Clipping Distortion Performance of Nonsquare $M$ -QAM OFDM Systems on Nonlinear Time-Variant Channels

Orthogonal frequency-division multiplexing (OFDM) systems usually make use of a set of square
$M$
quadrature-amplitude modulation (
$M$
-QAM) constellations to obtain a good tradeoff between throughput and symbol-error robustness. However, the switch to the next constellation increases the number of bits per modulation symbol by two. The introduction of nonsquare
$M$
-QAM constellations in such systems brings extra advantages such as smoother transition among bit rates and a reduction of the peak-to-average ratio of the OFDM signal. Therefore, this paper unfolds analytical expressions to evaluate the symbol-error performance of nonsquare
$M$
-QAM OFDM on nonlinear time-variant additive white Gaussian noise channels, taking clipping distortion into account. Cross and overlaid
$M$
-QAM are considered. Analytical performances are evaluated and compared with computational simulations, which show good agreement.