Analysis of satellite link budget

Dr.Rashid Saeed Aalaa Hussein Ibrahim Ibrahim_khider@hotmail.com aalaahussien@yahoo.com Sudan University of science & technology, school of electronics engineering, post graduate studies, September 2013.

Abstract:
The communication link between a satellite and the Earth Station (ES) is exposed to a lot of impairments such as noise, rain and atmospheric attenuations. It is also prone to loss such as those resulting from antenna misalignment and polarization. It is therefore crucial to design for all possible attenuation scenarios before the satellite is deployed. This paper presents the rudiments of a satellite link design by using MATLAB.

KEYWORDS: Satellite communications, Link analysis, Link design, EIRP, SNR, CNR. I. INTRODUCTION The satellite link is essentially a radio relay link, much like the terrestrial microwave radio relay link with the singular advantage of not requiring as many re-transmitters as are required in the terrestrial link. Transmission of signals over a satellite communication link requires Line-of-Sight (LoS) communication, but since theoretically three equidistant satellites in the geosynchronous orbit can effectively cover over 90 percent of the earth surface, the need for multiple retransmissions is removed. Satellite communication specialists, radio and broadcast engineers are in the business of determining the factors required for optimal link availability and quality of performance. These factors can be divided into two broad categories; the conduit factors and the content factors. The conduit factors include such factors as: earth-space and space-earth path (a.k.a. uplink and downlink) effect on signal propagation, quality of earth station equipments, and the impact of the propagation medium in the frequency band of interest, et cetera. The content factors deal mainly with the type of message transmitted and the devices involved in its transformation from one form to another for suitability for transmission over a microwave medium. These include, but are not limited to: satellite functionality, nature and peculiarities of the precise nature of information, data protocol, timing, and the telecommunications interface standards that apply to the service. It is for these reasons that a proper engineering methodology is required to guarantee timely deployment and effective and efficient exploitation of satellite communication applications and devices. These in turn must guarantee delivery of objectives for quality, reliability and availability. The remaining part of this tutorial paper presents the various component parts necessary for designing a robust satellite link with appreciable availability and required signal/noise ratios.

PtGt is called the Effective Isotropic Radiated Power or EIRP because an isotropic radiator with an equivalent power equal to PtGt would produce the same flux density in all directions.

II. BASIC LINK ANALYSIS Link analysis basically relates the transmit power and the receive power and shows in detail how the difference between these two is accounted for. To this end the fundamental elements of the communications satellite Radio Frequency (RF) or free space link are employed. Basic transmission parameters, such as antenna gain, beam width, free-space path loss, and the basic link power equation are exploited. The concept of system noise and how it is quantified on the RF link is then developed, and parameters such as noise power, noise temperature, noise figure, and figure of merit are defined. The carrier-to-noise ratio and related parameters used to define communications link design and performance are developed based on the basic link and system noise parameters introduced earlier. The flux density and link equation can be used to calculate the power received by an earth station from a satellite transmitter with output power Pt watts and driving a lossless antenna with gain Gt, the flux density in the direction of the antenna bore sight at a distance R meters is given by:

Thus

Equation (4) is known as the link equation and it is essential in the calculation of power received in any radio link. The term (4R/)2 is known as the Path Loss (Lp). It accounts for the dispersion of energy as an electromagnetic wave travels from a transmitting source in three-dimensional space. A measure of the attenuation suffered by a signal on the EarthSpace path. For a real antenna, however, the physical aperture area Ar, the effective aperture area Ae, and the aperture efficiency are related by the equation (5). (5) For real antenna equations (2) and (4) become (6) and (7):

The link equation expressed in equation (4) may be read as presented in equation (8).

III. SIGNAL ATTENUATION The path loss component of equation (9) is the algebraic sum of various loss components such as: losses in the atmosphere due to attenuation by air, water vapor and rain, losses at the antenna at each side of the link and possible reduction in antenna gain due to antenna misalignment (due to poor operation of the AOC3 satellite subsystem). This needs to be

IV. SOURCES OF INTERFERENCE With many telecommunication services using radio transmission, interference between services is inevitable and can arise in a number of ways. The Satellite Users Interference Reduction Group (SUIRG) categorizes satellite communication interference into five main groups, these are: 1. User error (Human error and equipment failure) 2. Crossbow Leakage 3. Adjacent satellites 4. Terrestrial services 5. Deliberate interference However, for the purpose of satellite link design, interference may be considered as a form of noise and hence, system performance is determined by the ratio of wanted to interfering powers. In this case the wanted carrier to the interfering carrier power or C/I ratio [2]. The single most important factor controlling interference is the radiation pattern of the earth station antenna. A. Downlink and Uplink Interference Ratios Consider two satellites, SC as the wanted satellite and SI as the interfering satellite. The carrier power received at an earth station is given by equation (11): [C] = [EIRB C ] + [GR ] - [FC ] - [Lac ] (11) [*] denotes values are in decibels. Where, EIRPC Equivalent Isotropic Radiated Power from satellite SC; GR Bore-sight (on-axis) receiving antenna gain; FC footprint contour of the satellite transmit antenna and Lac free space loss. An equation similar to equation (11) may be used for the interfering carrier power, albeit with the introduction of an additional term: [PD], which incorporates the polarization discrimination. Also the receiving antenna gain at the earth station is determined by the off-axis angle , giving: [ I ] = [EIRPI ] + [ GR () ] +[ FI ]- [Lac ] + [PD ] (12) Assuming that the free-space loss is the same for both the carrier and interference signals, and then from equations (11) and (12) we have that:

B. Carrier to Noise Ratio (C/N) One of the objectives of any satellite communication system is to meet a minimum carrier to noise (C/N) ratio for a specified percentage of time. The C/N ratio is function of the system noise temperature, which is very important in understanding the topic of carrier to noise ratio. V. SYSTEM NOISE A. Noise temperature Noise temperature provides a way of determining how much thermal noise active and passive devices generate in the receiving system. The most important source of noise in receiver is thermal noise in the pre-amplification stage. The noise power is given by the Nyquist equation as (16): Pn =kT pBn (16)

Where Pn delivered to load with matched impedance to source noise; k Boltzman constant = 1.39 x 10-23 J/K = - 228.6 dBW/K/Hz; Tp Noise temperature of source in Kelvin; Bn Noise bandwidth in which the temperature is measured in Hz. The term kTp is noise power spectral density and is constant for all radio frequencies up to 300 GHz. A low noise amplifier is usually desired. An ideal noiseless amplifier has a noise temperature of 0 K. Gallium Arsenide field effect transistors (GaAsFET) are normally used as amplifiers in satellite

Fig. 1. Simplified earth station receiver [2]. Communication systems because they can be used to achieve noise temperatures of 30 K to 200 K without physical cooling. GaAsFET can be built to operate at room temperature with a noise temperature of 30 K at 4 GHz and 100 K at 11 GHz; other conventional amplifiers give higher values. A simplified ES receiver is presented in Fig. 1. Since the RF amplifier in a satellite communication receiver must generate as little noise as possible, it is called a low noise amplifier (LNA). The mixer and local oscillator form a frequency conversion stage that down-converts the radio frequency signal to a fixed intermediate frequency (IF), where the signal can be amplified and filtered accurately. BPF is the band pass filter, used for selecting the operational frequency band of the ES. The receiver shown in Fig. 1 employs a single stage down frequency conversion.

Fig. 2. Double conversion super-heterodyne ES receiver [2].

Many earth station receivers use the double superheterodyne configuration shown in Fig. 2, which has two stages of frequency conversion. The front end of the receiver is usually mounted behind the antenna feed and converts the incoming RF signals to a first IF in the range 900 MHz to 1400 MHz. This allows the receiver to accept all the signals from a satellite in a 500 MHz bandwidth at C or Ku band for example. The noise is further reduced in IF low noise block converter (LNB). The second IF amplifier has a bandwidth matched to the spectrum of the transponder signal. The noise temperature of a source located at the input of a noiseless double conversion receiver shown in Fig. 2 is given by equation (17):

Where Gm, Gif, Grf Mixer, IF and RF amplifier gains respectively; Tm, Tif, Trf their equivalent noise temperatures.

Where Gm, Gif, Grf Mixer, IF and RF amplifier gains respectively; Tm, Tif, Trf their equivalent noise temperatures. VII. LINK BUDGET The link budget determines the antenna size to deploy, power requirements, link availability, bit error rate, as well as the overall customer satisfaction with the satellite service. A link budget is a tabular method for evaluating the power received and the noise ratio in a radio link [2]. It simplifies C/N ratio calculations The link budget must be calculated for an individual transponder, and must be recalculated for each of the individual links. Table 1 below shows a typical link budget for a C band downlink connection using a global beam GEO satellite and a 9m earth station antenna. Link budgets are usually calculated for a worst-case scenario, the one in which the link will have the lowest C/N ratio or lowest tolerable availability. TABLE 1. CBAND GEO SATELLITE LINK BUDGET IN CLEAR AIR
C band satellite parameters Transponder saturated output power 20 W Antenna gain on axis 20 dB Transponder bandwidth 36 MHz Downlink frequency band 3.7 4.2 GHz Signal FM TV analogue signal FM TV signal bandwidth 30 MHz Minimum permitted overall C/N in receiver 9.5 dB Receiving C band earth station Downlink frequency 4 GHz Antenna gain on axis at 4GHz 49.7 dB Receiver IF bandwidth 27 MHz Receiving system noise temperature 75 K Downlink power budget Pt satellite transponder output power, 20 W 13 dB Bo transponder output backoff -2dB Gt satellite antenna gain, on axis 20 dB Gr earth station antenna gain 49.7 dB LP free space path loss at 4GHz -196.5 dB Lant = edge of beam loss for satellite antenna -3 dB La = clear air atmospheric -0.2 dB Lm = other losses -0.5dB Pr = received power at earth station -119.5 dBW Downlink noise power budget in clear air k = Boltzmanns constant -228.6 dBW/K/Hz Ts = system noise temperature, 75 K 18.8 dBK Bn = noise bandwidth 27 MHz 74.3 dBHz N = receiver noise power -135.5 dBW C/N ratio in receiver in clear air C/N =Pr N = -119.5 (-135.5) 16.0 dB 16.0 dB

VIII. SATELLITE LINK DESIGN METHODOLOGY: The design methodology for a one-way satellite communication link can be summarized into the following steps. The return link follows the same procedure.

Methodology: Step 1. Frequency band determination. Step 2. Satellite communication parameters determination. Make informed guesses for unknown values. Step 3. Earth station parameter determination; both uplink and downlink. Step 4. Establish uplink budget and a transponder noise power budget to find (C/N)up in the transponder Step 5. Determine transponder output power from its gain or output back off. Step 6. Establish a downlink power and noise budget for the receiving earth station Step 7. Calculate (C/N) down and (C/N) 0 for a station at the outermost contour of the satellite footprint. Step 8. Calculate SNR/BER in the baseband channel. Step 9. Determine the link margin. Step 10. Do a comparative analysis of the result vis--vis the specification requirements. Step 11. Tweak system parameters to obtain acceptable (C/N) 0 /SNR/BER values. Step 12. Propagation condition determination. Step 13. Uplink and downlink unavailability estimation. Step 14. Redesign system by changing some parameters if the link margins are inadequate. Step 15. Are gotten parameters reasonable? Is design financially feasible? Step 16. If YES on both counts in step 15, then satellite link design is successful Stop. Step 17. If NO on either (or both) counts in step 15, then satellite link design is unsuccessful Go to step 1.

Implementation and results in matlab:

IX. CONCLUSION A number of factors have to be taken into consideration in the design of a robust satellite link. We have presented the most salient of these factors and examined how they are interrelated vis--vis satellite link design for the provision of optimal service availability. The transmitted and received power of the link between the satellite and earth stations must be accounted for,

losses due to the link and communication equipments must be taken into consideration et cetera. The link ratios, which include carriertonoise and Bit error rate are good indicators of the feasibility of the system design. The system availability is another factor of high interest, and must therefore be taken into account. Frequency re use enhances the capacity of the satellite, which makes it a vital element for optimizing the link. A sample link budget was outlined to illustrate the process. We have summarized in the satellite link design methodology the most salient points necessary for achieving a robust satellite link design with desired characteristics. REFERENCES [1] Dennis Roddy, Satellite Communications, 3rd edition, McGraw Hill, USA, 2001, ISBN: 0-07-120240-4 [2] Timothy Pratt et al., Satellite Communications Copyright2003, ISBN: 9814-12-684-5 [3] Gerard Maral and Michel Bousquet, Satellite Communication Systems, 5th edition, John Wiley, UK, 2002 [4] International Telecommunications Union, Handbook on satellite communications, 3rd edition, April, 2002, ISBN: 978-0-471-22189-0. [5] J. A. Pecar, The New McGraw-Hill Telecom Factbbok, McGraw-Hill, New York, 2000, ISBN: 0-07-135163-9.