We will see throughout this chapter several novel schemes to reduce interference of signals. This is an issue for many forms of wireless technology as the spectrum is unlicensed and shared ( we will discuss this more in the next section). Because of the fact that there may be several devices emanating RF energy in a shared space, interference will occur.  

Take Bluetooth and 802.11 Wi-Fi; both operate in the shared 2.4 GHz spectrum but remain functional even in congested environments. Bluetooth Low Energy (BLE), as we will see, will randomly select one of 40-2 MHz channels as a form of frequency hopping. We see in the following figure eleven free channels (three being advertising) on BLE that have a 15% chance of collision (especially since 802.11 does not hop between channels). The new Bluetooth 5 specification provides techniques such as slot availability masks to lock out Wi-Fi areas from the frequency hop list. Other techniques exist as well, which we will explore later. Here we show the ILM band for Zigbee and Bluetooth Low Energy. Also shown is a possible contention with three Wi-Fi channels in the 2.4 GHz spectrum.