[Success Story]
Satellite Radio Gets Serious
Sirius Satellite Radio delivers 100 channels of CD-quality music, sports, and news to auto and home receivers.
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.
DIVERSITY FLAVORS 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.
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