Large Random Access Systems with Retransmission Diversity

We consider that infinitely many users randomly access a basestation through a common channel of infinitely wide bandwidth with the number of users per dimension kept a constant. Retransmission diversity gain for demodulation of a transmitted packet is acquired via jointly using signals collected from multiple retransmissions of the packet. All users' channel profiles are considered in packet success probability. Through analyzing a large dominant random access system, sufficient conditions for stability of the original large random access system are obtained. Lower bounds on the steady throughput and spectral efficiency are also obtained. Through changing transmission probability as a means for medium access control, the maximum throughput as well as the maximum spectral efficiency can be achieved. The results are applied to large CDMA random access systems in flat fading channels. It is shown that the effect of medium random access as well as channel uncertainty is captured by the notion of effective interference. Numerical examples demonstrate that exploitation of retransmission diversity substantially improves the throughput and spectral efficiency. If retransmission diversity is exploited, performance gap between the MF and MMSE detectors in the high offered load regime is small. I. INTRODUCTION

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