On Channel Reciprocity to Activate Uplink Channel Training for Downlink Wireless Transmission in Tactile Internet Applications

TL;DR

This paper establishes the channel reciprocity requirement to activate uplink channel training for improved downlink data rates in multi-antenna systems, especially beneficial for short-packet Tactile Internet communications.

Contribution

It derives novel closed-form bounds for data rates and identifies the minimum channel reciprocity needed for uplink training to outperform downlink training, guiding strategy selection.

Findings

Minimum channel reciprocity decreases with shorter blocklengths.

Uplink training outperforms downlink training at lower reciprocity levels.

Results assist in latency reduction for Tactile Internet applications.

Abstract

We determine, for the first time, the requirement on channel reciprocity to activate uplink channel training, instead of downlink channel training, to achieve a higher data rate for the downlink transmission from a multi-antenna base station to a single-antenna user. We first derive novel closed-form expressions for the lower bounds on the data rates achieved by the two channel training strategies by considering the impact of finite blocklength. The performance comparison result of these two strategies is determined by the amount of channel reciprocity that is utilized in the uplink channel training. We then derive an approximated expression for the minimum channel reciprocity that enables the uplink channel training to outperform the downlink channel training. Through numerical results, we demonstrate that this minimum channel reciprocity decreases as the blocklength decreases or the number of transmit antennas increases, which shows the necessity and benefits of activating the uplink channel training for short-packet communications with multiple transmit antennas. This work provides pivotal and unprecedented guidelines on choosing channel training strategies and channel reciprocity calibrations, offering valuable insights into latency reduction in the Tactile Internet applications.