Multiple Input Multiple Output FSO Communication System with Q-ary PPM Mapper

Free Space Optics (FSO) or optical wireless communication is a promising solution for the need to very high data rate point-to-point communication. In FSO Communication links, the atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. This intensity fluctuation, also known as scintillation is one of the most important factors that degrade the performance of an FSO communication link even under the clear sky condition. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of scintillation. In the paper, the modulation format is Q-ary PPM across lasers, with intensity modulation. Ideal photodetectors are assumed, with and without background radiation. It is found that MIMO system provides significant performance not only under weak turbulence but also strong turbulence. The performance results are evaluated in terms of symbol error probability for several system parameters.
Free Space Optical (FSO) communication is a telecommunication technology that uses light propagating in free space to transmit data between two points. FSO communication links have some distinct advantages over conventional microwave and optical fiber communication systems by virtue of their high carrier frequencies that permit large capacity, enhanced security, high data rate and so on. Such links are suitable for 1–2 Gb/s rates over distances in the range of 1–5 km.It is a promising solution for the need to very high data rate point-to-point communication.

Short-range line-of-sight optical links have the potential for providing high-bandwidth access to a larger wired network, for linking intranets within corporate campuses, or even for links between aircraft. Such optical transmission, often called free-space optics (FSOs), is attractive for several reasons, notably the license-free nature of the spectrum, the highly directive nature of the radiation (providing spatial isolation), relatively low infrastructure costs, and rapid redeployment. The

Two primary challenges are attached to free-space optical communication. First, the narrow beam width (on the order of a few mill radians typically) implies the need for careful pointing, and building vibration or sway can induce signal strength fading in the link. Second, is the need to combat link fading due to scattering and scintillation.

To address both challenges, we consider the use of (no coherent) optical arrays, analogous to the use of antenna array technology for microwave systems, as a means of combating fading. Specifically, we envision M separate lasers, assumed to be intensity-modulated only, together with N photodetectors, assumed to be ideal no coherent (direct detection) receivers. Due to laser incoherence across the array, their powers add at the Receiver array. The sources and detectors are physically situated so that the fading experienced between source-detector pairs is statistically independent and, thus, diversity benefits can accrue from the multiple-input/multiple-output (MIMO) channel.

Optical Wireless Communication has emerged as a viable technology for next generation indoor and outdoor broadband wireless application. Applications range from short range wireless communication links providing network access to portable computers to last –mile links bridging between end users and existing fiber optic communications backbones, and even laser communications is also called wireless infrared communication, while outdoor optical wireless communication is commonly known as free space optical (FSO) communication.

The applications of free-space optics are many and varied. Such as,
•Metro network extensions: Carriers can deploy FSO to extend existing metropolitan-area fiber rings, to connect new networks, and, in their core infrastructure, to complete Sonet rings.
•Last-mile access: FSO can be used in high-speed links that connect end-users with Internet service providers or other networks. It can also be used to bypass local-loop systems to provide businesses with high-speed connections.
•Enterprise connectivity: The ease with which FSO links can be installed makes them a natural for interconnecting local-area network segments that are housed in buildings separated by public streets or other right-of-way property.
•Fiber backup: FSO may also be deployed in redundant links to back up fiber in place of a second fiber link.
•Backhaul: FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
•Service acceleration: FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid. In applying wireless infrared communication, on-directed links, which don’t require precise alignment between transmitter and receiver, are desirable. They can be categorized as either line-of-sight (LOS) or diffuse links. LOS required an unobstructed path for reliable communication, whereas diffuse links rely on multiple optical paths from surface reflections.
On the other hand FSO communication usually involves directed LOS and point-to-point laser links from transmitter to receiver through the atmosphere. FSO communication over few kilometer distances has been demonstrated at multi-Gbps data rates.
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