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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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