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AT86RF232
If, instead, the mean LQI over a large number of transmissions is computed, and the
mean LQI is quantized to an LQI value of the figure, the corresponding frame error rate
is not strictly equal to the true error rate.
The values are taken from received frames of PSDU length of 20 octets on
transmission channels with reasonable low multipath delay spreads. If the transmission
channel characteristic has higher multipath delay spread than assumed in the example,
the PER is slightly higher for a certain LQI value.
Since the packet error rate is a statistical value, the PER shown in Figure 8-13 is based
on a huge number of transactions. A reliable estimation of the packet error rate cannot
be based on a single or a small number of LQI values.
8.6.2 Request an LQI Measurement
The LQI byte can be obtained after a frame has been received by the radio transceiver.
One additional byte is automatically attached to the received frame containing the LQI
value. This information can also be read via Frame Buffer read access, see
Section 6.2.2 . The LQI byte can be read after IRQ_3 (TRX_END) interrupt.
8.6.3 Data Interpretation
According to IEEE 802.15.4 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:
1.
The received signal power as indicated by received signal strength indication
(RSSI) value or energy detection (ED) value of the Atmel AT86RF232 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 6dB 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 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 increased signal strength, that is 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.
Zi gBee networks often require the identification of the “best” routing between two
nodes. Both, the LQI and the RSSI/ED can be used for this, dependent on the
optimization criteria. If a low packet error rate (corresponding to high throughput) is the
optimization criteria then the LQI value should be taken into consideration. If a low
transmission power or the link margin is the optimization criteria then the RSSI/ED
value is also helpful.
Combinations of LQI, RSSI and ED are possible for routing decisions. As a rule of
thumb RSSI and ED values are 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. This is because RSSI/ED does not say
anything about the possibility to decode a signal. It is only an information about the
received signal strength whereas the source can be an interferer.
99
8321A – MCU Wireless – 10/11
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