Category: principles Of Communication

Spread Spectrum Modulation A collective class of signaling techniques are employed before transmitting a signal to provide a secure communication, known as the Spread Spectrum Modulation. The main advantage of spread spectrum communication technique is to prevent “interference” whether it is intentional or unintentional. The signals modulated with these techniques are hard to interfere and cannot be jammed. An intruder with no official access, is never allowed to crack them. Hence these techniques are used for military purposes. These spread spectrum signals transmit at low power density and has a wide spread of signals. Pseudo-Noise Sequence A coded sequence of 1s and 0s with certain auto-correlation properties, called as PseudoNoise coding sequence is used in spread-spectrum techniques. It is a maximum-length sequence, which is a type of cyclic code. Narrow-band Signal Narrow-band signals have the signal strength concentrated as shown in the frequency spectrum in the following figure. Here are the features of narrow-band signals − Band of signals occupy narrow range of frequencies. Power density is high. Spread of energy is low and concentrated. Though the features are good, these signals are prone to interference. Spread Spectrum Signals The spread spectrum signals have the signal strength distributed as shown in the following frequency spectrum figure. Here are the features of spread spectrum signals − Band of signals occupy a wide range of frequencies. Power density is very low. Energy is widespread. With these features, the spread spectrum signals are highly resistant to interference or jamming. Since, multiple users can share the same spread spectrum bandwidth without interfering with one another, these can be called as multiple access techniques. Spread spectrum multiple access techniques use signals which have a transmission bandwidth whose magnitude is greater than the minimum required RF bandwidth. Spread spectrum signals can be classified into two categories − Frequency Hopped Spread spectrum (FHSS) Direct Sequence Spread spectrum (DSSS) Frequency Hopped Spread Spectrum This is frequency hopping technique, where the users are made to change the frequencies of usage, from one to another in a specified time interval, hence it is called as frequency hopping. For example, a frequency was allotted to sender 1 for a particular period of time. Now, after a while, sender 1 hops to the other frequency and sender 2 uses the first frequency, which was previously used by sender1. This is called as frequency reuse. The frequencies of the data are hopped from one to another in order to provide secure transmission. The amount of time spent on each frequency hop is called as Dwell time. Direct Sequence Spread Spectrum Whenever a user wants to send data using this DSSS technique, each and every bit of the user data is multiplied by a secret code, called as chipping code. This chipping code is nothing but the spreading code which is multiplied with the original message and transmitted. The receiver uses the same code to retrieve the original message. This DSSS is also called as Code Division Multiple Access (CDMA). Comparison between FHSS and DSSS/CDMA Both the spread spectrum techniques are popular for their characteristics. To have a clear understanding, let us take a look at their comparisons. FHSS DSSS/CDMA Multiple frequencies are used Single frequency is used Hard to find the user’s frequency at any instant of time User frequency, once allotted is always the same Frequency reuse is allowed Frequency reuse is not allowed The sender need not wait The sender has to wait if the spectrum is busy Power strength of the signal is high Power strength of the signal is low It is stronger and penetrates through the obstacles It is weaker compared to FHSS It is never affected by interference It can be affected by interference It is cheaper It is expensive This is the mostly used technique This technique is not frequently used Advantages of Spread Spectrum Following are the advantages of Spread Spectrum. Cross-talk elimination Better output with data integrity Reduced effect of multipath fading Better security Reduction in noise Co-existence with other systems Longer operative distances Hard to detect Hard to demodulate/decode Harder to jam the signals Although spread spectrum techniques were originally designed for military uses, they are now being used widely as commercial purpose. Learning working make money

Discuss Principles of Communication In this tutorial, the basic concepts of communications along with the important concepts of analog and digital communications have been covered. This tutorial is helpful for a beginner who wants to acquire knowledge on the communication systems. There are a few topics in this tutorial covering the concepts of digital communications, which are elaborately discussed in our Digital Communication tutorial. Learning working make money

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