What is a modulation? Need of modulation

Guru

Introduction

Within the telecommunications industry, modulation is an essential and creative technique used to carry invisible signals across the airways. The art of modifying waves to transmit information is a process that we frequently take for granted in our day-to-day interactions with contemporary technology. We set out to demystify modulation in this blog, covering its importance, different varieties, and crucial function in the smooth transfer of data.

The process of changing some characteristic (amplitude, frequency, phase,etc.) of a carrier by the signal (audio or video) is called ‘modulation’.‘Modulation’ means modification, variation or change. We modify the carrier according to the signal and hence the name.This is an important process of wireless communication.

Understanding the Basics

The process of altering a carrier wave in order to embed information is fundamental to modulation.Acting as the data conveyor is this carrier wave, which is usually a high-frequency signal. Effective air transmission of information can be achieved by adding information and adjusting specific properties of this carrier wave.

NEED FOR MODULATION

In carrier (wireless) transmission, modulation is a necessity. This is explained
below:
  • The first and the foremost reason is that the original sound produced by microphone (or video camera in case of video signal) is very weak and it has a very low frequency. The energy contained by the signal is proportional to its frequency. Thus due to losses in energy, the signal will die after some distance. So, it cannot travel long distance.Therefore, the low frequency signal is made to sit on high frequency ‘carrier’. Such an arrangement enables the signal to travel long distances before it dies out. At the receiver the signal is separated out and the carrier is grounded. The phenomenon can be illustrated by the following analogy. 
  • Suppose a man travels ‘on foot’ to deliver a message. Naturally he will take a long time to reach the destination; moreover, he cannot travel long distance. But if the man is provided a horse, the message can reach longer distance in shorter time. At the destination, the ‘receiver’ will take out the message and will leave the horse.
  • Assume here, the message as a signal, horse as a carrier and the receiver as the radio or TV receiver. This explains the principle of radio transmission and reception.
  • The next reason describes the height of the antenna needed.The transmitting antenna should have a height equal to the wavelength.This condition gives best results. We know that
V = fλ
Where V = velocity of radio waves = `3times10^8` m/s
f = frequency
λ = wave length
  • If the frequency of the signal is 20 kHz, the length of the antenna
l=λ=`frac VF`= `frac{3times10^8;m/s}{20times10^3}`=15000 m=15 km
i.e., if the sound produced at mike is to be transmitted as such,
we need an antenna of 15 km height, which is totally impractical.
  • If f = 1 MHz

now length of the antenna l = λ = `frac{3times10^8}{1times10^6}`=300 m

 
i.e., if the frequency of the signal is raised to 1 MHz, it can be transmitted through a 300 m high antenna. This is a practical height. Therefore we can ‘modulate’ the signal according to the requirement and need. In other words, the signal is superimposed on a high frequency carrier.
  • The last and the most important reason is that modulation permits the transmission without wire. We can receive audio/video signals from any corner of the world through wireless communication. We can witness a match being played at France, sitting in our bedroom. Imagine the length of wire needed if wireless communication were not possible.

Challenges and Innovations

While modulation has revolutionized communication, challenges persist. Signal degradation due to interference, noise, and the limitations of available frequency bands are constant concerns. Engineers continually innovate to overcome these challenges, developing advanced modulation techniques and error-correction methods.

TYPES OF MODULATIONS

Modulation is an important process in all wireless (carrier) communications.In this, the signal is superimposed on a high frequency carrier wave. Some characteristic (amplitude, frequency, phase; etc.) of the carrier wave is changed in accordance with the instantaneous value of the signal. A sine wave may be represented by
e=`E_m` sin(ωt + φ)
e=instantaneous value of modulated wave
`E_m`=maximum amplitude
ω=angular velocity
φ=phase relation
Accordingly, modulation is of three types (see the above equation)
  • Amplitude modulation: By changing amplitude of the carrier.
  • Frequency modulation: By changing frequency of the carrier.
  • Phase modulation: By changing phase of the carrier.

However, the complete classification of modulation processes are given below:

1.Amplitude modulation (AM)
  • Single sideband AM (SSBAM)
  • Double sideband AM (DSBAM)
  • Frequency division multiplexing (FDM)

In India, for sound, amplitude modulation is used

2.Frequency modulation (FM)
     In India, for television signals, frequency modulation is used.
3.Phase modulation.

Other modulation processes are:

1.Pulse modulation (used in telephone and telegraphy)–these may be:
  • Pulse amplitude modulation (PAM)
  • Time division multiplexing (TDM)–used in long play records
  • Pulse time modulation (PTM) 
  • Pulse division multiplexing (PDM)
  • Pulse code modulation (PCM)

2.Digital modulation (DM)–They may be:

  • Differential PCM (DPCM)
  • Adoptive PCM (ADPCM)
  • Data modulation (DM)
  • Adoptive data modulation (ADM)

Note: 1. The modulations may also be:

  • analog modulation
  • Digital modulation

Amplitude Modulation

  • Amplitude Modulation (AM) is a modulation technique in which the instantaneous amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the analog modulating signal to be transmitted.
  • The modulating signal is an analog baseband signal which is random and has a low frequency, while the carrier signal is always a sinusoidal wave with high frequency.
  • The variations in amplitude of carrier signal represent the information carried.
  • The amplitude of the carrier wave is varied in accordance with the modulating signal while
    the frequency and phase of the carrier signal remains unchanged.
  • The modulating signal seems to be superimposed on the carrier signal.
  • The amplitude variations in the peak values of the carrier signal exactly replicate the
    modulating signal at different points of time which is known as an envelope.
  • Modulation Index is given by `mu=frac{A_m}{A_c}`
The amplitude modulation is often referred as linear modulation. The frequency and phase modulations are known as non linear, angular or exponential modulation. While there may be many forms of exponential modulations but only two i.e., frequency and phase modulations are practical. In particular, both linear as well as non-linear modulations are continuous wave (CW) type modulations.

Frequency Modulation

  • Frequency Modulation (FM) is a modulation technique in which the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the analog modulating signal to be transmitted.
  • Only the frequency of the carrier signal is varied while the amplitude and phase of the carrier are kept constant.
  • The original frequency of the carrier signal is called the center or resting frequency and denoted as 𝑓𝑐.
  • The amount by which the frequency of the carrier wave changes or shifts above or below the resting frequency is called frequency deviation ∆f. This means ∆𝑓 ∝ 𝑚(𝑡).
  • The total variation of frequency of FM wave from the lowest to highest is termed as carrier
    swing (CS),
𝐶𝑆 = 2 ∆f
  • Modulation Index
`mu_f=frac{triangle_f}{f_m}`=Frequency deviation/Modulating frequency

Phase Modulation

  • Phase Modulation (PM) is a modulation technique in which the phase of the carrier signal is varied in accordance with the instantaneous amplitude of the analog modulating signal to be transmitted.
  • After phase modulation, amplitude and frequency of the carrier signal remain unaltered.
  • The modulating signal is mapped to the carrier signal in the form of variations in the instantaneous phase of the carrier signal.
  • Phase modulation and frequency modulation are closely related to each other.
  • In both the cases, the total phase angle 𝜙 of the modulated signal varies.

 

Pulse Modulation

  • Pulse Modulation may be used to transmit analog information such as continuous speech or data.
  • Continuous waveforms are sampled at regular intervals.
  • It has the advantage of ability to use constant amplitude pulses.
  • Pulse modulation may be subdivided into two categories, analog and digital. 
  • In analog, the indication of sample amplitude may be indefinitely variable.
  • In digital, a code which indicates the sample amplitude to the nearest predetermined level is sent.
  • Pulse-amplitude and pulse-time modulation are both analog, while the pulse code and delta modulation systems are both digital.

 

Pulse Amplitude Modulation

  • Pulse Amplitude Modulation (PAM) is the simplest form of pulse modulation.
  • The signal is sampled at regular intervals and each sample is made proportional to the amplitude of the signal at the instant of sampling.
  • Disadvantage - PAM does not use constant-amplitude pulses.
  • Hence it is not used frequently.
  • In PAM, the amplitude of the pulses of the carrier pulse train is varied in accordance with the modulating signal.

Pulse Width Modulation or Pulse
Duration Modulation

  • Pulse Width Modulation (PWM), also called Pulse Duration Modulation (PDM) is a system in which the width or duration of each pulse is made proportional to the instantaneous value of analog signal.
  • The starting time and amplitude of each pulse are constant.
  • Disadvantage - Pulses are of varying width and hence of varying power content.
  • The transmitter must be powerful enough to handle the maximum-width pulses.

 

Pulse Position Modulation

  • Pulse Position Modulation (PPM), is a system in the position of each pulse in relation to the position of a recurrent reference pulse is varied according to the instantaneous sampled value of the modulating signal.
  • The amplitude and width of the pulses are constant.
  • Advantage over PWM – Requires constant transmitter power output
  • Disadvantage – Dependence of transmitter-receiver synchronization

 

Pulse Code Modulation

  • Pulse Code Modulation (PCM), is a digital process in which the message is sampled and rounded off to the nearest value of a finite set of allowable values.
  • The rounded values are coded.
  • PCM generator produces a series of numbers or digits.
  • Each one of these digits, in binary code, represents the amplitude of the signal sample at that instant. 
  • Signals are transmitted as binary code.

Digital Modulation Schemes

  • In digital communications, the modulating signal consists of binary data or M-ary version of it.
  • When it is required to transmit digital signals, the amplitude,frequency or phase of the sinusoidal carrier is varied in accordance with the incoming digital data.
  • Since, the digital data is in discrete steps, the modulation of the bandpass sinusoidal carrier is also done in discrete steps.
  • Due to this reason, this type of modulation is known as digital modulation.
  • Digital modulation schemes are classified as
  1. Amplitude Shift Keying (ASK)
  2. Frequency Shift Keying (FSK)
  3. Phase Shift Keying (PSK)

Amplitude Shift Keying

  • Amplitude Shift Keying (ASK) represents digital data as variations in the amplitude of a carrier wave.
  • ASK signal may be generated by simply applying the incoming binary data and the sinusoidal carrier to the two inputs of a product modulator.

Frequency Shift Keying

  • In Frequency Shift Keying (FSK), the digital information is transmitted through discrete frequency changes of a carrier signal.
  • The simplest FSK is binary FSK (BFSK).
  • BFSK uses a pair of discrete frequencies to transmit binary information (0s and 1s).

Phase Shift Keying

  • Phase Shift Keying (PSK) conveys data by changing (modulating) the phase of constant frequency carrier.
  • Each symbol (pattern of bits) is represented by a particular phase.
  • The simplest form of PSK is Binary PSK (BPSK).
  • It uses phases 0° and 180°.
  • It is widely used for wireless LANs, RFID and Bluetooth Communication.

Why FSK and PSK are preferred over
ASK?

Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) are often preferred over Amplitude Shift Keying (ASK) in certain communication systems due to several advantages. Each modulation scheme has its strengths and weaknesses, and the choice between them depends on the specific requirements and characteristics of the communication channel. Here are some reasons why FSK and PSK are sometimes preferred over ASK:
  • Because of the constant amplitude of FSK or PSK, the effect of non-linearities, noise and interference is minimum on signal detection.
  • These effects are more pronounced in ASK.

1. Noise Immunity:

A general trend can be considered to FSK and PSK being more tolerant of the noise than that has been observed with respect to the ASK system. Signal detection also suffers frequent errors due to limitations on faced by amplitude variations in ASK. FSK and PSK, using frequency and phase variations respectively give noise immunity.

2. Bandwidth Efficiency:

When comparing FSK and PSK versus ASK, such systems show better bandwidth efficiencies. This becomes even more critical where the spectrum supply is limited. The said task is achieved by the FSK through altering of the frequency carrier signal and it possible for PSK to achieve this repair with its phase while varying. Both techniques can transfer more information in a given bandwidth than that of an ASK.

3. Power Efficiency:

In ASK, larger dynamic range for power is desirable because there the amplitude of the carrier must be changed. However, FSK and PSK can modulate without having to vary the amplitude substantially hence are perhaps more power-efficient.

4. Spectral Efficiency:

FSK and PSK are more spectrum efficient so that they can achieve higher spectral efficiency. This is more critical in the functioning of wireless communication which require that unaltered multiple signals coexist for an operation within a single frequency band.

5. Binary Phase Shift Keying (BPSK) in PSK:

A particular form of PSK constituted by BPSK is usually simple and sturdy. It is widely applied to communication systems and as well it finds application in many noisy environments.

6. Ease of Detection:

In many instances, demodulation for FSK and PSK tends to be easier than one of ASK. ASK demodulation is less practical due to the circuitry needing more complexity.

7. Frequency Hopping Spread Spectrum (FHSS):

Foreclosures are one of the major reasons to live in 87 condominiums wherefore these living conditions have put a great impact on many ground floor home-buyers due to lit couple trying out this layout on various wall alterations.Indeed, it is widely applicable in FHSS systems that have the virtue of security and intereferencelessness. The decentralized nature of FHSS systems reduces their vulnerability.

Conclusion

Modulation is the unseen force that shapes our connected world, from the crackling radio waves that carry music through the air to the internet's lightning-fast data exchange. The skill of modifying waves will surely continue to be essential in broadening the scope of international communication as technology advances.
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