A low-power frequency modulated-ultra-wideband (FM-UWB) receiver for short-range communications, capable of simultaneously demodulating multiple FM-UWB signals located at the same frequency, is presented in this paper. The proposed receiver utilizes an "approximate zero IF" architecture that uses a low-power, free-running ring oscillator as the RF LO to first convert the input signal into baseband, where it is then amplified and demodulated. Moving the most power-hungry blocks from RF to IF results in reduction of power consumption by more than one order of magnitude compared to previous implementations using the delay-line demodulator. Integrated in a 65-nm CMOS technology, the whole receiver chain consumes 423 mu W from a 1-V supply while achieving -70-dBm sensitivity at a data rate of 100 kb/s at 4 GHz. Communication with up to four FM-UWB users, operating in the same band, is demonstrated, making this receiver suitable for high-density wireless sensor networks.