1. Digital Speech Transmission and Recovery

2. Overall System Output (speaker) Channel (coax cable) Receiver Circuit Input (microphone) Transmitter Circuit Input (microphone) Transmitter Circuit

3. What is Spread Spectrum? This is the process of taking a signal that has been digitized and replacing the 1’s and 0’s with their own bit sequence (chip). This is the process of taking a signal that has been digitized and replacing the 1’s and 0’s with their own bit sequence (chip). Example: 1 = (101), 0 = (001), then a coded sequence such as 1100 would become 101 101 001 001. Example: 1 = (101), 0 = (001), then a coded sequence such as 1100 would become 101 101 001 001.

4. Spread Spectrum Communication Schemes Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum Code Division Multiple Access (CDMA) Code Division Multiple Access (CDMA)

5. Direct Sequence Spread Spectrum 1 Transmitter  1 Receiver 1 Transmitter  1 Receiver The transmitter spreads the sequence and transmits it across a channel. The transmitter spreads the sequence and transmits it across a channel. The receiver then decodes the spread signal and returns the original message. The receiver then decodes the spread signal and returns the original message.

6. Code Division Multiple Access Multiple Transmitters  1 Receiver Multiple Transmitters  1 Receiver Choice of unique sequences to spread the data bits Choice of unique sequences to spread the data bits Ability of receiver to distinguish between data streams Ability of receiver to distinguish between data streams

7. Benefits of Spread Spectrum Communications Security Security Multiple users on a single channel Multiple users on a single channel

8. Project Goals Implement a direct sequence spread spectrum communication system Implement a direct sequence spread spectrum communication system Obtain a low level CDMA scheme Obtain a low level CDMA scheme Transmit and receive speech signals through both systems Transmit and receive speech signals through both systems Learn about coding schemes in digital communications Learn about coding schemes in digital communications

9. Design Options Modulate the signal out to high frequencies, or transmit across the baseband Modulate the signal out to high frequencies, or transmit across the baseband What spreading sequence we choose and how long should it be What spreading sequence we choose and how long should it be Choice for voice coding Choice for voice coding

10. Coding and Setup Coding and Setup The system was implemented using C language. The system was implemented using C language. We wrote the filter code in TI assembly. We wrote the filter code in TI assembly. The transmission to the receiver was done over a coaxial cable. The transmission to the receiver was done over a coaxial cable.

11. Original Transmitter X x(t) X(n) Spreading Sequence Channel

12. Final Transmitter T Delta Modulation Spreading Sequence Conversion 8 Channel Input D / A 6 kHz LPF 1

13. Transmitter Sampling Rate: 44.1 kHz Sampling Rate: 44.1 kHz After decimating by eight: 5.512 kHz. After decimating by eight: 5.512 kHz. After spreading sequence: 44.1 kHz. After spreading sequence: 44.1 kHz.

14. Modulation Scheme We transmit with a baseband BPSK scheme. We transmit with a baseband BPSK scheme. Baseband transmission was valid because we only implemented two users. Baseband transmission was valid because we only implemented two users.

15. Delta Input  Spreading Sequence Spreading Sequence Output An Example Spreading Sequence: 1,-1,-1,1,-1,-1,1,1 1 0

16. Voice Coding We used a delta modulation scheme We used a delta modulation scheme We chose delta modulation because it allows the transmission of voice with a spreading sequence We chose delta modulation because it allows the transmission of voice with a spreading sequence

17. Delta Modulation Input to Transmitter Delta Modulation Time Amplitude

18. Delta Modulation 64/8 Samples 16 Bits/Sample 1 Chip of 8 Bits = 1024 Outputs 64/8 Samples 1 Bit/Sample 1 Chip of 8 Bits = 64 Outputs

19. Delta Modulation Diagram Quantizer Delay Delta To Interpolator From Decimator From DLL To Spreading Sequence Conversion

20. Original Receiver LPF Matched Filter Channel LPF Matched Filter Channel Sampler D/AY(t)

21. Final Receiver T Matched Filter Delta Modulation Delay-Lock Loop 8 Output Channel D / A 6 kHz LPF 1

22. Matched Filter It emphasizes the power of the correct spreading sequence and cancels out the power of the undesirable sequence. It emphasizes the power of the correct spreading sequence and cancels out the power of the undesirable sequence. The matched filter coefficients are equal to the desired spreading sequence. The matched filter coefficients are equal to the desired spreading sequence.

23. Delay-Lock Loop Sampler Early Sample Late Sample On-time Sample + X Offset Logic _ Ensures that we choose the correct sample From Matched Filter To Delta Modulation

24. Delay-Lock Loop w/o Noise Magnitude 1 Time 0 Encoded dataInput to DLL

25. Delay-Lock Loop w/ Noise Magnitude 1 Time 0

26. Tests Input: Sinusoidal Signal, Voice Input: Sinusoidal Signal, Voice 1 Transmitter  Oscilloscope 1 Transmitter  Oscilloscope 1 Transmitter  1 Receiver 1 Transmitter  1 Receiver 2 Transmitters  1 Receiver 2 Transmitters  1 Receiver

27. Performance Delta Modulation: 0-3.0 dB attenuation Delta Modulation: 0-3.0 dB attenuation Overall System: 3.68 dB attenuation Overall System: 3.68 dB attenuation Sound Sound Delta Modulation Delta Modulation 1 Transmitter  1 Receiver 1 Transmitter  1 Receiver 2 Transmitters  1 Receiver 2 Transmitters  1 Receiver