Cognitive Relay Networks Research Articles

MIMO Based Cooperative Communication and Joint Maximum Likelihood Detection for Cognitive Radio System

In this work, novel cooperative communication strategies like Decode and Forward based relay schemes, joint maximum likelihood signal detection in combinational Multiple Input Multiple Output (MIMO) cooperative relay cognitive networks are used. The joint cooperative signal detection using Maximum-Likelihood (ML) detector at each relays, and associated path links improves the quality of services at the destination node, which can be dynamically altered using different relay combinations and associated iterative computational complexity accumulated during ML detection. Here the relay performs ML decoding and transmit the detected symbols to the destination and also to the multiple antennas equipped for cooperative communication. The same ML detection at the destination combines the MIMO based diversity to narrow down the error probability among different orthogonal transmissions. The combination of iterative signal detection and relay assisted decoding scheme shows superior error rate performance when compared to non-iterative hard decoded signal detection. The simulations results proved the influence of MIMO assisted cooperative communication and the associated performance penalty gaps that come with imperfect channel state information at the destination node.

Cognitive Buffer-Aided Mixed RF/FSO Backhauling Network With Switch-Based Rate Adaptation

The cognitive relay networks can effectively mitigate the congestion problem of electromagnetic spectrum. In this context, data buffering at the relaying terminal and free space optical communication can be incorporated to further improve the performance of such networks. To explore this novel scenario, we develop a network model for the mixed RF/FSO cognitive relaying networks and design a transmission protocol to utilize the available resources. Furthermore, we derive the analytical outage probability for the secondary system under the proposed protocol. Monte-Carlo simulations demonstrate that the proposed model guarantees a reliable communication system and the presented theoretical analysis is correct.

Performance Study of Cognitive Relay NOMA Networks With Dynamic Power Transmission

The cooperative non-orthogonal multiple access (NOMA) networks with one pair primary user and one pair cognitive user share the same spectrum resource via a common relay is considered in this paper. We propose a dynamic power transmission scheme for both uplink and downlink NOMA transmission in cognitive relay networks, which preserves the quality of service for the primary user. The closed-form expressions of overall outage probability and average sum rate for the proposed dynamic power transmission scheme of cognitive relay NOMA networks are derived. Both developed analytical results and Monte Carlo simulations show that the proposed dynamic power control scheme can dramatically enhance performance gain for the proposed networks, compared to other existing NOMA power allocation schemes.

Combining Power Allocation and Superposition Coding for an Underlay Two-way Decode-and-forward Scheme

Combining Power Allocation and Superposition Coding for an Underlay Two-way Decode-and-forward Scheme

Outage Minimization of Energy Harvesting-Based Relay-Assisted Random Underlay Cognitive Radio Networks With Interference Cancellation

This paper incorporates interference cancellation in the outage analysis of a wireless energy harvesting cognitive relay network. Based on an interference threshold, primary receivers are assumed to be able to cancel a fraction of the strongest interferers in the primary network which often dominates the total interference. To achieve this, the coefficient of cancellation is adapted into the analysis to reduce the level of interference. The rationale is to improve the successful transmission probability of the primary network by cancelling interference at its receivers. Interestingly, this reduces the overhead incurred by the secondary network to guarantee the primary outage constraint. In this work, the optimal relay selection range is derived and deployed to minimize the secondary outage probability. Analytical results show that this approach can be used to significantly reduce the secondary outage probability which in turn improves the network performance. However, this is at the cost of reducing the energy harvesting success probability of the relays within the secondary network.

Secrecy Outage Performance of Cognitive Radio Network with Selection Combining at Eavesdropper

Background: Based on the idea of cooperative communication, recently a lot of attention has been drawn to cooperative spectrum access for the secure information transmission in a Cognitive Radio Network (CRN). Security is one of the most important aspects of these networks, as due to their open and dynamic nature, they are extremely vulnerable to malicious behavior. Cooperative cognitive radio has emerged as a dynamic spectrum access technique, where an unlicensed (secondary) user is allowed to simultaneously access the licensed channels dedicated to a Primary User (PU), as long as the Quality of Service (QoS) of primary communication is not affected. Method: This paper investigates the secrecy outage performance of threshold-based cognitive decode-andforward relay network, with interference constraints from primary licensed user. Threshold-based relaying is considered where; the source message is successfully decoded by the relay, only if the received SNR satisfies the particular threshold. Outage probability expressions have been derived for the worst-case scenario, where only the eavesdropper can achieve the advantage of diversity. The Selection Combining (SC) diversity scheme is employed only at the secondary eavesdropper. Results: The system secrecy performance is better for SC diversity scheme at the eavesdropper than Maximal Ratio Combining (MRC) diversity scheme, as MRC has better diversity performance than SC. We have shown that the improvement in desired secrecy rate, predetermined threshold, eavesdropper channel quality and interference constraints affect the secrecy performance of the cognitive radio system. The outage probability decreases accordingly with an increase in the maximum tolerable interference level at primary destination. The outage probability of Optimal relay Selection (OS) scheme is derived for a multi-relay system, when either the Instantaneous Channel State Information (ICSI) or the Statistical Channel State Information (SCSI) is available. We have shown that the secrecy performance of the OS with ICSI of the system is better than with SCSI. Also, the OS improves the performance of the multi-relay system, when the number of relays is increased. Conclusion: The secrecy outage probability of threshold-based DF underlay cognitive relay network is evaluated. Both interference and maximum transmit power constraints are considered at secondary source and secondary relay. Also, the relay can successfully decode the message, only if it meets the pre-defined threshold. We have investigated the performance of MRC and SC diversity schemes at the secondary eavesdropper and have shown that the system secrecy performance is better for SC than MRC, as MRC has better diversity performance than SC. We have shown that the system secrecy performance is significantly affected by the required secrecy rate, pre-defined threshold, interference constraints and choice of diversity scheme (MRC/SC) at the eavesdropper. The outage probability of OS scheme is derived for a multi-relay system, when either the ICSI or the SCSI is available. We have shown that the secrecy performance of the OS with ICSI of the system is better than with SCSI. Also, the OS improves the performance of the multi-relay system, when the number of relays is increased.

Power Adaptation for Enhancing Spectral Efficiency and Energy Efficiency in Multi-Hop Full Duplex Cognitive Wireless Relay Networks

This work investigates the energy efficiency (EE) and the spectral efficiency (SE) performance of multi-hop full duplex cognitive relay networks (MH-FDCRNs) operating in the spectrum sharing mode, under the influence of interference from the primary source. We formulate three distinct optimization problems for finding the optimal power allocation (OPA) for the secondary nodes in MH-FDCRN: (i) EE optimization with minimum SE requirement; (ii) SE optimization with minimum EE requirement; and (iii) EE-SE trade-off optimization. The impact of residual self-interference (RSI) arising due to full duplex operation at the relay nodes and the inter relay interference (IRI) arising due to frequency re-use are considered. For the EE/SE optimization problems, we transform the original non-convex optimization problems to their convex forms by expressing the numerator of the objective function as the difference of concave functions, and by using parametric transformation. For the EE-SE trade-off optimization problem, we first transform the original multi objective optimization problem into a single objective form; and then into its convex form by introducing an auxiliary variable. Computationally efficient algorithms are proposed to solve the considered problems. The convergence properties of the proposed algorithms are established through mathematical analysis as well as through computer simulation studies. We prove that the points of convergence of the proposed iterative algorithms are the Karush-Kuhn-Tucker (KKT) points of the initial non-convex problems. With the help of detailed numerical results, the best trade-off among EE and SE can be achieved by proper selection of priority factor, compared to the individual optimization approaches.

Power allocation algorithm of full duplex cognitive relay network based on energy harvesting

Power allocation algorithm of full duplex cognitive relay network based on energy harvesting

Physical layer security and energy efficiency driven resource optimisation for cognitive relay networks

In this study, a resource allocation problem considering physical layer security and power consumption for cognitive relay networks is studied. In a secondary network, the ratio of secret rate to power consumption is defined as a secret rate per watt (SRW). Fixed circuit power, dynamic circuit power, and transmit power are all considered in the power consumption model. Under the constraints of the maximum total transmit power of secondary user transmitters and relay nodes, minimum secret rate requirement of secondary user receiver and tolerable interference threshold of each primary user receiver, the authors propose a SRW maximisation algorithm to maximise SRW. The resource allocation problem is formulated as a non-linear fractional programming and it is transformed into an equivalent subtraction based on the Dinkelbach method. They incorporate the non-convex constraints into the objective function to convert the feasible region into a convex set and achieve an optimal resource allocation scheme with the nested loop iteration algorithm through the Lagrange dual theory and the difference of convex programming. By the Dinkelbach method and the nested loop iteration algorithm, the optimal SRW is obtained. Simulation results demonstrate the effectiveness and the out-performance of the proposed algorithm comparing with other algorithms.

Secrecy Outage Analysis of UAV Assisted Relay and Antenna Selection for Cognitive Network Under Nakagami-${m}$ Channel

In view of the scarcity of spectrum resources in wireless communication, it is studied in this paper over the two-hop cognitive secrecy transmission scheme of decoding and forwarding (DF) unmanned aerial vehicles (UAVs) assisted relay with energy harvesting under Nakagami-m channel. It is worth noting that the terminal node is equipped with multiple antennas and the optimal antenna selection can be adopted for signal reception. Meanwhile, the UAV assisted relay uses time switching (TS) and power splitting (PS) energy harvesting techniques and adopts optimal secrecy capacity selection scheme. It is successfully derived for the exact closed form expressions of non-zero secrecy capacity probability and optimal secrecy outage probability while the eavesdropper has no channel state information (CSI). Moreover, the system asymptotic secrecy outage probability demonstrates the secrecy diversity order and secrecy diversity gains of the UAV assisted relay cognitive network under Nakagami-m channel. Furthermore, joint optimization of power splitting factor and other parameters can effectively increase the system secrecy capacity and improve the system secrecy performance. Simulation results verify the correctness of the theoretical derivation and the effectiveness of the proposed scheme, which demonstrate that the proposed algorithm outperforms the other compared schemes.

Joint power and subcarrier allocation for SWIPT in cognitive relay system

In this paper, we study power and subcarrier allocation in decode-and-forward (DF) cognitive relay network based on orthogonal frequency division multiple access (OFDMA) technology, in which power splitting (PS) is used for the secondary users (SUs) supporting simultaneous wireless information and the power transfer (SWIPT). We attempt to maximize the energy efficiency (EE) of the secondary system subject to the constraints of minimum system rate, maximum transmission power and minimum harvested energy. The optimal power allocation (OPA) algorithm is proposed to determine the optimal solution of relay transmission power, power splitting ratio and subcarrier allocation. To tackle the original non-convex optimization problem, the Dinkelbach method is used for fractional programming and the problem is decomposed into two solvable sub-problems. The Lagrangian dual algorithm is utilized to determine the optimal transmission power and the power splitting ratio. The Hungarian algorithm is then introduced to determine the optimal subcarrier allocation. Compared with some existing algorithms, the simulation results show that the proposed OPA scheme is able to achieve higher energy efficiency when certain communication quality is required.

Performance evaluation of a two-way relay network with energy harvesting and hardware noises

In this paper, we propose a Two-Way Cognitive Relay Network (TWCRN) where the secondary users operate on an underlay mode to access the licensed bands. In the proposed protocols, two secondary sources transmit their data to a relay in the first time slot, and then the relay would forward the received information to both sources in the remaining time. Moreover, the relay is self-powered by harvesting energy from ambient Radio Frequency (RF) signals, using the Time Switching (TS) and the Power Switching (PS) method. This paper concentrates on evaluating the performance of the secondary networks under the impact of hardware impairments and co-channel interference from the primary networks. In particular, based on the secondary transmitters’ constraint power, we derive the closed-form expressions of the outage probability and the throughput over Rayleigh fading channels in two cases: TS and PS. We also investigate the energy efficiency issue and the locally optimal position of the relay to maximize the system throughput, which provides much information to install the relay location. Finally, our derivations are verified by Monte Carlo simulation.

Joint beamforming and time switching designs for energy-constrained cognitive two-way relay networks

In this paper, we investigate a joint beamforming and time switching (TS) design for an energy-constrained cognitive two-way relay (TWR) network. In the network, the energy-constrained secondary user (SU) relay employs TS protocol to harvest energy from the signals sent by the circuit-powered primary user (PU) transmitter, and then exploits the harvested energy to perform information forwarding. Our aim is to maximize the sum rate of SUs under the constraints of the data rate of PU, the energy harvesting and the transmit power of the SU relay. To determine the beamforming matrix and TS ratio, we decouple the original non-convex problem into two subproblems which can be solved by semidefinite relaxation and successive convex optimization methods. Furthermore, we derive closed form expressions of the optimal solutions for each subproblem, which facilitates the design of a suboptimal iterative algorithm to handle the original sum rate maximization problem. Simulation results are presented to illustrate the effectiveness and superior performance of the proposed joint design against other conventional schemes in the literature.

Optimization Algorithm for Spectrum Sensing Delay Time in Cognitive Radio Networks Using Decoding Forward Relay

Using decode-and-forward relaying in the cognitive radio networks, the spectrum efficiency can improve furthermore. The optimization algorithm of the spectrum sensing estimation time is presented for the cognitive relay networks in this paper. The longer sensing time will bring two aspects of the consequences. On the one hand, the channel parameters are estimated more accurate so as to reduce the interferences to the authorized users and to improve the throughput of the cognitive users. On the other hand, it shortens the transmission time so as to decease the system throughput. In this time, it exists an optimal sensing time to maximize the throughput. The channel state information of the sub-bands is considered as the exponentially distributed, so a stochastic programming method is proposed to optimize the sensing time for the cognitive relay networks. The computer simulation results using the Matlab software show that the algorithm is effective, which has a certain engineering application value.

Analysis of energy harvesting cognitive relay network with cooperative spectrum sensing

In this study, the authors investigate the performance of an energy harvesting cognitive cooperative multiple relays network with spectrum sensing, where secondary user accesses the licensed spectrum according to the spectrum sensing results. Particularly, if the spectrum hole is detected, the secondary nodes harvest energy from the ambient environment, otherwise, the secondary source and relays transmit with the energy collected from the primary signals. Taking the imperfect spectrum sensing results into account, exact closed-form expressions for the total outage probability and capacity of the secondary network are derived under the context of Rayleigh fading channels, where the proposed modified selective cooperative decode-and-forward relaying transmission schemes are adopted. Compared with the conventional cognitive relay network, the authors must undertake the correlations among the received signal-to-noise ratios or signal-to-interference-plus-noise ratios caused by both the presence of primary user's interference and energy harvesting for the system understudy. It is shown that spectrum sensing phase and data transmission phase are mutual influence and mutual restraint.

End-to-end performance of Hybrid DF/AF (HDAF) relayed underlay cognitive radio networks

In the presented work, the performance analysis of an underlay cognitive relay network with a hybrid-decode/amplify-and-forward relaying protocol is undertaken. Since the system is an underlay system, the secondary network will communicate simultaneously with the primary network. Hence interference temperature will act as a constraint to keep the secondary communication from interfering with the primary transmission. To ensure this, secondary transmit powers need to be kept below a pre-defined threshold level. Severe multi-path fading and shadowing or a long distance between the secondary source and the destination make a direct link between the two infeasible. These nodes can only be linked by a relay between the two. All the radio links in the system are characterized by the extended generalized-K fading distribution. The closed form expressions for the outage probability and ergodic channel capacity of the system are derived. The system outage performance and capacity with hybrid-decode/amplify-and-forward relay is compared to the performance with amplify-and-forward and decode-and-forward relays. Optimum relay position for best outage performance is also investigated. Further, the results of the performance analysis of the system are also illustrated by the numerical plots.

A Hop-by-Hop Relay Selection Strategy in Multi-Hop Cognitive Relay Networks

In this paper, a hop-by-hop relay selection strategy for multi-hop underlay cognitive relay networks (CRNs) is proposed. In each stage, relays that successfully decode the message from previous hop form a decoding set. Taking both maximum transmit power and maximum interference constraints into consideration, the relay in the decoding set which has the largest number of channels with an acceptable signal-to-noise ratio (SNR) level to the relays in the next stage is selected for retransmission. Therefore, relay selection in each stage only relies on channel state information (CSI) of the channels in that stage and does not require the CSI of any other stage. We analyze the performance of the proposed strategy in terms of end-to-end outage probability and throughput, and show that the results match those obtained from simulation closely. Moreover, we derive the asymptotic end-to-end outage probability of the proposed strategy when there is no upper bound on transmitters’ power. We compare this strategy to other hop-by-hop strategies that have appeared recently in the literature and show that this strategy has the best performance in terms of outage probability and throughput. Finally it is shown that the outage probability and throughput of the proposed strategy are very close to that of exhaustive strategy which provides a lower bound for outage probability and an upper bound for throughput of all path selection strategies.

Best Relay Selection Schemes for NOMA Based Cognitive Relay Networks in Underlay Spectrum Sharing

Cognitive Radio and the emerging Non-Orthogonal Multiple Access (NOMA) techniques hold the potential to fulfill the increasing demands of radio spectrum by 5G-and-B5G wireless systems. It is a usual practice in underlay cognitive radio networks to take assistance from intermediate relays to reach the remote destination. Putting together, NOMA based Cognitive Relay Networks (NCRNs) have recently gained tremendous research attention to improve the spectrum utilization efficiency. This article studies the application of best relay selection (BRS) in downlink scenario of NCRNs in which a base station (BS) being unable to communicate directly with the far user U2 takes assistance from the near user U1 and from the best Decode-and-Forward (DF) relay selected from the potential NCRN operating in underlay environment. Three BRS schemes are proposed selecting a relay which (i) maximizes signal-to-noise ratio (SNR) on relayU2 link, (ii) minimizes interference on relay-primary user (PU) link, (iii) maximizes the quotient of relay-U2 link's SNR and relay-PU link's interference power. Scheme 3 considers the existence of U1 -U2 link as well whereas the other two schemes do not. To characterize the performance of the proposed schemes assuming Rayleigh channel model, closed-form expressions of probability distribution function (PDF) of received SNR at U2, average number of reliable relays, outage probability and bit error rate are obtained. For insight analysis, analytical results are validated through simulations which reveal that relay selection in NCRNs is different from non-cognitive networks and is feasible in low-to-medium SNR regions. The signal from near user further improved the outage performance.

Buffer-Aided Relaying With Direct Link Transmission and Spectrum Sharing

Cognitive relay networks are designed to increase access to under-utilized spectrum. The direct link communication can be exploited to decrease the outage probability with a buffer-aided relaying network. To this end, a novel transmission policy is proposed to combine the benefits of direct communication and buffer-aided relaying. In addition, a modification to the hybrid interweave-underlay spectrum access policy is presented to restrict the amount of interference caused due to a miss-detection event. Assuming independent and not necessarily identical fading channels, the analytical outage probability and the end-to-end average packet delay for the secondary system are derived. Monte-Carlo simulations show that adopting the proposed policy, compared to the conventional link selection techniques, results in a significantly larger outage gain, a lower average packet delay, a higher empty buffer probability, and a lower full buffer probability. Also, the theoretical and Monte-Carlo results coincide in the provided simulation examples which verifies correctness of the theoretical analysis.

Optimal power allocation for energy‐efficient full‐duplex cognitive relay networks under primary interference

This study investigates optimal power allocation (OPA) for maximising the energy efficiency (EE) of dual-hop underlay full-duplex cognitive relay network, under the influence of interference from primary source. The OPA for the EE optimisation is constrained on the maximum transmit power of the secondary nodes and the interference threshold power which can be tolerated at the primary destination. The authors convert the initial non-convex optimisation problem to a concave fractional programming form and propose an iterative algorithm using parametric transformation to find the OPA vector. Further, they prove that the point of convergence of the proposed iterative algorithm is a Karush–Kuhn–Tucker point of the initial non-convex problem. They numerically demonstrate the EE improvement achieved by the proposed algorithms over equal power allocation strategy.