An inter-modulation distortion (IMD) compensation method for thermal memory effect using a multistage RC-ladder circuit has been proposed. The IMD caused by the thermal memory effect on an InGaP/GaAs HBT amplifier was compensated for by inserting a multistage RC-ladder circuit in the base bias circuit of the amplifier. Since heat flux owing to self-heating in the transistor can be approximated with a multistage thermal RC-ladder circuit, the canceling of IMD by an additional electrical memory effect generated from the RC-ladder circuit is predicted. The memory effects cause asymmetrical characteristics between upper and lower IMD. The IMD caused by the memory effects is expressed as a vector sum of each origin. By adjusting an electrical reactance characteristic for sub-harmonics affected by the thermal memory effect in the amplifier circuit, the asymmetric characteristic is symmetrized. The parameters of the RC-ladder circuit were estimated so that the adjusted electrical reactance characteristic is reproduced in simulation. A fabricated InGaP/GaAs HBT amplifier with the thermal memory effect compensation circuit exhibited a symmetrized and suppressed IMD characteristics.

Distortion characteristics caused by the thermal memory effect in power amplifiers were accurately predicted using a multi-stage thermal RC-ladder network derived by simplifying the heat diffusion equation. Assuming a steep gradient of heat diffusion near an intrinsic transistor region in a semiconductor substrate, the steady state temperature, as well as the transient thermal response at the transistor region, was estimated. The thermal resistances and thermal capacitances were adjusted to fit a temperature distribution characteristic and a step response characteristic of temperature in the substrate. These thermal characteristics were calculated by thermal FDTD simulation. For an InGaP/GaAs HBT, a step response characteristic for a square-wave voltage signal input was simulated using a large-signal model of the HBT connecting the multi-stage thermal RC-ladder network. The result was verified experimentally. Additionally, for an RF-amplifier using the HBT, the 3rd-order intermodulation distortion caused by the thermal memory effect was simulated and this result was also verified experimentally. From these verifications, a multi-stage thermal RC-ladder network can be used to accurately design super linear microwave power amplifiers and linearizers.

VLBI is an important application of ATM technology because it can transmit huge amounts of data. A single VLBI experiment typically generates data (which must be recorded and transported until they are cross-correlated) of tera-bit order at each separated observing site. Conventional VLBI not only requires manpower but also limits the maximum observation data rate. Therefore, a realtime VLBI using a private ATM network was developed recently, but it could not be utilized for regular VLBI experiment. Since utilization of public ATM is most realistic solution for realtime VLBI between ordinary observing sites, we have developed an interface equipment that connects VLBI observation and processing equipment to a public ATM network and demonstrated a successful experiment. This equipment supports VLBI's standard bit rates as 128 Mbps and 256 Mbps, though data rate for user's payload in 155.52 Mbps (STM-1/OC-3) ATM network is actually only 119.5 Mbps. It can easily step to higher networks as 622 Mbps.

The Communications Research Laboratory (CRL), the National Astronomical Observatory (NAO), the Institute of Space and Astronoutical Science (ISAS), and the Telecommunication Network Laboratory Group of Nippon Telegraph and Telephone Corporation (NTT) have developed a very-long-baseline-connected-interferometry array, maximum baseline-length was 208 km, using a high-speed asynchronous transfer mode (ATM) network with an AAL1 that corresponds to the constant bit-rate protocol. The very long baseline interferometry (VLBI) observed data is transmitted through a 2.488-Gbps [STM-16/OC-48] ATM network instead of being recorded onto magnetic tape. By combining antennas via a high-speed ATM network, a highly-sensitive virtual (radio) telescope system was realized. The system was composed of two real-time VLBI networks: the Key-Stone-Project (KSP) network of CRL (which is used for measuring crustal deformation in the Tokyo metropolitan area), and the OLIVE (optically linked VLBI experiment) network of NAO and ISAS which is used for astronomy (space-VLBI). These networks operated in cooperation with NTT. In order to realize a virtual telescope, the acquired VLBI data were corrected via the ATM networks and were synthesized using the VLBI technique. The cross-correlation processing and data observation were done simultaneously in this system and radio flares on the weak radio source (HR1099) were detected.

Admission control is a procedure to guarantee a given level of Quality of Service (QoS) by accepting or rejecting arrival connection requests. There are many studies on backlog or loss rate evaluation formulas for admission control at a single node. However, there are few studies on end-to-end evaluation formulas suitable for admission control. In a previous paper, the authors proposed a new stochastic network calculus for many flows using an approach taken from large deviations techniques and obtained asymptotic end-to-end evaluation formulas for output burstiness and backlog. In this paper, we apply this stochastic network calculus to a heterogeneous tandem network with many forwarding flows and cross traffic flows constrained by leaky buckets, and obtain a simple evaluation formula for the end-to-end backlog. In this formula, the end-to-end backlog can be evaluated by the traffic load at the bottle neck node. This result leads us to a natural extension of the evaluation formula for a single node.