MIMO equalization optimized for baud rate clock recovery in coherent DP-QPSK metro systems
The present invention is directed to a MIMO equalization system and method, optimized for baud rate clock recovery in coherent symbol-spaced DP-QPSK Metro systems. According to this method, the Mueller & Muller timing function is extended to cope with controlled ISI induced signals, while decoupling between MIMO equalization and clock recovery loops, using a midpoint output of the equalizer for timing estimation, instead of its final output. At least a portion of controlled Inter-Symbol Interference (ISI) is kept intact and the controlled ISI is compensated by an MLSE, right after carrier timing synchronization.
1. A BCR-MIMO equalization system optimized for baud rate clock recovery in coherent symbol-spaced DP-QPSK Metro systems, comprising:
a) a first filter connected to the X polarization path, for receiving the sampled signal from the X polarization path and filtering out a first portion of the X polarization sampled signal;
b) a second filter connected to the Y polarization path, for receiving the sampled signal from the Y polarization path and filtering out a second portion of the Y polarization sampled signal;
c) a delay element in said X polarization path, for delaying the sampled signal in the X polarization path to have timing alignment with said second portion;
d) a delay element in said Y polarization path, for delaying the sampled signal in the Y polarization path to have timing alignment with said first portion;
e) a first adder in said X polarization path for adding said second portion to the delayed signal in the X polarization path;
f) a second adder in said Y polarization path for adding said first portion to the delayed signal in the Y polarization path;
g) a first Timing Error Detection (TED) unit, connected to the output of said first adder, for signal timing estimation and timing synchronization of the signal in the X polarization path;
h) a second Timing Error Detection (TED) unit, connected to the output of said second adder, for signal timing estimation and timing synchronization of the signal in the Y polarization path;
i) a third filter connected to the output of said first adder, for reconstructing the signal received in said X polarization path;
j) a fourth filter connected to the output of said second adder, for reconstructing the signal received in said Y polarization path; and
k) an error correction and adaptation unit, for adapting the taps of said filters to minimize the clock timing error in each path.
2. A system according to claim 1 , in which the delay of each delay element is half the length of each filter.
3. A system according to claim 1 , in which polarization mixing dependency is removed by the first (g xy ) and the second (g yx ) filters, while maintaining temporal timing dependency.
4. A system according to claim 1 , in which compensating time dependent ISI by the third and the fourth filters.
5. A system according to claim 1 , in which timing estimation performed prior to equalization, based on the signals at the input to the third (g xx ) and the fourth (g yy ) filters.
6. A system according to claim 1 , in which the taps of the filters are adapted using m-LMS, while sampling the signal at a constant phase.