Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 224

By IQ mixing, the baseband signals i(t) and q(t) are retrieved. The car-
rier harmonics superimposed on these signals have to be eliminated by
means of a lowpass filter before the signals are applied to the demapper.
The demapper simply reverses the mapping procedure, i.e. it samples
the baseband signals i(t) and q(t) at the middle of the symbol and so recov-
ers the data stream data(t).
Fig. 13.16. illustrates the processes of IQ modulation and demodulation
in the time domain and in the form of constellation diagrams for the QPSK
method. The signal in the first line represents the input data stream data(t).
The second and the third line show the signals i(t) and q(t) at the modula-
tion end. The fourth and the fifth line are the voltage characteristics after
the I and the Q mixer of the modulator, the sixth line the characteristic of
iqmod(t). The phase steps between the symbols are clearly visible. The
amplitude does not change (QPSK). In the last line, the corresponding con-
stellation diagrams are shown. Lines 7 and 8 show the digitally recovered
signals i(t) and q(t) at the demodulation end. It can be seen that, in addition
to the baseband signals, the traces contain the carrier at double the fre-
quency. The latter has to be eliminated both in the I and the Q path by
means of a lowpass filter prior to demapping. In the case of analog mixing,
harmonics would be superimposed in addition which would also be sup-
pressed by the lowpass filters.
data(t)
i(t)
q(t)
imod(t)
qmod(t)
iqmod(t)
idemod(t)
qdemod(t)
vector(t)
Fig. 13.16. IQ modulation and demodulation (mapping table different to examples
before)