Adaptive overlap-and-add technique in MB-OFDM based UWB receiver design

Traditional OFDM based transmission system uses Cyclic Prefix (CP) in an OFDM symbol in order to maintain orthogonality of transmission. Present days´ Ultra-Wideband (UWB) systems use Multi-Band OFDM (MB-OFDM) techniques for transmission in application like Wireless Personal Area Network (WPAN). UWB based systems are power limited by the regulation of FCC. CP introduces correlation in the transmitted data sequence and hence introduces ripples in the power spectral density (PSD) of the transmitted data. This in turn reduces the range of data transmission. However, use of zero-pad suffix (ZPS) will have a flat PSD and hence does not suffer from the range degradation problem. In the receiver, ZP removal requires use of a technique called as overlap and add (OLA) in order to capture the multipath energy of the channel and maintain the orthogonality in the received data. During transmission, the length of the ZP is fixed and equal to the channel length. During reception, in traditional OFDM receiver, the OLA is done using a ZP length of same as channel length as transmitted. In UWB receiver the FFT window gets smeared due to multipath fading and hence estimation of true FFT window start point does affect the OLA process and hence the overall system performance. In this paper, we propose a method which adapts OLA length in UWB receiver depending on the current band of reception and the band wise estimated true FFT window start point. The proposed method is more beneficiary for high delay spread channel like channel model 4 (CM4). In CM4, the technique can achieve around 1 dB E_b/N_o gain at 10^-2 of BER, while simulated over uncoded MB-OFDM based UWB system.