# Advanced Physical-Layer Technologies for Beyond 5G Wireless Communication Networks
## 3 Brief Review of Technical Papers  
NOMA and cognitive-radio (CR) networks were envisioned as key enabling techniques for 5G networks. The primary approach in NOMA is to remove orthogonality between allocated resource blocks, and it enables the simultaneous transmission of multiple users. Consequently, such an approach improves transmission rate and user fairness in the system, and provides superior spectrum utilization efficiency. Similarly, flexible spectrum allocation and efficient spectrum utilization can be achieved using CR technology [9; 10]. In the first paper, titled Performance Optimization of Hybrid Satellite-Terrestrial Relay Network Based on CR-NOMA [11], the authors proposed a joint relay-and-antenna selection scheme on the basis of the NOMA-CR scenario to achieve the maximal communication rate of the secondary user under a condition in which the primary user maintained optimal outage performance. Simulation results proved the effectiveness and superiority of the proposed scheme, and indicated the impact of parameter configurations on system outage performance. The integration of MIMO and relaying systems is a promising solution for the reliability and coverage improvements in 5G networks. However, the benefits of these technologies can only be achieved using proper system design. In the second paper, titled Separable MSE-Based Design of Two-Way Multiple-Relay Cooperative MIMO 5G Networks [12], the authors considered an interesting problem of jointly optimizing terminal precoders/decoders and relay forwarding matrices on the basis of the sum mean square error (MSE) criterion in MIMO two-way relay systems. Rather than attempting to iteratively solve the optimization problem, a relaxed version of the original minimal sum MSE nonconvex optimization problem was derived, which enabled the construction of twosimpler problems that admitted the closed-form, albeit suboptimal, solution. The proposed technique exhibited performance gain over the previous iterative method proposed in [13]. The development of hybrid satellite-terrestrial relay networks (HSTRNs) is a driving force in revolutionizing satellite communications in the modern era. The use of HSTRNs is required for the seamless integration of terrestrial cellular and satellite communications. Moreover, more users are simultaneously served using NOMA, and different performance types or services are observed among these users [14]. In the third paper, titled Hybrid Satellite-Terrestrial Relay Network: Proposed Model and Application of Power Splitting Multiple Access [15], the authors considered the requirements of both performance and energy efficiency (EE) for the future satellite communications, and provided an indepth performance evaluation of NOMA-enabled HSTRNs with a wireless-powered (i.e., energy harvesting) terrestrial relay. The proposed analytical expressions and derived results provided design insights for the implementations of HSTRNs in the 5G/6G era. Spectral efficiency (SE) is a highlighted feature of 5G/B5G MIMO systems. In the fourth paper, titled Spectral-Efficiency Augmentation in Uplink Massive MIMO Systems by Increasing Transmit Power and Uniform Linear Array Gain [16], the authors examined a massive MIMO system to increase the SE in each cell and consequently the area throughput of the proposed system. The presented analytical modeling indicated the possibility of maximizing area throughput by determining appropriate parameters in terms of average cell density, available bandwidth, and SE. The effectiveness of the proposed model was validated using simulation results implemented in real-time scenarios. The findings of the paper are useful in ensuring the efficient transmissions in future massive MIMO systems. Hybrid precoding strategies provide a potential solution to the path loss experienced by mmWave-MIMO systems. Furthermore, improved spectrum utilization is a key requirement in 5G/B5G systems. Consequently, the design of spectrally efficient precoders for partially connected hybrid structures is important.