Spatial-to-Spectral Harmonic-Modulated Arrays for 6G Multi-Beam MIMO

TL;DR

The paper tackles the challenge of scalable, multi-beam MIMO for 6G by introducing spatial-to-spectral harmonic-modulated arrays (SHAs) that map spatial directions to frequency bins, enabling concurrent beams without hardware replication. It analyzes how harmonic-modulation LO (HM-LO) waveforms shape bandwidth, gain, and noise, and demonstrates that comb-like HM-LO spectra can eliminate harmonic loss and provide uniform per-beam gain. A formal DOF framework is developed, showing how two and three spatial-to-spectral DOFs can independently steer multiple beams (via HM-JPTA architectures), and it discusses two scalable pathways to realize more than three DOFs (multi-phase mixers and narrowband phase shifters) at the cost of hardware replication. The work highlights SHA applicability to multi-user MIMO, joint communication and sensing (JCAS), and interference cancellation, and offers design guidelines for implementing SHA-based MB-MIMO solutions in future 6G networks, balancing performance and hardware complexity.

Abstract

This article presents an overview and analysis of spatial-to-spectral harmonic-modulated arrays (SHAs). Compared to traditional analog or digital beamforming arrays, SHAs enable concurrent multi-beamforming without requiring substantial hardware replication. SHAs replace the need for hardware replication with frequency-domain multiplexing. Furthermore, SHAs have the potential to become key contributors to future 6G networks by enabling scalable multi-user communications, joint communication and sensing, and spatial interference mitigation. In addition, an analysis of the SHA's harmonic-modulation waveform and its effects on gain, noise and bandwidth is presented. A comb-like modulation waveform for SHAs that minimizes spectral inefficiency is proposed. Further, an analysis of the SHA's capability to independently steer multiple beams is presented. This capability is quantified in terms of the SHA's spatial-to-spectral degrees of freedom. Lastly, this work introduces a novel SHA architecture that provides three spatial-to-spectral degrees of freedom with minimal hardware replication.

Figures (10)

• Figure 1: (a) A phased array typically forms a single beam and transmits/receives a signal centered at a single frequency $f_0$. (b) An SHA forms multiple beams at several frequencies $f_1, f_2, f_3 ...$ for transmit or receive operation.
• Figure 2: SHAs generally consist of LTV elements, such as mixers. Additionally, SHAs have a phase-frequency profile that can be determined by true-time delay elements and/or phase shifters.
• Figure 3: (a) A generic representation of an SHA with output frequencies $f_1, f_2, f_3$. (b) The SHA's phase-frequency relationship, $\Delta\phi(f)$, is such that at $f_1, f_2, f_3$ the progressive phase shift across the array is $\Delta\phi_1, \Delta\phi_2, \Delta\phi_3$. (c) Due to the frequency-dependence of $\Delta\phi(f)$, a beam is formed at each frequency $f_1, f_2, f_3$ pointed toward angle $\theta_1, \theta_2, \theta_3$, respectively.
• Figure 4: (a) The phased array and (b) true-time delay array are two common beamforming architectures. Additionally, the (c) joint phase-time array has been previously explored to enable frequency-dependent beamforming. The remaining architectures are SHAs. (d) The time-modulated array is an example of an SHA where the waveform a square wave. In this paper, we generalize to (e) the harmonic-modulated array, where that waveform is an arbitrary periodic waveform. Additionally, this article explores architectures that provide multiple spatial-to-spectral degrees of freedom such as (f) the harmonic-modulated JPTA.
• Figure 5: (a) A block diagram of an $N$-channel TMA. (b) The TMA output spectrum. At each frequency $f_m$, the TMA forms a beam centered at a unique angle $\theta_m$.