Motivated by the current interest in ultra-reliable, low-latency,machine-type communication systems, we investigate the tradeoff betweenreliability, throughput, and latency in the transmission of information overmultiple-antenna Rayleigh block-fading channels. Specifically, we obtainfinite-blocklength, finite-SNR upper and lower bounds on the maximum codingrate achievable over such channels for a given constraint on the packet errorprobability. Numerical evidence suggests that our bounds delimit tightly themaximum coding rate already for short blocklengths (packets of about 100symbols). Furthermore, our bounds reveal the existence of a tradeoff betweenthe rate gain obtainable by spreading each codeword over all availabletime-frequency-spatial degrees of freedom, and the rate loss caused by the needof estimating the fading coefficients over these degrees of freedom. Inparticular, our bounds allow us to determine the optimal number of transmitantennas and the optimal number of time-frequency diversity branches thatmaximize the rate. Finally, we show that infinite-blocklength performancemetrics such as the ergodic capacity and the outage capacity yield inaccuratethroughput estimates.
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