A Random Access Protocol Incorporating Multi-packet Reception, Retransmission Diversity and Successive Interference Cancellation

Abstract

This paper presents a random access protocol assisted by a set of signal processing tools that significantly improve the multi-packet reception (MPR) capabilities of the system. A receiver with M antennas is mainly used to resolve collisions with multiplicity $K\le M$. The remaining unresolved conflicts (with multiplicity $K>M$ or with decoding errors) are processed by means of protocol-induced retransmissions that create an adaptive multiple-input multiple-output (MIMO) system. This scheme, also known as NDMA (network diversity multiple access) with MPR, can achieve in ideal conditions a maximum throughput of M packets/time-slot. A further improvement is proposed here, where the receiver attempts to recover the information immediately after the reception of each (re)transmission. This is different from conventional NDMA, where this decoding process only occurs once the adaptive MIMO channel is assumed to become full-rank (i.e., once the estimated number of required retransmissions has been collected). The signals that are correctly decoded at every step of the proposed algorithm are used to mitigate interference upon the remaining contending signals by means of successive interference cancellation (SIC). The use of SIC not only improves signal reception, but most importantly reduces the required number of retransmissions to resolve a collision. Significantly high throughput figures (depending on channel/load conditions) that surpass the nominal rate ($T>M$) are reported. To the best of our knowledge this is the first random access protocol that achieves this throughput figure. Spatial and time correlation, as well as imperfections of SIC operation are also considered. In ideal conditions, the effects of SIC are found to be equivalent to a splitting tree operation. The inclusion of SIC in NDMA-MPR also opens the possibility of backwards compatibility with legacy terminals. The protocol achieves the highest throughput in the literature with minimum feedback complexity (identical to conventional NDMA ). This is a significant result for future highly dense and 5G wireless networks.