TrellisCoded Modulation
A very successful method of reducing the power requirements without an increase in the requirements on bandwidth was published by Gottfried Ungerboeck in 1987 [Ung87]. Trellis coded modulation (TCM) allows the achievement of significant coding gains over conventional uncoded multilevel modulation without compromising bandwidth efficiency. TCM schemes employ redundant nonbinary modulation in combination with a finitestate encoder which controls the selection of modulation signals to generate coded signal sequences.
We start our lesson with an illustrative example of this method
of combining coding and modulation. Trellis encoder example with
an 8state finitestate machine driving a 3bit to 8PSK signal
mapper is shown in Fig. 1. We use the same notation as in [Ung87],
i.e. a denotes a complexvalued discrete channel signal transmitted
at modulation time nT. Uncoded 4PSK modulation is regarded as a
reference system.
Fig. 1 Realization of 8PSK code by means of systematic
convolutional encoder with feedback.
The encoder shown in Fig. 9 consists of two parts, the first of
which is a finitestate machine (FSM) with a total of eight states.
State of the FSM is defined by the contents of the delay cells.
The second part is called a signal mapper. Its function is a memoryless
mapping of the three bits (y_{n}^{2}, y_{n}^{1},
y_{n}^{0}) into one of the eight symbols of an 8PSK
signal set. The FSM accepts two bits (x_{n}^{2},
x_{n}^{1}) at each symbol time n and moves from
a state S_{n} to one of four successor states S_{n+1}.Assuming
that the encoder in Fig. 1 operates continuously, there are four
choices at each time n, which allows us to transmit two information
bits (x_{n}^{2}, x_{n}^{1}) per
symbol, the same as the reference uncoded 4PSK system.
