ECE 5233 Satellite Communications. Prepared by: Dr . Ivica Kostanic Lecture 9: Satellite link design (Section 4.3). Spring 2014. Outline . Thermal noise in satellite systems Noise temperature and noise figure of a device System level noise figure and noise temperature Examples.
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Dr. Ivica Kostanic
Lecture 9: Satellite link design
Important note: Slides present summary of the results. Detailed derivations are given in notes.
Radio spectrum extends up to 300GHz
Note: PSD graph is generated for T=300K
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”
Note: accuracy dependant on size of Y
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 physical temperature
Available output noise
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
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
Signal to noise ration at the output of the RX antenna
Depends on the RX only
Signal to noise
Consider the system shown in the figure
Assume that the system is at the temperature of 290K and with bandwidth of B=10MHz
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.