Short-Packet Communications over Multiple-Antenna Rayleigh-Fading Channels

TL;DR

This paper analyzes the fundamental limits of short-packet, low-latency communication over multiple-antenna Rayleigh-fading channels, providing bounds that guide optimal system design for reliable machine-type communication.

Contribution

It derives tight finite-blocklength bounds on the maximum coding rate, revealing the tradeoff between diversity and channel estimation, and identifies optimal antenna and diversity configurations.

Findings

Finite-blocklength bounds tightly estimate achievable rates for short packets.

Tradeoff between diversity gain and channel estimation overhead is quantified.

Optimal number of antennas and diversity branches for maximum rate is determined.

Abstract

Motivated by the current interest in ultra-reliable, low-latency, machine-type communication systems, we investigate the tradeoff between reliability, throughput, and latency in the transmission of information over multiple-antenna Rayleigh block-fading channels. Specifically, we obtain finite-blocklength, finite-SNR upper and lower bounds on the maximum coding rate achievable over such channels for a given constraint on the packet error probability. Numerical evidence suggests that our bounds delimit tightly the maximum coding rate already for short blocklengths (packets of about 100 symbols). Furthermore, our bounds reveal the existence of a tradeoff between the rate gain obtainable by spreading each codeword over all available time-frequency-spatial degrees of freedom, and the rate loss caused by the need of estimating the fading coefficients over these degrees of freedom. In particular, our bounds allow us to determine the optimal number of transmit antennas and the optimal number of time-frequency diversity branches that maximize the rate. Finally, we show that infinite-blocklength performance metrics such as the ergodic capacity and the outage capacity yield inaccurate throughput estimates.