ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 9: Satellite link design (Section 4.3) Spring 2014

Florida Institute of technologies Page 2  Thermal noise in satellite systems  Noise temperature and noise figure of a device  System level noise figure and noise temperature  Examples Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies Thermal noise  Generated as a consequence of random electron motion at non zero temperature  Dominant source of noise in microwave- portion of spectrum  Other types of noise in electronic circuits oShot noise – random motion of charge in solid state devices and tubes oFlicker noise – low frequency noise in solid state circuits oQuantum noise – consequence of discrete nature of charge oPlasma noise – random motion of charge in ionized plasma  Different noise types have different origins but similar power spectral density -> they can all be treated as thermal noise  Power spectrum density of thermal noise form a black body (one sided): Page 3 Radio spectrum extends up to 300GHz Satellite service Note: PSD graph is generated for T=300K

Florida Institute of technologies Thermal noise in RF communication Page 4 Consider Since frequency smaller than 40GHz, hf/kT is small. Note 1: T is temperature in K Note 2: The noise if flat in spectral domain – “white noise” PDF of thermal noise in amplitude domain Note 1: noise has normal distribution in amplitude domain (CLT) Note 2: filter noise is also Gaussian (i.e. normally distributed) Note 3. power of the noise is limited by the equivalent bandwidth of the system

Florida Institute of technologies Equivalent noise temperature of a device  Noise temperature of the device – used to characterize noise sources internal to the device  Each device is characterized either by noise temperature or noise figure  In satellite communication – noise temperature more convenient Page 5  Measurement of equivalent noise temperature – Y factor method Note: accuracy dependant on size of Y

Florida Institute of technologies Noise temperature of waveguides  Waveguides are part of RF front end  Waveguides have associated losses  Losses attenuate both signal and noise that enter the waveguide Page 6 All components on the same temperature – thermal equilibrium Available input noise Available output noise Solving for equivalent noise temperature Note: Two ways of minimizing equivalent noise temperature of a waveguide 1.Reduce losses 2.Reduce physical temperature

Florida Institute of technologies Noise figure Page 7 Available power at the input If the network were noise free Due to sources internal to network One may write System may be modeled as a noise free but one assumes that the PSD of the input is increased by the factor of F relative to the PSD on the room temperature Noise figure/Noise temperature

Florida Institute of technologies Noise temperature of cascaded devices  At the Rx signal travels through multiple components  Each component has associate noise temperature  Of great interest is to determine equivalent “end to end” noise temperature – system temperature Page 8 One may extend the process to arbitrary number of components Using relationship between noise temperature and noise figure: Note 1: System noise figure depends most heavily on the first component in Rx chain Note 2: Noise figure values in above equations are in linear domain

Florida Institute of technologies G/T ratio for earth stations  G/T ratio – figure of merit for the RX  Usually given in dB/K  Small satellite terminals may have negative G/T value Page 9 Signal to noise ration at the output of the RX antenna Signal Noise Signal to noise Depends on the RX only

Florida Institute of technologies Example Consider the system shown in the figure a)Compute the overall noise figure of the system. b)If the noise power from the antenna is kT a B where T a = 15K, find the output noise power in dBm. c)What is the two sided PSD of the thermal noise? d)If the required SNR at the output is 20dB, what is the minimum signal power at the input? Assume that the system is at the temperature of 290K and with bandwidth of B =10MHz Answers: a)2.55 b)-98.7dBm c)6.8e-18mW/Hz d)-84.66dBm Page 10

Florida Institute of technologies Examples Example Earth station has a diameter of 30m, overall efficiency of 68% and it is used for reception of a signal at 4150MHz. The system noise temperature is 79K when the antenna points at 28 degrees above horizon. a)What is the G/T ratio under these conditions? b)If heavy rain causes system temperature to increase to 88K, what is the new G/T value? Answer: a)G/T = 41.6dB/K b)G/T = 41.2dB/K Page 11