With the growing demand for high-bandwidth applications, traditional optical networks are reaching their capacity limits. Multi-band elastic optical networks (MB-EONs) have emerged as a cost-effective and promising solution to enhance spectral efficiency by utilizing spectrum slots across multiple bands beyond the C-band. However, the performance of MBEONs is significantly affected by physical layer impairments (PLIs), including amplified spontaneous emission (ASE), nonlinear interference (NLI), and inter-channel stimulated Raman scattering (ISRS), which complicate dynamic resource allocation and degrade the quality of transmission (QoT). To address these challenges, we propose an ISRS-Enhanced PLI-Aware (IE-PLIA) routing algorithm for MB-EON resource allocation, explicitly accounting for PLIs under the influence of ISRS. Unlike prior approaches, IE-PLIA dynamically adjusts routing costs based on link conditions, self-channel interference (SCI), and cross-channel interference (XCI) variations induced by ISRS. Furthermore, we investigate the network performance under various traffic load conditions to determine when transitioning from single-band to multi-band EONs becomes advantageous. Our results show that IE-PLIA significantly lowers the blocking probability, achieving a 60% reduction compared to the benchmark algorithm in the Pan-European network at 2000 Erlangs.

Current signal processing algorithms excel at impairment compensation when the parameters of optical fiber systems are precisely defined. However, their effectiveness diminishes considerably in the presence of imprecise parameters. By integrating ‘knowledge application from physics to neural networks (NN)’ with ‘information feedback from NNs to physics’, this paper proposes a physics-regulated digital backpropagation (PR-DBP) algorithm, which shows great promise for impairment compensation with imprecise fiber parameters. The PR-DBP employs an optimization-estimation-initialization loop structure. The optimization process provides the network’s adaptability by minimizing the loss value as in conventional NNs. The estimation process dynamically tracks physical parameters by extracting information from NNs to the physical domain. The initialization process offers a physics-based global control over the neural network, thereby mitigating the risk of overfitting by preventing an excessive focus on loss minimization. Moreover, a phase-shift weight function is applied to further improve algorithmic efficiency. Numerical analyses indicate that the PR-DBP significantly outperforms conventional methods in optical fiber systems with imprecise parameters, achieving a bit error rate reduction by an order of magnitude. As a mutually reinforcing part of impairment compensation, a high-accuracy and adaptive fiber parameter estimation is also demonstrated.

Visible light communication (VLC) is a technology that complements radio frequency (RF) to fulfill the ever-increasing demand for wireless data traffic. The ubiquity of light-emitting diodes (LEDs), exploited as transmitters, increases the VLC market penetration and positions it as one of the most promising technologies to alleviate the spectrum scarcity of RF. However, VLC deployment is hindered by blockage causing connectivity outages in the presence of obstacles. Recently, optical reconfigurable intelligent surfaces (ORISs) have been considered to mitigate this problem. While prior works exploit ORISs for data or secrecy rate maximization, this paper studies the optimal placement of mirrors and ORISs, and the LED power allocation, for jointly minimizing the outage probability while keeping the lighting standards. We describe an optimal outage minimization framework called JointMinOut and present solvable heuristics. We provide extensive numerical results and show that the use of ORISs may reduce the outage probability by up to 67% with respect to a no-mirror scenario and provide a gain of hundreds of kbit/J in optical energy efficiency with respect to the presented benchmark.

Visible light communication (VLC) complements radio frequency in indoor environments with large wireless data traffic. However, VLC is hindered by dramatic path losses when an opaque object is interposed between the transmitter and the receiver. Prior works propose the use of plane mirrors as optical reconfigurable intelligent surfaces (ORISs) to enhance communications through non-line-of-sight links. Plane mirrors rely on their orientation to forward the light to the target user location, which is challenging to implement in practice. This paper studies the potential of curved mirrors as static reflective surfaces to provide a broadening specular reflection that increases the signal coverage in mirror-assisted VLC scenarios. We study the behavior of paraboloid and semi-spherical mirrors and derive the irradiance equations. We provide extensive numerical and analytical results and show that curved mirrors, when developed with proper dimensions, may reduce the shadowing probability to zero, while static plane mirrors of the same size have shadowing probabilities larger than 65%. Furthermore, the signal-to-noise ratio offered by curved mirrors may suffice to provide connectivity to users deployed in the room even when a line-of-sight link blockage occurs.

Visible light communication (VLC) is a growing technology that can complement radio frequency to fulfill the increasing demand for wireless data services. VLC exploits the already deployed illumination infrastructure based on light-emitting diodes to communicate data. While mirrors have been traditionally used for interior decoration, among other applications, to what extent can they be deployed to improve VLC performance? In this paper, we propose optimizing the placement of mirrors to minimize the outage area, a common problem in VLC when line-of-sight link blockage occurs. We compare this optimal performance with a single-rectangular mirror array, a common mirror shape that is found indoors. We also study the most helpful mirror placement under different conditions. We show that our outage minimization algorithm, MinOut, easily provides threefold outage reductions. Besides, we demonstrate that a single-rectangular mirror array with orientation mobility can provide similar or even better performance than an optimal distributed mirror placement without mobility.

In this paper, we develop an object detection method that takes advantage of the blockage status of line-of sight (LOS) links between the user devices and transceivers on the ceiling to locate an object. The target objects are modeled as cylinders with random radii. Simulation results show that the root-mean-squared error can be less than 2 cm and 1 cm for estimating the center and the radius of the object, respectively.

Optical wireless communications (OWC) and the internet of things (IoT) are two recent paradigms that are expected to be ubiquitous in the near future. In this paper, we introduce an early concept of how OWC can play a critical role in IoT applications in the context of real-time location-based services. OWC systems such as visible light communications (VLC) use light-emitting diodes (LED), already present in most user electronics as data transmission nodes. Studies on indoor OWC positioning generally assume that the location of the nodes, the LEDs that are connected to the backbone communications network, is provided to the user equipment (UE). In this paper, we consider a scenario where no a priori knowledge of the LED and UE locations is available, and the UE uses a single photodetector to measure the received optical intensity. We use these measurements to extract the distance between the UE and LEDs, and form geometric relations to find their locations. Once the LED locations have been found, we can track the UE using an extended Kalman filter. The joint source LED localization and UE tracking process is enhanced by using measurements that are collected by other members of the OWC-IoT network. Results show that the root mean square error of the LED localization can be decreased to 10 centimeters, and this leads to a user tracking accuracy better than 20 centimeters using the proposed method.

Visible light communication is a solution for high-security wireless data transmission. In this paper, we first analyze the potential vulnerability of the system from eavesdropping outside the room. By setting up a signal to noise ratio threshold, we define a vulnerable area outside of the room through a window. We compute the receiver aperture needed to capture the signal and what portion of the space is most vulnerable to eavesdropping. Based on the analysis, we propose a solution to improve the security by optimizing the modulation efficiency of each LED in the indoor lamp. The simulation results show that the proposed solution can improve the security considerably while maintaining the indoor communication performance.

Gait impairment in multiple sclerosis (MS) can result from muscle weakness, physical fatigue, lack of coordination, and other symptoms. Walking speed, as measured by a number of clinician-administered walking tests, is the primary measure of gait impairment used by clinical researchers, but inertial gait features from body-worn sensors have been proven to add clinical value. This paper seeks to understand and differentiate the physiological significance of four such features with proven value in MS to facilitate adoption by clinical researchers and incorporation in gait monitoring and analysis systems. In addition, this information can be used to select features that might be appropriate in other forms of disability. Two of the four features are computed using the dynamic time warping (DTW) algorithm: The "DTW Score" is based on the usual DTW distance, and the "Warp Score" is based on the warping length. The third feature, based on kernel density estimation (KDE), is the "KDE Peak" value. Finally, the "Causality Index" is based on the phase slope index between inertial signals from different body parts. Relationships between these measures and the aforementioned gait-related symptoms are determined by applying factor analysis to three common, clinical walking outcomes, then correlating the inertial measures as well as walking speed to each extracted factor. Statistically significant differences in correlation coefficients to the three extracted clinical factors support their distinct physiological meaning and suggest they may have complimentary roles in the analysis of MS-related walking disability.

Multiple sclerosis (MS) interrupts communication between the brain and other parts of the body causing functional deterioration. Gait impairment is a common finding in MS, one caused by several neurological symptoms. We perform an event-specific analysis to study the variable impact of MS on gait components. Our results show that the mid-swing to heel strike (HS) phase of a gait cycle is the most indicative of motor problems. We apply the Hilbert-Huang transform to inertial gait data, corresponding to this phase, to extract the spectral features and study their relationships with the patient-reported outcomes. A number of strong and statistically significant dependencies were found, many having to do with activities of daily living and MS walking scale, leading to the conclusion that the disturbance in mid-swing to HS is specific to deterioration in physical functions. Spearman correlations coefficients and adjusted R2obtained using stepwise linear regression models are reported. We conclude that event-specific gait features can be used to quantify the precise impact of MS symptoms on gait phases and identify markers of balance, stability, or fall risk, etc. We believe that this information supplements on-going MS research and could be used to develop personalized disease-modifying therapies and exercises.

In this paper, we introduce a new uplink visible light indoor positioning system that estimates the position of the users in the network-side of a visible light communications (VLC) system. This technique takes advantage of the diffuse components of the uplink channel impulse response for positioning, which has been considered as a destructive noise in existing visible light communication positioning literature. Exploiting the line of sight (LOS) component, the most significant diffusive component of the channel (the second power peak (SPP)), and the delay time between LOS and SPP, we present a proof of concept analysis for positioning using fixed reference points, i.e. uplink photodetectors (PDs). Simulation results show the root mean square (RMS) positioning accuracy of 25 cm and 5 cm for one and 4 PDs scenarios, respectively.

Visible light communication is an emerging field in indoor wireless connection, and it is a candidate technique to provide high-speed data transmission. In this paper, we propose a joint optimal waveform design system to support multiple users. In this system, transmitted waveforms and minimum mean squared error filters are jointly optimized to minimize the inter-symbol interference and multiple access interference. The adaptive approach is proposed so that the data rate can be maximized for current channel conditions. From the numerical results, the proposed optimally designed waveforms can support higher data rates than time division multiplexing access and optical orthogonal codes for multiple users over dispersive indoor channels. Furthermore, the optimal waveforms are more power efficient for illumination than optical orthogonal codes, and can achieve to 80% of the maximal illumination efficiency.

Tracking of user equipment in indoor areas is becoming ubiquitous with the development of location-based services. The use of an RF received signal strength map in wireless positioning systems, a so-called site survey, is a well- studied technique. In this paper, we propose a hybrid method that combines the trilateration technique used in Wi-Fi and an extended Kalman filtering for visible light with crowdsourced measurements to create a received light-signal strength map. How to efficiently create such a site survey is an open challenge. Our aim is to offer a solution to this problem that is able to automate the map creation step using data sourced from the users. The results show that the proposed method outperforms Wi-Fi-only based trilateration methods. The accuracy of the proposed method is around 5 cm when the resolution of the fingerprint map is 1 dm.

We study the performance of multi-hop networks composed of millimeter wave (MMW)-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in the cases with and without hybrid automatic repeat request (HARQ). Taking the MMW characteristics of the RF links into account, we derive closed-form expressions for the network outage probability. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we present mappings between the performance of RF- FSO multi-hop networks and the ones using only the RF- or the FSO-based communication, in the sense that with appropriate parameter settings the same outage probability is achieved in these setups. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, the HARQ can effectively improve the outage probability#x002F;energy efficiency, and compensate the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual- hop networks, outage probability 10^{-4} and code rate 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to the cases with no HARQ, by 13 and 17 dB, respectively.

The desire to obtain accurate location-based services has increased with the use of mobile devices. In this study, a visible light based indoor localization method that works with both diffusing and nondiffusing lamps is proposed. The received power from the light emitting diodes (LEDs) is used as sensor input, and an extended Kalman filter (EKF) is used for state estimation. The algorithm relies on knowledge of the expected power distribution in the room. The proposed method is robust to low SNR, nonuniform power distributions, and intermittent measurements. The results show that a tracking error around 1 centimeter can be achieved.

This paper studies the performance of multi-hop and mesh networks composed of millimeter wave-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in cases with and without hybrid automatic repeat request (HARQ). Using the central limit theorem as well as other state-of-the-art approximation schemes, we derive closed-form expressions for the networks' outage probability and ergodic achievable rates. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we determine the minimum number of the transmit antennas in the RF link such that the same rate is supported in the RF- and the FSO-based hops. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, HARQ can effectively improve the outage probability/energy efficiency, and compensate for the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability of 10-4and code rate of 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to cases with open-loop communication, by 13 and 17 dB, respectively.

Multiple sclerosis (MS) is a neurological disorder that disrupts the communication within the brain, and between the brain and body. MS symptoms may vary over time. So we propose to do longitudinal assessments of a patient's gait characteristics using inertial data, in order to evaluate his/her gait for an extended period of time.

In multiuser multiple input single output (MISO) visible light communication (VLC) systems using LEDs with limited rise-times, it is essential to find effective M-ary modulation schemes to increase the bit rate. In this paper, we propose an adaptive M-ary pulse amplitude modulation (M- PAM) algorithm to support multiple users. The proposed algorithm can adjust the modulation constellation size for each user to maximize the bit rate under different channel environments such as shadowing, light dimming, and the impact of multiple access interference. In our MISO approach, multiple LED lamps coordinate to provide users with maximum data rates. We compare optical code division multiplexing access (OCDMA) using our adaptive M-PAM with time division multiplexing access (TDMA). The OCDMA technique can offer a higher bit rate when the number of users is larger than the length of the OCDMA code.

Visible light communication (VLC) systems using the indoor lighting system to also provide downlink communications require high-average optical powers to satisfy the illumination needs. This can cause high-amplitude signals common in higher-order modulation schemes to be clipped by the peak power constraint of the light emitting diode (LED) and lead to high-signal distortion. In this paper, we introduce Hadamard coded modulation (HCM) to achieve low error probabilities in LED-based VLC systems needing high-average optical powers. This technique uses a fast Walsh-Hadamard transform (FWHT) to modulate the data as an alternative modulation technique to orthogonal frequency division multiplexing (OFDM). HCM achieves a better performance for high-illumination levels because of its small peak to average power ratio (PAPR). The power efficiency of HCM can be improved by reducing the DC part of the transmitted signals. The resulting so-called DC-reduced HCM (DCR-HCM) is well suited to environments requiring dimmer lighting.

In multiuser MISO (multiple-input single output) visible light communication (VLC) systems, it is essential to find effective means for allocating the transmitted power from the various LEDs fairly among users that are randomly dispersed in large indoor environments. Distributed algorithms are a natural choice because, unlike centralized power allocation algorithms, distributed schemes do not require extensive computation complexity and computation time. In this paper we propose and compare two distributed algorithms: a partial jointly optimized power allocation and a weighted distributed power allocation. The system uses code division multiple access (CDMA) plus optical on- off keying (OOK) with a minimum mean squared error (MMSE) receiver to diminish the effect of multiple-access interference in the indoor VLC system. We compare the performance of the proposed distributed algorithms with an optimal centralized approach. From the numerical results, both distributed algorithms proposed in this paper have less than a 2 dB power penalty compared with the centralized approach, and the computational complexity can be reduced by 75% for a small indoor environment and by 93% for a large indoor environment.

This paper proposes a new two-wavelength design for large-enterprise access links. The goal of this design is to lower power consumption and equipment costs without having a significant impact on performance. In our design, one wavelength is used as part of a lower-rate static circuit for general-purpose IP traffic, while the second wavelength is dynamically configured into a high-rate circuit for large dataset transfers whenever needed. A few provider-router ports are shared among a larger number of customers given that large dataset transfers are relatively infrequent. This leads to potential start-time delays, but results in significant power and cost savings. Using analytical models, we provide a quantitative power-and-cost comparison of our dynamic solution with the static always- on high-rate single-circuit solution used currently. The number of shared provider-router ports is kept large enough based on the number of customers to ensure that the probability of start-time delay exceeding a threshold is less than 1%.

Orthogonal frequency division multiplexing (OFDM) is a modulation technique susceptible to source, channel and amplifier nonlinearities because of its high peak-to-average ratio (PAPR). The distortion gets worse by increasing the average power of the OFDM signals since larger portion of the signals are affected by nonlinearity. In this paper we introduce Hadamard coded modulation (HCM) that uses the fast Walsh-Hadamard transform (FWHT) to modulate data as an alternative technique to OFDM in direct-detection wireless optical systems. This technique is shown to have a better performance for high average optical power scenarios because of its small PAPR, and can be used instead of OFDM in two scenarios: 1) in optical systems that require high average optical powers such as visible light communications (VLC), and 2) in optical wireless systems unconstrained by average power, for which HCM achieves a lower bit error rate (BER) compared to OFDM. The power efficiency of HCM can be improved by removing a part of the signal's DC bias without losing any information. In this way, the amplitude of the transmitted signal is decreased and the signals become less susceptible to nonlinearity. Interleaving can be applied on HCM to make the resulting signals résistent against inter-symbol interference (ISI) effects in dispersive channels by uniformly distributing the interference over all symbols.

Interconnecting devices in an indoor environment using the illumination system and white light emitting diodes (LED) requires adaptive networking techniques that can provide network access for multiple users. Two techniques based on multilevel signaling and optical orthogonal codes (OOC) are explored in this paper in order to provide simultaneous multiple access in an indoor multiuser network. Balanced incomplete block designs (BIBD) are used to construct multilevel symbols for M-ary signaling. Using these multilevel symbols we are able to control the optical peak to average power ratio (PAPR) in the system, and hereby control the dimming level. In the first technique, the M-ary data of each user is first encoded using the OOC codeword that is assigned to that user, and then it is fed into a BIBD encoder to generate a multilevel signal. The second multiple access method uses sub-sets of a BIBD code to apply multilevel expurgated pulse-position modulation (MEPPM) [1] to the data of each user. While the first approach has a larger Hamming distance between the symbols of each user, the latter can provide higher bit-rates for users in VLC systems with bandwidth-limited LEDs.

In long-haul DWDM or elastic subwavelength modulated systems with periodic dispersion compensation and amplification, all-optical communications give rise to severe physical impairments, due to fiber dispersion and nonlinearity, together with noise due to amplified spontaneous emission (ASE), that adversely affect system performance. Many signal processing techniques, including forward error correction, constrained coding, and predistortion/equalization, have been developed to mitigate these physical impairments and fully exploit the system capacity of long-haul DWDM systems. However, existing equalizers either only mitigate linear impairments or are too complex to be useful, such as backpropagation. This paper proposes a model-centric nonlinear equalizer for coherent systems based on a 2D discrete-time model of physical impairments in long-haul time and wavelength channel systems with periodic dispersion compensation and amplification using inverse Volterra theory. Different from backpropagation, which is hard to implement in hardware, this equalizer can function on the most basic discrete-time signal processing device. The nonlinear equalizer is effective in suppressing linear and nonlinear physical impairments in a long-haul fiber-optic communication system, particularly for high launched power levels where fiber nonlinearity dominates.

One of the major problems facing optical networking is to intelligently assign physical resources such as lightpaths and regenerators to connection requests, namely the grooming, routing and wavelength assignment (GRWA) problem. Due to the high computation complexity, many heuristic methods have been proposed to solve the problem. We first propose an extended Dijkstra shortest path algorithm to solve GRWA for dynamic network while considering regeneration, quality of transmission (QoT), mixed-line-rate (MLR) and traffic grooming. This generalized adaptive shortest path (GASP) algorithm requires that each node maintain global view of network state information. In order to perform GRWA in a distributed fashion, which allows for greater network scalability, and gives individual domains more control over their data, we then apply an ant colony optimization (ACO) technique, a metaheuristic optimization algorithm used to solve dynamic problems, to real-time optical networks. We compare the two proposed algorithms and show that the ACO algorithm outperforms the GASP algorithm in terms of connection request blocking probability and network throughput while maintaining a reasonable computation complexity.

Virtualization improves the efficiency of networks by allowing multiple virtual networks to share a single physical network's resources. Next-generation optical transport networks are expected to support virtualization by accommodating multiple virtual networks with different topologies and bit rate requirements. Meanwhile, Optical Orthogonal Frequency-Division Multiplexing (OOFDM) is emerging as a viable technique for efficiently using the optical fiber's bandwidth in an elastic manner. OOFDM partitions the fiber's bandwidth into hundreds or even thousands of OFDM subcarriers that may be allocated to services. In this paper, we consider an OOFDM-based optical network and formulate a virtual network mapping problem for both static and dynamic traffic. This problem has several natural applications, such as e-Science, Grid, and cloud computing. The objective for static traffic is to maximize the subcarrier utilization, while minimizing the blocking ratio is the aim for dynamic traffic. Two heuristics are proposed and compared. Simulation results are presented to demonstrate the effectiveness of the proposed approaches.

We present an M-ary spectral amplitude code (SAC) modulation technique to improve the performance of free-space optical (FSO) communication systems. Although this approach can be used in any dispersive FSO system, in this paper we focus on non-line of sight (NLOS) ultraviolet (UV) systems relying on atmospheric scattering. Spectral amplitude encoding is applied on a broadband UV source using the same code families for the M-ary alphabet as used previously in SAC optical code division multiple access (OCDMA) systems. A differential structure using two photomultiplier tubes is utilized in conjunction with various demodulation algorithms to decode the received signal. Intersymbol interference (ISI), received beam divergence and shot noise are considered as the main factors limiting the system performance. An upper bound on the bit error probability is presented and compared with simulation results for various geometries and for different code parameters. The maximum bit rate for a fixed bit error probability is calculated in terms of the link length, and results for different alphabet sizes are shown. By sacrificing spectral efficiency without becoming more susceptible to ISI, the proposed system can support higher rates and longer distances for the same performance compared with on-off keying systems.

In this paper, two new modulation schemes using multilevel pulse-position modulation (PPM) for application in unipolar optical wireless systems are presented. Balanced incomplete block designs (BIBD) are used for constructing the symbol alphabets. Each symbol is obtained by combining multiple codewords of a BIBD code. In one scheme the symbols have equal energies, and therefore, no threshold is needed to make a decision on the received signal. The other modulation has better performance yet higher complexity. Since cyclic BIBDs are used for constructing the symbols, the transmitters and receivers have simple structures, and can be implemented using shift registers. These schemes can achieve high spectral-efficiencies, and are therefore suitable for systems with bandlimited sources or highly dispersive channels, where intersymbol interference (ISI) has a significant impact on the performance. We also show that using the same receiver structure, the constellation size can be increased by including the complements of the codewords. The performance of the proposed schemes are compared to other modulation schemes for both LED-based non-dispersive and dispersive free-space optical (FSO) systems.

In long-haul fiber-optic communication systems, the system performance is affected adversely by both severe physical impairments and amplified spontaneous emission (ASE) noise. Constrained coding, which avoids waveforms in the transmitted signal that are more likely to be detected incorrectly, has been proved to be an effective approach to suppress some physical impairments. However, the constrained coding schemes proposed in the literature are limited to the suppression of only some “resonant” sequences and their performance is evaluated in the absence of ASE noise. Various error correction codes also have been developed to reduce errors due to ASE noise but their performance is very vulnerable to the strong nonlinear impairments. This paper develops a novel concatenation scheme with the inner code being a constrained code based on Total Impairment Extent Rank (TIER) and the outer code being a low-density parity-check (LDPC) code. The TIER code ranks the bit patterns by order of all physical impairments imposed on them and constrains the bit patterns with large physical impairments. It is based on a discrete-time analytical model of physical impairments in long-haul fiber-optic communication systems. Compared with the current constrained codes, the TIER code offers more flexibility and better effectiveness. The TIER-LDPC concatenation scheme combines the strength of the TIER code in correcting errors due to physical impairments and that of the LDPC code in correcting memoryless errors due to ASE noise. Simulation results in the presence of ASE noise show that the TIER-LDPC concatenation scheme performs significantly better than the LDPC code alone in the case of severe impairments.

Translucent fiber-optic networks are carefully planned to achieve high capacity utilization efficiency as required by society's ever-increasing traffic demand. Existing research treats the problem of resource placement largely as a static design problem, which is solved with linear programming (LP) to find the optimal solution. The dynamic operational problem (grooming, regeneration, routing, and wavelength assignment) is approached using heuristic methods with the goal of improving the overall network performance given an existing network infrastructure. Our work combines these two approaches and solves a real-time dynamic traffic scenario with integer linear programming (ILP), seeking to maximize the overall network throughput. The traffic is served in a time-slotted fashion so that the network throughput is optimized at each time slot given the existing network state. The solution is compared with results from existing heuristic methods. We incorporate physical impairment limitations into our network model, and consider several grooming options.

Wireless Mesh Networks (WMNs) are designed to provide broadband wireless services; however, capacities of WMNs are fundamentally limited in bandwidth because of the scarcity of RF spectral resources. A solution is to design a WMN using multiple wireless technologies. Since the optical spectrum remains unlicensed and under-utilized, we propose to augment RF WMNs with free space optical (FSO) systems. FSO links do not interfere with RF links and have higher capacity than RF links. In this paper, we study the throughput improvement of FSO-augmented RF WMNs. We address two questions. First, given a fixed number of FSO links, where should we install them to achieve the maximum improvement of network throughput for given traffic demands? Second, how do we route and schedule the flows in the hybrid FSO/RF network to achieve this throughput? We jointly formulate the above problems as one mixed integer linear programming problem. The results show that the throughput of the network increases dramatically by properly allocating FSO links. We also provide a computationally efficient algorithm to evaluate upper and lower bounds on the throughput of this hybrid FSO/RF network. The network administrator can use these bounds to determine the tradeoff between the number of FSO links (which are expensive) and the throughput improvement.

Wavelength Division Multiplexing (WDM)-based optical networks are ideal candidates for core backbone networks because of their ability to carry large amounts of traffic. Optical amplification has increased the reach of long-haul optical links. Nevertheless, it is impossible today to construct a truly optical network without converting optical signals to electrical signals and regenerating them, because of the deleterious effects of physical impairments such as amplifier noise, dispersion, and non-linear effects such as four-wave mixing and cross-phase modulation. Being expensive devices, these regenerators are expected to be sparsely located and used in such a network called as a translucent optical network. In this paper, we consider the routing and wavelength assignment (RWA) problem so that the Quality of Transmission (QoT) for connections is satisfied, and the network-level performance metric of blocking probability is minimized. Cross-layer heuristics that are based on dynamic programming to effectively allocate the sparse regenerators are developed, and extensive simulation results are presented to demonstrate their effectiveness.

Hybrid free space optical (FSO)/radio frequency (RF) communication systems provide an attractive solution for high-throughput wireless connectivity. Such systems promise high data rates (Gbps) due to the FSO channel, weather permitting. The reliability of these systems is enhanced by the more robust RF channel. A critical question is how to efficiently transmit data using hybrid systems with highly variable channel conditions. In this paper, a hybrid FSO/RF communication system is modeled as two independent parallel channels. Under the assumptions of quasi-static channels and perfect channel knowledge at the transceivers, we propose an adaptive design resulting in a capacity-approaching and seamless joint system. In our design, information bits are jointly encoded and decoded. The encoder rate, modem transmission rate, and modulation scheme of each channel are dynamically adjusted according to the channel state. Analytical and simulation results give insight into the relationship between the adaptive parameters and the performance of the hybrid system.

In wavelength-division multiplexed (WDM) systems, system performance is adversely affected by severe physical impairments due to fiber loss, dispersion and nonlinearity. Fiber modeling is a prerequisite for the development of physical impairment mitigation techniques to improve system performance. The distance between two interacting symbols in time and wavelength, i.e., the range of influence (RoI) of each physical impairment, plays an important role in the development of these mitigation techniques. This paper develops a two-dimensional (time and wavelength) discrete-time input-output model of WDM systems based on the Volterra series transfer function (VSTF) method. This model takes into account multiple channel effects, fiber loss, and frequency chirp, which are ignored in the current literature. Furthermore, with this model we define coefficients that capture intersymbol interference (ISI), self phase modulation (SPM), intrachannel cross phase modulation (IXPM), intrachannel four wave mixing (IFWM), cross phase modulation (XPM) and four wave mixing (FWM) to characterize the impact of these impairments individually on the system output. By investigating these impairment coefficients, the RoIs for different physical impairments are determined.

Wireless sensor networks (WSN) intended for critical applications often rely on energy harvesting batteries, yet also require backup batteries to ensure uninterrupted operation. An optimal algorithm for resource allocation in such WSN used for state estimation is presented. The resulting state estimate approaches a minimum accuracy constraint with efficient use of the harvesting capability to minimize the use of the backup power. Power usage is fairly distributed among the sensors to prolong system lifetime. The algorithm is based on a stochastic Lyapunov optimization used with a standard Kalman filter estimator. The framework can achieve arbitrarily close to optimal power efficiency over time without requiring knowledge of either the channel or harvesting statistics. Asymptotically optimal efficiency is obtained at the expense of an increase in latency for the system to converge to the desired estimation accuracy.

To mitigate various physical impairments of long-haul dense wavelength division multiplexing (DWDM) systems and exploit their system capacity, there is a need to develop a two-dimensional (time and wavelength) discrete-time input-output model which can become the foundation of signal processing for optical communications. As the first step, this paper develops a model for single channel multipulse multispan systems based on the Volterra series transfer function (VSTF) method. This model is suitable for high-bit-rate time-division multiplexing (TDM) transmission in the pseudo-linear regime and is easily extendable to the multichannel case. We overcome the well-known triple integral problem and reduce it to a simple integral. This model takes into account fiber losses, frequency chirp and photodetection, which are ignored in the literature. Furthermore, with this model we introduce the intersymbol interference (ISI), self phase modulation (SPM), intrachannel cross phase modulation (IXPM) and intrachannel four wave mixing (IFWM) coefficients to characterize the impact of these effects on the system performance. The model is in excellent agreement with SSF (split-step Fourier) simulation. To illustrate its application, we develop a constrained coding scheme based on the system model to suppress the impact of various impairments.

While many BSN applications require that sensor nodes be able to operate for extended periods of time, they also often require the wireless transmission of copious amounts of sensor data to a data aggregator or base station, where the raw data is processed into application-relevant information. The energy requirements of such streaming can be prohibitive, given the competing considerations of form factor and battery life requirements. Making intelligent decisions on the node about which data to store or transmit, and which to ignore, is a promising method of reducing energy consumption. Artificial neural network (ANN) classifiers are among several competitive techniques for such data selection. However, no systematic metrics exist for determining if an ANN classifier is suited for a particular resource constrained computing environment of a typical BSN node. An especially difficult task is assessing, at the design stage, which classifier architectures are feasible on a given resource-constrained node, what computational resources are required to execute a given classifier, and what classification performance might be achieved by a particular classifier on a given set of resources. This paper describes techniques for quantifying and predicting the performance of ANN classifiers on wearable sensor nodes using scalable synthetic test data. Additionally, the paper shows a comparison of synthetic data with gait data collected using an inertial BSN node, and classification results of the gait data using a cerebellar model arithmetic computer (CMAC) architecture show excellent agreement with theoretical predictions.

Resiliency is a crucial performance measure for high-speed all-optical networks. Previous work has shown that any algorithm designed for such purpose should take into account the physical layer characteristics of these environments, in other words it must employ a cross-layer design. In particular, cross-layer p-Cycle design has been developed as a desirable approach. In this paper, we propose a classification of network traffic resiliency requirements and present a multi-class link protection scheme. In particular we allow pre-computed but not pre-crossconnected protection routes for some portion of the network traffic. We evaluate the performance of these algorithms in the face of physical layer impairments (PLIs) and show that this classification of traffic can result in higher resiliency or, equivalently, lower vulnerability ratio in the network.

Cognitive radio offers an attractive means of utilizing spectral resources opportunistically, yet relies on an agreement that the secondary users do not interfere with the primary users. This system-wide constraint makes distributed resource allocation difficult. In this paper, sum rates for a multiuser cognitive radio system under received power constraints are derived and optimal and suboptimal power assignments are found for orthogonal resource sharing and simultaneous transmission scenarios. By allocating a portion of the primary user constraints to each basestation, a novel rate-maximizing distributed technique by which each multi-antenna basestation controls the parameters of its own secondary users is proposed and evaluated. Though suboptimal, the computational burden of the proposed algorithm is minimal compared to the NP-hard optimal solution.

Their high recovery speed and efficiency have made p-Cycles a premier approach to link protection. Recent research has discussed their capabilities and provided different methods for p-Cycle selection in communication networks. In this work, we consider the p-Cycle approach to resilient all-optical network design and present an ILP formulation that preconfigures p-Cycles in such an environment. Our proposed formulation strives to find p-Cycles that have good recovery capability in the face of physical layer impairments (PLIs) and are capacity-efficient at the same time. Our numerical results show a trade-off between these two performance measures. We evaluate the performances of different outcomes of the formulation using our accurate physical layer model. We show that applying our previously proposed cross-layer RWA algorithm, HQ, greatly improves the performance.

Link protection schemes for WDM-based optical networks have been extensively researched. Most work to date has ignored the physical layer impairments (PLIs) that could be dominant in transparent optical networks with long links. In this paper, we first evaluate the performance of two link protection schemes - namely, p-cycles and generalized loopback - when PLIs are considered. Our evaluation shows that a choice of p-cycles that merely tries to optimize the wavelength usage can be more susceptible to failures in a realistic environment. In particular, we show that the Hamiltonian p-cycle is not the optimal choice for p-cycle selection in all-optical networks. This is mainly due to the interference caused by the long lightpaths that appear in the network after a failure occurs. We show that although a selection of smaller p-cycles can cause higher blocking probability, it is less vulnerable to failures. We also compare the performance of p-cycles to the generalized loopback link cover approach in these environments. Finally, we apply a cross-layer routing and wavelength assignment algorithm to these schemes that significantly enhances performance in physically impaired optical networks.

Wavelength routed optical networks can experience increased call blocking due to insufficient quality of transmission. The routing and wavelength assignment algorithm must verify the quality of the lightpath before accepting it. In this paper, analytical expressions for the total blocking probability are derived for first fit wavelength assignment for networks suffering from transmission impairments. Physical layer degradations considered include amplifier noise and crosstalk between WDM channels. The technique effectively predicts the performance for wavelength selection techniques that consider a single candidate channel or all channels for quality of transmission compliance. The analysis is also applicable to first-fit algorithms with different static channel orderings.

Protection against failure in high bandwidth optical networks can prevent significant potential losses of data but at the same time it requires a considerable fraction of the network resources to be reserved for it. Moreover, in optical networks suffering from physical layer impairments, inefficient use of network resources leads to significant increase in the blocking probability and higher vulnerability due to degradation of the QoT (quality of transmission) in the network. These trade offs call for the need to consider the physical layer impairments in designing higher layer protection schemes, i.e., a QoT-aware or cross-layer design. In this paper, we look at the performance of several link protection and link and path restoration algorithms in all-optical networks with realistic physical layer impairments, and propose a new cross-layer restoration method that exhibits both low blocking probability and low vulnerability ratio.

Performance bounds of free space optical (FSO) systems with multiple laser transmitters and multiple avalanche photodetectors (APDs) are evaluated for terrestrial, line-of- sight communication. Specifically, analytical upper bounds on average uncoded error rate in turbulent multiple-input multiple- output (MIMO) channels with Q-ary pulse position modulation (PPM) are derived for arbitrary observable densities, and are used to quantify APD-based performance improvements over pin detector arrays in non-fading, lognormal, and negative exponential channels. Moreover, it is shown that despite the high complexity of APD array statistics, these performance gains can be accurately predicted using a jointly-Gaussian model for APD array outputs. Both maximum likelihood (ML) and sub-optimal combining rules are derived, and a simple equal gain combiner (EGC) that performs close to optimum is demonstrated. Finally, APD gain selection is discussed.

In WDM all-optical networks where electrical regeneration is not available, physical impairments due to propagation in the fibers, amplifier noise, and leaks between channels and in the switches cannot be removed at the physical layer. These effects cause calls, especially between physically distant nodes, to be rejected because they cannot meet minimum Quality of Transmission (QoT) requirements, as measured by signal bit-error rates. It is possible to mitigate physical layer effects at the network layer using appropriate Routing and Wavelength Assignment (RWA) algorithms. We present new RWA algorithms which account for physical impairments in their design and increase QoT and fairness among users without sacrificing low blocking probabilities in metropolitan-sized networks. We also present RWA algorithms that can sharply decrease blocking probabilities in regional-sized networks using optional channel coding. All algorithms are evaluated through simulation in realistic scenarios and shown to successfully mitigate crosstalk effects and to perform better in terms of QoT and network access fairness than traditional algorithms.

As an optical signal propagates along a lightpath to its destination in wavelength-routed optical networks (WRONs), the quality of transmission (QoT) is degraded by transmission impairments such as crosstalk and amplified spontaneous emission noise. Consequently, the signal’s bit error rate at the destination’s receiver can become unacceptably high. Recently, many BER-aware connection provisioning algorithms have been proposed that incorporate physical layer impairments in terms of increased blocking rate from the BER constraint. However, they have a high computational complexity compared to traditional connection provisioning schemes because of the complexity of BER estimation. The delay involved in the computation greatly affects the performance in distributed networks because the latency worsens the contention and makes instantaneous network status impossible to obtain. In this paper, distributed connection provisioning schemes are compared using two BER estimation procedures and several wavelength assignment algorithms. Simulation results show the effects of delay in connection provisioning and measure the performance in terms of blocking probability.

The use of an optical preamplifier in a two laser system for atmospheric line-of-sight optical communication is investigated. Using orthogonal repetition coding is shown to provide diversity. Two different intensity modulation methods, on-off keying (OOK) and binary pulse position modulation (PPM) are analyzed for systems corrupted by background radiation and amplified spontaneous emission noise. Using an optical preamplifier in a MISO channel results in better performance than using an electrical amplifier.

In this paper, we consider the problem of routing and wavelength assignment (RWA) in all-optical networks, where signals propagate in the optical domain (with no electrical regeneration) from end-to-end. In such networks, physical impairments are an issue and can cause calls to be blocked if a minimum quality of transmission (QoT) measured in terms of bit-error rate (BER) cannot be guaranteed for all calls, at all times. We focus on the routing component of RWA to incorporate QoT: we propose a novel RWA algorithm that finds a route based on both the network utilization and the physical impairments experienced over the tentative route. Our algorithm is fully decentralized and is shown using simulations to perform better than previously proposed algorithms on a regional-sized network.

Crosstalk originating from optical switching devices and demultiplexers can be the dominant physical impairment in large all-optical networks. When a centralized network controller must estimate the impact of crosstalk and other physical impairments on the quality of a lightpath, this can add significant latency. In this paper, QoS-aware wavelength assignment algorithms are proposed that consider both bit error rate and latency guarantees. A technique called wavelength ordering is presented to alleviate physical impairments and also decrease the processing delay due to bit-error-rate (BER) estimation. Simulations presented show that the wavelength ordering algorithm outperforms other wavelength assignments when both BER and response latency guarantees are enforced.

As an optical signal propagates along a lightpath to its destination in distributed wavelength-routed optical networks, the signal's quality is degraded by transmission impairments, such as node crosstalk, which is induced by power leaking in the network nodes. Consequently, the bit error rate of the received signal at the destination can be unacceptably high. In this paper, a new adaptive wavelength assignment scheme based on wavelength spectrum separation technique is proposed to combat physical impairments in distributed optical networks. The proposed algorithm outperforms traditional techniques as measured by the blocking probability, adding little complexity.

There has been a significant amount of recent research on routing and wavelength assignment for DWDM networks suffering from physical layer transmission impairments. These algorithms attempt to increase the quality of transmission or service and are called QoT (quality of transmission) or QoS aware algorithms. However, protection has not been considered in this context so far, as far as we are aware. In this paper, we investigate the effect of physical layer impairments on dedicated path protection schemes. In dedicated path protection, every connection has resources reserved and dedicated on both a primary and a backup lightpath. While there is no difference in network performance whether the backup path is lit or kept dark in networks that are not transmission-impaired, lighting the backup path has an adverse effect on the network in transmission-impaired networks. By considering two different RWA algorithms - a QoT-aware one and a QoT-unaware one, we study the blocking performance and the vulnerability of the connections to failures for the two cases - dark backup and lit backup. Our results show that there are significant penalties in backup lit case, and that the QoT-aware algorithm considerably outperforms the QoT-unaware algorithm in terms of both blocking and vulnerability to failures.

In all-optical networks with no wavelength converters, signals are switched optically inside the nodes and therefore propagate over hundreds or thousands of kilometers with no electrical regeneration. Over such distances, physical impairments accumulate and can lead to serious signal degradation, resulting in poor quality of transmission (QoT) as measured by signal bit-error rates. The role of routing and wavelength assignment (RWA) algorithms is to accommodate incoming calls in optical networks over a route and a wavelength. RWA algorithms block calls if a continuous wavelength from source to destination cannot be found (wavelength blocking), or when the QoT of the call is not acceptable (QoT blocking). In this paper, we present an analytical method to evaluate blocking probability in all-optical networks, accounting for several physical layer impairments: intersymbol interference (ISI), amplifier noise (both are static effects that only depend on the network topology only) and node crosstalk (a dynamic effect that depends on the network status). We successfully validate our model through simulations on large scale networks with realistic physical layer parameters.

Design techniques for sub-optimal multiuser detectors for multichannel communication over optically amplified fiber optic links with square-law receivers are investigated. A new regression based technique is proposed for designing affine and quadratic multiuser detectors. The proposed techniques are compared to high-complexity asymptotically optimal (AO) detectors and two standard low-complexity techniques, viz. single user detection (SUD) and affine minimum mean square error (AMMSE) detection. Benefits of the proposed design technique over SUD and AMMSE based techniques are demonstrated using typical two user systems.

As an optical signal propagates along a lightpath to its destination in wavelength-routed optical networks (WRONs), the quality of service (QoS) will be degraded by transmission impairments such as crosstalk, which is induced by signals traversing the same optical crossconnects, demultiplexers, and fiber segments. Consequently, the signal's bit error rate at the destination's receiver might become unacceptably high. Recently, many QoS-aware wavelength assignments (WAs) have been proposed that corporate physical layer blocking; these have a high computational complexity compared to traditional WAs because of QoS estimation. In this paper, a new dynamic WA scheme using a wavelength spectrum separation technique is proposed to combat crosstalk occurring in WRONs. Our simulation results show that the new WA scheme has much lower complexity than QoS-aware WA schemes yet has a performance close to the QoS-aware WA schemes.

Multiple transmitter and receivers can be used to combat link outage due to the atmosphere turbulence and scattering in an urban 'last-mile' free space optical communication system. We investigate the use of an optical preamplifier in the resulting multiple-input-multiple-output (MIMO) system for atmospheric line-of-sight optical communication, with signal repetition across the laser array. The optically preamplified system has better performance than previous MIMO system relying on electronic amplification. Our focus is on determining symbol error probability for uncoded transmission.

In all-optical networks with no wavelength conversion, signals must travel on the same wavelength over possibly very long distances. During transmission, the QoS of signals as measured by their Bit Error Rates is degraded not only by the propagation through fibers, but also by small optical leaks from other signals called crosstalk that occur in the nodes and cannot be removed at the physical layer. We present a set of Routing and Wavelength Assignment algorithms that mitigate the crosstalk effects on all-optical network operation. These algorithms incorporate QoS information at both the routing and the wavelength assignment steps and account for dynamic crosstalk to yield better performance in terms of average BER and fairness among network users without sacrificing blocking probabilities, as shown through simulation.

Optimal and suboptimal multiuser noncoherent detection for square-law receivers is studied in this paper. Great potential is found for the multiuser square-law detector when compared with the conventional single-user square-law detector. We study the optimal detector and two detectors with simpler structures: the asymptotically optimal detector and the pairwise linear detector. The saddle-point approximation method is used to study the error performance and its asymptotic behavior as noise reduces. Due to difficulty in optimizing the detectors at an arbitrary noise level, we propose to use the asymptotic efficiency for detector optimization. For a low-error-rate system like a fiber optic communication system, the asymptotic efficiency is found to be an efficient way to design detectors. Numerical results show that the asymptotically optimized detectors perform as well as the optimal detector, even for nonzero noise levels of practical interest. Despite their exponential complexity, these detectors are applicable to wavelength-division multiplexed systems in which only a few neighboring channels produce strong interference

We study the use of multiple laser transmitters combined with multiple photodetectors for atmospheric, line-of-sight optical communication, and focus upon the use of multiple-pulse-position-modulation as a power-efficient transmission format, with signal repetition across the laser array. Ideal (photon counting) photodetectors are assumed, with and without background radiation. The resulting multiple-input/multiple-output channel has the potential for combating fading effects on turbulent optical channels, for which both log-normal and Rayleigh-fading models are treated. Our focus is upon symbol error probability for uncoded transmission, and on capacity for coded transmission. Full spatial diversity is obtained naturally in this application.

The use of multiple laser transmitters combined with multiple photodetectors (PDs) is studied for terrestrial, line-of-sight optical communication. The resulting multiple-input/multiple-output channel has the potential for combatting fading effects on turbulent optical channels. In this paper, the modulation format is repetition Q-ary PPM across lasers, with intensity modulation. Ideal PDs are assumed, with and without background radiation. Both Rayleigh and log-normal fading models are treated. The focus is upon both symbol-/bit-error probability for uncoded transmission, and on constrained channel capacity.

In fiber-optic wavelength-division multiplexing (WDM) communications systems, the nonlinearity of the channel and the limitations of optical filters result in received channel signals that are correlated. Furthermore, the square-law nature of the detector converts the additive fiber amplifier noise to a data-dependent asymmetric and non-Gaussian noise. The optimal multiuser detector for a Gaussian approximation is compared to the actual statistics and found to give a close performance estimate. Through bounds on the error probability and the asymptotic multiuser efficiency, the optimal multiuser detector based on a Gaussian approximation is found to improve significantly performance in highly correlated data cases. This result is applied to a practical optical WDM system.

Fading due to atmospheric turbulence is a primary problem in free-space-optical links, and to address this, we consider the use of (noncoherent) optical arrays, analogous to the use of antenna array technology for microwave systems. Specifically, we envision M separate lasers, assumed to be intensity-modulated only, together with N photodetectors, assumed to be ideal noncoherent (direct detection) receivers. The sources and detectors are physically situated so that the fading experienced between source-detector pairs is statistically independent, and thus diversity benefits can accrue from the MIMO channel. Whereas classic single pulse PPM is known to be efficient from an average power perspective, the required peak power from the lasers must become arbitrarily large in this process. This motivates study of multipulse modulation.

In wireless code division multiple access (CDMA) communications systems, there has been interest in processing the transmitted down-link signal in order to shift signal processing to the transmitter where power and computational resources are plentiful, thus simplifying receiver operation and reducing the power it requires. Multiuser interference (MUI) and multipath effects observed by the receiver are anticipated and suppressed at the transmitter; channel equalization and multiuser detection are therefore not required. This paper introduces two methods that are able to combat both degradations, yet allow the receiver to remain as simple as a single user receiver for a perfect channel. For mild multipath channels, the performance of the algorithms is excellent, within a few decibels of the single user ideal channel case, at the cost of additional computation at the base station at which complete knowledge of the channels and the receiver codewords is required. One method, the decorrelating prefilter, is most flexible and applicable to existing systems yet less powerful than other previously published methods. The second, the jointly optimized sequences algorithm, has a performance on average superior to published methods. In addition to theoretical analysis and simulation of the algorithms' potential, these algorithms have also been implemented and tested on a software radio testbed and experimental data are shown. The jointly optimized sequences performed particularly well even in severe multipath and multiuser interference environments.

Described is a decorrelating filter that separates signals in a multiuser environment by relying on the relative delays to be sufficiently distinct. The input signal is fractionally sampled to allow for the differentiation of the user delays. Both zero-forcing and minimum mean-square-error versions of this filter are derived and probability of error expressions are given. The algorithm successfully separates unknown digital signals by using the known received pulse shapes and the symbol rate. A delay-division multiple-access (DDMA) scheme based on this signal decorrelator is proposed that will allow signals to be transmitted without spreading the signal spectrum. It is shown that in a noisy environment the signal-to-noise ratio penalty for DDMA can be severe yet the bandwidth advantage is very pronounced compared to spread-spectrum systems and is similar to other bandwidth efficient schemes. The performance of a code-division multiple-access (CDMA) system using this signal decorrelator is also given. The decorrelator can be used as a blind multiuser detector or as a preprocessor to enhance the performance of a CDMA system.

A multistage multiuser detector (MMD) is presented for frequency-hopping/code-division multiple access (FH/CDMA.) The MMD reduces the bit-error rate (BER) over the conventional detector by exploiting prior knowledge of the addresses and energies of the user signals. This detector is a conservative multiuser detector which is robust to unknown users and has a complexity that is only linear in the number of users. The performance analysis of the synchronous MMD includes both theoretical and simulation BERs for the noiseless case as well as simulation results that include the presence of noise. The MMD is then extended to the fully asynchronous case which makes this work the first to propose an FH/CDMA multiuser detector for asynchronous communications. This asynchronous MMD is compared via simulation to the conventional detector.

A multiuser decoder/demodulator is presented that uses successive cancellation within the Viterbi (1990) decoder of each user in a code-division multiple access (CDMA) communication system. The decisions of high power users take into account decisions on several lower power users symbols and several hypothesis trellis paths are considered. The method is shown to perform significantly better than the traditional interference cancellation detector. The number of hypotheses and the number of low power users to cancel can be set as parameters so that the method provides for flexible computational complexity.

We present three detector/estimators (DEs) which allow multiuser detection and parameter estimation without a side channel in a dynamic asynchronous code-division multiple-access (CDMA) system in which users are entering and leaving the system. These DEs optimally detect a new user given only the chip rate and the spreading factor of the new user. Two of these DEs, the maximum-likelihood detector/estimator (MLDE) and the generalized maximum-likelihood detector/estimator (GMLDE), produce maximum-likelihood estimates of the new user's signature sequence, delay, and amplitude, which are then incorporated into a multiuser detector. The third DE, the cyclic detector/estimator (CDE), is the most computationally efficient of the three processors. This DE detects the new user by testing for cyclostationarity and then uses suboptimal schemes to estimate the new user's signature sequence, delay, and amplitude. Simulations indicate that all three DEs reliably detect a new user for an E/sub s///spl sigma//sup 2/ (symbol-energy-to-noise ratio) of 5 dB. The MLDE and GMLDE produce signature sequence and delay estimates with probability of error less than 0.07 for an E/sub s///spl sigma//sup 2/ of 10 dB, and the CDE produces signature sequence and delay estimates with probability of error less than 0.13 for an E/sub s///spl sigma//sup 2/ of 15 dB.

Code division multiple access (CDMA) is a powerful multiplexing scheme, particularly suited to the optical domain with its broad spectrum. Unfortunately, analysis of optical CDMA (OCDMA) systems has been almost exclusively restricted to systems employing a correlation detector, which suffer high degradation when received user powers are dissimilar. This study examines the performance of an optimum single-user detector and two multiuser detectors for OCDMA in the presence of additive light intensity noise and Poisson detection. The optimized single-user detector is the maximum likelihood detector given the interference from other users is a random process with known distribution. One multiuser detector is based on a local search algorithm for maximizing the likelihood function. The second multiuser detector is a generalization of the optical multistage detector to accept soft-decision from the previous stage. Approximations to the error probabilities of the OCDMA system based on either random signature sequences or deterministic codes are derived for each detector using a large deviations theory approach. The asymptotic multiuser efficiency is found to be related to the large deviations rate function. The analysis shows that the correlation detector performance decreases rapidly as the number of users increases, while the optimized single-user detector and the two multiuser detectors proposed perform considerably better and show less sensitivity to unequal received user powers.>

Considers a multiuser detection scheme for optical direct sequence code division multiple access (OCDMA) systems, referred to as the multistage detector. Previous works in this area have proposed two detectors: the correlation detector that is simple, but has poor performance for large number of users, and the optimal (minimum probability of error) detector that has exponential complexity in the number of users. Efficient multiuser detection algorithms consider the interfering user codes at the expense of electronic speed processing, unlike the optical processing achievable with the correlation detector. The multiple access system using the proposed multistage detector is shown to be of high performance and low complexity, compared to the conventional correlation detector and the optimal detector. The model studied includes multiple access interference as well as the Poisson characteristics of the optical direct detection process. An approximation to the probability of error is derived for the multistage detector, and it is compared to the actual error probability of the correlation detector, to a lower bound to the error probability of the optimal detector, and to simulation results for the multistage detector. The probability of error is calculated using a characteristic function method. Results are presented for a random code case that show a significant improvement in the performance of the OCDMA system using this detector over the correlation detector.>