Application of space-time codes to OFDM UWB systems with under-sampled receivers

In this paper, the application of space-time codes (STCs) to orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) systems with under-sampled receivers is investigated. In this system, the total available bandwidth is divided into M_t smaller sub-bands. OFDM modulation is adopted in each sub-band, and data are transmitted in all the sub-bands simultaneously. The transmitter properly encodes the data in sub-bands to gain the rich frequency diversity, enabling a low-cost receiver with reduced sampling rate, only one M_t-th of the Nyquist rate. We first show that this system can be analogized to a multiple-input single-output (MISO) system with M_t transmit antennas, by which the application of STC becomes a natural consequence. We then consider employing low-rate space-time trellis codes (STTCs), which can jointly attain the maximal diversity gain and coding gain based on the two design criteria from the MISO analogy. Simulation results with proposed STTCs have verified the superior performance to the previously proposed coding scheme.