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Low Earth orbit (LEO) satellite communication (SATCOM) networks have gradually been recognized as an efficient solution to enhance ground-based wireless networks. As one of the main characteristics of LEO SATCOM, the beam-edge area could be covered by multiple satellite nodes. In this case, user terminals (UTs) located at the beam-edge have the chance to connect one or more LEO satellites. To develop the diversity gain of multiple nodes in the overlapping area, we propose two high spectral efficiency cooperative transmission strategies, i.e., directly combining (DC) and selection combining (SC). In the DC scheme, signals arrived at the UT simultaneously could be combined into one enhanced signal. For downlink time division multiplexing, the SC scheme enables the UT to select the strongest signal path. Further, as there exists a significant channel gain difference of the beam-center and beam-edge areas, UTs in these two areas can be allocated in one resource block. In this case, we derive co-carriers based on DC and SC, respectively. To deeply analyze the novel methods, we study the ergodic sum-rate and outage probability while the outage diversity gain is further provided. Simulation results show that the co-carrier-based DC method has the ability to provide a higher ergodic sum-rate while the SC method performs better in terms of the outage probability.

This paper investigates the impact of non-ideal environmental factors, including hardware impairments, random user distributions, and imperfect channel conditions, on the performance of distributed reconfigurable intelligent surface (RIS)-assisted air–ground fusion networks. Using an unmanned aerial vehicle (UAV) as an aerial base station, performance metrics such as the outage probability, ergodic rate, and energy efficiency are analyzed with Nakagami-m fading channels. To highlight the superiority of RIS-assisted air–ground networks, comparisons are made with point-to-point links, amplify-and-forward (AF) relay scenarios, conventional centralized RIS deployment, and fusion networks without hardware impairments. Monte Carlo simulations are employed to validate theoretical analyses, demonstrating that in non-ideal environmental conditions, distributed RIS-assisted air–ground fusion networks outperform benchmark scenarios. This model offers some insights into the improvement of wireless communication networks in emerging smart cities.

Abstract In this letter, a hybrid non‐orthogonal multiple access (NOMA) scheme for satellite–terrestrial systems is proposed, which is capable of integrating both power domain NOMA (PD‐NOMA) and code domain NOMA (CD‐NOMA) design modes. The stochastic geometry method is considered to model the position of the terrestrial NOMA user. Specifically, the closed‐form expressions of exact outage probability with imperfect interference cancellation are derived. In order to get more insights, the asymptotic outage probabilities for high SNR conditions are also discussed. Numerical results are provided to confirm the validity of theoretical analysis and reveal the effects of critical parameters on system performance.

Abstract In this paper, an intelligent reflecting surface (IRS) assisted cooperative non‐orthogonal multiple access and mobile edge computing (NOMA‐MEC) system is proposed, where a base station sends signals to users through the virtue of the IRS. When the direct links between the base station and the user are blocked, the offloading tasks will be refracted to the near‐end user through the IRS and forwarded to the remote user, where the near‐end user is equipped with an MEC server working at full duplex or half duplex mode. To evaluate the potential performance of IRS aided cooperative NOMA‐MEC, new closed‐form expressions of the offloading outage probability is deduced for a pair of users. The asymptotic expressions of offloading outage probability are provided in the high signal‐to‐noise ratio regime. Simulation results are presented to substantiate the analyses and demonstrate that: (i) The offloading outage behaviors of IRS‐aided cooperative NOMA‐MEC systems are superior to that of NOMA‐MEC without IRS and IRS‐aided OMA‐MEC; (ii) As the value of parameter Q increases, the IRS‐aided NOMA‐MEC system is capable of achieving the enhanced offloading outage performance; (iii) By comparing with NOMA‐MEC without IRS and IRS‐aided OMA‐MEC, the energy efficiency and system throughput of IRS‐aided NOMA‐MEC has obvious advantages in delay‐limited transmission mode.

Active reconfigurable intelligent surface (ARIS) has sparked more attention due to its capability to overcome the impact of double fading. This paper introduces using an ARIS to aid non-orthogonal multiple access (NOMA) communications over cascade Rician fading channels, where the direct links between the base station and users are seriously blocked. By applying ARIS, it can amplify the superposed signals to overcome the double pass loss effect caused by passive RIS. Both ARIS and NOMA can have synergistic impacts on sixth-generation communication systems. New approximated and asymptotic expressions in terms of outage probability and ergodic data rate of the k-th user are deduced for ARIS-NOMA systems. Based on asymptotic analytical results, we further calculate the diversity order and high signal-to-noise ratio slope of the k-th user. Finally, the system throughput of ARIS-NOMA is discussed in the delay-constrained transmission mode. Monte Carlo numerical results are performed to verify that: (1) the outage behaviors of ARIS-NOMA are better than that of ARIS-assisted orthogonal multiple access (OMA); (2) as the impact of thermal noise caused by ARIS becomes larger, the communication performance from the base station to ARIS, then to users, becomes worse; (3) the ARIS-NOMA systems have the ability to provide the improved ergodic data rate relative to ARIS-OMA.

Abstract In this paper, a reconfigurable intelligent surface (RIS) assisted downlink non‐orthogonal multiple access (NOMA) network is considered, where a base station communicates with a pair of users with the assistance of a RIS. The performance of RIS‐assisted downlink NOMA networks is investigated by exploiting the coherent phase shifting design. In particular, the central limit theorem based Gaussian approximation is introduced to model the sum of independent and identically distributed random variables and derive the approximate expressions of the outage probability for two users. Furthermore, the upper bounds for the outage probability are obtained based on the property of the Bessel function. Additionally, both the asymptotic outage probabilities and the asymptotic upper bounds at high signal‐to‐noise ratio are derived and the diversity order achieved by the network is obtained. Simulation results are provided to validate the theoretical findings and demonstrate that RIS‐NOMA can achieve superior outage performance compared to RIS‐assisted orthogonal multiple access and conventional full‐duplex decode‐and‐forward relaying schemes. Moreover, it can be observed that the outage performance of RIS‐NOMA can be tremendously enhanced with increasing the number of reflecting elements.

Abstract Reconfigurable intelligent surface (RIS) has stimulated their potential applications for improving the performance of wireless communication networks. In this paper, the performance of RIS aided non‐orthogonal multiple access (NOMA) networks with hardware impairments over Rician fading channels is investigated. More specifically, the exact and asymptotic expressions of outage probability for a pair of users, that is, the nearby user n and distant user m are derived, where the imperfect successive interference cancellation (ipSIC) and perfect SIC (pSIC) are taken into consideration. According to the approximate analyses, the diversity orders of user n with ipSIC/pSIC and user m are obtained in the high signal‐to‐noise radio regime. It indicates that the diversity orders are in connection with reflecting elements and Rician factors except the channel ordering. The impact of these parameters on outage behaviors of RIS‐NOMA networks is also analysed. In addition, the system throughput of RIS‐NOMA networks with ipSIC/pSIC is surveyed in detail. Monte Carlo simulations are present to verify the correctness of theoretical analyses that: (1) The outage probability of user n with pSIC is superior to that of orthogonal user, while the outage probability of user m is inferior to that of orthogonal user and (2) as the number of reflecting elements and Rician factors increases, the outage behaviors of RIS‐NOMA networks are enhanced carefully.

As a significant component of non-terrestrial networks, satellite communication has gradually evolved into a heterogeneous topology where the network nodes are classified as low Earth orbit (LEO) and geostationary Earth orbit (GEO) nodes. Considering the advantage of low transmission delay of LEO satellites in a heterogeneous network, ground users are arranged to access LEO satellites in the first priority. By fully understanding the equivalent channel gain difference in the beam-edge and beam-center users, we develop non-orthogonal multiple access (NOMA) in an LEO network where different users are allocated in the same resource block. As a complementary strategy, the beam-edge user has the ability to connect the GEO satellite if the LEO links are not available. That is, the two users are served cooperatively by LEO and GEO satellites in an orthogonal manner. In this paper, we provide deep insights for the uplink transmission performance of NOMA and cooperative orthogonal multiple access methods. As a general metric, the analytical expressions of outage probability are derived and the diversity orders are also provided. The simulation results show that NOMA is capable of providing remarkable performance in low signal-to-noise ratio regions and is more promising when the channels are assumed to be ordered.

This paper analyzed the performance of an ambient-backscatter-(AmBC)-assisted non-orthogonal multiple access (NOMA) system, where a backscatter device (BD) broadcasts its signal to numerous users. More specifically, the realistic assumptions of imperfect successive interference cancellation (ipSIC) and residual hardware impairments (RHIs) for AmBC–NOMA systems were taken into consideration. We further derived the closed-form and asymptotic expressions of outage probability for the BD and the d-th user. Based on the asymptotic expressions, the diversity orders of the BD and the d-th user were obtained in the high SNR regime. Furthermore, throughput and energy efficiency are further discussed for AmBC-assisted orthogonal multiple access (OMA) systems in the delay-limited transmission model. The numerical results revealed that: (i) AmBC–NOMA systems have the ability to achieve better outage behavior than AmBC–OMA; (ii) due to the existence of the backscatter link, the error floors of outage probability for the BD and the d-th user appear at a high signal-to-noise ratio; (iii) AmBC–NOMA systems are able to achieve higher energy efficiency and throughput than AmBC–OMA systems.

This paper investigates a satellite-terrestrial system employing the non-orthogonal multiple access (NOMA) scheme over Shadowed-Rician fading channels. The effects of both imperfect channel state information (CSI) and imperfect successive interference cancellation (SIC) on the NOMA based satellite-terrestrial system are taken into account. First, the outage performance of the proposed system is evaluated, and the closed-form expressions for the exact and asymptotic outage probabilities are obtained. Next, the expressions of the ergodic sum rate and system throughput are derived in the presence of channel estimation errors and residual interference. Moreover, we analyze the impacts of different residual interference levels on the energy efficiency. Finally, the numerical simulation results are conducted to verify the correctness of theoretical analysis and the advantages of the proposed NOMA scheme. At medium-high SNR region, the NOMA users are capable of outperforming orthogonal multiple access (OMA) users in terms of outage behaviors and system throughput with imperfect CSI and imperfect SIC. The ergodic sum rate decreases as the level of residual interference ω increases, and is even lower than that of OMA once ω goes beyond a certain value. Additionally, the energy efficiency of NOMA scheme is sensitive to the imperfect SIC.

Abstract In this paper, a non-orthogonal multiple access (NOMA)-based mobile edge computing (MEC) system is proposed, where paired users (the mth user and the nth user) offload tasks to the MEC server under imperfect channel state information (ipCSI) condition. To evaluate the effect on performance under ipCSI condition in NOMA-MEC system, this study derives new exact and asymptotic expressions of the offloading outage probability for two users under ipCSI and perfect channel state information (pCSI) conditions. On the basis of the theoretical derivation, the diversity orders of the nth user under ipCSI and pCSI conditions are zero and n, respectively, while those for the distant user are all zeros. In addition, we also investigate the system throughput and energy efficiency of NOMA-MEC in delay-limited transmission. Numerical results show that (1) the offloading outage behaviors of NOMA-MEC are better than those of time division multiple address (TDMA), (2) the offloading outage performance of paired users under ipCSI is worse than that under pCSI, and (3) the NOMA-MEC system achieves higher throughput and energy efficiency than does the TDMA-MEC system.

Power domain non-orthogonal multiple access (PD-NOMA) has been widely recognized a spectrum-efficient multi-user transmission method when the channel gains across different users vary obviously. Due to the long travel distance between the space and the Earth in satellite communication scenarios, the advantages from PD-NOMA will disappear since the signals experience almost the same path loss. However, in hybrid satellite networks, terrestrial users equipped with different size antennas may lead to various receiving gains, which are equivalent to diverse channel gains. In this paper, by deeply exploiting such a characteristic, we propose a power domain multiplexing based co-carrier transmission method in a hybrid satellite network. Superposing a weak signal on a strong one, two signals are enabled to be transmitted on the same carrier with the bandwidth of the strong signal. To guarantee the signals are decodable at both sides, a user pairing strategy is investigated in detail, and the performance gain of the new method is further analyzed. Simulation results show that a carrier resource can be saved in the proposed method with the sacrifice of a marginal bit-error-rate performance.

An unmanned aerial vehicle (UAV) swarm together with a large-scale heterogeneous Internet of Things (IoT) network consisting of macrocells and energy-constrained IoT transmitters (IoT-Ts) is investigated. The UAVs are utilized as flying robot swarms that intelligently transfer energy to the energy-constrained IoT-Ts on the ground. Each IoT-T has an associated IoT device (IoT-D) that is placed at a fixed distance in a random direction. The transmission probability of the energy-constrained IoT-Ts is derived by considering one-slot charging and two-slot charging according to three dimensional (3D) locations, respectively. The coverage probability of each type of IoT-D is investigated. The energy efficiency is derived by considering the transmission power of the active IoT-Ts and the effect of the association biasing factor, and the energy efficiency is also maximized by deploying the optimal density of IoT-Ts. Simulation results are examined to validate the accuracy of our theoretical analysis. Results illustrate the insightful effects of the network parameters, and the helpful guidelines for practical UAV swarms and IoT system design.

Satellite communication networks have gradually been recognized as an effective way to enhance the ground-based wireless communication. Considering the weight restriction of payloads, multi-antenna technologies have recently come into use on satellite platforms, and are capable of generating beams flexibly to provide services. To avoid incurring interferences, adjacent beams are designed to take different spectral resources. Unfortunately, this may limit the simultaneously accessed terminals since the spectrum cannot be fully used. In this paper, we propose a spectrum-saving transmission method in a satellite star network, where terminals communicate with each other through the hub station. Taking advantage of the great transmission capability differences of the hub station and terminals, we could allocate them the same spectral resources. Specifically, it is not necessary to use exclusive frequency bands for terminals.The proposed method can play a significant role when large numbers of users need to access the system with limited spectrum resource. To give a deep insight into the spectrum-saving method, the expressions of ergodic sum-rate are provided, and the impact of the number of accessed terminals is further analyzed. Simulation results validate the advantage of the proposed method in terms of bit error rate and ergodic sum rate.

This paper proposes a self-energy recycling full-duplex cooperative non-orthogonal multiple access (SRF-NOMA) system, where a nearby user can be employed as a decode-and-forward, amplify-and-forward, and quantize-map-forward relay with self-energy recycling protocol to assist a distant user. The relay harvests energy from dedicated energy signal sent by a base station, while it reuses energy from loop self-interference (LI) signal. To characterize the performance of the system proposed, new expressions of exact and asymptotic outage probabilities for two users are derived. Based on analytical results, the diversity orders achieved by the nearby and distant user are one and two, respectively. We confirm that the diversity orders of the distant user are obtained with direct link and relaying link, since the energy from LI is harvested for information transmission at the relay. Numerical results are presented to validate the accuracy of the derived results and demonstrate that: 1) when the DF or AF protocol is employed, the outage behavior of the SRF-NOMA system is superior to orthogonal multiple access; 2) in the high signal-to-noise ratio region, DF relay in the SRF-NOMA system is capable of enhancing the outage performance compared to AF relay; and 3) the distant user in the SRF-NOMA system with QMF relay is able to obtain a better performance than that of DF and AF relays.

This paper studies the application of cooperative techniques for pattern division multiple access (PDMA) in downlink (DL) network. More particularly, the performance of joint decode-and-forward (DF) relaying and PDMA (DF-PDMA) is investigated over Rayleigh channel, where the direct links are existent between the base station and users. To characterize the performance of the considered scenarios above, the closed-form expressions for outage probability and sum data rate are derived. Based on the derived results, the diversity orders achieved by users are obtained. The analysis results manifest that those users with DF relaying can achieve higher diversity order, in addition, the users selecting the PDMA pattern with higher column weight can further obtain higher diversity order. Simulation results unveil that DF-PDMA outperforms PDMA and orthogonal multiple access with DF relaying (DF-OMA) in terms of outage probability and system throughput. It is also worth noting that DF-PDMA can provide better fairness compared to DF-OMA.

This paper proposes a novel spatial-temporal deep neural network (STDNN) that is applied to multi-view gait recognition. The STDNN comprises a temporal feature network (TFN) and a spatial feature network (SFN). In TFN, a feature sub-network is adopted to extract the low-level edge features of gait silhouettes. These features are input to the spatial-temporal gradient (STG) network that adopts a STG unit and a long short-term memory unit to extract the STG features. In SFN, the spatial features of gait sequences are extracted by multilayer convolutional neural networks from a gait energy image. The SFN is optimized by classification loss and verification loss jointly, which makes inter-class variations larger than intra-class variations. After training, the TFN and the SFN are employed to extract temporal and spatial features, respectively, which are applied to multi-view gait recognition. Finally, the combined predicted probability is adopted to identify individuals by the differences in their gaits. To evaluate the performance of the STDNN, extensive evaluations are carried out based on the CASIA-B, OU-ISIR, and CMU MoBo data sets. The best recognition scores achieved by STDNN are 95.67% under an identical view, 93.64% under a cross-view, and 92.54% under a multi-view. State-of-the-art approaches are compared with the STDNN in various situations. The results show that the STDNN outperforms the other methods and demonstrates the great potential of the STDNN for practical applications in the future.

Low latency and a massive connection have become the requirements of energy internet wireless communication. Effective capacity analysis of non-orthogonal multiple access (NOMA) networks with short packets is of vital importance in energy internet communication planning and design. Low-latency communications are one of the main application scenarios in next-generation wireless networks. This paper focuses on the effective capacity of NOMA networks, where the finite blocklength, delay exponent, and transmission error probability are taken into account. New exact and asymptotic expressions of effective capacities are derived for arbitrarily ordered users with a finite blocklength. Based on the analytical results, the high Signal-to-Noise Ratio slopes of effective capacity in NOMA networks are carefully attained. The numerical results validate that (a) non-orthogonal users are capable of obtaining a larger effective capacity when the blocklength decreases, and that (b), as the value of the error probability and delay exponent increases, the effective capacity of non-orthogonal users worsens.

This paper studies the application of cooperative techniques for non-orthogonal multiple access (NOMA). More particularly, the fixed gain amplify-and-forward (AF) relaying with NOMA is investigated over Nakagami-m fading channels. Two scenarios are considered insightfully: 1) the first scenario is that the base station (BS) intends to communicate with multiple users through the assistance of AF relaying, where the direct links are existent between the BS and users and 2) the second scenario is that the AF relaying is inexistent between the BS and users. To characterize the performance of the considered scenarios, new closed-form expressions for both exact and asymptomatic outage probabilities are derived. Based on the analytical results, the diversity orders achieved by the users are obtained. For the first and second scenarios, the diversity order for the nth user are μ(n + 1) and μn, respectively. Simulation results unveil that NOMA is capable of outperforming orthogonal multiple access (OMA) in terms of outage probability and system throughput. It is also worth noting that NOMA can provide better fairness compared with conventional OMA. By comparing the two scenarios, cooperative NOMA scenario can provide better outage performance relative to the second scenario.

Pattern division multiple access (PDMA) is a promising non-orthogonal multiple access scheme for the fifth generation mobile communication system. Link performance estimation technique is the key to ensure accurate performance evaluation of the PDMA uplink system. Although the belief propagation (BP) receiver with near-optimal performance is normally considered for PDMA uplink system, it remains a big challenge for the estimation of the link performance for the PDMA uplink system with the BP receiver. In this paper, we first propose a novel link performance estimation technique for the PDMA uplink system with the BP receiver. The performance of the BP receiver is estimated by utilizing the genie-aided interference cancellation receiver as an upper bound with a single fitting parameter. Then, we provide a simple search process of finding the fitting parameter. Finally, the simulation results show that the proposed method achieves greater link performance estimation accuracy over the conventional method in the PDMA uplink evaluation.

In this paper, we investigate a downlink cooperative non-orthogonal multiple access (NOMA) network with decode-and-forward relaying, where two scenarios of user relaying with direct link and user relaying without direct link are discussed in detail. More particularly, the performance of cooperative NOMA system under the assumption of imperfect channel state information (ipCSI) is studied over Nakagami-m fading channels. To evaluate the outage performance of the above discussed two scenarios, the closed-form expressions of outage probability for a pair of users are derived carefully. The diversity orders of users are achieved in the high signal-to-noise region. An error floor appears in the outage probability owing to the existence of channel estimation errors under ipCSI conditions. Simulation results verify the validity of our analysis and show that: (1) NOMA is superior to conventional orthogonal multiple access; (2) The best user relaying location for cooperative NOMA networks should be near to the base station; and (3) The outage performance of distant user with direct link significantly outperforms distant user without direct link by comparing the two scenarios.