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Abstract-This term paper is a project on analog and digital communication systems and it is explanation of the advantages and disadvantages of these communication systems with comparison and application of these communication systems.
The purpose of a communication system is to transmit message intelligence of a source to a user. Thus communication may be defined as a process of establishing connection or link between two points for information exchange. The electronic equipment used for communication purpose are referred to as communication equipments. Different communication equipments assembled together form a communication system.
Typical examples of communication systems are line telephony, radio broadcasting, point to point communication and mobile communication, computer communication, radar communication, television broadcasting, radio telemetry etc.
Analog and digital signals
In communication systems, a signal means a time varying electrical signal which contains all information or intelligence. The analog signal is that type of signal which varies smoothly and continuously with time. This means that analog signals are defined for every value of time and they take on continuous values in a given time interval. Thus, we can say that analog messages are characterized by data whose values vary over a continuous range. The signal depicted in fig. 1 is an analog signal. For an analog signal, the name derives from the fact that such a signal is analogous to the physical signal that it represents. The vast majority of signals in the world around us are analog. For example the temperature or the atmospheric pressure of a certain location may vary over a continuous range and may assume an infinite number of possible values. Similarly, a speech waveform is an analog signal since it has amplitudes that vary over a continuous range. The analog signals can be easily generated. For instance, pressure variations in air produced by sound waves can be converted into corresponding current or voltage variations with the help of microphone. Similarly, a photodiode can be used to convert light intensity variations into corresponding current or voltage variations.
Fig. 1 analog signal
An alternative form of signal representation is that of a sequence of numbers, each number representing the signal magnitude at an instant of time. The resulting signal is called digital signal. Digital messages are constructed with finite number of symbols. For example, the printed language consists of 26 letters, 10 digits, a space and several punctuation marks. Thus, any text is a digital message constructed from about 50 symbols.
Now, since a digital signal is represented only by digits, therefore, any number system can be used for representation of a digital signal. However, in practice, binary number system is used to represent a digital signal. In a binary number system, each digit in the number takes on one of only two possible values, denoted 0 and1. Correspondingly, the digital signals in binary systems need have only two voltage levels which may be labelled low and high. Fig. 2 shows a digital signal.
Fig2. Digital signal
The fundamental purpose of an electronic communications system is to transfer information from one place to another. Thus, electronic communications can be summarized as the transmission, reception and processing of information between two or more locations using electronic circuits. The original source information can be in analog form, such as the human voice or music or in digital form, such as binary-coded numbers or alphanumeric codes. Analog signals are time varying voltages or currents that are continuously changing, such as sine and cosine waves. An analog signal contains an infinite number of values. Digital signals are voltages or current that change in discrete steps or levels. The most common form of digital signal is binary, which has two levels. All form of information, however, must be converted to electromagnetic energy before being propagated through an electronic communication system.
Communication between human beings probably began in the form of hand gestures and facial expressions, which gradually evolved into verbal grunts and groans. Verbal communications using sound waves, however, was limited by how loud a person could yell. Long distance communication probably began with smoke signal or tom-tom drums and that using electricity began in 1837 when Samuel Finley Breese Morse invented the first workable telegraph. Morse applied for a patent in 1838 and was finally granted it in 1848. He used electromagnetic induction to transfer information in the form of dots, dashes and spaces between a simple transmitter and receiver using a transmission line consisting of a length of metallic wire. In 1876, Alexander Graham Bell and Thomas A. Watson were the first to successfully transfer human conversation over a crude metallic wire communications system using a device they called the telephone.
Elements Of Communication System
Figure 3 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.
Fig3. Elements of a communication system
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.
Types of communication systems
There are mainly two types of communication system.
Analog communication system
Digital communication system
Analog communication is that type of communication in which the message or information signal is transmitted in analog in nature. This means that in analog communication the modulating signal is an analog signal. This analog message signal may be obtained from sources like speech, video shooting etc. Presently all the AM, FM radio transmission and TV transmission are of analog communication type.
Fig. 4 analog communication system
Analog modulation methods are employed in these systems. Any wave has three significant characteristics viz. Amplitude, frequency and phase and modulation is process of impressing information to be transmitted on a high frequency wave, called the carrier wave, by changing its one of the characteristics. Modulation may also be defined as the process of altering some characteristics of the carrier wave in accordance with the instantaneous value of some other wave called the modulating wave.
Need for modulation
Short operating range- the energy of any wave depends on its frequency- the larger the frequency of the wave, the greater the energy associated with it. Obviously the audio signals having small frequency and consequently small power cannot be transmitted over large distance when radiated directly into the space. However, modulated wave can be transmitted over long distances.
Poor radiation efficiency- at audio frequency, radiation is not practicable as efficiency of radiation is poor. However, electrical energy can be radiated efficiently at high frequencies.
Mutual interference- if low frequency signals are transmitted directly from different sources all of them would be mixed up and completely blanket in the air. However, by modulation different messages of different frequency levels can be transmitted without any interference.
Huge antenna requirement- for efficient radiation of a signal, the length of transmitting and receiving antenna should be at least one quarter wavelength. Thus, for transmitting a signal of frequency 2 kHz, an antenna of length 37.5 km will be required, practically impossible. On the other hand, for transmitting a signal of frequency of 2 MHz, an antenna of about 37.5 meters would be required which can be easily constructed.
Solution lies in modulation that enables a low frequency signal transmission over long distances through space with the help of a high frequency carrier wave. These carrier waves need reasonably sized antenna and produce no interference with other transmitters operating in the same area.
Types of modulation
In analog communication systems, we use the sinusoidal signal as the frequency carrier. And as the sinusoidal wave can be represented in three parameters; amplitude, frequency and phase, these parameters may be varied for the purpose of transmitting information giving respectively the modulation methods:
a) Amplitude Modulation (AM) - the amplitude of the carrier waveform varies with the information signal
b) Frequency Modulation (FM) - the frequency of the carrier waveform varies with the information signal
c) Phase Modulation (PM) - the phase of the carrier waveform varies with the information signal
The process of varying amplitude of the high frequency or carrier wave in accordance with the intelligence to be transmitted, keeping the frequency and the phase of the carrier wave unchanged, is known as the amplitude modulation. In this process the modulating signal is superimposed upon the radio frequency carrier by applying both to nonlinear impedance. The principle of amplitude modulation is illustrated in fig. 5. In this process the amplitudes of both positive and negative half cycles of carrier wave are varied in accordance with the modulating signal. The carrier then consists of sine waves whose amplitude follow the amplitude variations of the modulating wave. The carrier is contained in an envelope formed by the modulating wave. From fig. 5 it is obvious that the amplitude variations of the carrier wave are at the signal frequency and the frequency of the amplitude modulated wave is the same as that of the carrier wave.
Fig. 5 amplitude modulation process
Modulation index- The degree of modulation is an important parameter and is known as the modulation index. It is the ratio of the peak amplitude of the modulation signal to the peak amplitude of the carrier signal,.
The modulation index, m is also referred as percent modulation, modulation factor and depth of modulation. It is a number lying between 0 and 1 and is typically expressed as a percentage. The modulation index can be determined by measuring the actual values of the modulation voltage and the carrier voltage and computing the ratio.
Total power dissipated in amplitude modulation is (1+m2/2) times the power dissipated by a carrier wave.
Maximum power in the amplitude modulated wave will occur for m=1, thus total maximum power will be 1.5 times the power dissipated by carrier.
Limitations of amplitude modulation
Low efficiency- in amplitude modulation, the useful power that lies in the sidebands, is quite small, so the efficiency of AM system is low.
Limited operating range- transmitters employing amplitude modulation have small operating range. This is due to low efficiency. Hence information cannot be transmitted over long distance.
Noisy reception- in case of AM, the reception is generally noisy. This is because a radio-receiver cannot distinguish between the amplitude variations that represent noise and those contain the desired signal.
Poor audio quality- in order to attain high fidelity reception, all audio frequencies up to 15 kHz must be reproduced and this necessitates the bandwidth of 30 kHz while the AM broadcasting stations are assigned bandwidth of only 10 kHz to minimise the interference from the adjacent broadcasting stations. Therefore in AM broadcasting stations audio quality is usually poor.
In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous value of the input signal amplitude.
Suppose the baseband data signal (the message) to be transmitted is xm(t) and the sinusoidal carrier is , where fc is the carrier's base frequency and Ac is the carrier's amplitude. The modulator combines the carrier with the baseband data signal to get the transmitted signal:
In this equation, is the instantaneous frequency of the oscillator and is the frequency deviation, which represents the maximum shift away from fc in one direction, assuming xm(t) is limited to the range Â±1.
Although it may seem that this limits the frequencies in use to fc Â± fÎ”, this neglects the distinction between instantaneous frequency and spectral frequency. The frequency spectrum of an actual FM signal has components extending out to infinite frequency, although they become negligibly small beyond a point.
Applications of FM
Broadcasting- FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.
Magnetic tape storage- FM is also used at intermediate frequencies by all analog VCR systems, including VHS, to record both the luminance (black and white) and the chrominance portions of the video signal. FM is the only feasible method of recording video to and retrieving video from Magnetic tape without extreme distortion, as video signals have a very large range of frequency components - from a few hertz to several megahertz, too wide for equalizers to work with due to electronic noise below âˆ’60Â dB. FM also keeps the tape at saturation level, and therefore acts as a form of noise reduction, and a simple limiter can mask variations in the playback output, and the FM capture effect removes print-through and pre-echo. A continuous pilot-tone, if added to the signal - as was done on V2000 and many Hi-band formats - can keep mechanical jitter under control and assist time base correction. These FM systems are unusual in that they have a ratio of carrier to maximum modulation frequency of less than two; contrast this with FM audio broadcasting where the ratio is around 10,000. Consider for example a 6Â MHz carrier modulated at a 3.5Â MHz rate; by Bessel analysis the first sidebands are on 9.5 and 2.5Â MHz, while the second sidebands are on 13Â MHz and âˆ’1Â MHz The result is a sideband of reversed phase on +1Â MHz; on demodulation, this results in an unwanted output at 6âˆ’1 = 5Â MHz The system must be designed so that this is at an acceptable level.
Radio- As the name implies, wideband FM (W-FM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" (although for many years the BBC called it "VHF radio", because commercial FM broadcasting uses a well-known part of the VHF band - the FM broadcast band.
FM receivers employ a special detector for FM signals and exhibit a phenomenon called capture effect, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically however, frequency drift or lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner.
An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and demultiplexing before and after the FM process. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.
A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier. This gives FM another advantage over other modulation schemes that require linear amplifiers, such as AM and QAM.
Limitations of frequency modulation
FM transmitting and receiving equipment particularly used for modulation and demodulation are more complex and more costly.
A much wider channel typically 200 kHz is required in FM as against only 10 kHz in AM broadcast. This forms serious limitation of FM.
In FM, the area of reception is small as it is limited to only one line of sight.
Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180Â° or -180Â°. PM is used, As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the phase of the carrier signal:
where Î”Î¸ is the peak phase deviation
In the design of large and complex digital systems, it is often necessary to have one device communicate digital information to and from other devices. One advantage of digital information is that it tends to be far more resistant to transmitted and interpreted errors than information symbolized in an analog medium. This accounts for the clarity of digitally-encoded telephone connections, compact audio disks, and for much of the enthusiasm in the engineering community for digital communications technology. However, digital communication has its own unique pitfalls, and there are multitudes of different and incompatible ways in which it can be sent. Hopefully, this chapter will enlighten you as to the basics of digital communication, its advantages, disadvantages, and practical considerations.
The digital communication involves the transmission of information in digital form from a source that generates the information to one or more destinations. A complete system for communication of signals in digital form has many aspects besides simply sending signals that correspond to the binary 1s and 0s. Multiple levels of coding, formatting and protocol precede the actual connection of the data bit signal to the physical channel at the transmitter; the receiver reverses these operations to recover the original information. The advantage of this apparently unnecessary complexity is that the system becomes reliable, flexible and can automatically handle many problems without operator intervention. Such problems include smooth start up, recovery of equipment from power failure and detection of error in individual bits. In digital communication the signals representing information are restricted to a specific, limited group of values. However, this set of values can be used to represent, completely and faithfully, any analog information which is not restricted to only a specific set of values. The advantage of digital signals is their ability to provide virtually error free result and potential for signal improvement through additional processing that they offer.
Elements of a digital communication system
The whole digital communication system is divided as per the figure shown below. These are the basic elements of any digital communication system and It gives a basic understanding of communication systems. We will discuss these basic elements
Fig. 6 digital communication system
Information Source and Input Transducer:
The source of information can be analog or digital, e.g. analog: audio or video signal, digital: like teletype signal. In digital communication the signal produced by this source is converted into digital signal consists of 1â€²s and 0â€²s. For this we need source encoder.
In digital communication we convert the signal from source into digital signal as mentioned above. The point to remember is we should like to use as few binary digits as possible to represent the signal. In such a way this efficient representation of the source output results in little or no redundancy. This sequence of binary digits is called information sequence.
Source Encoding or Data Compression: the process of efficiently converting the output of wither analog or digital source into a sequence of binary digits is known as source encoding.
The information sequence is passed through the channel encoder. The purpose of the channel encoder is to introduced, in controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission on the signal through the channel.
e.g. take k bits of the information sequence and map that k bits to unique n bit sequence called code word. The amount of redundancy introduced is measured by the ratio n/k and the reciprocal of this ratio (k/n) is known as rate of code or code rate.
The binary sequence is passed to digital modulator which in turns convert the sequence into electric signals so that we can transmit them on channel (we will see channel later). The digital modulator maps the binary sequences into signal wave forms , for example if we represent 1 by sin x and 0 by cos x then we will transmit sin x for 1 and cos x for 0. ( a case similar to BPSK)
The communication channel is the physical medium that is used for transmitting signals from transmitter to receiver. In wireless system, this channel consists of atmosphere, for traditional telephony, this channel is wired , there are optical channels, under water acoustic channels etc.
we further discriminate this channels on the basis of their property and characteristics, like AWGN channel etc.
The digital demodulator processes the channel corrupted transmitted waveform and reduces the waveform to the sequence of numbers that represents estimates of the transmitted data symbols.
This sequence of numbers then passed through the channel decoder which attempts to reconstruct the original information sequence from the knowledge of the code used by the channel encoder and the redundancy contained in the received data
The average probability of a bit error at the output of the decoder is a measure of the performance of the demodulator - decoder combination. This is the most important point; We will discuss a lot about this BER (Bit Error Rate) stuff in coming posts.
At the end, if an analog signal is desired then source decoder tries to decode the sequence from the knowledge of the encoding algorithm. And which results in the approximate replica of the input at the transmitter end
Finally we get the desired signal in desired format analog or digital.
Optical data communication
A modern alternative to sending (binary) digital information via electric voltage signals is to use optical (light) signals. Electrical signals from digital circuits (high/low voltages) may be converted into discrete optical signals (light or no light) with LEDs or solid-state lasers. Likewise, light signals can be translated back into electrical form through the use of photodiodes or phototransistors for introduction into the inputs of gate circuits.
Fig. 7 digital transmitter and receiver
Transmitting digital information in optical form may be done in open air, simply by aiming a laser at a photo detector at a remote distance, but interference with the beam in the form of temperature inversion layers, dust, rain, fog, and other obstructions can present significant engineering problems:
Fig. 8 transmitter and receiver interference
One way to avoid the problems of open-air optical data transmission is to send the light pulses down an ultra-pure glass fibre. Glass fibres will "conduct" a beam of light much as a copper wire will conduct electrons, with the advantage of completely avoiding all the associated problems of inductance, capacitance, and external interference plaguing electrical signals. Optical fibres keep the light beam contained within the fibre core by a phenomenon known as total internal reflectance.
An optical fibre is composed of two layers of ultra-pure glass, each layer made of glass with a slightly different refractive index, or capacity to "bend" light. With one type of glass concentrically layered around a central glass core, light introduced into the central core cannot escape outside the fibre, but is confined to travel within the core:
Fig. 9 three layers of glass
These layers of glass are very thin, the outer "cladding" typically 125 microns (1 micron = 1 millionth of a meter, or 10-6 meter) in diameter. This thinness gives the fibre considerable flexibility. To protect the fibre from physical damage, it is usually given a thin plastic coating, placed inside of a plastic tube, wrapped with Kevlar fibres for tensile strength, and given an outer sheath of plastic similar to electrical wire insulation. Like electrical wires, optical fibres are often bundled together within the same sheath to form a single cable.
Optical fibres exceed the data-handling performance of copper wire in almost every regard. They are totally immune to electromagnetic interference and have very high bandwidths. However, they are not without certain weaknesses.
One weakness of optical fibre is a phenomenon known as micro bending. This is where the fibre is bend around too small of a radius, causing light to escape the inner core, through the cladding:
Fig. 10 optical fibre inside process
Not only does micro bending lead to diminished signal strength due to the lost light, but it also constitutes a security weakness in that a light sensor intentionally placed on the outside of a sharp bend could intercept digital data transmitted over the fibre.
Another problem unique to optical fibre is signal distortion due to multiple light paths, or modes, having different distances over the length of the fibre. When light is emitted by a source, the photons (light particles) do not all travel the exact same path. This fact is patently obvious in any source of light not conforming to a straight beam, but is true even in devices such as lasers. If the optical fibre core is large enough in diameter, it will support multiple pathways for photons to travel, each of these pathways having a slightly different length from one end of the fibre to the other. This type of optical fibre is called multimode fibre:
Fig. 11 light travelling in fibre
A light pulse emitted by the LED taking a shorter path through the fibre will arrive at the detector sooner than light pulses taking longer paths. The result is distortion of the square-wave's rising and falling edges, called pulse stretching. This problem becomes worse as the overall fibre length is increased:
Fig 12 pulse stretching in optical fiber
However, if the fibre core is made small enough (around 5 microns in diameter), light modes are restricted to a single pathway with one length. Fibre so designed to permit only a single mode of light is known as single-mode fibre. Because single-mode fibre escapes the problem of pulse stretching experienced in long cables, it is the fibre of choice for long-distance (several miles or more) networks. The drawback, of course, is that with only one mode of light, single-mode fibres do not conduct as much light as multimode fibres. Over long distances, this exacerbates the need for "repeater" units to boost light power
Advantages of digital communication system
The digital communication systems are simpler and cheaper compared to analog communication systems because of the advancements made in the IC technologies.
In digital communication, the speech, video and other data may be merged and transmitted over a common channel using multiplexing.
Using data encryption, only permitted receivers may be allowed to detect the transmitted data. This property is of its most importance in military applications.
Since the transmission is digital and the channel encoding is used, the noise does not accumulate from repeater to repeater in long distance communications.
Since the transmitted signal is digital in nature, a large amount of noise interference may be tolerated.
Since in digital communication, channel coding is used, the errors may be detected and corrected in the receivers.
Digital communication is adaptive to other advanced branches of data processing such as digital signal processing, image processing and data compression etc.
Disadvantages of digital communication
Although digital communication offers so many advantages as mentioned above, it has some drawbacks also. However, the advantages of digital communication outweigh the disadvantages given below:
Because of analog to digital conversion, the data rate becomes high and therefore, more transmission bandwidth is required for digital communication.
Digital communication needs synchronisation in case of synchronous modulation.
Frequency Shift Keying - FSK
The two binary states, logic 0 (low) and 1 (high), are each represented by an analogue waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a different frequency.
Below figure shows the basic representation.
Â Â Â
Fig. 13 FSK process
With binary FSK, the centre or carrier frequency is shifted by the binary input data. Thus the input and output rates of change are equal and therefore the bit rate and baud rate equal.
The frequency of the carrier is changed as a function of the modulating signal (data), which is being transmitted. Amplitude remains unchanged. Two fixed-amplitude carriers are used, one for a binary zero, the other for a binary one.
You can see from the movie below howÂ the FSK wave form is generated. Note when the edge of a a new logic level enters the transmitter the frequency of the output.
How the Waveform is generated.
The general analytic expression for FSK is;
si (t) = Acos2Ï€ Æ’i t 0 â‰¤ t â‰¥ T and i = 1,....,M
Æ’i = (Æ’0 + 2i - M) Æ’d
Æ’0 denotes the carrier frequency.
Generation of these waveforms maybe accomplished with a set of M separate oscillators, each tuned to the frequency
It can be observed below that the error probability for a given signal-to-noise ratio decrease as M increases, contrary to other modulation scheme (i.e. PSK and QAM), but on the other hand the bandwidth efficiency decrease as M increases, it value being given by;
The FSK Transmitter
Below shows a block diagram of a FSK modulator where the input signal M equaled to eitherÂ 2-, 4- or 8-level impulses separated by the baud period, T.
It is first filtered by v(t) to control the bandwidth of the base band signal which, in turn, partially controls the FSK signal spectrum. The filter output signal level is then adjusted and input to a phase modulator.
The phase modulator centers the signal at frequency. Different choices of the low-pass filter characteristic and signal gain, a, control the signal bandwidth and inter symbol interference (ISI) on the base band signal. A common filter characteristic uses a rectangular pulse shape. It does not cause ISI but the bandwidth is relatively wide.
Another choice is to use a Nyquist filter that introduces controlled ISI but complicates the demodulator timing recovery. More aggressive filtering, such as Gaussian filters, provide very good bandwidth control but require ISI compensation in the demodulator. Note that base band-filtering-induced ISI is different from multi-path-induced ISI that causes distortion on the FM signal rather than the base band.
fig. 14 psk process
Uses of FSK
Today FSK Modems are used for short haul data communication over private lines or any dedicated wire pair. These are many usedÂ for communication between industrial applications like railroad signaling controls and mobile robotic equipment. The short haul modem offers the following specs;
- Speeds of up to 9600 bps
- Full-duplex or half duplex operation.
- Distance up to 9.5 miles
In the past FSK was used in the Bell 103 and Bell 202. These were the first data modem but due to there low bit rate there not being used any more. The Bell 103 had a data rate of only 300 bauds. This modem was predominant until the early 1980s
Phase Shift Keying - PSK.
Phase shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information.
The simplest from of PSK has only two phases, 0 and 1. It is therefore a type of ASK with Â¦ (t) taking the values -1 or 1, and its bandwidth is the same as that of ASK. The digital signal is broken up time wise into individual bits (binary digits).
The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low)
If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called bi-phase modulation.
If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes.
The general analytic expressionÂ for PSK is;
Â Â Â Â Â Â Â Â Â Â Sin (2pft)
Â Â Â Â Â Â Â Â Â Â Sin (2pft + p)
It can be observed below that the error probability for a given signal-to-noise ratio increase as M increases, contrary to other modulation scheme (i.e. FSK), but on the other hand the bandwidth efficiency increases as M increases, it value being given by;
Rs / B = log2M / T * T = log2M
Below shows error probability of coherently demodulated FSK where P(e) is the probability of error.
Binary Phase Shift Keying (BPSK):
- Use alternative sine wave phase to encode bits
- Simple to implement, inefficient use of bandwidth
Fig. 15 phase sepapration
Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK. PSK can be expanded to a M-ray scheme, employing multiple phases and amplitudes as different states.
Filtering can be employed to avoid spectral spreading.
Quadrature Phase Shift Keying (QPSK):
- Multilevel modulation technique: 2 bits per symbol
- More spectrally efficient, more complex receiver
Fig. 16 state with respect to phase
Fig. 17 quadrature phase shift keying
Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency. Output waveform is sum of modulated; Â± Cosine and Â± Sine wave.
Variants of QPSK
- Conventional QPSK has transitions through zero (i.e.. 180o phase transition). Highly linear amplifier required.
- In Offset QPSK, the transitions on the I and Q channels are staggered. Phase transitions are therefore limited to 90o
- Ï€/4-QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations.
Uses of PSK
Binary Phase Shift Keying (BPSK)
- BPSK is mainly used in deep space telemetry and also cable modems Quadrature Phase Shift Keying (QPSK) and it variants.
- CDMA, (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications.
-TETRA, Terrestrial Trunked Radio) is a set of standards developed by the European Telecommunications Standardization Institute (ETSI) that describes a common mobile radio communications infrastructure throughout Europe This infrastructure is targeted primarily at the mobile radio needs of public safety groups (such as police and fire departments), utility companies, and other enterprises that provide voice and data communications services.
- PHS, (Personal Handy-phone System) Developed by the Nippon Telegraph and Telephone Corporation, the Personal Handy-phone is a lightweight portable wireless telephone that functions as a cordless phone in the home and as a mobile phone elsewhere. The Personal Handy-phone also handles voice, fax, and video signals. The phone is now being marketed in other Asian countries.
- LMDS, (Local Multipoint Distribution System) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. LMDS is an alternative to installing optic-fiber all the way to the user or to adapting cable-TV for broadband Internet service
Quadrature Amplitude Modulation - QAM
QAM is the encoding of information into a carrier wave by variation of the amplitude of both the carrier wave and a 'quadrature' carrier that is 90Â° out of phase with the main carrier in accordance with two input signals. Alternately, this can be regarded (using complex number notation) as simple amplitude modulation of a complex-valued carrier wave by a single complex-valued signal.
What this actually means is that the amplitude and the phase of the carrier wave are simultaneously changed according to the information you want to transmit.
- The incoming binary data stream is passed through a serial to parallel converter of appropriate bandwidth.
- The data is then passed through a logic block which the translation between the binary input data and the constellation point to allow Gray coding. (An ordering of 2n binary numbers such that only one bit changes from one entry to the next>
- Digital-to-Analogue (D/A) conversion is then requires, followed by pulse-shaping, before up-converting the signal to the carrier frequency.
- It is often advantageous to perform the pulse-shaping prior to the D/A conversion by over sampling the transmitted data and by using further lookup tables containing the pulse-shaping law in order to produce a digital representation of the smoothing signal.
This will still need to be filtered after D/A in order to remove aliasing errors caused by over sampling.
Fig. 18 generating of QAM
In order to demodulate the signal the reverse process is followed:
- The incoming waveform is sampled at the correct instants provided by the clock recovery circuit, A/D converted and a decision is made as to what the most likely constellation point to have been transmitted.
- The position of the constellation point is then passed through a logic block which performs the reverse operation to the one in the transmitter, leading to the encoded information.
- This is passed through a parallel to serial converter leading to the output data stream
We chose 16 as an example this time. That is, we consider combining bits in groups of four, yielding 16 possible values. These values change every 4T so we can expect a bandwidth that is one-fourth that of BPSK. If we use 16 ray- PSK, the points in signal space would be equally spaced on the circumference of a circle, and the angular spacing between adjacent points would be 22.5 degrees.
In this case, both the amplitude and the phase vary, so the points no longer lie on the circumference of a single circle. The signal space diagram consists of 16 points in a uniform square array. The Individual signals are of the form:
The index i takes the values of 0 to 15. The equation can be re-written to the following form:
The following is a mathematical implementation of a 16-QAM modulator scheme:
fig. 19 generating of QAM
We have implemented a 16-QAM modulator- demodulator Demonstration. We have used a Pseudo-Random signal as an input
COMPARISON OF ANALOG AND DIGITAL COMMUNICATION
Analog communication systems, amplitude modulation (AM) radio being a typifying example, can inexpensively communicate a band limited analog signal from one location to another (point-to-point communication) or from one point to many (broadcast). Although it is not shown here, the coherent receiver provides the largest possible signal-to-noise ratio for the demodulated message. An analysis of this receiver thus indicates that some residual error will always be present in an analog system's output. Although analog systems are less expensive in many cases than digital ones for the same application, digital systems offer much more efficiency, better performance, and much greater flexibility.
Efficiency: The Source Coding Theorem allows quantification of just how complex a given message source is and allows us to exploit that complexity by source coding (compression). In analog communication, the only parameters of interest are message bandwidth and amplitude. We cannot exploit signal structure to achieve a more efficient communication system.
Performance: Because of the Noisy Channel Coding Theorem, we have a specific criterion by which to Formulate error-correcting codes that can bring us as close to error-free transmission as we might want. Even though we may send information by way of a noisy channel, digital schemes are capable of error-free transmission while analog ones cannot overcome channel disturbances; see this problem for a comparison.
Flexibility: Digital communication systems can transmit real-valued discrete-time signals, which could be analog ones obtained by analog-to-digital conversion, and symbolic-valued ones (computer data, for example). Any signal that can be transmitted by analog means can be sent by digital means, with the only issue being the number of bits used in A/D conversion (how accurately do we need to represent signal amplitude). Images can be sent by analog means (commercial television), but better communication performance occurs when we use digital systems (HDTV). In addition to digital communication's ability to transmit a wider variety of signals than analog systems, point-to-point digital systems can be organized into global (and beyond as well) systems that provide efficient and flexible information transmission. Computer networks, explored in the next section, are what we call such system today. Even analog-based networks, such as the telephone system, employ modern computer networking ideas rather than the purely analog systems of the past.
Consequently, with the increased speed of digital computers, the development of increasingly efficient algorithms, and the ability to interconnect computers to form a communications infrastructure, digital communication is now the best choice for many situations.
Both analog and digital communications are two important forms of communication. Digital communication is important because of its advantages over analog communication. Now day's digital communication is preferred over analog communication. We all know 1G was first time used in 1990's and it was based on analog communication. But, after that 2G, 3G and now 4G all are based on digital communication. But still analog communication is used in many fields because of number of uses it has.
As usual a large number of people deserve my thanks for the help they provided me for the preparation of this term paper.
First of all I would like to thank my teacher Mr. Gursharanjeet Singh for his support during the preparation of this topic. I am very thankful for his guidance.
I would also like to thank my friends for the encouragement and information about the topic they provided to me during my efforts to prepare this topic. At last but not the least I would like to thank seniors for providing me their experience and being with me during my work.