As an alternative to the transmitted reference (TR-) ultra-wideband (UWB), the dual-carrier frequency shifted reference (FSR-) UWB  and its high-rate multi-carrier counterparts  have been recently studied. In this paper, we consider a lowrate multi-carrier differential modulation and its low-complexity receiver designs. We prove that such a multi-carrier system actually results in a pulse position modulation, where the peak of the transmitted waveform changes location according to the value of the data symbol. Based on this discovery, we develop a simple energy detector by detecting the pulse position in time domain. In addition, the multi-carrier nature of this modulation also enables a differential demodulator in frequency domain. We derived the analytical performance for this receiver. Simulations are carried out to evaluate the performance of these time- and frequency-domain low-complexity receivers.