User's Manual

43
Document Number: 0100SM1401 Issue: 05-15
BER & Fade Margin
The BER specification used by the Q data radios
shows ‘1 x 10E -6 BER’. This means one bit of data
in a million bits of data will error at the BER threshold
level. The BER threshold values can vary depending
on the over the air data rate (RF speed). The BER
thresholds can be found in the table shown.
Fade Margin is the amount by which a received
signal level may be reduced without causing system
performance to fall below a specified threshold
value, in this case, the specified threshold value is
defined by the radio’s Bit Error Rate (BER).
20dB to 30dB fade margins are typical design
targets, however, 15db may still be acceptable if
this is all that is possible.
Fade margin is required to maintain the radio link quality if the signal fades, factors that can influence signal fading include:
• Reflections
• Rain
• Maintenance Issues
When a Q data radio system is configured for Dynamic mode, the 20 dB (typical) fade margin calculation may be performed
using the lowest data rate, for best receiver sensitivity. (8 kbps in a 12.5 kHz channel or 14 kbps in a 25 kHz channel) This
helps to ensure a strong reliable path when conditions are poor
The amount of fade margin a radio link has can be calculated by performing a theoretical system gain calculation.
Factors that can influence system gain include: Tx power of Transmitting Radio, Coaxial Cable, Antenna Gains, Free Space
Loss, Rx sensitivity of receiving radio. See the example below.
The example is showing the factors within a radio link that can influence the signal strength of the transmitting radio at the
receiving radio (total system gain). Once the total system gain is calculated, the result is -88 dB. This means the received
signal strength (RSSI) at the receiving radio should be -88dBm.
As the BER threshold of the receiving radio is at -113 dBm, with an RSSI value of -88, the fade margin, before 1 bit in a
million bits of data become lost, is 25 dB. In other words, the RSSI level at the receiving radio can fluctuate up to -25 dB
before data loss may start to occur in over the air transmissions.
Regulatory
Region
Channel Bandwidth
(KHz)
RF Speed (Kbps)
BER threshold
(10^6)
FCC
12.5
8
16
24
32
-113
-110
-107
-100
ACMA/
ETSI
12.5
8
16
24
32
-113
-110
-107
-100
25
14
28
42
56
-111
-109
-106
-99
Part E – Radio Planning and Design
Total System Gain = -88 dB
BER = -113 dBm @ 10 ^ 6
Fade Margin = + 25 dB
Receiving Radio
BER = -113 dBm @ 10 ^ 6
Transmitting Radio