Data Sheet

CC1101
SWRS061B Page 38 of 93
17.4 Carrier Sense (CS)
Carrier Sense (CS) is used as a sync word
qualifier and for CCA and can be asserted
based on two conditions, which can be
individually adjusted:
• CS is asserted when the RSSI is above a
programmable absolute threshold, and de-
asserted when RSSI is below the same
threshold (with hysteresis).
• CS is asserted when the RSSI has
increased with a programmable number of
dB from one RSSI sample to the next, and
de-asserted when RSSI has decreased
with the same number of dB. This setting
is not dependent on the absolute signal
level and is thus useful to detect signals in
environments with time varying noise floor.
Carrier Sense can be used as a sync word
qualifier that requires the signal level to be
higher than the threshold for a sync word
search to be performed. The signal can also
be observed on one of the GDO pins by
setting IOCFGx.GDOx_CFG=14 and in the
status register bit PKTSTATUS.CS.
Other uses of Carrier Sense include the TX-if-
CCA function (see Section 17.5 on page 39)
and the optional fast RX termination (see
Section 19.7 on page 45).
CS can be used to avoid interference from
other RF sources in the ISM bands.
17.4.1 CS Absolute Threshold
The absolute threshold related to the RSSI
value depends on the following register fields:
• AGCCTRL2.MAX_LNA_GAIN
• AGCCTRL2.MAX_DVGA_GAIN
• AGCCTRL1.CARRIER_SENSE_ABS_THR
• AGCCTRL2.MAGN_TARGET
• For a given AGCCTRL2.MAX_LNA_GAIN
and AGCCTRL2.MAX_DVGA_GAIN setting the
absolute threshold can be adjusted ±7 dB in
steps of 1 dB using
CARRIER_SENSE_ABS_THR.
The MAGN_TARGET setting is a compromise
between blocker tolerance/selectivity and
sensitivity. The value sets the desired signal
level in the channel into the demodulator.
Increasing this value reduces the headroom
for blockers, and therefore close-in selectivity.
It is strongly recommended to use SmartRF
®
Studio to generate the correct MAGN_TARGET
setting.
Table 26 and Table 27 show the typical RSSI
readout values at the CS threshold at 2.4
kBaud and 250 kBaud data rate respectively.
The default CARRIER_SENSE_ABS_THR=0 (0
dB) and MAGN_TARGET=3 (33 dB) have been
used.
For other data rates the user must generate
similar tables to find the CS absolute
threshold.
MAX_DVGA_GAIN[1:0]
00 01 10 11
000 -97.5 -91.5 -85.5 -79.5
001 -94 -88 -82.5 -76
010 -90.5 -84.5 -78.5 -72.5
011 -88 -82.5 -76.5 -70.5
100 -85.5 -80 -73.5 -68
101 -84 -78 -72 -66
110 -82 -76 -70 -64
MAX_LNA_GAIN[2:0]
111 -79 -73.5 -67 -61
Table 26: Typical RSSI Value in dBm at CS
Threshold with Default MAGN_TARGET at 2.4
kBaud, 868 MHz
MAX_DVGA_GAIN[1:0]
00 01 10 11
000 -90.5 -84.5 -78.5 -72.5
001 -88 -82 -76 -70
010 -84.5 -78.5 -72 -66
011 -82.5 -76.5 -70 -64
100 -80.5 -74.5 -68 -62
101 -78 -72 -66 -60
110 -76.5 -70 -64 -58
MAX_LNA_GAIN[2:0]
111 -74.5 -68 -62 -56
Table 27: Typical RSSI Value in dBm at CS
Threshold with Default MAGN_TARGET at 250
kBaud, 868 MHz
If the threshold is set high, i.e. only strong
signals are wanted, the threshold should be
adjusted upwards by first reducing the
MAX_LNA_GAIN value and then the