25kHz 반송주파수와 5kHz 심볼율을 갖는 수중음향통신용 QPSK 수신기 전단부 설계

Abstract

In this paper, a front-end of QPSK receiver with 25 kHz carrier frequency, 5 kHz symbol rate, and any excess-bandwidth of pulse shaping filter is designed using the uniform sampling theorem for bandpass signal and introducing the sub-filters and sub-sampling sequences. The designed front-end of QPSK receiver is composed of an A/D converter, which takes samples at 20kHz rate, a serial-to-parallel converter, which converts a sampled bandpass sequence to 4 paralleled sub-sample sequences, 4 sub-filter blocks, which act as a frequency shifter and lowpass filter for complex sequence at the same time, 4 synchronized switches, and 2 adders. Although the designed front-end requires 4 lowpass filters, which is two times bigger than the conventional design, it dramatically reduces the computational complexity by more than 50% for frequency shifting and lowpass filtering operations since the conventional front-end requires a frequency shifting and two lowpass filtering operations to get one lowpass complex sample and the proposed front-end requires only four filtering operation to get four lowpass complex samples, which is equivalent to one filtering operation for one sample.