What is the power spectral density limit for a UWB device under FCC Part 15 regulations?

The power spectral density (PSD) limit for a UWB device under FCC Part 15 regulations is -41.3 dBm/MHz (measured as the average power in a 1 MHz bandwidth), which is the same limit applied to unintentional radiators (noise). This limit was carefully chosen to allow UWB devices to share spectrum with existing licensed services without causing harmful interference, due to the ultra-wide distribution of the UWB signal energy across a large bandwidth. The FCC Part 15.517 (indoor UWB devices) and 15.519 (outdoor handheld UWB devices) specify: for indoor UWB devices operating in 3.1-10.6 GHz: PSD limit = -41.3 dBm/MHz EIRP (average). Total power in 500 MHz bandwidth = -41.3 + 10 x log10(500) = -41.3 + 27 = -14.3 dBm (37 microwatts). Peak power limit: 0 dBm in a 50 MHz bandwidth (this limits the peak-to-average ratio of the UWB signal). Below 3.1 GHz: the PSD limit drops to -51.3 dBm/MHz or lower to protect GPS (1.575 GHz), cellular, and PCS bands. Above 10.6 GHz: the limit drops to -51.3 dBm/MHz to protect radionavigation and fixed satellites. The outdoor handheld limit is the same -41.3 dBm/MHz in 3.1-10.6 GHz, with additional restrictions on fixed outdoor installations (which are not permitted). The measurement procedure (per FCC Part 15.31) uses: an antenna at 3 meters from the device, a spectrum analyzer with 1 MHz resolution bandwidth, and average detection (not peak). The RF design implication is: the UWB PA (if used) must produce very low output power (typically less than -10 dBm total), and much of the link budget relies on the antenna gain and receiver sensitivity rather than transmit power.