CC2520_12 Datasheet, PDF (84/133 Pages) Texas Instruments – 2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER

CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
To find the actual signal power P at the RF pins with reasonable accuracy, an offset has to be added to the
RSSI value.
P = RSSI - OFFSET [dBm]
The offset is an empirical value which is found during characterization and is approximately 76 dBm for the
CC2520 reference design. E.g. reading a RSSI value of -10 from the RSSI register means that the RF input
power is approximately -86 dBm.
It is configurable how CC2520 updates 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 will contain the largest value since the
energy scan was enabled.
20.6 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 [2]. The LQI value is required by the IEEE 802.15.4 standard [2] to
be limited to the range 0 through 255, with at least 8 unique values. CC2520 does not provide a LQI value
directly, but reports several measurements that can be used by the microcontroller to calculate a LQI value.
The RSSI value may be used by the MAC software to calculate the LQI value. This approach has the
disadvantage that e.g. a narrowband interferer inside the channel bandwidth will increase the RSSI and thus
the LQI value although it actually reduces the true link quality. CC2520 therefore also provides an average
correlation value for each incoming frame, 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 CC2520 does not do chip
decision.
As described in section 20.3.4, 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 CC2520.
Software must convert the correlation value to the range 0-255 as defined by [2], 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.
84
WWW.TI.COM