Contents Telektronikk - Telenor

- service/source coding
- interleaving
- physical level encryption
- channel coding
- burst building/formatting (duplexing
scheme)
- modulation
- multiplexing
- power control.
Most of these mechanisms must be harmonised
both in the transmitter and the
receiver side of the air interface. When
examining the capabilities of these mechanisms
in combination, the overall system
performance and the performance as
seen from each user should be considered,
as will be returned to in Section 4.1.
In addition, a number of other features
can be identified, like the equalisation
and the diversity scheme.
Error control techniques can be classified
as forward error control or automatic
repeat request. Some performance measures
relevant for error control are the
throughput (and goodput), integrity, and
delay. In this context, goodput is the rate
of error free information (user level) that
is transferred. Naturally, automatic repeat
request may introduce long delays, especially
when the radio signal conditions
are poor. This fact can be compared with
the more constant delay that forward
error control will introduce. Another
aspect is that the automatic repeat request
is more likely to keep the error control
within a given limit. These capabilities
make the different error control techniques
suitable for various services, like
automatic repeat request for data services
and forward error control for services
having real time requirements.
Returning to the portion specific for
mobile systems (the radio interface), several
channels are usually defined, as
depicted in Figure 5. Several of these
channel types have different usage patterns
and ways of control. Some are oneway
in the sense that the information
flows either to or from the base station,
e.g. like the broadcast control channel
usually carrying information from the
base station to the mobile stations. The
structure of these channels has so far
been fixed during the system specification
phase.
Various performance characteristics can
be relevant for the different channels. For
instance, when one entity (e.g. the base
station) controls the information transmission,
the maximum transfer rate and
delay could be of most interest. When
several entities access the channel, stability
considerations should be included as
well.
One observation from the preceding discussion
is that the different mechanisms
should be dynamically adjusted for the
various services and the environments,
see e.g. [1], [4]. Utilising such a flexibility
will support the introduction of
TGMS into the application areas as the
need for an overall trade-off between the
various mechanisms could be alleviated.
However, the complexity will increase,
implying a change of the cost, which
then invites for the definition of other
trade-off problems as well.
3.3 Mobility procedures
In addition to the usual call handling procedures
(call set-up, call release, authentication,
supplementary services, etc.), a
number of more or less mobile specific
procedures can be defined. Such procedures
mostly result from the fact that the
users are moving. They include:
- handover (switching an established
connection between channels in different
base stations or within a base station)
- location update (performed when a
user/terminal is crossing a location
area boundary, which can be defined
by a set of base station coverage areas)
- paging (searching for a mobile)
- registration/deregistration (linking/delinking
a user to/from a certain terminal)
- attachment/detachment (informing the
network of a terminal/user status, e.g.
power on/off for a mobile terminal).
A number of trade-off problems (optimisation
problem formulations) can be
identified, several related to the division
of the coverage area and the decisions of
which criteria to use for initiating the relevant
procedures.
A location area can be defined as the area
within which a mobile station can roam
without informing the fixed network
about its whereabouts. Entering a new
location area, the mobile station sends a
location update (registration) to the network.
When a call for a mobile station
arrives, the network has to search for
(named paging) the mobile station in the
location area where it was last registered.
Traffic
channels
Control
channels
Common
control
channels
User
specific
channels
Dedicated
control channel
Figure 5 Categories of some possible channels in a
mobile communications system
In a network with cell sizes of the same
range and without overlap, minimising
the location update traffic requests large
location areas, while minimising the paging
traffic requests small location areas.
By a suitable weighing between location
updates and paging messages, this can be
formulated as an optimisation problem,
see e.g. [12]. Such problems are expected
to become more complex in multi-layer
cellular networks.
The shape and the location of a cell may
have a major impact on the handover
traffic, e.g. as shown in [10]. This effect
is more pronounced for smaller cells
where the distinct paths (routes for
mobile stations), their orientation and
traffic flows become important. For a
larger cell the approximation that the
moving directions of the mobile stations
are uniformly distributed over (0,2π)
could be more justified. Some possible
distributions for the dwelling times in
cells are studied in [6]. Under suitable
approximations (ned total call duration,
random change of direction within a cell,
etc.), it was found that a ned cell
dwelling time can be assumed. As also
pointed out, it is doubtful whether this
result is valid for smaller cells. Then, any
regularity in the mobile stations’ activity
patterns should be considered.
The influence from the cell geometry on
the handover rates together with the
grouping of cells and the location update
rates can also be studied. The routes followed
by the mobile stations could be
taken into account when planning the
base stations’ coverage areas. Another
point is to introduce hystereses for the
decisions when a handover should be initiated
in order to avoid multiple hand-
Broadcast
control
channel
Paging
channel
Set-up
control
channel
User
packet
channel
Slow associated
control channel
Fast associated
control channel
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