ON DIVERSITY RECEPTION OF 16 STAR-QAM IN RICIAN FADING

Abstract

Growth in mobile radio communications leads to increasing requirements for high data rate transmission that can be met by bandwidth efficient modulation schemes. In this thesis, a spectrally efficient multilevel quadrature amplitude modulation scheme called 16 STAR-QAM has been studied thoroughly in terms of the bit error rate (BER) performance in Rician fading. Differential encoding of 16 STAR-QAM produces 2 amplitude transitions and 8 phase transitions in the modulated carrier. An uncorrelated L-branch diversity receiver which differentially detects and decodes the 16 STAR-QAM signal, can be decomposed into two independent units: one for phase detection and the other for amplitude detection. Expressions for evaluating average BER of 16 STAR-QAM have been derived for three kinds of channel and system models: frequency-nonselective Rician fading, with matched receive filter; frequency-nonselective Rician fading, with IF Gaussian receive filter; and frequency-selective Rician fading, with matched receive filter. In frequency-nonselective Rician fading, a time varying channel introduces multiplicative noise characterized by a complex-valued Gaussian random process, which results in an error floor in the BER curve; and the error floor level is decided by the fading bandwidth of the Doppler spectrum. LOS Doppler shift increases bit error rate, even after a correction scheme is employed, especially at large signal-to-noise ratios. Diversity reception greatly improves the DER performance of 16 STAR-QAM in frequency-nonselective Rician fading. Moreover, 16 STAR-QAM has shown better error performance than 16 DPSK. If a bandwidth constrained IF filter is used in the receiver, intersymbol interference will he introduced clue to the limited filter bandwidth. In using a, realizable IF Gaussian filter, an optimum receiver performance is obtained with a filter bandwidth of about 1.2 times the symbol rate. In frequency-selective slow Rician fading, a wide sense stationary uncorrelated scattering (WSSUS) channel is characterized by the autocorrelation function of its impulse response-a power delay profile. Three types of delay profile has been studied: one-sided exponential, double spike and Gaussian profile. Intersymbol interference induced by frequency-selectivity is shown to degrade the system performance significantly and eventually, limits the data capacity of a channel. The time delay between the LOS and multipath components are also included in the delay profiles. The diversity technique is still capable of improving BER performance in frequency-selective Rician fading, but when the delay spread is comparable to the symbol period, it is no longer sufficient in combating the distortion introduced by the propagation channel.