Page 52 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 2
The following conclusions can be drawn from the
figure:
– The value of the RF subcarrier in the first option
is significantly higher in comparison with the
second one, which tightens the requirements for
the electronic and optoelectronic components of
the DU and RU equipment and, as a result, their
cost.
– The option with transmission in the RF band is
realized with the least number of transformations
on the RU, which minimizes its cost, and,
consequently, the cost of the fronthaul network
equipment.
However, the fiber-optic transmission of the
MMW-band signals has a serious limitation due to
the dispersion effect of a standard single-mode
optical fiber [15]. Namely, according to the
conducted modeling experiments, in which the
options for the transmission of optical signals with
quadrature amplitude modulation (QAM) at RF of
40 GHz and IF of 15 GHz are compared [16, 21, 25]
the maximum permissible distance between the
DU and RU, even at such a relatively low MMW
frequency, decreases by more than 4 times
compared to transmission at the IF, reaching units
of km, which is not always provided in real C-RAN
fronthauls. It is understood that the situation will
become much more critical for B5G NG-RANs using
RF subcarriers above 100 GHz.
That’s why we believe that the optimal approach
would be IF-over-Fiber transmission in spite of this
option requires an interface at the RU that has to
perform RF conversion. Nevertheless,
transmission in the IF-band provides flexibility, as
there is a simple possibility of RF conversion at a
RU, both in the FR1 and in the FR2 (see
Introduction).
To effectively implement it in the downlink of Fig.
1 (b), a MMW frequency up-converter is used in the
RU scheme, an important element of which is a
reference oscillator (RO), the optimal design
principle of which will be investigated in Section 4.
3. MICROWAVE PHOTONICS APPROACH
TO DESIGN ELECTRONICS
APPARATUSES
Microwave photonics is a multidisciplinary
scientific branch combining optical, RF,
microwave, MMW, and terahertz fields [15, 26, 27
and refs. cited therein]. This field in the last 30
years has attracted immense interest and
38 © International Telecommunication Union, 2021
generated many new R&Ds from both the scientific
community and the commercial sector. Emerging
applications for a mobile communication network
of RoF architecture, sub-terahertz wireless
systems, radar, and electronic warfare systems
indicate that MWP is a subject of importance. By
common opinion, MWP opens the way to super-
wide bandwidth characteristics at lower size,
weight, and power as compared with traditional
means. Fig. 2 depicts typical arrangements of MWP
subassembly. As it follows, a photonic processing
unit (PPU) is inserted between two optoelectronics
modules that are electrical-to-optical (E/O) and
optical-to-electrical (O/E) converters. The first one
usually includes a semiconductor laser source
(SLS) and electro-optic modulator (EOM) as E/O
converter. The second one typically contains a pin-
photodetector (PD) followed by a low-noise
electronic amplifier (EA). Typical functions of the
PPU are transmission, switching, distribution,
filtration, time-delaying, and up/down frequency
conversion of RF signals in an optical domain. An
example of the last operation in relation to the RF
multiplier circuit including in the reference
oscillator of 102 GHz for the NG-RAN RU (see Fig.
1 (b)) will be discussed in Section 4.
E/O O/E
SLS EOM PPU PD EA RF
RF
Fig. 2 – Conceptual diagram of MWP subassembly (Optical
connections are painted in red, electrical connections in
black)
4. COMPARATIVE COMPUTER-AIDED
DESIGN OF MMW FREQUENCY
MULTIPLIER
In the section, we propose and describe two
alternative schemes for implementing a frequency
multiplier from 4.25 to 102 GHz using an MWE or
MWP approach. The goal of the simulation is to
select an optimal design concept for developing a
high-order RF multiplier of NG-RAN’s reference
oscillator using the same input and output powers
and frequencies. For both cases, Cadence AWRDE
software with an additional introduction of the
previously proposed models of optoelectronic
devices is implemented. To ensure the practical
orientation of the simulation experiments, the
parameters of the active and passive schematic
elements are selected based on the data sheets of
the specific manufactured devices. Table 1 lists the
common reference data for the simulation.