Far-Field vs. Near-Field Propagation Channels: Key Differences and Impact on 6G XL-MIMO Performance Evaluation

TL;DR

This paper examines the differences between near-field and far-field propagation channels in XL-MIMO systems, highlighting the importance of using near-field transceiver techniques to maintain performance as the system scales.

Contribution

It provides a comprehensive comparison of NF and FF channel models and demonstrates the necessity of NF techniques for XL-MIMO performance in the near-field region.

Findings

Maximum beam gain loss under FF technique is less than 3 dB in NF region.

Achievable rate loss of beam training is less than 3% when using FF techniques.

NF transceiver techniques are necessary for optimal XL-MIMO performance.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) is regarded as a promising technology for next-generation communication systems. However, this will expand the near-field (NF) range, rendering more users more likely to be located in the NF region. In this paper, we aim to answer two questions: What are the new characteristics of the NF channel? Is it necessary to develop new transciver techniques to maintain system performance within the NF region? To this end, we first review current NF channel models and analyze the differences between the existing 3GPP TR 38.901 channel model and the NF channel model, including the spherical wavefront and spatially non-stationarity. Then, we provide examples on how these differences affect the XL-MIMO system performance in terms of beamforming gain and achievable rate. Simulation results demonstrate that, when using far-field (FF) technique under the NF channel, the maximum normalized beam gain loss is less than 3 dB for most users in the NF region defined by Rayleigh distance. Moreover, the achievable rate loss of beam training is less than 3% compared to that realized by NF technique. Finally, we demonstrate the necessity of employing NF transceiver techniques based on simulation results.