Trellis Coding

Trellis coding is the process of altering the QAM constellation to provide better performance in a noisy environment. Trellis uses a coding algorithm that allows modems to compensate for minor phase or amplitude variations that are caused by noise on the transmission path. The combination of modulation technique and code allows operation at a higher data rate for a given SNR than could be accomplished without the Trellis code. The net coding gain (equivalent improvement in noise margin) from Trellis encoding is usually estimated to be about 3 dB, although theoretically it could be slightly higher.

Trellis coding is less effective when noise is random, such as erratic impulse spikes, or when a high level of white noise (random noise) is present. Trellis coding can differentiate between correct and incorrect symbols by establishing a pattern or history of the effects of an interfering noise source. Therefore, Trellis coding is more effective when noise has recurring, predictable patterns.

A significant advantage of Trellis coding is that it does not expand bandwidth requirements or increase transmit power requirements to maintain acceptable error rates. Trellis coding does require more complex transceivers and Trellis-capable chipsets might have a slightly higher internal power requirement.

It is often debated whether Trellis adds a significant amount of latency to the transmission path. The general view is that although a small amount of data buffering is required for the coding/decoding process, the newer hardware designs have fast processing speeds that minimize latency. The added latency, although theoretically present, is considered negligible in present designs.

The end result of using Trellis coding/decoding is that you transmit at faster line rates with lower error rates, thus providing faster overall throughput in a moderately noisy environment. Trellis coding offers no advantage in a noise-free environment. The benefit or liability of turning on Trellis coding has to be compared to the achieved coding gain, and whether improved transmission quality is needed in a particular situation. DSL transmission on longer telco loops is generally improved by using Trellis coding.

When RS FEC and Trellis coding are both used at the same time, the coding gain is not simply additive. That is, if 3 dB is gained by using RS alone, and 3 dB is gained by using Trellis alone, the combined coding gain is not 6 dB. Using both systems is ultimately more effective than using only RS or Trellis, so it is generally recommended that both be used in DSL service when supported by the system hardware and software.

A variation of Trellis coding, Overlapped pulse amplitude modulation (PAM) Transmission with Interlocking Spectrum (OPTIS), was recently developed for HDSL version 2 (HDSL2). This transmission technology creates a more noise friendly power spectral density for both the upstream and downstream frequencies. It also reduces crosstalk onto other services through the use of the interlocking upstream and downstream frequencies. It uses Trellis-coded PAM(TC-PAM) to create a flexible PSD that boosts the upstream power in the 200-300 kHz range, at the same time notching the downstream power at the same point. This is done so that the transmit PSD is highest in the region where the receiver has the best SNR.

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