Network Planning and Diagnostics
Wireless Communication Stability Considerations
SKF Wireless Machine Condition Sensor
63
Service Manual
Material
Signal attenuation
Metal frame glass with reflective coating (wall or
window)
6 dB
Interior office wall
6 dB
Glass window with security wire
8 dB
Concrete wall
10 to 15 dB
The wireless sensor’s sensitivity is approximately -90 dBm. For planning purposes, the
gateway’s transmitter power starts at 10 dBm and the antenna adds 2 dBi. Each
obstacle subtracts from the signal strength.
Link Budget Calculation Example
You can estimate the Receive Signal Strength Index (RSSI) at the sensor as shown in
this example. The gateway and a sensor are 50 m apart and there are two concrete
walls in between them.
P+F Gateway radio power
10 dBm
Antenna gain
+2 dBi
FSPL (see note below)
-74 dB
2 concrete walls
-15 dB * 2
=
-92 dBm (RSSI)
Because the estimated RSSI of -92 dBm is below the sensitivity of the receiver in the
sensor (-90 dBm), the connection will be marginal to bad. (Without adding the two
concrete walls, the RSSI of -62 dBm is a good connection.)
FSPL
(dB) is Free Space Path Loss.
For 2.4 GHz, FSPL
(dB) = 20log
10
(d) + 40.19, where d = distance
between the RF source and destination in meters.
For 50 m, FSPL = 20log
10
(50) + 40.19 = 74 dB.
The system margins can be determined by comparing this sensitivity with the received
signal level. Even with good signal strength, stability can be negatively influenced by
wireless networks or other noise emitters in the 2.4 GHz band. You can check the radio
RSSI
and
Stability
on the gateway’s
Diagnostics
>
Wireless Communication
>
Details
web page, which is discussed later in this chapter.