communications - access schemes 1 introduction to space systems and spacecraft design space systems...
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Communications - Access Schemes
1Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Got link with Satellite
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
Communications - Access Schemes
Have assigned frequency & BW
How do we work with multiple ground stations to use the satellite service?
Multiple Access Methods
f1 f2
BW – bandwidth
How can we service all three without interference?
Communications - Access Schemes
FDMA – Frequency Division Multiple Access
f1 f2
BW – bandwidth
Multiple Access Methods
TDMA – Time Division Multiple Access
CDMA - Code Division Multiple Access
Sharing same frequency bandwidth
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Communications - Access Schemes
Customer A
Customer BCustomer C
CBA
f1 f2
CBA
CBA
Frequency
Time
FDMA
TDMA
CDMA
f1 f2Frequency
CBA
Division by freq. bands
All three share same freq. & time
Division by time
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Code Division Multiple Access
Introduction to Communications
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
Baseband
Data B
MultiplyMixer
X /Amplifier Amplifier
Code B
DivideBaseband
Data B
Code B
f1-f2
BasebandData A
MultiplyMixer
X /Amplifier Amplifier
Code A
DivideBaseband
Data A
Code A
f1-f2
BasebandData C
MultiplyMixer
X /Amplifier Amplifier
Code C
DivideBaseband
Data C
Code C
f1-f2
Multiple channels over same freq. band
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Communications - Access Schemes
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Communications - Access Schemes
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Communications - Access Schemes
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Communications - Access Schemes
Transmit this signal
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Communications - Access Schemes
Process signal received
BasebandData A/B
MultiplyMixer
X /Amplifier Amplifier
Code A/B
DivideBasebandData A/B
f1-f2
Code A/B
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Communications - Access Schemes
Process signal received
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Communications - Access Schemes
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Communications - Access Schemes
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Data Data
Communications System
Move
Convert to Baseband Data# bits for ADCSample rate
Choose a frequency for carrier to send on fcavailability of equipment?can use band?
Choose a modulation scheme (BPSK, QPSK, FSK, AFSK ----)How much power can you get from Tx to Rx?How much BER can your tolerate?Need to use FEC?Do link equations ---- got enough margin?Choose equipment
antennaspolarization
Communications - Access Schemes
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Other Communication Considerations1. Contact time with satellite (distance between gs & sat)
LEOGEOMolnyia
2. Doppler shift on signals to and from satellite
V
Ground Station
Satellite
From Satellitefr > fc when approachingfr <fc when leaving
To Satellite? when approaching? when leaving
Communications - Access Schemes
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How do you calculate doppler shift?
fr > fc
fr < fc
fr = fc
fr = freq. at Tr at gsfc = carrier freq. at Tx on sat
Communications - Access Schemes
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Communications Design Guidelines for University
Satellites
Communications - Access Schemes
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Communications - Access Schemes
Questions?