Khairi Ashour Hamdi

This paper considers an intelligent reflecting surface (IRS)-assisted multiple-input single-output multi-user non-orthogonal multiple access (NOMA) network operating in the terahertz band subject to co-channel interference (CCI). The IRS is examined under various configurations: (i) IRS partitioning with discrete phase shifts, (ii) IRS with random phase shifts, and (iii) IRS with discrete phase shifts for one user and random phase shifts for the remaining users. It aims to assess the impacts of various factors, including beam misalignment, imperfect successive interference cancellation (SIC), CCI, phase quantization errors, and random phase shifts. Thus, it derives accurate expressions for the ergodic rate, outage probability and diversity order, which are verified via numerical and simulation results. The results show that the impacts of beam misalignment and imperfect SIC are more dominant compared to those resulting from random phase shifts, phase quantization errors and CCI. Despite achieving lower performance compared to the discrete phase shifting scheme, the random phase shifting scheme can be a promising candidate for multi-user NOMA, as it avoids the costly channel estimation and computational complexity that are proportional to the numbers of IRS elements and users. The results also reveal that partitioning the IRS under discrete phase shifts outperforms a combination of discrete and random phase shifts. The system performance in the high-signal-to-noise ratio regime is also analyzed, which shows that the performance reaches a limit due to inter-user interference and CCI.

This letter presents a new expression for evaluating the ergodic capacity in general fading channels. Its broad applicability is validated by applying it to various communication systems, including: (i) systems with equal gain combining (EGC) diversity, (ii) multi-user rate-splitting multiple access (RSMA) systems with EGC, and (iii) intelligent reflecting surface (IRS)-assisted systems employing EGC with arbitrary numbers of IRS elements and diversity branches. The IRS is considered under different transmission schemes such as intelligent transmission, transmission with imperfect phase shifts, and blind transmission. Subsequently, the preciseness of the expression in various fading channels is verified through numerical and simulation results.

This letter considers a multi-user non-orthogonal multiple access (NOMA) system in the presence of arbitrary co-channel interference (CCI) signals. Taking into account the precise statistics of the self-interference signals of NOMA users, it derives a new unified expression for the bit error rate (BER) of an arbitrary number of users. The users may employ various modulation schemes and orders, including M-ary pulse amplitude modulation with arbitrary orders, M-ary quadrature amplitude modulation with arbitrary orders, or a combination of both with arbitrary orders. The newly derived expression is then applied to a multi-user NOMA in two different interference scenarios: (i) fixed located CCI, and (ii) Poisson point process-based randomly located CCI. The preciseness of the BER expression for different number of users and modulation schemes/orders is verified via numerical and simulation results.

This paper considers intelligent reflecting surface (IRS)-assisted communication systems in the presence of arbitrary interfering signals. The IRS reflects both the desired and interference signals through the same fading channels towards the user equipment. In this scenario, the signal-to-interference-plus-noise ratio might become a ratio of correlated random variables. Therefore, this paper aims to identify practical scenarios in which it is acceptable to disregard or necessary to consider the correlation between the desired and interference signals sharing the same fading channels. Furthermore, in the presence of correlated desired and interference signals, the analytical expressions presented in the existing literature are inadequate for evaluating the system performance. To address this limitation, new accurate expressions for the ergodic capacity, outage probability, and bit error rate are derived, which are applicable to both correlated and uncorrelated scenarios. The accuracy of the newly derived expressions is then validated by Monte-Carlo simulations. In contrast to the existing literature, it is revealed that when interferers have line-of-sight (LOS) components, the desired and interference signals are always correlated. It is also revealed that, in the presence of LOS interference, for a very large number of reflecting elements, the system performance reaches a limit, which cannot be improved by increasing the number of reflecting elements.

This paper considers simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted terahertz communications, in which a multi-antenna full-duplex base station serves two half-duplex users (U1 and U2) in the downlink and uplink, respectively. It is assumed that U1 and U2 are located in the reflection and transmission spaces. The STAR-RIS is considered under energy splitting (ES) and mode switching (MS) protocols to provide simultaneous full-space coverage to both users, but at the expense of inter-user interference (IUI). The paper aims to evaluate the impacts of various practical factors on the system performance, including IUI, quantization errors due to discrete phase shifters of the STAR-RIS, beam misalignment due to highly-directive antennas, and random fog. Thus, it derives accurate expressions for the ergodic capacity, outage probability, and symbol error rate. Numerical and simulation results reveal that the system performance is more severely impacted by beam misalignment and random fog than by other factors. It is also observed that the ES protocol outperforms the MS protocol in the downlink, but the situation is reversed in the uplink due to coupled phase shifts. Moreover, in the high-signal-to-noise ratio regime, the system performance is restricted due to IUI signals.

The dramatic growth of mobile data demand may impose a heavy traffic burden in indoor environments such that conventional radio frequency wireless networks might not be enough to ensure a high quality of service (QoS). Applications such as texting, 4k video streaming, and virtual reality have substantial differences in terms of data rate requirements. Visible light communication (VLC) has emerged recently as a complementary technology in heterogeneous networks, as it supports a high data rate and uses an unlicensed spectrum. In hybrid WiFi-VLC networks, users can be assigned to WiFi or VLC access points (AP) in downlink to maximize the minimum user data rate. In this paper, we propose an intelligent network selection that is capable of reducing network complexity and enhancing users’ QoS. A federated Q-learning (FQL) algorithm is proposed, where each VLC AP performs local Q-learning and updates the global model at the WiFi AP. A new local reward function is proposed to maximize user satisfaction with limited shared information. Furthermore, a deep neural network (DNN) is proposed to reduce the complexity of the FQL. The output of the DNN is adjusted to assign some users directly to WiFi or VLC and create an initial policy for the FQL. To quantify the improvements achieved, the proposed federated Q-learning assisted by knowledge transfer (FQL-KT) is compared against state-of-the-art benchmark approaches, such as centralized Q-learning (CQL), FQL, signal strength strategy (SSS), and exhaustive search. Numerical simulation results show that the proposed FQL-KT significantly outperformed the other approaches in terms of convergence speed, maximizing the minimum user data rate, fairness, and minimizing the outage probability.

This paper considers intelligent reflecting surface (IRS)-assisted wireless communication systems having an arbitrary number of reflecting elements in additive white generalized Gaussian noise and general fading channels. It presents unified expressions for the precise bit error rate (BER) analysis of different modulation schemes, including M-ary pulse amplitude modulation (MPAM), M-ary quadrature amplitude modulation (MQAM), and M-ary phase shift keying modulation (MPSK). By applying the unified expressions, we analyze the BER performance in different IRS transmission schemes: intelligent transmission, where the phases of the fading channels are known at the IRS; transmission with imperfect phase shifts, where the phases are partially known and/or the IRS is able to apply discrete phase shifts; and blind transmission, where the phases are unknown at the IRS. The preciseness of the expressions is then verified by Monte-Carlo simulations. Although an IRS with blind transmission achieves a higher BER, it can be a promising candidate for low-cost and low-data rate communications through an ambient IRS that does not require implementation complexity and channel estimation overhead.

This letter considers a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided non-orthogonal multiple access (NOMA) system in the presence of unbalanced transmission and reflection (T&R) users. It takes into account the correlated T&R phase shifts and proposes various phase shift configuration strategies, namely blind phase shifting (BPS), STAR-RIS partitioning (SPS), secondary user STAR-RIS partitioning (SSPS), and near-field broadcasting (NBS). The ergodic sum rate is then analyzed under the mentioned strategies. Numerical and simulation results demonstrate the NBS strategy’s superiority over other strategies. However, its effectiveness is constrained by the distance and size of the STAR-RIS, making it more suitable for short-range applications. The results also reveal that the SPS strategy outperforms the BPS and SSPS strategies in low-power scenarios and with limited STAR-RIS elements allocated for the secondary user. The SSPS strategy shows substantial improvement in high-power scenarios, making it well-suited for systems such as millimeter-wave and terahertz wireless communications.

This paper considers a reflecting surface (RS)-aided ambient backscatter communication (AmBC) system, where the RS can be either an intelligent RS (IRS) or a dumb RS (DRS). The former uses channel state information to apply phase shifts and form beams towards the reader, while the latter randomly reflects signals without applying phase shifts. The paper aims to evaluate the effects of ambient sources that act as simultaneous energy and interference sources, as well as the potential benefits of RSs on the bit error rate (BER) performance and coverage of AmBC systems. Therefore, it presents accurate BER expressions for on-off keying modulation using a simple non-coherent energy detection scheme under an IRS with ideal/non-ideal phase shifts and a DRS. Numerical and Monte-Carlo simulation results demonstrate that an IRS under both ideal and non-ideal phase shifts outperforms a DRS; however, the latter is cost-efficient and suitable for low-power applications, such as Internet of things communications. The results also show that an RS significantly enhances the limited coverage of conventional AmBC systems. Finally, it is observed that potential blockages limit the BER in the high-signal-to-noise ratio and large number of reflecting elements regimes, which can be improved by increasing the number of antennas.