Space-division multiplexing and MIMO processing
of multiple parallel optical paths, is left. The general technique is referred to a space-division multiplexing (SDM), and is the obvious choice for any interconnection technology if the capacity of a single serial channel reaches a technological barrier. Space-division multiplexed system can be implemented by using multiple parallel SMFs, in this form however no significant reduction of cost-per-bit can be expected and therefore around 2009 a new research effort in fiber optics communication started with the aim of identifying cost efficient and scalable high capacity SDM fiber systems  Two fiber types of particular interest emerged: The multicore fibers (MCFs), where multiple fiber guiding cores are introduced in a common cladding area and multimode fibers (MMFs), where the fiber modes are exploited to transmit multiple parallel channels. Both fiber types are well known but the design was now revisited and optimized for high capacity transmission of parallel channels. All SDM transmission systems can essentially be divided in two categories depending on the way the multiple parallel channels are processed at the receiver. In uncoupled systems, each path is processed individually and any crosstalk from other channels will appear as an impairment limiting either the reach or the capacity of the individual channels. The second approach is based on joint processing of the signals from the multiple parallel paths. The transmission system can then be described as a multiple-input multiple-output (MIMO) channel and well known digital signal processing (DSP) techniques developed for wireless communication can be adopted.