Technical information on WorldSpace (9806B)

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. This enables a weaker input signal to be used at the receiver than the OFDM process usually employed for terrestrial mobile radio applications.

WorldSpace broadcasts via three 3-beam satellites and a fourth (backup) satellite. Each beam has a transmission capacity of 192 channels with a data rate of 16 kbit/s. These can also be combined; for example, eight channels are required for music broadcasts in CD quality.

The receivers

Implementing this system meant having to redesign not only the transmitter, but also the receiver technology. The'StarMa' chip concept developed by MICRONAS INTERMETALL 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'StarMa' chips

The'StarMa'-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.

Function and Features

• The tuner performs the carrier selection and the conversion of the 1.5 GHz 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.
• 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 INTERMETALL.
• The chip set is controlled via a I2C bus; the digital audio data are exchanged via a I2S bus.
• A DC converter integrated in the MAS 3506D ensures the stable 3 V power supply of the chip set, allowing radio operation of typically 12 hours with two monocells.

The 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. In this respect, the INTERMETALL chips go well beyond WordSpac's stringent specifications.

Due consideration was also given to the requirements of the equipment manufacturers and the future radio listeners. In particular, the ICs were optimized in terms of economic system design and - of decisive importance for portable units - low energy consumption. For example, power consumption of the receiver is reduced automatically with lower signal bit rates.

The first radio models equipped with the'StarMa' chips from MICRONAS INTERMETALL are currently undergoing trials. The full-scale production receivers will be available in time for the start of broadcasting in spring 1999.