Free space communication

Laser communication systems based on free-space propagation through the atmosphere suffer drawbacks because of factors like atmospheric turbulence and attenuation by rain, snow, haze, or fog. Nevertheless, free-space laser communication systems were developed for many appHcations (89—91). They employ separate components, such as lasers, modulators, collimators, and detectors. Some of the most promising appHcations are for space communications, because the problems of turbulence and opacity in the atmosphere are absent. 

Pigure 10 shows the typical commercial performance of LEDs used for optical data communication. Both free-space emission and fiber-coupled devices are shown, the latter exhibiting speeds of <10 ns. Typically there exists a tradeoff between speed and power in these devices, however performance has been plotted as a function of wavelength for purposes of clarity. In communication systems, photodetectors (qv) are employed as receivers rather than the human eye, making radiometric power emitted by the devices, or coupled into an optical fiber, an important figure of merit. 

Until recently optical communications were restricted by the lack of fast monochromatic light sources and sensitive photodetectors. Prospects for optical communications improved considerably about two decades ago when a powerful light source became available with the invention of the laser. After that, the transmission medium was the bottleneck of an optical communication system. At that time an intensive search for a new transmission medium was started, particulary because free space propagation could be ruled out for civil use as a consequence of the relative frequent occurrence of atmospheric disturbances. 

The first QKD prototype operated over a distance of 32 cm in free space [147], Since then, experimental techniques have undergone a tremendous progress and today QKD systems at almost a commercial level are offered [165], A number of problems must have been solved. Communication distance has reached several tens of kilometers in optical fibers [166-169]. Also, free-space systems are being developed with earth-satellite communication on mind [170], It should be noted that only a few systems presented until now have exhibited parameters that would ensure a secure key generation. 

Teitelbaum M.E., Yarlagadda S., O Brien D., Wetzel E., Goossen K.W. (2008), Normal incidence free space optical data porting to embedded communication links. IEEE Transactions on Components and Packaging Technologies, vol. 31, no. 1, pp. 32-38... 

Lee, S. S., E. Motamedi, and M. C. Wu. 1997. Surface-micromachined free-space fiber optic switches with integrated microactuators for optical fiber communication systems. Pp. 85-88 in Proceedings of the 1997 International Conference on Solid-State Sensors and Actuators (Transducers 97), Chicago, 111., June 16-19,1997. New York Institute of Electrical and Electronics Engineers. 

The first one is a four-element patch antenna array for off-body communication in the 60 GHz band. The antenna radiating elements were fabricated by laser cutting a 0.07-mm-thick flexible copper foil, deployed on a cotton substrate with a Shieldlt ground plane. Antenna performance was experimentally tested in free space and in... 

The second proposed antenna for on-body communication is a planar Yagi-Uda, providing end-fire radiation for on-body propagation. In free space, the antenna is matched over the complete 60 GHz band and exhibits a maximum gain of 11.8 dBi at 60 GHz. Human body proximity affects performance depending on the antenna-body separation distance. However, the antenna also performs in a satisfactory way when deployed on a human subject, as well as under bending conditions. 

A typical problem in the design of a radio frequency communications system requires the calculation of the power available at the output terminals of the receive antenna. Although the gain or loss characteristics of the equipment at the receiver and transmitter sites can be ascertained from manufacturer s data, the effective loss between the two antennas must be stated in a way that allows for the characterization of the transmission path between the antennas. The ratio of the power radiated by the transmit antenna to the power available at the receive antenna is known as the path loss and is usually expressed in decibels. The minimum loss on any given path occurs between two antennas when there are no intervening obstructions and no ground losses. In such a case when the receive and transmit antennas are isotropic, the path loss is known as free space path loss. 

Note that, from Eq. (16.10), in free-space conditions the path loss rate due to the distance is 20 dB/decade or 6 dB/octave. Moreover, although it has been said that the free-space model describes an ideal propagation condition, it can indeed be accurately used in satellite communication systems and short hne-of-sight radio links. 

Radio frequency (RF) communication is usually understood to mean radio and television broadcasting, cellular radio communication, point-to-point radio communication, microwave radio, and other wireless radio communication. For such wireless radio communication, the communication channel is the atmosphere or free space. It is in this sense that RF distortion is treated in this section, although RF signals are also transmitted through other media such as coaxial cables. 

In wireless radio communication, electromagnetic waves are radiated into the atmosphere or free space via a transmitting antenna. For efficient radiation, the antenna length should exceed one-tenth of the wavelength of the electromagnetic wave. The wavelength X is given by... 

In Eq. (20.11) c = 3 10 m/s, is the free-space velocity of electromagnetic waves and / is the frequency in Hz. Wireless radio communications covers a wide range of frequencies from about 10 kHz in the very low frequency (VLF) band to above 300 GHz in the extra high frequency (EHF) band. Depending on the... 

Channel (1) The medium along which data travel between the transmitter and receiver in a communication system. This could be a wire, coaxial cable, free space, etc. 

Free space optical interconnection networks are preferable over other methods, especially for cases of relatively high interconnection volumes. Free space systems make full use of parallelism since all three spatial dimensions are involved. Flowever, dynamic systems are necessary for telecommunication applications, especially for optical fiber communications. 

D.P. Hutchinsoit R.K. Richards, All-weather long-wavelength infrared free space optical communications. J. Opt Fibta- Comm. Res. 4(3), 214-224 (2007)... 

Usually, a large field of use for MWIR and LWIR detectors is overlooked— the free-space optical (FSO) telecommunicafions. This is certainly unusual when one bears in mind that communications themselves are one of the pillars of civilization itself, actually one could maintain that communications were those that created civilization in the first place. 

Development, test and evaluation of MEMS micro-mirrors for free-space optical communications, Proc. SPIE 5550, pp. 299-312 (2004). 

We close this chapter with further statements on liquid crystals as a preferred material for optical and electro-optical applications. To date, liquid crystals and related optical technologies have been incorporated in the design and fabrication of filters, lens, waveguides, diffractive and reflective elements, routers and interconnects, etc., of various forms, shapes, and functions used in optical communication system" as well as in free-space beam steering systems. Their compatibility with almost all optoelectronic materials as well as polymers and organic materials allows even more possibilities and flexibility in the emerging field of flexible displays" and polymer cholesteric liquid crystal flake/fluid display. 

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