What is the type of the wifi interference to ZigBee? Multiplicative or additive?
Because the two RF sources are not synchronized, by default they are additive, and random to each other. I might add that cell-phones and cordless phones operate in the same frequency bands. That is why cordless phones usually have a channel switch option if you have interference. For WiFi and cell phones that might have close or same channels, the ID code of the device is used to separate and identify them so they only respond to data meant for them. Remember that smart phones and cell towers can detect "crowding" of any band and request another channel or tower, including local WiFi (handled by the smart phone or tablet). ZigBee does not have much power or self awareness if it is using a crowded channel, and could be swamped out. ZigBee is best used with a line-of-sight antenna over short range. Add-on software to select the most quiet band is recommended. ZigBee also has a 915MHZ band but the data rate is slower.NOTE: All of these communication types use data encryption so if severe interference or snooping occurs any data or voice sent cannot be deciphered.