Optimization of Cooperative Spectrum Sensing based on Improved Energy Detector with Selection Diversity in AWGN and Rayleigh Fading M. Ranjeeth, Sipra Behera
Srinivas Nallagonda
S. Anuradha
Department of ECE Department of ECE Department of ECE National Institute of Technology M.V.S.R Engineering College National Institute of Technology Warangal, India 506004 Osmania University, Hyderabad, India 501510 Warangal, India 506004 Email:
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Abstract—In this paper, an optimized cooperative spectrum sensing (CSS) network, in which each cognitive radio (CR) uses multiple antennas and an improved energy detector is proposed. The performance is evaluated by optimizing the network parameters such as number of CR users and normalized detection threshold. We have derived the suitable expressions for optimum value of normalized threshold in Rayleigh fading channel for single and multiple antenna case and optimum number of CRs for both AWGN and Rayleigh fading channels. Finally, optimal values of number of CRs and normalized detection threshold is calculated for different values of average sensing channel SNR, normalized threshold, and multiple antennas at each CR. Comparison between conventional energy detector (CED) and improved energy detector (IED) is also provided. Index Terms—Cooperative spectrum sensing, improved energy detector, fading, antenna diversity, optimization.
I. I NTRODUCTION In present days, the uses of radio spectrum have become crowded due to the increasing in the number of communication networks and services. Various reports on spectrum utilization have shown the inefficient usage of spectrum statistics. Hence, new method of allocation policies has to be introduced to make best usage of the spectrum [1]. Cognitive Radio (CR) is one of the best solutions to make the proper utilization of spectrum [2]. These CRs are allowed to access the untapped and licensed frequency bands without causing significant harmful interference to primary users (PUs). Spectrum sensing is used to sense the spectrum to decide the vacant bands of the spectrum in CR system. Spectrum sensing is limited due to the single CR present in the channel, fading and shadowing present in the environment. Cooperation of multiple CR users were used to sense the spectrum that will increases the detection probability of the primary user, this concept is called as cooperative spectrum sensing (CSS) [3]. The cooperative spectrum sensing is provides better immunity to fading and shadowing present in the environment. Different detection schemes are available to make final decision about the primary user, among all these detection techniques, conventional energy [4] detection is used frequently, due to system complexity is less and non-coherent in nature. The conventional energy detector (CED) [5] utilizes
square of the magnitude of the received data sample, which is equal to the energy of the received data sample. It is shown in [6] that the performance of a cognitive radio network can be improved by utilizing an improved energy detector in the CRs, where the conventional energy detector is modified by replacing the squaring operation of the received signal amplitude with an arbitrary positive power (p). Sometimes it is required to optimize the performance of the network by optimizing the system parameters to minimize the complexity of the network. In [7], optimized performance of cooperative spectrum sensing is achieved by optimizing the values for number of CR users, normalized threshold value and amplitude of the received signal (p) in Rayleigh fading environment for single antenna at each CR. In [8], multiple antennas at each CR and an improved energy detector (IED) is considered as detection scheme to evaluate the performance of total error probability in Rayleigh fading channel. Optimization of cooperative spectrum sensing with conventional energy detector (CED) in cognitive radios networks is considered in [9]. In [5], they have provided the detection probability equations for different fading environments using an improved energy detector but they have not performed the optimization technique. This has motivated us to do the optimization on network parameters to get the optimized performance of cooperative spectrum sensing using IED as detection scheme over AWGN and Rayleigh fading channels. Finally, with this paper our contribution to an existing literature is providing an optimum value of number of CR users to make the final decision about the primary user at fusion center in AWGN and Rayleigh fading environment for different values of average SNR, multiple number of antennas (M ) at each CR, amplitude of the received signal (p) and normalized threshold value. Optimum value of normalized detection threshold is calculated in Rayleigh fading channel. Comparison between CED and IED also provided for different cases. Selection combining (SC) deiversity scheme is used at fusion center (FC) to make final decision about primary user. The rest of the paper is organized as follows: Section II describes about the proposed cooperative spectrum sensing
system model. Section III present simulation results and discussion followed by conclusion in section IV. II. S YSTEM M ODEL Fig. 1 shows the proposed CSS system with N cognitive radios (CRs), a fusion center (FC) and a primary user (PU). Each CR consists of multiple antennas (M ) and each of the FC and PU uses single antenna. The channel present between PU and CR is called as sensing channel (S-channel), which senses the PU activity with multiple antennas and stores the information with it. The channel present between CR user and FC is called as reporting channel (R-channel), the information stored by the CRs will tranfer to FC. Fusion Center collects the total information from all the CRs and final decision made at FC using selection combining (SC) diversity scheme. Improved energy detector (IED) is used as detection scheme for the detection of PU. The CR uses selection combining scheme to make final decision about PU from the obtained test statistics by an IED. For the detection of the spectrum hole, there are two hypotheses, H0 and H1 , in the i-th CR, where i = 1, ..., N which indicates PU’s absence and presence, respectively. The received signal at i-th CR, yi (t) can be written as: ( ni (t) : H0 , yi (t) = (1) hi (t)s(t) + ni (t) : H1 . where s(t) is the received signal with energy Es and ni (t) is the noise value at i-th CR. Additive white Gaussian noise (AWGN) with normal distributed is assumed at each CR. Schannel fading coefficient for i-th CR is denoted as hi . It is considered that each CR with multiple antenna contains the IED to detect the spectrum hole and the expression at i-th antenna to make local decision about the PU is given by [7], [8] wi = yip , p>0 (2) It can be observed from (2), if p = 2, substitute in (2) then wi expression is used to measure the energy of the received signal like conventional energy detector (CED). For IED, p value should be more than 2 i.e., p > 2 to achieve more detection probability than CED. Each CR uses an IED to get the decision statistic given in (2) for all (i = 1, 2, ..., M ) antennas. Selection combining (SC) scheme is operated over all wi values and selects largest value of all wi values denoted as Z and compared with detection threshold λ as, to decide about the PU. [7], [8] Z>λ
: H1 ,
Z
