STM isn't the only game in town when it comes to turbo codes. Advanced Hardware Architectures has been evangelizing the technology for a number of years, with some success from companies like Comtech, in Canada. In fact, Comtech recently won a Canadian government contract based on its use of AHA's turbo product-code (TPC) technology.
According to Dave Williams, product marketing manager at AHA, "We're doing iterative decoding based on block codes, which doesn't have an error floor, thereby giving us BERs beyond what we can simulate." He continues, "Turbo convolutional codes (TCCs) [such as those used by STM] tend to be expensive, but the cost can be hidden within an overall system solution. In addition, TPC technology is not under license."
AHA has recently brought out its Astro-OC3 TPC encoder/decoder, which is capable of 155-Mbit/s data rates (Fig. 4). At the heart of the device is the soft-in-soft-out decoder, which allows the system to say that it reserves the right to change its answer pending further information. The likelihood of whether or not the answer is a one or a zero is determined iteratively.
Key specifications also include an encode latency of less than 10 clocks, flexible coding rates from 0.25 to 0.98 (essential in instances such as poor weather), as well as 32-bit CRC insertion and checking with programmable packet length.
Because turbo codes are relatively new, with respect to availability, silicon is hard to come by. There are, however, a number of things to look out for when evaluating them, including how real it actually is, the data rate, flexibility, latency, and error floors. Many researchers have tried to figure out TCC's error floor, but it's too complex an issue. All they can do is run live data and check for errors. No mathematical models exist. The problem stems from the fact that TCCs are data dependent. Under many circumstances, error floors can just crop up. One advantage TPCs have, though, is that the technologies have varying strengths according to the application. For instance, TCCs show slightly better performance at higher BERs and at lower code rates.
Regardless of whether TPCs or TCCs are used, turbo codes will become an integral part of satellite-system design in the coming years, and they won't stop there. Any application that demands optimal use of bandwidth, such as wireless LANs, will be a prime target for the technology.
The rapid development of satellite systems over the past number of years has not taken place in a vacuum, but in fact reflects and contributes to many cultural changes that also have taken place. The fall of the Berlin Wall, for example, opened the floodgates to the East-West cooperation that's so essential to reducing the overall cost of getting the satellite infrastructure in place to begin with. American companies are launching satellites from Russian-controlled territory, and vice-versa.
The improved communication that the first wave of satellite systems brought about helped cement the bonds of cooperation, as nothing breaks down barriers like open communication. This is a self-propagating phenomenon, with the latest example being the push from within China to join the global community. As companies like Globestar connect the most remote parts of China's regions to the outside world, first by voice and later through data on the Internet, the effects on the world community can barely be imagined. Now could be a good time to begin learning Mandarin.