ECE 5233 Satellite Communications

1. ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 10: Satellite link design (Section 4.4) Spring 2014

2. Outline • Example of noise temperature calculations • Heterodyning • Downlink link budget • Example link budget in C-band Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Single conversion receiver Block diagram of SC super heterodyne RX • LNA – Low Noise Amplifier – first signal amplifying stage • BPF1 – band filter, selection of the band of operation • Mixer – down conversion to IF stage • BPF2 – channel filter, section of the appropriate satellite channel • IF amplifier – main signal amplification on RX chain • Demod – demodulation of the signal • BB – base band processing Note 1: LNA noise temperature is the most dominant term Note 2. band selection filter insertion losses need to be small to minimize noise in mixer Noise power at the output of the IF stage

4. Single conversion receiver –Ex 4.3.1 Suppose we have 4GHz receiver with following gains and temperatures: Ta = 25K, Trf = 50K, Tif = 1000K, Tm = 500K, Grf = 23dB, Gif = 30dB . Calculate system noise temperature assuming no losses in the mixer. Recalculate the system noise temperature assuming 10dB mixer losses. How can the system temperature be minimized in the 10dB mixer lose case. Answers: Ts = 82.5K Ts = 127.5K Increase LNA gain Note: spreadsheet allows for insertion losses and noise temperatures of the filtering components.

5. RX with antenna feed losses (Ex 4.3.2) The system in Ex. 4.4.2 has an LNA with a 50dB gain. A section of lossy wave-guide with attenuation of 2dB is inserted between antenna and LNA. Find the new noise temperature if the physical temperature of the waveguide is 300K. Answers: Ts = 176.5 K Note: To increase sensitivity of the front end, the receiver may be cryogenically-cooled. For example using liquid Nitrogen, the physical temperature may be brought down to 77K.

6. Super-heterodyne receiver • Heterodyning – translation of signal from RF to IF frequency • Reason – to facilitate filtering and demodulation • May be single conversion or double conversion • Opposite to heterodyning – direct conversion receivers Note: double conversion RX has two IF stages Block diagram of a SH RX

7. Downlink link budget • Downlink – satellite to ground • Downlink link budget – usually limiting budget in fixed satcomm • Link budget components • Satellite TX (PA power, backoff, antenna, pointing) • Path losses (FSPL, additional losses due to elements, fade margin) • Ground RX (antenna, G/T ratio, system noise figure) • Link budget – evaluates various system design trade-offs • Date rate / capacity • Reliability • Component size and cost

8. Components of the link budget table • Transmitter side • TX components • Principle result: EiRP • Receiver side • RX components • Principle results • RxSensitivity • System temperature • RSLA • Propagation modeling • Free space loss • Environmental losses • Other system losses • Link reliability and margin calculations Elements of the radio link

9. Satellite transmission – link budget components • Transponder output power • Transponder backoff • Dependent on Peak to Average Power Ratio (PAPR) of the modulated signal • Backoff ensures that transponder PA works in linear region of power amplification • Satellite antenna gain at the edge of coverage region • Usually the coverage region is defined through 3dB beamwidth – gain to the edge is 3dB lower than the maximum gain of the antenna EiRP – effective isotropic radiated power [dBm] PT – transponder power [dBm] Gt – transmit antenna gain on axis [dB] Bo – backoff [dB] Lant – edge of beam loss of antenna gain [dB] PAPR for a given modulation scheme is usually specified through CCDF curves

10. Propagation losses • Components of propagation losses • Free Space Path Loss (path length) • Atmospheric losses • Design margin (misalignment, severe weather, drift in component parameters) • Propagation losses - calculated for worst case Satellite Note: All losses are frequency dependent. Earth station

11. Earth station – link budget components Signal to noise ratio requirement • Antenna gain • Function of efficiency and size • System noise temperature • Determined from the Rx front end • Receiver sensitivity • Minimum required RX power for proper link operation • Determined from noise power and required S/N ratio • Required S/N ratio • Determined from the modulation type • Ultimately driven by bandwidth efficiency requirement • Obtained from receiver operating curves R – information rate B – bandwidth of the channel IM – implementation margin

12. Downlink link budget example Typical downlink LB spreadsheet Note: Link budget as shown here has four distinct sections Transmitter Receiver Propagation Budget results