CC2420Z-RTR1 datasheet(49/92 Pages) TI

49 / 92 page

CC2420

SWRS041B

Page 49 of 89

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RF Level [dBm]

Figure 27. Typical RSSI value vs. input power

24 Link Quality Indication

The

link

quality

indication

(LQI)

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.

CC2420 therefore also provides

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

CC2420 does not do chip

decision.

described

the

Frame

check

sequence section on page 38, 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

quality

frames

detectable by

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.

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