Modulation Modulation is performed by causing the baseband modulating signal to vary a parameter of a carrier wave. An analogue carrier is defined by three parameters: amplitude, Ac, fre- quency, fc, and phase, O. (2.5) Thus, there are three basic modulation methods: 1. Amplitude modulation (AM) 2. Frequency modulation (FM) 3. Phase modulation (PM) When modulation is employed, a modulator is needed at the sending end of a channel, and a demodulator at the receiving end recovers the baseband signal from the modulated carrier. The combination of modulator and demodulator at a terminal is often referred to as a modem. Amplitude Modulation The simplest form of modulation is amplitude modulation. The modulator causes the enve- lope of the carrier wave to follow the waveform of the modulating signal, and the demodu- lator recovers it from this envelope. Chapter 2 Data Transmission If a sine carrier is modulated to a depth, m, by a modulating signal, x(t), the AM wave can be mathematically given as s(t) = =(1+mx(t))sin(2ft+0) (2.6) In modern communication systems, AM is not a preferred modulation mechanism. It makes inefficient use of the transmitted power because information is transmitted only in the sidebands but the majority of the power is contained in the carrier. More efficient variations of AM eliminate the carrier and generate only the sidebands. These schemes are known as double sideband suppressed carrier modulation (DSBSC) and single sideband suppressed carrier (SSBSC). The SSBSC signal requires the minimum pos- sible bandwidth for transmission. Consequently, this method is used whenever its complex- ity is justified by the saving in bandwidth signal. Frequency and Phase Modulation Frequency modulation (FM) and phase modulation (PM) are both forms of angle modula- tion. The angle-modulated signal can be mathematically represented as s(t) = (2.7) Frequency modulation follows equation 2.7 with the condition that the derivative of the phase O'(t) is proportional to the information signal x(t), which can be math