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

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

Optimal Transmission of Short-Packet Communications in Multiple-Input Single-Output Systems

We design the optimal transmission strategy, which maximizes the average achievable data rate of the multiple-input single-output system that adopts short-packet communications. In this system, the N A -antenna access point (AP) transmits to the single-antenna user with finite blocklength T after estimating the AP-user channel via downlink training and uplink feedback. For this system, we determine the optimal allocation of the finite resource (e.g., the total transmit power and a finite number of symbol periods) for downlink training, uplink feedback, and data transmission to maximize the average data rate. Specifically, we derive an approximate closed-form lower bound on the average data rate, an explicit result for the optimal number of symbol periods for downlink training, an easy-to-implement method to find the optimal number of symbol periods for uplink feedback, and a simple expression for the optimal power allocation between data transmission and downlink training. By using numerical results, we demonstrate the effectiveness of our analytical solutions and examine the impact of system parameters, e.g., N A and T, on the optimal strategy.This work was supported by the Australian Research Council under Discovery Project Grant DP180104062