User guide

TD 92422GB
18 February 2010 / Ver. E
System Planning
Ascom IP-DECT System
33
A.5 Unsynchronized Systems
If two systems operating in the same cell/coverage area are synchronized with each other
they will dynamically share the available 120 channels capacity. However, if the two
systems are unsynchronized, each call and/or dummy bearer in any of the two systems will
steal two channels from the dynamically shared resources. If more unsynchronized systems
are installed, the issue will grow.
A multiple base station system (including repeaters to these systems) has all base stations
synchronized to a central clock i.e. a base stations in such system will not disturb other
base stations in the same synchronized system. If two of these systems (unsynchronized)
or one resedential base station are installed within the coverage of the system, the
theoretical capacity in the shared radio resource will obviously decrease. In practice and to
some extent, reuse of the channels will be done depending on the position of portable
devices and base stations.
Imagine that you have one system and that you add another one. Then think about what
will happen when additional resedential base stations are installed instead of one multiple
base station system or a combination of these in one single coverage area? Add to this the
normal radio environment conditions like sliding collisions, interference from for example
reflections, portable devices in the outer part of the coverage area with low field strength,
also hearing high field strength from neighbour base stations, etc. All in all it creates load
on the base stations and the radio environment will sooner or later be drained out of free
bearers.
It is obvious that if many DECT systems are placed within the same coverage area an
increased disturbance will be experienced.
The disturbances described above are normally not a problem since DECT as a standard is
designed for coexistence with multiple systems and that any portable device or base
station that experiences a disturbance or error on the slot will communicate to the other
part (portable device/base station) that they should switch to another non-disturbed slot.
However a handover from one slot to the other does take some time and the more system
disturbances experienced the harder it will be to find a stable and non-disturbed slot.
If the dynamical channels selection algorithms in the different systems are not tuned well,
clicks, cracks, disturbances and dropped calls as well as blocked calls will be the result. If
the algorithms are well tuned the user experience will not be as critical, however the radio
capacity will still be lower.
How this in all affects the end-users depends on how high traffic it is, how dense the cell
is with other systems and how important the communication is to the users work process.
If the radio environment (in worst case) is so extremely interfered or busy that the dummy
bearer partly can not find an available slot, then the users will experience that it is not
possible to neither make nor to receive calls and the portable devices will show “No
system” in the display until a dummy bearer is found and the portable device has made a
new registration to the system.
A.6 Planning Considerations
There is no doubt that a DECT multiple base station system is preferred compared to a
high number of single cell systems. How smooth a lot of single cell systems will work
depends on the traffic capacity need. With low call rate more systems may be used, with
high call rate fewer system can be used. As the call rate increase sliding collisions appear
more frequently in unsynchronized systems.
A recommendation is that when a multiple base station system is to be installed, an
inventory of existing systems shall be made and consideration to move subscription of the
residential portable devices to the multiple base station system or to replace them all with