Recent Advances of 6G Ultra-Massive MIMO Technologies in Spatial and Beam Domains

TL;DR

The paper analyzes 6G ultra-massive MIMO by examining channel characterization and modeling in both spatial and beam domains, highlighting the challenges of high-dimensional CSI, near-field effects, and RF-chain bottlenecks. It demonstrates how transforming to beam-domain representations via unitary transforms exploits sparsity and reduces complexity through models like the Beam Domain Channel Model (BDCM). It surveys channel estimation, multiplexing, and precoding techniques across domains, and discusses future directions including array configurations, holographic MIMO, RIS integration, joint beam division in time/frequency, and large-dimensional computing. The work suggests that beam-domain processing can substantially lower computational and energy costs while maintaining performance, facilitating practical ultra-massive MIMO deployment for 6G.

Abstract

To explore the full potential of ultra-massive multiple-input multiple-output (MIMO) communication systems, it is fundamental to understand new ultra-massive MIMO channel characteristics and establish pervasive channel models. On this basis, large dimensional spatial-temporal transmission and random access technologies need to be investigated and evaluated for better practical implementation. Firstly, this paper reviews recent advances of ultra-massive MIMO technologies in the traditional spatial domain, including wireless channel characterization and modeling, channel estimation, spatial multiplexing, and precoding. Secondly, considering the dramatic increase of base station (BS) antennas and access users in ultra-massive MIMO systems, the confronted high dimensional complexity and computing burden of these ultra-massive MIMO technologies are indicated. To provide efficient and systematic solution, the emerging tendency to transform related technologies from the traditional spatial domain to beam domain is introduced. The utilities of large sparsity merit, reduced energy consumption, and improved usage of radio frequency (RF) chains in the beam domain channel are elaborated. At last, future challenges of ultra-massive MIMO communication systems are discussed.

Figures (5)

• Figure 1: Key PHY technologies of ultra-massive MIMO.
• Figure 2: Wireless channel in the beam domain.
• Figure 3: Power variations of different virtual beams.
• Figure 4: Channel capacities of various channel models.
• Figure 5: Working flowchart of ultra-massive MIMO.