Other Parts Discussed in Thread: CC2531, CC2530, Z-STACK, CC2591
Hello!
In our application the Zigbee network is composed of one coordinator and several end devices. The poll rate of the end device is 10 seconds for low power consideration as well as the indirect transfer timeout of the coordinator is 18 seconds. The coordinator maintains the connection status by sending "Heart Beat" packet to each end device and wait for its acknowledgment. The coordinator will do this one by one, which means it will not poll the next end device until the acknowledge packet of current end device is received or the indirect transfer timeout occurs. If no acknowledge is received within the past 15 minutes the connection between the coordinator and the end device will be treated as "Lost".
We setup this network and observe the stability of the communication between the coordinator and the end devices. It works fine most of the time however sometimes the loss of the connection is detected. We logged the RSSI and correlation value from the data packet (with correct FCS result only) to trace the signal quality and found that during the lost period and a short period before the lost, the RSSI value is stable while the correlation value become worse. For example, the RSSI value could be about -75dbm all the time, but the correlation value could be changed slowly from 108 to 59, and then the connection is lost. After a while (several minutes or half an hour or even longer) the end device is re-joined the correlation value could be changed from a poor value back to 108 slowly. In another example when the connection is lost, the last RSSI is -58dbm while the last correlation value is 85.
The following description is from TI "CC253x System-on-Chip Solution for 2.4-Ghz IEEE 802.15.4 and Zigbee Applications User's Guide" Section 23.10.4
“The RSSI value can 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 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 far as I can see, if the correlation value is decreased by a narrowband interferer, the RSSI should be increased. But in our cases the RSSI value keeps as a constant value, sometimes even 2-3 dB lower, it does not look like a narrowband interferer right? So would you please give me some advice about the possible reason and, which is more expected, guide me some way to find out the actual reason and to improve the communication's stability, since it is very important in our application.
By the way, our application is based on ZStack-CC2530-2.5.1a Znp mode.
Regards.
Zhou Liang
