APPLICATIONS APPLICAT IONS OF PHASE MODULATION
Abstract :
An Introductory or initial report on the applications of phase modulation, the topic of the term paper for the current 3rd semester.
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INTRODUCTION TO THE TOPIC
wave to be radiated in order to increase the radiation efficiency. Modulation:It is defined as the process of alternating some of the characteristics such as amplitude, frequency or phase angle of the carrier signal of higher frequency in accordance with the low frequency modulating signal.
Carrier wave:It is a high frequency constant amplitude and non interrupted wave generated by radio frequency oscillator these waveforms are inaudible it means we can’t listen these radio waveforms even through loud speakers.
Need of modulation:- Low frequency signals can’t be transmitted over long distances if radiated directly in space because of interference with similar frequency range waveforms . We require modulation because of following reasons
1) Short operating range:- The energy of any wave depends upon its frequency so larger the frequency of the wave the greater the energy associated with it. For small signal frequency power is lower so it can be transmitted over large distances.
2) Poor radiation efficiency:- At audible frequency the radiation efficiency is poor so we require to increase the frequency of the
3) Mutual interference:- The frequency of noises such as thermal noises and electromagnetic noises lies in the range of audible frequency so it can be easily interfered by these noises. When we modulate It over a carrier signal its frequency increases and thus it becomes immune to noises.
4) Huge antenna requirement:For efficient radiation of signal the length of transmitting and receiving antenna should be at least one quarter of the wave length that is l=1/4(velocity/ frequency (hz)) so we see that the length of the antenna is inversely proportional to the frequency of modulating signal thus high frequency of modulated signal leads to small length of antenna and vice versa.
Types of modulation:-
Modulation is mainly done in three ways:-
1) Amplitude modulation(AM):modulation(AM):-In In amplitude modulation the amplitude of the carrier signal is varied in accordance with the modulating signal .Example is shown below
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3) Phase modulation(PM):modulation(PM):- In phase modulation the phase angle of the carrier signal is varied in accordance with the modulating signal .The diagram is shown below
2) Frequency Modulation (FM):- In frequency modulation the frequency of the carrier signal is varied in accordance with modulating signal. Example is shown below
Modulating Index (m):It is the ratio of the extent by which the amplitude, frequency or phase angle of the carrier wave is varied to its original amplitude, frequency or phase angle when the carrier waveform is unmodulated .It is expressed in percentage.
Now we come to our main topic ”phase modulation”
The question arises, What is phase modulation???
Answer to this is that li ke frequency modulation, phase modulation is also a technique of modulation in which information is represented as variation in the instantaneous phase of the carrier wave.
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It would become more clear from the following example-
Let us consider that signal which we want to send is represented by s(t) and carrier on which signal is to be modulated is represented by
c(t)=Ac sin (ωct +Фc)
thus the modulated signal comes comes out to be
c(t)=Ac sin (ωct +s(t)+Фc)
this shows that how the phase of the carrier wave is modulated according to signal which is to be sent.
In phase modulation there are 2 regions of particular interest:
The phase of a wave is a specific point or benchmark along that wave. A phase shift is an observable repositioning of this benchmark between successive transmissions. A pulse Doppler radar, in its simplest form, provides a reference signal by which changes in the frequency phase of successively received pulses may be recognized. The known phase of the transmitted signal allows measurement of the phase of the received signal. The Doppler shift associated with the echo from which the return originated is calculated from the time rate of change of phase. The phase of a wave, measured in degrees, where 360 degrees equals one wavelength, indicates the current position of the wave relative to a reference position. For example, look at figure 2-18. At time T1 (fig. 2-18, view A), the position of the wave along the vertical line was as shown, while at time T2 (fig. 2-18, view B), the position of the wave along the vertical line was as shown. Notice that the wavelength did not change from T1 to T2. However, the wave’s position relative to the vertical line changed 1/4 wavelength, or 90 degrees. This change is the phase shift.
1) For small amplitude signals, PM is similar to amplitude modulation (AM) and exhibits its unfortunate doubling of baseband of baseband bandwidth and poor efficiency.
2) For a single large sinusoidal signal, PM is similar to FM, and its bandwidth is approximately 2(m+1)f m where f m= ωm /2π and m is the modulation index.
Phase modulation is similar in practice to frequency modulation (FM). (FM). When the instantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, the instantaneous phase changes. But PM and FM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear; that is, phase and frequency do not vary in direct proportion.
Phase shift or phase shifting:-
Modulation index:-
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The modulation index of a phase modulated carrier wave is given by
m=ΔƟ wher where e ΔƟ is the the pea peak k phas phase e deviation.
Phase shift keying:-
keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. This modulation is the most robust of all the PSKs since it takes the highest level of noise or distortion to make the demodulator reach demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications when bandwidth is limited.
it is the digital version of phase modulation. It conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). 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 of binary bits. 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 coherent (and referred to as CPSK). Quadrature phase-shift keying (QPSK):Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phaseshift 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). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.
Sometimes known as quaternary or quadriphase PSK, 4-PSK, or 4-QAM[ 4-QAM[6], 6], QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the BER — twice the rate of BPSK. Analysis shows that this may be used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed.
Applications of phase modulation: Binary phase-shift keying (BPSK):1) Phase modulation is used in Wireless technology:BPSK (also sometimes called PRK, Phase Reversal Keying, or 2PSK) is the simplest form of phase shift
The wireless LAN standard, IEEE 802.11b, 802.11b, uses a variety of different PSKs depending on the data-rate
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required. At the basic-rate of 1 Mbit/s, Mbit/s, it uses DBPSK (differential BPSK). To provide the extended-rate of 2 Mbit/s, DQPSK is used. In reaching 5.5 Mbit/s and the full-rate of 11 Mbit/s, QPSK is employed, but has to be coupled with complementary code keying. keying.
The higher-speed wireless LAN standard, IEEE 802.11g has eight data rates: 6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s. The 6 and 9 Mbit/s modes use OFDM modulation where each sub-carrier is BPSK modulated. The 12 and 18 Mbit/s modes use OFDM with QPSK. The fastest four modes use OFDM with forms of quadrature of quadrature amplitude modulation. modulation. Because of its simplicity BPSK is appropriate for low-cost passive transmitters, and is used in RFID standards such as ISO/IEC 14443 which has been adopted for biometric for biometric passports, credit cards and many other applications.
Bluetooth 2 will use π / 4-DQPSK at its lower rate (2 Mbit /s) and 8-DPSK at its higher rate (3 Mbit /s) when the link between the two devices is sufficiently robust. Bluetooth 1 modulates with Gaussian minimum-shift keying keying,, a binary scheme, so either modulation choice in version 2 will yield a higher data-rate. A similar technology, IEEE 802.15.4 (the wireless standard used by Zig Bee) Bee) also relies on PSK. IEEE 802.15.4 allows the use of two frequency bands: 868–915 MHz using BPSK and at 2.4 GHz using OQPSK.
2) Phase modulation is also used in signal transmission system system in army . It helps to transmit video signal along with the audio signal. Thus phase modulation is used in communication system by army by making the use of special receiver devices which can receive and demodulate both audio and video signal simultaneously.
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