SATELLITE COMMUNICATIONS Master of Management and Economics of Telecommunication Networks University of Athens - 006 The Link Budget by E. Rammos ESA Senior Advisor Satcom Courses University of Athens ER 006 - Slide nr 1 The Link Budget Forward Link Return link HUB Client Satcom Courses University of Athens ER 006 - Slide nr
The Link Budget The Link Budget Relates the Signal-to-Noise Ratio to the principal Characteristics of the Equipment on the Satellite and in the Ground Station The Telecommunications Equation allows the calculation of the received power as a function of the transmitted power and the antenna characteristics Satcom Courses University of Athens ER 006 - Slide nr 3 The Link Budget Transmission side Pe = Transmit Power Ge = Transmit Antenna Gain Se = Transmit Antenna Equivalent Surface Reception side Pr = Received Power Gr = Receive Antenna Gain Sr = Receive Antenna Equivalent Surface Satcom Courses University of Athens ER 006 - Slide nr 4
The Link Budget Isotropic antenna power flux per unit surface at distance D: PFD = Pe 4πD Satcom Courses University of Athens ER 006 - Slide nr 5 The Link Budget Taking into account the Transmit Antenna Gain Ge the PFD becomes: PFD = GePe 4πD The product GePe is called the EIRP (Equivalent Isotropically Radiated Power) Satcom Courses University of Athens ER 006 - Slide nr 6
The Link Budget At the receiver side the receive antenna intercepts from the incoming wave a power: Pr = SrGePe 4πD Satcom Courses University of Athens ER 006 - Slide nr 7 The Link Budget The antenna Gain is related to the effective area by: Gr = 4πSr is the wave wavelength Satcom Courses University of Athens ER 006 - Slide nr 8
The received power is therefore: Pr = PeGeGr ( 4πD) Pe = Pr GeGr ( 4πD ) And the transmission attenuation α is: Satcom Courses University of Athens ER 006 - Slide nr 9 In db the attenuation is given by: Pe αdb = 10log Pr And therefore the overall expression becomes: D αdb = + 0 log( ) Ge Gr For fixed antenna gain the attenuation varies as: D αdb = + 0 log( ) This is the so called Free Space Attenuation Free Space Attenuation (about 06db in Ku-Band for GEO orbit) Satcom Courses University of Athens ER 006 - Slide nr 10
Free Space Loss as a function of Frequency Attenuation (db) 5 0 15 10 05 00 195 190 185 180 1 10 100 Frequency (GHz) Satcom Courses University of Athens ER 006 - Slide nr 11 The Antenna gain with respect to an isotropic antenna is given by: G = n4πa n is the antenna efficiency, typically 0.6 or 60% A is the Area of the Antenna If the antenna is circular with diameter D the gain is given by: πd G = n( ) Satcom Courses University of Athens ER 006 - Slide nr 1
Gain as a function of the Diameter Gain (db) 70 60 50 40 30 0 10 0 10 100 1000 Diameter (cm) Ku-Band C-Band L-Band Satcom Courses University of Athens ER 006 - Slide nr 13 The 3db beamwidth of the antenna is given by: ϑ3 db = 70 D For a good antenna of 65 % efficiency the gain is approximately: G = 3000 ϑ 3db Satcom Courses University of Athens ER 006 - Slide nr 14
3db Beamwidth 3db BW (deg) 0 18 16 14 1 10 8 6 4 0 0 100 00 300 400 500 Diameter (cm) Ku-Band C-Band L-Band Satcom Courses University of Athens ER 006 - Slide nr 15 Satcom Courses University of Athens ER 006 - Slide nr 16
In real cases the various losses need to be taken into account and a loss coefficient A is added to the equation Pe = Pr ( 4πD) AGeGr and in db D αdb = + 0 log( ) Ge Gr + A Satcom Courses University of Athens ER 006 - Slide nr 17 A = Atr Aprop Apol Apoint Arec Atr = losses between transmitter output and antenna (transmission lines, duplexers, filters ) Aprop=propagation losses in the atmosphere and ionosphere Apol = polarisation losses (pol. mismatch ) Apoin= antenna pointing losses Arec = losses between receive antenna and receiver (lines, duplexer, filters ) Satcom Courses University of Athens ER 006 - Slide nr 18
Respecting the order of the various phenomena the overall equation is written as: D Pe = Pr+ Arec Gr + Apol + Aprop + + 0log + Apoin Ge + Ae Example: For the following assumptions: GEO satellite transmitting at a frequency of 1 GHz Receiver sensitivity -108dBW Transmit antenna 1.3m diameter (55% efficiency) Receive antenna 1m diameter (55% efficiency) Polarisation loss 1dB Pointing loss 3 db Transmission loss 1 db Reception loss 1db Find the required Transmit power (in W) Satcom Courses University of Athens ER 006 - Slide nr 19 Frequency (GHz) 1 Wavelength (m) 0,05 Receive Antenna Trans mit Antenna Diameter (m) 1 1,3 Efficiency 0,55 0,55 Gain (db) 39,38 41,66 Notations Negative terms Pos itive terms Required Power at receiver Pr -108 Reception losses Are c 1 Receive Antenna Gain Gr -39,38 Polarisation losses Apol 1 Free Space Loss +0logD/ 05,17 Pointing Lo s s Apoin 3 Transmit antenna Gain Ge -41,66 Trans mit los s Atr 1 To tal -189,05 11,17 Trans mit Power (db),1 Transmit Power (W) 16,99 Satcom Courses University of Athens ER 006 - Slide nr 0
Receiver Noise Figure F It is the ratio of the noise power Ns at the receiver output to the receiver output when only a noise source at temperature To=300K is connected at the input. F = Ns GkToB G is the receiver gain B is the receiver frequency bandwidth K = 1.379.10-3 W/HzK is the Boltzman constant (and in db it is equal to -8.6 dbw/hzk) Satcom Courses University of Athens ER 006 - Slide nr 1 Receiver Equivalent Noise Temperature Te It is the temperature of a noise source at the input of an ideal receiver that would generate at the ideal receiver output the same noise power generated by the real receiver. If at the input of the real receiver is connected a noise source at temperature To then the noise power at the output is: Ns = GkToB + GkTeB Satcom Courses University of Athens ER 006 - Slide nr
The Noise Figure and the equivalent noise Temperature are related by: F =1+ Typical values of Te are: Te To For ground stations 10 to 100K For satellite receivers can be much higher Te (K) 7 35 75 300 900 3000 F (db) 0,1 0,5 1 3 6 10 Satcom Courses University of Athens ER 006 - Slide nr 3 The total Equivalent Noise Temperature of a series of receivers, each of gain G i (i=1,,3..) and of equivalent noise temperature of T ei is given by: T e = T T T 3 1 + e + e e + G1 G1G... Satcom Courses University of Athens ER 006 - Slide nr 4
Antenna Noise Temperature T A All bodies radiate energy. Received by the antenna this is an external noise source. If No is the received spectral density (W/Hz) then: N 0 = k T A It depends on the bodies generating the noise and the antenna characteristics. For antennas pointed to the satellites it is mainly due to: the sky noise the earth radiation temperature Satcom Courses University of Athens ER 006 - Slide nr 5 The sky noise is more important for frequencies above Ghz Generated by the non ionised regions of the atmosphere. The clouds, rain etc also generate noise. For frequencies between 1 to 15 GHz it typically below 40K. If the sun is within the antenna beam then the noise may increase by thousands degrees! The earth radiation noise temperature It has about the value of the physical temperature (around 90K) It depends on the antenna radiation pattern and the antenna orientation Satcom Courses University of Athens ER 006 - Slide nr 6
Global Noise Temperature For a receiver with Equivalent Noise Temperature Te, connected to an antenna with antenna Temperature T A via a line with line losses L at a physical temperature T L the global noise temperature at the receiver input is: T 1 T = + TL (1 ) L L A + Te The line loss adds about 7K noise temperature for each tenth of db of losses Satellite antenna looking at the earth have T A equal to about 300K Satcom Courses University of Athens ER 006 - Slide nr 7 Signal to Noise Ratio at the output of the receiver in the band B is given by: Pr N = PeGeGr AkTB If the signal is a carrier C=Pr and if No=N/B (W/Hz) is the spectral noise density then: ( 4π D C Gr PeGe 1 1 = ( )( ) ( ) N 4 D T k A 0 π ) Satcom Courses University of Athens ER 006 - Slide nr 8
The Signal to Noise Ratio in db is given by: ( C N 0 ) dbhz 4πD Gr = 10log( PeGe) 0log( ) + 10log 10logA+ 8.6 T EIRP Free Space Quality Losses Losses Factor Satcom Courses University of Athens ER 006 - Slide nr 9 According the Shannon theorem, for a transmission without error, of a rate R (bits/sec) in a bandwidth B: R B log (1 + C N For digital transmission at a rate R bit/sec the energy per bit E is related to the C/N by: C = N E No R B ) Satcom Courses University of Athens ER 006 - Slide nr 30
Satcom Courses University of Athens ER 006 - Slide nr 31 The total link from a transmit station via the satellite to an other receive station includes two links: The Uplink from the transmit station to the satellite, characterised by a signal to noise ration of (C/No) U The Downlink from the satellite to the receive station, characterised by a signal to noise ratio (C/No) D The total link signal to noise ration is given by ( C ) N 1 T C C 1 1 = ( ) U + ( ) D N N Satcom Courses University of Athens ER 006 - Slide nr 3
Propagation losses in the atmosphere Absorption from the gases in the atmosphere - Oxygen around 60 GHz - Water vapor around.5 GHZ Diffusion by rain etc creates interferences Increase of the Noise contribution by the sky Degradation of polarisation isolation Absorption due to rain: complex phenomenon depending on rain drops diameter and distribution, and increasing with the frequency (up to about 10GHz). Rain statistics very important for dimensioning of link. Satcom Courses University of Athens ER 006 - Slide nr 33 Satcom Courses University of Athens ER 006 - Slide nr 34
Operational Constraints Choice of Frequency Band - Coverage zone on the earth - Small earth station requirement (e.g. TVRO, SIT) - Volume limitation on the launcher Propagation Conditions - Rain attenuation statistics (e.g. SE Asia, Africa) Frequency regulations - As defined by International regulations Satcom Courses University of Athens ER 006 - Slide nr 35 Frequency Bands The radio-frequency spectrum, as defined by the International Telecommunications Union (ITU), is said to extend from 3 khz to 3,000 GHz The spectrum is divided into nine bands as listed below (ITU Radio Regulations 1998, V.1, Article RRS, Nomenclature, Section I - Frequency and Wavelength Bands Satcom Courses University of Athens ER 006 - Slide nr 36
Band letters Unofficial, non-standard and consequently imprecise designations of frequency bands Often divided into subbands, designated by suffix subscript letters. For instance, the K band usually is divided into at least two subbands, designed Ku for frequencies in the range 10 14 GHz and Ka for frequencies in the range 4 36 Satcom Courses University of Athens ER 006 - Slide nr 37 Radio Spectrum Regulations The repartition of the radio frequencies between the various services is defined in the frame of the International Telecommunications Union (ITU) The Comite Consultatif International des Radiocommunications (CCIR) prepares the recommendations for the technical characteristics The CCIR recommendations are made for limiting the interferences between the various systems. Several bands are shared between space and terrestrial systems For Space Systems the World Administrative radio Conference (WARC) allocates frequency bands to be used by various services and administrations Satcom Courses University of Athens ER 006 - Slide nr 38
Radio Regulations -systems may operate at designated frequency bands (depending in part on regional availability and on the regulatory allocation of the country in which the system operates) Satellite Services include - Fixed Satellite Services (FSS) - Broadcast Satellite Services (BSS) - Mobile Satellite Services (MSS) Satcom Courses University of Athens ER 006 - Slide nr 39