Amplify-and-Forward Relay Transmission in Uplink Non-Orthogonal Multiple Access Networks

We develop an analytical framework to derive the meta distribution and moments of the conditional success probability (CSP), which is defined as success probability for a given realization of the transmitters, in large-scale co-channel uplink and downlink non-orthogonal multiple access (NOMA) networks with one NOMA cluster per cell. The moments of CSP translate to various network performance metrics such as the standard success or signal-to-interference ratio (SIR) coverage probability (which is the 1-st moment), the mean local delay (which is the −1st moment in a static network setting), and the meta distribution (which is the complementary cumulative distribution function of the success or SIR coverage probability and can be approximated by using the 1st and 2nd moments). For the uplink NOMA network, to make the framework tractable, we propose two point process models for the spatial locations of the inter-cell interferers by utilizing the base station (BS)/user pair correlation function. We validate the proposed models by comparing the second moment measure of each model with that of the actual point process for the inter-cluster (or inter-cell) interferers obtained via simulations. For downlink NOMA, we derive closed-form solutions for the moments of the CSP, success (or coverage) probability, mean local delay, and meta distribution for the users. As an application of the developed analytical framework, we use the closed-form expressions to optimize the power allocations for downlink NOMA users in order to maximize the success probability of a given NOMA user with and without latency constraints. Closed-form optimal solutions for the transmit powers are obtained for two-user NOMA scenario. We note that maximizing the success probability with latency constraints can significantly impact the optimal power solutions for low SIR thresholds and favor orthogonal multiple access.

Wireless power transmission in the next-generation wireless networks is the subject that attracts a lot of attention from academia and industry. In this work, we study and analyze the performance of relay selection on uplink non-orthogonal multiple access (NOMA) networks with wireless power transmission. Specifically, the considered system consists of one base station, multiple power-constrained relays and a pair of NOMA users. The best relay (with highest energy harvested from the base station) is chosen to cooperate with two users which use NOMA scheme to send messages to the base station. To analyze the performance, based on the statistical characteristics of signal-to-noise ratio (SNR) and signal-to-interference-plus-noise ratio (SINR), using the Gaussian-Chebyshev quadrature method, the closed-form expressions of outage probability and throughput for two users are derived. In order to understand more details about the behavior of this considered system, the numerical results on outage probability and throughput of a given system are provided following the system key parameters, such as the transmit power, the number of relays, time switching ratio and energy conversion efficiency. In the end, the theoretical result is also verified by using the Monte-Carlo simulation. The simulation results demonstrate that the performance of the system is improved by increasing the number of relays.

In this paper, a novel approach is introduced to study the achievable delay-guaranteed secrecy rate, by introducing the concept of the effective secrecy rate (ESR). This study focuses on the downlink of a non-orthogonal multiple access (NOMA) network with one base station, multiple single-antenna NOMA users and an eavesdropper. Two possible eavesdropping scenarios are considered: 1) an internal, unknown, eavesdropper in a purely antagonistic network; and 2) an external eavesdropper in a network with trustworthy peers. For a purely antagonistic network with an internal eavesdropper, the only receiver with a guaranteed positive ESR is the one with the highest channel gain. A closed-form expression is obtained for the ESR at high signal-to-noise ratio (SNR) values, showing that the strongest user’s ESR in the high SNR regime approaches a constant value irrespective of the power coefficients. Furthermore, it is shown the strongest user can achieve higher ESR if it has a distinctive advantage in terms of channel gain with respect to the second strongest user. For a trustworthy NOMA network with an external eavesdropper, a lower bound and an upper bound on the ESR are proposed and investigated for an arbitrary legitimate user. For the lower bound, a closed-form expression is derived in the high SNR regime. For the upper bound, the analysis shows that if the external eavesdropper cannot attain any channel state information (CSI), the legitimate NOMA user at high SNRs can always achieve positive ESR, and the value of it depends on the power coefficients. Simulation results numerically validate the accuracy of the derived closed-form expressions and verify the analytical results given in the theorems and lemmas.

This paper investigates the secure transmission in hybrid automatic repeat request (HARQ)-assisted non-orthogonal multiple access (NOMA) networks, where a security-required (SR) user is paired with an opportunity-served (OS) user to perform NOMA. Different from the previous works where NOMA and HARQ are separately considered for secure transmissions, in this paper, both NOMA and HARQ are utilized to enhance the security of the SR user. We derive the closed-form expressions for connection outage probability (COP), secrecy outage probability (SOP), reliable and secure transmission probability (RSP) and effective secrecy throughput (EST) of the SR user in both maximum ratio combining (MRC) and selection combining (SC) schemes. The security-reliability trade-off (SRT) results of the SR user in both MRC and SC schemes are also provided. Analysis and simulation results show that HARQ improves secrecy performance in MRC and SC schemes in terms of RSP, especially in low transmit power region. Besides, since every retransmission is utilized in the MRC scheme, it strictly outperforms the SC scheme in terms of the SRT metric. However, the SC scheme has the potential to achieve higher EST than the MRC scheme by optimizing the transmit power or the power allocation factor. Significantly, the quality of service (QoS) of the OS user can be guaranteed by pairing an SR user who is far away from the transmitter and the secrecy performance of the SR user can be enhanced by pairing an OS user.

In this work, non-orthogonal multiple access (NOMA) in coordinated direct and relay transmission (CDRT) is introduced, where a base station (BS) directly communicates with user equipment 1 (UE1) while communicating with user equipment 2 (UE2) only through a relay. The main challenge of nonorthogonal CDRT can be solved by using the inherent property of NOMA that allows a receiver to obtain side information such as other TIE's data for interference cancellation. Analytical expressions for outage probability and ergodic sum capacity are provided. It is shown that the proposed NOMA in CDRT provides remarkable performance gain compared with NOMA in non-coordinated direct and relay transmission (nCDRT); the sum capacity scaling of the proposed scheme is log ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> as signal-to-noise-ratio (SNR) ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> increases, but 1/2log ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> for NOMA in nCDRT. Exact and closed-form expressions for outage probability of each stream for UE1 and UE2 are respectively derived, and it is shown that the achievable diversity orders for each stream are same as one.

In this paper, we focus on non-orthogonal multiple access (NOMA) network with power-domain user multiplexing at the transmitter side and successive interference cancellation (SIC) on the receiver side. The main problem presented in NOMA network is the design of the precoder. It has to be applied to users with weak channel conditions to effectively mitigate interference between cells. We propose a coordinated operation access scheme between OMA and NOMA that removes the precoder and increases the performance of the system. The channel gain is analyzed by order statistics obtaining its probability density function. System level simulation results are provided. We show, under multiple configurations, that the transmission rate achieved by our scheme is higher by more than 42% compared to NOMA on the edge of the cell.

In this paper, a unified framework of non-orthogonal multiple access (NOMA) networks is proposed, which can be applied to code-domain NOMA (CD-NOMA) and power-domain NOMA (PD-NOMA). Since the detection of NOMA users mainly depend on efficient successive interference cancellation (SIC) schemes, both imperfect SIC (ipSIC) and perfect SIC (pSIC) are taken into considered. To characterize the performance of this unified framework, the exact and asymptotic expressions of outage probabilities as well as delay-limited throughput for CD/PD-NOMA with ipSIC/pSIC are derived. Based on the asymptotic analysis, the diversity orders of CD/PD-NOMA are provided. It is confirmed that due to the impact of residual interference (RI), the outage probability of the n-th user with ipSIC for CD/PD-NOMA converges to an error floor in the high signal-to-noise ratio (SNR) region. Numerical simulations demonstrate that the outage behavior of CD-NOMA is superior to that of PD-NOMA.

This paper investigates the reliability and security performance of the downlink non-orthogonal multiple access (NOMA) networks over Nakagami-m fading channels, where the base station (BS) aims to communicate with multi-antenna NOMA users in the presence of a multi-antenna eavesdropper. To be more practical, a detrimental factor at both transmitter and receiver is considered, namely in-phase and quadrature-phase imbalance (IQI). To further improve the reliability and security of the considered networks, the selection combining (SC) algorithm at the receiver is taken into account. More specifically, the exact analytical expressions for the outage probability (OP) and the intercept probability (IP) are derived in closed-form. To obtain a better understanding of the influence for the IQI parameters on the system performance, the asymptotic behaviors for the outage probabilities (OPs) in the high signal-to-noise ratio (SNR) region are analyzed. Based on the asymptotic results, the diversity order of the considered system are obtained and discussed. The numerical results are presented to verify the validity of the theoretical analysis.

In this article, a multiuser cooperative nonorthogonal multiple access (NOMA) network is considered. In order to improve their battery lifetime, the near-user (NU) Internet of Things (IoT) nodes relay information to the far-user (FU) IoT nodes using only the energy harvested from the ambience. The harvested energy at all the NU IoT nodes is stored in energy buffers. To improve the throughput performance, the best NU and best FU (BNBF) user selection scheme is employed. Moreover, unlike conventional orthogonal multiple access (OMA) networks, an adaptive NOMA network is considered in this article that switches between direct NOMA, NOMA with relaying, and OMA modes to maximize throughput. To improve accuracy and reduce the computational complexity, the energy buffer states at the NU IoT nodes are modeled using a continuous state-space Markov chain (CSMC) instead of discrete state-space Markov chain (DSMC). We derive the limiting distributions of the stored energy in energy buffers with either the best effort policy (BEP) or the on–off policy (OOP) applied for buffer energy management. Expressions for throughput are derived for both harvest-store-use (HSU) and harvest-use (HU) architectures. The Monte Carlo simulations are used to validate the derived analytical expressions.

This article proposes a novel framework of resource allocation in multi-cell intelligent reflecting surface (IRS) aided non-orthogonal multiple access (NOMA) networks, where an IRS is deployed to enhance the wireless service. The problem of joint user association, subchannel assignment, power allocation, phase shifts design, and decoding order determination is formulated for maximizing the achievable sum rate. The challenging mixed-integer non-linear problem is decomposed into an optimization subproblem (P1) with continuous variables and a matching subproblem (P2) with integer variables. In an effort to tackle the non-convex optimization problem (P1), iterative algorithms are proposed for allocating transmission power, designing reflection matrix, and determining decoding order by invoking relaxation methods such as convex upper bound substitution, successive convex approximation, and semidefinite relaxation. In terms of the combinational problem (P2), swap matching-based algorithms are developed for achieving a two-sided exchange-stable state among users, BSs and subchannels. Numerical results demonstrate that: i) the sum rate of multi-cell NOMA networks is capable of being increased by 35% with the aid of the IRS; ii) the proposed algorithms for multi-cell IRS-aided NOMA networks can enjoy 22% higher energy efficiency than conventional NOMA counterparts; iii) the trade-off between spectrum efficiency and coverage area can be tuned by judiciously selecting the location of the IRS.

In this letter, we investigate the resource allocation problem, including user pairing and power allocation, for downlink non-orthogonal multiple access (NOMA) networks. Different from most of the existing studies focusing on maximizing throughput, we aim to maximize users’ quality-of-experience (QoE) considering demand diversity. We apply mean opinion score (MOS) to evaluate user’s QoE level, and the optimization target is to maximize the sum MOSs of all users, which is a complex non-convex problem. To tackle this problem, we firstly propose a linear searching algorithm to get the optimal sum MOSs for a specific 2-user NOMA system. Then, we derive the corresponding power allocation strategy. Furthermore, we extend it to a general NOMA system with $2{K}$ users, and design a fast and low complexity scheme. Results show that our proposed scheme can significantly improve the users’ sum MOSs in the NOMA networks, compared with other existing schemes.

In this paper, a downlink single-cell non-orthogonal multiple access (NOMA) network with uniformly deployed users is considered and an analytical framework to evaluate its performance is developed. Particularly, the performance of NOMA is studied by assuming two types of partial channel state information (CSI). For the first one, which is based on imperfect CSI , we present a simple closed-form approximation for the outage probability and the average sum rate, as well as their high signal-to-noise ratio (SNR) expressions. For the second type of CSI, which is based on second order statistics (SOS) , we derive a closed-form expression for the outage probability and an approximate expression for the average sum rate for the special case two users. For the addressed scenario with the two types of partial CSI, the results demonstrate that NOMA can achieve superior performance compared to the traditional orthogonal multiple access (OMA). Moreover, SOS-based NOMA always achieves better performance than that with imperfect CSI, while it can achieve similar performance to the NOMA with perfect CSI at the low SNR region. The provided numerical results confirm that the derived expressions for the outage probability and the average sum rate match well with the Monte Carlo simulations.

Secure communication of intelligent reflecting surface-aided NOMA in massive MIMO networks

In this paper, we anaylze the secrecy performance for a wireless-powered Internet of Things (IoT) relaying-enabled non-orthogonal multiple access (NOMA) network. In the considered system, a source node transmits signal to a user through a power-constrained IoT relay, which puts forward its own data together with the decoded data under the NOMA technique to two users in the presence of an eavesdropper. In addition, the scenario that the relay can receive both the energy and information from the source through a time-switching mode is considered. Assuming all the channels are subject to Nakagami- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$m$</tex> fading, the closed-form expressions for the secrecy outage probability of the two users are obtained, which are also verified by Monte-Carlo simulations.

In this letter, we carry out the performance analysis of the simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) in the hybrid automatic repeat request (HARQ) assisted cognitive non-orthogonal multiple access (NOMA) networks, where a security-required user (SRU) with secure transmission requirements is paired with a quality of service (QoS)-sensitive user (QSU) with low delay requirements to perform NOMA. To characterize the performance of secure transmission, we derive the analytical expressions for connection outage probability (COP), average number of transmission (ANT) and secrecy outage probability (SOP) for the SRU in randomized retransmission NOMA (RR-NOMA) scheme, while a cognitive power allocation scheme is developed to meet QSU’s QoS requirement. To obtain deeper insights, the asymptotic expressions of COP and SOP are also derived. Furthermore, some significant results are drawn from simulations: 1) There exist a balance between COP and ANT, COP and SOP, which are denoted by the security-reliability balance (SRB) and reliability-delay balance (RDB), respectively. 2) HARQ is benefit to improve the RDB and SRB performance of SRU. 3) The increase in the number of STAR-RIS elements <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> improves the system reliability performance, while the secrecy performance deteriorates.