8Psk-Tcm Coder - Safran CORTEX Series User Manual

With 4D-8PSK-TCM, the combination of convolutional coding, multiphase modulation and
multidimensional techniques offers a substantial power gain together with bandwidth conservation or
reduction, in comparison to their separate utilization as it is done frequently with binary or quaternary
modulations (i.e., sequential implementation of convolutional coding). The result is an improvement of
the performances in terms of BER versus signal to noise ratio for the same or better bandwidth
efficiency, compared with the uncoded (or even convolutionally coded) OQPSK or QPSK modulations.
Example: Assuming the bit rate of input data equal to 100 Mbps, the 4D-8PSK-TCM channel symbol
rate is 50 Msps for 2 bits/channel-symbol or 40 Msps for 2.5 bits/channel-symbol.
The performances of 4D-8PSK-TCM scheme can be substantially improved when concatenated with
Reed Solomon (R-S) on bytes block coding with a proper interleaving so as to ensure a good word
error splitting (at the output of the convolutional trellis decoder) and then good working conditions for
the R-S decoder.
3.3.7.2 4D-8PSK-TCM Coder
The 4D-8PSK Trellis-Coded Modulator consists of a serial to parallel converter, a differential coder, a
trellis encoder (convolutional coder), a constellation mapper and an 8PSK modulator (see Figure 19).
Note that in this figure, wi (with index i = 1, ... , m) represent the uncoded bits and xj (with index j = 0,
... , m) are the coded bits. The trellis encoder is based on a 64 state systematic convolutional coder
and can be considered as the inner code if an outer block code is introduced. Carrier phase ambiguity
is resolved by the use of a differential coder located prior to the trellis encoder. Spectral efficiencies of
2, 2.25, 2.5, and 2.75 bits/channel-symbol are achieved with four possible architectures of the
constellation mapper. The output switch addresses successively one of the four symbols ( Z(0) – Z(3) )
from the constellation mapper to the 8PSK modulator (which is in fact a QPSK modulator for the 2 first
efficiencies values).
The present standard is based on the use of a 4D-8PSK-TCM characterized by the following
parameters:
 Size of the constellation: M = 8 phase states (8PSK)
 Number of signal set constituents: L = 4 (shown as Z(0) ... Z(3) in Figure 19)
 Number of states for the trellis encoder: 64
 Rate of the convolutional coder used for the construction of the trellis: R = ¾
 Rate of the modulation: Rm = m/(m+1) selectable to 8/9, 9/10, 10/11, or 11/12
 Efficiency of the modulation:
 Reff = 2 bits per channel-symbol (for Rm = 8/9)
 Reff = 2.25 bits per channel-symbol (for Rm = 9/10)
 Reff = 2.5 bits per channel-symbol (for Rm = 10/11)
 Reff = 2.75 bits per channel-symbol (for Rm = 11/12)
In fact, the serial to parallel conversion is a byte (8 bits) to symbol (8 to 11 bits) conversion that will be
described later.
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Ref. DTU 100782
Is.Rev 3.5
Date: June 1, 2021
Safran Data Systems
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