LE 1M PHY

LE 1M PHY

The LE 1M PHY is the mandatory physical layer for all BLE devices, providing a raw data rate of 1 Mbps using GFSK modulation. Present since the first BLE specification (Bluetooth 4.0), it is the baseline PHY that every compliant BLE controller must support, making it the universal common denominator for interoperability.

Technical Characteristics

The LE 1M PHY uses Gaussian Frequency Shift Keying (GFSK) with a symbol rate of 1 megasymbol per second. Each symbol encodes one bit, yielding an on-air data rate of 1 Mbps. The modulation index is between 0.45 and 0.55, and the Gaussian filter has a bandwidth-time product (BT) of 0.5, which provides a good balance between spectral efficiency and inter-symbol interference.

The receiver sensitivity requirement for LE 1M PHY is -70 dBm per the Core Specification, though practical implementations from vendors like Nordic Semiconductor achieve -96 to -98 dBm on chips like the nRF52840. This translates to a typical line-of-sight range of 100--200 meters at +0 dBm TX power.

Throughput

The raw 1 Mbps data rate does not translate directly to application throughput. After accounting for packet headers, CRC, inter-frame spacing, and protocol overhead, the maximum application-layer throughput is approximately 236 kbps with DLE enabled (251-byte payloads). Without DLE, the maximum is around 120 kbps due to the 27-byte legacy PDU limit. Effective throughput also depends on the connection interval and the number of packets exchanged per connection event.

When to Use LE 1M PHY

LE 1M PHY is the right choice when maximum compatibility is required, as it is supported by every BLE 4.0+ device. It also offers better range than LE 2M PHY -- roughly 1.5x -- because the lower symbol rate allows the receiver to collect more energy per bit. For applications where neither maximum range (LE Coded PHY) nor maximum throughput (LE 2M PHY) is critical, LE 1M PHY provides a solid middle-ground option with universal device support.