Technical Information on WorldSpace (9914b)

The system

The WorldSpace system is one of the first commercial digital radio projects to be realized on a large scale. The audio data are coded in the MPEG 2.5/Layer 3 standard which was specially developed for WorldSpace, and are transmitted in compressed form in the L-band (1467...1492 MHz). The up-link is implemented by frequency-division multiplexing (FDM), and the down-link by time-division multiplexing (TDM). This ensures that several up-links can be operated in parallel without mutual interference and that optimum use can be made of the satellite transponders. Quaternary PSK is used as the modulation process and features better efficiency for satellite transmission circuits than the COFDM process frequently employed for terrestrial digital radios.

WorldSpace broadcasts via three satellites, each of which covers three target regions; a fourth satellite is provided for backup. Per region, a transmission capacity of 192 prime-rate channels with a data rate of 16 kbit/s is available. Depending on quality requirements, from one to eight prime-rate channels can be combined into one radio channel so that pure news programs, for example, can be run on reduced channel capacity.

The receiver technology

Implementing this system meant having to redesign not only the transmitter and transmission technology, but also the receiver technology. The 'StarMan' chip concept developed by MICRONAS is a very compact, complete solution for a digital satellite radio. It consists of two highly integrated circuits for the channel or source decoding, as well as an L-band tuner constructed on the basis of proven AF components.

The 'StarMan' ICs include the entire range of functions required for the demodulation and decoding of the satellite radio signals, as well as a complete analog back-end.

The tuner performs the carrier selection and the conversion of the 1.5 Hz satellite signal to an IF of 1.8 MHz.

The analog IF signal is demodulated in the DRD 3515A channel decoder, and a combined error correction using the Viterbi and Reed Solomon methods is carried out. Service components contained in the signal are split off and can be transmitted (via a special interface) to an external unit (e.g. a PC).

The MAS 3506D source decoder decodes the audio data stream compressed on the transmitter side with the MPEG 2.5/Audio Layer 3 method. The decoded signals are then returned to the DRD 3515A for D-A reconversion and amplification.
The DRD 3515A features a complete audio output stage including a D-A converter and an output amplifier for stereo headphones and small loudspeakers. Via a digital interface defined by WorldSpace, data can be transferred to a PC or other external units.

MICRONAS' developers attached special importance to the performance data for the quaternary PSK demodulation, almost reaching the theoretically possible limits. Even with an extremely low signal-to-noise ratio, this guarantees reliable and practically interference-free reception.

The DRD 3515A also contains a descrambler unit, which can be used for blocking undesirable advertising (e.g. for alcohol in Islamic countries), as well as for pay services. The corresponding descrambling algorithm was developed, to a large extent, by MICRONAS. The chip set is controlled via an I2C bus; the digital audio data are exchanged via an I2S bus.

A DC voltage converter integrated in the MAS 3506D ensures that the chipset is provided with a stable 3-V supply. The units are optimized for low energy consumption; for example, the power consumption of the receivers is automatically reduced when the signal bit rate falls. Both battery and solar-powered operation of the receivers is possible, therefore.

Partners of the WorldSpace project

• Fraunhofer Institute for Integrated Circuitry, Applied Electronics Center (IIS-A): development of the transmission system
• MICRONAS and SGS Thomson Microelectronics: development and manufacture of the ‚StarMan' ICs for the receivers
• Hitachi, JVC, Matsushita (Panasonic), and Sanyo: radio receivers
• Solaria Corp.: solar technology for the receivers
• Alcatel, Arianespace, Matra Marconi Space, and TIW: satellite technology