3G基站的两个关键部件--高速数据转换器和放大器

第三代移动电话主要依靠宽带码分多址(WCDMA)技术,信号处理技术的发展、体系结构的进步以及新开发的高速数据转换器和运算放大器正在使无线基础结构的设计师们降低成本,更好地实现第三代无线通信系统的目标。     

Effect of ohmic contacts on buffer leakage of GaN transistors

The effect of ohmic contacts on the buffer leakage of GaN transistors is presented. The buffer leakage for AlGaN/GaN high-electron mobility transistors and GaN MESFETs grown on the same underlying buffer was observed to be different. Controlled experiments show that the increased buffer leakage is due to the nature of the alloyed ohmic contacts and can be minimized if they are screened by the Si doping or by the two-dimensional electron gas.     

Novel algorithm for time division multiple access in broadband ISDN passive optical networks

For shared access to a broadband communication network using the asynchronous transfer mode (ATM) a passive optical network (PON) has previously been proposed. A novel algorithm to evaluate the time division multiple access (TDMA) protocol which controls the traffic from the various subscribers towards the shared access equipment (upstream direction) is proposed. It uses counters to estimate the moment of a data packet arrival at the subscriber side. By this ‘look-ahead’ technique no extra upstream channels are needed to initiate the polling by the subscriber. The algorithm guarantees fair access for all subscribers and minimizes the necessary overhead. Both the maximum access delay and overhead may be readily calculated, and the obtained results were supported by numerical simulation. Applicability in the standard broadband network environment has been proved by hardware design simulation.     

A two-wavelength, passively self-injection locked, CW Ti3+:Al2O3 laser

We have demonstrated that passive self-injection control is very effective in a CW Ti³⁺:Al₂O₃ laser to produce high efficiency (η~15%), narrow-line, two-wavelength emission. Light reinjection is achieved with an original high-resolution two-wavelength selector combining an interference wedge and a grazing incidence grating. The device operates outside the main nonselective laser cavity; thus avoiding insertion losses and optical damage problems. A 1.5 W, background free, total output power has been obtained simultaneously at two narrow laser lines (fwhm: 8 pm), independently tunable over 50 mn. The two laser lines can be emitted in a single beam or in two separated beams. Self-injection wavelength control has been demonstrated both for linear and for ring laser cavities. In the latter case, unidirectional behavior is also obtained in the same way. Single-step (5s²S_1/2-5P²P_1/2,3/2 ) and two-step (5s²S_1/2-5p²P_3/2-5d² D_3/2,5/2) excitation of atomic rubidium vapor has been performed with our original laser device     

Sliding mode closed-loop control of FES: controlling the shank movement

Functional electrical stimulation (FES) enables restoration of movement in individuals with spinal cord injury. FES-based devices use electric current pulses to stimulate and excite the intact peripheral nerves. They produce muscle contractions, generate joint torques, and thus, joint movements. Since the underlying neuromuscular-skeletal system is highly nonlinear and time-varying, feedback control is necessary for accurate control of the generated movement. However, classical feedback/closed-loop control algorithms have so far failed to provide satisfactory performance and were not able to guarantee stability of the closed-loop system. Because of this, only open-loop controlled FES devices are in clinical use in spite of their limitations. The purpose of the reported research was to design a novel closed-loop FES controller that achieves good tracking performance and guarantees closed-loop stability. Such a controller was designed based on a mathematical neuromuscular-skeletal model and is founded on a sliding mode control theory. The controller was used to control shank movement and was tested in computer simulations as well as in actual experiments on healthy and spinal cord injured subjects. It demonstrated good robustness, stability, and tracking performance properties.     

Convergence of Broadcast and New Telecom Networks

The demand of the new telecom industry for cost efficient provision of mobile multimedia services is faced with the reality of scarce radio resources. The requirement of spectrum efficiency has driven the development of various digital radio technologies that have been optimized for specific services, namely for broadcast or for mobile communication. However, existing and emerging multimedia services exhibit challenging requirements in terms of asymmetry, interactivity, real-time, and multicast communication. This paper describes an IP based multi-radio infrastructure that enables the co-operation of existing radio networks to combine their capabilities to ensure a spectrum efficient provision of high-quality mobile multimedia services. Further the need for a dynamic allocation of spectrum to radio services is motivated. The basic functionality and the architecture of a multi-radio system is outlined, with a special emphasis on the cooperation between different radio systems. Further an evolution path for the convergence of broadcast and new telecom is desribed, starting from today's systems and leading to a fully coordinated system.     

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm² were observed for 10×1200 μm lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95°C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T₀ of 114 K     

Timing jitter and pump-induced amplitude modulation in thecolliding-pulse mode-locked (CPM) laser

The timing jitter and spurious amplitude modulation of colliding-pulse mode-locked (CPM) lasers were measured. The absolute jitter (the jitter of the laser alone) varied between 5 and 10 ps RMS in a 50-500-Hz bandwidth. The smallest measured relative jitter (timing fluctuations between the CPM and a radio-frequency (RF) synthesizer synchronized to the CPM) was 1.8 ps RMS in a 2-Hz to 1-kHz bandwidth. Separate from the jitter, spurious modulation in the CW pump laser mixes with the CPM pulse train to produce a set of discrete amplitude-modulated sidebands in the power spectrum of the CPM output. The frequencies of these sidebands change with cavity length, and the sidebands can be eliminated by operating the pump laser in a single longitudinal mode