Optimization mechanisms in multi-dimensional and flexible PONs: challenging issues and possible solutions

The current Passive Optical Network (PON) has faced many challenges when heterogeneous access techniques and multiple resource multiplexing ways are involved. Thus, we require a unified control and management of time, spectrum and mode, which is a necessary standardization for PONs. For this end, the multi-dimensional and flexible PON (Mflexi-PON) will emerge as an ideal solution of supporting the heterogeneous access with different resource multiplexing ways. But a high-efficient Mflexi-PON has its own problem. In term of the physical layer of Mflexi-PONs, some signals include a series of orthogonal subcarriers, or transmission modes should have the orthogonal property. Once multiple subcarriers have the same phase, the instantaneous power of a signal will exceed the average value, thus leading to a high Peak-to-Average Power Ratio (PAPR). Furthermore, the signal or transmission mode must be kept strictly orthogonal so that it can be correctly demodulated by the receiver side, which results in the high sensitivity of frequency offset and phase noise. In addition, we need to solve these problems under various resource multiplexing approaches. Thus in this paper, we first review previous solutions for the next-generation PON and discuss some challenging issues for Mflexi-PONs. Next, some promising solutions are proposed for optimization mechanisms in this new networking paradigm, mainly including system architecture, MAC (Media Access Control) layer allocation of multi-dimensional resources, physical-layer performance improvement, and cross-layer optimization.