Performance Evaluation of Adaptive Modulation System over Mobile Satellite Channels

The performance of an adaptive five-mode M-ary quadrature amplitude modulation (QAM) system under constant power constraint over time-varying land mobile satellite channel is evaluated in this paper. The mathematically-tractable closed-form expressions of performance for outage probability, bit error rate (BER) and average spectral efficiency are deduced by a simple Rician channel model in which the amplitude of the line-ofsight path is represented by the Nakagami distribution. Two adaptive modulation schemes (AMS) are compared, one with the optimal switching thresholds to maximize the average throughput under the constraint of average BER and another with fixed thresholds defined by the given BER tolerance for each modulation. Numeric simulation results show that the M-QAM AMS with optimum switching levels offers performance gains of ~2dB compared to fixed mode at given average spectrum efficiency, and nearly 0.3bps higher average throughput for different Rician K-factor.