Method of measuring the error rate of an optical transmission system and apparatus for implementing the method

• US 6,744,496 B2
• Filed: 10/10/2002
• Issued: 06/01/2004
• Est. Priority Date: 10/11/2001
• Status: Expired due to Term

• Current Assignee: Inria, Alcatel SA (Nokia Corporation)
• Original Assignee: Inria, Alcatel SA (Nokia Corporation)
• Inventors: Benveniste, Albert, Cerou, Frederic, Manzanedo, Maria, Moustakides, George, Penninckx, Denis, Noirie, Ludovic, Audouin, Olivier
• Primary Examiner(s): Nguyen, Tu T.
• Application Number:
  US10/267,573
• Publication Number:
  US 20030117613A1
• Time in Patent Office: 600 Days
• Field of Search: 356/73.1, 398/140-172, 398/182-201, 371/5.1, 371/5.2, 371/20.1, 375/355, 375/214, 375/286, 375/350
• US Class Current: 356/73.1
• CPC Class Codes: H04B 10/07953 Monitoring or measuring OSNR, BER or Q
• CPC Class Codes: H04L 1/203 Details of error rate determination, e.g. BER, FER or WER

Abstract

A method of measuring the error rate of an optical transmission system transmitting a signal by

detecting the signal, and

asynchronously sampling the signal at a frequency independent of the bit rate of the signal to obtain K samples of the signal at respective times t₁ to t_K, where K is an integer greater than or equal to 2.

The eye diagram of the signal,

the error rate of the signal and the bit time of the signal are then computed.

Claims

1. A method of measuring the error rate of an optical transmission system transmitting a signal, said method comprises

detecting said signal,

asynchronously sampling said signal at a frequency independent of a bit rate of said signal to obtain K samples of said signal at respective limes t₁, to t_K where K is an integer greater than or equal to 2,

computing a bit time of said signal,

computing an diagram of said signal using the computed bit time of said signal, and

computing an error rate of said signal.

2. A method according to claim 1, wherein an absolute bit time is computed from an approximate value T₀ that is known initially.

3. A method according to claim 2, wherein a simultaneous computation of the bit time and the eye diagram comprises:

choosing a sub-sample of K/N samples of said signal where N is an integer power of 2,

separating said sub-sample into two parts,

computing two eye diagrams from respective parts of the sub-sample using the value T₀ for the bit time,

computing two histograms from the two eye diagrams by digitizing time and intensity,

determining the time period δ

between the two histograms,

determining the bit time T₁ from the equation;

$T_1=T_0-2<br><br>\mathrm{\delta <br><br>}<br><br>\frac{T_0}{t_k},\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}$

repeating the above operations by substituting N/2 for N until a sub-sample of K/2 samples is obtained.

4. A method according to claim 1, wherein computing the hit time relating to the sampling frequency comprises:

applying a non-linear function to a series of samples of the signal to obtain a series of substantially periodic values y_k for k varying from 1 to K,

dividing the series into M sub-series, each of L elements, where L and M are integers,

computing a discrete Fourier transform of each sub-series, which yields a function Y_i for i varying from 1 to M,

defining a periodogram function as the ratio with respect to M of the sum of the squares of the moduli of the functions Y_i for i varying from 1 to M, and

determining the frequency f which maximizes the periodogram function.

5. A method according to claim 4, wherein the computing of the eye diagram comprises:

computing a discrete Fourier transform at the frequency f of the series y_k which yields a function Z_k for k varying from 1 to K, and

obtaining a time associated with each sample of the signal from the equation;

${\mathrm{\tau <br><br>}}_k=\frac{\arg <br><br>\left(Z_k\right)}{2<br><br>\mathrm{\pi <br><br>}}.$

6. A method according to claim 5, wherein the Fourier transform is computed over a sliding window centered on y_k.

7. A method according to claim 1, wherein the error rate is computed by modeling the statistical distributions of levels of said signal by means of P Gaussian distributions where P is an integer greater than or equal to 2.

8. A method according to claim 7, wherein when P is equal to 8, an average and a variance of the eight Gaussian distributions are estimated by applying an expectation-maximization algorithm.

9. Apparatus for measuring the error rate of an optical transmission system transmitting a signal, said apparatus comprising:

means for detecting a signal that has been transmitted by the optical transmission system,

means far asynchronously sampling said detected signal at a frequency independent of a bit rate of said signal to obtain K samples of said signal at respective times t₁ to t_k where K is an integer greater than or equal to 2,

means for digitizing samples obtained at an output of said sampling means, and

software for processing said digitized samples to compute a bit time, an eye diagram using the computed bit time, and an error rate of said signal.

10. Apparatus according to claim 9, wherein said software models statistical distributions of levels of the signal in order to compute the error rate of the signal.

11. Apparatus for measuring the error rate of an optical transmission system transmitting a signal, said apparatus comprising:

a detector that detects a signal that has been transmitted by the optical transmission system,

a sampler head that asynchronously samples said detected signal at a frequency independent of a the bit rate of said signal to obtain K samples of said signal at respective times t₁ to t_K where K is an integer greater than or equal to 2,

a digitizer that digitizes samples obtained at an output of said sampler head, and

a computer executing software that processes said digitized samples to compute a bit time, an eye diagram using the computed bit time, and an error rate of said signal.

12. Apparatus according to claim 11, wherein said software models statistical distributions of levels of the signal in order to compute the error rate of the signal.