In this paper, we consider the problem of Space-Time (ST) coding for Multiple-Input-Multiple-Output (MIMO) and Decode-and-Forward (DF) Impulse-Radio Ultra-Wideband (IR-UWB) systems. In particular, we propose three novel transmission schemes that are suitable for such systems. The first scheme is a multiplexing scheme that transmits at a rate higher than that of spatial multiplexing with the same complexity. The second scheme is a unipolar fully diverse and totally-real scheme that is suitable for Pulse Position Modulation (PPM). The rate of this scheme exceeds that of full-rate ST codes with a lower decoding complexity. The third scheme corresponds to a 2×2 full-rate and fully diverse ST code that has a non-vanishing coding gain when associated with Pulse Amplitude Modulation (PAM). These diversity schemes are appealing since they are tolerant to the asynchronization between the relays in DF networks.
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