Performance of low-cost radio-over-fibre systems

The research presented in this thesis has focused on the use of radio-over-fibre (RoF) technology for improving the quality of mobile/wireless coverage within buildings. The primary aim was to minimise overall system costs by employing commercially available components. For this purpose, a distributed antenna system using low-cost vertical-cavity surface-emitting lasers (VCSELs) operating at 850 nm and multimode fibre (OM1/OM2) has been designed and implemented.

A detailed link budget analysis has been performed which allows for the prediction of maximum achievable ranges for the transmission of different wireless systems over the RoF link, while taking into account practical restrictions that are important for bidirectional link operation (e.g. crosstalk and noise emissions). The analysis indicates that when optimised component parameter values are utilised, reasonable cell sizes may be achieved for systems such as GSM, UMTS and WLAN. The link budget predictions were verified for the transmission of ‘real’ WLAN signals over the designed RoF link and complete coverage of a standard office room was demonstrated.

The majority of previous research into low-cost RoF links has primarily involved characterisation of the optical path. In this investigation, signal strength and throughput measurements were conducted for the combined optical and wireless paths in order to verify the operation of the complete fibre-fed WLAN system. Throughput values close to 5 Mbps for IEEE 802.11b and 20 Mbps for IEEE 802.1 lg were recorded. Additionally, the transmission of different combinations of emulated mobile/wireless systems in a dual-band configuration over another radio-over-fibre link (also employing 850 nm VCSELs and MMF) has been successfully demonstrated.

Experimental investigations have been carried out for the first time to analyse the performance of WLAN-over-fibre networks using different MAC mechanisms such as fragmentation and the use of RTS/CTS in the presence of hidden nodes. Finally, scenarios involving multiple clients accessing a single remote antenna unit and multiple remote antenna units being fed by a single access point have been demonstrated.