Contents Telektronikk - Telenor

110
radio
interface
tion inside a building. In the second view
a street situation is studied. Most likely
the first view would consider a smaller
area compared to the street situation.
Therefore, more details may be included.
In the same way we want the model to
adapt to the scale. For instance, we could
choose to regard some phenomena in
more detail while others could be lumped
together or considered as outside the
modelled part. In this way base stations
covering small areas could, for instance,
be considered to be outside the model.
Then, any handover calls from these base
stations to the base stations included in
the model could be regarded as new calls
with a modified call duration. Several
classes of mobile stations could also be
regarded as one class while studying a
particular class in more detail. Figure 7
shows some examples. In the first view
we have chosen to consider the outside
base stations as interference sources,
another solution is to include them as
secondary servers, i.e. they would handle
any overflow traffic for some services. In
view 2 the base stations inside the building
are not taken into account explicitly.
In the more general case, a part of the
area could be modelled with a greater
detail while other parts are represented in
a more coarse way. In addition, we could
use more coarse models in order to get
some overall results quickly.
The requested output data or results we
want to get from such a model are also
included in the requirements to the
base
stations
control
Figure 8 Two view points for estimating the service quality
variables: First as seen from the mobile stations, and
second, related to the fixed network capabilities
model. They can be divided into two categories,
see Figure 8. In the first category
we find variables related to how the
user/mobile station experiences the services.
Here, these are commonly named
service quality variables. Some service
quality variables are:
- blocking probability for new calls
- probability that a handover will be
blocked
- probability that a call will be released
due to blocking of a handover request
- probability that a certain base station
handles the call as seen from a given
location (may influence the quality of
the connection as seen from the user
and could be related to other variables
like the outage probability).
The second type of variables are those
used for the dimensioning of the fixed
network. Examples of such variables are
the call arrival rates and the service times
for a base station. These will give requirements
to the call handling capacity
related to that base station, e.g. processing
capacity, number of transceivers and
the capacity of trunks and signalling
links connected to the base station. The
amount of traffic served by a base station
is usually closely related to the income
(and charges) of the calls using that base
station and the accounting between the
operators/providers.
Finding the handover traffic between any
pair of base stations is also requested as
this traffic implies a load on the signalling
network as well as load
for some elements representing
the functional
entities in the fixed net-
data
base
switching and
transmission
connection and
signalling network
work. The delay requirements
for handover
procedures are
expected to be more
pronounced as
smaller cells are
involved (relative to
the velocities of the
mobile stations). The
combination of the handover
traffic and the related
procedures would then be useful when
studying alternative architectures for the
structures of base station inter-
connections.
In addition to using the analysis
results of such a model when
studying time delay variables, the output
could be used as input of mobility variables
when signalling and processing for
the relevant fixed network entities are
examined. The teletraffic model can also
be seen in relation to a dependability
study. For instance, what will be the
effects and which measures should be
taken when a set of base stations gets
reduced traffic handling capability. These
effects may not be intuitive in a hierarchical
cell structure with overlapping coverage
areas.
This discussion shows that the teletraffic
performance could be used both as a
“stand-alone” result or as a module/step
in a wider analysis. Anyway, it is necessary
for the model to provide estimates
for the relevant variables and allow the
analyst to change the input data in order
to take into account the aspects studied.
The model must be capable of considering
the relevant radio conditions and the
service characteristics experienced by the
base stations. Using this model, dimensioning
of the fixed network part may be
performed in order to achieve the stated
service goals. For some activity patterns
of the mobile stations, we should then be
able to estimate the demands for radio
resources, processing capacities, signalling
links, etc. The influence from
changes in the mobile stations’ activity
patterns on the capacities of the fixed
installations can be studied in this way.
4.2.2 Describing the base
station coverage
For an outdoor environment, predicting
the path loss can be seen as a three step
process:
- step 1: predict the link parameters
from the terrain data, distance, terrain
profile, etc.
- step 2: consider system variables like
frequency, antenna heights, etc., and
- step 3: include losses caused by isolated
phenomena, like shadowing, etc.
A medium scale prediction model for the
total path loss will then result, i.e. aspects
like multipath fading are not taken into
account. The multipath effect is often
related to variations in the signal over
very short distances (fraction of wavelengths).
It seems difficult to include
such variations in a teletraffic model in a
direct way. However, as the statistical
description of this effect is related to the
environment, it could be taken into
account in an indirect way.
Several models for radio signal propagation
have been described in a number of