Benefit of Delay on the Diversity-Multiplexing Tradeoffs of MIMO Channels with Partial CSI

TL;DR

This paper investigates how minimal causal CSI at the transmitter can significantly improve the reliability-delay tradeoff in MIMO channels, highlighting the benefits of short delays and bounded worst-case decoding times.

Contribution

It demonstrates that even one bit of causal CSI per coherence interval can substantially enhance reliability with minimal delay, expanding understanding of delay and codeword length as separate design parameters.

Findings

Short expected delays improve reliability with causal CSI.

Bounded worst-case delays provide reliable communication.

Causal CSI enables reliability gains similar to ARQ systems.

Abstract

This paper re-examines the well-known fundamental tradeoffs between rate and reliability for the multi-antenna, block Rayleigh fading channel in the high signal to noise ratio (SNR) regime when (i) the transmitter has access to (noiseless) one bit per coherence-interval of causal channel state information (CSI) and (ii) soft decoding delays together with worst-case delay guarantees are acceptable. A key finding of this work is that substantial improvements in reliability can be realized with a very short expected delay and a slightly longer (but bounded) worst-case decoding delay guarantee in communication systems where the transmitter has access to even one bit per coherence interval of causal CSI. While similar in spirit to the recent work on communication systems based on automatic repeat requests (ARQ) where decoding failure is known at the transmitter and leads to re-transmission, here transmit side-information is purely based on CSI. The findings reported here also lend further support to an emerging understanding that decoding delay (related to throughput) and codeword blocklength (related to coding complexity and delays) are distinctly different design parameters which can be tuned to control reliability.