Page 604 - 5G Basics - Core Network Aspects

2 Transport aspects

7.3.3 Multi-level modulation

There are many multi-level modulation schemes, for example, quadrature phase shift keying (QPSK), M-ary
quadrature amplitude modulation (M-QAM), discrete multi-tone (DMT), OFDM and so on. They serve to
increase the transmission capacity of a single channel provided that the signal-to-noise ratio (SNR) is high
enough. For an RoF signal, it is considered that multi-level modulation can be generally performed in both
the electrical domain and the optical domain.

In the electrical scheme, multi-level modulation is carried out when a modulating electrical signal is
generated. Figure 7-9 shows an example of multi-level modulation performed in the electrical domain. As
shown in Figure 7-9, at first, serial binary data is converted to parallel data for multi-level modulation with a
binary-to-multi-level converter. The converted data modulates an electrical carrier at fele with an electrical
I/Q modulator to generate a modulating electrical signal. The modulating electrical signal is put into an E/O
converter, shown in Figures 7-1 and 7-2, to generate a desired RoF signal as shown in Figure 7-9-a and -b,
respectively.

Bias E/O converter
f opt
f f opt
Modulating electrical signal ele + Light source RoF signal

Electrical I/Q modulator

Binary-to-multi-level
converter

Serial binary data a)

Bias E/O converter
f opt
f f opt
Modulating electrical signal ele + Light source External modulator RoF signal

Electrical I/Q modulator

Binary-to-multi-level
converter

G Suppl.55(15)_F7-9
Serial binary data b)

Figure 7-9 – Schematic block diagram of multi-level modulation performed in the electrical domain: a)
with direct modulation; and b) with external modulation

In the optical scheme, multi-level modulation is carried out with an optical I/Q modulator. Figure 7-10
shows an example of multi-level modulation performed in the optical domain. As shown in Figure 7-10, at
first, two-tone or multi-tone light is generated with a reference frequency of fele/2. A DEMUX extracts two
optical frequency components at fopt-fele/2 and fopt+fele/2, where fopt is the central frequency of optical
carrier. On the other hand, serial binary data is converted to In-phase (I) data and quadrature-phase (Q)
data for multi-level modulation with a binary-to-multi-level converter. The converted data modulates one
optical frequency component at fopt+fele/2 with an optical I/Q modulator. The modulated optical
component at fopt+fele/2 is combined with the other optical unmodulated component at fopt-fele/2 via a
MUX to generate a desired RoF signal.

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