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subscribers. Service structure contains information
about traffic behaviour, resources usage relative
information and the maximum values of supported
packet delay. All service parameters defined will
ensure the tool versatility to create new services and
to change the existing services parameters. These
parameters are:
• Source bit rate – average UL and DL source bit
rate;
• Channel bit rate – average UL and DL physical
source rate. For this value estimation the signalling
channel has been taken into account, as well as
CRC and tail bits attachment, coding and final rate
matching into UMTS standard channel bit rates;
• Switching mode – packet or circuit switched
mode;
• Maximum transfer delay – a quality of service
parameter. If the delay experimented by a packet
exceed this value, the session will be dropped.
• Source traffic model – one of three available
models. Choice depends on switching mode and
service characteristics;
• Source traffic model parameters – values that
define traffic behaviour.
3.2 Traffic Models
In order to generate the UMTS multi-service traffic,
three source models had been implemented. Traffic
models choice not only considers resources usage
behaviour, but also the flexibility to create new
services and implementation viability on a
reasonable computational platform. On this basis,
the models that had been implemented are:
• Poisson – model indicated to circuit switched
services. In this model calls are generated
according to a Poisson process and call duration
may be described by several probability
distributions, which depends on service
applications type.
• ON-OFF – a simple packet services model, that
includes inactivity periods and activity periods into
a packet session. For both UL and DL direction
packets generation configuration, several
parameters are provided. A complete model
description can be found in (Cota, 2004).
• ETSI – the implementation of ETSI TR 101-112
(UMTS, 1998) model for packet-switched traffic.
In this model it is considered that a session
consists of a sequence of packet calls. During a
packet call, several packets may be generated with
different sizes. This model considers only traffic in
DL direction.
3.3 Mobility
In mobile systems, the terminal mobility has an
important influence on service performance. Due to
the UMTS characteristics, this influence must be
considered and quantified on simulation tools, in
order to achieve an accurate performance evaluation.
On the other hand, mobility inclusion brings an
increased complexity of simulation model. Based on
these facts, a simple but flexible mobility model was
included.
The simulation tool considers five classes that
describe the existing mobility types in an urban
scenario:
• Static;
• Pedestrian;
• Low speed vehicular;
• Medium speed vehicular;
• High speed vehicular.
The model adopted to describe subscriber’s
motion direction and speed was the Gauss-Markov
Mobility Model (Liang, 1999) that was designed to
adapt to different levels of randomness via two
tuning parameters. At fixed intervals of time,
movement occurs by updating the speed and
direction of each terminal. Specifically, the value of
speed and direction at one instance is calculated
based upon the value of speed and direction at the
previous
instance and a random variable using
equations (Cota, 2004).
However, this mobility model has a limitation
due the fact that he does not consider the urban
morphology. In order to become a more realistic
model, an improvement was made, considering the
vector data of streets and crossroads. Thus, it is
considered that vehicles moves along streets and
may turn at cross streets with a given probability.
However, streets change only occurs if new street
support terminal speed. Speed limitation can be
configured to each road of the database.
3.4 User Profiles
To evaluate the demand of WCDMA resources, it is
necessary to identify different types of customers,
since their calling patterns, usage and mobility
behaviour may differ. Thus, system subscribers
population would be classified according to their
user profiles, which includes information about
service usage, mobility and placement.
User profile contains a service list with the busy
hour call attempts (BHCA) respected values.
Placement information refers to location limits
during simulation, e.g., a high-speed vehicular
subscriber cannot transit to an indoor environment.
ICETE 2004 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS
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