Page 588 - 5G Basics - Core Network Aspects

2 Transport aspects

microwave photonics (MWP). The current implementations of RoF technologies are described in
[b-APT/ASTAP/REPT-03] and [b-APT/ASTAP/REPT-11].
Figure 5-1 shows the basic concept of an RoF system. In this Supplement, RoF is defined as the fibre-optic
transmission of a waveform for radiocommunication services without any intentional essential change to
that waveform during fibre-optic transmission. The waveform includes the essential physical information
for radiocommunication services, such as the format of the radio wave and payload. Note that the carrier
frequency of the radio signal will not affect processing at the baseband, and is thus considered nonessential
here. Therefore, the RoF signal should be regarded as an analogue signal carrying the same radio signal
when viewed from the optical domain, although the radio-frequency (RF) carrier frequency of the RoF
signal may be different from that of the original radio signal. As shown in Figure 5-1, the RoF system
consists of components for electrical-to-optical (E/O) and optical-to-electrical (O/E) conversions and of an
optical fibre for transmission. RoF has two major features as follows.

– Preservation of the waveform: the waveform of the radio signal is essentially preserved during the
fibre-optic transmission under ideal or close-to-ideal conditions.

– Tolerance to electromagnetic interference: RoF signals in the fibre are not affected by frequency
interference from the proximate radiocommunication signals.
From the technical point of view, a distribution technology for legacy RF video is considered to be a type of
RoF technology, but one which possesses only a downlink function. Since the RoF system should be
generally treated as an analogue transmission system, the overall signal-to-noise power ratio and the
overall dynamic range should be increased to maximize the potential of the two RoF features listed above
by properly managing the noise figure and nonlinearity of the system.
An alternative method of transmission is digital fibre-optic transmission. Digitized radio over fibre (D-RoF) is
a kind of digital RoF as explained in clause 6.2. The D-RoF technology is an alternative attractive candidate
for transmitting the waveform, especially in cases where both distortion and poor sensitivity hamper
analogue transmission under conditions of higher noise figure and nonlinearity. Here, we have to pay
attention to the fact that its realization strongly depends on the performance of the digital signal
processing (DSP) function, which is influenced by performance of analogue-to-digital converters (ADCs) and
digital-to-analogue converters (DACs). It is also difficult to remove quantization noise due to digitization,
which itself causes distortion in the radio waveform. Furthermore, each time domain sample is digitized to
many quantized bits for binary transmission in D-RoF, so the bandwidth efficiency of D-RoF can be much
lower than that of analogue RoF. Digital interfaces for mobile base stations, such as the Common Public
Radio Interface [b-CPRI] and Open Base Station Architecture Initiative [b-OBSAI], make good use of the
concept of D-RoF technology.

The same waveform

RF signal Lightwave modulated by RF signal RF signal

Optical fibre

RF signal source E/O conversion O/E conversion

No interference from
RF: radio frequency radio signal outside of fibre
E/O: electrical-to-optical
O/E: optical-to-electrical
G Suppl.55(15)_F5-1

Figure 5-1 – Basic concept of a RoF system

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