Joint Beamforming and Power Splitting Control in Downlink Cooperative SWIPT NOMA Systems (1706.02851v1)

Abstract: This paper investigates the application of simultaneous wireless information and power transfer (SWIPT) to cooperative non-orthogonal multiple access (NOMA). A new cooperative multiple-input single-output (MISO) SWIPT NOMA protocol is proposed, where a user with a strong channel condition acts as an energy-harvesting (EH) relay to help a user with a poor channel condition. The power splitting (PS) scheme is adopted at the EH relay. By jointly optimizing the PS ratio and the beamforming vectors, the design objective is to maximize the data rate of the "strong user" while satisfying the QoS requirement of the "weak user". It boils down to a challenging nonconvex problem. To resolve this issue, the semidefinite relaxation (SDR) technique is applied to relax the quadratic terms related with the beamformers, and then it is solved to its global optimality by two-dimensional exhaustive search. We prove the rank-one optimality, which establishes the equivalence between the relaxed problem and the original one. To further reduce the high complexity due to the exhaustive search, an iterative algorithm based on successive convex approximation (SCA) is proposed, which can at least attain its stationary point efficiently. In view of the potential application scenarios, e.g., IoT, the single-input single-output (SISO) case of the cooperative SWIPT NOMA system is also studied. The formulated problem is proved to be strictly unimodal with respect to the PS ratio. Hence, a golden section search (GSS) based algorithm with closed-form solution at each step is proposed to find the unique global optimal solution. It is worth pointing out that the SCA method can also converge to the optimal solution in SISO cases. In the numerical simulation, the proposed algorithm is numerically shown to converge within a few iterations, and the SWIPT-aided NOMA protocol outperforms the existing transmission protocols.

• The paper introduces a cooperative MISO SWIPT NOMA protocol where a strong user acts as an energy-harvesting relay to support a weak user.
• It employs semidefinite relaxation and an iterative SCA algorithm to jointly optimize beamforming and power splitting, achieving high data rates.
• Results demonstrate that the joint optimization outperforms traditional OMA and noncooperative NOMA, with significant benefits for IoT and 5G applications.

Joint Beamforming and Power Splitting Control in Downlink Cooperative SWIPT NOMA Systems

The paper "Joint Beamforming and Power Splitting Control in Downlink Cooperative SWIPT NOMA Systems" addresses a sophisticated signal processing challenge involving simultaneous wireless information and power transfer (SWIPT) in a cooperative non-orthogonal multiple access (NOMA) scenario. Specifically, the authors propose a new cooperative multiple-input single-output (MISO) SWIPT NOMA protocol that harnesses the potential of a user with a strong channel condition to act as an energy-harvesting relay for aiding a user with a weaker channel condition. The core methodology focuses on optimizing both power splitting (PS) and beamforming vectors to maximize the data rate of the strong user while adhering to the quality-of-service (QoS) requirement for the weak user.

Problem Formulation and Methodology

The main problem tackled in the paper is inherently non-convex, presenting significant mathematical challenges. To navigate these complications, the semidefinite relaxation (SDR) technique is employed to handle the non-convexity introduced by the quadratic terms associated with beamformers. In pursuit of global optimality, the authors perform an exhaustive two-dimensional search. An important result demonstrated is the SDR tightness, highlighting the equivalence between the relaxed and original problems.

To counterbalance the computational inefficiencies of exhaustive search, an iterative algorithm leveraging successive convex approximation (SCA) is proposed. This method efficiently achieves at least a stationary point while maintaining low complexity. The simplicity of the problem in single-input single-output (SISO) settings allows for a golden section search (GSS)-based algorithm, offering a semiclosed-form solution by exploiting unimodal properties with respect to the PS ratio.

Results and Contributions

The paper presents several important contributions:

1. Proposal of a Cooperative SWIPT-Aided NOMA: A novel strategy where a strong user serves as an EH relay for a weak user, providing a sustainable relay transmission without depleting its battery.
2. Beamforming and Power Splitting in MISO: The paper reveals that by jointly optimizing the beamforming vectors and PS ratio, system performance is greatly enhanced when compared to traditional OMA and noncooperative NOMA setups.
3. Iterative Algorithm Development: The SCA-based algorithm emerges as a pivotal tool, striking a balance between attaining stationary solutions and computational practicality.
4. SISO Case Analysis: A semiclosed-form global optimal solution via GSS underscores practical benefits in IoT scenarios, elucidating achievable performance improvements in less complex antenna configurations.

Implications and Future Work

From a theoretical perspective, the work provides key insights into SWIPT NOMA systems, particularly how energy harvesting and efficient beamforming can be construed to optimize resource allocation in wireless networks. Practically, it paves the way for more energy-efficient and robust communications in environments such as IoT networks and next-generation mobile architectures.

Future developments in this research domain could explore deeper integration of NOMA with other advanced technologies like massive MIMO and intelligent reflecting surfaces, further pushing the boundaries of spectral and energy efficiency. Consideration of real-world constraints and hardware impairments should be incorporated into the optimization models to make solutions even more applicable. This will be crucial for scaling the proposed strategies in larger, denser networks with a myriad of devices, reflecting realistic 5G (and beyond) scenarios.