Reconfigurable intelligent surface (RIS) and non-orthogonal multiple access (NOMA) are two promising technologies for future wireless communication networks. This article proposes a new transmission design for short-packet communications (SPCs) in a downlink RIS-assisted NOMA system, considering hardware impairments at the transceiver nodes, nonoptimal continuous phase shifts (CPSs) and discrete phase shifts (DPSs) at the RIS, and imperfect successive interference cancellation (i-SIC) for a pair of NOMA users. Using the method of moment matching, we approximate the end-to-end channel gain with a Gamma distribution and derive the closed-form approximate expressions for the average block error rates (BLERs) for NOMA users, along with the diversity order and the asymptotic expressions for the average BLERs at the high signal-to-noise ratio (SNR) region. Inspired by the asymptotic average BLER, the minimum blocklength is derived, and the optimal power allocation factor is obtained. Then, the system throughput is obtained to characterize the transmission efficiency of the RIS-assisted short-packet NOMA network. Analytical results of the proposed scheme indicate that: 1) the phase estimation error of the RIS has a significant impact on the system performance, and thus, the nonoptimal CPSs should be used in the theoretical analysis and 2) the DPSs with 3-bit quantization can effectively achieve a comparable performance to the optimal CPS scheme in practical applications. The numerical results validate our theoretical analysis and clearly show the performance improvement of transmission reliability and effectiveness of the proposed scheme, as compared to existing transmission schemes.

In this article, we investigate the robust relay and jammer selection for secure communications in wireless-powered cooperative nonorthogonal multiple access (CoNOMA) networks under imperfect channel state information (CSI) and imperfect successive interference cancellation (SIC). The proposed secrecy communication scheme consists of two transmission phases. In the first transmission phase, a relay is selected from M candidates to assist in the transmission from the base station (BS) to a pair of nonorthogonal multiple access (NOMA) users. A wirelesspowered jammer is selected from J candidates based on the criterion of maximizing the generated interference to the eavesdropper (Eve), and the selected jammer harvests energy from the transmitted signals of the BS. In the second transmission phase, the selected relay serves NOMA users, and the selected wireless-powered jammer emits artificial noise (AN) to confuse the Eve. To assess the transmission security and reliability of the proposed scheme, closed-form expressions for the connection outage probability (COP), secrecy outage probability (SOP), effective secrecy throughput (EST), and secrecy diversity order for a pair of NOMA users are derived under imperfect CSI and imperfect SIC, revealing that our robust relay and jammer selection scheme improves the transmission performance of NOMA users. Numerical results verify the correctness of our theoretical analysis and demonstrate the performance superiority of the proposed scheme to existing schemes.

This paper investigates the secrecy communications of a downlink wireless-powered cooperative non-orthogonal multiple access (NOMA) system with a source node, a relay, a pair of near and far users, and an eavesdropper (Eve). A wireless-powered relay assists the transmission from the source to the far user, and the Eve intends to wiretap the confidential message transmitted to the near user. We propose a dynamic power allocation and friendly jamming based secure transmission scheme. Specifically, the optimal power allocation factors at the source node for the paired NOMA users are obtained to guarantee the perfect successive interference cancellation performance and improve the transmission reliability of the pair of NOMA users. In the direct transmission phase, the source transmits a superimposed signal to the near user and the wireless-powered relay, and the far user serves as a friendly jammer to emit artificial noise to confuse the Eve. In the cooperative transmission phase, the relay utilizes the harvested energy to assist the transmission to the far user. By applying the Gaussian-Chebyshev and Gaussian-Laguerre quadratures, closed-form expressions for the secrecy outage probability of the near user and the connection outage probability of the far user are derived. Simulation results verify the correctness of our theoretical analysis. The superiority of the proposed scheme is also demonstrated for improving the secrecy performance, as compared to the existing cooperative NOMA schemes. © 2024 IEEE.

Impartial cooperation is considered in simultaneous wireless information and power transfer (SWIPT)-assisted downlink non-orthogonal multiple access (NOMA) systems with random user distribution. Unlike conventional SWIPT-assisted cooperative NOMA transmissions, we consider random user locations and investigate a transmission model in which both the near and far users of a NOMA system can serve as energy harvesting relays and forward information to each other. To assess the impact of user placement on the system performance, three user pairing schemes are designed. Closed-form expressions for the outage probability, diversity order, diversity-multiplexing trade-off (DMT), and system throughput are derived under different user location distributions, revealing that the novel SWIPT-assisted impartial cooperative NOMA improves these metrics of the near user, and does not degrade those of the far user, compared with the conventional schemes. Numerical results validate our analysis and confirm that our approach is more efficient in resource utilization, as compared to existing schemes.

To allieviate the heavy traffic burden over backhaul links and improve the user’s quality of service (QoS), edge caching plays an important role in wireless networks. This paper investigated the optimal designs of content placement and transmission in wireless caching networks. The contents to be cached and requested were encoded into individual layers by scalable video coding (SVC), and different sets of layers can provide different viewing qualities to end users. The demanded contents were provided by helpers caching the requested layers, or by the macro-cell base station (MBS) otherwise. In the content placement phase, this work formulated and solved the delay minimization problem. In the content transmission phase, the sum rate optimization problem was established. To effectively solve the nonconvex problem, the methods of semi-definite relaxation (SDR), successive convex approximation (SCA), and arithmetic-geometric mean (AGM) inequality were adopted, after which the original problem was transformed into the convex form. The numerical results show that the transmission delay is reduced by caching contents at helpers. Moreover, the fast convergence of the proposed algorithm for solving the sum rate maximization problem is presented, and the sum rate gain of edge caching is also revealed, as compared to the benchmark scheme without content caching. © 2023 by the authors.

In this paper, we investigate the robust resource allocation and relay selection in wireless-powered full-duplex (FD) multi-relay networks, where a self-energy recycling relay node is selected from N candidates to assist the transmission from the source node to the destination node. With imperfect channel state information (CSI), an optimization problem is formulated to minimize the outage probability, and the optimal values of time and power allocation coefficients are obtained. Moreover, on the basis of the optimal time and power allocation coefficients, a two-stage relay selection scheme is proposed. In addition, the analytical expressions of outage probability and diversity order are derived to characterize the performance of the proposed scheme. Finally, simulation results are provided to verify the theoretical analysis and demonstrate the effectiveness of the proposed scheme in different scenarios. © 2023 IEEE.

随着全球数字化经济的飞速发展,各行各业对高质量的通信需求也在不断地增加,无论是多样化的移动业务还是个性化的智能设备均在逐年地呈指数型增长,因此,来自大数据流量和设备接入量的挑战也在不断激励着无线网络的发展和通信技术的革新。但是,面对5G和未来6G的海量设备连接需求和日益稀缺的频谱资源,传统的正交多址接入（Orthogonal Multiple Access,OMA）技术已不能满足通信需求,在5G的候选接入技术中,非正交多址接入（Non-Orthogonal Multiple Access,NOMA）技术被认为是潜力巨大的新一代移动通信接入技术,与OMA技术相比,NOMA技术最大的特点在于多个用户可以复用在同一时频资源块上,这不仅可以增加用户接入的数量,也可以提高频谱资源利用率。因此,NOMA技术可以更好的应对未来海量设备连接需求和高质量通信需求的挑战。随着移动应用的多元化发展,越来越多的移动应用对时延和计算能力有着很高的要求,比如虚拟现实、增强现实、无人驾驶等。但是,目前移动设备依然受限于计算硬件和电池容量技术的发展,运行此类应用的移动设备在性能方面还是会受到极大的限制,而移动边缘计算（Mobile Edge Computing,MEC）技术可以有效地解决上述难题。MEC将云服务器设立在蜂窝小区的基站处,移动设备可以直接将计算任务需求交给MEC服务器来处理,而不需要再将计算任务发送到远端的服务器中心处理,这不仅有效解决了时延问题,还可以减轻移动设备的计算任务复杂度。特别地,目前的研究现状更多关注于MEC下行场景中传输的时延与能耗问题而忽略了对无线传输方式的优化,基于上述分析,本文研究了MEC场景下基于NOMA的用户配对和功率分配技术方案。在NOMA系统中,用户配对和功率分配问题均是影响系统性能的关键因素,由于用户配对和功率分配是相互耦合的联合优化问题,本文将该问题拆分为两个子问题分别进行求解并最终找到联合解决方案。针对用户配对问题,以最大化蜂窝小区内所有用户的数据和速率为目的,本文提出了一种在MEC场景下基于深度强化学习（Deep Reinforcement Learning,DRL）算法的下行NOMA系统用户配对方案。首先,将用户配对问题转化为DRL模型;其次,根据所有用户的信道状态信息构建出用户配对矩阵;最后,通过深度Q网络（Deep Q-Network,DQN）算法找到最合适的用户配对方案。仿真结果表明,提出的方案可以有效提高所有用户的数据和速率。针对功率分配问题,本文在已提出的用户配对方案基础上,研究了一种基于DRL算法的下行两用户NOMA系统功率分配方案。首先,将功率分配问题转化为DRL模型,并根据用户的信干噪比和数据速率构建出特征矩阵;其次,利用异步优势演员-评论家（Asynchronous Advantage Actor Critic,A3C）算法求解出最优功率分配方案;最后,提出了DQN-A3C用户配对和功率分配联合优化方案。仿真结果表明,相比已有的优化方案,所提方案可以有效提高NOMA系统的性能。最后,本文对全文的研究内容做了详细的总结,并对还未研究的后续内容做了展望。

This paper investigates a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted downlink non-orthogonal multiple access (NOMA) system, consisting of a base station (BS), a STAR-RIS, a transmission user and a reflection user, where STAR-RIS uses mode switching protocols. Under the proposed transmission model, the base station serves the two users via the STAR-RIS and a direct link exists from the base station to the reflection user. By using the Beaulieu series, the statistical property of the Nakagami-m fading cascaded channel is characterized. Based on this, closed-from expressions of outage probability and diversity order for the reflection user and transmission user are obtained, respectively. In addition, to ensure the maximum gain in system throughput, a component allocation scheme is proposed, and the optimal allocation interval between the transmission and reflection components is obtained. Numerical results verify the theoretical analysis and demonstrate the superiority of the proposed scheme in terms of outage probability and system throughput compared to existing schemes. © 2023 IEEE.

This paper proposes transmission mode switching for the simultaneous wireless information and power transfer enabled non-orthogonal multiple access (NOMA) systems with imperfect successive interference cancellation (SIC) and random user locations. The system is composed of a base station (BS) and multiple near and far users. The location of the near and far users follows a homogeneous Poisson point process. The near users adopt incremental relay protocol and the simultaneous wireless information and power transfer enabled system can switch between cooperative and non-cooperative transmission mode based on 1-bit feedback of the far users. Due to the decoding errors of practical SIC process, imperfect SIC is considered at the near user. Under the proposed mode switching scheme, the closed-form expressions for the outage probability and system throughput are obtained considering random user locations. Numerical results demonstrate the correctness of the theoretical analysis and show that the proposed scheme outperforms the traditional cooperative and non-cooperative NOMA schemes in terms of outage and throughput performance. © 2023 IEEE.

This paper proposes a novel video caching and transmission scheme in cache and full-duplex (FD) relay assisted wireless networks, where parts of the contents to be requested are stored in the local caches of relay nodes. To provide diversified viewing qualities to multimedia users, we combine scalable video coding (SVC) with edge caching in this work. With the aid of SVC, each video is encoded into multiple layers, including a base layer (BL) and multiple enhancement layers (ELs). The super layers, each of them containing one BL and several successive ELs, are cached and transmitted to users to bring diversified viewing experiences. Under the proposed caching and transmission policies, we conduct comprehensive performance analysis. By using the tool of stochastic geometry, we obtain the analytical expressions for successful transmission probabilities (STPs) in cache hit and miss cases. After obtaining STPs, the cache-aided data rate (CADR) is derived. Simulation results validate the derived STPs and demonstrate that the proposed SVC-based random caching scheme with FD relays has great potential to increase the data transmission rate. © 2023 IEEE.