Abstract

Over the last couple of years well-known equalization concepts have been adapted to optical transmission, like Adaptive Threshold (AT), Feed Forward Equalization (FFE), Decision Feedback Equalization (DFE) and Viterbi Equalization.¹ In addition, Forward Error Correction (FEC) has become essential to reduce the required receiver optical input power or optical signal-to-noise ratio.² Dynamic changes, for instance due to PMD, must be tracked by adaptation of the equalizer coefficients as well as the sampling phase. Different types of adaptation algorithms were proposed for optical systems, e.g., based on eye monitoring or RF spectrum properties. These methods require additional measurement and control circuits.³ The main goal of adaptive equalization is to minimize the receiver output BER. In systems where FEC is applied to the transmitted data, BER can be estimated online using the number of bits corrected by FEC.⁴ This paper will show that a 3-tap FFE can be adjusted by estimated conditional BERs (i.e., error counts for different sequences of transmitted bits), which can provide low cost FFE adaptation in FEC systems.

© 2002 Optical Society of America