Introduction the utilization of broadband wireless access network


Today, users are
enjoying very fast network response together with being connected at all times,
which has caused a steady growth in the demand for broadband access. Broadband Wireless Access Network (BWAN) can be an economical option to supply a very high-speed connection
since it calls for considerably less prerequisites when compared with wired
alternatives like xDSL and cable modem networks 86. As a result, especially
in crowded city areas, the “last mile” portion of the networks is being
increasingly implemented as wireless. It is statistically reported in several
countries that personal computers are substantially widespread in rural areas.
However, studies have shown that the majority of such computers are yet
connected to internet via slow networks and using outdated technologies

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broadband access to such areas using wired connections entails excessive costs.
A viable alternative is wireless access. In this study, the focus is on the
utilization of broadband wireless access network
for deserted and difficult to access regions where it is necessary to supply a
multitude of Base Stations (BSs). Meanwhile,
each BS demands for far less data rate than highly occupied areas. In addition,
wireless networks using Radio-over-Fiber (RoF) instead of BWANs are suggested
in recent studies with the advantage of more efficient network design 12.
MM-wave bands can be used to implement BWAN in frequencies like 36 or 60 GHz.
Many studies have specifically focused on the use of mm-wave bands for BWANs
based on RoF to resolve spectral congestion in sub-microwave frequencies and
improve efficiency 30, 33, 34, 38, 40, 42.

CS?? is
usually equipped with both Laser Diode (LD) and Photo Detector (PD) in most of
RoF systems which causes complexities in CS architecture 2940.  While Wavelength Division Multiplexing (WDM)
is extensively applied to RoF architectures, it is restrictively used to facilitate
the link between CS and BSs 9, 29.

applications necessitate simultaneous usage of wired and wireless networking.
Although alternatives such as RoF systems and Passive Optical Networks (PON)
seem to be suitable, it is of high interest to send both baseband and Radio
Frequency (RF) signals over one wavelength and one fiber while maintaining
performance. To achieve this, it is common to limit changes by making the RoF
connection using current PON system. This makes the integration of the link
with current optical distribution network very simple. The architecture put
forward in this study is based on a recent 100G-PON network with high
loss-budget properties and without any amplifiers. Transmitters of class 10G
are helpful in accomplishment of a cheap and less complicated link. Such a
transmitter calls for complex modulation and high spectrum efficiency such as
4-level pulse amplitude modulation (PAM4).


1.1  Radio-over-Fiber

fiber connections are employed to transmit RF signals from head-end to other
stations called Remote Antenna Units (RAUs). This technique is called
Radio-over-Fibre (RoF). RF signals in narrowband systems and WLAN, RF signals
are processed with transformations like multiplexing, frequency up-conversion
and carrier modulation. After these processing is performed in base station or
the RAP, the signals are instantly transmitted to the antenna. Using RoF, the processing of RF signals can be
concentrated in one place (head-end) and the next step is using optical fiber.
This keeps the signal loss at low levels (0.3 dB/km for 1550 nm, and 0.5
dB/km for 1310 nm wavelengths) when sending out RF signals to RAUs. RoF concept
is illustrated in Figure 1.1.



Figure 1.1: The
Radio over Fibre System Concept


In such an
architecture, RAUs require to optoelectronically convert and amplify the
signals, and thus can be greatly simplified. Integrating RF signal processing
tasks in one location allows for equipment to be shared, resources to be
actively adjusted and the whole system to be easily operated and maintained.
These advantages, particularly in the case of broadband wireless
communication systems with extensive coverage, mean much more efficient installation and
operation of the system 8.

An early RoF
architecture is shown in Figure 1.2. This system, for instance, can be employed
for the transmission of GSM signals. LD in the head-end is modulated using the
RF signal. The signal that is obtained from this is modulated over intensity
and is sent to the BS (RAU) via the length of the fiber. There, the signal is
directly detected in the PIN photodetector which results in recovery of the
signal. The antenna in the next step performs the amplification and radiation
functions. Similarly, the uplink signal is transmitted from the RAU to the

Figure 1.2 ????

This technique is the
most basic type of RoF link and is termed as Intensity Modulation with
Direct Detection (IM-DD).

Although RF
signal in Figure 1.5 is transmitted at its frequency, this is not generally
required. Using Local Oscillator (LO) signals, for example, the uplink carrier
can be transformed in the RAU down to an IF. This lets low-frequency components to be applied to the uplink path in the
RAU which result in lower costs. LO, instead of being located in the RAU, can
be sent from the head-end to the RAU by means of RoF. Using such an
architecture, the LO can be employed in order to down-convert the uplink

In this type of system, with the RAU significantly
simplified, the role of downlink part of the RoF becomes very essential because
it transmits high frequency signals. This is much more difficult as it
necessitates large link bandwidth and components with high frequency. In this
way, the signals are prone to be disturbed by dysfunctions in transmitter,
receiver and transmission link signal. Other methods, involving signal
frequency up-conversion over distribution, are common as well which Remote
Heterodyning (RHD) and harmonic up-conversion, among others, are discussed in
Chapter 3.

1.2  Advantages

Compared to electronic distribution of signals,
RoF systems boast some benefits that are discussed in the following.

Low Attenuation Loss

Microwave signals with high frequency, when
distributed via electrical systems either in free space or using transmission
lines, bring complications and large expenses. In the case of free space,
absorption and reflection cause higher losses as frequency increases 5.
Losses are also intensified in transmission lines since higher frequencies lead
to larger impedance 11. Thus, transmission of high frequency signals using
electrical systems in long distances inevitably requires costly equipment for
signal recovery. Using transmission lines for transporting mm-wave signals is
not practical even in short distances. Instead, low intermediate frequency (IF)
or baseband signals, may be transmitted from the head-end to the base station
1. At each BS, such signals go through up-conversion to microwave or mm-wave
frequencies, amplification, and finally transmission. This architecture is
similarly utilized in narrowband mobile communication
systems which can be seen in Figure 1.3.

Because of the requirement for high performance
LOs in each BS, this configuration results in complicated BSs with
unsatisfactory performances. Nevertheless, with benefits of optical fibers such
as small loss, RoF systems are alternatives to realize both low-loss
transmission of mm-waves, and less complex RAUs.

Attenuation losses in the current glass
(silica)-based Single Mode Fibres (SMFs) fall in the range lower than 0.2
dB/km for the 1550 nm window and lower than 0.5 dB/km for the 1300 nm window.
Recently, Polymer Optical Fibres (POFs) have been made available that offer
attenuation values in the range 10–40 dB/km in the 500–1300 nm zones 12, 13.
Compared with other technologies such as coaxial cable, these figures
are a few orders of magnitude smaller in higher frequencies. In 14, a ½ inch
coaxial cable (RG-214) working in over-5 Ghz region is reported to attenuate in
the range >500 dB/km. Thus, optical transmission of microwaves leads to many
times higher distances and much lower powers.

1.2.2 Large

The bandwidth
available by the optical fibres is tremendous. Although three windows with low
attenuation are currently in use 15, the efforts are still going on to
explore much more from a single optical fibre. To obtain wider bandwidth many lines
of investigation are pursued including the search for a fibre with low
dispersion, employing the Erbium Doped Fibre Amplifier (EDFA) in the 1550 nm
window, and combining novel methods such as Optical Time Division Multiplexing
(OTDM) with Dense Wavelength Division Multiplex (DWDM).

There are
other advantages for the use of optical fibres with great bandwidth. The
increased bandwidth makes fast processing of signal possible, which can be more
complicated or infeasible in electronic systems. This means that performing challenging
microwave operations is readily available using optical techniques 16. For
example, to filter mm-waves, the electrical signal can first be converted and
filtered into an optical signal, then employing some optical devices like the
Mach Zehnder Interferometer (MZI) or Fibre Bragg Gratings (FBG) to perform
filtering, and finally transforming the resulting signal back into electrical form

In addition,
inexpensive low bandwidth optical devices like modulators or laser diodes are
available by processing with optical methods, while maintaining the ability to
manage high bandwidth signals 18. There are still limitations in electronic
systems regarding the bandwidth which seriously hinder the use of extensive
bandwidth accessible in the optical domain and are called “electronic
bottleneck”. This issue is usually resolved by effective multiplexing. The
aforementioned OTDM and DWDM methods are applied in digital optical systems. Similarly,
to improve the use of bandwidth offered by optical fibres in analogue systems, Sub-Carrier
Multiplexing (SCM) is used. In this method, multiple microwave subcarriers, modulated
with either digital or analogue data, are merged. The result is employed to
modulate the optical signal, and to transmit it on a single fibre 19, 20. RoF
techniques are made cos-effective this way.

1.2.3 Easy Installation and

RAUs in the
RoF technology become simpler by placing the complicated and costly devices at
the head-end. The majority of RoF systems, for example, remove the LO and its
devices at the RAU. This way, the RAU is composed of limited components
including a photodetector, an RF amplifier, and an antenna. Modulation
equipment and switching devices are located at the head-end and are shared
between multiple RAUs. RAUs in this architecture are smaller and simpler which results
in lower costs to install and maintain the system. This is of critical
importance for mm-wave systems in which many RAUs are usually necessary. Also,
remote RAUs are an issue that lead to high operational costs for maintenance 8,
11. Smaller number of RAUs alleviate this issue and reduces environmental harms
as well.

1.2.4 Reduced Power Consumption

a result of simplifying RAUs, the power consumed by the system is decreased. Many
if the high-end devices are placed at the central head-end. Sometimes, RAUs can
work in passive mode as in some 5 GHz Fibre-Radio systems using pico-cells. Decreased
use of power is very important, especially when RAUs are located in difficult
to access areas and have separate power sources.