We propose a new bit timing recovery (BTR) scheme, called perturbed sampling BTR (PSBTR), that can operate near the symbol rate in high-bit-rate wireless systems. A peculiar sample clock, the duty factor of which is not 50%, is used in the PSBTR scheme. We call this type of clock a perturbed sample clock and use it for clock recovery. In PSBTR, there is no cycle slip of the sample clock, and the PSBTR circuit is mostly digital. We examine the performance of the PSBTR scheme under additive white Gaussian noise (AWGN) by computer simulation and experiment, and from these results, clarify the relationship between the performance and circuit parameters of the PSBTR circuit. The overall results indicate that the PSBTR scheme performs well and can be employed as a BTR scheme for high-bit-rate wireless systems.

We have developed an InP-based monolithic optical frequency discriminator consisting of a temperature-insensitive optical filter and dual photodiodes. This integrated device detects the optical frequency deviation of the input light as differential photocurrent from the dual photodiodes, and the photocurrent is fedback to the light source for frequency stabilization through a differential amplifier. The FSR and extinction ratio of the filter are 50 GHz and 20 dB. The total opto-electronic conversion efficiency is 40%. In a frequency stabilization experiment using the developed discriminator, the frequency fluctuation of a DFB laser was reduced to less than 10 MHz.

After a short introduction to the different requirements to and techniques for wavelength conversion, focus is on cross-gain and cross-phase modulation in SOA based converters. Aspects like jitter accumulation, regeneration and conversion to the same wavelength is discussed. It is predicted that jitter accumulation can be minimised while also assuring a high extinction ratio by using a 9-10 dB ratio between the signal and CW power. Using this guideline simulations show that 20 cross-gain modulation converters can be cascaded at 10 Gbit/s with only 20 ps of accumulated jitter and an extinction ratio of 10 dB. The regenerative capabilities of the cross-phase converters are described and verified experimentally at 20 Gbit/s. By controlling the input power to an EDFA, the noise redistribution and improvement of the signal-to-noise ratio is demonstrated. In a similar experiment at 2.5 Gbit/s, the regeneration causes a reduction of the required input power to an in-line EDFA of 6 dB for a power penalty of 1 dB at a bit error rate of 10-9. If two converters are concatenated the power requirement is reduced 8 dB. Obviously, the power reduction allows for longer spans between in-line EDFAs. A simple scheme for regeneration without wavelength conversion is assessed at 2.5 Gbit/s resulting in 4.5 dB lower required EDFA input power. The scheme is characterised by a quasi-digital transfer function that is ideal for regeneration. A combination of cross-gain and cross-phase conversion is used to perform conversion to the same wavelength at 20 Gbit/s. The insertion penalty for this dual-stage converter is below 2 dB and is mainly caused by extinction ratio degradation from the cross-gain converter. Finally, a new device for all-optical wavelength conversion has been proposed and 2.5 Gbit/s operation has been simulated with good results.

Coaxial-core erbium-doped fiber amplifiers (EDFA's) having a property of self-regulated gain spectrum are developed. The operation of a coaxial-core EDFA is based on the partial separation of the light paths for different wavelength channels in the directionally-coupled waveguides of a coaxial-core geometry. The degree of channel equalization depends on the geometrical and optical parameters of the coaxial-core EDFA and on relative channel power levels. A numerical analysis based on the coupled-mode theory and on the rate equation shows that, under fully optimized conditions, a coaxial-core EDFA provides equalization rates in excess of -0.4 dB per dB of input-power imbalance in the case with two WDM channels. A cascade experiment demonstrates the effect of coaxial-core EDFA's toward channel-power equalization in fiber links with a small number of WDM channels.

We analyze the dispersion-managed optical transmission system for the non-return-to-zero (NRZ) pulse format. First, we investigate the physical image of dispersion management by computing small-signal-based transfer functions, and summarize the dependence of transmission performance on system parameters. Next, the Q-map is computed numerically to design long-distance large-capacity dispersion-managed transmission systems for a single channel in a more detailed manner. It is shown that the third-order dispersion of fibers negatively influences transmission performance, and third-order dispersion compensation is proved to be an effective method for extending the transmission distance of high bit-rate systems. Utilizing these results, guidelines can be derived for the optimal design of long-distance large-capacity NRZ transmission systems.

After a short introduction to the different requirements to and techniques for wavelength conversion, focus is on cross-gain and cross-phase modulation in SOA based converters. Aspects like jitter accumulation, regeneration and conversion to the same wavelength is discussed. It is predicted that jitter accumulation can be minimised while also assuring a high extinction ratio by using a 9-10 dB ratio between the signal and CW power. Using this guideline simulations show that 20 cross-gain modulation converters can be cascaded at 10 Gbit/s with only 20 ps of accumulated jitter and an extinction ratio of 10 dB. The regenerative capabilities of the cross-phase converters are described and verified experimentally at 20 Gbit/s. By controlling the input power to an EDFA, the noise redistribution and improvement of the signal-to-noise ratio is demonstrated. In a similar experiment at 2.5 Gbit/s, the regeneration causes a reduction of the required input power to an in-line EDFA of 6 dB for a power penalty of 1 dB at a bit error rate of 10-9. If two converters are concatenated the power requirement is reduced 8 dB. Obviously, the power reduction allows for longer spans between in-line EDFAs. A simple scheme for regeneration without wavelength conversion is assessed at 2.5 Gbit/s resulting in 4.5 dB lower required EDFA input power. The scheme is characterised by a quasi-digital transfer function that is ideal for regeneration. A combination of cross-gain and cross-phase conversion is used to perform conversion to the same wavelength at 20 Gbit/s. The insertion penalty for this dual-stage converter is below 2 dB and is mainly caused by extinction ratio degradation from the cross-gain converter. Finally, a new device for all-optical wavelength conversion has been proposed and 2.5 Gbit/s operation has been simulated with good results.

Coaxial-core erbium-doped fiber amplifiers (EDFA's) having a property of self-regulated gain spectrum are developed. The operation of a coaxial-core EDFA is based on the partial separation of the light paths for different wavelength channels in the directionally-coupled waveguides of a coaxial-core geometry. The degree of channel equalization depends on the geometrical and optical parameters of the coaxial-core EDFA and on relative channel power levels. A numerical analysis based on the coupled-mode theory and on the rate equation shows that, under fully optimized conditions, a coaxial-core EDFA provides equalization rates in excess of -0.4 dB per dB of input-power imbalance in the case with two WDM channels. A cascade experiment demonstrates the effect of coaxial-core EDFA's toward channel-power equalization in fiber links with a small number of WDM channels.

We propose a query processing method for amalgamated knowledge bases. Our query processing method is an extension of the magic sets technique for query processing in amalgamated knowledge bases, augmented with the capabilities of handling amalgamated atoms. Through rewriting rules in a given amalgamated knowledge base, our method offers the advantages associated with top-down as well as bottom-up evaluation. We discuss how to handle amalgamated atoms, consider how to check whether an amalgamated atom is satisfiable in a fact set and how to extend a fact set by inserting an amalgamated atom. We also give the transformation procedures for amalgamated knowledge databases and show the correctness of our method.

We have developed an InP-based monolithic optical frequency discriminator consisting of a temperature-insensitive optical filter and dual photodiodes. This integrated device detects the optical frequency deviation of the input light as differential photocurrent from the dual photodiodes, and the photocurrent is fedback to the light source for frequency stabilization through a differential amplifier. The FSR and extinction ratio of the filter are 50 GHz and 20 dB. The total opto-electronic conversion efficiency is 40%. In a frequency stabilization experiment using the developed discriminator, the frequency fluctuation of a DFB laser was reduced to less than 10 MHz.

WDM transmission experiments over cascaded sections of optical links including wavelength converting 2R-transponders have been carried out in a loop testbed. Using dispersion compensated links and simple direct modulated transponder lasers, up to 11 cascaded crossconnects and 1750 km trunk lines have been bridged with 2.5 Gbit/s NRZ signals. The limitations are given mainly due to the accumulated jitter as it is shown by numerical simulations. The results indicate, that 2R-transponders are a useful approach to a flexible WDM network design using bitrate-transparent wavelength conversion.

We analyze the dispersion-managed optical transmission system for the non-return-to-zero (NRZ) pulse format. First, we investigate the physical image of dispersion management by computing small-signal-based transfer functions, and summarize the dependence of transmission performance on system parameters. Next, the Q-map is computed numerically to design long-distance large-capacity dispersion-managed transmission systems for a single channel in a more detailed manner. It is shown that the third-order dispersion of fibers negatively influences transmission performance, and third-order dispersion compensation is proved to be an effective method for extending the transmission distance of high bit-rate systems. Utilizing these results, guidelines can be derived for the optimal design of long-distance large-capacity NRZ transmission systems.

We have developed a new type of phase interpolation direct digital synthesizer (DDS) with a symmetrically structured delay generator. The new DDS is similar to a sine output DDS in that it produces lower spurious signals, but it does not require a sine look-up table. The symmetrically structured delay generator reduces the periodic jitter in the most significant bit (MSB) of the DDS accumulator. The symmetrical structure enables the delay generator to produce highly accurate delay timing and eliminates the need to adjust the circuit constants. Experimental results confirm frequency synthesizer operation in which the spurious signal level is reduced to less than that of the accumulator.

The concept of controlled resource sharing and dynamic quality of service (QoS) on the next generation Internet has attracted much attention recently. It is suggested that, by imposing real-time revision of shared resource allocated to individual media streams or data flows according to user/application QoS demand and resource availability, more balanced and efficient multimedia services can be provided. In this paper, we present an Adaptive Control Framework (ACF), which is developed for controlled resource sharing and dynamic QoS in real-time multimedia service. We discuss main elements of ACF including 1) Control schemes applicable in the framework, and 2) Control mechanisms used in ACF. It is clearly shown in this paper that, with control schemes and mechanisms incorporated in ACF and supportive algorithms and protocols for ACF applications on the Internet, more flexible service and better overall performance in terms of packet loss, latency, signal-noise ratio and re-synchronization delay, can be offered.

This study is aimed to explore a fast convergence method of blind equalization using higher order statistics (cumulants). The efforts are focused on deriving new theoretical solutions for blind equalizers rather than investigating practical algorithms. Under the common assumptions for this framework, it is found that the condition for blind equalization is directly associated with an eigenproblem, i. e. the lag coefficients of the equalizer can be obtained from the eigenvectors of a higher order statistics matrix. A method of blind phase recovery is also proposed for QAM systems. Computer simulations show that very fast convergence can be achieved based on the approach.

Novel memory and several logic circuits utilizing the chaotic oscillations produced in SQUIDs are proposed. First, the oscillation modes that can be produced in a SQUID circuit are analyzed. The results of simulation for the SQUID show that there exist four types of oscillations: periodic, subharmonic, chaotic and relaxation oscillations. The bifurcation diagram of oscillation waveforms reveals that the hysteresis phenomena in the relation between the terminal voltage or the current and the external flux appear and that these phenomena can be used for a memory operation. Secondary, novel digital circuits such as memory, Exclusive-OR and full adder circuits are proposed by utilizing the chaotic oscillations. In these digital circuits the chaotic oscillations are made correspond to the logic "1," while the periodic and subharmonic oscillations are made to the logic "0." In order to investigate how these digital circuits perform their functions, computer simulations are made. The simulation results show that the right operations can be achieved.

A novel dual-band internal antenna similar in size to the single-band internal antenna for cellular handsets is proposed. Our approach to realize a small and low-profile dual-band internal antenna is to use the dominant mode (TM10 mode) and the higher-order mode (TM30 mode). In order to use this approach for recent dual-band cellular systems it is necessary to lower the resonant frequency of the higher-order mode (TM30 mode). This motivated our development of a new antenna configuration with a slot on the radiation element of a quarter-wavelength shorted microstrip antenna to lower the resonant frequency of the TM30 mode. In this paper, the experimental and the analytical results for this antenna are presented. In the results, by adjusting the location and the length of the slot, the dual-frequency operation can be achieved with the frequency ratio (TM30 mode/TM10 mode) from 2 to 3. In addition, the enhancement of bandwidth is presented.

A loop architecture of DQDB with slot reuse (LDQDB-SR) segmented by erasure nodes was studied to overcome the performance limitation due to the nature of the unidirectional bus architecture of DQDB with slot reuse. The LDQDB-SR adopts the destination slot release and an inter-segment bandwidth regulation based on the distributed queuing system of DQDB. This network suffers not only from severe throughput deterioration due to a high regulation cost and an excessive transit delay but also from unfairness in bandwidth sharing, especially under an overload condition. In this paper, we introduce an enhanced loop architecture of DQDB with slot reuse (ELDQDB-SR) to improve the performance of LDQDB-SR. The ELDQDB-SR uses a quota-based inter-segment bandwidth regulation mechanism to effectively control the bandwidth use of each segment. Each station selects the bus that minimizes the number of erasure nodes on the path to destination stations. Fairness control methods of DQDB are reviewed and the alpha-tuning mechanism is modified to achieve a fair bandwidth distribution among stations within each segment. Simulation results show that the ELDQDB-SR gives an enhanced throughput level and also maintains good fairness under overload conditions.

In wireless ATM networks, the condition of the transmission channel fluctuates significantly as a result of many causes such as multipath, shadow phasing, and so forth. Several schemes have been proposed to take care of transmission errors. Some schemes are suitable for favorable channel conditions, while others are more suitable for unfavorable channel conditions. Thus, using a fixed error control scheme regardless of channel conditions is destined to be inefficient. This paper presents a dynamic error control scheme that dynamically selects an appropriate error control scheme according to the conditions of the transmission channel. Using this method, we can decrease both errors and the overhead of redundancy.

In this paper, we consider the Integrate-and-Fire Model (ab. IFM) with two periodic inputs. The IFM outputs a pulse-train which is governed by a one dimensional return map. Using the return map, the relationship between the inputs and the output is clarified: the first input determines the global shape of the return map and the IFM outputs various periodic and chaotic pulse-trains; the second input quantizes the state of the return map and the IFM outputs various periodic pulse-trains. Using a computer aided analysis method, the quantized return map can be analyzed rigorously. Also, some typical phenomena are confirmed in the laboratory.

49-GHz operation for a state-of-the-art static frequency divider using FETs is achieved with high-performance 0.1-µm-gate InAlAs/InGaAs/InP HEMTs and high-speed double-layer interconnections with a thick low-permittivity BCB inter-layer dielectric film. An experiment shows that the propagation delay for the upper-layer line in the double-layer interconnections is less than half of that for the conventional single-layer interconnections directly on InP-substrate. The frequency divider with the double-layer interconnections is about 20% faster than the conventional one with the single-layer interconnections. A delay time analysis reveals that this speed increase is due to the decrease in interconnection propagation delay.