EDUCATION HOLE PRESENTS

ELECTRONICS ENGINEERING

Unit-V

Fundamentals of Communication Engineering ........................................................................ 3 Elements of a Communication System ................................................................................................................. 3 Elements of a communication system .................................................................................................................. 3

Need of modulation ......................................................................................................................... 4 (i) To separate signal from different transmitters ................................................................................................ 4 (ii) Size of the antenna ......................................................................................................................................... 4 Types of modulation ............................................................................................................................................. 4

Electromagnetic spectrum ................................................................................................................ 5 Electromagnetic typical applications .................................................................................................................... 5

Radio Waves (communications) ....................................................................................................................... 6 Satellite signals (Microwaves) ............................................................................................................................... 6 Infrared Radiation (remote controls, toasters) ..................................................................................................... 6 Ultraviolet ............................................................................................................................................................. 6 X-rays .................................................................................................................................................................... 7 Gamma Rays ......................................................................................................................................................... 7

Terminologies in communication systems ........................................................................................ 7 (i) Transducer .................................................................................................................................................... 7 (ii) Signal ........................................................................................................................................................... 7 (iii) Noise ........................................................................................................................................................... 8 (iv) Transmitter ................................................................................................................................................ 8 (v) Receiver ....................................................................................................................................................... 8 (vi) Attenuation ................................................................................................................................................ 8 (vii) Amplification ............................................................................................................................................. 8 (viii) Range ........................................................................................................................................................ 8 (ix) Bandwidth .................................................................................................................................................. 8 (x) Modulation .................................................................................................................................................. 8 (xi) Demodulation ............................................................................................................................................. 9 (xii) Repeater .................................................................................................................................................... 9

Basics of signal representation and analysis ..................................................................................... 9 Magnitude and Phase Information of the FFT ...................................................................................................... 9

Fundamentals of amplitude ........................................................................................................... 10

Amplitude modulation basics ......................................................................................................... 11

Amplitude modulation advantages & disadvantages ...................................................................... 12

Fundamentals of angle modulation ................................................................................................ 12 Modulation techniques ....................................................................................................................................... 13 Amplitude Phase Shift Keying (APSK).................................................................................................................. 13 Orthogonal Frequency Division Multiplexing (OFDM) ........................................................................................ 14 Determining Spectral Efficiency .......................................................................................................................... 15 Other Factors Affecting Spectral Efficiency ........................................................................................................ 16

Implementing Modulation And Demodulation ............................................................................... 17 Demodulation techniques .................................................................................................................................. 17

Fundamentals of Communication Engineering

Elements of a Communication System

Elements of a communication system

The above figure depicts the elements of a communication system. There are three essential parts of any communication system, the transmitter, transmission channel, and receiver. Each parts plays a particular role in signal transmission, as follows:

The transmitter processes the input signal to produce a suitable transmitted signal suited to the characteristics of the transmission channel. Signal processing for transmissions almost always involves modulation and may also include coding. The transmission channel is the electrical medium that bridges the distance from source to destination. It may be a pair of wires, a coaxial cable, or a radio wave or laser beam. Every channel introduces some amount of transmission loss or attenuation. So, the signal power progressively decreases with increasing distance. The receiver operates on the output signal from the channel in preparation for delivery to the transducer at the destination. Receiver operations include amplification to compensate for transmission loss. These also include demodulation and decoding to reverse the signal procession performed at the transmitter. Filtering is another important function at the receiver. The figure represents one-way or simplex (SX) transmission. Two way communication of course requires a transmitter and receiver at each end. A full-duplex (FDX) system has a channel that allows

simultaneous transmission in both directions. A half-duplex (HDX) system allows transmission in either direction but not at the same time.

Need of modulation

(i) To separate signal from different transmitters

Audio frequencies are within the range of 20 Hz to 20 kHz. Without modulation all signals at same frequencies from different transmitters would be mixed up. There by giving impossible situation to tune to any one of them. In order to separate the various signals, radio stations must broadcast at different frequencies.

Each radio station must be given its own frequency band. This is achieved by frequency translation as a result of modulation process.

(ii) Size of the antenna

For efficient transmission the transmitting antennas should have length at least equal to a quarter of the wavelength of the signal to be transmitted. For an electromagnetic wave of frequency 15 kHz, the wavelength λ is 20 km and one-quarter of this will be equal to 5 km. Obviously, a vertical antenna of this size is impractible. On the other hand, for a frequency of 1 MHz, this height is reduced to 75m.

Also, the power radiated by an antenna of length l is proportional to (l/λ)2. This shows that for the same antenna length, power radiated is large for shorter wavelength. Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the electromagnetic wave. This is achieved by the process of modulation.

Types of modulation

A sinusoidal carrier wave can be expressed as

ec = Ec cos(ωc t + θ)

Its three distinct characteristics are

(i) Amplitude (Ec)

(ii) angular frequency (ωc)

(iii) phase angle (θ).

Either of these three characteristics can be varied in accordance with the modulating signal. These result in three types of modulation.

(i) Amplitude modulation (AM)

(ii) Frequency modulation (FM)

(iii) Phase modulation (PM)

Electromagnetic spectrum The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays. Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space.

Microwave: Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies.

Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the dust between stars.

Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light.

Ultraviolet: Ultraviolet radiation is emitted by the Sun and is the reason skin tans and burns. "Hot" objects in space emit UV radiation as well.

X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Hot gases in the Universe also emit X-rays.

Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all is the Universe.

Electromagnetic typical applications Wavelength of the ElectroMagnetic spectrum continually changes

• high frequency = short wavelength • high frequency = high energy

• high energy = more dangerous

Radio Waves (communications)

• TV and FM radio (short wavelength) • Direct line of sight with transmitter (do not diffract) • Medium wavelength – travel further because they reflect from layers in the atmosphere

Satellite signals (Microwaves)

• Frequency of microwaves pass easily through atmosphere and clouds

Cooking (Microwaves)

• Microwaves are absorbed by water molecules. • These water molecules become heated > heat food • Dangers: microwaves are absorbed by living tissue Internal heating will damage or kill

cells

Infrared Radiation (remote controls, toasters)

• Any object that radiates heat radiates Infrared Radiation • Infrared Radiation is absorbed by all materials and causes heating • It is used for night vision and security cameras as Infrared Radiation is visible in daytime

or night-time • Police use it to catch criminals, army use it to detect enemy • Dangers: damage to cells (burns)

Ultraviolet

• Dangers: o over-exposure to UVA and B damages surface cells and eyes and can cause

cancer. There is a problem with current sunscreens which protect against skin

burning from high UVB but give inadequate protection against free radical damage caused by UVA.

Dark skins are not necessarily safer from harm. Sun exposure for the skin is best restricted to before 11am and after 3pm

in the UK in summer months. • Benefits:

o sanitary and therapeutic properties have a marked effect on architecture, engineering and public health and have done so throughout history.

o UVC is germicidal, destroying bacteria, viruses and moulds in the air, in water and on surfaces.

o UV synthesises vitamin D in skin, controls the endocrine system and is a painkiller.

o Used in state of the art air-handling units, personal air purifiers and swimming pool technology.

o Used to detect forged bank notes: they fluoresce in UV light; real bank notes don’t. Used to identify items outside visible spectrum areas, known as 'black lighting'.

X-rays

• X-rays detect bone breaks • X-rays pass through flesh but not dense material like bones • Dangers: X-rays damage cells and cause cancers. Radiographer precautions include

wearing lead aprons and standing behind a lead screen to minimise exposure

Gamma Rays

• Gamma Rays cause and treat cancers • In high doses, gamma can kill normal cells and cause cancers • Gamma can be used to kill mutated cells though too.

Terminologies in communication systems (i) Transducer: Any device that converts one form of energy into another can be termed as a transducer. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc) into corresponding variations in the electrical signal at its output. (ii) Signal: Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. Sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature. Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’

corresponds to a high level of voltage/ current. There are several coding schemes useful for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD)*. American Standard Code for Information Interchange (ASCII)** is a universally popular digital code to represent numbers, letters and certain characters. (iii) Noise: Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system. (iv) Transmitter: A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. (v) Receiver: A receiver extracts the desired message signals from the received signals at the channel output. (vi) Attenuation: The loss of strength of a signal while propagating through a medium is known as attenuation. * In BCD, a digit is usually represented by four binary (0 or 1) bits. For example the numbers 0, 1, 2, 3, 4 in the decimal system are written as 0000, 0001, 0010, 0011 and 0100. 1000 would represent eight. (vii) Amplification: It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength. (viii) Range: It is the largest distance between a source and a destination up to which the signal is received with sufficient strength. (ix) Bandwidth: Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. (x) Modulation: The original low frequency message/information signal cannot be transmitted to long distances because of reasons. Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. As will be explained later, there are several types of modulation, abbreviated as AM, FM and PM.

(xi) Demodulation: The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation. (xii) Repeater: A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency.

Basics of signal representation and analysis

Frequency-domain analysis is a tool of utmost importance in signal processing applications. Frequency-domain analysis is widely used in such areas as communications, geology, remote sensing, and image processing. While time-domain analysis shows how a signal changes over time, frequency-domain analysis shows how the signal's energy is distributed over a range of frequencies. A frequency-domain representation also includes information on the phase shift that must be applied to each frequency component in order to recover the original time signal with a combination of all the individual frequency components.

A signal can be converted between the time and frequency domains with a pair of mathematical operators called a transform. An example is the Fourier transform, which decomposes a function into the sum of a (potentially infinite) number of sine wave frequency components. The 'spectrum' of frequency components is the frequency domain representation of the signal. The inverse Fourier transform converts the frequency domain function back to a time function. The fft and ifft functions in MATLAB allow you to compute the Discrete Fourier transform (DFT) of a signal and the inverse of this transform respectively.

Magnitude and Phase Information of the FFT

The frequency-domain representation of a signal carries information about the signal's magnitude and phase at each frequency. This is why the output of the FFT computation is complex. A complex number, , has a real, , and an imaginary part, , such that . The magnitude of is computed as , and the phase of is computed as . You can use MATLAB functions abs and angle to respectively get the magnitude and phase of any complex number.

Use an audio example to develop some insight on what information is carried by the magnitude and the phase of a signal. To do this, load an audio file containing 15 seconds of acoustic guitar music. The sample rate of the audio signal is 44.1 kHz.

Fs = 44100; y = audioread

Use fft to observe the frequency content of the signal.

NFFT = length(y); Y = fft(y,NFFT); F = ((0:1/NFFT:1-1/NFFT)*Fs).';

The output of the FFT is a complex vector containing information about the frequency content of the signal. The magnitude tells you the strength of the frequency components relative to other components. The phase tells you how all the frequency components align in time.

Fundamentals of amplitude

Amplitude modulation, AM tutorial includes:

• Amplitude modulation introduction • AM theory & equations • AM spectrum & bandwidth • AM modulation index • Amplitude modulation efficiency • Single sideband modulation • Single sideband suppressed carrier

In order that a steady radio signal or "radio carrier" can carry information it must be changed or modulated in one way so that the information can be conveyed from one place to another. There are a number of ways in which a carrier can be modulated to carry a signal - often an audio signal and the most obvious way is to vary its amplitude.

Amplitude Modulation has been in use since the very earliest days of radio technology. The first recorded instance of its use was in 1901 when a signal was transmitted by a Canadian engineer named Reginald Fessenden. To achieve this, he used a continuous spark transmission and placed a carbon microphone in the antenna lead. The sound waves impacting on the microphone varied its resistance and in turn this varied the intensity of the transmission. Although very crude, signals were audible over a distance of a few hundred metres. The quality of the audio was not good particularly as a result of the continuous rasping sound caused by the spark used for the transmission.

Later, continuous sine wave signals could be generated and the audio quality was greatly improved. As a result, amplitude modulation, AM became the standard for voice transmissions.

Currently amplitude modulation is primarily used for broadcasting, but it is still used for some forms of two way radio communications. Its main radio communications use is for local aviation

related VHF two way radio links. It is used for ground to air radio communications as well as two way radio links for ground staff as well.

Amplitude modulation basics

When an amplitude modulated signal is created, the amplitude of the signal is varied in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line with the modulating signal.

Amplitude Modulation, AM

Amplitude modulation, AM is the most straightforward way of modulating a signal. Demodulation, or the process where the radio frequency signal is converted into an audio frequency signal is also very simple. An amplitude modulation signal only requires a simple diode detector circuit. The circuit that is commonly used has a diode that rectifies the signal, only allowing the one half of the alternating radio frequency waveform through. A capacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used for demodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low.

Amplitude modulation advantages & disadvantages

Like any other system of modulation, amplitude modulation has several advantages and disadvantages. These mean that it is used in particular circumstances where its advantages can be used to good effect..

Advantages Disadvantages

• It is simple to implement • It can be demodulated using a circuit

consisting of very few components • AM receivers are very cheap as no

specialised components are needed.

• An amplitude modulation signal is not efficient in terms of its power usage

• It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency

• An amplitude modulation signal is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

In view of its characteristics advantages and disadvantages, amplitude modulation is being used less frequently. However it is still in widespread use for broadcasting on the long, medium and short wave bands as well as for a number of mobile or portable communications systems including some aircraft communications.

Fundamentals of angle modulation

A sine wave carrier can be modulated by varying its amplitude, frequency, or phase shift. The basic equation for a carrier wave is

ν = Vc sin(2Πft ± θ)

where Vc = peak amplitude, f = frequency, and θ = phase angle

Impressing an information signal on a carrier by changing its frequency produces FM. Varying the amount of phase shift that a carrier experiences is known as phase modulation (PM). Varying the phase shift of a carrier also produces FM. FM and PM are collectively referred to as angle modulation. Since FM is generally superior in performance to AM, it is widely used in many areas of communication electronics.

Modulation techniques The main goal of modulation today is to squeeze as much data into the least amount of spectrum possible. That objective, known as spectral efficiency, measures how quickly data can be transmitted in an assigned bandwidth. The unit of measurement is bits per second per Hz (b/s/Hz). Multiple techniques have emerged to achieve and improve spectral efficiency.

Amplitude Phase Shift Keying (APSK) Amplitude phase shift keying (APSK), a variation of both M-PSK and QAM, was created in response to the need for an improved QAM. Higher levels of QAM such as 16QAM and above have many different amplitude levels as well as phase shifts. These amplitude levels are more susceptible to noise. Furthermore, these multiple levels require linear power amplifiers (PAs) that are less efficient than nonlinear (e.g., class C). The fewer the number of amplitude levels or the smaller the difference between the amplitude levels, the greater the chance to operate in the nonlinear region of the PA to boost power level. APSK uses fewer amplitude levels. It essentially arranges the symbols into two or more concentric rings with a constant phase offset θ. For example, 16APSK uses a double-ring PSK format (Fig. 5). This is called 4-12 16APSK with four symbols in the center ring and 12 in the outer ring.

5. 16APSK uses two amplitude levels, A1 and A2, plus 16 different phase positions with an offset of θ. This technique is widely used in satellites.

Two close amplitude levels allow the amplifier to operate closer to the nonlinear region, improving efficiency as well as power output. APSK is used primarily in satellites since it is a good fit with the popular traveling wave tube (TWT) PAs.

Orthogonal Frequency Division Multiplexing (OFDM) Orthogonal frequency division multiplexing (OFDM) combines modulation and multiplexing techniques to improve spectral efficiency. A transmission channel is divided into many smaller subchannels or subcarriers. The subcarrier frequencies and spacings are chosen so they’re orthogonal to one another. Their spectra won’t interfere with one another, then, so no guard bands are required (Fig. 6).

6. In the OFDM signal for the IEEE 802.11n Wi-Fi standard, 56 subcarriers are spaced 312.5 kHz in a 20-MHz channel. Data rates to 300 Mbits/s can be achieved with 64QAM.

The serial digital data to be transmitted is subdivided into parallel slower data rate channels. These lower data rate signals are then used to modulate each subcarrier. The most common forms of modulation are BPSK, QPSK, and several levels of QAM. BPSK, QPSK, 16QAM, and 64QAM are defined with 802.11n. Data rates up to about 300 Mbits/s are possible with 64QAM. The complex modulation process is only produced by digital signal processing (DSP) techniques. An inverse fast Fourier transform (IFFT) generates the signal to be transmitted. An FFT process recovers the signal at the receiver. OFDM is very spectrally efficient. That efficiency level depends on the number of subcarriers and the type of modulation, but it can be as high as 30 bits/s/Hz. Because of the wide bandwidth it usually occupies and the large number of subcarriers, it also is less prone to signal loss due to fading, multipath reflections, and similar effects common in UHF and microwave radio signal propagation. Currently, OFDM is the most popular

form of digital modulation. It is used in Wi-Fi LANs, WiMAX broadband wireless, Long Term Evolution (LTE) 4G cellular systems, digital subscriber line (DSL) systems, and in most power-line communications (PLC) applications. For more, see “Orthogonal Frequency-Division Multiplexing (OFDM): FAQ Tutorial.”

Determining Spectral Efficiency Again, spectral efficiency is a measure of how quickly data can be transmitted in an assigned bandwidth, and the unit of measurement is bits/s/Hz (b/s/Hz). Each type of modulation has a maximum theoretical spectral efficiency measure (Table 2).

SNR is another important factor that influences spectral efficiency. It also can be expressed as the carrier to noise power ratio (CNR). The measure is the BER for a given CNR value. BER is the percentage of errors that occur in a given number of bits transmitted. As the noise becomes larger compared to the signal level, more errors occur.

Some modulation methods are more immune to noise than others. Amplitude modulation methods like ASK/OOK and QAM are far more susceptible to noise so they have a higher BER

for a given modulation. Phase and frequency modulation (BPSK, FSK, etc.) fare better in a noisy environment so they require less signal power for a given noise level (Fig. 7).

7. This is a comparison of several popular modulation methods and their spectral efficiency expressed in terms of BER versus CNR. Note that for a given BER, a greater CNR is needed for the higher QAM levels.

Other Factors Affecting Spectral Efficiency While modulation plays a key role in the spectral efficiency you can expect, other aspects in wireless design influence it as well. For example, the use of forward error correction (FEC) techniques can greatly improve the BER. Such coding methods add extra bits so errors can be detected and corrected.

These extra coding bits add overhead to the signal, reducing the net bit rate of the data, but that’s usually an acceptable tradeoff for the single-digit dB improvement in CNR. Such coding gain is common to almost all wireless systems today. Digital compression is another useful technique. The digital data to be sent is subjected to a compression algorithm that greatly reduces the amount of information. This allows digital signals to be reduced in content so they can be transmitted as shorter, slower data streams. For example, voice signals are compressed for digital cell phones and voice over Internet protocol (VoIP) phones. Music is compressed in MP3 or AAC files for faster transmission and less storage. Video is compressed so high-resolution images can be transmitted faster or in bandwidth-limited systems.

Another factor affecting spectral efficiency is the use of multiple-input multiple-output (MIMO), which is the use of multiple antennas and transceivers to transmit two or more bit streams. A single high-rate stream is divided into two parallel streams and transmitted in the same bandwidth simultaneously. By coding the streams and their unique path characteristics, the

receiver can identify and demodulate each stream and reassemble it into the original stream. MIMO, therefore, improves data rate, noise performance, and spectral efficiency. Newer wireless LAN (WLAN) standards like 802.11n and 802.11ac/ad and cellular standards like LTE and WiMAX use MIMO. For more, see.”

Implementing Modulation And Demodulation In the past, unique circuits implemented modulation and demodulation. Today, most modern radios are software-defined radios (SDR) where functions like modulation and demodulation are handled in software. DSP algorithms do the job previously assigned to modulator and demodulator circuits. The modulation process begins with the data to be transmitted being fed to a DSP device that generates two digital outputs, which are needed to define the amplitude and phase information required at the receiver to recover the data. The DSP produces two baseband streams that are sent to digital-to-analog converters (DACs) that produce the analog equivalents.

Demodulation techniques

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer program in a software defined radio) that is used to recover the information content from the modulated carrier wave. These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator. There are several ways of demodulation depending on how parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency or phase. For example, for a signal modulated with a linear modulation, like AM (Amplitude Modulated), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (Frequency Modulation) demodulator or a PM (Phase Modulation) demodulator. Different kinds of circuits perform these functions.

Many techniques—such as carrier recovery, clock recovery, bit slip, frame synchronization, rake receiver, pulse compression, Received Signal Strength Indication, error detection and correction, etc. -- are only performed by demodulators, although any specific demodulator may perform only some or none of these techniques.