جستجوی مقالات مرتبط با کلیدواژه « cognitive radio » در نشریات گروه « برق »

By performing cooperative spectrum sensing in a cognitive radio network, although the network throughput increases with the increase in the number of secondary users, but at the same time, it also causes an increase in energy consumption. This makes it necessary to provide a system that is able to create a tradeoff between throughput and energy consumption. In contrast to the conventional method of spectrum sensing based on one detection threshold, spectrum sensing with double thresholds avoids reporting unreliable data to the fusion center, thus potentially leading to greater energy saving. In this paper, a double threshold spectrum sensing cognitive radio network with a non-ideal reporting channel is optimized. The values of the threshold and the sensing time are jointly optimized to maximize the throughput of the network, provided that the network energy consumption and the amount of interference with the primary users are limited. The optimization problem is formulated and a numerical method is presented to solve it. The simulation results show a flexible system that can simultaneously provide higher throughput and lower energy consumption than the conventional sensing method. These results, while confirming the higher tolerance against the error of the reporting channel, show a significant energy saving of up to 70% by guaranteeing the throughput efficiency greater than 1.

Cognitive radio is well known as an ultra-wideband communication system that intelligently optimizes the available frequency spectrum by implementing the dynamic spectrum monitoring method. The receiver of such a system requires an ultra-wideband low noise amplifier (from 50 MHz to 10 GHz). In this work, a low power ultra-wideband low noise amplifier is proposed by quasi-differential structure and active inductors. Using active circuits as inductors, in addition to expanding the amplifier bandwidth and reducing the chip area, have an inherent gain and due to their high quality factor, have the ability to adjust the inductance and frequency. In addition, the quasi-differential structure increases the transconductance of transistors and reduces both power consumption and noise figure of this circuit. The simulation results in a 0.18 µm CMOS process show that the proposed LNA achieved NF changes 3-7 dB and input matching less than -10 dB in the 0.05-10 GHz band. The voltage gain changes from 16.5-19 dB and third-order intercept point is -6.7 dBm. The power consumption of the main circuit is 1.98 mW with 1.8 V power supply.

The spectral sensor provides one of the components of opportunistic access to frequency resources. These sensors must measure the ambient spectrum with high accuracy in a short time. It is important to use methods and algorithms that can be implemented in practice; therefore, the complexity of the calculations should be reduced as much as possible. Detection performance is hampered by issues such as shadow effects, blurring, and multi-directionally; therefore, to overcome these problems and the unavailability of information about the signals received by the primary users and the power level and noise of the received signals, methods such as using several antennas in the spectral sensor, resisting spectroscopic methods against indeterminate noise, are used. In this paper, different criteria for decision making and determining the appropriate threshold by extracting special values ​​of the covariance matrix are investigated. Also, by calculating a few moments from the detector to the maximum to minimum values ​​of the MME covariance matrix by the proposed standard GEV distribution, simulations are performed and the results are compared with the Tracy-Widom distribution. The results of the simulations with a limited number of 4 antennas indicate the probability of signal detection in SNR with -30dB, which has increased the accuracy by 3dB compared to the Tracy-Widom distribution. To evaluate the efficiency of this method for hardware execution, the results of the proposed algorithm have been reviewed and implemented by an FPGA array processor along with a processor with XILINX ARM architecture, which is called the systems embedded in the SOC chip.

In this paper, a new CPW (Coplanar Waveguide) fed printed UWB antenna integrated with a narrowband antenna is designed and analyzed for cognitive radio applications. The ultra-wideband (UWB) and narrowband (NB) antennas provide bandwidth from 2.2 GHz to 11GHz and from 4.8 GHz to 5.7 GHz respectively and reasonable isolation between the antennas. A prototype of the integrated antenna was fabricated and its performance was verified. The dimensions of the FR4 substrate are 40 × 48 × 1.6 mm3.The antenna represents good radiation characteristics, stable gain and good time domain manner to deliver the requirements of the current wireless communication standards. Also, two external tuning circuits were designed to show the ability of the narrowband antenna for reconfiguration.

This paper presents performance evaluations for two spectrum-sharing cognitive-radio systems employing cooperative amplify-and-forward relaying in which the best relay is selected for the transmission. In these systems, a secondary user (SU, unlicensed user) shares the spectrum of a primary user (PU, licensed user), in such a way that the interference to the PU is always kept below a predefined threshold. To this end, a power control for the SU is used. In these systems, the SU employs an amplify-and-forward relaying with several relays, where the best relay who has the best channel condition is selected for the transmission. The considered systems are different in the employed relay selection schemes, such that: 1) In the first system, partial relay selection is employed, in which the partial channel state information (CSI) is used to select the best relay. 2) In the second system opportunistic relay selection is employed, in which the full CSI is used to select the best relay. To analyze the system performances, at first the cumulative distribution function (CDF) of the received signal-to-noise-ratio (SNR) for each considered system is derived. Then, using the obtained CDF expressions, closed-form expressions are obtained for three performance metrics: outage probability, the average bit-error-rate (BER), and the average channel capacity. Finally, using the numerical results obtained from the mathematical formulas derived in this paper, we evaluate and compare the systems performances in different cases under different conditions. In addition, to verify the accuracy of the performed analysis, Monte-Carlo simulation results are also provided.

Ever increasing development of wireless devices and wireless networks have increased the value of spectral space‎. ‎Many efforts have been conducted to increase spectral utilization‎. The radio environment mapping opens new gates for developing low cost wireless devices. ‎In this paper‎, ‎a new method is proposed for increasing spectral utilization in distributed networks‎. ‎In this method‎ ‎distributed Kalman filter, which is modified to increase estimation accuracy, is used to estimate position‎, ‎velocity and power of primary transmitters‎. ‎These data are used to select spectrum holes optimally and increase spectral utilization compared to centralized methods‎. Obtained r‎esults are evaluated through practical implementations and simulations‎. ‎Innovations of this research include introducing and employing a linear model for estimating position of a transmitter using received power in line of sight (LoS) and non-line of sight (NLS) conditions, ‎‎‎modifying extended kalman filter and‎ ‎implementation of distributed spectrum sensing; advantages of this method are illustrated compared to centralized methods.‎‎

The conventional power grid has several drawbacks and a new powerful smart grid perspective has been recently introduced. The smart grid principle, allowing to efficiently manage an electrical grid network, needs to exploit a communication network for interconnecting the Smart Grid devices. An increasing interest is toward wireless communications due to their higher flexibility. Within this context cognitive radio (CR) techniques has been introduced aiming to exploit more efficiently the radio spectrum resources. In neighborhood area network (NAN), mesh grids can be considered as one of possible network topologies. In such networks no base station is required and data will be sent to gateway by means of nodes themselves. Hence, routing is one of the main issues in such networks. Routing in such networks should be done by a protocol which maximizes throughput against cognitive radio drawbacks and Packets delay in such protocol needs to be minimum and suitable for smart grids applications. CORPL has been introduced as a routing protocol to meet some of these goals. In this paper by CORPL functionality would be evaluated under burst and poisson traffic. It will be shown that by increasing active nodes, CORPL functionality would be decreased. Then average upper limit for delay would be mathematically modeled and to reduce that a multigate scheme would be introduced.

In this paper, a more realistic cognitive radio network is proposed which considers several secondary links, some secondary relays, and one primary link. In the network, there are two steps of transmission from secondary sources to secondary destinations. At the first step, when primary link is idle, all of the secondary sources simultaneously broadcast their information to the relays. At the second step, the relays that successfully decode the information employ a new cooperative beamforming method and forward them to the secondary destination without causing harmful interference to the primary destination irrespective of whether primary link is idle or not. It is assumed that the channel state information between relays and primary destination isn’t perfectly known at the relays and they have erroneous estimation of the channel. The proposed cooperative beamforming method can maximize the received signal power at the secondary destination subject to keep received interference plus noise power at the primary destination under the predefined threshold. Results of simulations confirm validity of the method and improved performance compared to Zero-Forcing beamforming. The proposed cooperative beamforming can remain the interference plus noise power at the primary destination below the threshold even if the error in channel estimation is maximum.

In this paper the QoS-aware channel allocation problem formulated as an optimization problem with two conflicting objectives; spectrum utilization and fairness among secondary users (SUs) subject to channel availabilities constraints. Any possible channel allocation which could be a solution of the optimization problem, encoded as a binary chromosome. By having coded available spectrum opportunities instead of all channel-user combinations, the search space is significantly reduced. Designing the QoS-aware channel assignment scheme is based on NSGA-II Algorithm to find the optimum allocation of these two objectives jointly and finally the set of Pareto optimal solutions achieved by proposed algorithm in discrete space of feasible solutions. Simulation results demonstrate the trade-off between spectrum utilization and fairness and the Pareto optimum points. Binary Integer Programming (BIP) confirms the results of the proposed evolutionary scheme in small-scale instances while our scheme outperforms BIP method significantly in computational for large-scale ones.