Coded QPSK using balanced incomplete block design

In this paper, a new M-ary modulation scheme based on quadrature phase shift keying (QPSK) for application in bipolar energy constrained communication systems is presented. Balanced incomplete block designs (BIBD) are used for constructing the symbol alphabets. Each symbol is obtained by combining two bipolar BIBD codewords, one in-phase and the other quadrature-phase. Simple transmitter and receiver structures using shift registers are proposed. Due to the fixed cross-correlation property of BIBD codes, symbols have equal energies, and therefore, no threshold is needed to make a decision on the received signal. Because of the large distance between symbols, this scheme is more power efficient than standard QPSK. Because of the low spectral-efficiency of this constellation, it has a poor performance in dispersive channels, and therefore, an interleaver and deinterleaver are used to lower its error probability. The performance of various modulation schemes are compared using numerical results for an additive white Gaussian noise (AWGN) channel and a bandwidth limited channel.