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Presentation Transcript

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

Important note: Slides present summary of the results. Detailed derivations are given in notes.

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

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”

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

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

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

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

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

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

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

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