Multiplex communications – Communication over free space
Reexamination Certificate
1998-07-01
2002-08-27
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
C370S204000
Reexamination Certificate
active
06442145
ABSTRACT:
TECHNICAL FIELD
The present invention relates to multi-mode wireless optical communication systems and the communication and/or coexistence of communication between different kinds of devices, operating in different modes within such communication systems.
BACKGROUND OF THE INVENTION
With the rapidly increasing number of workstations and personal computers (e.g. desktop or handheld ones) in all areas of business administration, fabrication etc., there is also an increasing demand for flexible and simple interconnection of these systems. There is a similar need as far as the hook-up and interconnection of peripheral devices such as keyboards, computer mice, printers, plotters, fax machines, scanners, displays, modems, and so forth, is concerned. The use of electrical interconnects becomes a problem with increasing number of systems communicating with each other, and in many cases in which the location of systems, or the configuration of subsystems, must be changed frequently. It is therefore desirable to gain flexibility by eliminating electrical interconnects for such systems and using wireless communication instead.
The use of optical signals for wireless transfer of digital data between systems and devices has received increased interest during recent years and has lead to applications in commercial products. One example is the optical remote control of electronic consumer devices. Another example is the communication between information systems in an office environment. In an optical communication system digital data to be transferred between a transmitting system and a receiving system are transformed into modulated optical signals which are radiated by a light source —in particular an infrared (IR) light source—of the transmitting system and are received, converted to electrical signals and then into digital data by the receiving system. The optical signals might directly propagate to the optical receiver of the receiving system or they might indirectly reach the receivers after changes of the direction of propagation due to processes like reflections or scattering at surfaces. Today, the former case is realized in portable computers and peripheral devices where the data transfer takes place between an optical transmitter and a receiver which are close together at a distance on the scale of 1-3 m and properly aligned. The latter case is typical for applications in an office environment in which undisturbed direct transmission of optical signals between transmitters and receivers several meters away from each other is impractical or even impossible due to unavoidable perturbations of the direct path. One known approach to achieve a high degree of flexibility is to radiate optical signals form the transmitting system to the ceiling or walls of an office where they are reflected or diffusely scattered. Thus, the radiation is distributed over a certain zone in the surroundings of the transmitter. The distribution of the light signals spreading from the ceiling depends on many details which are characteristic for the particular environment under consideration. However, essential in this context is mainly that the transmission range, i.e. the distance between transmitting system and receiving system, is limited to some final value, hereafter called the transmission range, since the energy flux of the transmitted radiation decreases with increasing distance or propagation and the receiver sensitivity is limited due to a minimum signal-to-noise ratio. Typical known systems, operating at levels of optical power which are limited by the performance of the light sources and safety requirements for light exposure, have demonstrated transmission ranges of several meters for data rates of 1 Mbps.
The latter example illustrates basic features of wireless optical communication and indicates fields of applications where it is favorably applied in contrast to another competitive method of wireless communication, the radio frequency (RF) transmission. Wireless optical communication allows data transmission which is short range, whereas RF transmission is potentially long range. Furthermore, optical wireless communication in an office environment is localized since typical boundaries of an office such as walls and ceilings are not transparent for sight but generally for RF waves. That is why possible interferences between different communication systems are easier to control and a simpler way for achieving data security is possible for a wireless communication system which is based on optical radiation rather than RF transmission. RF transmission is even restricted by communications regulations and licenses whereas optical wireless communication systems are not.
Crucial performance parameters of a wireless optical communication system are the achievable data rate and the distance between the systems exchanging data. In an office environment, it can be necessary to communicate data over distances exceeding the transmission range of a single optical transmitter. However, the transmission range of a single optical transmitter can be extended within the concept of wireless communication, for example by introducing optical repeaters. One example of such an extended system has been proposed in U.S. Pat. No. 4,402,090 entitled “Communication System in which Data are Transferred Between Terminal Stations and Satellite Stations by Infrared Systems”. In this patent, a system is described which provides a plurality of satellite stations, i.e. stations usually fixed at the ceiling of a large room. Terminals can optically interact with satellites within their transmission range, and data can be distributed via intersatellite communication thus enabling the distribution of data over distances beyond the transmission range of a single transmitter.
When designing a wireless optical communication system, one has to be aware of unavoidable ambient light, such as daylight or light from lamps, which always reaches the optical detectors, unless the system is restricted for the use in a completely dark environment. The IR energy in ambient light (fluorescent and incandescent lamps, sun light) can lead to dominant noise in the optical receiver. Thus, ambient light influences the signal-to-noise ratio of the receiver and, therefore, affects the transmission range. Further details on the effect of ambient light are outlined in the pending PCT patent applications PCT/EP 94/01196, published on Oct. 26, 1995 (Publication No. WO 95/28777). The appearance of ambient light is mostly statistical and often difficult to control and its intensity can drastically change, as it is apparent for lamps being switched on/off, or sunlight. A further realistic effect which statistically affects the signal-to-noise ratio and thus the transmission range is the occurrence of optical path obstructions influencing the receiver signal. In an office environment, for example, moving users can change the strength of the transmitted signals and the influence of unavoidable ambient light as well.
In present light-based wireless communication systems, first obvious attempts have been made to handle the ambient-light problem. Usually, low frequency (≦500 KHz) AC signals, which can be attributed to common room illumination, are suppressed with electrical filters after the conversion of light to electrical signals. Optical filters are used to restrict the spectrum of undesired ambient light. However, a significant portion of daylight is spectrally in the same range as the optical radiation of the light sources appropriate for wireless communication systems.
As described in the above mentioned PCT patent applications PCT/EP 94/01196, and in another PCT application PCT/EP 94/00577, published on Aug. 31, 1995 (Publication No. WO 95/23461), it is possible to provide an infrared wireless communication system which efficiently copes with basic problems, such as incident ambient light, for example, of commercially available systems. In PCT/EP 94/01196 a scheme is provided which allows the dynamic optimization of wireless optical communication systems
De Lange Martin
Gfeller Fritz R.
Hirt Walter
Nguyen Brian
Reid Scott W.
Ton Dang
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