Page 590 - 5G Basics - Core Network Aspects

2 Transport aspects

an E/O converter in the optical transceiver. The generated downlink analogue RoF signal is transmitted over
the fibre-optic link. At the remote antenna end, the received downlink 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 downlink RF signal. For the uplink, a received
uplink RF signal modulates an optical carrier using another E/O converter in the optical transceiver. The
generated uplink analogue RoF signal is transmitted over the fibre-optic link. At the local office end, the
received uplink 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 RF-band subcarrier signal, is demodulated with
the RF-band demodulator to recover the uplink payload data.
In an IF-band RoF transmission scheme such as that shown in Figure 6-1-b, the system consists of an IF-
band modulator, an IF-band demodulator, a pair of optical transceivers, a fibre-optic link, an IF-to-RF up-
converter, an RF-to-IF down-converter, and a reference frequency generator. Because the IF is typically
much lower that the RF, the IF-band RoF transmission scheme offers a much improved optical bandwidth
efficiency compared to the RF-band RoF transmission scheme. For the downlink, an IF-band carrier is
modulated by downlink payload data using the IF-band modulator at the local office end. The generated
downlink IF-band subcarrier signal modulates an optical carrier using an E/O converter in the optical
transceiver. The generated downlink analogue RoF signal is transmitted over the fibre-optic link. At the
remote antenna end, the received downlink 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
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 modulates an optical carrier using another E/O converter in the optical
transceiver. The generated uplink analogue RoF signal is transmitted over the fibre-optic link. At the local
office end, the received uplink 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 IF-band subcarrier signal, is
demodulated using the IF-band demodulator to recover the uplink payload data. In an IF-band RoF and
reference frequency transmission scheme, such as that shown in Figure 6-1-c, the system configuration is
the same as that of the IF-band RoF transmission scheme shown in Figure 6-1-b, except that the reference
frequency is provided from the local office end and is delivered to the remote antenna site.

Since the main parts of the radio transceiver, such as electrical modulator and demodulator, can be located
at the local office end, the configuration of each equipment at the remote end becomes simpler. In some
cases (for example, those involving no change in the frequency band of interest), this architecture can
easily offer an upgrade to the latest radiocommunication service without any change of configuration at
the antenna site. Since the main part of the radio transceiver is located at the operator's site, it is also easy
to repair or renew the unit. This also offers a step towards a future optical access network (OAN), which
can realize higher-speed transmission and provide multi-granular bandwidth resources, such as an OAN
based on orthogonal frequency division multiplexing (OFDM). In the configurations shown in Figures 6-1-a
and 6-1-c, no reference frequency generator is required at the remote end, resulting in a simpler
configuration of remote equipment. In the configurations shown in Figure 6-1-b and -c, the reference
frequency value can be also decreased if a sub-harmonic mixing technique is used in the IF-to-RF and
RF-to-IF converters.

6.1.2 Equivalent low-pass (equivalent baseband) signal(s) transmission
Figure 6-2 illustrates general and fundamental architectures for transmitting orthogonal equivalent low-
pass (equivalent baseband) signals, such as (non-binary) in-phase and quadrature-phase (I/Q) baseband
signals. In Figure 6-2, it is assumed that equipment on the left side of the fibre-optic link is located in the
local office and equipment on the right side is located at the remote antenna.

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