The Bluetooth® Low Energy (BLE) physical layer (PHY) contains the analog communications circuitry responsible for translation of digital symbols over the air. It is the lowest layer of the protocol stack, and provides its services to the Link Layer.
The radio uses the 2.4 GHz ISM (Industrial, Scientific, and Medical) band to communicate and divides this band into 40 channels on 2 MHz spacing from 2.4000 GHz to 2.4835 GHz, starting at 2402 MHz:
The 40 channels are divided into 3 Advertising Channels (Ch. 37, 38, 39), and 37 Data Channels (Ch. 0-36).
Advertising Channel Usage
- Device Discovery
- Connection Establishment
- Broadcast Transmissions
Data Channel Usage
- Bidirectional communication between connected devices
- Adaptive frequency hopping used for subsequent connection events
Modulation & Data Rate
When transmitting data, the BLE radio transmits at 1 Mbps, with 1 bit per symbol. The radio is optimized for sending small chunks of data quickly.
BLE protocol overhead limits overall data throughput to significantly less than 1 Mbps.
The BLE radio uses Gaussian frequency-shift keying (GFSK), whereby the data pulses are filtered with a Gaussian filter before being applied to alter the carrier frequency, in order to make the frequency transitions smoother.
Co-existence with 802.11/Wi-Fi®
Since the advertising channels form the basis for how BLE operates, they have been assigned center frequencies that minimize overlapping with the most common 802.11 channels, as shown: