trellis encoding (TC)

Trellis coding is a coded, digital modulation method used in various high-speed modems according to V.32, V.32bis, V.32terbo, V. 34 and V.fast. In combination with other modulation techniques, trellis code modulation is also used in modern network concepts such as Gigabit Ethernet and in DSL procedures.

In the trellis method, data to be backed up is linked with a backup polynomial at the transmit end; redundancy bits are thus constantly inserted into the data stream. For example, in four-bit trellis coding, the four data bits are coded into five bits and transmitted, with the fifth bit being calculated from the four data bits and representing the backup polynomial. This bit serves redundancy purposes and links the temporal sequence of the transmitted data bits.

As a result, the signal transitionsare no longer free, but are subject to a certain regularity, with the help of which signal sequences containing errors are filtered out on the receiving side. Compared to pure quadrature modulation, the trellis method achieves a 3 dB better signal-to- noise ratio. Trellis coding is also used for ISDN.