VCAT-LCAS in a clamshell

Virtual concatenation (VCAT) is a standardized layer 1 inverse multiplexing technique that can be applied to the optical transport network (OTN), synchronous optical network (SONET), synchronous digital hierarchy (SDH), and plesiochronous digital hierarchy (PDH) component signals. By inverse multiplexing, it combines multiple links at a particular layer into aggregate links to achieve a commensurate increase in available bandwidth on the aggregate link. While any inverse multiplexing scheme is about "more bandwidth", the VCAT/LCAS is a general technique that can enable a fairly broad range of network features such as (1) right sizing bandwidth for data applications, (2) bandwidth extraction from a mesh network, (3), bandwidth on demand and IP traffic engineering, and (4) provides new forms of protection-restoration and graceful degradation. In this paper, the authors have given a quick overview of VCAT/LCAS technology and just a few examples of its applications.     

Optimized breakdown probabilities in Al/sub 0.6/Ga/sub 0.4/As-GaAs heterojunction avalanche photodiodes

Recently, it has been shown that the noise characteristics of heterojunction Al/sub 0.6/Ga/sub 0.4/As-GaAs avalanche photodiodes (APDs) can be optimized by proper selection of the width of the Al/sub 0.6/Ga/sub 0.4/As layer. Similar trends have also been shown theoretically for the bandwidth characteristics. The resulting noise reduction and potential bandwidth enhancement have been attributed to the fact that the high bandgap Al/sub 0.6/Ga/sub 0.4/As layer serves to energize the injected electrons, thereby minimizing their first dead space in the GaAs layer. We show theoretically that the same optimized structures yield optimal breakdown-probability characteristics when the APD is operated in Geiger mode. The steep breakdown-probability characteristics, as a function of the excess bias, of thick multiplication regions (e.g., in a 1000-nm GaAs homojunction) can be mimicked in much thinner optimized Al/sub 0.6/Ga/sub 0.4/As-GaAs APDs (e.g., in a 40-nm Al/sub 0.6/Ga/sub 0.4/As and 200-nm GaAs structure) with the added advantage of having a reduced breakdown voltage (e.g., from 36.5 V to 13.7 V).     

Bandwidth‐satisfied routing in multi‐rate MANETs by cross‐layer approach

Previous quality‐of‐service (QoS) routing protocols in mobile ad hoc networks (MANETs) determined bandwidth‐satisfied routes for QoS applications. Since the multi‐rate enhancements have been implemented in MANETs, QoS routing protocols should be adapted to exploit them fully. However, existing works suffer from one bandwidth‐violation problem, named the hidden route problem (HRP), which may arise when a new flow is permitted and only the bandwidth consumption of the hosts in the neighborhood of the route is computed. Without considering the bandwidth consumption to ongoing flows is the reason the problem is introduced. This work proposes a routing protocol that can avoid HRP for data rate selection and bandwidth‐satisfied route determination with an efficient cross‐layer design based on the integration of PHY and MAC layers into the network layer. To use bandwidth efficiently, we aim to select the combination of data rates and a route with minimal bandwidth consumption to the network, instead of the strategy adopted in the most previous works by selecting the combination with the shortest total transmission time. Using bandwidth efficiently can increase the number of flows supported by a network. Copyright 2010 John Wiley & Sons, Ltd.     

A Double Rhombus Antenna Fed by 180$^{circ}$ Phase Shifter for Ultrawideband Phased Array Applications

A double rhombus antenna fed through a microstrip 180deg phase shifter is presented. The 180deg phase shifter is a novel design of microstrip-to-coplanar waveguide (CPW)-to-microstrip transition. The proposed phase shifter is obtained without changing the layer of the second microstrip line for an easy embedding into the feedline. Via holes are used to transfer the current from the top to the bottom substrate layer and vice versa. The antenna with the phase shifter is operating in a wide bandwidth from 5.7 and 18.6 GHz (106%), and provides stable endfire radiation patterns. This design is tested in a modified two-element array configuration and proved to have a wide usable bandwidth.     

Monte Carlo simulations of the bandwidth of InAlAs avalanche photodiodes

We present a Monte Carlo simulation of the bandwidth of an InAlAs avalanche photodiode with an undepleted absorber. The carrier velocities are simulated in the charge layer and the multiplication region. It is shown that the velocity overshoot effect is not as significant as simpler models have suggested. At high electric field intensity, the electron effective saturation velocity is only slightly higher when impact ionization is significant, compared with when impact ionization is absent. The simulated 3 dB bandwidth is consistent with experiments for gains up to 50.     

Design of ultra-fast dual-wavelength resonant-cavity-enhancedSchottky photodetectors

A systematic optimization procedure for the design of RCE Schottky photodetectors to achieve maximum quantum efficiency and high speed operation at 1.3 and 1.55 μm wavelengths is presented. The quantum efficiency formulation used includes the structural parameters of the photodetector and takes into account the wavelength dependence of the top and bottom mirrors reflectivities. The results have shown that the value of the thickness of the antireflection coating layer has a major influence in selecting the width of the photodetector to simultaneously achieve maximum quantum efficiency and high bandwidth at the two wavelengths. Simulated values of 270 and 40 GHz were obtained, respectively, for the 3-dB carrier-transit time-limited bandwidth and bandwidth-efficiency product for an RCE Schottky photodetector with a 0.02-μm gold layer     

Buffer and bandwidth allocation for TCP data traffic: experimental results

This paper discusses the dimensioning of buffers and the bandwidth allocation for data traffic in the ATM network. Data traffic is notoriously complex and bursty, making such dimensioning a difficult task. However, the COMBINE project, when dimensioning their InterWorking Units (IWUs), adopted a Poissonian packet arrival model, based upon the argument that burstiness at timescales higher than that of a packet arrival are a problem to be tackled by flow control at higher layers. This paper presents experimental results from the COMBINE testbed that show that this hypothesis was justified and that good TCP goodput was obtained based upon this dimensioning approach, due to TCP's ability to adapt to network congestion. However, it is also shown that it was the TCP algorithm that was ultimately responsible for controlling the packet loss ratio in the network and not the bandwidth allocation or buffer size. The results highlight the importance of taking into account the mutual influence between the ATM layer and the transport layer congestion control algorithms.     

Wideband reflectarrays using artificial impedance surfaces

The gain bandwidth of a microstrip reflectarray can be substantially improved by replacing the usual layer of near-resonant patches with a simple artificial impedance surface consisting of closely spaced electrically small patches on a grounded dielectric substrate. It is shown that this approach can lead to reflectarray gain bandwidths (1 dB) in excess of 20% with a single layer of printed elements     

Optimization of microwave properties for ultrahigh-speed etched and unetched lithium niobate electrooptic modulators

Simultaneous phase velocity and characteristic impedance matching of the ultrahigh-speed electrooptic modulators is presented by using the finite-element method (FEM). It is also shown that the dielectric loss in the silica buffer layer is larger than that in the lithium niobate substrate and when these dielectric losses are included, the resulting bandwidth is reduced significantly. It is also shown that for an etched LN structure, it is relatively easier to match both N/sub m/ and Z/sub c/ simultaneously and the resulting optical bandwidth is also greater.     

导带不连续性对InGaAsP/InGaAs UTC PD带宽的影响

研究了导带不连续性对InGaAsP/InGaAs单行载流子高速光探测器(UTC-PD)的3dB带宽的影响。研究结果表明,由于导带不连续性的存在,使得3dB带宽降低。导带不连续性越大,3dB带宽越低。通过增加隧穿系数、收集层厚度、收集层掺杂浓度和减小吸收层掺杂浓度可以在一定程度上消除导带不连续性带来的不利影响。研究结果还表明,增加隧穿系数、收集层厚度和减小吸收层掺杂浓度这三种方法在消除导带不连续性不利影响的同时又有各自的缺点,而适当增加收集层掺杂浓度是最为有效的一种方式。该研究结果可以为UTC-PD的设计提供理论指导,特别是采用UTC结构的波导型光电二极管。     

IEEE 802.11 medium access control enhancements based on simultaneous multiple‐input multiple‐output bandwidth sharing

The demand for higher data rate has spurred the adoption of multiple‐input multiple‐output (MIMO) transmission techniques in IEEE 802.11 products. MIMO techniques provide an additional spatial dimension that can significantly increase the channel capacity. A number of multiuser MIMO system have been proposed, where the multiple antenna at the physical layer are employed for multiuser access, allowing multiple users to share the same bandwidth. As these MIMO physical layer technologies further evolve, the usable bandwidth per application increases; hence, the average service time per application decreases. However, in the IEEE 802.11 distributed coordination function‐based systems, a considerable amount of bandwidth is wasted during the medium access and coordination process. Therefore, as the usable bandwidth is enhanced using MIMO technology, the bandwidth wastage of medium access and coordination becomes a significant performance bottleneck. Hence, there is a fundamental need for bandwidth sharing schemes at the medium access control (MAC) layer where multiple connections can concurrently use the increased bandwidth provided by the physical layer MIMO technologies. In this paper, we propose the MIMO‐aware rate splitting (MRS) MAC protocol and examine its behavior under different scenarios. MRS is a distributed MAC protocol where nodes locally cooperate with one another to share bandwidth via splitting the spatial channels of MIMO systems. Simulation results of MRS protocol are obtained and compared with those of IEEE 802.11n protocol. We show that our proposed MRS scheme can significantly outperform the IEEE 802.11n in medium access delay and throughput. Copyright © 2012 John Wiley & Sons, Ltd.     

Delta-doped avalanche photodiodes for high bit-rate lightwavereceivers

The authors estimate the GB (grain bandwidth) product limits and the noise performance of a new SAGM-APD (separate avalanche, grating, and multiplication avalanche photodiode) structure: the δ-doped SAGM-APD. It is shown that GB products in excess of 140 GHz for a 0.2-μm-thick multiplication layer and possibly larger GB products for smaller widths can be obtained. While recent calculations have predicted increased GB products for this δ-doped SAGM-APD structure, the authors explicitly prove using conventional theory that this is possible only with a concomitant increase in the multiplication noise. It is further demonstrated that it is essential to optimize the width of the multiplication layer for a given bit-rate to achieve minimum multiplication noise consistent with a GB product high enough to accommodate the requisite frequency response at the optimum gain. It is shown that the δ-doped SAGM-APD structure is a very good candidate for high bit-rate receiver applications     

Interwoven convoluted element frequency selective surfaces with wide bandwidths

The interweaving of adjacent convoluted elements of crossed dipoles is shown to give reductions in operating frequencies of almost 15 times for single layer surfaces. The frequency stability with oblique incidence is observed to be significantly better than for simple crossed dipoles. At normal incidence the fractional reflection bandwidth increases to more than 60% and the common bandwidth for oblique incidence up to 45/spl deg/ is 46%.     

Distributed probabilistic medium access with multipacket reception and Markovian traffic

Enabling multipacket reception (MPR) at the physical layer is a promising way to achieve higher bandwidth efficiency while reducing the complexity of the medium access control layer in distributed wireless networks. We study distributed probabilistic access where transmitting nodes access the shared wireless medium with a probability based on the node’s information about the aggregate traffic carried by the network. We model bursty traffic by rate-controlled two-state Markov sources and introduce a parameter that describes the “burstiness” level of the offered traffic. A throughput-optimal medium access strategy utilizing limited feedback is then described and its performance is examined for traffic with different levels of burstiness. It is shown that the bursty nature of the traffic in data networks allows for improvement of the bandwidth efficiency. Bounds on the system throughput are proposed and the queuing delay is analyzed.     

A Bandwidth Guaranteed Integrated Routing Algorithm in IP over WDM Optical Networks

In this paper, we have developed an integrated online algorithm for dynamic routing of bandwidth guaranteed label switched paths (LSPs) in IP-over-WDM optical networks. Traditionally, routing at an upper layer (e.g., IP layer) is independent of wavelength routing at the optical layer. Wavelength routing at the optical layer sets up a quasi-static logical topology which is then used at the IP layer for IP routing. The coarse-grain wavelength channels and the pre-determined virtual topologies with respect to some a priori assumed traffic distribution are barriers to efficient resource use and inflexible to changing traffic. We take into account the combined knowledge of resource and topology information at both IP and optical layers. With this added knowledge, an integrated routing approach may extract better network efficiencies, be more robust to changing traffic patterns at the IP layer than schemes that either use dynamic routing information at the IP layer or use a static wavelength topology only. LSP set-up requests are represented in terms of a pair of ingress and egress routers as well as its bandwidth requirement, and arrive one-by-one. There is no a priori knowledge regarding the arrivals and characteristics of future LSP set-up requests. Our proposed algorithm considers not only the importance of critical links, but also their relative importance to routing potential future LSP set-up requests by characterizing their normalized bandwidth contribution to routing future LSP requests with bandwidth requirements. Moreover, link residual bandwidth information that captures the link's capability of routing future LSPs is also incorporated into route calculation. Extensive simulation was conducted to study the performance of our proposed algorithm and to compare it with some existing ones, such as the integrated minimum hop routing algorithm and the maximum open capacity routing algorithm. Simulation results show that our proposed algorithm performs better than both routing algorithms in terms of the number of LSP set-up requests rejected and the total available bandwidth between router pairs.     

Radio resource management across multiple protocol layers in satellite networks: a tutorial overview

Satellite transmissions have an important role in telephone communications, television broadcasting, computer communications, maritime navigation, and military command and control. Moreover, in many situations they may be the only possible communication set‐up. Trends in telecommunications indicate that four major growth market/service areas are messaging and navigation services (wireless and satellite), mobility services (wireless and satellite), video delivery services (cable and satellite), and interactive multimedia services (fibre/cable, satellite). When using geostationary satellites (GEO), the long propagation delay may have great impact, given the end‐to‐end delay user's requirements of relevant applications; moreover, atmospheric conditions may seriously affect data transmission. Since satellite bandwidth is a relatively scarce resource compared to the terrestrial one (e.g. in optical transport networks), and the environment is harsher, resource management of the radio segment plays an important role in the system's efficiency and economy. The radio resource management (RMM) entity is responsible for the utilization of the air interface resources, and covers power control, handover, admission control, congestion control, bandwidth allocation, and packet scheduling. RRM functions are crucial for the best possible utilization of the capacity. RRM functions can be implemented in different ways, thus having an impact on the overall system efficiency. This tutorial aims to provide an overview of satellite transmission aspects at various OSI layers, with emphasis on the MAC layer; some cross‐layer solutions for bandwidth allocation are also indicated. Far from being an exhaustive survey (mainly due to the extensive nature of the subject), it offers the readers an extensive bibliography, which could be used for further research on specific aspects. Copyright © 2005 John Wiley & Sons, Ltd.     

Thin wideband electromagnetic wave absorber for oblique incidenceapplications

A thin tapered absorber is described that is capable of effecting wide band absorption of electromagnetic waves at oblique incidence. The absorption mechanism for the taper is shown to be inherently wide band and seems not to be constrained by the bandwidth/reflectivity limitations that apply to thin planar materials. Predicted reflectivity data for the tapered absorbing layer are shown     

Design, modeling, and characterization of monolithically integratedInP-based (1.55 μm) high-speed (24 Gb/s) p-i-n/HBT front-endphotoreceivers

High-speed, long-wavelength InAlAs/InGaAs OEIC photoreceivers based on a p-i-n/HBT shared layer integration scheme have been designed, fabricated and characterized. The p-i-n photodiodes, formed with the 6000 Å-thick InGaAs precollector layer of the HBT as the absorbing layer, exhibited a responsivity of ~0.4 A/W and a -3 dB optical bandwidth larger than 20 GHz at λ=1.55 μm. The fabricated three-stage transimpedance amplifier with a feedback resistor of 550 Ω demonstrated a transimpedance gain of 46 dBΩ and a -3 dB bandwidth of 20 GHz. The monolithically integrated photoreceiver with a 83 μm p-i-n photodiode consumed a small dc power of 35 mW and demonstrated a measured -3 dB optical bandwidth of 19.5 GHz, which is the highest reported to date for an InAlAs/InGaAs integrated front-end photoreceiver. The OEIC photoreceiver also has a measured input optical dynamic range of 20 dB. The performance of individual devices and integrated circuits was also investigated through detailed CAD-based analysis and characterization. Transient simulations, based on a HSPICE circuit model and previous measurements of eye diagrams for a NRZ 2³¹-1 pseudorandom binary sequence (PRBS), show that the OEIC photoreceiver is capable of operation up to 24 Gb/s     

High-responsivity and low-operation-voltage edge-illuminatedrefracting-facet photodiodes with large alignment tolerance forsingle-mode fiber

We demonstrate a novel edge-illuminated refracting-facet photodiode (RFPD), in which the incident light parallel to the up-side surface is refracted at an angled facet and absorbed in a thin absorption layer. Although the absorption layer is thin, the absorption length is effectively increased by making the light transit at a certain angle to the absorption layer, resulting in an increase in internal quantum efficiency. The fabricated RFPDs with an absorption layer thickness of 1.5 μm have a responsivity as high as 0.95 A/W even at a bias voltage of 0.5 V for a flat-ended single-mode fiber. The 1-dB-down misalignment tolerances for vertical and horizontal directions are as large as 9.5 and 33 μm, respectively. A 3-dB bandwidth of more than 6 GHz is obtained     

Ultrafast all-optical switching with an asymmetric Fabry-Perotdevice using low-temperature-grown GaAs: material and device issues

For future telecommunications systems to take full advantage of the optical fiber bandwidth, it will be necessary to have components responding at picosecond speeds. The only way currently known to achieve these speeds is using all-optical switching. By using low-temperature-grown GaAs (LT-GaAs) in a compact asymmetric Fabry-Perot device, we have achieved ultrafast all-optical switching with large bandwidth, high contrast ratio, low insertion loss, and low switching energy. In this paper, we discuss the dependence of the switch performance on the mirror bandwidth and reflectivity, and the LT-GaAs layer thickness and growth conditions. We develop guidelines for the optimization of the device design to maximize the bandwidth and contrast ratio