The Mobile broadcasting services, delivering high-quality digital broadcasting services to the mobile users, like the personal digital assistants (PDAs) and more than 70 million car owners nationwide is expected to be in service by the year 2004. This brand-new multi-channel broadcasting service is expected to be a large growing market.

Mobile Broadcasting Corporation (MBCO) was established in the year 1998, as a company delivering satellite digital audio broadcasting service to the mobile users for the first time in Japan. The services are delivered using a broadcasting satellite at a significantly shorter wavelength called as S Band (2.6GHz) and has the following characteristics:

• Enable to cover throughout Japan with a single frequency.
• Miniaturization of receivers is possible, as the receiving antennas are small.

Credit: MBSAT

The above image shows the outline diagram of the entire broadcasting system. The broadcast waves delivered from the broadcasting center are transmitted to MBSAT (Mobile Broadcasting Satellite), the exclusive satellite using the Ku band (12~13GHz) where they get converted to S band and on amplifying the power are broadcasted throughout Japan with the help of a large-sized parabolic antenna. The broadcast from the MBSAT is directly received in some places, but in cities where the broadcast cannot be received directly due to the shadow of buildings, an attempt is being made to install 'Gap-Fillers' to improve the reception in those areas. Simultaneously, adoption of effective error correction coding or multi-path features make it possible to provide a digital broadcasting service of a superior audio and visual quality in any environment, be it inside the house or an office, be it outdoor or a moving car.

Broadcasting reception terminal

System of receivers
We assume a various reception terminals to meet the demands.

The terminal used as a mobile broadcasting receiver can be largely classified as a personal portable terminal, and a terminal that can be mounted in a vehicle (a car).

The personal portable terminal can be further classified as a portable TV, PDA, radio, smart phone, portable game, and mobilephone*. (*Note* -- The future mobile telephone, which is being developed, should be a step ahead of 2nd generation LSI. It should have low electricity consumption.)

The terminal that can be mounted in a car can be further classified as an external connection type (hide-away type) for the existing car navigation system, a navigation built-in type that has a tuner in the car navigation system, a built-in radio type, or an add-on stereo that can be fitted to the existing car stereo system.

A cradle letting a portable terminal be able to use in automobile environment is also under consideration.

Moreover, there are household or interior immovable terminals like PCMCIA card that can be inserted in a card slot of a PC. We also assume small size units that can be used in ships and boats.

Antenna
The antenna required for receiving mobile broadcasting, if compared with a parabola antenna for the satellite broadcasting, is much smaller in size, portable and can be connected to personal portable terminal. For vehicles or cars a flat patch antenna or 3 dimensional helical antenna is used, or mobile type small sized helical antenna or rod antenna (within range of gap-filler) is used.Further, a portable device has a built-in patch antenna.

Broadcasting satellite system

The MBSAT (Mobile Broadcasting Satellite) plays a central role for the mobile broadcasting service. The MBSAT receives Ku band uplink signal transmitted from the Broadcasting Center and transmits it by S band and Ku band downlink signals to service areas all over Japan.

An artist's concept of the MBSAT spacecraft in orbit. Credit: MBSAT

The MBSAT is a three-axis attitude stabilized geostationary satellite designed and manufactured by Space Systems / Loral, USA (SS/L). The MBSAT design is based on the SS/L 1300 bus which has a lot of operation heritage in the orbit.

The MBSAT has following features:

The MBSAT provides high EIRP downlink signal so as to enable the mobile terminals to receive satellite downlink signal by a small omni directional antenna all over Japan. In order to achieve high EIRP, high power S band transmitters and a 12m-aperture S band high gain antenna are mounted on the MBSAT.

Ku band high power transmitters and a Ku band antenna are used to transmit the broadcasting signal to ground gap-filler system which enables the mobile terminals to receive broadcasting signals at place where the satellite downlink signal is shaded due to tall buildings.

The 12m-aperture S band high gain antenna has a deployable reflector which consists of mesh, nets and truss structures. While the satellite is in a launch phase, the reflector is stowed and mounted on the satellite body in order to be stored inside the launch vehicle fairing. Once the satellite reaches the orbit, reflector holddown systems are released and the reflector is deployed by electrical motors. The reflector is supplied by TRW Astro, USA with design heritage from several programs, including the on-orbit program.

In order to prevent the large antenna from being excited by the disturbance force of the thruster firing, a low thrust plasma thruster engine is used for NS station keeping. The plasma thruster engine has high specific impulse and it reduces fuel mass for the station keeping. The plasma thruster engine has a lot of on-orbit operation heritage for Russian satellites.