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Scalable voip mobility intedration and deployment- P16: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | 150 Chapter 5 Carrier the other from the center frequency and uses that drift to derive the original data. For car radios which are usually analog that drift becomes the signal that is sent to the speakers. There is a third type phase modulation PM or OM as O is the custom mathematical symbol for phase measured in degrees from 0 to 360 which works by altering the phase of the signal or the offset of the carrier in time. Figure 5.16 shows phase modulation and how it compares to frequency modulation. Note how the phase modulated signal is offset in this example of simple of phase modulation the modulated appears to be reflected vertically the result of its phase having been altered by 180 degrees when the data signal is non-zero whereas the frequency modulated signal clearly has been sped up or slowed down for non-zero data. Because the phase of a signal is the offset from the carrier in degrees and the amplitude is just a distance the peak of the carrier is offset from the origin an amplitude and phase modulated signal can be represented using polar coordinates. In the descriptions here I will try to avoid unnecessary mathematics but the concept that the type of modulation applied to a carrier can be represented by polar coordinates is powerful for those interested in www.newnespress.com Introduction to Wi-Fi 151 -1.0 1.0 0.6 0.2 -0.2 -0.6 Carrier A cos 2pfc t Final Modulated Signal A cos 2p fc Data t Figure 5.15 Analog Frequency Modulation understanding just what radios do to the signal to be able to transmit the data. The polar coordinate representation is one of the two common methods for representing complex numbers and the designs of radios are always though of as involving complex math. A complex number is type of number that can represents the square roots of negative numbers something not allowed in real numbers. For those interested in the details the appendix at the end of this chapter explores the mathematics. The term baseband refers to the signal .
