Radio-over-Fiber (RoF): The existing alternative to Femtocells

Recently while going through NTT Docomo Technical Journal, I came across an article on Radio over Fibre. This is the first time I have come across RoF but apparently this is a common way to provide indoor coverage before Femtocells.

My intention here is not to compare this with Femtocells as I can think of advantages and disadvantages of both of them.

I found the following extract in the book Femtocells: Technologies and Deployment:

Active Fibre DAS (Radio over Fibre)

Active fibre DAS is the most efficient in term of performance. Optical fibres are used to make the link between the MU and the RU. They can cover very long distances (up to 6 km) and support multiple radio services. With such a system the RU directly converts the optical signal into radio signal and vice versa. The other advantage is that optical fibre is very cheap and easy to install. Radio over fibre is now the most common technique used for indoor radio coverage. As detailed in [16], radio over fibre is today the optimal solution to extending indoor coverage, because it provides scalability, flexibility, easy expandability, and also because the signal degradation is very low compared with DAS using standard connections.

The following is from Wikipedia:

Radio over Fiber (RoF) refers to a technology whereby light is modulated by a radio signal and transmitted over an optical fiber link to facilitate wireless access. Although radio transmission over fiber is used for multiple purposes, such as in cable television (CATV) networks and in satellite base stations, the term RoF is usually applied when this is done for wireless access.

In RoF systems, wireless signals are transported in optical form between a central station and a set of base stations before being radiated through the air. Each base station is adapted to communicate over a radio link with at least one user's mobile station located within the radio range of said base station.

RoF transmission systems are usually classified into two main categories (RF-over-Fiber ; IF-over-Fiber) depending on the frequency range of the radio signal to be transported.

a) In RF-over-Fiber architecture, a data-carrying RF (Radio Frequency) signal with a high frequency (usually greater than 10 GHz) is imposed on a lightwave signal before being transported over the optical link. Therefore, wireless signals are optically distributed to base stations directly at high frequencies and converted to from optical to electrical domain at the base stations before being amplified and radiated by an antenna. As a result, no frequency up/down conversion is required at the various base station, thereby resulting in simple and rather cost-effective implementation is enabled at the base stations.

b) In IF-over-Fiber architecture, an IF (Intermediate Frequency) radio signal with a lower frequency (less than 10 GHz) is used for modulating light before being transported over the optical link. Therefore, wireless signals are transported at intermediate frequency over the optical.

Access to dead zones

An important application of RoF is its use to provide wireless coverage in the area where wireless backhaul link is not possible. These zones can be areas inside a structure such as a tunnel, areas behind buildings, Mountainous places or secluded areas such a jungle.

FTTA (Fiber to the Antenna)

By using an optical connection directly to the antenna, the equipment vendor can gain several advantages like low line losses, immunity to lightening strikes/electric discharges and reduced complexity of base station by attaching light weight Optical-to-Electrical (O/E) converter directly to antenna.