Abstract
We consider the problem of efficient ultra-massive multiple-input multiple-output (UM-MIMO) data detection in terahertz (THz)-band non-orthogonal multiple access (NOMA) systems. We argue that the most common THz NOMA configuration is power-domain superposition coding over quasi-optical doubly-massive MIMO channels. We propose spatial tuning techniques that modify antenna subarray arrangements to enhance channel conditions. Towards recovering the superposed data at the receiver side, we propose a family of data detectors based on low-complexity channel matrix puncturing, in which higher-order detectors are dynamically formed from lower-order component detectors. We first detail the proposed solutions for the case of superposition coding of multiple streams in point-to-point THz MIMO links. We then extend the study to multi-user NOMA, in which randomly distributed users get grouped into narrow cell sectors and are allocated different power levels depending on their proximity to the base station. We show that successive interference cancellation is carried with minimal performance and complexity costs under spatial tuning. We derive approximate bit error rate (BER) equations, and we propose an architectural design to illustrate complexity reductions. Under typical THz conditions, channel puncturing introduces more than an order of magnitude reduction in BER at high signal-to-noise ratios while reducing complexity by approximately 90%.
Highlights
Terahertz (THz) communications [1], [2] are currently accelerating research on future sixth-generation (6G) wireless mobile communications, following the successful deployment of millimeter-wave communications [3] in fifthgeneration (5G) networks
Since high-frequency non-orthogonal multiple access (NOMA) [16] is likely to be conducted over line-of-sight (LoS) ultra-massive multipleinput multiple-output (UM-MIMO) links, we argue that studying THz multiple access techniques reduces to studying the superposition coding of multiple data streams over a single point-to-point link
The main paper contributions are: 1) We study the performance of NOMA systems under THzspecific UM-MIMO channel models [31] and applying recently-reported adaptive spatial tuning techniques [10] that enhance THz channel conditions
Summary
Terahertz (THz) communications [1], [2] are currently accelerating research on future sixth-generation (6G) wireless mobile communications, following the successful deployment of millimeter-wave (mmWave) communications [3] in fifthgeneration (5G) networks. A variety of probabilistic shaping and index modulation techniques [22] can be explored for adaptive array usage, especially in plasmonic THz solutions where antenna elements (AEs) can independently tune the frequency of operation by simple material doping or electrostatic bias [7] Such reconfigurability can simultaneously accomplish resource allocation, user clustering, and beamforming. 2) We build on the distance-dependant THz propagation features and propose simple low-complexity power allocation (through AE allocation per SA) and user clustering NOMA solutions that exploit the distance-based path-loss between the users and the BS to mitigate intra-cluster interference.
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