Abstract:
An indirect optical free-space communications system provides broadband transmission of high-speed data in transportation vehicles such as aircraft. The system includes a transmitter with a light source that can be modulated and a receiver with a photo detector, adapted to receive light that is emitted from the transmitter and to convert it into an electric signal. The transmitter and the receiver are directed onto at least one common surface, which reflects the light emitted from the transmitter before it reaches the receiver. The free space communications system is cellular and includes several cells, each of which has a transmitter and at least one corresponding receiver. The cells are configured in such a way that crosstalk with a respective adjoining cell is prevented. The cells are thus independent of one another.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to an indirect optical free-space communications system for broadband transmission of high-speed data and a method for broadband transmission of high-speed data. 
     Communication between computers or other technical systems is important not only in office rooms or computer rooms but increasingly also in the interior of transportation means because data communication is also increasingly important there. For example, larger transportation means, such as buses, trains, airplanes, ships, etc. are equipped with displays, earphone connections, input terminals and/or receivers or data stations, so the passenger can be entertained or informed during the trip and service personnel can exchange data with central data stations, etc. In such applications, very large volumes of data are usually transmitted by a central data memory and/or transmitter to one or more data receivers or vice-versa. 
     The data transmission, which may be in analog or digital form, is accomplished via electric cable connections between the transmitter and receiver. However, this is associated with a number of disadvantages. 
     First, cable must be installed to each terminal, i.e., each receiver/transmitter and/or each data station in the interior must be equipped with a suitable plug and the cable must be provided with a suitable plug connection at the location of the connection. The receiver/transmitter therefore has only limited flexible mobility and also needs a standardized plug connection, and the number of possible receivers is predetermined by the number of plugs on the cable or by the number of cables. The location of the data station is also limited by the predetermined spatial arrangement. This is problematical where the number and location of receivers is not predetermined and must be kept flexible. However, equipment with a great many cables and plug connections is expensive and susceptible to trouble. 
     Electric cables are also exposed to interference due to electromagnetic radiation (EMF). There is a drastic loss of quality during transmission of analog signals in particular but interference also occurs in the case of digital transmission. This problem is especially difficult to handle in transportation means because in a moving vehicle, unlike a fixed building, the environment and the incident radiation cannot be kept constant over the transmission link, so that incident radiation can at most be reduced by overdimensional shielding during travel. However, such highly shielding cables are heavy, expensive and immobile. Furthermore, the transportation means itself may cause a high level of electromagnetic contamination, i.e., EMF. 
     In addition, an electric cable is not only exposed to incident radiation but also emits radiation itself. This in turn causes EMF with respect to other electronic systems within the transportation means or with respect to other transportation means that happen to be in the vicinity. Furthermore, such a system is also susceptible to interception. 
     Another disadvantage is the limited data rate which can be transmitted due to the limited bandwidth of electric cables. The higher the data rate, the more difficult is its transmission over an electric connection owing to damping and dispersion and the greater is the problem of EMF and shielding. 
     To overcome the problem of EMF, an attempt is made to perform the data transmission through optical fibers by means of modulated light signals. A light source is usually amplitude modulated. Frequency and phase modulation are also possible. The light of the transmission source is transmitted to the receiver over a glass or plastic light source conductor that replace the electric cable. The receiver contains a photodetector for converting the optical signal back into an electric signal. Very high data rates can be achieved in this way and EMF is ruled out. 
     However, inflexibility with regard to the number and location of the stations to be connected in the interior remains a problem, especially since the optical fibers and plug connections cost more in general than the corresponding electric connections and are more difficult to install. Furthermore, it is difficult to provide free plug connections for stations to be connected later and/or it can be done only at great expense. 
     To solve the problem of cabling, there have been attempts to use wireless communication systems such as those that have long been used in the field of wireless transmission. By analogy with radio and wireless communications, all types of data can be transmitted by wireless transmission. The possible data transmission rate, however, depends on the frequency of the electromagnetic radiation used. Recently there are small microwave transceivers having frequencies in the gigahertz range, which in practice allow data communication in the range of a few Mb/s. However, a disadvantage here is that the data transmission rate is limited in principle, and there is still the problem of EMF radiation and emission of EMF into other systems of the transportation means as well as the problem of security against interception. In the case of transportation means that are used mainly in various countries of the world, such as airplane or ships, the local regulations for the frequency bands and types of modulation that are used must be taken into account, which greatly restricts the choice of available frequency bands and complicates their international use. 
     An alternative approach is a wireless but optical data transmission system in which the optical radiation modulated with data is transmitted directly into the room and is received by a photodetector (e.g., IrDA standard). This method is used, for example, for data transmission between a mobile computer and a printer. However, the disadvantage of this method is that there must be a direct line-of-sight connection between the transmitter and receiver, and the distance between the two must be relatively small, typically &lt;1-2 meters. The radiation must be within a certain angle range of the receiver, e.g., in the range of less than ±15° and in direct visual contact with it. 
     IR data transmission systems using scattered IR radiation which have a much greater range and acceptance angle and are suitable for connecting computers in office rooms, for example, are described by F. R. Gfeller, U. Bapst, Proceedings of the IEEE, Vol. 67, No. 11, November 1979. 
     Another system is described in German Patent DE 101 07 538 B4. Such a system allows a considerable bandwidth in the range of a few Mb/s to approx. 100 Mb/s and ranges of several meters without the requirements of beam alignment due to the use of scattered radiation, where the bandwidth is limited essentially by the multipath propagation which occurs with multiple reflections. 
       FIG. 1  shows such a known optical free-space communication system for broadband transmission of high-speed data. It includes a transmitter  1 , which has a modulable light source and at least one receiver  5   a ,  5   b ,  5   c ,  5   d  comprising a photodetector to receive light transmitted by the transmitter  1  and converted into an electric signal. The transmitter  1  and receivers  5   a ,  5   b ,  5   c ,  5   d  are designed so that the light transmitted by the transmitter  1  is detectable and/or detected by the receiver  5   a ,  5   b ,  5   c ,  5   d  after scattering or reflection on a surface  7  inside a transportation means. The surface  7  is situated inside, i.e., in the interior space of a transportation means, e.g., on a ceiling  8 . The light source used may be, for example, one or more LEDs, laser diodes, edge-emitting laser diodes and/or VCSEL lasers, and there may be bidirectional communication between the transmitter and receiver. 
     A bidirectional design is preferable because of the greater flexibility. Therefore, when speaking of transmitters below, this is understood to refer, for example, in particular to the transmitter or transceiver which is attached to the body of the transportation means, for example, and the term data station is understood to refer, for example, in particular to the receiver or transceiver that is facing the user, i.e., is mounted on the passenger seat and/or on the multimedia unit attached to the seat, on a handheld device or the like. To be able to differentiate this, the following text therefore refers simply to a distributor and a data station. 
     The object of the present invention is to improve upon the known optical free-space communication system in such a way that the bandwidth is increased. In particular, high data rates in the range of Gbit/s or more on the whole should be transmissable in the interior of transportation means such as airplane or motor vehicles. 
     This object is achieved by the optical free-space communication system for broadband transmission of high-speed data as claimed and by the method for broadband transmission of high-speed data as claimed. Other advantageous features, aspects and details of the invention are derived from the dependent claims, the description and the drawings. 
     The inventive indirect optical free-space communication system is used for broadband transmission of high-speed data and includes a transmitter having a modulable light source and a receiver having a photodetector to receive light emitted by the transmitter and converted into an electric signal. The transmitter and receiver are directed at at least one shared surface that reflects light emitted by the transmitter before reaching the receiver. The free-space communication system has a cellular design and includes several cells, each of which comprises at least one transmitter and at least one receiver, and the cells are designed to prevent crosstalk with any of the neighboring cells, so the cells are independent of one another. 
     High data rates in the range of Gbit/s or even higher can be transmitted optically with this system from one transmitter to multiple receivers (unidirectional) and/or from one transmitter to multiple distributed receivers (unidirectional broadcast) or between transceivers (bidirectional). This system can be used in the interior of transportation means or vehicles, where it allows secure transmission of high data rates at a reduced cost and with increased bandwidth. 
     With this communication system it is possible to easily transmit data at a high rate in a transportation means, e.g., an automobile, airplane, train, ship, satellite or the like without requiring a direct line-of-sight connection between the transmitter and receiver and without requiring cabling between the transmitter and receiver for data transmission. This invention also overcomes the problem of the EMF and ensures an increased security against interception. 
     Through this invention, multiple transmitters with respective receivers, e.g., in an airplane having two or more aisles between the rows of seats can utilize the full bandwidth of approximately 100 Mbit/s, for example, independently of one another; this corresponds to doubling the total useable bandwidth. Two or more “cells” of a communication system based on the same physical medium can thus be implemented, and the system does not have any crosstalk due to the relatively good alignment of the optical signal. This is possible only in the optical range used here because such a sharp delineation would not be possible with radio waves because of the non-directional propagation at lower frequencies. 
     The surface to which the distributors, i.e., transmitters and data stations, i.e., receivers are directed is advantageously situated in the interior space of a transportation means in which the data transmission takes place, preferably in an automobile, airplane, train, ship or satellite. In this way the data transmission can take place in mobile systems in a particularly simple manner. The cost is reduced and comfort is increased because there are no restrictions or at least there are only minor restrictions with regard to the reception site. 
     Means for beam shaping of the respective transmitter and/or means for limiting the reception opening angle of the respective receiver are advantageously provided to prevent crosstalk. Therefore the optical beam propagation is shaped in a suitable way so as to yield only spatially limited signal propagation. For example this also takes into account the fact that the data rates are limited to approximately 100 Mbit/s due to multipath propagation. 
     The indirect optical free-space communication system is preferably designed so that the reflection or scattering ranges of the shared surfaces do not overlap mutually within neighboring cells. Therefore independent communication cells can be achieved, e.g., along an aisle in an airplane or in transportation means in general. 
     For example, in an airplane with two aisles, the ceiling panels along the two aisles may reflect signals from separate transmitters or transceivers, so that there is no overlapping of the surfaces reflecting signals of the transmitters and receivers of the two aisles where said surfaces are reflecting signals, i.e., within the line of sight vision of the receivers. 
     For example, areas of an airplane along its aisle may reflect the optical signals separately, so that the neighboring shared surfaces of different distributors and data stations of an area do not have any overlap with those of the other area and can communicate independent of one another so that different, independent communication cells that do not have any crosstalk are formed along an aisle. The surfaces reflecting optical signals can be delineated either through a suitable choice of the transmission and reception lenses or by fade-in using optically nontransmitting beam apertures at the border of the mutually independent surfaces reflecting signals. 
     Therefore one or more shading elements is also preferably provided, serving to shield the light reflected by the shared surface of one cell with respect to a receiver belonging to a different cell. 
     The light sources of neighboring cells especially advantageously have different wavelengths. In this way the surfaces reflecting signals and/or the surfaces in the field of vision of the receivers can easily overlap at the interfaces, for example. The different wavelengths of the light are 810 nm and 960 nm; these wavelengths are especially easy to separate by simple filters of the receiver. Therefore, corresponding light sources, preferably LEDs or lasers of the two wavelength ranges, are used at the transmission end. 
     The different measures are especially preferably combined, which yields special advantages. 
     Although the two aisles of an airplane, for example, are separated by the beam guidance as independent communication cells because of the shadowing that occurs in the borderline area due to the baggage receptacles in the middle area, slightly overlapping cells with different wavelengths are implemented along the aisles. The full reflection/adjustment of the field of vision is accomplished preferably in such a way that although overlapping may be possible (but need not be possible) in the borderline area of neighboring cells, there is no overlapping with the next-but-one cell. Therefore, only two different labelings L 1  and L 2  are needed along an aisle, alternating approximately in the arrangement L 1 -L 2 -L 1 -L 2 -L 1 -L 2 . This reduces the equipment complexity due to the wavelengths required to a number limited to two, especially since filtering need not be very sharp, but on the other hand, almost any data speed can be achieved due to the alternating use. 
     This means that the cells are preferably arranged in a row, with the wavelengths of the light alternating in neighboring cells. 
     However, the cells are advantageously also separated from one another spatially. The area of a cell is a surface of a wall or an object in the interior of a space in which data is transmitted. 
     The light source of the respective transmitter includes, for example, one or more LEDs, laser diodes, edge-emitting laser diodes and/or VCSEL lasers. 
     The respective receiver may include a light source and the respective transmitter may include a photodetector to perform bidirectional communication. During operation a single transmitter of a cell may supply a plurality of receivers with identical or different data at the same time. 
     According to another aspect of the invention, a method for indirect optical transmission of high-speed data is made available. In the method light is generated by means of a light source, and is modulated at the frequency of the data to be transmitted. The modulated light is transmitted onto a surface that reflects and/or scatters the light and the light scattered and/or reflected by the surface is received so that it travels from the light source to the receiver by an indirect route. The modulated light is connected into an electric signal, whereby the transmission and reception of the light take place in multiple independent cells. Each cell includes at least one transmitter and at least one receiver, and crosstalk with a neighboring cell is actively prevented. 
     The advantages achieved above with respect to the inventive system are also applicable for the inventive method and vice-versa. 
     The light emitted by the transmitter of a cell is limited in its beam angles such that it does not reach receivers of other cells. 
     For example, the reception beam angle of the respective receiver of a cell may also be limited in such a way that the light from the light sources of neighboring cells does not reach the receiver. 
     In particular, the light reflected from the surface of a cell may also be shaded with respect to a receiver belonging to another cell. 
     Light of different wavelengths is advantageously used in neighboring cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of invention will now be described with reference to the drawing figures, in which: 
         FIG. 1  shows an example according to the state of the art with beam guidance of a data communications link without a line-of-sight connection; 
         FIG. 2  shows an inventive arrangement in a schematic diagram according to an embodiment in which an airplane with two aisles are operated as separate communication cells; 
         FIG. 3  shows an inventive arrangement in a schematic diagram in which independent crosstalk-free cells are operated along an aisle of an airplane; 
         FIG. 4   a  shows an inventive arrangement according to  FIG. 3  wherein the crosstalk is prevented by suitable beam guidance; 
         FIG. 4   b  shows an inventive arrangement according to  FIG. 3  wherein the crosstalk is prevented by suitable beam apertures or shading elements; 
         FIG. 5   a  shows an inventive arrangement in a schematic design in which several independent crosstalk-free cells are operated along the aisle of an airplane by using two different transmission wavelengths; 
         FIG. 5   b  shows a schematic design of two neighboring cells of different transmission wavelengths; 
         FIG. 6  shows a preferred embodiment of the invention through a combination of the aforementioned methods. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic sectional view of a known optical free space communication system that is improved upon by the present invention and that is described in detail in German Patent DE 101 07 538 B4. 
     A “distributor” or transmitter  1  which is mounted on the transportation means, e.g., on a holder  17 , is arranged in such a way that the light emitted by it strikes a surface  7  at a distance on a ceiling  8  of a transportation means. The transportation means is, for example, an airplane, a motor vehicle, a motor vehicle, a train, bus, streetcar, satellite or the like. Data stations  5   a ,  5   b ,  5   c ,  5   d ,  40  and the respective multimedia stations  4   a ,  4   b ,  4   c ,  4   d  are arranged at a distance from the distributor  1  and receive the light reflected and/or scattered by the surface  7 . Thus the light goes by an indirect route from the distributor  1  to the data stations; an oblique hatched region illustrates the light emitted by the distributor and a region  6   a ,  6   b ,  6   c ,  6   d  having oblique hatching in the other direction represents the reception area of the respective data stations. The overlapping area with crosshatching in  FIG. 1  denotes the light that is emitted by the distributor  1  and reaches the data stations  5   a - 5   d ,  40  in the respective reception areas  6   a - 6   d . Due to the fact that the light is scattered at the surface  7 , there must not be any direct line of sight contact between the distributor  1  and the data stations  5   a - 5   d ,  40 , i.e., there may even be an obstacle between the two. The reference numerals  3   a - 3   e  in this diagram indicate passenger seats. 
     The known free-space communication system according to  FIG. 1  is part of a preferred embodiment of the invention which reflects the known system. 
     The surface  7  may be a part of the interior paneling in the transportation means, for example, e.g., the roof liner of a motor vehicle or the side paneling of the doors, the paneling of the roof pillars or IR reflecting window surfaces, etc. It is also possible to use as the surfaces  7 , e.g., in an airplane or a train the ceiling, the floor, the side walls, paneling parts of the interior, etc. In general terms, any surface onto which a light source can be directed and which will then reflect or scatter at least a portion of the incident radiation is suitable. 
     In waiting rooms or transportation means, many such surfaces are available and can be used as part of the optical free-space communication system in the manner according to the present invention. However, it is also possible to provide a special reflection or scattering element having a structured surface for deflecting the beams of light as part of the system. 
     In the case of bidirectional transmission, transmission units are also integrated into the data stations  5   a ,  5   b ,  5   c ,  5   d ,  40 . The areas  6   a ,  6   b ,  6   c ,  6   d  are therefore at the same time also the corresponding beam angles of the transmission units integrated into the data stations. The two beam angles of the receiver and transmitter of a data station need not be identical; in that case differentiation of the transmission direction is the responsibility of the protocol. However, in this basic diagram according to  FIG. 1  they are not differentiated for reasons of simplicity. 
     A portable computer or handheld device  40  which is designed as a data station and/or has an inventive input unit and/or transmission unit for communication via the surface  7  is installed at the additional seat  3   e.    
     According to a preferred embodiment of the invention, the distributor  1  consists of one or more LEDs, edge-emitting laser diodes, laser diodes or VCSEL laser diodes as the light source. The output power of the light source here is limited at the lower end by the fact that depending on the geometric factors of the transportation means and the reflexivity of the surfaces in the interior, a sufficient quantity of light is able to reach the data stations via reflection. The output power of the light source of the distributor  1  is limited at the upper end by the limit of eye safety, i.e., so that the possibility of eye damage is ruled out during operation. Depending on the specific embodiment of the transportation means, it is possible to work with a low output power accordingly. 
     However, if the required output powers of traditional LEDs or VCSEL laser diodes is in the near infrared, i.e., usually at a wavelength &lt;1 μm, which is higher than the limit set by law, then LEDs or VCSEL laser diodes which emit at a wavelength having a much higher damage limit with regard to retinal tolerability, e.g., 1.55 μm may be used. 
     The distributor  1  which is mounted on the transportation means by means of the holder  17  may emit, e.g., along the fuselage of an airplane, so that it directs signals at one or more ceiling panels  8  along an aisle. It may thus be mounted across the fuselage, so that it directs optical signals at a single ceiling panel from head rack to head rack, for example, or may be arranged at any angle in between with oblique signal reflection characteristics accordingly. For example, it may also be accommodated together with the optical signal equipment for the compartment in a shared housing. In the case of LEDs as the cabin optical signal means, this would actually be especially advantageous. 
     The receiver which is included at least in the data stations  5   a - 5   d  and/or in the handheld device  40  but also in the distributor  1  consists of one or more photodetectors, the surface of which is limited by the capacitance and thus the possible data rate. The restriction at the lower end is determined by the sensitivity. For detection of the radiation reaching the detector by reflection in the interior of the transportation means, the detector surface usually amounts to at least a few mm 2 . 
     Using LEDs allows wide-area signal radiation of the reflecting surfaces and/or surfaces or inside walls of the transportation means because of the high divergence, although the output power here is limited. If necessary, several LEDs may be used to generate a higher power output. 
     VSCEL laser diodes that are small in size and can be manufactured especially inexpensively have a much higher output power at a much higher efficiency. In other words, low currents occur, typically &lt;10-20 mA at less than 5 V. The emission characteristic of this VCSEL, however, differs significantly from the bundled characteristic, where divergence is typically in the range of a few degrees. VCSEL laser diodes may also be controlled directly by electronic driver modules and/or standard TTL drivers on the basis of the low voltages and extremely low currents. This optionally takes place with the use of a voltage divider. Therefore, the power supply is very simple and an extremely high data rate is made possible, e.g., greater than 1 Gb/s, in addition to the low capacity of the VCSEL elements. 
       FIG. 2  shows an inventive arrangement with spatially separate regions of two aisles  61  and  62  that form the cells Z 1 , Z 2 . The respective elements of the two cells Z 1 , Z 2  are labeled as follows:  5   a ,  5   b  represent the data stations of the aisle  61 , whereas the data stations of the aisle  62  are labeled as  7   a  and  7   b . A distributor  11  is assigned as a transmitter or transceiver to the aisle  61 . Another distributor  12  as a transmitter or transceiver is assigned to the aisle  62 . The distributor  11  and/or the transmitter for the aisle  61  and the respective receiver and/or data stations  5   a ,  5   b  are aimed at the shared surface  8   a . The distributor  12  for the aisle  62  and the respective receivers and/or data stations  7   a ,  7   b  are directed at the shared surface  8   b.    
     The data stations  5   a ,  5   b  and  7   a ,  7   b  each have a field of view and/or reception angle  9   a ,  9   b  and/or  10   a ,  10   b , which corresponds here at the same time to the respective emission characteristics of transmitters optionally contained in the data stations  5   a ,  5   b ,  7   a ,  7   b.    
     The distributors  11  and  12  each have an emission characteristic  11   a  and  12   a , respectively, which corresponds here at the same time to the field of view of receivers optionally contained in the distributors  11 ,  12 . 
     The data stations  5   a ,  5   b  and  7   a ,  7   b  are each mounted at a seat  3   a ,  3   b  and  13   a ,  13   b  respectively, whereby the group of seats  3   a ,  3   b  belongs to the aisle  61  and the group of seats  13   a ,  13   b  belongs to the aisle  62 . 
     The two communication cells are independent of one another and cannot have crosstalk because there is shielding of the shared surface  8   a  for the components of the aisle  61  due to the central baggage compartment  30  and conversely there is shadowing of the surface  8   b  for the components of the aisle  62 . 
     This results in two cells of a communications network which are assigned to the two aisles  61  and  62  and can be operated independently and without mutual crosstalk. 
     As a result of this measure, the total data rate transmitted can be doubled in comparison with a simple system. 
       FIG. 3  shows another preferred embodiment of the invention in which several independent cells are provided and operated along an aisle  61  and/or  62 . Therefore, it is possible to achieve even a much higher data rate, which is required, for example, with transportation means having a great many data stations, in particular in airplanes having a very large number of seats. 
     In the example shown here, the seats  3   a ,  3   b ,  3   c ,  3   d  are assigned to a first cell Z 1  and additional seats  14   a ,  14   b ,  14   c ,  14   d  are assigned to a second cell Z 2 . Crosstalk, for example, is prevented by the fact that the surfaces reflecting the optical signals of each individual cell Z 1 , Z 2  are adequately separated, so that the field-of-view areas of the seats  14   a - 14   d  do not see the areas of the cell Z 1  that are reflecting and vice versa. 
     A different approach is to shape the lenses of the distributors in such a way that there is a definite separation of the surfaces reflecting signals, as explained in greater detail below. 
       FIG. 4   a  shows the interior, i.e., the cabin of a passenger airplane. A data station  5   a ,  7   a  and  17   a  is provided at each row of seats or seats  3   a ,  14   a ,  15   a . A number of distributors  11 ,  12 ,  19  are mounted on the ceiling panel or wall panel, emitting signals across the axis of fuselage and/or across the longitudinal direction of the cabin with a beam angle  11   a ,  12   a ,  19   a . Thus a number of separate cells are formed, specifically three in this example. 
     The data stations  5   a  with its field of view and/or reception beam angle  9   a  is assigned precisely to the distributor  11 . The data station  7   a  with its field of view  10   a  is assigned to the distributor  12  and the data station  17   a  with its field of view  18   a  is assigned to the distributor  19 . The distributors  11 ,  12  and  19  with their beam angles  11   a ,  12   a ,  19   a  form different independent cells Z 1 , Z 2 , Z 3 . In this way, the beam shaping of the optical medium which can be shaped in a very defined manner allows clear-cut separation of the shared surfaces and thus crosstalk-free creation of separate communication cells. 
       FIG. 4   b  shows another embodiment of the invention in which shading elements  40  are mounted on the ceiling panels of the cabin limiting the beam propagation. This results in shaded areas that restrict the field of view  9   a ,  10   a ,  18   a  of the respective data station  5   a ,  7   a ,  17   a  so that here again separate cells Z 1 , Z 2  and Z 3  are formed. Beam propagation of the individual distributors is limited in particular by the shading elements  40  so that each distributor emits signals at a respective surface element that is shaded with respect to the incident light from other distributors. The data stations  5   a ,  7   a  and  17   a  provided at the seats  3   a ,  14   a  and  15   a  belong to independent communication cells Z 1 , Z 2  and Z 3  that are separate from one another, each having different distributors and/or transmitters that are separate from one another and different surface elements that reflect and/or scatter the light and are separated from one another by the shading elements  40 . The fields of view  9   a ,  10   a  and  18   a  are each directed exactly at such a surface element. 
       FIG. 5   a  shows an especially advantageous embodiment of the invention in which multiple crosstalk-free cells Z 1 , Z 2 , Z 3 , Z 4  are operated along the aisle  61  of the airplane, using two different transmission wavelengths L 1  and L 2 . In this example, the beams and the fields of view of the respective cells are not specially shaped or limited, but instead they may even overlap optically. The individual cells Z 1 , Z 2 , Z 3 , Z 4  are designed so that the wavelengths used alternate along the rows of cells. In this example, the cell Z 1  is operated at the wavelength L 1  while the cell Z 2  adjacent thereto is operated at the wavelength L 2  which is different from the wavelength L 1 . In the next cell Z 3  but one, which is adjacent to the cell Z 2 , again the first wavelength L 1  is used because this then generally originates from the area of the field of view  90  of the data stations of the first cell Z 1 . In this way, almost any alignment of independent communication cells can be implemented with only two different wavelengths that are used in alternation. In the present example, seats  3   a - d  are assigned to the first cell Z 1 , seats  14   a - 14   d  are assigned to the second cell Z 2 , seats  15   a - 15   d  are assigned to the third cell Z 3  and seats  27   a - 27   d  are assigned to the fourth cell Z 4 . All the cells are situated along the aisle  61 . The second aisle  62  may be designed like the first aisle  61 , i.e., having cells Z 1  through Z 4 . 
     A particular example follows. 
     In an airplane comprised of two aisles and three rows of seats, the outer rows of seats each having two seats and the inner row of seats having four seats, so that the airplane has a total of 336 seats arranged in such a way, for example, the ceiling panels of the two aisles are used as the shared surface for reflection/as the field of vision. All three rows of seats are alternated between the two wavelengths. The data stations of the middle row of seats are aligned so that each is directed at the more proximate aisle panels. 
     In this way,  12  seats are always assigned to one communication cell. Independent communication cells are formed along each aisle by the alternation of the wavelength  14  used, yielding a total of 28 cells. 
     If each cell has a data rate of 100 Mbit/s, for example, then in the entire airplane a data rate of 2.8 Gbit/s can be transmitted independently for each. A rate of 8 Mbit/s can be assigned to each individual seat. However, each of the  12  seats of a 100 Mbit/s cell can also be wired in such a way that it is able to utilize the full data rate of 100 Mbit/s as needed if the other seats are not transmitting any data and/or the data rate is shared differently. 
       FIG. 5   b  shows a detailed view of the arrangement described here with overlapping cells. The distributor  11 , which is a transmitter or a transceiver, emits signals at the surface  8   a  with light sources in the form of LEDs of the wavelength L 1 . The surface  8   a  is seen by the receiver of the data station  5   a  through its field of view  9   a . The light reflected by the surface  8   a  passes through a lens  41  that defines the field of view  9   a  and strikes a photodetector  43 , whereby a filter  42   a  is installed between the lens  41  and the photodetector  43 . The filter  42   a  allows light of wavelength L 1  to be transmitted and blocks light of wavelength L 2 . This filter may of course also be mounted on the lens  41  or on the photodetector  43 , e.g., in the form of dielectric layers and/or through a suitable choice of materials or the color of lens  41 . 
     Another distributor  12  directs signals at the surface  8   b  which overlaps with the surface  8   a , whereby light sources of the wavelength L 2  are used. The overlap is not obligatory but instead there may also be a separation between the surfaces reflecting signals  8   a  and  8   b . The data station  7   a  assigned to the distributor  12  is designed like the data station  5   a , but the filter  42   b  blocks the light of the wavelength L 1  while it allows the light of wavelength L 2  emitted by the distributor  12  to be transmitted. 
       FIG. 6  shows a combination of the various features of the inventive communication system according to yet another preferred embodiment. This combination has a great many independent cells and thus an extremely enhanced data rate in comparison with the use of only a single cell. The cells Z 1  and Z 1   a  in the neighboring aisles  61  and  62  may have the same wavelength, but there is no crosstalk between them because the baggage compartments  30  in the middle (see  FIG. 2 ) of the airplane optically shade the shared surfaces of the two cells Z 1 , Z 1   a  from one another, and the respective receivers cannot see the shared surfaces of the other cells. 
     Along the aisle  61  there is another cell Z 2  which is adjacent to the cell Z 1 , whereby the optionally overlapping optical regions of the cells Z 1  and Z 2  are separated to prevent crosstalk by using a second wavelength. By analogy with that, the two cells Z 1  and Z 2  are arranged in the neighboring aisle  62 . 
     The two next cells Z 3  and Z 4  in the aisle  61  are shaded with respect to one another and with respect to the cells Z 1  and Z 2  by a shading element  45  such that there cannot be any crosstalk here. The shading element  45  is, for example, a class divider in the passenger cabin, i.e., a curtain. Likewise there is no crosstalk between cells Z 1  and Z 2  due to this shading element  45  on the one hand but also due to the separation of cells Z 1  and Z 2 . Similarly, the cells Z 3   a  and Z 4   a  which are also protected from crosstalk with respect to cells Z 1   a  and Z 2   a  by the shading element  45  are arranged in aisle  62 . The two neighboring cells Z 3  and Z 4  and/or Z 3   a  and Z 4   a  have different wavelengths L 1  and L 2 . 
     The next cells Z 5  and Z 6  in the aisle  61  and/or cells Z 5   a  and Z 6   a  in aisle  62  are separated from cells Z 4  and Z 4   a  by a great spatial separation, i.e., the shared surfaces of the cells are each outside of the field of view of the receivers of the other cell so that no crosstalk takes place here either. The neighboring cells Z 5  and Z 6  and/or Z 5   a  and Z 6   a  in turn differ in their wavelengths L 1  and L 2 . 
     Accordingly, the entire fuselage of the airplane can be equipped with independent crosstalk-free cells through a suitable combination or selection of the inventive measures such that a very broadband data network can be implemented with a multiple of the nominal bandwidth of an individual cell. Through the inventive measures, the advantageous properties of the optical transmission medium are utilized in particular—in the field of wireless transmission, such multiple occupancy of the same transmission band is impossible because of the completely different propagation behavior here. 
     The proposed data transmission system is therefore suitable for transmission of very high data rates, so that it causes a low EMF and nevertheless has a very high flexibility. A direct line-of-sight connection between the transmitter and receiver is not necessary so there is a very high flexibility with respect to their spatial arrangement. 
     The transmission and reception modules may be integrated directly into data stations such as displays, miniature computers, etc. The number of transmission and reception elements is not limited by cables, plugs or the like and can be kept flexible. 
     In addition, increased security against interception is ensured, thus offering an advantage over traditional wireless approaches. Due to the low possibility of penetration of optical radiation with respect to most materials, modulated light radiation cannot penetrate out of the transportation means. In the case of windows, no radiation can be detected at a distance of a few meters away from the transportation means owing to the high divergence and relatively low intensity of the scattered radiation. To further suppress even this residual radiation, the window surfaces may also be coated in such a way that the wavelength of the transmitter radiation, which is usually in the near-infrared, cannot be transmitted through the window without restricting the transmission of the windows in the visible range. Certain coatings, in particular dielectric layers may be provided for this purpose, for example. 
     In summary, the present invention relates to an optical free-space data communication system for broadband transmission of high-speed data, e.g., video data comprising at least one transmitter and at least one receiver, whereby the data transmission within a transportation means, e.g., a motor vehicle, plane, train, boat, satellite and the transmitter has at least one light source, e.g., LED, VCSEL or the like which is modulated at the frequency of the data to be transmitted and optionally encoded, and the receiver contains at least one photodetector that receives the light scattered and/or reflected on the walls in the interior of the transportation means without requiring a direct line-of-sight connection between the transmitter and receiver, and this modulated light signal is converted back into an electric signal. 
     The invention provides the following advantages. 
     No cabling is necessary so the weight is reduced. Plugs, cable distributors or the like are emitted. Therefore increased flexibility is achieved. The damping is low. A transmission of high data rates is possible with this system, but there is only little or no EMF. Furthermore, no line-of-sight connection is required, which is why the system can be placed flexibly. Integration into displays or the like is possible. Furthermore, increased security against interception is provided.