS-CDMA) schemes. JD receivers attempt to mitigate the effects of multiple access interference and inter-symbol interference in frequency-selective channels. In order to accommodate the fluctuating quality of the multipath fading channel, adaptive-rate transmission was introduced. When the instantaneous channel quality was high, the transmission rate was increased and when the channel conditions were inferior, a more robust transmission mode was employed in order to maintain a performance target. Two methods of rate adaptations were investigated, namely the Adaptive Quadrature Amplitude Modulation (AQAM) based JD-CDMA system and the Variable Spreading Factor (VSF) JD-CDMA scheme. For both methods, the output signal to interference plus noise ratio (SINR) of the JD receiver was used as the criterion for rate adaptation. In the AQAM scheme where there were five choices of modulation modes, namely no transmission (NOTX), BPSK, 4-QAM and 64-QAM, it was demonstrated that for the target BER of 0.01%, the performance gain of the AQAM scheme over fixed-mode schemes was in the range of 1-2 dB for a Bad Urban (BU) channel, 4-6 dB for a Typical Urban (TU) channel and 11-14 dB for a Rural Area (RA) channel.

A range of interference cancellation (IC) receivers were also investigated and their performance was compared with that of JD. For a twin-mode AQAM scheme supporting 8 users transmitting over a seven-path BU channel, the bits-per-symbol (BPS) throughput of the JD was the highest, where a throughput of 1.9 BPS was achieved at E_s/N₀ - 14 dB compared to the 1.55 BPS and 1.02 BPS achieved by the parallel IC and successive IC receivers, respectively. For the triple-mode VSF scheme employing spreading factors of Q₁ = 64, Q₂ = 32 and Q₃ = 16, the JD outperformed the two IC receivers in terms of BER and BPS throughput performance with a normalized throughput of 3.5 compared to 2.7 and 2 for the PIC and SIC receivers, respectively, at E_s/N₀ = 16 dB. It was concluded that flexible burst-by-burst adaptive CDMA schemes constitute attractive schemes for future mobile communications standards, which can be backwards compatible with existing standards.

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