COMMUNICATION SYSTEM

UNIT-III

IV SEMESTER

ECC-206

Department of Instrumentation & Control Engineering, BVCOE New Delhi

Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Digital Modulation Technique

Base Band Pulse Transmission:

• Input data is represented in the form of discrete PAM signals (line codes).
• The signals are transmitted over a low pass channel ( as baseband signals have large power at low frequencies).
• They can be transmitted over a pair of wires or coaxial cables.
• It is not possible to transmit the baseband signals over radio links or satellites because impractically large antennas.

Hence the spectrum of message signal has to be shifted to higher frequencies.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• This is achieved by using baseband digital signal to modulate a sinusoidal carrier.
• Transmitted over bandpass channel e.g. microwave radio link or satellite channel.

There are three basic signaling schemes used in passband data transmission;

• Amplitude Shift Keying (ASK)
• Phase Shift Keying (PSK)
• Frequency Shift Keying (FSK)

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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The modulation technique may be classified into two categories.

• Coherent Techniques: In coherent digital modulation techniques, we have to use phase synchronized carrier to be generated at the receiver to recover the information signal.

• Non-coherent Techniques: In the non-coherent technique, no phase synchronized local carrier is needed at the receiver. These techniques are less complex. However, the performance is inferior to that of coherent techniques.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Block Diagram of Digital Communication

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Advantages of Digital Communication

• Storage capacity
• Inexpensive
• Use of repeaters
• Privacy and security through the use of encryption
• Data Compression, error detection and error correction are possible
• Flexible hardware implementation
• Easier and efficient multiplexing by TDMA and CDMA

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Disadvantages of Dig. Comm.

• Bandwidth is high for channel
• Synchronization is compulsory
• Higher power consumption due to multiple stages
• Complex circuit

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Bit rate Baud Rate:

• As the time domain signal width increases , then frequency domain signal width decreases

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• BW max= Rb

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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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.

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• Any modulated signal has a high frequency carrier. The binary signal when ASK modulated, gives a zero value for Low input while it gives the carrier output for High input.

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• The following figure represents ASK modulated waveform along with its input

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�ASK Modulator

• The ASK modulator block diagram comprises of the carrier signal generator, the binary sequence from the message signal and the band-limited filter.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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ASK Demodulator�

There are two types of ASK Demodulation techniques. They are −

• Asynchronous ASK Demodulation/detection
• Synchronous ASK Demodulation/detection

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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. Otherwise, it is known as Asynchronous.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Asynchronous ASK Demodulator

• The Asynchronous ASK detector consists of a half-wave rectifier, a low pass filter, and a comparator. Following is the block diagram for the same.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Synchronous ASK Demodulator�

• Synchronous ASK detector consists of a Square law detector, low pass filter, a comparator, and a voltage limiter. Following is the block diagram for the same.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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BANDWIDTH

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Frequency Shift Keying�

• Frequency Shift Keying FSK is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. FSK is a scheme of frequency modulation.

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• 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.

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• The following image is the diagrammatic representation of FSK modulated waveform along with its input.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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FSK Modulator�

The FSK modulator block diagram comprises of two oscillators with a clock and the input binary sequence. Following is its block diagram.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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FSK Demodulator�

• There are different methods for demodulating a FSK wave. The main methods of FSK detection are asynchronous detector and synchronous detector. The synchronous detector is a coherent one, while asynchronous detector is a non-coherent one.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�Asynchronous FSK Detector

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�Synchronous FSK Detector

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�Phase Shift Keying

• 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.

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• PSK is of two types, depending upon the phases the signal gets shifted. They are −

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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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°.

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• BPSK is basically a Double Side Band Suppressed Carrier DSBSC modulation scheme, for message being the digital information.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�BPSK Modulator

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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BPSK Demodulator�

• The block diagram of BPSK demodulator consists of a mixer with local oscillator circuit, a bandpass filter, a two-input detector circuit. The diagram is as follows.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�Differential Phase Shift Keying

• In Differential Phase Shift Keying DPSK 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.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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DPSK Modulator�

• DPSK is a technique of BPSK, in which there is no reference phase signal. Here, the transmitted signal itself can be used as a reference signal.
• PSK encodes two distinct signals, i.e., the carrier and the modulating signal with 180° phase shift each. 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. The output of the XNOR gate along with the carrier signal is given to the balance modulator, to produce the DPSK modulated signal.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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DPSK Demodulator�

• In DPSK demodulator, the phase of the reversed bit is compared with the phase of the previous bit. Following is the block diagram of DPSK demodulator.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• From the above figure, it is evident that the balance modulator is given the DPSK signal along with 1-bit delay input. That signal is made to confine to lower frequencies with the help of LPF. 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.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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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.
• QPSK is a from of PSK, in which two bits are modulated at once.
• It selects one of four possible carrier phase shift [0, 90,180, 270]
• QPSK allows the signal to carry twice as much information as ordinary PSK using the same BW.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• 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. The base band signal which is band limited is obtained and this signal is used to regenerate the binary message bit stream.
• In the next stage of demodulation, the bit clock rate is needed at the detector circuit to produce the original binary message signal. If the bit rate is a sub-multiple of the carrier frequency, then the bit clock regeneration is simplified. To make the circuit easily understandable, a decision-making circuit may also be inserted at the 2nd stage of detection.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• QPSK is used for satellite transmission, cable Modem, Cellular phone system etc.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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QPSK Modulator

• The QPSK Modulator uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit.

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• At the modulator’s input, the message signal’s even bits (i.e., 2nd bit, 4th bit, 6th bit, etc.) and odd bits (i.e., 1st bit, 3rd bit, 5th bit, etc.) 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). The PSKQ signal is anyhow phase shifted by 90° before being modulated.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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�QPSK Demodulator

• The QPSK Demodulator uses two product demodulator circuits with local oscillator, two low pass filters, two integrator circuits, and a 2-bit parallel to serial converter. Following is the diagram for the same.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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The two product detectors at the input of demodulator simultaneously demodulate the two BPSK signals. The pair of bits are recovered here from the original data. These signals after processing, are passed to the parallel to serial converter.

• 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 data is a 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.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Line Coding

• Line coding is defined as the process of converting binary data to a digital signal.

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• Binary Data

0 1 0 0 1 1 Line coding

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Important Characteristics of Line Coding

• Signal level
• Data level
• Pulse rate
• Bit rate

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• Signal Level: the no of values allowed in a particular signal is defined as the no. of signal level.
• Data Level: The no. of values to represent data is called as the no. of data level.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• Pulse rate: is defined as the no. of pulses per second, and a pulse is defined as the minimum amount of time required to transmit a symbol.
• Bit rate: is defined as the no. of bits per second.

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If one pulse corresponds to one bit then pulse rate equals to bit rate, but if pulse carries more than 1 bit then pulse rate is lower than 1 bit then pulse rate is lower than bit rate.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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DC component: Over one cycle period of a waveform, if all the +ve voltage are cancelled by –ve voltages then DC component of waveform is zero.

• In line coding, the signal with non-zero DC component is treated as distorted one and it can create errors in received signals.
• The signal with a DC component cannot pass through a transformer. Hence the signals with zero DC component are preferred.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Self Synchronization

• If receiver bit intervals corresponds exactly to the sender’s bit interval, only then it is possible to receive a signal correctly.
• Clock frequency of Tx and Rx should be same.
• If the clock freq. of Tx and Rx are different then data at Tx and Rx will be different.
• So to match data at Tx and Rx, clock frequency should be same and synchronized.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Basic Pulse Shaping Techniques

• Unipolar; Polar; Bipolar

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Basic Pulses

• NRZ (Not Return to Zero)

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• RZ (Return to Zero)

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• Manchester Pulse ( At half duration there is transition from high to low, or low to high)

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Line Coding Techniques:

Digital Data [ 1 1 0 0 1 0]

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Information Theory

• Information is the source of a communication system, whether it is analog or digital.
• Information theory is a mathematical approach to the study of coding of information along with the quantification, storage, and communication of information.

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Conditions of Occurrence of Events

If we consider an event, there are three conditions of occurrence.

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• If the event has not occurred, there is a condition of uncertainty.

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• If the event has just occurred, there is a condition of surprise.

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• If the event has occurred, a time back, there is a condition of having some information.

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These three events occur at different times. The difference in these conditions help us gain knowledge on the probabilities of the occurrence of events.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Information Theory

• Information is the source of a communication system, whether it is analog or digital.
• Information theory is a mathematical approach to the study of coding of information along with the quantification, storage, and communication of information.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Uncertainty

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Measure of Information

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Properties of Information

• More uncertainty about message then information is more.

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• Receiver knows message being transmitted then information is zero.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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• I1 is the information of message m1 and I2 of m2 then combined information will be I1+I2

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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ENTROPY

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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properties

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Question 1: For a discrete memoryless source there are three symbols with P1=2 and P2=P3. Find the entropy of the source.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Shannon Fano Coding Algorithm

• The message are first written in order of decreasing probability.
• Then divide the messages set into two equiprobable subset X and Y.
• The message of 1^st set X is given bit 0 and message in 2^nd subset is given bit 1.
• The procedure is now applied for each set separately till end.
• Finally we get the code word for respective symbols.
• Calculation

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Numerical

Question: Find the code words occurring in the probability {1/2, ¼, 1/8, 1/8} for symbols S1,S2,S3,S4. Find the efficiency and redundancy of codes.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Find the code words occurring in the probability {0.4,0.2,0.12,0.08,0.08,0.08,0.04} for symbols X1,X2,X3,X4,X5,X6,X7. Find the efficiency and redundancy of codes

Condition 1. First subset consists of X1 and Second subset has remaining symbols

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Find the code words occurring in the probability {0.4,0.2,0.12,0.08,0.08,0.08,0.04} for symbols X1,X2,X3,X4,X5,X6,X7. Find the efficiency and redundancy of codes

Condition 1. First subset consists of X1 and Second subset has remaining symbols

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Condition 2. First subset consists of X1 and X2 and Second subset has remaining symbols

Huffman Coding Algorithm

• The source symbol are arranged in order of decreasing probability.
• Then two lowest probability are assigned bit 0 and 1.
• Then combine two symbols and move combined symbols as high as possible.
• Repeat the above step until end.
• Code for each symbol is found by moving backward.
• Calculation.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Calculation

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Example: Alphabet with probability ={0.4,0.2,0.2,0.1,0.1} for symbols {S1,S2,S3,S4,S5}. Find Huffman codes and also find efficiency and variance.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Qst: Apply the Hoffman coding procedure for the following message:

[x]=[x1, x2, x3, x4, x5, x6]

[P]= [ 0.30, 0.25, 0.15, 0.12, 0.08, 0.10]

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Qst: Apply the Hoffman coding procedure for the following message:

[x]=[x1, x2, x3, x4, x5, x6, x7, x8, x9]

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[P]= [0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11]

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: Communication System , Instructor: Manish Talwar

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