Radio over multimode fiber for wireless access

A radio over fiber link is a promising technology for antenna remoting applications. Typically, the radio over fiber link employs a single mode fiber. However, the signal power at the remote antenna is very small. The main reason is large power loss in the E/O and O/E convertor, but the coupling efficiency of a E/O convertor can be improved with multimode fiber (MMF), so we propose to use a ROF link with a vertical-cavity surface-emitting laser with a graded index MMF to transport optical signals. A multimode fiber has a larger core radius compared to a SMF. A larger core radius allows more optical power coupled into a fiber. With simple butt-coupling techniques, the coupling efficiency can be 90% and simplicity leads to reduction in cost of the link. Normally, the MMF is used in short distance digital applications with a bandwidth distance product of about 500 MHz.km, so it is good for local area picocells. Our approach is to transmit passband signals such as QPSK and FSK through the ROF link. Our simulation shows that a 900 MHz carrier can transport through a link of 1.22 km long. In this paper, we investigate the feasibility of using a MMF for antenna remoting in local area picocells and compare the tradeoff between coupling efficiency and bandwidth.