Today I will explain about the modulation techniques used in telecommunication.Later I will discuss about the De-modulation. Why modulation is important in the telecommunication system & which type of modulation is more important. The major types of modulation and demodulation techniques used in wireless voice, data, and video systems.
What is modulation?
➤Single sideband modulation—An amplitude-modulated emission with only one sideband being used.
VHF/UHF/SHF Systems
Wireless Local Area Network (WLAN) System Signal Modulation Techniques
Video Modulation
A video modulator is a circuit that converts composite video signal and audio to an RF signal, which then can be connected to a television antenna input.
Two types of modulating techniques are used for video signals:
➤Analog—Most analog television systems use a modulation called vestigial sideband.
Vestigial sideband is an AM signal with most of one sideband filtered out to save
bandwidth (only the carrier and one sideband are needed to recover the video).
➤Digital—Codec is an assembly consisting of an encoder and a decoder in one piece of
equipment. It converts the analog video signals to digital and digital signals to analog.
Mobile Telephony Systems
Mobile telephony uses the following types of coding for transmission of voice, data, and
video over long distances:
➤CDMA is a coding scheme used as a modulation/demodulation technique in which multiple channels are independently coded for transmission/reception over a single wideband channel. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudorandom digital sequence.
In CDMA systems, all users transmit in the same bandwidth simultaneously. Communications systems following this concept are spread spectrum systems. In this transmission technique, the frequency spectrum of a data signal is spread using a code uncorrelated with that signal. As a result, the bandwidth occupancy is much higher than required.
Demodulation
Demodulation (also known as detection) is the recovery from a modulated carrier of a signal
having substantially the same characteristics as the original modulating signal.
Conventional radio waves are usually detected by heterodyning (i.e., coherent reception/ detection). In this method of reception/detection, the received signal is mixed, in some type of nonlinear device, with a signal from a local oscillator to produce an intermediate frequency (i.e., beat frequency) from which the modulating signal is recovered (i.e., detected).
Propagate information information signals over mettalic or fiber cables or through earth's atmosphere, it is often necessary to encode the information on to a higher-frequency carrier signal.The information modulates the carrier by changing either its amplitude,frequency, or phase. Modulation is simply the process of changing some property of the carrier is accordance with the information. The modulation is performed in a transmitter in a circuit called a modulator. A carrier that has been acted upon by an information signal is called a modulated wave or modulated signal. Very simple we can say that Modulation is the process of altering the amplitude, frequency, or phase of a carrier signal in a measured way that allows for data to be added to it.
Why modulation is necessary in Telecommunication
There are two reasons why modulation is necessary in telecommunication system.
1. It is extremely difficult to radiate low frequency signals through Earth's atmosphere in the form of electromagnetic energy.
2. Information signals often occupy the same frequency band and, if signal from two or more sources are transmitted at the same time, they would interfere with each other.
Types of Modulation
Amplitude Modulation
wave is varied in accordance with some characteristic of the modulating signal.
Varying the frequency of a constant amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal.
Phase Modulation
High Frequency (HF) Radio Systems
I will discuss three basic types of modulation-
Amplitude modulation in below figure is the process in which the amplitude of a carrier
wave is varied in accordance with some characteristic of the modulating signal.
wave is varied in accordance with some characteristic of the modulating signal.
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| AM Modulation wave varies |
Amplitude Modulation reception is the reverse process of AM modulation. A conventional AM receiver simply convert as amplitude-modulated wave back to the original source information. An AM receiver must be capable of receiving, amplifying, and demodulating an AM wave. It must be capable of bandlimiting the total radio frequency spectrum to a specific band of frequencies. This process called the tuning the receiver.
Frequency Modulation
Frequency modulation see in below figure is the process in which the frequency of a carrierFrequency Modulation
wave is varied in accordance with some characteristic of the modulating signal.
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| Wave varied in FM |
Phase modulation see in below figure is the process in which the phase of a carrier wave is
varied in accordance with some characteristic of the modulating signal.
varied in accordance with some characteristic of the modulating signal.
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| Wave varies in Phase Modulation |
Varying the phase of a constant amplitude carrier directly proportional to the apmlitude of the modulating signal at a rate equal to the frequency of the modulating signal.
The difference between frequency and phase modulation lies in which property of the carrier (the frequency or the phase) is directly varied by the modulating signal and which properties is indirectly varied. Whenever the frequency of a carrier is varied.the phase is also varied and vice versa.
The various types of radio systems used today rely on different modulation techniques. The most common wireless systems and the modulation techniques applied within them are described below.
The difference between frequency and phase modulation lies in which property of the carrier (the frequency or the phase) is directly varied by the modulating signal and which properties is indirectly varied. Whenever the frequency of a carrier is varied.the phase is also varied and vice versa.
The various types of radio systems used today rely on different modulation techniques. The most common wireless systems and the modulation techniques applied within them are described below.
Modulation techniques used within HF radio systems are as follows:
➤Amplitude modulation (AM)—Implies the modulation of a carrier wave by mixing it in a nonlinear device with the modulating signal to produce discrete upper and lower sidebands, which are the sum and difference frequencies of the carrier and signal. The resulting envelope of the modulated wave is an analog of the modulating signal.➤Single sideband modulation—An amplitude-modulated emission with only one sideband being used.
Modulation techniques used within VHF/UHF/SHF systems are as follows:
➤Frequency modulation (FM)—The instantaneous frequency of a sine wave carrier is caused to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal.
➤Phase shift keying (PSK)—The phase of the carrier is discretely varied in relation to either a reference phase or the phase of the immediately preceding signal element, in accordance with data being transmitted.
➤ Pulse code modulation (PCM)—A signal is sampled, and the magnitude (with respect to a fixed reference) of each sample is quantized and digitized for transmission over a common medium.
➤Frequency modulation (FM)—The instantaneous frequency of a sine wave carrier is caused to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal.
➤Phase shift keying (PSK)—The phase of the carrier is discretely varied in relation to either a reference phase or the phase of the immediately preceding signal element, in accordance with data being transmitted.
➤ Pulse code modulation (PCM)—A signal is sampled, and the magnitude (with respect to a fixed reference) of each sample is quantized and digitized for transmission over a common medium.
➤ Binary phase shift keying (BPSK)—Two different phase angles are used. In BPSK, the two angles are usually out of phase by 180 degrees.
➤ Quadrature phase shift keying (QPSK)—Four different phase angles are used. In QPSK, the four angles are usually out of phase by 90 degrees.
➤Complementary code keying (CCK)—A standard adopted by the IEEE 802.11 standards committee as the basis for the high-rate physical layer extension to deliver data rates of 11 Mb/s at 2.4 GHz. CCK is a variation on M-ary orthogonal keying modulation, which uses an inphase and quadrature modulation architecture with complex symbol structures.
➤Quadrature amplitude modulation (QAM)—Two carriers are amplitude modulated. The name quadrature originates from the stipulation that a phase shift, when required, must be an integral multiple of Ï€ radians (i.e., one-quarter of a cycle).
In video modulators, audio plus video is modulated onto an RF (VHF or UHF) carrier in the range of 30 MHz to 900 MHz.
➤Analog—Most analog television systems use a modulation called vestigial sideband.
Vestigial sideband is an AM signal with most of one sideband filtered out to save
bandwidth (only the carrier and one sideband are needed to recover the video).
➤Digital—Codec is an assembly consisting of an encoder and a decoder in one piece of
equipment. It converts the analog video signals to digital and digital signals to analog.
video over long distances:
Wideband CDMA (WCDMA) is a high-speed third generation mobile wireless technology with the capacity to offer higher data speeds than CDMA. WCDMA can reach speeds of up to 2 Mb/s for voice, video, data, and image transmission. WCDMA was adopted as a standard by the ITU under the name International Mobile Telecommunications 2000 direct spread.
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| CDMA = Code division multiple access |
➤TDMA is a communications technique that uses a common channel (multipoint of
broadcast) for communications among multiple users by allocating unique time slots to
different users.
TDMA is used extensively in satellite systems, LANs, physical security systems, and combat-net radio systems. GSM became the global system for mobile communications. It uses TDMA with the addition of frequency hopping.
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| TDMA |
Demodulation (also known as detection) is the recovery from a modulated carrier of a signal
having substantially the same characteristics as the original modulating signal.
Conventional radio waves are usually detected by heterodyning (i.e., coherent reception/ detection). In this method of reception/detection, the received signal is mixed, in some type of nonlinear device, with a signal from a local oscillator to produce an intermediate frequency (i.e., beat frequency) from which the modulating signal is recovered (i.e., detected).
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