1 Introduction:
The next generation of communication networks is anticipated to deliver significantly higher
data rates and reduced latency. To achieve these goals, a range of advanced wireless
technologies has emerged, including Coordinated Multipoint (CoMP) Transmission,
Distributed Antenna Systems (DAS), Millimeter Wave (MMW) communication, Software
Defined Radio (SDR), Software Defined Networks (SDN), and Cognitive Radio (CR). The
simultaneous coexistence of these technologies within communication networks creates what
is known as a heterogeneous network.
Figure 1: Generation of Millimeter Wave in Radio-over-Fiber (RoF) Transmission System
In parallel with advancements in wireless technologies, optical technologies have also
evolved. One promising solution for next-generation heterogeneous networks is the Radio-
over-Fiber (RoF) transmission system. Specifically, Millimeter Wave Radio-over-Fiber
(MMW-RoF) represents an integrated approach that combines both wireless and optical
technologies. The MMW-RoF transmission system benefits from utilizing both extremely
high-frequency millimeter waves and optical fiber for signal transmission.
The RoF system is typically composed of three main sections: the Central Station (CS), the
Base Station (BS), and the Optical Distribution Network (ODN), also referred to as the
Optical Fiber Network (OFN). Complex signal processing occurs in the CS, while simple
Optical-to-Electrical (O/E) and Electrical-to-Optical (E/O) conversions take place at the BS.
The ODN or OFN serves as the optical fiber link for transmitting signals in the RoF system.
Figure 1: RoF system topology by Zin[4].
The RoF system topology begins with the input of a radio frequency (RF) signal. This signal
is optically modulated using an optical modulator and a continuous wave laser source. The
modulated signal is transmitted via optical fiber, and at the receiving end, it is demodulated
and converted back into an electrical signal using a photodetector. Thus the radio frequency
output signal is extracted.
Millimeter waves (mm-waves) contains a spectrum of extremely high frequencies (EHF),
ranging from 30 GHz to 300 GHz. This wide spectrum allows for a large range of frequencies
to be distributed to mm-wave bands, enabling high data rates of up to 10 Gbps. Additionally,
spectrum distribution for mm-waves can be coordinated globally, making them ideal for
international telecommunications. Compared to microwave and ultra-wideband (UWB)
technologies, mm-waves offer the advantage of supporting higher bit rates and better
coordination for worldwide spectrum allocation. However, the coverage range for mm-wave
signals does not dynamically change, as these waves are limited by shorter transmission
ranges due to higher frequency. Despite this, international harmonization for mm-waves is
likely, creating the foundation for broader applications of mm-wave signals in future
communication systems. Mm-wave bands have been proposed for high-capacity wireless
systems using Radio-over-Fiber (RoF) technology. According to Beans, the 60 GHz
frequency band has attracted globally in telecommunications interest. This interest is
primarily due to the fact that the 60 GHz frequency matches with the oxygen absorption peak,
resulting in high atmospheric attenuation of over 15 dB/km.
High attenuation in millimeter wave (mm-wave) frequencies allows for the reduction in cell
sizes (picocells) [5] and the frequency reuse distance in cellular systems, thereby increasing
wireless capacity. The mm-wave frequency range also creates opportunities for global
standardization and commercial production. In North America, for instance, there are 7 GHz
of unlicensed spectrum that overlap with the unlicensed spectra around 60 GHz in Europe,
Japan, and Australia. The overall performance of a Radio-over-Fiber (RoF) system is
determined by the performance of its Central Station (CS), Base Station (BS), and Optical
Distribution Network (ODN). RoF transmission systems combine the advantages of both
optical fibers and extremely high frequencies (EHF), such as millimeter waves, to support
high data rates. Recently, mm-wave bands have been proposed for broadband wireless
systems using RoF technology [5]. Figure 2 depicts an overview of the subsystems involved
in the RoF link, by Weiβ [12]. The extremely high radio frequency carrier wave is generated
optically and is known as the photonic generation of millimeter waves. This optical carrier is
then used to modulate the transmitter's data. The optically modulated signal is transmitted
through a fiber-optic channel, and upon reception, the signal is detected and converted back
into an electrical signal. The millimeter wave frequency signal is then transmitted wirelessly.
Figure 2: Block diagram of RoF subsystems described by Weiβ[12].
The primary concern in the architectural design of telecommunication networks is ensuring
that the network is practically possible, less complex, and cost-effective. RoF systems need to
be designed with a focus on reducing costs and minimizing the operational complexities of
individual components such as the CS, BS, and ODN. However, certain challenges, such as
the requirement for high-speed optical components, complex mm-wave generation methods,
and a larger number of BS, must be considered as drawbacks of RoF systems. For an optimal
RoF design, simplicity in system architecture should be prioritized. Fiber Bragg Gratings
(FBG) are used to extract the light wave carrier from the received downlink signal, which can
then be reused for uplink transmission [11].
The first RoF design for mm-wave generation, proposed by Al Dabbagh [11], utilized a phase
modulator. The performance of this design was compared with RoF systems using a Mach-
Zehnder (MZ) modulator for optical modulation, highlighting different outcomes based on
the modulator choice.
