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Bluetooth® Low Energy (BLE) Physical Layer

Last modified by Microchip on 2023/11/09 08:53

BLE/PHY

The Bluetooth® Low Energy (BLE) physical layer (PHY) contains the analog communications circuitry responsible for the translation of digital symbols over the air. It is the lowest layer of the protocol stack and provides its services to the link layer.

Graphic showing Physical Layer in stack

Frequency Bands

The radio uses the 2.4 GHz Industrial, Scientific, and Medical (ISM) 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:

PHY channel assignment

Channel Arrangement

The 40 channels are divided into three advertising channels (Ch. 37, 38, and 39), and 37 data channels (Ch. 0-36).

Advertising Channel Usage

  • Device discovery
  • Connection establishment
  • Broadcast transmissions

Data Channel Usage

Modulation and 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:

Overlap of Phy channels