SAN MATEO, Calif. When Agere Systems unveiled a satellite digital radio chip set recently, the company's onetime Lucent engineers put on display not only their technical ingenuity but also the complexities implicit in this new broadcasting architecture.
The eight-chip set, designed for Sirius Satellite Radio, dynamically combines signals broadcast from ground antennas and three satellites. The combined signals coded orthogonal frequency-division multiplex (COFDM) for the ground and time-division multiplex quadrature phase-shift keyed (TDM-QPSK) for the satellites get the redundancy needed for a moving vehicle to enjoy robust reception of up to 100 channels of digital-quality radio, Agere said.
Sirius (New York) said that by yearend it will be delivering 50 channels of commercial-free music and up to 50 channels of news, sports, talk, comedy and children's programming. It's one of two similar but competing digital systems that combine satellite radio broadcasting with terrestrial repeaters. The other is from XM Satellite Radio (Washington), which said it will begin service this summer.
Both launch dates represent delays. And while some industry observers are not surprised by such postponements, they are already lowering their market expectations, slashing by half the estimated 2001 satellite radio subscriber base.
"Because so many factors are involved in building such a completely new product, you don't expect them service providers to get everything together as scheduled, and you don't expect them to start services when they are only halfway ready," said Michelle Abraham, senior analyst of multimedia at Cahners In-Stat Group. She now predicts the combined Sirius/XM subscriber base could reach "under 100,000 this year," half her original estimate.
Complex problem, solution
For its part, Agere's eight-chip set will be designed into radio receivers to be manufactured by Alpine, Clarion, Delphi Delco, Jensen, Kenwood, Panasonic and Visteon. Agere (Allentown, Pa.) recently began shipping engineering samples to all seven radio receiver manufacturers. Its production chip sets will start rolling out in early fall.
In contrast, STMicroelectronics finished in February the production design of its two-chip set composed of a channel decoder and a source decoder. The chips are now in the hands of XM's satellite radio manufacturers, including Sony, Pioneer, Alpine and Delphi Delco, for final validation, testing and integration into XM-ready radios, STMicro officials said.
Sirius' choice of a satellite radio system architecture, different from that of XM, forced Agere to develop a chip set much more complex than STMicroelectronics' design for XM Satellite Radio, Agere sources said. A Sirius spokeswoman confirmed that the late completion of Agere's chip set, subsequently delaying radio receiver production, was part of the reason Sirius postponed its service launch.
Agere officials said the chip company was not given access to satellites until Jan. 3, when Sirius finally got all of its satellites in operation, to start testing broadcast signals. "Although we had been simulating our chips, just so many variables are involved in the real-world satellite broadcasting signals," said Rob Franzo, director of the automotive products group at Agere.
XM refused to discuss any technical aspects of its service but Franzo, in an interview, compared the competing systems. His version of the XM system could not be confirmed independently. The biggest difference between Sirius and XM satellite radio, Franzo said, lies in the way the two companies configure their satellites and terrestrial repeaters.
Sirius uses three geosynchronous satellites located higher in space, requiring fewer repeaters on the ground. By contrast, Franzo said, XM Satellite Radio depends on two geostationary satellites, located at lower elevations. As a result, XM needs about 1,500 or more ground-based repeaters about 10 times the number used by Sirius to reach its audience, he said.
To broadcast signals in urban areas, Sirius and XM both need ground repeaters to overcome line-of-sight problems, Franzo said. Sirius needs fewer ground repeaters than XM, he said, because its satellites are positioned at higher elevations.
Contrasting the two architectures, Franzo said, "Sirius is a satellite radio system with terrestrial gap fillers, while XM Satellite Radio is more like a terrestrial radio system composed of a network of terrestrial repeaters with satellite gap fillers."
Unlike a satellite TV signal to a home's set-top, which can be tweaked by adjusting the angle of the satellite dish, a digital satellite radio's signal, transmitted in 2.3-GHz S-band, must be received in a moving vehicle. The receiver, then, must know how to turn it into a usable signal.
Three geosynchronous satellites one ascending, another at a high elevation angle and the third located below the equator and idling allow Sirius to achieve spatial diversity and time diversity, essential to offering robust satellite broadcast signals. Further, when satellite signals based on the two TDM-QPSK bands are combined with terrestrial signals from the ground-based repeaters using the COFDM band, the Sirius system also achieves diversity in frequency bandwidth and modulation schemes, Franzo said.
In essence, the Sirius satellite radio system uses three independent transmission signals, two from satellites at any given moment and one terrestrial. Each contains an audio bandwidth of 4.4 Mbits/second.
Three copies of an encoded, encrypted and modulated audio bit stream reach a satellite radio receiver via three paths. The redundant signals, once demodulated, can be proportionally combined for optimal quality. The chip set implements three concurrent receivers to demodulate two TDM-QPSK signals sent from satellites, and one COFDM signal broadcast from ground antennas.
Agere's eight-chip set includes two chips for RF-to-intermediate frequency (RF/IF) demodulation and a high-performance, dual-channel analog-to-digital converter. Once signals are converted to digital, they are separated into three paths two TDM-modulated streams and one COFDM-modulated stream and run through demodulation chips.
The TDM demodulation chip is integrated with a maximum-ratio combiner, which identifies and combines the strongest signals from three separate sources to achieve optimal quality.
After the broadcast signals are demodulated, a concatenated decoder handles decryption, error correction and channel selection. Finally, a digital audio processor decodes satellite perceptual-audio-coding-compressed music and programs descriptive text such as channel name, song title and artists. Agere also focused on the chip set's wideband RF design in such a way that when radio receivers demodulate a 12.5-MHz spectrum in the 2.3-GHz S-band, they aren't desensitized by nearby radio devices such as PCS, 802.11b or Bluetooth.
Sirius has alliances to install three-band (AM, FM, SAT) radios in Ford, Chrysler, BMW, Mercedes, Mazda, Jaguar and Volvo vehicles as well as Freightliner and Sterling heavy trucks.
XM Satellite Radio has lined up as its automobile partners General Motors, America Honda Motor, American Isuzu, Suzuki and Freightliner. Both companies are planning to offer up to 100 digital channels for a monthly subscription fee of $9.95 for XM and $12.95 for Sirius.
The U.S. government has ordered Sirius and XM to work on an interoperable radio design so that consumers can choose either service provider.