Spread Spectrum

1
Spread Spectrum

• Daniel Kraus
• Supervisor Richard Guinea
• Assessor Brian Donovan

2
What is Spread Spectrum ?

• Shannons Law
• Direct sequence
• Usage 3-G (CDMA) , GPS (position location)

C W log2 (1 S/N)
Data
X
Spread signal
PN code
3
System configuration
70 MHz Radio Link
Receiver
Transmitter
DATA
DATA
4
Transmitter
Carrier
PN code generator
Clock
XOR
Data Source
5
Receiver
On time
( )2
Carrier recovery
Data processing
Early
PN generator
VCO

-
Late
On time
Early
Late
Differencing Amplifier
6
Transmitter block diagram
7
Receiver block diagram
8
Despreading - artwork
9
Design

• Based on 1993 design published in Electronic
  Wireless World
• Incomplete documentation
• Some bugs in design
• 435 Mhz part of original design omitted
• Added code selection
• Demodulating part redesigned completely
• Virtually impossible to simulate

10
Construction

• Digital low frequency circuitry tested on bread
  board
• Separate PCBs for functional blocks
• Final version - separate boards for transmitter
  and receiver
• PCB made of standard FR4 , manufactured in
  college facilities

11
Troubleshooting noise

• Major effect on functionality
• Digital switching affects analog circuits
• Decoupling used on each chip
• Big capacitors at inputs of power supplies
• Ferrites used to separate digital and analog
  power nets
• Ground plane underneath RF circuitry

12
Results conclusions

• 3 PCBs on prospect
• 2.14m half-wavelength antenna
• Nice to have power amplifiers and channel
  coders (another project ? )
• 3 unsuccessful projects before this

13
Some figures

• 7bit gt 27-1 127 bit PN sequence
• Process gain Gp10log10(BWRF/Rinfo)24dB
• Data rate 31.25Kb/s
• Maximum input carrier 7dBm
• Receiver sensitivity achieved 50dBm (-69dBm)
• BER 3.4510-2 (48dB attenuator)
  8.1510-2(free space)
• Theoretical transmitted distance 150m for 48dB
  (1700m for 70dB)
• Material cost 200 euro