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3.9.5.2
5/1531-ANF 901 14 Uen D3 2013-11-28
2
Base
station
A
Figure 15: Time delay spread.
The base station at corner A reaches the cordless phone at corner B by
means of a direct signal (signal 1) and by means of a reflected indirect
signal (signal 2). Generally there are many reflected signals reaching the
cordless phone. In general the paths travelled by these signals are not
equal which means in turn that they will arrive at the cordless phone at
different times.
A DECT signal consists of frames of 420 bits transmitted every 10 ms.
The bit length for DECT is 0.868 µs. Mutes, interrupts in you speech
conversation, and clicks on the line will occur if the time difference
between the various received signals is of the order of 1/10 bit length. If
this occurs, the receiver has difficulty in distinguishing between the
different transmitted bits. Therefore, the base stations in a metal hall
must be sited in a way that the time delay spread is minimized. This
means in turn that you must locate the base stations such that the
number of reflections is minimized
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 char-
acteristics for reflective environment are:
Reflecting wall
2
1
B
ASE STATION PLANNING
B
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