Since 1987 both the Japanese government -- the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications (MPT), the Electronic Navigation Research Institute (ENRI), and the National Space Development Agency (NASDA) -- and the private sector -- KDD and Nippon Telephone and Telegraph Corp. (NTT) -- have been conducting mobile satellite communications experiments with Japan's experimental satellite ETS-V, located at 150 degrees E. In 1994, the more powerful EST-VI spacecraft will be placed at 154 degrees E to continue the existing EST-V tests and/or to accommodate additional mobile satellite communications experimentation.
Each participant is carrying out its own experiments. CRL and NTT are testing LMSS using vehicle-mounted and portable terminals. CRL, ENRI, KDD and NTT are taking their maritime mobile satellite services (MMSS) measurements with their own shipboard terminals. Here, ENRI is focusing its effort in the RDSSs for navigation applications and NTT is more interested in its domestic offshore MMSSs. Aeronautical mobile satellite services (AMSSs) are explored by CRL, ENRI and KDD. Note that the international MMSSs are provided by KDD which serves as the Japanese signatory in Inmarsat.
More advanced mobile satcom experiments have been planned for the Communications and Broadcasting Engineering Test Satellite (COMETS) beginning in 1997. These experiments will include not only the standard aeronautical, maritime and land terminals but also portable VSAT and handheld personal units. Using COMETS, it is possible to test a wide range of novel/enhanced services from low-bit-rate voice and message to high-bit-rate still pictures and compressed video since the spacecraft is equipped with multiple spot beams, high-power travelling wave tube amplifiers (TWTAs) and SSPAs, and an on-board regenerative repeater.
In mobile terminal development, NHK has designed three 12 GHz direct broadcast satellite (DBS) receivers suitable for installation inside trains, ships, buses or personal automobiles. KDD has also described a "handy phone" design for personal communications services (PCS) applications. The Advanced Telecommunication Research Institute International (ATR) is actively involved in the development of small, lightweight and low-power terminal units for mobile usage.
So far, there are no LEO satellite system license applications in Japan. However, CRL is well positioned to tackle a host of LEO problems through its experimental work in small satellites, a cluster of satellites and intersatellite link (ISL) development. Details of these activities are given in the subsequent sections.
NHK has developed mobile receivers for direct satellite broadcasting at 12 GHz to trains, ships, buses and cars. The design uses adaptive array antenna with very stable electronic tracking and has the ability to cope with signal blocking by a combination of techniques involving space diversity, mechanical control, and high-speed mechanical searching for satellite signal recapture. The characteristics of these three types of receivers are summarized in Table 3.5.
Characteristics of Three Types of Receivers
KDD is in the process of developing a "Personal Handy Phone" (PHP) for global PCS. This PHP is compatible with ISDN standards and provides a common air interface for aeronautical applications. It offers RDSS, the ability to perform authentication for any fault connections, and automatic interference elusion by burst position change and/or carrier frequency change. The main features of the PHP are given below:
(superscript *) TDD = Time division diplexed
(superscript **) QPSK = Quadrature phase shift keyed
(superscript ***) ADPM = Adaptive differential pulse code modulation
ATR believes that technology can significantly reduce the size and power consumption of future mobile terminals. Examples include three-dimensional MMICs; line unified field effect transistors (LUFETs); multilayered MMICs (made by stacking metal and dielectric layers on GaAs substrate); device technologies such as a metal coplanar waveguide modulator (CPW), optical and MMIC receivers; antenna technologies such as active arrays, self-diplexing, slot-coupled microstrip, and conformal arrays; and digital BFNs. Key parameters of these units are given in Table 3.6.
Key Parameters of Future Mobile Terminals
The Japanese MPT has not established any LEO guideline, since there is as yet no proposed LEO system in Japan. This situation may change soon. A senior official of the Space Systems Division within CRL expressed keen interest in cooperating with the U.S. on LEO activities. At the moment CRL's main interest in small LEO is on a store-and-forward operation ISL.
The use of a cluster of satellites co-located at the same orbital slot is intended to improve on-orbit satellite system reliability. CRL is currently developing station- keeping techniques using a laboratory simulator to measure the relative distance and motion for up to four satellites co-located at the same orbital slot. These satellites could be placed as far apart as 20 km.