Introduction Today, users are enjoying very fast

IntroductionToday, users areenjoying 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 connectionsince it calls for considerably less prerequisites when compared with wiredalternatives like xDSL and cable modem networks 86. As a result, especiallyin crowded city areas, the “last mile” portion of the networks is beingincreasingly implemented as wireless. It is statistically reported in severalcountries that personal computers are substantially widespread in rural areas.However, studies have shown that the majority of such computers are yetconnected to internet via slow networks and using outdated technologies8788.

Providingbroadband access to such areas using wired connections entails excessive costs.A viable alternative is wireless access. In this study, the focus is on theutilization of broadband wireless access networkfor deserted and difficult to access regions where it is necessary to supply amultitude 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 suggestedin 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 BWANsbased on RoF to resolve spectral congestion in sub-microwave frequencies andimprove efficiency 30, 33, 34, 38, 40, 42.CS?? isusually equipped with both Laser Diode (LD) and Photo Detector (PD) in most ofRoF systems which causes complexities in CS architecture 2940.

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 While Wavelength Division Multiplexing (WDM)is extensively applied to RoF architectures, it is restrictively used to facilitatethe link between CS and BSs 9, 29.Manyapplications 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 RadioFrequency (RF) signals over one wavelength and one fiber while maintainingperformance.

To achieve this, it is common to limit changes by making the RoFconnection using current PON system. This makes the integration of the linkwith current optical distribution network very simple. The architecture putforward in this study is based on a recent 100G-PON network with highloss-budget properties and without any amplifiers. Transmitters of class 10Gare helpful in accomplishment of a cheap and less complicated link. Such atransmitter calls for complex modulation and high spectrum efficiency such as4-level pulse amplitude modulation (PAM4). 1.1  Radio-over-FiberOpticalfiber connections are employed to transmit RF signals from head-end to otherstations called Remote Antenna Units (RAUs).

This technique is calledRadio-over-Fibre (RoF). RF signals in narrowband systems and WLAN, RF signalsare processed with transformations like multiplexing, frequency up-conversionand carrier modulation. After these processing is performed in base station orthe RAP, the signals are instantly transmitted to the antenna. Using RoF, the processing of RF signals can beconcentrated 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 0.5dB/km for 1310 nm wavelengths) when sending out RF signals to RAUs. RoF conceptis illustrated in Figure 1.

1.  Figure 1.1: TheRadio over Fibre System Concept In such anarchitecture, RAUs require to optoelectronically convert and amplify thesignals, and thus can be greatly simplified. Integrating RF signal processingtasks in one location allows for equipment to be shared, resources to beactively adjusted and the whole system to be easily operated and maintained.These advantages, particularly in the case of broadband wirelesscommunication systems with extensive coverage, mean much more efficient installation andoperation of the system 8.An early RoFarchitecture is shown in Figure 1.2. This system, for instance, can be employedfor the transmission of GSM signals.

LD in the head-end is modulated using theRF signal. The signal that is obtained from this is modulated over intensityand is sent to the BS (RAU) via the length of the fiber. There, the signal isdirectly detected in the PIN photodetector which results in recovery of thesignal. The antenna in the next step performs the amplification and radiationfunctions.

Similarly, the uplink signal is transmitted from the RAU to thehead-end.Figure 1.2 ????This technique is themost basic type of RoF link and is termed as Intensity Modulation withDirect Detection (IM-DD).Although RFsignal in Figure 1.5 is transmitted at its frequency, this is not generallyrequired. Using Local Oscillator (LO) signals, for example, the uplink carriercan be transformed in the RAU down to an IF. This lets low-frequency components to be applied to the uplink path in theRAU which result in lower costs. LO, instead of being located in the RAU, canbe sent from the head-end to the RAU by means of RoF.

Using such anarchitecture, the LO can be employed in order to down-convert the uplinksignals. In this type of system, with the RAU significantlysimplified, the role of downlink part of the RoF becomes very essential becauseit transmits high frequency signals. This is much more difficult as itnecessitates large link bandwidth and components with high frequency. In thisway, the signals are prone to be disturbed by dysfunctions in transmitter,receiver and transmission link signal. Other methods, involving signalfrequency up-conversion over distribution, are common as well which RemoteHeterodyning (RHD) and harmonic up-conversion, among others, are discussed inChapter 3.1.2  AdvantagesCompared to electronic distribution of signals,RoF systems boast some benefits that are discussed in the following.

1.2.1       Low Attenuation LossMicrowave signals with high frequency, whendistributed via electrical systems either in free space or using transmissionlines, 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 leadto larger impedance 11.

Thus, transmission of high frequency signals usingelectrical systems in long distances inevitably requires costly equipment forsignal recovery. Using transmission lines for transporting mm-wave signals isnot practical even in short distances. Instead, low intermediate frequency (IF)or baseband signals, may be transmitted from the head-end to the base station1. At each BS, such signals go through up-conversion to microwave or mm-wavefrequencies, amplification, and finally transmission. This architecture issimilarly utilized in narrowband mobile communicationsystems which can be seen in Figure 1.

3.Because of the requirement for high performanceLOs in each BS, this configuration results in complicated BSs withunsatisfactory performances. Nevertheless, with benefits of optical fibers suchas small loss, RoF systems are alternatives to realize both low-losstransmission 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.

2dB/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 offerattenuation 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 figuresare a few orders of magnitude smaller in higher frequencies. In 14, a ½ inchcoaxial cable (RG-214) working in over-5 Ghz region is reported to attenuate inthe range >500 dB/km. Thus, optical transmission of microwaves leads to manytimes higher distances and much lower powers.

1.2.2 LargeBandwidthThe bandwidthavailable by the optical fibres is tremendous.

Although three windows with lowattenuation are currently in use 15, the efforts are still going on toexplore much more from a single optical fibre. To obtain wider bandwidth many linesof investigation are pursued including the search for a fibre with lowdispersion, employing the Erbium Doped Fibre Amplifier (EDFA) in the 1550 nmwindow, and combining novel methods such as Optical Time Division Multiplexing(OTDM) with Dense Wavelength Division Multiplex (DWDM).There areother advantages for the use of optical fibres with great bandwidth. Theincreased bandwidth makes fast processing of signal possible, which can be morecomplicated or infeasible in electronic systems. This means that performing challengingmicrowave operations is readily available using optical techniques 16.

Forexample, to filter mm-waves, the electrical signal can first be converted andfiltered into an optical signal, then employing some optical devices like theMach Zehnder Interferometer (MZI) or Fibre Bragg Gratings (FBG) to performfiltering, and finally transforming the resulting signal back into electrical form17.In addition,inexpensive low bandwidth optical devices like modulators or laser diodes areavailable by processing with optical methods, while maintaining the ability tomanage high bandwidth signals 18. There are still limitations in electronicsystems regarding the bandwidth which seriously hinder the use of extensivebandwidth accessible in the optical domain and are called “electronicbottleneck”. This issue is usually resolved by effective multiplexing. Theaforementioned 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-CarrierMultiplexing (SCM) is used. In this method, multiple microwave subcarriers, modulatedwith either digital or analogue data, are merged. The result is employed tomodulate the optical signal, and to transmit it on a single fibre 19, 20. RoFtechniques are made cos-effective this way.

1.2.3 Easy Installation andMaintenanceRAUs in theRoF technology become simpler by placing the complicated and costly devices atthe head-end. The majority of RoF systems, for example, remove the LO and itsdevices at the RAU. This way, the RAU is composed of limited componentsincluding a photodetector, an RF amplifier, and an antenna. Modulationequipment and switching devices are located at the head-end and are sharedbetween multiple RAUs. RAUs in this architecture are smaller and simpler which resultsin lower costs to install and maintain the system. This is of criticalimportance 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 harmsas well.1.2.4 Reduced Power ConsumptionAsa result of simplifying RAUs, the power consumed by the system is decreased.

Manyif the high-end devices are placed at the central head-end. Sometimes, RAUs canwork in passive mode as in some 5 GHz Fibre-Radio systems using pico-cells. Decreaseduse of power is very important, especially when RAUs are located in difficultto access areas and have separate power sources.

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