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Part 3 covers parametric estimation and wavelets. Part 5 introduces Phase Shift Keying (PSK) and its variants BPSK, QPSK, QAM, and GMSK. It will be published Thursday, March 20.
5.3 Modulation
In many communication systems, especially in radio systems, the information ("digital" or "analog"), or the baseband signal to be transmitted is modulated, i.e. "encoded" onto a carrier signal. The carrier is chosen depending on the type of media available for the transmission, for instance, a limited band in the radio frequency spectrum, or an appropriate frequency for transmission over cables or fibers. If we assume that the carrier signal is a plain cosine signal there are three parameters of the carrier that can be modulated and used for information transfer. These parameters are the amplitude a(t), the frequency f(t) and the phase Φ(t). The modulated carrier is then
The corresponding modulation types are denoted amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM). FM and PM are also referred to as angular modulation and are related in that the frequency is the phase changing speed; in other words, the derivative of the phase function is the frequency. Hence, frequency modulation can be achieved by first integrating the baseband signal and then feeding it into a phase modulator. This method is called Armstrong's indirect FM (Miller and Beasley, 2002).
In a digital radio communication system, it is common to modulate either by changing f(t) between a number of discrete frequencies, frequency shift keying (FSK) or by changing Φ(t) between a number of discrete phases, phase shift keying (PSK). In some systems, the amplitude is also changed between discrete levels, amplitude shift keying (ASK). So, if we are to design a modulation scheme which is able to transmit M different discrete symbols (for binary signals, M = 2), we hence have to define M unique combinations of amplitude, frequency and/or phase values of the carrier signal. The trick is to define these signal points, an(t), fn(t) or Φn(t), where n ∈ {0, ..., M −1}, in a way that communication speed can be high, influence of interference low, spectral occupancy low, and modulation and demodulation equipment can be made fairly simple.
5.3.1 Amplitude shift keying (ASK)
The concept of ASK is rather straightforward. Using this modulation method, the amplitude of the carrier can be one out of M given baseband amplitude functions an(t), corresponding to the M information symbols used in the system.
The carrier frequency f(t) = fc and the phase shift Φ(t) = Φ are constant. The amplitude functions an(t) are defined over a finite period of time, i.e. t0 ≤ t < t0 + T , where T is the symbol time. A simple example would be a system using binary symbols, i.e. M = 2 and square pulses for amplitude functions according to the below (see Figure 5-5)
symbol "0":
symbol "1":
Figure 5-5. Baseband signal and modulated signal for ASK, M = 2
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