Page 598 - 5G Basics - Core Network Aspects
P. 598
2 Transport aspects
7.1.2 External modulation
External modulation creates the desired optical signal (RoF signal) using an external modulator coupled
with a CW light source, as shown in Figure 7-2. Figure 7-2-a and -b represent the external modulation
schemes for subcarrier signal(s) transmission and for equivalent low-pass (baseband) signal(s) or digital
signal(s) transmission, respectively. Once again, a bias current or voltage may be added to the modulating
electrical signal for a linear operation if needed. The power, phase, frequency or state-of-polarization of
optical carrier can be modulated in proportion to the waveform of modulating electrical signal. Although
this scheme requires an external modulator in addition to the light source, the modulated optical signal will
be of higher quality, because the intrinsic imperfections of the light source are avoided.
t Bias E/O converter
t
f opt
f ele
Modulating electrical signal + Light source External modulator Modulated optical signal (RoF signal)
... ...
f f
0 f ele 0 f opt –2 f ele f opt – f ele f opt + f ele f opt +2 f ele
a)
Modulating electrical signal Bias E/O converter Modulated optical signal (RoF signal)
f opt
+ Light source External modulator
f f
0 0 f opt
b) G Suppl.55(15)_F7-2
Figure 7-2 – Schematic block diagram of external modulation: a) for subcarrier signal(s) transmission; and
b) for equivalent low-pass (baseband) signal(s) or digital signal(s) transmission
7.2 Optical-to-electrical conversion
7.2.1 Optical incoherent detection
Optical incoherent detection is a power detection scheme. The square-law function of photodetector is
used for detecting the envelope of received optical signal (RoF signal), which corresponds to the power of
optical signal. Therefore, if the optical signal power is modulated in proportion to the modulating signal,
then incoherent detection can recover the original signal, as shown in Figure 7-3. Figure 7-3-a and -b
represent the optical incoherent detection schemes for subcarrier signal(s) transmission and for digital
signal(s) transmission, respectively. In most cases, a bias voltage should be added to the output port of
photodetector for a linear operation. An electrical filter is used to extract only a desired electrical signal.
588