Feedback Communication Systems with Limitations on Incremental Redundancy

This paper explores feedback systems using incremental redundancy (IR) with noiseless transmitter confirmation (NTC). For IR-NTC systems based on {\em finite-length} codes (with blocklength $N$) and decoding attempts only at {\em certain specified decoding times}, this paper presents the asymptotic expansion achieved by random coding, provides rate-compatible sphere-packing (RCSP) performance approximations, and presents simulation results of tail-biting convolutional codes. The information-theoretic analysis shows that values of $N$ relatively close to the expected latency yield the same random-coding achievability expansion as with $N = \infty$. However, the penalty introduced in the expansion by limiting decoding times is linear in the interval between decoding times. For binary symmetric channels, the RCSP approximation provides an efficiently-computed approximation of performance that shows excellent agreement with a family of rate-compatible, tail-biting convolutional codes in the short-latency regime. For the additive white Gaussian noise channel, bounded-distance decoding simplifies the computation of the marginal RCSP approximation and produces similar results as analysis based on maximum-likelihood decoding for latencies greater than 200. The efficiency of the marginal RCSP approximation facilitates optimization of the lengths of incremental transmissions when the number of incremental transmissions is constrained to be small or the length of the incremental transmissions is constrained to be uniform after the first transmission. Finally, an RCSP-based decoding error trajectory is introduced that provides target error rates for the design of rate-compatible code families for use in feedback communication systems.Comment: 23 pages, 15 figure

Provision of Guaranteed QoS with Hybrid Automatic Repeat Request in Interleave Division Multiple Access Systems

Provision of guaranteed quality of service (QoS) in wireless communication has always been a demanding task. QoS can be ensured by the mechanism of repeat request, however real time systems can tolerate only a finite delay. In this paper we investigate the problem of provision of guaranteed QoS by hybrid automatic repeat request (HARQ) schemes, based on soft outputs of a channel decoder. Focus is on adaptive interleavedivision multiple access (IDMA) transmission. Such a system is able to provide virtually arbitrary bit error rates (BER). The proposed HARQ schemes use the reliability information provided by the channel decoder to decide whether a packet satisfies the quality of service (QoS) requirements. QoS can be specified in terms of a minimum required throughput/data rate or a minimum required BER. Effect of truncation of repeat requests on the bit error rate and packet throughput is investigated. An adaptation scheme based upon the bit error rate in the accepted packets is proposed and the effect of the adaptation on the effective throughput is demonstrated