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2 Transport aspects
digital IF-band modulator at the local office end. The generated downlink IF-band D-RoF signal is
transmitted over the fibre-optic link. At the remote end, the received downlink D-RoF signal is optically
detected using an O/E converter in the optical transceiver. The detected electrical signal, which is the same
as the modulating digitized IF-band subcarrier signal, is D/A converted to generate the desired downlink IF
signal. Then, the downlink IF-band signal is frequency up-converted using the IF-to-RF up-converter and a
reference frequency to the desired downlink RF signal. The characteristics of the reference frequency
should be designed to be satisfied with the frequency stability of the downlink RF signal. For the uplink, a
received uplink RF signal is frequency down-converted using the RF-to-IF down-converter to an IF-band
subcarrier signal. The generated uplink IF-band subcarrier signal is A/D converted and then the generated
digitized IF-band subcarrier signal modulates an optical carrier using another E/O converter in the optical
transceiver. The generated uplink IF-band D-RoF signal is transmitted over the fibre-optic link. At the local
office end, the received uplink D-RoF signal is optically detected using another O/E converter in the optical
transceiver. The detected electrical signal, which is the same as the uplink digitized IF-band subcarrier
signal, is digitally demodulated using the digital IF-band demodulator to recover the uplink payload data.
In a digitized I/Q baseband signals transmission scheme such as that shown in Figure 6-4-c, the system
basically consists of a digital I/Q modulator, a digital I/Q demodulator, a pair of optical transceivers, a fibre-
optic link, a DAC, an ADC, an I/Q-to-RF up-converter, an RF-to-I/Q down-converter, and a reference
frequency generator. For the downlink, the digital I/Q modulator generates digitized I/Q baseband signals
from the downlink payload data at the local office end. The generated downlink digitized I/Q baseband
signals modulate an optical carrier using an E/O converter in the optical transceiver. The generated
downlink baseband D-RoF signals are transmitted over the fibre-optic link. At the remote end, the received
downlink D-RoF signals are optically detected using an O/E converter in the optical transceiver. The
detected electrical signals, which are the same as the modulating digitized I/Q baseband signal, are D/A
converted to generate the desired downlink I/Q baseband signals. Then, the downlink I/Q baseband signals
are frequency up-converted with the I/Q-to-RF up-converter and a reference frequency to the desired
downlink RF signal. The characteristics of the reference frequency should be designed to be satisfied with
the frequency stability of the downlink RF signal. For the uplink, a received uplink RF signal is frequency
down-converted using the RF-to-I/Q down-converter to an I/Q baseband signals. The generated uplink I/Q
baseband signals are A/D converted and then the generated digitized I/Q baseband signals modulate an
optical carrier using another E/O converter in the optical transceiver. The generated uplink D-RoF signals
are transmitted over the fibre-optic link. At the local office end, the received uplink optical signals are
optically detected using another O/E converter in the optical transceiver. The detected electrical signals,
which are the same as the uplink digitized I/Q baseband signals, are digitally demodulated with the digital
I/Q demodulator to recover the uplink payload data.
From the above, these digitized radio signal(s) (D-RoF) transmissions require DACs and ADCs.
6.3 Relay transmission (repeater)
Figures 6-5 and 6-6 illustrate general and fundamental architectures for relay transmission (repeater) for
analogue and digital RoF systems, respectively. Both subcarrier and equivalent low-pass signal transmission
are possible as the relaying signal over a fibre-optic link. In Figures 6-5 and 6-6, it is assumed that
equipments at both ends of the system are equivalent and play the same role for both directions of
transmission of the RF signals.
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