pbs finds fiber educational for its satellite network

Like many companies that rely on satellite technology, the TV broadcaster has turned to fiber optics to link disparate sites and solve transmission problems.

Stephen Hardy Editor in Chief

The Public Broadcasting System (pbs), like many television broadcasting companies, relies on satellite communications to distribute programming to its television stations across the continental United States, in Alaska and Hawaii, and in territories such as the Virgin Islands. pbs maintains regional earth stations around the country, but its main uplink facility is in Springfield, VA. Located 7 mi southwest of the Capital Beltway that surrounds Washington, DC, the Springfield site contains three C-band and an equal number of Ku-band transmit/ receive satellite antennas.

Satellite communications works well for pbs. However, the technology has its limitations. For example, Ku-band transmissions are susceptible to weather-related interference--which is not an insignificant problem in an area of the country such as Springfield that is regularly visited by violent thunderstorms. To solve the problem, the engineers at pbs decided to set up a duplicate, remotely controlled Ku-band site in Independence Hill, VA, approximately 22 mi from Springfield. The challenge facing pbs was to quickly ship signals from the control center at Springfield to the remote site when inclement weather threatened. In a move that reflects a growing trend within the broadcast industry, pbs turned to fiber-optic technology to provide this critical link.

Direct connection

According to John Kling, supervisor for the Springfield site, a fiber link had clear advantages over the other primary alternative, microwave radio. A microwave radio link would have presented several hurdles, from obtaining the necessary licenses for transmission and tower siting to the assignment of a transmission frequency. A microwave link also would have required several hops to detour the signal around areas of high microwave traffic between Springfield and Independence Hill, he says.

The fiber link therefore promised a more straightforward solution to the problem faced by pbs. As initially envisioned, a fiber-optic transmitter provided by Ortel Corp. (Alhambra, CA) would convert the digital satellite signal to optical wavelengths for transmission over approximately 40 km of singlemode fiber. The cable would be provided by a combination of Bell Atlantic and GTE, which split the territory to be covered by the link. An Ortel receiver at Independence Hill would reconvert the optical signal to electricity for transmission by the 6-m Ku-band antenna at Independence Hill.

The finished network follows the initial network vision--except for the fiber link. A direct fiber line between the two phone companies proved difficult to create, and the final fiber pathway ended up consuming 70 km and approximately 114 splices, according to Kling. The extra distance led to the installation of a repeater--consisting of an additional Ortel receiver and transmitter--at one of the central offices between Springfield and Independence Hill. Nevertheless, the link (which in-cludes a second fiber for redundancy) has been up and running for almost a year, providing a 70-MHz pathway between the two sites.

A growing niche

The pbs application is unique in that microwave radio was fiber`s principal competitor. According to John Rinks, vice president and general manager of Ortel`s satellite communications/government business sector, the managers at most satellite earth stations confront a choice between coaxial cable and fiber when looking to link their antennas with their equipment rooms. Coaxial cable has traditionally been the medium of choice, according to Rinks. However, the technology suffers from relatively high signal loss--as much as 11 dB every 100 ft at L-band frequencies--which in turn has limited the effective length of coaxial-cable runs. The higher the frequency at which the satellite equipment operates, the greater the rate of signal loss.

Thus, antennas have historically needed to be close to their corresponding equipment rooms. When earth stations expand, site managers face a dilemma: Find sufficient acreage close to the existing equipment room for the new antennas, or site the antennas in a different spot and build a new equipment room to service them. Fiber optics provides an alternative to this conundrum (see photo). Because of its comparatively low loss--as little as 0.1 dB every 100 ft at L-band, according to Rinks--antennas can be situated hundreds of meters away from the central equipment room, easing distance limitations and obviating the need for additional equipment rooms.

The benefits of fiber come with a higher price tag than coaxial-cable networks, however. As is the case with other applications, the cost of the attendant optoelectronics can make fiber-based links relatively expensive. Thus, earth station managers must weigh such factors as distance requirements, frequency, and the potential need to build additional equipment rooms when determining the most economical transmission technology for their application.

"I would say, in general, that anything over 500 m is a slam dunk for fiber. And between 150 m and 500 m, it depends on these other [considerations]," says Rinks. "Under 150 m, coax typically is cheaper."

Other factors also may enter the equation. For example, fiber`s low susceptibility to electromagnetic interference and the effects of lightning strikes can be important attributes in certain applications. Military users also appreciate fiber`s resistance to signal interception.

In general, Rinks sees seven applications where satellite system users are considering fiber links to supplement their networks:

Teleport and gateways: These large earth

stations are used to transmit high-bandwidth signals across countries or oceans. Companies such as Norwegian Telecom, Singapore Telecom, IDB Systems, IMPSAT in Colombia, and the Washington International Teleport are known to have used fiber links in their applications.

Cable-TV headends: These applications

are generally receive-only, where satellite stations receive programming for distribution to cable-TV subscribers. Cox Cable and Viacom have installed fiber-optic technology to support such applications.

Broadcast earth stations: In addition to

pbs, Direct TV and espn also use fiber optics for uplink applications.

Satellite control centers: These sites

control the use of large satellite fleets. Hughes Communications Inc./ panamsat is one company that is using fiber optics in such an application.

Local multipoint distribution system:

This emerging technology represents a significant growth market for fiber, according to Rinks. Field trials of fiber technology in tandem with local multipoint distribution system equipment are now under way, he reports.

Military communications: The armed

forces have long been fans of fiber for the reasons stated earlier, according to Rinks.

Multidwelling units: Large apartments

and condominium complexes also represent a growth market, says Rinks. Zoning or community restrictions frequently prevent individual residents from owning their own direct broadcast satellite dishes. The usual solution has been to install a single dish, then divide the signal among the residents. Coaxial cable has been used for such applications in the past, but doesn`t work well for applications serving more than 100 units, says Rinks. Again, the losses associated with coaxial cable frequently require the use of amplifiers in such networks. A growing number of building contractors and facilities managers are therefore turning to hybrid fiber/coaxial-cable architectures, in which a fiber backbone runs vertically to each floor. Coaxial cable branches run off the backbone to service individual users.

Military applications have been the traditional leader of what Rinks considers a niche market worth between $25 million and $40 million worldwide. Until recently, the low-profile nature of military work and the relatively small size of the market have meant that few companies have addressed fiber systems for satellite applications. However, the growing number of commercial applications has led to greater revenue expectations and new entrants in the field, he says.

Satisfied customers

These new entrants are being enticed by an increasing appreciation of the benefits of fiber among satellite technology users, says Rinks. "Basically, anybody who`s using a dish is giving serious consideration to fiber optics," he explains.

Certainly, Kling is glad that pbs considered fiber. He expects to use the link to the remote site 10 to 15 times a year. "It`s worked out very well; it`s been very reliable," he says. "We don`t seem to have any problem with it whatsoever."

The bottom line is that the benefits of fiber are leading the technology into new applications. "There`s fiber content even in these satellite and wireless applications," concludes Rinks. "So there`s a role for fiber just about anywhere you want to look." u