What are the Modulation Techniques?
Modulation Techniques:
In order to transmit digital signals over Radio systems. It is necessary to transfer the information to the Radio frequency carrier.
Digital, information can be imposed upon the carrier by modifying the
amplitude, frequency, phase or a combination of these characteristics, The choice of the modulating scheme is made after considering a number of conflicting requirements, which include susceptibilities to noise interference, fading, non linearities, spectrum efficiency (i.e. Bits/sec/Hz) and equipment complexities with associated cost aspect. The spectrum efficiency is a ratio of bit speed (say R bits per second) and band width say B Hz. This ratio i.e, R/B is known as the spectrum efficiency for the particular modulation technique adopted for the purpose of modulation of the RF carrier. The following sections describe the most commonly adopted digital modulation schemes.
In general, for amplitude modulation, the amplitude of the carrier is varied in proportion to the amplitude of the modulating signal and the carrier
frequency does not change The special cases-of digital modulating signals are referred to as amplitude shift keying. A number is usually added as per the number of the digital symbol states. Ti&us binary signals produce 2 ASK and 4 level signals produce 4 ASK. The ASK signals are generally expressed mathematically as:
X (t) = g (t) x A x cos {2pi fc t)
where g ft) is the random digital signal. A binary ASK modulator is symbolized in Fig. 2.1 where the binary bits cause switching between carrier ‘ON’ and ‘OFF’ states.
The power spectral density of the resultant 2 ASK signal as the same as that of the random data signal but mirrored about the carriers.
In the normal ASK signal the presence of DC component in the modulating signal results in the presence of a carrier component, which contains no information in the output signal and is a waste of available transmitted power.
2.1 Suppressed Carrier ASK
If the DC component is removed from the random signal, the resultant Signal is referred to as double side band suppressed carrier amplitude modulated signal often abbreviated to the word DSB.
2.2 Single Sideband ASK
The modulating process produces both upper and lower sidebands and the spectrum occupancy of the signal doubles. Since either of the sidebands of ASK signals contains the information to be transmitted, spectrum efficiency can be improved considerably by elimination of one of the sidebands, such a system is known as single sideband suppressed carrier amplitude modulation (SSBSCAM) usually abbreviated to SSB. To separate the sidebands a perfect high or low pass filter is required with a cut off at the carrier frequency.
2.3 Vestigial Sideband ASK
An alternative method to overcome the difficulties associated with SSB signals is to transmit a small part (vestige) of the other sideband. This is known as vestigial sideband amplitude modulation VSBAM often abbreviated to VSB.
3.0 Frequency Shift Keying
In frequency modulation, the frequency of the carrier is varied in proportion to the amplitude of the modulating signal and the carrier amplitude remains
constant; Since for ‘digital modulation the baseband signal takes on only one of the two values, the frequency of the modulation also will take one of the two values and the modulation prosess can be thought of as a keying operation. In general, the binary FSK signal can be mathematically expressed by.
X(t) = A Cos (2p fc t+2pi*fd ò g (t) dt + f).
where A. and fc are the carrier amplitude and frequency, g(t) is a random
binary waveform with levels + 1 and -1 and -0 is an arbitrary phase. The
instantaneous frequency is given by the derivative of the phase of X(t), namely by fc + fd g(t) which is equal to the two shift frequencies f 1 and f 2
where f1= fc – fd and f2 = fc + fd
Figure 3.1 illustrates a simple modulator consisting of two oscillators and a switch (key). This form of FM is referred to as Frequency Shift Keying (FSK).
3.1 Demodulation of FSK
There are two methods of demodulation of FSK. They are
– Coherent detection
– Incoherent detection.
3.1.1 Coherent detection
The Coherent detection is illustrated in Fig. 3.4
3.1.2 Incoherent Detection
If the phase of the incoming wave is not known, we must resort to incoherent forms of detection. An incoherent demodulator is illustrated in Fig. 3.5.
It may be seen that for a given BER requirement, the Eb/No (and hence C/N)
requirement is more for incoherent detection compared to that of coherent
detection i.e. Coherent detection is superior to incoherent detection.
3.2 M-ARY FSK
M-ARY FSK (MFSK) -is-a way to trade bandwidth for signaling speed.
Instead of sending data using binary signals with one of two frequencies, the signaling alphabet is expanded to include M possible frequencies. This process will normally increase the speed between the lowest and the highest freq. and therefore the bandwidth can be expected to increase. However, since increased information is sent with each signal element, the baud rate can be decreased to partially counteract the increase in bandwidth. For example, if it were necessary to send 1000 bps of, data, this could be one by sending a binary FSK pulse every millisecond. Alternatively, a 4 ary FSK burst could be sent every 2 ms, representing a decrease in baud rate by a factor of two. (Baud rate is a unit of signaling speed and it is the number of symbols (pulses)/ second in the Channel. If each symbol represents one bit, then baud rate is same as bit rate, it each
symbol represents more than one bit then baud rate is less than bit rate.
Baud rate= Bit Rate/No, of Bits per Symbol).
The performance of MFSK for the various values of M is shown in Fig.3.7.
In the Fig.3.7 it may please be noted that ordinate is the symbol error
probability and not the bit error probability. This is an important distinction,
since a single symbol error can cause more than one bit error. We should
also note that constant energy (E) does not imply constant signal power. As
Main causes, the symbol period increases, so proportionately less signal
power is required to achieve the same signal to noise ratio. Also shown on
the figure is a theoretical bound for M——>¥ which is obtained from the Shannon channel capacity theorem.
C.E. Shannon has shown that a given communication channel has a maximum rate of information ‘C’, Known as .the channel capacity. If the information rate R, is less than C, one can approach arbitrarily small error probabilities by intelligent coding techniques. If the information rate R is greater than the channel capacity ‘C’, errors can not be avoided regardless of the coding technique employed.
We consider the band limited channel operating in the presence of additive white Gaussian noise. In this case, the channel capacity is given by:
C= B log2 (1 + S/N)
Where C is the capacity in bits per second, B is the bandwidth of channel in Hz and S/N is the signal to noise ratio.
The signal power S is the energy per bit multiplied by the number of bits per second. The noise power is No multiplied by the system bandwidth. If we take the limit as the bandwidth approaches infinity.
4.0 PSK Modulation
In general for Phase modulation, the phase of the carrier is varied in proportion to the amplitude of the input signal and the amplitude and frequency remains constant. The special cases of Digital Modulation of signals are referred to as Phase Shift Keying (PSK) Modulation with a number in the front indicating the number of levels of the digital signal. 2 PSK, 4 PSK and 8 PSK are modulation methods that have been adopted for some digital radio systems.
4.1 Correspondence between carrier phase and bit stream state
Since a bit stream is a series of binary digits, it requires a 2 Phase PSK system, where phases of a ‘O’ and ‘p’ radians correspond to the Os and 1 s of the bit stream. The number of phases used is limited only by the requirement that each phase be distinguishable from the others, which result in an increased capacity for information transmission, n bit streams required 2n combinations of n bits.
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