One of the principal challenges anticipated for the forthcoming sixth-generation (6G) wireless networks is the imper-ative need to design advanced multiple access techniques capable of enabling massive connectivity. In this direction, rate splitting multiple access (RSMA) has been reported as a promising approach. In this work, we investigate the ergodic rate (ER) performance of an uplink RSMA network which consists of two sources. Specifically, analytical closed-form expressions for the sources' ERs and the system ergodic sum rate (ESR) are derived under the case of perfect successive interference cancellation$(\text{pSIC})$and perfect channel state information (pCSI) as well as under the more realistic scenario of imperfect SIC (ipSIC) and imperfect CSI (ipCSI). Furthermore, an asymptotic analysis for the high signal-to-noise ratio regime is presented, which provides useful insights into the sources' behavior in all considered cases. The accuracy of the provided analytical expressions is validated by simulation results, which not only explore how different system parameters affect the extracted expressions, but also reveal the detrimental impact of ipSIC and ipCSI on system performance.

One of the main challenges that the upcoming sixth-generation (6G) wireless networks will encounter is the necessity to design sophisticated multiple access techniques that besides being capable of supporting massive connectivity, they can also fulfill the heterogeneous requirements of 6G services, namely further-enhanced mobile broadband (feMBB), extremely reliable and low-latency communication, and ultra-massive machine-type communication (umMTC). To this end, this work investigates the coexistence of multiple feMBB and umMTC wireless sources in a network. In order to enhance the achievable connectivity, each orthogonal resource block of the network is assigned to one feMBB and multiple umMTC sources. FeMBB sources are assumed to constantly transmit, while umMTC are considered to access the network in a probabilistic manner. If more than one umMTC sources attempt to access the network in a given resource block, no umMTC transmission is permitted, however, when precisely one umMTC source endeavors to access the medium, rate-splitting multiple access is employed to concurrently serve both feMBB and umMTC transmissions. For such a communication scenario, we derive novel closed-form expressions for sources’ outage probabilities (OPs), ergodic rates (ERs), system throughput, and ergodic sum rate under both the ideal case of perfect channel state information (pCSI) and perfect successive interference cancellation (pSIC) and the more realistic scenario of imperfect CSI (ipCSI) and imperfect SIC (ipSIC). Furthermore, a high signal-to-noise ratio analysis is provided revealing deeper insights for sources’ asymptotic behavior under all considered cases. Simulation results corroborate the accuracy of the derived analytical expressions, investigate the impact of different system parameters on sources’ OP and ER performance, and illustrate the detrimental impact of ipCSI and ipSIC on system performance compared to the ideal case of pCSI and pSIC.

In this work, we address a novel framework for next-generation distributed radio access. In contrast with existing studies, where all remote radio heads (RRHs) in a distributed network are directly connected to a central unit (CU), an alternative architecture for advanced flexibility is proposed. In our setup, only one of the RRHs, namely the$primary$RRH, communicates directly with the CU, while the connectivity between the rest RRHs, namely$secondary$RRHs, and the CU is achieved through the primary RRH via free-space optical links. Assuming that users exploit non-orthogonal multiple access (NOMA) for their transmissions, we introduce two successive interference cancellation (SIC) cooperation schemes, depending on the one-directional or bidirectional communication between the RRHs, as well as, a four-step centralized algorithm for efficient user-RRH association and decoding order operations is proposed. The feasibility of the suggested schemes is adequately demonstrated by deriving analytical expressions for users' outage probabilities and providing valuable insights into the high signal-to-noise ratio regime. Furthermore, the performance of the proposed system under various weather conditions is investigated via simulation and analytical results. The comparison with a benchmark scheme, where all RRHs are directly connected to the CU and cooperate with each other via ideal links, is provided and it is revealed that although the performance of the proposed system model is weather dependent, in most of the practical cases it achieves similar performance with the ideal benchmark.

A core vision of the future sixth-generation wireless networks is to serve, in the most spectral and energy efficient way, a massive number of devices and/or Internet-of-Things (IoT) sensors, plenty of which are expected to be low-powered or even battery-free. In this direction, non-orthogonal multiple access (NOMA) and ambient backscatter communications (AmBC) are considered as two key promising technologies. In this work, we present a novel analytical framework for studying the performance of uplink NOMA-based AmBC systems. Specifically, analytical expressions for both NOMA-users’ and IoT backscatter device’s (BD) outage probabilities (OPs) are derived under both perfect and the more realistic case of imperfect successive interference cancellation (SIC). Likewise, system’s performance at the high signal-to-noise ratio (SNR) regime is investigated as well as expressions for the system’s overall average throughput are also derived. A performance comparison between the proposed setup and a conventional orthogonal multiple access (OMA)-based AmBC system is provided as well as it is proved that the proposed system’s average throughput is equal or even greater compared to a conventional uplink NOMA system with two NOMA-users and no BDs.

In this paper, we analyze the performance of holographic surface assisted wireless systems that employ non-orthogonal multiple access (NOMA) approaches. In this direction, we present a comprehensive system model that captures the particularities of holographic surfaces as well as the impact of multipath fading. Building upon the system model, we quantify the outage performance of the downlink scenario, by providing novel closed-form expressions for its outage probability. Simulation results validate the authenticity of the presented analysis, illustrate the performance gain of the proposed NOMA scheme over conventional orthogonal multiple access schemes and reveal the feasibility and efficiency of the proposed approach.