The contents of this article are taken from different sites.
INTRODUCTION
Communication involves the transmission of information from one point to another through a succession of processes. The source originates a message which is converted by an in put transducer into electrical waveform referred to as base band or message signal. The transmitter modifies the base band signal for efficient transmission. The channel is a medium through which the transmitter output is sent. The signal is contaminated on the channel by undesirable signals usually called noise. The receiver responses the signal received from the channel by undoing the signal modification made at the transmitter and the channel. An output transducer converts the waveform to its original waveform.
Digital messages are constructed with a finite number of symbols. For example printed language consists of 26 alphabets 10 letters a spacebar and a number of punctuation marks. Thus text message consists of about 50 symbols. Human speech is also a digital message, because it is made up of finite vocabulary in language. A digital symbol constructed with M symbols is called an M-ary message.
Digital messages are transmitted using a finite set of electrical waveforms. The transmitter according to its input transmits the appropriate waveform. The task of the receiver is to extract the message from a distorted or noisy signal at the channel output. The principle feature of digital communication system is that during a finite interval of time, it sends a waveform from a finite set of possible waveforms, in contrast to an analog communication which sends a waveform from an infinite variety of waveform shapes with theoretically infinite resolution. In a digital communication system, the objective of the receiver is not to reproduce a transmitted waveform with precision, it is instead to determine from a noise perturbed signal, which waveform from a finite set of waveform had been sent by the transmitter.
1.1.1 Baseband Signals
In context of communication system, a signal of primary interest is the message signal delivered by a source of information. This signal is also referred to as a baseband signal, with the term baseband used to designate the band of frequencies representing the message signal. Baseband signal can be of analog or digital type. In analog time signal time takes on values in continuum, and so does the amplitude of the signal. Analog baseband signals arise when a physical waveform such as light or acoustic is converted into an electrical signal. In digital signal on the other hand, both time and signal’s amplitude takes on discrete values. The output of a digital computer is an example of baseband signal of digital type. In this particular example only two values commonly represented by 0 and 1 are used so the signal is called a binary signal. Analog signal of primary interest is the transmitted signal, the characterization of which is determined by the type of channel used in the communication system. In the context we speak of baseband or passband. In baseband transmission, as the name implies the band of transmission frequencies supported by the channel closely matches the band of frequencies occupied by the message signal.
1.1.2 Passband Signals
In a passband transmission, the transmission band of the channel is centered at a frequency much higher than the maximum frequency component of the message signal. In the latter case, the transmitted signal is said to be a passband signal, the generation of which is accomplished in the transmission using a process known as modulation.
1.2 Modulation Techniques
Baseband signals are usually modified to facilitate transmission. This process is known as modulation. One of the parameters of a carrier is varied in proportion to the baseband signal. This results in amplitude modulation (AM), phase modulation (PM) or frequency modulation (FM) depending upon the parameters used. At the receiver the modulated signal is passed through a reverse process in order to construct the original signal.1.2.1 FSK
When the data is transmitted by varying the carrier frequency it is called Frequency Shifting Keying (FSK). FSK modulation is characterized by the information being contained in the frequency of the carrier. FSK is characterized in Cartesian coordinate space, with each axis representing a frequency tone from the M-ary set of orthogonal tones. The amplitude and phase of the carrier remain constant.
Figure 1.1: Frequency Shift Keying
In phase shift keying (PSK),it is the phase changes of the carrier that represents the binary 1s and 0s of a digital signal. A binary0 is 180 degree out of phase.
Figure 1.2: Phase Shift Keying
1.2.3 ASK
In amplitude shift keying (ASK),it is the amplitude difference or the presence or absence of a sine wave that represents the binary 1s and 0s of a digital signal. A fixed amplitude carrier represents a binary 1 and its absence is binary 0.
Figure 1.3: Amplitude Shift Keying
No comments:
Post a Comment