All electronic design engineers face design challenges, but few have to face the enormous undertaking that confronted the engineers who decided to build a next-generation satellite system to deliver high-quality digital audio to U.S. vehicles and homes.
It's not easy or inexpensive. But if you haven't already experienced it, the outcome is more than spectacular, far better than you would expect from radio (see "Test Report," p. 44). This is the story about how Sirius Satellite Radio (www.sirius.com) developed and launched its system.
WHAT IS IT?
Sirius Radio is one of two satellite radio companies licensed by the FCC to deliver digital audio by satellite. This service, more formally known as Digital Audio Radio Service (DARS), was established in 1992. After many applications and reviews, the FCC whittled the number of applicants down and granted broadcast rights to Sirius and its competitor XM Satellite Radio (www.xmradio.com). These companies paid for this spectrum—Sirius $89 million and XM $92 million. The license authorizes broadcast rights in the microwave S-band at 2.3 GHz. After many years of system development, satellite launches, and receiver design, both companies launched their services in 2002.
The primary application for this service is constant coast-to-coast coverage of radio for cars. We have all experienced the problem of trying to listen to radio on a long trip. Both AM and FM stations fade in and out as we drive into and out of their coverage area. And signal reception is generally poor as well as variable. With the DARS systems, radio coverage throughout the 48 continental states is solid and continuous.
Sirius and XM each offer 100 channels of digital radio, mostly music channels but also news, sports, and other entertainment formats. Special car radios have been designed, and as of last year, were offered as an option by most major vehicle manufacturers. Sirius receivers are available as an option in Ford/Lincoln/Mercury vehicles as well as those of DaimlerChrysler, BMW, Jaguar, Porsche, Mercedes-Benz, Volvo, Mazda, and several others. XM radios are available primarily in GM vehicles. At $12.95 per month, the Sirius service offers the benefit of commercial-free music 24/7. After-market auto receivers and home receivers are also available at most popular consumer electronic stores, such as Best Buy, Circuit City, Good Guys, Sound Advice, Tweeters, and Crutchfield by mail order.
THE SPACE SEGMENT
There are two basic ways to cover a given area with satellites. The traditional approach and the one used by most space communications systems is to put up a geostationary satellite over the desired area. Geostationary satellites are put into a circular orbit around the equator about 22,300 miles (yes, miles) from earth. In such an orbit, the satellite speed matches the rotation of the earth, so the satellite is always overhead to any observer or station on earth. Sirius' competitor XM Radio uses this system with two satellites providing full U.S. coverage. The equatorial geostationary orbit is unique and currently jammed with satellites side by side only a few degrees apart. As with the frequency spectrum, we're simply running out of space.
Sirius takes the other approach of using elliptical orbits. The company has three elliptical orbits over the U.S. They are geosynchronous, meaning that their rotational period is 24 hours just like a geostationary satellite. The satellite apogee (high point) is 29,200 miles over Canada and the perigee (low point) is 14,900 miles. The orbits function in a way where two satellites are over the U.S. at all times. The satellites are spaced eight hours from one another, and each satellite is over the U.S. for about 16 hours. All three transmit the same data.
The elliptical orbits offer the advantage of a very high angle of coverage. With a conventional geostationary satellite, the line-of-sight path runs at a very low angle of elevation above the equator (about 30°) to the south from the U.S. Because microwave transmissions are direct-line-of-sight, signals from geostationary satellites encounter many more obstacles like trees and buildings. With elliptical orbits, the satellites are more directly overhead (always above 60°) and thereby avoid most earth obstacles. Yet at such distances, the attenuation from satellite to earth is enormous. Typical signal strength in the U.S. is −102 dBi, meaning that a hot receiver is needed.
All communications satellites are space-based repeaters that receive an uplinked signal, which is translated to another frequency and retransmitted back to earth. In the Sirius system, the digitized music and talk is uplinked from studios in New York City and retransmitted back to earth.
Mike Ledford, VP of engineering for Sirius, explains the system. Sirius is assigned 12.5 MHz of spectrum from 2320 to 2332.5 MHz centered on 2.32625 GHz (2326.25 MHz). This spectrum is roughly divided into thirds. One third is assigned to transmitting satellite #1 (TDM1) centered at 2322.3 MHz, one third to the terrestrial repeater network (more on that later) centered on 2326.25 MHz, and one third to transmitting satellite #2 (TDM2) centered on 2330.2 MHz. This gives each satellite roughly 4-MHz bandwidth.
The satellites use quadrature phase shift keying (QPSK) modulation. It's the most common for satellite communications, due to its robustness against signal degradation over long distances with minimal interference.
The receiver can receive and decode one, two, or all three signals simultaneously and recombine them internally into one signal. The receiver also accounts for phase delays (the satellites have large and constantly changing distances from the receivers), frequency shift (Doppler effects), and absolute time delays. (TDM1 is broadcast with an approximate 4-second delay from TDM2 and buffered internally, a Sirius patent.) Also, it accommodates a huge dynamic range. The receiver must work with a range of signals that are separated by more than a billion times the power of one another.
Ledford goes on to say that the data rate is approximately 7.5 Mbits/s. After accounting for overhead, including forward-error-correction coding (Reed-Solomon outer code and convolutional inner code) and encryption, we're left with an audio bit stream of about 4.4 Mbits/s. The bits stream may be broken down into as many substreams as required by the commercial system. At the moment, the 4.4-Mbit/s bit stream has 100 channels, averaging 44 kHz each. But, each bit stream may be assigned its own unique bandwidth.
Talk channels, which require far less bandwidth than music, may be assigned a 24-kHz bandwidth and music channels may be assigned a 64k bandwidth. It can be any combination Sirius decides as long as it doesn't exceed the total bandwidth of around 4.4 Mbits/s. When combined with modern Perceptual Audio Codecs (PACs) and statistical multiplexing, the sound quality that the average listener perceives is far superior to today's FM radio stations. When combined with a further level of tuning, which considers the genre and fidelity of the original recordings (i.e., music recorded from albums made in the '50s or earlier hardly need the bandwidth of a modern classical recording), Sirius can offer a truly delightful experience.
Sirius uses the Space Systems/Loral model 1300 satellite (Fig. 1). This 8300+ pound device costs roughly $100 million a pop and is launched into orbit by a three-stage Russian-built Proton rocket from Kazakhstan, also at about $100 million a shot, not including insurance. With three satellites in orbit and one spare on the ground for emergencies, Sirius has invested over $700 million on just the space segment of the system, not including ground stations.
With transmission distances so great and the desired coverage so broad, a receiver needs all the help it can get to pick up a usable signal. The techniques that usually take care of this problem are called receiver diversity. The Sirius system employs spatial, frequency, and time diversity to make sure an ample signal is always available.
The spatial diversity technique is patented and implemented in the form of at least two satellites in view at all times. Here, the receiver chooses the stronger signal. Frequency diversity comes from the use of three transmitting frequencies within the 12.5-MHz band. Time diversity is implemented by a special system patented by Sirius. This technique receives and stores four seconds of the signal in a receiver memory chip before feeding it to the speakers. Should you drive into a tunnel, beneath an underpass, or through a heavily wooded area, the stored data is usually sufficient to prevent the loss of a signal.
Even though Sirius' high-angle satellites provide an unusual high availability of service, it's just not enough in some areas. This is especially true in large cities with tall buildings and hundreds of obstructions that either block the signals completely or introduce multiple paths that erode signal strength. The Sirius system covers such gaps with an estimated 105 terrestrial repeaters in 50 U.S. cities.
The digital radio signal is uplinked to the Telstar 6 geostationary satellite operating in the 12-GHz Ku band. This satellite transmits the digital content to the terrestrial repeaters that rebroadcast them over a smaller area within the city on 2326.25 MHz as indicated earlier. The terrestrial repeaters use COFDM (coded orthogonal frequency division multiplexing) as their modulation scheme because it's more robust in complex multipath environments.
Finally, all space systems need some telemetry, command, and control. This is handled at the New York City studios, and the commands are sent to Mount Vernon, N.J., where they're uplinked to the satellites. Monitoring is accomplished at two fully automated and remotely controlled listening outposts in Quito, Ecuador and Utibe, Panama.
While the space segment of the Sirius system is by far the most expensive and complex, the receivers also presented a major challenge, along with substantial cost. Sirius hired Lucent to design the chip set that would form the basis for all auto and home radios. At one time, Lucent, now Agere Systems (www.agere.com), had as many as 100 designers working on the horrifically complex radio chip set.
Although the design is a superheterodyne, it's nothing like those we're most familiar with because of the diversity functions and other features. The chip set, originally packaged in seven chips, now uses four chips in its latest incarnation. Most of it is made with 0.14-µm biCMOS. An even newer version will use fewer chips that take advantage of the continuing smaller feature size and newer chip processing technologies.
Figure 2 shows a simplified block diagram of the Agere chip set. Note that there are two antenna inputs. Of course, the satellite antenna is the main one. But if you live in an area where the signal only comes through via the terrestrial repeaters, a different antenna is needed. The input chip houses the gallium-arsenide (GaAs) low-noise amplifier (LNA) and the down-conversion mixers. After the usual IF filtering and amplification in the second chip, the signal is digitized in the third chip. Resulting data is then sent to the baseband chip for all processing. The baseband chip includes an ARM core plus Agere's DSP16000 core. A 4-Mbyte by 16 SDRAM and a 256k by 16 flash memory handle all storage chores.
The receivers themselves are made by a variety of manufacturers, including Alpine, Clarion, Delphi Delco, Jensen, Kenwood, Panasonic, Pioneer, Sony, and Visteon. Your first Sirius receiver will probably come with a new vehicle purchase. But after-market auto and home receivers are now available, too (Fig. 3).
A critical part of the satellite receiver is its antenna. It's built into the vehicle when you purchase a factory-installed model. After-market models, including home receivers, require that you add an antenna. Vehicle antennas, which use left-hand circular polarization (LHCP), have a gain in the 2-dBic range. Terrestrial repeater antennas, on the other hand, utilize linear vertical polarization and have a typical gain of 3 dBic. The antennas normally come with a built-in GaAs LNA.
Is Sirius a success? Technically, for sure. It's a real triumph of electronic design and implementation. Give Agere a hand as well for a brutally difficult but successful chip design. As a business enterprise, the final word is still out. As Sirius' founder Rob Briskman told me, the total up-front investment so far is about $2 billion, not your usual venture-capital funded deal.
Nevertheless, subscriptions are increasing daily. Once most vehicles get their Sirius radio options, the service will grow significantly. Auto manufacturers are rolling out more models with satellite radio options each year, and, as more consumers become familiar with their options, you'll begin to see more after-market auto and home satellite radios. XM even has a portable satellite radio boom box that's sure to be popular. The system sells itself if you can get a chance to test it.
So while the main objective of Sirius and XM right now is to become profitable from their existing systems, the future holds some interesting possibilities for the technology. With a satellite system that can downlink digital data streams reliably anywhere in the U.S., it seems likely that we will also see other digital services develop. As Sirius' Mike Ledford says, "think video to the back seat."