In the upcoming 6G era, multiple access (MA) will play an essential role in achieving high throughput performances required in a wide range of wireless applications. Since MA and interference management are closely related issues, the conventional MA techniques are limited in that they cannot provide near-optimal performance in universal interference regimes. Recently, rate-splitting multiple access (RSMA) has been gaining much attention. RSMA splits an individual message into two parts: a common part, decodable by every user, and a private part, decodable only by the intended user. Each user first decodes the common message and then decodes its private message by applying successive interference cancellation (SIC). By doing so, RSMA not only embraces the existing MA techniques as special cases but also provides significant performance gains by efficiently mitigating inter-user interference in a broad range of interference regimes. In this article, we first present the theoretical foundation of RSMA. Subsequently, we put forth four key benefits of RSMA: spectral efficiency, robustness, scalability, and flexibility. Upon this, we describe how RSMA can enable ten promising scenarios and applications along with future research directions to pave the way for 6G.

We investigate the effect of intermediate resonators (i.e., intermediate receiver (i-Rx) or relay) on the power transfer efficiency (PTE) for nonradiative wireless power transfer (WPT) with transmitter (Tx), intermediate-resonators, and end receiver (e-Rx). Specifically, we consider WPT systems with two different types of an intermediate resonator: 1) WPT relay systems, where the relay has no load resistor and just forwards the power from the Tx to the e-Rx, and 2) WPT i-Rx systems, where both i-Rx and e-Rx have a load resistor each and the power transmitted by the Tx is consumed at each Rx. Using an equivalent circuit model, we derive a closed-form solution for representing the optimal coupling coefficients between the Tx and the intermediate resonator for a given placement of the intermediate resonator and the e-Rx, i.e., k _12,opt for WPT i-Rx systems and k _1r,opt for WPT relay systems, respectively. The analytical result indicates that the quality factors of resonators have a great effect on determining their optimal positions. We also provide performance comparisons between the considered WPT systems. From the result, it is observed that k _12,opt is always larger than k _1r,opt , which indicates that the optimal position of the Tx is closer to the i-Rx rather than the relay. Moreover, in this case, WPT i-Rx systems can attain a higher PTE than WPT relay systems. Performing experiments under a variety of scenarios, we verify that the analytical results are in concordance with the measured ones.

In this paper, we study simultaneous wireless information and power transfer (SWIPT) for Internet of Things (IoT) sensor networks. The transmitters (e.g., access point) employ hybrid beamforming and each IoT receiver adopts a power splitting method that divides the received signal into two parts for information recovery and energy harvesting. We propose a novel strategy for SWIPT in which the transmitters have the option to either send a private or a common message. The private message is recovered only by a designated IoT receiver while common messages are recovered by all the receivers. While requiring the receivers to recover a common message results in additional rate constraints, the overall system performance benefits by mitigating interference. We propose SWIPT schemes that minimize the total transmit power by properly selecting message configurations, designing hybrid beamforming vectors, and adjusting the power splitting ratio at the receivers to satisfy the individual rate and energy harvesting constraints. In particular, we develop tractable and efficient two-stage algorithms that, in the first stage determine the message configurations and in the second stage find the beamforming vectors (for both analog and digital components). Numerical simulations demonstrate that the proposed schemes significantly outperform conventional schemes that transmit private messages only using digital beamforming.

We consider reconfigurable intelligent surface (RIS) aided sixth-generation (6G) terahertz (THz) communications for indoor environment in which a base station (BS) wishes to send independent messages to its serving users with the help of multiple RISs. For indoor environment, various obstacles such as pillars, walls, and other objects can result in no line-of-sight signal path between the BS and a user, which can significantly degrade performance. To overcome such limitation of indoor THz communication, we firstly optimize the placement of RISs to maximize the coverage area. Under the optimized RIS placement, we propose 3D hybrid beamforming at the BS and phase adjustment at RISs, which are jointly performed at the BS and RISs via codebook-based 3D beam scanning with low complexity. Numerical simulations demonstrate that the proposed scheme significantly improves the average sum rate compared to the cases of no RIS and randomly deployed RISs and also outperforms several benchmark schemes. It is further shown that the proposed codebook-based 3D beam scanning efficiently aligns analog beams between BS–user links or BS–RIS–user links and, as a consequence, achieves the average sum rate close to that of coherent beam alignment requiring global channel state information.

This paper studies simultaneous wireless information and power transfer (SWIPT) for unmanned aerial vehicle (UAV)-enabled Internet of Things (IoT) networks. Specifically, it is assumed that a single UAV wishes to simultaneously send common and private data streams as well as energy to multiple IoT nodes, in which the common stream should be recovered by all nodes while private streams are recovered only by the dedicated nodes. In addition, it is assumed that each node uses a power splitting method that divides the received signal into two parts for energy harvesting and information decoding. Under this setting, by applying a concave-convex procedure (CCCP) method, we propose a novel algorithm that maximizes the minimum rate of private streams of IoT nodes by properly allocating the transmit power of each stream, adjusting the power splitting ratios at IoT nodes, and designing the trajectory of the UAV, while the common rate and energy harvesting constraints at each node should also be satisfied. Moreover, to enable multi-user communication, a superposition coding in conjunction with successive interference cancellation (SIC) decoding is considered for SWIPT. The performance of the proposed scheme is evaluated and compared with benchmark schemes in various aspects by numerical simulations.

We study cooperative communication with an active reconfigurable intelligent surface (RIS) for the K -user rank-deficient multiple-input multiple-output (MIMO) interference channel. Specifically, we assume that transmitters and receivers use M antennas each, and the channel matrix between each transmitter and each receiver has rank L ₂ whereas the channel matrix between each transmitter or each receiver and the RIS has rank L ₁ , where L ₁ , L ₂ ≤ M . In the absence of the RIS, the multiplexing gain from MIMO is severely limited when L ₂ is small. We develop a novel cooperative transmission technique utilizing the RIS to overcome the channel rank deficiency and manage inter-user interference, and the key idea is to neutralize interfering links using signals reflected from the RIS or to reflect incident waves at the RIS to increase the rank of effective channel matrices, depending on the system configuration parameters. We analyze the achievable sum degrees of freedom (DoF) and sum rate, and derive an upper bound on the sum DoF, which is tight under certain conditions. The results show that using an active RIS can significantly improve both the sum rate and sum DoF compared to the network without the RIS, especially when L ₁ is small and/or L ₂ is large.