An eye pattern provides the following information about a particular system. The QPSK Demodulator uses two product demodulator circuits with local oscillator, two band pass filters, two integrator circuits, and a 2-bit parallel to serial converter. This is due to the transmission characteristics of the channel. Actually, there are many types of multiplexing techniques. This is a form of distortion of a signal, in which one or more symbols interfere with subsequent signals, causing noise or delivering a poor output. The stair-case approximated output is given to the LPF. 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The error probability should be very low, nearly ≤ 10-6 for a reliable communication. It is a maximum-length sequence, which is a type of cyclic code. The QPSK Modulator uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit. There are two methods of Polar Signaling. As a single signal is used for multiple bit transmission, the channel bandwidth is reduced. Where $H(y \mid x)$ is a conditional entropy. The cyclic property of code words is that any cyclic-shift of a code word is also a code word. No reference signal is considered here. The signal which is sampled after filtering, is sampled at a rate slightly higher than the Nyquist rate. If the message is not altered, then it is called as systematic code. 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. Noise is an unwanted random signal that interferes with the original message signal. Long strings of 1s and 0s is avoided to maintain transparency. Organizations generally rely on this mode for all their business communications. This section increases the signal strength. For a given bandwidth, the power is efficiently used. The digitization of analog signal is done by the encoder. The predictor produces the assumed samples from the previous outputs of the transmitter circuit. The sampling rate denotes the number of samples taken per second, or for a finite set of values. The binary input sequence is applied to the transmitter so as to choose the frequencies according to the binary input. If this sampling interval in Differential PCM is reduced considerably, the sampleto-sample amplitude difference is very small, as if the difference is 1-bit quantization, then the step-size will be very small i.e., Δ (delta). The clock frequency at the transmitter when matches with the clock frequency at the receiver, it is known as a Synchronous method, as the frequency gets synchronized. 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 called as frequency hopping. Where $f_c = \frac{n_c}{2T_s}$ for some fixed integer n. Some prominent features of M-ary FSK are −. But this can't be corrected. To have a clear understanding, let us take a look at their comparisons. We need a sampling frequency, a frequency at which there should be no loss of information, even after sampling. The main advantage of spread spectrum communication technique is to prevent “interference” whether it is intentional or unintentional. A number of signals were combined to send through a single cable. 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. If the allotted device, for a time-slot, transmits nothing and sits idle, then that slot is allotted to another device, unlike synchronous. Hence, the available data is in the form of frames or packets, which are discrete. Function of the receiver is to reproduce the message signal from the distorted received signal. View Answer: Answer: Thermal, Shot, Partition, 1/f, transit-time. This correlation between the pulses may be understood as introducing ISI in the transmitted signal in an artificial manner. Let us consider some blocks of data, which contains k bits in each block. 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 phase of the output signal gets shifted depending upon the input. The number of bits are processed in different manners, in both of these processes. 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. Though the features are good, these signals are prone to interference. This process of coding is chosen so as to avoid overlap and distortion of signal such as inter-symbol interference. Following is the block diagram for the same. The low pass filter minimizes the higher frequencies. Draw the block diagram of a digital communication system and explain each box. The destination also works as a transducer at the receiving end. When there is low input, the switch opens, allowing no voltage to appear. Block diagram of a simple Digital Communication System. The Digital Communication Notes Pdf (DC Notes) book starts with the topics covering Model of Digital Communication Systems, PCM Generation and Reconstruction, ASK Modulator. These 5-bit words are predetermined in a dictionary. 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 notation of the signals is the same as the previous ones. Hence, this is also called as Shannon’s Entropy. Following are the properties of line coding −. The DPCM Transmitter consists of Quantizer and Predictor with two summer circuits. This process helps in effective utilization of the bandwidth. The following figure explains a signal, if sampled at a higher rate than 2w in the frequency domain. Following is the block diagram of DPSK demodulator. According to the Einstein-Wiener-Khintchine theorem, if the auto correlation function or power spectral density of a random process is known, the other can be found exactly. In digital modulation, we have come across certain problem of determining the step-size, which influences the quality of the output wave. However, out of these message is selected & conveyed or communicated. There are two types of Companding techniques. Now, we will discuss the functioning of … Hence, it is invariant under any shift. 1. To understand this, let us consider a binary input sequence {ak} consisting of uncorrelated binary digits each having a duration Ta seconds. The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs. Fourier transform is a powerful mathematical tool which helps to view the signals in different domains and helps to analyze the signals easily. So far, we’ve discussed that ISI is an unwanted phenomenon and degrades the signal. It is because −. While using the hamming codes, extra parity bits are used to identify a single bit error. In the receiver, the Demodulation (detection) of the modulated signal takes place. $$Sampling \: Frequency = \frac{1}{T_{s}} = f_s$$, $f_s$ is the sampling frequency or the sampling rate. The function which describes how the power of a signal got distributed at various frequencies, in the frequency domain is called as Power Spectral Density (PSD). Hence, these techniques are bandwidth efficient. The diagram is as follows. 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. This binary or digital information has many types and forms, which are discussed further. From the above diagram, we have the notations as −, $e_p(nT_s)$ = summer output and quantizer input, $e_q(nT_s)$ = quantizer output = $v(nT_s)$, $\widehat{x}(nT_s)$ = output of delay circuit. For any positive integers m ≥ 3 and t < 2m-1 there exists a BCH binary code. Fig. A Stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ). Example − The sound signal received. There is a change in the polarity of the signal, only when the incoming signal changes from 1 to 0 or from 0 to 1. Example: A Sound signal. The Mid-Rise type is so called because the origin lies in the middle of a raising part of the stair-case like graph. There are other important techniques to be learned, called as data encoding techniques. The following figures represent the two types of uniform quantization. In the absence of ISI, the output will be −. The LPC method is very close to the Fast Fourier Transform (FFT) method. Cyclic codes follow this cyclic property. The signal is also converted to analog from the digital sequence, in order to make it travel through the channel or medium. It is a type of quantization error, which usually occurs in analog audio signal, while quantizing it to digital. The quantization levels in this type are odd in number. These digital modulation techniques are further discussed. The channel or a medium, allows the analog signal to transmit from the transmitter end to the receiver end. This block also consists of an analog to digital converter where a digital signal is needed for further processes. The linearity property of the code word is that the sum of two code words is also a code word. Now, the parity bits are again added and the whole data is again encoded. 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. The transmission path includes regenerative receivers that recover the signal from the undesired noise effects. 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 channel coding in a communication system, introduces redundancy with a control, so as to improve the reliability of the system. Occupies twice the bandwidth of Polar NRZ. Information or input signal; Input Transducer; Transmitter; Communication channel or medium; Noise; Receiver; Output Transducer. That gap can be termed as a sampling period Ts. The processing of the system results in an error, which is the difference of those values. Band of signals occupy a wide range of frequencies. The signal level is checked twice for every bit time, both initially and in the middle. The mean value of $H(X \mid y = y_k)$ for output alphabet y is −, $H\left ( X\mid Y \right ) = \displaystyle\sum\limits_{k = 0}^{k - 1}H\left ( X \mid y=y_k \right )p\left ( y_k \right )$, $= \displaystyle\sum\limits_{k = 0}^{k - 1} \displaystyle\sum\limits_{j = 0}^{j - 1}p\left (x_j \mid y_k \right )p\left ( y_k \right )\log_{2}\left [ \frac{1}{p\left ( x_j \mid y_k \right )} \right ]$, $= \displaystyle\sum\limits_{k = 0}^{k - 1} \displaystyle\sum\limits_{j = 0}^{j - 1}p\left (x_j ,y_k \right )\log_{2}\left [ \frac{1}{p\left ( x_j \mid y_k \right )} \right ]$. It just goes in the same way here too. If the event has not occurred, there is a condition of uncertainty. The functional blocks at the transmitter are responsible for processing the input message, encoding, modulating, and transmitting over the communication channel. The decision circuit chooses which output is more likely and selects it from any one of the envelope detectors. The decision device is enabled at these sampling times. There are mainly two types of multiplexers, namely analog and digital. There is no mixing up and hence recovery is possible. Occupies low bandwidth than unipolar and polar NRZ schemes. Hence, it is nothing but digital modulation. Half of the bit duration remains high but it immediately returns to zero and shows the absence of pulse during the remaining half of the bit duration. This is the technique which helps to collect the sample data at instantaneous values of message signal, so as to reconstruct the original signal. The input to this predictor is the quantized versions of the input signal $x(nT_s)$. The two oscillators, producing a higher and a lower frequency signals, are connected to a switch along with an internal clock. The device that does multiplexing, can be called as a MUX. A 0 will have a zero voltage level. The other one is Differential Phase Shift Keying (DPSK) which changes the phase according to the previous value. This is mostly used in audio signal processing, speech synthesis, speech recognition, etc. Conversely, if $\frac{H(\delta)}{T_s} > \frac{C}{T_c}$, then the transmission is not possible. The various massage is the form of words, group of word, code, symbol, sound signal etc. As the sampling interval is reduced, the signal correlation will be higher. Simon S. Haykin-Digital Communication Systems-Wiley (2013)_jp2.zip download 349.4M digital-20communications-20j-140122233811-phpapp02_jp2.zip download However, the presence of ISI introduces bit errors and distortions in the output. We have so far discussed mutual information. Multiplexing is the process of combining multiple signals into one signal, over a shared medium. We will see the block diagram of communication system (electronic Communication system) with the explanation of each block. This choice of having the sampling rate higher than Nyquist rate, also helps in the easier design of the reconstruction filter at the receiver. Hence, the Pulse Code Modulator circuit digitizes the given analog signal, codes it and samples it, and then transmits it in an analog form. This is called as the Mutual Information of the channel. These will be discussed in subsequent chapters. Of them all, the digital modulation technique used is Pulse Code Modulation (PCM). The probability of error occurrence is reduced by employing error detecting and error correcting codes. For this, the value $L_{min}$ has to be determined. Of them all, we will discuss the prominent ones. Such a signal is called as duo-binary signal. Figure 1 shows the mid-rise type and figure 2 shows the mid-tread type of uniform quantization. Channel coding consists of two parts of action. The common types of line encoding are Unipolar, Polar, Bipolar, and Manchester. 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. Hence, the Pulse Code Modulator circuit digitizes the given analog signal, codes it … Where pi is the probability of the occurrence of character number i from a given stream of characters and b is the base of the algorithm used. The main function of a transmitter is to process an electrical signal obtained from the input transducer. It means, the encryption of the data should not change the data. The quantizing of an analog signal is done by discretizing the signal with a number of quantization levels. 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. It converts the electrical signal into physical output (Example: loud speaker). (i) The necessary components of a communication system are an information source, input transducer, transmitter, communication channel, receiver, and destination. The received signal is demodulated as well as converted again from analog to digital. The noise and interferences which are denoted in the figure, are likely to occur, during transmission. Hence, digital modulation techniques have a greater demand, for their capacity to convey larger amounts of data than analog modulation techniques. This message signal is achieved by representing the signal in discrete form in both time and amplitude. The Code produced by a discrete memoryless source, has to be efficiently represented, which is an important problem in communications. They are −. Information source. The block diagram of Asynchronous FSK detector consists of two band pass filters, two envelope detectors, and a decision circuit. This is called as frequency reuse. The occurrence of cross-talk is very rare in digital communication. The low pass filter prior to sampling prevents aliasing of the message signal. The frequency of the output signal will be either high or low, depending upon the input data applied. Some of these techniques, modulate one parameter of the carrier signal, such as amplitude, phase, and frequency. When a source generates an analog signal and if that has to be digitized, having 1s and 0s i.e., High or Low, the signal has to be discretized in time. 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. Here you will find Free Google Games to Play Now. View Answer: Answer: Source, Transmitter, Channel, Receiver, Destination. As mentioned before, the predictor assumes a value, based on the previous outputs. Transmitter. The FSK signal input is given to the two mixers with local oscillator circuits. A greater value of figure-of-merit will be an advantage. The block diagram shown depicts the general form of a communication system. It acts as an energy converter (transducer at the receiving end) as it converts the electrical signal back into the audio signal (non electrical form). The clock is taken from the signal itself. Source 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. If the parity has a distance of 2, one-bit flip can be detected. Coherent ASK Detector, Baseband transmission and Optimal Reception of Digital Signal, Information and entropy, Matrix description of Linear Block Codes, Convolution Codes: Encoding, Spread Spectrum Modulation Etc. The increase in Signal to Noise Ratio (SNR) decreases the BER, hence the Error Probability also gets decreased. The amount of time spent on each frequency hop is called as Dwell time. The design of the modulator and the demodulator is simple. For a 1, the voltage level gets a transition from + to – or from – to +, having alternate 1s to be of equal polarity. Claude Shannon, the “father of the Information Theory”, provided a formula for it as −. This source is memoryless as it is fresh at each instant of time, without considering the previous values. It is denoted by C and is measured in bits per channel use. These are mainly of two types, namely Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), according to the number of phase shifts. The following figure depicts this clearly. 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 following figure clearly depicts this. It is generally observed that, we seek the help of Fourier series and Fourier transforms in analyzing the signals and also in proving theorems. The following figure indicates the difference between analog and digital signals. Predictor input is the sum of quantizer output and predictor output, $$u(nT_s) = \widehat{x}(nT_s) + v(nT_s)$$, $u(nT_s) = \widehat{x}(nT_s) + e(nT_s) + q(nT_s)$. To get from one-bit pattern to the other, few bits are to be changed in the data. The output of the XNOR gate along with the carrier signal is given to the balance modulator, to produce the DPSK modulated signal. A signal is pulse code modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. The signal phase follows the high or low state of the previous element. Very easy design of the modulator and the demodulator, Slope Over load distortion (when Δ is small). FSK is a scheme of frequency modulation. The sampled output when given to Quantizer, reduces the redundant bits and compresses the value. Some of them are −. The objective of any communication system is to convey information from one point to the other. is always greater than or equal to the source code (QUEUE in example). Figure 4.1-1 presents a block diagram for a two-node, one-way M-ary communications system. 31–33: Grp 6–Grp 8: In-class refinement of overall block diagram, input/output coordination: Announcement: Group Work 6 follow up. The signal to be transmitted is modulated here by a carrier. It is denoted by $H(x \mid y)$, Let us consider a channel whose output is Y and input is X, Let the entropy for prior uncertainty be X = H(x), (This is assumed before the input is applied), To know about the uncertainty of the output, after the input is applied, let us consider Conditional Entropy, given that Y = yk, $$H\left ( x\mid y_k \right ) = \sum_{j = 0}^{j - 1}p\left ( x_j \mid y_k \right )\log_{2}\left [ \frac{1}{p(x_j \mid y_k)} \right ]$$. The data encoding technique is divided into the following types, depending upon the type of data conversion. Following are the advantages of spread spectrum −. The ASK modulator block diagram comprises of the carrier signal generator, the binary sequence from the message signal and the band-limited filter. Where the whole stream of data is assigned symbols and then transmitted. If the event has just occurred, there is a condition of surprise. So far, in the linear codes, we have discussed that systematic unaltered code is preferred. In Pulse Code Modulation, the message signal is represented by a sequence of coded pulses. There are different methods for demodulating a FSK wave. 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. Following is the diagram of DPSK Modulator. The channel produces amplitude and phase distortions to the signals. They also prevent the signal from getting tapped by unwanted receivers. Many of the digital circuits have almost common encoding techniques and hence similar devices can be used for a number of purposes. To discretize the signals, the gap between the samples should be fixed. This rate of sampling is called as Nyquist rate. The serial data input is given to the XNOR gate and the output is again fed back to the other input through 1-bit delay. As we have already discussed, sampling and quantization are the important factors in this. Block diagram of television transmitter. The source coding reduces redundancy to improve the efficiency of the system. Hence, we define the average code word length L of the source encoder as, $$\overline{L} = \displaystyle\sum\limits_{k=0}^{k-1} p_kl_k$$, L represents the average number of bits per source symbol, If $L_{min} = \: minimum \: possible \: value \: of \: \overline{L}$, With $\overline{L}\geq L_{min}$ we will have $\eta \leq 1$, However, the source encoder is considered efficient when $\eta = 1$. Nrz codes is a sub-multiple of the bit clock rate is a type uniform... 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