By James Careless

Satellite customers do not want to lose any transmission signals–but it happens. The challenge facing service providers and equipment manufacturers is providing turnkey solutions to combat such problems. While certain signal glitches remain unfixable, others can be corrected. From rain fade issues to technical errors, satellite technology advancements are transporting more content through space without it being lost in space.

Standing in the way of this delivery is interference. After all, at the core of any satellite business is signal delivery. If unsuccessful at getting voice, data and video transmitted to the client’s destination, then those clients will not be expanding their business relations at contract renewal time. Regardless if caused by man or nature, interference can seriously diminish the power of a satellite transmission, or block it entirely. In either case, finding a way to "clear up the static" is important for satellite users, and a source of profit for equipment manufacturers and content distributors.

Nature’s Onslaught

Rain attenuation, solar energy and heavy static are examples of how Mother Nature can interfere with satellite transmissions. Attenuation, or rain fade as it is more commonly referred to, occurs when a passing electromagnetic wave gives up energy to the liquid in a raindrop. The loosely bound molecules of water absorb energy from the electromagnetic signals. Airborne water can reflect or absorb satellite transmissions and this rain fade is what causes many Ku-band DBS receives to go dark during heavy storms. On the ground, frozen water in the form of ice and snow can collect inside a satellite antenna and attenuate its transmit/receive capabilities to nothing.

To combat such obstacles posed by Mother Nature, manufacturers have brought to market equipment to thaw nature’s chill. Even though rain fade is an unavoidable fact of satellite life, especially in more tropical regions of the world where torrential downpours more readily occur, snow and ice, however, can be managed. One such company fighting the elements is Surface Heating Systems Ltd. (SHS) out of the United Kingdom. SHS has designed an innovative de-icing system used by Andrew Corp. for its new 3.7m antenna.

"This proved to be different to the normal system manufactured, as each petal has to be split into three to allow for the strengthening bars that run across each petal," says Michael Cummings, SHS director. "We achieved this by manufacturing light weight, self-adhesive heating panels, which are supplied pre-connected, ensuring that the installation time and subsequent cost is kept to a minimum."

W.B. Walton Enterprises also sells a range of protective snow shield and hot air heaters to keep antennas free of frozen water. "Snow in your reflector can take you off air," says Richard Gomrick, Walton’s vice president of sales and marketing. "Think of it as a form of rain fade, which protective covers and heat can dispel."

Energy bursts generated by the sun can seriously interfere with satellite RF (radio frequency) transmissions, or scramble them entirely. In the most extreme cases, solar storms can damage or disable satellites themselves. Such was the apparent fate of Telstar 401 in 1997. It failed after being hammered by a solar ejection of magnetically charged helium and hydrogen carrying an electrical charge estimated at 1 million amps.

The Human Factor

Adding to the cacophony of natural noise is man-made interference. Sometimes it emanates from power lines and electrically driven equipment; other times it occurs due to human error.

In many cases, man-made interference occurs because proper transmission management is rocket science and requires continual scrutiny. When corners are cut in this arena, business begins to significantly suffer. In a world where geostationary satellites are positioned just two degrees apart in orbit, it is common for one satellite’s downlink transmission to overlap those of its next door neighbor. This is known as Adjacent Satellite Interference (ASI). Meanwhile, misaligned earth stations often overlay uplink signals on top of others. If the two competing uplinks are in adjacent channels, the result is signal-reducing interference.

"Should both signals be on the same frequency, chances are that neither will get through," says Mark Miller, Viasat’s chief technology officer.

Unfortunately, the explosive growth of VSAT (Very Small Aperture Terminal) earth station deployment in developing countries has resulted in a forest of misaligned dishes. "70,000 to 90,000 new VSATs are being deployed every year, says David Hartshorn, secretary general of the Global VSAT Forum (GVF), the world body that represents satellite service suppliers and users. "Many of these technicians may be able to point a dish, but they are not trained to do so in a way that prevents interference, or even ensures that they’re bouncing off the right satellite."

To mitigate this problem, the Global VSAT Forum and its members have begun offering a VSAT Installer Training Course. It teaches technicians the best practices to implement when monitoring and proper instruction to earth station installers, using standards developed by GVF member companies. Admittedly, proper training will not deter rogue carriers who deliberately and illegally uplink to other people’s satellites, but it will go a long way to reducing unintentional man-made interference.

Planning For Static

When engineers design a satellite transmission/reception system, they do so by creating link budgets. Essentially, a link budget takes into account the characteristics of the particular transmission band to be used (C-band or Ku-band), the antenna’s overall diameter and signal delivery performance, the distance to be traveled, and any known interference factors. The goal is to come up with a link budget that minimizes the power and equipment complexity needed to deliver the signal–thus keeping costs down–while maximizing the reliability of the signal transmission.

A link budget calculator is available at http://www.satsig.net/linkbugt.htm.

Obviously, certain factors in the link budget equation are not open to negotiation, such as the transmission characteristics of each frequency bandwidth. But others are. Specifically, those factors that can be altered by improving existing technology, or even by taking a better approach to the whole issue of signal delivery.

The Ka-band is quickly becoming a popular choice for two-way broadband transmission via satellite. The problem is that Ka-band millimeter-long RF waves are very susceptible to rain fade.

To deal with this, Ka-band users typically rely on high signal strengths–to compensate for energy loss due to signal absorption–and adjustable data rates. The idea here is that slower data rates stand a better chance of getting through the static, since extra error correction (double-checking that the right bits are received, and resending those that do not make it) can be factored into such transmissions.

Telesat Canada, for example, is a big believer in adjustable data rates and on-demand Uplink Power Control. "If you are sending a T1 [1.544 Mbps] transmission to a satellite, and rain fade is making it indecipherable due to missed data packets, then the logical response is to dial down to half T1 speeds," says Paul Bush, Telesat’s vice president of broadcasting and corporate development. "Meanwhile, if rain fade is devouring too much of your signal strength, it makes sense to increase it through dynamic ‘Uplink Power Control’. The key here is not to arbitrarily boost your uplink power; that could result in interference to adjacent channels, and cost more money in electricity than necessary. Instead, you boost power as needed to get through the rough spots, then drop it down when conditions improve."

Having spent 30 years coping with interference issues, Telesat’s expertise has become a salable product for this Canadian satellite operator. "In fact, 8 to 10 percent of our business is in consulting to other satellite operators and users," says Bush.

Improving Error Management

With the exception of times when signals just cannot punch through the static, the major problem facing satellite users is corrupted transmission. Yes, the data stream did get through, but some of the information got lost or garbled in transit due to interference. In such cases, the only thing to do is to find where the error occurred and resume the transmission at that point, or begin all over again.

The problem is that retransmission wastes time and money; especially because satellite bandwidth is not cheap. Clearly, this is not an option satellite users want to choose, nor one that satellite service operators wish to rely on.

So what can a satellite user do to protect its data? One option is incorporating signal equipment that assists in making sure satellite signals transmitted make a seamless trip from origin to destination. Kencast’s Fazzt Digital Delivery System, for example, uses a proprietary forward error correction algorithm. Fazzt can detect and reconstruct corrupt data packets so well, that retransmission are virtually eliminated.

The reason Fazzt can deliver such powerful error correction is due to the system’s architecture, says Bill Steele, Kencast’s chairman and CEO. "We add some metadata," Steele adds. "Say that you need to deliver a music album, a file of 100 Mbps, to 1,000 homes. With Fazzt, we actually may send 104 Mbps, with the extra 4 Mbps being metadata computed by processing the original 100 Mbps file. The extra 4 Mbts of packets are not like any of the 100 Mbts of original packets. Now imagine that one of the 1,000 homes loses (randomly or in an extended burst)1.5 Mbps of original data due to rain attenuation. At the receiving end, a client with Fazzt software will identify the missing 1.5 Mbps and reconstruct them using ‘any’ 1.5 Mbps of the metadata. The metadata can also be employed to ensure that the whole file as reconstructed is valid. After an incoming file is validated by Fazzt, the chance there is a single bit error is typically only 1 in 2 to the power 32,000, comparable to winning the Powerball lottery at 1:84 million odds–and winning it 1,200 times in a row."

Spreading The Risk

Think of a typical satellite transmission as a thin line in the sky. This line can be blocked on its way into space by a single interfering source. But what if this single line could be divided into tens or even hundreds of lines, and then be spread across a range of frequencies? Such a spread spectrum signal could be far more resistant to single frequency interference, and thus more likely to arrive intact.

This is the strategy underlying Aloha Networks’ Spread Aloha Multiple Access (SAMA) technology. Under SAMA, satellite signals are spread across a wider bandwidth with a lower power spectral density (power per unit bandwidth). In addition, SAMA signals from multiple remote stations can be sent simultaneously across this same bandwidth. The result is a highly efficient method of sharing the bandwidth among a large number of intermittent, bursts traffic users. The lower power spectral density allows smaller antennas to be deployed without impacting adjacent satellites.

"Spread spectrum transmissions of the SkyDSL Network reduce the risk of generating adjacent satellite interference because the spectrum is spread throughout a wider range of frequencies thereby requiring lower power per bandwidth for equivalent data rates," says Martin Jaffe, Aloha Networks’ vice president. "And this power per unit bandwidth is the quantity regulated by the FCC."

Viasat also believes in using spread spectrum transmissions. In addition, Viasat’s Arclight VSAT transmission system reduces bandwidth costs by allowing users to send and receive signals through one single channel. Designed for VSAT networks where channel use is controlled on a demand basis, the key to Arclight is Viasat’s proprietary Code Reuse Multiple Access (CRMA) and Asymmetrical Paired Carrier Multiple Access (PCMA) protocols.

CRMA uses coding management similar to that used in cellular telephone networks to efficiently allocate data pathways. PCMA then employs spread spectrum signal management to allow two-way data transmissions to share the same channel, by acting as a "traffic cop" in managing the back-and-forth traffic of different data streams.

Ka-band interference management is a major priority for ND Satcom. Its BBI Network Management System is the backbone of SES Astra’s Broadband Interactive Service (BIS). A two- way satellite broadband system provided to European consumers in the Ka-band, BIS offers up to 2 Mbps of data via satellite, a rate backed up by BBI’s triply redundant baseband systems, and spread spectrum Ka-band signal reception.

No Static At All?

At the end of the day, some satellite transmission problems are unsolvable. For instance, the 22,300 miles between the average earth station and a geostationary satellite in orbit means that some signal delay or latency is inevitable, because everyone is bound by the speed of light.

Still, intelligent link budget design, innovative transmission management tools, and properly aligned, well-engineered hardware can go a long way to reduce the problems of signal interference. It may not always be possible to cut through the static, but it is possible to live with it.

As Via Satellite’s senior contributing editor, James Careless has covered all aspects of the global satellite industry for more than six years.