Fundamental limits of electronic dispersion compensation in optical communications with direct photodetection

Introduction: Owing to the decreasing cost of powerful integrated circuits, electronic signal processing in digital optical communication systems is receiving increasing scientific and industrial interest. In 10 Gbit=s optical links, such as those used in metropolitan and wide area networks, chromatic dispersion (CD) compensation plays an important role and affects significantly the communication system cost. Electronic dispersion compensation (EDC) techniques are appealing to replace costly optical dispersion compensation units and to guarantee robustness and adaptivity. Various EDC solutions are possible, such as electronic equalisation [1] and maximum likelihood sequence detection (MLSD) [2]. In this Letter, we evaluate the information rate (IR) of an optical fibre communication system with direct photodetection. The IR is the supremum of the achievable transmission rates, i.e. the rates at which reliable communication can be achieved [3, 4]. The IR is evaluated accounting for the modulation format, the fibre characteristics, and the receiver front end. The fibre is affected by CD and white Gaussian amplified spontaneous emission (ASE) noise. For a given value of CD, we compute a lower bound on the IR against optical signal-to-noise ratio (OSNR). The OSNR required to achieve a desired value of IR is also derived against CD. In particular, the IR can be fixed at the rate of a typical error correction code (ECC). In the considered communication scenario, our results show that the theoretical OSNR penalty, with respect to a scenario with no dispersion (back-to-back), is at most 2 dB at CD values up to 6800 ps=nm. Moreover, our results suggest that the OSNR penalty flattens for increasing values of CD. The price to be paid for a limited OSNR penalty is an increase in EDC signal processing complexity with the amount of CD.