In the recent decades, free-space optical (FSO) communication technology has gained significant importance owing to its promising unique features: high user capacity, license-free spectrum, ease and quick deploy-ability. However, the performance of FSO communication systems depends on the uncontrollable terrestrial atmospheric effects. The second main challenge is that the FSO system performance degrades as a line-of-Sight (LoS) technology due to the misalignment transmitter and receiver. The third challenge is the consideration of time value of money, which is central to most engineering economic analyses in employing communication systems. The opportunity cost of making one choice over another must also be considered. This paper presents a proposed FSO system design model for mitigating the three performance challenges we mentioned. The results show an enhancement by 83.34% in system efficiency in case of moderate scintillation (using gamma-gamma model). This proposed hybrid model is proved to be applicable to any atmospheric channel conditions.

In this paper, we present a polarization-assisted L-ary differential phase-shift keying multipulse pulse-position modulation (PA.LDPSK-MPPM) technique that is secured in the physical layer by a discrete-chaos system. The all-optical PA.LDPSK-MPPM scheme benefits from the polarization as an additional degree of freedom which greatly reduces the system complexity relative to prior implementations. The discrete-chaos scrambling is based on a message-seeded two-dimensional chaotic map tailored for independent perturbation of the occupied time-slot positions (MPPM information) and their relative phase shift (LDPSK information). Synchronized and non-synchronized implementations of the chaotic PA.LDPSK-MPPM technique are proposed with expressions for the corresponding spectral efficiencies being determined and compared with prior LDPSKMPPM setups. The performance of PA.LDPSK-MPPM under gamma-gamma (GG) free-space optical (FSO) fading channels is analytically verified to outperform the prior designs for different FSO channel states which is supplemented by Monte Carlo (MC) simulations. The system security is numerically examined against various types of attacks, including brute-force, differential, and statistical attacks.

A hybrid two-level multipulse pulse-position modulation-M-ary differential phase-shift keying (2L-MPPM- MDPSK) technique is proposed which achieves high power and spectral efficiencies. The proposed 2L-MPPM-MDPSK scheme is shown to have the highest spectral efficiency in comparison with other hybrid modulation schemes. Additionally, the proposed technique has the advantage of removing the complex variable delay-line required for the direct- detection of other hybrid MPPM-MDPSK systems. The performance of 2L-MPPM-MDPSK is quantified in free-space optical (FSO) channels modeled with an exponentiated Weibull (EW) fading distribution. Closed-from expressions for both bit-error rate (BER) and outage probability upper-bound are derived and verified using Monte Carlo (MC) simulation. At the same data rate and bandwidth, 2L-MPPM-MDPSK techniques outperform traditional MPPM-MDPSK scheme by approximately 2dB at a BER of 10-4.

High Efficiency Video Coding (HEVC) provides more compression than its predecessors. One of the modules that contributes to higher compression rates is the Motion Estimation module, which consists of Integer and Fractional pixel motion estimation. The Fractional Motion Estimation (FME) process performs interpolations to find sample values at fractional-pixel locations, which can be computationally demanding. In this paper, we propose an interpolation-free method for FME based on Artificial Neural Networks (ANNs). Our proposed method is implemented in HEVC reference software (HM-16.9). According to our results, ANNs can accomplish FME task with an average increase of 2.6% in BDRate and an average reduction of 0.09 dB in BD-PSNR.

A multicode optical code-division multiplexing (OCDM) is proposed to support the dynamic changes in the requested traffic demand in OCDM networks by adapting the number of allocated codes according to the requested transmission rate. In order to support time-variant data rates and multiservice transmissions; a multicode variable-weight 2D one-coincidence frequency hopping code/optical orthogonal code (OCFHC/OOC) is employed as the signature code in the proposed system. Two multicode assignment methods, namely, random multicode assignment (RMA) policy and uniform multicode assignment (UMA) policy, are presented. For each code assignment method, we drive the probability density function (pdf) of the number of active codes in each wavelength group. The bit error probability (BEP), the probability of degradation, and the blocking probability are derived as network performance key parameters. The proposed system has a simplified routing and code assignment process with a lower blocking probability. Our numerical results indicate that the network teletraffic capacity and blocking probability can be improved significantly using UMA policy and call admission control.

Optical code-division multiplexing (OCDM) systems use short pulses compared with bit-duration to achieve high transmission rate. The use of short pulses poses several problems as a result of group velocity dispersion (GVD), intersymbol interference (ISI) (due to avalanche photodiode (APD) buildup time), and receivers limited bandwidth. In this paper, an OCDM system employing code-shift keying (CSK) and overlapping pulse-position modulation (OPPM) signaling is proposed and theoretically investigated. By using CSK while maintaining same data rate, the chip duration can be increased to counteract the GVD effect in 2D OCDM systems. Moreover, by increasing the chip duration, the chip rate is decreased and the stringent requirement on receiver bandwidth is relaxed. In addition, using overlapping property in OPPM allows for further chip duration increase. We consider using correlation receivers with hard-limiters and APDs at the receiver side. The bit error probability (BEP) of the proposed system is derived taking into account the impacts of APD noise, thermal noise, GVD, ISI, and multiple-access interference (MAI). A performance comparison between OOK-, PPM-, OPPM-OCDM and the proposed system is carried out. Our results reveal that the use of CSK/OPPM-OCDM with data rate constraint allows the reduction of MAI, GVD and ISI effects with improved spectral efficiency.

A blind (non-data-aided) SNR estimator using the statistical moments of the received signal is proposed. The proposed envelope-based non-data-aided estimator works for any time-domain Gaussian-distributed signal (e.g., OFDM signals). A closed-form expression for the estimated SNR as a function of the moments of the received signal and the Nakagami-m parameter, is derived. Interestingly, the obtained expression shows that the proposed estimator operation and performance is independent of the constellation of the received signal. Moreover, the existence of the closed-form expression results in lower implementation complexity. Furthermore, to enable theoretical performance analysis, a general mathematical expression is derived for the even moments of the received signal in terms of SNR and the Nakagami-m parameter. The performance of the proposed estimator is evaluated based on the mean-squared-error, under different conditions of the channel. An extension of our SNR estimation method into multiple antennas configurations is provided. Our results reveal that the proposed estimator works better in low SNR conditions, which is attractive to applications such as cognitive radio spectrum sharing scenarios.

Fractional Frequency Reuse (FFR) is an efficient method to mitigate Inter Cell Interference in multicellular Orthogonal Frequency Division Multiple Access (OFDMA) systems. In this paper, we analyze the downlink worst case Signal to Interference Ratio for FFR schemes. A closed form expression is derived analytically for the worst SIR, outage probability, and Spectral Efficiency (SE). The proposed analytical technique is used to configure a FFR solution for the downlink of OFDMA cellular system. The analysis is performed using two-tiers cellular network with uniform user density and for three different cases of FFR, namely, Frequency Reuse Factor (FRF) = 3, FRF=4 and sectored FFR. The inner radius configuration depends on equalizing the worst SIR for both inner and outer edges of the cell. Numerical results show that sectored FFR yields the highest SE and low outage probability. Sectored FFR highly balances the needs of interference reduction and resource efficiency.

We propose the implementation of spectral-amplitude-coding optical code division multiple access (SAC-OCDMA) as a contention resolution technique in optical burst switched (OBS) networks. The new system architecture is presented in details where an all-optical methodology for cancelling multiple access interference is proposed. Performance evaluation of the proposed system in both MAC and optical layers is introduced where the overall burst error rate of the system is evaluated in three cases: full, partial, and no code conversion capabilities taking into account the receiver dark current, thermal, and shot noises at the egress nodes. Our results reveal that a considerable improvement in the performance of each core node in the system is achieved by using SAC-OCDMA instead of WDM in the optical layer underneath an OBS based MAC layer. We also conclude that a slight increase in the employed number of code converters enhances the overall system performance noticeably. Finally, optimum values for the number of codes, which lead to minimum overall burst error rate, are reached at different traffic conditions.

An enhanced mathematical model is introduced to study and evaluate the performance of a core node in an optical burst switched network. In the proposed model, the exact Poisson traffic arrivals to the OBS node is approximated by assuming that the maximum allowed number of arrivals to the OBS node, in a given time slot, is two (instead of infinity). A detailed state diagram is outlined to illustrate the problem, and then a mathematical model based on the equilibrium point analysis (EPA) technique is presented. The steady-state system throughput is derived from the model which is built in the absence of wavelength conversion capability. Our proposed model is aided by a simulation work which studies the performance of an OBS core node under the assumption of Poisson traffic arrivals (the exact case) and calculates the steady- state system throughput. The results obtained from the proposed mathematical model are consistent with that of simulation when assuming Poisson traffic arrivals and this consistency holds for a wide range of traffic load.

In this work we develop a fair resource allocation scheme for a multi-class time-slotted optical overlapped code- division multiple-access (OV-CDMA) network. The resource management scheme is fair in the sense that the users have their power and rate allotted according to their quality of service (QoS) requirements. In addition, no class of users can dominate over the other classes. A unified framework is proposed, which consists of finding a single control parameter based on which the optimal transmission rate and power are obtained for every class of users. Analytical results show that the maximum transmission rate for a given class of users is only achievable when the users in that class are transmitting with the highest possible laser transmission power. In addition, we have demonstrated that the optimal transmission rates can be obtained via the solution of the rate characteristic polynomial.

A queuing subsystem is proposed to the round robin receiver/transmitter (R/sup 3/T) optical code-division multiple-access (OCDMA) protocol. The corresponding state diagram and its mathematical model are outlined. We prove that significant improvement in terms of system throughput and blocking probability can be achieved by only adding a single buffer at each user node. Furthermore, the modified R/sup 3/T protocol can support higher traffic loads with acceptable delay and good efficiency.

This paper addresses the problem of resource allocation in a multiservice optical network based on an overlapped-CDMA system. A joint transmission power and overlapping coefficient (transmission rate) allocation strategy is provided via the solution of a constrained convex quadratic optimization problem. The solution to this problem maximizes the aggregate throughput subject to peak lasers transmission power constraints. The optimization problem is solved in a closed form, and the resource allocation strategy is simple to implement in an optical network. Results are presented showing a total agreement between the derived analytical solution and the one obtained using a numerical search method. In addition, analytical and numerical results show that the proposed resource allocation strategy can offer substantial improvement in the system throughput.

A media access control (MAC) protocol for optical code-division multiple-access (CDMA) packet networks with variable length data traffic is proposed. This protocol exhibits a sliding window with variable size. A model for interference level fluctuation and an accurate analysis for channel usage are presented. Both multiple access interference (MAI) and photodetector's shot noise are considered. Both chip-level and correlation receivers are adopted. The system performance is evaluated using traditional average system throughput and average delay. Finally, we apply error control codes (ECCs) in order to enhance the overall performance. Our results indicate that the performance can be enhanced to reach its peak using ECC with optimum number of correctable errors. Furthermore chip-level receivers are shown to give much higher performance than that of correlation receivers. Also, it has been shown that MAI is the main source of signal degradation.

In this paper we propose a new multirate optical network based on a hybrid S-ALOHA/overlapped-CDMA system as an effective way of integrating multi-class traffic. The key issue in this proposal is to exploit the potential of the optical overlapped CDMA using fiber Bragg grating when jointly used with the S-ALOHA protocol in a link layer. The newly proposed system is modeled using general Markov chain from which both the system throughput and the average packet delay are derived. Our system is then compared to the classical S-ALOHA/variable processing gain (VPG) CDMA system. Numerical results show that our system outperforms the latter especially at high transmission rates

We consider an optical code-division multiple-access (OCDMA) network that supports multiple traffic classes. Each class has different processing gain and performance requirements. In this paper, a new method is proposed to analyze the cutoff rates for a multiclass, multirate optical frequency-hopping code-division multiple-access system using fiber Bragg gratings and direct detection. This approach exploits the linear structure of passive OCDMA systems and the nominal time required to accomplish the encoding-decoding operations in such systems. A system model is presented and analyzed, based on a newly introduced bit-overlap procedure. An expression for the cutoff rate of a single-class system is derived. In addition, for a multiclass system, an expression that relates the cutoff rates of the offered classes is introduced, and it is called the service curve. It is shown that for a required quality of service guarantee, a number of active users, and a given probability of hit, the system's data rate can be increased beyond the nominal limit imposed by the physical constraint of the encoder-decoder set.

An upper bound on the users strength in direct-detection optical code-division multiple-access communication systems, which employ pulse-position modulation techniques, is developed. This upper bound is consistent with the lower bound derived previously, as long as the transmitted information per photon is less than some positive threshold. The concept of users strength was introduced by the author in a previous paper, where it provides a measure to the maximum number of simultaneous users that can communicate simultaneously with arbitrary small probability of error.

An optical CDMA network that supports multiple traffic classes is considered. Each class has different processing gain (PG) and performance requirements. In this paper, we generalize the method that we have previously proposed to analyze the cutoff rates for a single-class multirate optical frequency hopping code division multiple access system (OFFH-CDMA), to the case of multiclass multirate OFFH-CDMA system. This approach exploits the linear structure of passive optical CDMA systems and the nominal time required to accomplish the encoding-decoding operations in such systems. A system model is presented and analyzed based on a newly introduced bit-overlap procedure. An expression that relates the cutoff rates of the offered classes is introduced and it is termed the service curve. It is shown that for a required quality of service (QoS) guarantee, a number of active users, and a given probability of hit, the system's data rates can be increased beyond the nominal limits imposed by the physical constraint of the encoder-decoder sets.

A random-access protocol for an optical direct-detection code-division-multiple access packet network is proposed. A mathematical description of the protocol is presented using a detailed state diagram. The steady-state system throughput is derived based on the equilibrium point analysis technique. The effect of several design parameters on the above performance measure has been examined with the aid of a set of numerical examples. In our numerical calculations chip-level models have been adopted at the receiver side.

New protocol for optical code-division multiple access (CDMA) networks is proposed. Our proposed protocol is based on selective retransmission technique. A mathematical model is presented using a detailed state diagram. The protocol is analyzed using equilibrium point analysis (EPA), and its performance is examined using traditional throughput and average delay for several network parameters. We also compare the performance of the proposed protocol to the R/sup 3/T protocol based on go-back-n technique. Results show that a higher performance is achieved by the proposed protocol at the expense of system complexity.

The link layer of an optical direct-detection code-division-multiple access packet network is considered. Two of them need pretransmission coordination, whereas the third one does not. Both system throughput and average packet delay are derived and investigated for two different receiver models, namely the correlation and chip-level receivers. Both multiple-access interference and photodetector's shot noise are taken into account in the analysis. The case where the number of users exceeds the available number of CDMA codes is numerically investigated. Our results reveal that the proposed protocols yield competitive system throughputs when used with the correlation receivers. Further, significant improvement in the throughput is achieved when using chip-level receivers along with the second or third protocol.

A new method is proposed to analyze the cutoff rate of a variable-bit-rate (VBR) optical fast frequency hopping code division multiple access system (OFFH-CDMA) using fiber Bragg gratings and direct detection. This approach exploits the linear structure of passive optical CDMA systems and the nominal time required to accomplish the encoding-decoding operations in such systems. A system model is presented and analyzed based on a newly introduced bit-overlap procedure. An expression for the cutoff rate of a VBR OFFH-CDMA system is derived. It is shown that for a required quality of service (QoS) guarantee, the system's data rate can be increased beyond the nominal limit imposed by the physical constraint of the encoder-decoder set.

The complexity in the hardware implementation of traditional optical code-division multiple-access correlation receivers with double optical hardlimiters is discussed. A comparison with the implementation of chip-level receivers is presented as well. In addition, the bit error probabilities and the throughput capacities for both chip-level and correlation systems (without hardlimiters) are derived and evaluated under code-correlation constraints equal to one and two. Our results reveal that chip-level receivers are much simpler and their performances are competitive with that of traditional correlation receivers with double optical hardlimiters. Further, the throughput capacity of chip-level systems can be increased by almost a factor of 3.4 when increasing the code-correlation constraint from one to two.

This paper analyses a new transmitter-receiver architecture based on a modified version of unipolar-bipolar correlation proposed for noncoherent multirate optical fast frequency hopping CDMA (OFFH-CDMA) system. The system assigns a frequency-shifted version (FSV) of the code used to transmit data bit "1" in order to transmit data bit "0". For the original system, we show that due to the nature of multimedia network, the fluctuation in the MAI average causes a threshold drift and thus an increase in the probability of error. This paper also provides a stochastic description of the MAI average amplitude fluctuation using a predefined multimedia probability density function. A system model is presented. From the theoretical analysis and numerical results, it is shown that the proposed system has good performance without dynamic estimation of the detection threshold, thus is independent of both the number of users and the distribution of those users in the offered multimedia classes.

A comparison between the performance of traditional optical code-division multiple-access correlation receivers and that of the previously proposed chip-level receivers is presented. The performance is measured in terms of both throughput and efficiency capacities. Signature code correlations bounded by either one or two are employed. The bit error probabilities for chip-level receivers, with code correlations bounded by two, are derived. That with code correlations bounded by one is recited from previous literature. Our results reveal that chip-level systems are much more efficient and their throughput capacities are much higher than that of traditional correlation systems. Further, the throughput capacity of chip-level systems can be increased by almost a factor of 3.4 when increasing the code correlation constraint from one to two.

Direct-detection optical code-division multiple access (CDMA) systems employing overlapping pulse-position modulation (OPPM) schemes are proposed. A union upper bound on the bit error rate is derived taking into account the effect of both multiple-user interference and receiver shot noise. The photodiodes' dark currents are neglected in order to have some insights on the problem. Performance characteristics are then compared to the traditional on off keying (OOK) and pulse-position modulation (PPM) schemes. It is shown that under the constraints of both throughput and chip time, the OPPM system outperforms both traditional systems.

A multiple-user interference reduction technique is proposed for optical code-division multiple-access (CDMA) systems. Data symbols from each user are encoded using a pulse-position modulation (PPM) scheme before multiplexing. Modified prime sequences are adopted as the signature codes in the multiplexing process. An interesting property of this code is the uniformity of the cross correlation among its sequences. This property is the main key in constructing the multiple-access interference canceler. In addition to its simplicity, this canceler offers a great improvement in the error probability as compared to the system without cancellation. A simple modification to this canceler that enhances its performance is proposed as well.

A new technique of interference cancellation for synchronous optical CDMA communication systems is proposed We utilized the properties of modified prime sequences under the assumption of a Poisson shot noise model for the PIN photodiode and synchronization between users' symbols. The interference cancellation technique is based on estimating the multiuser interference (MUI) through the special correlation properties of the used codes. The estimated MUI is then used to adapt a threshold level to optimally detect the concerned user's information and to suppress the MUI. Detailed derivations of the closed forms for the bit error rate are presented for both cases of with and without the suggested MUI suppression. Numerical results clearly show the advantages of the proposed technique and proves that it outperforms a system without interference cancellation.

The performance of optical code-division multiple-access (CDMA) communication systems utilizing number-state light field is evaluated. Lossy direct-detection optical channels are assumed. Both on-off keying (OOK) and pulse-position modulation (PPM) schemes are investigated. For OOK, the exact bit error rate is evaluated taking into account the effect of both multiple-user interference and transmission loss. Upper and lower bounds on the bit error probability for PPM-CDMA systems are derived under the above considerations. The effect of both background and thermal noise is neglected in our analysis. Performance comparison between the number-state and coherent-state OOK/PPM-CDMA networks is also presented. Our results demonstrate that the number-state system requires less than half the energy consumed by the coherent-state one in order to attain the same performance. Lower bounds on the maximum number of simultaneous users are derived for both number- and coherent-state PPM-CDMA systems with asymptotically zero error rate.>

Direct-detection optical synchronous code-division multiple-access (CDMA) systems with M-ary pulse-position modulation (PPM) signaling are investigated. Optical orthogonal codes are used as the signature sequences of our system. A union upper bound on the bit error rate is derived taking into account the effect of the background noise, multiple-user interference, and receiver shot noise. The performance characteristics are then discussed for a variety of system parameters. Another upper bound on the probability of error is also obtained (based on Chernoff inequality). This bound is utilized to derive achievable expressions for both the maximum number of users that can communicate simultaneously with asymptotically zero error rate and the channel capacity. Our results show that under average power and bit error rate constraints, there always exists a pulse position multiplicity that permit all the subscribers to communicate simultaneously.>

Upper and lower bounds on the capacity and cutoff rate for direct-detection optical overlapping-pulse-position modulation channels are derived. It is shown that these bounds are asymptotically tight, in the sense that the difference between the upper and lower bounds converges to zero as the pulse position multiplicity M and/or the average photon count per pulse Q goes to infinity. The tightness of these bounds for finite values of M and and is investigated by providing some numerical examples. Orders of magnitude of the rate of convergence of the difference between the bounds are also estimated.>

The paper considers a binary hypothesis testing system in which two sensors simultaneously observe a discrete-time finite-valued stationary ergodic Markov source and transmit M-ary messages to a Neyman-Pearson central detector. The size M of the message alphabet increases at most subexponentially with the number of observations. The asymptotic behavior of the type II error rate is investigated as the number of observations increases to infinity, and the associated error exponent is obtained under mild assumptions on the source distributions. This exponent is independent of the test level /spl epsiv/ and the actual codebook sizes M, is achieved by a universally optimal sequence of acceptance regions, and is characterized by an infimum of informational divergence rate over a class of infinite-dimensional distributions. Important differences-due to the observations being Markov-between the asymptotically optimal distributed tests and their nondistributed counterparts are highlighted. The converse results require a blowing-up lemma for stationary ergodic Markov sources, which is also proven.>

The effect of feedback on the performance of a distributed Neyman-Pearson detection system consisting of n sensors, two-stage local quantizers, and a global detector is investigated. Two feedback schemes are discussed, only one of which yields an asymptotic gain in performance, as measured by an appropriate error exponent.>

The asymptotic performance of a multiterminal detection system comprising a central detector and two remote sensors that have access to discrete, spatially dependent, and temporally memoryless observations is discussed. It is assumed that prior to transmitting information to the central detector, each sensor compresses its observations at a rate which approaches zero as the sample size tends to infinity, and that, on the basis of the compressed data from both sensors, the central detector seeks to determine whether the true distribution of the observations belongs to a null class II or an alternative class Xi . Under the criterion that stipulates minimization of the type-II error rate subject to an upper bound epsilon on the type-I error rate, it is shown that, in the case of simple distribution classes ( mod Pi mod = mod Xi mod =1), the error exponent achievable by such a system has a simple characterization, is independent of the value of epsilon , and is insensitive to changes in compression rate as long as the asymptotic rate on one of the sensors is zero.>