Traceback equalization for non-uniformly synthesized optical QAM signals

Optical quadrature-amplitude modulation (QAM) enables high-spectral-efficiency transmission, where high-order optical multilevel signals can be generated, combined from multiple low-order multilevel signals (like binary streams). This signal combining approach is useful because it is difficult to directly generate multilevel signals at high symbol rate beyond ~10 Gbaud. Electrical combining is one of the possible ways, where electrical multilevel signals are generated combining binary sequences first, and then IQ modulator is driven with the signals. Another one is optical synthesis approach, where optical signals are combined in optical domain by using integrated serial or parallel modulators to create higher order multilevel signals. A difficulty still remained is how to uniformly combine electrical/optical signals to form multilevel signals, especially at high symbol rate. Due to imperfectness of the combiners and driving conditions of modulators, the signal sources would be independently distorted, shaped into different waveforms, which is critically cause intersymbol interference especially when higher-order and higher-speed multilevel signaling are targeted. In this report, it is proven that conventional adaptive FIR equalizers cannot linearly compensate for the non-uniformly distorted signals. A novel equalizing scheme called TraceBack Equalization (TBE) is proposed to cope with the problem.