The Hierarchical Cell Structure

The trend over the past 15 years has been to infill macro sites (up to 35 km radius) with
micro sites (500-meter radius) and to infill micro sites with pico sites (100-meter
radius). This infilling is called a hierarchical cell structure. The handover algorithms are
optimized to move traffic from the macro sites to the micro sites to the pico sites,
depending on factors such as mobility, signal strength, and traffic loading.
Suppose you are driving down Oxford Street, the main shopping street in London,
as you make your journey, you might typically be handed over from several micro
sites. Because the traffic is heavy, you will be a slowly moving user and are probably
only driving at walking pace. As you turn into Hyde Park (wide open space), you
might be handed over to a macro site. If you get out of your car in Oxford Street and
walk into a shop, you would be handed over from a micro site to an indoor pico site.
Hierarchical cell structures can provide an almost infinite amount of bandwidth by
increasing network density. Cellular networks are therefore very adaptive—able to support
a mobile in a 35 km cell, a user in a 100-meter picocell, and handover users from cell to cell, without any detectable gap or loss of quality. Power control and handover algorithms
continue to improve over time. We become expert at optimizing network performance
to provide wide area and local area coverage.
There is, however, a cost associated with this flexibility. Power control and handover
algorithms absorb signaling bandwidth. As this is part of the overall bandwidth budget,
then we will always have significant power control and signaling overheads.
Typically, at least 20 percent of our bandwidth is used for signaling. An additional
40 percent is absorbed by channel coding, much of which is needed to counteract the
wide area radio impairments—delay spread from multipath, fast fading, the need to
support high mobility (Doppler spread), interleaving to counter burst errors. Not only
does this absorb bandwidth (RF power), it also introduces tens of milliseconds of processing
delay.