Ultra-wideband impulse radio (UWB-IR) merges low complexity system design with good localization capabilities enabling low-data-rate/location-tracking (LDR/LT) sensor network design. However power consumption of the sensor nodes is a fundamental problem. Therefore, a simple wake-up scheme for asymmetric network topologies is investigated, where a static wireless backbone communicates with many semi-active sensor nodes. At sensor side only a low power wake-up detector is active during idle times to reduce power consumption. The wireless backbone consists of distributed UWB-IR devices and triggers the sensor nodes by means of beamforming, exploiting the large UWB-IR bandwidth to highly concentrate energy in a specified area. Conditions for basic feasibility of the scheme are derived and promising performance results based on realistic simulations are presented