Sharif Digital Repository

In this paper, we consider a relay-assisted uplink non-orthogonal multiple access (NOMA) system where two radio frequency (RF) users are grouped for simultaneous transmission, over each resource block, to an intermediate relay which forwards the amplified version of the users' aggregated signals in the presence of multiuser interference to a relatively far destination. In order to cope with the users' ever-increasing desire for higher data rates, a high-throughput free-space optics (FSO) link is employed as the relay-destination backhaul link. Dynamic-order decoding is employed at the destination to determine the priority of the users based on their instantaneous channel state information... 

Network-on-Chip (NoC) is a precious approach to handle huge number of transistors by virtue of technology scaling to lower than 50nm. Virtual channels have been introduced in order to improve the performance according to a timing multiplexing concept in each physical channel. The incremental effect of virtual channels on power consumption has been shown in literatures. The issue of power saving has always been controversial to many designers. In this paper, we introduce a new technique which tries to adaptively mange the number of virtual channels in order to reduce the power consumption while not degrading the performance of the network without any reconfiguration. Our experimental results... 

Inherent properties of UWB signals prevent both coherent and noncoherent processing with conventional methods used in narrow-band RADARs. In this paper, we show how to exploit the Hough transform, first developed to detect lines in an image distorted by noise, to detect moving objects from successive scans that are corrupted by noise and clutter. We discuss why conventional methods fail to process UWB pulses noncoherently/coherently and investigate different aspects of using the Hough transform in noncoherent integration of UWB signals. Simulation results to support our approach are also presented. © 2008 EuMA  

In this paper, the problem of power allocation and optimal decoding order of users for the uplink channel of a multirate direct-sequence code-division multiple-access system with linear successive interference cancellation (IC) is addressed. First, the closed-form expressions of the required received powers at the base station for providing all users with their demanded rates and signal to interference-plus-noise ratios (SINRs) are derived. Then, it is shown that, unlike the case when the IC is perfect, in this case, optimum ordering of users at the receiver, which minimizes the total transmitted power, is a function of both their requested SINRs and path gains. Finally, in searching for the... 

In this paper, analysis of the throughput and delay in a spectrally phase-encoded optical code-division multiple-access (OCDMA) system, using a simplified slotted packet network protocol is conducted. Steady-state throughput and average packet delay of the system are derived. In evaluating the performance of the system, only multiple access interference is considered and other sources of noise are neglected. Comparing different systems with the fixed bit rate and chip duration leads us to conclude that increasing the code length will improve the performance of networks with small average activity; but in highly active networks, decreasing the code length will result in a significant... 

Both secure and error control coding are very extensive subjects, each with a variety of subdisciplines. A secure channel coding (joint encryption-channel coding) scheme provides both data secrecy and data reliability in one process to combat problems in an insecure and unreliable channel. In this paper, a joint encryption-channel coding scheme is developed, based on concatenated turbo codes for more efficient and secure transmission of LEO satellite data. Reliability and security are achieved by adapting the pseudo-random puncturing strategy with a change of distance between satellites and ground stations in the communication séance, an issue further burdened by reducing energy consumption... 

In this paper, the application of regular Low-Density Parity-Check (LDPC) codes in Multi-Carrier Frequency-Hopping (MC-FH) CDMA systems is studied. To this end, different well-known constructions of regular LDPC codes are considered and the performance of LDPC coded MC-FH-CDMA systems, based on these constructions, are evaluated and compared in a frequency-selective slowly Rayleigh fading channel. These results are compared with those previously reported for super-orthogonal convolutionally coded MC-FH-CDMA systems. The simulation results indicate that the LDPC coded MC-FH-CDMA system significantly outperforms the uncoded and super-orthogonal convolutionally coded schemes. To alleviate the... 

In this paper, the performances of both uncoded and coded multiple access TH-UWB systems, introduced in [1-3] in multipath Rayleigh fading channels, are evaluated. The receiver is a selective diversity combining receiver, known as SRake. Based on a Gaussian distribution assumption for the multiple access interference at the output of the SRake receiver and by using a virtual branch technique, as introduced in [4,5], the bit error rates for uncoded and coded schemes are derived. The performance analysis shows that the effective order of diversity achieved by the coded scheme is the product of the number of branches of the SRake receiver and the Hamming distance of the code applied.... 

In this paper, we develop a new error-resistant secure lossless source coding scheme for discrete sources. Recent results indicate that the same turbo principle that provides sub-optimal strategy for channel coding, can be used to obtain efficient source coding schemes. We extend the source turbo-coding idea to include security for transmission of the compressed data and also to ensure the lossless recovery of information at the receiver. Compression and security are achieved by adapting the random puncturing strategy to the statistics of the source. Lossless compression is guaranteed by finely puncturing the encoded data of a parallel-concatenated turbo code while verifying the integrity of... 

One of the most important problems in teleoperation systems is time delay and packet loss in the communication channel, which can affect transparency and stability. One way to overcome the time delay effects in a teleoperation system is to predict the master-side motion. In this way, when data is received in the slave side, it will be considered as the current position of the master robot and, thus, complete transparency could be achieved. The majority of the previous works regarding operator position prediction have considered known and constant time delay in the system; however, in the real applications, time delay is unknown and variable. In this paper, an adaptive online prediction... 

It's well known that "hidden- and exposed-terminal" are among the main problems in ad-hoc WLAN networks. In addition, there are scenarios where the desired destination is located in the range of other transmitters, so that the efforts on setting up communication with this terminal will fail due to collisions occurred between desired control packets and unwanted received data packets at destination. In such scenarios, conventional protocols can not address the problem efficiently, resulting in throughput and channel utilization degradation. In this paper, by using the same PHY of IEEE 802.11 and making slight changes in its MAC layer, a new MAC protocol is presented to address such problems.... 

In this paper, a new method is proposed utilizing a diversity scheme to reduce power consumption in large scale sensor networks. Sensor networks are composed of large number of battery powered nodes. Energy consumption is the most important design objective in sensor networks while delay and throughput are taken less into account. Wireless transmission is the other important characteristic of these networks. Small-scale fading decreases wireless communication performance. In a fading channel higher SNRs is needed and consequently more energy is consumed in fading channels. Another important characteristic of sensor networks is the necessity of fault tolerant protocols. Node-to-node links are... 

In this paper, the problem of scheduling commercial messages during the peak of viewing time of a TV channel is formulated as a combinatorial auction-based mathematical programming model. Through this model, a profitable and efficient mechanism for allocating the advertising time to advertisers is developed by which the revenue of TV channels is maximised while the effectiveness of advertising is increased. We developed a steady-state genetic algorithm to find an optimal or a near optimal solution for the proposed problem. A computational experiment was conducted for evaluating the efficiency of the proposed algorithm. A set of test problems with different sizes were generated, using the... 

A probabilistic method is developed here to model the beacon period length (BPL) defined in the ECMA-368 and ECMA-387 standards for the distributed reservation protocol. The model computes the probability mass function (PMF) of the BPL in a superframe which will be used in modeling of throughput, delay, fairness, power and other major distributed medium access control layer parameters of the emerging ultra wideband and 60- GHz millimeter wave wireless personal area networks. The model relates the PMF of the BPL to the network density, antenna beamwidth, and the transmission range of the devices assuming that the devices are distributed in an outdoor environment based on Two-dimensional... 

Lattice Reduction (LR) has been proposed as a method to enhance the performance of MIMO detectors such as ZF, MMSE and V-BLAST. Until recently, the application of LR to the superior K-Best tree-search detection algorithm was not practical due to the significant increase in complexity of K-Best as a result of the distortion of tree symmetry caused by LR. However, in our recently published work we developed an innovative K-Best algorithm to accommodate tree-asymmetry with no additional complexity. In this work, we build on this result and perform a detailed analysis of the effect of various LR algorithms on the performance of LR-aided K-Best. We show that LLL and Seysen provide equivalent... 

In this paper, we address the problem of recovering degraded images using multivariate Gaussian mixture model (GMM) as a prior. The GMM framework in our method for image restoration is based on the assumption that the accumulation of similar patches in a neighborhood are derived from a multivariate Gaussian probability distribution with a specific covariance and mean. Previous methods of image restoration with GMM have not considered spatial (geometric) distance between patches in clustering. Our conducted experiments show that in the case of constraining Gaussian estimates into a finite-sized windows, the patch clusters are more likely to be derived from the estimated multivariate Gaussian... 

Network-on-Chip (NoC) is a key element in the total power consumption of a chip multiprocessor. Dynamic Voltage Scaling is a promising method for power saving in NoCs since it contributes to reduction in both static and dynamic power consumptions. In this paper, we propose a novel scheme to reduce on-chip network power consumption when the number of Virtual Channels (VCs) with active allocation requests per cycle is less than the number of total VCs. In our method, we introduce a reconfigurable arbitration logic which can be configured to have multiple latencies and hence, multiple slack times. The increased slack times are then used to reduce the supply voltage of the routers in order to... 

In a MIMO (Multiple Input Multiple Output) radar system, proper design of transmit signal and receive filter is an advantageous tool to improve the detection performance. Thus, in this paper, we consider the problem of transmit code and receive filter design, in order to maximize the Signal to Noise Ratio (SNR) at the receiver while enforcing a similarity constraint between the transmit space-time code (STC) and a reference STC which has a desirable ambiguity function. We will show that by solving such constrained optimization problem through successive iterations, our proposed method leads to satisfactory results  

In this paper, we consider the uplink of the narrowband multicell multiuser massive multiple-input multiple-output (MIMO) systems. In traditional approaches, orthogonal pilots among users of all (interfering) cells are utilized to estimate their channels assumed to be constant during channel coherence time. However, in multicell scenarios, such schemes suffer from pilot contamination. In order to address this issue, based on asymptotic pairwise orthogonality of the users' channels in massive regime, blind channel estimation schemes have been proposed. On the other hand, such schemes rely on low interference or high signal to noise ratio (SNR) levels, to achieve reliable performance. Inspired... 

Capacity approximation of multi-hop unicast and multi-hop multicast channels is one of unsolved problems in information theory. Recently some researches investigate Degrees of Freedom (DoF) characterization and Interference Alignment schemes for these channels. However most of them assumed perfect instantaneous Channel State Information (CSI) at the relays and the transmitters. Due to practical limitations, like the delay and the rate limitation in the feedback links and the fading channels, it is difficult to provide perfect instantaneous CSI at the transmitters and even the relays. Achievable DoF by the IA schemes collapses greatly with imperfect CSI. It has been shown that the delayed CSI...