Recent improvements in front- and back-end data converters have accelerated the acceptance of digital or software-defined radios in the commercial world. These improvements have filled an existing void after the availability of low-cost DSPs facilitated the transition for designing such radios. That has prompted cellular communications technology to take the digital route. Now, similar efforts are under way to speed the development of AM and FM digital radio broadcasts in the U.S. and the rest of the world, using the existing terrestrial in-band system. In Europe, though, out-of-band digital audio broadcasting (DAB) via satellites has been impacting the airwaves for nearly a decade. Several international consortiums are spearheading that effort across the globe, as digital alternatives to the terrestrial mode of transmission.
Thanks to the U.S.-based WorldSpace Corp., satellite digital radio has been in service in Africa for over a year. This was made possible by the launch of the AfriStar satellite. Additionally, satellite DAB broke ground in Asia last month via the AsiaStar satellite, which was launched in March. Primarily, WorldSpace's digital radio transmission and receiver technology is aimed at stationary receivers in the homes and offices of developing countries.
Furthermore, satellite radio broadcasting is in the works in the U.S. But unlike WorldSpace, XM Satellite Radio and Sirius Satellite Radio are aggressively racing to beam their signals toward automobiles. In short, they are preparing to provide satellite-to-car or mobile radio broadcasting services by early next year.
In 1997, the U.S. FCC licensed the S-band spectrum to XM and Sirius for national satellite radio broadcasts. These service providers have aligned themselves with major analog/mixed-signal and DSP chip suppliers to generate digital radio transmission and reception circuits in compliance with their system architectures and broadcast services.
For instance, Lucent Technologies is developing RF, analog/mixed-signal, and DSP chips for the Sirius digital radio receiver for satellite-to-car receiver sets. "We have been working with Sirius for two-and-a-half years, developing the system and the chips," notes Rob Franzo, director of automotive products at Lucent. "It's a broadband solution with double downconversion," he adds. The incoming 2.32-GHz RF is downconverted to a 300-MHz IF, which is further converted to a 75-MHz signal for sampling. Then, this signal is digitized using a very high-performance analog-to-digital converter (ADC).
By implementing a pipelined architecture in deep-submicron CMOS, Lucent is readying a 14-bit, 60-Msample/s ADC for this application as part of the chip-set solution for the satellite receiver. It will be followed by a digital downconverter (DDC) that will format the digital data for linking with the Lucent DSP.
In this design, the carrier frequency uses a 12.5-MHz bandwidth to incorporate two satellite signals that employ quadrature phase-shift keying (QPSK) modulation, time-division multiple-access (TDMA) digital encoding, and a single terrestrial signal employing coded orthogonal frequency-division multiplexing (COFDM) techniques. No further details on the chips were provided, however, and the developer didn't give out any hints about when the chip set will be ready for delivery.
On the other hand, STMicroelectronics has lined up a deal with XM. Under an agreement with the satellite radio service provider, STMicroelectronics is developing a chip set for mobile and home radio receivers, which includes the RF/analog front end and the DSP-based baseband processor. In essence, STMicroelectronics is extending the experience that it gained by designing the WorldSpace digital satellite receiver using the 0.25-µm CMOS process.
Fortunately, the merger of two leading technology developers of terrestrial digital radio broadcasting will soon make DAB a reality in the U.S. Today, radio broadcasting in the U.S. implements analog signals with amplitude modulation (AM) and frequency modulation (FM) techniques. The transition toward digital modulation will transform terrestrial radio broadcasters into providers of enhanced-quality sound for AM radio and CD-quality audio for FM users, as well as data for a variety of consumer applications. Without diminishing the quality of current analog transmissions, the new DAB system will initially deliver a hybrid solution. It will ensure that the presently installed base of traditional analog broadcasts won't be obsoleted immediately. This solution provides a smooth transition toward ultimately pure DAB service.
To expedite this effort, Lucent Digital Radio and USA Digital Radio recently unwrapped plans to combine forces and operate as a single entity under the name iBiquity Digital Corp. This merger is expected to speed up the standardization effort and pave the way for rapid development and commercial deployment of AM and FM in-band on-channel (IBOC) digital radio in the country.
Conversion Without Disruption
IBOC digital audio broadcasting uses the current radio spectrum to transmit AM and FM analog simultaneously with high-quality digital signals. As a result, it will enable broadcasters and listeners to convert from analog to digital radio without service disruption. Plus, current dial positions of existing stations will be maintained with IBOC.
The emerging digital radio broadcast standard, coupled with the emerging multiprotocol, multistandard cellular/PCS phone, is forcing analog and mixed-signal IC suppliers to develop integrated analog front-end chips for these applications. So when the standards are established and the market is ready to take off, the analog front ends will be there to facilitate the digital radio revolution. In conjunction with DSPs, these IC suppliers are developing software-radio architectures that can be reprogrammed for different modulation formats and transmission standards. The end result is one software radio for multiple standards.
Toward that end, analog front-end IC suppliers are spawning faster ADCs and digital-to-analog converters (DACs) with higher resolution, better dynamic range, and lower noise performance. Above all else, they're achieving wider input bandwidths at higher frequencies (Fig. 1). It's the data-converter's sampling/update rate, its spurious-free dynamic range (SFDR), and its signal-to-noise ratio (SNR), in conjunction with the speed of the DSP, that will determine where digitization will begin—at RF, IF, or the baseband.