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Copy of Communications Topic Presentation: Pulse Code Modulation
Transcript of Copy of Communications Topic Presentation: Pulse Code Modulation
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 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 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 S(t)
The width and amplitude of the pulse are constant
Position is determined by the amplitude of the modulating signal. Pulse modulation consists of switching the carrier on and off as required. Drawbacks s(t) -modulated signal for Morse code of 'B' s(t) - modulated signal, made up of pulses Uses direct photon detection with a high-order pulse-position modulation
Photons act to transmit the signal The number of pulses per frame gives the number of controllable channels available Receiver must be properly synchronized to align the local clock with the beginning of each symbol Inherently sensitive to multipath interference
This occurs when the receiver's signal has 1+ echoes of each transmitted signal T 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 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 Advantages R/C PPM
Single Conversion Superheterodyne Receiver Deep Space Optical PPM 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 Conclusion 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