Pulse or digital communications – Repeaters – Testing
Patent
1995-06-19
1997-03-18
Chin, Stephen
Pulse or digital communications
Repeaters
Testing
H04B 1707
Patent
active
056129734
ABSTRACT:
A communication and/or measurement system includes a transmitter that modulates a pseudo-random noise signal with a message signal to produce a wideband signal for transmission. A receiver, which demodulates the wideband signal to recover the message signal, includes an "analog" feedback shift register that reproduces the noise signal based on samples of the received signal. The analog feedback shift register ("AFSR") is a generalization of a linear feedback shift register ("LFSR"). The AFSR is characterized by a function that agrees with the function that characterizes the LFSR, at the points at which that function is defined. Further, the AFSR characterizing function has stable fixed points at these values, i.e., it has a slope of less than one in these regions. Specifically, the AFSR's function has stable fixed points at integer values and unstable fixed points at half-integer values and, the stable fixed points act as attractors. The AFSR thus produces a sequence that relaxes to the nearest integer-valued sequence. One example of such a function is: ##EQU1## where the .alpha..sub.i 's are the coefficients of the maximum length polynomial used to produce the wide-band noise signal and y.sub.n-1 is the state of the (n-i).sup.th stage of the register. As long as the samples of received signal that are fed to the AFSR fall within the basins of attraction that surround the stable values, the AFSR can accurately determine the expected next state of the shift register. While this can be done explicitly, the AFSR merges the symbol parsing, acquisition, tracking and update rules into a simple governing equation. The AFSR will thus entrain and produce a binary-valued pseudo-random noise signal. In one embodiment the signal y.sub.n is coupled to the register by a coupling circuit that sums (1-.epsilon.)y.sub.n and .epsilon.S.sub.n, to produce the update signal y'.sub.n where .epsilon. is a coupling factor selected between 0 and 1, and S.sub.n is a sample of the received signal.
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Chin Stephen
Kim Kevin
Massachusetts Institute of Technology
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