Coexistence refers to the functioning of different wireless devices and standards in the same frequency band. The various IEEE 802.11 wireless standards, for example, are packed into the 2.4-GHz unlicensed Industrial-Scientific-Medical (ISM) frequency band. IEEE 802.15.4 wireless sensor networks share the 2.4-GHz ISM unlicensed frequency band. Bluetooth is yet another wireless standard operating in that same frequency band.
While it be ideal to have different frequencies for each wireless standard, as with radio and television broadcast channels, frequency spectrum is limited. The growing number of wireless standards makes it difficult, if not impossible, to allocate separate frequency spectrum for each standard. In addition, new applications being added to the same crowded frequency spectrum, such as Internet-of-Things (IoT) wireless devices and machine-to-machine (M2M) devices in the 2.4-GHz ISM band, make the task of achieving wireless coexistence even more challenging.