Frequency-diversity coded OFDM for ultra-wideband systems with under-sampling-rate receivers

Orthogonal frequency division multiplexing (OFDM) has been proposed for use as the physical layer of ultra-wideband (UWB) systems for high-rate, short-range personal area networking (PAN). For ultra-wideband systems, there is a constraint on the maximum power spectral density for the transmitted signal. Therefore, the bandwidth of the transmitted spectrum must be spread widely by a bandwidth expansion scheme so that the transmitted power spectral density can be kept as low as possible. In this paper, frequency expansion of the UWB system is achieved by using a simple frequency-diversity coding scheme. A major issue for the frequency-diversity coding scheme is that the receiver must sample the baseband received signal using high-sampling-rate analog-to-digital converters (ADCs) for discrete signal processing (DSP). However, such high-sampling-rate ADCs and DSP are expensive and have high power consumption. One advantage of the proposed frequency-diversity coding scheme is that the sampling rate of the baseband ADCs and DSP can be less then the Nyquist rate. The aliasing phenomenon occurs due to the reduced sampling rate, yet it, however, appears as transmission diversity to the receiver. The performance of the frequency-diversity coded OFDM system with an under-sampling-rate receiver is analyzed by evaluating the pairwise error probability. From the analysis of the pairwise error probability, design criteria for the frequency-diversity coded OFDM are obtained. A practical construction of frequency-diversity codes is proposed based on linear block codes. Simulation and analytical results for frequency-diversity coded OFDM systems are presented. The results show that a significant diversity/coding gain can be achieved with the under-sampling-rate receiver.