Modulation is a process of combining an input signal

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Modulation is a process of combining an input signal and a carrier at frequency fcto produce a signal whose bandwidth is usually centered on fc. Input signal can be either an analog or a digital signal. In the analog case the amplitude, frequency, or phase of the carrier varies continuously in response to the message waveform. However, in the binary digital case these three parameters switch between one of two possible values, depending on whether a 0 or 1 pulse is transmitted. Examples of signal modulation are shown in Figure 2-5. The left part shows modulation of analog signals. An amplitude-modulated(AM) signal is a high-frequency wave whose peak amplitudes vary in accordance with an input signal. Analogously, in frequency modulation(FM), the instantaneous frequency of the carrier is varied proportionally to the amplitude of the input signal. In phase modulation(PM), the phase angle of the carrier is varied proportionally to the amplitude of the input signal. For digital input signals, the amplitude of the carrier sine wave is shifted from one level to another depending on whether a logic 1 or a logic 0 is sent, which is called amplitude-shift keying (ASK). Similarly, in frequency-shift keying(FSK) the frequency can take on one of two predetermined constant frequencies. In phase-shift keying(PSK) the phase can be one value or another and the signal transitions the phase shifts by 180°. ModulatorDestinationError Control EncoderSource Decoder (Decompress)Source Encoder (Compress)Error Control DecoderInformationSourceDemodulatorTransmitterReceiverCommunicationChannelFigure 2-4: Components of a wireless transmission and receiving system.
Ivan Marsic Rutgers University 8PSK is a preferred modulation technique in wireless communications. The absolute phase of the waveform is not relevant in PSK; only changes in the phase encode data. PSK is not susceptible to the noise degradation that mostly affects amplitude and thus ASK, nor does it have the bandwidth limitations of FSK. This implies that a receiver can detect small variations in the signal. Therefore, instead of using only binary variations (BPSK), one can establish, for example, quadruple (QPSK) and octal phase variations (8-PSK). One can represent the phase and amplitude relationships by a constellation or phase-state diagram, as illustrated in Figure 2-6(a) and Figure 2-6(b), where different combinations for amplitude and phase represent different symbols. Thus, QPSK works with four symbols, represented by binary numbers as “00,” “01,” “10,” and “11.” Generally, in M-ary PSK (Figure 2-6(c)), the carrier phase takes on one of Mpossible values, namely, θi= 2(i1)π/M, and it works with Msymbols. The simplest form, BPSK, uses two carrier waves, shifted by a half cycle (i.e., half wavelength) relative to each other. One wave, the reference wave is used to encode a “0,” and the half-cycle shifted is used to encode a “1.” Since absolute phase is difficult to determine, the phase is usually

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