technology oriented way, so lectures will tend to slice across a text. There is no Yes — a wide field, Digital Communications covers all of these fields. Need to. Engineering Class handwritten notes, exam notes, previous year questions, PDF free download. Digital. Communication. Course Notes for ECE Frank R. Kschischang of previous versions of these notes who took the time to send me written comments.

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This section provides the lecture notes for the course. 1, Introduction to digital communication (PDF) 6, Channels, modulation, and demodulation (PDF). DIGITAL COMMUNICATIONS. LECTURE NOTES. caite.info (III YEAR – II SEM). ( ). Prepared by: caite.infoitha, Professor. Mrs. P. Swetha, Assistant. Request PDF on ResearchGate | Digital Communication: Lecture Notes | The modern world is dependent on digital communications. Until relatively recently.

The bandwidth is normally a difference between two numbers. Adaptive equalization — It consists of tapped delay line filter with set of delay elements. Contents of the rightmost K-1 shift register stages define the states of the encoder. The three basic elements of every communication systems are Transmitter, Receiver and Channel. The range of frequencies contained in a composite signal is its bandwidth. Periodic signal:

Sixteen unique sequences can be obtained from four bit words. Although the Hamming weight of our chosen code set is 3. The Hamming distance is the number of disagreements between two binary sequences of the same size. This number is 3 among the 10 codewords we have chosen. A block of k information bits are coded to become a block of n bits.

The passband signal bandwidth is double that of the baseband signal. The bandwidth is always a positive quantity so the bandwidth of this signal is fm. Hamming Weight: Block codes are referred to as n. This signal extends in range from -fc. The information signal is called the baseband signal. The code takes k information bits and computes n-k parity bits from the code generator matrix.

A parity-check polynomial h X of order k also plays a role in designing the code. The number of errors that we can correct is given by Creating block codes: The block codes are specified by n.

All Hamming codes are able to detect three errors and correct one. This enables to reduce the complexity and also the number of computations involved in their implementation. Chaudhuri-Hocquenghem BCH codes and are very powerful linear non-binary block codes capable of correcting multiple random as well as burst errors.

They have an important feature that the generator polynomial and the code symbols are derived from the same finite field. So we want to have a code set with as large a Hamming distance as possible since this directly effects our ability to detect errors. The symbol x. An R-S code is described by a generator polynomial g x and other usual important code parameters such as the number of message symbols per block k.

A large number of R-S codes are available with different code rates. Reed-Solomon Codes: Reed Solomon R-S codes form an important sub-class of the family of Bose. Most block codes are systematic in that the information bits remain unchanged with parity bits attached either to the front or to the back of the information sequence. A simple convolutional encoder is shown below fig 3.

Convolutional codes: It can be noted that the block length n of an n. The code rate. The information bits are fed in small groups of k-bits at a time to a shift register. Closely related to K is the parameter m. Number of encoded symbols per block: Contents of the rightmost K-1 shift register stages define the states of the encoder. A new input bit causes a transition from one state to another. A convolutional encoder may be defined as a finite state machine.

The transition of an encoder from one state to another. Representing convolutional codes compactly: State diagram The encoded bits are labeled on the branches of the tree. Given an nput sequence.

The tree diagram representation shows all possible information and encoded sequences for the convolutional encoder. For instance. Structural properties of convolutional codes: All state transitions at each time step are explicitly shown in the diagram to retain the time dimension.

The trellis diagram of a convolutional code is obtained from its state diagram. It is interesting to note that the trellis diagram. Trellis diagram for the encoder in Fig.

The selection of a proper line code can help in so many ways: One possibility is to aid in clock recovery at the receiver. The DC component in a line code is called the bias or the DC coefficient. Two possibilities for the pulse p t These codes are explained here: It has the advantage of being compatible with TTL logic..

Another advantage is to get rid of DC shifts. Some common types of line encoding in common-use nowadays are unipolar. This is why most line codes try to eliminate the DC component before being transmitted on the channel. A clock signal is recovered by observing transitions in the received bit sequence. MLT-3 and Duobinary encoding. Unipolar is the simplest line coding scheme possible.

Such codes are called DC balanced. Unipolar coding uses a positive rectangular pulse p t to represent binary 1. Using the assumption that in a regular bit In NRZI there are two possible pulses. Inverted NRZI: The drawback of polar NRZ. A transition from one pulse to the other happens if the bit being transmitted is a logic 1. The rms value of polar signals is bigger than unipolar signals. Although transitions allow the signal to be self-clocking. Manchester encoding: In Manchester code each bit of data is signified by at least one transition.

In addition. USB ports. Manchester encoding is therefore considered to be self-clocking. Correlative receiver For a matched filter operating in real time to be physically realizable. The N elements of the observation vector x are first multiplied by the corresponding N elements of each of the M signal vectors s1.

For causal system The inner products are corrected for the fact that the transmitted signal energies may be unequal. Then for an AWGN channel a possible realization of sample function x t of the received random process X t.

The vectors x and w are sampled values of the random vectors X and W respectively. Based on the observation vector x. The relation between them is as shown in the fig The receiver has to observe the signal x t and make a best estimate of the transmitted signal si t or equivalently symbol mi The transmitted signal si t. When the received signal x t is applied to the bank o N correlators. Such a point is referred as transmitted signal point or message point.

The collection of M message points in the N Euclidean space is called a signal constellation. Detection of known signals in noise Assume that in each time slot of duration T seconds. In short it is called ISI. The maximum likelihood detector provides solution to this problem. But when channel bandwidth is close to signal bandwidth.

This overlapping is called Inter Symbol Interference. Similar to interference caused by other sources. Such a channel disperses or spreads a pulse carrying digitized samples passing through it. Inter symbol Interference Generally. Illustrating the effect of noise perturbation on location of the received signal point In the detection problem. ISI causes degradations of signal if left When the channel bandwidth is greater than bandwidth of pulse.

This waveform is generally termed as Line codes. The eye pattern is experimental method that contains all the information concerning the degradation of quality. The received wave is applied to the vertical deflection plates of an oscilloscope and the sawtooth wave at a rate equal to transmitted symbol rate is applied to the horizontal deflection plates.

The Spreading of pulse is greater than symbol duration. Degradation of quality occurs in each process modulation. In base band transmission best way is to map digits or symbols into pulse waveform. First let us have look at different formats of transmitting digital data. In this chapter main objective is to study the effect of ISI. This problem of ISI exists strongly in Telephone channels like coaxial cables and optical fibers. The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.

When the effected of ISI is excessive. Any non linear transmission distortion would reveal itself in an asymmetric or squinted eye. Example of eye pattern: Adaptive equalization — It consists of tapped delay line filter with set of delay elements. Adaptive equalizer can adjust its coefficients continuously during the transmission of data. Post channel equalization Achieved prior to data transmission by training the filter with the guidance of a training sequence transmitted through the channel so as to adjust the filter parameters to optimum values.

Training mode 2. Tap spacing is equal to symbol duration T of transmitted signal In a conventional FIR filter the tap weights are constant and particular designed response is obtained. Decision directed mode Mechanism of adaptation In the adaptive equaliser the Ci's are variable and are adjusted by an algorithm Two modes of operation 1.

This training sequence has maximal length PN Sequence. The difference between resulting response y nT and desired response d nT is error signal which is used to estimate the direction in which the coefficients of filter are to be optimized using algorithms. In digital communications. Coherent Binary M. Which gives an output Eb volts for symbol 1 and 0 volt for symbol 0.

The resulting binary wave [in unipolar form] and sinusoidal carrier are applied to a product modulator. The desired BASK wave is obtained at the modulator output.

Generation and Detection: FSK Bandwidth: The incoming noisy BFSK signal x t is common to both correlator. The detector consists of two correlators. The transmitted signal is nothing but required BFSK signal.

These two frequencies are combined using an adder circuit and then transmitted. When we have symbol 1 the upper channel is switched on with oscillator frequency f1.

QPSK is an expanded version from binary PSK where in a symbol consists of two bits and two orthonormal basis functions are used. Each symbol carries same energy.

Energy per Symbol and T: Probability of error: Thus the two binary PSK systems may be characterized as follows.

The correlator outputs x1 and x2 produced in response to the received signal x t are each compared with a threshold value of zero. The signal energy per bit E 2 The in-phase channel output x1 and the Q-channel output x2 may be viewed as the individual outputs of the two coherent binary PSK systems. The in-phase channel output: More phases than amplitudes. Since only 4 phase angles have 2 different amplitudes.

The I-channel makes a decision on one of the two bits constituting a symbol di bit of the QPSK signal and the Q-channel takes care of the other bit. With 16 signal combinations. What are Non Coherent Receivers? Receivers in which the carrier used in the receiver is of different frequency and phase of the transmitted one is called coherent receivers. Reconstruction Filter are the components of a Digital Communication System.

What are Coherent Receivers? Receivers in which the carrier used in the receiver is of same frequency and phaseof the transmitted one is called coherent receivers. Write the two methods of channel coding? What are the components of a Digital Communication System? What are the advantages of Digital Communication? If a finite —energy signal g t contains no frequencies higher than W hertz. Define Bandwidth Bandwidth is simply a measure of frequency range. Mathematically it is expressed as.

Define Power signal A power signal.

State sampling theorem. Define quadrature sampling. If a finite energy signal g t contains no frequencies higher than W hertz. What is the disadvantage of uniform quantization over the non-uniform quantization? SNR decreases with decrease in input power level at the uniform quantizer but non-uniform quantization maintain a constant SNR for wide range of input power levels.

The signal is compressed at the transmitter and expanded at the receiver. The conversion of analog sample of the signal into digital form is called quantizing process. This type of sampling is called quadrature sampling. What you mean by non. Name the types of uniform quantizer? This type of quantization is called as robust quantization. The combination of a compressor and expander is called a compander. Define quantizing process. State NRZ unipolar format Non uniform quantizer is characterized by a step size that increases as the separation from the origin of the transfer characteristics is increased.

Define compander. What is aliasing? The phenomenon of a high. This is called as companding. Step size is not uniform. What is meant by prediction error? The difference between the actual sample of the process at the time of interest and the predictor output is called a prediction error. What is linear code? A code is linear if the sum of any two code vectors produces another code vector. What is code rate? Code rate is the ratio of message bits k and the encoder output bits n.

Why do we go for Gram-Schmidt Orthogonalization procedure? Consider a message signal m. It is defined by r i. State NRZ bipolar format Binary 0 is reporesented by no pulse and binary one is represented by the alternative positive and negative pulse. State NRZ polar format Binary 1 is represented by a positive pulse and binary 0 is represented by a Negative pulse. The justification for this separation lies in the Gram-Schmidt orthogonalization procedure which permits the representation of any set of M energy signals.

Define code efficiency It is the ratio of message bits in a block to the transmitted bits for that block by the encoder i. A random binary sequence is a sequence in which the presence of a binary Why RS codes are called maximum distance separable codes?

What are the conditions to satisfy the hamming code No. Define a random binary sequence. Give the parameters of RS codes: Reed Solomon codes. The no. These are non binary BCH codes. This process of suppressing channel induced distortion is called channel equalization. The noise can be internal to the system or external to the system.

The correlator outputs define the signal The equalization in digital communication scenario is illustrated Therefore the hamming distance between x and Y is two. Need for equalization in digital communication The two principal causes of distortion in a digital communication channels are Inter Symbol Interference ISI and the additive noise. What is Signal constellation diagram? Suppose that in each time slot of duration T seconds. Hence at the receiver the distortion must be compensated in order to reconstruct the transmitted symbols.

What is hamming distance? The hamming distance between two code vectors is equal to the number of Elements in which they differ.. What is meant by a matched filter? Matched filter is used for detection of signal in base band and pass band transmission. In non coherent detection the local carrier generated at the receiver not be phase locked with the carrier at the transmitter. In coherent detection the local carrier generated at the receiver is phase locked with the carrier at the transmitter.

Distinguish between Coherent and Noncoherent receiver.

It is simple. M is called a signal constellation.. Hence it is also called synchronous detection. How do we get eye pattern? What you infer from this? The eye pattern is obtained by applying the received wave to the vertical deflection plates of an oscilloscope and to apply a saw tooth wave at the transmitted symbol rate to the horizontal deflection plate.

Flag for inappropriate content. Related titles. Jump to Page. Search inside document. Introduction The purpose of a Communication System is to transport an information bearing signal from a source to a user destination via a communication channel.

Block diagram of Communication System. The receiver has the task of operating on the received signal so 2 3 www. The Compressor and expander take together constitute a Compander.

In other words, channel codes 23 24 www. This is given by the following equation: Rahul Pal. Abhishek Bose.

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