User's Manual
41
Document Number: 0100SM1401 Issue: 10-14
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
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 -116 dBm, with an RSSI value of -88, the fade margin, before 1 bit in a
million bits of data become lost, is 28 dB. In other words, the RSSI level at the receiving radio can fluctuate up to -28 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