Bluetooth does not operate on a fixed channel, but instead hops from one channel to another in a pseudo-random manner across all of its 79 channels, which span the 2.4 GHz band.
Bluetooth signals are a potential source of interference for ZigBee and WiFi networks. They can swamp a received signal, which prevents the receiving device from extracting the data contained with the signal. For this reason, Bluetooth sources should be separated as far as sensibly possible from all ZigBee and WiFi devices.
For Bluetooth to cause any interference with a ZigBee signal, it must meet two criteria. Firstly, the Bluetooth signal must be sufficiently strong to completely swamp the ZigBee signal, because ZigBee can still function correctly in the presence of radio noise. Secondly, the Bluetooth signal must be transmitting on exactly the same radio frequency and at exactly the same time as the ZigBee signal.
As the ZigBee radio frequency is fixed, and the Bluetooth hops from one channel to another in a pseudo-random sequence, a Bluetooth signal will be transmitted on the same frequency as a ZigBee signal.
Bluetooth transmits 1600 times per second, and hops across 79 channels. On average, it will transmit on the same frequency as a ZigBee signal 20 times per second. As Bluetooth hops from one channel to another in a pseudo-random manner, it could transmit on the same frequency as a ZigBee signal more often or less often than this during one second.
The ZigBee transmissions also occur randomly in time, so for any conflict to occur, Bluetooth has to be transmitting on the same frequency as a ZigBee device at exactly the same time as the ZigBee device is receiving data.
Both Bluetooth and ZigBee transmit for very short periods of time, typically between 1 and 2 milliseconds, and infrequently.
The randomness in timing of the transmissions, together with the very short transmission periods, ensures that the likelihood of a Bluetooth signal interfering with the ZigBee transmissions is very low, and typically less than 1 in 10,000 ZigBee transmissions.
If interference does occur, the ZigBee standard includes an automatic retry facility. In addition, the control system monitors the status of all devices and expected communications within the network. The control system also initiates retries and reports errors if unsuccessful.
Other Background Noise
In addition to the Bluetooth signals, which are discussed above, the main other source of radio noise is from external WiFi devices. Several thousand separate WiFi sources were identified over a two week period.
Most of these were from passing vehicles. The spectrum analyzer was located about 100 feet from a busy road carrying commuter traffic and school-run traffic in the mornings and in the late afternoon. Despite the large number of these WiFi transmissions, they were attenuated sufficiently to not cause any interference with the system.
Other sources of radio signals in the 2.4 GHz band within the home include microwave ovens, hand-held phones, video senders, and microphones. Sources outside of the home include cars, and car alarms, drones, and adjacent properties.