Naser G. Tarhuni

Multiclass optical orthogonal codes for multiservice optical CDMA networks

Optical code division multiple access (CDMA)-based networks are an interesting alternative to support various traffic types of multimedia applications with highly variable performance targets. Generally, multilength codes are designed to support multirate services, while the multiweight codes are designed to support differentiated quality of service (QoS) for multimedia applications. However, existing optical orthogonal codes (OOCs) are limited to single class or multiclass with restricted weight and length properties. Therefore, there exists a lack of flexibility in the existing OOCs to support arbitrary rate and QoS. This paper presents a proposal of generation procedure and performance analysis of joint multiweight multilength strict OOCs. The approach used in this paper is to apply a methodology strongly relying on developed analytical theory that is supported by computer optimization, because it has turned out that it is mathematically intractable to construct unconstraint joint multilength multiweight OOCs using pure algebraic techniques. The generated code set fulfills the conditions of strictly OOCs, namely, the maximum nonzero shift autocorrelation and the maximum cross correlation constraints of one. The mark position difference (MPD) approach is used to generate in a flexible way the multiclass code set. The MPD results in the simple evaluation of multiclass code set cardinality. Furthermore, the multiple-access interference (MAI) in a multiclass OOC system is evaluated by modeling the interference per class as a Poisson distribution to simplify performance evaluation with acceptable accuracy.

Performance analysis of random uniform power allocation for wireless networks in Rayleigh fading channels

When there is no feedback channels in wireless sensor networks or when the sensors are deaf (cannot listen to the feedback channels), the conventional and practical method for power allocation is to use fixed power transmission. This method has many limitations such as near–far problems where the relatively far away sensors are blocked by near sensors and also the unnecessarily high power consumption for good channel sensors. One recent proposal to overcome such limitations is to use uniform random power allocation for terminals. In that work the system performance is analysed for static channels (i.e. snapshot assumption), where it shows that there is a considerable obtained gain when using uniform random power allocation over fixed power. An important question is about the performance of such transmission power strategy in case of a more realistic channel scenario such as Rayleigh fading channels. This paper answers this question. Detailed mathematical analysis as well as Monte-Carlo simulations for the system performance in Rayleigh fading channels are given in this paper. We show that the performance of uniform power allocation is much worse in case of fading channels than in static channels. Almost there is no gain of using uniform random power over fixed power in terms of average probability of outage. However, uniform random power may considerably reduce the total power consumption at small degradation of the average performance. This can be critical feature in energy constrained networks. Copyright

Multi-access-interference mitigation using power control in optical-CDMA star networks

This paper investigates optical power control to enhance capacity of optical CDMA star coupled networks. We consider a network in which the nodes encode their optical signal by prime codes, and the resulting error rate performance is inspected with and without power control. Network impairments, such as fiber attenuation, star coupler loss, thermal noise and shot noise, are taken into account while inspecting the number of supported users. We use a well known centralized power control algorithm to adjust the laser power levels. In short range optical networks, as for instance local area networks (LANs) and metropolitan area networks (MANs), the thermal and shot noise levels can be very small compared to the signal level, thus, noise-free or multiple access interference (MAI) limited case is of interest. In our analysis, the star network is partitioned into access part and broadcasting part. We proof that in the MAI noise dominated scenario, the broadcasting part of the network is irrelevant with respect of the optimum power control problem. In this case, the users can apply a lookup table containing information about the access part of the network to set their optimum transmission power levels. We note that whenever a node is switched on or off, the other active nodes should update their power levels in order to maintain or upgrade their respective error rate limits.

Multi-class optical-CDMA network using optical power control

In this paper we use optical power control to support multirate transmission over temporal optical CDMA networks. We apply the centralized power control algorithm to set the transmit power of the users’ optical sources in order to satisfy a given target QoS. In addition, optical amplifiers are included to enhance the overall system performance while the Amplified Spontaneous Emission (ASE) is considered as the main noise source. The objective function defined as the sum of the transmitted optical power from all nodes is minimized subject to a signal-to-interference (SIR) constraint. Moreover, the network feasibility, defined as the ability to evaluate a power vector that satisfy the target SIR, is discussed in terms of the spectral radius of the network interference matrix. Next, the spectral radius of the network interference matrix is investigated and modeled as a truncated Gaussian distribution. Last, a rate reduction algorithm, categorized in terms of the number of nodes involved in the process of rate reduction, is proposed to increase the network feasibility. As more nodes are added to the rate reduction campaign, the network feasibility is significantly enhanced. For typical network parameters we find by simulating 104 random network realizations that a threenode rate reduction results in 99% network feasibility. 280 Tarhuni, Elmusrati, and Korhonenn

Probabilistic model for MV spark-gap characteristics with lightning induced overvoltage superimposed on AC voltage

In this paper, the breakdown probability of MV spark-gaps is modeled using the Gaussian distribution function under an impulse voltage test in accordance with the IEC 60060-1 standard. The model is presented in the form of the well-known Gaussian tail probability. Accordingly, a modified probabilistic model is introduced to study the effect of impulse voltage superimposed on the AC voltage on the breakdown probability of MV spark-gaps. The modified model is verified using experimental data, where the experimental setup is arranged to generate a range of impulse voltages superimposed on the ac voltages. The results show evidence of the efficacy of the proposed probabilistic model. Furthermore, the proposed model is used to evaluate single-phase, two-phase and three-phase spark-gap breakdown probabilities in the case of lightning induced overvoltages. Finally, these breakdown probabilities are used along with the simplified Rusck expression to evaluate the performance of MV overhead lines above a perfectly conducting ground under lightning-induced voltages using a statistical approach.

STATE-OF-POLARIZATION ENCODING FOR OPTICAL CODE-DIVISION MULTIPLE-ACCESS NETWORKS

In this paper we propose the use of the State of Polarization (SOP) encoding in order to realize an optical CDMA system. The SOP of the optical beam is alternately switched between two orthogonal SOPs according to a given codeword. Multiple users can use the network simultaneously each with a distinct codeword. Gold and Hadamard codes can be applied with minimal interference in the latter case. The optical beam power is constant in such case, therefore, self-phase and cross-phase effects can be eliminated.

Random Power Control for Uncorrelated Rayleigh Fading Channels

When the channel measurement update rate is very slow compared to the channel fluctuations, the performance of conventional power control algorithms is greatly reduced. In this paper we introduce novel method to update the transmit power in such extreme situations. We propose to use random power allocation with certain distribution function and then using the channel measurements to update the statistics of this random power rather the power value itself. The truncated inverted exponential distribution is one candidate for the power distribution which is used in this paper. This distribution has one parameter which can be easily updated according to the channel quality. The signal-to-interference and noise ratio (SINR) is used as an indicator for the channel quality. The probability of outage when using the proposed power allocation is also given in mathematical form. Simulations show that, one may improve the average SINR when using the proposed algorithm compared to the conventional distributed power control algorithm.

DWT and Bayesian technique for enhancing earth fault protection in MV networks

In this paper, a Bayesian selectivity technique is introduced to identify the faulty feeder in compensated medium voltage (MV) networks. The proposed technique is based on a conditional probabilistic method applied on features extracted from the residual currents only using the Discrete Wavelet Transform (DWT). DWT enhances to localize initial transients generated in the network due to the fault event. The absolute sum of a window of the DWT detail coefficient is used to detect the fault. The conditional probability provides the selectivity decision. The fault cases occurring at different locations in a compensated 20 kV network are simulated by ATP/EMTP concerning practical fault case such as arcing faults. Test results corroborate the efficacy of proposed technique.

Nonlinear Power Control for Asynchronous Fiber-optic CDMA Networks

In this paper we investigate the effect of photodetector nonlinearity on the optimum transmit power setting in an asynchronous optical CDMA network. Network impairments as well as thermal noise, shot noise and multiple access interference (MAI) are taken into account. Unlike in traditional systems, the optical detector generated shot noise adds a new self-interference normally not considered previously as part of the MAI. We formulate a centralized power control algorithm and propose novel, iterative, Nonlinear Optical Power Control (NOPC) algorithm. Furthermore, convergence properties of the proposed algorithm are studied. We prove that the iterative NOPC converges to a unique point. Solution feasibility is then formulated in terms of the employed network parameters, MAI and target SIR. Finally, simulation results are given illustrating the theoretical analysis.

Polarized Optical Orthogonal Code for Optical Code Division Multiple Access Systems

In this paper a Polarized Optical Orthogonal Code (Polarized-OOC) is proposed by exploiting the polarization property of the fiber and the chip’s polarization state. The polarized-OOC code is generated using the concept of Mark Position Difference (MPD) set. Polarized-OOC code cardinality is shown to be two times that of the conventional OOC which reflects an increase in the number of supported users. Furthermore, since the correlation properties of the constructed code are the same as that of conventional OOC, error rate performance is evaluated in the same way as in conventional OOC. Also, a simple procedure for fiber-induced polarization rotation compensation is introduced. We then use simulations to show that relative polarization axis misalignment between the desired user and interfering users leads to violation of the correlation properties of the proposed code. 126 Tarhuni, Elmusrati, and Korhonen