David Austerberry /
12.01.2010
Originally featured on BroadcastEngineering.com
Satellites for broadcast
Technology is evolving to meet new demands.

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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For downlinks, again antenna diameter is a factor. For occasional use links, all these parameters will dictate the maximum video data that can be used in the given conditions.

Coding

DVB-S2 has developed upon the earlier DVB-S modulation scheme to provide greater efficiency, and permits the data rate of the channel to approach the Shannon (theoretical) limit for the link. The expanded toolkit for error correction and modulation can improve efficiency by 50 percent to 80 percent.

If this is combined with a change of video coding from MPEG-2 to MPEG-4 AVC, then HD channels can be transmitted in similar transponder slots to that formerly used for SD contribution links.

Building a network

An acquisition-to-viewer network for the delivery of news or sports could include three or more satellite hops. (See Figure 1 on page 8.) The SNG and occasional use circuits for sports and special events provide connectivity if there is no fiber available at the venue. The first hop could be to a teleport then via fiber to the broadcast facility. Backhaul uses a single channel per carrier, as opposed to the multiplexes used for distribution.

Distribution to headends or turnaround teleports can multiplex bouquets of channels together for uplinking, with channels possibly being remultiplexed into different groups for onward relay or DTH.

Service provision

Although many teleports are small businesses, there are a few global operators who can provide broadcasters with a complete contribution and distribution service. For occasional use circuits, this takes a lot of the headaches out of running daily operations for outside broadcasts and news. Intercontinental backhaul can require negotiations with telecom regulators, several teleports and fiber providers, as well as booking satellite time.

These service providers are also providing file delivery and content management services. The global distribution of content is migrating from videotape to files, and such content management services can simplify the provision of programming across continents.

IP over satellite

A recent advance is the provision of Internet Protocol over satellite links. The terminal equipment presents as an Ethernet connection, so media files or general production data can be delivered over long distances as simply as plugging a laptop into a broadband Internet connection.

Such technology helps to ease integration of satellite hops into media networks, leveraging the ubiquity of IP. It can also be multiplexed with conventional video channels for general production applications alongside the broadcast stream.

Summary

Television is competing with other visual media, but it has a niche for live programming, news and sports. Episodic programming is distributed across the world on compressed timescales compared with a few years ago. Transporting videotape by courier services does not meet either of these demands, and this is where the strengths of global satellite and fiber networks play. Together they provide flexible services for permanent circuits and occasional use. Satellite excels beyond the reach of fiber provision, especially for contributions from remote locations. Satellite also provides for cost-effective distribution of content to headends, one-to-many.

Satellite and fiber both form key components in live production and the distribution of programs as files. Advances in satellite technology are keeping pace with broadcasters' demands for more circuits and for the upgrade to HD.

The demands for capacity are being met by Ka band, by improved modulation and video coding. And finally, interconnectivity is simplified as satellite links adopt the ubiquitous Internet Protocol.



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