We would like to present an alternative constellation
mapping to the conventional square 16-QAM modulation scheme presented in Fig. 2.102. This
has also been called the Star-QAM modulation scheme where the 16 states have been evenly split
across 2 circles of different radii [9].
This solution of constellation mapping provides two immediate benefits. First, it allows for efficient
differential encoding and decoding methods, which now give the receiver designer a choice to use
coherent detection or noncoherent detection. Second, it allows for less-complicated receive structures,
practically speaking, when we consider signal processing functions such as AGC, carrier recovery for
coherent detection, constellation stabilization, demapping from states to bits, and so forth.
2.3 MODULATION SCHEME COMPARISONS
In this section, we will provide an interim summary of the modulation schemes provided in this chapter.
We say interim since the details of the various choices available to the communication system
designer have yet to be discussed and are available in Chap. 4. In the meantime we will present a
preview of the comparisons as well as on what grounds they should be compared.
First we will discuss some goals of digital communication system designs.
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