Modulation systems used in satellite communications INTRODUCTION:
Radio-Frequency transmission requires that the baseband signal be modulated on a carrier in the desired frequency band. Baseband signals are the basic information carrying signals, such as out put from a voice coder or a video camera. Modulation is a proces processs in which which some some charac characteri teristi sticc of a high-f high-freq reque uency ncy carrie carrierr is varied varied in accordance with the baseband signal. For example sinusoidal wave f(t): f(t) = Acos{2 π fc(t)+ θ
Modulation can be achieved by altering the amplitude, A, frequency fc or phase
θ
of
the wave in accordance with the information. When its amplitude A is varied, the wave is said to be Amplitude Modulated; similarly when the frequency fc or phase
θ
is varied, the signal is said to be frequency or phase modulated respectively. The sinusoidal is a high frequency wave and hence this type of modulation is called continuous wave or sinusoidal modulation. On the other hand the reverse process can be used for recovering the information signal from the modulated signal and is known as demodulation. Generally modulation can be defined as a process in which the information is altered into a form, which is more efficient for transmission. In order to design a system there are number of system level consideration when studying the applicability of modulation to a given application. 1- To distinguish the carrier-to-noise ratio (CNR) of the radio frequency passband sign signal al from from the the sign signal al-to -to-n -noi oise se ratio ration n (SNR (SNR)) of an anal analog ogue ue base base band band message signal after demodulation, for example an audio frequency analogue signals. 2- Spectral accupancy of the system: it is essential that the bandwidth of the modulated signal be as small as possible. 3- Sensitivity of modulation scheme to signal imparements: signal imparements are caused by a number of factors such as thermal noise, intermodulation noise, multipath noise, intra or inter-system interfernce and signal fading 4- Hardware complexity: Hardware required for modulation and demodulation be compatible with the permited cost and complexity.
There are different catagories of modulation systems have been used in satellite communications such as: linear modulation scheme, non linear modulation scheme
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and a digital communication schemes. Linear modulation scheme in which the baseband signal is linearly related to the modulation signal. Linear modulation schemes have been only of limited use in satellite communication RF links. However, at least one of the schemes, the single side band (SSB) has a potential application in satellite systems. The other schemes amplitude modulation (AM) and double sideband suppressed carrier (DSB-SC) are not of great practical use for satellite communications. The non linear modulation scheme which is known as frequency modulation (FM) will be discussed after that and in last the digital modulation schemes will be discussed.
AMPLITUDE MODULATION
Amplitude modulation is generated using a product modulator. Conventionally the source signal is multiplied by sinusoidal carrier. The amplitude of the modulated carrier follows the amplitude of the source signal. Or a carrier is said to be amplitude modulated when the amplitude of the carrier varies in accordance with the message signal. An amplitude modulated signal may be represented as
(1)
where m(t) is the message signal and
ω
c is the angular frequency of the carrier. An
examination of the above equation shows that the amplitude of the carrier varies in accordance with the signal. Assume m(t) is a sinusoidal with amplitude Am and angular frequency ωm then
(2)
The spectral characteristic of v(t) can be obtained by expanding the above equation. It can be shown that the spectrum consists of the carrier (ƒc), an upper side band (ƒc+ƒm) and a lower side band (ƒc-ƒ m). In a more general representation, the upper
and lower side bands have the same spectral shape as that of the message signal m(t).
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An amplitude modulated signal may be generated either by the use of a non-linear device or by switching the modulating signal at the carrier rate. One of the main advantages of amplitude modulation is its simple demodulation capability. An amplitude-modulated carrier can be detected simply by the use of an envelope detector. The rectified signal contains message components, together with other undesirable high-frequency components. Such undesirable components, being at a significantly higher frequency than the carrier frequency, are easily filtered out with a low pass filter. A simple RC circuit with a large time constant is commonly used. There are some major shortcomings in amplitude modulation for satellite communications. Variation in amplitude can make the message superimposed because the amplitude modulation system is susceptible to signal fluctuation. RF signal in satellite communication links suffer various types of amplitude perturbation caused either during propagation or by inherent limitation of equipment. The consequent degradation to an amplitude modulated signal in unacceptable. It should be noted that the amplitude modulation is not in general use in Earth-space satellite links. However, amplitude modulation in the form of ‘on-off’ keying has been considered advantageous in a laser inter-satellite link (ISL) because the scheme can be readily implemented with lasers, and signal fluctuations in (ISL)’s are minimal.
Double side band suppressed carrier (DSB_SC )
The (DSB_SC) is not in used for satellite communication so I will not go in detail. However, the concept is useful in understanding single side band modulation discussed next. In (DSB_SC) modulation scheme the carrier is suppressed and only side bands are transmitted, and the amplitude of such a wave does not follow the signal amplitude. Unlikely in amplitude modulation the information is carried only in the side bands and therefore power in the carrier remains un-utilized. DSB-SC using a different type of modulation, known as synchronous detection. In this scheme the modulated signal is multiplied with a carrier which is synchronized in frequency and phase to the transmitted carrier. One of the main problems in using this scheme is the difficulty of generating a synchronized carrier at a receiver thousands of kilometers away.
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Single side band modulation
Single side band SSB has a potential application in satellite communication. Its exhibits characteristics make the scheme a potential candidate for applications where bandwidth is at a premium, such as mobile satellite services. In an amplitude modulation system the baseband information is contains redundancy. In SSB this redundancy is removed by filtering out one of the sidebands at the modulator. The bandwidth of the RF carrier is therefore the same as that of the baseband signal. For example telephone signals can be transmitted in a bandwidth of 4-5 kHz. In contrast the other modulation schemes require considerably more bandwidth and the spectral advantage offered by SSB makes it a potential candidate for frequency-limited applications. Therefore the single side band scheme is called a single side band suppressed carrier (SSB-SC) scheme when the carrier is suppressed. A multiplex voice signal into a composite baseband is a most common application of SSB modulation is satellite communication. A SSB signal can be represented by
(3)
where (t) is a signal derived by shifting all the frequency components of m(t) by 90 degree. The term (t) is also called Hilbert transform of m(t). the amplitude of a SSB modulated carrier is constant, the inherent simplicity of the process of demodulating amplitude modulated signal is not possible in the case of SSB modulated signal. A SSB modulated signal can be recovered by using synchronous detection. It can be generated either by an open-loop or a closed-loop scheme. For the open-loop system, receivers use a high stable local oscillator such that the maximum frequency error is 10-30 Hz. In closed-loop a low level pilot is transmitted together with the signal. The pilot is recovered at the receiver. Errors in phase( ∆θ) and frequency ( ∆ω ) of the recovered carrier give the demodulated signal terms cos(ω t - ∆θ) and cos(ω t - ∆ω ) respectively. There are two aspects considered is assessing the suitability os SSB for satellite communication - the required carrier-to-noice ration and the occupied bandwidth. The use of companders offer a signal-to-noice ration advantage on 15-20 dB. (for datails
see Satellite Communication Systems by M.Richaria) with this advantage the SSB
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transmission begins to appear attractive. The scheme is known as Amplitude Companded Single Side Band ACSSB transmission. Regarding the occupied bandwidth RF bandwidth of a SSB transmission is the same as the baseband bandwidth. So the 4-5 KHz RF bandwidth is adequate for a single telephone channel transmission.
FREQUENCY MODULATION
Frequency modulation has been widely used in satellite communications for both telephony and video transmission. It is in use in both single channel per carrier SCPC and multiple channels per carrier MCPC. FM is extensively treated in the technical communication literature and its applications in all forms of radio communications are truly prolific. [2]
Fig: (1) http://upload.wikimedia.org/wikipedia/commons/9/94/Amfm2.gif
Frequency modulation is created by varying the frequency of a sinusoidal carrier with the amplitude of the message signal this voltage-to-frequency conversion process results in a bandwidth expansion in the RF channel, which can be treated for signalto-noise ratio improvement in the base band signal. Frequency modulation systems are well suited for those cases where the baseband signal is in analog form fig:(1). It also offers advantages for transmission of digital data in applications where simple receiversare essential. An example application for digital transmission is Inmarsat paging system. However, Inmarsat paging system requires simple, low cost and rugged receivers. In more general frequency modulation is a class of angle modulation and it can be represented as (4)
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when m(t) change the phase θ(t) of the signal, the wave is said to be phase modulated. A frequency modulated wave is produced when the derivative of phase angle
θ(t)
of
the above equation is changed in accordance with m(t). Consider m(t) as a sinusoid, β sin(ω mt). differentiating m(t) and substituting in equation (4) we obtain the equation of a frequency modulated wave as
(5)
and the instantaneous frequency of the wave will be
(6)
the maximum variation in frequency is
or
(7)
β is the modulation index of the frequency carrier.
Carson bandwidth rule
Carson’s bandwidth rule defines the approximate bandwidth requirements of communications system components for a carrier signal that is frequency modulated by a continuous or broad spectrum of frequencies rather than a single frequency. Carson's rule does not apply well when the modulating signals contain discontinuities, such as a square wave. Carson's rule originates from John Renshaw Carson's 1922 paper. Carson's bandwidth rule is expressed by the relation CBR = 2 ( f + f m) where CBR is the bandwidth requirement, f is the peak frequency deviation, and f m is the highest frequency in the modulating signal.
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For example, an FM signal with 5 kHz peak deviation, and a maximum audio frequency of 3 kHz, would require an approximate bandwidth 2(5+3) = 16 kHz. Carson's bandwidth rule is often applied to transmitters, antennas, optical sources, receivers, photo
detectors, and
other
communications system components.
Theoretically any FM signal will have an infinite number of sidebands and hence an infinite bandwidth but in practice all significant sideband energy (98% or more) is concentrated within the bandwidth defined by Carson's rule. ( Wikipedia, Digital and Analog Communication Systems, 6th Edition", Prentice-Hall, Inc., 2001. ISBN 0-13-081223-4)
Bandwidth of frequency modulated wave
In order to test the performance of FM system a definition of the occupied bandwidth becomes indispensable. For practical purpose the received signal distortion is acceptable provided that 98% of the energy is contained in the modulated spectrum. It can be shown that for a sinusoidal baseband, this condition is satisfied when
(8)
where ∆ƒ is the peak carrier deviation and ƒ m is the frequency of the sinusoid. The bandwidth obtained by using equation (8) is called Carson’s bandwidth. Equation (8) gives the bandwidth of a signal modulated with a sinusoid but in practice the message signal is more complex. The FM waveform of such a signal is gaussian and the bandwidth B containing 98% of the side band is
(9)
where ∆ƒ rms is the deviation of the power spectral density. When the spectral shape of the signal is unknown, the occupied bandwidth can be estimated by using the largest frequency of significant power in the message signal as ƒ rule.
m,
when applying Carson’s
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Noise characteristic at the output of a FM demodulator
The power spectral density of noise is given by
(10)
where A is amplitude of RF carrier,
α
is a constant, ω is angular frequency of the
baseband and N o is noise spectral density at the input. From the equation (10) it can be seen that the noise power spectral density PSD at the output of a FM detector is proportional to ƒ2 where ƒ = frequency. A PSD plot of such a noise is parabolic in shap and therefore G(ƒ) is often reffered to as par abolic noise. If the response of the post-detection filter is chosen as inversily shaped, the mean value of the received noise at the output decreases. However, this type of response also cuases attenuation to higher frequency components of the message waveform. Now consider that the spectral density of the message signal at the transmitter is shapped as ƒ square and an invers parabolic filter is used at the out put of the demodulator. With such an arrangement the frequency components of the output message signals are unaffected and at the same time the mean value of the noise is redused. This concept is universally used in FM satellite communication systems. The process of shapping the transmitted waveform is called pre-emphasis and its inverse operation at the receiver is called de-emphasis.
Block Diagram of FM detection process
Fig: (2) An FM receiver A block diagram of FM detection process, for more details go to [2]
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Threshold effect in FM
Figure (3)
The relationship between the input carrier-to-noise ration (C/N) and the output signalto-noise ratio (S/N) of FM demodulator. Output signal-to-noise ration shows a rather abrupt degradation beyond a threshold. This sudden loss of output signal quality is known as the threshold effect figure (3). The FM threshold is defined arbitrarily as the magnitude of input carrier-t o-noise ratio where the deviation between the extrapolated and the actual output signal-to-noise ratio is 1 dB (fig: (3)) this point is rather abrupt and in practice can quite easily be estimated subjectively.
FM threshold effect occurs when C/N is low
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Figure (4) (Lecture # 10 Satellite Communication, Prof Clive Parini)
Higher the noise level at discriminator input more likely this effect to occur at each zero crossing of the modulating signal. Thus higher output noise can cause threshold effect. Can also think of this in terms of rotating phaser: fig:(5)
Figure (5)
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Noise causes resultant signal to sweep around the origin ø(t) so increases or decreases by 2 π. Whenever a carrier vector rotates by more than 2 π radians as a result of noise, the modulator produces a noise spike of considerable energy. As the carrier-to-noise ration is progressively reduced, rotation of carrier vector due to noise increases until eventually the effect become dominant causing the threshold effect. It can be shown that the onset of threshold occurs at larger C/N as the modulation index of the carrier is increased. As we know in FM the large improvement in signal-to-noise ratio made possible by using a large modulation index. However, the threshold phenomenon limits the potential of FM precisely when needed most, that is at low carrier-to-noise ratio. Hence a number of techniques have been developed for improving the FM threshold to obtain the maximum benefit of FM. The phase locked loop and FM demodulator using feedback are the two commonly used techniques, can be seen in block diagrams below figure (6) and (7). [For more details go to(2)]
Figure (6) The main elements of phase lock loop
Figure (7) A block diagram of FM demodulator using feedback
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DIGITAL MODULATION
Digital Signals use the same principle to modulate a carrier as do analog signals. An analog carrier, signal is modulated by a digital bit stream. Digital signal consist of a pulse stream containing 0’s and 1’s which cannot be transmitted directly as radio signals. To transmit a digital signal as a radio signal requires the use of some form of modulation and demodulation techniques. It can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. Amplitude, phase, and frequency modulation schemes are all applicable to digital communication. The digital equivalents of these modulation schemes are known as been Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). Additionally, some modulation shames have been developed specifically to optimize digital modulation. These are hybrid phase/amplitude schemes
called
Quadrature
Amplitude
Modulation
(QAM).
In
satellite
communication, PSK is the most commonly used modulation scheme. FSK has also used in certain applications where receiver simplicity is essential. ASK schemes are not generally used in Earth-space links because of the uncertainties associated with the amplitude of the received signal.
