A Shifted Constellation of QPSK: Constellation Design and Dynamic Demodulation Threshold Optimization

This paper proposes an improved QPSK modulation scheme which is called special QPSK. The key aspect of special QPSK is that the synchronous sequence is hidden in the information bits and does not occupy spectrum resource. In order to implement frame synchronization, correlation acquisition has been considered. Based on this approach, we derive the closed-form expression of expected peak values for the cases signals aligned or not aligned. Then the constellation of the proposed scheme is optimized by minimizing the bit error rate with the constraint for acquisition performance. Finally, at the receiver, we proposed a dynamic threshold demodulation optimization method and solved the optimal dynamic threshold. Simulation results confirm that the signal channel utilization of our proposed special QPSK is better than that of conventional \pi/4 -QPSK modulation scheme. At the same time, special QPSK can also achieve a good bit error rate performance which is nearly the same as that of QPSK by using the dynamic threshold demodulation method.