Joint Pilot Design and Uplink Power Allocation in Multi-Cell Massive MIMO Systems

TL;DR

This paper introduces a continuous optimization framework for pilot design and uplink power allocation in multi-cell Massive MIMO systems, significantly reducing pilot contamination and improving system fairness and capacity.

Contribution

It models pilot signals using a basis with continuous power variables and develops an algorithm for near-optimal solutions, advancing beyond prior combinatorial approaches.

Findings

Proposed algorithms closely approximate exhaustive search solutions.

New pilot structure and optimization yield large capacity gains.

Framework serves as a benchmark for practical pilot design.

Abstract

This paper considers pilot design to mitigate pilot contamination and provide good service for everyone in multi-cell Massive multiple input multiple output (MIMO) systems. Instead of modeling the pilot design as a combinatorial assignment problem, as in prior works, we express the pilot signals using a pilot basis and treat the associated power coefficients as continuous optimization variables. We compute a lower bound on the uplink capacity for Rayleigh fading channels with maximum ratio detection that applies with arbitrary pilot signals. We further formulate the max-min fairness problem under power budget constraints, with the pilot signals and data powers as optimization variables. Because this optimization problem is non-deterministic polynomial-time hard due to signomial constraints, we then propose an algorithm to obtain a local optimum with polynomial complexity. Our framework serves as a benchmark for pilot design in scenarios with either ideal or non-ideal hardware. Numerical results manifest that the proposed optimization algorithms are close to the optimal solution obtained by exhaustive search for different pilot assignments and the new pilot structure and optimization bring large gains over the state-of-the-art suboptimal pilot design.