Contents Telektronikk - Telenor

Although it seems tempting to define a
lot of classes, the drawbacks are the
dimension of the problem and that the
required input data must be given for
each class.
5.2.2 Routing probabilities
The mobile stations follow some routes
as defined in the set of space elements.
That is, given that a mobile station stays
in a space element, the probabilities for
entering each of the elements as the next
one are assumed to be given. Implicitly,
it is then said that no historical data of
the route that a mobile station has followed
are kept (Markov routing). One way
of including the historic information is to
define a separate class for these mobile
stations. There could be classes of mobile
stations having predetermined routing
and others that follow alternative routes
through the relevant area, e.g. the routes
could be described by any combination
of branches/trees and loops. The routing
probabilities can be estimated by measuring
or predicting the physical motion of
mobile stations (taking into account the
number of active stations, i.e. those engaged
in calls). This should be done for
each class. If there is no indication that
active mobile stations follow a route different
from the others, the estimate could
be based on data for all the mobile stations
of that class.
5.2.3 Call initiating processes
Each mobile station class can have its
separate description of processes to initiate
new calls. These processes can also be
different for the various services, and they
may vary between the space elements.
To estimate these processes, actual measurements
could be done. Another way
would be to use a general call initiating
process and modify this by considering
how the relevant space elements invite
the users to make calls for the various
services. Making consistent measurements
of the call initiating process may
be difficult. In most models of cellular
systems new calls are assumed to be initiated
according to a Poisson process for
each cell. How well such an assumption
is for a fraction of a cell should be validated.
5.2.4 Space element dwelling times
The time a mobile station stays in a space
element is named the dwelling time for
that element. The distribution of the
dwelling times may be estimated by predictions
and measurements. Here, we
may have to consider the possibility that
a mobile station had stayed in an element
for a while before the call was initiated.
That is, there may be a difference in the
remaining dwelling time for a mobile station
after it has initiated a call in that
space element and the dwelling time if it
already had an ongoing call when the
space element was entered.
These distributions will depend on how
the mobile stations move. For an element
that contains many mobile stations and
where each of these have an independent
behaviour regarding the arrival, departure,
number of turns and routing, the
dwelling times could perhaps be approximated
by a negative exponential distribution,
ref. [6]. For elements covering a
highway with heavy vehicular traffic, an
Erlang-k distribution or a truncated
Gaussian distribution might be more
appropriate as assumptions of the
dwelling times.
Dwelling times may differ for the various
classes of mobile stations. In fact, the
velocities of the mobile stations may be
one for describing the factors for dividing
these into classes.
5.2.5 Coverage
The base stations’ coverage for space
elements are assumed to be known by
predictions or measurements. Each class
of mobile stations may have its separate
set of base stations. This set is based on
which base stations the mobile stations of
that class are allowed to use (authorised).
Other reasons could be the available
power, signalling processing used, supported
services, etc.
As seen from a space element, a mobile
station of a given class will then have a
set of base stations available. These base
stations are assumed to be sorted in a list,
called a coverage list. A number of criteria
for sorting the base stations could be
applied. All classes of mobile stations do
not have to use the same set of criteria;
neither must they arrange the criteria in
the same way. For example, one criterion
could be that the emitted power should
be as low as possible, resulting in selection
of a small-cell base station (pico of
microcell) before a macrocell base station.
For another class of mobile stations
the emitted power may not be a main
constraint and we could rather wish to be
connected to the same base station as
long as possible (to limit the number of
handovers) which could result in choosing
a macrocell base station before a
microcell base stations. The operators
would naturally have an influence on
how such lists are arranged, for instance,
by applying appropriate charging policies.
A natural order would be to let most
mobile stations look for the small-cell
base stations first. This would often lead
to an increase of the traffic handling
capacity (capacity reuse) as lower power
would be emitted. In addition, the quality
of the received radio signal could be
higher. If the first base station in a list
does not have sufficient free capacity to
serve the call, the mobile station should
look at the next one in the coverage list.
The information necessary for mobile
stations to select the “best” possible base
station could be sent in a broadcast channel
from each base station. Then, the
mobile stations must scan these broadcast
channels for (some of) the relevant base
stations in order to make a decision of
which one to use.
5.2.6 Service usage
Mobile stations from a given class may
use a set of services that differ from the
other classes. On the other hand we could
define one class for every service that is
used. Each class can also use the services
in its own way, i.e. defined by different
values and distributions compared to the
other classes.
The service usage may be dependent on
the space elements, that is, in which environment
a call is made. For instance, it
seems natural that a telephone call made
when standing in a crowd is shorter than
if the same call were made sitting in a
comfortable chair. On a larger scale, city
areas and rural areas might have different
characteristic values.
5.3 Characterising the services
5.3.1 Holding times
It must be possible to define different
total holding times for the various services.
These holding times may also vary
between the classes of mobile stations
and depend on the environment in which
the call is initiated. One factor that might
influence this is how the mobile user
experiences his/her surroundings (nonhuman
users are also included). The
holding times can be measured for longer
calls, like telephone calls, and predicted
for shorter calls, like short message services
when the message lengths are
given. Whether it is appropriate to allow
for such services to compete for the same
radio capacity is not further dealt with
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