With the advent of the coherent age the implementation of massive digital signal processors (DSP) co-integrated with high speed AD and DA converters became feasible allowing for the realization of huge flexibility of transponders. Today the implementation of variable transponders is mainly based on variable programming of DSP to support different modulation formats and symbol rates. Modulation formats with high flexibility are required such as pragmatic QAM formats and hybrid modulation formats. Furthermore, we report on an implementable probabilistically shaping technique allowing for adjusting the bitrate. We introduce fundamental characteristics of all modes and describe basic operation principles. The modifications of the operational modes are enabled simply by switching between different formats and symbol rates in the DSP to adjust the transponders spectral efficiency, the bitrate and the maximum transmission distance. A fine granularity in bitrate and in maximum transmission distance can be implemented especially by hybrid formats and by probabilistically shaped formats. Furthermore, latter allow for ~+25% increase of the maximum transmission distance due their operation close to the Shannon limit as a consequence of their 2D Gaussian like signal nature. If the flexibility and programmability of transponders is implemented, it can be utilized to support different strategies for the application. The variability in symbol rate is mainly translated into variability in bitrate and in bandwidth consumption. Contrary the variable spectral efficiency translates into a variation of the maximum transmission reach and of the bitrate. A co-adjustment of both options will lead to a superior flexibility of optical transponders to address all requirements from application, transponder and fiber infrastructure perspective.
On the following pages we address some actual topics of analog and digital electronic equalisation for 10 and 40 Gbit/s transmission. Analog circuits currently overcome the bandwidth limits for 40Gb/s operation. With first 5 tap feed forward equaliser (FFE) SiGe chips CD and PMD tolerance enhancement can be demonstrated. According to numerical simulations, the combination of the FFE with a one-stage decision feedback equaliser (DFE) should be a good choice to mitigate signal degradation caused by various effects. At 10 Gb/s more complex signal processing based on digitalsignal-processing (DSP) is already implemented in first products as maximum likelihood sequence estimator (MLSE, also referred to as Viterbi equaliser VE) or in a lab prototype of soft error correction (soft FEC). Detailed numerical
studies on the performance of the VE reveal that opposite to the analog FFE+DFE, low electrical bandwidth cannot be handled by the standard MLSE scheme and an extension of the processing algorithm is needed. Though the combination of an MLSE based equaliser and soft-FEC has the potential to lead to further improvement, numerical analysis of turbo equalisers combining distortion mitigation and error correction in an iterative way indicate that with a FEC overhead in the range of 7% only moderate equalisation improvement seems to be possible.