The digitization of analog signals involves the rounding off of the values which are approximately equal to the analog values. Name five types of internal noise. However, for a Low input, a negative pulse represents the data, and the zero level remains same for the other half of the bit duration. To understand this, let us consider a binary input sequence {ak} consisting of uncorrelated binary digits each having a duration Ta seconds. This source coding theorem is called as noiseless coding theorem as it establishes an error-free encoding. The diagram is as follows. The input sequence is much higher than the Nyquist rate. That gap can be termed as a sampling period Ts. Figure 1 shows the mid-rise type and figure 2 shows the mid-tread type of uniform quantization. µ-law has mid-tread at the origin. In this type of Polar signaling, a High in data, though represented by a Mark pulse, its duration T0 is less than the symbol bit duration. A prism which can turn different wavelengths into a single line, can be used at the output of MUX and input of DEMUX. This is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°. The MUX allocates the same slot to each device at all times. In this type of unipolar signaling, a High in data, though represented by a Mark pulse, its duration T0 is less than the symbol bit duration. The most occurring loss, as we have discussed, is the ISI. $S_i(t) = \sqrt{\frac{2E}{T}} \cos \left (w_o t + \phi _it\right )$ $0 \leq t \leq T \quad and \quad i = 1,2 ... M$, $$\phi _i \left ( t \right ) = \frac{2 \pi i}{M} \quad where \quad i = 1,2,3 ... \: ...M$$, Some prominent features of M-ary PSK are −. The high signal closes the switch, allowing a carrier wave. In analog multiplexing, the most used technique is Frequency Division Multiplexing (FDM). In general, Multi-level (M-ary) modulation techniques are used in digital communications as the digital inputs with more than two modulation levels are allowed on the transmitter’s input. They are −. There are other important techniques to be learned, called as data encoding techniques. In this type of unipolar signaling, a High in data is represented by a positive pulse called as Mark, which has a duration T0 equal to the symbol bit duration. Digital circuits are easy to design and cheaper than analog circuits. In the next stage of demodulation, the bit clock rate is needed at the detector circuit to produce the original binary message signal. Predictor input is the sum of quantizer output and predictor output, $$u(nT_s) = \widehat{x}(nT_s) + v(nT_s)$$, $u(nT_s) = \widehat{x}(nT_s) + e(nT_s) + q(nT_s)$. The clock synchronization problem in NRZ-I encoding is avoided by assigning an equivalent word of 5 bits in the place of each block of 4 consecutive bits. Block Diagram of Basic Communication System: Fig. The Narrow-band signals have the signal strength concentrated as shown in the following frequency spectrum figure. The source can be an analog signal. The ratio k/n is the code rate. The difference between an input value and its quantized value is called a Quantization Error. Digital communication is one of the most commonly used mode of communication in the current scenario. Following is the block diagram of DPCM transmitter. The occurrence of cross-talk is very rare in digital communication. the basic elements of a spread spectrum digital communication system with a binary information sequence at its input at the transmitting end and its output at the receiving end. Spread of energy is low and concentrated. Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. These will be discussed in subsequent chapters. The modulation of BPSK is done using a balance modulator, which multiplies the two signals applied at the input. These are the properties of Mutual information. These 5-bit words are predetermined in a dictionary. A source from which the data is being emitted at successive intervals, which is independent of previous values, can be termed as discrete memoryless source. Hence, the clock rate is double the data transfer rate and thus the modulation rate is also doubled. The FSK signal is passed through the two Band Pass Filters (BPFs), tuned to Space and Mark frequencies. The following figure indicates a continuous-time signal x (t) and a sampled signal xs (t). $0 \leq t \leq T_s \quad and \quad i = 1,2,3... \: ..M$, $0 \leq t \leq T \quad and \quad i = 1,2 ... M$, Hard to find the user’s frequency at any instant of time, User frequency, once allotted is always the same, Sender has to wait if the spectrum is busy, Stronger and penetrates through the obstacles. The DPCM Transmitter consists of Quantizer and Predictor with two summer circuits. So far, we’ve discussed that ISI is an unwanted phenomenon and degrades the signal. The height of the eye opening, at a specified sampling time, defines the margin over noise. Data Communication Model. The signals modulated with these techniques are hard to interfere and cannot be jammed. This residual effect is called as Inter Symbol Interference. Both the spread spectrum techniques are popular for their characteristics. This technique is used to transmit a signal over a single communication channel, by allotting one slot for each message. In a communication system, the transmitter is located at one place, the receiver is located at some other place (maybe far or near) separate from the transmitter and the channel is that physical medium that connects them. $\widehat{x}(nT_s)$ is the predicted sample, $e(nT_s)$ is the difference of sampled input and predicted output, often called as prediction error, $u(nT_s)$ is the predictor input which is actually the summer output of the predictor output and the quantizer output. In the demodulation process the original message signal is recovered back from the modulated signal. It removes the redundant bits (unnecessary excess bits, i.e., zeroes). The block diagram of Synchronous FSK detector consists of two mixers with local oscillator circuits, two band pass filters and a decision circuit. The signal is un-altered as the pulse needs a high disturbance to alter its properties, which is very difficult. The resultant signal is once again digitized by sampling and quantizing so that the pure digital output is obtained without the loss of information. Analog data to Analog signals − The modulation techniques such as Amplitude Modulation, Frequency Modulation and Phase Modulation of analog signals, fall under this category. Basic block diagram of communication system. This synchronous demodulator is a bit complex than asynchronous type demodulators. We can observe from the above pattern that the over-lapping of information is done, which leads to mixing up and loss of information. The analog multiplexing techniques involve signals which are analog in nature. In the absence of noise, the encoded receiver input will be the same as the encoded transmitter output. The block diagram of Asynchronous FSK detector consists of two band pass filters, two envelope detectors, and a decision circuit. Depending on Signal specification or technology, the communication system is classified as follows: (1) Analog Analog technology communicates data as electronic signals of varying frequency or amplitude. The following figure shows an equalizer in the receiver portion of the communication system. $e_p(nT_s) = x(nT_s) - \widehat{x}(nT_s)$, $= x(nT_s) - [\widehat{x} [[n - 1]T_s] + v[[n-1]T_s]]$, $v(nT_s) = e_q(nT_s) = S.sig. The Mid-Rise type is so called because the origin lies in the middle of a raising part of the stair-case like graph. The reverse process, i.e., extracting the number of channels from one, which is done at the receiver is called as de-multiplexing. This section increases the signal strength. Therefore input transducer is simply an energy converter at the input end, having the function to convert non-electrical signals into electrical signals. Multiplexing is the process of combining multiple signals into one signal, over a shared medium. It is used to produced required message which has to be transmitted. and odd bits (i.e., 1st bit, 3rd bit, 5th bit, etc.) These are few of the techniques used in digital communications. The effects of noise and crosstalk are reduced by using this technique. By recovering the band-limited message signal, with the help of the mixer circuit and the band pass filter, the first stage of demodulation gets completed. Occupies low bandwidth than unipolar and polar NRZ schemes. For example, a frequency was allotted to sender 1 for a particular period of time. These techniques help in Error Detection and Error Correction. Aliasing can be referred to as “the phenomenon of a high-frequency component in the spectrum of a signal, taking on the identity of a low-frequency component in the spectrum of its sampled version.”, The corrective measures taken to reduce the effect of Aliasing are −. The spectral line present at the symbol rate can be used as a clock. All of these are discussed in subsequent chapters. These combinations act as demodulators and the decision circuit chooses which output is more likely and selects it from any one of the detectors. The spacing between the two adjacent representation levels is called a quantum or step-size. Actual eye patterns are used to estimate the bit error rate and the signal-to-noise ratio. What technological innovations and development are necessary to realize 5G – the fifth generation of wireless communications technology? A theorem called, Sampling Theorem, was stated on the theory of this Nyquist rate. The rate at which data can be communicated is called the data rate. µ-law is used in North America and Japan. When the effect of ISI increases, traces from the upper portion to the lower portion of the eye opening increases and the eye gets completely closed, if ISI is very high. These waveforms can be transmitted over a cable. Discussed also source encoding and channel encoding Then, pulse waveforms are assigned that represented these symbols. The decoder circuit decodes the pulse coded waveform to reproduce the original signal. The channel coding in a communication system, introduces redundancy with a control, so as to improve the reliability of the system. The probability of error is much reduced. Set DC source to some value say 1 V with the help of multimeter and connect it to the A/D converter input and observe the output LED’s. Then it is passed to a shaper circuit, which is a comparator or a Schmitt trigger circuit, to recover the original binary data as the output. 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. NRZ Codes has 1 for High voltage level and 0 for Low voltage level. These three events occur at different times. For any positive integers m ≥ 3 and t < 2m-1 there exists a BCH binary code. For this we need source encoder. To achieve this perfect sampling, a periodic pulse train has to be derived from the received pulses, which is done by the timing circuit. If the wavelength increases, the frequency of the signal decreases. M.Tech from C-DAC-Mohali. Hence, this is also a good sampling rate. But why we need to convert non electrical signals into electrical signals? The output of the XNOR gate along with the carrier signal is given to the balance modulator, to produce the DPSK modulated signal. Name five types of internal noise. This is a very useful method for encoding speech at a low bit rate. M is the number of conditions, levels, or combinations possible with N bits. A signal is pulse code modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. Any modulated signal has a high frequency carrier. The sampling theorem states that, “a signal can be exactly reproduced if it is sampled at the rate fs which is greater than twice the maximum frequency W.”. Hz making the signals orthogonal to each other. These two are connected to two band pass filters. Transmitter. In TDM, the time frame is divided into slots. The basic television Broadcast transmitter block diagram is shown in figure (a). The quantizing of an analog signal is done by discretizing the signal with a number of quantization levels. It is used to produced required message which has to be transmitted. Hence, the maximum rate of the transmission is equal to the critical rate of the channel capacity, for reliable error-free messages, which can take place, over a discrete memoryless channel. Following is the diagrammatic representation. The result will be as shown in the above figure. These are the techniques mostly used for converting digital data into digital signals by compressing or coding them for reliable transmission of data. The two oscillators, producing a higher and a lower frequency signals, are connected to a switch along with an internal clock. The FSK signal input is given to the two mixers with local oscillator circuits. M represents a digit that corresponds to the number of conditions, levels, or combinations possible for a given number of binary variables. In order to overcome these problems, the signals are digitized using different techniques. They are −. As an Amazon Associate I earn from qualifying purchases. The signal droop is caused at places where the signal is non-zero at 0 Hz. The source could be in continuous form as in the case of analog communication and as discrete signals as in the case of digital communication. The carrier generator, sends a continuous high-frequency carrier. In the coming chapters, we will learn about the concepts of Digital communications, in detail. Digital data to Analog signals − The modulation techniques such as Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), etc., fall under this category. Sample is a piece of data taken from the whole data which is continuous in the time domain. 9. A Quantizer is a logarithmic function that performs Quantization (rounding off the value). The destination also works as a transducer at the receiving end. Channel coding consists of two parts of action. 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 continued as it was. Digital-to-Analog signals is the next conversion we will discuss in this chapter. A 0 will have a zero voltage level. The binary signal when ASK modulated, gives a zero value for Low input while it gives the carrier output for High input. A square law detector is one whose output voltage is proportional to the square of the amplitude modulated input voltage. In Manchester encoding, to send the data, the clocks with double speed is required rather than NRZ coding. Low pass filter is used for many reasons, but the prominent reason is noise elimination for out-of-band signals. Claude Shannon, the “father of the Information Theory”, provided a formula for it as −. The maximum average mutual information, in an instant of a signaling interval, when transmitted by a discrete memoryless channel, the probabilities of the rate of maximum reliable transmission of data, can be understood as the channel capacity. To make the circuit easily understandable, a decision-making circuit may also be inserted at the 2nd stage of detection. Hence, these techniques are used for military purposes. To find the process of obtaining this FSK modulated wave, let us know about the working of a FSK modulator. Announcement: Group Work 8 follow up. We further designate digital systems as being binary systems if only two possible symbols exist, and M-ary systems otherwise, where M is the number of possible alternative symbols. It designates each quantized level by a binary code. analog: audio or video signal, digital: like teletype signal. The Mid-tread type is so called because the origin lies in the middle of a tread of the stair-case like graph. The device that does multiplexing, can be called as a MUX. BPSK is basically a Double Side Band Suppressed Carrier (DSBSC) modulation scheme, for message being the digital information. The digitization of analog signal is done by the encoder. This choice of having the sampling rate higher than Nyquist rate, also helps in the easier design of the reconstruction filter at the receiver. The number of bits necessary to produce a given number of conditions is expressed mathematically as. The probability of the occurrence of BER is the Error Probability. They are −. Mutual information can be expressed in terms of entropy of the channel output. For the signals to get digitized, we have the sampling and quantizing techniques. In the next chapter, let us discuss about the concept of Quantization. When x (t) is multiplied by a periodic impulse train, the sampled signal xs (t) is obtained. The channel encoder, does the coding for error correction. In the data being transmitted, the left most bits of the code word correspond to the message bits, and the right most bits of the code word correspond to the parity bits. Solution: 2. Cyclic codes follow this cyclic property. The distortions which might occur during the transmission, are corrected by adding some redundant bits. The main methods of FSK detection are asynchronous detector and synchronous detector. The information is replaced without any loss. Mutual information of a channel is symmetric. Depending on … 1. The transmission losses of the channel and other factors affecting the quality of the signal, have to be treated. Acceptance Angle and Numerical Aperture (NA) (Opti... Current Affairs 7 August 2018 in Hindi/ Daily Curr... Current Affairs 6 August 2018 in Hindi/ Daily Curr... Intelligence Agencies of the World/ Intelligence A... North american islands list Islands in north america. Uniform quantization is achieved at µ = 0, where the characteristic curve is linear and no compression is done. Both the Narrow band and Spread spectrum signals can be understood easily by observing their frequency spectrum as shown in the following figures. That signal is made to confine to lower frequencies with the help of LPF. If the bit rate is a sub-multiple of the carrier frequency, then the bit clock regeneration is simplified. $R_x(\tau) = \lim_{T_p \rightarrow \infty}\frac{1}{T_p}\int_{\frac{{-T_p}}{2}}^{\frac{T_p}{2}}x(t)x(t + \tau)dt$, Since $x(t)$ consists of impulses, $R_x(\tau)$ can be written as, $R_x(\tau) = \frac{1}{T}\displaystyle\sum\limits_{n = -\infty}^\infty R_n\delta(\tau - nT)$, Where $R_n = \lim_{N \rightarrow \infty}\frac{1}{N}\sum_ka_ka_{k + n}$, Getting to know that $R_n = R_{-n}$ for real signals, we have, $S_x(w) = \frac{1}{T}(R_0 + 2\displaystyle\sum\limits_{n = 1}^\infty R_n \cos nwT)$, Since the pulse filter has the spectrum of $(w) \leftrightarrow f(t)$, we have, $= \frac{\mid F(w) \mid^2}{T}(\displaystyle\sum\limits_{n = -\infty}^\infty R_ne^{-jnwT_{b}})$, $= \frac{\mid F(w) \mid^2}{T}(R_0 + 2\displaystyle\sum\limits_{n = 1}^\infty R_n \cos nwT)$. This is called as the Mutual Information of the channel. If there is no noise present, then the modulator output equals the demodulator input. The data encoding technique is divided into the following types, depending upon the type of data conversion. The output from these two BPFs look like ASK signal, which is given to the envelope detector. The Equalizer is feasible to get implemented. They are −. The word Companding is a combination of Compressing and Expanding, which means that it does both. Switching & Transmission Prof. Murat Torlak Digital Modulation Overview: In digital wireless communication … The linearity property of the code word is that the sum of two code words is also a code word. It is generally observed that, we seek the help of Fourier series and Fourier transforms in analyzing the signals and also in proving theorems. Spread spectrum multiple access techniques uses signals which have a transmission bandwidth of a magnitude greater than the minimum required RF bandwidth. This method was used during the early days of space communication. This equation shows that the ith bit transmitted is correctly reproduced. The basic operations in the receiver section are regeneration of impaired signals, decoding, and reconstruction of the quantized pulse train. The step-size error that may occur at the transmitter is called granular noise, which is eliminated here. This is called as Channel coding theorem. Let the binary code word assigned to symbol Sk, by the encoder having length lk, measured in bits. In Differential Phase Shift Keying (DPSK) the phase of the modulated signal is shifted relative to the previous signal element. The output quality of the waveform is moderate. The communication that occurs in our day-to-day life is in the form of signals. The device which does de-multiplexing is called as DEMUX. But if we use more than 3 voltage levels, we can send more bits per signal. Following is the diagrammatic representation of BPSK Modulated output wave along with its given input. $S_m(t) = A_mcos (2 \pi f_ct) \quad A_m\epsilon {(2m - 1 - M) \Delta, m = 1,2... \: .M} \quad and \quad 0 \leq t \leq T_s$, Some prominent features of M-ary ASK are −. If there occurs a transition at the beginning of the bit interval, then the input bit is 0. Now, after a while, sender 1 hops to the other frequency and sender 2 uses the first frequency, which was previously used by sender 1. Main building blocks of the communication system in electronics are-, Now we will understand the functioning of each block of the block diagram -. They are further divided into FDM, WDM, and TDM. The phase of the carrier signal, takes on M different levels. If the event has occurred, a time back, there is a condition of having some information. If the parity has a distance of 2, one-bit flip can be detected. These signals, such as sound signals, generally, are analog in nature. There are a few techniques which have paved the basic path to digital communication processes. This is a random variable for $H(X \mid y = y_0) \: ... \: ... \: ... \: ... \: ... \: H(X \mid y = y_k)$ with probabilities $p(y_0) \: ... \: ... \: ... \: ... \: p(y_{k-1)}$ respectively. The following figures represent MUX and DEMUX. But the same ISI if used in a controlled manner, is possible to achieve a bit rate of 2W bits per second in a channel of bandwidth W Hertz. The predictor circuit is eliminated here and hence no assumed input is given to the demodulator. Here we will learn what is Communication System and how we can convey messages from one place to another electronically. The mean value of $H(X \mid y = y_k)$ for output alphabet y is −, $H\left ( X\mid Y \right ) = \displaystyle\sum\limits_{k = 0}^{k - 1}H\left ( X \mid y=y_k \right )p\left ( y_k \right )$, $= \displaystyle\sum\limits_{k = 0}^{k - 1} \displaystyle\sum\limits_{j = 0}^{j - 1}p\left (x_j \mid y_k \right )p\left ( y_k \right )\log_{2}\left [ \frac{1}{p\left ( x_j \mid y_k \right )} \right ]$, $= \displaystyle\sum\limits_{k = 0}^{k - 1} \displaystyle\sum\limits_{j = 0}^{j - 1}p\left (x_j ,y_k \right )\log_{2}\left [ \frac{1}{p\left ( x_j \mid y_k \right )} \right ]$. Hence, the available data is in the form of frames or packets, which are discrete. The type of quantization in which the quantization levels are unequal and mostly the relation between them is logarithmic, is termed as a Non-uniform Quantization. In digital modulation, we have come across certain problem of determining the step-size, which influences the quality of the output wave. It just goes in the same way here too. In the transmitter section of PCM, a low pass anti-aliasing filter is employed, before the sampler, to eliminate the high frequency components, which are unwanted. It means, the encryption of the data should not change the data. Instead of a pulse train, PCM produces a series of numbers or digits, and hence this process is called as digital. ASSIGNEMENT SET # 1 Spring 20182019B 3 EE LE 290:DIGITAL SYSTEMS Feb 8, 2018 1. The quantization levels in this type are odd in number. A Stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ). Following is the representation of the structure of code word, according to their allocation. The communication systems have been developed for communicating useful information from one place to other The delta demodulator comprises of a low pass filter, a summer, and a delay circuit. In the linear block codes, the parity bits and message bits have a linear combination, which means that the resultant code word is the linear combination of any two code words. Hence, these are a few error correcting codes, which are to be detected at the receiver. This sampling frequency, can be simply called as Sampling rate. The noise present in a channel creates unwanted errors between the input and the output sequences of a digital communication system. We can broadly classify the channels into two parts-. Block diagram of a simple Digital Communication System. In DPSK demodulator, the phase of the reversed bit is compared with the phase of the previous bit. Therefore at the input side, first we convert these non electrical signals into electrical signals then process these electrical signals and finally convert them back into the original form (non electrical) at the receiving end. Both sampling and quantization result in the loss of information. This technique uses various frequencies to combine streams of data, for sending them on a communication medium, as a single signal. The error probability should be very low, nearly ≤ 10-6 for a reliable communication. The following figure explains a signal, if sampled at a higher rate than 2w in the frequency domain. This video lecture introduces Basic Building Blocks of Communication System in Electronics. These spread spectrum signals transmit at low power density and has a wide spread of signals. It is also called as Shannon’s first theorem. View Answer: Answer: Source, Transmitter, Channel, Receiver, Destination. are separated by the bits splitter and are multiplied with the same carrier to generate odd BPSK (called as PSKI) and even BPSK (called as PSKQ). So far, in the linear codes, we have discussed that systematic unaltered code is preferred. This message signal is achieved by representing the signal in discrete form in both time and amplitude. The following figure represents an analog signal. Wavelength Division multiplexing is an analog technique, in which many data streams of different wavelengths are transmitted in the light spectrum. The receiving filter output $y(t)$ is sampled at time $t_i = iT_b$ (with i taking on integer values), yielding −, $y(t_i) = \mu \displaystyle\sum\limits_{k = -\infty}^{\infty}a_kp(iT_b - kT_b)$, $= \mu a_i + \mu \displaystyle\sum\limits_{k = -\infty \\ k \neq i}^{\infty}a_kp(iT_b - kT_b)$. The analysis estimates the values of a discrete-time signal as a linear function of the previous samples. The resultant pattern will look like the following figure. (2) Digital In digital technology, the data are generated and processed in two states: High (represented as 1) and Low (represented as 0). The transmitted M number of signals are equal in energy and duration. The word binary represents two bits. Though the features are good, these signals are prone to interference. 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The early days of space communication have come across certain problem of determining the step-size is very,!

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