Overlay Cognitive Radio Networks Research Articles

Cognitive Multi-Hop Multi-Branch Relaying: Spectrum Leasing and Optimal Power Allocation

In this paper, for an overlay cognitive radio network in which the primary user’s transmission is assisted by a cognitive multi-hop multi-branch network, we propose and analyze a spectrum leasing scheme that achieves full diversity in both the primary and secondary networks. Among the available branches, the best one is selected to establish a multi-hop relay link between the primary users. In return, the secondary users of the selected branch are rewarded with a fraction of spectrum to transmit their own signals over a nonorthogonal multiple-access channel. We jointly optimize the branch selection and the spectrum allocation such that the sum-rate of the secondary users is maximized under the constraint that a minimum quality-of-service (QoS) requirement in terms of the achievable rate is satisfied in the primary network. We also optimize the power allocation to the secondary users such that the asymptotic outage probability of the primary network is minimized. The analytical results show that this spectrum leasing scheme guarantees the full diversity of order $M$ in both the primary and secondary networks, where $M$ is the number of cognitive branches. The simulation results also show that optimizing the allocation of power significantly decreases the risk of spectrum leasing failure.

Wireless Information and Power Transfer for IoT Applications in Overlay Cognitive Radio Networks

This paper proposes and investigates an overlay spectrum sharing system in conjunction with the simultaneous wireless information and power transfer (SWIPT) to enable communications for the Internet of Things (IoT) applications. Considered is a cooperative cognitive radio network, where two IoT devices (IoDs) exchange their information and also provide relay assistance to a pair of primary users (PUs). Different from most existing works, in this paper, both IoDs can harvest energy from the radio-frequency (RF) signals received from the PUs. By utilizing the harvested energy, they provide relay cooperation to PUs and realize their own communications. For harvesting energy, a time-switching (TS) based approach is adopted at both IoDs. With the proposed scheme, one round of bidirectional information exchange for both primary and IoT systems is performed in four phases, i.e., one energy harvesting (EH) phase and three information processing (IP) phases. Both IoDs rely on the decode-and-forward operation to facilitate relaying, whereas the PUs employ selection combining (SC) technique. For investigating the performance of the considered network, this paper first provides exact expressions of user outage probability (OP) for the primary and IoT systems under Nakagami-m fading. Then, by utilizing the expressions of user OP, the system throughput and energy efficiency are quantified together with the average end-to-end transmission time. Numerical and simulation results are provided to give useful insights into the system behavior and to highlight the impact of various system/channel parameters.

Energy-Efficiency Optimization of UAV-Based Cognitive Radio System

Unmanned aerial vehicles (UAVs) equipped with data transmission and sensing facilities are gaining more popularity in different applications due to its miniaturization and mobility. In this paper, a UAV-based overlay cognitive radio (CR) network is investigated in which the UAV is used as a secondary user (SU). This paper proposes an efficient energy management solution to improve the performance of the UAV. When SUs opportunistically utilize the licensed spectrum of the primary network, spectrum sensing is needed to determine whether to transmit data or not, so the sensing time and secondary transmission power should be jointly optimized. We formulate this non-convex optimization problem subject to multiple constraints, which seeks to investigate on the effect of the sensing time and transmission power on the performance of the system. The problem is difficult to tackle, then we propose an algorithm applying the techniques of alternating optimization and dichotomy method. In addition, we compare the proposed algorithm with the particle swarm optimization (PSO) algorithm to verify its performance. Numerical results show that our proposed algorithm outperforms the PSO algorithm and significantly enhances the energy efficiency of the UAV-based CR system.

Physical Layer Security in Overlay Cognitive Radio Networks With Energy Harvesting

In this correspondence paper, we investigate the security and efficiency of data transmission for overlay cognitive radio networks, which consist of a pair of primary users, multiple pairs of energy-limited secondary users and multiple eavesdroppers. Specifically, to achieve a better tradeoff between the security of primary transmission and the efficiency of secondary transmission, we propose an optimal relay selection (ORS) based on two energy harvesting (EH) protocols, i.e., the time switching relay (TSR) protocol and power splitting relay (PSR) protocol. Building in this, the exact closed-form expressions for the secrecy outage probability (SOP) of primary transmission and ergodic rate of secondary transmission are derived with the two EH protocols, respectively. Moreover, to deeply analyze the secrecy performance, two asymptotic expressions for SOP of primary transmission are also derived under high main-to-eavesdropper ratio and the infinity number of secondary user conditions, respectively. Through the simulation analysis, we find that the ORS based on EH protocols can achieve better performance than that of ORS without EH protocol in terms of tradeoff. Moreover, PSR protocol is superior to TSR protocol.

Spectrally-Constrained Sequences: Bounds and Constructions

We investigate spectrally-constrained sequences (SCSs), which are applicable to the communication and radar systems operating over non-contiguous carriers or frequency slots. Typical examples of such systems are overlay cognitive radio or cognitive radar networks. First, we derive the periodic- and aperiodic-correlation lower bounds for single-channel SCSs and multi-channel SCSs by convex optimization in the frequency domain. Each of these bounds reduces to a Welch bound when the number of forbidden carriers is set to zero. We then propose systematic constructions of optimal unimodular single-channel SCSs with the aid of cyclic difference sets and the theory of maximal-length shift register sequences.

Enhanced Spectrum Sharing and Cognitive Radio Using Asynchronous Primary and Secondary Users

We show that timing mismatch between the primary and the secondary users results in improving the cognitive radio networks (CRNs) performance. We perform oversampling to exploit the advantages of asynchrony in CRNs. We design detectors for asynchronous transmission to boost the signal received by the primary user in the selfless overlay CRN paradigm. Accordingly, more power is conserved for the secondary user transmission without the performance of the primary user or spectrum license holder being degraded. Simulation results present a green CRN with an almost 6-dB power gain compared with the current CRN frameworks.

QoS‐driven power and time allocation scheme for spectrum leasing in overlay cognitive radio networks

The coexistence of a primary user (PU) with a secondary user (SU) is studied in an overlay cognitive radio network (CRN) which is proposed as an efficient technique to treat with the problem of spectrum underutilisation. A cooperative spectrum leasing process under the decode and forward protocol is considered among the PU and SU. Assuming users' specific Quality of Service requirements, a joint power and time allocation scheme for maximising the PU's cooperative effective capacity subject to minimum SU's and PU's effective capacities and maximum SU's instantaneous power transmission constraints is proposed. Its performance is close to the optimal one, that is computed using the exhaustive search scheme. A two-step solution is applied considering the fundamentals of convex optimisation theory and closed-form power and time expressions are extracted. As simulation results prove, the proposed scheme outperforms a less sophisticated scheme considering power allocation under constant time. Finally, the proposed algorithm is incorporated in a multi-user CRN scenario where efficient matchings among the PUs and the SUs are investigated targeting to the increase of PUs' cooperative performance applying the matching theory.

Secondary users selection and sparse narrow-band interference mitigation in cognitive radio networks

Spectrum scarcity is a critical problem that may reduce the effectiveness of wireless technologies and services. To address this problem, different spectrum management techniques have been proposed in the literature such as overlay cognitive radio (CR) where the unlicensed users can share the same spectrum with the licensed users. The main challenges in overlay CR networks are the identification and detection of the Primary User (PU) signals in a multi-source narrow-band interference (NBI) scenario. Therefore, in this paper, we investigate the performance of an orthogonal frequency division multiplexing (OFDM) overlay CR network with Secondary Users (SUs) and subcarriers selection schemes. Three approaches for SUs and subcarriers Selection named Direct, Distributed and Incremental selection techniques are proposed in this paper to increase the expected signal to interference and noise ratio based on full or partial knowledge of the channel state information (CSI). We also show that Distributed selection techniques provide all the SUs equal chances to be selected without affecting the selection diversity gain. General as well as simplified outage probability expressions are derived and extensive simulations are conducted to evaluate the performance of the proposed techniques and support the theoretical derivations. To accommodate more SUs, a new approach for asynchronous NBI estimation and mitigation in CR networks is investigated. Without any prior knowledge of the NBI characteristics and based on sparse signal recovery theory, the proposed approach allows the PU to exploit the sparsity of the SUs interference to recover it and approach the interference-free limit over practical ranges of NBI power levels.

Cooperation-based multi-hop routing protocol for cognitive radio networks

In the scope of cognitive radio networks, typical routing protocols avoid areas that are highly congested with primary users, leaving only a small fragment of available links for secondary route construction. In addition, wireless links are prone to channel impairments such as multipath fading which renders the quality of the available links highly fluctuating. In this paper, we propose Undercover: a multi-hop routing protocol for cognitive radio networks in which we integrate the collaborative beamforming technique with layer 3 routing. Specifically, our protocol revisits a fundamental assumption taken by the state of the art routing protocols designed for overlay cognitive radio networks; this assumption is that secondary users cannot use the spectrum when primary users are using it. In Undercover, we allow a group of secondary users, each with a single antenna, to collaborate together and transmit in the regions of primary users activity. This is done through nulling out transmission at primary receivers via beamforming. Moreover, Undercover is designed to enhance the transmission quality at the secondary destinations whenever possible. To account for the excessive levels of interference typically incurred due to cooperative transmissions, we allow our protocol to be interference-aware. Thus, cooperative transmissions are penalized in accordance with the amount of negatively affected secondary flows. We evaluate the performance of our proposed protocol via NS2 simulations which show that our protocol can enhance the network goodput by a ratio reaches up to 250% compared to other state-of-art cognitive routing protocols with minimal added overhead.

Non-orthogonal multiple access protocol for overlay cognitive radio networks using spatial modulation and antenna selection

In this paper we propose a novel spectrum sharing protocol for overlay cognitive radio networks using non-orthogonal multiple access (NOMA), spatial modulation (SM) and antenna selection (AS). The proposed protocol allows a secondary transmitter (ST) to transmit simultaneously to both a primary receiver (PR) and a secondary receiver (SR) using SM. The usage of NOMA and SM will increase the spectral efficiency for both PR and SR with reduced detection complexity than the case without NOMA in which the detectors are required to jointly detect both SM symbols at each receiver. The application of AS at ST with regards to PR provides higher quality transmission for PR without affecting the performance of SR. The performance of the proposed protocol is investigated by derivations of upper bounds on the average symbol error probabilities at PR and SR and by Monte Carlo simulations. Analytical and simulation results show that the proposed protocol offers efficient spectrum utilization over spectrum sharing protocols proposed recently that uses SM – to convey the primary data to PR through the amplitude phase modulation technique and the secondary data to SR through the index of the active antenna.

A network steganographic approach to overlay cognitive radio systems utilizing systematic coding

Network steganography is an information hiding technique that utilizes network protocols to facilitate hidden communication. The aim is to embed secret information bits into regular network traffic (as a carrier) so that confidential data can be transmitted covertly between two communicating parties. In this paper, the concept of network steganography is extended for overlay cognitive radio networks utilizing cooperative relaying protocols with systematic channel codes. In the considered model, the cognitive node relays the primary data according to a time-slotted decode-and-forward cooperative protocol with maximum-ratio combining (MRC) at the primary destination. We propose a steganographic approach in which the cognitive node embeds confidential cognitive data at the wavelet transform domain into primary data code words. In this approach, the embedding operation is designed to preserve the statistical properties of the cover data in terms of decoded BER after MRC decoding of code words at the destination node which includes both the effect of wireless channel errors and errors due to embedding distortion. From this point of view, the proposed scheme may be seen as a lossless steganography scheme that embeds secret data into coded data strings. We provided implementation examples using both systematic BCH and turbo channel coding. The results show that the proposed scheme can provide diversity gain using stego-based MRC at the primary receiver and covert confidential cognitive data communications simultaneously.

Performance Analysis of a Cognitive Radio Network With an Energy Harvesting Secondary Transmitter Under Nakagami- ${m}$ Fading

In this paper, we consider an overlay cognitive radio network, in which a secondary transmitter (ST) is willing to relay the information of a primary transmitter toward a primary receiver. In return, ST can access the licensed band to send its own information superimposed with the primary signal to a secondary receiver. The power-limited ST uses a power splitting protocol to harvest energy from its received signal to increase its transmit power. We analyze the performance of the primary and the secondary systems under independent Nakagami-m fading by deriving their corresponding outage probabilities in integral-based expressions. In addition, by considering the high signal-to-noise ratio, we obtain very tight closed-form approximations of the outage probabilities. Thereafter, by further analyzing the approximations, we reveal novel insights on the diversity orders and coding gains of the two systems. Our analytical results are validated through extensive Monte-Carlo simulations.

Joint Power and Time Allocation Scheme with QoS Constraints in Overlay Multi-user Cognitive Radio Networks

This paper studies the case of an overlay cognitive radio network where the primary user leases spectral resources to the secondary user in exchange for cooperation, considering that both type of users have specific quality of service requirements. We investigate the problem of joint power and time allocation for the secondary access during the cooperative phase, with a view to optimizing the effective capacity of the primary user given an average energy constraint for the secondary user. Afterwards, the optimal power allocation of the secondary user for its own transmission phase is investigated in order to maximize the effective capacity of the secondary link. The proposed joint power and time allocation mechanism is compared with an optimal time/constant power allocation scheme and a less sophisticated baseline allocation scheme, i.e. power allocation under constant time and its superiority is proven for various network parameters. The reference model of one primary---one secondary user is extended to a general multi user cognitive radio network through the proposed pairing mechanism based on matching theory. Particularly, considering the remarks of the reference scenario, we propose two different matching schemes (with/without consideration of primary users' quality of service requirements) and we confirm their superiority compared to other matching mechanisms.

Quality of Service Analysis for the Real-Time Secondary Users in Cognitive Radio Cellular Networks

Cognitive radio technology can improve the spectrum utilization in wireless networks by providing the capability for the secondary users to opportunistically use the frequency spectrum which is originally dedicated to the licensed users. In this paper the quality of service parameters for the real-time (RT) secondary users are analyzed in an overlay cognitive radio cellular network. RT packets are delay sensitive and can tolerate a limited amount of waiting time. Otherwise they will be useless and hence will be dropped. On the other hand, the availability of the frequency channels for SUs in cognitive radio networks (CRNs) depends on the activity of licensed users. Therefore, the support of RT services in CRNs requires elaborate considerations in various functionalities of the network. Here a practical cellular structure for the network is considered. Furthermore, a packet scheduling scheme is proposed and an analytical framework is developed for the performance analysis of RT secondary users. The effect of errors of spectrum sensing is also taken into account in the performed analysis. The analytical results are verified by simulations. The obtained results are very helpful for the support of RT traffic in cellular CRNs.

Coverage probability in cognitive radio networks powered by renewable energy with primary transmitter assisted protocol

This paper studies the opportunistic spectrum access (OSA) of the secondary users in a large-scale overlay cognitive radio (CR) network powered by renewable energy to improve both the energy and spectral efficiency of data transmission. Particularly, with energy harvesting module and energy storage module, the primary transmitters (PTs) and secondary transmitters (STs) are assumed to be able to collect ambient renewables and store them in batteries for future use. Upon harvesting sufficient energy, the corresponding PTs and STs (denoted by eligible PTs and STs) are then allowed to access the spectrum according to their respective medium access control (MAC) protocols. For the primary network, an Aloha type of MAC protocol is considered, under which the eligible PTs make independent decisions to access the spectrum with probability ρp. For the secondary network, a threshold-based Opportunistic spectrum access (OSA) scheme, namely the primary transmitter assisted (PTA) protocol, is investigated, under which an eligible ST is allowed to access the spectrum only if the maximum signal power of the received pilots sent from the active primary transmitters (PTs) is lower than a certain threshold Nta. By applying tools from random walk theory and stochastic geometry, and assuming infinite battery capacity for energy storage module, we characterize the transmission probabilities of PTs and STs, respectively. With the obtained results of transmission probability, we then evaluate the coverage (transmission non-outage) performance of the overlay CR network powered by renewable energy. Simulations are provided to validate our analysis.

Performance Analysis of Spatial Modulation in Overlay Cognitive Radio Communications

We study the application of spatial modulation (SM) in overlay cognitive radio (CR) networks, in which the primary and secondary networks work concurrently over the same spectrum band. We assume that the direct link between the primary transmitter and receiver is not available, i.e. broken, and that the CR transmitter assists the primary network as a relay to amplify-and-forward the transmitted symbols of the primary. SM is used in the secondary to split the transmission space into two amplitude-phase modulation (APM) and spatial domains. In the proposed scheme, the secondary transmitter retransmits the primary symbols in APM domain, while its own information is transmitted by the index of transmitting antenna, similar to space shift keying, without causing any interference to the primary receiver. We analyze the performance of the optimal detectors at both the primary and secondary receivers for the case of phase shift keying modulation in terms of the average symbol error rate (ASER). We also study the asymptotic behavior of the ASER at both the primary and secondary at high signal-to-noise ratios. Simulation results show that the SM can be effectively used in overlay CR systems.

TCP over scarce transmission opportunity in cognitive radio networks

Transmission control protocol (TCP) is the most popular transport layer protocol for applications that require reliable and ordered data delivery essentially. In this paper we consider the deployment of TCP to secondary users (SUs) in overlay cognitive radio networks (CRNs), and address its performance degradation; in CRNs, SU’s transmissions are frequently disrupted by the detection of primary user’s transmission, and which makes the SU experience consecutive retransmission-timeout and its exponential backoff. Subsequently, the TCP in SU does not proceed with the transmission even after the disruption is over or the SU hands over to other idle spectrum. To tackle this problem, we propose a cross-layer approach called TCP-Freeze-CR; lower layer protocols send the overlying TCP two different cross-layer signals, freeze on the detection of primary user’s transmission, and unfreeze after handing over to an idle spectrum. Moreover we consider a practical situation where either secondary transmitter (ST) or secondary receiver (SR) detects primary user’s transmission; therefore additional message exchanges are needed between ST and SR to retrieve and resynchronize to other idle spectrum, i.e., spectrum synchronization. This situation is more complex than the case where both ST and SR detect primary user’s transmission. Hereby, we develop a spectrum synchronization procedure coupled with TCP-Freeze-CR into a finite state machine. All of our proposals are implemented and evaluated on a real CRN consisting of 6 software radio platforms. In the implementation, we deploy 802.15.4 implementation as a target physical layer protocol, and couple it with TCP-Freeze-CR using Unix Domain Socket. The experimental results illustrate that standard TCP suffers from significant performance degradation in CRNs, and show that TCP-Freeze-CR can greatly alleviate the degradation; e.g., for 1200 s, ST with TCP-Freeze-CR can send about 10 times more packets than ST with standard TCP.

Energy–Spectral Efficiency Tradeoff in Cognitive Radio Networks

In this paper, we propose a general framework to evaluate the tradeoff between energy efficiency (EE) and spectral efficiency (SE) in cognitive radio networks (CRNs). The proposed framework is discussed in three typical CRN paradigms: underlay CRNs (UCRNs), overlay CRNs (OCRNs), and interweave CRNs (ICRNs). The EE–SE relation for three CRNs is expressed in the closed-form formulation, in which the optimal (suboptimal) EE solution as the function of SE is deduced with the corresponding limits. The theoretical analysis and numerical results indicate that the EE–SE relation in CRNs is not contrary, i.e., an optimal EE–SE tradeoff can be achieved. The proposed framework provides a useful guidance in the design of practical green CRNs.

Performance analysis and power allocation strategy in single-antenna cognitive overlay systems

A power allocation strategy is proposed for single-antenna overlay cognitive radio networks, in which the secondary user helps transmit the signal of the primary user while concurrently conveying its own signal by means of a superposition coding technique. The study commences by deriving analytical expressions for the bit error rates (BERs) of the primary and secondary users. A power allocation strategy is then proposed for minimizing the total power consumption of the two users while simultaneously satisfying their respective BER constraints. The analytical BER formulas are not convex, and hence the optimization process presents a significant challenge. Accordingly, two more tractable BER approximations for the primary and secondary users are proposed to transfer the non-convex problem into a convex one. The simulation results confirm the effectiveness of the proposed power allocation strategy under various channel environments.

Filter‐and‐forward transceiver design for cognitive two‐way relay networks

In this study a new approach to filter-and-forward (FF) relay design in an overlay cognitive radio (CR) network with frequency selective channels is proposed. In this system, to implement two-way relaying scheme, a secondary multi-antenna node acts as a relay and helps two primary users (PUs) communicate with each other and in return the secondary node superposes its signal to filtered signals of PUs. The authors intend to design secondary transceiver under two criteria, one from the viewpoint of improving the performance of secondary network and the other to guarantee quality of service (QoS) of primary network. The first criterion is to maximise the signal-to-interference-plus-noise ratio (SINR) of secondary receiver subject to constraints on QoS of PUs and the relay transmitted power. For the other one, the goal is to achieve max-min fairness among PUs through maximising the worse signal-to-interference-plus-noise ratio (SINR) of PUs subject to constraints on QoS of secondary receiver and transmit power of relay. The authors employ an alternating optimisation algorithm to find the beamforming matrices and the allocated power to secondary receiver's message in these non-convex problems. Simulation results show that the proposed FF approach has better performance compared with simple AF relaying strategy.