Link Budget

Link Budget

A link budget is the accounting of all gains and losses in a BLE radio link, from the transmitter through the propagation path to the receiver. It determines the maximum distance over which two BLE devices can reliably communicate and is the fundamental engineering tool for range planning in wireless product design.

Link Budget Formula

The basic link budget equation calculates the received power as TX Power plus TX Antenna Gain minus Path Loss plus RX Antenna Gain. For communication to succeed, the received power must exceed the RX sensitivity of the receiver. The maximum allowable path loss (MAPL) is therefore TX Power + TX Antenna Gain + RX Antenna Gain - RX Sensitivity.

For example, a BLE device transmitting at +0 dBm with a 0 dBi antenna, communicating with a receiver at -96 dBm sensitivity and 0 dBi antenna, has an MAPL of 96 dB. At 2.4 GHz in free space, this translates to approximately 200 meters of range.

Path Loss Models

Free-space path loss at 2.4 GHz increases with distance according to a logarithmic formula: PL(dB) = 40.05 + 20 * log10(d), where d is in meters. Indoors, the log-distance model with a path loss exponent of 2.5--3.5 is more appropriate, reducing practical range to 20--50 meters through walls and obstacles. Dense commercial environments with metal shelving or concrete walls push the exponent to 4.0 or higher.

Improving Link Budget

Engineers have several tools to improve the link budget. Increasing TX power by +6 dBm doubles range. Replacing a PCB antenna with a higher-gain chip antenna or external antenna can add 2--5 dBi. Using LE Coded PHY with S=8 coding adds approximately 12 dB of coding gain, effectively quadrupling range. Clean PCB layout, proper RF matching, and separation from noisy digital circuits improve effective sensitivity.

Design Margin

Production link budgets should include a fade margin of 10--15 dB to account for multipath fading, body effects, manufacturing variation, and environmental changes. If the calculated MAPL is 96 dB and you need 15 dB of margin, design for 81 dB of path loss, which corresponds to roughly 50 meters indoors. This margin ensures reliable operation across the full population of manufactured devices and deployment environments.