The Benefits of Sectorization and Downtilt Antennas

Sectorization helps to provide more capacity and delivers a better downlink RF link
budget (more directivity) and better uplink selectivity (which improves sensitivity).
Many Node Bs also use electrical downtilt. We discuss smart antennas in Chapter 13;
these antennas can adaptively change the coverage footprint of an antenna either to
null out unwanted interference or to minimize interference to other users or other adjacent
Node Bs. Electrical downtilt has been used in GSM base stations from the mid-
1990s (1995 onward). By changing the elevation of the antenna or the electrical
phasing, the vertical beam pattern can be raised or lowered. Figure 11.6 shows how the
beam pattern can be adjusted to increase or decrease the cell radius. This can be used
to reduce or increase the traffic loading on the cell by reducing or increasing the
physical footprint available from the Node B. Adaptive downtilt can be used to change
coverage as loading shifts through the day�"for example, to accommodate morning
rush hour traffic flows or evening rush hour loading.
For in-building coverage, an additional option is to have a distributed RF solution in
which a Node B is positioned in a building and then the incoming/outgoing RF signals
are piped over either copper feeder (rather lossy) or optical fiber to distributed antennas.
The optical fiber option is preferable in terms of performance but requires linear
lasers to take the (analog) RF signal and modulate it onto the optical fiber and a linear
laser to remodulate the optical signal back to RF at the antenna.

As we shall see in our next section on system planning, the 3G air interface is well
suited to a fairly dense low-powered network (less noise rise is produced by adjacent
Node Bs and less interference is visible at the Node B receiver). This places a premium
on the need to design a small form factor (sub-30 kg) Node B product.