Rainbow Electronics RC2100 - Manual (Page 17)

RC2100
 2005 Radiocrafts AS RC2100 Data Sheet (rev. 1.0) Page 17 of 22
RSSI and Link Quality Indicator
The module provide a digital Received Signal Strength Indicator (RSSI) through the ‘S’
command and a Link Quality Indicator (LQI) through the ‘Q’ command. The module returns an
8 bit character indicating the signal strength or link quality for the last received data packet ,
not the instantaneous value. The signal strength and the link quality indicator can be used as
an indication of the fading margin.
The RSSI value is a 2’s complement value and is a measure of the input power. To refer the
measurement to actual input power in dBm use the following equation giving an approximate
value:
P = RSSI - 45 [dBm]
The -45 dB offset may be found empirically in an actual application taking into account any
loss in the antenna input.
The LQI is based on a correlation value from the DSSS receiver. The LQI is an unsigned 7-bit
value expressing a kind of “chip error rate” measurement. A value of approximately 110 is the
best quality, while 50 is approximately the lowest quality where frames still can be received.
A narrow band interferer will give a high RSSI value even if the link quality actually is bad. A
better measurement of the true link quality is therefore provided by the LQI. A combination of
RSSI and LQI can also be used to evaluate the channel.
Clear Channel Assessment
The operation of pin 23 can be configured by setting CCA_ENABLE in the configuration
memory. If CCA_ENABLE = 1 the pin is a Clear Channel Assessment (CCA) output. The
channel is clear if CCA is high. The CCA is based on an RSSI measurement and search for a
data frame. If no frames are detected and the RSSI is below the threshold limit, the channel is
assessed to be clear.
The carrier sense limit can be adjusted by setting CCA_LEVEL. The default value is 0xE0 (-
32 decimal coded as 2’complement). The default value corresponds to approximately -77
dBm. The threshold is approximately given by:
CCA Threshold = CCA_LEVEL – 45 [dBm]
If CCA_ENABLE = 0 pin 23 is used as a PA_EN (active low) output. This signal can be used
to control an external power amplifier and T/R switch.
The CCA signal at pin 23 (when CCA_ENABLE = 1) can be used to check if the channel is
free before transmission when implementing a Carrier Sense Multiple Access / Collision
Avoidance (CSMA/CA) scheme.