Electrical and Computer Engineering

Label stacking is used for hierarchical addressing to reduce the size of lookup tables and to increase the speed of the routing process. We propose an optical label stacking using spectral-amplitude codes (SAC) as labels to accomplish ultrafast packet forwarding. We discuss the advantages of this label architecture compared to other proposals in the literature and present experimental results. We experimentally examine two types of optical packets, one with separable SAC labels and the other one with SAC-encoded payloads. In the first case, the SAC label is a collection of spectral tones modulated at the packet rate (low rate), and the payload is on a separate wavelength modulated at the data rate (fast rate). In the second case, the payload data modulates the collection of wavelengths that constitute the code. We implement a network with two forwarding nodes, and we transmit the packets with two labels in the label stack over 80 km of fiber and measure the bit error rate (BER) after two hops. We achieve error-free transmission (BER < 10 −9 ) for the packets with SAC labels and SAC-encoded payload at payload bit rates of 10 and 2.5 Gb/s, respectively. This is the first experimental demonstration of optical label stacking to our knowledge.

In optical code-division multiple access (OCDMA), the optical bandwidth is accessed simultaneously by multiple users, leading to beat noise in photodetection. The choice of the optical source can have important impact on that noise. In this paper, we compare two optical sources, a broad-band erbium-doped fiber source and a multilaser source. Experimental results for a wavelength-time OCDMA system are presented for up to four simultaneous users. Bit-error-rate curves are measured at 1.25 Gb/s with a chip rate of 10 Gchip/s. It is shown that the multilaser source outperforms the broad-band source for the design parameters of this system. Index Terms-Beat noise, code-division multiple access, multiple access interference (MAI), optical fiber communications, wavelength-time coding.

We propose and demonstrate experimentally, for the first time, a prototype for all-optical ultra-wideband (UWB) transceiver at 500 Mb/s. We report 1) UWB pulse optimization that takes into account the transmitter RF front end and the U.S. Federal Communications Commission (FCC) spectral mask, 2) a new approximate match filter receiver using optical signal processing, and 3) modulation at 500 Mb/s. Our previous optimization of UWB pulse shapes was based only on the FCC spectral mask, without taking into account the frequency response of the RF components (amplifier and antenna) in the UWB transmitter. Here, we modify our pulse optimization technique to ensure that the equivalent isotropic radiated power (EIRP) from the transmitter meets FCC specifications. For the RF hardware used, we achieve 63.6% efficiency over the FCC mask, which yields an 11.6-and a 5.9-dB improvement over Gaussian monocycle and doublet pulses, respectively. We also introduce simple optical signal processing at the receiver that allows the incoming RF signal to be matched against a square pulse whose duration is adapted to the channel. The exact matched filter would require a new optimized pulse that would include not only hardware frequency response but channel effects that vary with antenna placement as well. The proposed approximation allows a simple variation of the pulse duration: an increase to account for pulse expansion in the channel but an upper limit to combat multipath effects. Finally, we demonstrate the optimized pulse and approximate match filter receiver at 500 M/s. We attain a bit error rate at a 65-cm separation line of sight (LOS) link with simple on-off keying and no forward error correction.

We develop a methodology for numerical optimization of fiber Bragg grating frequency response to maximize the achievable capacity of a spectral-amplitude-coded optical code-division multiple-access (SAC-OCDMA) system. The optimal encoders are realized, and we experimentally demonstrate an incoherent SAC-OCDMA system with seven simultaneous users. We report a bit error rate (BER) of 2.7 × 10 −8 at 622 Mb/s for a fully loaded network (seven users) using a 9.6-nm optical band. We achieve error-free transmission (BER < 1 × 10 −9 ) for up to five simultaneous users.

The authors propose a method to optimize the RF gain in narrowband radio-over-fiber links employing a Mach-Zehnder modulator followed by an erbium-doped fiber amplifier (EDFA) for amplification. Optimization is achieved by control of the modulator bias in order to improve the signal optical-modulation depth (OMD). Thus, for a given modulation amplitude, the optical signal has a reduced mean optical power and can access the small signal gain of the EDFA. This unsaturated gain is higher than the saturated one, thereby significantly increasing the RF gain of the link. Simultaneous optimization of OMD is also desirable to reduce detector saturation and fiber-induced nonlinear effects. They derive an analytical expression to describe optimum operating conditions for the modulator bias and validate their results through numerical simulation and experimental work. The proposed optimum modulator operating point is experimentally proven to be applicable to multicarrier signals like those used in 802.11a/g protocols.

In this paper, we design, analyze, and demonstrate experimentally U.S. Federal Communications Commission (FCC)compliant power-efficient ultrawideband (UWB) waveforms generated by optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs the frequency-to-time conversion. The waveform is inscribed in the frequency domain by the fiber Bragg grating (FBG). A significant challenge for this approach is elimination of an unwanted, positive rectangular pulse superimposed on the desired waveform. Our innovative use of balanced photodetection eliminates this pedestal, assuring compliance with the FCC mask at low frequency. Three UWB pulses with duration of 0.3, 0.6, and 1.2 ns are designed and tested experimentally. Whereas an excellent match between the optimized and measured pulses is achieved for the simpler, shorter duration waveforms, the noise in the fabrication process of FBGs limits the generation of the more complex, longer duration waveforms.

This paper proposes an all-fiber fast optical frequency-hop code division multiple access (FFH-CDMA) for high-bandwidth communications. The system does not require an optical frequency synthesizer allowing high communication bit rates. Encoding and decoding are passively achieved by Bragg gratings, Multiple Bragg gratings replace a frequency synthesizer, achieving a hopping rate in tens of GHz. A main lobe sine apodization can be used in writing the gratings to enhance the system capacity and the spectrum efficiency. All network users can use the same tunable encoder/decoder design. The simultaneous utilization of the time and frequency domains offers notable flexibility in code selection. Simulations show that the encoder efficiently performs the FFH spread spectrum signal generation and that the receiver easily extracts the desired signal from a received signal for several multiple access interference scenarios. We measure the system performance in terms of bit error rate, as well as auto-to cross-correlation contrast. A transmission rate of 500 Mb/s per user is supported in a system with up to 30 simultaneous users at 10-9 bit error rate. We compare FFH-CDMA to several direct sequence-CDMA systems in terms of bit error rate versus the number of simultaneous users. We show that an optical FFH-CDMA system requires new design criteria for code families, as optical device technology differs significantly from that of radio frequency communications

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. In this work, 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.

We propose and demonstrate experimentally a prototype for ultra-wideband (UWB) waveform generator based on optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs frequency-to-time conversion. A U.S. Federal Communications Commission (FCC)-compliant power efficient pulse shape is inscribed in the frequency domain by a fiber Bragg grating (FBG) with an excellent match between optimized and measured pulses. Two other popular UWB pulse shapes (Gaussian monocycle and doublet pulses) are achieved by proper tuning of two FBG-based variable optical filters. A balanced photodetector removes an unwanted rectangular pulse superimposed on the desired waveform, assuring compliance at low frequency. Index Terms-Balanced detection, fiber Bragg grating (FBG), spectral pulse shaping, ultra-wideband (UWB).

We propose fast optical frequency-hop code division multiple access (FH-CDMA) for high bandwidth local area networks. Encoding and decoding are achieved by tunable fiber Bragg gratings. Frequency hopping offers many advantages compared to the previously proposed optical CDMA multiple access techniques. The simultaneous utilization of the time and frequency domains offers notable flexibility in the selection of codes, however code families previously developed for radio frequency (RF) communications are not directly applicable to an optical FH-CDMA system. We propose codes to meet the special constraints imposed by our encoding device and present theoretical and simulation results for their performance

We propose, to the best of our knowledge, a novel in-service live fiber-to-thehome (FTTH) passive optical networks (PONs) management solution. Our solution uses a modified direct-sequence (DS) optical code-division multiplexing (OCDM) system and overcomes the optical time-domain reflectometry (OTDR) point-to-multipoint shortcomings. Our solution addresses various service provisioning and network maintenance challenges in PONs, alleviates their complexity, and reduces their operational cost. In addition, our system exploits passive devices (or encoders) to demark service provider ownership and responsibility from that of customers. Our OCDM-based management solution easily scales up from FTTH time-division multiplexing (TDM)-PON to WDM-PON and TDM/WDM-PON to support as many as a thousand customers, all using only one monitoring wavelength. We address the coding system and develop capacity curves for different PON scenarios.

We propose a new multirate optical communication system using optical fast frequency hopping CDMA (OFFH-CDMA) for multimedia applications in which different quality of services (QoS) are required. In this system, each user needs only to transmit the minimum required power to achieve a desired signal to interference ratio (SIR). We assign different power levels to each rate through an average interference-based power control algorithm using variable optical attenuators. Such an approach minimizes interference and at the same time provides variable QoS constraints for different traffic types. The simulation shows a great improvement in the system capacity

This letter 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 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.

A serious problem facing wavelength-division multiplexed (WDM) networks with fiber amplifier cascades is transient cross-gain saturation or gain dynamics of fiber amplifiers. Attention has been focused primarily on circuit-switched scenarios. When the number of WDM channels transmitted through a circuit-switching network varies, channel addition/removal will tend to perturb signals at the surviving channels that share all or part of the route. Even more serious bit error rate deterioration can arise in WDM packet switched burst mode networks. In this paper, we present experimental and theoretical results demonstrating the effect of fast power transients in erbium-doped fiber amplifiers (EDFAs) on packetized traffic transmitted through a chain of five EDFAs. Traffic of a local-area network has been transmitted over three channels. The effect of EDFA cross-gain saturation due to the burstiness of the traffic has been observed in a continuous-wave monitoring channel. The stabilizing effect of gain clamping of the first EDFA in the cascade has been investigated. The experimental results are extended to eight-channel WDM system using a large signal numerical analysis.

This paper studies via simulation the stabilizing effect of all-optical gain-clamping (AOGC) in a chain of erbium-doped fiber amplifiers (EDFA) fed by wavelength-division multiplexing (WDM) burst-mode packet traffic. AOGC is necessary to suppress swings of output power and optical signal-to-noise ratio (OSNR). A case study is selected, in which only the first EDFA in a cascade of six amplifiers is clamped using a ring laser configuration. A numerical model which solves the transcendental equation for the average inversion at each EDFA is used for the analysis. The traffic is generated on the eight WDM channels by ON-OFF time-slotted sources, with statistically independent ON and OFF durations, randomly generated by a truncated Pareto distribution with infinite variance. The simulation model includes the generation of amplified spontaneous emission within each amplifier and the propagation of the lasing power generated in the AOGC EDFA through the cascade. It is shown that the sizable power and OSNR swings arising in an unclamped cascade of EDFA's can be effectively suppressed when a lasing signal a few decibels above the aggregate signal power develops in the AOGC EDFA and propagates along the cascade.

[1] succeeded in reducing the set of coupled first-order nonlinear partial differential equations determining the wavelength-dependent, time-varying amplifier gain into a single ordinary differential equation (ODE). In this paper, we further simplify the ODE bringing into greater evidence the physical meaning of the amplification process, and greatly enhancing the utility of the ODE as an analysis and design tool. We find that the gain dynamics of a doped-fiber amplifier are completely specified by its total number of excited ions r, whose time behavior is described by a simple first-order differential equation. We exploit this new understanding of amplifier gain dynamics: 1) to develop an equivalent circuit model for amplifier gain dynamics, 2) to identify that channel addition causes much faster transients than channel dropping in wavelength division multiplexing networks, and 3) to demonstrate that gain excursions can be significant in multichannel packet switching applications, which unlike timemultiplexed signals are characterized by bursts and lulls in communications. We are also able to revisit the most significant previously published results on both steady-state and dynamic analysis of doped-fiber amplifiers with a much more concise and more intuitive derivation.

We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interferernce for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as 30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers.

We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate that the adaptive MMSE is able to reject an IEEE 802.11a OFDM interferer, even for SIR as severe as -30 dB. The adaptive MMSE exhibits only a 6 dB penalty in the 16-user case relative to the single user case. The practical RAKE receivers were incapable of effectively rejecting either strong narrowband interference or heavily loaded wideband interference. Even more moderate levels of interference caused significant performance degradation of RAKE receivers.

Previous UWB channel characterizations have been reported for various frequency bands predominantly for office environments. We present residential characterizations from 2 to 8 GHz, covering the spectrum under consideration by the FCC for UWB overlay systems. A high degree of multipath fragmentation was found in the NLOS paths (about five times that of LOS).