satellite Transmission fundamentals

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Information about satellite Transmission fundamentals

Published on March 5, 2014

Author: ajal4u



Transmission fundamentals

system noise temperature in satellite communication galaxy sun ionosphere troposphere SATELLITE TRACKING, TELEMETRY AND COMMAND 1

Receiving station architecture LNA block and first conversion Receiver station diagram block

Receiving station architecture Second conversion and IF2 amplifier block

Noise   Noise is an electronic signal that gets added to a radio or information signal as it is transmitted from one place to another. It is not the same as interference from other information signals.

Noise   Noise is the static you hear in the speaker when you tune any AM or FM receiver to any position between stations. It is also the “snow” or “confetti” that is visible on a TV screen. The noise level in a system is proportional to temperature and bandwidth, the amount of current flowing in a component, the gain of the circuit, and the resistance of the circuit.

Signal-to-Noise Ratio    The signal-to-noise (S/N) ratio indicates the relative strengths of the signal and the noise in a communication system. The stronger the signal and the weaker the noise, the higher the S/N ratio. The S/N ratio is a power ratio.

External Noise  External noise comes from sources over which we have little or no control, such as:  Industrial sources   Atmospheric sources   motors, generators, manufactured equipment The naturally occurring electrical disturbances in the earth’s atmosphere; atmospheric noise is also called static. Space  The sun radiates a wide range of signals in a broad noise spectrum.

Internal Noise  Electronic components in a receiver such as resistors, diodes, and transistors are major sources of internal noise. Types of internal noise include:    Thermal noise Semiconductor noise Intermodulation distortion

Expressing Noise Levels  The noise quality of a receiver can be expressed in the following terms:     The noise factor is the ratio of the S/N power at the input to the S/N power at the output. When the noise factor is expressed in decibels, it is called the noise figure. Most of the noise produced in a device is thermal, which is directly proportional to temperature. Therefore, the term noise temperature (TN) is used. SINAD is the composite signal plus noise and distortion divided by noise and distortion contributed by the receiver.

Noise in Cascaded Stages   Noise has its greatest effect at the input to a receiver because that is the point at which the signal level is lowest. The noise performance of a receiver is determined in the first stage of the receiver, usually an RF amplifier or mixer.

The Earth is Curved ! • • • • • Radio waves above 30 MHz travel in straight lines Ways must be found to get signals beyond horizon Ionospheric reflection uses hf band, 2 – 30 MHz Microwave link uses line of sight between towers Chain of repeaters can take the signal thousands of miles • Satellite communications uses a repeater in the sky • Single link via GEO satellite can reach round one third of the earth’s surface. 11

Ionospheric layers multipath Tx Rx Earth Fig. HF Radio Communication 12

Tx Rx Earth Fig. LOS Microwave Communications 13

GEO satellite Altitude 35,680 km Tx Rx Earth Fig. Satellite Communications 14

noise @ Satellites 1. 2. 3. 4. Thermal Noise Intermodulation noise Crosstalk Impulse Noise

Thermal Noise  Thermal noise due to agitation of electrons  Present in all electronic devices and transmission media  Cannot be eliminated  Function of temperature  Particularly significant for satellite communication

Thermal Noise  Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N 0 = kT ( W/Hz ) • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803 x 10-23 J/K • T = temperature, in Kelvin's (absolute temperature)

Thermal Noise  Noise is assumed to be independent of frequency  Thermal noise present in a bandwidth of B Hertz (in watts): N = kTB or, in decibel-watts N = 10 log k + 10 log T + 10 log B = −228.6 dBW + 10 log T + 10 log B

Noise Terminology  Intermodulation noise – occurs if signals with different frequencies share the same medium o Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies  Crosstalk – unwanted coupling between signal paths  Impulse noise – irregular pulses or noise spikes o Short duration and of relatively high amplitude o Caused by external electromagnetic disturbances, or faults and flaws in the communications system o Primary source of error for digital data transmission

Comm. Subsystem—Design Typical System Noise Temperatures

Expression Eb/N0  Ratio of signal energy per bit to noise power density per Hertz Eb S / R S = = N0 N0 kTR  The bit error rate for digital data is a function of Eb/N0 o Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected o As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0

Other Impairments  Atmospheric absorption – water vapor and oxygen contribute to attenuation  Multipath – obstacles reflect signals so that multiple copies with varying delays are received  Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Propagation  Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal  Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave  Scattering – occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal or less

R= Reflection D= Diffraction S= Scattering

Effects of Multipath Propagation  Multiple copies of a signal may arrive at different phases o If phases add destructively, the signal level relative to noise declines, making detection more difficult  Intersymbol interference (ISI) o One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Fading  Time variation of received signal power caused by changes in the transmission medium or path(s)  In a fixed environment: o Changes in atmospheric conditions  In a mobile environment: o Multipath propagation

Types of Fading  Fast fading  Slow fading  Flat fading  Selective fading  Rayleigh fading  Racian fading Transmit beam θτ Receive beam θρ

Error Compensation Mechanisms 1. Forward error correction 2. Adaptive equalization 3. Diversity techniques

1.Forward Error Correction  Transmitter adds error-correcting code to data block o Code is a function of the data bits  Receiver calculates error-correcting code from incoming data bits o If calculated code matches incoming code, no error occurred o If error-correcting codes don’t match, receiver attempts to determine bits in error and correct

2.Adaptive Equalization  Can be applied to transmissions that carry analog or digital information o Analog voice or video o Digital data, digitized voice or video  Used to combat intersymbol interference  Involves gathering dispersed symbol energy back into its original time interval  Techniques o Lumped analog circuits o Sophisticated digital signal processing algorithms

3.Diversity Techniques  Space diversity: o Use multiple nearby antennas and combine received signals to obtain the desired signal o Use collocated multiple directional antennas  Frequency diversity: o Spreading out signal over a larger frequency bandwidth o Spread spectrum  Time diversity: o Noise often occurs in bursts o Spreading the data out over time spreads the errors and hence allows FEC techniques to work well o TDM o Interleaving





Major problems for satellites 1. Positioning in orbit 2. Stability 3. Power 4. Communications 5. Harsh environment

1.Positioning • This can be achieved by several methods • One method is to use small rocket motors • These use fuel - over half of the weight of most satellites is made up of fuel • Often it is the fuel availability which determines the lifetime of a satellite • Commercial life of a satellite typically 1015 years

2.Stability • It is vital that satellites are stabilised – to ensure that solar panels are aligned properly – to ensure that communications antennae are aligned properly • Early satellites used spin stabilisation – Either this required an inefficient omnidirectional aerial – Or antennae were precisely counter-rotated in order to provide stable communications

Stability (2) • Modern satellites use reaction wheel stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible • including: – eddy current stabilisation – (forces act on the satellite as it moves through the earth’s magnetic field)

Reaction wheel stabilisation • Heavy wheels which rotate at high speed often in groups of 4. • 3 are orthogonal, and the 4th (spare) is a backup at an angle to the others • Driven by electric motors - as they speed up or slow down the satellite rotates • If the speed of the wheels is inappropriate, rocket motors must be used to stabilise the satellite - which uses fuel

3.Power • Modern satellites use a variety of power means • Solar panels are now quite efficient, so solar power is used to generate electricity • Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite • Nuclear power has been used - but not recommended

5.Harsh Environment • Satellite components need to be specially “hardened” • Circuits which work on the ground will fail very rapidly in space • Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully

Alignment • There are a number of components which need alignment – Solar panels – Antennae • These have to point at different parts of the sky at different times, so the problem is not trivial

Antenna alignment • A parabolic dish can be used which is pointing in the correct general direction • Different feeder “horns” can be used to direct outgoing beams more precisely • Similarly for incoming beams • A modern satellite should be capable of at least 50 differently directed beams

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