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Abstract: Recently, there has been growing interest in opportunistically utilizing the 2.4 GHz ISM-band. Numerous spectrum occupancy measurements covering the ISM-band have been performed to analyze the spectrum usage. However, in these campaigns the verification of the correctness of the obtained occupancy values for the highly dynamic ISM-band has not been presented. In this paper, we propose and verify channel occupancy rate (COR) estimation utilizing energy detection mechanism with a novel adaptive energy detection threshold setting method. The results are compared with the true reference COR values. Several different types of verification measurements showed that our setup can estimate the COR values of 802.11 traffic well, with negligible overestimation. The results from real-time real-life measurements also confirm that the proposed adaptive threshold setting method enables accurate thresholds even in the situations where multiple interferers are present in the received signal.
Notes: Copyright IEICE 2012. Please check also IEICE's other publications in IEICE Transactions Online top page http://search.ieice.org/
Abstract: We design an efficient sensing order selection strategy for a distributed cognitive radio (CR) network, where two or more autonomous CRs sense the channels sequentially (in some sensing order) for spectrum opportunities. We are particularly interested in the case where CRs with false alarms autonomously select the sensing orders in which they visit channels, without coordination from a centralized entity. We propose an adaptive persistent sensing order selection strategy and show that this strategy converges and reduces the likelihood of collisions among the autonomous CRs as compared to a random selection of sensing orders. We also show that, when the number of CRs is less than or equal to the number of channels, the proposed strategy enables the CRs to converge to collision-free channel sensing orders. The proposed adaptive persistent strategy also reduces the expected time of arrival at collision-free sensing orders as compared to the randomize after every collision strategy, in which a CR, upon colliding, randomly selects a new sensing order.
Abstract: Although establishing cooperation in a wireless network is a dynamic process, most game theoretic coalition formation models proposed in the literature are static. We analyze a dynamic coalition formation game based on a Markovian model for the spectrum sharing problem in an interference channel. Our model is dynamic in the sense that distributed transmitter-receiver pairs, with partial channel knowledge, reach stable coalition structures (CSs) through a time-evolving sequence of steps. Depending on an interference environment, we show that the game process either converges to the absorbing state of the grand coalition or to the absorbing state of internal and external stability. We also show that, due to myopic links, it is possible that the core of the game is non-empty, but links cannot form the grand coalition to utilize the core rate allocations. We then formulate a condition for the formation of the stable grand coalition. Using simulation we show that coalition formation yields significant gains in terms of average rates per link for different network sizes. We also show average maximum coalition sizes for different distances between the transmitters and their own receivers. Finally, we analyze the mean and variance of the time for the game to reach the stable coalition structures.
Abstract: The localization algorithm based on the double-thresholding (LAD) method was originally proposed for detecting and localizing narrowband (NB) signals with respect to the search bandwidth. Its weakness is that the localized signal is often split into several parts, especially when the signal-to-noise ratio (SNR) is low. This may lead to the illusion of unoccupied frequencies in the middle of the signals. In this paper, an extension of the LAD method, namely the two-dimensional LAD (2-D LAD), is proposed to solve that problem. In addition to offering low computational complexity, the proposed method is able to operate at lower SNR values than the original 1-D LAD method.
Notes: Copyright IEICE 2010. Please check also IEICE's other publications in IEICE Transactions Online top page http://search.ieice.org/
Abstract: We formulate the problem of distributed throughput-efficient sensing in cognitive radio (CR) networks as a dynamic coalition formation game based on a Markovian model. The proposed coalition formation enables the CRs to increase their achievable throughput, under the detection probability constraint, while also taking into account the overhead in sensing reports combining. The dynamic model of coalition formation is used to express and model the behavior of the coalition forming CRs over time. In the proposed game, CRs form coalitions either to increase their individual gains (selfish coalition formation) or to maximize the overall gains of the group (altruistic coalition formation). We show that the proposed coalition formation solutions yield significant gains in terms of reduced average false alarm probability and increased average throughput per CR as compared to the non-cooperative solutions. Given a target detection probability for a coalition, we adopt a weighted target detection probability for individual CRs in a coalition. We find that the weighted target detection probability for individual CRs results in increased average throughput per CR as compared to when each CR is assigned the same target detection probability in a coalition.
Abstract: In cooperative spectrum sensing, information from several cognitive radios (CRs) is used for detecting the primary user. To reduce sensing overhead and total energy consumption, it is recommended to cooperate only with the CRs that have the best detection performance. However, the problem is that it is not known a priori which of the CRs have the best detection performance. In this letter, we are proposing three methods for selecting the CRs with the best detection performance based only on hard (binary) local decisions from the CRs. Simulations are used to evaluate and compare the methods. The results indicate that the proposed CR selection methods are able to offer significant gains in terms of system performance.
Abstract: In direct communication, terminals that are close to each other can communicate directly without traffic going through centralized controller such as a base station (BS). This brings several advantages. We study direct communication with localized distribution, so that users tend to gather around some areas (clusters/hot-spots) within the cell such as buildings. Previous analysis about clustering has focused on one dimensional scenarios. Here we present theoretical analysis of direct communication with two dimensional clustering. Additional analysis is presented for direct communication with correlated clusters. With correlated clusters some pairs of source and destination clusters are more probable than other pairs. According to our best knowledge, this is the first time that theoretical analysis is presented about clustering and correlated clusters in two dimensional scenarios. Simulations confirm the validity of the analysis. In addition to the exact results, we also suggest using the point-based approximation to rapidly and easily obtain results. The numerical results show that the gains from direct communication, in terms of blocking probability and carried traffic, depend on the offered traffic. Additionally, correlation in cluster selection is shown to significantly improve performance. Point-based approximation is shown to be very useful when the number of clusters is large.
Notes: Copyright IEICE 2009. Please check also IEICE's other publications in IEICE Transactions Online top page http://search.ieice.org/
Abstract: A localization algorithm based on double-thresholding (LAD) has been recently proposed for localizing concentrated signal(s) in the frequency or time domain. The LAD method blindly detects and separates concentrated signals and estimates their characteristics. This letter presents a theoretical performance analysis of the LAD method and an enhancement of the LAD method: the LAD with adjacent cluster combining (ACC). The good detection performance of the LAD ACC method is confirmed both theoretically and numerically.
Abstract: Separation or classification of signal-present samples from noise-only samples is studied. The false-alarm probability implies how many noise-only samples are wrongly classified as outliers, and typically it should be smaller than some upper limit. The noise distribution parameters are not known a priori and have to be estimated. Multiple outliers have a strong influence to that estimation and may lead to uncontrollable false-alarm probability. The false-alarm probability control can be improved by robust estimators and/or by forward-detection methods. In this article, the false-alarm probability of the forward methods is analyzed. The forward consecutive mean excision (FCME) algorithm is enhanced to allow better false-alarm control. It is proposed that the forward method using the cell-averaging (CA) constant false-alarm rate (CFAR) technique can be applied for locating the outliers. The results show that its false-alarm probability stays close to the required value even in the presence of multiple outliers.
Abstract: We analyze the impact of a uniform quantizer on the false-alarm probability of a total power radiometer. Different possibilities to set the detection threshold are discussed. The main emphasis is on methods that use the estimated noise level. In particular, we analyze the cell-averaging (CA) constant false-alarm rate threshold setting strategy. The numerical results show that the CA strategy offers the desired false-alarm probability.
Abstract: The paper presents a novel performance analysis of a frequency sweeping channelized radiometer when the signal to be detected is a frequency hopping signal. Often technology limits the instantaneous bandwidth that can be used. When frequency sweeping (search strategy) is used, the center (carrier) frequency of the receiver is changed rapidly to increase the probability of intercept (POI). Conventional logical OR, sum and maximum based methods are used to combine the channelized radiometer outputs. Exact results are derived for the sum based channelized radiometer. A novel accurate approximation of the performance of the sweeping maximum based channelized radiometer is presented. An efficient method is presented for accurately calculating the likelihood ratio used in optimal detection. The effects of fading are analyzed. Numerical results show that although sweeping increases POI, the final probability of detection is not increased if the number of hops observed is large. When the number of hops observed is small, sweeping can increase performance in the case of fading channel.
Notes: This the extended journal version of the previous conference paper "Detection of Frequency Hopping Signals With a Sweeping Channelized Radiometer" (ASILOMAR 2004). For the final published version, please follow the DOI.
Abstract: The channelized radiometer is a well-known intercept receiver. We analyze its performance when different constant false alarm rate (CFAR) strategies are used to set a detection threshold based on values of some reference cells. The studied conventional methods are cell-averaging (CA) and order statistics. The proposed iterative methods for setting the detection threshold are forward consecutive mean excision (FCME) with the CA scaling factors in (final) detection decision (FCME+CA), backward consecutive mean excision (BCME) with the CA scaling factors in detection (BCME+CA) and a method that uses the CA scaling factors in both censoring and detection (CA+CA). The results show that iterative CFAR methods can improve detection performance compared to the baseline methods.
Abstract: Efficient decision fusion technique for cooperative spectrum sensing based on the logical OR-rule is considered for dynamic spectrum sharing between primary users (PUs) and secondary users (SUs). Due to the use of the OR-rule, the fusion center (FC) only needs to know if any of the local decisions was "1". Therefore, it can be a waste of resources to communicate each local decision in an orthogonal channel. To reduce the resource consumption, we consider that the SUs that had local decision "1" transmit simultaneously in the same channel using continuous wave (CW) signaling without a phase synchronization and that the FC will declare the PU is present if it detects the summed CWs. In this signaling, even with very large channel gain to noise power ratio (CNR) between the SUs and the FC the system may not reach error-free signaling due to destructive interference in the summed CWs. To address this problem, we propose power allocation to reduce the effects of the destructive interference, as with this technique it is possible to guarantee that the CWs are not going to fully cancel each other. The proposed method is very simple and although it is suboptimal, it could provide good detection performance. Numerical results show that it is possible to practically reach the upper bound with error-free signaling by the proposal, even with moderate CNR values.
Abstract: This paper presents queueing analysis of opportunistic access in cognitive radios. The primary (licensed user) has priority over the secondary user and it does not need to care about the secondary user transmissions. A time slotted system is assumed, so that the secondary user can perform spectrum sensing at the beginning of the slot to know if it is occupied by primary or not. If the slot is free, it can be utilized for secondary transmissions. This leads to no interference with primary user communication, assuming perfect sensing. Finding waiting time and queue length of this type of system has not, according to our best knowledge, been performed before. We perform theoretical analysis by applying M/D/1 priority queueing scheme. The results were used to evaluate the performance of the cognitive network. Simulation are used to validate the results, and simulation results demonstrate a high degree of accuracy for the derived expressions. Results indicate that the performance of the secondary user depends on the data traffic characteristics of the primary user, and under high arrival rate for the primary, the average waiting and average queueing length of the secondary user grow especially when the combined arrival rate approach the queue utilization factor.
Abstract: Congested radio frequencies call for efficient and flexible spectrum use and, hence, spectrum sensing. As cognitive radios need channel information for achieving better use of radio frequencies, military systems require information about unknown signals. In this paper, a novel method for computing signal-to-noise ratios (SNR) of several unknown narrowband signals without a priori information is proposed. The presented frequency domain method is an extension of the localization algorithm based on double-thresholding (LAD). The main point is to produce fast and cost-efficient estimators with adequate accuracy. The proposed method is verified via computer simulations and tested also with real-life radio channel measurement data. The results verify that the proposed method gives sufficient approximations of SNR values with low overall computational complexity.
Abstract: New technologies will require effective spectrum use. Opportunistic spectrum usage that is one application of so called cognitive radio techniques enables the use of unused frequencies. One possible way to locate these free frequency bands is to use so called spectrum sensing. In this paper, energy detection based spectrum sensing methods called the forward consecutive mean excision (FCME) and forward cell averaging (CA) methods are studied in the situations where the noise power is unknown. The detection and false alarm probabilities of the studied methods are also of interest. Numerical results show that the investigated approaches have good performance.
Abstract: A localization algorithm based on double-thresholding (LAD) is a computationally simple method for localizing narrowband signals in the frequency domain. The method does not need any a priori information about the narrowband signal. The localization is based on two thresholds. The lower threshold is used to compose adjacent signal samples into clusters whereas the upper threshold is used to detect signals. The LAD can be applied in narrowband signal detection as well as in interference suppression. The simulation results show that the LAD gives quite good localization accuracy and the LAD is able to determine correct number of narrowband signals even over 95% of the cases.
Abstract: A method to set the threshold of a power-law detector that uses a nonorthogonal transform is presented. The mean, variance and skewness of the decision variable in the noise-only case are derived and these are used to find a shifted log-normal approximation to the distribution of the decision variable. The accuracy of this method is verified through simulations. Simulation results are presented for the case where the windowed fast Fourier transform (FFT) is used. The results show that the performance loss associated with
windowing (windowing loss) is 0.6-1.7 dB depending on the parameters of the power-law detector and the signal to be detected.
Abstract: The focus of this thesis is on the binary signal detection problem, i.e., if a signal or signals are present or not. Depending on the application, the signal to be detected can be either unknown or known. The detection is based on some function of the received samples which is compared to a threshold. If the threshold is exceeded, it is decided that signal(s) is (are) present. Energy detectors (radiometers) are often used due to their simplicity and good performance. The main goal here is to develop and analyze energy based detectors as well as power-law based detectors.
Different possibilities for setting the detection threshold for a quantized total power radiometer are analyzed. The main emphasis is on methods that use reference samples. In particular, the cell-averaging (CA) constant false alarm rate (CFAR) threshold setting method is analyzed. Numerical examples show that the CA strategy offers the desired false alarm probability, whereas a more conventional strategy gives too high values, especially with a small number of reference samples.
New performance analysis of a frequency sweeping channelized radiometer is presented. The total power radiometer outputs from different frequencies are combined using logical-OR, sum and maximum operations. An efficient method is presented for accurately calculating the likelihood ratio used in the optimal detection. Also the effects of fading are analyzed. Numerical results show that although sweeping increases probability of intercept (POI), the final probability of detection is not increased if the number of observed hops is large.
The performance of a channelized radiometer is studied when different CFAR strategies are used to set the detection threshold. The proposed iterative methods for setting the detection threshold are the forward consecutive mean excision (FCME) method with the CA scaling factors in final detection decision (FCME+CA), the backward consecutive mean excision (BCME) method with the CA scaling factors in detection (BCME+CA) and a method that uses the CA scaling factors for both censoring and detection (CA+CA). Numerical results show that iterative CFAR methods may improve detection performance compared to baseline methods.
Finally, a method to set the threshold of a power-law detector that uses a nonorthogonal transform is presented. The mean, variance and skewness of the decision variable in the noise-only case are derived and these are used to find a shifted log-normal approximation for the distribution of the decision variable. The accuracy of this method is verified through simulations.
