Modulation Systems Used In Satellite Communications Computer Science Essay

transition Systems Used In Satellite communication theory computer Science EssaySatellite is the angiotensin-converting enzyme of the greatest means of communication carrying a large chuck of voice and data stream from bingle parcel of the horizon to the different as comp bed to other medium. In this modern age, communication major planet networks ar an indispensable part of the major telecommunication organisations. Satellite interconnects the nodes and provides nigh better advantages in application than the traditional communication corpses such as interconnecting large traffic nodes (e.g. telephone ex kinds), mobile communication and provision of residue to end connections directly to utilisers, television and sound broadcast directly to the public 1. To provide the optimum quality of services (QoS) different types of design proficiencys needs to be penury for different purposes same distinct types of inflection and coding (channel and source) techniques be u tilize for specific purpose, link budget calculations, selection of radio frequency (RF) etc. sepa localise dominating factors atomic number 18 permitted earth station size and complexity, the size and the shape of the service area etc 2.The objective of this paper is to describe different types of modulations placements employ for transmit communication theory. In this report, first I am difficult to give some basic in mastermindation tie in to the modulation systems and then(prenominal) bring all viable modulation schemes used in transmit communications.modulationIn general view, modulation is the process where the characteristics ( word form, frequency or amplitude) of the tall frequency mailman are varied in accordance with the baseband signal. The baseband signal carries the basic information such as the yield of video camera, voice coder or digital television. In practical point of view, it is more convenient to carry the modulation at the intermediate frequency-t ypically 70MHz 1. Satellites are usually take at Gigahertz range of frequency and the modulation signals are up-converted to the transmission frequency and amplified before going on air 1.The term modulation is withal used to describe certain types of baseband treat equal thrill amplitude modulation (PAM) or Pulse Code modulation (PCM) 2.modulation Systems for Satellite CommunicationsWe cornerstone divide the modulation systems in two broad categories likeAnalogue Modulation andDigital ModulationAnalogue modulation is Amplitude Modulation, Frequency Modulation and the phase Modulation. Digital Modulation is divided into several categories and individually of those uses in specific purpose.Analogue ModulationLet consider a sinusoidal wind f(t)f(t) = A cos 2**f c(t)+..(A)Where A is the amplitude, f c is the attack aircraft pallbearer frequency and theta is the phase of the wave with respect to the information signals. When the amplitude of the pallbearer wave is changed, th en the wave is called amplitude modulate and scheme is k this instant as the amplitude modulation. Likewise when frequency or phase of the wave is altered then it called frequency modulated and phase modulation respectively and techniques are known as frequency and phase modulation respectively 1.Amplitude Modulation As mentioned above, a carrier wave is said to be amplitude modulated when the amplitude of the carrier varies with respect to the baseband signal. An amplitude modulated signal can be given asV(t)= A1+m(t)cos (2**fc) (1)In equation (1), m(t) is the message signal and the fc is the carrier frequency. A1+m(t) is the amplitude of the carrier and it varies accordance with the signal m(t). If m(t) is the sinusoidal signal and Am and fm is the amplitude and the frequency of the m(t) respectively, then it can be written as fol wretchedsV(t)= A1+Am sin2*pi*fmcos (2**fc) .(2)The spectral efficiency of V(t) can be obtain by expanding equation (3) and it can be shown that thither are two side bands in the spectrum like upper and first-class honours degreeer. The upper and lower sidebands consists of homogeneous spectral shape as that of the message signal m(t) 1. The main advantages of the AM is that amplitude modulated carrier can be easily sight by using a simple hold in detector. An envelope detector is the simple large time aeonian RC circuit. But the difficulty is that AM modulation is rarely use for air communications because 1An amplitude modulated signal is very susceptible to the signal editionIt requires very high Signal to Noise ratio for desire amount of quality.AM is used in laser inter-satellite links (ISL) in an alternate form called ON-OFF keying.There are two forms of AM systemsDouble Side Band Suppressed common carrier (DSB-SC)Single Side Band Modulation (SSB)In DSB-SC the carrier frequency is suppressed and only side bands are transmitted. In this case the amplitude of the wave does not follow the amplitude of the signal and as a c onsequence of that it pores the simplicity of the demodulation in FM. DSB-SC is not used in the satellite communications but this technique is good for understanding the SSB modulation 1.SSB is another resource form of the AM systems. This technique is suitable for the application where the bandwidth is the premium. In AM modulation, both the side bands consist of baseband information and transmission of same information double showing the redundancy. In this case, SSB modulation, this redundancy is cut-off by removing one of the sideband so the bandwidth of the RG signal is same as the bandwidth of the baseband signal. Sometimes the carrier is also removed then it is called SSB-SC (suppressed carrier) 1. SSB is detected at the receiver end using synchronous detector. There are two factors playing an important role to make the SSB suitable for the satellite communications occupied bandwidth and the required carrier to noise ratio. The most common application of the SSB in the sate llite communication is to multiplex voice signals into a manifold baseband signal. other form of SSB is using in the mobile satellite communication is known as the ACSSB (Amplitude Companded SSB) because in mobile satellite service efficiency of the bandwidth is desired.Frequency ModulationThe FM systems are widely used in satellite communications. FM system is appropriate for those cases where the baseband signals are in additive form. In FM, frequency of the carrier signal is varied with the message signal1. This scheme exploits where simple receivers are essential for transmission of the digital data4. An example is Inmarsat Paging system because an important requirement of the leaf system is the need for simple, low-cost and rugged receiver 1. The most common methods of modulation used in the fixed-satellite service are FM for analogue signals 5.FM has been largely used in satellite communications. It is particularly convenient when a single carrier per transponder is used a nd where the constant envelope of the FM signals allows the power amplifiers to operate at saturation, thus making maximum use of the available power.5Digital ModulationIn digital modulation techniques baseband signals can be modulated by altering the one or more of its basic parameter amplitude, frequency and phase. According to this there are three basic modulation schemes in digital modulation like Amplitude coggle Keying (ASK), Frequency Shift Keying (FSK) and variant Shift Keying (PSK). There are many variations and combination of these techniques. The combination of ASK and PSK is also known as the QAM (Quadrature Amplitude Modulation).Table 1 below showing list of some digital modulation schemes. Some of the schemes can be derived from more than one parent schemes 3.AbbreviationsAlternate AbbreviationsDescriptive namesFSKBFSKMFSKFSKFrequency Shift Keying (Generic name)Binary Frequency Shift KeyingM-ary Frequency Shift KeyingPSKBPSKQPSKOQPSK/4-QPSKMPSKPSK4PSKSQPSK build Shif t Keying (Generic Name)Binary Phase Shift KeyingQuadrature Phase Shift KeyingOffset QPSK, Staggered QPSK/4 Quadrature Phase Shift KeyingM-ary Phase Shift KeyingCPMSHPMMHPMLRECCPFSKMSKLRCLSRCGMSKTFMCORPSKFFSKContinuous Phase ModulationSingle h (modulation index) phase modulationMulti h phase modulationRectangular pulse of length LContinuous phase frequency exchange keying minimal phase shift keying, Fast frequency shift keyingRaised cosine of pulse of length LSpectrally brocaded cosine pulse of length LGaussian Minimum shift keyingTamed frequency shift keyingCorrelative PSKQAMASKQuadrature amplitude modulationAmplitude shift keyingQORCQOSRCSQORCQ2PSKUF-OQPSKTSI OQPSKSQAMXPSKQuadrature overlapped raised cosine modulationQuadrature overlapped square raised cosine modulationStaggered QORCQuadrature Quadrature phase shift keyingInter-symbol interference/Jitter free OQPSKTwo symbol interleaved OQPSKSuperposed QAMCross-correlated QPSKTable Digital Modulation using in satellite commiunicat ions.As the advance in technology and limitation of analogue modulation it is now most of the satellite using digital modulation techniques and providing some advantages over analogue modulation like better slaying, error detection and correction efficiency, signal to noise ratio etc. So in this paper focus are mainly given to the divers(a) types of digital modulation systems.We can divide digital modulation systems into two large categories 1 unending Envelop Modulation andNon- Constant Envelop Modulation andThe constant envelop class is generally considered as the most suitable for the satellite communications because it minimizes the effect of non-linear amplification in the high power amplifier like TWTA (Travelling Wave Tube Amplifier) or KTA (Klystron Tube Amplifier). In this case the generic FSK is not suitable for satellite communication because it has very low bandwidth efficiency as compared to PSK systems 5.In this section of the report a brief description of different types of digital modulation systems using in satellite communication is given.PSK BPSK QPSKIn PSK system, envelop is constant but the phase changes discontinuously from symbol to symbol. There are two types of classic PSK schemes like BPSK (Binary PSK) and QPSK (Quadrature PSK). Generally, PSK modulation systems with M-ary signals can be used and its known as MPSK 5. MPSK is more suitable than MFSK because it provides better Power Nyquest efficiency than MFSK.In mobile satellite communication, the most commonly used digital modulation system have been BPSK and different forms of QPSK. It is not surprising that higher order modulation are providing the better performance and efficient as well but they are more sensitive to the channel impairments. Recently, higher order PSK, 16-QAM is using for this purpose. 1PSK modulation systems easily represented in the I-Q plane where I and Q axes are normal to each other. One point in I-Q plane is providing the information about the signal stat e 1. PSK scheme include two basic techniques like BPSK and QPSK. In BPSK one binary code is represented by the two phases 0 and 180 where in QPSK two binary codes is represented by the four phases 0, 90, 180 and 270. In multi level PSK, the higher the order the more requirement of power to achieve the same amount of performance 5. PSK is using for satellite communication because PSK signal can be transmitted through the non-linear channel of a satellite and it depends on the proper choice of the modem filters 5.Other types of modulation systems use in satellite communications are Quadrature schemes where the modulated signals are represented in terms of two channels like I and Q. Three schemes of Quadrature modulation are given below.OQPSK, cancel QPSK or OKQPSK (offset keying QPSK), also known as the SQPSK (Staggered QPSK), is a modified form of QPSK. In the OQPSK, the value of Q and I channel not changing at the same instant because the value of the Q-channel baseband is delayed by the duration of one symbol. By using this property, OQPSK signals provide the reduction of the non-linear distortion when passing through non-linear element. So in this case, a better performance can be expect in the non-linear satellite channel as compared to the QPSK 7. In OQPSK systems, maximum phase change is extra to 90. Its one of the reasons why OQPSK is advantageous for satellite channels and is used in satellite links 1. MSK (minimum shift keying) is another coherent frequency shift keying modulation systems with modulation index 0.5. It also shows the non-linear property through a non-linear satellite channel and the signal has constant envelop.5 MSK is spectrally more efficient than the QPSK and OQPSK because its spectrum has wider main lobe than QPSK and OQPSK 9.QAMIt is seen that as there is a very particular(a) bandwidth available for the satellite channels, the research for spectrally efficient modulation technique is essential for this application. Here QAM is u sed with the modified signal constellations for broadband satellite communication systems. 8 Because of its constant envelop to combat the non linearity of the satellite channel M-ary phase shift keying is conventionally used in digital satellite communications systems. But when higher spectral efficiency is expected it cost more power than the other schemes. The main objective of modifying the QAM signal constellation is to reduce its sensitivity to the channel non linearity 8.FSKAs described above, in Frequency Shift Keying systems, frequency is sack in accordance with the message state change 1. Binary Phase Shift Keying (BFSK) is the simplest form of the FSK allowed only two frequency states and this system can be extended to M level of frequency shifting then it is known as the M-ary FSK. In M-ary FSK N numbers of bits are combined to form a symbol. 1M-FSK is a power efficient modulation technique which power efficiency can be increased by increasing the number of frequencies (M) at the cost of additional complexity and menialer bandwidth efficiency. This modulation system is found appropriate for some low rate and low power applications such as Global paging via satellite communications 10. M-FSK can be used for in land mobile satellite communications where the numbers of user are stationary. It is valid for low data rate application like paging via satellites 10.OFDM (Orthogonal Frequency Division Multiplexing) ModulationOFDM is a mature digital multi-carrier modulation technique has been used in physical layer of broadband radio set air interface standard like IEEE 802.11/Wi-Fi and 802.16/WiMax as well as digital video broadcast-Terrestrial DVB-T 11. Simultaneously, OFDM modulation is attracting more attention for satellite communications and now it is using for DVB-TH. OFDM modulation systems also use in Military satellite communication.Coded Orthogonal Frequency division Multiplexing Modulation (C-OFDM)A COFDM modulation system is subway to the frequency selective fading. This type of fading is seen in the wideband mobile communications. This system is useful for the digital audio and video broadcasting over a long distance. In prox, C-OFDM systems would be one of the candidates for direct sound broadcast satellite systems 1.Trellis Coded Modulation (TCM)Error correction coding and modulation are combined in Trellis coded modulation systems. To transmit information over the power-limited and band-limited channel, the modulation and the channel coding must be optimized jointly 12.TCM with Octal Phase Modulation or 8-PSK8-PSK is a constant amplitude modulation system with higher bandwidth efficiency. The first TCM application to satellite transmission occurred with the 8-PSK trellis codes. Using a 72 M Hz transponder bandwidth, transmission at up to 155.52 Mbits/s have been realized 5. TCM with 8-PSK provides high bit rate which is essential in future for the high bit rate application like images, TV and HDTV services over the satellite transmission 5. There are various forms of TCM like PTCM (pragmatic) and PPTCM (Punctured PTCM).The PTCM codes are interesting for satellite transmissions. As an example, a new coding systems using the PTCM 2/3-8PSK code concatenated with Reed -Solomon code in the INTELSAT IDR service and is expected to increase the energy up to 25% base on the standard A earth station and INTELSAT VII space segment conditions 5. As the demand is increasing for higher bandwidth efficiency, the QAM trellis codes could find the potential applications in HDTV satellite transmissions in the future. But the main limitation of trellis codes applying in the QAM using in the satellite communications is their non-constant amplitude and more distortion occur when pass through the NLA (Non-linear amplifier) 13. A precise amplitude control must be used in the receiver end 5.Trellis coded 16-PSK and 16-QAM modulation systems are used for satellite communications. But the fact is that when the modu lation level increases, the constant envelop M-ary PSK modulation systems are inferior to the QAM systems. On the other hand, QAM suffers more distortion in the non-linear satellite communications channels 14. QAM is suitable for geostationary orbit satellite channel with only Gaussian impairments because of being amplitude and phase modulated signal, QAM is more sensitive to the effects of interference and fading than MPSK 15.Single Channel Per Carrier FM Modulation with syllabic Companding (SCPC FM)The main advantage of using SCPC FM modulation with syllabic Companding in satellite communication is to increase the limited capacity and this capacity increment is fully depends on the syllabic companding. The compadors in the transmission side take a wide volume of speech signals and reduce by the compressor with small amount of deviations. As a consequence of that in the same bandwidth it would be possible to employee more channels for transmission. And at the receiver end , the spe ndthrift returns the original voice signals and during the speech pauses suppress the noise link16. This bandwidth efficiency can also be applied to the domestic satellite systems where the use of a small earth station and a large number of routes operating with few channels are important 16. In future for the larger satellite (IntelSat iva or V), the inefficient use of bandwidth of satellite channel by small carriers, especially in the transponder one of the factors which limits the total achievable capacity of bandwidth. In this case, the use of a SCPC companded FM would significantly increase the total capacity of the satellite bandwidth without increasing the total segment cost of the earth stations like other modulation systems 16. From research it can be shown that the use of SCPC FM with compandors will significantly improve the economic scenario associated with the small earth stations.Wavelet Packet Modulation (wpm)WPM is a multicarrier modulation system like OFDM using Dis crete Wavelet Transform (DWT). DWT is a transformation technique which is a presentation of the composite signal in time and frequency domain. So in WPM, packets social organisation is divided into time and frequency domain. So when any interference is realize, in TDMA or FDMA system all packets are degraded but in case of WPM, packets are keep away from the interference with the help of providing the appropriate packet structure 17. Both WPM and OFDM are multicarrier modulation system but the difference is OFDM uses FFT to combine the transmission where WPM use DWT and Bit Error Rate (BER) of WPM is much better than the OFDM. Similarity between these modulation systems is High Peak to Average Power Ratio (PAPR). For improving the performance of OFDM, single carrier OFDM (SC-OFDM) is proposed where diminish PARP was the main goal. It is seen that the PARP is also high in WPM so SC-OFDM can be used to improve efficiency of the WPM. SC-WPM also can be used by exploiting the principa l of SC-OFDM 17. Some experiment shows that the WPM is the effective modulation systems for satellite communications and with lower PARP, SC-WPM would enable the broadband satellite communications 17. PARP performance of SC-WPM is superior to WPM and OFDM. The BER performance of the WPM is better than OFDM.Multi-Level Gaussian Frequency Shift Keying (MGFSK)MGFSK is a modulation systems use in the satellite communications. It also exploiting the technique of narrow band FM which has constant envelop throughout the signal. MGFSK is suitable for satellite communications where the transponders are in saturations and it is also useful for the transmitter where output amplifier is also saturated 18. As compared to the 8PSK bandwidth efficiency (3 bit/s/Hz), MGFSK providing bandwidth efficiency is 6 bit/s/Hz 18. BW efficiency of MGFSK is very identical to the 64 QAM but 64 QAM is not feasible to use in satellite communication because it requires highly linear and well-equalised satellite channels 18. The key applications of MGFSK are in those satellites which trucking of ISP backbone traffic, satellite news gathering and soldiers satellite applications.Pulse Code Modulation (PCM)Pulse code modulation system is another technique which is employed in the satellite communications. Here in PCM the main aim is to coding the analogue signal for digital representation and then transmitter by using digital techniques. In PCM, analogue signals are sampled in a periodic manner of time in a rate of Nyquest Sampling rate (twice the highest of baseband frequency) and then quantized in agreed manner. To keep the error in certain level, quantization step should keep as low as possible. 5 Different types of pulse code modulation are seen like DPCM and ADPCM.Other Modulation Systems use in Satellite CommunicationsDelta modulation (AM), Pulse amplitude modulation (PAM), Pulse Time Modulation (PTM) etc. Spread Spectrum modulation systems are closely related to the PCM 1.Conclusion In this report it has been trying to make an overview of the modulation system used in the satellite communication channels. A different type of modulation systems has been described. The selection of modulation systems is depends on the various factors like type of the channels, constraints imposed by the earth stations (for fixed satellite, direct broadcast services and mobile satellite services), hardware limitations etc 1. One innate characteristic of the satellite communications channels is the trade-off between the bandwidth and the power to obtain the received signal with certain level of quality. In case of the bandwidth limited channels, spectrally efficient modulation systems are used where penalty is paying for additional carrier power. In the power limited links, bandwidth efficient modulation is using which related to the hardware constraints as well.References1. Raicharia, M., 1995 Satellite Communication Systems Design Principles, 2nd Edition, 1999, ISBN0-333-74722-4, Macmillan Press Ltd,Baisingstoke, RG21 6XS.2. Couch, L W., Digital and Analogue Communication Systems, 5th Edition. 1997, Prantice Hall, Upperside river, NJ-07458, ISBN 0-13-522583-3.3 . Fuqin, Xiong., August, 1994 Modem techniques in satellite communications,IEEE Communications Magazine, p. 84-98.4. Fthenakis, E., Manual of the Satellite Communications, McGraw-Hill Inc, 1984.5. Handbook of the Satellite Communications (HSC), 3rd Edition, pp 242-275,http//www.scribd.com/doc/7047432/Handbook-on-Satellite-Communications, Last Visited 25.03.106. 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