In a carrier communication system, the baseband signal of a low-frequency spectrum is translated to a high frequency spectrum. This is achieved through modulation. The aim of this topic is to explore the reasons for using modulation. Modulation is defined as a process by virtue of which, some characteristic of a high frequency sinusoidal wave is varied in accordance with the instantaneous amplitude of the baseband signal.
Two signals are involved in the modulation process. The baseband signal and the carrier signal. The baseband signal is to be transmitted to the receiver. The frequency of this signal is generally low. In the modulation process, this baseband signal is called the modulating signal. The waveform of this signal is unpredictable. For example, the waveform of a speech signal is random in nature and cannot be predicted. In this case, the speech signal is the modulating signal.
The other signal involved with the modulation is a high frequency sinusoidal wave. This signal is called the carrier signal or carrier. The frequency of the carrier signal is always much higher than that of the baseband signal. After modulation, the baseband signal of low frequency is transferred to the high frequency carrier, which carries the information in the form of some variations. After the completion of the modulation process, some characteristic of the carrier is varied such that the resultant variations carry the information.
The carrier signal is represented by the equation:
ec = Ec sin(ωct + ϕ) ---------------- (1)
In Equation (1), c is an indicator that this equation represents the carrier signal. The components of this equation arc as follows:
- ec: Instantaneous amplitude of the carrier
- Ec: Amplitude of the carrier
- Angular frequency of the carrier, such that ωc = 2πfc, Where fc is the frequency carrier, also called the central frequency
- ϕ: Initial phase of the carrier signal
Equation (1) has three parameters namely, amplitudes (Ec), frequency (ωc), and phase (f). In principle, these parameters have constant values for a particular sinusoidal wave. According to the definition of Modulation, some characteristic of the carrier signal is varied in accordance with the modulating signal. After modulation any one of the three parameters of the carrier signal, namely, frequency, or phase, is varied keeping the remaining two constant.
The baseband signal is then carried by these variations. The type of the modulation is decided by the parameter chosen to vary.
For example, if amplitude of the carrier is chosen to vary in accordance with the instantaneous amplitude of the baseband signal, keeping frequency and phase constant, the resulting modulation called amplitude modulation. Frequency modulation and phase modulation are also obtained in a similar way.
Low-frequency baseband sign it is thus translated to a high frequency carrier such that the information is coded in the variations in one of the parameters of the carrier. At the receiver side, these variations are detected through the demodulation process to recover the original baseband signal.
The following can be summarized with reference to modulation.
- The baseband signal is known as the modulating signal.
- The baseband signal is a low-frequency signal.
- The carrier signal is always a high frequency sinusoidal wave.
- During the modulation process, the modulating, signal varies the frequency, y, amplitude, or phase of the carrier in accordance with its instantaneous amplitude.
- After modulation, the carrier is said to be modulated by the modulating.
- The output of the modulator is called the modulated signal.
The process of modulation in a communication system increases its cost and complexity. This may be considered as a disadvantage. However, modulation is extensively used in most communication systems. There is a definite need for using modulation. There can be problems if modulation is not used. Scrutinizing these problems can explain why modulation is required.
The baseband signal will be transmitted as it is. If modulation is not employed however, the system designer could confront the fallowing problems:
- Antenna Height
- Narrow Banding
- Poor radiation and penetration
- Diffraction angle