This paper presents a comparative study of four 2
n-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

) is taken as a robustness criterion.