Enhancing Performance of Uplink URLLC Systems via Shared Diversity Transmissions and Multiple Antenna Processing

In this work we investigate the reliability aspects of uplink multi-user MIMO communication over a preallocated pool of time-frequency resources shared by a group of ultra-reliable low-latency devices. To achieve sufficient diversity, users perform multiple transmissions of their packets over a shared pool of time-frequency resources in a non-orthogonal manner. The preallocation allows users to employ fast, grant-free type of access, while sharing improves the overall spectral efficiency. The multiple transmit opportunities enhance the robustness of communication through incremental redundancy. On the base station side, we consider the performance of a minimum mean square error (MMSE) receiver, chosen for its relative simplicity.
In addition to a baseline scheme in which devices randomly select the resources without coordination, we consider two other approaches based on preassigned access patterns: i) one in which all resources are utilized evenly and with equal power, and ii) another, where the spectrum is divided into high and low contention portions and users benefit from having few reliable transmissions and few diversity resources. In particular, we focus on evaluating the performance limits of the schemes as the number of antennas at the base station grows.