LE 2M PHY

LE 2M PHY

The LE 2M PHY is an optional physical layer introduced in Bluetooth 5.0 that doubles the on-air data rate to 2 Mbps compared to the mandatory LE 1M PHY. It achieves this higher rate by increasing the symbol rate while maintaining the same GFSK modulation scheme, making it ideal for throughput-sensitive and power-constrained applications.

Technical Characteristics

LE 2M PHY uses GFSK at 2 megasymbols per second with one bit per symbol. The modulation index remains between 0.45 and 0.55. Because each bit occupies half the air time compared to 1M PHY, the receiver has less energy per bit, resulting in reduced RX sensitivity. The specification requires -65 dBm sensitivity for 2M PHY versus -70 dBm for 1M PHY, though practical implementations typically achieve -93 to -95 dBm.

Throughput and Power Benefits

The primary advantage of LE 2M PHY is reduced radio-on time. A packet that takes 200 us to transmit at 1M takes only 100 us at 2M. This translates to two key benefits:

• Higher throughput: With DLE and LE 2M PHY, application throughput can reach approximately 450 kbps, nearly double the 1M PHY maximum.
• Lower power per byte: Because each packet occupies the air for half the time, the radio returns to sleep sooner. For a fixed amount of data, LE 2M PHY can reduce the energy per byte by 30--40% compared to 1M PHY.

Range Trade-off

The reduced sensitivity of LE 2M PHY means shorter range -- roughly 65--80% of the range achievable with LE 1M PHY under the same TX power and antenna conditions. For applications like fitness trackers, smart locks, and medical devices where the central is typically within a few meters, this range reduction is irrelevant and the power savings are welcome.

PHY Switching

BLE 5.0+ devices can dynamically switch between 1M and 2M PHY during an active connection using the PHY Update Procedure. A device might use LE 2M PHY for bulk data transfer (e.g., firmware OTA updates) and switch back to LE 1M PHY for normal sensor readings to maximize range. The PHY negotiation is handled transparently by the link layer without dropping the connection.