Hitless rate and bandwidth switching in dynamically recon gurable coherent optical systems

Much of the latest research in optical communications is directed towards increasing network efficiency, optimizing bandwidth usage and reducing power consumption. Emerging paradigms, such as flexible frequency grid, sophisticated optical switching methods and spectral defragmentation, attribute adaptive properties to future networks, enabling them to adjust to the changing transmission and traffic conditions. To support these features, next-generation optical transponders must be capable of adjusting their transmission rate and bandwidth occupation according to the time-varying traffic demands. However, conventional rate-switching requires service interruption, which is highly undesirable from the operators’ standpoint. Our contributions in the proposed thesis consist of two methods for hitless online rate switching that allow to reduce power consumption, associated with digital signal processing, during low traffic periods. The first method is designed for transponders with binary-signal-driven transmit-side modulators, and targets primarily static bandwidth allocation scenarios. Here, rate reduction is achieved by symbol repetition, and a conventional receive-side equalizer block is replaced by a multi-branch structure, where each branch operates at a specific transmission rate. Synchronization loss is avoided by using the output of the active branch as a training sequence for the target-rate branch. The method can also support dynamic bandwidth allocation, provided there is a cooperation mechanism between the optical transponders and the reconfigurable add-drop multiplexers. Here, signal spectral adjustment can be obtained via optical filtering by flex-grid-enabled wavelength-selective switching. The second method targets transponders with transmit-side digital signal processing. Rate changing is achieved by increasing/reducing the number of samples per transmitted symbol, while maintaining a fixed digital-to-analog conversion rate. Changing the symbol rate also implies in corresponding signal bandwidth adjustment, so that the method can support dynamic bandwidth allocation features of the next-generations elastic optical networks. We show that if the rate and bandwidth switching is performed in sufficiently small discrete intermediate steps, the receive-side dynamic equalizer successfully tracks signal changes, maintaining system synchronism.