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1.2. MULTIPLE CELLS
9
traffic in Erlangs for a given blocking probability has a non-linear relationship with the
number of channels per cell such that for more channels there is a disproportionate increase
in the traffic that may be supported. We now observe an important aspect of cellular radio,
i.e. for a mobile radio system that employs clusters of cells. If the radio link equipment is
capable of operating with a low SIR, the cluster size becomes small and the carried traffic
high. Another important point to note is that a cell becomes smaller in the presence of
cochannel interference. By this we mean that the area around the cell site where the SIR is
high enough to yield a sufficiently low bit error rate (BER) is decreased due to the presence
of the interfering cells. This is illustrated in Figure 1.5. We also note that as the levels of
interference power alter, so does the SIR, and so does the effective cell boundary for an
acceptable BER. The cell boundaries shown in Figure 1.5 relate to a specific BER. For a
higher BER, the cell size increases and vice versa. It is important to avoid the simple notion
that a cell has a fixed area. It is better to think of it as breathing, i.e. changing its size
as the traffic conditions within the network vary. Cell breathing is a feature of both GSM
and cdmaOne, although it is more acute in the CDMA system. For analysis reasons we
generally consider fixed cells and often worse case conditions.
Newcomers to cellular radio often consider spectral efficiency in terms of the number of
channels, N, a carrier can support in a given bandwidth. This notion is related to modulation
efficiency in terms of bits per second per Hertz of RF bandwidth. Since cellular radio
must operate in an interference-limited environment, the crucial factor is not the modulation
efficiency. For example, employing quadrature amplitude modulation (QAM), where each
symbol carries multiple bits, gives a high modulation efficiency [10, 14]. However, QAM
requires a high SIR value and hence large cluster sizes, resulting in low values of carried
traffic per cell site, for a given bandwidth allocation. The choice of modulation and multiple
access scheme is complex and will be addressed at a later stage. What we must note is that,
given a modulation and multiple access scheme resulting in a cluster size of M, the number
of users on the network is greatly increased if the cells, and thereby the clusters, are small.
This is because each cluster carries a traffic of MA
c
Erlangs, where A
c
is the carried traffic
at each BS, and if a cluster occupies an area S
c
then the traffic carried per km
2
is MA
c
=
S
c
Erlangs/km
2
for a bandwidth W. Using small cells, often called microcells, means S
c
is
small and the traffic density that may be supported is high.
1.2.1 Hexagonal cells
These types of cells are conceptual. The cell site is located at the centre of each hexagon,
and the hexagonal cells are tessellated to form clusters [15]. Although these cells are fic-
titious, they are often used for comparing the performances of different cellular systems.
Figure 1.6 shows clusters of tessellated hexagonal cells. Observe that for hexagonal cells
there are always six near cochannel cells, irrespective of the cluster size. This is because