Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 344

Fig. 20.7. shows the case of QPSK embedded in 64QAM. The LP informa-
tion is carried by the discrete constellation point and the HP is described
by the quadrant. A cloud of 8 times 8 points in a quadrant as a whole thus
corresponds virtually to the total constellation point of the QPSK in this
quadrant.

64QAM, =1 64QAM, =2 64QAM, =4

16QAM, =1

16QAM, =2 16QAM, =4

Fig. 20.8. Possible Constellations with hierarchical modulation

A 64QAM modulation enables 6 bits per symbol to be transmitted.
However, since the quadrant information, as QPSK, diverts 2 bits per
symbol for the HP stream, only 4 bits per symbol remain for the transmis-
sion of the LP stream. The gross data rates for LP and HP thus have a fixed
ratio of 4:2 to one another. In addition, the net data rates are dependent on
the code rate used. QPSK embedded in 16QAM is also possible. The ratio
between the gross data rates of LP and HP is then 2:2. To make the QPSK
of the high priority path more robust, i.e. less susceptible to interference,
the constellation diagram can be spread at the I axis and the Q axis. A fac-
tor Α of 2 or 4 increases the distance between the individual quadrants of
the 16QAM or 64QAM diagrams. The higher Α is, the more insensitive the
high priority path becomes and the more sensitive the low priority path be-
comes since the discrete constellation points move closer together. Fig.
20.8. shows the 6 possible constellations with hierarchical modulation, i.e.