Communications - Access Schemes

1Introduction to Space Systems and Spacecraft DesignSpace Systems Design

2

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

5

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

6

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

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

7

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

8

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

9

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

10

Communications - Access Schemes

Transmit this signal

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

11

Communications - Access Schemes

Process signal received

BasebandData A/B

MultiplyMixer

X /Amplifier Amplifier

Code A/B

DivideBasebandData A/B

f1-f2

Code A/B

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

12

Communications - Access Schemes

Process signal received

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

13

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

14

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

15

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

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

16

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

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

1717

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

18

Communications Design Guidelines for University

Satellites

Communications - Access Schemes

Introduction to Space Systems and Spacecraft DesignSpace Systems Design

19

Communications - Access Schemes

Questions?