Signal Multiplexing Techniques for GNSS: The Principle, Progress, and Challenges Within a Uniform Framework

Signal multiplexing techniques are those that enable the efficient transmission of multiple signals through a single modulator, up converter, power amplifier chain, and antenna aperture, without mutual interference. In the construction of new-generation global navigation satellite systems (GNSSs), signal multiplexing has encountered many challenges as the number of signals to be multiplexed increases and signal elements become more diverse and complex. In the past 15 years, many novel and advanced constant envelope multiplexing (CEM) techniques have been proposed. However, increased requirements for adaptability and flexibility of signal multiplexing techniques for future evolutional GNSSs have resulted in the need for further improvement in generalized CEM design theory. The uniform CEM mathematical framework and more general CEM design approaches can offer a good starting point to meet these challenges. In this article, we provide a comprehensive tutorial on the important concepts, recent advances, representative applications, and the remaining challenges of GNSS signal multiplexing techniques. To provide readers with a global overview of multiplexing techniques in GNSS and to foster new research ideas, a mathematical cornerstone and intrinsic conceptual relationship of assorted multiplexing techniques along with the high adaptability and high flexibility challenges of CEM design will be two key discussions.