Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 220

for i(t) and q(t), any desired amplitude and phase can be generated for
iqmod(t).
The data stream data(t) is converted to the two modulation signals i(t)
for the I path and q(t) for the Q path by means of a mapper. This is shown
in Fig. 13.13. for QPSK modulation. The mapping table is the rule accord-
ing to which the data stream data(t) is converted to modulation signals i(t)
and q(t). In the case of QPSK, two bits (corresponding to bit 0 and bit 1 in
the mapping table) are combined to form a dibit. For dibit combination 10,
for example, the mapper outputs the signals i(t)= -1 V and q(t)= -1 V ac-
cording to the mapping table shown here.

Q

QPSK

I

I

+ iqmod(t)

q(t)

Q

90°

Carrier lo(t)

Fig. 13.11. IQ Modulator, I and Q Paths active and identical Amplitudes (QPSK)

The bit combination 11 yields i(t)=+1 V and q(t)= -1 V in this example.
The allocation of bits to modulation signals, defining how the bit stream is
to be read and converted by the mapper, is merely a matter of definition. It
is important that the modulator and the demodulator, i.e. the mapper and
the demapper, use the same mapping rules. Fig. 13.12. also shows that in
this case the data rate is halved after the mapper. QPSK can transmit
two bits per state. Two bits each are combined to form a dibit that deter-
mines the state of the mapper output signals i(t) and q(t). Therefore, in this
case, i(t) and q(t) have half the data rate of data(t). i(t) and q(t) in turn
modulate the carrier signal and, in the case of QPSK, switch it only in
phase. There are four possible constellations for iqmod(t): 45°, 135°, 225°
and 315°. The information is contained in the phase of the carrier. Now