Chapter 2

Constant Envelope Modulations

2.1 The Need for Constant Envelope

Digital communication systems operate in the presence of path loss and atmospheric-induced fading. In order to maintain sufficient received power at the destination, it is required that a device for generating adequate transmitter output power based on fixed- but-limited available power be employed, examples of which are traveling-wave tube amplifiers (TWTAs) and solid-state power amplifiers (SSPAs) operated in full- saturation mode to maximize conversion efficiency. Unfortunately, this requirement introduces amplitude modulation-amplitude modulation (AM-AM) and amplitude modulation-phase modulation (AM-PM) conversions into the transmitted signal. Because of this, modulations that transmit information via their amplitude, e.g., quadrature amplitude modulation (QAM), and therefore need a linear amplifying characteristic, are not suitable for use on channels operated in the above maximum transmitter power efficiency requirement.1 Another consideration regarding radio frequency (RF) amplifier devices that operate in a nonlinear mode at or near saturation is the spectral spreading that they reintroduce due to the nonlinearity subsequent to bandlimiting the modulation prior to amplification. Because of the need for the transmitted power spectrum to fall under a specified mask imposed by regulating agencies such as the FCC or International Telecommunications Union (ITU), the modulation must be designed to keep ...

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