Link Quality Indication

Link Quality Indicator (LQI) is used in Zigbee networks to indicate how strong the communications link is. LQI is a computed value, based on the received signal strength as well as the number of errors received.

According to the IEEE 802.15.4 specification (section 6.7.8) "The minimum and maximum LQI values (0x00 and 0xFF) should be associated with the lowest and highest IEEE 802.15.4 signals detectable by the receiver, and LQ values should be uniformly distributed between these two limits." From section E.2.3: "The LQI (see 6.7.8) measures the received energy and/or SNR for each received packet. When energy level and SNR information are combined, they can indicate whether a corrupt packet resulted from low signal strength or from high signal strength plus interference."

The MAC layer computes an 8-bit “link quality index” (LQI) for each received packet from the 2.4 GHz radio. The LQI is computed from the raw “received signal strength index” (RSSI) by linearly scaling it between the minimum and maximum defined RF power levels for the radio. This provides an LQI value that is based entirely on the strength of the received signal. This can be misleading in the case of a narrowband interferer that is within the channel bandwidth – the RSSI may be increased even though the true link quality decreases. To the module it will appear that the RSSI is very high (normally a good thing) when in fact it is the interferer that is very strong, not the message.

For more information see:
 * Z-Stack Developer's Guide
 * [Calculation and Usage of LQI and RSSI]