POLO: Phase-Only Localization in Uplink Distributed MIMO Systems

TL;DR

POLO introduces a low-complexity, phase-only localization method for uplink distributed MIMO systems, using differential carrier-phase measurements and AP selection strategies to efficiently estimate user positions with minimal computational effort.

Contribution

The paper presents POLO, a novel localization framework that reduces ML localization complexity in D-MIMO systems by using differential phase measurements and AP selection variants.

Findings

Significant reduction in localization computational cost.

Comparable coverage performance to exhaustive search.

Effective AP selection strategies for different latency and coverage needs.

Abstract

We propose a low-complexity localization framework for uplink distributed MIMO (D-MIMO) systems, targeting the challenge of minimizing the highly spiky maximum-likelihood (ML) cost function that arises in sparsely deployed phasecoherent access points (APs) with narrowband transmission. In such systems, ML-based localization typically relies on dense grid search, incurring prohibitive computational complexity. To address this, we introduce phase-only localization (POLO), an approach that leverages differential carrier-phase measurements from selected APs to generate a compact set of candidate user positions. The ML cost function is then evaluated only at these candidates, reducing complexity significantly. A key challenge is to devise an AP selection mechanism that reduces the number of candidate points while maintaining reliable coverage. We propose two variants: POLO-I, which selects three APs to provide closed-form candidate positions with low computational cost, and POLO-II, which selects four APs using an alternative strategy that enhances coverage at marginally higher runtime. Comprehensive analytical and simulation results show that POLO achieves a favorable coverage-complexity trade-off, reducing cost by orders of magnitude relative to exhaustive grid search with only marginal loss in coverage. By characterizing this tradeoff under diverse AP configurations, we also provide practical guidelines for selecting between POLO-I and POLO-II depending on latency and coverage requirements.