In this paper a concept for an energy-efficient multi-frequency localization system is proposed. The system design aims at extremely light-weight, and thus energy-efficient tags. The localization system is realized by a Wireless Sensor Network comprising signal strength and phase measurements. The concept is based on signals for phase-based ranging. The proposed signals have to meet several constraints: low-power signal generation, efficient signal pro-cessing, and, most important, precise range estimation. The performance of the range estimation is assessed utilizing the Cramér-Rao Lower Bound.