Pulse or digital communications – Receivers – Automatic baseline or threshold adjustment
Reexamination Certificate
2000-07-12
2004-08-31
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Automatic baseline or threshold adjustment
Reexamination Certificate
active
06785344
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communications systems and more particularly to data differentiation in communications systems.
RELATED ART
Several modem communications systems employ packet-multiplexed transmission, in which multiple data sources transmit over a shared medium using a time-division multiple-access (TDMA) protocol to a central master location. For example, one such system is a passive fiber optic network described in ITU Standard G.983 (the “Standard”), which is incorporated by reference herein in its entirety.
FIG. 1
depicts in block diagram form a passive fiber optic network
100
described in the Standard. Multiple remote stations (depicted as ONU
104
-
1
to
104
-n) are data sources that each communicate with a receiver at the master location (the OLT
102
). ONUs transmit packets of information during time slots assigned by the OLT, so that packets originating from different ONUs are interleaved in time on the transmission medium. If the ONUs are at different distances from the OLT, the signal level at the OLT can vary substantially depending on which ONU is sending the data. Thus the decision threshold for distinguishing between ONE and ZERO levels in digital data streams must be recalibrated by the OLT every time a different ONU transmits to the OLT because the analog signal strength from each ONU is distinct at the OLT. If different source transmissions are interleaved rapidly on a communication fiber, for example on a packet-by-packet or cell-by-cell basis, rapid threshold recalibrations are necessary.
Establishing a correct threshold value, for example at a level close to the average of the ONE and ZERO levels, is important to minimize impairments due to both amplitude noise and timing errors. If the threshold is placed too close to a ONE or ZERO level, then amplitude noise in the system will increase the frequency of incorrectly detected bits. In addition, improper threshold adjustment causes transitions to occur at incorrect times (so called “pulse width distortion”), with the effect that bits of one signal level will have reduced duration while bits at the other logic level will have excessive duration. This pulse width distortion complicates clock recovery and data timing.
Generally, the threshold should be optimally set halfway between the analog signal levels corresponding to digital ONE and ZERO levels. For example, for ATM cells transmitted at 155 megabits/s, each cell is transmitted in 2.7 microseconds, so that the threshold must be reset in a much shorter time (e.g., tens of nanoseconds). As the signaling rate increases above 155 megabits/s, the time to establish the threshold level becomes correspondingly shorter. In the fiber optic passive optical network defined in the Standard, the optical input signal strength to the OLT can vary by as much as 19 dB. Because the extinction ratio (ratio of logical ONE to ZERO levels) is only 10 dB in the Standard, the logical ONE level associated with one ONU may be smaller than the logical ZERO level associated with a different ONU.
FIG. 2
depicts a block diagram of a conventional TDMA receiver
200
, used for example by OLT
102
, that converts optical signals into binary data. The receiver
200
accepts an analog signal input consisting of a digitally encoded optical signal. The conventional photodiode
202
converts the optical input into an electrical current, and the conventional low-noise trans-impedance amplifier
204
converts the electrical current into an analog voltage. A conventional filter
206
, such as a low pass filter, filters the output signal from the amplifier
204
to reduce noise and improve sensitivity, and provides the filtered output to a conventional comparator
212
(also known as a quantizer or limiting amplifier). Threshold setting device
208
is coupled to receive the filtered signal at node
214
from filter
206
and determines and provides a threshold value to the conventional sample-and-hold or track-and-hold amplifier
210
. The amplifier
210
provides the threshold value to an input of comparator
212
, possibly fixing the value for the duration of an input data packet. The comparator
212
compares the analog voltage against a reference value, the threshold, and outputs a digital signal (either a logic level ONE or logic level ZERO) depending on whether the analog voltage is greater than or less than the threshold.
Typically in the system described in the Standard, data is transmitted by a source in packets. The useful data in a data packet is preceded by a sequence of signal bits (typically a repetitive ONE-ZERO sequence) which carries no useful information. These additional bits, referred to as the preamble, are used to establish the threshold value to be used by the threshold setting device of
FIG. 2
for determining the threshold. Because the preamble transports no useful information, the length of the preamble should be minimized to improve the transmission rate of useful data.
One conventional threshold setting device
208
uses a pair of peak detectors to measure the ONE and ZERO levels of the input signal, and then uses the average of these values as the threshold (so called “peak detector”). See for example the following publications or issued patents which are each incorporated by reference herein in their entirety: M. Nakamura, N. Ishihara, and Y. Akazawa, “A 156 Mbs CMOS Optical Receiver for Burst-mode Transmission”, IEEE J. Sol. St. Circuits, 33, 117901187 (1998); Y. Ota, R. G. Swartz, V. D. Archer, S. K. Korotky, and A. E. Dunlop, “High-speed, Burst-mode Packet Capable Optical Receiver and Instantaneous Recovery for Optical Bus Operation”, J. Lightwave Technology 12, 325-330 (1994); U.S. Pat. No. 5,475,342, issued Dec. 12, 1995 to Nakamura et al., and entitled “Amplifier for Stably Maintaining a Constant Output”; U.S. Pat. No. 5,430,766, issued Jul. 4, 1995 to Ota et al., and entitled “Burst Mode Digital Data Receiver”; and U.S. Pat. No. 5,875,050, issued Feb. 23, 1999 to Ota, and entitled “Burst Mode Digital Optical Receiver”.
The peak detector provides very fast threshold detection, but has several difficulties associated with it. For example, the peak detector must operate at the input signal bit rate, which at high rates (e.g., 155 megabits/s or above) requires high slew rate within the peak detection block for accurate peak determination. High slew rate becomes increasingly difficult to achieve at higher bit rates. The peak detector is also fully susceptible to noise corruption because the peak detector must operate at high speed compared to the bit rate to provide high-fidelity signal tracking. Signal noise during peak detection, which can not be filtered, will result in imperfect threshold adjustment. Another example of the difficulty with the peak detector is the cost of peak detection can be higher than other approaches, because it requires two amplifiers (a first for ONE level detection and second for ZERO level detection).
Another conventional threshold setting device uses temporal averaging circuits to measure the average of the ONE and ZERO levels during the preamble (so called “temporal averaging detector”). The temporal averaging detector uses low pass filters (e.g., resistor-capacitor circuits) or integrators (amplifiers with capacitors) to average the signal level over many bit periods. They generally work by charging a capacitor over many bit periods. See for example, U.S. Pat. No. 5,539,779 issued Jul. 23, 1996 to T. Nagahori, and entitled “Automatic offset control circuit for digital receiver”, which is incorporated by reference herein in its entirety. The temporal averaging detector reduces noise due to the averaging process, but it is typically slow because the preamble must be averaged over many bits. The averaging is typically achieved using a capacitor charging circuit, which can be quite slow. Typical temporal averaging detectors require 24 to 32 bits of preamble.
Thus what is needed is a threshold determination system that provides for rapid and reliable threshold determination at hig
Deri Robert J.
Jiang Jing Wen
Chin Stephen
Fenwick & West LLP
Kim Kevin
Terawave Communications, Inc.
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