Description varix 200/varix 14 (DECT)
2 - 25
2 Planning a cordless communications network
2.9
Glossary
2.9.1
Bit-error rate
In reality, there is no such thing as perfect wireless data transmission. Data transmission is sub-
ject to bit errors which invariably occur when the signals are sent via the wireless loop. To be
able to achieve transmissions without too severe interference, we require more precise infor-
mation on these bit errors. The measurement ”Bit-error Rate” (BER) is useful in this respect. By
calculating the ratio of the number of bit errors which occur to the total number of bits trans-
mitted within a specified period, we obtain a value which describes the transmission quality
of a data transmission path.
Fig. 2.14
Equation 1
For a useable voice connection, a BER of 1·10
-3
is required. The delay spread (see chapter
2.9.3) and the BER are related: the bit error rate is proportional to the square of the delay
spread and inversely proportional to the bit period. The BER thus enables us to deduce infor-
mation on the delay spread, which, in turn, is directly related to the phenomenon of multiple-
path signal transmission.
Fig. 2.15
Equation 2
t
b
:
the DECT standard specifies a bit period of 868 ns.
t
ds
:
the delay spread time, if this has achieved a value of 50 ns, the maximum BER of
1·10
-3
is clearly exceeded and it will not be able to establish a connection via this
path.
A rule to remember: the higher the observed bit error rate, the higher the delay spread due to
multiple signal paths.
Number of bit errors occurring in given period
BER = ---------------------------------------------------------------------------------------------------------------------------------------------------
Number of bits transmitted in the same period
BER
t
dS
t
b
--------
2
=
