On Filter and Code Design for Energy Efficient Fiber-Optic Communications

Licentiatavhandling, 2017

The advent of the Internet not only changed the communication methods significantly, but also the life-style of the human beings. The number of Internet users has grown exponentially in the last decade, and the number of users exceeded 3.4 billion in 2016. Fiber links serve as the Internet backbone, hence, the fast grow of the Internet network and the sheer of new applications is highly driven by advances in optical communications. The emergence of coherent optical systems has led to a more efficient use of the available spectrum compared to traditional on-off keying transmission, and has made it possible to increase the supported data rates. Transmission over fiber suffers from several impairments, which can be compensated for to a large extent using sophisticated digital signal processing (DSP). This comes at the cost of increasing the receiver complexity, which in turn leads to a higher energy consumption. Therefore, improving the energy efficiency of fiber-optic systems is of upmost interest. In this thesis, we attempt to improve the energy efficiency of coherent optical systems from an algorithmic design perspective. Chromatic dispersion (CD) is one of the major impairments that limits the transmission reach of optical systems. CD can be compensated for in the DSP of the receiver. To improve the energy efficiency of CD compensation, one should take implementation considerations into account when designing the CD compensation filter. In this thesis, we design a new CD compensation filter that is more robust to quantization errors. This allows to reduce the required word length for the filter implementation and the energy consumption of the CD compensation. To achieve high spectral efficiencies and improve the transmission reach, coding in combination with a higher order modulation, a scheme known as coded modulation (CM), has become indispensable in fiber-optic communications. In the recent years, graph-based codes such as low-density parity-check codes and soft decision decoding (SDD) have been adopted for long-haul coherent optical systems. SDD yields very high net coding gains but at the expense of a high decoding complexity, which brings implementation challenges at very high data rates. Hard decision decoding (HDD) is an appealing alternative that reduces the decoding complexity. This motivates the focus on this thesis on HDD. In particular, we derive achievable information rates (AIRs) for CM with HDD for both bit-wise and symbol-wise decoding, and show that bit-wise HDD yields significantly higher AIRs. We also design nonbinary staircase codes using density evolution. Finite length simulation results of binary and nonbinary staircase codes corroborate the conclusions arising from the AIR analysis, i.e., for HDD binary codes are preferable.