Joint Beam Management and Power Allocation in THz-NOMA Networks

Abstract

This paper investigates how to apply non-orthogonal multiple access (NOMA) as an add-on in terahertz (THz) networks. In particular, prior to the implementation of NOMA, it is assumed that there exists a legacy THz system, where spatial beams have already been configured to serve legacy primary users. The aim of this paper is to study how these pre-configured spatial beams can be used as a type of bandwidth resources, on which additional secondary users are served without degrading the performance of the legacy primary users. A joint beam management and power allocation problem is first formulated as a mixed combinatorial non-convex optimization problem, and then solved by two methods with different performance-complexity tradeoffs, one based on the branch and bound method and the other based on successive convex approximation. Both analytical and simulation results are presented to illustrate the new features of beam-based resource allocation in THz-NOMA networks and also demonstrate that those pre-configured spatial beams can be employed to improve the system throughput and connectivity in a spectrally efficient manner. Index TermsNon-orthogonal multiple access (NOMA), Terahertz (THz), the branch and bound method, successive convex approximation, beam management, power allocation. I. INTRODUCTION Terahertz (THz) communications and non-orthogonal multiple access (NOMA) are two key enabling technologies for the envisioned sixth generation (6G) mobile network [1], [2]. On the one hand, the use of THz communications is promising because a huge amount of bandwidth in the THz spectrum is available for communications [3]-[6]. On the other hand, the use of NOMA transmission can significantly improve spectral efficiency and support massive connectivity, by encouraging intelligent spectrum cooperation among mobile users [7], [8]. The two communication techniques are naturally complementary to each other. For example, using Z. Ding and H. V.

Potentials and Limits of Using Preconfigured Spatial Beams as Bandwidth Resources: Beam Selection Versus Beam Aggregation

“…As a result, g H m f i = 0, for m = i. As shown in [7], these preconfigured beams, f m , can be exploited as a type of bandwidth resources and used to serve additional secondary users, similarly to OFDM subcarriers. In this paper, a simple case with a single secondary user, denoted by U S , is focused.…”

“…As shown in [7], with s S m , 1 ≤ m ≤ M , independently encoded, the optimal resource allocation strategy is to select a single beam to serve the secondary user, and the performance of beam selection is analyzed in the following section.…”

“…For the simulations conducted for this paper, an exhaustive search is used to find the optimal choice of |D|. An important future direction is to design an efficient way to optimize D by applying advanced optimization tools, such as the branch and bound algorithm [7].…”

“…An alternative is to directly use those preconfigured beams for serving the new users, which makes the implementation of NOMA and the admission of the new users transparent to the legacy network. As shown in [7], these preconfigured spatial beams indeed have the potential to be used as bandwidth resources, similar to orthogonal frequency-division multiplexing (OFDM) subcarriers, but the benefits of using these beams can be severely limited by inter-beam interference.…”

“…Note that this is an important case to illustrate the key difference between beam-based resource allocation and conventional subcarrier allocation. In particular, instead of the water-fillingbased solution which is optimal for subcarrier allocation, the use of beam selection, i.e., selecting a single beam to serve the secondary user, is optimal for this case, if the secondary user's signals on different beams are independently encoded, as shown in [7]. In this letter, the performance realized by beam selection is analyzed by using the outage probability as the metric, where our developed analytical result shows that beam selection can ensure that the outage probability achieved by the secondary user goes to zero at high signal-to-noise ratio (SNR), despite the strong interference from those primary Z. Ding is with the School of Electrical and Electronic Engineering, the University of Manchester, Manchester, UK.…”