Frequency reuse is an attractive way for increasing transmission capacity of digital radio systems. Propagation impairments such as multipath fading or antenna imperfections require the introduction of new techniques to fight against the degradation of crosspolarization discrimination. This paper presents the performance of new kinds of cross-polarization interference cancellers for a dual 16 QAM system over dispersive radio channels. The structure of the studied cancellers is a decision feedback one. Topics include a multipath propagation model, canceller description, and the theoretical and dynamic simulated performance of the receiver. For asynchronous clocks for the horizontally and vertically polarized channels, the results show that the receiver performance is critically sensitive to the timing delay between the main and the interfering signals.

This paper presents a comparative study of four 2n-state quadrature amplitude modulation techniques, namely, 16, 32, 64, and 128 QAM, in a digital radio system environment. The effects of filtering, interference, amplifier nonlinearities, and selective fading are investigated using computational and simulation methods. Increase of the carrier-to-noise ratio (CNR) at a fixed symbol error probability(10^{-3}) is taken as a robustness criterion.