BLE PHY Layer: 1M, 2M, and Coded PHY Explained

BLE PHY Layer

The Physical Layer (PHY) determines how bits are modulated onto the 2.4 GHz radio. Bluetooth Low Energy offers three PHY modes — 1M, 2M, and Coded — each representing a different trade-off between data rate, range, and power. Selecting the right PHY is one of the highest-leverage decisions in BLE hardware design.

1M PHY

The LE 1M PHY is the baseline mode, defined in Bluetooth 4.0 and supported by every BLE device ever made. It transmits at 1 Megasymbol per second using GFSK modulation with a BT product of 0.5 (moderate spectral shaping).

Effective data throughput with DLE (Data Length Extension) enabled reaches roughly 800 kbps in ideal conditions. Range is typically 10–30 m indoors and up to 100 m outdoors with line-of-sight.

2M PHY

The LE 2M PHY doubles the symbol rate to 2 Msps, halving time-on-air for the same payload. This directly reduces energy per byte, making 2M PHY more power-efficient than 1M when throughput is the bottleneck (e.g., streaming audio, OTA updates).

The trade-off is slightly reduced sensitivity (~2–3 dB) due to the wider symbol bandwidth. In clean indoor environments the difference is negligible; in noisy or multi-wall environments, 1M PHY may edge out 2M in reliability.

Coded PHY

The LE Coded PHY adds Forward Error Correction (FEC) over 1M symbols, trading data rate for sensitivity gain. Two coding schemes exist:

• S=2 (500 kbps effective): 3 dB sensitivity gain, 2× range in free space
• S=8 (125 kbps effective): 6 dB sensitivity gain, 4× range in free space

Coded PHY is targeted at building automation, industrial sensors, and asset tracking over distances of 200–1000 m. The mandatory preamble and FEC tail add overhead, so each packet takes 4–8× longer to transmit than 1M PHY — plan your duty cycle and connection interval accordingly.

Data Rate vs Range Comparison

Use the Range Calculator and Power Estimator to model your specific environment.

Link Budget and Sensitivity

The link budget is the sum of all gains and losses in the radio path:

Link Budget (dB) = TX Power − Cable/Connector Loss + TX Antenna Gain
                   + RX Antenna Gain − Path Loss − Fade Margin

RX Sensitivity defines the minimum detectable signal power. Typical BLE values range from −90 dBm (1M) to −103 dBm (Coded S=8). TX Power is configurable from −40 dBm (minimal range, minimal current) to +20 dBm (maximum range, high current).

RSSI readings at the receiver reflect the received signal strength in dBm. Subtract from sensitivity to find the fade margin — anything above 10 dB margin is considered robust.

PHY Selection Strategy

PHY negotiation uses the LL_PHY_REQ / LL_PHY_RSP LL control procedure (BT 5.0+). Both peers must support the PHY for it to be used; if the central requests 2M but the peripheral only supports 1M, the link stays at 1M.

Guidelines by application: