A simple multipath delay time detection method for π/4 shift QPSK in digital mobile/portable communications

A simple in-service measurement method of multipath delay spread applicable to a π/4-shift QPSK (quadrature phase-shift keying) is proposed. The basic idea lies in applying the in-service delay time detection method proposed previously for a BPSK (binary phase-shift keying) signal to a π/2-shift BPSK signal which is converted from a π/4-shift QPSK signal by passing a frequency-doubler circuit. Then, by measuring both in-phase and quadrature channel outputs, a bit stream corresponding to the doubled phase shifts is obtained from one channel output and multipath delay time information is obtained from the other channel output. To confirm the effectiveness of the proposed method, computer simulation has been performed assuming few-ray multipath models, and a fairly good result was obtained. However, it was also shown that this method is sensitive to bandwidth restriction to some extent