Altera Viterbi Compiler - Manual (Page 20)

Trellis Coded Modulation
Trellis coded modulation (TCM) combines modulation and encoding processes to achieve better
efficiency without increasing the bandwidth.
Bandwidth-constrained channels operate in the region R/W > 1, where R = data rate and W = bandwidth
available. For such channels, digital communication systems use bandwidth efficient multilevel phase
modulation. For example, phase shift keying (PSK), phase amplitude modulation (PAM), or quadrature
amplitude modulation (QAM).
When you apply TCM to a bandwidth-constrained channel, you see a performance gain without
expanding the signal bandwidth. An increase in the number of signal phases from four to eight requires
approximately 4dB in additional signal power to maintain the same error rate. Hence, if TCM is to
provide a benefit, the performance gain of the rate 2/3 code must overcome this 4dB penalty. If the
modulation is an integral part of the encoding process and is designed in conjunction with the code to
increase the minimum Euclidian distance between the pairs of coded signals, the loss from the expansion
of the signal set is easily overcome and significant coding gain is achieved with relatively simple codes.
Any bandwidth-constrained system benefits from this technique, for example, satellite modem systems.
The TCM Viterbi decoder only supports N = 2 (only mother code rates of 1/2).
Half-Rate Convolutional Codes
A 1/2 rate convolutional code encodes one information bit and leaves the second information bit
uncoded.
Figure 3-2: Half-Rate Convolutional Code
With an eight-point signal constellation (e.g. eight-PSK), the two bits select one of the four subsets in the
signal constellation. The remaining information bit selects one of the two points within each subset.
Uncoded Bit
Input
c
2
c
0
GB
GA
c
1
3-2 Trellis Coded Modulation
UG-VITERBI
2014.12.15
Altera Corporation Viterbi IP Core Functional Description
Send Feedback