The paucity of orthogonal resource blocks has led to focused research on non-orthogonal multiple access (NOMA) signaling. This paper considers a multiuser downlink hybrid non-orthogonal multiple access (NOMA) framework with direct links to near-users (NUs) as well as far-users (FUs). It is shown that intelligent power allocation, together with mode switching and user selection, is the key to high spectral efficiency (SE) and energy efficiency (EE). To enhance the system throughput, intelligent NU and FU selection, and switching among NOMA Mode (NM), cooperative NOMA mode (CNM), and OMA mode (OM) have been considered. Switching to OM when both NM and CNM fail ensures performance equivalent to OM. The closedform expressions for the outage probability, mode probability, throughput, and EE are derived with mode switching and user selection. It is demonstrated that choosing the optimal target rate and NOMA power allocation coefficient is essential to maximize the EE. Only statistical channel knowledge is used for this purpose. Furthermore, we demonstrate that the proposed framework outperforms known schemes in terms of throughput and EE. In particular, the energy savings with the proposed scheme can be very significant. Monte Carlo simulations validate the derived expressions.

In this letter, we analyze the performance of a downlink non-orthogonal multiple access (NOMA) network that uses co-phasing at an intelligent reflecting surface (IRS) to communicate to a near-user (NU) and a far-user (FU) concurrently. Both users combine the direct and reflected signals. Considering cross-channel interference (CI) and a finite number of phase levels at the IRS, we derive closed-form expressions for the outage probability. Closed form expressions are also presented for throughput, energy efficiency (EE) and optimal power allocation. A low complexity procedure is suggested to determine the optimum partitioning so as to maximize the FU throughput while ensuring a desired throughput at the NU. The proposed IRS-NOMA (IN) system outperforms conventional NOMA (CN) and orthogonal multiple access (OMA) systems in terms of throughput and EE, and permits a broader range of NOMA power allocations. Simulations validate the derived expressions.

In this letter, we consider a downlink multi-user (MU) non-orthogonal multiple access (NOMA) network. We demonstrate that utilizing knowledge of the channel gains to the users to determine the NOMA power allocation coefficients can dramatically improve throughput performance. Considering practical imperfect successive interference cancellation, expressions are derived for the optimum power allocation that ensures the minimum outage probability for the signalling scheme. The level of successive interference cancellation at each user and the decoding order are specified. It is shown that the proposed decoding order and the power allocations result in the maximum throughput. Expressions are derived for the throughput with these power allocations. Channel knowledge is exploited to determine the minimum power required for non-outage, and an expression is derived for the average value of this minimum power requirement. Computer simulations validate the derived expressions.

In this letter, we consider a secondary network (SN) in which an ambient backscatter device (BD) utilizes the secondary transmitter (ST) signal to communicate its own information to the secondary destination (SD). Optimizing performance of such networks is complicated by signal reflections by the BD. It is shown in this work how the secondary transmit power and the reflection coefficient of the BD can both be jointly optimized using a simple restricted one dimensional search to satisfy the quality of service (QoS) constraints of SN as well as the primary network (PN) while maximizing performance of the backscatter link, which is termed the tertiary network (TN). Only statistical channel knowledge is used for this purpose. It is seen that careful optimization can improve spectral efficiency. Simulations validate the derived analytical expressions.

This paper investigates a cooperative non-orthogonal multiple access (Co-NM) based network consisting of a multiantenna source, a full-duplex energy harvesting (EH) near user (NU) internet-of-things (IoT) node and multiple distant user (DU) IoT nodes. The source shares a direct link to the NU, while the NU augments the harvested energy by a limited amount of its battery energy to relay the information to the selected DU. Considering time-switching (TS) or power-splitting (PS) protocol, practical nonlinear EH, successive interference cancellation error, and opportunistic Co-NM/OMA (OM) switching, closed-form expressions are derived for the outage probability and throughput of both DU and NU. We demonstrate that the proposed opportunistic Co-NM/OM switching can ensure a performance similar to OM at the NU without loss in DU throughput. Also, a joint optimal choice of battery energy and PS/TS parameter helps in attaining a  maximum energy efficiency (EE). Moreover, Co-NM/OM switching ensures higher EE compared to Co-NM and OM.

The performance of an underlay coordinated direct and relay transmission (CDRT) based non-orthogonal multiple access (NOMA) network is limited by the random nature of the transmit powers due to the interference temperature limit (ITL) imposed by the primary network. In this paper, we show that combining the signals at both near and far users can significantly enhance throughput. Considering the practical scenario of imperfect successive interference cancellation, the performance of near and far user nodes is analyzed in terms of throughput and outage probability. The traditional case of static ITL is considered first. Then, analysis is presented for the case when the ITL depends on the primary channel state information, and it is shown that the gain in performance can be very large over static ITL. Closed-form expressions for NOMA power pportioning are presented for a special case. Further, it is shown that how optimum parameters can be chosen to maximize the performance. Simulations validate the derived analytical expressions.

High spectral and energy efficiencies are vital for the implementation of smart cities. To this end, this paper investigates the performance of an amplify-and-forward (AF) relay-aided coordinated direct and relay transmission (CDRT) protocol in a downlink non-orthogonal multiple access (NOMA) based Internet-of-things (IoT) network in which source shares the direct as well as the relayed links to the IoT near-user (NU) and the far-user (FU). Different from existing literature, we exploit incremental relaying (IR) along with combining at the NU (and not just at the FU) to achieve a 20% increase in FU throughput while ensuring the desired performance at NU. Considering practical imperfect successive interference cancellation (SIC), we analyze the throughput performance. Both NU and FU accrue large throughput gains, and doubling of the energy efficiency (EE) is achieved over the non-incremental AF CDRT NOMA scheme and its relayed OMA counterpart. We accomplish this by intelligently exploiting feedback bits in the second phase of signalling to avoid unwanted relaying, which saves energy and improves EE, which is very much required from the perspective of IoT-based energy-efficient smart cities. It is seen that the proper choice of the power allocation coefficient and the target information rate is crucial for maximizing the sum throughput and EE of the considered IoT Network. Finally, we validate the correctness of the theoretical analysis and the superiority of the proposed scheme through Monte Carlo simulations.

This paper investigates a cooperative non-orthogonal multiple access network where a near user (NU) harvests energy using time-switching (TS) or power-splitting (PS) protocol to assist information transmission to the far user (FU). Considering nonlinear energy harvesting and opportunistic cooperative mode (CM) to non-cooperative mode (NCM) switching, expressions are derived for FU and NU outage probabilities and throughput. We demonstrate that CM/NCM switching can guarantee NU’s performance similar to OMA without any degradation in FU throughput. Also, the choice of optimum PS/TS parameter is essential to maximize FU throughput while ensuring desired NU throughput. CM/NCM switching ensures higher energy efficiency.

This paper investigates the performance of a framework for low-outage downlink non-orthogonal multiple access (NOMA) signalling using a coordinated direct and relay transmission (CDRT) scheme with direct links to both the near-user (NU) and the far-user (FU). Both amplify-and-forward (AF) and decode-and-forward (DF) relaying are considered. In this framework, NU and FU combine the signals from BS and R to attain good outage performance and harness a diversity of two without any need for feedback. For the NU, this serves as an incentive to participate in NOMA signalling. For both NU and FU, expressions for outage probability and throughput are derived in closed form. High-SNR approximations to the outage probability are also presented. We demonstrate that the choice of power allocation coefficient and target rate is crucial to maximize the NU performance while ensuring a desired FU performance. We demonstrate performance gain of the proposed scheme over selective decode-and-forward (SDF) CDRT-NOMA in terms of three metrics: outage probability, sum throughput and energy efficiency. Further, we demonstrate that by choosing the target rate intelligently, the proposed CDRT NOMA scheme ensures higher energy efficiency (EE) in comparison to its orthogonal multiple access counterpart. Monte Carlo simulations validate the derived expressions.