Sharif Digital Repository

In this study, the authors show that the coding advantage (CA) of quasi-orthogonal space-frequency block codes (QOSFBCs) could be decomposed into two parts, namely intrinsic CA (ICA) and extrinsic CA (ECA). Then, by using the ICA, the authors demonstrate that the optimum precoder of the QOSFBCs could be analytically derived without any requirement of the exhaustive search. Then, by improving the ECA, the authors enhance performances of the QOSFBCs efficiently - up to 4dB. Finally, the authors show that a very fast optimisation process for the QOSFBCs is viable over any arbitrary frequency-selective channel  

In this paper, we study the degrees of freedom (DoF) of a frequency-selective K-user interference channel in the presence of an instantaneous relay (IR) with multiple receiving and transmitting antennas. We investigate two scenarios based on the IR antennas’ cooperation ability. First, we assume that the IR receiving and transmitting antennas can coordinate with each other and that the transmitted signal of each transmitting antenna can depend on the received signals of all receiving antennas, and we derive lower and upper bounds for the sum DoF of this model. In an interference alignment scheme, we divide receivers into two groups called clean and dirty receivers. We design our scheme such... 

Monolayer band-reject frequency selective surfaces (FSS) based on curved coupled microstrip line (CCML) resonators are proposed. Each unit cell comprises a single- or multi-loop CCML resonator, featuring single-wideband or multi-band resonance, respectively. Design guidelines for achieving various responses are demonstrated and it is shown there is a great flexibility attaining desired frequency responses versus structural parameters. For instance, it is shown that the entire microwave X-band may be rejected, using a simple single-loop structure. The FSS is particularly useful for protecting buildings against parasitic microwave signals. A visual transparency factor for the FSS, when regular... 

In this paper, we study the sum mean square error (SMSE) performance of all users by applying joint MIMO filter-and-forward relay and post-processing at base stations (BSs) for uplink coordinated multi-point (CoMP) reception in case of users-to-relay and relay-to-BSs frequency-selective fading channels. In order to combat the interference due to frequency-selectivity of channels, MIMO relay passes its received signals through a finite impulse response (FIR) filter. For further reducing SMSE, each BS applies a post-processing to its received signals. The goal is minimizing the SMSE subject to transmitted power constraint of MIMO relay. An iterative algorithm is proposed to jointly optimize... 

The achievement of our previously proposed space-time coding algorithm entitled full-rate linear-receiver space-time block code (FRLR STBC) has motivated us to propose, in this paper, a new class of high-rate space-time-frequency block codes (STFBCs) over frequency-selective Rayleigh fading channels. We have called these codes FRLR STFBCs with interleaving (FRLR STFBCs-I). FRLR STFBCs-I could achieve a full-diversity property over quasi-static channels. Simulation results also verify that the proposed schemes exhibit proper performances in comparison with the recently proposed STFBCs. The most outstanding characteristic of the newly introduced high-rate codes is the linear complexity of the... 

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... 

The problem of aperiodic excitation of periodic structures with periods larger than a half-wavelength is revisited. A large number of antennas and other electromagnetic wave propagation problems fall within this category. Because of overlapping branch cuts, the conventional path deformation techniques employed in application of the array scanning method for this type of problem fail when the period is larger than a half-wavelength. A new method based on the subdivision of the integration path and using the double exponential quadrature formula is introduced to alleviate this problem and apply the array scanning method to structures with an arbitrary spatial period. To demonstrate the... 

In this paper, joint dynamic frequency selection and transmit power control problem with the goal of power minimization for a target SINR is investigated for cognitive radio networks. The optimization problem is formulated and three algorithms are proposed: A centralized algorithm based on graph coloring, and two distributed algorithms based on game theory; one for coordinated and the other for non-coordinated scenarios. Performance of the proposed algorithms in terms of power minimization and convergence speed is evaluated through simulations. Comparing the performance with the optimum solution, all three algorithms show near-optimum power minimization performance for the simulated... 

We consider a wireless network with one source, multiple relays and multiple destinations. The source and the relays are assumed to utilize multiple antennas. To reduce the effects of the frequency selective source-to-relay and relay-to-destination channels, and to compensate for the different delays of the various signal paths in the network and for multi-user interference and noise, the source, the relays and the destinations apply linear filtering to their signals. The proposed scheme generalizes the recently proposed filter-and-forward relaying scheme. We propose an iterative algorithm to solve the problem of maximizing the quality-of-service at the destinations. Simulation results... 

In this paper, by applying a new permutation to the full-diversity block circular delay diversity (BCDD) space-frequency codes (SFCs), we show that their performance could be enhanced. Furthermore, we propose a new method for performing the permutation in the absence of partial channel state information at the transmitter. This new permutation, which is done based on artificial delay and power profiles (ADPPs), could improve the performance of the BCDD SFCs more than 3 dB as simulation results demonstrate  

In this paper, we consider a broadband secondary transmitter-receiver pair which interferes with N narrowband primary users and study the effect of cognition and cooperation on the maximum stable throughput. In our study we focus on four transmission protocols as well as two channel types, i.e., flat fading and frequency selective fading. In the cooperative protocols, the broadband transmitter relays the packets of the primary users which have not correctly decoded at the primary receiver. The analysis includes random packet arrivals at the transmitters which may impact on the maximum stable throughput. Moreover, sensing errors at the secondary user are considered. In this paper, we derive... 

A cooperative space-frequency (SF) coded orthogonal frequency division multiplexing system is considered and its performance over quasi-static frequency-selective Rayleigh fading channels is evaluated. An expression for exact outage error probability is derived and its tight closed-form lower bound is presented. The tightness of the lower bound is demonstrated through Monte Carlo simulation. Asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed SF coded cooperation protocol is further confirmed by computer simulation using a previously introduced SF... 

In this paper, a non-cooperative distributed frequency selection algorithm is proposed for cognitive radio networks in which in addition to interference minimization, the minimization of implementation complexity and probability of interruption of secondary users are also addressed. The convergence and interference reduction property of the algorithm is also demonstrated using game theoretic concepts. In order to evaluate the performance of the algorithm, the associated optimization problem is formulated into a variant of graph colouring problem, and a centralized algorithm is also proposed as an approximation of the optimal solution. Finally, simulation results demonstrate that the proposed... 

In this paper, the impact of both time offsets (TOs) and carrier frequency offsets (CFOs) on the average output signal-to-interference-plus-noise ratio (SINR) in a frequency selective channel is investigated with a novel approach. First, TOs are partitioned into five cases based on how they occur. Then the mathematical models for different cases are derived. Considering CFOs and benefiting from the obtained models, closed-form expression for the SINR in each case is derived. It is proved that, SINR is independent of the observed subcarrier for the given TO and CFO. Besides, it is shown that the performance of the affected system deteriorates rapidly as the number of subcarriers increases. It... 

In this paper, new sequences (λn,ρn) of capacity achieving low-density parity-check (LDPC) code ensembles over the binary erasure channel (BEC) is introduced. These sequences include the existing sequences by Shokrollahi as a special case. For a fixed code rate R, in the set of proposed sequences, Shokrollahi's sequences are superior to the rest of the set in that for any given value of n, their threshold is closer to the capacity upper bound 1 - R. For any given δ, 0 < δ < 1 - R, however, there are infinitely many sequences in the set that are superior to Shokrollahi's sequences in that for each of them, there exists an integer number n0, such that for any n > n0, the sequence (λn, ρn)... 

Time-delay in sensor measurements can be a frequent cause of instability and performance degradation in a robotic system. In this paper, motion tracking of rigid manipulators in presence of constant and known delay in sensors is investigated. By using non-minimal model of a manipulator, a dynamically smooth controller based on the Linear Matrix Inequality (LMI) approach is proposed which guarantees asymptotic tracking of desired joint angles and velocities in presence of delayed measurements. For a given controller the maximum amount of delay that preserves system stability is computed by solving an LMI optimization and also by numerical simulations, and the results are compared. Finally, a... 

Regarding the dynamic nature of cognitive radio networks and the delay of the central decision algorithms due to collecting necessary information in a multi-hop wireless network, distributed resource allocation algorithms are of crucial importance. In this paper we propose a new distributed resource allocation algorithm that properly selects link/frequency in a multi-hop cognitive radio network for delay sensitive applications. This algorithm, uses the locally collected information from neighboring nodes to make suitable routing decisions regarding the dynamic environment and the behavior of the neighbors. We consider the delay as the routing criterion, consisting the queueing delay and the...