Pulse or digital communications – Receivers
Reexamination Certificate
1999-12-20
2004-05-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Reexamination Certificate
active
06735259
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This is the first application filed for the present invention.
MICROFICHE APPENDIX
Not Applicable.
TECHNICAL FIELD
The present invention relates to high-speed data communications systems and in particular to a method and apparatus for optimizing the performance of a data communications system.
BACKGROUND OF THE INVENTION
It is well known that signals suffer degradation between the transmitter and receiver, due to various sampling and quantizing effects, and channel effects.
The sampling and quantizing effects comprise the distortion inherent in quantization of a received data signal, which may be a round-off or truncation error, errors introduced by saturation of the quantizer circuitry, and timing jitter. Generally, saturation may be avoided by using automatic gain control (AGC), which extends the operating range of the quantizer. Jitter includes any deviation of the position (i.e. the phase relationship) of the sampling clock with respect to the received data signal, and its effect is equivalent to a frequency modulation of the data signal. Timing jitter is generally controlled with very good power supply isolation and stable clock references.
Signal corruption introduced by the channel is due to such factors as noise, inter-symbol interference, dispersion, etc. The degradation of the recovered data signal quality with the channel induced errors is called “threshold effect”.
If the channel noise is small, there will be no problem detecting the presence of a waveform, and the only errors present in reconstruction of the data signal will be due to sampling and quantizing noise. On the other hand, if the channel noise is large, the resultant detection errors cause reconstruction errors. Thermal noise interference from circuit switching transients can cause errors in detecting the signal pulses carrying the digitized symbols (data).
Intersymbol interference is due to the bandwidth of the channel. A band-limited channel tends to spread the pulses, and if the width of the pulse exceeds a symbol duration, overlap with neighboring pulses may occur.
Dispersion is the chromatic or wavelength dependence of a parameter, such as, for example, the distortion caused by different wavelengths of light within a pulse travelling at different speeds through a fiber. The pulse distortion in a fiber optic system may also be caused by some parts of the light pulses following longer paths (modes) than other parts.
The quality of a data signal is expressed in terms of a Bit Error Rate (BER) which is the ratio between the number of erroneous bits counted at a site of interest over the total number of bits received.
In the last decade, transmission rates of data signals have increased dramatically. For high rate transmission, such as at 10-40 Gb/s, signal corruption introduced by the transmission channel is a critical parameter. The demand for receivers with high sensitivity has increased progressively with the transmission rates. The receiver's task is to determine which symbol was actually transmitted. For a given BER, the system performance is dependent upon the slicing level, defined also as threshold level, which is used to discriminate high and low levels of a received data signal. For example, a slicing level variation of only 8% can result in a variation of the receiver sensitivity of up to about
1
dB. Data signal recovery errors may develop as a result of an incorrect slicing level, or incorrect sampling clock/data signal timing (i.e. phase relationship) being selected.
Current optical receivers comprise an avalanche photodiode (APD), or a high performance PIN photodiode, typically coupled to a transimpedance amplifier. The transimpedance amplifier is a shunt feedback amplifier acting as a current-to-voltage transducer. The received signal is then amplified, and a data decoder (e.g. a single channel super-decoder) extracts a “clean” data signal from the amplified received data signal. Generally, binary data decoders are provided with a fixed slicing level selected such as to provide the best error rate at a predetermined signal power level. However, a fixed slicing level cannot account for the effects of aging of the components, temperature variations, etc. As a result, higher power levels need to be transmitted to account for the above factors, which in turn diminish the length of the transmission channel.
As the requirement for essentially error free operation for fiber systems become more stringent, systems which allow bit detection errors to occur during a normal data signal recovery mode of operation are increasingly less acceptable. Driven by customer demand, sophisticated performance monitors are provided at the receiver site, which perform optimization routines for lowering the BER of the recovered data signal.
It is known to generate a control code at the transmission site which is then transmitted with the payload data over the communication link. Error detection is based, in general, on comparison between the transmitted and the received control code. Error correction is based on various algorithms which compensate for the specific error detected in the control code. This method is known as forward error correction (FEC).
A data decoder including a performance monitor is disclosed in U.S. Pat. No. 4,097,697 (Harman, issued on Jun. 27, 1978 and assigned to Northern Telecom Limited). This patent discloses a data decoder including a first differential amplifier which recovers the data signal by comparing the incoming signal with a fixed slicing level. A second differential amplifier compares the incoming signal with an offset slicing level to produce an errored signal. Both differential amplifiers are clocked by a recovered clock signal. The recovered data signal and the errored signal are compared to each other and the result is used to determine the degradation of the incoming signal.
U.S. Pat. No. 4,799,790 (Tsukamoto et al., issued Jan. 24, 1989 and assigned to Anritsu Corporation) discloses a device comprising a transmitter for launching signals of various wavelengths into a reference or test fiber, and a receiver. At the receiver, the phase difference between two adjacent wavelengths is measured for both the reference and test path for determining the delay of the respective wavelength.
None of the above patents is concerned, however, with providing a simple device and method for detecting and correcting errors in the recovered data signal using information in the data path itself. The receiver circuits described in the above patents rely on duplicate channels and pseudo-error detection.
The extent of signal degradations may be directly measured using an eye closure diagram, which is the graphic pattern produced on an oscilloscope when a baseband signal is applied to the vertical input of the oscilloscope and the symbol rate triggers the instrument time base. For a binary signal, such an eye diagram has a single eye which is open or closed to an extent determined by the signal degradation. For data recovery with low BER, an open pattern is desired. Changes in the eye opening size indicate intersymbol interference, amplitude irregularities, or timing problems.
U.S. Pat. No. 4,823,360 (Tremblay et al., issued Apr. 18, 1989 and assigned to Northern Telecom Limited) discloses a device for measuring chromatic dispersion of an optical fiber based on a baseband phase comparison method, using the eye closure diagram of the data signal received over the transmission link. The device described in this U.S. patent evaluates the transmission link performance using three slicing levels for recovering data. Two of the slicing levels are obtained by measuring on the eye diagram the level of “long 0s” and “long 1s”, respectively, for a preset error rate, and the third slicing level is provided in a selected relationship to the other two to produce recovered data signals.
The technique described in Tremblay et al. is based on generating “pseudo-errors” on separate pseudo-error channels. The pseudo-errors give some idea of how error performance vari
Oberhammer Wolfgang
Roberts Kim B.
(Ogilvy Renault)
Chin Stephen
Daniels Kent
Nortel Networks Limited
Odom Curtis
LandOfFree
Method and apparatus for optimization of a data... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for optimization of a data..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for optimization of a data... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3237990