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Transmit Power Control (TPC)

Transmit Power Control (TPC) is a regulatory requirement and system function that dynamically reduces a device’s transmit power to the minimum necessary level for reliable communication. It is primarily mandated in specific frequency bands to minimize interference with other systems.

Scope and Application

TPC applies to wireless communication systems operating in shared spectrum environments, most notably:

5 GHz Wi-Fi (UNII-2A and UNII-2C bands)
Outdoor wireless links in compliance with DFS rules
Licensed point-to-point and point-to-multipoint systems

Regulatory Requirements

TPC is required under:

FCC §15.407: For devices operating in the 5.25–5.35 GHz and 5.47–5.725 GHz bands
ETSI EN 301 893: For 5 GHz WLAN equipment in Europe
ISED RSS-247: For 5 GHz RLAN devices in Canada

Technical Requirements

Purpose

TPC is designed to reduce interference risks by ensuring that the transmission power does not exceed what is needed to maintain the communication link.

Implementation

Devices must either:

Support adaptive power control based on link conditions, or
Operate at least 3 dB below the maximum allowed power level in affected bands

Interaction with DFS

TPC complements DFS by further reducing the probability of interfering with radar systems sharing the same band.

Certification and Compliance

TPC functionality must be verified during type approval testing when operating in affected bands. Regulatory bodies typically require:

Documentation of power adjustment behavior
Spectral analysis showing compliance with maximum EIRP (Equivalent Isotropically Radiated Power) limits
Conformance with reduction thresholds (e.g., 3 dB below the limit if no TPC function is present)