ROF net-Transmission of RF Signals over Optical Fibers Using Radio Over Fiber Technol
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22-02-2011, 10:51 AM
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ROF net-Transmission of RF Signals over Optical Fibers Using Radio Over Fiber Technology
Abstract- Radio over fiber technology is a combination of wireless and fiber optic networks, is an essential and emerging technology in the field of wireless communications, which can be used in a range of applications including the extension of existing radio coverage and capacity. Obviously, we know that the low loss, light weight, large bandwidth, small size and some other important features made the optical fiber flexible and reliable. Adding to this, the insensitivity of optical fibers to the electromagnetic radiation makes it a key factor to be a backbone of a wireless network. The link between the radio base station (RBS) and the antenna has previously been a copper coaxial cable. Using the optical fiber instead of it makes the hardware much easier and we can also design new architectures.
Key words- Central Office or Exchange (CO), Base Station (BS), Multimode fiber (MMF), Single mode fiber (SMF).
The optical wireless networking can be achieved using radio frequency (RF) signal. The optical wireless networking provides a vast bandwidth with in an indoor environment to set up high speed multimedia services. The optical signal transmission and detection offers immunity from fading and security. Earlier ROF systems were used to transmit microwave signals and to obtain the mobility functions in central office or exchange (CO). That is, the ROF systems would like to take the modulated microwave as input, which subsequently transported over a distance to RS (Remote Site) in the form of optical signals. The signals are then regenerated into RF signals at the RS and are radiated by antennas.
However, recent ROF systems are designed to perform added radio system functionalities besides transportation and mobility functions. The data modulation signal processing and frequency conversion are the other included functions. But the input of a multifunctional ROF system depends on the ROF technology and the functionality desired. Figure 1. shows a typical RF signal being transported over an analog optical fiber link. The input to ROF system may be a baseband signal, IF signal or an actual RF signal. The RF signal modulates the optical source in transmitter, it means the RF signal is converted into an optical signal and is transported over an optical fiber. And at the receiving end the optical signal is demodulated to a RF signal, means the optical signal will be converted into an RF signal. Usually a single optical fiber can carry information in only one direction (simplex); consequently, we require two fibers for bi-directional communication (duplex). But the recent advances in WDM make it possible for bi-directional communication. The WDM is the technology, in which the different wavelengths are combined together and to transport the multiplexed signal over optical fiber and it greatly increases the utilization of available bandwidth and it also maximizes the data throughput.
Observing the figure2 we come to know that the CO (Central Office) acts as an interface between an external network (High capacity MAN or LAN) and the wireless network where the different Base Stations (BS) provides wireless coverage to the Mobile Units (MU).
Generally, ROF system consists of a central office (CO) and a Remote Site (RS) linked up by analog optical fiber. The ROF architecture varies according to the planned service application. For example, if we choose GSM network, the CO could be the Mobile Switching Center (MSC) and RS be the BS. And when we take the example of Wireless Land Area Network (WLAN), the CO could be head end while the RS is the Remote Access Point (RAP).
In recent, the dominant market in ROF technology is to distribute RF signal over optical fiber to extend the capacity of radio systems; so called Distributed Antenna Systems (DAS). At this instant, the fiber based DAS installations are being increasingly used in a variety of locations including public, buildings, airports, stadiums and others. However, in building wireless systems the radio signal are converted to optical signal at the Base Station and then transmitted via an optical fiber and at the other end the RF signal is regenerated from optical signal. The figure 3 is the architecture used in the many applications of ROF systems. This architecture consists of CO’s, BS connected through optical fiber links. As shown in this diagram, CO1 is connected to several Remote Nodes (RN) via an optical ring, while the CO2 is connected to RN’s via a star tree arrangement. In both arrangements individual wavelengths would be demultiplexed from WDM signal by being dropped from optical WDM MAN via Remote Nodes, which direct optical signal to remote antenna BS’s for detection of signal and radio distribution. In the upstream direction the radio signals are converted to optical signal, that is frequency up conversion and then the signal is directed to CO for the next stage of processing.
III. ROF LINK CONFIGURATIONS
The transportation of optical signals over the optical link can be achieved in different ways. They are distributed on the basis of frequency bands. The three fundamental techniques are shown in figure 4. Commonly, the ROF analog links are multichannel in nature and they require high power to digital schemes because of increased Carrier to Noise Ratio (CNR). The performance, including CNR and capacity of ROF systems employing analog optical links is limited by the noise of the various optical and electrical components in the link as well as by device nonlinearities which brings in the intermodulation and distortion products that create interference with other radio channels.
At moment, ROF is the most straight forward radio signal distribution technology, as there is no case of up or down conversion of frequency when the wireless signals are transported directly over the optical link. The downstream signal transmission via RF over fiber is shown in Figure 5. This type of configuration energizes the central control and remote monitoring of radio signal distribution via fiber network and decreases complexity at BS, but the transmission distance will be seriously effected by the fiber chromatic dispersion.
The wireless data taken from trunk network are modulated onto a number of low IF carriers and then together to form a Sub Carrier Multiplexed (SCM) signal. At the CO, the SCM signal is up converted into the RF signal using Local Oscillator (LO) and then modulated onto an optical carrier; it means the RF signal is converted into an optical signal to transmit over an optical fiber. At the Remote BS, the received optical signal (modulated), is detected, amplified, filtered and directed to antenna for wireless transmission. The reverse operation of the downstream transmission must be carried out for the process of upstream transmission of signal from BS to the CO. The second scheme can be implemented to distribute the radio signal at a lower IF; hence the name “IF over fiber”. And the figure 6 shows the hardware to be used for the downstream signal transmission via IF over fiber. The most important advantage of this type of scheme is that the readily available microwave hardware can be utilized at BS. The need for frequency conversion at the BS increases complexity of BS architecture, as the frequency of wireless application enter into millimeter wave frequency region. At this instant, the BS requires LO’s and mixers for frequency conversion process, which limits ability to reconfigure radio network with provision of radio channels or implementation of required changes in IF frequency. IF radio over fiber utilized the multimode fiber (MMF) for transmission as it costs low.
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