Achievement Award
Invention of Multiport Amplifier and Contributions to Multiple Beam Mobile Satellite Communications Development
Shunichiro Egami , Makoto Kawai

Shunichiro Egami
Makoto Kawai		        
　The salient feature of satellite communications is broad service coverage. Broader coverage usually results in lower satellite antenna gain. In contrast, a multiple beam system achieves both higher satellite antenna gain and broader service coverage, which makes it possible to use smaller terminals and to accommodate a larger number of users through frequency reuse. In the 1970s through 1990s, significant advances were made in communications satellite and launching technology, enabling the launching of a multiple beam communications satellite with large multiple beam antennas. However, in implementing the multiple beam system, it was necessary to cope with varying, uneven traffic distribution among beams and to assure reliable operation of all beams. The multiport amplifier, which was contrived by the award recipients, was an important invention that solved those inherent problems with the multiple beam system.
　The concepts of a Multiport Directional Coupler (MDC) and a Multiport Amplifier (MPA), which were invented by the award recipients, are shown in Figure 1. The figure shows configurations of 2, 4, 8 port MDC and MPA. The concept of MDC is an extension of the 2 port 3 dB directional coupler to 2n port (n: natural number). MPA is a 2n port amplifier with 2n amplifiers connected by the symmetrical 2n port MDC at the input and output sides. The number of ports is 2n. In practice, a lower number of ports can be selected by using dummies to terminate unnecessary ports.
　The input power to each MPA port is divided to each amplifier by the input side MDC, and the output of each amplifier is combined to the corresponding output port by the output side MDC. The non-corresponding ports are isolated with no output power. The maximum output power of each MPA port is the sum of the maximum output power of each amplifier and it can be freely assigned to the output ports with an arbitrary ratio. Moreover, while failure of one or two amplifiers will cause a slight decrease in total output power, it does not cause fatal loss of any beams.


Fig.1 Multiport Directional Coupler (MDC） and Multiport Amplifier (MPA)

　The award recipients proposed applying MPA to the domestic mobile satellite communications system which was planned in the 1980s. Figure 2 shows the configuration of the mobile satellite forward link payload of the Engineering Test Satellite, ETS-VI, which implemented MPA for the first time. MPA with 5 ports composed of 8 amplifiers was used for the 5-beam coverage of ETS-VI.
　The input to each port of MPA is distributed equally to 8 amplifiers with the same performance levels, and the power or number of channels of each output port can be selected freely within the limitation of the MPA maximum output power. And, although a few amplifier failures will decrease the maximum MPA output power, they do not bring about fatal specific beam failure.


Fig. 2　Configuration of ETS-VI mobile satellite forward link payload

　MPA was space-qualified on the Engineering Test Satellite, ETS-VI, which was launched in 1994. It was implemented in the NTT communications satellites N-STAR a and N-STAR b, which were launched in 1995 and 1996, respectively, enabling introduction of Japan’s first mobile satellite communications service. Next-generation N-STAR implemented 3 units of 4-port MPA and is now providing the DoCoMo Widestar service.
　In Japan, MPA was also used in MTSAT (2005, 2006) and WINDS (2008). Overseas, immediately after ETS-VI, it was adopted for the multiple beam mobile satellites AMSC/MSAT (1995〜1996) and Inmarsat-3 (1996〜1998). Recently, it has also been used in a multiple beam array feed which irradiates a large reflector designed to handle varying, uneven distribution of the array elements that transmit power.
　As described above, the award recipients have made noteworthy contributions to the advancement of the satellite communications technology. Their outstanding achievements are highly deserving of the IEICE Achievement Award.
 
References
(１)　S. Egami and M. Kawai, “Multiport Power Combining Transmitter for Multibeam Satellite Communications,” IEICE Trans. on Comm., Vol.69, No.2, pp.206-212, 1986.
(２)　S. Egami and M. Kawai, "An Adaptive Multiple Beam System Concept," IEEE Journal on Selected Areas in Communications, Vol. SAC-5, No.4, pp.630-636, 1987.
 

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