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.
Email
Comments
Print
Reuse
