Quadrature-quadrature phase-shift keying

Quadrature-quadrature phase-shift keying (Q²PSK) is a spectrally efficient modulation scheme which utilizes available signal space dimensions in a more efficient way than two-dimensional schemes such as QPSK and MSK (minimum-shift keying). It uses two date shaping pulses and two carriers, which are pairwise quadrature in phase, to create a four-dimensional signal space and increases the transmission rate by a factor of two over QPSK and MSK. However, the bit error rate performance depends on the choice of pulse pair. With simple sinusoidal and cosinusoidal data pulses, the E_b/N₀ requirement for P _b(E)=10^-5 is approximately 1.6 dB higher than that of MSK. Without additional constraints, Q²PSK does not maintain a constant envelope, however, a simple block coding can provide a constant envelope. This coded signal substantially outperforms MSK and TFM (time-frequency multiplexing) in bandwidth efficiency. Like MSK, Q ²PSK also has self-clocking and self-synchronizing ability. An optimum class of pulse shapes for use in Q²PSK format is presented. One suboptimum realization achieves the Nyquist rate of 2 b/s/Hz using binary detection