Communication Systems Test - 1

Modulation is a process of varying one or more properties of a carrier signal (High-frequency signal) with a modulating signal (Low-frequency signal). Hence, Modulation is the process of combining signal and radiofrequency.

Need for Modulation:

• It is needed in order to match the characteristics of the input signal to that of channel characteristics.
• Low-frequency signals can be multiplexed with high-frequency signals to travel long distances.
• Wireless transmission of low pass signal is made possible due to modulation.
• Practically realization of antenna size can be achieved.
• Some modulation technique reduces the effect of noise on the signal.

The AM, FM, and PM output waveforms are as shown:

In Frequency modulation, the frequency of the carrier is varied according to the amplitude of the message signal and the amplitude of the carrier remains constant.

CONCEPT:

The bandwidth of Transmission Medium:

• Similar to message signals, different types of transmission media offer different bandwidths.
• The commonly used transmission media are wire, free space, and fiber optic cable.
• Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz.
• Such cables are normally operated below 18 GHz.
• Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.
• This range of frequencies is further subdivided and allocated for various services as indicated in the given table.
• Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.
• Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

EXPLANATION:

• We know that the frequency band of FM broadcast is 88 MHz to 108 MHz. Hence, option 1 is correct.

CONCEPT:

Mixing up of signals from different transmitters:

• Another important argument against transmitting baseband signals directly is more practical in nature.
• Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously.
• All these signals will get mixed up and there is no simple way to distinguish between them.
• This points out a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.
• The above arguments suggest that there is a need for translating the original low-frequency baseband message or information signal into a high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.
• In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it.
• The carrier wave may be continuous (sinusoidal) or in the form of pulses.
• A sinusoidal carrier wave can be represented as,

⇒ c(t) = A_csin (ω_ct + ϕ)

Where c(t) = signal strength (voltage or current), A_c = amplitude, ω_c = angular frequency and ϕ = initial phase of the carrier wave

• During the process of modulation, any of the three parameters, viz A_c, ω_c, and ϕ of the carrier wave can be controlled by the message or information signal.
• This results in three types of modulation:
  1. Amplitude modulation (AM)
  2. Frequency modulation (FM)
  3. Phase modulation (PM)
• Similarly, the significant characteristics of a pulse are pulse amplitude, pulse duration or pulse width, and pulse position (denoting the time of rise or fall of the pulse amplitude).
• Hence, different types of pulse modulation are:
  1. Pulse amplitude modulation (PAM)
  2. Pulse duration modulation (PDM) or pulse width modulation (PWM)
  3. Pulse position modulation (PPM)

EXPLANATION:

• We know that the carrier wave in the modulation may be continuous (sinusoidal) or in the form of pulses. Hence, option 3 is correct.

Modulation is a process of varying one or more properties of a carrier signal (High-frequency signal) with a modulating signal (Low-frequency signal).

Hence, Modulation is the process of combining signal and radio frequency.

Need for Modulation:

1. It is needed in order to match the characteristics of the input signal to that of channel characteristics.

2. Multiplexing is made possible due to modulation.

3. Wireless transmission of low pass signal is made possible due to modulation.

4. Practically realization antenna size can be achieved

5. Some modulation technique reduces the effect of noise on the signal.

The AM, FM, and PM output waveforms are as shown:

In Frequency modulation, the frequency of the carrier is varied according to the amplitude of the message signal and the amplitude of the carrier remains constant.

CONCEPT:

Elements of a Communication System:

• Irrespective of the nature of the communication system, every communication system has three essential elements:

1. Transmitter
2. Medium or channel
3. Receiver

• In a communication system, the transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter and the channel is the physical medium that connects them.
• Depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.
• The purpose of the transmitter is to convert the message signal produced by the source of information into a suitable form for transmission through the channel.
• If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to an electrical form before giving it as an input to the transmitter.
• When a transmitted signal propagates along the channel it may get distorted due to channel imperfection.
• Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal.
• The receiver has the task of operating on the received signal. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.
• There are two basic modes of communication:
  1. Point-to-point
  2. Broadcast

Point to point communication:

• In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver.
• A telephone call is an example of such a mode of communication.

Broadcast communication:

• In the broadcast mode, there are a large number of receivers corresponding to a single transmitter.
• Radio and television are examples of broadcast modes of communication.

EXPLANATION:

• In the broadcast mode, there are a large number of receivers corresponding to a single transmitter.
• Radio and television are examples of broadcast modes of communication. Hence, option 3 is correct.

CONCEPT:

• In the field of communication different types of propagation method are used for electromagnetic wave.
  • Ground wave propagation: If the radio waves have a frequency less than 2 MHz, then this method is used to communicate from one place to another via earth surface.
    • It gives the best efficiency at receiving end.
  • Sky wave propagation: If the radio frequency is between 2 MHz to 30 MHz then it can be communicated via sky wave by ionosphere reflection of a wave toward earth.
    • This type of propagation is used by short wave broadcast service.
    • Ionosphere act as a reflecting surface for this propagation.
    • It extends its height from 65 to 400 km above the earth surface.
  • Space wave Propagation: It is also known as line-of-sight (LOS) propagation. If the frequency is above 40 MHz then for communication we use this method.
    • Space wave is used in line-of-sight as well as space communication.
    • Because of LOS type of communication sometimes it is blocked by objects in between.
    • Examples are- Television Broadcast, microwave links, satellite communication etc.

EXPLANATION:

• Reflection through different layers of the atmosphere is used in Sky wave mode of propagation.

CONCEPT:

Amplitude modulation:

• In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal.
• Here we explain the amplitude modulation process using a sinusoidal signal as the modulating signal.
• Let c(t) = Acsin(ωct) represent carrier wave and m(t) = Amsin(ωmt) represent the message or the modulating signal.
• So the modulated signal can be represented as,

⇒ cm(t) = [Ac + Am.sin(ωmt)].sin(ωct)

⇒ cm(t) = [Ac + μAc.sin(ωmt)].sin(ωct)

Where Ac = amplitude of the carrier wave, Am = amplitude of the modulating signal, ωc = angular frequency of the carrier wave, ωm = angular frequency of the modulating signal, and μ = modulation index

• The modulation index is given as,

\(⇒μ=\frac{A_m}{A_c}\)

• In practice, μ is kept ≤ 1 to avoid distortion.
• Using the trignomatric relation 2sinA.sinB = cos(A - B) - cos(A + B), we can represent the modulated signal as,

\(⇒ c_m(t)=A_csin(ω_ct)+\frac{μ A_c}{2}cos(ω_c-ω_m)t-\frac{μ A_c}{2}cos(ω_c+ω_m)t\)

• Here (ωc - ωm) and (ωc + ωm) are respectively called the lower side and upper side frequencies.
• The modulated signal now consists of the carrier wave of frequency ωc plus two sinusoidal waves, each slightly different from ωc, known as sidebands.

• As long as the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.

EXPLANATION:

• The modulation index in amplitude modulation is given as,

\(⇒μ=\frac{A_m}{A_c}\)

Where A_m = amplitude of the modulating signal and Ac = amplitude of the carrier wave

• In practice, μ is kept ≤ 1 to avoid distortion. Hence, option 2 is correct.

CONCEPT:

Elements of a Communication System:

• Irrespective of the nature of the communication system, every communication system has three essential elements:

1. Transmitter
2. Medium or channel
3. Receiver

• In a communication system, the transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter and the channel is the physical medium that connects them.
• Depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.
• The purpose of the transmitter is to convert the message signal produced by the source of information into a suitable form for transmission through the channel.
• If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to an electrical form before giving it as an input to the transmitter.
• When a transmitted signal propagates along the channel it may get distorted due to channel imperfection.
• Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal.
• The receiver has the task of operating on the received signal. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.
• There are two basic modes of communication:
  1. Point-to-point
  2. Broadcast

Point to point communication:

• In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver.
• A telephone call is an example of such a mode of communication.

Broadcast communication:

• In the broadcast mode, there are a large number of receivers corresponding to a single transmitter.
• Radio and television are examples of broadcast modes of communication.

EXPLANATION:

• We know that depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless. Hence, option 3 is correct.

CONCEPT:

Attenuation:

• The loss of strength of a signal while propagating through a medium is known as attenuation.

Amplification:

• It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier.
• Amplification is necessary to compensate for the attenuation of the signal in communication systems.
• The energy needed for additional signal strength is obtained from a DC power source.
• Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.

Demodulation:

• The process of retrieval of information from the carrier wave at the receiver is termed demodulation.
• This is the reverse process of modulation.

EXPLANATION:

• We know that the loss of strength of a signal while propagating through a medium is known as attenuation. Hence, option 1 is correct.

CONCEPT:

The bandwidth of Transmission Medium:

• Similar to message signals, different types of transmission media offer different bandwidths.
• The commonly used transmission media are wire, free space, and fiber optic cable.
• Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz.
• Such cables are normally operated below 18 GHz.
• Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.
• This range of frequencies is further subdivided and allocated for various services as indicated in the given table.
• Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.
• Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

EXPLANATION:

• Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. Hence, option 1 is correct.