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We determine, for the first time, the requirement on channel reciprocity toactivate uplink channel training, instead of downlink channel training, toachieve a higher data rate for the downlink transmission from a multi-antennabase station to a single-antenna user. We first derive novel closed-formexpressions for the lower bounds on the data rates achieved by the two channeltraining strategies by considering the impact of finite blocklength. Theperformance comparison result of these two strategies is determined by theamount of channel reciprocity that is utilized in the uplink channel training.We then derive an approximated expression for the minimum channel reciprocitythat enables the uplink channel training to outperform the downlink channeltraining. Through numerical results, we demonstrate that this minimum channelreciprocity decreases as the blocklength decreases or the number of transmitantennas increases, which shows the necessity and benefits of activating theuplink channel training for short-packet communications with multiple transmitantennas. This work provides pivotal and unprecedented guidelines on choosingchannel training strategies and channel reciprocity calibrations, offeringvaluable insights into latency reduction in the Tactile Internet applications.

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