Use of a BPSK demodulator for a QPSK signal with identical I & Q sequences except small time misalignments

The possibility of using existing QPSK (quadrature phase shift keying) transmitter design and the BPSK (binary phase shift keying) demodulator with Costas carrier recovery at the White Sands Ground Terminal was studied for the space station Freedom Ku-band downlink at 50 Mb/s data rate. To generate the QPSK signal, the in-phase (I)-channel takes the data bit sequence and the quadrature-phase (Q)-channel takes the copy of the data sequence with implementation time delay t₀. The bit error probability (BEP) degradation due to time misalignment t₀ and phase jitter at the Costas loop in a BPSK demodulation for the QPSK signal was derived. It was observed that the BEP difference between two systems, the BPSK signal with BPSK demodulation and the QPSK signal with BPSK demodulation, is small when t₀ is less than 10% of the data bit time interval and the closed-loop bandwidth is less than 1% of the data rate. However, it was observed that the degradation in the BEP increases and gets worse for time misalignment t₀ larger than 10% of the bit time interval even when closed-loop bandwidth is small, such as 1% of the data rate.