2 request a lqi measurement, 3 data interpretation, Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
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8266A-MCU Wireless-12/09
ATmega128RFA1
can be associated with an expected packet error rate. The PER is the ratio of erroneous
received frames to the total number of received frames. A PER of zero indicates no
frame error whereas at a PER of one no frame was received correctly.
The radio transceiver uses correlation results of multiple symbols within a frame to
determine the LQI value. This is done for each received frame. The minimum frame
length for a valid LQI value is two octets PSDU. LQI values are integers ranging from 0
to 255.
The following figure shows an example of a conditional packet error rate when receiving
a certain LQI value.
Figure 9-19. Conditional Packet Error Rate versus LQI
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
50
100
150
200
250
P
E
R
LQI
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 a 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
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.
9.5.6.2 Request a 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
accessible Frame Content" on page 78
. The LQI byte can be read after the
TRX24_RX_END interrupt.
9.5.6.3 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.