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Communication is the process of transfer of information from sender to receiver. Figure shows a basic structure of a communication system. The transmitter converts the information into signals suitable for the communication channel. While the signals propagate through the channel, noise signals arise. These signals along with noise will reach the receiver end, where message signal is filtered from the transmitted signal along with noise. To some extent, the noise signals can be filtered out and the message signal can be reproduced.
Basically there are three elements in a communication system- transmitter, communication channel, and receiver.
The information cannot be transmitted in its raw format through the communication channel. Transmitter unit is used to convert raw information into the format that is understandable by the communication channel. Different methods like modulation and coding may be used depending on the requirement. Mobile phones and AM radio transmitters are some of the most familiar transmitters. Modulation is employed to superimpose a low frequency message signal on to a high frequency carrier to protect the signal from getting dispersed.
Communication channel is defined as the medium through which the signal is send from transmitter to receiver. When signal is propagated through the channel, it gets affected by noise. And also channel attenuation degrades the signal strength, so signal power decreases with distance. In radio communication systems, air is the medium and in satellite communication systems, both air and vacuum are the medium.
Signals sent through the communication channel reaches the receiver, where it is decoded or demodulated to extract the message. Since channel attenuation degrades the signal power, amplifier is used at the receiver to compensate for the transmission losses. Both selectivity and sensitivity of a receiver should be high only then the message signal can be extracted from the received signal.
Selectivity is defined as the ability of a receiver to select the exact message signal while rejecting other noise signals. Selectivity is measured in terms of ratio in decibels (dBs) by comparing the signal strength received to that of similar signal strength on another frequency.
Sensitivity is the ability of a receiver to pick up weak signals which get affected by channel attenuation. It is defined as the minimum magnitude of input signal required to produce a specified output signal. That is, minimum input signal required to produce a specified signal-to-noise S/N ratio at the output port of the receiver.
Noise is any unwanted signals that interfere with the information, which includes atmospheric changes, lighting and thunderstorms, other communication systems etc. that can cause noises in the transmitted signal. Different techniques can be used to minimize the noise in the signal but it cannot be completely removed.
Communication protocol is defined as the set of rules that the devices agree for communication. When messages are exchanged between two systems, parameters like rate of transmission, synchronous or asynchronous, half-duplex or full-duplex mode should be agreed to before transmission. Detecting and correction of transmission errors and encoding and decoding data will also come under communication protocol. It is implemented in both hardware and software.
Elements in communication protocol
Syntax in communication protocol defines the structure of data, that is, how each data is arranged in the transmitted data. For example, transmitted data may contain 12 bits in which first four bits represent sender’s address, next four bits shows the receiver’s address and the final four bits are for data. Syntax defines this structure only after which the receiver can understand and be able to fetch the corresponding data.
Transmitted data contains digital representation of any real time variable. Semantic defines the meaning of a particular data and how it is to be interpreted. For example, in the receiver’s address part, if data is coded as 0010 it means that data has to be moved to third module.
‘When to send’ and ‘at what rate’ comes under timing protocol. If the sender produces data at one rate and receiver process at a lower rate, then there is a possibility of data loss causing incorrect interpretation or system failure.
In this method, signal is directly transmitted from the transmitter to the receiver. Most common example of baseband communication is direct conversation between two people. In this method, the signal amplitude gets distorted with the distance. If the second person is standing 200 meters away from the first person, he cannot hear what the other person is saying.
Baseband signal communication is not commonly used for distance communication. Low frequency base band signals, having low energy, if transmitted directly will get distorted. So baseband signal must be modulated with high frequency signal to increase the range of transmission. Modulation is defined as the superimposition of low frequency baseband signal over high frequency carrier signal by varying different parameters of the carrier signals. Based on the types of parameters that are varied in proportion to the baseband signal, modulation are of different types.
In this method, amplitude of the carrier signal is varied with respect to the message signal. Carrier frequency used for amplitude modulation will be at least ten times more than baseband frequency. Instantaneous amplitude of the carrier signal is varied in proportion to the message signal. Since, frequency of the carrier signal is very high, the energy of the message signal improves and the information can be transmitted to a longer distance without distortion.
Figure shows the waveforms of carrier signal, modulating signal or message signal and amplitude modulated signal. We can see that the amplitude of the modulated waveform changes in proportion to the message signal without varying any other parameters. This technology is applied in amplitude modulation.
Amplitude Modulation
Frequency modulation method is also used to improve the range. In this method, instantaneous frequency of the carrier wave is varied in proportion to the amplitude of the message signal. One important advantage of frequency modulation over amplitude modulation is that, it can be used for both analog and digital signals. But amplitude modulation can only be used in analog signals.
Frequency Modulation
Radio communication system implies wireless transmission of electromagnetic signals through space. Radio signals have a frequency range from 30 kHz to 300 GHz which is less than the frequency of visible light. Various modulation methods like Amplitude, frequency, phase or pulse width modulations are used to transmit information using radio wave communication.
Radio communication system
(a). Transmitter and Modulation
At the transmitter end, message signal is modulated with radio frequency carrier signal. Depending on the application and communication requirements, suitable type of modulation technique is used. Modulated high frequency signal is amplified and fed to the antenna for transmission.
(b). Antenna
In radio wave communication system, antennas are used at both transmitter and receiver end. At the transmitter end, output from the transmitter is fed into the antenna which launches the radio waves into space. At the receiver end, antenna picks up as much of the transmitter's power as possible. Size and construction of antenna depends on the frequency that it deals with.
An antenna consists of an arrangement of metallic conductors. High frequency electric current fed to these cause free electrons to vibrate at very high frequency resulting in the electromagnetic radiation.
(c). Propagation
The behavior of radio waves when they are transmitted from one point to the other is defined by propagation. Phenomenon such as reflection, refraction, diffraction, absorption, polarization and scattering affect the propagation of radio waves along with, presence of water vapor in the troposphere and ionization of the upper atmosphere. Frequency of transmitted signal also influences the propagation of the radio wave. Signals at different frequencies behave differently when transmitted.
(d). Receiver and demodulation
Received signal contains modulated wave along with high frequency noise content. Since the received signal is very feeble the signals are amplified. Receiver contains resonant circuit to select a particular frequency and discarding others. Modulated wave thus received is fed to demodulator to obtain the message signal. The message signal is then amplified to the required level.
Fiber optic communication is now the most preferred communication method in telecommunication systems. Because of its advantages in many areas, it has already replaced wired communication. A few main advantages are listed below.
In this method, electric signal is first converted into optical signal and passed through the optical fiber. The light signal reaches the receiver where it is converted back to electrical signals. Light signal is send through a long, thin strand of very pure glass about the diameter of a human hair. Many such strands are arranged in bundles and covered with a jacket for protection.
Total internal reflection
Total internal reflection is the basic principle used in optical fibers. From the source, light enters the core of the optical fiber. When the light passes from a medium with high refractive index (core) to another medium with a lower refractive index (cladding), it bends or refracts away from the normal. As the angle of the beam through the core becomes greater than a particular angle (critical angle) the incident light will get reflected. This process happens throughout the optical cable therefore, light signal is transmitted through it by multiple reflections.
Construction of Fiber Optic Cable
An optical fiber essentially consists of three layers Core, Cladding and Buffer coating. The rest of the layers are provided in order to increase the flexibility, strength and protection from external stresses.
Core is a thin glass/silica at the center of the optical fiber through which light travels. A Glass material with high refractive index is used for this purpose.
Core is surrounded by a medium, with lesser refractive index. Ray of light incident on the core-cladding interface is reflected back into the core. Cladding ensures that no light signal escapes from the optical fiber.
The entire structure is protected by a plastic coating. It is composed of multiple layers and materials in order to protect from external shocks, moisture, surrounding materials etc. The sheath ensures safe operating conditions to the fiber by providing the necessary strength and flexibility.
Fiber-Optic System
Fiber optic communication system consists of transmitter, optical fiber, optical regenerator and finally a receiver.
Transmitter is the first stage of the optical fiber communication system. It consists of a light source which converts electric signals into light signals and a focusing lens is used to focus the light beam into the optical fiber. Both Lasers and LEDs can be used as a light source. Lasers have more power than LEDs, but its characteristics vary with changes in temperature.
Light signal from the transmitter is given into the optical fiber. Signal is propagated through it by multiple internal reflections.
When light passes through the optical fiber, the signal may get distorted due to the presence of impurities in the core. Distance to which the light signal can propagate through the fiber depends on the purity of the glass and the wavelength of the transmitted light. Therefore, to improve the transmission distance, Optical regenerators must be used at regular intervals. One or more optical regenerators are used to boost the degraded light signals in the optical communication system. In certain systems, the feeble optical signals are converted back into electrical signals and the optical data is reconstructed as in the case of a transmitter.
Optical regenerators are also called laser amplifiers. They are optical fibers with a special coating (doping). When degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. Thus degraded light signal will get amplified and propagate further.
Optical receiver receives light signals which it converts back to electrical signals. Receiver uses a photocell or photodiode to detect the light and convert it to proportional electric signals.
Advantages of analog communication over digital communication
Block diagram of digital communication system
The source may analog or digital. The analog information will be converted into digital using sampling and quantization process.
It converts the digital signal generated at the source output into another signal in digital form. The symbol sequence is converted into binary sequence using the source encoder. It removes the redundancy in transmitting information. It will reduce the bandwidth requirement for transmitting information.
Source decoder performs inverse mapping. It reproduces the original digital source output. It converts the binary output of channel decoder into symbol sequence.
It map the incoming digital signal into channel input to reduce the effect of channel noise. It provides reliable communication over noisy channel. In source encoding, the redundancy is removed and in channel encoding, the redundancy is introduced in a controlled manner. Error control is the main aim of channel coding. It adds some extra bits to the output of source encoder. These bits do not convey any information. It helps the receiver to detect and correct the errors occurred in the information. The channel decoder performs error detection and correction.
For the transmission through communication channel, the modulator converts input bit stream into electrical waveform. It minimizes the effect of channel noise, to match the frequency spectrum of transmitted signal and channel characteristics.
The information bearing waveform produced by modulation is converted into bit stream using detector or demodulator.
It provides the connection between source and destination. There are different channels like coaxial cable, optical fiber, radio channel etc.
Pulse Code Modulation
It is a digital pulse modulation system. The analog signal is converted into digital signal. It converts any analog signal into digital signal. This is mainly used in telecommunication applications for the transmission of digital speech. The sampled analog signal can represented digitally using pulse code modulation. The amplitude of the analog signal is sampled at uniform levels. Each sample is quantized to the nearest value. It is a digital scheme used for transmitting analog data. The signals in PCM has two states such as logic 1(high) and logic 0(low). All forms of analog data such as videos, music etc. can be digitalizing using PCM. It is the simplest form of wave coding.
The PCM encoder has three forms. They are
The analog signal amplitude is sampled at regular time intervals. The sampling rate is greater than maximum frequency of the analog signal in cycle per second (Hertz). The sampling frequency should be twice the highest frequency of the analog signal. The signal will be restored at the destination using a low pass filter.
It is the process of assigning discrete values to all samples. The number of possible value depends on number of bits used to represent samples. The instantaneous amplitude of analog signal is rounded off to up or down to the nearest level. It is called quantization.
The sampled value is represented into binary numbers. The range is in between 0 to n. The value of n is chosen as power of 2 depending upon the accuracy required. The value of n increases will reduces the step size between adjacent quantization level and it will reduced the quantization noise.
Differential Pulse Code Modulation (DPCM)
In this difference between adjacent samples is encoded. The difference is taken between original and predicted signal. It reduces the number of bits per sample used for PCM.
At first, it iworks like PCM. At constant sampling frequency, the input is sampled and modulated using a modulation technique such as PAM. The sampled signal is stored in a predictor and it sends this signal to the differentiator. Predictor helps to compare the current sampled signal with previous sampled signal. This signal difference is given to the quantizing and coding phase of PCM. After that it is transmitted.
At the receiver, the difference signal is dequantized. The sample signal stored in predictor is added with the dequantized signal. It is then pass through a low pass filter which reconstructs the original signal.
Modulation techniques
Modulation is the process of varying the characteristics of carrier signal with the modulating signal. In digital modulation, the message signal is converted from analog into digital. In digital modulation techniques, the analog carrier signal is modulated by discrete signal. The carrier wave is switched on and off to create pulses such that signal is modulated. The most important digital modulation techniques are
Amplitude Shift Keying
It is the form of AM (amplitude modulation). In this modulation the carrier wave amplitude is varied according to the message signal which is in digital format.It is used for low band requirements. It is sensitive to noise. Binary symbol 1 is represented by transmitting a fixed amplitude carrier wave and fixed frequency for a bit duration of T seconds. If the signal value is 1 then the carrier signal will be transmitted. Otherwise the signal value of 0 will be transmitted.
Phase Shift Keying
In this, the phase of the carrier signal is varied according to the message signal. It is also called Binary Phase Shift Keying (BPSK). BPSK uses two opposite phase signals. One is 0 degree and the other signal is 180 degree. Determination of the state of each bit is based to the state of the preceding bit. When the phase of the wave does not change then the state of the signal will remain same. If the phase of the wave reverses, that is when phase changes by 180 degree, then signal state changes from 1 to 0 or 0 to 1.
Frequency Shift Keying (FSK)
The frequency of the carrier signal is varied according to the message signal. It used a pair of discrete frequencies such as logic 0 (low) and logic 1(high). The “1” is called the Mark frequency and “0” is called the space frequency.
In analog communication, the information signal is a continuous signal in both amplitude and time. But in digital communication, the information signal is converted into discrete messages.Any signal can be converted into analog signal. The physical variables such as sound, light etc can be converted into analog signal using a transducer.
Block diagram of communication system
The information signal is transmitted from sender to receiver. The information signal is converted into electrical signal using input transducer.
Transducer is a device used to convert one form of energy into another. The information signal such as voice signal is converted into electrical form using transducer. eg:- microphone converts sound signal into electrical form.
The information signal is modulated using a carrier signal and transmitted. Modulation is the process of superimposing carrier signal with information signal.
Channel connects between the sender and receiver. Copper wires, fiber optic cable free space are commonly used as channels. There are different types of noise present in the channel. This noise will interfered with the transmitted signal which causes distortion.
The noise is first removed and demodulated to produce the original signal. Demodulation is the process of extracting the original signal from the modulated signal.
The output transducer converts the electrical signal back to the information signal. Loudspeaker can convert the electrical signal into sound signal.
Modulation is the process of changing characteristics of carrier signal according to the modulating signal.
The message signal has low frequency. As frequency decreases, the wavelength increases.
Height = λ/4
So as wavelength increases, antenna height need to be increased. But it is practically impossible.
To reduce the antenna height, the low frequency message signal is converted into high frequency signal using any modulation technique. The information (message) cannot be transmitted through communication channel. So the low frequency message signal is modulated using high frequency carrier signal.
In one channel, several signals can be transmitted using multiplexing. Using modulation various signals can be allotted to different frequencies. It avoids the interference of signals.
At some frequencies, the effect of noise will be high and low at some other frequencies. If some frequency has high noise, then by using modulation technique the spectrum is shifted to another frequency where noise is less.
Modulation converts wideband signal into narrowband signal.
If the carrier signal is continuous, then the modulation is known as continuous wave modulation.
It is a modulation technique used for transmitting information using carrier wave. In this modulation technique, the amplitude of the carrier wave is varied according to the information signal.
In this modulation technique, the frequency of the carrier signal is varied according to the information signal.
In this modulation technique, the phase of the carrier wave is varied according to the information signal.
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