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Channel Charting-assisted Non-orthogonal Pilot Allocation for Uplink XL-MIMO Transmission

Haohong Che, Li You, Jue Wang, Zhenzhou Jin, Chenjie Xie, Xiqi Gao

TL;DR

The paper tackles the pilot overhead challenge in near-field XL-MIMO uplink by introducing a channel charting (CC) framework that leverages polar-domain features of near-field channels. It develops a near-field CC (NCC) pipeline, including a polar-domain codebook, feature extraction, and Isomap-based charting, to map UTs into a low-dimensional space where pilot-sharing UTs are optimally separated. Building on this, the NCC-assisted pilot allocation (NCC-PA) algorithm assigns non-orthogonal pilots to maximize CC-map distances between UTs sharing the same pilot, enhancing channel orthogonality and reducing estimation error. Simulation results show NCC-PA achieves lower MSE and higher sum-rate than FCC-based and random pilot schemes, especially at higher SNR and longer pilot lengths, indicating significant practical gains for XL-MIMO networks.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) is critical to future wireless networks. The substantial increase in the number of base station (BS) antennas introduces near-field propagation effects in the wireless channels, complicating channel parameter estimation and increasing pilot overhead. Channel charting (CC) has emerged as a potent unsupervised technique to effectively harness varying high-dimensional channel statistics to enable non-orthogonal pilot assignment and reduce pilot overhead. In this paper, we investigate near-field channel estimation with reduced pilot overhead by developing a CC-assisted pilot scheduling. To this end, we introduce a polar-domain codebook to capture the power distribution of near-field XL-MIMO channels. The CC-assisted approach uses such features as inputs to enable an effective low-dimensional mapping of the inherent correlation patterns in near-field user terminal (UT) channels. Building upon the mapped channel correlations, we further propose a near-field CC-assisted pilot allocation (NCC-PA) algorithm, which efficiently enhances channel orthogonality among pilot-reusing UTs. Numerical results confirm that the NCC-PA algorithm substantially elevates the wireless transmission performance, offering a marked improvement over the conventional far-field CC-PA approach.

Channel Charting-assisted Non-orthogonal Pilot Allocation for Uplink XL-MIMO Transmission

TL;DR

The paper tackles the pilot overhead challenge in near-field XL-MIMO uplink by introducing a channel charting (CC) framework that leverages polar-domain features of near-field channels. It develops a near-field CC (NCC) pipeline, including a polar-domain codebook, feature extraction, and Isomap-based charting, to map UTs into a low-dimensional space where pilot-sharing UTs are optimally separated. Building on this, the NCC-assisted pilot allocation (NCC-PA) algorithm assigns non-orthogonal pilots to maximize CC-map distances between UTs sharing the same pilot, enhancing channel orthogonality and reducing estimation error. Simulation results show NCC-PA achieves lower MSE and higher sum-rate than FCC-based and random pilot schemes, especially at higher SNR and longer pilot lengths, indicating significant practical gains for XL-MIMO networks.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) is critical to future wireless networks. The substantial increase in the number of base station (BS) antennas introduces near-field propagation effects in the wireless channels, complicating channel parameter estimation and increasing pilot overhead. Channel charting (CC) has emerged as a potent unsupervised technique to effectively harness varying high-dimensional channel statistics to enable non-orthogonal pilot assignment and reduce pilot overhead. In this paper, we investigate near-field channel estimation with reduced pilot overhead by developing a CC-assisted pilot scheduling. To this end, we introduce a polar-domain codebook to capture the power distribution of near-field XL-MIMO channels. The CC-assisted approach uses such features as inputs to enable an effective low-dimensional mapping of the inherent correlation patterns in near-field user terminal (UT) channels. Building upon the mapped channel correlations, we further propose a near-field CC-assisted pilot allocation (NCC-PA) algorithm, which efficiently enhances channel orthogonality among pilot-reusing UTs. Numerical results confirm that the NCC-PA algorithm substantially elevates the wireless transmission performance, offering a marked improvement over the conventional far-field CC-PA approach.
Paper Structure (10 sections, 9 equations, 3 figures, 1 table, 2 algorithms)

This paper contains 10 sections, 9 equations, 3 figures, 1 table, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Model and Problem Statement
  3. NCC-assisted Non-orthogonal Pilot Allocation
  4. Channel Charting
  5. Overview of CC
  6. Feature Extraction
  7. Chart Coordinates
  8. PA Algorithm Design
  9. Simulation Results
  10. Conclusion

Figures (3)

  • Figure 1: Illustration of CC in an XL-MIMO setup with 128 UTs: (a) The purple triangle denotes the BS equipped with a 256-element ULA, where each colored marker indicates a UT. (b) A 2D CC map created using far-field features $\bm{F}$. (c) The same CC map using near-field features $\bm{\Xi}$.
  • Figure 2: MSE performance comparison against pilot length for different SNR values.
  • Figure 3: Sum rate performance comparison against pilot length for different SNR values.