Principles of Communication – Resources The following resources contain additional information on Principles of Communication. Please use them to get more in-depth knowledge on this. Useful Video Courses 17 Lectures 2.5 hours 46 Lectures 2 hours 17 Lectures 57 mins 6 Lectures 52 mins 103 Lectures 16 hours 5 Lectures 39 mins Learning working make money
Principles of Communication – Quick Guide Principles of Communication – Introduction The word communication arises from the Latin word “commūnicāre”, which means “to share”. Communication is the basic step for the exchange of information. For example, a baby in a cradle, communicates with a cry that she needs her mother. A cow moos loudly when it is in danger. A person communicates with the help of a language. Communication is the bridge to share. Communication can be defined as the process of exchange of information through means such as words, actions, signs, etc., between two or more individuals. Need for Communication For any living being, while co-existing, there occurs the necessity of exchange of some information. Whenever a need for exchange of information arises, some means of communication should exist. While the means of communication, can be anything such as gestures, signs, symbols, or a language, the need for communication is inevitable. Language and gestures play an important role in human communication, while sounds and actions are important for animal communication. However, when some message has to be conveyed, a communication has to be established. Parts of Communication System Any system which provides communication, consists of the three important and basic parts as shown in the following figure. The Sender is the person who sends a message. It could be a transmitting station from where the signal is transmitted. The Channel is the medium through which the message signals travel to reach the destination. The Receiver is the person who receives the message. It could be a receiving station where the signal transmitted is received. What is a Signal? Conveying an information by some means such as gestures, sounds, actions, etc., can be termed as signaling. Hence, a signal can be a source of energy which transmits some information. This signal helps to establish communication between a sender and a receiver. An electrical impulse or an electromagnetic wave which travels a distance to convey a message, can be termed as a signal in communication systems. Depending on their characteristics, signals are mainly classified into two types: Analog and Digital. Analog and Digital signals are further classified, as shown in the following figure. Analog Signal A continuous time varying signal, which represents a time varying quantity can be termed as an Analog Signal. This signal keeps on varying with respect to time, according to the instantaneous values of the quantity, which represents it. Example Let us consider, a tap that fills a tank of 100 liters capacity in an hour (6 am to 7 am). The portion of filling the tank is varied by the varying time. Which means, after 15 mins (6:15 am) the quarter portion of the tank gets filled, whereas at 6:45 am, 3/4th of the tank is filled. If you try to plot the varying portions of water in the tank, according to the varying time, it would look like the following figure. As the resultant shown in this image varies (increases) according to time, this time varying quantity can be understood as Analog quantity. The signal which represents this condition with an inclined line in the figure, is an Analog Signal. The communication based on analog signals and analog values is called as Analog Communication. Digital Signal A signal which is discrete in nature or which is non-continuous in form can be termed as a Digital signal. This signal has individual values, denoted separately, which are not based on the previous values, as if they are derived at that particular instant of time. Example Let us consider a classroom having 20 students. If their attendance in a week is plotted, it would look like the following figure. In this figure, the values are separately stated. For instance, the attendance of the class on Wednesday is 20 whereas on Saturday is 15. These values can be considered individually and separately or discretely, hence they are called as discrete values. The binary digits which has only 1s and 0s are mostly termed as digital values. Hence, the signals which represent 1s and 0s are also called as digital signals. The communication based on digital signals and digital values is called as Digital Communication. Periodic Signal Any analog or digital signal, that repeats its pattern over a period of time, is called as a Periodic Signal. This signal has its pattern continued repeatedly and is easy to be assumed or to be calculated. Example If we consider a machinery in an industry, the process that takes place one after the other is a continuous and repeat procedure. For example, procuring and grading the raw material, processing the material in batches, packing a load of products one after the other etc., follow a certain procedure repeatedly. Such a process whether considered analog or digital, can be graphically represented as follows. Aperiodic Signal Any analog or digital signal, that doesn’t repeat its pattern over a period of time, is called as Aperiodic Signal. This signal has its pattern continued but the pattern is not repeated and is not so easy to be assumed or to be calculated. Example The daily routine of a person, if considered, consists of many types of works which take different time intervals for different works. The time interval or the work doesn’t continuously repeat. For example, a person will not continuously brush his teeth from morning to night, that too with the same time period. Such a process whether considered analog or digital, can be graphically represented as follows. In general, the signals which are used in communication systems are analog in nature, which are transmitted in analog or converted to digital and then transmitted, depending upon the requirement. But for a signal to get transmitted to a distance, without the effect of any external interferences or noise addition and without getting faded away, it has to undergo a process called as Modulation, which is discussed in the next chapter. Principles of Communication – Modulation A signal can be anything like a sound wave which
Delta Modulation The sampling rate of a signal should be higher than the Nyquist rate, to achieve better sampling. If this sampling interval in a Differential PCM (DPCM) is reduced considerably, the sample-to-sample amplitude difference is very small, as if the difference is 1-bit quantization, then the step-size is very small i.e., Δ (delta). What is Delta Modulation? The type of modulation, where the sampling rate is much higher and in which the stepsize after quantization is of smaller value Δ, such a modulation is termed as delta modulation. Features of Delta Modulation An over-sampled input is taken to make full use of a signal correlation. The quantization design is simple. The input sequence is much higher than Nyquist rate. The quality is moderate. The design of the modulator and the demodulator is simple. The stair-case approximation of output waveform. The step-size is very small, i.e., Δ (delta). The bit rate can be decided by the user. It requires simpler implementation. Delta Modulation is a simplified form of DPCM technique, also viewed as 1-bit DPCM scheme. As the sampling interval is reduced, the signal correlation will be higher. Delta Modulator The Delta Modulator comprises of a 1-bit quantizer and a delay circuit along with two summer circuits. Following is the block diagram of a delta modulator. A stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ). The output quality of the waveform is moderate. Delta Demodulator The delta demodulator comprises of a low pass filter, a summer, and a delay circuit. The predictor circuit is eliminated here and hence no assumed input is given to the demodulator. Following is the block diagram for delta demodulator. Low pass filter is used for many reasons, but the prominent one is noise elimination for out-of-band signals. The step-size error that may occur at the transmitter is called granular noise, which is eliminated here. If there is no noise present, then the modulator output equals the demodulator input. Advantages of DM over DPCM 1-bit quantizer Very easy design of modulator & demodulator However, there exists some noise in DM and following are the types of noise. Slope Over load distortion (when Δ is small) Granular noise (when Δ is large) Adaptive Delta Modulation In digital modulation, we come across certain problems in determining the step-size, which influences the quality of the output wave. The larger step-size is needed in the steep slope of modulating signal and a smaller stepsize is needed where the message has a small slope. As a result, the minute details get missed. Hence, it would be better if we can control the adjustment of step-size, according to our requirement in order to obtain the sampling in a desired fashion. This is the concept of Adaptive Delta Modulation (ADM). Learning working make money
Principles of Satellite Communications A satellite is a body that moves around another body in a mathematically predictable path called an Orbit. A communication satellite is nothing but a microwave repeater station in space that is helpful in telecommunications, radio, and television along with internet applications. A repeater is a circuit which increases the strength of the signal it receives and retransmits it. But here this repeater works as a transponder, which changes the frequency band of the transmitted signal, from the received one. The frequency with which the signal is sent into the space is called Uplink frequency, while the frequency with which it is sent by the transponder is Downlink frequency. The following figure illustrates this concept clearly. Now, let us have a look at the advantages, disadvantages and applications of satellite communications. Satellite Communication − Advantages There are many Advantages of satellite communications such as − Flexibility Ease in installing new circuits Distances are easily covered and cost doesn’t matter Broadcasting possibilities Each and every corner of earth is covered User can control the network Satellite Communication − Disadvantages Satellite communication has the following drawbacks − The initial costs such as segment and launch costs are too high. Congestion of frequencies Interference and propagation Satellite Communication − Applications Satellite communication finds its applications in the following areas − In Radio broadcasting. In TV broadcasting such as DTH. In Internet applications such as providing Internet connection for data transfer, GPS applications, Internet surfing, etc. For voice communications. For research and development sector, in many areas. In military applications and navigations. The orientation of the satellite in its orbit depends upon the three laws called as Kepler’s laws. Kepler’s Laws Johannes Kepler (1571-1630) the astronomical scientist, gave 3 revolutionary laws, regarding the motion of satellites. The path followed by a satellite around its primary (the earth) is an ellipse. Ellipse has two foci – F1 and F2, the earth being one of them. If the distance from the center of the object to a point on its elliptical path is considered, then the farthest point of an ellipse from the center is called as apogee and the shortest point of an ellipse from the center is called as perigee. Kepler’s 1st Law Kepler’s 1st law states that, “every planet revolves around the sun in an elliptical orbit, with sun as one of its foci.” As such, a satellite moves in an elliptical path with earth as one of its foci. The semi major axis of the ellipse is denoted as ‘a’ and semi minor axis is denoted as b. Therefore, the eccentricity e of this system can be written as − $$e = frac{sqrt{a^{2}-b^{2}}}{a}$$ Eccentricity (e) − It is the parameter which defines the difference in the shape of the ellipse rather than that of a circle. Semi-major axis (a) − It is the longest diameter drawn joining the two foci along the center, which touches both the apogees (farthest points of an ellipse from the center). Semi-minor axis (b) − It is the shortest diameter drawn through the center which touches both the perigees (shortest points of an ellipse from the center). These are well described in the following figure. For an elliptical path, it is always desirable that the eccentricity should lie in between 0 and 1, i.e. 0 < e < 1 because if e becomes zero, the path will be no more in elliptical shape rather it will be converted into a circular path. Kepler’s 2nd Law Kepler’s 2nd law states that, “For equal intervals of time, the area covered by the satellite is equal with respect to the center of the earth.” It can be understood by taking a look at the following figure. Suppose that the satellite covers p1 and p2 distances, in the same time interval, then the areas B1 and B2 covered in both instances respectively, are equal. Kepler’s 3rd Law Kepler’s 3rd law states that, “The square of the periodic time of the orbit is proportional to the cube of the mean distance between the two bodies.” This can be written mathematically as $$T^{2}:alpha::a^{3}$$ Which implies $$T^{2} = frac{4pi ^{2}}{GM}a^{3}$$ Where $frac{4pi ^{2}}{GM}$ is the proportionality constant (according to Newtonian Mechanics) $$T^{2} = frac{4pi ^{2}}{mu}a^{3} $$ Where μ = the earth’s geocentric gravitational constant, i.e. Μ = 3.986005 × 1014 m3/sec2 $$1 = left ( frac{2pi}{T} right )^{2}frac{a^{3}}{mu}$$ $$1 = n^{2}frac{a^{3}}{mu}:::Rightarrow :::a^{3} = frac{mu}{n^{2}}$$ Where n = the mean motion of the satellite in radians per second The orbital functioning of satellites is calculated with the help of these Kepler’s laws. Along with these, there is an important thing which has to be noted. A satellite, when it revolves around the earth, undergoes a pulling force from the earth which is the gravitational force. Also, it experiences some pulling force from the sun and the moon. Hence, there are two forces acting on it. They are − Centripetal force − The force that tends to draw an object moving in a trajectory path, towards itself is called as centripetal force. Centrifugal force − The force that tends to push an object moving in a trajectory path, away from its position is called as centrifugal force. So, a satellite has to balance these two forces to keep itself in its orbit. Earth Orbits A satellite when launched into space, needs to be placed in a certain orbit to provide a particular way for its revolution, so as to maintain accessibility and serve its purpose whether scientific, military, or commercial. Such orbits which are assigned to satellites, with respect to earth are called as Earth Orbits. The satellites in these orbits are Earth Orbit Satellites. The important kinds of Earth Orbits are − Geo Synchronous Earth Orbit Medium Earth Orbit Low Earth Orbit Geosynchronous Earth Orbit Satellites A Geo-Synchronous Earth Orbit (GEO) satellite is one which is placed at an altitude of 22,300 miles above the Earth. This orbit is synchronized with a side real day (i.e., 23hours 56minutes). This orbit
Digital Modulation So far we have gone through different modulation techniques. The one remaining is digital modulation, which falls under the classification of pulse modulation. Digital modulation has Pulse Code Modulation (PCM) as the main classification. It further gets processed to delta modulation and ADM. Pulse Code Modulation A signal is Pulse Code modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. The output of a Pulse Code Modulation (PCM) will resemble a binary sequence. The following figure shows an example of PCM output with respect to instantaneous values of a given sine wave. Instead of a pulse train, PCM produces a series of numbers or digits, and hence this process is called as digital. Each one of these digits, though in binary code, represent the approximate amplitude of the signal sample at that instant. In Pulse Code Modulation, the message signal is represented by a sequence of coded pulses. This message signal is achieved by representing the signal in discrete form in both time and amplitude. Basic Elements of PCM The transmitter section of a Pulse Code Modulator circuit consists of Sampling, Quantizing and Encoding, which are performed in the analog-to-digital converter section. The low pass filter prior to sampling prevents aliasing of the message signal. The basic operations in the receiver section are regeneration of impaired signals, decoding, and reconstruction of the quantized pulse train. The following figure is the block diagram of PCM which represents the basic elements of both the transmitter and the receiver sections. Low Pass Filter (LPF) This filter eliminates the high frequency components present in the input analog signal which is greater than the highest frequency of the message signal, to avoid aliasing of the message signal. Sampler This is the circuit which uses the technique that helps to collect the sample data at instantaneous values of the message signal, so as to reconstruct the original signal. The sampling rate must be greater than twice the highest frequency component W of the message signal, in accordance with the sampling theorem. Quantizer Quantizing is a process of reducing the excessive bits and confining the data. The sampled output when given to Quantizer, reduces the redundant bits and compresses the value. Encoder The digitization of analog signal is done by the encoder. It designates each quantized level by a binary code. The sampling done here is the sample-and-hold process. These three sections will act as an analog to the digital converter. Encoding minimizes the bandwidth used. Regenerative Repeater The output of the channel has one regenerative repeater circuit to compensate the signal loss and reconstruct the signal. It also increases the strength of the signal. Decoder The decoder circuit decodes the pulse coded waveform to reproduce the original signal. This circuit acts as the demodulator. Reconstruction Filter After the digital-to-analog conversion is done by the regenerative circuit and the decoder, a low pass filter is employed, called as the reconstruction filter to get back the original signal. Hence, the Pulse Code Modulator circuit digitizes the analog signal given, codes it, and samples it. It then transmits in an analog form. This whole process is repeated in a reverse pattern to obtain the original signal. Learning working make money