Broadband signal transmission using radio over fibre system.

摘要

Radio over fibre (ROF) has great potential to be the medium of choice to distribute radio frequency (RF) signal to remote stations for broadband applications. It is a technology where microwave signals are distributed by optical components and techniques. It has emerged as a cost effective approach for reducing radio system costs because it simplifies the remote antenna sites and enhances the sharing of expensive radio equipment located at the Central Stations (CS). This thesis explores broadband transmission over the ROF system. A system is proposed, where directly modulated wireless local area network (WLAN) and externally modulated baseband (BB) signals, are simultaneously transmitted. Both modulations are studied for their impact on error vector magnitude (EVM), bit rate, signal-to-noise ratio (SNR) and power penalty for the BB signal to obtain a bit error rate (BER) of 10-9 measured. Then, another alternative simultaneous transmission system using an RF combiner is explored for a comparison with the first system. A vector signal generator is utilised to generate the WLAN signals and a pulse pattern generator (PPG) is used to generate the 1.25Gbps BB signal. A real time spectrum analyser and a bit error rate tester are used to analyse the WLAN and baseband signals respectively. Various setups for WLAN transmission over the ROF system are also studied with the intention to optimise it with respect to the direct and external modulations in the system. Laser diode is used in the direct modulation whereas the Mach-Zehnder modulator (MZM) is the essential component for the external modulation. Results reveal that, directly modulated 6 Mbps WLAN 802.11g signal at 2.4 GHz can coexist with externally modulated 1.25 Gbps BB signal using MZM in a 2 km ROF system. In comparison, system with RF combiner has about 3.5dB optical power penalty for a BER of 10 -9 over the first system, however, it requires more RF power for transmission. The proposed systems have great potential to be used in future multiservice access networks in order to enable ubiquitous high speed connectivity.