Chapter 3

Quasi-Constant Envelope Modulations

In Chap. 2, we restricted our consideration to bandwidth-efficient modulations that were strictly constant envelope, thus rendering themselves maximally power efficient when transmitted over a nonlinear channel operating in saturation. As a compromise between the two extremes of constant and nonconstant envelope, we turn our attention to modulations that deviate slightly rom the former but make up for the attendent small loss in power efficiency by offering a more significant improvement in bandwidth efficiency. The most promising modulation in this category is Feher-patented quadriphase-shift-keying (FQPSK) [1], whose generic form finds its roots in cross-correlated PSK (XPSK), introduced in 1983 [2], and which has recently been given a more insightful interpretation [3], thereby allowing further enhancements [3,4]. Since the basic form of FQPSK and its predecessor, XPSK, are well documented in several of Feher's textbooks and papers [5-8], our focus here will be on the recent advancements [3,4] that allow additional improvements in power and bandwidth efficiencies. Nevertheless, we shall present a brief review of FQPSK in its originally conceived form, since it provides insight into the new interpretation and enhancements that followed. Before proceeding with the technical details, we present a brief historical perspective as well as the current state of the art regarding the practical application of FQPSK in government- and commercially ...

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