Interference is one of the biggest challenges any wireless technology encounters in providing reliable data communications. Since wireless technology devices such as Bluetooth, Wi-Fi, and 802.15.4 share a transmission medium, if a packet in transit collides with another packet in transit at the exact same time and on the same channel, the packet may be damaged or lost. To overcome interference and find a clear transmission path that avoids packet collisions, Bluetooth technology uses a form of frequency-hopping spread spectrum (FHSS) called adaptive frequency hopping (AFH). Bluetooth divides the frequency band into smaller channels (like 40 channels for Bluetooth Low Energy) and quickly jumps between those channels when transmitting packets. To further reduce the probability of interference, Bluetooth adjusts its jump order. Noisy and busy channels are tracked and avoided at all times when sending packets.
Adaptive Frequency Hopping (AFH)
One of the main challenges of radio communication is collisions, which are especially serious in busy radio environments. Earlier we discussed collisions and explained that collisions occur when two or more devices transmit data on the same radio channel during overlapping time periods. If different radio technologies, such as Bluetooth and Wi-Fi, use one radio spectrum at the same time, they can interfere with each other. Bluetooth technology reduces the risk of conflict by using spread spectrum technology. When two devices are connected, a specific technique called adaptive frequency hopping is required. During each connection event, a pair of connected devices can use their radios to exchange packets at precise time intervals. But beyond that, at the beginning of each connection event, frequency hopping occurs, using a channel selection algorithm to determine a radio channel from the set of available channels. Each connected device will then switch to the selected channel. Over time and over a series of connection events, communication will take place through a frequently changing series of different channels spread over the 2.4 GHz frequency band, greatly reducing the probability of collisions. Of the 40 channels used by Bluetooth Low Energy, 37 channels (called general purpose channels) can be used when connecting communications. Frequency hopping contributes significantly to the reliability of communication between connected devices, and Bluetooth further improves communication reliability. In certain environments, some Bluetooth radio channels may not work properly, possibly because they are affected by interference, while other channels work reliably. As wireless communication devices are added and removed from the environment, reliable and unreliable channels may gradually change. The connected master produces a channel map that classifies each well-working channel as “used” or “unused”. The channel map can be shared with the second device using link layer procedures so that they can share information about which channels will be used and which will not be used. The device monitors the operation of each channel using specific implementation techniques. The channel graph is updated when it is determined that one or more previously working channels are no longer applicable. Conversely, if a previously unhealthy channel is found to be healthy now, the channel status is also updated in the channel graph. After that, channel map updates will be shared with the second device. In this way, Bluetooth ensures that it only uses known good channels, avoids problematic channels, and keeps the channel graph updated so that it always reflects the best subset of channels in use. This is the adaptive capability of the Bluetooth adaptive frequency hopping system.
Excerpted from Bluetooth SIG
