Non-orthogonal multiple access (NOMA) has the potential to provide higher throughput than conventional orthogonal multiple access (OMA), which has been considered as a key technology for 5G. NOMA in satellite communication system can provide anytime, anywhere access with improved spectral efficiency and system capacity because of its ubiquitous coverage. However, the characteristics of the satellite channels are different from that of the terrestrial network, i.e., huge time delay and Doppler shift. In this paper, different from the existing works, which mainly focus on the performance of NOMA in static terrestrial base stations and Geostationary orbit (GEO) scenarios, we investigate the performance analysis of downlink NOMA in dynamic low earth orbit (LEO) satellite communication system with Doppler shift considered. We combine NOMA and orthogonal frequency division multiplexing (OFDM) for better spectral efficiency. Our channel model includes both small scale model and large scale model. For simplify, we only consider two users in one spot beam. Besides, we express the performance analysis of downlink NOMA in LEO satellite communication system, like ergodic capacity, outage probability (OP) and mutual information. However, in traditional NOMA scheme, the proceeding error decision of the high power user will cause the deterioration of the subsequent detection performance. Therefore, a symmetrical coding (SC) scheme for different modulation mode is proposed for low power users to get better performance. Finally, simulation results validate the performance of the NOMA scheme is better than that of the OMA scheme. The proposed SC scheme can achieve a prominent increase performance contrasted to the traditional NOMA scheme.

/pdf/the-performance-analysis-of-downlink-noma-in-leo-satellite-4z7x08hepc.pdf

The Performance Analysis of Downlink NOMA in LEO Satellite Communication System

As the renewable energy sources (RES) continues to grow in power system due to emissions decarbonisation and sustainability policies, a fast and reliable connection between RES components and equipment is crucial to ensure quality power delivery. This review provides a detailed insight into the full potential in 6G for the acceleration of RES with the faster and stable data bandwidth. The potential applications of 6G wireless networks for faster data processing in RES include P2P energy trade market, vehicular network, wireless energy transfer, energy management system, smart grid, self-healing, smart batteries and AI-based weather forecasting. This paper also lists the advantages and challenges of 6G in RES sectors, the practical implementations and the potential applications. It was found out that although the 6G is a visionary application in the RES sector to improve the security, connectivity, integration and sensory data processing of RES, 6G still possesses various technical limitations, including incompatibility issues of older devices, higher power consumption and higher operating cost. Future research should focus on the improvements of 6G in RES in terms of cost-effectiveness, compatibility issues and power consumption to balance between the advancement and limitations. This review serves as a visionary for 6G wireless communication to be applied in the RES sector in the near future. 6G in the RES sector encountered several critical limitations needing an intensive development, including still the issues of security and privacy, prone to hacking or data bleaching, required advanced hardware support and requirements for ultra-fast communications with low latency. • Due to climate change and emissions decarbonisation policy, the power system has shifted to RES-based generation. • 6G technology may revolutionise the RES sector with a fast, reliable and secured connection. • Potential applications and recommendations of 6G in the RES sector are discussed. • AI and blockchain frameworks improve remote data privacy and secure energy trading. 

Future outlook on 6G technology for renewable energy sources (RES)

Help from space: grant-free massive access for satellite-based IoT in the 6G era

Non-orthogonal multiple access (NOMA) and coordinated multi-point (CoMP) are two fundamental techniques considered for the fifth generation (5 G) of wireless communications. In this paper, a hybrid satellite-unmanned aerial vehicle (UAV) relay network (HSURN) is proposed where the UAV relays (URs) employ CoMP transmission to serve the terrestrial users (UEs). Furthermore, all UEs associated with the CoMP-URs form a single NOMA cluster. For this model, an optimization problem is formulated subject to the minimum quality of services (QoSs) requirements of the UEs, transmission power budgets and, successive interference cancellation (SIC), to select URs and allocate their transmission powers for the energy efficiency (EE) maximization. With this insight, first, a computationally efficient sub-optimal UR selection scheme is proposed. Then, the powers are allocated to the selected URs via the Lagrange multipliers optimization (LMO) method. Due to the non-convex nature of the considered problem, it is relatively difficult to be solved. Hence, a metaheuristic teaching-learning-based optimization (TLBO) algorithm is employed to achieve an efficient solution. Simulation results are provided to verify the effectiveness of the proposed sub-optimal relay selection scheme and the TLBO-based power allocation method compared to the LMO conventional method. Besides, the obtained results also reveal that the CoMP-NOMA transmission in the proposed scenario significantly improves the spectral efficiency (SE) and outage probability (OP) of the system compared to non-comp NOMA transmission case. 

/pdf/relay-selection-and-power-allocation-for-energy-efficiency-2isjyrsz.pdf

Relay Selection and Power Allocation for Energy Efficiency Maximization in Hybrid Satellite-UAV Networks With CoMP-NOMA Transmission

This paper investigates a satellite-terrestrial system employing the non-orthogonal multiple access (NOMA) scheme over Shadowed-Rician fading channels. The effects of both imperfect channel state information (CSI) and imperfect successive interference cancellation (SIC) on the NOMA based satellite-terrestrial system are taken into account. First, the outage performance of the proposed system is evaluated, and the closed-form expressions for the exact and asymptotic outage probabilities are obtained. Next, the expressions of the ergodic sum rate and system throughput are derived in the presence of channel estimation errors and residual interference. Moreover, we analyze the impacts of different residual interference levels on the energy efficiency. Finally, the numerical simulation results are conducted to verify the correctness of theoretical analysis and the advantages of the proposed NOMA scheme. At medium-high SNR region, the NOMA users are capable of outperforming orthogonal multiple access (OMA) users in terms of outage behaviors and system throughput with imperfect CSI and imperfect SIC. The ergodic sum rate decreases as the level of residual interference $\varpi $ increases, and is even lower than that of OMA once $\varpi $ goes beyond a certain value. Additionally, the energy efficiency of NOMA scheme is sensitive to the imperfect SIC.

/pdf/joint-effects-of-imperfect-csi-and-sic-on-noma-based-2lhqs9lg9s.pdf

Joint Effects of Imperfect CSI and SIC on NOMA Based Satellite-Terrestrial Systems

Due to the spectrum scarcity in the space information networks, non-orthogonal multiple access (NOMA) is envisioned as a promising technique in satellite-terrestrial communications networks as its improved system capacity and spectral efficiency. In this paper, to further improve the spectrum utilization efficiency in satellite-terrestrial networks, bandwidth compression (BC) design is embedded in the NOMA scheme, which contributes to the non-orthogonality both in power and frequency domains, named BC-NOMA. However, with such advantages provided by BC-NOMA, the inter-carrier interference (ICI) and the intra-group interference (IGI) are severe, which degrade the reliability and the capacity. Therefore, the impacts of BC and power domain multiplexing on the system capacity are first investigated. To cancel the mixed internal interference, an iterative and successive interference cancellation (ISIC) approach is proposed. Secondly, the symmetrical coding (SC) is designed with BC-NOMA to avoid the error propagation when using ISIC, which benefits much for the low power user (LUE). Furthermore, the closed-form expression of error probability for SCNOMA is derived and the system and individual capacities are also analyzed. Results show that the BC-NOMA system can achieve a quasi-optimal performance compared with the orthogonal subcarrier NOMA (ONOMA) system and balance the fairness.

Power Multiplexing NOMA and Bandwidth Compression for Satellite-Terrestrial Networks

For the future sixth-generation (6G) wireless communication networks, improved metrics are expected to provide connectivity of massive devices, which brings new challenges for 6G networks extending to modern radio access for Internet of Things (IoT) applications. We consider a 6G enabled nonterrestrial network working in remote areas in this article, where an on-demand unmanned aerial vehicles (UAVs) provides the connectivity services. To improve the energy efficiency (EE), a method combining nonorthogonal multiple access (NOMA) and spatial modulation (SM) techniques is proposed and termed spatial NOMA (S-NOMA). Particularly, by employing multiple input multiple output (MIMO), SM only activates partial transmit antennas in per symbol interval, which can provide large data rate with less interantenna interference (IAI). Moreover, a power allocation optimization method subject to EE for S-NOMA scheme is proposed. Specifically, the antenna selection bits are determined by all users, which improves EE for all users instead of the selected one. Besides, the capacity expressions of S-NOMA are derived, then the EE performance of S-NOMA is analyzed. In addition, simulation results show that the proposed S-NOMA with energy-efficient power allocation performs better EE performance compared with the conventional NOMA.

Energy-Efficiency Power Allocation Design for UAV-Assisted Spatial NOMA

Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) communication systems are investigated in its vicinity, where a STAR-RIS is deployed within a predefined region for establishing communication links for users. Both beamformer-based NOMA and cluster-based NOMA schemes are employed at the multi-antenna base station (BS). For each scheme, the STAR-RIS deployment location, the passive transmitting and reflecting beamforming (BF) of the STAR-RIS, and the active BF at the BS are jointly optimized for maximizing the weighted sum-rate (WSR) of users. To solve the resultant non-convex problems, an alternating optimization (AO) algorithm is proposed, where successive convex approximation (SCA) and semi-definite programming (SDP) methods are invoked for iteratively addressing the non-convexity of each sub-problem. Numerical results reveal that 1) the WSR performance can be significantly enhanced by optimizing the specific deployment location of the STAR-RIS; 2) both beamformer-based and cluster-based NOMA prefer asymmetric STAR-RIS deployment.

Joint Location and Beamforming Design for STAR-RIS Assisted NOMA Systems

Non-orthogonal multiple access (NOMA) has been regarded as a promising technique in the future generation wireless communications due to its superior spectral efficiency. However, according to the key conception of NOMA, the distant user (DU) which is far from the base station will be allocated more power than near user (NU) to guarantee fairness. With the utilization of successive interference cancellation (SIC), the signal of NU can be decoded after subtracting the signal of DU, which will deteriorate the performance of NU due to the error bit delivery. In this paper, the analysis and optimization on bit error rate (BER) performance of symmetrical coding (SC) based NOMA (SC-NOMA) scheme is studied under Nakagami-m fading channel. First, closed-form BER expressions of DU and NU are derived simultaneously in SC-NOMA scheme. Second, based on the closed-form expressions, a power allocation algorithm is proposed in SC-NOMA to achieve a better BER performance, and the trade-off between BER performances of NU and DU can be achieved with the proposed algorithm. Finally, simulation results are conducted to verify the validity of analysis. Results show that the proposed SC-NOMA can achieve a better BER performance compared with conventional NOMA.

On the Analysis and Optimization of BER Performance of Symmetrical Coding Based NOMA

In this paper, the system combining Non-Orthogonal Multiple Access (NOMA) with Spectrally Efficient Frequency Division Multiplexing (SEFDM) is mainly discussed. First, the mathematical model of NOMA–SEFDM system is given to further improve bandwidth efficiency. The widely used structure of Successive Interference Cancellation (SIC) is succeeded and the performance of NOMA–SEFDM system is investigated with Maximum Likelihood (ML) detector. Moreover, for BPSK modulation, considering the two users’ transmit symbols as a four-point constellation, the feasibility of iterative detection in NOMA–SEFDM system is verified, which achieves a quasi-optimal performance while the number of iterations is moderate while behaves with low computing complexity.

An Improved Detector Design for Combining Power Domain NOMA to Spectrally Efficient FDM

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