Design of the Radio Access Network

Once an RF plan has been developed, the next step in the design effort is to
design a network that will connect the various base stations to their Radio
Network Controllers (RNCs). This means that we must determine the number
of RNCs required, we must determine a suitable placement for the
RNCs, and we must design a transmission network from RNCs to the various
base stations and between RNCs (for inter-RNC soft handover).
For many GSM Base Station Controllers (BSCs), the main capacity limitations
are in the numbers of base stations, cells, or transceivers that can
be supported. In some cases, there are limits in terms of Erlangs, but such
capacity limits are rarely encountered in real networks.With UMTS, however,
the capacity of most RNCs is more tightly linked to the traffic mix.
While one still finds limitations in terms of total base stations, cells, or RF
carriers, the traffic handling limitations play a major role. Traffic limits typically
include, total throughput, total Iub interface capacity, and total busy
hour call attempts (BHCA) for voice calls. Therefore, when determining the
number of RNCs required, we need to make sure that none of these limits
is exceeded. This means that the RNC network design must be done in close
cooperation with the RF network design.To make things more complicated,
there is often a trade off between one limit and another. For example, if
fewer voice Erlangs are used, then the RNC is likely to be able to support a
greater data traffic demand.

In order to simplify the dimensioning effort, a good place to start is with
dimensioning of the Iub interface. For many vendors, the total Iub interface
capacity is likely to be the most constraining factor. Moreover, the Iub interface
is common for voice and packet data. Once we have determined the Iub
capacity demand from each of the base stations, we can sum that capacity
and determine the minimum number of RNCs needed. In practice, we
should add an additional 25 to 35 percent to the RNC capacity that we have
determined for three reasons. First, when allocating base stations to RNCs,
we need to consider location areas/routing areas. It is to common assign
such registration areas such that they align with RNC boundaries. This
means that we need some flexibility in how we allocate base stations to
RNCs. Second, intra-RNC soft handover is preferable to inter-RNC soft
handover as it helps to minimize Iub transport requirements and it helps to
minimize the total switching demand on the RNC. Thus, we would like to define RNC boundaries such that they do not align with areas of high traffic.
This also means that we need flexibility in how base stations are
assigned to RNCs. Third, we never want to find ourselves in a situation
where the addition of one or two extra sites (or even RF carriers) would
require the addition of a new RNC. In other words, we need to leave some
room for growth.