Comparison Between the Performance of QPSK, SQPSK, QPR, and SQPR Systems Over Microwave LOS Channels

Abstract
This paper studies the effectiveness of adaptive equalizers used with QPSK, SQPSK, QPR, and SQPR modems over frequency selective microwave LOS channels. A power series model is employed to represent the channel model, giving special attention to the channels with a null in the passband. The effect of staggering the symbols on the performance of modems with and without an adaptive equalizer is provided. With a deep null in the passband of the channel, the performance of QPR modems is slightly worse than SQPR modems, while QPSK systems significantly outperform SQPSK systems. The general conclusion that a decision feedback equalizer is required for a robust performance by each modem is reached.