Welcome to Masayoshi Tanaka  Website  

Research activities of wireless & satellite communications in the following laboratories.
-  College of Industrial Technology, Nihon University, 
     Department of Electrical and Electronic
     Engineering.​
-  NTT Electrical Communications Laboratories.
  

The rapid advance of information technology is the reason for the growing demand for high-speed access to the Internet, anytime, anywhere, at a reasonable cost to the consumer. 
 

An interactive multimedia multibeam satellite communications system  (IMMSC) is one of the promising network access systems because it can be used to construct a broadband access system more easily, more rapidly and in a much wider area, compared with other systems, such as fiber-to-the-home (FTTH), CATV, or ADSL.
 

During the past several years, more than a dozen satellite systems with dedicated bent pipe/processing payloads, LEO, MEO, GEO constellation, and fixed, moving, or hopping beams have been proposed. The DVB Return Channel System via Satellite (DVB-RCS) is one of them and employs a user terminal supporting a two-way multimedia system. 
 

In IMMSC, user uplink traffic is generally lower than that of user downlink; i.e., the traffic is asymmetrical. The bandwidth of the return link (user terminal to hub station) can thus be shared in a group of seven or eight beams and can be dynamically assigned to match the return traffic demand. User terminals can employ a multi-carrier system like Multi-Frequency TDMA (MF-TDMA) as their uplink scheme, which enables bandwidth-on-demand and is a very efficient adaptation to widely varying multimedia transmission requirements. 
 

A multi-carrier system however must possess linearity since its carriers undergo common amplification. Linearity is generally critical in satellite-on-board devices, especially in a high power amplifier (HPA), which is required to operate as near the saturation region as possible to raise efficiency within the range that satisfies system requirements.

 

Hence, degradation in linearity lowers the quality of communications or decreases the maximum number of access channels. In addition, actual antenna isolation among beams is limited, and as a result, beam couplings exist and leaks from adjacent beams do occur. At the same time, traffic is not uniform across all beams, because users are distributed geographically. Traffic itself, moreover, is characterized by variation over time. 
 

Therefore, leaks from adjacent beams change depending on the traffic distribution. However, there have been no reports to fully investigate how beam couplings affect transmission characteristics or proposals for countermeasures to it.

 

Hence, we placed the main focus on the return-link transmission system of IMMSC in which couplings among multi beams become a problem. The total amount of undesired signal leakage from adjacent beams is analyzed when the traffic distribution varies. On the basis of this analysis, a theoretical study on the change in characteristics is performed and the items that must be considered in a system design are investigated.