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.Comment: 6 pages, 3 figures, Submitted to IEEE ICC 2018 Worksho