Last month's column began with a short reference to the first transatlantic satellite transmission over Telstar (1965). Of course, the more interesting end of the transmission loop is the reception, without which there is not much value to the effort. It is easy to forget sometimes how low the power level of early international satellites was.
While touring the Teleglobe Canada facility near Montreal a number of years ago, I was confronted with the primitive nature of technology at the dawn of satellite transmission. The facility's first antenna for transatlantic work was 108ft in diameter, with a beam so small the antenna was hunting constantly to keep the beacon from the satellite in the center of the pattern. Three huge C-band amplifiers were connected to an immense antenna, two of which were needed to saturate the transponder despite the enormous gain of an antenna 108ft in diameter. The LNA was bigger than a shoebox.
By the 1990s, orders of magnitude less power were enough. A single small solid-state amplifier was all that was necessary to saturate a modern transponder.
Much of the credit for this goes to the improvements in spacecraft, including their enormous size difference. Telstar could have been put on top of a file cabinet, but modern synchronous satellites have solar arrays as large as a 737, more than 110ft. This size difference produces immensely more power, which permits much higher transmit power (in the range of 25kW), especially when combined with larger antennas on modern spacecraft.
In addition to the improvements in technology in spacecraft, transmission systems have been revolutionized in the digital era. Early deployments used analog modulation, requiring high power to achieve noise-free reception. But, beginning in the early 1990s, deployment of digital compression changed everything. The first European Telecommunications Standards Institute (ETSI) codecs were deployed on both sides of the Atlantic and allowed transmission of SD pictures at 34Mb/s, with improvements allowing rates as low as 8Mb/s for some news coverage.
Transmissions normally were sent over links using quadrature phase shift keying (QPSK), allowing 2 bits to be transmitted per symbol. Noise immunity was good, but as satellites and ground systems became more phase stable and lower in noise, modulation of more bits per symbol became both possible and highly desirable for economic reasons.
Increasing satellite capacity
Satellite capacity is sold on the basis of two factors: the power required by the spacecraft and the bandwidth occupied. To increase the data throughput one can increase the bandwidth, but that increases price, so high-order digital modulation standards have evolved to allow more bits to be transmitted in the same RF bandwidth.
But there is a tradeoff that is actually quite insidious. More bits transmitted per hertz will be more susceptible to noise, requiring more power, which of course costs more. The equation is established in part because each transponder can handle only a fixed amount of power. Two carriers in one transponder means less power available to each, and nonlinearly loading a transponder may not allow for easy math in calculating the effective cost of the space segment.
Thus, the trick is to design an end-to-end system that can use less bandwidth and at the same time survive in noisy environments. That is exactly where the innovation is coming, along with improvements in compression for sound and picture that will permit fewer bits to represent the content without compromising the quality.
Holistically, compression and modulation work hand in hand to deliver improved performance and more affordable business platforms. The current standard most often used for backhaul internationally (and domestically) is DVB-S2, promulgated by the DVB consortium in Europe. DVB-S2 offers modulation at higher orders, including 8PSK and 16/32APSK. The former uses eight phases of the carrier to represent the data, allowing double the information density provided by QPSK. Similarly, 16APSK and 32APSK offer up to 16 bits per hertz.
As noted earlier, higher-order modulation is more susceptible to noise, including black-body thermal noise and that created by other sources of interference. One method of reducing the impact is to use more resilient forward error correction. But as before, contributing more of the payload to error correction steals from the delivered content. There is simply no free lunch. Thus, “tuning” modern systems is sensitive to many factors, all of which must be considered in harmony to achieve good link performance.
For instance, if a service uses a fixed modulation standard and content bit rate, a larger antenna and more capable LNA/receiver decoder (IRD) may be needed to achieve adequate margin for fade during weather events. If the operator has the luxury of planning the full link, it needs to take into account the capability of all of the hardware and that of the spacecraft. What works on one transponder may not work as well on another in the same spacecraft.
There is a temptation to break this Gordian knot simply by pushing out more power or putting up a larger transmit antenna to achieve adequate results. Before resorting to that approach, it is recommended that broadcasters see the earlier paragraphs related to the cost of the space segment. More power may well cost them more money.
Modern IRDs for broadcast use must adapt a huge range of standards. First, the modulation standard (actually multiple variants with different error-correction schema) must be supported. Second, each service may use MPEG-2 or MPEG-4 (AVC or H.264). Audio may be PCM, MPEG compressed, AC3 or Dolby E. And, of course, the video may be SD or HD, though the time is approaching when delivery of SD content will dwindle to insignificant values, especially with advances in compression technology. Outputs may include upconversion and downconversion capability as needed du jour.
One final comment on the subject must be made. Digital Satellite Newsgathering (DSNG) adds additional complexity, particularly with flyaway antennas for use in remote locations. Communications channels and data connections often must be accommodated. Careful band planning is required, and specific knowledge of the communications plans of networks and location providers is critical if a remote transmission is to be free from hair pulling and wailing at the moon.
John Luff is a television technology consultant.
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