How does AFH work in BLE?

The central device monitors packet error rates per channel and classifies channels as good or bad. It updates the 37-bit channel map and sends it to the peripheral. Both devices then skip bad channels during hopping. Minimum 2 channels must remain enabled. The map is updated periodically as interference conditions change.

How many channels does BLE use?

BLE has 40 total channels: 37 data channels (0-36) for connected communication and 3 advertising channels (37-39). The channel map applies only to data channels. Each channel is 2 MHz wide, spanning 2400-2480 MHz.

Wi-Fi channels 1/6/11 occupy 22 MHz each, overlapping approximately 15 of 37 BLE data channels. AFH marks these as bad when Wi-Fi is active, leaving 22+ clean channels. Hardware coexistence interfaces (PTA) can coordinate BLE and Wi-Fi timing on combo chips for even better performance.

Wireless Protocols

BLE Channel Map

The BLE Channel Map is a 37-bit bitmask where each bit represents one of the 37 BLE data channels (0-36). A '1' marks a channel as available; a '0' marks it as blocked. The central device maintains this map based on measured interference and communicates it to the peripheral, enabling Adaptive Frequency Hopping (AFH) that skips channels degraded by Wi-Fi, microwave ovens, or other 2.4 GHz interference.

Category: Wireless Protocols

Size: 37 bits (5 bytes)

Understanding BLE Channel Map

BLE's 40 channels span 2400-2480 MHz: 37 data channels (2 MHz each, ch 0-36) and 3 advertising channels (37/38/39). During a connection, both devices hop across data channels using a pseudo-random sequence. The channel map controls which channels are included in this hopping sequence.

The central monitors PER (Packet Error Rate) per channel. Channels with high PER (overlapping active Wi-Fi) are marked bad. At least 2 channels must remain enabled. The map updates are sent via LL_CHANNEL_MAP_IND, taking effect after a specified connection event.

Channel Mapping

BLE data channels: 0-36 (2 MHz spacing)
f = 2402 + 2×ch (MHz), ch = 0..36

Wi-Fi overlap (ch 1/6/11):
~15 of 37 BLE channels overlap Wi-Fi
AFH disables these → 22+ clean channels remain

BLE Channel Structure

Channel Type	Count	Frequencies	Purpose
Data (0-36)	37	2404-2478 MHz	Connected data, AFH
Advertising (37)	1	2402 MHz	Discovery (Wi-Fi ch 1 gap)
Advertising (38)	1	2426 MHz	Discovery (Wi-Fi ch 6 gap)
Advertising (39)	1	2480 MHz	Discovery (above Wi-Fi)

Common Questions

Frequently Asked Questions

How does AFH work?

Central monitors PER per channel, classifies good/bad, updates map, sends to peripheral. Both skip bad channels. Minimum 2 channels must remain.

How many channels?

40 total: 37 data (0-36) + 3 advertising (37-39). Channel map applies to data channels only. Each is 2 MHz wide.

Wi-Fi coexistence?

Wi-Fi ch 1/6/11 overlap ~15 BLE data channels. AFH marks these bad, leaving 22+ clean channels. Combo chips use PTA for timing coordination.

Related Terms

← BLE Advertising BLE Coded PHY →

BLE Coexistence

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