Conventional Multiple Access Schemes Research Articles

Analysis and Compensation of Tx and Rx IQ Imbalances in Uplink IM-OFDMA Systems

Index modulation-based orthogonal division multiple access (IM-OFDMA) has recently been proposed as a potential technique for future wireless communication systems due to its superior spectral efficiency and error performance over conventional multiple access schemes. However, its performance is still under investigation by researchers in light of a variety of scenarios and assumptions. Following this direction, in this paper, the individual and joint effects of the transmitter (Tx) and receiver (Rx) in-phase and quadrature imbalances (IQI) on the bit-error-rate (BER) performance of the uplink IM-OFDMA scheme are investigated by considering whether each user's RF front-end is identical or not. Moreover, to reduce the detrimental effect of IQI, a preamble-based estimation and compensation method is proposed for IM-OFDMA. Closed-form expressions for the average BER of IM-OFDMA are obtained by considering the physical effect of the Tx and Rx IQI. Also, using Monte Carlo simulations, the derived expressions are verified under different system configurations. Analytical and simulation results reveal that Tx and Rx IQI cause an error floor in the BER performance of IM-OFDMA schemes. However, the proposed estimation and compensation method not only reduces the impact of IQI but also leads to a better BER performance compared to the case of no IQI, which mainly refers to the frequency diversity caused by the IQI.

Joint Power Allocation and Rate Control for Rate Splitting Multiple Access Networks With Covert Communications

Rate Splitting Multiple Access (RSMA) has recently emerged as a promising technique to enhance the transmission rate for multiple access networks. Unlike conventional multiple access schemes, RSMA requires splitting and transmitting messages at different rates. The joint optimization of the power allocation and rate control at the transmitter is challenging given the uncertainty and dynamics of the environment. Furthermore, securing transmissions in RSMA networks is a crucial problem because the messages transmitted can be easily exposed to adversaries. This work first proposes a stochastic optimization framework that allows the transmitter to adaptively adjust its power and transmission rates allocated to users, and thereby maximizing the sum-rate and fairness of the system under the presence of an adversary. We then develop a highly effective learning algorithm that can help the transmitter to find the optimal policy without requiring complete information about the environment in advance. Extensive simulations show that our proposed scheme can achieve non-saturated transmission rates at high SNR values with infinite blocklength. More significantly, our proposed scheme can achieve positive covert transmission rates in the finite blocklength regime, compared with zero-valued covert rates of a conventional multiple access scheme.

Max-Min Fairness Precoder Design for Rate-Splitting Multiple Access: Impact of Imperfect Channel Knowledge

Rate-splitting multiple access (RSMA) is a key enabling technology for beyond 5G networks due to its robustness against imperfect channel knowledge. In this paper, we consider both the imperfect channel state information (CSI) at the receiver (CSIR) and imperfect CSI at the transmitter (CSIT) and propose a max-min fairness optimization framework relying on RSMA to guarantee each user's minimum level of quality of service (QoS). First, we formulate the max-min fairness optimization problem that has non-convexity. To transform the non-convex problem into a convex problem, we apply the semi-definite relaxation (SDR) and convex-concave procedure (CCP) to the proposed algorithm. From simulation results, we confirm that RSMA is still a robust scheme over conventional multiple access schemes for both imperfect CSIT and CSIR.

Joint beamforming design and resource allocation for double-IRS-assisted RSMA SWIPT systems

Being able to achieve high passive beamforming gains cost-effectively, intelligent reflecting surface (IRS) is envisioned as a promising technique in assisting simultaneous wireless information and power transfer (SWIPT). Different from existing works that consider deploying a single IRS or applying conventional multiple access schemes like space division multiple access (SDMA) and non-orthogonal multiple access (NOMA), we consider a double-IRS-assisted multi-user SWIPT system applying the rate-splitting multiple access (RSMA) scheme in this paper. To maximize the minimum achievable rate of all users and guarantee their minimum energy harvesting amount, we jointly optimize the transmit beamforming for both the common and private messages at the access point of the system, the reflecting beamforming of the two IRSs, as well as the power splitting coefficients and the individual common rates of all users. Although the considered optimization problem is difficult to solve due to its coupling variables and non-convex structure, we propose an efficient alternative optimization-based algorithm to obtain a high-quality solution to it. Simulation results validate the effectiveness of the proposed algorithm and demonstrate the superior performance of deploying two IRSs and applying RSMA, as compared to the baseline schemes that deploy a single IRS or use SDMA/NOMA.

Massive Access of Static and Mobile Users via Reconfigurable Intelligent Surfaces: Protocol Design and Performance Analysis

The envisioned wireless networks of the future entail the provisioning of massive numbers of connections, heterogeneous data traffic, ultra-high spectral efficiency, and low latency services. This vision is spurring research activities focused on defining a next generation multiple access (NGMA) protocol that can accommodate massive numbers of users in different resource blocks, thereby, achieving higher spectral efficiency and increased connectivity compared to conventional multiple access schemes. In this article, we present a multiple access scheme for NGMA in wireless communication systems assisted by multiple reconfigurable intelligent surfaces (RISs). In this regard, considering the practical scenario of static users operating together with mobile ones, we first study the interplay of the design of NGMA schemes and RIS phase configuration in terms of efficiency and complexity. Based on this, we then propose a multiple access framework for RIS-assisted communication systems, and we also design a medium access control (MAC) protocol incorporating RISs. In addition, we give a detailed performance analysis of the designed RIS-assisted MAC protocol. Our extensive simulation results demonstrate that the proposed MAC design outperforms the benchmarks in terms of system throughput and access fairness, and also reveal a trade-off relationship between the system throughput and fairness.

Hierarchical Multiple Access (HiMA) for Fog-RAN: Protocol Design and Resource Allocation

We introduce a set of multiple access protocols, called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hierarchical multiple access (HiMA)</i> , which are based on non-orthogonal multiple access (NOMA) and time-division multiple access (TDMA), optimized for the hierarchical network scenario. The proposed protocols can be efficiently utilized in various network configurations with an hierarchical form, such as relay networks, cloud-radio access networks (C-RANs), and fog-radio access networks (F-RANs). In particular, C-RANs and, more recently, F-RANs are regarded as promising paradigms to fully utilize the edge of the networks. Therefore, the implementation of novel multiple access protocols to properly exploit these configurations is critical for the fifth generation and beyond of wireless access. Furthermore, the resource allocation problem is formulated for each protocol with respect to the timeslot duration and power. As a result two fairness metrics are optimized, namely max-min rate fairness and proportional fairness. Finally, numerical results reveal the effectiveness of the joint design in the hierarchical network and an interesting trade-off is identified between fairness and achievable rate. Interestingly, despite NOMA being a very promising alternative to conventional multiple access schemes, the protocol that is solely based on NOMA does not always outperform the rest.

Efficient data confidentiality scheme for 5G wireless NOMA communications

Non-Orthogonal Multiple Access (NOMA) has recently emerged as a promising multiple access technique for current and future mobile communication networks. Unlike conventional multiple access schemes, NOMA exploits the power domain to service a large number of users at different power levels, using the same time–frequency resources. However, this technology suffers from major security threats since users are able to decode the messages of other paired users utilizing the same resources. In this paper, we propose an efficient and lightweight cipher scheme for NOMA systems at the physical layer. Unlike previous schemes in the literature, the proposed solution combines cryptography and Physical Layer Security (PLS) to provide robust security at minimum cost by adopting a structure that requires a single round with a single operation. Moreover, it is generic in the sense that any simple cipher operation such as permutation, substitution, phase shuffling, or pseudo-random masking, can be utilized in order to secure the underlying NOMA frame symbols. Also, we propose a dynamic key derivation scheme that benefits from the dynamic properties of wireless channels. Simulation results and cryptanalysis show that the proposed solution achieves the desired security level and strikes a good balance between performance and security level.

Impact of fixed power allocation in wireless energy harvesting NOMA networks

SummaryThe time switching‐based relaying (TSR) scheme is considered in energy harvesting protocol to implement with its advantage to nonorthogonal multiple access (NOMA) system. In particular, decode‐and‐forward (DF) mode is proposed to employ in relay to forward signal to serve two far NOMA users. There are two main metrics including outage probability and ergodic rate, which are derived in exact expressions with respect to varying performance under impacts of energy harvesting fractions. To evaluate system performance, outage event and related capacity are illustrated, and we tailor performance gap among two NOMA users and such gap can be controlled by selecting of appropriate power allocation factors assigned for each user to obtain optimal performance. By examining node arrangement, target rates and varying transmit signal to noise ratio (SNR), it can be further achieved performance in several situations of such NOMA. As important result, the considered NOMA system outperforms than the conventional multiple access scheme, and this expected result is confirmed in numerical result and theoretical results. We also explore impacts of transmit power at source, noise power, the other key parameters of energy harvesting scheme to exhibit outage, and ergodic performance. Simulation results are presented to corroborate the proposed methodology.

Time Shared Half/Full-Duplex Cooperative NOMA With Clustered Cell Edge Users

This letter proposes time sharing (TS)-based half/full-duplex (HD/FD) cooperative non-orthogonal multiple access (CRNOMA) schemes, namely, TS-HD-CRNOMA and TS-FD-CRNOMA to efficiently utilize the spectrum of unpaired users. A scenario with more cell edge users (CEUs) than cell center users is considered. Both schemes pair a CCU with multiple clustered CEUs in a TS fashion via a relay, thereby efficiently utilizing the frequency bands of multiple CEUs, which may remain underused in conventional CRNOMA. Ergodic sum capacity of the schemes is analytically derived, validated through simulations, and compared with conventional multiple access schemes to show the achieved capacity gains.

Non-orthogonal multiple access for a full-duplex cooperative network with virtually paired users

Non-orthogonal multiple access (NOMA) is recognized as a promising radio access technology for next generation wireless networks. In this paper, a novel full-duplex (FD) cooperative relaying scheme for a NOMA based system (termed FD-NOMA-VP) is proposed, where K similar gain near users and relay-aided N similar gain far users are distributed into different NOMA cluster. In each cluster, one near user is virtually paired (VP) with multiple far users over non-overlapping frequency bands. In FD-NOMA-VP, the near user directly communicates with the base station, whereas far users communicate via a dedicated FD relay, in each cluster. The performance of FD-NOMA-VP is studied comprehensively in terms of the ergodic sum capacity, outage probability, and outage sum capacity along with analytical derivations under both perfect and imperfect interference cancellation (IC) scenarios. The results demonstrate that FD-NOMA-VP shows significant performance gains compared to the conventional multiple access scheme under perfect IC, while the residual interference has a substantial impact on the performance gain under imperfect IC. In addition, a strong agreement between analytical and simulation results affirms the correctness of our analysis.

Multiple Access for Visible Light Communications: Research Challenges and Future Trends

The ever-increasing demand of mobile Internet and multimedia services poses unique and significant challenges for current and future generation wireless networks. These challenges are mainly related to the support of massive ubiquitous connectivity, low latency, and highly efficient utilization of spectrum resources. Therefore, it is of a paramount importance to address them in the design and deployment of future wireless networks. To this end, this paper provides a comprehensive overview of multiple access schemes in the context of visible light communications (VLC). Specifically, we initially provide a thorough overview of frequency-domain multiple access techniques for single- and multi-carrier systems, which is then followed by an in-depth discussion on the technical considerations of optical code-division multiple access techniques and their adoption in indoor VLC applications. Furthermore, we address space-division multiple access and, finally, we revisit and analyze a new promising technology, namely, non-orthogonal multiple access (NOMA). It is shown that NOMA exhibits significant benefits in VLC systems that can outperform conventional multiple access schemes, rendering it a particularly effective solution. Furthermore, it is demonstrated that it can coexist with the above optical multiple access schemes, which can maximize the performance and efficiency of future VLC systems. However, it is also shown that the potential of NOMA in VLC systems requires efficient addressing of a series of related challenges and constraints, such as fast and effective successive interference cancellation techniques, compensation and mitigation of LED non-linearity, and imperfect and/or outdated channel state information.

Code division multiple-access techniques for use in optical-fibre local-area networks

Optical fibre offers a virtually limitless communications bandwidth when used as the physical medium for a local-area network (LAN). However, conventional multiple-access schemes are limited in their ability to exploit this potential. Direct-sequence spread-spectrum systems are a class of code division multiple-access (CDMA) systems that encode data using high-rate sequences to spread data over a much wider bandwidth than normally required for transmission. The paper first considers the problems to be overcome in the design of future high-speed optical networks and then suggests the application of CDMA techniques as a possible solution. There follows an introduction to basic CDMA principles which is then extended to cover the particular coding, modulation and processing considerations to be taken into account when applying CDMA to an optical medium. These considerations are then used to develop a classification scheme which forms the basis for a review of current approaches in the application of CDMA techniques to optical-fibre local-area networks.