AWQPSK: An optimum modulation technique for spread spectrum communication

Quadrature phase shift keying (QPSK) and minimum shift keying (MSK) are the two most used M-ary modulation techniques in direct sequence (DS) spread spectrum communication systems. A modulation technique is introduced that can compete well with QPSK and MSK in many applications. This new modulation technique made up of a superposition of one QPSK signal and two amplitude weighted QPSK signals, is called amplitude weighted quadrature phase shift keying (AWQPSK). It is found to have the same probability of error as QPSK and MSK techniques. It has a higher bandwidth efficiency in bits/sec/Hz than QPSK and MSK. It has 99.99 percent of its energy within the bandwidth and its sidelobes are 63 db down from the main lobe. Intersymbol interference (ISI) was simulated on an HP 9845 computer and was shown to be smaller than the ISI in a QPSK or an MSK signal. Two different implementation schemes are presented.