Wireless Communications

5Minimum Shift Keying

Abstract

Given its important role played in 2G cellular systems, minimum shift keying (MSK) is worth a dedicated chapter for its discussion to serve as an in-depth record. The reader can skip this chapter without losing reading continuity. The drawback of QPSK is its phase discontinuity such that the phase shift can jump by as much as 180° from one symbol to the next, causing spectrum spillover to the adjacent channels. In MSK, a different philosophy is employed. The information-modulated phase is no longer a constant over a symbol duration. But rather, it varies with time as a straight line. In doing so, two degrees of freedom are used to control the phase trajectory: the slope of the straight line, and its intercept on the -axis. The two parameters are chosen so that the variation of the phase is a continuous function of time. Clearly, two consecutive symbols need to jointly control the two phase parameters. That is the characteristics of the MSK.

5.1 Introduction

Digital modulation/demodulation is a key technology to cellular mobile communication. A cellular system is a communication system that supports a large number of voice channels. Any out-of-band radiation (leakage) will produce interference with adjacent channels, thereby degrading the system performance. To ensure the necessary transmission quality, the signal to adjacent interference should ...