ABSTRACT
This chapter presents a simplified mathematical model to evaluate the performance of any given circular constellation of the M-level quadrature amplitude modulation (M-QAM) technique in terms of probability of error (POE) or symbol error rate (SER). With the aim to work on memoryless nonlinear satellite channels, the model is derived as a generalized form for both linear and nonlinear channels in the presence of additive white Gaussian noise (AWGN). The analysis provides the means to calculate the optimal ring ratio (RR) and phase difference (PD) for several possible candidates of 16-and 32-QAM circular constellations. The effects of RR and PD on the POE performance are investigated in the analysis. In the 16-QAM system, the analytical formulation has been extended for total degradation (TD) performance measure as a function of input back-off (IBO) of the nonlinear amplifier. To overcome the nonlinear distortion, data predistortion is taken into account. A POE performance comparison between different constellations for both 16-and 32-QAM systems has also been presented in this chapter. The analytical results are validated by simulation.
The rapid evolution of global information technology demands high-data-rate transmission via satellites in the presence of available bandwidth, which in turn requires a spectrally efficient modulation technique. In this case, high-level modulation techniques (M-ary) are the favorable candidates. Among all the existing M-ary modulation techniques M-level quadrature amplitude modulation (M-QAM) offers the maximum power/spectral efficiency [Pro02] and appears to be a potentially attractive modulation scheme for satellite communications [Ibn04].
