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ECE 5233 Satellite Communications

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

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

  3. 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 • Shot noise – random motion of charge in solid state devices and tubes • Flicker noise – low frequency noise in solid state circuits • Quantum noise – consequence of discrete nature of charge • Plasma 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): Satellite service Radio spectrum extends up to 300GHz Note: PSD graph is generated for T=300K

  4. Thermal noise in RF communication Consider PDF of thermal noise in amplitude domain Since frequency smaller than 40GHz, hf/kT is small. 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 Note 1: T is temperature in K Note 2: The noise if flat in spectral domain – “white noise”

  5. 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 • Measurement of equivalent noise temperature – Y factor method Note: accuracy dependant on size of Y

  6. 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 All components on the same temperature – thermal equilibrium Solving for equivalent noise temperature Available input noise Note: Two ways of minimizing equivalent noise temperature of a waveguide Reduce losses Reduce physical temperature Available output noise

  7. Noise figure 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 Available power at the input Noise figure/Noise temperature If the network were noise free Due to sources internal to network

  8. Noise temperature of cascaded devices One may extend the process to arbitrary number of components • 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 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

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

  10. Example Consider the system shown in the figure • Compute the overall noise figure of the system. • If the noise power from the antenna is kTaB where Ta= 15K, find the output noise power in dBm. • What is the two sided PSD of the thermal noise? • 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: • 2.55 • -98.7dBm • 6.8e-18mW/Hz • -84.66dBm

  11. Examples Example 4.3.4. 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. • What is the G/T ratio under these conditions? • If heavy rain causes system temperature to increase to 88K, what is the new G/T value? Answer: • G/T = 41.6dB/K • G/T = 41.2dB/K