Performance of adaptive modulation with optimal switching thresholds for distributed antenna system in composite channels

In this paper, the performance of distributed antenna system (DAS) with adaptive modulation (AM) over a composite fading channel which takes path loss, Rayleigh fading, and log-normal shadowing into account is studied. Based on target bit error rate (BER), the AM scheme for DAS with average BER constraints is presented. The optimum switching thresholds (STs) for attaining maximum spectrum efficiency (SE) are derived by using Lagrange optimization method. An effective iterative algorithm based on Newton method for finding the optimal STs is proposed. With these thresholds, the closed-form expression of SE and average BER are derived for performance evaluation. Simulation results for SE and BER are in good agreement with the theoretical analysis. The results show that DAS-AM with optimal STs has higher SE than that with conventional fixed thresholds. Moreover, the proposed AM can fulfill the target BER for different signal-to-noise ratios (SNRs).