A System Design and an Analysis on Satellite Mobile Communication Link

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

A System Design And An Analysis On Satellite Mobile Communication Link

1 ARTI J JANSARI , 2 PROF.KUNAL ACHARYA

1 Department of Electronics & communication, L.D.Engineering College, Ahmedabad.

2Department of Electronics & communication, L.D.Engineering College, Ahmedabad.

,

ABSTRACT : A satellitemobile communication link between user terminal is analyzed based on a simple fundamental equations by systematic approach in this paper. With the given fundamental design parameter, the important parameters are calculated step by step and three communication characteristics such as carrier-to-noise density ratio, Energy per bitto noise density ratio, carrier-to-noise ratio (CNR) at the satellite and gateway station are analyzed. It gives very useful information to the system engineers for designing and analyzing the overall satellite mobile communication system in the conceptual design phase.

Key word— satellite ,RF link

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

1. INTRODUCTION

Geostationary Earth Orbit (GEO) have been used to support all forms of communication via satellite,voice, data, multimedia services,higher gain, voice conference etc. Geostationary Earth Orbit satellite is in a circular orbit 35,876km above the earth’s surface and rotates in the equatorial plane of the earth. It will therefore rotate at exactly the same angular speed as the earth. The major advantages of these systems is their unchanging position with respect to the earth surface, thus no control overhead is required to track the satellites. For this link design s band frequency is used between user terminal and satellite station and C band frequency is used between satellite station and gateway station.The overall receiver noise temperatures available today range from as little as about 30ÆK to thousands of degrees Kelvin. Power output of the transponders in differing communication satellite may vary by an order of magnitude or more. For satellite mobile application, mesh reflector antenna is used between user terminal and satellite station. In practical case, it necessary to consider additional losses due to system design such asatmospheric loss, polarisation loss, antenna mismatches loss etc.

2. FUNDAMENTALDESIGN PARAMETERS

2.1 Transmission basis

2.1.1 Effective Isotropic Radiated Power

The power radiated by an isotropic antenna fed from a radio frequency source of power PT. In direction where the value of transmission gain is GT.

EIRP PT GT

The product PTGT is called Equivalent Isotropic Radiated Power (EIRP).It is express in W. The EIRP serves as a single

parameter ‘figure of merit’ for the transmit portion of the communication link.

2.1.2 Power Flux Density

The power flux density, usually expressed in Watts/m2, at the distance r from the transmit antenna with a gain GT, is defined as the power flux density which is shown in figure 2.1.

Power flux density Φ = (W/m2)

Figure 2.1: power flux density

Where PT, GT and EIRP are the transmit power, transmit antenna gain, and effective radiated power, all expressed in dB. Φ is called power flux density.

2.1.3 Saturation flux density

If you transmit an uplink signal with sufficient power to produce the pfd/sat specified above you will saturate the transponder. This is applicable only if you are using one carrier in the transponder. In such a case the input and output back offs would both be zero. For two carriers an input back off of 0.5 dB is suggested, with an output back off of about 1.5 dB due to the production of second order intermodulation products (A+B) which are hopefully absorbed in the satellite before the output filter. In many cases 3 or more carriers are transmitted through the transponder and to avoid unacceptable intermodulation interference levels, it is necessary to operate the transponder power backoff.

2.2 Losses

In practical case, it necessary to consider additional losses due to system design.

a)Losses occurs in atmosphere

In the atmosphere, the attenuation of waves, express by LA, is during the presence of gaseous component in troposphere, water and ionosphere. The overall effect on the power of received signal can be taken into account by replacing LFS,

L LFS LA

Where L is path loss and LFS is free space loss. LFS is not a loss in sense of power being absorbed. It accounts for the way energy spread out as an electromagnetic wave away from a transmitting source in three dimensional spaces.

b)Losses in transmitting and receiving component

The loss occurs between the transmitter and antenna, denoted by LFTX. At the output of the transmission amplifier, power PTX is,

PTX PT LFTX

Where PT is transmitting power in watts and LFTX is transmitter feeder loss.The loss LFRX occurs between the antenna and the receiver, at the input of receiver, signal power PRX is,

PRX PR/LFRX

Where PR is receiving power in watts and LFRX is Receiver feeder loss.

c)Imperfect alignment of the antennas

The loss of antenna gain which can be inserted in the form of a misalignment loss LT on transmission and misalignment loss in reception. These losses are a function of misalignment angle of transmission αT and reception αR.

LT 12(αT /θ3dB) 2

LR 12(αR /θ3dB) 2

d)Polarisation Mismatch loss

When the receiving antenna is not oriented with the polarisation of received wave, the polarisation mismatch loss occurs. In link with circular polarisation, the transmitted wave circularly polarised only on the axis of antenna and elliptical of this axis. Propagation through the atmosphere can also change circular in to elliptical polarisation. In a linear polarisation, the wave can be rotate of its plane of polarisation as it propagates through the atmosphere.

2.3System Noise Temperature

The four sources of noise in the front-end area are the receiver front end, the receiver antenna, the connecting elements between them and noise entering from the free space path. The receiver antenna, receiver front end, and the connecting elements between them are the subsystems that must be designed to minimize the effects of noise on the performance of the satellite link. Both the ground terminal antenna/receiver and the satellite antenna/receiver are possible sources of noise degradation. The received carrier power at the receive antenna terminals, PR, if is very low (picowatts) then very little noise introduced into the system at that point is needed to degrade the performance.

The major contributor of noise at radio frequencies is thermal noise, caused by the thermal motion of electrons in the devices of the receiver. The noise introduced by each device in the system is quantified by the introduction of an equivalent noise temperature. The equivalent noise temperature, Te, is defined as the temperature of resistor producing a noise power per unit bandwidth that is equal to that produced by the device.

The noise power, nN, is defined by the Nyquist formula as

nNk Te BN watts

Where k Boltzmann’s Constant = 1.39 × 10−23 Joules/K

−228.6 dBw /K/Hz

Te equivalent noise temperature in K

BNnoise bandwidth, in Hz

Antenna losses are absorptive losses produced by the physical structure main reflector, sub reflector, struts, etc., which effectively reduce the power level of the radiowave. Antenna losses are usually specified by an equivalent noise temperature for the antenna. The antenna loss is usually included as part of the antenna aperture.

In clear condition ,at the frequencies greater than 2 GHz the greater contribution is that of the non-ionized region of atmosphere which being an absorbent medium ,is noise source. In the absence of meteorological formation the antenna noise temperature contains contributions due to the sky and surrounding ground.The antenna noise temperature TA of receiver can be expressed as

TA TSKYTGROUND

System noise temperature is

TS TA /LFRX T0 (11/LFRX) TR

2.4 Figure of Merit (G/T ratio)

It is defined as the ratio of receiver antenna gain to the receiver system noise temperature. Regarding the antenna gain GR and equivalent input noise temperature TS can be denoted as,

(G/T) GR − TS

GR − 10 log10 (TS)

Where TS is the system noise temperature. The (G/T) is a single parameter measure of the performance of the receiver system, and is analogous to EIRP as the single parameter measure of performance for the transmitter portion of the link. (G/T) values cover a wide range in operational satellite systems, including negative dB values. The lower values are typically found in satellite receivers (uplinks) with broad beam antennas where the gain may be lower than the system noise temperature expressed in dB.

2.5 Link performance parameter

2.5.1 Carried-to-noise Ratio

The ratio of average RF carrier power C to the noise power N in the same bandwidth is called carrier-to-noise ratio. It express in (C/N). It is the primary parameter of interest for defining the overall system performance in a communications system. It can be defined at any point in the link, such as at the receiver antenna terminals, or at the input to the demodulator.

The (C/N) can be expressed in terms of the EIRP, G/T, and other link parameters developed earlier. Consider the link with a transmit power PT, transmit antenna gain GT, and receive antenna gain GR .In practical case, we define the losses on the link by two components, the free space path loss.

LFS (4πR/λ) 2

The received power at the input,

PR PTGTGR(1/LFSL0)

EIRP (G/T) (LFSΣ other losses) 228.6 BN

Where the EIRP is in dBW, the bandwidth BN is in dBHz, k = −228.6 dBw/K/Hz.

The (C/N) is the single most important parameter that defines the performance of a satellite Communications link. The larger the C/N, the better the link will perform. Some modern communications systems that employ significant coding can operate at much lower values. Spread spectrum systems can operate with negative C/N values and still achieve acceptable performance. The performance of the link will be degraded in two ways: if the carrier power, C, is reduced, and/or if the noise power, Nn, increases. Both factors must be taken into account when evaluating link performance and system design.

2.5.2 Carrier-to-Noise Density

A related parameter to the carrier-to-noise ratio often used in link calculations is the carrier to- noise density, or carrier-to-noise density ratio, C/No. The carrier-to-noise density is defined in terms of noise power density, No, defined by Equation

N0

The carrier-to-noise density behaves similarly to the carrier-to-noise ratio in terms of system Performance. The larger the value, the better the performance. The C/No tend to be much larger in dB value than the C/N because of the large values for BN that occur for most communications links.

2.5.3 Energy-Per-Bit to Noise Density

For digital communications systems, the bit energy, Eb, is more useful than carrier power in describing the performance of the link. The bit energy is related to the carrier power from,

Eb C Tb

Where C is the carrier power and Tb is the bit duration in s. The energy-per-bit to noise density ratio, Eb/No, is the most frequently used parameter to describe digital communications link performance. Eb/ No is related to (C/N0) by

Eb/N0 Tb (C/N0)

(1/Rb) (C/N0)

Where Rb is the bit rate, in bits per second (bps). This relation allows for a comparison of link performance of both analog and digital modulation techniques, and various transmission rates, for the same link system parameters.

2.5.4 Uplink

Satellite link performance evaluation for an uplink includes additional considerations and parameters. If we represent the uplink parameters by the subscript U.

Satellite telephone handsets are restricted to transmitter power level below 1 w because of the risk of EM radiation hazard. In mobile system the uplink from the satellite telephone is usually the link with lowest C/N ratio.

Where LU is the sum of other losses on the uplink. Uplink performance is often specified in terms of a power flux density requirement at the satellite receiver antenna to produce a desired satellite output transmit power.

2.5.5 Downlink

++228.6

Where LD is the sum of other losses on the downlink. When input backoff is employed for multiple carriers or for linear operation, a corresponding output backoff must be included in the link performance equations. The downlink EIRP that is the EIRP from the satellite, resulting from operation at an output backoff of BOo is given by

EIRPDM EIRPDS− BOo

Where EIRPDS is the downlink EIRP for a single carrier saturated output. BOo is on linearly related to BOi.

3 COMMUNICATION SATELLITE SYSTEM DESIGN

The RF communication links of communication mobile satellite systems between two handheld terminal have been designedand analyzed using the fundamental parameters and losses in this section.

Input data of return link are given below.

3.1 Return link

At user terminal

Transmitting Amplifier Power = PT(MAX) 2 watt

Antenna Gain = GTMAX 3 dBi

Distance = R 40000Km

Atmospheric Wave Attenuation = LA 0.3 dB

At satellite station

Antenna Diameter = D 12 meter

Antenna Noise Temperature = TA 290 ˚K

Absolute Temperature = T0 290˚K

Noise Figure = NF 3 dB

Transmitting Amplifier Power = PT(MA 15 Watt

Antenna Diameter = DSL 0.7 meter

At gateway station

Antenna Diameter = D 6 meter

Maximum Pointing Error at Earth Station= αR 0.1

Antenna Noise Temperature = TA 65˚K

Absolute Temperature = T0290 ˚K

Noise Figure = NF2.2 dB

Calculation

Input data of forward link are given below.

3.2 Forward link

At gateway station

Transmitting Amplifier Power = PT(MAX) 0.35 watt

Antenna Diameter = D 6 meter

Distance = R 40000Km

At satellite station

Antenna Diameter = D 0.7 meter

Antenna Noise Temperature = TA 290 ˚K

Absolute Temperature = T0 290˚K

Noise Figure = NF 3 dB

Transmitting Amplifier Power = Pt(MAX) 230 watt

Antenna Diameter = DSL12 meter

At user terminal

Antenna Diameter = D 0.1 meter

Maximum Pointing Error at Earth Station= αR 0.1˚

Antenna Noise Temperature = TA 65˚K

Absolute Temperature = T0 290˚K

Noise Figure = NF 2.2 dB

Calculation

4 CONCLUSIONS

A satellite RF communication link between user terminal is analyzed based on a simple fundamental equations by systematic approach in this paper. Ingeneral, the above procedures are applicable to LEO (Low Earth Orbit), MEO (Medium Earth Orbit),and GEO satellite systems, and also are valid in satellite and ground station since the two pointswhich communicate with each other are symmetrical in the sense of uplink and down link. The keyparameters are operating frequency (up-link and down-link), transmitter power output, antenna gain,satellite altitude, receiver noise temperature, andsystem . Other important parameters may be calculated by combiningthe key parameters.

It is very informative to estimate the overall mobile satellite communication systems. However, themore detailed analysis will be required by taking RF circuits into account.

References

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[2]‘Pratt’, second edition satellite communication,1993.

[3]‘Shingo Ohmari’ ,Mobile satellite communication,1993.

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[5] M.Vilaca, The INMARSAT 4 Mobile Satellite System in 8th International workshop on signal processing for space communication SPSC 2003 ,catania Italy ,24-26 September,2003.

[6]Lutz, E., D. Cygcn, M. Oippold. E Dolainsky, and W. Papkc,' The Land MobileSatellits Communication Channel' - Recording. Stati.tinics, and C h m d Madcl, IEEE Trans. on Vehicular Tech. , ,% I .a.No . 2. May 1991, pp.315-386.

[7] Masafumi SHIGAKI, Katsuyoshi SHIMADA, Nokazu HAMAMOTO,Yukio HASHIMOTO, Toshiyuki IDE, and Shinichi YAMAMOTO,'A handheld terminal for Mobile Satellite System ',21st international satellite system conference AIAA 2003 -2403.

[8]Louis J. Ippolito, Jr. Satellite Communications Systems Engineering ,'Atmospheric Effects, Satellite Link Design and System Performance' ,2000.

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

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