Radio Over Fibre or Fibre Wireless Systems

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Introduction

Technology has undergone and continues to undergo dynamic revolution every day. The world has moved from analogous technological equipments to digital equipments, and this has revolutionized the communication sector, as well. The Internet is the other feature of today’s communication systems, which has made globalization a reality. The world has become one big village connected from every corner via the Internet and this scenario enhances communication. With the increasing rate of urbanization and general modernization, more and more people are finding themselves in need of the Internet, which translates to rampant demand for the Internet distribution systems. The existing systems of transmission are no longer sufficient to meet these demands efficiently, which is why this paper is proposing Radio over Fibre, (ROF) technology as the most prudent approach in satisfying these demands. It does this by discussing several articles on this subject and then making recommendations for future research projects on the matter in question.

Radio-over-fibre technologies have numerous advantages including their capacity to be multifaceted. They are usable for different purposes simultaneously including the provision of unregulated access to broadband wireless communications for different communication applications such as in the last mile solutions, for the extension of radio coverage and capacities, and back haul. With the imminent revolution of technology, the world needs systems that are much faster than the present technology. Moreover, the world population is on the rise, which means more users, hence, higher demand. Presently, Wireless LAN is capable of transmitting data of up to 54 Mbps at 2.4 GHz and 5GHz. WiMAX, on the other hand, is under modification to make it capable of transmitting from 2-66 GHz (Opatic 2011, p. 5). In creating efficient systems, innovators should take into account factors such as the losses that are incurred by high-operating systems in an indoor set up due to the loss to the building walls. With this in mind, they should come up with radio cells that are smaller in size and operating frequencies that are higher than existent ones. These optical fibres are used in the transmission of radiofrequency (RF) signals to remote areas. Recently, they have been modified to minimize the cost of installation in remote sites (RS), as well as their visual impact on the site of installation. They also ensure the least levels of loss in terms on energy and space required for installation. Moreover, they reduce the burden of searching for a site for installation, and by extension, the cost of leasing such a site. These fibres are also small in size, and exceptionally light. Their minimum weight makes them highly portable for mobility purposes. They have unregulated bandwidth characteristics, which mean that mobile terminals which are located indoors can utilize this technology if they require fast, multimedia services (Al-Raweshidy 2010, p. 13). Their cable is affordable making them even more attractive in terms of saving costs and increasing profitability. The fibres are not sensitive to electromagnetic radiation, and so they could be set up anywhere without adverse effects of the surrounding’ radiations. Another advantageous feature of these fibres is their immunity to fading and physical security.

The proposed radio over fibre optical link would replace the existing copper coaxial cables as the connection between a radio-based station (RBS) and the antenna, which is normally located in an extremely remote site. The optical fibre transmits the signals in different wavelengths within the fibre, and then at the antenna, there is equipment to re-engineer these signals back into radiofrequency signals through various processes. The radio over fibre technologies would reduce the processes necessary for the re-engineering of signals back into radiofrequency states to as few as two. These are amplification and optoelectronic conversion. The RoFs have wavelength diversity, which refers to their reusable capacity. It is possible to reapply them in different areas of the same building using the original communication equipment and similar wavelength. This is a cost-effective feature, which most organizations that are in need of indoor communication systems favour (Ng’oma 2005, p. 67). They satisfy user mobility, and finally, they can propagate large amounts of data depending on traffic demands. This feature demonstrates a landmark differentiation from existing communication systems that can only allocate tens of gigabits (Fibre Optic LANs) or several tens megabits (mobile users) at a go, thereby leading to slow internet, or other inconveniences for clients. In contrast, these systems are capable of transmitting terahertz of bandwidth.

They work by transmitting regulated microwave signals across the link or network from central office, or exchange (CO) to the remote site, (RS) or remote antenna units (RAU). These microwave signals need to be present at the input end of the RoF whereby they are transformed into electric signals for the transmission stage, and finally, at the remote terminus, there is equipment that can be used to reconvert the signals back to radiofrequency mode. The central office mentioned above is one of the modifications present in RoFs that make them cost effective and user friendly. It is possible to centralize them in a remote locality so that they are the main supplier for the local remote site. However, for this to happen, several measures need to be set in place. RoFs are multipurpose. Besides their use in transmission and in enhancing mobility, these systems are also usable in modulating data, processing signals, and converting frequencies (up and down). Some of the signals they process include base band data, modulated Ifs and actual modulated RF signals. To deal with the issue of remoteness, these systems are multiplex, which means that they can transmit several bundles of data in different directions in one fibre simultaneously. They are, therefore, better than simplex systems that transmit data in one fibre and one direction, or duplex systems, which can only transmit in two directions simultaneously in one fibre. They have de-multiplexing traits, which enable them to regulate and merge several wavelengths for transmission through one fibre, which ensures that they do not waste any of the available bandwidth. The process of transmitting radiofrequency signals over optical fibre is known as Intensity Modulation with Direct Detection (IMDD).

RoFs being a new technology inadvertently require a different architectural design. However, they comply with the generally accepted standard of transmission. They feature a central office, and an optic fibre link or network that connects the central office with the remote area units. However, specific wireless applications have customized architectural designs. For instance, GSM has as their central office a Mobile Switching Centre (MSC) and their RAU is a remote site, (RS). On the other hand, Wireless Local Area Networks have the head end as their central office and this links with a base station, (BS) using optical fibres.

Cost-effective nature

Transporting microwave signals through electric transmission is extraordinarily expensive. One has to consider losses that this form of the system is susceptible to, because of reflection or absorption, which escalates with an increase in frequency in free space. The answer to this issue would be to use commercial single mode fibres (SMFs), which are made from glass, silica, and have minimal attenuation losses. Another available option is Polymer Optical Fibres, POFs.

Unregulated (large) bandwidth

Optical fibres have the capacity of more than 45 THz of data in bandwidth yet the existing systems only use 1.6Hz. However, this should soon change because improvements are underway that will ensure that optical fibre phases out the existing systems. Some of the factors that are going to make this possible include the fact that low-dispersion fibre is now available in the form of Erbium Doped Fibre Amplifier (EDFA). It is also enhanced by using multiple techniques such as Optical Time Division Multiplexing (OTDM) and the Dense Wavelength Division Multiplex techniques among others (Rahman, Lee, Youngil, & Ki-Doo 2009, p. 426). The result of these improvements is the high capacity of transmission of microwave signals, and efficiency during the processing of signals as previous electronic processes such as up- and down- converting can be carried out optically. However, it is difficult to utilize the large capacities of bandwidth that are made available by optical systems because they cannot be sustained by the electric system, which happens to be the transmitter. This situation is known as “an electronic bottleneck” and to solve the same, it is necessary to multiplex.

Immunity to radiofrequency interference

Optical fibres do not get affected by surrounding electromagnetic waves. This implies that it can be implemented at any location without fear or doubt. During microwave transmissions, signals are transmitted in the form of light. This feature also prevents unauthorized access of data in transit and so it is more secure.

Easy installation and maintenance

Optical fibres are easier to install and maintain because, in Radio over Fibre systems, the complex equipment and hardware are located at the head-end or the centralized offices. Therefore, the Remote Area Units are easier to manage as they are only minimally equipped to carry out one or two transmission processes such as maybe amplification and optoelectronic conversion. The result is that, at the RAU, the only equipments necessary for processing received signals are a photo detector, a radiofrequency amplifier, and an antenna. It is necessary to cut costs because though cheap to maintain, RAUs are required in large numbers and so it is prudent to recover costs in the installation and maintenance sector, and then transfer these funds to the installation of more RAUs. Finally, the smaller the remote area unit, the less adverse effects on the environment, and so to be environmentally friendly, they should be built small.

Less energy consumption

Because the head-end is centralized, remote area units have few processes to run, and so utilize minimal power or energy because there are also few types of equipment at the RAU. Moreover, since they are mostly located in far-removed areas, this element is a merit because, probably such a location does not benefit from the power grid.

Multipurpose

Radio over Fibre systems is highly flexible, and they offer various options to any user. They are tolerant to chromatic dispersion as another trait that fosters flexibility, and it is necessary to acknowledge this as innovators. It is also possible to be dynamic about allocating resources. For instance, a shopping mall would require more power and allocation during off peak hours, and ensure that we satisfy this need.

However, these systems have several shortcomings that would require correction before they can attain optimal effectiveness. These limitations include amplified spontaneous emissions (ASE), noise, distortion, non-linearity, multipath propagation, pulse dispersion, and inter-symbol interference. It is necessary to note that RoFs are analogous and so anything that affects analogue systems is relevant to them. It is not surprising; therefore, that noise should be an issue (Al-Raweshidy 2010, p. 6). The causes of this noise are plenty, and they may include the laser’s Relative intensity noise (RIN), phase noise, the shot noise that photodiode causes, thermal noise from the amplifier, and dispersion in the fibre. This scenario is not a particularly promising condition as is evidenced by Single Mode Fibres (SMF). Here, chromatic dispersion adversely affects fibre length and causes de-correlation of phases. This phenomenon is another source of noise. Noise should be at minimum, especially in mobile communication devices.

Dispersion

This refers to the widening of the pulse duration as it is propagated through a fibre. The problem arises when it extends so much as to interfere with adjacent bits or pulses, causing inter-symbol interference. This sets limits on the amount of spacing allocated to each bit while maximizing the transmission rate of the channel. Inter-mode dispersion refers to the scenario whereby numerous modes transmitting the same signal travel at a different speed in the same fibre. The disruption caused by this state can lead to distortion of data in transition. Material or chromatic dispersion’s basis is the dispersive medium. It is affected by the refractive index (differs according to the medium through which transition is occurring, e.g. from water to air, through water or air, or ice or rocks.) in the event of the occurrence of multiple wavelengths in a single signal, it is obvious that some will travel at higher velocities than others will. (Ng’oma 2005, p. 45). This is because the lasers in existence cannot regulate wavelengths before propagation to ensure that they are equal in magnitude and velocity. Wavelength dispersion results from propagation of different wavelengths. This is because different factors affect this transmission through the fibre, for instance, indices, shape, core, and cladding of the fibre. Consequently, it is difficult to regulate the outcome of any individual propagation. The dispersion solution could take the form of dispersion shifting and the utilization of fibre that has zero dispersion at 1300-1700 nm.

Non-linearity

If the wavelength division multiplexing, (WDM) of an optical communication system is non-linear, it could lead to distortion and cross channel interference. This is due to the effect on spacing between adjacent wavelength channels, which limits the maximum output of any such channel at an individual level. The maximum bit rate also diminishes.

Cross talk

This can either be heterodyne, meaning that it is occurring between signals that are at different wavelengths, or homodyne, which occurs between signals which are in the same nominal wavelength. Homodyne cross talk is again stratified into phase correlated and non-correlated signals. If not managed at the appropriate time, this can result in the compounding of an entire system’s error rate to the bit error rate (BER) floor.

Numerous societal institutions, including large buildings owned by different corporations and organizations, residential houses, and shopping centres use these systems (Opatic 2011, p. 6). Examples of optical systems include technology that has been invented to enable transmission through the air and water, through rocks, and walls, as well as through reinforced concrete among others.

Conclusion

The architectural designs of RoF systems can operate numerous radio services and standards. They are flexible, reliable, and cost-effective. They simplify any system’s communication channels and are located near clients. They eradicate the complications that are part of the existing electrical systems such as noise production, complexity, expensive nature, low capacity, and space consumption among others. The advantages of these systems outweigh any shortcoming they may have in the meantime. This phenomena is because most of these shortcomings are already being worked upon by innovators and other interested parties. In the meantime, it would be prudent to look into the advantages of optical systems, especially RoFs, in a more specific setting to establish their worth. Though they will still be analogous, they will be advantageous because the complexity of the equipment close to the antennae will be minimal, the equipment shall, therefore, have minimal visual impact on those around it, and the necessary operational costs will be manageable. Installation costs will reduce the loss of data that occur when it is transmitted via electric cables will reduce, and climate will be a null issue.

Reference List

Al-Raweshidy, H. A., 2010. Optical Fibre Technologies and Radio over Fibre Strategic Research for Future Networks. eMobility: White Papers, 1-19.

Ng’oma, A. S., 2005. Radio-over-Fibre Technology for Broadband Wireless Communication Systems. Eindhoven: Technische Universiteit Eindhoven.

Opatic, D., (2011). Radio over Fibre Technology for Wireless Access. Web.

Rahman, K. M., Lee, H. J., Youngil, L. P., & Ki-Doo, K. N., 2009. Radio over Fibre as a Cost Effective Technology for Transmission of WiMAX Signals. World Academy of Science, Engineering and Technology, 56, pp. 424-428.

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