LQI, or Link Quality Index is a value used to compute Link Cost for Zigbee and other stacks. Link Cost can be used for creating network routes, selecting network parents, determining neighbors and managing routing tables. In both the Zigbee and IEEE 802.15.4 standards, LQI is required, but unlike RSSI (Receive Signal Strength Indication -- a standardized value), it is left to the manufacturer to determine how to implement LQI. On-chip radios differ significantly, so the mechanisms to evaluate LQI must be specific to the radio.
Per the Zigbee Specification, (R21):
The ZigBee routing algorithm uses a path cost metric for route comparison during route discovery and maintenance. In order to compute this metric, a cost, known as the link cost, is associated with each link in the path and link cost values are summed to produce the cost for the path as a whole.
The most common way to provide a Link Cost is to make use of the LQI specified in the IEEE 802.15.4-2003 MAC and PHY:
The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using receiver ED, a signal-to-noise ratio estimation, or a combination of these methods. The use of the LQI result by the network or application layers is not specified in this standard.
The LQI measurement shall be performed for each received packet, and the result shall be reported to the MAC sublayer using PD-DATA.indication (see 184.108.40.206) as an integer ranging from 0x00 to 0xff. The minimum and maximum LQI values (0x00 and 0xff) should be associated with the lowest and highest quality IEEE 802.15.4 signals detectable by the receiver, and LQ values in between should be uniformly distributed between these two limits. At least eight unique values of LQ shall be used.
LQI is derived from low-level radio information, basically a correlation between automatically-sensed radio errors and bit or packet error rates. Silicon Labs' EFR32 hardware is a significantly different architecture from EM2xx and EM3xx, and LQI is computed differently between the two designs.
In the Silicon Labs EM3xx parts, the LQI value is an unsigned 8-bit integer (int8u) ranging from 0 to 255, with the maximum value representing the best possible link quality. The relationship between LQI and link “cost” for purposes of route/neighbor maintenance in the stack is such that LQI values of 200 map to the lowest costs of 1, 3 and 5, while LQIs below 200 represent links with high error rates, so the worst-case cost of 7 is assigned. The LQI value of 200 represents approximately 80% reliability of receiving the packet intact. Note that the LQI measurement is based on the chip error rate (similar to Bit Error Rate [BER]) of the current packet being received from the previous hop of the inbound route, so that it provides information specific to the link-layer connection to the neighboring device relaying the current packet to the local device. This LQI data is also used by the stack to determine connectivity between neighboring nodes and to select parent devices when joining the network as a ZigBee End Device.
In the Silicon Labs EFR32MG1 and EFR32MG1x families, the bit error rate is not available to the RAIL layer, so LQI is derived from RSSI values. The RSSI value is a signed 8-bit integer (int8s) ranging from approximately -100 (based on the receive sensitivity rating for the chip you’re using) to 127, with each value representing the energy level (in dBm) at the radio’s receiver.
The RSSI measurement is based on the peak (highest) energy level detected by the radio on the current frequency over the first 8 symbol periods (about 128 microseconds) of the current packet being received. A drawback to using RSSI is that signal energy at a given frequency can come from any transmitter/interferer on that frequency, whether it is another nearby node on the same network, a device from a different PAN on the same channel, or a non-ZigBee, non-802.15.4 interferer, such as a WiFi transmitter or microwave.
We are planning further improvements to the way LQI is calculated to improve consistency across platforms. We will be implementing hysteresis in the LQI value to reduce short-term fluctuation which may have the effect on some routers of reducing their preference for EFR32 parts in a heterogeneous network with EM3xx parts, although the EFR32 parts are generally more capable. We are also examining other radio data which could provide more LQI/Link Cost consistency with EM3xx parts.