Clear Channel Assessment (CCA)
The Clear Channel Assessment
to indicate if the current channel is free or
busy. The current CCA state is viewable on
is in the RX state, the TX
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 sent on the SPI interface)
requirements can be programmed:
requirements can be programmed:
Always (CCA disabled, always goes to TX)
If RSSI is below threshold
Unless currently receiving a packet
Both the above (RSSI below threshold and
Link Quality Indicator (LQI)
The Link Quality Indicator is a metric of the
current quality of the received signal. If
value is automatically added to the last byte
d after the payload. The value can
LQI gives an estimate of how easily a received
signal can be demodulated by accumulating
the magnitude of the error between ideal
the received signal over the
64 symbols immediately following the sync
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
Forward Error Correction with Interleaving
Forward Error Correction (FEC)
has built in support for Forward Error
Correction (FEC). To enable this option, set
employed on the data field and CRC word in
order to reduce the gross bit error rate when
operating near the sensitivity limit.
Redundancy is added to the transmitted data
n such a way that the receiver can restore the
original data in the presence of some bit
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
packet error rate (PER) is related to BER by:
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 en
produce occasional errors even in otherwise
good reception conditions. FEC will mask such
errors and, combined with interleaving of the
coded data, even correct relatively long
periods of faulty recepti
The FEC scheme adopted for
convolutional coding, in which
most recent input bits, forming a code stream
able to withstand a certain number of 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.
transmit at the same effective data
using FEC, it is necessary to use twice as high
rate. This will require a higher
receiver bandwidth, and thus reduce
sensitivity. In other words
reception by using FEC and the degraded
sensitivity from a higher receiver
will be counteracting factors.
Data received through radio channels will
often experience burst errors due to
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