Free Space Optical Wireless Network
Fiber network architecture building blocks can be applied to create “free-space optical wireless network” over the atmosphere with vastly improved and disruptive characteristics for urban and suburban accesses. The prime application will be terrestrial over short distances such as between top of buildings in a densely populated city, but aircraft to ground or satellites and satellites to ground are also possibilities. Communications over rain, fog, and snow are feasible for short but interesting ranges, up to four times the optical depth (or visibility) of the channel. Thus, availability of ranges of around 1 km is high for the 1.5 μm telecom band (to the point of near all weather), and with additional developments in longer infrared wavelengths (e.g., 3.2 μm) that yield longer optical depths, ranges over foul weather can be extended. For both clear and foul weather, atmospheric turbulence impairs transmission and must be properly dealt with. Temperature fluctuations give rise to many random “weak” lenses, and the resulting diffraction effect causes “hot” and “cold” spots of the optical field in the receiving plane. The receiver aperture typically only collects a small area of that field. Hence, it is not uncommon for the link to suffer dropouts of 10–20 dB for 0.1–100 mS every 0.1–1 S. At high data rates, these dropouts represent a large number of bits. Coding and interleaving are possible as a mitigation technique, but at the expense of very long delays, and thus will not work well with typical transport layer protocols such as TCP, leading to serious throughput loss (up to 99 %). Thus, the telecom-based communication network architecture must be modified to deal with this serious impairment to realize a usable system. The channel can be abstractly modeled as an “on–off” channel. When the received power level is high enough for the receiver to demodulate with low error probability, the channel is considered “on.” When fades due to turbulence are deep and the receiver can no longer demodulate with high reliability, the channel is “off.” A two-state continuous parameter Markov Process can be used to characterize the channel with its transition rates given by the strength of the turbulence and the transverse wind velocity blowing the turbules across the channel. These channel fades will not yield satisfactory performance for any serious network usages, and especially when used as part of a short-range open air high-speed (large delay-bandwidth product) edge network interconnected to the wired Internet. In the Physical Layer, transmitter and receiver diversities can be used to reduce the length, duration, and frequency of occurrence of the fades are possible mitigation techniques. Small, low cost optics can be used in transmitter and receiver arrays with the elements spaced more than an intensity and phase coherence length apart (typically 1–20 cm). For strong turbulence, which occurs frequently during day time, additional innovations in higher layers of the network are needed. In the network layer, when the network is multiply connected, multipath routing will increase reliability at the expense of some network throughput. Since this technique couples the resources of the link layer and the network layer, this chapter explores network connection topologies with maximum number of independent paths for the same array sizes of the transmitter and receiver arrays to maximize path diversity performance. Innovations in the transport layer are also needed. Unless great expense is invested in many diversity paths, there will still be occasional residual dropouts. If TCP is used, as in most applications over the Internet, it will interpret these dropouts as congestions at the routers and trigger “window-closing” and “slow-start” resulting in devastating effects on network throughput. A new mechanism must be used in the transport layer to differentiate fading from router congestion and embedded in the ARQ protocol. There will be the necessity for an integrated physical layer to transport layer architecture.
KeywordsMicrowave Coherence Expense Hunt Huygens
Research partially supported by DARPA and ONR and the manuscript is a much expanded version of .
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