Analysis of adaptive modulation with antenna selection under channel prediction errors

Abstract

Abstract-The performance of an adaptive modulation system in Rayleigh fading channels exploiting spatial diversity through transmit antenna selection is analysed for the case of delay constrained networks. The system combines maximal ratio combining at the receiver and a transmit antenna selection system which switches between available antennas and modulation schemes at the transmitter. In a practical system, a delay between the channel being sampled at the receiver and acted upon by the transmitter will tend to degrade system performance. In this paper, a channel prediction scheme is employed at the receiver to provide estimates of future best transmission states, which includes selecting the best transmission antenna as well as the best supported M-QAM modulation scheme. The Shannon capacity for optimal rate and constant power is derived and presented, and used as a benchmark to evaluate the spectral efficiency of the discrete rate system optimised for instantaneous BER (bit error rate) and constant power constraint. [5]. However, the performance of adaptive modulated schemes is limited by CSI (channel state information) imperfections particularly in the form of delay in the feedback channel, which causes outdated channel information at the transmitter, thus reducing capacity. The use of imperfect channel estimates for SISO channels is investigated in [6], [7] while the effect of feed back delay in adaptive modulation (AM) systems over a Nakagami fading SISO has been considered in [8], showing the degradation of BER (bit error rate) with feedback delay. To mitigate the effect of feedback delay, channel prediction has been employed for use in SISO channels using a pilot symbol aided modulation (PSAM) technique [9], while [10], [11], [12], [13] consider multiple antenna schemes. This paper investigates the issues relating to degraded CSI for an un-coded TAS/MRC (transmit antenna selection with maximal ratio combing) system with prediction, which adaptively adjusts modulation format based upon outdated CSI, in order to maximise spectral efficiency at a target BER. The predicted future CSI is used at the receiver to select the best switching parameters for transmission. Although the use of a feedback channel is necessary, in practice only a low bandwidth channel is required since it is simply the indices of the best predicted antenna and rate that need to be fed back. TAS [14], is beneficial in reducing

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“…Since then, the technique has been investigated widely, with the main purpose of matching transmission rates to varying channel conditions, for example in [ [112], and even for the realistic scenario where only outdated channel information is available [113], erroneous [114], or even in multipleinput-multiple-output orthogonal space-time block code (OSTBC) environment [115].…”