Modulation Systems Used In Satellite Communications Computer Science Essay

Satellite is the one of the greatest means of communication carrying a large chuck of voice and data stream from one part of the horizon to the other as compared to other medium. In this modern age, communication satellite networks are an indispensable part of the major telecommunication systems. Satellite interconnects the nodes and provides some better advantages in application than the traditional communication systems such as interconnecting large traffic nodes (e.g. telephone exchanges), mobile communication and provision of end to end connections directly to users, television and sound broadcast directly to the public [1]. To provide the optimum quality of services (QoS) different types of design techniques needs to be consider for different purposes like distinct types of modulation and coding (channel and source) techniques are used for specific purpose, link budget calculations, selection of radio frequency (RF) etc. Other dominating factors are: 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 systems using for satellite communications. In this report, first I am trying to give some basic information related to the modulation systems and then bring all possible modulation schemes used in satellite communications.

Modulation:

In general view, modulation is the process where the characteristics (phase, frequency or amplitude) of the high frequency carrier are varied in accordance with the baseband signal. The baseband signal carries the basic information such as the output 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-typically 70MHz [1]. Satellites are usually operate 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 also used to describe certain types of baseband processing like Pulse amplitude modulation (PAM) or Pulse Code modulation (PCM) [2].

Modulation Systems for Satellite Communications:

We can divide the modulation systems in two broad categories like

Analogue Modulation and

Digital Modulation

Analogue modulation is Amplitude Modulation, Frequency Modulation and the phase Modulation. Digital Modulation is divided into several categories and each of those uses in specific purpose.

Analogue Modulation:

Let consider a sinusoidal wave f(t):

f(t) = A cos {2*Ï€*f c(t)+α}……………………………..(A)

Where A is the amplitude, f c is the carrier frequency and theta is the phase of the wave with respect to the information signals. When the amplitude of the carrier wave is changed, then the wave is called amplitude modulated and scheme is known 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 as

V(t)= A{1+m(t)}cos (2*Ï€*fc) ………………………………(1)

In equation (1), m(t) is the message signal and the fc is the carrier frequency. A{1+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 follows

V(t)= A{1+Am sin2*pi*fm}cos (2*Ï€*fc) ……………….(2)

The spectral efficiency of V(t) can be obtain by expanding equation (3) and it can be shown that there are two side bands in the spectrum like upper and lower. The upper and lower sidebands consists of same 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 detected by using a simple envelop detector. An envelope detector is the simple large time constant RC circuit. But the difficulty is that AM modulation is rarely use for satellite communications because [1]:

An amplitude modulated signal is very susceptible to the signal fluctuation

It 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 systems:

Double Side Band Suppressed 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 consequence of that it reduces 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 alternative 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 twice 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 satellite communication is to multiplex voice signals into a composite baseband signal. Another 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 Modulation:

The FM systems are widely used in satellite communications. FM system is appropriate for those cases where the baseband signals are in analogue form. In FM, frequency of the carrier signal is varied with the message signal[1]. This scheme exploits where simple receivers are essential for transmission of the digital data[4]. An example is Inmarsat Paging system because an important requirement of the paging 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 and where the constant envelope of the FM signals allows the power amplifiers to operate at saturation, thus making maximum use of the available power.[5]

Digital Modulation:

In 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 Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase 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].

Abbreviations

Alternate Abbreviations

Descriptive names

FSK

BFSK

MFSK

FSK

Frequency Shift Keying (Generic name)

Binary Frequency Shift Keying

M-ary Frequency Shift Keying

PSK

BPSK

QPSK

OQPSK

Ï€/4-QPSK

MPSK

PSK

4PSK

SQPSK

Phase Shift Keying (Generic Name)

Binary Phase Shift Keying

Quadrature Phase Shift Keying

Offset QPSK, Staggered QPSK

Ï€/4 Quadrature Phase Shift Keying

M-ary Phase Shift Keying

CPM

SHPM

MHPM

LREC

CPFSK

MSK

LRC

LSRC

GMSK

TFM

CORPSK

FFSK

Continuous Phase Modulation

Single h (modulation index) phase modulation

Multi h phase modulation

Rectangular pulse of length L

Continuous phase frequency shift keying

Minimum phase shift keying, Fast frequency shift keying

Raised cosine of pulse of length L

Spectrally raised cosine pulse of length L

Gaussian Minimum shift keying

Tamed frequency shift keying

Correlative PSK

QAM

ASK

Quadrature amplitude modulation

Amplitude shift keying

QORC

QOSRC

SQORC

Q2PSK

UF-OQPSK

TSI OQPSK

SQAM

XPSK

Quadrature overlapped raised cosine modulation

Quadrature overlapped square raised cosine modulation

Staggered QORC

Quadrature Quadrature phase shift keying

Inter-symbol interference/Jitter free OQPSK

Two symbol interleaved OQPSK

Superposed QAM

Cross-correlated QPSK

Table: Digital Modulation using in satellite commiunications.

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 performance, error detection and correction efficiency, signal to noise ratio etc. So in this paper focus are mainly given to the various types of digital modulation systems.

We can divide digital modulation systems into two large categories [1];

Constant Envelop Modulation and

Non- Constant Envelop Modulation and

The 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 QPSK:

In 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 it’s 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. [1]

PSK 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 state [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, offset 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 expected in the non-linear satellite channel as compared to the QPSK [7]. In OQPSK systems, maximum phase change is limited to 90. It’s 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].

QAM:

It is seen that as there is a very limited bandwidth available for the satellite channels, the research for spectrally efficient modulation technique is essential for this application. Here QAM is used with the modified signal constellations for wideband 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].

FSK:

As described above, in Frequency Shift Keying systems, frequency is shifting 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. [1]

M-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 smaller 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) Modulation

OFDM is a mature digital multi-carrier modulation technique has been used in physical layer of broadband wireless 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 resistance 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 future, 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-PSK

8-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 capacity 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 modulation 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 expender returns the original voice signals and during the speech pauses suppress the noise link[16]. 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 Discrete 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 structure 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 decreasing 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 principal 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 similar 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 military 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 Communications

Delta 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 fundamental 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.

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