User guide

TD 92422GB
18 February 2010 / Ver. E
System Planning
Ascom IP-DECT System
34
portable devices subscribed in the multiple base station system shall be made. In an office
environment with high call traffic only very few DECT systems may be operational. In a low
traffic environment a few resedential base stations may be used within the enterprise and
will most certainly not be a problem.
If unsynchronized systems are installed, the radio units should be placed as far as possible
apart from each other to limit the interference. Base stations from different systems shall if
possible not be placed directly beside or underneath each other.
A.7 Impact on Air Synchronization
Figure 16. Impact on air synchronization
The signal used for air synchronization in the IP-DECT system might be weak, normally
about -90 dBm. In the above example in
figure 16, RFP-B tries to synchronize to RFP-A, but
RFP-B is also affected by an alien DECT system with much higher signal strength then the
synchronization source (RFP-A).
According to ETSI EN 300 175-2 a DECT radio has a signal /interfering requirement for the
same channel of 11dB. This means that the signal you want to use in the receive direction
must be 11 dB stronger than the interfering signal on the same channel. In the example
above the alien RFP with a potential interfering signal have a 40 dB stronger signal. Signals
from the alien RFP must be -101 dB to guarantee that the -90 dBm signal can be received
without major loss.
But is it not 120 available duplex channels in DECT you may ask. Yes it is, but to be able to
avoid one of the “used” channels it must be possible to detect a signal in this channel.
RFP-A has no chance of discovering an interfering signal from the alien RFP as the signal
from the alien RFP is very week (-100 dBm) at RFP-A.
As described with the above example one single alien DECT system may result in a
situation where the air synchronization has no chance to operate properly. The only
chance to guarantee successful operation in this case is to place the IP-DECT base stations
closer to each other or to remove the alien DECT system. If it can be assumed that signals
from alien DECT systems have signal strength less then -50 dBm, the “distance” between
IP-DECT base stations must be -39 dBm. This result in a much denser IP-DECT
infrastructure. If other DECT systems exist on the site you should try to remove as many of
these as possible.
IP-DECT RFP-A Alien RFP
IP-DECT RFP-B
-90 dBm -50 dBm
-100 dBm