CC2430-RTR1-F64Z datasheet(182/234 Pages) TI

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CC2430

CC2430 PRELIMINARY Data Sheet (rev. 1.03) SWRS036A

Page 182 of 232

14.24 Link Quality Indication

The

link

quality

indication

(LQI)

measurement is a characterization 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 eight unique

values.

Software

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.

CC2430 therefore also provides

an average correlation value for each

incoming packet, based on the eight first

symbols following the SFD. This unsigned

7-bit value can be looked upon as a

measurement of the “chip error rate,”

although

CC2430

does not do chip

decision.

described

the

Frame

check

sequence section on page 175, the

average correlation value for the eight first

symbols is appended to each received

frame together with the RSSI and CRC

OK/not OK when MDMCTRL0L.AUTOCRC

is set. A correlation value of approx. 110

indicates a maximum quality frame while a

value of approx. 50 is typically the lowest

quality frames detectable by

CC2430.

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.

14.25 Clear Channel Assessment

The clear channel assessment signal is

based on the measured RSSI value and a

programmable

threshold.

The

clear

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

sense

threshold

level

programmed by RSSI.CCA_THR. The

threshold value can be programmed in

steps of 1 dB. A CCA hysteresis can also

programmed

the

MDMCTRL0H.CCA_HYST control bits.

All three CCA modes specified by [1] are

implemented in

CC2430. These are set in

MDMCTRL0L.CCA_MODE, as can be seen

in the register description. The different

modes are:

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

Clear channel assessment is available on

the RFSTATUS.CCA RF register bit.

RFSTATUS.CCA is active high. This

RFIF.IRQ_CCA.

Implementing CSMA-CA may easiest be

done by using the STXONCCA command

strobe given by the CSMA-CA/strobe

processor, as shown in the Radio control

state machine section on page 179.

Transmission will then only start if the

channel is clear. The TX_ACTIVE status

bit in the RFSTATUS RF register may be

used to detect the result of the CCA.

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