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The generation of microwaves from optical signals suffers from thermal and shot noise inherent in the photodetection process. This problem is more acute at lower pulse repetition rates where photodiode saturation limits the achievable signal-to-noise ratio. In this paper, we demonstrate a 10-15-dB reduction in the 10-GHz phase noise floor by multiplication of the pulse repetition rate. Starting with a 250-MHz fundamentally mode-locked erbium (Er):fiber laser, we compare the following two different approaches to repetition rate multiplication: 1) Fabry-Perot cavity filtering and 2) a cascaded, unbalanced Mach-Zehnder (MZ) fiber-based interferometer. These techniques reduce the phase noise floor on the 10-GHz photodetected harmonic to -158 and -162 dBc/Hz, respectively, for Fourier frequencies higher than 100 kHz.

This paper addresses the problem of resource allocation in a multiservice optical network based on an overlapped code-division-multiple-access 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 of this problem maximizes the aggregate throughput subject to peak laser 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. Simulation 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. [ABSTRACT FROM PUBLISHER]

In this paper, a new multirate optical network based on a hybrid slotted-ALOHA (S-ALOHA)/overlapped code division multiple access (O-CDMA) system is proposed as an effective way of integrating multiclass traffic. The key issue in this proposal is to exploit the potential of the optical O-CDMA using fiber Bragg gratings when jointly used with the S-ALOHA protocol in a link layer. The newly proposed system is modeled using a general Markov chain from which both the system throughput and the average packet delay are derived. The system stability passing through a general finite user model is also envisaged. The system is then compared to the classical S-ALOHA/variable processing gain (VPG) CDMA system. Numerical results show that the proposed system outperforms the latter one, especially at high transmission rates. In addition, stability measures demonstrate that the overlapped system tends to be more stable than the VPG system. [ABSTRACT FROM PUBLISHER]

This letter proposes a simple analytical model to accurately quantify the degree of fading envelope correlation among the received signals at the antenna elements of an adaptive array operating in a frequency-selective environment. The adaptive array is assumed to be at an elevated base station with scattering being confined to a local region around the subscriber station. This corresponds to the so-called "local scattering" or "macrocell" assumption and it results in most of the received energy at the base station array being concentrated within a given beamwidth. Our formulation provides closed-form, finite sum expressions for the covariance and correlation coefficients as a function of spatial and frequency separation. [ABSTRACT FROM PUBLISHER]

Maximum-ratio combining (MRC) is a simple and effective combining scheme for adaptive antenna arrays to combat noise, fading, and to a certain degree, cochannel interference. However, it requires estimation of the spatial signature (i.e., the channel gain and phase at each antenna element) of the desired signal across the array. Assuming that this estimate is obtained by correlation using a known training sequence of K symbols embedded in the useful signal, we proceed to develop a fully analytical assessment of the impact of estimation error on the output signal-to-noise ratio (SNR) of the array. The originality of the approach revolves around the derivation of the distribution of the normalized SNR, that is the real SNR normalized to the ideal (i.e., perfect estimation) SNR. The end result is a set of distributions which can potentially reduce or in certain cases eliminate the need for simulation to determine certain design parameters such as array size, training sequence length, etc. These are then applied to find closed-form expressions for the outage probability and the error probability in differential phase-shift keying and quarternary phase-shift keying after training in uncorrelated Rayleigh fading. [ABSTRACT FROM PUBLISHER]

Analyzes a new transmitter-receiver architecture based on a modified version of unipolar-bipolar correlation proposed for a noncoherent multirate optical fast frequency hopping code-division multiple-access (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 the multimedia network, the fluctuation in the multiple-access interference (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; in addition, the signal-to-interference ratio is derived. From the theoretical analysis and numerical results, it is shown that the proposed system has good performance without dynamic estimation of the detection threshold, and thus is independent of both the number of users and the distribution of those users in the offered multimedia classes. [ABSTRACT FROM PUBLISHER]

This paper addresses the problem of real-time multimedia transmission in fiber-optic networks using code division multiple access (CDMA). We present a multirate optical fast frequency hopping CDMA (OFFH-CDMA) system architecture using fiber Bragg gratings (FBGs). In addition, we argue that, in multimedia applications, different services have different quality of service (QoS) requirements; hence, the user only needs to use the minimum required power to transmit the signal, such that the required signal-to-interference ratio (SIR) is met. We show that a variable bit rate optical communication system with variable QoS can be implemented by way of power control with great efficiency. Present-day multirate optical CDMA systems concentrate on finding the code structure that supports a variable rate system, neglecting the importance of the transmission power of active users on the multiple access interference (MAI) and, therefore, on the system capacity. We assign different power levels to each rate through a power control algorithm using variable optical attenuators, which minimizes the interference and, at the same time, provides variable QoS constraints for different traffic types. Although we are using a code family that preserves good correlation properties between codes of different lengths, simulations show a great improvement in the system capacity when power control is used [ABSTRACT FROM PUBLISHER]

In this letter, we propose a class of irregular structured low-density parity-check (LDPC) codes with low error floor and low encoding complexity by designing the parity check matrix in a triangular plus dual-diagonal form. The proposed irregular codes clearly lower the error floor and dramatically improve the performance in the waterfall region of error-rate curves. Being characterized by linear encoding complexity, the encoders of the proposed codes attain throughputs over 10 Gbit/s. [ABSTRACT FROM PUBLISHER]

This paper investigates the signal-to-interference ratio (SIR) performance of a single chip rate variable processing gain optical fast frequency hopping CDMA (OFFH-CDMA) system, using direct detection. A fiber-optic CDMA communication network is considered. A system model that allows multirate transmission is presented. In addition, the SIR is derived. A comparison between the exact value of the SIR and the approximation one assuming random sequences is also discussed [ABSTRACT FROM PUBLISHER]