When Pilots Should Not Be Reused Across Interfering Cells in Massive MIMO

TL;DR

This paper analyzes when pilot reuse is detrimental in massive MIMO systems, especially in scenarios with low mobility or fixed user sets, and proposes optimal pilot assignment strategies to improve sum-rate performance.

Contribution

It provides a closed-form solution for optimal pilot assignment in specific scenarios, demonstrating when less aggressive reuse improves system performance.

Findings

Less aggressive pilot reuse yields higher sum-rate in certain scenarios.

Optimal pilot assignment can be derived analytically for specific system parameters.

Performance gains are significant when user mobility is low or fixed user sets are considered.

Abstract

The pilot reuse issue in massive multi-input multi-output (MIMO) antenna systems with interfering cells is closely examined. This paper considers scenarios where the ratio of the channel coherence time to the number of users in a cell may be sufficiently large. One such practical scenario arises when the number of users per unit coverage area cannot grow freely while user mobility is low, as in indoor networks. Another important scenario is when the service provider is interested in maximizing the sum rate over a fixed, selected number of users rather than the sum rate over all users in the cell. A sum-rate comparison analysis shows that in such scenarios less aggressive reuse of pilots involving allocation of additional pilots for interfering users yields significant performance advantage relative to the case where all cells reuse the same pilot set. For a given ratio of the normalized coherence time interval to the number of users per cell, the optimal pilot assignment strategy is revealed via a closed-form solution and the resulting net sum-rate is compared with that of the full pilot reuse.