Unit VI Satellite Communication Link Design Contents of

Unit VI Satellite Communication Link Design

Contents of Unit VI �Satellite Communication Link Design Introduction, Basic Transmission Theory, System Noise Temperature and G/T Ratio, Design of Downlinks, Satellite Systems using Small Earth Stations, Uplink Design, Design for Specified C/N: Combining C/N and C/I Values in Satellite Links, System Design Examples.

LINK BUDGET

LINK BUDGET �A satellite link is defined as an… Earth station - satellite - Earth station connection. The Earth station - satellite segment is called the uplink and The satellite - Earth station segment is called the downlink. �The Earth station design consists of the Transmission Link Design, or Link Budget, and the Transmission System Design. �The Link Budget establishes the resources needed for a given service to achieve the performance

Some IMP issues in Link Design �Weight ($25, 000 per KG) -- half of weight is fuel. �Choice of Frequency band (& Multiple Access Technique) �Atmospheric propagation effects �Antenna type and related issues.

LINK BUDGET �Performance objectives for digital links consist of: v BER for normal operating conditions v Link Availability, or percentage of time that the link has a BER better than a specified threshold level

Basic Transmission Theory �The Calculation of the power received by an earth station from a satellite transmitter is fundamental to the understanding of Sat Comm. �Two approaches: Ø Use of Flux Density Ø Link Equation

Flux density produced by an isotropic source. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright © 2003 John Wiley & Sons. Inc. All rights reserved. Flux density crossing the surface of sphere is F= Pt/4�� R 2

Power received by an ideal antenna with area A m 2. Incident flux density is F = Pt/4 R 2 W/m 2. Received power is Pr = F X A = Pt. A/4 R 2 W. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt

Finally, �Pr = (EIRP x Rx Antenna Gain) --- Link Equation Path Loss In d. B, Pr = EIRP + Gr - Lp (Lp+La+Lta+Lra)

A satellite link. LNA, low noise amplifier. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright © 2003 John Wiley & Sons. Inc. All

Different Types of losses:

Free Space Loss FSL = 10 log (4 pr/l)2 in d. BW , FSL = 32. 4 + 20 log r + 20 log ƒ e. g. , ES to satellite is 42, 000 km, ƒ is 6 GHz, what is FSL? » FSL = 32. 4 + 20 log 42000 + 20 log 6000 = 200. 4 d. B » Very large loss!! e. g. , EIRP = 56 d. BW, receive antenna gain 50 d. B » PR = 56 + 50 - 200. 4 = -94. 4 d. BW = 355 p. W • Other sources of losses – Feeder losses – Antenna misalignment losses – Fixed atmospheric and ionospheric losses – Effects of rain • PR = EIRP + GR - Losses, in d. BW

Path Loss �Depends on: �Distance and frequency �About 200 d. B at C-band �About 206 d. B at Ku-band

Atmospheric Losses in the signal can also occur through absorption by atmospheric gases such as oxygen and water vapor. This characteristic depends on the frequency, elevation angle, altitude above sea level, and absolute humidity. At frequencies below 10 GHz, the effect of atmospheric absorption is negligible. Its importance increases with frequencies above 10 GHz, especially for low elevation angles.

Atmospheric Losses �Table shows an example of the mean value of atmospheric losses for a 10 -degree elevation angle.

Atmospheric Attenuation

Atmospheric Attenuation

Atmospheric Absorption 1% of the time, rain attenuation exceeds 0. 3 d. B (99% of the time, it is less than or equal to 0. 3 d. B) 0. 5% of the time, it exceeds 0. 5 d. B 0. 1% of the time, it exceeds 1. 9 d. B

Rain Effects � An important climatic effect on a satellite link is the rainfall. Rain results in attenuation of radio waves by scattering and by absorption of energy from the wave. � Rain attenuation increases with the frequency, being worse for Ku-band than for C-band. Enough extra power must be transmitted to overcome the additional attenuation induced by rain to provide adequate link availability.

Numericals on it… �Ex 4. 2. 1 and 4. 2. 2 to calculate Pr

System Noise Temp and G/T Ratio �Noise Power : Pn= k. Tp. Bn () �Noise Power at Demodulator : N = Pno : k. Tp. Bn. Grx �Signal Power received at Receiver = C= Pr. G �Therefore: C/N = Pr. G = __Pr__ k. T p. Bn. Grx k. Tp. Bn rx rx

Carrier to Noise Ratio In the link equation, by unfolding the k. TB product under the logarithm, the link equation becomes: C/N = EIRP - L+ G - 10 log(k) - 10 log(T) - 10 log(B) The difference, G - 10 log. T, is the figure of merit: (17) C/N = EIRP - L+ G/T - 10 log(k) - 10 log(B) Where: L = transmission losses G/T = figure of merit of the receiver k = Boltzmann constant B = carrier occupied bandwidth (18)

Carrier to Noise Ratio Because the receiver bandwidth (B) is often dependent on the modulation format, isolate the link power parameters by normalizing out the bandwidth dependence. The new relation is known as Carrier-to-Noise Density ratio (C/No). C/No = EIRP - L + G/T - 10 log(k) (19) Note that: C/N = C/T - 10 logk. B (20) Expressing C/T as a function of C/N, and replacing C/N with the right side of the link equation, results: C/T = EIRP - L + G/T (21)

Numericals: Sample Question #1. A C-band earth station has an antenna with a transmit gain of 54 d. B. The transmitter output power is set to 100 W at a frequency of 6. 100 GHz. The signal is received by a satellite at a distance of 37, 500 km by an antenna with a gain of 26 d. B. The signal is then routed to a transponder with a noise temperature of 500 K, a bandwidth of 36 MHz, and a gain of 110 d. B. Calculate the path loss at 6. 1 GHz. Wavelength is 0. 04918 m. Answer: Path loss = 20 log ( 4 p R / l) = 20 log ( 4 p ´ 37, 500 ´ 103 / 0. 04918) d. B Lp = 199. 6 d. B a. b. Calculate the power at the output port (sometimes called the output waveguide flange) of the satellite antenna, in d. BW. Answer: Uplink power budget gives Pr = Pt + Gr - Lp d. BW = 20 + 54 + 26 – 199. 6 = -99. 6 d. BW c. Calculate the noise power at the transponder input, in d. BW, in a bandwidth of 36 MHz. Answer: N = k Ts BN = -228. 6 + 27 + 75. 6 = -126. 0 d. BW. Calculate the C/N ratio, in d. B, in the transponder. Answer: C/N = Pr – N = -99. 6 + 126. 0 = 26. 4 d. B d e. Calculate the carrier power, in d. BW and in watts, at the transponder output. Answer: The gain of the transponder is 110 d. B. Output power is Pt = Pr + G = -99. 6 + 110 = 10. 4 d. BW or 101. 04 = 11. 0 W.

Double conversion earth station receiver. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright © 2003 John Wiley & Sons. Inc. All

Noise model of receiver. The noisy amplifiers and downconverter have been replaced by noiseless units, with equivalent noise generators at their inputs.

Noise model of receiver. All noisy units have been replaced by one noiseless amplifier, with a single noise source Ts as its input.

Combined Together: �Pn = k. Tp. Bn. Grx -where Grx = GRFGm. GIF Tp = Ts = [ Tin +Trf + Tm/Grf + Tif /(Gm. Grf)] succeeding stages contribute less and less noise to total system noise temperature

Numericals. . . �Numericals on Ts calculation and Pn calculation… Ex 4. 3. 1 and 4. 3. 2

Noise Figure and Noise Temperature �NF= SNR(in) / SNR(out) �T = To(NF-1) �G/T Ratio From the equation of C/N = Pr/N It can be seen that, C/N α G/T Ratio Numericals…

Design of Downlinks �The design of any satellite comm is based on two objectives: 1. Meeting a specified C/N ratio for a specified time. 2. Carrying a maximum revenue earning traffic at min cost.

Link Budget Example: Downlink Budget

Numericals…

Satellite Systems using Small Earth Stations (DBS) �Same as of normal downlink design + consideration of Sky Rain attenuation… + One numerical…

Uplink Design �Design same as of Downloads… �Difference is that the noise power calculation are not for the receiver but for the transponder.

Link Budget Example: Uplink Budget

Link Budget Example: Uplink Budget � The repeater in this design is a simple bent pipe that does not alter or recover data from the transmission from the uplink. The noise on the uplink (e. g. , N in the denominator of C/N) will be transferred directly to the downlink and added to the downlink noise. � In a baseband processing type of repeater, the uplink carrier is demodulated within the satellite and only the bits themselves are transferred to the downlink. � In such case, the uplink noise only produces bit errors (and possibly frame errors, depending on the modulation and multiple access scheme) that transfer over the re-modulated carrier. � This is a complex process and can only be assessed for the particular transmission system design in a digital processing satellite.

Over all Design (Downlink + Uplink) �Adding the C/N ratio and C/I Ratio to get over all C/N Ratio. �Reciprocal C/N Formula

System Design Examples � Ex 4. 8. 1 – Satcomm in Ku Band (Uplink +Downlink +Attn) � Ex 4. 8. 2 – Satcomm in LEO oribit (Appln: PCS) Satellite Uplink = Mobile – Gateway + Gateway – Downlink = Gateway –Satellite + Mobile – Gateway Inbound – Link between Mobile to Gateway Out bound – Link between Gateway to Mobile Outbound Uplink Inbound Uplink. (Total 4 links) Refer Figure on