Link Quality Indication; Radio-Control State Machine - Texas Instruments CC253x User Manual

RXFIFO Access
The status bit RSSI_VALID should be checked before reading the RSSI value register. RSSI_VALID
indicates that the RSSI value in the register is in fact valid, which means that the receiver has been
enabled for at least eight symbol periods.
To find the actual signal power P at the RF pins with reasonable accuracy, an offset must be added to the
RSSI value.
P = RSSI – OFFSET [dBm]
For example, with an offset of 73 dB, reading an RSSI value of –10 from the RSSI register means that the
RF input power is approximately –83 dBm. For the correct offset value to use, see the data sheet
(Appendix C).
There are two ways the radio can update the RSSI register after it has first become valid. If
FRMCTRL0.ENERGY_SCAN = 0 (default), the RSSI register contains the latest value available, but if this bit
is set to 1, a peak search is performed, and the RSSI register contains the largest value since the energy
scan was enabled.

23.10.4 Link Quality Indication

The link quality indication (LQI) is a measurement of the strength and/or quality of the received frame as
defined by the IEEE 802.15.4 standard [1]. The LQI value is required by the IEEE 802.15.4 standard [1] to
be limited to the range 0 through 255, with at least eight unique values. The radio does not provide an LQI
value directly, but reports several measurements that can be used by the microcontroller to calculate an
LQI value.
The RSSI value can be used by the MAC software to calculate the LQI value. This approach has the
disadvantage that, for example, a narrowband interferer inside the channel bandwidth can increase the
RSSI and thus the LQI value, although the true link quality actually decreases. The radio therefore also
provides an average correlation value for each incoming frame, based on the first eight symbols following
the SFD. This unsigned 7-bit value can be looked on as a measurement of the chip error rate, although
the radio does not do chip decision.
As described in Section
each received frame, together with the RSSI and CRC OK or not-OK, when FRMCTRL0.AUTOCRC is set. A
correlation value of approximately 110 indicates a maximum-quality frame, whereas a value of
approximately 50 is typically the lowest-quality frame detectable by the radio.
Software must convert the correlation value to the range 0–255 as defined by [1], for instance 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.

23.11 Radio-Control State Machine

The FSM module is responsible for maintaining the TXFIFO and RXFIFO pointers, control of analog
dynamic signals such as power up and power down, control of the data flow within the RF Core,
generation of automatic acknowledgment frames, and control of all analog RF calibration.
234
CC253x Radio
23.9.7, the average correlation value for the first eight symbols is appended to
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SWRU191F – April 2009 – Revised April 2014
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