فهرست مطالب

  • Volume:3 Issue: 3, 2012
  • تاریخ انتشار: 1390/10/11
  • تعداد عناوین: 7
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  • Hamid Arezumand, Hamed Sadeghi Page 1
    Fast reliable spectrum sensing (SS) is a crucial problem in the cognitive radio systems. To address this issue, cyclostationarity-based detection methods, which are generally more complex but more reliable than energy detection methods, have been proposed. This paper presents a new method to detect the presence of the second-order cyclostationarity in the OFDM-based primary user (PU) signals. The proposed method has a low computational complexity, while it presents a performance close to the well-known GLRT-based Dandawaté-Giannakis''s method. Moreover, since the proposed low-complexity method is robust against the noise uncertainty, it can be a good alternative to the energy detection method. We further propose some cooperative spectrum sensing methods to improve the detection performance. Extensive simulation results confirm the superiority of the proposed schemes.
    Keywords: Cooperative, Cyclostationarity, Based, Low, Complexity Spectrum Sensing, Soft Combination
  • Vahid Jamali Kooshkghazi, Nima Reisi, Mahmoud Ahmadian, Soheil Salari, K. N. Toosi Page 11
    Energy detection has been adopted as an alternative spectrum sensing method for cognitive radios due to its low computational complexity and not requiring a priori information about the signal to be detected. However, noise uncertainty and hidden terminal problem make energy detector practically challenging specially in low signal-to-noise ratio (SNR) regime. Collaboration among multiple cognitive radios has been recognized as a practical strategy to improve the reliability of spectrum sensing. In this paper, a cooperative spectrum sensing framework is proposed to blindly determine the occupancy of a wideband spectrum. Specifically, contrary to conventional energy detector, the proposed method does not require any knowledge of noise variance to detect the presence of primary signals. Moreover, diversity achieved by cooperation enables the framework to maintain a reasonable performance even in low SNR values. Simulation results confirm the effectiveness of our proposed method in improving both the probabilities of detection and false alarm.
    Keywords: cognitive radio, cooperative spectrum sensing, energy detection, Generalized Bayesian estimator
  • Saeid Balaneshin Kordan, Bahman Abolhassani Page 19
    In this paper, we propose a new algorithm of spectrum sensing based on algorithmic information theory. Since the proposed algorithm is applied in time domain, resulting in an overall reduction on the system computational complexity. In this paper, we investigate two cases of wideband and narrowband spectrum sensing problems. To sense the spectrum, we propose to use some measures based on algorithmic information theory such as Lempel-Ziv Complexity (LZC), Higuchi fractal dimension (HFD), and Algorithmic mutual information (algorithmic MI). LZC and HFD are reliable and promising measures that calculate the complexity of a time series signal in a straight forward manner. On the other hand, algorithmic MI calculates the algorithmic mutual information between two time series signals. Our proposed algorithm is blind in the sense that it requires no prior knowledge of the channel, primary users’ signals, and noise variance. In simulation section, it is shown that our proposed algorithm has better performance in contrast with the other complexity based detectors such as Shannon entropy and spectral entropy based detectors.
    Keywords: Cognitive Radio, Blind Spectrum Sensing, Lempel, Ziv Complexity (LZC), Higuchi Fractal Dimension (HFD), Algorithmic mutual information, Entropy
  • Mahshad Hamedanian, Abbas Mohammadi, Abdolali Abdipour Page 27
    This paper presents a strategy to combine the antenna selection technique and the adaptive modulation method for multiple input multiple output (MIMO) systems. The MIMO architecture significantly improves the performance of wireless systems at the expense of a radio frequency (RF) hardware complexity. However, the advantages of MIMO systems can be mainly maintained with simple hardware using antenna selection technique. Moreover, in order to improve the efficiency, the adaptive modulation method can be implemented. As a practical system design limitation, the perfect channel state information (CSI) may not be achievable. Accordingly, the effect of imperfect channel state information (ICSI) in the proposed systems has been investigated.
    Keywords: MIMO, antenna selection, adaptive modulation, CSI imperfection, spectral efficiency, bit error rate
  • Joint Power Adaptation and Interference Avoidance for Unlicensed Wireless Systems: A Game Theoretic Approach
    Seyed Mohsen Beheshti, Behrad Mahboobi, Mehrdad Ardebilipour Page 35
    In multiuser wireless systems, waveform adaptation is one of the interference management methods in which the users adaptively change their transmitting waveforms in signal space to enhance the system performance subject to some Quality-of-Service (QoS) requirements. In this paper, the uplink scenario of a Multi Carrier Code Division Multiple Access (MC-CDMA) is considered under slow and frequency selective fading. An admissibility test under maximum received power constraint is presented to check that if there exist feasiblepowers and waveforms for network users to hold the QoS constraint of each user above a predetermined level. In our set-up, a number of licensed (fixed) and unlicensed (agile) users share some orthogonal carriers to send data to a common receiver (or co-located independent receivers), subject to specific target SINRs for users. We analyze this system as a non-cooperative separable game and show the existence of Nash equilibrium point for this joint power control and codeword adaptation game.An iterative distributed algorithm based on better response strategy is presented which at the receiversresults in an ensemble of well-known Generalized Welch Bound Equality (GWBE) optimal waveforms and orthogonal waveforms for oversized users in addition to optimal transmission power for each user. We investigate the properties of fixed point and the effect of algorithm on the performance of fixed users’ network. Finally the algorithm will be confirmed by the numerical examples.
    Keywords: Unlicensed communication, interference avoidance, MC, CDMA, waveform adaptation, transmitter optimization
  • Neda Banivaheb, Keivan Navaie Page 47
    This paper studies the impact of using multihop wireless adhoc network in conjunction with cellular networks under a probabilistic routing strategy. Our objective is to obtain the outage probability of the two-dimensional hybrid CDMA network under path loss. In the proposed network the base stations are placed on a regular grid and users are distributed randomly according to a homogeneous Poisson point process. To obtain the outage probability of the proposed hybrid network, we study the outage probability of a cellular CDMA wireless network on both uplink and downlink as well as the outage probability of a multihop ad hoc CDMA network. We validate our analytical expressions via Monte Carlo simulations. Our results demonstrate that in order to improve the performance of a hybrid wireless network with a large density of users, it is beneficial to use multihop ad hoc communication.
    Keywords: hybrid networks, stochastic geometry, point process
  • Farid Samsami, Ghosheh Abed Hodtani, S. Bagher Hosseini Karani, Hossein Khoshbin Ghomash Page 55
    In this paper, a new method for estimating the direction of arrival of Asynchronous DS-CDMA signals in multipath fading channel is proposed. In the proposed method, first, for removing the effects of undesired paths, we project the coherent signals of the undesired paths perpendicularly to the under process signal, next, the signal is passed through a filter bank.Byusing beam forming filters, the effectof other users on the desired users signal is decreased and hence, the search area is decreased almost to one tenth. In this algorithm, searching all angles is not required,to estimate the number of sources does not require any information criteria, andalso, the number of users can exceed the number of antenna arrays contrary to many of the conventional methods. Simulation results are illustrated to confirm the efficiency of the method.
    Keywords: Direction of arrival estimation, orthogonal projection, beam forming, frequency, selective fading channel