Introduction to application in communication systems and type of modulation

Introduction

The purpose of a communication system is to transport a signal representing a message that is generated by a source of information over a channel and deliver a reliable estimate of that signal to a user. For example, the message signal may be a speech signal, and the channel may be a cellular telephone channel or a satellite channel. 

Modulation provides the means for:

(1) SShifting the range of frequencies contained in the message signal into another frequency range suitable for transmission over the channel.

(2) Performing a corresponding shift back to the original frequency range after reception of the signal. 

Technically, modulation is defined as the process by which some characteristic of a carrier wave is varied in accordance with the message signal. The message signal is referred to as the modulating wave, and the result of the modulation process is referred to as the modulated wave. In the receiver, demodulation is used to recover the message signal from the modulated wave. Demodulation is the inverse of the modulation process. of the modulation process. 

Types of modulation

The specific type of modulation employed in a communication system is determined by the form of carrier wave used to perform the modulation. The two most commonly used forms of the carrier are a sinusoidal wave and a periodic pulse train.

Also, we may identify two main classes of modulation: 

1. continuous-wave or CW modulation and 
2. Pulse modulation. 

1. Continuous- wave modulation

Continuous-wave modulation can be further categorized into the following sub-classes:

• Amplitude modulation: In this the carrier amplitude is varied with the message signal. 
• Angle modulation: In this the angle of the carrier is varied with the message signal. 

The amplitude modulation can itself be implemented in several different forms. For a given message signal, the frequency content of the modulated wave depends on the form of amplitude modulation used. To specify, we have the following four types:

• • Full amplitude modulation or double sideband-transmitted carrier.
  • Double sideband-suppressed carrier modulation.
  • Single sideband modulation
  • Vestigial sideband modulation. 

If the amplitude of the message signal is scaled by a certain factor, then the amplitude of the modulated wave is scaled by exactly the same factor. In this strict sense, full amplitude modulation fails to meet the definition of linear modulation with respect to the message signal for reasons that will become apparent later. Still, the departure from linearity in the case of full amplitude modulation is of a rather mild sort, in a way that many of the mathematical procedures applicable to the analysis of linear modulation may be retained. On the other hand, angle modulation is a nonlinear modulation process. 

2. Pulse modulation.

Let us consider a carrier wave:

c(t) = Σp(t-nT) 

that consists of a periodic train of narrow pulses, where T is the period and p(t) denotes a pulse of relatively short duration as compared with the period T and centered on the origin. When some characteristic parameter of p(t) is varied in accordance with the message signal, we have pulse modulation. 

Below is the diagram of a sinusoidal modulating wave. 

Depending on how pulse modulation is actually accomplished, we may distinguish the following two subclasses:

1. Analog pulse modulation.

In analog pulse modulation a characteristic parameter such as the amplitude, duration, or the position of a pulse is varied continuously with the message signal. Therefore, we speak of pulse-amplitude modulation, pulse-duration modulation, and pulse position modulation as different realizations of analog pulse modulation. This type of pulse modulation may be viewed as the counterpart of continuous wave modulation. 

2. Digital pulse modulation

In digital pulse modulation the modulated signal is represented in coded form. This representation can be accomplished in a number of different ways. 

The standard method involves two operations. At first, the amplitude of each modulated pulse is approximated by the nearest member of a set of discrete levels that occupies a compatible range of values. This operation is called quantization, and the device for performing it is called a quantizer. 

Secondly, the quantizer output is coded, for example, in binary form. This particular form of digital pulse modulation is known as pulse code modulation or PCM. Quantization is a nonlinear process that results in a loss of information, but the loss is under the designer’s control in that it can be made as small as desired simply by using a large enough number of discrete (quantization) levels. In any event, PCM has no continuous wave counterpart. 

Benefits of modulation

The benefits of modulation are as follows:

1. Modulation is used to shift the spectral content of a message signal so that it lies inside the operating frequency band of a communication channel.
2. Modulation permits the use of multiplexing.
3. Modulation makes it possible for the physical size of the transmitting and receiving antenna to assume a practical value. 

Reference

Introduction to application in communication systems and type of modulation