Sharif Digital Repository

A cognitive radio provides access to a licensed spectrum for unlicensed users opportunistically provided that they are not harmful for licensed users (primary users). We call a cognitive radio user (CR user) harmful, when it generates interference for primary users (PU) or occupies spectrum holes excessively. This paper discusses the spectrum management policy against harmful CR users. For this, we design a jammer signal which ruins transmission of harmful CR users and forces them to leave the spectrum without damaging the communication of PUs. This signal is designed using group subspace technique for both synchronous and asynchronous channels. Our simulation results show that the designed... 

We propose a novel inverse method that utilizes a set of data to construct a simple equation that governs the stochastic process for which the data have been measured, hence enabling us to reconstruct the stochastic process. As an example, we analyze the stochasticity in the beat-to-beat fluctuations in the heart rates of healthy subjects as well as those with congestive heart failure. The inverse method provides a novel technique for distinguishing the two classes of subjects in terms of a drift and a diffusion coefficients which behave completely differently for the two classes of subjects, hence potentially providing a novel diagnostic tool for distinguishing healthy subjects from those... 

Smart environments are complex systems encompassing multitude of devices with complex interconnections and interactions. User must not only be able to figure out the devices’ functionalities, but also understand the architectural and semantic relations between various components. Providing such information leads to better understanding and user satisfaction, if users get the appropriate mental models. One way to train users about such smart environments is Interactive Realistic Virtual Reality based Simulations (IRVRS): multimedia rich collaborative 3D virtual environments. IRVRS also facilitate easy and inexpensive access to population masses, thus they can be used for conducting large... 

Traditionally, the frequency spectrum is licensed to users by government agencies in a fixed manner where the licensee has exclusive right to access the allocated band. However, with increasing demand for the spectrum and scarcity of vacant bands, a spectrum policy reform seems inevitable. Meanwhile, recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. Spectrum-sharing between a primary licensee and a group of secondary users has been... 

The effective utilization of the spectrum has become an essential goal in the communications field, which is addressed by the Cognitive Radio (CR) systems. The primary task in a CR system is to sense the spectrum to identify its holes to be exploited by the secondary users. In this paper, we tackle the compressed spectrum sensing problem in a cooperative manner. The CRs distributed in an area take the samples of the signal that has been reached to them through a wireless fading channel. The spectrum has the block-sparse structure. Moreover, the spectrum observed by different CRs in an area share the same block-sparse support. Therefore, we suggest to exploit the joint block-sparsity... 

Power control mechanisms play a critical role in underlay cognitive radio networks where primary users' quality of service should not be degraded by the secondary users transmission. Consequently, the inflicted interference power of the secondary transmitters on the primary receivers should be less than certain threshold levels that is required to be considered in designing power control algorithms for such networks. In this paper, we first propose a power reduction algorithm for the set of secondary transmitting links in order to guarantee that the members of this set do not violate the interference constraints set by primary users while maintaining their specific SINR quality of service... 

A joint pre- and post-coder (JPROC) is proposed for MIMO OFDM-based cognitive radio systems over correlated fading channels. The main aim is to have a trade-off between licensed networks and un-licensed networks in the underlay scenario. To handle the un-desired interferences on the main networks, the eigen-values of this JPROC should be small in each licensed network, as much as possible, whereas un-licensed users have higher rates. Frequency selective MIMO fading channels are coded with orthogonal space-time-frequency block codes (OSTFBCs) to improve the bad effects of these small eigen-values on the signal to noise ratio (SNR) in the main network. On the other hand, the system is operated... 

Hidden Primary User problem caused by fading and shadowing severely affects the detection rate of the cognitive radio systems with a single spectrum sensor. Cooperative Spectrum Sensing has been introduced to tackle this problem by using the spatial diversity of spectrum sensors. It is shown that the use of soft decision algorithms in fusion center has a better performance than hard decision algorithms. The problem of soft decision based on sensor measurements perfectly matches the Machine Learning paradigm. In this brief, a novel fast soft decision algorithm is proposed based on Machine Learning theory for wideband Cooperative Spectrum Sensing, which finds a decision boundary to classify... 

The impact of adding correlation to a population of neurons on the information and the activity of the population is one of the fundamental questions in recent system neuroscience. In this paper, we would like to introduce topology-based correlation at the level of storing patterns in a recurrent network. We then study the effects of topological patterns on the activity and memory capacity of the network. The general aim of the present work is to show how the repertoire of possible stored patterns is determined by the underlying network topology.Two topological probability rules for pattern selection in recurrent network are introduced. The first one selects patterns according to a...