Category: principles Of Communication

Analyzing Signals To analyze a signal, it has to be represented. This representation in communication systems is of two types − Frequency domain representation, and Time domain representation. Consider two signals with 1 kHz and 2 kHz frequencies. Both of them are represented in time and frequency domain as shown in the following figure. Time domain analysis, gives the signal behavior over a certain time period. In the frequency domain, the signal is analyzed as a mathematical function with respect to the frequency. Frequency domain representation is needed where the signal processing such as filtering, amplifying and mixing are done. For instance, if a signal such as the following is considered, it is understood that noise is present in it. The frequency of the original signal may be 1 kHz, but the noise of certain frequency, which corrupts this signal is unknown. However, when the same signal is represented in the frequency domain, using a spectrum analyzer, it is plotted as shown in the following figure. Here, we can observe few harmonics, which represent the noise introduced into the original signal. Hence, the signal representation helps in analyzing the signals. Frequency domain analysis helps in creating the desired wave patterns. For example, the binary bit patterns in a computer, the Lissajous patterns in a CRO, etc. Time domain analysis helps to understand such bit patterns. Learning working make money

Principles of Communication Tutorial Job Search 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. Audience This tutorial is prepared for beginners who are interested in the basics of communications and who aspire to acquire knowledge regarding analog and digital communications. Prerequisites To benefit from this tutorial, basic knowledge of the terms involved in Electronics and Communications would be an added advantage. Learning working make money

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. Learning working make money

Analog Pulse Modulation After the continuous wave modulation, the next division is Pulse modulation. Pulse modulation is further divided into analog and digital modulation. The analog modulation techniques are mainly classified into Pulse Amplitude Modulation, Pulse Duration Modulation/Pulse Width Modulation, and Pulse Position Modulation. Pulse Amplitude Modulation Pulse Amplitude Modulation (PAM) is an analog modulating scheme in which the amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message signal. The pulse amplitude modulated signal, will follow the amplitude of the original signal, as the signal traces out the path of the whole wave. In natural PAM, a signal sampled at the Nyquist rate is reconstructed, by passing it through an efficient Low Pass Frequency (LPF) with exact cutoff frequency The following figures explain the Pulse Amplitude Modulation. Though the PAM signal is passed through an LPF, it cannot recover the signal without distortion. Hence to avoid this noise, flat-top sampling is done as shown in the following figure. Flat-top sampling is the process in which sampled signal can be represented in pulses for which the amplitude of the signal cannot be changed with respect to the analog signal, to be sampled. The tops of amplitude remain flat. This process simplifies the circuit design. Pulse Width Modulation Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) or Pulse Time Modulation (PTM) is an analog modulating scheme in which the duration or width or time of the pulse carrier varies proportional to the instantaneous amplitude of the message signal. The width of the pulse varies in this method, but the amplitude of the signal remains constant. Amplitude limiters are used to make the amplitude of the signal constant. These circuits clip off the amplitude, to a desired level and hence the noise is limited. The following figures explain the types of Pulse Width Modulations. There are three variations of PWM. They are − The leading edge of the pulse being constant, the trailing edge varies according to the message signal. The trailing edge of the pulse being constant, the leading edge varies according to the message signal. The center of the pulse being constant, the leading edge and the trailing edge varies according to the message signal. These three types are shown in the above given figure, with timing slots. Pulse Position Modulation Pulse Position Modulation (PPM) is an analog modulating scheme in which the amplitude and width of the pulses are kept constant, while the position of each pulse, with reference to the position of a reference pulse varies according to the instantaneous sampled value of the message signal. The transmitter has to send synchronizing pulses (or simply sync pulses) to keep the transmitter and receiver in synchronism. These sync pulses help maintain the position of the pulses. The following figures explain the Pulse Position Modulation. Pulse position modulation is done in accordance with the pulse width modulated signal. Each trailing of the pulse width modulated signal becomes the starting point for pulses in PPM signal. Hence, the position of these pulses is proportional to the width of the PWM pulses. Advantage As the amplitude and width are constant, the power handled is also constant. Disadvantage The synchronization between transmitter and receiver is a must. Comparison between PAM, PWM, and PPM The comparison between the above modulation processes is presented in a single table. PAM PWM PPM Amplitude is varied Width is varied Position is varied Bandwidth depends on the width of the pulse Bandwidth depends on the rise time of the pulse Bandwidth depends on the rise time of the pulse Instantaneous transmitter power varies with the amplitude of the pulses Instantaneous transmitter power varies with the amplitude and width of the pulses Instantaneous transmitter power remains constant with the width of the pulses System complexity is high System complexity is low System complexity is low Noise interference is high Noise interference is low Noise interference is low It is similar to amplitude modulation It is similar to frequency modulation It is similar to phase modulation Learning working make money

Principles of Communication – Noise In any communication system, during the transmission of the signal, or while receiving the signal, some unwanted signal gets introduced into the communication, making it unpleasant for the receiver, questioning the quality of the communication. Such a disturbance is called as Noise. What is Noise? Noise is an unwanted signal which interferes with the original message signal and corrupts the parameters of the message signal. This alteration in the communication process, leads to the message getting altered. It is most likely to be entered at the channel or the receiver. The noise signal can be understood by taking a look at the following example. Hence, it is understood that noise is some signal which has no pattern and no constant frequency or amplitude. It is quite random and unpredictable. Measures are usually taken to reduce it, though it can’t be completely eliminated. Most common examples of noise are − Hiss sound in radio receivers Buzz sound amidst of telephone conversations Flicker in television receivers, etc. Effects of Noise Noise is an inconvenient feature which affects the system performance. Following are the effects of noise. Noise limits the operating range of the systems Noise indirectly places a limit on the weakest signal that can be amplified by an amplifier. The oscillator in the mixer circuit may limit its frequency because of noise. A system’s operation depends on the operation of its circuits. Noise limits the smallest signal that a receiver is capable of processing. Noise affects the sensitivity of receivers Sensitivity is the minimum amount of input signal necessary to obtain the specified quality output. Noise affects the sensitivity of a receiver system, which eventually affects the output. Types of Noise The classification of noise is done depending on the type of the source, the effect it shows or the relation it has with the receiver, etc. There are two main ways in which noise is produced. One is through some external source while the other is created by an internal source, within the receiver section. External Source This noise is produced by the external sources which may occur in the medium or channel of communication, usually. This noise cannot be completely eliminated. The best way is to avoid the noise from affecting the signal. Examples Most common examples of this type of noise are − Atmospheric noise (due to irregularities in the atmosphere). Extra-terrestrial noise, such as solar noise and cosmic noise. Industrial noise. Internal Source This noise is produced by the receiver components while functioning. The components in the circuits, due to continuous functioning, may produce few types of noise. This noise is quantifiable. A proper receiver design may lower the effect of this internal noise. Examples Most common examples of this type of noise are − Thermal agitation noise (Johnson noise or Electrical noise). Shot noise (due to the random movement of electrons and holes). Transit-time noise (during transition). Miscellaneous noise is another type of noise which includes flicker, resistance effect and mixer generated noise, etc. Signal to Noise Ratio Signal-to-Noise Ratio (SNR) is the ratio of the signal power to the noise power. The higher the value of SNR, the greater will be the quality of the received output. Signal-to-noise ratio at different points can be calculated by using the following formulae − $$Input : SNR = (SNR)_I = frac{Average : power : of : modulating : signal}{Average : power : of : noise : at : input}$$ $$Output : SNR = (SNR)_O = frac{Average : power : of : demodulated : signal}{Average : power : of : noise : at : output}$$ $$Channel : SNR = (SNR)_C = frac{Average : power : of : modulated : signal}{Average : power : of : noise : in : message : bandwidth}$$ Figure of Merit The ratio of output SNR to the input SNR can be termed as the Figure of merit (F). It is denoted by F. It describes the performance of a device. $$F = frac{(SNR)_O}{(SNR)_I}$$ Figure of merit of a receiver is − $$F = frac{(SNR)_O}{(SNR)_C}$$ It is so because for a receiver, the channel is the input. Learning working make money

Digital Modulation Techniques Digital Modulation provides more information capacity, high data security, quicker system availability with great quality communication. Hence, digital modulation techniques have a greater demand, for their capacity to convey larger amounts of data than analog ones. There are many types of digital modulation techniques and we can even use a combination of these techniques as well. In this chapter, we will be discussing the most prominent digital modulation techniques. Amplitude Shift Keying The amplitude of the resultant output depends upon the input data whether it should be a zero level or a variation of positive and negative, depending upon the carrier frequency. Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary data in the form of variations in the amplitude of a signal. Following is the diagram for ASK modulated waveform along with its input. Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated, gives a zero value for LOW input and gives the carrier output for HIGH input. Frequency Shift Keying The frequency of the output signal will be either high or low, depending upon the input data applied. Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of the carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency modulation. Following is the diagram for FSK modulated waveform along with its input. The output of a FSK modulated wave is high in frequency for a binary HIGH input and is low in frequency for a binary LOW input. The binary 1s and 0s are called Mark and Space frequencies. Phase Shift Keying The phase of the output signal gets shifted depending upon the input. These are mainly of two types, namely BPSK and QPSK, according to the number of phase shifts. The other one is DPSK which changes the phase according to the previous value. Phase Shift Keying (PSK) is the digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. PSK technique is widely used for wireless LANs, bio-metric, contactless operations, along with RFID and Bluetooth communications. PSK is of two types, depending upon the phases the signal gets shifted. They are − Binary Phase Shift Keying (BPSK) This is also called as 2-phase PSK (or) Phase Reversal Keying. In this technique, the sine wave carrier takes two phase reversals such as 0° and 180°. BPSK is basically a DSB-SC (Double Sideband Suppressed Carrier) modulation scheme, for message being the digital information. Following is the image of BPSK Modulated output wave along with its input. Quadrature Phase Shift Keying (QPSK) This is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°. If this kind of techniques are further extended, PSK can be done by eight or sixteen values also, depending upon the requirement. The following figure represents the QPSK waveform for two bits input, which shows the modulated result for different instances of binary inputs. QPSK is a variation of BPSK, and it is also a DSB-SC (Double Sideband Suppressed Carrier) modulation scheme, which send two bits of digital information at a time, called as bigits. Instead of the conversion of digital bits into a series of digital stream, it converts them into bit-pairs. This decreases the data bit rate to half, which allows space for the other users. Differential Phase Shift Keying (DPSK) In DPSK (Differential Phase Shift Keying) the phase of the modulated signal is shifted relative to the previous signal element. No reference signal is considered here. The signal phase follows the high or low state of the previous element. This DPSK technique doesn’t need a reference oscillator. The following figure represents the model waveform of DPSK. It is seen from the above figure that, if the data bit is LOW i.e., 0, then the phase of the signal is not reversed, but is continued as it was. If the data is HIGH i.e., 1, then the phase of the signal is reversed, as with NRZI, invert on 1 (a form of differential encoding). If we observe the above waveform, we can say that the HIGH state represents an M in the modulating signal and the LOW state represents a W in the modulating signal. Learning working make money

Principles of Optical Fiber Communications The digital communication techniques discussed so far have led to the advancement in the study of both Optical and Satellite communications. Let us take a look at them. Fiber Optics An optical fiber can be understood as a dielectric waveguide, which operates at optical frequencies. The device or a tube, if bent or if terminated to radiate energy, is called a waveguide, in general. Following image depicts a bunch of fiber optic cables. The electromagnetic energy travels through it in the form of light. The light propagation, along a waveguide can be described in terms of a set of guided electromagnetic waves, called as modes of the waveguide. Working Principle A fundamental optical parameter one should have an idea about, while studying fiber optics is Refractive index. By definition, “The ratio of the speed of light in a vacuum to that in matter is the index of refraction n of the material.” It is represented as − $$n = frac{c}{v}$$ Where, c = the speed of light in free space = 3 × 108 m/s v = the speed of light in di-electric or non-conducting material Generally, for a travelling light ray, reflection takes place when n2 < n1 . The bent of light ray at the interface is the result of difference in the speed of light in two materials that have different refractive indices. The relationship between these angles at the interface can be termed as Snell’s law. It is represented as − $$n_1sinphi _1 = n_2sinphi _2$$ Where, $phi _1$ is the angle of incidence $phi _2$ is the refracted angle n1 and n2 are the refractive indices of two materials For an optically dense material, if the reflection takes place within the same material, then such a phenomenon is called as internal reflection. The incident angle and refracted angle are shown in the following figure. If the angle of incidence $phi _1$ is much larger, then the refracted angle $phi _2$ at a point becomes Π/2 . Further refraction is not possible beyond this point. Hence, such a point is called as Critical angle $phi _c$. When the incident angle $phi _1$ is greater than the critical angle, the condition for total internal reflection is satisfied. The following figure shows these terms clearly. A light ray, if passed into a glass, at such condition, it is totally reflected back into the glass with no light escaping from the surface of the glass. Parts of a Fiber The most commonly used optical fiber is single solid di-electric cylinder of radius a and index of refraction n1. The following figure explains the parts of an optical fiber. This cylinder is known as the Core of the fiber. A solid di-electric material surrounds the core, which is called as Cladding. Cladding has a refractive index n2 which is less than n1. Cladding helps in − Reducing scattering losses. Adds mechanical strength to the fiber. Protects the core from absorbing unwanted surface contaminants. Types of Optical Fibers Depending upon the material composition of the core, there are two types of fibers used commonly. They are − Step-index fiber − The refractive index of the core is uniform throughout and undergoes an abrupt change (or step) at the cladding boundary. Graded-index fiber − The core refractive index is made to vary as a function of the radial distance from the center of the fiber. Both of these are further divided into − Single-mode fiber − These are excited with laser. Multi-mode fiber − These are excited with LED. Optical Fiber Communications The communication system of fiber optics is well understood by studying the parts and sections of it. The major elements of an optical fiber communication system are shown in the following figure. The basic components are light signal transmitter, the optical fiber, and the photo detecting receiver. The additional elements such as fiber and cable splicers and connectors, regenerators, beam splitters, and optical amplifiers are employed to improve the performance of the communication system. Functional Advantages The functional advantages of optical fibers are − The transmission bandwidth of the fiber optic cables is higher than the metal cables. The amount of data transmission is higher in fiber optic cables. The power loss is very low and hence helpful in long-distance transmissions. Fiber optic cables provide high security and cannot be tapped. Fiber optic cables are the most secure way for data transmission. Fiber optic cables are immune to electromagnetic interference. These are not affected by electrical noise. Physical Advantages The physical advantages of fiber optic cables are − The capacity of these cables is much higher than copper wire cables. Though the capacity is higher, the size of the cable doesn’t increase like it does in copper wire cabling system. The space occupied by these cables is much less. The weight of these FOC cables is much lighter than the copper ones. Since these cables are di-electric, no spark hazards are present. These cables are more corrosion resistant than copper cables, as they are bent easily and are flexible. The raw material for the manufacture of fiber optic cables is glass, which is cheaper than copper. Fiber optic cables last longer than copper cables. Disadvantages Although fiber optics offer many advantages, they have the following drawbacks − Though fiber optic cables last longer, the installation cost is high. The number of repeaters are to be increased with distance. They are fragile if not enclosed in a plastic sheath. Hence, more protection is needed than copper ones. Applications of Fiber Optics The optical fibers have many applications. Some of them are as follows − Used in telephone systems Used in sub-marine cable networks Used in data link for computer networks, CATV Systems Used in CCTV surveillance cameras Used for connecting fire, police, and other emergency services. Used in hospitals, schools, and traffic management systems. They have many industrial uses and also used for in heavy duty constructions. Learning working make money

M-ary Encoding The word binary represents two-bits. M simply represents a digit that corresponds to the number of conditions, levels, or combinations possible for a given number of binary variables. This is the type of digital modulation technique used for data transmission in which instead of one-bit, two or more bits are transmitted at a time. As a single signal is used for multiple bit transmission, the channel bandwidth is reduced. M-ary Equation If a digital signal is given under four conditions, such as voltage levels, frequencies, phases and amplitude, then M = 4. The number of bits necessary to produce a given number of conditions is expressed mathematically as $$N = log_{2}M$$ Where, N is the number of bits necessary. M is the number of conditions, levels, or combinations possible with N bits. The above equation can be re-arranged as − $$2^{N} = M$$ For example, with two bits, 22 = 4 conditions are possible. Types of M-ary Techniques In general, (M-ary) multi-level modulation techniques are used in digital communications as the digital inputs with more than two modulation levels allowed on the transmitter’s input. Hence, these techniques are bandwidth efficient. There are many different M-ary modulation techniques. Some of these techniques, modulate one parameter of the carrier signal, such as amplitude, phase, and frequency. M-ary ASK This is called M-ary Amplitude Shift Keying (M-ASK) or M-ary Pulse Amplitude Modulation (PAM). The amplitude of the carrier signal, takes on M different levels. Representation of M-ary ASK $$S_m(t) = A_mcos(2pi f_ct)::::::A_mepsilon {(2m-1-M)Delta ,m = 1,2….M}:::and:::0leq tleq T_s$$ This method is also used in PAM. Its implementation is simple. However, M-ary ASK is susceptible to noise and distortion. M-ary FSK This is called as M-ary Frequency Shift Keying. The frequency of the carrier signal, takes on M different levels. Representation of M-ary FSK $$S_{i} (t) = sqrt{frac{2E_{s}}{T_{S}}} coslgroupfrac{Pi} {T_{s}}(n_{c} + i)trgroup ::::0leq tleq T_{s}:::and:::i = 1,2…..M$$ where $f_{c} = frac{n_{c}}{2T_{s}}$ for some fixed integer n. This is not susceptible to noise as much as ASK. The transmitted M number of signals are equal in energy and duration. The signals are separated by $frac{1}{2T_s}$ Hz making the signals orthogonal to each other. Since M signals are orthogonal, there is no crowding in the signal space. The bandwidth efficiency of an M-ary FSK decreases and the power efficiency increases with the increase in M. M-ary PSK This is called as M-ary Phase Shift Keying. The phase of the carrier signal, takes on M different levels. Representation of M-ary PSK $$S_{i}(t) = sqrt{frac{2E}{T}} cos(w_{0}t + emptyset_{i}t)::::0leq tleq T_{s}:::and:::i = 1,2…..M$$ $$emptyset_{i}t = frac{2Pi i} {M}:::where::i = 1,2,3…:…M$$ Here, the envelope is constant with more phase possibilities. This method was used during the early days of space communication. It has better performance than ASK and FSK. Minimal phase estimation error at the receiver. The bandwidth efficiency of M-ary PSK decreases and the power efficiency increases with the increase in M. So far, we have discussed different modulation techniques. The output of all these techniques is a binary sequence, represented as 1s and 0s. This binary or digital information has many types and forms, which are discussed further. Learning working make money

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