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OVERVIEW OF DIFFERENT MODULATION TECHNIQUESBASICS OF MODULATION By Ravi Kumar (21-Aug-2012)

CONTENTSModulationModulation TechniquesAnalog ModulationDigital ModulationDemodulation and DetectionModulation SummaryFactors Affecting Choice of ModulationModulation Trade-OffsComparisons of Digital ModemModem Block DiagramReferences

MODULATION

Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted.

The three key parameters of a periodic waveform are its amplitude (volume), its phase (timing) and its frequency (pitch).

Modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted.

Modulation of a sine waveform is used to transform a baseband message into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal)

MODULATION

The technique of superimposing the message signal on the carrier is known as modulation.

That is, modulation is the process by which a parameter of one signal (carrier) is varied in proportion to the second signal (message signal).

Let m(t) = message (or information) signal c(t) = carrier signal s(t) = modulated signal (transmitted signal)

m(t) s(t) Modulating Modulated

c(t) CarrierModulator

MODULATION

The carrier c(t) is a pure sinusoidal signal generally given as

Examination of c(t) indicate that there are 3 parameters which may be varied:AmplitudeFrequency Phase

These parameters can be varied in Analog or Digital form. When varied in digital form, it is referred to as Shifting and Keying

MODULATION

Modulation is the process of facilitating the transfer of information over a medium.

E.g., sound transmission in air has limited range for the amount of power your lungs can generate. To extend the range of your voice can reach, we need to transmit it through a medium other than air, such as a phone line or radio.

The process of converting information so that it can be successfully sent through a medium is called modulation.

It is important that we do not confuse mixing with modulation. Mixing occurs when two or more signals are simply combined in a linear network. Modulation requires one signal to control a variable of another signal.

MODULATION TECHNIQUES

All the modulation techniques vary a parameter of a sinusoid to represent the information which we wish to send.

A sinusoid has three different parameters that can be varied known as amplitude, frequency and phase.

Modulation is a process of mapping such that it takes your voice, converts it into some aspect of a sine wave and then transmits the sine wave, leaving the actual voice behind.

The sine wave on the other side is remapped back to a near copy of your sound.


The medium is the thing through which the sine wave travels. So wire is a medium and the sine wave is called the carrier.

The information to be sent, which can be voice or data is called the information signal.

Once the carrier is mapped with the information to be sent, it is no longer a sine wave and we call it as the signal.

The signal has the unfortunate luck of getting corrupted by noise as it travels.


Difference between Analog and Digital

There are three parts to a communication system.The information, also called the basebandThe mediumThe carrier

Information can be divided into two forms, digital or analog.

Analog signal is considered continuous. Its signal amplitude can take on any number of values between the signal maximum and minimum.


Voice is analog and can take any number of volume levels between its dynamic-range which is the range of volumes your vocal cords can produce.

Digital devices convert analog voice to a digital signal by process of sampling and quantization. The analog signal is first sampled and then quantized in levels and then each level is converted to a binary number.

For example, we may quantize your voice in 16 levels. Each of these levels can be represented by four bits.

Perhaps you remember when your telephone system went to the tone dialing. It went from being a pure analog system to a digital system based on sampling and quantization.


The medium is the thing the signal travels through. It can be air, space or wires of all sorts.

Each of these mediums offers its own unique set of advantages and distortions that determine what is used as a carrier.

A short wire in a chip for example may not need a carrier at all.

A signal through space such as for satellite transmission may need a very high frequency carrier that can overcome space loss and other atmospheric losses.


If medium is considered as road, then carrier is the truck that carries the information hence we call it as carrier. It is a sinusoid in our case.

Depending on the medium, carrier will have a frequency appropriate to the medium. It can be light frequencies as in optical fiber or a microwave frequency as for mobile communications.

An electromagnetic carrier can be of any frequency depending on the medium and the communication needs.

Most mediums dictate what type of carrier (its frequency, amplitude) can propagate through it and the type of distortions it will suffer while travelling through it.


Anything that is wireless is analog - always. Wired signals can be digital or analog.

Communications inside a computer are examples of pure digital communications, digital data over digital medium.

LAN communications are digital data over analog medium. The AM and FM radios are examples of analog data over analog medium.

When two signals are mixed, they combine without the creation of any additional frequencies. When two signals are modulated they are said to beat with each other, creating additional frequencies called beat frequencies.


Signal modulation can be divided into two broad categories: analog modulation and digital modulation. Analog or Digital refers to how the data is modulated onto a sine wave.

If analog audio data is modulated onto a carrier sine wave, then this is referred to as analog modulation.

If analog audio data is sampled by an analog to digital converter (ADC) with the resulting digital bits modulated onto a carrier sine wave, this is digital modulation because digital data is being encoded.

Both analog modulation and digital modulation are performed by changing the carrier wave amplitude, frequency, or phase (or combination of amplitude and phase simultaneously) according to the message data.


Analog Modulation

The aim of analog modulation is to transfer an analog baseband signal, for example an audio or TV signal, over an analog bandpass channel at a different frequency (over a limited radio frequency band or cable TV network channel).

Using the message signal to vary amplitude, frequency, phase leads to three basic types of analog modulation schemes respectively known asAmplitude ModulationFrequency ModulationPhase Modulation

These types of modulation are carrier/continuous wave modulation. Frequency and Phase Modulation are also known as Angle Modulation.


Amplitude Modulation (AM) is a technique used in electronic communication for transmitting information using radio waves in which the amplitude of the carrier wave is varied in accordance with the amplitude of the information signal.

Amplitude Modulation is used whenever a shift in the frequency components of a given signal is desired.E.g., transmitting voice signal (3 kHz) through electromagnetic wave requires that 3 kHz be raised several orders of magnitude before transmission.

If the two modulated signals are sinusoidal, the beat frequencies will be the sum and the difference of the original frequencies.

AM radio broadcast transmissions contain two signals of primary importance to the user: the carrier signal and the audio signal, or the program signal.


The carrier frequency is the frequency to which the radio receiver is tuned for station selection.

For example, the AM radio band (broadcast band) is legally designated from 535 to 1605 kHz.

If your favorite local radio station broadcasts on 830 kHz, this means that the carrier frequency being used for transmission is 830 kHz.

The audio or program signal is riding on this carrier frequency.


When the AM signal was broadcast, the program signal modulated the amplitude, or the level of the carrier; this process formed an envelope of carrier amplitude, having the same shape as the program signal.

The beat frequencies, contained within the AM waveform will be the sum and the difference of the carrier and its program signal.


Amplitude Modulation (AM) is the oldest method of transmitting human voice electronically.

In an analog telephone conversation, the voice waves on both sides are modulating the voltage of the direct current loop connected to them by the telephone company.

Amplitude Modulation (AM) is also widely used to alter a carrier wave to transmit data.

For example, in AM radio, the voltage (amplitude) of a carrier with a fixed center frequency (the stations channel) is varied (modulated) by the analog audio signal.

MODULATION TECHNIQUESIn AM, the voltage (amplitude) of the carrier is varied by the incoming signal. In this example, the modulating wave implies an analog signal.


Examples

E.g1: If the program signal was a constant 5 kHz tone, with a carrier frequency of 600 kHz, the beat frequencies would be 595 kHz (diff freq) and 605 kHz (sum freq).

In a typical AM broadcast, the program signal will contain vocal and music information, making up a very wide range of frequencies.

The highest frequency of this range of frequencies, will determine the maximum separation of the beat frequencies from the carrier.


E.g2: If the highest frequency in the program signal was limited to 1 kHz, then the beat frequencies would be 599 and 601 kHz.

However as we can see from the earlier example, when the highest frequency is limited to 5 kHz, the width (distance from the carrier frequency) increases.

The range of beat frequencies above (and below) the carrier frequency are called sidebands.

The width of the sidebands is closely monitored at AM broadcast stations because, if they become too wide, they can interfere with adjacent stations.

MODULATION TECHNIQUESAmplitude Modulation Signal waveform


There are four kinds of Amplitude Modulation techniques namely;

Conventional Amplitude Modulation Carrier + Upper Sideband + Lower Sideband

Double Sideband Modulation (DSB) Suppressed Carrier (SC)/Reduced Carrier (RC) Upper Sideband + Lower Sideband

Single Sideband Modulation (SSB) Only one Sideband (Upper or Lower)

Vestigial Sideband (VSB) Upper Sideband + portions of the Lower Sideband

MODULATION TECHNIQUESSpectrum and Waveforms of Conventional-, DSBSC- & SSBSC-AM SignalsLSB = Lower SidebandUSB = Upper SidebandFc = Carrier FrequencyFm = Information Frequency


Vestigial Sideband (VSB) Upper Sideband + portions of the Lower SidebandA vestigial sideband is a sideband that has been only partly cutoff or suppressed.

Television broadcasts (in analog video formats) use this method if the video is transmitted in AM, due to large bandwidth used.

It may also be used in digital transmission, such as the ATSC standardized 8-VSB.

The video baseband signal used in TV in countries that use NTSC or ATSC has a bandwidth of 6 MHz.

To conserve bandwidth, SSB would be desirable, but the video signal has significant low frequency content (avg brightness) and has rectangular synchronizing pulses. The engineering compromise is VSB modulation.


In VSB the full upper sideband of bandwidth W2 = 4 MHz is transmitted, but only W1 = 1.25 MHz of the lower sideband is transmitted along with a carrier.

This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies.

The absence of the lower sideband components at high frequencies must be compensated for, and this is done by the RF and IF filters.


Frequency Modulation (FM) is a method of transmitting information by varying the frequency of the carrier wave in accordance with the amplitude of the input signal, the amplitude of the carrier remaining unchanged.

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.

FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape-recording systems and some video-transmission systems.

In radio systems, FM with sufficient bandwidth provides an advantage in cancelling naturally-occurring noise.


Frequency Modulation (FM) radio signals also have a carrier signal and a program signal. However, the program signal does not ride on the carrier frequency, it is contained within frequency variances modulated into the carrier signal.

Because the program signal is not dependant on carrier amplitudes (as are AM transmissions), FM radio is largely immune to many forms of interference.

MODULATION TECHNIQUESA signal modifies the frequency of a carrier in FMIn FM, the frequency of the carrier wave is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

MODULATION TECHNIQUESA low-frequency message signal (top) may be carried by an AM or FM radio waveDifference between Amplitude and Frequency Modulation


Phase Modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.

PM is not very widely used for radio transmissions, because it tends to require more complex receiving hardware and there can be uncertainty problems in determining whether the signal has changed phase by +180 deg or -180 deg.

Phase is a property which compares the starting point of two signals. Two signals are said to be in phase when the timing of their starting points coincide.

For small amplitude signals, PM is similar to AM and exhibits its unfortunate doubling of baseband bandwidth and poor efficiency.

For a single large sinusoidal signal, PM is similar to FM and its bandwidth is approximately.

MODULATION TECHNIQUESTop fig shows the modulating signal superimposed on the carrier wave. Bottom fig shows the resulting phase-modulated signalIn PM, the angle of the carrier wave is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

MODULATION TECHNIQUESDifference between Amplitude, Frequency and Phase Modulation


Digital Modulation

The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

The purpose of digital modulation is to convert an information-bearing discrete-time symbol into a continuous-time waveform.

The objective of a digital communication system is to transport digital data between two or more nodes.


In radio communications this is usually achieved by adjusting a physical characteristic of a sinusoidal carrier, either the frequency, phase, amplitude or a combination of thereof.

This is performed in real systems with a modulator at the transmitting end to impose the physical change to the carrier and a demodulator at the receiving end to detect the resultant modulation on reception.

In digital modulation, an analog carrier signal is modulated by a discrete signal.

Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion.


Symbols, Bits and Bauds

A symbol is quite apart from a bit in concept although both can be represented by sinusoidal or wave functions.

Where bit is the unit of information, the symbol is a unit of transmission energy. It is the representation of the bit that the medium transmits to convey the information.

Imagine bits as widgets, and symbols as boxes in which the widgets travel on a truck. We can have one widget per box or we can have more.

Packing of widgets (bits) per box (symbols) is what modulation is all about.

MODULATION TECHNIQUESIn communications, the analog signal shape, by pre-agreed convention, stands for a certain number of bits and is called a symbol.

Digital information travels on analog carrier.


A symbol is just a symbol. It can stand for any number of bits, not just one bit.

The bits that it stands for are not being transmitted, what is transmitted is the symbol or actually the little signal packet. The frequency of this packet is usually quite high.

A baud is same as the symbol rate of a communication system. So if we send 200 bauds, then we are send 200 symbols per second.

All digital information is transmitted as a series of zeros (0) and ones (1). These two symbols are called Bits.

The number of Bits could be considered also as a unit to express the amount of information being transmitted. As an example, if a million symbols (0 & 1) were transmitted, it would imply 1 Mega Bit.


Bit Error Rate (BER): Better accuracy of the transmitted digital signal is measured by BER. Simply put Bit Error Rate is:

The number of Error BitsBER= ---------------------------- The total number of Bits

A lower Bit Error Rate implies that the signal has been more accurately transmitted and demodulated.

A Bit Error Rate of one error in 10,000 Bits transmitted is quite normal for modulated signals. After error correction is applied, the Error further falls down to one part in 100,000 Million Bits.


To help transmit digital bits over any significant distances, different modulation schemes have been devised.

The requirements for broadcasting a satellite signal would be rather than different from the requirements for transmitting a digital signal over a cable network.

The most fundamental digital modulation techniques are based on keying.

Keying is a family of modulation forms where the modulating signal takes one of two (or more) values at all times. The goal of keying is to transmit a digital signal over an analog channel.


There are three basic types of digital modulation techniques known as:Amplitude-Shift Keying (ASK)Frequency-Shift Keying (FSK)Phase-Shift Keying (PSK)

Amplitude-Shift Keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.Amplitude of the carrier is switched between two or more levels according to the digital data.


Any digital modulation scheme uses a finite number of distinct signals to represent digital data.

ASK uses a finite number of amplitudes, each assigned a unique pattern of binary digits. Usually each amplitude encodes an equal number of bits.

Each pattern of bits forms the symbol that is represented by the particular amplitude.

The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the amplitude of the received signal and maps it back to the symbol it represents, thus recovering the original data.

Frequency and Phase of the carrier are kept constant.


In ASK, the amplitude of the carrier is changed in response to information and all else is kept fixed.

Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant.

On-Off Keying (OOK) is a special form of ASK, where one of the amplitudes is zero as shown below.Baseband Information Sequence - 0010110010Binary ASK (OOK) Signal


ASK in the context of digital signal 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.Below fig shows a binary ASK signal (lower), together with the binary sequence which initiated it (upper). Neither of the signal has been band limited.An ASK signal (below) and the message (above)


There are sharp discontinuities shown at the transition points. These result in the signal having an unnecessarily wide bandwidth.

Band limiting is generally introduced before transmission, in which case these discontinuities would be rounded off.

The band limiting may be applied to the digital message or the modulated signal itself.

Like Amplitude Modulation (AM), Amplitude-Shift Keying (ASK) is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc.


Both ASK modulation and demodulation processes are relatively inexpensive.

The ASK technique is also commonly used to transmit digital data over optical fiber.

For LED transmitters, binary 1 is represented by a short pulse of light and binary 0 by the absence of light.

Laser transmitters normally have a fixed bias current that causes the device to emit a low light level. This low level represents binary 0, where a higher-amplitude light wave represents binary 1.


ASK techniques are most susceptible to the effects of non-linear devices which compress and distort signal amplitude.

To avoid such distortion, the system must be operated in the linear range, away from the point of maximum power where most of the non-linear behavior occurs.

Despite this problem in high frequency carrier systems, Amplitude-Shift Keying is often used in wire-based radio signaling, both with or without a carrier.

MODULATION TECHNIQUESASK is a simple technique that uses two voltage levels to represent 0 and 1.


Frequency-Shift Keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.

The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information.

With this scheme the 1 is called the mark frequency and the 0 is called the space frequency.

In FSK, the instantaneous frequency of the carrier is switched between 2 or more levels according to the baseband digital data.


Frequency-Shift Keying (FSK) is a method 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 specific frequency and logic 1 is represented by a wave at a different frequency. In FSK, the bit affect the frequency of a carrier sinusoid.


In FSK, we change the frequency in response to information, one particular frequency for a 1 and another frequency for a 0.

In the example below, frequency f1 for bit 1 is higher than f2 used for the bit 0.

MODULATION TECHNIQUESAn example of binary FSKFor digital signals, FSK uses two frequencies for 0 and 1 as in this example.


FSK is used in all single-channel, radiotelegraph systems that use automatic printing systems.

The advantage of FSK over on-off keyed CW is that it rejects unwanted signals (noise) that are weaker than the desired signal.

This is true of all FM systems. Since a signal is always present in the FSK receiver, automatic volume control methods may be used to minimize the effects of signal fading caused by ionosphere variations.


The terms coherent and incoherent are frequently used when discussing the generation and reception of digital modulation.

When linked to the process of modulation the term coherence relates to the ability of the modulator to control the phase of the signal, not just the frequency.

For example, FSK can be generated both coherently with an IQ modulator and incoherently with simply a Voltage Controlled Oscillator (VCO) and a digital voltage source as shown below.


With the system in figure 1(a) the instantaneous frequency of the output waveform is determined by the modulator (within a tolerance set by the VCO and data amplitude etc) but the instantaneous phase of the signal is not controlled and can have any value.

Alternatively coherent generation of modulation is achieved as shown in below figure 1(b). Here the phase of the signal is controlled, rather than the frequency.


When a coherent modulator is used to generate FSK the exact signal frequency and phase are controlled.

The modulated shown in figure 1(b) offers the possibility to shape the resultant carrier phase trajectory at baseband either with analog filtering or digital signal processing and a DAC.

This can be used to generate both constant amplitude and amplitude modulated signals.

Use of the term coherent with respect to the act of demodulation refers to a system that makes a demodulation decision based on the received signal phase, not frequency. The additional information available results in an improved BER performance.


Phase-Shift Keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits.

Usually each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.


The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data.

This requires the receiver to be able to compare the phase of the received signal to a reference signal such a system is termed as coherent (CPSK).

In PSK, the phase of the carrier signal is switched between 2 or more values in response to the baseband digital data.


In PSK, we change the phase of the sinusoidal carrier to indicate information. Phase in this context is the starting angle at which the sinusoid starts.

To transmit 0, we shift the phase of the sinusoid by 180 degrees. Phase shift represents the change in the state of the information in this case.Binary PSK Carrier (180 degree phase shifts at bit edges)

MODULATION TECHNIQUESFor digital signals, PSK uses two phases for 0 and 1 as in this example.


PSK is a digital modulation scheme that conveys data by changing, or modulating the phase of a reference signal (the carrier wave)PSK is a technique which shifts the period of a wave.This wave has a period of p, noted above. Also notice that the start of the waves period is at 0.This is the same wave as the first, but its phase has been shifted. Notice that the period starts at the waves highest point 1.


We have shifted this wave by one quarter of the waves full period.We can shift it another quarter, if we want to, so the original wave would be shifted by half its period.

And we could do it one more time, so that it would be shifted three quarters of its original period.

This means we have 4 separate waves. So we can let each wave stand for some binary value. Since there are 4, we can let each wave signify 2 bits (00, 01, 10, 11).

Bit value amount of shift00 None01 10 11


This technique of letting each shift of a wave represent some bit value is phase shift keying. But the real key is to shift each wave relative to the wave that came before it.


Alternatively, instead of operating with respect to a constant reference wave, the broadcast can operate with respect to itself.

Changes in phase of a single broadcast waveform can be considered the significant items. In this system, the demodulator determines the changes in the phase of the received signal rather than the phase (relative to reference wave) itself.

Since this scheme depends on the difference between successive phases, it is termed as Differential Phase-Shift Keying (DPSK).

DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme).


Difference between ASK, FSK and PSK Modulation


Nyquist and Root-Raises Cosine Filters The Nyquist bandwidth is the minimum bandwidth that can be used to represent a signal.

It is important to limit the spectral occupancy of a signal, to improve bandwidth efficiency and remove adjacent channel interference.

Root raised cosine filters allow an approximation to this minimum bandwidth.Nyquist bandwidth on the QPSK spectrum


Quadrature Phase-Shift Keying (QPSK) is effectively two independent BPSK systems (I-In phase and Q-Out of phase) and therefore exhibits the same performance but twice the bandwidth efficiency.

Sometimes this is known as quaternary PSK, quadriphase PSK, 4-PSK, or 4-QAM (although the root concepts of QPSK and QAM are different, the resulting modulated radio wave are exactly the same.)

QPSK uses four points on the constellation diagram, equispaced around a circle.With 4 phases, QPSK can encode two bits per symbol to minimize the BER sometimes misperceived as twice the BER of BPSK.


Constellation Diagram is a graphical representation of the complex envelope of each possible symbol state.The X-axis represents the in-phase component and the y-axis the quadrature component of the complex envelope.The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them.QPSK Constellation Diagram


Even though if we create the BPSK modulated signal by stringing together the appropriate packet of signals, in real systems, we can not create a modulated carrier this way.

What we have at our disposal are oscillators that can produce continuous sine and cosine waves. We can not just use a certain part of the signal as if it was sitting on a shelf for us to grab the needed piece.

We need a way to create a signal packet of a particular phase when needed out of a free-running sine or cosine.

This is where Quadrature Modulation with I and Q channels come into play.


I and Q channels are not just concepts but also how modulators are designed. However, the signal created by I and Q channels is not what is transmitted, it is the sum or the difference of these two and that is the real modulated signal.

In QPSK, the bit transmission in I - & Q-channels occur simultaneously.

MODULATION TECHNIQUESQPSK uses four phases 90 degrees apart to represent four values in each two bits (0-4) of input.


QPSK signal in the time domain: The modulated signal is shown below for a short segment of a random binary data-stream.Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.


The two carrier waves are a cosine and a sine wave, as indicated by the signal space analysis. Here the odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quadrature component.

The total signal the sum of the two components is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period.

The topmost waveform alone matches the description given for BPSK.

The binary data that is conveyed by this waveform is: 11 00 01 10.The odd bits, highlighted here, contribute to the in-phase component: 11 00 01 10The even bits, highlighted here, contributed to the quadrature-phase component: 11 00 01 10

MODULATION TECHNIQUESAn arbitrary modulated signal which shows phase shifts at each time tick.


More advanced modulation techniques convey multiple bits of information simultaneously by providing multiple states in each symbol of transmitted information. This helps transmit more digital data.

Quadrature Phase-Shift Keying (QPSK) conveys 2 bits per symbol and is prevalent in satellite communication.

Digital (DVB-S) satellite broadcasts universally use Phase Modulation-actually QPSK

Satellite transmissions have a few unique characteristics

The signal has to travel an extremely large distance (36000 km) from the ground to the satellite and then another similar distance back to the earth.


The signal from the satellite experiences an attenuation of approximately 200 dB before it reaches a dish antenna on the ground.

The satellite transmission is subjected to a broadband noise which is practically uniform at all frequencies.

Since multiple channels are broadcast from the same satellite, the modulation technique should not be prone to Inter Channel Interference.

A satellite transponder has a fairly large bandwidth. Full transponders often have a bandwidth of 72 MHz with some broadcasters utilizing only a half transponder bandwidth of 36 MHz.


The trend now is shifting towards a half transponder bandwidth of 27 MHz.

This is still a fairly wide bandwidth, particularly when compared with the 7 or 8 MHz allotted to a channel on a cable system.

Hence a Digital Modulation technique used for Satellite Broadcasting (DVB-S) can use a fairly large bandwidth but should be capable of preserving the signal and maintaining a low BER even for very low signal strength.

The QPSK Modulation system provides an ideal solution for this.

Quadrature Phase-Shift Keying is a very simple but robust form of Digital Modulation. While the name sounds extremely elaborate it is fairly straight forward.


The word Quadrature simply means Out of Phase by 90 degrees.

Hence as shown in below fig, QPSK provides for 4 different states or possibilities for encoding a Digital Bit.

This is because 2 components are used one In Phase (I) & the other Out of Phase or Quadrature (Q). This doubles the number of possible variations, from 2 to 4, that simple PSK offers.

MODULATION TECHNIQUES DVB-S Transmission System


Constant Envelope ModulationQPSK is part of a class of signals called constant-envelope signals.

There is no rigorous definition of a constant envelope signal. One definition is; when sampled at the symbol rate, the sampled value of amplitude is constant.

Another is that there are no discontinuous phase changes. Yet another is that the max and min amplitude attained by the signal over one period is constant.

The sine wave is an ideal constant envelope signal.

QPSK is not technically a constant envelope because of its discontinuous phase shifts but is considered nearly so.


Types of QPSK

Conventional QPSK has transitions through zero (i.e. 180 deg 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 90 degrees.In -QPSK the set of constellation points are toggled each symbol, so transitions through zero can not occur. This scheme produces the lowest envelope variations.All QPSK schemes require linear power amplifiers.


Offset QPSK (OQPSK) is a minor but important variation on QPSK.

In OQPSK, the Q channel is shifted by half a symbol time so that I and Q channel signals do not transition at the same time.

The result of this simple change is that phase shifts at any one time are limited and hence offset QPSK is more constant envelope than straight QPSK.

In high power amplifiers and for certain satellite applications, Offset QPSK offers better performance.

Although in a linear channel its BER is the same as QPSK, in non-linear applications, its BER is lower when operating close to the saturation point of the transmitting amplifier. Offset QPSK is also called as Staggered QPSK (SQPSK).

MODULATION TECHNIQUESQPSK modified to become OQPSK

MODULATION TECHNIQUES I and Q channel mappings of an Offset QPSK signal, the symbol transitions do not occur at the same time

MODULATION TECHNIQUESOQPSK All phase shifts are 90 degreesQPSK Note the 180 degree phase shiftThe phase jumps at the symbol transition for OQPSK are smaller


Note that the OQPSK signal never transitions more than 90 degrees. QPSK on the other hand goes through phase change of 180 degrees for some transitions.

The larger transitions are a source of trouble for amplifiers and to be avoided if possible.

In satellite transmission, QPSK reigns supreme, it is easy to build and operate.

How OQPSK differs from QPSK: The Q channel of OQPSK is delayed by a half symbol time, staggering the two quadrature channels.


In OQPSK, I- channel (or Q-channel) bit stream is offset by one bit period w.r.t. the Q-channel (or I-channel) prior to multiplication by the carrier.


Quadrature Amplitude Modulation (QAM) is both an analog and digital modulation scheme.

It conveys two analog messages, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the ASK digital modulation scheme or AM analog modulation scheme.

The two carrier waves, usually sinusoids are out of phase with each other by 90 degrees and are thus called quadrature carriers or quadrature components.

The modulated waves are summed and the resulting waveform is a combination of both PSK and ASK or in the analog case of PM and AM.


In the digital QAM case, a finite number of at least two phases and two amplitudes are used.

PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant.

In digital QAM, the input stream is divided into groups of bits based on the number of modulation states used.

For example, in 8QAM each three bits of input, which provides 8 values (0-7) alters the phase and amplitude of the carrier to derive 8 unique modulation states.

In 64QAM, each 6 bits generates 64 modulation states; in 128QAM, each 7 bits generates 128 states, and so on.

MODULATION TECHNIQUESAnalog QAM modulates 2 carriers 90 degrees out of phase with each from 2 analog input streams. The modulated carriers are combined and transmitted.In this 8QAM example, 3 bits of input generate 8 different modulation states (0-7) using 4 phase angles on 90 degree boundaries and 2 amplitudes: one at 50% modulation; the other at 100% (4 phases x 2 amplitudes = 8 modulation states). QAM examples with more modulation states become extremely difficult to visualize.


The modulation equation for QAM is a variation of the one used for PSK. The generalized PSK allows changing both the amplitude and the phase.

In PSK all points lie on a circle so the I and Q values are related to each other. PSK signals are constant envelope because of this.

All points have the same amplitude. If we allow the amplitude to change from symbol to symbol, then we get a modulation called QAM.

It can be considered as linear combination of two DSB-SC signals. So it is a AM and a PM modulation at the same time.

MODULATION TECHNIQUES QAM Constellation


QAM is used extensively as a modulation scheme for digital telecommunication systems.

Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel.

QAM modulation is being used in optical fiber systems as bit rates increase 16QAM and 64QAM can be optically emulated with a 3-path interferometer.


CATV Transmission QAM

QAM systems utilize changes of both, PSK and ASK to increase the number of states per symbol.

Each state is defined with a specific variation of both amplitude and phase.

This means that the generation and detection of symbols is more complex than a simple phase detection as in QPSK employed for Satellite Transmissions (DVB-S) because in QAM the amplitude changes have also to be detected.

QAM modulation is ideal for use in CATV networks.


A cable system provides different transmission characteristics compared to satellite transmissions.

A system such as QAM must be able to address the following needs, if it is to be successfully employed for Digital Modulation in a CATV system.

The bandwidth allocated per channel is restricted just 6 to 8 MHz (depending on the TV system such as PAL, NTSC or SECAM, employed).

Hence the digital modulation system must densely pack the digital data in a small bandwidth (unlike a satellite based transmission).


The signal levels are significantly higher than for satellite transmissions. Since the Carrier (signal strength) is larger, the Carrier to Noise (C/N) ratio is always fairly good in a CATV network.

A large number of channels are modulated and carried simultaneously on the same cable. Hence the modulation scheme should provide good Inter Channel Interference suppression.

Since the Phase and Amplitude are varied in QAM modulation, a large number of states or possible discrete values can be created to provide dense digital modulation. Hence designers have created 16 Bit and 64 Bit QAM Modulation.


Below Fig. 3a & 3b shows graphically 16QAM and 64QAM.

Each time the number of states or options per symbol is increased, the bandwidth efficiency also increases. This bandwidth efficiency is measured in bits per second/Hz.


As higher density modulation schemes are adopted, the Decoder or Demodulator gets progressively more complex.

A benefit of digital technology is that higher complexity does not necessarily mean a higher cost to the customer, since Large Scale Integration (LSI) ICs can be mass produced at reasonable cost, if a large demand exists.

MODULATION TECHNIQUESDVB-C Transmission System


Orthogonal Frequency-Division Multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies.

OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networks.

OFDM is a digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies.

FDM is the process by which the total bandwidth available to the system is divided into a series of non overlapping frequency sub-bands that are then assigned to each communicating source and user pair.


OFDM is a Frequency Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method.

Although FDM implies multiple data streams, orthogonal FDM carries only one data stream broken up into multiple signals. Hundreds or thousands of carriers, known as "subcarriers," are used for a single data channel. The data is divided into several parallel data streams or channels, one for each sub-carrier.

Each sub-carrier is modulated with a conventional modulation scheme (such as QAM or PSK) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.


Lower Speeds Easier DetectionThe multiple subcarriers enable the receiver to more easily detect the signals in environments with multipath and other interference.

In addition, each subcarrier can transmit a lower-speed signal, all of which are aggregated at the receiving side into the original high-speed signal.

Lower speed signals are also more easily decoded at the receiving end.

OFDM subcarriers can be modulated by any method, although QAM and QPSK are typically used. Coded OFDM (COFDM) adds forward error correction.


Terrestrial Transmission OFDM

The biggest concern for proper reception of terrestrial broadcast is multi path distortion, or Ghosts.

This happens when a signal arrives at the receiving antenna from multiple paths or directions. These multiple signals add up at the antenna, creating multiple images or Ghosts on the TV screen.

This distortion is most pronounced in densely populated cities particularly those with high rise buildings.


Analog transmission can not prevent Ghosts. The only hope is to realign the antenna to minimize the extent of ghosts.

Hence it was a top priority for engineers to device a Digital Modulation scheme that would eliminate any possibilities of ghosts images.

Further the terrestrially transmitted television signal should preferably not interfere with other terrestrial transmissions such as those for wireless radio etc.

To overcome these problems engineers have created a modulation scheme that appears to be extremely complex.


OFDM is a type of Frequency Multiplexing. In frequency multiplexing, multiple carriers are used at different frequencies as shown

Each carrier is separated by an unused band of frequencies called a Guard Band. Of course, the guard band is a waste of the bandwidth resource.

A Digital Terrestrial transmission (DVB-T) for a single television channel can utilize up to 8000 separate carriers.

Even a Digital Audio broadcast which requires much smaller amount of data to be transmitted compared to a Television channel, employs 1500 separate carriers.


To fit these large number of carriers into the typical 8 MHz bandwidth allocated for terrestrial broadcasts, engineers employed a further Orthogonal variation.

Orthogonal here refers to a phase difference of 90 degrees between two adjacent carriers as shown in below fig.


Using OFDM modulation, two adjacent carriers will overlap without causing any interference because the two carriers are out of phase by 90 degrees.

The overlapping of carriers avoid wastage of frequency bandwidth.

OFDM causes less interference to analog transmissions than an analog signal would, because it doesnt have the same strong carrier and subcarrier elements.

Also because there is a specific spacing between carriers of the same phase (guard interval), the signal is immune to multi path reflections or Ghosts.

Further, OFDM Modulation can be used in so called Single Frequency Networks (SFN), where a chain of transmitters can all use the same frequency for transmission.

MODULATION TECHNIQUESDVB-T Transmission System

DEMODULATION & DETECTION

Demodulation is the process of extracting the original information-bearing signal from a modulated carrier wave.

Techniques:There are several ways of demodulation depending on how parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency and phase.

For example, for a signal modulated with a linear modulation like AM, we can use a synchronous detector

On the other hand, for a signal modulated with an angular modulation, we must use an FM or a PM demodulator. Different kinds of circuits perform these functions.


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

Detection/Reception is the process of extracting the symbols from the waveform.

Basically there are two types of detection,Coherent detectionIn-coherent detection

DEMODULATION & DETECTION Digital Detection Techniques


Coherent Detection

An estimate of the channel phase and attenuation is recovered. It is then possible to reproduce the transmitted signal and demodulate.

Requires a replica carrier wave of the same frequency and phase at the receiver.

The received signal and replica carrier are cross-correlated using information contained in their amplitudes and phases.

Also known as synchronous detection.


Carrier recovery methods include:Pilot Tone (such as Transparent Tone in Band)Less power in the information bearing signalHigh peak-to-mean power ratio.Pilot Symbol Assisted ModulationLess power in information bearing signalCarrier Recovery (such as Costas loop)The carrier is recovered from the information signal.*A Costas loop is a phase-locked loop used for carrier phase recovery from suppressed-carrier modulation signals, such as from double-sideband suppressed carrier signals.

Coherent detection applicable toPhase Shift Keying (PSK)Frequency Shift Keying (FSK)Amplitude Shift Keying (ASK)


In-coherent Detection: Requires no reference wave, does not exploit phase reference information (envelope detection).

In the transmitter, each symbol is modulated relative to the previous symbol, for example in differential BPSK:0 = no change 1 = +180 degrees

In the receiver, the current symbol is demodulated using the previous symbol as a reference. The previous symbol act as an estimate of the channel

In-coherent detection is less complex than coherent detection (easier to implement), but has worse performance.

This is because the In-coherent detection system has two sources of error: a corrupted symbol and a corrupted reference (the previous symbol).

MODULATION SUMMARY

PSK is often used, as it provides a highly bandwidth efficient modulation scheme.

QPSK modulation is very robust, but requires some form of linear amplification. OQPSK and -QPSK can be implemented and reduce the envelope variations of the signal.

High level M-ary schemes (such as 64QAM) are very bandwidth-efficient, but more susceptible to noise and require linear amplification.

Constant envelope schemes (such as GMSK) can be employed since an efficient non-linear amplifier can be used.

Coherent reception provides better performances than differential, but requires a more complex receiver.

FACTORS AFFECTING CHOICE OF MODULATION

Signal-to-noise ratio (SNR)Portability of error or Bit Error Rate (BER)Power EfficiencyPower Efficiency is a measure of how much received power is needed to achieve a specified BER.As BER increases, PE decreases since transmitted power is wasted on more bad data.Bandwidth Efficiency (or Spectral Efficiency)Defined as the ratio of the bit rate to the channel bandwidthPerformance in multipath environmentEnvelope fluctuations and channel non-linearityImplementation cost and complexity

No modulation scheme possesses all the above characteristics; hence trade-offs are made when selecting modulation/demodulation schemes.

FACTORS AFFECTING CHOICE OF MODULATION

For example, in wireless communications it is important to select Modem based on the following requirementsHigh Spectral Efficiency High Power EfficiencyHigh Fading Immunity

These factors are affected by baseband pulse shape and phase transition characteristics of the signal.

MODULATION TRADE-OFFS

Similar to most engineering Trade-Offs, different Digital Modulation schemes too result in trade-offs.

Relatively simple modulation such as QPSK offer excellent BER performance at even very low signal strengths. QPSK however requires a large bandwidth.

QAM is very bandwidth efficient, but require strong signal strength for good BER. This is particularly so for the more dense bandwidth schemes such as 64QAM.

Channel data rate and range are inversely related (as range increases, data throughout decreases).

Alternatively, a large signal strength should be maintained, but providing booster amplifiers in the signal path, as is normal in a CATV network.

MODULATION TRADE-OFFS

Ensuring a strong signal strength is not easy for terrestrial transmissions (DVB-T).

The size of the receiving antenna cannot also be increased beyond reasonable limits. In view of this DVB-T employs OFDM Digital Modulation.

OFDM modulation requires a complex decoder, capable of receiving up to 8000 simultaneous carriers.

The level of each of these carriers is maintained very low, to insure that they do not interfere with other terrestrial transmissions.

In fact OFDM is so robust that it can be transmitted simultaneously, with an Analog TV transmission at the same frequency, without any interference.

COMPARISONS OF DIGITAL MODEM

For practical application, the choice of digital modem depends on:Bandwidth EfficiencyPower EfficiencyError PerformanceComplexity of implementation and CostProbability of symbol error or Probability of bit error is related to:Power EfficiencyBandwidth Efficiency (Spectral Efficiency)Usually transmitted power and complexity increases with increase in bandwidth efficiency.The linear or nonlinear nature of the channel also affect the choice of digital modem.

COMPARISONS OF DIGITAL MODEM

MODULATION AND DEMODULATION (MODEM)

REFERENCES

http://en.wikipedia.org/wiki/Modulation

www.complextoreal.com/chapters/mod1.pdf

http://www.ecgf.uakron.edu/ugweje/web/Research/Publication/NASAseminarLecture3.PDF

www.berk.tc/combas/digital_mod.pdf

www.scatmag.com/technical/Digital%20Modulation_sept07.pdf

www.pcmag.com/encyclopedia/

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