RF over Fiber: Optical Transport

So far in this chapter we have looked at how the characteristics of copper determine the
characteristics and quality of the radio signal. We can also take an RF signal and convert
it to an optical signal using a linear laser. We need the linearity to preserve the
phase and amplitude components of our RF signal. The reason we might choose to
convert our RF signal to an optical signal is that the loss per kilometer is much lower in
an optical fiber when compared to a copper cable.
Typically, the loss in an optical fiber cable is less than 1 dB per kilometer. The loss
over a 7/8 copper feeder (a copper waveguide inside a plastic outer sheath) is between
1 and 3 dB per 100 meters, and the loss increases with frequency. Putting an RF signal
over optical fiber is not unique to the cellular radio industry but is also widely used in
cable TV.
In cellular industry applications, the RF signal at the base station is converted to an
optical signal and then sent down the fiber. The signal is delivered to a fiber-optic
antenna (which looks rather like a simple detector). A photodiode then converts the
optical signal back to RF. The RF signal is then amplified and fed to the antenna. On the
uplink, the signal is received at the antenna and then amplified before the laser diode.
The laser diode converts the signal to the optical domain for transmission to the laser
and is then combined at the base station/hub with other fiber optic signals.
Because the optical link is linear, it effectively becomes transparent to the network;
the handset is sending an over-the-air signal, which just happens to be channeled
through an optical fiber feed for part of its journey. RF over fiber can be used to solve
tricky installation problems. An option for in-building coverage is to install a number
of pico base stations.
Often, however, the architect wants to minimize visibility of all external hardware,
that is, base stations and antennas. An application example is the Bluewater shopping
center in Kent, United Kingdom. This is a small shopping center by U.S. standards but
an enormous shopping center by U.K. standards. There are five mobile phone shops in
the complex, so coverage has to be good. The problem is that the mall is on two levels,
so it is difficult to achieve consistent coverage. The solution was to install 60 GSM
transceivers (400 simultaneous phone calls) and to use 64 core and 8 core fiber to connect
the base station to small distributed antennas. 324