Comparison of different modulation techniques

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Abstract And Introdution

Modulation is the process in which one charateristic of a signal is varied according to some characteristic of a signal known as carrier is varied according to the instantaneous value of another signal known as modulating signal . In case of telecommunications, modulation may be used for the conveyance of a message. Whereas in radio communications , reception of electrical signals is best when the receiver and the transmitter are resonant tuned. Therefore if we keep the frequency content within the message signal very very possibly close to the frequency of resonance of the two will be ideal. Sometimes usage of sinusoid waveform with high frequency is used as a carrier signal so as to convey a low frequency signal. Key parameters of a sine wave are :- it's amplitude ("volume"), it's phase ("timing") and it's frequency ("pitch"). All this can be modified according to a low frequency info. signal in order to obtain a modulated signal.

The signals containing intelligence or information that are to be transmitted are known as modulating signals.This modulating signals containing info. are also all as baseband signals. Also the carrier frequency is greater than the modulating frequencies and the signals which results from the process of modulation is known as modulation signal. A device that is capable of doing both operations known as a modem (short-form of "Modulator-Demodulator").


Digital Modulation - The aim of this is the transfer of a digital bit stream over the analog channel's passband, for example In the public switched tele. networks (here a bandpass filter auss limiting of the frequency range to within 300 and 3400 Hz), or maybe over a radio limit frequencied band.

Analog Modulation - The aim of this is the transfer of an analog baseband (lowpass) signal, as in case of an audio signal , over an analog channel's passband, for instance suppose a limited radio frequencied band.

Both the above forms of modulation facilitate multiplexing on the basis of frequency ,that is, frequency division multiplxing (FDM), wherein many low pass info. signals are transmitted simultaneously over a same shared physical media, with the usage of separate passband channels.

Digital Baseband Modulation methods' aim , also known as line coding, is the transfer of a digital stream of bits over the baseband channel, which is typically a non-filtered wire of copper ,for example a serial bus.

Pulse Modulation methods's aim is the transfer of a narrowband analog signal, as for instance a phone call made over the wide baseband channel or, maybe in some schemes, as a stream of bits over another sytem involving digital transmission.

Analog Modulation Methods

In as of analog modulation, modulation is continuously applied as a response to analog info. signals.

A low-frequency info. signal (top) an be carried by AM or FM radio wave.

Th most common analog modulation techniques are as follows:

Amplitude modulation (AM) (wherein the amplitude of a signal known as the carrier signal is varied according to the instantaneous amp. of the baseband signal)

o Double-sideband modulation (or the DSB)

  • Double-sideband modulation having unsuppressed carrier (DSB-WC) (generally used in the AM radio broadcast band)
  • Double-sideband with suppressed-carrier System(DSB-SC)
  • Double-sideband with reduced carrier transmission System(DSB-RC)

Single-sideband modulation Technique(SSB),

  • SSB with carrier
  • SSB with suppressed carrier transmission (SSB-SC)

Vestigial sideband modulation System(VSB)

Quadrature amplitude modulation (or QAM)

Angle modulation

  • Frequency modulation (FM) (wherein the frequency of a signal known as the carrier signal is varied according to the instant. frequency of the basband signal)
  • Phase modulation (PM) (wherein the phase shift of a signal known as the carrier signal is varied according to the instant. phase shift of the signal)

Digital Modulation Methods

In this, a carrier signal(analog in nature) is modulated by a digital stream of bits. Digital modulation methods may be considered somewhat as (D to A)digital-to-analog conversion.The corresponding process of demodulation may be termed as analog-to-digital conversion. These changes in carrier are result from a finite number of M alternative available symbols (this is the modulation alphabet).

As per the definition of the digital signal, the modulated signal is a digital signal, and as per the another definition, this modulation is some form of digital-to-analog conversion.

Amplitude Modulation

This is a technique which is used in electronic communication, and most common cases for the transmission of info. through the radio carrier wave. This works by a variation of the strength of the signal which is transmitted in relation with the info. being sent. As for example, on changing the signal strength it can be be used to reflect the sounds that are to be reproduced by the speaker.

In mid-1870s, a form of AM—initially known as "undulatory currents"—was the first method for successful production of quality audio over the telephone lines. To begin with the 1906 audio demonstrations by Reginald Fessenden's, it was the original method that was used for transmission of audio radio, and it remains in use even today in many forms used for communication—"AM" requires the frequent use for the reference to mediumwave broadcast bands.

Forms Of Amplitude Modulation

In radio communication all that is modulated is a continuous form of a radio signal produced generally by radio transmitter. In the basic form, AM produces a signal in which a power concentrated at the frequency of the carrier, which are in 2 adjacent sidebands. The process used is called heterodyning. In this each sideband has a bandwidth equal to that of the baseband signal and is also a mirror image of other. An AM that results in 2 sidebands and a carrier can also be called double sideband amplitude modulated signal (DSB-AM). AM is very inefficient when power usage is considered and much of this is wasted. At least 2/3 of this power is concentrated within the carrier signal, that does not carry any useful info.; the power that is remaining is split among the two identical sidebands, although only 1 of these is required as they contain exactly the similar info. .

Double Sided Band With Supr Essed Carrier

In this type of transmission (DSB-SC) and modulation, as opposed to that in AM, the carrier wave is'nt transmitted; and thus, a greater percentage of the power which is dedicated to this is distributed among the sidebands. This implies that there is an increase of the cover in the case of a DSB-SC, as compared to AM, if the same power is used.

Single-Sideband Modulation(Ssb)

This is nothing new but a refined of AM , that can more efficiently use the electrical power and bandwidth. This is also very closely related to (VSB)vestigial sideband modulation.

Vestigial Sideband Modulation

This form is a sideband whose only some of the part has been cut off or has been suppressed. This method is used in television broadcasts , in case if the video is being transmitted in AM, which may be due to the large bandwidth used. It use may also be in case of digital transmission, for example the ATSC standardized 8-VSB.Another example is the Milgo 4400/48 modem which also used vestigial sideband and also made use of phase-shift keying in order to provide 4800-bit/s transmission over a 1600Hz channel.

Modulation Index

This quantity, may also be called as modulation depth, directly indicates that by how much the variable that has been modulated varies around the 'original' level assigned to it.In case of AM, this relates to the carrier amplitude's variation and maybe defined as:

Frequency Modulation

Frequency modulation (FM) conveys information(in the case of telecommunication) over the wave called carrier by varying the frequency (as opposed to that in amplitude modulation, wherein the amplitude of carrier was varied whereas its frequency remained constant). When it comes to analog applications, the carrier's instantaneous frequency is directly proportional to the input signal's instantaneous value. The data which is in digital format can be sent by shifting the frequency of the carrier among a discrete set of values that are discrete, a technique which is called frequency-shift keying.

Modulation Index

This quantity indicates that by how much the variable that has been modulated varies around the unmodulated level. It also relates to variations that are present in the frequency of the carrier:

wherein - highest frequency component that is present in the baseband signal xm(t),

- Peak frequency-deviation

If , the kind of modulation is known as narrowband FM, and it has a bandwidth of approximately .

If , the kind modulation is known as wideband FM and it has a bandwidth of approximately .

Whereas wideband FM's use more of the bandwidth. These can also improve the signal-to-noise ratio quite significantly.

While considering a tone-modulated FM signal, if we hold the modulation frequency constant and the index of modulation is increased,the part of the bandwidth that is non-negligible(of the FM signal) will increase, but we'll find that the spacing between spectras would remain the same. We thus find that some of the components in the spectra decrease in strength while others increase. If the deviation in the frequency is held constant and the frequency of modulation is increased, we‘ll see that the spacing between the spectra would increase.

Carson's Rule

It states that approx. all (~98%) the power of an (FM)frequency-modulated wave signal lies in a bandwidth of wherein - peak deviation of instant. freq. from the central carrier frequency .

Phase Modulation

This is the form of modulation which represents info. as variations in the carrier wave's instantaneous phase.

As opposed to its more known counterpart, frequency modulation (FM), the PM is quite not widely useful in radio transmissions. This is due to the fact that it tends to require a very very complex receiving set up of hardware and many ambiguous problems may arise in the determination of whether PM is used


Modulation Index

This quantity indicates that by how much the variable that has been modulated varies around the unmodulated level. It also relates to variations that exist in the phase of the carrier signal:

Wherein Δθ - peak phase deviation.

Comparison Between Fm And Pm

1. a. Frequncy Deviation is proportional to modulating voltage.

b. Phase Deviation is proportional to modulating voltage.

2. a. Associated with the change in fc , there is some phase change

b. Associated with the phase change ,there is some change in fc

3. a. mf is related to both modulating voltage and frequency fc

b. mp is proportion only to the modulating voltage.

4. a. It is possible to receive FM on a PM receiver.

b. It is possible to receive PM on a FM receiver.

5. a. Noise immunity is better than AM and PM.

b. Noise immunity is better than AM but worse than FM.

6. a. Amplitude of FM wave is constant.

b. Amplitude of PM wave is also constant.

7. a. Signal to noise ratio is better than that of PM.

b. Signal to noise ratio is inferior to that of FM.

8. a. FM is widely used.

b. PM is used in some mobile systems.

9. a. Frequency Deviation is proportional to modulating voltage . only.

b. Frequency Deviation is proportional to modulating voltage . and the modulating frequency also.

Comparison Between Am And Fm

1. a. Amplitude of FM wave is constant. It is independent of the . . modulation index . b. Amplitude of AM wave will change with modulating . . voltage.

2. a. Transmitted power remains constant. It is independent of mf

b. Transmitted power is dependent on modulation index.

3. a. All transmitted power is useful.

b. Carrier power and one sideband power are useless.

4. a. FM receivers are immune to noise.

b. AM receivers are not immune to noise.

5. a. It is possible to decrease noise further by increasing . . deviation.

b. This feature is absent in AM.

6. a. The bandwidth depends on modulation index.

b. It is not dependent on modulation index.

7. a. Bandwidth is large , hence wide channel is required.

b. Bandwidth is much less than FM.

8. a. Spacewave is used for propagation . So radius of . transmission is limited to line of sight

b. Ground wave and skywave propagation is used. Thus, a . larger area is covered than FM.

9. a. It is possible to operate several transmitters on one frequency

b. Not possible to operate more channels on the same frequency

10. a. FM transmission and reception equipments are complex.

b. AM equipments are less complex.

11. a. Number of sidebands having significant amplitude depends . . on the modulation index.

b. Number of sidebands in AM will be contant and equal to 2.

12. a. Info. is contained in frequency variation of the carrier

b. Info. is contained in the amplitude variation of the carrier.