ZigBee Testing Q&A

　　ZigBee's anti-interference feature is mainly to resist co-channel interference, that is, interference from other technologies sharing the same frequency band. The main features are as follows:
　　1. Clear Channel Assessment (CCA):
Determine whether the channel is clear. The IEEE 802.15.4 physical layer provides CCA capabilities in the collision avoidance mechanism, that is, if
the channel is occupied by other devices, the transmission is allowed to exit without considering the communication protocol of the channel.
There are currently three main clear channel assessment modes: (1) Determine the signal energy of the channel. If the signal energy is lower than a certain threshold, the channel is considered clear; (2) Determine the characteristics of the wireless channel. This characteristic mainly includes two aspects, namely, spread spectrum signal and carrier frequency; (3) Combine the first two modes, detect signal strength and signal characteristics at the same time, and give a channel clear judgment.
　　2. Dynamic channel selection:
　　The coordinator in the ZigBee personal area network (PAN) (the central node of the network, responsible for the organization and maintenance of the network). First, scan all channels, then confirm and join a suitable PAN (no need to create a new PAN), reduce the number of PANs in the same frequency band, and reduce potential interference. If the interference source appears on an overlapping channel, the upper-layer software of the coordinator shall apply the channel algorithm to select a new channel.
　　3. Channel algorithm:
　　When the network is initialized or responding to an interrupt, ZigBee devices will first scan a series of channels listed in the channel table parameters for dynamic channel selection, and set the channel table parameters according to the idle channels mentioned above to enhance the coexistence performance of the network.
During severe interference, ZigBee does not change channels, but relies on its low duty cycle, conflict-free algorithm (each device listens to the channel before sending data) to reduce data loss caused by transmission conflicts.

　　4. Direct Sequence Spread Spectrum (DSSS) and Fast Frequency Change FA:
　　Direct sequence spread spectrum technology has a certain anti-interference effect (as shown in the figure below). Under the same other conditions, the transmission distance is greater than that of frequency hopping technology (when the transmission power is 0dBm, Bluetooth has a range of 10m, while ZigBee can reach a range of 30-50m indoors). The so-called fast frequency change is to change the frequency to avoid the influence of a known interference source or signal source.

 
　　As can be seen from the above figure, ZigBee's bit error rate can reach 10 -9 when the signal-to-noise ratio is 4dB. To achieve the same bit error rate, Wi-Fi must reach 10dB and Bluetooth must reach 16dB. ZIGBEE's anti-interference performance is significantly higher than Wi-Fi and Bluetooth technologies.
　　5. Response retransmission and frame buffering:
　　ZigBee's MAC layer and application layer (APS part) have automatic request retransmission ARQ and frame buffering functions. When transmitting a data frame to a device, if the receiving device is busy or dormant and cannot receive the frame, the network's main coordinating device temporarily buffers the frame until the receiving end receives the frame.
　　Although the modulation method is simple, ZigBee has shown good anti-interference performance in the 2.4GHz ISM band, which is very suitable for low-power, low-data transmission applications. At present, the main test standard used by ZiBee products in CE certification is EN300328, while the test requirements of Part 15C are mainly used in FCC.