Satellites for broadcast
Dec 1, 2010 12:00 PM, By David Austerberry
Technology is evolving to meet new demands.
Figure 1. Satellites are integral for the delivery of live content where fiber is not available.
Select figure to enlarge.
One of broadcast television's compelling propositions is live video. Whether it is news, sports or special events, the immediacy and picture quality of television still has the advantage over more recent consumer delivery platforms. Viewers expect to see news and sports live from the other side of the world as much as from their own backyard.
The rise of global media brands means that the same programming can be seen across all continents. Much of this programming comes from a few clusters of content factories, notably in Los Angeles. This used to slowly promulgate across the world on videotape, but pressures like piracy are driving content owners to publish across the world within smaller release windows.
Global brands now want to deliver content and version for the locale in much shorter timescales. Episodic programming used to take a year or so to spread around the world. Now files are delivered, and programs are dubbed or subtitled in less than a week after airing in the United States.
In order to achieve these telescoped timescales, every process must be accelerated. If content is distributed as a file, it cuts down delays of transport like customs clearance and removes some of the QC steps. Onward file transport to dubbing houses rather than shipping tapes around all helps to trim days off turnaround of program language versions.
Satellite content delivery
The globalization of media has developed hand-in-hand with the development of satellite services for backhaul, distribution and delivery of content.
For viewers, satellites are known for DTH delivery and the backhaul of news and sports from around the world. But for broadcasters, satellites can offer all manner of services, especially when fiber links are not available.
For most operations, it's not a straight choice of fiber or satellite, but using the two in combination to exploit the strengths of each. Fiber may be the carrier of choice between metropolitan centers, but satellite offers advantages for truly global reach and for occasional use where fiber is not available.
Geosynchronous is the predominate satellite class for broadcast applications. If they are positioned above the equator, then they appear (geo) stationary in the sky. With this fixed position, they don't need the complex tracking required by the lower orbit devices that are used for applications like remote sensing of the earth's surface and weather.
The geosynchronous orbit is at a distance of 36,000km from the earth's surface, so the round-trip delay for radio waves is about 0.2s. Add to that encode/decode steps, and the familiar delays of satellite links result. With the popularity of studio anchors interviewing the “man on the spot,” satellite is second choice over fiber, especially if there are two hops. However, fiber is not always available.
Geosynchronous satellites are typically positioned in an operational box of about 0.05 degrees, so the dish only needs minor adjustments to keep it on station. Satellite operators also issue bulletins detailing the exact position of their satellites day-to-day. For occasional use, finding a satellite is aided with a beacon from the satellite as well as GPS to give an accurate location for a mobile earth station.
Which band
Satellite service providers offer services in C, Ku and Ka bands. Other bands like X are primarily reserved for military satellites. The frequency allocations for the bands vary according to region.
C-band saw the earliest use by broadcast application and is also used for terrestrial microwave links. C-band uplink frequencies are 5.850GHz to 6.425GHz and downlink 3.4GHz to 4.8GHz.
The shorter wavelength Ku band offers smaller dish sizes, an important factor for vehicle-mounted dishes or flyaways, and is now the favorite for broadcast links. Ku-band uplink frequencies are 14GHz to 14.5GHz, and downlinks are 10.7GHz to 12.7GHz.
The demands for ever more capacity has led to the opening up of the Ka band. The higher frequency band, 26GHz to 40GHz, has a much wider frequency range than the lower frequency C and Ku bands, so it potentially has much greater capacity. The Ka band offers great savings in link cost; it can be 10 to 20 times cheaper per bit of data transferred. However, the equipment is currently more expensive than Ku- and C-band terminals.
In general, the shorter the wavelength, the more a link will be subject to attenuation by weather conditions such as rain and snow. Attenuation by rain in the millimetric Ka band is about three times that of centimeter-wavelength Ku band.
Rainfade is more of a problem in tropical areas, where daily heavy rainfall is common. The solution is to use higher power during spells of atmospheric attenuation and to specify larger diameter dishes than would be used in dryer regions. For a fixed installation, diversity earth stations that are geographically separated and linked by fiber offer a better chance of dodging storm clouds, albeit at a cost.
So although there is great capacity available in the Ka band, it is difficult to achieve uptimes over 99 percent. Where the service requires higher reliability under all weather conditions, then Ku and, better still, C-band remain the better choices.
Beams
A geosynchronous satellite can cover about one-third of the earth's surface, but for most applications, smaller, shaped beams are used. For example, in DTH applications, a spot beam may be used to limit coverage to one country. For backhaul, a zoned beam may be configured to cover a landmass like a continent and not waste coverage on empty ocean.
International agreements control the available orbital slots for the geosynchronous satellites and also the frequency allocations in order to avoid adjacent satellite interference. Geosynchronous satellites are spaced two degrees apart around the equator. More than one satellite can be co-sited in one orbital slot, as long as each uses different frequency bands.
This two-degree spacing means a maximum of 180 slots around the equator. Tighter spacing would not be practical, as limits on beam widths would mean adjacent satellites would interfere with each other. Just like the terrestrial UHF band, the competition for spectrum is heating up, but there is a finite resource limited by available orbital slots and the frequency bands allocated by international treaties to television contribution and distribution.
The only ways to expand capacity are to use what's there more efficiently and to move some traffic up to the Ka band region, weather permitting.
Link budget
The data rate of a satellite link depends on several factors: power, bandwidth and the maximum allowed bit error rate (BER). The uplink power is a combination of the dish diameter and the high power amplifier (HPA) rating. The bandwidth will ultimately be limited by the transponder bandwidth, but usually many channels share a transponder. So, in most circumstances, the bandwidth can be considered to be fixed. The BER will be affected by circuit noise, rainfade and other atmospheric conditions, and the modulation technique.
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