Power spectral density of a UWB signal with discrete quantized pulse positions

The FCC specification for ultra-wideband (UWB) emissions states that the effective isotropic radiated power (EIRP) cannot exceed -41.3 dBm per 1 MHz bandwidth in the frequency region of 3.1 to 10.6 GHz. Hence, to achieve adequate transmit power in a UWB data link it is necessary to use a wide-bandwidth modulation scheme. This requirement presents a pair of technical challenges which are addressed in this paper. The first challenge is to realize a wide-bandwidth radiated transmitter pulse shape which adequately covers the UWB band from 3.1 to 10.6 GHz. The second challenge is to structure the modulation such that the discrete line component of the power spectral density (PSD) of the transmitted signal is minimized. This is necessary as the discrete spectral components are essentially wasted power and limit the output transmitted power due to the FCC EIRP mask. In this paper the radiated EIRP of a UWB signal based on pulse position modulation (PPM) is explored. Previous researchers have derived the PSD under the supposition that the PPM pulse delay is continuous. However, simpler and more insightful expressions are possible for the PPM PSD if the pulse delays are restricted to quantized steps, as is assumed in this paper. A network-analysis approach for determining the EIRP of the unmodulated radiated pulses is given based on spectral measurements of an experimental UWB transceiver pair. These EIRP characteristics are applied to the derived equations for the continuous and discrete portions of the PSD of the UWB signal. From this, insights emerge into optimum PPM transmitter implementation that maximizes transmit power and minimizes power losses due to the discrete spectral lines. As demonstrated in this paper, such optimization necessitates the joint design of the UWB transmitter radiated pulse shape and PPM structure.