Radio over fiber systems

Abstract

The three main types of Radio Over Fiber (ROF)communication systems, namely analogue ROF, baseband ROF and digitized ROF are investigated. Optical fibers are increasingly replacing copper wires. In long-haul, high-bit-rate communication systems optical fiber has already become the dominant mode of transmission due to its enormous bandwidth and low loss. ROF facilitate the seamless integration of optical and wireless communication systems. Since the RF spectrum is limited, wireless systems rely on re-using the frequencies at different geographic locations, but the ever-increasing tele-traffic demands require ever-reduced cell-sizes. This enables wireless systems to provide high data rates for a reduced number of users by assigning each of them a larger fraction of the total bandwidth. Furthermore, higher frequencies, expanding to the microwave and mmwavelength bands are capable of supporting increased data rates. Since high-frequency signals travel shorter distances due to their higher path-loss, the cell sizes have to be further reduced. This reduction in cell size implies that more Radio Access Units (RAUs) are required for the increased number of cells, which are located close to each other. Conceiving these RAUs relying on complex signal processing is costly. Therefore, it is desirable to have simple RAUs that are connected to a central unit where all the signal processing tasks are carried out. In this scenario, ROF plays an important role in connecting these RAUs to the central unit. The major factors that makes ROF suitable is its transparency to the type of RF signal being transported, the large available bandwidth of fiber and its low attenuation. The first type of ROF communication investigated is baseband ROF (BROF), where electronic baseband data is directly transmitted over the fiber after Electronic-to-Optical (EO) conversion. Baseband optical communications generally transmit optical pulses that have a Gaussian timedomain profile associated with the most compact spectrum for transmitting baseband digital data. Optical pulsed laser sources are capable of generating narrow Gaussian pulses that may be used for high-rate systems relying on ON-OFF keying. All-optical regeneration of signals is investigated, where the signals transmitted over large distances may be regenerated at regular intervals. The second type of ROF communication is analogue ROF (AROF), where the analogue RF signal is transmitted over the fiber using an optical carrier. Finally, digitized ROF (DROF) communication is discussed, which digitizes the RF signal at the transmitter and after transmitting it over the fiber, it converts the digital signal back to analogue at the receiver. The thesis is concluded with the comparative study of the pros and cons of BROF, AROF and DROF techniques