III. Gm-C Filtering - Epublications - Université de Limoges

I.1.b.ii Digital modulations
In digital systems, the carrier is modulated by a digital baseband signal. Modulations
similar to the analog ones are possible, modulating the amplitude (ASK for Amplitude Shift
Keying), the phase (PSK for Phase Shift Keying) or the frequency (FSK for Frequency Shift
Keying). Time representations of such modulations are given in Figure 5.
Figure 5. ASK (a), FSK (b) and PSK (c) modulations
In many applications, the binary data stream is divided by sets of two (or more) bits
and is modulated using a single carrier. As said in [I.1], this is possible because sine and
cosine are orthogonal functions so each set of two bits has a single representation in this
space. Such modulation is called IQ modulation, I and Q standing for In-phase and
Quadrature-phase.
The major advantage of IQ modulation is that symbol rate becomes half the bit rate.
Hence, the required bandwidth is divided by two, which makes it a very popular solution.
As it may be seen in Figure 6, assuming X and Y are two simultaneous bits forming a
set and ωc is the carrier frequency, the output S(t) is then:
( ω t)
Y sin(
ω t)
)
S( t)
= X cos c +
c . (I.3)
Figure 6. I-Q modulation
Nowadays, most used IQ modulations are QAM and QPSK. QAM actually consist in
coding the signal with different amplitudes: for instance, +A and –A for each I and Q path.
Such coding is called 4QAM. PSK uses phase to code the signal, as depicted in Figure 7 for
example.
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