Kenta Umebayashi

On the Selection of the Best Detection Performance Sensors for Cognitive Radio Networks

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.

Study on Soft Decision Based Cooperative Sensing for Cognitive Radio Networks

SUMMARY In this paper, we propose a soft decision based cooperative sensing method for cognitive radio (CR) networks for opportunistic frequency usage. To identify unused frequency, CR should exploit sensing technique to detect presence or absence of primary user and use this information to opportunistically provide communication among secondary users while performance of primary user should not be deteriorated by the secondary users. Because of multipath fading or shadowing, the detection of primary users may be significantly difficult. For this problem, cooperative sensing (CS), where gathered observations obtained by multiple secondary users is utilized to achieve higher performance of detection, has been investigated. We design a soft decision based CS analytically and analyze the detector in several situations, i.e., signal model where single-carrier case and multi-carrier case are assumed and two scenarios; in the first scenario, SNR values of secondary users are totally equal and in the second scenario, a certain SNR difference between secondary users is assumed. We present numerical results as follows. The first scenario shows that there is little difference between the signal models in terms of detection performance. The second scenario shows that CS is superior to non-cooperative sensing. In addition, we presents that detection performance of soft decision based CS outperform detection performance of hard decision based CS.

On the Measurement of Duty Cycle and Channel Occupancy Rate

Duty cycle (DC) and channel occupancy rate (COR) measurements are important to characterize the availability of white space for cognitive radio systems (CRSs). The COR is related to the real occupancy of channels considered by a CRS but despite its importance it has not yet been widely considered for spectrum use measurements. Spectrum use measurements have typically used detection with a single threshold. In order to improve sensitivity, we utilize the localization algorithm based on double-thresholding (LAD) with the adjacent cluster combining (ACC) to spectrum use measurements and propose a modified version of it. We theoretically analyze the probability of false alarm after the LAD processing in a realistic case with correlation among samples and theoretically analyze the probability of detection after the LAD ACC processing. We propose COR estimation algorithm based on hard decision fusion of the frequency domain decisions. Measurement and theoretical results confirm the accurate DC and COR estimation and the significant sensitivity gains with the proposed algorithms.

Study on Cooperative Sensing in Cognitive Radio based AD-HOC Network

In this paper, we propose a soft decision based cooperative sensing method for cognitive radio (CR) in ad-hoc networks. A CR can sense a surrounding radio environment and adapt to the environment by the reconfigurability to achieve more effective frequency utilization. Observing the environment, CR should detect presence or absence of primary user and exploits this information to opportunistically provide communication among secondary users while performance of primary user should not be deteriorated by the secondary users. Because of multi- path fading or shadowing, the detection of primary users may be significantly difficult. For this problem, cooperative sensing (CS), where gathered observations obtained by multiple users is utilized to achieve higher performance of detection, has been investigated. We propose a soft decision based CS where test statistic used in traditional single user energy detection is treated as soft decision information. The detector is designed analytically, i.e., the statistic of the performance, detection probability and given false alarm, can be obtained analytically. By computer simulations, we have presented as follows; validity of analysis and that the CS has better performance compared to non-cooperative sensing. In addition, we have presented the detection performance of cooperative sensing when there are considerable differences regarding signal to noise ratio among users.

Frequency and Time Domain Channel Models for Nanonetworks in Terahertz Band

Time and frequency domain channel models are proposed for nanonetworks utilizing the terahertz band (0.1-10 THz) for wireless communication. Nanonetworks are formed by tiny nanodevices which consist of nanoscale (molecular scale) components. Channel models capturing the unique peculiarities of the THz band are needed for designing proper physical layer techniques and for accurate performance analysis. Existing channel models have included the free space path loss and the molecular absorption loss, which is significant in the THz band. This paper theoretically analyzes scattering including multiple scattering referring to a sequence of scattering events from small particles, such as aerosols. Both the frequency and the impulse responses are derived. It is shown that the small particle scattering can result into significant additional loss that needs to be taken into account with the loss depending on the density and size distribution of the particles. It is shown that multiple scattering leads to a long tail in the impulse response. As most of the physical layer proposals for nanonetworks are based on the on-off keying, the channel response to pulse waveforms is specifically considered.

Analysis of cognitive radio networks with imperfect sensing

Recently, cognitive radio access has received much attention. Spectrum sensing methods are often used for finding free channels to be used by cognitive radios (secondary users). State diagram based approach can be used for analyzing the effects of imperfect spectrum sensing (with false alarms and misdetections). The state diagram consists of two-tuples like (1,2) meaning one primary user and two secondary users present. We note that state dependent transition rates are very important for accurate modeling. This is because for example in state (3,0) (all channels occupied by primary users) collisions happen with increased probability. Our contribution is as follows. Explicit expressions for state dependent transition rates are presented for the case with three channels. However, the approach can be used also for more channels. Primary termination probability is used for evaluating the level of interference to primary users caused by secondary users. Secondary success probability is used to find out how often does a secondary call start and terminate successfully. Simulation and analysis results agree very well.

Improved Channel Occupancy Rate Estimation

Channel occupancy rate (COR) is the fraction of the time that a channel is occupied, i.e., contains signal(s) in addition to noise. Estimation of COR is important, e.g., in cognitive radio systems, which can use this information for intelligently adapting their spectrum use to the operating environment. For COR estimation, both the ability to operate with weak signals (sensitivity) and closeness of the estimate to the true COR value (accuracy) are important. In this paper, an improved COR estimation (iCOR) method is proposed enabling the use of high false alarm probabilities to improve sensitivity without the overestimation usually associated with high false alarm probabilities. The iCOR method is compared with the conventional method in terms of worst-case root-mean-square error (RMSE), which refers to the RMSE for the COR level yielding the maximum RMSE. To fairly compare different COR estimation methods, it is required that the RMSE for strong signals equals a target value and the considered methods are compared by their RMSE for weaker signals. Comprehensive theoretical analysis is performed and both exact results and approximations are derived. Experimental results verify the theoretical analysis and show significant sensitivity gains from the iCOR method (around 5 dB).

A method of non-data-aided carrier recovery with modulation identification

A non-data aided carrier recovery technique using modulation format identification is proposed. This technique can also be interpreted as a modulation identification method that is robust against static phase and frequency offsets. The performance of the proposed technique is studied and analytical expressions derived for the mean acquisition time to detect lock in the cases of M-PSK, M=2,4,8, and 16-QAM modulation, with respect to frequency offset and signal-to-noise ratio. The results are verified with Monte Carlo simulations. The main advantage of the proposed method lies in its simpler implementation and faster lock detection, when compared to conventional methods.

Study on efficient decision fusion in OR-rule based cooperative spectrum sensing

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.

Optimum welch FFT segment size for duty cycle estimation in spectrum awareness system

In order to realize practical dynamic spectrum access (DSA), implementing spectrum sensing with reasonably low cost is very challenging due to the required detection accuracy and quickness. For this issue, we have introduced an extended DSA, smart spectrum access (SSA), where any useful information in terms of spectrum utilization, such as statistics regarding spectrum utilization, is used not only for accomplishing the requirements of spectrum sensing but also for enhancing the performance of the DSA. In this approach, obtaining the statistics of the spectrum utilization is an important issue for which we have developed a spectrum awareness system prototype. One issue in the spectrum awareness is the Welch FFT segment size design which is used for spectrum analysis. This issue involves a trade-off between the spectrum usage detection accuracy and available frequency resolution. For this issue, we derive an optimum segment size based on analysis and show that the optimum segment size depends on signal-to-measurement bandwidth ratio and duty cycle. The optimum segment size derived by the analysis is validated with numerical simulation and experimental results.