In this paper, we introduce a stochastic shape optimization technique for base-station placement in cellular wireless communication networks. We formulate the data-driven facility location problem in a gradient-based framework and propose an algorithm that computes stochastic gradients efficiently via nearest-neighbor evaluations on Voronoi diagrams. This enables the use of Shannon-entropy objectives that promote balanced coverage and yield more than two orders of magnitude reduction in per-iteration runtime compared to a conventional integral-based optimization that assumes full knowledge of the under-lying density, making the proposed approach practical for real deployments. We highlight the requirements of facility location balancing problems with the introduction of the Adjusted Entropy Ratio and show a significant improvement in load balancing compared to the baseline algorithms, particularly in scenarios where baseline algorithms fall short in subdividing crowded areas for more equitable coverage. A downlink telecom evaluation with realistic propagation and interference models further shows that the proposed method configuration substantially improves user-rate fairness and load balance. Our results also show that Self-Organizing Maps (SOMs) provide an effective initialization by capturing the structure of the users’ location data.

In this paper, we explore the general principles of time-division multiple-access passive optical networks (PON) through the lens of quantum light signals. We investigate the optimal quantum receiver for various quantum light signals and delve into the fundamental constraints imposed by their quantum nature. This paper evaluates the optimal error probability for the Fock, coherent and phase-randomized coherent states. Furthermore, we investigated the power splitter/combiner channel, which is the main element in a PON, from the quantum viewpoint using the scattering matrix formalism. We demonstrated that under the low-loss condition, the error probability of the Fock quantum passive optical network (QPON) can surpass the coherent QPON.

Future heterogeneous wireless networks (HetNets) should provide massive connectivity for a large number of devices. In such networks, the problem of user association and multiple access management is of paramount importance. This paper formulates a statistical representation of the user association problem using users' probability density function. In other words, this paper is a bridge between the dynamic mixed-integer formulation of the user association used in relaxation, game theory, and reinforcement learning approaches and the static user association considered in analytical stochastic geometry methods. To this aim, a novel representation independent of the number of users is derived from the combinatorial user association formulation. Interestingly, we show that for fair user association, the statistical representation is a multi-objective optimization. The first objective is to maximize the network throughput with fairness consideration, and the second objective is to optimize the load balancing in terms of the Shannon entropy. Based on this representation, we introduce an algorithm to optimize user association using the first-order derivative formula. We propose a method that can optimize individual base stations' bias factors inside each tier of a HetNet. Numerical results show that the statistical representation closely tracks the stochastic behavior of the dynamic association. The proposed optimization approach improves the 10% outage rate of a two-tier network by 19% and enhances the load balancing by reducing the load of macro base stations by 22%.

Elastic Optical Network (EON) is a promising solution to address the high capacity, low latency, and flexibility requirements of the upcoming 5th-generation (5G) networks. Furthermore, Multi-Core Fibers (MCFs) and Space Division Multiplexing (SDM) technique can be utilized to overcome the capacity limitation of the conventional Single Mode Fibers (SMFs). On the other hand, Quantum Key Distribution (QKD) is an effective solution to address the security issues in 5G transport networks. In this paper, we investigate the performance of QKD over elastic optical networks with multi-core fibers and address the resource allocation problem for quantum and classical channels of QKD (QChs and CChs) and conventional data channels (DChs). To do so, we calculate the background noise caused by different noise sources and accordingly calculate the Secret Key Rate (SKR) in quantum channels. Then, we propose an Integer Linear Programming formulation and a heuristic algorithm to allocate network resources (spectrum, core, and links) to QChs, CChs, and DChs, with the objective of maximizing the secret key rate and minimizing the number of utilized frequency slots (FSs). Finally, we evaluate the proposed ILP and heuristic algorithm in terms of SKR and the number of utilized FSs. In our simulations, we consider core and metro topologies, fixed and distance adaptive launch power for classical signals, different fiber specifications, and different assumptions regarding the relative locations of quantum and classical channels in a multi-core fiber.

Barcoding photons, atoms, and any quantum states can provide a host of functionalities that could benefit future quantum communication systems and networks beyond today's imagination. As a significant application of barcoding photons, we introduce code division multiple-access (CDMA) communication systems for various applications. In this context, we introduce and discuss the fundamental principles of a novel quantum CDMA (QCDMA) technique based on spectrally encoding and decoding of continuous-mode quantum light pulses. In particular, we present the mathematical models of various QCDMA modules that are fundamental in describing an ideal and typical QCDMA system, such as quantum signal sources, quantum spectral encoding phase operators, M×M quantum broadcasting star-coupler, quantum spectral phase decoding operators, and the quantum receivers. Following the above discussions, we then elaborate on a QCDMA system with M users. In describing a QCDMA system, this paper considers a unified approach where the input continuous-mode quantum light pulses can take on any form of pure states such as quantum coherent (Glauber) states and quantum number (Fock) states. The mathematical presentation is independent of the form of the input pure quantum states. We show that the spectrally encoded quantum states of the light at the quantum star-coupler output are not, in general, factorized states, except for input Glauber states. For input number states, one can observe features like entanglement and quantum interference. Moreover and interestingly, as a consequence of Heisenberg's uncertainty principle, the quantum signals sent by photon number states obtain complete phase uncertainty at the time of measurement. Therefore, at the receiver output, the multiaccess inter-signal interference vanishes. Due to Heisenberg's uncertainty principle, the received signal intensity at the photodetector's output, right at the time of measurement, changes from coherent detection scheme for input Glauber states to incoherent detection scheme for input number states. We also would like to highlight that the mathematical models and tools developed in this paper, in the context of QCDMA, become very useful for developing and analyzing other quantum multiple-access techniques based on wavelength, space and time domain. Furthermore, our mathematical model is valuable in signal design and data modulations of point-to-point quantum communications, quantum pulse shaping, and quantum radar signals and systems where the inputs are continuous mode quantum signals. QCDMA may open up novel possibilities for various quantum technologies such as data privacy, distributed quantum computation, quantum internet, and anti-jamming quantum communication systems.

In this paper, a new approach toward the optimization of Access Point (AP) placement in cellular Visible light Communication (VLC) networks is proposed based on the projected gradient algorithm. The objective of the optimal placement problem is to maximize the average throughput of the network subject to constraints on the minimum illumination level and the minimum rate of the users. This optimization framework gives the enhanced AP deployment in case of users with static, nomadic, or completely mobile behavior. Taking the distribution of users, the receiver's field of view, reflection from walls and interference from neighboring APs into account makes the deployment problem complicated. To solve the arising intricate optimization problem, we derive analytical expressions for gradients of the objective and use them in the gradient ascent algorithm. The proposed adaptive gradient projection method then realizes the constraints. This method relaxes the projection onto a high dimensional space to planar projections, which are implemented using efficient tools from computational geometry. The proposed method eliminates the need for an intractable exhaustive search to find the optimal placement of VLC APs in cellular VLC networks, while it gives either exact optimum or very close approximations to the optimal placement. It is shown, with the aid of properties of convolution of parametric concave functions, that in some practical cases the objective function is unimodal and has no local optimum. Simulation results show a significant improvement in the throughput, SINR and outage probability of the system when the access points are deployed according to their optimized placement, determined by the algorithm. On the other hand, proper constraints on the minimum achievable rate and minimum illumination level improve the worst-case performance of the network to the desired extent.

Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely, absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations, as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path and, then, examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beam-expander-and-collimator (BEC) at the transmitter side and aperture averaging lens (AAL) at the receiver side are experimentally investigated. We show that the use of a transmitter BEC and/or a receiver AAL suits single-lobe distributions, such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of 10-3s and larger which implies to the slow fading turbulent channels.

In this paper, we investigate multi-quality of service (QoS) routing and spectrum assignment in elastic optical networks (EONs) by utilizing network state information in terms of bandwidth usage and holding time of established connections. Toward this aim, two types of traffic, namely big data traffic (BDT) requests, and fixed rate flow requests (FLRs) are considered, where BDTs can tolerate a pre-determined initial delay while FLRs must be served immediately without any delay. We assume that both frequency and time usage of optical bandwidth is slotted, and accordingly optical resources are defined in a two-dimensional (2D) time/frequency domain, where the smallest resource is specified with its time and frequency slot number. Furthermore, we define two controlling coefficients determining the ratio of time and spectrum slots which can be assigned to BDTs. By selecting various values for these coefficients, network manager will be able to support different class of services and change the relative priorities of different classes. In addition, for each coefficient we obtain a threshold determining the upper bound on the number of time or frequency slots allocated to BDTs. Our results highlight the relation between the blocking probability and the value of these thresholds. In particular, the results show that assigning smaller resource blocks will result into lower blocking probability for both FLRs and BDTs. In addition, the maximum initial delay plays an important role in the blocking probability of BDTs.

In this paper, we analytically study the performance of multiple-input multiple-output underwater wireless optical communication (UWOC) systems with ON-OFF keying modulation. To mitigate turbulence-induced fading, which is amongst the major degrading effects of underwater channels on the propagating optical signal, we use spatial diversity over UWOC links. Furthermore, the effects of absorption and scattering are considered in our analysis. We analytically obtain the exact and an upper bound bit error rate (BER) expressions for both optimal and equal gain combining. In order to more effectively calculate the system BER, we apply Gauss-Hermite quadrature formula as well as approximation to the sum of lognormal random variables. We also apply the photon-counting method to evaluate the system BER in the presence of shot noise. Our numerical results indicate an excellent match between the exact and upper bound BER curves. Also, a good match between the analytical results and numerical simulations confirms the accuracy of our derived expressions. Moreover, our results show that spatial diversity can considerably improve the system performance, especially for channels with higher turbulence, e.g., a 3×1 multiple-input single-output transmission in a 25 m coastal water link with a log-amplitude variance of 0.16 can introduce 8 dB performance improvement at the BER of 10-9.

In this paper, we formulate an integer linear programming (ILP) to perform multicast routing and spectrum assignment (MRSA) in elastic optical networks, which serves jointly a set of multicast requests. In this formulation, all physical layer restrictions including modulation level assignment, maximum number of multicast capable nodes (MCNs), and maximum splitting degree (MSD) of MCNs, are considered. In addition, we modify the proposed joint ILP to serve multicast requests one-by-one, which is referred to as a separate ILP. Furthermore, we present three heuristic algorithms for MRSA, namely distance-based MRSA (DMRSA), congestion-based MRSA (CMRSA), and mixed CMRSA/DMRSA, which are applicable in both static and dynamic operation scenarios. In CMRSA and DMRSA, the link length and the amount of occupied spectrum are considered as the cost function of multicast routing, respectively; and in mixed CMRSA/DMRSA, a combination of normalized link length and normalized occupied spectrum is considered as the cost function. The comparison of ILPs and heuristic algorithms in static operation reveals that the joint ILP, as the benchmark, gives the optimum solution while has the most computational complexity. Furthermore, the separate ILP has lower complexity at the cost of consuming slightly more spectrum. Unless the DMRSA method, which has the worst performance, the gap between the other two heuristic algorithms and the ILPs is negligible. Furthermore, simulation results of dynamic operation scenarios reveal that mixed CMRSA/DMRSA outperforms other two heuristics algorithms in terms of blocking probability.

To cope with the unprecedented growth of mobile data traffic, we investigate the performance gains obtained from unifying coverage-centric 4G mobile networks and capacity-centric fiber-wireless (FiWi) broadband access networks based on data-centric Ethernet technologies with resulting fiber backhaul sharing and WiFi offloading capabilities. Despite recent progress on backhaul-aware 4G studies with capacity-limited backhaul links, the performance-limiting impact of backhaul latency and reliability has not been examined in sufficient detail previously. In this paper, we evaluate the maximum aggregate throughput, offloading efficiency, and in particular, the delay performance of FiWi enhanced LTE-Advanced (LTE-A) heterogeneous networks (HetNets), including the beneficial impact of various localized fiber-lean backhaul redundancy and wireless protection techniques, by means of probabilistic analysis and verifying simulation, paying close attention to fiber backhaul reliability issues and WiFi offloading limitations due to WiFi mesh node failures as well as temporal and spatial WiFi coverage constraints. We use recent and comprehensive smartphone traces of the PhoneLab data set to verify whether the previously reported assumption that the complementary cumulative distribution function of both WiFi connection and interconnection times fit a truncated Pareto distribution is still valid. In this paper, we put a particular focus on the 5G key attributes of very low latency and ultra-high reliability and investigate how they can be achieved in FiWi enhanced LTE-A HetNets. Furthermore, given the growing interest in decentralization of future 5G networks (e.g., user equipment assisted mobility), we develop a decentralized routing algorithm for FiWi enhanced LTE-A HetNets.

Because of opportunistic spectrum access, cognitive radio networks is a promising solution for spectrum scarcity problem. In cognitive radio networks, users need to communicate through control channels in order to cooperate for sharing sensing information, channel sharing and spectrum management. As a result control channels security is of a great importance. If control channels get jammed, network's performance degrades dramatically; therefore, the jammers primary target in cognitive radio networks is to demolish control channels in order to disturb the network's performance. Traditionally hopping sequences were used to battle this problem but considering cognitive radio networks intelligence, dynamic spectrum access and learning algorithms all can be hired to confront jammer. In this paper, by using game theoretic concepts, stochastic games and by defining states, actions and players' reward, we introduce a stochastic game with multiple users to make control channels resilient against sweep attacks. Win or Learn Fast (WoLF) Q learning algorithm has been used to solve the designed game and variable learning rate are used to make sure that the designed game converges. By simulation we show that in the presence of dynamic channel variations and imperfect sensing the proposed algorithm outperforms the existing ones in terms of jammer resistivity.

In this paper, we analyze the performance of code acquisition system in atmospheric optical code division multiple access (OCDMA) communications using optical orthogonal codes. Memory introduced by temporal correlation of optical fading process precludes us from using the Markov chain model for a code acquisition analysis. By considering this issue, we discuss how to extend the applicability of the Markov chain model to the atmospheric OCDMA communications. We analyze and compare the performance of correlator and chip level detector (CLD) structures in the acquisition system. In our analysis, we consider the effects of free space optical channel impairments, multiple access interference, and receiver thermal noise in the context of semi-classical photon-counting approach. Furthermore, we evaluate the performance of various two stage schemes that utilize different combinations of active correlator, matched filter, and CLD in search and verification stages, and we find the optimum acquisition scheme among them. Numerical results show significant improvement in reducing the acquisition time and required power for synchronization using our optimum scheme in the wireless OCDMA communications.

In this paper, we characterize the performance of relay-assisted underwater wireless optical code division multiple access (OCDMA) networks over turbulent channels. In addition to scattering and absorption effects of underwater channels, we also consider optical turbulence as a log-normal fading coefficient in our analysis. To simultaneously and asynchronously share medium among many users, we assign a unique optical orthogonal code (OOC) to each user in order to actualize OCDMA-based underwater network. The most significant challenge in underwater optical communication is in the ability to extend the short range of its coverage. In order to expand the viable communication range, we consider multi-hop transmission to the destination. Moreover, we evaluate the performance of a relay-assisted point-to-point UWOC system as a special case of the proposed relay-assisted OCDMA network. Our numerical results indicate significant performance improvement by employing intermediate relays, e.g., one can achieve 32 dB improvement in the bit error rate (BER) of 10-6using only a dual-hop transmission in a 90 m point-to-point clear ocean link.

To cope with the unprecedented growth of mobile data traffic, we investigate the performance gains obtained from unifying coverage-centric 4G mobile networks and capacity-centric fiber-wireless (FiWi) broadband access networks based on data-centric Ethernet technologies with resulting fiber backhaul sharing and WiFi offloading capabilities. Despite recent progress on backhaul-aware 4G studies with capacity-limited backhaul links, the performance-limiting impact of backhaul latency and reliability has not been examined in sufficient detail previously. In this paper, we evaluate the maximum aggregate throughput, offloading efficiency, and in particular the delay performance of FiWi enhanced LTE-A heterogeneous networks (HetNets), including the beneficial impact of various localized fiber-lean backhaul redundancy and wireless protection techniques, by means of probabilistic analysis and verifying simulation, paying close attention to fiber backhaul reliability issues and WiFi offloading limitations due to WiFi mesh node failures as well as temporal and spatial WiFi coverage constraints.

In this paper, we introduce and investigate a cellular underwater wireless optical code division multiple-access (OCDMA) network based on optical orthogonal codes (OOC). The structures, principles, and performance of the underwater wireless OCDMA network in various water types are presented. Since underwater wireless optical links are considered for high-bandwidth underwater communications at short ranges, we will place a set of optical base transceiver stations (OBTS) each in the center of a hexagonal cell to cover a larger underwater area. The OBTSs are connected via fiber optic to an optical network controller (ONC) which operates as the core of the network. An integral expression for system error probability is obtained for both uplink and downlink schemes under a specific channel model expressed in this paper, among many channel models in underwater optical communication systems. Furthermore, an experimental prototype of underwater an OCDMA system including the OBTS, ONC, transmitters, and receivers are implemented using field programmable gate arrays (FPGA).

In this paper, we present an analytical approach in obtaining the probability density function (pdf) of the random decision variable Y, which is formed at the output of the power-cubic all-optical nonlinear preprocessor followed by the photodetector, with applications in ultrafast optical time-division multiplexing and optical code-division multiple-access systems. Our approach can be used to accurately evaluate the performance of ultrafast pulse detection in the presence of Gaussian noise. Through rigorous Monte Carlo simulation, the accuracy of the widely used Gaussian approximation of decision variable Y is refuted. However, in this paper, we show that the so-called log-Pearson type-3 (LP3) pdf is an excellent representation for the decision variable Y. Three distinguishable parameters of the LP3 pdf are obtained through an analytical derivation of three moments of the decision variable Y. Furthermore, toward a more realistic model, in addition to amplified spontaneous emission Gaussian noise, the effects of shot and thermal noises are also included. Finally, using the presented analytical approach, it is shown that the power-cubic preprocessor outperforms its quadratic counterparts, i.e., second-harmonic-generation and two-photon-absorption devices, in the high-power regime where shot and thermal noises can be neglected.

In this paper, we shed some light on the latency and reliability issues of mobile backhaul networks, which have been largely ignored in the past, and examine their impact on LTE-A heterogeneous networks (HetNets). Specifically, we propose a backhaul-aware user association algorithm for fiber-wireless (FiWi) enhanced LTE-A HetNets. The performance limiting factors of state-of-the-art fiber backhaul infrastructures are highlighted and a variety of solutions are described. To mitigate the vulnerability of the backhaul against fiber cuts, we introduce different advanced protection techniques. Accounting for the given conditions of the backhaul in terms of delay and reliability, we present a distributed load-balancing algorithm for user association in FiWi-LTE HetNets. The proposed algorithm is analyzed and evaluated numerically by comparing its performance with state-of-the-art alternative approaches in terms of average delay, blocking probability, average achievable throughput, and service interruption percentage. The obtained results demonstrate that our algorithm outperform its counterparts in terms of delay and service interruption percentage, while its average achievable throughput is the same as that of a backhaul-unaware alternative solution. In addition, the blocking probability of the proposed backhaul-aware load-balancing method is shown to be higher than that of backhaul-unaware ones.

In this paper, we analyze the exact node- and network-wise operation scenarios in elastic optical networks (EONs). First, the busy/idle patterns of optical spectrum in a stand-alone node are modeled by utilizing a continuous Markov chain. In the node-wise perspective, four operation scenarios are investigated, which are determined based on the spectrum allocation methods. We present an algorithmic procedure to derive the global balance equations of the corresponding Markov chains for all operation scenarios. Furthermore, the network-wise operation is assessed by analyzing the end-to-end blocking probability for two operation modes, with and without spectrum conversion capabilities at the intermediate nodes. Because the computational complexity of exact model increases exponentially versus the number of spectrum slots, we present two approximate alternatives. The results of the exact models, approximations, and verifying simulations are compared for small scale problem. Comparison reveals that the exact model and simulation match very well. In addition, the accuracy of both approximations is acceptable. The approximate solutions are also examined under large scale scenarios by considering simulation results as a benchmark. The accuracy of the first approximation is not degraded in large scale cases, as opposed to the second one, which is only applicable to small scale problems.

In this study the authors present an in-depth study, analysis and discussion on maximum likelihood (ML)-based receiver for a typical spectrally encoded spread-time CDMA in the presence of multiple narrowband interference (NBI) signals in an additive white Gaussian noise channel. Furthermore, the authors demonstrate that by combining useful properties of ML-based receiver and three-level codes, that is, codes with values of −1, 0, +1, the authors can introduce a new strategy in which superior performance with respect to previous receiver structure based on two-level codes, that is, codes with values of −1, +1, can be attained. With the help of an example, the authors drive, first, the required signal-to-noise+interference power ratio (SNR) in a multiple access communication environment with multiple NBIs and, second, SNR attenuating parameter owing to multiple NBIs for two- and three-level codes. Finally, the authors relate the above parameters to a number of simple descriptive factors such as bit error rate. The results are validated by an extensive simulation on the aforementioned system.

In the above titled paper (ibid., vol. 58, no. 5, pp. 1589-1598, May 10), Fig. 4 was reproduced incorrectly. The correct Fig. 4 is presented here.

In this paper we study and evaluate the performance of a spectral-phase encoded optical CDMA (SPE-OCDMA) system using advanced receiver structures based on second harmonic generation (SHG) effect imposed in a Thick or Thin crystal. This receiver structure is employed as the nonlinear pre-processor prior to the conventional low-speed photodetector. In our performance evaluation amplified spontaneous emission (ASE) of the optical amplifiers, receiver front end thermal noise and photodetector shot noise which are effective in low power conditions, and multiple access interference (MAI) noise which is the dominant source of noise in high power conditions in any coherent OCDMA communications system have been considered. We begin by studying the statistical behavior of Thick crystals in an optically amplified digital lightwave communication system in the context of a SPE-OCDMA communication system. The error probability for Thick crystal receiver structure is evaluated using Saddle-Point approximation. Furthermore, we obtain a closed form approximation for the probability density function (pdf) of the decision variable in SPE-OCDMA systems with Thin crystal SHG receiver. In this analysis, to obtain approximate probability density function, we employ Gram-Charlier expansion based on the corresponding first order moments of the decision variable. The first three moments are obtained in a closed form approximation and consequently a closed form expression for the error probability is obtained by integrating the approximated pdf of the decision variables for transmitting bit "1" and "0" in Thin crystal receiver structure. The precision of the approximation is verified by the Monte-Carlo simulation. Finally the performance of SPE-OCDMA network for both Thin and Thick SHG crystals are analytically evaluated and discussed for different number of interfering users and different code-lengths and different speed of the conventional photodetectors. We deduce that the performance of Thin SHG crystal receiver is more sensitive to the transmitted power level especially in low power conditions. It is concluded that in low power conditions where ASE, thermal and shot noises are the dominant noise sources, Thick SHG crystal receiver outperforms the Thin crystal receiver. However, in high power conditions where MAI noise is the most effective noise term, Thin SHG crystal receiver structure outperforms the Thick crystal receiver.

Autocorrelation receiver (AcR) is usually used as the general receiver for transmitted reference (TR) communication systems on a quite intuitive basis. In this paper, we apply the principle of statistical invariance to obtain the optimum TR receiver on a decision-theoretic basis, and it is shown that the autocorrelation receiver is optimal only in a special case. Performance of the optimum receiver and the suboptimum AcR for some important modulation schemes is also evaluated. The results imply that allocating a part of the transmitted energy and bandwidth to reference pulses would not help improve the performance of the system and the optimal performance might be achieved via much simpler non-TR transceiver structures. However, it is shown that differential TR and some other generalizations might be used to boost the performance, and numerical results are provided to assess the degree of performance improvement achieved in each case.

Considering the space of constant-weight sequences as the reference set for every optical orthogonal code (OOC) design algorithm, we propose a classification method that preserves the correlation properties of sequences. First, we introduce the circulant matrix representation of optical orthogonal codes and, based on the spectrum of circulant matrices, we define the spectral classification of the set Sn,wof all (0, 1)-sequences with length n, weight w, and the first chip “1”. Then, as a method for spectrally classifying the set Sn,w, we discuss an algebraic structure called multiplicative group action. Using the above multiplicative group action, we define an equivalence relation on Sn,win order to classify it into equivalence classes called multiplicative partitions which are the same as the spectral classes. The algebraic properties of the proposed partitioning such as the number of classes and the size of each class are investigated and in the case of primen, a novel formula for the number of classes is derived. Finally, we present and prove the autocorrelation, intraclass and interclass cross-correlation properties of our proposed classification of the space Sn,wthat decrease the computational complexity of search algorithms in designing and constructing(n, w, λa,λc)-OOC.

A binary differential impulse radio-ultra wide band (IR-UWB) communication scheme over a single-mode optical fibre is examined. For a receiver structure, the conventional electrical receiver as well as an optical receiver structure, which is similar to the optical receiver used for digital, optically phase-modulated differential phase shift keying, are considered. The optical receiver can alleviate the IR-UWB receiver implementation challenges and it is studied for the first time in the context of IR-UWB. Considering various important noises, for example, phase noise, laser intensity noise, thermal noise and shot noise, analytical expressions for the error probability of the aforementioned receivers are derived. The mathematical models for optical components including laser diode and single-mode fibre, along with the analytical expressions for the receiver's error probability, are used to evaluate the overall performance of an UWB communication system over a fibre transmission medium. Furthermore, the electrical receiver is compared with the optical receiver and it is shown that the performance of the optical receiver can be as good as that of the electrical receiver and even better. The impact of wireless channel fading, bias current of laser diode and the coherence time of laser diode on the UWB over fibre system performance is also examined.

In this paper we present and investigate a digital design and implementational details of a wireless optical CDMA (OCDMA) system based on generalized optical orthogonal codes (OOCs). A detailed digital implementation analysis and transmitter/receiver architectures in the context of the above design are fully studied. We analyze various subsystems for a wireless OCDMA system including detector and receiver structure, acquisition, and tracking blocks as well as evaluating their performance in a typical wireless OCDMA communication system. By analyzing two main parts of the system, namely, chip-level detector and OCDMA receiver blocks we evaluate the performance of the overall system. Analytical and simulation results are presented to depict system performance. Results on the design and implementation of such systems in this paper indicate the viability and the importance of OOC based wireless OCDMA technology to satisfy certain applications need in a wider communication system worldwide. Finally we describe an experimental prototype where we highlight the main aspects of the implementation of the system.

Detection of ultrashort light pulses with conventional bandlimited photodetectors can lead to substantial performance degradation in digital lightwave communication systems. All-optical nonlinear processing of the received field prior to the optical to electrical conversion can greatly overcome this problem. In this letter, through a decision-theoretic framework and using a basic invariance property of the photodetector, we obtain the optimal zero-memory nonlinear element and evaluate its performance. Possible suboptimum nonlinearities are also investigated and their performance is examined. The results in this letter strongly justify the intuitive use of power-law devices, such as those implemented via two photon absorption (TPA) or second harmonic generation (SHG), in the detection of ultrashort light pulses and show how by using them almost optimal performance can be achieved.

In this paper we introduce and propose novel signaling methods and receiver structures based on advanced binary optical logic gates for fiber-optic code division multiple access (FO-CDMA) systems using all-optical signal processing. In the proposed system the users of the network are categorized into multiple classes. Users of each class transmit at the same power level but different from the levels of the other classes' users. Using a combination of optical OR, AND and XNOR logic gates for the receiver structure we show that such a network not only takes the full advantages of all-optical signal processing but also demonstrates a considerable throughput efficiency when compared to ordinary FO-CDMA systems. The proposed receiver structure mitigates the effect of interfering users from the other classes by rejecting some specified power level combinations from the other classes. The depth of interference cancellation is a function of the corresponding number of power levels and the number of stages applied to the optical logic gates in use. In our analysis we choose the generalized form of optical orthogonal codes (OOC), i.e., OOCs with cross-correlation value greater than one, as the signature sequence. We begin by emphasizing on two-level systems, that is, when the users can select one out of two power levels for signal transmission. However, for multilevel FO-CDMA we obtain a closed-form relation for the upper bound on the probability of error.We will show that under the ideal case the increase in throughput resulting from the proposed multilevel system is proportional to the number of classes or power levels in use. Our analytical results are compared to the results of an extensive system simulation. The numerical closeness between, the analytical and system simulation, indicates the accuracy with which we have modeled mathematically our proposed signaling using advanced binary optical logic gates in FO-CDMA.

In this letter we introduce an effective tool to demonstrate signal power behavior of a typical spectrally encoded spread-time (SE/ST) CDMA system using probabilistic approach. By defining power level profile (PLP), as the probability of stretching instantaneous power from a pre-specified threshold, we can find novel upper bounds on the signal power profile. We discuss various properties owing to PLP and elaborate on the main parameters affecting the PLP value. We present three theorems, propose some simple upper bounds on the PLP, and give an insight on the main contributions of the results. We introduce a fundamental parameter, namely ß, as a key design parameter which relates the most important quantities affecting the PLP behavior. Finally, we propose a corollary demonstrating an approach to guarantee that the PLP value is set at zero. It is shown that using suitable distributions, we can also obtain some distribution gain, thereby enhancing the overall system performance.

In this paper we study and present an in-depth analysis on the operability and the viability of a typical wireless optical CDMA (OCDMA) local area network. Three receiver structures for OCDMA systems, using optical orthogonal codes (OOC) with minimum auto and cross-correlations as signature sequence, namely, correlation, correlation with hard-limiter, and chip-level detection are studied and proposed for such a network. For the synchronization circuit design the performance of two algorithms for OOC based OCDMA networks, namely, simple serial-search and multiple-shift in the context of wireless OCDMA LAN are studied. Furthermore, we study a synchronization method based on matched filtering and show that it presents a much better performance in our wireless OCDMA system. The effect of sampling rate and its performance on tracking circuit is analyzed. Bit-error-rate (BER) analysis is performed by photon counting methodology. Multi-user interference (MUI), ambient light, and photodetector dark current are considered in our analysis. Our analysis strongly indicates the viability and practicality of such systems in certain important wireless optical communications systems.

In this letter we consider optimum code structure for positive optical code division multiple-access (optical CDMA) systems. Positive systems are a class of systems that operate with positive real numbers only. We consider the effect of multipleaccess interference in our model and show that code design for both On-Off Keying (OOK) and Binary PPM optical CDMA systems results in the same solutions. Furthermore, we show that a class of codes known as optical orthogonal codes (OOCs) are the best possible positive codes. In obtaining the results we define normalized divergence based on signal-to-multipleaccess interference ratio (SIR) for a multiple-access system in a useful manner and use it as our criterion to maximize the multiple-access capability of the codes. Finally, we demonstrate that BPPM/OOC can be considered as the closest counterpart of plusmn1 pseudorandom sequence in radio CDMA communication systems.

In this paper we study and analyze the performance of the RAKE receiver for a typical ultrawideband (UWB) spectrally-encoded spread-time (ST) CDMA in the presence of multiple Gaussian interference (GI) signals. We demonstrate that by combining useful properties of three-level codes, i.e., codes with values of -1, 0, +1, and RAKE receiver, we can introduce a new strategy in which it can result in superior performance when compared to previous results. In particular, following a discussion on three-level codes we can show that RAKE receiver with optimum multiplicative weights achieves a better performance when compared with simple two-level codes. With the help of an example we show various properties of RAKE receiver as well as three-level codes and relate these properties to a number of simple descriptive parameters of the communication media. Simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the above system.

In this paper we study the performance of spectrally-encoded spread-time (SE/ST) technique when applied to ultrawideband (UWB) signals in the context of wireless code-division multiple-access (CDMA) communication. In this technique UWB pulses are spectrally encoded with pseudorandom codes that are assigned uniquely to each user. We shall show that the SE/ST ultrawideband system provides an appropriate multi-user capacity in realistic UWB channels, along with the ability of narrowband interference (NBI) suppression. To this end, after obtaining a suitable statistical model for spectrally encoded spread-time signals, we investigate the performance of the system in the presence of multiple-access and narrowband interferers, and simple and accurate approximations will also be developed which are in good agreement with simulation results. Furthermore, the effect of variations of the main system parameters and some of the relevant trade-offs will be numerically demonstrated.

In this letter, we introduce a whole new approach in defining and representing optical orthogonal codes (OOCs), namely, outer-product matrix representation. Instead of applying commonly used approaches based on inner product to construct OOC codes, we use the newly defined approach to obtain a more efficient algorithm in constructing and generating OOC codes. The outer-product matrix approach can obtain a family of OOC codes with a cardinality closer to the Johnson upper bound, when compared with the previously defined accelerated greedy algorithm using the inner-product approach. We believe the new look introduced in this letter on OOCs could help to devise new approaches in designing and generating OOC codes, using the rich literature in matrix algebra

We propose and consider using a class of multiple-access sequences, namely, optical orthogonal codes (OOCs) in atmospheric optical code-division multiple-access systems. We obtain analytical solutions to the error probability for various channel models using positive-intrinsic-negative diode and avalanche photodiode photodetectors. In our analysis, the effects of atmospheric turbulence, ambient light, thermal noise, and multiuser interference are considered, in the context of a semiclassical photon-counting approach. The performance of the systems taking advantage of space diversity and error-correcting codes are also evaluated. Two common and widely used optical modulations, on-off keying and pulse-position modulation, are considered. Receiver structures based on correlator and chip level are used for OOC detection. Unlike the traditional chip-level receiver, here a generalized form of chip-level structure with two threshold levels is considered. Upper and lower bounds on the error probability for the above-chip-level receiver structure is obtained. From our analytical results, we can deduce that the chip-level receiver outperforms a simple correlator in the absence or weak atmospheric fading; however, in a strong fading environment, the simple correlator outperforms the chip-level receiver

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (CDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation$lambda = 1$were considered, we consider generalized OOCs with$1 leq lambda leq w$, where$w$is the weight of the corresponding codes. To enhance the performance of such systems, we propose that use of optical AND gate receiver, which, in an ideal case, e.g., in the absence of any noise source except the optical multiple-access noise, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with$lambda = 1,2,3,ldots,w$, with optical AND gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with$lambdageq 1$. In particular, we can obtain some simple relations between$ P_ e min$(minimum error rate),$L_min$(minimum required OOC length), and$N_max$(maximum number of interfering users to be supported), which are the most desired parameters for any optical CDMA system design. Furthermore, we show that in most practical cases, OOCs with$lambda = 2$or$3$perform better than OOCs with$lambda = 1$, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of$sqrt2lambda L$where L is the length of the OOCs used in systems.

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (OCDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation lambda=1 were considered, we consider generalized OOCs with 1leslambdalesw, where w is the weight of the corresponding codes. To enhance the performance of such systems, we propose the use of an optical and logic gate receiver, which, in an ideal case, e.g., in the absence of any noise source, except the optical multiple-access interference, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with lambda=1,2,3,...,w, with the optical and logic gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with lambdages1. In particular, we can obtain some simple relations between Pemin(minimum error rate), Lmin(minimum required OOC length), and Nmax(maximum number of interfering users to be supported), which are the most desired parameters for any OCDMA system design. Furthermore, we show that in most practical cases, OOCs with lambda=2,3 perform better than OOCs with lambda=1, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of radic2lambdaL where L is the length of the OOCs

In this paper, we introduce a new and advanced algorithm, namely, multiple-shift algorithm for code acquisition in optical code-division multiple access (CDMA) systems using unipolar optical orthogonal codes (OOCs) as signature sequences. We analyze the performance of the newly proposed algorithm and obtain a bound on its performance and show its advantage in reducing the mean time of synchronization when compared with other synchronization methods. The algorithm can be used with many different receiver structures, like active or passive correlator with or without hardlimiter(s). However, in this paper, we only consider the simple active correlator structure for further discussions and analysis.

In this paper, we focus on the reverse link traffic analysis of a code-division multiple-access (CDMA) cellular network in dynamic environments. In this respect, we propose a new and flexible traffic model, which takes into account the interference-limitedness attribute of CDMA capacity as well as its soft-handoff feature. This new traffic model is developed according to an interference-based call admission control (ICAC) method and a geographical structure with three regions. The main advantage of this traffic model is in its flexibility when we consider different traffic conditions including time-varying status of traffic in the neighboring cells.

In this paper, we propose a general mobility model suitable for wireless multimedia networks. Our model is based on splitting a region into subregions. Furthermore, we make an analogy between subregions as well as their inter-connections with a multi-class Jackson queueing network comprising of infinite-server nodes. The main attribute of such a network is due to its product-form stationary distribution. Using this model, we are able to obtain a closed analytical form for the spatial traffic distribution corresponding to a specific number of network-connected users with different classes of service and mobility in a typical region. Also, we show the flexibility obtained by the proposed mobility model in representing some general distributions such as sum-of-hyper-exponentials (SOHYP), hyper-Erlang and Cox which were previously suggested to model mobility-related statistical parameters, e.g., cell dwell time and channel holding time. Finally, we apply the proposed model to a few mobility scenarios and obtain the resultant active user's location density.

We study and examine the effect of fading channels on the performance of spread-time/code division multiple access (ST/CDMA). We consider two different models for fading channel and obtain the average signal to interference plus noise ratio (SIR) for ST/CDMA in both cases. We also obtain the SIR for spread-spectrum/CDMA (SS/CDMA) and compare the results with that of ST/CDMA. Depending upon the transmitted chip pulse shape, and in the absence of thermal noise, spread-time (ST) performance is equal to or greater than spread spectrum (SS). For example ST/CDMA has 0.62 dB higher SIR value compared to SS/CDMA with root raised cosine pulse shape with rolloff factor 0.22 used in W-CDMA. The spread-time technique was first proposed by Crespo et al. (1991, 1995), and US Patents no. 5177768, 5175744, 5175743, 5173923, and is considered as the time-frequency dual of spread spectrum technique.

We consider using a soft-input decoder for internally bandwidth efficient convolutionally coded fiber-optic code division multiple access (CDMA) communication systems using optical orthogonal codes (OOC). The convolution codes that are used for demonstrating the capabilities of the soft-input decoder are super orthogonal codes (SOC). These codes are near optimal and have a relatively low complexity. We evaluate the upper bounds on the bit error probability of the internally coded fiber-optic CDMA system using soft-input and hard-input decoders, assuming both on-off keying (OOK) and binary pulse position modulation (BPPM) schemes. It is shown that the soft-input decoder significantly outperforms the hard-input decoder.

One of the most prominent features of code division multiple-access (CDMA) cellular networks is their ability for applying soft-handoff. This type of handoff results in a better support of seamless communication services, capacity enhancement, QoS improvement, etc. Because of the existence of more parameters and flexibilities compared to hard-handoff, an optimum solution for soft-handoff management and admission control for new and handoff calls is necessary. We propose a new algorithm with two thresholds for admission of new and handoff calls, and with two decision boundaries for controlling of handoff traffic. We compare this algorithm with a simpler one with only one decision boundary, by employing a general and flexible 3-dimensional framework with respect to important traffic parameters including loss probability, carried traffic, blocking and dropping probabilities, and show that our algorithm can result in less loss probability as well as less blocking and dropping probabilities with more control over traffic growth in the cell.

We study and analyze multiple-shift algorithms in the context of ultra-wideband radio frame time-hopping CDMA system (UWB-FTH-CDMA). We consider major sources of noise, namely multiaccess interference and additive white Gaussian noise. For multiaccess interference, we consider a model in which mutual interference due to each user is based on a frame asynchronous but half-chip synchronous system. Since simple serial-search is a special case of the multiple-shift algorithm, we can analyze the advantage of the multiple-shift algorithm. It is shown that the multiple-shift algorithm is superior to the simple serial-search algorithm in reducing the required average acquisition time by the square root of average acquisition time of the simple serial-search algorithm.

We propose the use of pulse shaping for mitigating the effect of channel distortion on the performance of indoor wireless infrared (IR) CDMA systems. The bit error rate (BER) performance of an on-off keying (OOK)-CDMA system as a function of the average transmitted power is investigated. To this end, a photon-counting model is utilized and major noise such as shot noise, ambient light, dark current, and receiver thermal noise are taken into consideration. It is shown that by using appropriate pulse shapes, the system performance can be improved and the required transmitter power can be reduced.

We compute the outputs of matched filters for spread-time CDMA with and without the effect of window which is used for truncating the resulting pulse. We obtain the signal to interference plus noise ratio (SIR) in both cases and compare the results. We examine the effect of different multiuser detectors used for spread spectrum (SS) in spread-time (ST). It is shown that ST and SS have the same performance while ST has the ability of matching the transmitted spectrum to the channel. Furthermore we consider the effect of synchronization error in the absence of any window. It is shown that synchronization error in timing causes infinite ISI terms that do not converge.

Spread-time code division multiple access (ST-CDMA) was proposed as an alternative CDMA scheme to spread spectrum (SS) (see Crespo, P.M. et al., Proc. IEEE Globecom, p.836-40, 1991; US Patents No. 5177768, 5175744, 5175743, 5173923; IEEE Trans. Commun., vol.43, no.6, p.2139-48, 1995). ST-CDMA can be considered as the time-frequency dual of SS-CDMA. In ST-CDMA, pseudorandom (PN) sequences are assigned to each user, and the Fourier transform of the transmitted pulse for a given user is determined by modulating the amplitude of the desired spectrum by the user's PN sequence. The main advantage of ST-CDMA is the flexibility with which power-limited pulses can be designed. We examine the effect of fading channels on the performance of spread-time. We compute the average signal-to-interference-plus-noise ratio in two different fading channels both for ST-CDMA and SS-CDMA. We show that ST-CDMA has 2.1 dB better performance than SS-CDMA with a rectangular pulse chip.

In this paper, we investigate the use of M-ary infrared CDMA technique in wireless infrared networks. It is shown that this method is highly capable in providing the multiple access facility for downlinks of infrared wireless networks. The results show that the M-ary CDMA technique, which fully utilizes the properties of optical orthogonal codes, achieves better BER performance in comparison with ordinary infrared CDMA networks. The main advantages are the reduction in bandwidth, power, and the elimination of threshold-setting at the receiver.

Matsufuji and Imamura (see IEEE Trans. on Info. Theory, vol.46, no.4, p.1691-95, 2000) proposed a new method to generate an optimal set of bent sequences as a set of signature sequences for a CDMA system has been introduced. This method is designed to make the CDMA system secure against interception, for both an insider intruder (a user) and an outsider intruder. The security analysis of this system of Matsufuji et al. was based on computing the cardinality of the search space. In this paper after reforming the bent function decomposition, we reapply the above analysis to the system. Then for binary sequences, we consider another analysis to this system, which is based on the fact that all the sequences in the search space do not have equal worthiness for the intruder. Moreover, another application of the signature assignment algorithm to provide multi-level security is presented.

We propose using a soft-input decoder for the decoding of internally convolutional coded Poisson noise-dominated fiber-optic code-division multiple-access (CDMA) communication systems using optical orthogonal codes. We first compute the coded symbol log-likelihoods at the output of the Poisson noise-dominated channel, which is then used by a soft-input maximum-likelihood decoder, for a fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. Furthermore, we develop a discrete soft-output channel model for a Poisson noise-dominated channel, with which we evaluate the upper bound on the bit error probability of the internally coded Poisson noise-dominated fiber-optic CDMA system using a soft-input decoder. It is shown that the soft-input decoder significantly outperforms the hard-input decoder. Furthermore, the performance of the soft-input decoder is also evaluated in the presence of different values of dark current.

A new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-access (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC (/spl lambda/=1). To obtain the performance of such systems, we use a communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. Systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the performance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC (/spl lambda/=1), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard limiter, are also proposed. The performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, optimum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures.

We analyze, for the first time, the performance of a serial-search algorithm for optical code-division multiple access (CDMA) systems using optical orthogonal codes (OOC). For our analysis, we use advance photon-counting methods and we consider the effect of shot noise and dark current and multi-access interference (MAI). We introduce a new method for modeling the effect of MAI, then we use two approaches to consider this effect on the performance of the synchronization system. For the first approach, we consider the average effect of interference in our analysis, while for the second approach two specific forms of interference are taken into account where one form represents a heavily localized interference pattern and the other shows a uniformly distributed one. The effects of different parameters of the system including system threshold, number of active users in the network, and mean number of photons in each optical pulse on the performance of the synchronization system are evaluated and discussed.

We present a thorough analysis based on photon-counting techniques on temporal/spatial fiber-optic code-division multiple-access systems incorporating both post- and pre-optical amplifiers. In this analysis, we consider the effect of shot noise, thermal noise, and source extinction on system bit-error rate. Our results can be used to estimate the maximum tolerable amount of quantum fluctuations in the received signal.

We analyze the basic principles of the spread-space holographic code division multiple access (CDMA) technique. We describe the structure of the encoder and decoder and present a basic mathematical analysis based on spatial harmonic decomposition for the receiver's processing module, which is comprised of a Fourier transform lens and a holographic matched filter. Subsequently, we study two applications of optical holographic CDMA, namely, free-space holographic CDMA photonic switch and free-space (wireless) multiaccess optical (infrared) indoor communications. For both techniques, we describe the two-dimensional imaging techniques, namely user's code transmission, and highlight the basic parameters and components that need to be optimized. To obtain the bit-error rate (BER) of the proposed application, we first evaluate the probability density function of multiuser interference and then evaluate the BER as a function of the processing gain, number of users, the received power, and the optimum threshold. For BER /spl ap/ 10/sup -9/, the result shows an extraordinary number of users that can be supported via holographic CDMA, for both applications.

An ultra-wide bandwidth time-hopping spread-spectrum code division multiple-access system employing a binary PPM signaling has been introduced by Scholtz (1993), and its performance was obtained based on a Gaussian distribution assumption for the multiple-access interference. In this paper, we begin first by proposing to use a practical low-rate error correcting code in the system without any further required bandwidth expansion. We then present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates for the uncoded scheme. Furthermore, it indicates that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate, the coding scheme increases the number of users by a factor which is logarithmic in the number of pulses used in time-hopping spread-spectrum systems.

We analyze, for the first time, the performance of a simple serial-search algorithm for optical code-division multiple access (CDMA) systems using optical orthogonal codes (OOC). For our analysis, we use advanced photon-counting methods and we consider the effect of shot noise and dark current and multi-access interference. We use two approaches for considering the effect of multi-access interference on the performance of the synchronization system. One approach is the general method of considering the average effect of interference. In the other approach, we introduce a new concept for modeling interference; then, with the help of this model, we obtain results that can be regarded as upper and lower bounds on the performance of the synchronization system.

In a previous paper by R.A. Scholtz (see Proc. IEEE MILCOM '93, p.447-50, Oct. 1993), an ultrawide bandwidth time-hopping spread-spectrum code division multiple access system employing a binary PPM signaling was introduced, and its performance was obtained based on a Gaussian distribution assumption for the multiple access interference. A coded scheme of this system was proposed by the authors (see Proc. PIMRC '00, vol.2, p.1555-8, Sept. 2000), which showed much better performance in comparison to the uncoded scheme under the above Gaussian assumption. In this paper, we present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates.

We employ photon-counting techniques to analyze the performance of an all-optical fiber-optic code division multiple access (CDMA) network incorporating multiple optical amplifiers. Our analysis is based on obtaining characteristic functions indicating the photon-count of the output of a block of CDMA code pulse to its input count. The bit error rate (BER) results are obtained using two methods, namely, saddle-point and Gaussian approximations. From our BER analysis, we develop some insight into the optimum distribution of optical amplifiers and their corresponding gain setting parameter.

We compare various receiver structures for optical CDMA networks. Optical orthogonal codes (OOC) are utilized as signature sequences and the performance studied in this paper takes into account the effect of all major noise sources. The required mean number of photon count per chip time for reliable transmission of data bits for various receiver structures is investigated. Two general classes of receivers are investigated which use chip-time or bit-time electronic integration. The results show that receivers with chip-time integration have superior performance and therefore low-power transmission is possible at low speeds. However, the high-speed systems need bit-time integration and therefore should operate at higher levels of signal power.

We study an application of optical holographic CDMA for free-space (wireless) optical (infrared) indoor communications. First, we describe the principles of the spread-space holographic CDMA indicating the structure of the encoder and decoder and describe mathematically the receiver's processing module which is comprised of a Fourier transform lens and a holographic matched-filter. Then, we study the performance of an indoors infrared communication link using spot-diffused configuration. Free-space optical holographic CDMA processing gain (PG) can in principle be as high as 10/sup 6/-10/sup 9/ and each user can have different modulating signals (analog or digital), or different information rate with any signal shape. This high PG, can be interpreted as the high capacity of the system to support large number of users with very small bit error rate.

We propose to use a practical low rate error-correcting code in a time-hopping spread spectrum multiple access system employing a binary PPM signaling. To this end, superorthogonal codes, which are near optimal and have relatively low implementation complexity, are considered. The performance analysis shows that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate the proposed scheme increases the number of users by a factor which is logarithmic in the number of pulses used in a time-hopping spread spectrum system.

Indoors wireless infrared transmission is severely impaired by ambient noise. In addition, in a multiple user network, since many users share the same physical channel, considerable amount of multi-user interference will also be introduced. Infrared CDMA techniques are proposed to combat the effect of ambient and multi-user interference. Our analysis is based on photon counting approach and the bit error rate is given versus signal power. The results show that using infrared CDMA techniques, transmission can be performed well below the ambient noise power with transmission power that satisfies eye-safety power regulations.

An alternative code-division multiple-access (CDMA) scheme to spread spectrum (SS), called spread time (ST) is proposed for bandlimited multiple-access channels. ST-CDMA can be considered the time-frequency dual of SS-CDMA. In ST-CDMA pseudorandom (PN) sequences are assigned to each user, and the Fourier transform of the transmitted pulse for a given user is determined by modulating the phase of the desired transmitted spectrum by the user's PN-sequence. The transmitted data for a particular user can be recovered by sampling the output of a filter matched to the user's pulse. Implementations are described in which surface acoustic wave devices are used to perform the matched filtering or Fourier transformation. Averaged signal-to-interference plus noise ratio (SIR) and spectral efficiency are computed for both asynchronous ST and direct-sequence SS-CDMA systems, assuming an arbitrary channel transfer function H(f), which is the same between all pairs of users. The results are the same for SS and ST provided that the magnitude of the Fourier transform of the chip shape in the SS system is the same as the magnitude of the Fourier transform of the ST pulse shape. The main advantage of the ST technique Is the flexibility with which the transmitted spectrum can be selected. We derive the transmitted spectrum that maximizes the SIR subject to an average power constraint.>

A new technique for all-optical code division multiple access (CDMA) based on optical holography is described. In this technique the energy of an incoming information light signal is spread over a larger spatial domain by a two-dimensional spatial encoder which consists of patterns of (0, 1) or (1, -1). Subsequent decoding, which is obtained by a holographic filter, spatially despreads the energy of information light signal and brings it to a bright focused light spot. Experimental results obtained to demonstrate the encoding, decoding, and multiaccess capability of the proposed holographic CDMA technique are discussed and compared to other all-optical CDMA techniques. The processing gain for a holographic CDMA can be potentially as high as 10/sup 6/-10/sup 8/, which suggests that this technique can in principle support hundreds of thousands to a few millions of users.>

The authors provide an analysis of the performance of optical orthogonal codes in an optical code division multiple access (CDMA) network by considering the probability distribution of the interference patterns. It is shown that the actual performance is close to a previous estimate. A less structured temporal code in which the code words are allowed to overlap at two pulse positions is also considered. The bit error probability for this class of codes is obtained for two cases: with and without optical hard-limiting at the receivers. It is shown that this code may increase the number of users in the network considerably without a significant loss in the performance.>

The authors obtain lower and upper bounds on the peak intensity of a spectrally phase encoded ultrashort light pulse as a function of the length of spectral phase encoding. It is shown that using random phase codes for spectral encoding of ultrashort light pulses in a coherent ultrashort light pulse code-division multiple-access (CDMA) system is almost optimal. Furthermore, simple upper bounds and an asymptotic approximation on the bit error rate (BER) for a wide class of ultrashort light pulse CDMA systems are obtained. It is shown that, in spite of being simple, these upper bounds can be used to evaluate the performance of a coherent ultrashort light pulse CDMA system.>

An examination is made of fiber-optic code-division multiple-access (FO-CDMA), a technique in which low information data rates are mapped into very-high-rate address codes (signature sequences) for the purpose of achieving random, asynchronous communications free of network control, among many users. The need for a special class of signature sequences to achieve the multiple-access capability using fiber-optic signal processing techniques is discussed. A class of signature sequences called optical orthogonal codes (OOCs) that provide the auto- and cross-correlation properties required for FO-CDMA is introduced and used in an experiment to show the principles of FO-CDMA. The experiment demonstrates the auto- and cross-correlation properties of the codes. The concept of optical disk patterns, an equivalent way of representing the OOCs, is introduced. The patterns are used to derive the probability density functions associated with any two interfering OOCs. A detailed study of different interference patterns is presented, and the strongest and the weakest interference patterns are determined.>

For pt.I see ibid., vol.37, no.8, p.824-33 (1989). In Part I a technique based on optical orthogonal codes was presented to establish a fiber-optic code-division multiple-access (FO-CDMA) communications system. The results are used to derive the bit error rate of the proposed FO-CDMA system as a function of data rate, code length, code weight, number of users, and receiver threshold. The performance characteristics for a variety of system parameters are discussed. A means of reducing the effective multiple-access interference signal by placing an optical hard-limiter at the front end of the desired optical correlator is presented. Performance calculations are shown for the FO-CDMA with an ideal optical hard-limiter, and it is shown that using an optical hard-limiter would, in general, improve system performance.>

An optical orthogonal code (OOC) is a family of