Wireless techniques in optical transport
2009 14th OptoElectronics and Communications Conference
The field of optical communications is undergoing a transformation from analog to digital. Advanced signal processing techniques which have been widely used in wireless communications and local access loops are now being applied to long haul optical transmission networks. In this paper, we discuss the implications of such transformations and postulate a new paradigm for optical transport in future high speed optical backbone networks. Summary Since the beginning of fiber optic communications
... c communications more than 40 years ago, the capacity of information carried in a single fiber has increased by more than 5 orders of magnitude (10 5 ). This vast capacity increase was mostly propelled by the tremendous advancements in fiber optic component technologies. Unlike other technologies such as wireless and DSL (digital subscription line), in which the physical layer is tightly integrated with medium access control (MAC) and IP networking layers as a single platform, fiber optic transmission system remained mostly a separate isolated transport layer which is relatively independent from upper layer equipment providing networking functions such as routing and switching. Many optical transport layer equipment companies, with special knowhow in engineering physical layer optical transmission links, have been established to produce large-capacity and high-density WDM (wavelength division multiplexing) optical transmission systems. In a modern-day optical transport system, a short-distance client side interface with gray-optics (usually in the form of 850nm multimode fiber interface or 1310 single mode fiber interface) forms the demarcation point between the upper networking-layer equipment and the lower-layer optical transport equipment. Most of the optical transport equipment reframes the upper layer data with additional FEC (forward error correction) overhead to protect the signal for long haul transmission. The reframed signals then remodulate WDM-compliant optics, which are multiplexed in the wavelength domain before transmission. The main function of the optical transport layer is to provide pointto-point connectivity in most fiber optic networks. Throughout the history of fiber optic communications, simple on-off keying (OOK) with direct intensity detection has been the main modulation scheme of choice  (even for long haul WDM transmissions). Advanced signal processing techniques commonly adopted in other communication systems  have given way to the simplicity and economic benefit of OOK and direct detection for the following reasons: (1) In the past, the abundance of bandwidths inside the optical fiber reduced the value of more bandwidth efficient modulation schemes, which would increase the cost and complexity of transceiver designs.