Adaptive decision-aided algorithm for coherent optical communication systems

Internet traffic is growing exponentially worldwide, driven by multimedia applications such as video sharing and point-topoint file transfers. By 2014, demand is expected to increase by 64 exabytes (or billion gigabytes) per month, equivalent to over three million years’ worth of DVD-quality video.1 How to keep ever-greater volumes of data moving instantaneously from place to place without bottlenecks poses a challenge to the speed of both electrical and optical components. If the backbone networks were to fail, it would affect communication between continents and cities and consequently business, education, and other kinds of critical information sharing. Currently, keeping up the pace of data would require laying down more fiber to increase capacity. Yet according to the Shannon limit—a measure of the information transmission rate in the presence of noise—higher-order modulation formats are needed to accommodate the increased data traffic while keeping changes to the system configuration to a minimum in the interest of cost. On the receiver side, a local oscillator (LO) laser must be at the same frequency as the received signal to obtain the original intensity and phase information. But time-varying laser phase noise introduces distortions. Consequently, the LO laser should be locked to the received signal’s phase to recover the transmitted signal phase. Full information (i.e., the amplitude and phase) of the optical electric field can be preserved in digital coherent receivers. Accordingly, rather than using an optical phase-locked loop to lock the phase of the LO laser to the transmitter laser—which is sensitive to feedback delay in ultra-high-speed (e.g., 100Gbits/s) optical systems—an LO laser can run freely (i.e., require no extra Figure 1. Structure of the adaptive decision-aided algorithm. r(k): Received signal. ̨ and 1 ̨: Weights assigned to the previous and current phase reference. m, m̂*: The decision of the received symbol and its conjugate. T: Symbol duration (the minimum time interval between condition changes of a signal). V(k+1): Phase reference.