CC1101
SWRS061H Page 48 of 98
17.5 Clear Channel Assessment (CCA)
The Clear Channel Assessment (CCA) is used
to indicate if the current channel is free or
busy. The current CCA state is viewable on
any of the GDO pins by setting
IOCFGx.GDOx_CFG=0x09.
MCSM1.CCA_MODE selects the mode to use
when determining CCA.
When the STX or SFSTXON command strobe is
given while
CC1101
is in the RX state, the TX or
FSTXON state is only entered if the clear
channel requirements are fulfilled. Otherwise,
the chip will remain in RX. If the channel then
becomes available, the radio will not enter TX
or FSTXON state before a new strobe
command is sent on the SPI interface. This
feature is called TX-if-CCA. Four CCA
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
not currently receiving a packet)
17.6 Link Quality Indicator (LQI)
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 LQI status 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
word. LQI is best used as a relative
measurement of the link quality (a low value
indicates a better link than what a high value
does), since the value is dependent on the
modulation format.
18 Forward Error Correction with Interleaving
18.1 Forward Error Correction (FEC)
CC1101
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, i.e. when
PKTCTRL0.LENGTH_CONFIG=0. FEC is
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
in such a way that the receiver can restore the
original data in the presence of some bit
errors.
The use of FEC allows correct reception at a
lower Signal-to-Noise Ratio (SNR), thus
extending communication range if the receiver
bandwidth remains constant. Alternatively, for
a given SNR, using FEC decreases the bit
error rate (BER). The packet error rate (PER)
is related to BER by
lengthpacket
BERPER
_
)1(1
A lower BER can therefore 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 errors and, combined with
interleaving of the coded data, even correct
relatively long periods of faulty reception (burst
errors).
The FEC scheme adopted for
CC1101
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).
The convolutional coder is a rate ½ 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. This
means that in order 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. See Design Note
DN504 for more details [19].
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