CC1110Fx / CC1111Fx
13.10.7 Clear Channel Assessment (CCA)
Always (CCA disabled, always goes to
TX)
The Clear Channel Assessment CCA) is used
to indicate if the current channel is free or
busy. The current CCA state is viewable on
If RSSI is below threshold
Unless currently receiving a packet
P1_5,
P1_6,
or
P1_7
by
setting
IOCFGx.GDOx_CFG=1001.
Both the above (RSSI below threshold
and not currently receiving a packet)
MCSM1.CCA_MODE selects the mode to use
when determining CCA.
13.10.8 Link Quality Indicator (LQI)
When the STXor SFSTXONcommand strobe is
given while CC1110Fx/CC1111Fx is in the RX state,
the TX or FSTXON state is only entered if the
clear channel requirements are fulfilled. The
chip will otherwise remain in RX (if the channel
becomes available, the radio will not enter TX
or FSTXON state before a new strobe
command is being issued). This feature is
called TX-if-CCA. Note that when using this
function the register TEST1 on Page 226
should be set to 0x31.
The Link Quality Indicator is a metric of the
current quality of the received signal. If
PKTCTRL1.APPEND_STATUS is enabled, the
value is automatically added to the last byte
appended after the payload. The value can
also be read from the LQIstatus register. The
LQI gives an estimate of how easily a received
signal can be demodulated by accumulating
the magnitude of the error between ideal
constellations and the received signal over the
64 symbols immediately following the sync
Four CCA requirements can be programmed:
word. LQI is best used as
a
relative
measurement of the link quality (a high value
indicates a better link than what a low value
does), since the value is dependent on the
modulation format.
13.11 Forward Error Correction with Interleaving
13.11.1 Forward Error Correction (FEC)
errors and, combined with interleaving of the
coded data, even correct relatively long
periods of faulty reception (burst errors).
CC1110Fx/CC1111Fx has built in support for
Forward Error Correction (FEC). To enable this
option, set MDMCFG1.FEC_EN to 1. FEC is
only supported in fixed packet length mode
(PKTCTRL0.LENGTH_CONFIG=0). FEC is
employed on the data field and CRC word in
order to reduce the gross bit error rate when
The FEC scheme adopted for CC1110Fx/CC1111Fx
is convolutional coding, in which n bits are
generated based on k input bits and the m
most recent input bits, forming a code stream
able to withstand a certain number of bit errors
between each coding state (the m-bit window).
operating
near
the
sensitivity
limit.
Redundancy is added to the transmitted data
in such a way that the receiver can restore the
original data in the presence of some bit
errors.
The convolutional coder is a rate 1/2 code with
a constraint length of m=4. The coder codes
one input bit and produces two output bits;
hence, the effective data rate is halved. I.e. to
transmit at the same effective data rate when
using FEC, it is necessary to use twice as high
over-the-air data rate. This will require a higher
receiver bandwidth, and thus reduce
sensitivity. In other words, the improved
reception by using FEC and the degraded
sensitivity from a higher receiver bandwidth will
be counteracting factors.
The use of FEC allows correct reception at a
lower SNR, thus extending communication
range. Alternatively, for a given SNR, using
FEC decreases the bit error rate (BER). As the
packet error rate (PER) is related to BER by:
PER 1 (1 BER)packet_length
,
a lower BER can be used to allow longer
packets, or a higher percentage of packets of a
given length, to be transmitted successfully.
Finally, in realistic ISM radio environments,
transient and time-varying phenomena will
produce occasional errors even in otherwise
good reception conditions. FEC will mask such
13.11.2 Interleaving
Data received through radio channels will often
experience burst errors due to interference and
time-varying signal strengths. In order to
increase the robustness to errors spanning
SWRS033H
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TI [ TEXAS INSTRUMENTS ]
