AT86RF230_09 ATMEL [ATMEL Corporation], AT86RF230_09 Datasheet - Page 54

• Manufacturer Part Number: AT86RF230_09
• Description: Low Power 2.4 GHz Transceiver for ZigBee, IEEE 802.15.4, 6LoWPAN, RF4CE and ISM
• Manufacturer: ATMEL [ATMEL Corporation]

8.5.3 Data Interpretation
8.6 Clear Channel Assessment (CCA)
8.6.1 Overview
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AT86RF230
information can be read as an extra byte from the Frame Buffer (see section 9.1). The
LQI byte can be uploaded after the TRX_END interrupt.
A low LQI value is associated with low signal strength and/or high signal distortions.
Signal distortions are mainly caused by interference signals and/or multipath
propagation. High LQI values indicate a sufficient high signal power and low signal
distortions.
Note, the received signal power as indicated by received signal strength indication
(RSSI) value or energy detection (ED) value do not characterize the signal quality and
the ability to decode a signal. As an example, a received signal with an input power of
about 6 dB above the receiver sensitivity likely results in a LQI value close to 255 for
radio channels with very low signal distortions. For higher signal power levels the LQI
value becomes independent of the actual signal strength. This is because the packet
error rate for these scenarios tends towards zero and further increase of the signal
strength, i.e. by increasing the transmission power, does not decrease the error rate
any further. In this case RSSI or ED can be used to evaluate the signal strength and the
link margin.
ZigBee networks require to determine the “best” route between two nodes. Both, the
LQI and the RSSI/ED can be used for this, depending on the optimization criteria. As a
rule of thumb RSSI/ED is useful to differentiate between links with high LQI values.
Transmission links with low LQI values should be discarded for routing decisions even if
the RSSI/ED values are high.
The main features of the Clear Channel Assessment (CCA) module are:
• All 3 modes available as defined by IEEE 802.15.4-2003 in section 6.7.9
• Adjustable threshold of the energy detection algorithm
The CCA is used to detect a clear channel. There are three modes available as defined
in Table 8-4.
Table 8-4. Available CCA Modes
CCA Mode
1
2
3
Description
Energy above threshold.
CCA shall report a busy medium upon detecting any energy above the ED
threshold.
Carrier sense only.
CCA shall report a busy medium only upon the detection of a signal with the
modulation and spreading characteristics of IEEE 802.15.4. This signal may be
above or below the ED threshold.
Carrier sense with energy above threshold.
CCA shall report a busy medium only upon the detection of a signal with the
modulation and spreading characteristics of IEEE 802.15.4 with energy above
the ED threshold.
5131E-MCU Wireless-02/09