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Copy of Communications Topic Presentation: Pulse Code Modulation
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Transcript
- In PPM: The pulses of equal amplitude are generated at a rate controlled by the modulating signal's amplitude
- In applications that require low noise interference over long distances, PPM is well suited
- Demodulation is simple and requires little circuitry
- Pulse Position Modulation is a very process specific modulation technique
- When multipath interference is present, this method is rendered useless
- Applications are seen in deep space communication, fiber optic channels, and R/C systems
Conclusion
- Can be implemented non-coherently
- The receiver does not need to use a phase-locked loop (PLL) to track the phase of the carrier
Advantages
- Higher noise immunity since all the receiver needs to do is detect the presence of a pulse at the correct time
- Electronics required to decode the signal (esp. in Remote Controlled systems) are extremely simple, which leads to small, light-weight receiver/decoder units
- Uses direct photon detection with a high-order pulse-position modulation
- Photons act to transmit the signal
- Inherently sensitive to multipath interference
- This occurs when the receiver's signal has 1+ echoes of each transmitted signal T
- Receiver must be properly synchronized to align the local clock with the beginning of each symbol
Drawbacks
- Deep space communication requires antennae lengths that are both impractical and expensive, so other methods must be used
- Serially Concatenated Pulse-Position Modulation (SCPPM) coding is one solution
- Invented by NASA for deep space transmission from Mars
- Uses light to transmit signal using PPM
Applications
Pulse Position Modulation
Ross Yeager & Kyle Pace
History & Development
Sources:
- Cheng, Michael K.. Implementation of a Coded Modulation for Deep Space Optical Communications. <http://ita.ucsd.edu/workshop/06/papers/50.pdf>
- Science Lobby. <http://www.sciencelobby.com/general-theory/pulse-modulation.html>
- http://cnyack.homestead.com/files/modulation/modppm.htm
- Evolved alongside pulse code modulation and pulse width modulation
- Origins in telegraph time-division multiplexing which dates back to 1853
- Early '60s: Don Mathers and Doug Spreng (NASA) invented Pulse Position Modulation (used in R/C systems)
- Currently being used in deep space communications, R/C systems, as well as in other fiber optic communications
Definition
- Pulse Position Modulation:
- A form of signal modulation
- x message bits are encoded by using 1 of 2^x possible time shifts
- Time shifts are then repeated every T seconds
- Primary use: optical communications systems
- Digital form of modulation
R/C PPM
Single Conversion Superheterodyne Receiver
Techniques
Deep Space Optical PPM
s(t) - modulated signal, made up of pulses
s(t) -modulated signal for Morse code of 'B'
Pulse modulation consists of switching the carrier on and off as required.
- S(t)
- The width and amplitude of the pulse are constant
- Position is determined by the amplitude of the modulating signal.
- The number of pulses per frame gives the number of controllable channels available
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