Digital modulation and demodulation

digital flection and de chantingChapter 1 Digital Communications1.0 Digital Communication1.1 IntroductionCommunication biddingWhen we think of dialogue, we usu whole(prenominal)(a)y think of people talking or sense of hearing to separately a nonher(prenominal). This may happen face to face, or it may make it by the assistance of a teleph ane, radio, or television.Basically, conference is the broadcast of culture. liveness In our modern, complex world depends to a greater extent and more on the remove of k nowledge. The increasing dependency on the transfer of discipline has stimulated the produce of more and more converse schemas. This surge in dialogue and communion systems has been referred to as a technological revolution.This shows understand the transfer of tuition in a talk systemThe communion system give contain of at least the three parts shown. The enthrall aro purpose be as simplistic as the air that carries the sound of your voice, or as com plex as the broadcast ne twainrk required to slabber a television program round the world.The close to common problem encountered by the communication process is hurly burly. Interference is any force that disrupts or distorts the in make upation or message while it is being channeled. It could be interference, as in the case of normal conversation, or atmospheric weather channelizes, as In the case of radio or televisionThe biggest cause of interference, however, is a simple misinterpretation of the intended message. Cultural, economic, and political diversities allow people to receive the kindred message but interpret it differently.Communication SystemsCommunication system is a combination of processes and hardw ar used to accomplish the transfer of In dustation (communication).A system is a group of interrelated parts. We find that there be systems all around us. In nature, we enkindle also find examples of systems that seduce been created by people. An automobile, a washing machine, and an electric drill be examples.1.2 TYPES OF parleyBased on the requirements, the communication theory stinker be of different character fibresPoint-to- forefront communication In this type, communication takes place in the midst of twain end points. For instance, in the case of voice communication using teleph singles, there is peerless occupational group party and one called party. Hence the communication is point-to-point.Point-to-multipoint communication In this type of communication, there is one sender and multiple recipients. For example, in voice conferencing, one person will be talking but many an(prenominal) others spate listen. The message from the sender has to be multicast to many others.Broadcasting In a broadcasting system, there is a central location from which information is direct to many recipients, as in the case of audio or record book picture broadcasting. In a broadcasting system, the listeners argon passive, and there is no abstract communication path.In simplex communication, the communication is one-way only.In half-duplex communication, communication is twain(prenominal) ways, but only in one direction at a time.In full-duplex communication, communication is in both directions simultaneously.Simplex communication In simplex communication, communication is possible only in one direction. on that point is one sender and one pass catcher the sender and liquidator potnot change roles.Half-duplex communication Half-duplex communication is possible in both directions betwixt 2 entities (computers or persons), but one at a time. A walkie-talkie uses this approach. The person who wants to talk crusadees a talk button on his handset to start talking, and the other persons handset will be in receiving mode.When the sender finishes, he terminates it with an over message. The other person can press the talk button and start talking. These types of systems require limited channel curingwidth, so they ar low romaine lettucet systems.Full-duplex communication In a full-duplex communication system, the two parties-the caller and the called-can communicate simultaneously, as in a address system. However, note that the communication system allows simultaneous contagious disease of selective information, but when two persons talk simultaneously, there is no deedive communication The ability of the communication system to transport entropy in both directions defines the system as full-duplex.1.3 ANALOG VERSUS DIGITAL TRANSMISSIONIn analog communication, the show, whose bounteousness varies continuously, is communicable over the medium. Reproducing the analog bless at the receiving end is real nasty due to transmission impairments. Hence, analog communication systems be badly impact by noise.In a digital communication system, 1s and 0s are transmissible as voltage meters. So, even if the pulse is distorted due to noise, it is not very(prenominal) difficult to remark t he pulses at the receiving end. Hence, digital communication is lotstimes more immune to noise as compared to analog communication.1.4 Digital intonationFirstly, what do we mean by digital changeover? Typically the target res publica of a digital communication system is to transport digital info between two or more nodes. In radio communications this is usually achieved by adjusting a physical characteristic of a curving common pallbearer, the frequence, stage, bounteousness or a combination thereof. This is performed in real systems with a modulator at the transmitting end to impose the physical change to the letter letter crew cut and a sensing element at the receiving end to detect the resultant flexion on reception.* chanting is the process of varying some characteristic of a periodic wave with an external note.* flexion is utilized to send an information bearing hint over long distances.* Radio communication superimposes this information bearing luff onto a carrier wave wave place.* These high a great dealness carrier prefigures can be transmitted over the air easily and are capable of traveling long distances.* The characteristics ( amplitude, absolute frequency, or variety) of the carrier head are varied in accordance with the information bearing level.* In the survey of communication engineering, the information bearing channelise is also known as the modulating augur.* The modulating signal is a slowly varying signal as contrasted to the rapidly varying carrier frequency.The principal of a digital communication system is that during a finite interval of time, it sends a wave term from a finite set of possible wave forms, in contrast to an analog communication system, which sends a wave shape from an unlimited variety of waveform shapes, with theoretically protrudeer space resolution. In a DCS (digital communication system), the objective of the receiver is not to retch a transmitted waveform with precision. The objective is to determine from a noise-perturbed signal which waveform from the finite set of waveforms was sent by the vector.Why Digital? The primary advantage is the ease with which digital signals, compared with analog signals, is regene orderd. The shape of the waveform is affected by two basic mechanisms.As all transmission lines and circuits arrive some non-ideal frequency transfer knead, there is a distorting effect on the ideal pulse.Unwanted electrical noise or other interference further distorts the pulse waveform.Both of these mechanisms cause the pulse shape to let down.* With digital proficiencys, extremely low error rates producing high signal fidelity are possible through error sleuthing and fudge factor but similar procedures are not available with analog.* Digital circuits are more reliable and can be reproduced at a a theme cost than analog circuits.* Digital hardware lends itself to more flexible slaying than analog circuits.* The combination of digita l signals using Time Division Multiplexing (TDM) is simpler than combining analog signals using Frequency Division Multiplexing (FDM).Metrics for Digital Modulation Power Efficiency Ability of a intonation technique to pertain the fidelity of the digital message at low power levels clothes designer can increase noise resistivity by increasing signal power Power efficiency is a measure of how much signal power should be increased to achieve a particular BER for a given transition end Signal energy per patch / noise power unearthly density Eb / N0 Bandwidth Efficiency Ability to accommodate data within a limited bandwidth Tradeoff between data rate and pulse width Throughput data rate per hertz R/B bps per Hz Shannon Limit Channel capacity / bandwidth C/B = log2(1 + S/N)Disadvantages of Digital Systems* Digital systems tend to be very signal touch on intensive compared with analog.* Digital systems privation to allocate a material share of their resources to the task of sy nchronising at various levels. With analog signals synchronization is accomplished more easily.* One disadvantage of digital communication system is non-graceful degradation. When the SNR drops below a certain threshold, the character reference of service can change form very good to very poor. Most analog systems degrade more gracefully.FormattingThe goal of the starting signal ingrained processing step, change is to ensure that the source signal is harmonious with digital processing. Transmit arrange is a transformation from source information to digital signs. When data muscular contraction in addition to formatting is employed, the process is termed source coding.The digital messages are considered to be in the logical format of binary program star 1s and 0s until they are alter by pulse inflection into base band (pulse) waveforms. Such waveforms are then transmitted over a cable.No channel can be used for the transmission of binary digits without first transforming the digits to waveforms that are compatible with the channel. For base band channels, compatible waveforms are pulses.The conversion from a crooke of streams to a ecological succession of pulse waveforms takes place in the block labeled, modulator. The widening of a modulator is typically a sequence of pulses with characteristics that correspond to the digits being sent. subsequently transmission through the channel the pulse waveforms are recovered (de orderd) and notice to produce an estimate of the transmitted digits.Formatting in a digital Communication SystemSymbolsWhen digitally transmitted, the characters are first encoded into a sequence of bits, called a bit stream or base band signal. stem of K bits can then be combined to form spick-and-span digits, or images, from a finite or alphabet of M = 2K such symbols. A system using a symbol set size of M is referred to as M-array system.Waveform Representation of binary DigitsDigits are just abstractions way to describ e the message information. hence we need something physical that will oppose or carry the digits.Thus binary digits are represented with electrical pulses in order to transmit them through a base band channel. At the receiver, a determination must be made regarding the shape of pulse. The likelihood of correctly detecting the pulse is a function of the trustworthy signal energy (or area under the pulse).PCM Waveform TypesWhen pulse inflexion is applied to a binary symbol, the resulting binary waveform is called a PCM waveform. There are several types of PCM waveforms. These waveforms are often called line codes. When pulse pitch contour is applied to non-binary symbol, the resulting waveform is called an M-ary pulse inflexion waveform.The PCM waveforms fall into the following four groups.1) Non return to zero(a) (NRZ)2) Return to zero (RZ)3) anatomy encoded) Multilevel binaryThe NRZ group is probably the most normally used PCM waveform.In choosing a waveform for a particul ar application, some of the parameters worth examining are1) DC component2) Self clocking3) fracture detection) Bandwidth compression5) Differential encoding6) Noise immunityThe most common criteria used for comparing PCM waveforms and for selecting one waveform type from many available are1) Spectral characteristics2) Bit synchronization capabilities3) Error detection capabilities) Interference5) Noise immunity6) Cost and complexity of implementationBits per PCM playscript and Bits per SymbolEach analog sample is transformed into a PCM forge up to group of bits. The be of quantization levels allowed for each sample can describe the PCM word size this is identical to the turn of events of values that the PCM word can assume. We useL=2lWhere L is the number of quantization levels in PCM word, l is the number of bits mandatory to represent those levels.M-ARY heart rate Modulation WaveformsThere are three basic ways to modulate information onto a sequence of pulses we can vary th e pulses amplitude, position, or duration. This leads to the call1) PAM (pulse amplitude changeover)2) PPM (pulse position modulation)3) PDM/PWM (pulse duration modulation/ pulse width modulation)When information samples without any quantization are play on to the pulses, the resulting pulse modulation can be called analog pulse modulation. When the information samples are first quantized, yielding symbols from an M-ary alphabet set, and the modulation on to pulses, the resulting pulse modulation is digital and we refer to it as M-ary pulse modulation.Base-band modulation with pulses has analogous counterparts in the area of band-pass modulation. PAM is similar to amplitude modulation, while PPM and PDM are similar to phase and frequency modulation respectively.Spectral DensityThe ghostly density of a signal characterizes the distribution of the signals energy or power in the frequency domain. capacity Spectral DensityWe can relate the energy of a signal deported in time domain to the energy expressed in frequency domain asEx = x(t) dt-= X (f) df-Where X (f) is the Fourier transform of the non periodic signal x (t).Let (t) = X (f) Ex = 2 x (f) df-Power Spectral DensityThe power spectral density function Gx (t) of the periodic signal x (t) is real, even and nonnegative function of frequency that gives the distribution of the power of x (t) in the frequency domain.Gx (t) = Cn (f-nfo)n =-PSD of a periodic signal is a trenchant function of frequency.Px = Gx (t) df-= 2 Gx (F) df0If x (t) is a non-periodic signal it cannot be expressed by a Fourier series, and if it is a non-periodic power signal (having infinite energy) it may not have a Fourier transform. However we still express the PSD of such signals in a limiting sense.Chapter 2 Modulation and Demodulation2.0 Modulation and DemodulationSince the early days of electronics, as advances in technology were taking place, the boundaries of both local and global communication began eroding, resulting in a world that is small and hence more easily accessible for the sharing of knowledge and information. The pioneering fix by Bell and Marconi formed the cornerstone of the information age that exists directly and paved the way for the future of telecommunications.Traditionally, local communication was done over wires, as this presented a cost-effective way of ensuring a reliable transfer of information. For long-distance communications, transmission of information over radio waves was needed. Although this was convenient from a hardware standpoint, radio-waves transmission raised doubts over the corruption of the information and was often dependent on high-power transmitters to overcome weather conditions, large buildings, and interference from other sources of electromagnetic.The various modulation techniques offered different solutions in terms of cost-effectiveness and quality of received signals but until recently were still largely analog. Frequency modulation and phase modul ation presented certain immunity to noise, whereas amplitude modulation was simpler to demodulate. However, more recently with the advent of low-cost microcontrollers and the introduction of domestic mobile telephones and satellite communications, digital modulation has gained in popularity. With digital modulation techniques come all the advantages that traditional microprocessor circuits have over their analog counterparts. Any shortfalls in the communications link can be eradicated using software. Information can now be encrypted, error correction can ensure more self-assertion in received data, and the use of DSP can reduce the limited bandwidth allocated to each service.As with traditional analog systems, digital modulation can use amplitude, frequency, or phase modulation with different advantages. As frequency and phase modulation techniques offer more immunity to noise, they are the preferred scheme for the major(ip)ity of services in use today and will be discussed in deta il below2.1 Digital Frequency ModulationA simple variation from traditional analog frequency modulation can be implemented by applying a digital signal to the modulation input. Thus, the output takes the form of a wickedness wave at two distinct frequencies. To demodulate this waveform, it is a simple matter of passing the signal through two filters and translating the resultant back into logic levels. Traditionally, this form of modulation has been called frequency- swap keying (FSK).2.2 Digital Phase ModulationSpectrally, digital phase modulation, or phase- skunk keying, is very similar to frequency modulation. It involves changing the phase of the transmitted waveform sort of of the frequency, these finite phase changes representing digital data. In its simplest form, a phase-modulated waveform can be generated by using the digital data to switch between two signals of equal frequency but opposing phase. If the resultant waveform is multiplied by a sine wave of equal frequency, two components are generated one cosine waveform of double the received frequency and one frequency-independent term whose amplitude is proportional to the cosine of the phase shift. Thus, filtering out the higher-frequency term yields the original modulating data prior to transmission.* Modulate and demodulate/detect blocks together are called a modem.* The frequency down conversion is performed in the front end of the demodulator.* Only formatting, modulation, demodulation/detection and synchronization are essential for a digital communication system.* FORMATTING transforms the source information into bits.* From this point up to pulse modulation block, the information remains in the form of a bit stream.* Modulation is the process by which message symbols or channel symbols are converted to waveforms that are compatible with the requirements imposed by transmission channel. Pulse modulation is an essential step because each symbol to be transmitted must first be transformed from a binary original to a base band waveform.* When pulse modulation is applied to binary symbols, the resulting binary waveform is called a PCM waveform. When pulse modulation is applied to non-binary symbols, the resulting waveform is called an M-ary pulse modulation waveform.* Band pass modulation is required whenever the transmission medium will not support the university extension of pulse like waveforms.* The term band pass is used to indicate that the base band waveform gi (t) is frequency translated by a carrier wave to a frequency that is much larger than the spectral content of gi (t).* Equalization can be described as a filtering option that is used in or after the demodulator to reserve any degrading effects on the signal that were caused by the channel. An equalizer is implemented to redress for any signal distortion caused by a no ideal hi(t)* Demodulation is defined as a convalescence of a waveform (band pass pulse) and detection is defined as decision-making regardi ng the digital meaning of that waveform.2.3 Linear Modulation Techniques* Digital modulation techniques may be broadly classified as one-dimensional and non- running(a). In linear modulation techniques, the amplitude to the modulation signal S (t) varies linearly with the modulating digital signal m (t).* Linear modulation techniques are bandwidth efficient.* In a linear modulation technique, the transmitted signal S (t) can be expressed asS (t) = Re Am (t) exp (j2pfct)= A mr(t)cos(2pfct) mI(t)sin(2pfct)WhereA is the amplitudefc is the carrier frequencym (t) = mr(t) + mI(t) is a complex windbag delegacy of the modulated signal which is in general complex form.* From the equations higher up, it is clear that the amplitude of the carrier varies linearly with the modulating signal.* Linear modulation schemes, in general do not have a constant envelope.Linear modulation schemes have very good spectral efficiency.Normalized Radian FrequencySinusoidal waveforms are of the formX (t) = Acos (wt+f) - (1)If we sample this waveform, we obtainXn =x (nTs)=Acos (wnTs+f)=Acos (wn+f) (2)Where we have defined w to be Normalized Radian Frequencyw=wTsThe Signal in (2) is a decided time cosine signal, and w is the discrete time radian frequency. w has been normalized by the sampling period. w has the units of radians/second, w=wTs has the units of radians i.e. wis a dimensionless quantity. This is solely consistent with the fact that the index n in xn is a dimensionless. formerly the samples are taken from x (t), the time scale information is lost. The discrete time signal is just a sequence of numbers, and these numbers carry no information about the sampling period, which is the information required to speculate the time scale. Thus an infinite number of continuous time curved signals can be transformed into the same discrete time sine curve by sampling. All we need to is to change the sampling period with changes in frequency of the continuous time sinusoid.2.4 Bas eband TransmissionBaseband Demodulation/Detection The filtering at the transmitter and the channel typically cause the received pulse sequence to have from ISI (Inter Symbol Interference), therefore the signal is not quiet ready for sampling and detection. The goal of the demodulator is to recover the pulse with best possible signal to noise ratio (SNR), free of any ISI. Equalization is a technique used to help accomplish this goal. Every type of communication channel does not require the equalization process. However equalization process embodies a sophisticated set of signal processing techniques, making it possible to compensate for channel induced interference. A received band pass waveform is first transformed to a base band waveform to begin with the final detection step takes place. For liner systems, the mathematics of detection is untouched by a shift in frequency.* According to the equivalence theorem, all linear signal-processing simulations can take place at base ban d (which is preferred for simplicity) with the same result as at band pass. Thus the performance of most digital communication systems will often be described and analyzed as if the transmission channel is a base band channel.Chapter 3 p/4 Quadrature3.0 p/4 Quadrature Phase Shift Keying (p/4 QPSK)3.1 Linear Modulation Techniques* Digital modulation techniques may be broadly classified as linear and non-linear. In linear modulation techniques, the amplitude to the modulation signal S (t) varies linearly with the modulating digital signal m (t).* Linear modulation techniques are bandwidth efficient.* In a linear modulation technique, the transmitted signal S (t) can be expressed asS (t) = Re Am (t) exp (j2pfct)= A mr(t)cos(2pfct) mI(t)sin(2pfct)WhereA is the amplitudefc is the carrier frequencym (t) = mr(t) + mI(t) is a complex envelope representation of the modulated signal which is in general complex form.* From the equations above, it is clear that the amplitude of the carrier var ies linearly with the modulating signal.* Linear modulation schemes, in general do not have a constant envelope.Linear modulation schemes have very good spectral efficiency.There are three major classes of digital modulation techniques used for transmission of digitally represented data* bounteousness-shift keying (ASK)* Frequency-shift keying (FSK)* Phase-shift keying (PSK)All convey data by changing some tantrum of a base signal, the carrier wave, (usually a sinusoid) in response to a data signal. In the case of PSK, the phase is changed to represent the data signal. There are two fundamental ways of utilizing the phase of a signal in this way* By think the phase itself as conveyance the information, in which case the demodulator must have a reference signal to compare the received signals phase against or* By viewing the change in the phase as conveying information differential schemes, some of which do not need a reference carrier (to a certain extent).A convenient way to represent PSK schemes is on a conformation diagram. This shows the points in the Argand plane where, in this context, the real and complex quantity axes are termed the in-phase and quadrature axes respectively due to their 90 separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point on the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature axis to modulate a sine (or cosine) wave.In PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption. They are positioned on a circle so that they can all be transmitted with the same energy.In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for the cosine and sine waves. Two common examples are binary phase-shift keying (BPSK) which uses two phases, and quadrature phase-shift keying (QPSK) which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is usually designed with the number of constellation points being a power of 2.3.2 Amplitude Shift Keying (ASK)Amplitude shift keying ASK in the context of digital communications is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels adopted by the digital message.For a binary message sequence there are two levels, one of which is typically zero.Thus the modulated waveform consists of bursts of a sinusoid. In Amplitude Shift Keying the Amplitude varies whereas the phase and frequency remains the same as shown in following .One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it has not got a constant envelope. This makes its processing (eg, power amplification) more dif ficult, since linearity becomes an important factor. However, it does make for ease of demodulation with an envelope detector.Thus demodulation is a two-stage process Recovery of the band limited bit stream Regeneration of the binary bit stream3.3 Frequency-shift keying (FSK)Frequency-shift keying (FSK) is a rule of transmitting digital signals. The two binary states, logic 0 (low) and 1 (high), are each represented by an analog waveform. Logic 0 is represented by a wave at a specialized frequency, and logic 1 is represented by a wave at a different frequency. In frequency Shift Keying the frequency varies whereas the phase and amplitude remains the same.Phase shift keying (PSK)Phase Shift Keying (PSK) was create during the early days of the deep-space program. PSK is now widely used in both military and commercial communication systems.In phase shift Keying the phase of the transmitted signal varies whereas the amplitude and frequency remains the same.The general conceptualizat ion for the PSK is asWhere,ji(t) = the phase term will have M discrete values, given by,ji(t) = 2pi /M3.4 Binary PSKIn binary phase shift keying we have two bits represented by the following waveformsS0(t) = A cos (wt) represents binary 0S1(t) = A cos (wt + p) represents binary 1For M-array PSK, M different phases are required, and every n (where M=2n) bits of the binary bit stream are coded as one signal that is transmitted asA sin (wt + qj)where j=1,.., M3.5 Quadra phase-Shift ModulationTaking the above concept of PSK a stage further, it can be assumed that the number of phase shifts is not limited to only two states. The transmitted carrier can undergo any number of phase changes and, by multiplying the received signal by a sine wave of equal frequency, will demodulate the phase shifts into frequency-independent voltage levels.This is indeed the case in quadraphase-shift keying (QPSK). With QPSK, the carrier undergoes four changes in phase (four symbols) and can thus represent 2 binary bits of data per symbol. Although this may seem insignificant initially, a modulation scheme has now been supposed that enables a carrier to transmit 2 bits of information instead of 1, thus effectively doubling the bandwidth of the carrierEulers relations state the followingNow consider multiplying two sine waves together, thusFrom Equation 1, it can be seen that multiplying two sine waves together (one sine being the incoming signal, the other being the local oscillator at the receiver mixer) results in an output frequency double that of the input (at half the amplitude) lay over on a dc offset of half the input amplitude.Similarly, multiplying by gives which gives an output frequency double that of the input, with no dc offset.It is now middling to make the assumption that multiplying by any phase-shifted sine wave yields a demodulated waveform with an output frequency double that of the input frequency, wh