As a high-rate alternative of impulse radio (IR) ultra-wideband (UWB) communications, analog and digital multi-carrier UWB radios with coherent detection have been introduced in , , . In this paper, we investigate digital multi-carrier differential (MCD) modulation and demodulation schemes for UWB. In our approach, the differential encoding and decoding are carried Out across multiple digital carriers in the frequency domain. Hence, channel estimation is bypassed by using neighboring carriers as reference carriers. Compared with the transmitted reference (TR) UWB which relies on delaying and correlating received segments in time domain, our MCD-UWB avoids the need for the analog delay element. In addition, our digital carrier based approach does not incur any spectrum expansion. We also show that our approach enables variable data rates, and remains operational even in the presence of severe inter-frame interference (IFI). In addition, the small spacing between differentially encoded carriers guarantees that the data rate is not limited by the channel coherence bandwidth. Simulations are also carried out to support our theoretical analysis.