Interference reduction techniques in hybrid DS/FH spread spectrum multiple access systems

Abstract

In this thesis, we consider several interference reduction techniques in hybrid DS/FH pread spectrum multiple access (SSMA) systems. First, we consider partial successive interference cancellation (SIC) in hybrid DS/FH SSMA systems. We first show that the lowest BER attained by employing full interference cancellation in DS/SSMA systems can almost be achieved by employing a partial interference cancellation in hybrid DS/FH systems. The reduction in the number of cancellations translates into an alleviation of correlator speed requirements and a reduction in delays incurred in interference cancellations. The optimal number of frequency slots that minimizes the BER is investigated as a function of the number of interference cancellations. The effect of imperfect power control on the BER is also investigated. Second, we consider adaptive frequency hopping schemes in hybrid DS/FH SSMA systems for the reverse link transmission in slow fading channels. Conventional hopping schemes include random hopping and adaptive hopping over the frequency slot with the largest power gain. We propose new adaptive hopping schemes, where adaptive hopping patterns are determined based on non-iterative as well as iterative signal-to-interference ratio (SIR) calculations. These hopping schemes are derived from the problem of determining the optimal hopping pattern and its drastic approximation. We investigate performance improvements that proposed hopping schemes provide over conventional hopping schemes. Third, we consider truncated power adaptation of controlling received powers for effective SIC in DS/CDMA communications. Users with the channel power gains above the cutoff level vary their transmit powers, so that the received powers of all transmitting users yield a geometrical series. The ratio between adjacent terms that rules this geometrical series measures a disparity in received powers of users in SIC systems. We find that there exists an optimum ratio that minimizes BER ...