FUNDAMENTALS OF ELECTRONICS COMMUNICATIONS SYSTEMS ELX314-EE-T | 2022_1st_Term Technological University of the Philippines - Taguig campus ELECTRONICS ENGINEERING DIGITAL MODULATION ELECTRONICS ENGINEERING Introduction Digital Modulation is the transmittal of digitally modulated analog signals (carriers) between two or more points in a communication system. digital radio because digitally modulated signals can be propagated through Earth’s atmosphere and used in wireless communications. offers several outstanding advantages over traditional analog systems, such as ease of processing, ease of multiplexing, and noise immunity ELECTRONICS ENGINEERING Introduction Digital Modulation include systems where relatively high-frequency analog carriers are modulated by relatively lowfrequency digital information signals (digital radio) and systems involving the transmission of digital pulses (digital transmission) digital transmission systems transport information in digital form and require a physical facility between the transmitter and the receiver (i.e., metallic wires, coaxial cable, or an optical fiber) ELECTRONICS ENGINEERING Modulation Modulation is the process of varying a certain characteristic (usually amplitude, frequency, or phase) of a high-frequency carrier in proportion with the characteristic of a low-frequency modulating or information signal. ELECTRONICS ENGINEERING Modulation ELECTRONICS ENGINEERING Digital Modulation is ideally suited to a multitude of communications applications, including both cable and wireless systems. Applications include: 1. Relatively low-speed voice-band data communications modems, such as those found in most personal computers; 2. High-speed data transmission systems, such as broadband digital subscriber lines (DSL); 3. Digital microwave and satellite communications systems; 4. Cellular telephone Personal Communications Systems (PCS) ELECTRONICS ENGINEERING Simplified Block Diagram of a Digital Radio System ELECTRONICS ENGINEERING In the transmitter, the precoder performs level conversion and then encodes the incoming data into groups of bits that modulate an analog carrier. The modulated carrier is shaped (filtered), amplified, and then transmitted through the transmission medium to the receiver. In the receiver, the incoming signals are filtered, amplified, and then applied to the demodulator and decoder circuits, which extracts the original source information from the modulated carrier. The clock and carrier recovery circuits recover the analog carrier and digital timing (clock) signals from the incoming modulated wave since they are necessary to perform the demodulation process. ELECTRONICS ENGINEERING Information Capacity Is a measure of how much information can be propagated through a communications system and is a function of bandwidth and transmission time. is often expressed as a bit rate. Bit rate – refers to the rate of change of a digital information signal ELECTRONICS ENGINEERING Baud rate Baud – refers to the rate of change of a signal on the transmission medium after encoding and modulation occurred Baud – is a unit of transmission rate, modulation rate, or symbol rate (symbols per second) baud = 1/tS where baud = symbol rate (symbols per second) tS = time for one signaling element (seconds) ELECTRONICS ENGINEERING Information Theory Is a highly theoretical study of the efficient use of bandwidth to propagate information through ECSs and use to determine the information capacity of a data communication system. ELECTRONICS ENGINEERING Information CAPACITY Hartley’s Law IαBxt Example: A data signal has 10MHz bandwidth, what is the channel capacity ? ELECTRONICS ENGINEERING Information CAPACITY Shannon limit law I = B log2 (1 + S/N) or I = 3.32 B log (1 + S/N) Example: For a standard telephone circuit with a signal-to-noise power ratio of 30 dB and a bandwidth of 2.7 kHz, the Shannon limit for information capacity is ELECTRONICS ENGINEERING Information CAPACITY M-ary Encoding M represents a digit that corresponds to the number of conditions, levels, or combinations of a given number of bits. The number of bits necessary to produce a given number of conditions is expressed mathematically as N = log2 M where N = number of bits necessary M = number of conditions, levels, or combinations possible with N bits ELECTRONICS ENGINEERING Information CAPACITY M-ary Encoding For example, a digital signal with four possible conditions (voltage levels, frequencies, phases, and so on) is an Msystem where M = 4. ELECTRONICS ENGINEERING Information CAPACITY H. Nyquist, Binary signals can be propagated through an ideal noiseless transmission medium at a rate equal to two times the bandwidth of the medium. The minimum theoretical bandwidth necessary to propagate a signal is called the minimum Nyquist bandwidth or minimum Nyquist frequency. ELECTRONICS ENGINEERING Information CAPACITY H. Nyquist, For example, a standard telephone circuit has a bandwidth approximately 2700 Hz, which has the capacity to propagate 5400 bps through it. ELECTRONICS ENGINEERING Information CAPACITY H. Nyquist, Using multilevel signaling, the Nyquist formulation for channel capacity is fb = B log2 M where fb = channel capacity (bps) B = minimum Nyquist Bandwidth (hertz) M = number of discrete or voltage levels . To solve for the minimum bandwidth necessary to pass M-ary digitally modulated carriers B = (fb / log2 M) or B = fb / N ELECTRONICS ENGINEERING Amplitude-Shift Keying Simplest digital modulation technique A binary information signal directly modulates the amplitude of an analog carrier Sometimes called digital amplitude modulation (DAM) ELECTRONICS ENGINEERING Amplitude-Shift Keying (a) binary input (b) ASK output waveform ELECTRONICS ENGINEERING Amplitude-Shift Keying ASK waveform is the same as the rate of change of the binary input (bps) thus, the bit rate equals the baud. With ASK, the bit rate is also equal to the minimum Nyquist Bandwidth. B = fb / 1 = fb baud = fb / 1 = fb ELECTRONICS ENGINEERING Amplitude-Shift Keying Example: Determine the baud and minimum bandwidth necessary to pass a 20 kbps binary signal using amplitude shift keying. ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK another relatively simple, low-performance type of digital modulation is a form of constant-amplitude angle modulation similar to standard FM except the modulating signal is a binary signal that varies between two discrete voltage levels rather than a continuously changing analog waveform ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK The output frequency shifts between two frequencies: Δf (Frequency Deviation) is expressed mathematically as Δf = |fm - fs | / 2 ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK The baud for binary FSK, baud = fb / 1 = fb Bandwidth = 2(Δf + fb ) almost resembles to Carson’s Rule Example: Determine (a) the peak frequency deviation, (b) minimum bandwidth, and (c) baud for a binary FSK signal with a mark frequency of 49 kHz, a space frequency of 51 kHz, and an input bit rate of 2 kbps. ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK Example: Determine (a) the peak frequency deviation, (b) minimum bandwidth, and (c) baud for a binary FSK signal with a mark frequency of 49 kHz, a space frequency of 51 kHz, and an input bit rate of 2 kbps. Solution (a) the peak frequency deviation ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK Example: Determine (a) the peak frequency deviation, (b) minimum bandwidth, and (c) baud for a binary FSK signal with a mark frequency of 49 kHz, a space frequency of 51 kHz, and an input bit rate of 2 kbps. Solution (b) minimum bandwidth ELECTRONICS ENGINEERING Frequency-Shift Keying/Binary FSK Example: Determine (a) the peak frequency deviation, (b) minimum bandwidth, and (c) baud for a binary FSK signal with a mark frequency of 49 kHz, a space frequency of 51 kHz, and an input bit rate of 2 kbps. Solution (c) baud ELECTRONICS ENGINEERING Phase-Shift Keying is an M-ary digital modulation scheme similar to conventional PM except that the input is a binary digital signal and there are limited number of output phases possible. The input binary information is encoded into groups of bits before modulating the carrier. The number of bits in a group ranges from 1 to 12 or more. The number of output phases is defined by M (2^N=M) and determined by the number of bits in the group (N). ELECTRONICS ENGINEERING Phase-Shift Keying The simplest form of PSK where N=1 and M=2. One phase represents logic 1, and the other phase represents a logic 0. As the input digital signal changes state (i.e., from a 1 to a 0 or from a 0 to a 1), the phase of the output carrier shifts between two angles that are separated by 180°. ELECTRONICS ENGINEERING Phase-Shift Keying Truth table ELECTRONICS ENGINEERING Quarternary or Quadrature PSK QPSK is an M-ary encoding scheme where N=2 and M=4. Four output phases representing each two-bit input: 00, 01, 10 and 11. The four possible output phases (+45°, +135°, –45°, –135°) ELECTRONICS ENGINEERING Quarternary or Quadrature PSK Truth table ELECTRONICS ENGINEERING Quarternary or Quadrature PSK 8-PSK Three bits are encoded, forming tribits(N=3) and producing eight different phases (M=8). 16-QAM Is an M-ary system where M = 16 and n = 4. The input binary data are divided into four channels: I, I’, Q, and Q’. ELECTRONICS ENGINEERING ASK, FSK, PSK, and QAM Summary ELECTRONICS ENGINEERING Bandwidth Efficiency Bandwidth Efficiency / Information Density / Spectral Density often used to compare the performance of one digital technique to another is the ratio of the transmission bit rate to the minimum bandwidth required for a particular modulation scheme In bits / cycle EXAMPLE ELECTRONICS ENGINEERING For an 8-PSK system, operating with an information bit rate of 24 kbps, determine (a) baud, (b) minimum bandwidth, and (c)bandwidth efficiency. EXAMPLE ELECTRONICS ENGINEERING For a 16-PSK and a transmission system with 10 kHz bandwidth, determine the maximum bit rate. ELECTRONICS ENGINEERING PULSE MODULATION TECHNIQUES ELECTRONICS ENGINEERING PULSE MODULATION Many signals in modern communication systems are digital Additionally, analog signals are transmitted digitally Digitizing a signal results in reduced distortion and improvement in signal-tonoise ratios The process of transmitting signals in the form of pulses (discontinuous signals) by using special techniques. ELECTRONICS ENGINEERING PULSE MODULATION Pulse Amplitude Modulation Pulse Width Modulation Pulse Position Modulation Pulse Code Modulation Delta Modulation ELECTRONICS ENGINEERING Sampling The process of transmitting signals in the form of pulses (discontinuous signals) by using special techniques. The signal is sampled at regular intervals such that each sample is propotional to the amplitude of signal at that instant.This technique is called “sampling”. Analog signal is sampled every TS secs. Ts is referred to as the sampling interval. fs = 1/Ts is called the sampling rate or sampling frequency. ELECTRONICS ENGINEERING There are 3 sampling methods Ideal - an impulse at each sampling instant Natural - a pulse of short width with varying amplitude Flat top - sample and hold, like natural but with single amplitude value ELECTRONICS ENGINEERING There are 3 sampling methods Ideal - an impulse at each sampling instant ELECTRONICS ENGINEERING There are 3 sampling methods Natural - a pulse of short width with varying amplitude ELECTRONICS ENGINEERING There are 3 sampling methods Flat top - sample and hold, like natural but with single amplitude value ELECTRONICS ENGINEERING Pulse Width Modulation In this type, the amplitude is maintained constant but the width of each pulse is varied in accordance with instantaneous value of the analog signal. ELECTRONICS ENGINEERING Pulse Width Modulation ELECTRONICS ENGINEERING Pulse Width Modulation In pulse width modulation (PWM), the width of each pulse is made directly proportional to the amplitude of the information signal. ELECTRONICS ENGINEERING Digital Pulse Modulation The process of Sampling which we have already discussed in initial slides is also adopted in Digital pulse modulation. It is mainly of two types: Pulse Code Modulation(PCM) Delta Modulation(DM) ELECTRONICS ENGINEERING Pulse Code Modulation(PCM) In PCM, the available range of signal voltages is divided into levels and each is assigned a binary number. Each sample is represented by a binary number and transmitted serially The number of levels available depends upon the number of bits used to express the sample value The number of levels is given by: N = 2^m ELECTRONICS ENGINEERING Pulse Code Modulation(PCM) PCM consists of three steps to digitize an analog signal: 1. Sampling 2. Quantization 3. Binary encoding ELECTRONICS ENGINEERING Pulse Code Modulation(PCM) Analog to digital converter employs two techniques: 1. Sampling: The process of generating pulses of zero width and of amplitude equal to the instantaneous amplitude of the analog signal. The no. of pulses per second is called “sampling rate”. ELECTRONICS ENGINEERING Pulse Code Modulation(PCM) 2. Quantization: The process of dividing the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code. The levels obtained are called “quanization levels”. ELECTRONICS ENGINEERING Pulse Code Modulation(PCM) 3. Binary Coding ELECTRONICS ENGINEERING Delta Modulation In Delta Modulation, only one bit is transmitted per sample That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction ELECTRONICS ENGINEERING Delta Modulation This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time tn, then a single bit, say a “1”, is used to indicate the positive value. If the pulse is lower in value, resulting in a negative value, a “0” is used. This scheme works well for small changes in signal values between samples. If changes in amplitude are large, this will result in large errors. ELECTRONICS ENGINEERING Delta Modulation ELECTRONICS ENGINEERING Delta Modulation Components of Delta Modulation ELECTRONICS ENGINEERING Delta Modulation Distortions in DM system 1. If the slope of analog signal is much higher than that of approximated digital signal over long duration,than this difference is called Slope overload distortion. 2. The difference between quantized signal and original signal is called as Granular noise. It is similar to quantisation noise. ELECTRONICS ENGINEERING Delta Modulation THANK YOU