A password authentication method PERM has been developed for application to e-mail forwarding. This method is suitable for communications in insecure network environments such as the Internet. In particular, it can be adapted to Internet appliances and Java applets which have limited performance. The PERM method does not require password resettings and enables high-speed authentication processing with a small-sized program. Moreover, it does not use facilities or mechanisms for generating random numbers and writing them into and reading them out of an IC card or similar storage medium on the user's side.

This paper studies a set of lip synchronization mechanisms for heterogeneous network environments. The set consists of four schemes, types 0 through 3, which are classified into the single-stream approach and the multi-stream approach. Types 0 and 1 belong to the single-stream approach, which interleaves voice and video to form a single transport stream for transmission. On the other hand, types 2 and 3, both of which are the multi-stream approach, set up separate transport streams for the individual media streams. Types 0 and 2 do not exert synchronization control at the destination, while types 1 and 3 do. We first discuss the features of each type in terms of networks intended for use, required synchronization quality of each medium, physical locations of media sources and implementation complexity. Then, a synchronization algorithm, which is referred to as the virtual-time rendering (VTR) algorithm, is specified for stored media; MPEG video and voice are considered in this paper. We implemented the four types on an ATM LAN and on an interconnected ATM-wireless LAN under the TCP protocol. The mean square error of synchronization, total pause time, throughput and total output time were measured in each of the two networks. We compare the measured performance among the four types to find out which one is the most suitable for a given condition of the underlying communication network and traffic.

This paper presents an integrated network configuration of wired and wireless access systems for nomadic computing and discusses the virtual LAN on a wireless access system. Furthermore, different types of ad hoc networks are summarized to delineate nomadic computing styles. In terms of user mobility, the integrated network provides a seamless connection environment, so a user can move between wireless and wired networks without dropping data communication sessions. This function is critical for nomadic computing users. By defining the integrated network and employing a virtual LAN, a nomadic computing environment can be realized. This paper reviews the key issues to realize integrated networks. They are mobile management including mobile IP, virtual IP and Logical Office, a high performance MAC, and security control.

This paper develops a prediction model for evaluating the influence of propagation attenuation due to aircraft's flying across the earth-satellite link. This prediction model is based on the Aperture-field method of Huygens-Fresnel wave theory. Considering arriving and taking off course around airport, attenuation impairment is calculated for different types of aircrafts and flight directions. In order to verify this model's accuracy, numerical results are compared with measurement values. The calculations agree well with the measurements. Ground antenna directivity and anticipated impairment to digital broadcasting system such as Perfect TV are also discussed.

This paper studies the routing algorithms for multi-destination connections where each destination may require different amount of data streams. This asymmetric feature can arise mostly in a large and/or heterogeneous network environment. There are mainly two reasons for this. One is that terminal equipments may have different capabilities. The other is that users may have various interests in the same set of information. We first define the asymmetric multicast problem and describe an original routing method for this type of multicast. The method is then employed in the presented routing algorithms, which can be run in multi-cluster environment. The multi-cluster architecture is considered to be effective for running routing in the networks, where a variety of operating methods might be applied in different clusters but global network performance is required. Our algorithms are designed based on some classical Steiner tree heuristics. The basic goal of our algorithms is to make routing decisions for the asymmetric multicast connections with minimum-cost purpose. In addition, we also consider delay constraint requirements in the multicast connections and propose correspondent algorithms. We compare the performance between SPT (Shortest Path Tree)-based algorithms and the presented algorithms by simulations. We show that performance difference exists among the different types of the algorithms.

A fiber Bragg grating, which has periodical perturbation of the refractive index in the fiber core, acts as a wavelength selective reflection filter and steep optical spectrum can be realized by forming more than ten thousand of gratings along the fiber core. Owing to capability of making steep optical spectrum, fiber Bragg gratings has been expected to be introduced practical use as multiplexing or demultiplexing filters in dense WDM transmission systems. On the other hand, radiation mode loss, reflection side mode and temperature dependence of Bragg wavelength, should be improved to put the fiber Bragg grating to practical use in dense WDM transmission systems. In this paper, an optimum design and performance of the fiber Bragg grating for dense WDM systems are described. The photosensitive cladding fiber realized less than 0. 2 dB insertion loss at transmitted signal channels and less than 0. 1 dB splicing loss with standard single-mode fibers. An adequate apodization technique in the refractive index distribution suppressed reflection side modes. A temperature compensating package, which gives longitudinal strain with negative temperature dependence to a fiber Bragg grating, minimized temperature dependence of Bragg wavelength less than 0. 001 nm/. Thermal decay of Bragg grating was also investigated and adequate annealing condition was estimated to obtain sufficient stability for practical use in dense WDM transmission.

The microwave attenuation, which is the key factor for realizing very large bandwidths Ti:LiNbO3 optical modulators is fully studied and the causes and reduction techniques are discussed in detail. Practical realization of wide-band optical modulators with low microwave attenuation and low driving voltage is also discussed.

In the future broadband networks, multicast services such as video conferencing and distance learning will become increasingly important. To support these multimedia services, one solution is to form an AMT(ATM Multicast Tree)to connect all the conferencing members. In this paper, based on AMT survivability requirements, we investigate the self-healing of an AMT. Self-healing on AMT is a new challenge of survivability of multimedia services. The pre-assign way is a method we usually considered on protection. If we construct a disjoint backup tree, the low building probability and complicated loading on constructing is the first problem. Second, if only one link or node failed on an AMT, we need to reroute links and reserve bandwidth on whole backup tree. Moreover, since the AMT usually transmits video images, the restoration rate will be decreased because even only one branch of backup tree does not endure the required bandwidth. These enhance us to restore the AMT by dynamic restoration scheme. Two proposed dynamic restoration schemes are developed to provide prioritized restoration from a link or node failure. In the first scheme, we apply a link-based restoration scheme on the AMT. The restoration is based on the failed links of network and does not take whole AMT into account. In the second scheme, without changing the multicast services to the members, we allow reconfiguration of the AMT during the restoration phase. Reconfiguration of the AMT is based on a tree-based restoration concept. By computer simulations, we verify the characteristics of the proposed schemes and the results show that the second scheme outperforms the first.

High- and low-reflection Bragg gratings with a flat-top spectral response free from ripples are proposed. Add/drop filters are created based on gratings photoinduced on planar waveguides by using the new design schemes. The measured spectral responses for the high and low reflection gratings are in good agreement with the calculated ones, and show the flat-top spectral responses.

Future satellite communication systems will be developed at Ka-band (20/30 GHz) owing to the relatively wide frequency allocation and current freedom from terrestrial interference for multimedia services. A serious disadvantage of the Ka-band, however, is the very high atmospheric attenuation in rainy weather. Quasi-synchronous CDMA drastically reduces the effect of self-noise with several interesting features of CDMA for mobile communications such as flexible frequency reuse, the capability of performing soft-handover and a lower sensitivity to interference. This paper evaluates the performance of a quasi-synchronous CDMA return link for a Ka-band geostationary satellite communication system. For a fixed satellite channel whose characteristics depend on weather conditions, the signal envelope and phase for this channel is modeled as Gaussian. The bit error and outage probability, and the detection loss due to imperfect chip timing synchronization is analytically evaluated and the system capacity degradation due to the weather condition is estimated. Two cases of general and worst conditions are evaluated, in which i) rain attenuation ii) nonlinearity of transponder are considered. The two cases consist of the general case in which all users are affected by rainy weather, and the worst case in which only the user of interest, not multiple access interferers, is affected by rain attenuation. The results for the two cases of rainy weather clearly show that quasi-synchronous CDMA eases the power control requirements and has less sensitivity to imperfect power control. When dealing with the impact of the satellite transponder nonlinearity in addition to the rain attenuation, the shift of optimum amplifier operating point is shown so that [Eb/N0]sat, defined as the sum of the Eb/N0 value required to obtain a BER equal to Pb at a given output backoff (OBO) and the value of the OBO itself, tends to decrease, and higher BER impairment is given, since the rain attenuation results in the same effect as the additive input backoff (IBO) at the satellite transponder input. As the BER increases, the optimum [Eb/N0]sat and IBO decrease that result in the shift of optimum operating point.

This paper describes methods used in the design of a high speed burst modem applied for mobile communication systems. The modem has burst mode operations including burst mode AGC (automatic gain control), burst mode BTR (bit timing recovery), adaptive equalization, and diversity based on a selection algorithm to achieve a higher performance in multipath fading channels. Moreover, the performance of the burst modem, which is developed using analog signal processing devices, DSPs (digital signal processors), and FPGAs (field programmable gate arrays), is analyzed experimentally. Results show that the modem can suppress irreducible BER values below 1. 0e-6 and attains a 2 dB implicit diversity gain over multipath fading channels modeled by a two-ray impulse response system with independent Rayleigh fading.

Unique characteristics such as quenching of photoluminescence and improvement of photovoltaic effect were observed in acceptor polymer, (cyano-substituted poly (p-phenylene vinylene)), CN-PPV/donor polymer (poly(3-hexylthiophene), P3HT composites. By taking account of the difference in electronic energy states of both CN-PPV and P3HT, these characteristics are interpreted in terms of photoinduced charge transfer between CN-PPV and P3HT and formation of fractal network.

IDDQ testing, or current testing, is a powerful method which detects a large class of defects which cause abnormal quiescent current, by measuring the power supply current. One of the problems on IDDQ testing which prevent its full practical use in manufacturing is that the testing speed is slow owing to time-consuming IDDQ measurement. One of the solutions to this problem is test pattern compaction. This paper presents an efficient method for generating a compact test set for IDDQ testing of bridging faults in combinational CMOS circuits. Our method is based on the iterative improvement method. Each of random primary input patterns is iteratively improved through changing its values pin by pin selected orderly, so as to increase the number of newly detected faults in the current yet undetected fault set. While our method is simple and easy to implement, it is efficient. Experimental results for large ISCAS benchmark circuits demonstrate its efficiency in comparison with results of previous methods.

In this paper, we propose several novel methods to decrease propagation error for multiple access interference cancellation techniques in asynchronous DS/CDMA. To increase spectral efficiency, the system wherein transmitting signal power of each user is assigned with exponential law and multiple access interference successive cancellation is used in the receiver has been discussed. However, when the number of active users is increased, propagation error occurs in the receiver. Thus, the improvement effect of spectral efficiency in the system has been degraded. In this paper, we propose novel methods to decrease these propagation errors for the system. These novel methods are quasi-maximum likelihood method that means maximum likelihood in considering the signal of the next user when the signal of the arbitrary user is demodulated, feedback method that means the demodulation error of the stronger users in transmitting signal power is estimated after several users, demodulations and the error is corrected, and combination method that is a combination of quasi-maximum likelihood method and feedback method. And we evaluate their performances by computer simulation and show that the combination method is effective for the reduction of the propagation error.

From an information geometric viewpoint, we investigate a characteristic of the submanifold of a mixture or exponential family in the manifold of finite discrete distributions. Using the characteristic, we derive a direct calculation method for an em-geodesic in the submanifold. In this method, the value of the primal parameter on the geodesic can be obtained without iterations for a gradient system which represents the geodesic. We also derive the similar algorithms for both problems of parameter estimation and functional extension of the submanifold for a data in the ambient manifold. These theoretical approaches from geometric analysis will contribute to the development of an efficient algorithm in computational complexity.

A communication system which attains an efficient amalgamation of base station (BS) type cellular communication and non-BS type direct communication for mobile terminals (MTs), "cellularad-hoc united communication system," is proposed. In this system, whether a cellular system or an ad-hoc communication system is chosen by distance and whether the condition is good or not between communicating MTs. In other words, when two terminals are relatively near each other and are in sufficient conditions to communicate directly, they select the ad-hoc communication, while for other cases they use the cellular system. The proposed system was confirmed to have effective channel usage and low battery consumption from the simulation results. It can be also said that this system is suited especially for MTs which tend to communicate with closer partners such as operational mobile robots and autonomous carrier vehicles.

Performance of optical asynchronous code-division multiple-access (CDMA) systems with double optical hard-limiters is analyzed in the presence of avalanche photodiode (APD) noise and thermal noise. Optical orthogonal codes (OOC's) are employed as signature sequence codes. In the analysis, chips are assumed to be synchronous among users: the chip synchronous case. Thus, the performance results in the upper bound on the performance of the asynchronous system. The results show that the optical asynchronous CDMA systems with double optical hard-limiters have good performance in the presence of the APD noise and the thermal noise even when the number of simultaneous users is large.

MTDMA (Multimedia, Multimode TDMA) system has been developed for the 3rd generation mobile communications. An adaptive modulation technique is employed, which select 16 QAM or QPSK modulations fit for the O (Indoor Office)/P (Outdoor to Indoor and Pedestrian) communication environments. The maximum user rate of 4 Mbps is realized. Basic specification is described for O, P and V environments.

This paper proposes the Dual Zipfian Model addressing how to describe HTTP access trends in large-scale data communication networks, and discusses how to design the capacity of address cache tables in an edge router of the networks. We show that destination addresses of packets can be characterized by two types of Zipf's law. Fundamental concept of the Dual Zipfian Model is in complementary use of these laws, and we can derive the relationship between the number of accesses and the number of destination addresses. Experimental results show that the relation gives a good approximation. Applying this relation, we derive cache hit probabilities of the address cache table that incorporates high-speed address resolution. Using the probabilities, design issues including the capacity of the cache tables and aging algorithms of cache entries are also discussed.

This paper compares the BER performance of two types of pilot channel-based coherent Rake combining achievable by the use of weighted multi-slot averaging (WMSA) channel estimation filter in DS-CDMA transmission links. One is for the time-multiplexed pilot channel and the other is for the parallel pilot channel. The WMSA channel estimation filter weights and averages the received pilot over a period of several slots to improve the BER performance. We propose the WMSA channel estimation filters for time-multiplexed pilot and parallel pilot structures. Achievable BER performance under frequency-selective fading environments is computer simulated. The simulation results show that almost same BER performance can be achieved for both pilot channel structures when the same energy is allocated to the pilot.