Optical communications – Optical communication over freee space – Transmitter and receiver
Reexamination Certificate
2002-04-18
2004-11-02
Pascal, Leslie (Department: 2633)
Optical communications
Optical communication over freee space
Transmitter and receiver
C398S156000, C398S162000
Reexamination Certificate
active
06813446
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to optical wireless communications and, more particularly, to apparatus and methods for establishing and maintaining a reliable optical wireless data link between to transmitting and receiving units.
BACKGROUND OF THE INVENTION
Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communication of large databases, but has also enabled the digital communication of audio and video content. This multimedia communication requires high bandwidth communication, which is now carried out over a variety of facilities, including telephone lines (e.g., fiber optic and twisted pair), coaxial cable (e.g., as supported by cable television service providers), dedicated network cabling within an office or home location, satellite links, and wireless telephony.
Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over “wired” networks, including the telephone network, cable network or dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network.
Wireless communication facilities of course overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled. Furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, wireless telephone communications are subject to interference among the various users within a nearby area. Radio frequency data communication must be carried out within specified frequencies, and is also vulnerable to interference from other transmissions. Satellite transmission is also currently expensive, particularly for bidirectional communications (i.e., beyond the passive reception of television programming).
Recently, attention has turned to optical wireless networking for data communications. Using this technology, data is transmitted by modulating a light beam, in much the same manner as in the case of fiber optic telephone communications. A photo-receiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the waveguide of a fiber optic cable.
Hence, optical wireless networks could provide numerous important advantages over other conventional communications systems. First, high frequency light modulation can provide for high bandwidth data communication (e.g., ~100 Mbps—Gbps). This high bandwidth need not be shared among multiple users, especially when carried out over line-of-sight optical communications between transmitters and receivers. Without other users on the communications link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing of multiple communications signals. Bi-directional communication can also be readily implemented utilizing this technology. Furthermore, optical frequencies are not currently regulated, and as such no licensing is required for the deployment of extra-premises networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector. The costs and effort associated with routing and placing cables in congested, space constrained areas can be eliminated using optical wireless links. If reliable enough, modems using optical wireless links would be especially valuable in mobile product devices such as laptop computers and handheld organizers.
A common problem with some conventional optical wireless links, however, is that they utilize relatively wide, diffuse optical beams to facilitate the acquisition and maintenance of a light link. The ability to correctly aim a transmitted light beam at a receiver is of importance in optical communications technology. Wider beams can allow for greater tracking tolerance, because exact positioning of a transmitting beam on a receiver is not required to maintain a nominal communication link. The use of wider beams, however, either decreases the intensity (i.e., power) of the beam at the receiver or increases the power required to deliver a high data rate signal, and can result in severe limitations in the usable bandwidth of the data link(s) established, thus decreasing the usefulness of link for many communication applications.
Some conventional systems attempt to use narrower, more tightly focused optical beams (e.g., laser generated collimated beams) to provide greater communications bandwidth. When utilizing laser-generated collimated beams, which can have quite small spot sizes, the reliability and signal-to-noise ratio of the transmitted signal are degraded if the aim of the transmitting beam strays from an optimum point at the receiver. Considering that many contemplated applications of this technology are in connection with equipment that will not be precisely located, or that may move over time, it is necessary to be able to rapidly and reliably adjust the aim of the light beam.
Because the integrity of communications does rely on precise optical alignment, conventional solutions can also present problems in circumstances where transceiver units are subject to some vibration or sway (e.g., a building to building link, or a mobile to stationary link). Many conventional systems rely on a low bandwidth direct feedback channel between transceivers, such as a secondary telephone line modem, and some gross mechanical adjustment (e.g., a motorized mechanical assembly housing one or more of the transceivers) to maintain transceiver alignment. Such conventional systems can have problems responding when high frequency vibrations occur, and make it difficult, if not impossible, to successfully track and maintain communications with a moving transceiver. Finally, such conventional systems are often not able to translate changes in signal strength, which is a common method of measuring the integrity of a communications link, into usable positioning information for the mechanical assembly.
Thus, when either a high degree of transceiver mobility is required, or when transceivers may be subject to high frequency or small scale vibrations, conventional systems are typically incapable of providing reliable, high bandwidth communication.
SUMMARY OF THE INVENTION
Therefore, a versatile system for acquiring and maintaining reliable optical wireless links that provides for simple and cost-effective high performance optical communications, especially where fixed optical units are subject to high frequency vibrations or where opt
Keller Robert C.
Melendez Jose L.
Brady III W. James
Kempler William B.
Pascal Leslie
Tran Dzung D.
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