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Advance satellite communications technology has been achieved through Wideband InterNetworking engineering test Satellite (WINDS) and Engineering Test Satellite 9 (ETS-IX) projects in these two decades in Japan. WINDS achieved Gbps-class transmission capability, wide bandwidth active phased array antenna in Ka-band, and high speed onboard switching technologies. ETS-IX is currently being developed to demonstrate flexible resource management capability with wide bandwidth digital channelizer and digital beam former in Ka-band. In this chapter, very high capacity optical communications technology being developed for feeder link applications of a very high throughput satellite is reviewed. Satellite communications are thought to be beneficial for 5G coverage expansion to the ocean surface, air and space, and some trials to integrate to 5G. ETS-IX will be used as an experimental platform for such technical trials based on European Space Agency-National Institute of Information and Communications Technology (ESA-NICT) collaborations. Based on these achievements and experiences, and taking into account the recent trend of satellite communications technology for VHTS, LEO constellations, HAPS, and so on, we have studied next-generation satellite communications technology.

We propose a method for managing the spontaneous organization of sensor activity in ad hoc wireless sensor systems. The wireless sensors exchange messages to coordinate responses to requests for sensing data, and to control the fraction of sensors which are active. This method can be used to manage a variety of sensor activities. In particular, it can be used for reducing the power consumption by battery operated devices when only low resolution sensing is required, thus increasing their operation lifetimes.

The high data rate satellite networks are expected to play an important roll in the global communications networking in the 21st century. CRL has started the three-year-long research both on system concept and key technologies of the experimental high data rate (“Gigabit”) communications satellite. New technological challenges are required for providing the connectivity for the future growing multi-media applications in the optical fiber network era; a wide area coverage with the Ka band narrow spot beams for the global high data-rate access, high-speed and flexible satellite switches, and an interoperability with the terrestrial optical fiber networks.

The Communications Research Laboratory (CRL) has been carrying out research and development of a very low rate message communication system with an ultra-small earth station called the Attache Case Size Message Communication Terminal (AMCT). In order to minimize the size and power consumption of the AMCT, a transmission rate of 100 b/s is adopted. This is extremely low for satellite communications. As a result, the AMCT is similar in size to an attache case. In the base station, the fast Fourier transform (FFT) technique is used in order to search for a very narrow band signal from the AMCT and compensate for the frequency deviation of the signal. The experiments have been carried out through ETS-V satellite links to estimate the bit error rate (BER) and acquisition performance of this system. Also, the probability of completed message transmission is calculated using these experimental results. Based on these results, it is confirmed that a very low rate transmission system such as this system is applicable to satellite communications. >

Ad hoc wireless networks are expected to have many useful applications, such as emergency communications after accidents and natural disasters, distributed sensing in buildings and natural environments, versatile wireless monitoring in health care and support system, and to make our daily lives easier, more productive and more enjoyable. In order to fully realize the potential of wireless ad hoc networks, we need to be able to realize large-scale networks, which connect large numbers of ad hoc terminals over wide areas. Previous research on ad hoc networks has been based mainly on computer simulations and small-scale implementations, and there have been few successful implementations of large-scale ad hoc networks. We have made significant breakthroughs toward realizing real-world operation of large-scale ad hoc networks, by the testbed based approach. In this paper, we discuss the breakthroughs toward realizing large-scale ad hoc networks and inter-vehicle communications for vehicle safety, which is one of the promising applications of ad hoc networks.

The experimental results of a very-low-speed message communication system using a hand-held terminal are presented. The terminal is installed in an attache case and communicates with a base station through a satellite link at a transmission rate of 100 bps. The terminal transmits and receives messages of up to 40 characters using a packet transmitting mode. A description of the complete system and experimental methods is provided. These results confirm the applicability of a very-low-rate transmission system such as this system to satellite communications. >

The Communications Research Laboratory (CRL) is conducting experiments on advanced satellite communications using the Engineering Test Satellite VI (ETS-VI), which will be launched in mid 1993. CRL has developed three payloads for the projects. The first is the S-band intersatellite communications system which features a 19-element multibeam phased array antenna. The second is the millimeter-wave band intersatellite/personal communications system. The third is the bidirectional space laser communications system. These systems and their associated experimentation are described. >

An 18 GHz-band MMIC linearizer using a parallel diode with a bias feed resistance and a parallel capacitor has been proposed. This linearizer has weak positive gain deviation and its gain deviation can be controlled without changing phase deviation. By applying this linearizer to an 18 GHz-band power amplifier, an improvement of IMD3 of 20 dB has been achieved.

Despite many routing protocols proposed for mobile ad hoc networks few have considered link quality and route stability. Weak links contained in the routes often lead to route breaks. Multipath fading and mobility make link quality time-variant. When the link metric is associated with link quality the routes may change frequently. In times of transmission failure due to collision or fading, the routing packets are susceptible to loss owing to the lack of retransmission mechanism for the broadcast packets. For these reasons, the routes become unstable. In this paper, we investigate the long-term link quality and route stability in the on-demand routing protocols. We compare the link metric policies, optimize the route discovery and use the soft detection scheme to avoid false link breaks. The simulation results show that both packet error ratio (PER) and throughput in the enhanced on-demand routing protocol can be greatly improved under the Rayleigh fading environment