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Decision Feedback Threshold Optimization for Optical Interconnects

IP.com Disclosure Number: IPCOM000107717D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 2 page(s) / 87K

Publishing Venue

IBM

Related People

Li, CS: AUTHOR [+2]

Abstract

Disclosed is a circuit using a decision feedback mechanism to overcome the threshold uncertainty problem in both single-ended and differential optical interconnects so that a correct threshold can be derived from the incoming data stream regardless of its DC contents.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Decision Feedback Threshold Optimization for Optical Interconnects

       Disclosed is a circuit using a decision feedback
mechanism to overcome the threshold uncertainty problem in both
single-ended and differential optical interconnects so that a correct
threshold can be derived from the incoming data stream regardless of
its DC contents.

      For a single-ended optical interconnect system, it is usually
necessary to require a DC-balanced data stream in order to maintain
the correct operation of the AGC mechanism and to derive the correct
decision threshold.  DC-balanced data stream can usually be achieved
through various line coding techniques such as the 8/10 code.
However, line coding of digital data stream has the following
disadvantages:
o   The channel efficiency is reduced, and so is the effective trans
mission rate,
o   The complexity of both the transmitter and receiver is increased
due to the extra hardware required for encoding/decoding.
o   Increase in the latency of an interconnect, which might not be
tolerable in delay-sensitive interconnect applications.

      Fully differential-ended optical interconnect can solve this
problem by transmitting complementary signals so that the optimal
decision threshold can be maintained at the absolute zero.  However,
this approach has the following disadvantages:
o   Extra hardware is necessary to accommodate the complementary
data. The extra hardware can reduce the reliability.
o   The receiver is sensitive to the channel and device mismatch, but
not sensitive to the average DC power in the  input data stream. A
total channel mismatch of 40% induces a 2.2 dB power penalty (1).

      In this invention, a decision feedback mechanism has been used
to generate the optimal threshold for both single-ended and
double-ended receive, thereby removing the problems described above.
This mechanism is illustrated in the figure for a differential
receiver.  The output from the receiver first passes through a
decision circuit.  The dec...