Contents Telektronikk - Telenor

Teletraffic analysis of mobile communications systems
BY TERJE JENSEN
As the traffic in mobile telecommunications
systems increases the performance
analysis from a teletraffic point of
view becomes more important. The
complexity of such analyses is expected
to grow even more when the third generation
mobile communications systems
will be introduced.
After a brief overview of some aspects
in mobile communications systems, a
teletraffic model for such systems is
presented.
1 Introduction
Systems for mobile telecommunications
have been available for several decades.
During this time several system generations
can be identified, where each generation
aims at offering improved service
quality and enhanced capabilities, as seen
from the users’ as well as from the operators’
point of view. Such an evolution
seems to have the effect on the market
that the mobile services become even
more popular. This fact, combined with
the availability of standards, leads to
mass-production of equipment for mobile
communications systems. In addition, the
competition between operators has been
introduced in several countries. The
related cost as seen from the users has
mostly decreased when related to other
living expenses. Naturally, compared
with wireline services the wireless solutions
allow the users to be more flexible
when utilising the telecommunications
services. The combination of these factors
seems to attract more users. In fact,
as seen today in several countries, the
growth in the number of subscribers in
wireless communications systems is
higher than the growth seen in the fixed
networks (wireline connections).
Furthermore, a number of telecommunications
services are expected to be supported
by future wireless systems. It is
assumed that the users will request most
of the services that are supported in the
wireless network. On the other hand, the
radio frequency spectrum available for
mobile communications services is limited.
Therefore, the growth in the number
of users, if it is accompanied by a corresponding
growth in the offered traffic to
the system, results in a need for new
solutions for the system structure. In
addition, a demand is expected for using
the same mobile terminal in most of the
environments, like at home, in the office,
in the streets, and so forth. Other types of
terminals could be used for special services.
The phrase “any place, any time,
any form” is often seen in the discussion
of future systems.
The result may be that a number of operators
can provide the same service as
seen from a mobile station at a certain
location. The various providers can have
different purposes for covering an area,
e.g. a company would implement wireless
services to its employees, while a
telecom operator provides the services
for the public. Which mobile station that
can utilise the various operators’ equipment
could be controlled by regulation
rules and authorisations.
Such situations, together with the increased
traffic demand, are expected to
lead to a mixture of coverage areas for
the base stations (usually named cells).
That is, some base stations cover large
areas while others cover smaller areas.
There seems to be an emerging interest
for such solutions, as they give the operators
more flexibility in the way they provide
the service in an area. However, an
increase in the complexity will also
accompany such structures.
As seen from an operator, the primary
interest can be to increase the traffic handling
capacity of a mobile communications
system. This is often measured in
the number of user connections that can
be handled (e.g. a variant of the Erlang
measure). There are also several other
measures for the performance of a
mobile telecommunications system.
A question that arises is how to model
and how to analyse such mixed cell
structures for the estimation of some teletraffic
variables. These tasks can be relevant
in several of the system life phases,
e.g. during the pre-standardisation, standardisation,
equipment design and network
dimensioning/service deployment.
One of the objectives of this paper is to
give a brief introduction to what influences
the performance of such systems.
Another objective is to present a framework
for models and corresponding analyses
of mixed cell structures.
In the following, a review of the wireless
systems will be given as they evolve in
time and for the various application areas.
Then, some of the functionality will be
described and the performance questions
discussed. In the second half of the paper,
a model for future mobile telecommunications
systems will be presented and the
main steps in the analysis are outlined.
One example is included. A list of the
abbreviations used is given at the end.
2 Mobile telecommunications
systems
Before going into the functionality and
the system performance topics, a somewhat
broader view of mobile communications
systems will be taken on.
2.1 A historical perspective
Just after H.R. Hertz’ discovery toward
the end to the nineteenth century, the
possibility of transmitting speech by the
use of telephones without wires was
recognised [3]. However, this was more
like a conjecture at that time. The first
experiments with radio on land communicating
between moving radio units
seem to have been carried out in 1921 in
Detroit, USA. This was a broadcast service
(one-way) and followed by experiments
with two-way systems that were
conducted during the 1930s. However,
during World War II the need for such
systems on a larger scale was pronounced
with more weight. One purpose
was to control the operations of the military
units. After the war the work on
improving the services was intensified,
including the possibility to establish
connections between a mobile radio unit
and a terminal in the fixed network. It
seems that the public market was not
waiting for these systems and the
demand was limited. This may explain
the reluctance of introducing new public
mobile communications services in the
1960s and the 1970s.
The cellular concept (including handover
capabilities) was proposed in the early
1970s, but not utilised before the automatic
systems were introduced in the
beginning of the 1980s (late 1970s in
Japan). Before this, manually operated
systems had been in service for several
years. If we define the cellular concept
by the reuse of frequency in distant cells,
the first idea of this seems to have been
presented in the last part of the 1940s.
The Nordic Mobile Telephone (NMT)
system became available in 1981 and
resulted in a growth in the number of
users which exceeded most of the prognoses
for the countries where it was
introduced. For larger cities the capacity
soon limited the number of users and a
system in a higher frequency band was
designed. This was also experienced in
USA where the first commercial cellular
system was introduced in 1983, and by
1984 some cells were reported to be saturated.
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