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This paper studies overlay-based cognitive radio networks (ObCRNs) where transmission from a licensed transmitter (LT) to a licensed receiver (LR) is supported by an unlicensed transmitter (UT) who has its own communication to an unlicensed receiver (UR). To improve power efficiency of UT and diversity gain at UR and LR, UT operates in direct transmission (DT) or amplify-and-forward (AF) mode or decode-and-forward (DF) mode, and UR and LR combine signals with selection combining (SC) and maximum ratio combining (MRC) from LT and UT. Explicit outage probability expressions for ObCRNs with MRC/SC and DT/AF/DF are proposed to promptly rate its performance. Numerous results reveal the efficacy of the proposed solution and its flexibly-controlled performance. [ABSTRACT FROM AUTHOR]

In this article, we compute the Secrecy Outage Probability (SOP) and the Probability of Strictly Positive Secrecy Capacity (SPSC) for Cognitive Radio Networks with Adaptive Transmit Power and energy harvesting using multiple antennas. The secondary source harvests power using multiple antennas using node A signal. Node A is a node transmitting Radio Frequency signals. Then, it adapts its power to generate interference at primary receiver P R less than T. Our analysis takes into consideration the interference between primary and secondary nodes. In fact, the Signal to Interference plus Noise Ratio is analyzed at the secondary destination in the presence of interference from the primary transmitter. We derive the SOP and SPSC when the secondary source and destination have multiple antennas. The transmission is made from the best antenna of the secondary source to the best receiving antenna at the secondary destination. [ABSTRACT FROM AUTHOR]

With exponential growth in the number of wireless devices and limited available spectrum, the problem of spectrum sharing remains in forefront of the research community. Due to the large number of wireless sensing devices, even a small percentage of savings can translate into significant spectrum efficiencies. One of the main hurdles in efficient spectrum sharing in wireless sensor networks (WSN) is the problem of power management at the node level to promote longevity without polluting the spectrum, while promoting collaboration. This problem is even more challenging in extreme (harsh) environments where access to power and battery replacement and charging is limited, if not impossible. Passive sensor technology can be used to eliminate the need for batteries, but it suffers from short communication range. Recently, wireless energy transfer (WET) for powering remote sensor nodes in a WSN has drawn considerable research attention, since it can charge sensing circuits remotely and relieve the need for battery replacement. Modeling the charging and power utilization processes can help with smart transmission decisions, which can eliminate unnecessary transmissions and not only save limited battery power at the node level, but also efficiently utilize the shared spectrum. In this paper, we consider a general relay energy assisted scenario, where a transmitter is powered by an energy source through both direct and relay links. We model data and energy channels separately, transmit energy to power the transmitter battery and schedule data transmission based on stochastic models for data. We also consider various static, mobile and highly scattered channel models. We will set a threshold on required transmission energy and channel quality to decide whether the transmission can be successful (efficient use of spectrum) or the packet may not reach the destination (polluting the spectrum unnecessarily). An energy efficient scheduling method is proposed for the system model to determine whether to transmit data or stay silent based on the stored energy level and channel state. An analytical expression has been derived to approximate outage probability of the system in terms of energy and data thresholds. All theoretical results are validated by numerical simulations and verify the effectiveness of energy relaying and proposed energy efficient scheduling method in reducing the outage probability of the system [ABSTRACT FROM AUTHOR]

In this paper, we suggest to optimize both power allocation coefficients, harvesting and sensing durations for cognitive radio networks using non orthogonal multiple access (CR-NOMA). In the first slot, secondary transmitter harvests energy from radio frequency signal received from node A. In the second slot, secondary transmitter sense the channel to check if primary user is active or not. When primary user is idle, secondary transmitter transmits data to N NOMA users in the third slot. We derive and optimize the total throughput for Rayleigh channels. [ABSTRACT FROM AUTHOR]

One of the motivations towards the 5G cellular networks and beyond is the large increase in the supported data rate in both uplink and downlink. Deployment of small size cells, massive MIMO, wherein a large number of antennas are deployed at transmitters and receiver, are effective tools in these networks. Users under cell massive MIMO networks "mMIMO" can provide high data rate. However, the cell-edge user and users, under shadowing, can have a poor performance. Cell-Free networks can provide a satisfied data rate even for shadowed and cell-edge users. In this paper, cell-Free mMIMO networks are mathematically analyzed applying different cooperation mechanisms among the access points APs. Moreover, a new mathematical formula for the throughput, based on BER, is derived. Furthermore, closed formulas for the throughput-BER as well as energy efficiency (EE) performance are derived. Finally, the cognitive radio concept is proposed in order to limit the interference among users accessing the same resources in uplink. Simulation results show that the fully centralized cell-Free mMIMO network has a higher spectral efficiency and energy efficiency than the cellular mMIMO networks at different BER values especially when MMSE is applied. Furthermore, the cognitive radio theory can increase the SE and EE performance of the cell-Free networks when all cooperation mechanisms are applied. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the problem of transceiver beamforming and power allocation in the downlink of a multiple input multiple output Cognitive radio system. In this system, a base station (BS) services multiple primary users (PU) and multiple secondary users (SU). In order to perform transceiver beamforming and power allocation, three new algorithms are proposed. The first one is an iterative single-objective constrained algorithm, in which the signal to interference plus noise (SINR) of SUs are improved at each iteration subject to constraints on quality of services of PUs (PUs SINR should be above a predefined threshold) and total transmit power of BS. The second and third algorithms, are based on a multi-objective constraint optimization problem, in which the goal is to improve both PUs and SUs SINR at each iteration subject to previous constraints of the first algorithm. The difference between the second and third algorithms is that in the joint maximization of the second algorithm, the cross-influence of each user on the others is ignored, but in the third algorithm, the cross effect of each users SINR on the others is considered. The novelty of these algorithms is the independency of the transmit precoding vectors from the receive beamforming vectors. Therefore, the feedback of receive beamforming to the BS is not needed. The performances of three proposed algorithms are evaluated in simulations vis bit error rate of PUs and SUs, SINR of PUs and SUs and the percent of BS power allocation to SUs. [ABSTRACT FROM AUTHOR]

In this work, we show how a distributed sensing network consisting of very low-cost nodes can also be used to locate radio transmitters without prior knowledge of which waveform is used. This information can aid in increasing location awareness among cognitive radios, as well as provide assistance in locating offending transmitters. The low accuracy of the internal clocks of these low-cost receivers as well as the geographical distribution of the nodes result in significant challenges regarding the synchronization of the receivers in order to position the source with adequate accuracy. In this article, we synchronize the nodes to an arbitrary modulated RF signal, after which we calculate estimated time differences of arrival to an unknown transmitter. We describe the implementation as well as give results on measurement accuracy in various scenarios using a prototype network of nodes spread out in the city of Turku, Finland. In the individual distance measurements of receiver pairs, the errors in distances vary between 30 and 900 m, depending on channel conditions. [ABSTRACT FROM AUTHOR]

For the underlay spectrum sharing, a primary transmitter (PT) and a secondary transmitter (ST) can transmit simultaneously to a primary receiver (PR) and a secondary receiver (SR), respectively. The transmit power of ST is strictly controlled under the performance constraint of primary system. The strong interference from PT and the weak signal from ST make the communication quality of secondary system very poor, especially when the two systems are close. In this work, we adopt the successive interference cancellation (SIC) technique at either or both of PR and SR for the data detection. Four spectrum sharing schemes are studied, i.e., PR–DIR–SR–DIR, PR–DIR–SR–SIC, PR–SIC–SR–DIR, PR–SIC–SR–SIC. The transmit power of ST is properly set to minimize the secondary outage probability subject to a performance-loss constraint of primary system. Numerical results show that the SIC decoding can significantly improve the spectral efficiency, especially when PR performs the SIC decoding. [ABSTRACT FROM AUTHOR]

This paper studies the achievable total throughput of an energy harvesting cooperative cognitive radio (CR) network. A CR transmitter cooperates with a primary user (PU) transmission if PU is found to be present in the given channel while it transmits its own data in the absence of PU. The CR transmitter is an energy harvesting node which harvests simultaneously from non-RF signal as well as RF signal of PU. The CR transmitter uses the harvested energy on shared basis for cooperation and transmission. The same study is also extended for cooperation of multiple CRs in multiple PU band scenario. In cooperation in muti-band scenario, all CRs sense all PU channels and sensing informations are fused at fusion centre to know about the status of PU. Performance is investigated in terms of total network throughput for several parameters such as sensing time, energy splitting parameter, and energy allocation ratio etc. Novel analytical expressions for total useful throughput and optimal energy allocation ratio parameter under the considered network scenario are developed. Useful throughput and optimal energy allocation ratio parameter are also estimated for a target secondary network (CR network) throughput under a quality of service constraint of PU such as collision probability. It is observed that the value of optimal energy allocation parameter gets reduced if the number of CRs in cooperation increases or the number of PU channels increases. [ABSTRACT FROM AUTHOR]

Cognitive radio (CR) technology is designed to improve reliability in communication between users through efficient and dynamic spectrum exploitation. CRs address the problems in spectrum allocation and channel access and improve the rate of radio resource utilization. The flexibility of the CR networks (CRN) and communication medium exposes it to a variety of threats; primary user emulation attack (PUEA) is a malicious and denial-of-service kind of adversary that defaces CRN performance. This manuscript proposes an adaptive learning-based attack detection in CRN for detecting and mitigating PUEA by analyzing the received power of the transmitter. The learning process endorses some beneficial features by distinguishing low spectrum legitimate PU from an adversary. The learning process adopts cyclostationary feature analysis for distinguishing adversaries and low power PU in CR communications. The process of learning is further enhanced by estimating distance variance and communication time-based analysis for improving the rate of signal classification and SU communication rate. The experimental analysis proves the stability of the proposed detection method by improving the SU throughput, with lesser signal classification time and misdetection probability. [ABSTRACT FROM AUTHOR]

Cybersecurity has become a major concern in the modern world. A serious threat to wireless networks is eavesdropping. On the other hand, usable spectrum is diminishing due to the presence of various wireless services. To address the problem, highly spectral efficient methods have been introduced e.g. spectrum sharing. Cognitive radio networks could sense unused spectrum and make use of it. In this paper, the cognitive radio is to perform a second task, i.e. jamming the eavesdropper. This paper is significant in revealing the cognitive radio's energy efficiency in this setting, where a cognitive transmitter (CT) can transmit its own data when it senses the absence of primary transmitter (PT). If PT is present, CT is to jam an eavesdropper (EA) by transmitting artificial noise. Our main contribution is finding the CT's optimal energy efficiency. Through the proposed formulas, we have determined the fractions of time and power required by CT to achieve the optimal energy efficiency, subject to constraints like minimum required secrecy rate Rs, etc. Our major findings are: (1) only a small portion of a time frame (66 μs in our setting) is required for sensing. CT can utilize the remaining time for sending its data or jamming EA. (2) To achieve a target energy efficiency of 0.5 bps/Hz/J, PT should not be actively transmitting more than 35% of the time, and Rs should not be larger than 1.2 bps/Hz. [ABSTRACT FROM AUTHOR]

In this paper, we consider the physical-layer security for the primary system in an underlay cognitive radio model which comprises of a primary source–destination pair, a secondary receiver (SR), K secondary transmitters (STi, i = 1,...,K) and an eavesdropper (E). To protecting the transmission confidentiality of the primary transmission system against eavesdropping, we propose secondary transmitter (ST) aided opportunistic jamming transmission protocols. In our protocols, the selection of ST plays a crucial role. To this end, we propose the optimal secondary transmission selection (OSTS) and the Optimal Cooperative Jammer Selection (OCJS), respectively. For the non-security management, OSTS, OCJS schemes, we derive the closed-form expressions of the outage probability and the intercept probability for primary system, respectively. Furthermore, we also derive the closed-form expressions of the outage probability for the secondary user for the selective schemes. Numerical results show that the OCJS scheme has the best security performance and the conventional non-security management scheme has the worst security performance for primary system. In addition, the outage performances of OSTS and OCJS are better than the conventional non-security protocol in high secondary transmitting SNR region. [ABSTRACT FROM AUTHOR]

In this paper, we propose two cooperative spectrum sharing protocols for cognitive radio networks which consist of an automatic repeat request based primary user with a primary transmitter (PT) and a primary receiver (PR), and a bidirectionally communicating secondary user (SU) with two nodes, S1 and S2. PT transmits a packet to PR by accessing the spectrum alone or by the cooperation of SU or under the interference of SU in the underlay transmission, according to the acknowledgement/negative acknowledgement feedback from PR. The two protocols differ by the application of decode-and-forward and amplify-and-forward strategies at SU during cooperation. Analytical expressions are derived for the throughputs of both users for both protocols and the results are compared with the corresponding reference protocols given in the literature. The theoretical results which are in perfect match with the simulation results show the advantages of the proposed protocols. [ABSTRACT FROM AUTHOR]

This paper proposes a novel efficient precoding algorithm to maximize the rate of the cognitive link, where a pair of multiple-input multiple-output cognitive radio users shares the spectrum allocated to multiple primary users as long as the interference power is acceptable. In our method, the precoding vectors are firstly obtained by solving an unconstrained convex problem which is formulated by using the penalty function idea and has a closed-form solution. Then, power is allocated to different precoding vectors to satisfy both the transmit-power constraint and interference-power constraints through interior method. Moreover, an effective set of the penalty weights is presented. Our method has much lower complexity than the optimal solution and similar complexity to other low-complexity methods. Simulation results show that our method significantly outperforms the existing low-complexity methods and has almost the same performance as the optimal solution. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the joint resource allocation to provide secure information transmission in a five-node cooperative cognitive radio network, which contains a primary transmitter (PT), a primary receiver (PR), a secondary transmitter (ST), a secondary receiver (SR), and an eavesdropper (E). To ensure the information is securely transmitted, PT and PR use a part of the power to transmit artificial noise (i.e., jamming signal) to confuse the eavesdropper. Specifically, in the first phase, PT transmits its signal, which contains secrecy information and artificial noise, by using all of its power and bandwidth. In the second phase, ST accesses to the PT's licensed bandwidth as a trusted relay by allocating a fraction of the bandwidth and power to forward PT's information with decode-and-forward (DF) relaying protocol. As a reward, ST can utilize the remaining bandwidth and power to transmit its own information simultaneously. We study the joint optimization of the time, bandwidth, and power allocation to maximize ST's transmission rate while satisfying PT's secrecy transmission rate requirements. Numerical results demonstrate that our strategy can achieve a win-win result. [ABSTRACT FROM AUTHOR]

This paper investigated cognitive relay networks with practical distributed switch-and-stay combining (DSSC) protocol when considering the primary transmitter's interference to secondary network. The closed-form expressions for the outage probability is derived, the correctness of the analysis is validated by Monte Carlo simulations and readily allows us to evaluate the impact of the key system parameters on the performance. The analytical results reveal that the proposed DSSC scheme has optimal switch threshold, and it is not the threshold SNR for the outage, which is true for the model without the interference from primary transmitter, and increasing the switching threshold do not always improve the system performance. The asymptotic analysis also reveals that the outage diversity only achieves one, which is also different from the traditional relay network with DSSC protocol. [ABSTRACT FROM AUTHOR]

Multihop communication is widely used in cognitive radio networks to facilitate agile access to opportunistically available spectrum resources while adequately protecting primary users (PUs). However, the performance of this collaborative communication is sensitive to spectrum sensing accuracy and the unpredictable activities of PUs. To quantify the impact of these factors, this article studies the performance of amplify and forward based incremental relaying (IR) in an opportunistic spectrum sharing environment. Using rigorous mathematical analysis, closed form expressions are derived for the average spectral efficiency and the outage probability. The derived results show that imperfect spectrum sensing and the unpredictable activities of PUs degrade the quality of the direct link and thwart the relay from assisting the destination, hence causing significant performance losses. To recover some of these losses, this article proposes a novel selective IR protocol that allows the source to retransmit whenever the relay fails to assist the destination. Hence, creating a second chance of assistance. This protocol is shown to outperform IR while not sacrificing spectral efficiency. Computer simulations are used to verify the accuracy of the derived results. [ABSTRACT FROM AUTHOR]

This paper evaluates outage performance of reactive relay selection in cognitive radio networks over Nakagami- m fading channels under consideration of imperfect channel information on both interference and transmission channels, peak transmit power constraint, long-term power constraint, interference from primary users (PUs), and direct channel between secondary source and destination. Toward this end, power allocation for secondary transmitters is first proposed to satisfy both power constraints and account for imperfect Nakagami- m channel information and interference from PUs. Then, exact closed-form outage probability representation for secondary destination is suggested to promptly assess system performance and provide useful insights into performance limits. Various results show validity of the proposed expressions, substantial system performance degradation due to imperfect channel information and interference from PUs, error floor in secondary network, performance trade-off between secondary and primary networks, and considerable performance enhancement with respect to the increase in the number of secondary relays and larger fading severity parameter. [ABSTRACT FROM AUTHOR]

Multiple input multiple out (MIMO) cognitive radio offer the spatial degree of freedom that can be used to share the spectrum with less interference via Precoding Technique. Many precoding techniques in the literature assume that the underlying hardware is ideal. In the practical case, hardware adds many impairments at both the transmitter and receiver side. Such hardware impairments degrade the performance of the MIMO system. This impairment introduces additional interference to the primary user in minimum interference precoder of MIMO cognitive radio and spoils the objective of the precoder. This paper analyzes the performance of the cognitive radio with minimum interference MIMO precoding in the presence of hardware impairment. Detecting the presence of hardware impairment and its exact variance is required to mitigate the impairment by prewhitening. In general all prewhitening work, it is assumed that the transmitter knows the presence of impairments and the exact variance of it. This work presents support vector machine (SVM) based hardware impairment detection technique on the real-time acquired data by using national instruments, vector signal generator (VSG) 5673. The VSG is configured in 2 × 2 MIMO setup to acquire the transmitter data without impairment and with injected impairment like DC offset, phase noise, I/Q gain imbalance, quadrature skew and frequency offset. After detecting the presence of hardware impairment by SVM, the variance of the data is calculated using Error Vector Magnitude (EVM) to enable prewhitening. The results show that the detection of hardware impairment can be possible with 100% of accuracy and the prewhitening scheme completely removes the impact of it. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple (NOMA) access using successive interference cancellation and cognitive radio are two promising techniques for enhancing the spectrum efficiency and utilization for future wireless communication systems. This paper presents a NOMA-based cooperative hybrid spectrum sharing protocol for cognitive radio networks. A two phase decode-and- forward (DF) relaying scheme in a multi-relay scenario is considered. Each secondary transmitter is grouped into one of the two clusters: a non-cooperative cluster (NCC) and a cooperative cluster (CC). The cluster head (CH) of the CC working as the best DF relay for the primary system is permitted to transmit its own signal superimposed on the primary signal using a NOMA approach in exchange for cooperation. On the other hand, the CH of the NCC transmits in parallel with the primary system satisfying a predefined peak transmit power and peak interference power constraints that guarantee a given primary quality of the service requirement. It is demonstrated that the performances of both the primary and secondary systems increase with the increasing number of secondary nodes. The simulation and theoretical results affirm the efficacy of the proposed protocol compared to the traditional overlay and underlay models in terms of the outage probability and the ergodic capacity. [ABSTRACT FROM AUTHOR]

We consider a cellular scenario in which a base-station primary transmitter communicates to its primary receivers (PRs) using a set of multiple antenna relay stations (RS). One of several RS that is selected is assumed to employ antenna selection. The primary network allows one of its users to access the secondary network simultaneously in the same two phases (by using a selected RS) through dynamic spectrum sharing principles. Such a framework allows the cellular system to increase its frequency utilization. We derive expressions for the outage and ergodic rate performance of such spectrum sharing communication links. Systems performing spectrum sharing are assumed to have a peak power constraint and use peak interference control (to limit interference to the PR). For a fixed primary outage, it is demonstrated that increasing the number of users improves the performance at medium values of SNR, whereas increasing the number of RSs improves the performance at high SNRs. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate the performance analysis of an underlay MIMO cognitive radios employing space time block coding at secondary's user transmitter (SU-Tx) and generalized selection combining at secondary's user receiver. The transmit power strategy enforces the SU-Tx to adapt its power to be transmitted so as to protect primary user's receiver from harmful interference in order to satisfy primary user outage constraint. We derive closed form expressions for average post processing signal to noise ratio and ergodic capacity. Moreover, approximate expression for average bit error rate is also obtained. Based on the achieved results, we show that the average post processing SNR, Ergodic Capacity and average BER improve as number of best branches to be combined increases. Finally analytical results are affirmed through Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

A pattern-based cognitive communication system (PBCCS) that optimizes non-periodic RF waveforms for security applications is proposed. PBCCS is a cross-layer approach that merges the channel encoding and modulation. The transmitter encodes sequences of bits into continuous signal patterns by selecting the proper symbol glossaries. The cognitive receiver preprocesses the received signal by extracting a limited set of wavelet features. The extracted features are fed into an artificial neural network (ANN) to recover the digital data carried by the distorted symbol. The PBCCS system offers a flexible management for robustness against a high noise level and increases the spectral efficiency. In this study, the spectral efficiency and robustness of a PBCCS scheme for an additive white Gaussian noise (AWGN) channel is investigated. The results show that at an SNR of −5 dB, a 3-bit glossary achieves a bit error rate (BER) of 10. Also, the link spectral efficiency (LSE) of the proposed system is 2.61 bps/Hz. [ABSTRACT FROM AUTHOR]

We investigate a method to set the maximum allowable transmit power for a secondary base station in dynamic spectrum sharing among secondary users and primary users. In conventional methods, location information is assumed. Thus, the maximum allowable transmit power can be set by considering the shadowing between the secondary base station and the primary user receivers to satisfy a constraint. Specifically, the probability that the interference from secondary base station exceeds the acceptable level must be less than the constraint target probability. We assume that the location information is not available at the secondary network. Instead, the secondary base station uses the received signal strength from the primary user transmitter for distance estimation. In this case, we have to consider shadowing not only between the secondary base station and the primary user receivers, but also between the primary user transmitter and the secondary receiver(s). We also need to account for the uncertainty of the distance. In order to satisfy the constraint target probability, we proposed a two-step approach to setting the maximum allowable transmit power where a transmission decision margin and a transmit power margin are utilized. To reduce these margins, we also propose cooperative maximum allowable transmit power setting method utilizing also received signal strength values from several secondary users. Simulation results confirm the validity of the analysis and show the effectiveness of the proposed cooperative maximum allowable transmit power setting method, i.e., the capacity based on cooperative maximum allowable transmit power setting method is significantly better than that of non-cooperative maximum allowable transmit power setting method. In addition, we show a proper size of radius of additional separation area to protect primary users by the numerical results. [ABSTRACT FROM AUTHOR]

For multiple-input multiple-output cognitive radio systems, we propose an optimal single transmit and receive antenna selection scheme which maximizes the signal-to-interference-and-noise ratio. Considering peak interference power constraint, peak transmit power constraint, and interference from primary transmitter to cognitive receiver, we theoretically derive the exact system outage probability. It is shown that the theoretical results match simulation results. [ABSTRACT FROM AUTHOR]

The communication efficiency of primary networks in cognitive radio depends on wireless environments, such as obstacles (e.g. buildings), distances between transmitter and receiver, and limited transmit power. A cooperative model between primary and secondary networks has the potential to overcome these problems. In this paper, we propose and analyze the performance of a decode-and-forward scheme with relay ordering for secondary spectrum access. In this scheme, a primary transmitter communicates with a primary receiver with the help of two secondary transmitters. Each secondary transmitter relays primary signals from the primary transmitter to primary receiver, and follows an optimal order to ensure the best communication capacity of the primary network and to find opportunities to transmit its own signals. The performance of primary and secondary networks is evaluated by theoretical analysis in terms of outage probability. Monte Carlo simulations are presented to verify the theoretical analysis and to compare the performance of the proposed protocol with that of a direct transmission protocol and a decode-and-forward protocol with a relay selection scheme. [ABSTRACT FROM AUTHOR]

In this paper, considering the interferences from primary transmitter to secondary receiver and from secondary transmitter to primary receiver, we derive the upper and lower bounds of outage probability for underlay cognitive opportunistic multi-relay networks. Theoretical and simulation results show the upper and lower bounds converge to the exact outage probability at high interference-to-noise (INR) region. Because the interference from primary transmitter to secondary receiver is considered, an outage floor at high signal-to-noise ratio (SNR) occurs when INR increases proportionally with SNR. [ABSTRACT FROM AUTHOR]

Cognitive radio requires that a secondary user (SU) stops its transmission as soon as possible upon the arrival of a primary user (PU). In this paper, we propose a novel approach to quickly detect PUs while an SU transmission is in progress. The proposed method requires an SU transmitter to use OFDM to insert periodic zero-energy intervals in a selected subcarrier and to detect energy during each interval for PU detection. The length and periodicity of these intervals determine the performance of PU detection. We also discuss how to determine the energy detection threshold. [ABSTRACT FROM AUTHOR]