This paper proposes a security-aware computation offloading framework tailored for mobile edge computing (MEC)-enabled Internet of Things (IoT) networks operating in environments with aerial eavesdroppers (AEs) and ground eavesdroppers (GEs). It is envisaged that multiple ground nodes (GNs) should perform computation tasks partly locally and partly remotely by offloading a portion of these tasks to MEC servers. To facilitate this paradigm, an unmanned aerial vehicle (UAV) is deployed, serving as both an aerial MEC server and a relay for forwarding part of the tasks to a ground access point (AP) for computing. The computation offloading is further reinforced by incorporating a reconfigurable intelligent surface (RIS) unit in close proximity to the AP. Within this context, this paper provides an analysis of the secrecy outage probability (SOP) and formulates an optimization problem aimed at maximizing the minimum secure computation efficiency (SCE) by jointly optimizing transmit power allocation, time slot scheduling, task allocation, and RIS’s phase shifts. Given the non-convex nature of the problem, an iterative algorithm is introduced to address the fractional objective function and coupled optimization variables by employing Dinkelbach- and block coordinate descent (BCD)-based methods, respectively. The obtained results confirm the efficacy of the optimized scheme.

Abstract In this paper, a two‐way relay‐aided cognitive satellite terrestrial network (TR‐CSTN) model is proposed, where primary users are located at the edge of the base station. In the TR‐CSTN, one of satellite terminal users (STUs) is selected by the fusion center as the TR to forward information between two edge primary users with power of the TR. Meanwhile, these edge primary users share the licensed frequency band with the selected TR to send information to the satellite. Then, given the limited spectrum utilization and energy efficiency (EE) of the communication system, the cooperative spectrum sensing is employed to realize green communication. Specifically, the fusion center threshold, energy detection threshold, sensing duration and number of STUs are jointly optimized to enhance EE. Furthermore, considering that the node's energy shortage results in a short network lifetime, absolute EE gets improved. In detail, a power allocation scheme named normalized power aided Lévy flight trajectory‐based whale optimization algorithm (NP‐LWOA) is provided, which fulfills effective energy compensation among STUs to prolong the network lifetime notably. Finally, numerical results confirm the theoretical analysis and show the effectiveness of the TR‐CSTN and the NP‐LWOA in efficiently achieving the concept of green communication compared with other methods.

A multiple-input multiple-output wireless communication system is analytically studied, which operates with the aid of a large-scale reconfigurable intelligent surface (LRIS). LRIS is equipped with multiple passive elements with discrete phase adjustment capabilities, and independent Rician fading conditions are assumed for both the transmitter-to-LRIS and LRIS-to-receiver links. A direct transceiver link is also considered which is modeled by Rayleigh fading distribution. The system performance is analytically studied when the linear yet efficient zero-forcing detection is implemented at the receiver. In particular, the outage performance is derived in closed-form expression for different system configuration setups with regards to the available channel state information (CSI) at the receiver. In fact, the case of both perfect and imperfect CSI is analyzed. Also, an efficient phase shift design approach at LRIS is introduced, which is linear on the number of passive elements and receive antennas. The proposed phase shift design can be applied on two different modes of operation; namely, when the system strives to adapt either on the instantaneous or statistical CSI. Finally, some impactful engineering insights are provided, such as how the channel fading conditions, CSI, discrete phase shift resolution, and volume of antenna/LRIS element arrays impact on the overall system performance.

To boost energy saving for the general delay-tolerant IoT networks, a two-stage, and single-relay queueing communication scheme is investigated. Concretely, a traffic-aware N-threshold and gated-service policy are applied at the relay. As two fundamental and significant performance metrics, the mean waiting time and long-term expected power consumption are explicitly derived and related with the queueing and service parameters, such as packet arrival rate, service threshold and channel statistics. Besides, we take into account the electrical circuit energy consumptions when the relay server and access point (AP) are in different modes and energy costs for mode transitions, whereby the power consumption model is more practical. The expected power minimization problem under the mean waiting time constraint is formulated. Tight closed-form bounds are adopted to obtain tractable analytical formulae with less computational complexity. The optimal energy-saving service threshold that can flexibly adjust to packet arrival rate is determined. In addition, numerical results reveal that: 1) sacrificing the mean waiting time not necessarily facilitates power savings; 2) a higher arrival rate leads to a greater optimal service threshold; and 3) our policy performs better than the current state-of-the-art.

In this paper, a novel 3D cellular model consisting of aerial base stations (aBSs) and aerial user equipments (aUEs) is proposed, by integrating the coordinated multi-point (CoMP) transmission technique with the theory of stochastic geometry. For this new 3D architecture, a tractable model for aBSs' deployment based on the binomial-Delaunay tetrahedralization is developed, which ensures seamless coverage for a given space. In addition, a versatile and practical frequency allocation scheme is designed to eliminate the inter-cell interference effectively. Based on this model, performance metrics including the achievable data rate and coverage probability are derived for two types of aUEs: i) the general aUE (i.e., an aUE having distinct distances from its serving aBSs) and ii) the worst-case aUE (i.e., an aUE having equal distances from its serving aBSs). Simulation and numerical results demonstrate that the proposed approach emphatically outperforms the conventional binomial-Voronoi tessellation without CoMP. Insightfully, it provides a similar performance to the binomial-Voronoi tessellation which utilizes the conventional CoMP scheme; yet, introducing a considerably reduced computational complexity and backhaul/ signaling overhead.

In this paper, we investigate a coordinated direct and relay transmission for uplink (CDRT-UL) of a cooperative non-orthogonal multiple access (NOMA) system, in which two users communicate with a base station (BS) with the aid of a decode-and-forward relay. By incorporating both relay links and non-negligible direct links from two users to BS, the CDRTUL scheme forms an orthogonal structure at the BS receiver side, so that the BS receiver applies only linear combinations to recover the desired signals without abusing successive interference cancellation. Outage probability of the paired two users is derived in closed-form under independent but not necessarily identically distributed Nakagami-m fading channels. Asymptotic analysis in the high signal-to-noise ratio regime is also introduced that provides closed-form expressions, which indicates that both two users achieve the same asymptotic outage performance. It is also shown that an improved outage performance and the higher target rate can be achieved by the CDRT-UL scheme over those of the DF relay without direct links. The enhanced system performance achieved by the CDRT-UL scheme is corroborated by Monte Carlo simulations.

A new detection scheme for multiuser multiple-input multiple-output (MIMO) systems is analytically presented. In particular, the transmitting users are being categorized in two distinct priority service groups, while they communicate directly with a multi-antenna receiver. The linear zero-forcing scheme is applied in two consecutive detection stages upon the signal reception. In the first stage, the signals of one service group are detected, followed by the second stage, including the corresponding detection of the remaining signals. An appropriate switching scheme based on specific transmission quality requirements is utilized prior to the detection so as to allocate the signals of a given service group to the suitable detection stage. The objective is the provision of the reception quality for both service groups. The proposed approach can be implemented directly in cognitive radio communication assigning the secondary users to the appropriate service group. The exact outage probability of the considered system is derived in closed form. The special case of massive MIMO is further studied yielding some useful engineering outcomes; the effective channel coherence time and a certain optimality condition defining both the transmission quality and effective number of independent transmissions.

In this paper, the performance of an underlay multiple-input multiple-output (MIMO) cognitive radio system is analytically studied. In particular, the multiple antennas of the secondary transmitter operate in a spatial multiplexing transmission mode, while a zero-forcing (ZF) detector is employed at the secondary receiver. Additionally, the secondary system is interfered by multiple randomly distributed single-antenna primary users (PUs). To enhance the performance of secondary transmission, optimal power allocation is performed at the secondary transmitter with a constraint on the interference temperature (IT) specified by the PUs. To mitigate instantaneous excessive interference onto PUs caused by the time-average IT, an iterative antenna reduction algorithm is developed for the secondary transmitter and, accordingly, the average number of transmit antennas is analytically computed. Extensive numerical and simulation results corroborate the effectiveness of our analysis.

This paper proposes an iterative detector for uncoded OFDM signals in cooperative networks, where the information symbols are simply partitioned through a time-domain matrix at the OFDM transmitter. We analytically show that our proposed iterative detector at the partner node converges and can completely recover the user's data from its partitioned version, if sufficient redundancy is inserted in the user's data. For efficient use of the redundancy in the user's data, a coded cooperative transmission based on superposition modulation is proposed. Additionally, a closed-form input-output relationship for the partitioning and reconstruction algorithm in the proposed cooperative scenario is derived. We also obtain closed-form expressions for symbol error rate performance of the proposed coded cooperative scenario over Rayleigh frequency-selective fading channels. Numerical comparisons shed light on the relative merits of the proposed coded cooperation under various inter-user and uplink channel conditions.

A multi-user cognitive (secondary) radio system is considered, where the spatial multiplexing mode of operation is implemented amongst the nodes, under the presence of multiple primary transmissions. The secondary receiver carries out minimum mean-squared error detection to effectively decode the secondary data streams, while it performs spectrum sensing at the remaining signal to capture the presence of primary activity or not. New analytical closed-form expressions regarding some important system measures are obtained, namely, the outage and detection probabilities, the transmission power of the secondary nodes, the probability of unexpected interference at the primary nodes, and the detection efficiency with the aid of the area under the receive operating characteristics curve. The realistic scenarios of channel fading time variation and channel estimation errors are encountered for the derived results. Finally, the enclosed numerical results verify the accuracy of the proposed framework, while some useful engineering insights are also revealed, such as the key role of the detection accuracy to the overall performance and the impact of transmission power from the secondary nodes to the primary system.

We present a multi-user multiple-input multiple-output (MIMO) cognitive radio system consisting of a secondary receiver that deploys spatial multiplexing to decode signals from multiple secondary transmitters, under the presence of primary transmissions. The secondary receiver carries out minimum mean-squared error detection to decode the secondary data streams, while it performs spectrum sensing at the remaining signal to capture the potential presence of primary activity. Assuming Rayleigh fading as well as the realistic cases of channel fading time variation and channel estimation errors, we present novel closed-form expressions for important system measures, namely, the detection and false-alarm probabilities as well as the transmission power of the secondary nodes. The enclosed numerical results verify the accuracy of the presented analysis.

A dual-hop cognitive (secondary) relaying system incorporating collaborative spectrum sensing to opportunistically switch between data transmission and energy harvesting is introduced. The secondary relays, first scan the wireless channel for a primary network activity, and then convey their reports to a secondary base station (SBS). Afterwards, the SBS, based on these reports and its own estimation, decides cooperatively the presence of primary transmission or not. In the former scenario, all secondary relays start to harvest energy from the transmission of one or more primary nodes. In the latter scenario, the system initiates secondary communication via a best relay selection policy. The performance of the proposed scheme is thoroughly investigated by assuming realistic channel conditions, i.e., non-identical link-distances and outdated channel estimation, while its overall energy consumption is evaluated, indicating the efficiency of the switching approach.

Energy efficiency has become an encouragement, and more than this, a requisite for the design of the next-generation wireless communication standards. In this paper, a dual-hop cognitive (secondary) relaying system is considered, incorporating multiple amplify-and-forward relays, a rather cost-effective solution. First, the secondary relays sense the wireless channel, scanning for a primary network activity, and then convey their reports to a secondary base station (SBS). Afterward, the SBS, based on these reports and its own estimation, decides cooperatively the presence of primary transmission or not. In the former scenario, all the secondary nodes start to harvest energy from the transmission of primary nodes. In the latter scenario, the system initiates secondary communication via a best relay selection policy. Performance evaluation of this system is thoroughly investigated, by assuming realistic channel conditions, i.e., non-identical link distances, Rayleigh fading, and outdated channel estimation. The detection and outage probabilities as well as the average harvested energy are derived as new closed-form expressions. In addition, an energy-efficiency optimization problem is analytically formulated and solved, while a necessary condition in terms of power consumption minimization for each secondary node is presented. From a green communication standpoint, it turns out that energy harvesting greatly enhances the resources of secondary nodes, especially when primary activity is densely present.

The performance of an underlay cognitive (secondary) dual-hop relaying system with multiple antennas and hardware impairments at each transceiver is investigated. In particular, the outage probability of the end-to-end (e2e) communication is derived in closed-form, when transmit antenna selection with maximum ratio combining (TAS/MRC) is established in each hop. The practical scenario of spatial correlation among the antennas at the received signal is also considered. Simplified asymptotic outage expressions reveal some new useful engineering insights; namely the diversity and array order of the system, the balance on the number of transmit/receive antennas, and the impact of hardware impairments to the e2e communication.

The performance of a distributed multiple input- multiple output (MIMO) system with m single-antenna transmitters and a receiver with n ≥ m antennas is analytically investigated. Particularly, the transmitters simultaneously send their information streams to the receiver, operating under the spatial multiplexing regime. To this end, the receiver utilizes the zero forcing detection followed by the ordered successive interference cancellation technique. Composite fading/shadowed channels between receiver and transmitters are assumed, which are modeled as non-identical K faded channels (i.e., Rayleigh multipath fading and Gamma distributed shadowing). The receiver sequentially detects and decodes the streams, starting from the strongest to the weakest, whereas symbol ordering is implemented with regards to shadowing side information without taking into account the fast varying multipath fading fluctuations. New closed-form expressions regarding the outage probability and the average symbol error probability are derived in terms of finite sums of the Meijer's-G functions. Further, simplified expressions in terms of elementary functions are obtained in some specific asymptotic cases of interest, namely, in high signal-to-noise regions and when the receiver is equipped with very large antenna arrays (i.e., massive MIMO systems). Moreover, useful engineering insights are manifested, such as the diversity order of the considered system.

The performance of multiuser dual-hop relaying over mixed radio frequency/free-space optical (RF/FSO) links is investigated. RF links are used for the simultaneous data transmission from m single-antenna sources to the relay, which is equipped with n ≥ m receive antennas and a photo-aperture transmitter. The relay operates under the decode-and-forward protocol and utilizes the popular ordered V-BLAST technique to successively decode each user's transmitted stream. A common norm-based ordering approach is adopted, where the streams are decoded in an ascending order. After the V-BLAST decoding, the relay retransmits the initial information to the destination, which is equipped with a photo-detector, via a point-to-point FSO link in m consecutive timeslots. Analytical expressions for the end-to-end outage probability and average symbol error probability of each user are derived. Some engineering insights are manifested, such as the diversity order, the impact of the pointing error displacement on the FSO link and the severity on the turbulence-induced channel fading.

The performance of a dual‐hop system is studied and evaluated, where m single‐antenna source nodes communicate with a destination equipped with n ≥ m antennas via m single‐antenna relays. The decode‐and‐forward protocol is implemented at the relays whereas the classical ordered V‐BLAST scheme is performed at the destination. The analysis involves spatially independent non‐identically distributed Rayleigh channel fading channels, reflecting on distinct average received powers among the different source and/or relay nodes with the destination, suitable for practical applications. The identically distributed scenario is also included as a special case. Novel union closed‐form system bound expressions are derived in terms of the outage probability and the average symbol error probability. These bounds correspond to the cases of both the perfect and the imperfect channel state information at the relay and the destination nodes. The numerical results, accompanied with the equivalent simulation ones, reveal the accuracy of the proposed approach.

A multiuser dual-hop relaying system over mixed radio frequency/free-space optical (RF/FSO) links is investigated. Specifically, the system consists of m single-antenna sources, a relay node equipped with n≥ m receive antennas and a single photo-aperture transmitter, and one destination equipped with a single photo-detector. RF links are used for the simultaneous data transmission from multiple sources to the relay. The relay operates under the decode-and-forward protocol and utilizes the popular V-BLAST technique by successively decoding each user's transmitted stream. Two common norm-based orderings are adopted, i.e., the streams are decoded in an ascending or a descending order. After V-BLAST, the relay retransmits the decoded information to the destination via a point-to-point FSO link in m consecutive timeslots. Analytical expressions for the end-to-end outage probability and average symbol error probability of each user are derived, while closed-form asymptotic expressions are also presented. Capitalizing on the derived results, some engineering insights are manifested, such as the coding and diversity gain of each user, the impact of the pointing error displacement on the FSO link and the V-BLAST ordering effectiveness at the relay.

The analytical performance evaluation of various coherent diversity combining reception techniques in the presence of multiuser interference is presented into this paper. Particularly, the outage probability of the generalized selection combining (GSC) and its two extreme cases, namely the maximum ratio combining (MRC) and the selection combining (SC), are evaluated. The multiple interferers undergo independent and non-identically distributed Nakagami-m fading channels, with distinct average received powers and arbitrary fading scales. New analytical outage expressions are presented including several channel fading conditions regarding the desired user's signal. Although the proposed approach is formed as an infinite series representation, it includes only simple elementary and Gamma functions, while it delivers a rapidly converging formula achieving a sufficiently high precision level. Moreover, the derived computational complexity of this function is thoroughly discussed, while a corresponding comparison with another popular technique indicates the usefulness of the proposed approach. Finally, the accuracy of the approach is verified with the aid of selected numerical results accompanied with equivalent simulation ones.

The authors propose a new reception scheme based on the successive interference cancellation (SIC) approach by jointly performing zero forcing (ZF) and minimum mean squared error (MMSE) according to a defined threshold. A complexity analysis is provided although upper and lower complexity bounds are also derived. A detailed performance evaluation is presented, which explicitly indicates the usefulness of the proposed switching method across the conventional ZF–SIC and MMSE–SIC in terms of computational savings and/or error resilience. An alternative switching threshold which is based on the ergodic capacity is also presented, in case of capacity‐driven applications.