The future connectivity landscape, and notably, the 5G wireless systems will feature Ultra-Reliable Low Latency Communication (URLLC). The coupling of high reliability and low latency requirements in URLLC use cases makes the wireless access design very challenging, in terms of both the protocol design and of the associated transmission techniques. This paper aims to provide a broad perspective on the fundamental tradeoffs in URLLC, as well as the principles used in building access protocols. Two specific technologies are considered in the context of URLLC: massive MIMO and multi-connectivity, also termed interface diversity . This paper also touches on the importance of the proper statistical methodology for designing and assessing extremely high-reliability levels.
One central question in multi-antenna system is the acquisition of instantaneous CSI. It is one of the most severe limitations to achieve URLLC when exploiting multiple antennas in a mobile environment constrained by channel coherence time as well as extreme latency requirements. The most critical acquisition occurs at the antenna array when the CSI is used for transmission mode (CSIT). In frequency division duplex (FDD) systems, CSIT acquisition requires a feedback loop from the terminals inducing a significant latency as the number of links to report is large. In a massive MIMO system below 6 GHz where the terminals have a small number of antennas, this concerns the BS in downlink transmission. In time division duplex (TDD) systems, latency can still be reduced by exploiting channel reciprocity @cite_1 , but remains critical. In a mmWave system, with potentially many antennas at the terminal, the issue concerns both side of the communication links.
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Channel reciprocity in time-division duplexing (TDD) massive multiple-input multiple-output (MIMO) systems can be exploited to reduce the overhead required for the acquisition of channel state information (CSI). However, perfect reciprocity is unrealistic in practical systems due to random radio-frequency (RF) circuit mismatches in uplink and downlink channels. This can result in a significant degradation in the performance of linear precoding schemes, which are sensitive to the accuracy of the CSI. In this paper, we model and analyse the impact of RF mismatches on the performance of linear precoding in a TDD multi-user massive MIMO system, by taking the channel estimation error into considerations. We use the truncated Gaussian distribution to model the RF mismatch, and derive closed-form expressions of the output signal-to-interference-plus-noise ratio for maximum ratio transmission and zero forcing precoders. We further investigate the asymptotic performance of the derived expressions, to provide valuable insights into the practical system designs, including useful guidelines for the selection of the effective precoding schemes. Simulation results are presented to demonstrate the validity and accuracy of the proposed analytical results.