Frequency Hopping

Frequency Hopping

Frequency hopping is a spread-spectrum technique where BLE devices change the radio channel used for each connection event, distributing communication across the available spectrum to avoid persistent interference from Wi-Fi, microwave ovens, and other 2.4 GHz sources sharing the ISM band.

Adaptive Frequency Hopping (AFH)

BLE uses Adaptive Frequency Hopping (AFH) rather than simple pseudo-random hopping. The central device monitors link quality across all 37 data channels and maintains a channel map -- a 37-bit bitmask indicating which channels are usable. Channels experiencing high packet error rates due to Wi-Fi interference are marked as bad and excluded from the hopping sequence.

The channel map is updated periodically and communicated from the central to the peripheral via a link layer control message (LL_CHANNEL_MAP_IND). The peripheral must switch to the new map within six connection intervals after receiving the update.

Hopping Algorithm

BLE defines two channel selection algorithms. Algorithm #1, used since Bluetooth 4.0, applies a simple modular arithmetic formula to determine the next hop. Algorithm #2, introduced in Bluetooth 5.0, uses a more sophisticated pseudo-random generator that produces a more uniform distribution across the used channels, reducing the chance of consecutive collisions on the same frequency.

For both algorithms, the hop increment and channel map together determine the sequence. The result is that even in a dense 2.4 GHz environment -- an office with dozens of Wi-Fi access points -- BLE connections remain reliable because the radio spends at most one connection event on any given channel before moving on.

Impact on Reliability and Throughput

AFH significantly improves BLE reliability in congested environments. Without it, a BLE link operating on a channel overlapping with a busy Wi-Fi access point would experience continuous packet loss. With AFH, the offending channel is blacklisted and the link continues on clean spectrum. The trade-off is a slight reduction in available channels and therefore capacity, but in practice BLE connections function well with as few as 10--15 good channels out of 37. This adaptive approach is one of the key reasons BLE works reliably in the crowded ISM band.