User guide

TD 92422GB
18 February 2010 / Ver. E
System Planning
Ascom IP-DECT System
13
6.4.2 How to Identify Reflective Environment
A high time delay spread will only have influence if the delayed signal is strong. In office
environment we also have signals arriving to the receiver with high delay but since these
signals have travelled trough walls, been reflected in attenuation materials (wood, cement,
etc) the reflections are highly attenuated. These signals with low power level will not cause
any problem. However, if there are no walls and the reflective surfaces have low
attenuation, e.g. metal surfaces, the power level of the reflective signals will be high, this
is what we call reflective environment. So characteristics for reflective environment are:
• Although the signal strength is good there are frame errors. The frame error rate and
signal strength can be measured with the portable device. Check frame errors in both
uplink (towards the base station) and downlink (towards the portable device). For
information on how to measure the frame error rate, see the user guide for the site
survey tool.
• Often no problems when standing still but when you move around there will be
problems with mutes and clicks during speech.
• Characteristics for reflective environment are large open spaces (e.g. large buildings
greater than say 20 meters), metal walls, metal inventories and ceilings.
6.4.3 Locations for Base Stations in Reflective Environment
Apart from the time delay spread there are a number of general rules to consider when
placing a base station:
• Install a base station in line of sight
1
.
• It is possible to increase the number of base stations at those locations that are
important to the client. In a typical case, the speech coverage area is 5-20 meter. This
must be verified with site survey.
• Place the base station as low as possible without having something to be placed in
front of the base station.
• Ensure that the distance between the base station antennas and a metal wall is at least
30 centimetres to avoid interference with the impedance of the antennas.
• To get a strong direct signal, the use of a directional antenna may improve the
situation.
Note: All these considerations, depend on the dimensions of the hall and its reflective
characteristics.
6.4.4 Customer Acceptance
It is very important that the radio performance is verified before any agreement is signed.
Verify the radio performance with a site survey tool and always use the same type of
portables and also the same type of base station that is going to be installed.
Make sure that the customer understands what kind of problem that can occur because of
the reflective environment. Ask the customer to listen to the speech quality. If the speech
quality is not accepted by the customer, do not recommend an installation of the DECT
system in this reflective environment in order to avoid future severe problems.
1. The base station ‘sees’ the user