Page 596 - 5G Basics - Core Network Aspects

2 Transport aspects

In an RF-band RoF relay transmission scheme such as that shown in Figures 6-5-a and 6-6-a, the system
consists of two pairs of RF-band filters, a pair of optical transceivers, and a fibre-optic link. For digital RoF,
moreover, an ADC and DAC pair is required. On the transmitter side, a received RF signal modulates an
optical carrier using an E/O converter in the optical transceiver. The generated analogue RoF signal is
transmitted over the fibre-optic link. On the receiver side, the received uplink 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 RF-band subcarrier signal, becomes the desired RF signal. In this way, RF signals in both
directions are repeated through the fibre-optic link.

In an IF-band RoF relay transmission scheme such as that shown in Figures 6-5-b and 6-6-b, the system
consists of two pairs of RF-to-IF and IF-to-RF converters, two reference frequency generators, a pair of
optical transceivers, and a fibre-optic link. For digital RoF, moreover, an ADC and DAC pair is required. On
the transmitter side, a received RF signal is frequency down-converted using the RF-to-IF down-converter
to an IF-band subcarrier signal. The generated IF-band subcarrier signal modulates an optical carrier using
an E/O converter in the optical transceiver. The generated analogue RoF signal is transmitted over the
fibre-optic link. On the receiver side, the received 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 IF-band
subcarrier signal, is frequency up-converted using the IF-to-RF up-converter and a reference frequency to
the desired RF signal. The characteristics of the reference frequency should be designed to be satisfied with
the frequency stability of the downlink RF signal. In this way, both directions of RF signal transmission are
repeated through the fibre-optic link.
In an I/Q baseband signals relay transmission scheme such as that shown in Figures 6-5-c and 6-6-c, the
system consists of two pairs of RF-to-I/Q and I/Q-to-RF converters, two reference frequency generators, a
pair of optical transceivers, and a fibre-optic link. For digital RoF, moreover, an ADC and DAC pair is
required. On the transmitter side, a received RF signal is frequency down-converted using the RF-to-I/Q
down-converter to I/Q baseband signals. The generated I/Q baseband signals modulate an optical carrier
using an E/O converter in the optical transceiver. The generated multi-level or analogue baseband signals,
which are a form of analogue RoF signal, are transmitted over the fibre-optic link. On the receiver side, the
received optical signals are optically detected using an O/E converter in the optical transceiver. The
detected electrical signals, which are the same as the modulating I/Q baseband signals, are frequency
up-converted using the I/Q-to-RF up-converter and a reference frequency to the desired RF signal. The
characteristics of the reference frequency should be designed to be satisfied with the frequency stability of
the downlink RF signal. In this way, both directions of RF signal transmission are repeated through the fibre-
optic link.

It is obvious that the RF-band RoF relay transmission scheme can achieve the simplest system architecture,
compared with system architectures for the other schemes. With the IF-band RoF relay transmission
scheme, the requirement on the frequency response of E/O and O/E converters can be reduced, and the
optical bandwidth efficiency of the analogue RoF system can be improved. Moreover, the I/Q baseband
signals relay transmission scheme has the advantage that the optical transmission bandwidth required for
repeating a radio waveform is minimized, while this relay transmission will increase the optical complexity
because optical I/Q modulation and optical coherent detection will be required. These schemes should be
adequately selected according to system requirements and installation conditions.

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