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108
overs for mobile stations that move along
the border of two cells.
3.4 Increasing the capacity
In this section, system capacity means
the amount of traffic that a system is
capable of serving. Most systems of
today are interference limited in the
sense that the level of the interfering signals
will decide the number of calls that
can be connected. In order to increase the
system capacity, the level of the interfering
signals should be reduced. This can
be done by a more directed delivery of
the radio signal power from the transmitter
(e.g. the base stations) to the receiver
(e.g. the mobile station) while no radio
signal is transmitted when it is not needed.
One example is the utilisation of
smaller cells for base stations and the use
of sectorized cells. The result can be that
a lower power is transmitted and a closer
reuse of the frequency (measured in geographical
distance) can be achieved. For
areas with high traffic demand such solutions
are relevant. Therefore, a tempting
approach could be to reduce the coverage
area for each base station in order to
increase the reuse factor of the radio
capacity as measured per geographical
area, e.g. per square kilometre. However,
this could become an expensive solution
as the cost of the base station network in
principle is related to the inverse of the
square of the cell radius. In addition, it
could be difficult to cover an area completely
with such smaller cells.
However, as areas with a lower traffic
demand will remain, future wireless systems
must allow for larger coverage
areas as well. Another consideration is
that mobile stations usually should be
connected to a base station for a minimum
time. As the coverage areas are
decreased, mobile stations would move
relatively faster (as seen from a base sta-
large
scale
small
scale
medium
scale
Figure 6 Three scales could be studied when examining
the radio signal propagation
tion). This results in shorter handling
times in each base station and increases
of the signalling load for control of the
connections for these mobile stations.
One solution can be to introduce a mixture
of coverage areas for the base stations,
in a range from smaller to larger
ones. The velocities of the mobile stations
served will also influence the size
of the coverage areas.
In addition to the propagation/interference
considerations, we should provide
the capacity to the areas where it is most
requested. If fixed capacity allocation to
every base station is applied, the utilisation
of smaller coverage areas could
make the situation worse if the traffic
demand is not according to what was
assumed during the allocation process.
Another aspect is the change of the traffic
demand during the day and during the
week. It is expected that this will have a
higher variability for future systems as
the number of users increases. For
instance, a football match could gather
several thousands of persons. Some
events during the arrangement could trigger
several of the spectators to make
calls. During the rest of the week this
football arena could be more or less
empty with a very low traffic demand.
In order to deal with such varying traffic
demand a hybrid capacity allocation
scheme could be applied. In this context,
the term hybrid includes both the fixed
allocation and the dynamic allocation as
the two extremes. When a fully dynamic
scheme is utilised no capacity is fixed to
any of the base stations.
For a future wireless system with various
base stations (hierarchy), overlapping
coverage areas and a number of operators,
a fully dynamic scheme might be
difficult to implement. Another fact is
that were such a scheme applied, most of
the capacity would be allocated to the
base stations that were the first ones to be
tried by the mobile stations. That is, the
other base stations could suffer from this.
Although a fully dynamic scheme could
be possible in principle, some maximal
limits for the capacity allocated to a base
station will most likely be present in a
real case. For instance, the number of
transceiver units, capacity on the fixed
line side of the base stations, etc.,
would have to be decided for most cases.
In view of the presence of different and
overlapping coverage areas and differing
mobile stations’ service usages, the situa-
tion for a hybrid capacity allocation in a
future wireless system is more complicated.
In relation to the implementation of
hybrid allocation schemes, there are several
decisions that have to be made
regarding the algorithms and the sets of
criteria that can be used, see e.g. [7].
4 Some performance topics
From the outset we can examine several
measures as the quality of a wireless system.
We may also think of modifying the
relevant system mechanisms in order to
improve that specific performance measure.
However, it is usually the overall
system performance that is of most interest.
In addition, we may see this overall
performance from the users’ and from
the operators’ points of view.
4.1 Performance measures
Three mutually independent and multiplicative
radio signal propagation phenomena
seem to be relevant, see Figure
6:
- large scale signal loss according to a
relevant model, like free space, earth
reflection, and diffraction
- medium scale, e.g. because of shadowing,
often modelled as log normal distributed
loss
- small scale for multipath fading due to
scatters and reflections of the radio
signal.
The small scale is related to some fractions
of wave lengths while the medium
scale may concern some tens to hundreds
of wave lengths. For a signal with a frequency
of 1.8 GHz the wave length is
approximately 17 cm.
Propagation studies can be divided into
the examination of two different characteristics:
the multipath propagation characteristics
and the path loss characteristics.
The multipath characteristics will
influence the maximum possible bit rates
and state some requirements to the channel
coding/equaliser.
Several studies of radio propagation have
been presented. Many of the results indicate
that the propagation loss is not
inversely proportional to some fixed
power of the distance (slope). Often, the
loss is inversely proportional to a slope
that varies, depending on the distance
from the transmitter. The main cause for
this variation seems to be the multipath