User's Manual

Doc. No.
Rev.
Date
1.00
2011-12-08
Damm Cellular Systems A/S, Denmark
TETRAFLEX
®
V7.5 MANUAL - RX Diversity for BS41x
TetraFlex® 7.5 Manual 4-23
The improvement of sensitivity is considerably under fading conditions. If the two received
signals are close to being uncorrelated, the probability, that a low input signal level is present
on both inputs simultaneously becomes low, effectively removing the fading.
The sensitivity will however also be improved under static conditions, when the two signals
are fully correlated. Here close to a 3dB improvement in sensitivity is achieved. This is the
same result as if the two antennas were combined directly, however without the problem in
obtaining the correct phase of the combination.
The gain of combining under static conditions can also be seen from the result of the
combining process. If two equal signals are added, the two input signals are added,
effectively giving the double input voltage equal to an increase in input signal of 6dB. The
noise is also added. However the two noise signals are fully uncorrelated, and adding the
power of the two signals only gives an increase in noise of 3dB. The difference between the
6dB and the 3dB gives the resulting improvement of sensitivity under static conditions of 3dB.
The same improvement of 3 dB is achieved if two signals with fully correlated fading are
present. When the two received signals become uncorrelated, the sensitivity improvement
will increase.
In the following sections graphs are shown with typical measured RX sensitivity
improvements under various conditions.
4.5.2 Diversity gain versus correlation factor
This curve shows the diversity gain (RX sensitivity improvement) for an SCH/F channel
(signaling channel) with TU50 fading profile and an MER (Message Error Rate) of 4.7%.
For two fully correlated signals the gain is almost 3dB as described. When the correlation
factor approaches zero, the diversity gain is about 7.5 dB.
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0 0,2 0,4 0,6 0,8 1
Diversity gain
Correlation
Diversity gain/Correlation