CC2420 Chipcon AS, CC2420 Datasheet - Page 46

• Manufacturer Part Number: CC2420
• Description: 2.4 GHz RF Transceiver for IEEE 802.15.4 and ZigBee
• Manufacturer: Chipcon AS

Link Quality Indication
The
measurement is a characterisation of the
strength and/or quality of a received
packet, as defined by [1].
The RSSI value described in the previous
section may be used by the MAC software
to produce the LQI value. The LQI value is
required by [1] to be limited to the range 0
through 255, with at least 8 unique values.
Software is responsible for generating the
appropriate scaling of the LQI value for the
given application.
Using the RSSI value directly to calculate
the LQI value has the disadvantage that
e.g. a narrowband interferer inside the
channel bandwidth will increase the LQI
value although it actually reduces the true
link quality.
an average correlation value for each
incoming packet, based on the 8 first
symbols following the SFD. This unsigned
7-bit value can be looked upon as a
measurement of the “chip error rate,”
although
decision.
Clear Channel Assessment
The clear channel assessment signal is
based on the measured RSSI value and a
programmable
channel assessment function is used to
implement the CSMA-CA functionality
specified in [1]. CCA is valid when the
receiver has been enabled for at least 8
symbol periods.
Carrier
programmed by RSSI.CCA_THR. The
threshold value can be programmed in
steps of 1 dB. A CCA hysteresis can also
be
MDMCTRL0.CCA_HYST control bits.
All 3 CCA modes specified by [1] are
implemented in
MDMCTRL0.CCA_MODE, as can be seen in
the register description. The different
modes are:
Chipcon AS SmartRF ® CC2420 Preliminary Datasheet (rev 1.0), 2003-11-17
link
programmed
sense
CC2420
CC2420
quality
CC2420
threshold.
therefore also provides
threshold
does not do chip
. They are set in
indication
in
The
level
(LQI)
clear
the
is
SmartRF
As
sequence section on page 33, the average
correlation value for the 8 first symbols is
appended to each received frame together
with the RSSI and CRC OK/not OK when
MDMCTRL0.AUTOCRC is set. A correlation
value of ~110 indicates a maximum quality
frame while a value of ~50 is typically the
lowest
CC2420
Software must convert the correlation
value to the range 0-255 defined by [1],
e.g. by calculating:
LQI = (CORR – a) · b
limited to the range 0-255, where a and b
are found empirically based on PER
measurements as a function of the
correlation value.
A combination of RSSI and correlation
values may also be used to generate the
LQI value.
0
1
2
3
Clear channel assessment is available on
the CCA output pin. CCA is active high, but
the polarity may be changed by setting the
IOCFG0.CCA_POLARITY control bit.
Implementing CSMA-CA may easiest be
done by using the STXONCCA command
strobe, as described in the Radio control
state machine section on page 39.
Transmission will then only start if the
described
Reserved
Clear channel when received energy is below
threshold.
Clear channel when not receiving valid IEEE
802.15.4 data.
Clear channel when energy is below threshold
and not receiving valid IEEE 802.15.4 data
.
quality
in
frames
®
the
CC2420
detectable
Frame
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