A Cross-Layer Approach to Reducing Packet Delay in Polling-Based Multiuser Systems

We consider a system where users are polled for transmission by a central server, and the link layer of each user employs automatic repeat request (ARQ) to ensure error-free delivery of packets. We present a cross-layer approach for reducing average packet delay (queuing delay plus service time). It is known that forward error correction (FEC) schemes can potentially lower the average packet delay by reducing the packet error probability (PEP) and, hence, the number of retransmissions. However, more precise results in the literature are few and far between. In this paper, we first establish that relative to an uncoded system, it is sufficient to reduce the average service time (AST) using FEC to achieve lower average packet delay. We then study and quantify the reduction in AST that can be achieved using the best possible FEC codes. The specific findings and contributions from our work are as follows: 1) We provide several bounds on the reduction in AST using the best possible FEC codes; 2) we give a sufficient condition when no FEC scheme can reduce the AST; 3) for Gaussian channels, we find that a relatively high PEP $(\sim\!\!\text{10}^{-2})$, which is obtained using as high a coding rate as possible, typically results in sufficiently small AST; 4) the performance of optimum maximum-likelihood decoding can be approached by a lower complexity bounded distance decoder; and 5) average packet delay can be further reduced in certain cases by opportunistically combining and encoding several packets jointly.