WO2005048492A1

WO2005048492A1 - Space-time coded diffuse-ir networking with photon density waves

Info

Publication number: WO2005048492A1
Application number: PCT/IB2004/052250
Authority: WO
Inventors: Sel B. Colak
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-11-14
Filing date: 2004-11-01
Publication date: 2005-05-26
Also published as: CN1883139A; EP1687917A1; JP2007511947A; KR20060122853A

Abstract

A network and method of operating the same are provided as well as network components with an additional physical layer concept for IR communications to decrease the disadvantages of fading due to multipath signals. In one aspect Photon Density Waves (PDW) on the original IR signal are used. In a second aspect IR sources and detectors in array form with separately addressable components are used. In a third aspect space-time coding to increase the data communication rate by taking advantage of the multipath is used. In a fourth aspect the space-time coding is optimized adaptively for a given indoor environment by engineering the spectral composition of the PDW components. In a fifth aspect a set of tools to optimize the indoor channel utilization for any given environment is provided. These tools make use of moving the central frequency of modulation from the IR frequency to the PDW frequency. In a sixth aspect PDW excited IR arrays are implemented with space-time coding.

Description

Space-time coded diffuse-IR networking with photon density waves

The present invention relates to wireless telecommunications systems and methods of operating the same, especially wireless indoor telecommunications networks and more particularly infrared telecommunications networks especially for use indoors. The present invention particularly relates to shared resource networks such as Local Area Networks (LAN) and network components for such networks. The present invention also relates to infrared communication channel optimization for quasi-diffuse multi-spot wireless indoor networking as well as to network elements and apparatus.

Recently there has been growing interest in wireless telecommunications, e.g. radio or optical communication methods (J.M. Kahn, J.R. Barry, "Wireless Infrared Communications", Proceeding of the IEEE, vol.85, No.2, p. 265 (1997), M. Saffman, D. Z. Anderson, "Mode multiplexing and holographic demultiplexing communication channels on a multimode fiber", Optics Letters, vol.16, p.300, (1991)). In particular, infrared (IR) optical wireless communications have become of interest. General information relating to the application of IR to communications can be found in "Wireless Infrared Communications" by J. R. Barry, Kluwer Academic, 1994. IR wireless forms a practical basis for many applications and provides solutions for environments where wired links or Radio Frequency (RF) wireless may not give the best implementation. For wide applicability, a wireless link should be compact, consume little power, and be easy to align (or free from the need to align), yet robust against background noise and interference from the other users. As a transmission medium for indoor wireless communications, infrared has several advantages over radio, such as a large unregulated bandwidth for high bit rates and absence of interference between links operating in rooms separated by walls or opaque partitions. Furthermore infrared components can be inexpensive, small and can consume little power. One way to classify the infrared (IR) based wireless LAN (WLAN) technologies is to consider their propagation properties in the communication channel. There are at least three basic categories of infrared (IR) WLAN methods described in the prior art. These are a) Line-of-Sight (LOS), b) Diffuse (D) c) Multi-spot (MS) or Quasi-Diffuse Multi- Spot (QDMS) configurations. LOS-IR-WLAN gives the best channel bandwidth (BW) and signal to noise ratio (SNR). Therefore it has the highest data rate. However, this comes at the expense of careful alignment and focusing. Furthermore, it has the highest sensitivity to path loss due to obstructing objects. In an indoor environment movement of furniture or the erection of temporary walls may require re-routing of the beam with consequent network downtime and costs. Due to the latter problems, this method is not suitable for low-cost installation and environments with crowded channel traffic, such as indoor applications. Also eye safety can be a safety issue. Attractiveness of IR technology has recently increased because of the possibility of diffuse communications using reflections from indoor objects. In this way, line- of-sight (LOS) communications are not required as was the case previously. This avoids many of the alignment problems but can only support a lower data rate. With diffuse infrared a detector receives the data symbols modulated over an IR signal after multipath indoor . propagation from a source. If the multipath causes destructive interference at the detector for the data symbol time frame, the data will not be received or will have many errors. To eliminate this, an imaging receiver can be used. This is complicated and expensive for high volume use. D-IR-WLAN gives the worst BW due to the multi-path nature of the channel coming from numerous reflections from surrounding objects. It also has low SNR due to diverging radiation, long path lengths and reflection losses. Therefore it has the lowest data rate. However it does not have any installation difficulties and its link is highly stable against obstructions from moving objects. Therefore it is attractive to use this method for low data rate indoor applications at 10s of Mbps. QDMS, or as sometimes called, Multi-Spot Diffusing Configuration (MSDC) is a quasi-diffuse configuration using multi-beam transmitters emitting nearly collimated beams and an array of detectors with each one having a narrow Field-Of-View (FOV) - see S. Jivkova and M. Kavehrad, "Receiver Designs and Channel Characterization for Multi-Spot High-Bit-Rate Wireless Infrared Communications" IEEE Transactions on Communication, vol. 49, No.12, p.2145 (2001). In some cases, the detector array is formed into an imaging receiver - see J.M. Kahn, R. You, "Imaging Diversity Receivers for High-Speed Infrared Wireless Communication", IEEE Communications Magazine, p.88 (1998). QDMS-IR-WLAN provides, at the expense of some difficulties about multiple beams and their alignments, a gain in data-rate and stability. The more complex the installation, the more the gain in communication link performance. Due to its installation difficulties, this method is not ideal for low-cost markets such as in indoor communications. Future indoor networking will need high speed data transfers. This is especially needed for video content delivery. Infrared (IR) physical links have the following further advantages: (1) there are no regulations on the spectrum utilization as opposed to RF links, and (2) a fundamentally higher frequency data modulation is possible due to the much higher center frequency of IR radiation. A geometry of an IR channel for a schematic room is shown schematically in Fig. la. In this figure, a pair of transmitters (T) and detectors (D) are provided and their IR beams couple primarily via the ceiling, since this is the best reflecting surface available in most rooms. The beams and the Field of View (FOV) of the transmitters and detectors can have a generalized Lambertian pattern or approximation thereto. This pattern may be defined by [(n + 1) cosⁿ(a)]/2π, where "a" is the angle from the directivity vector of the transmitter/detector and "n" is an integer for setting the narrowness of the beam/FOV. More realistic beam/FOV forms and reflectivity functions do not change the qualitative results as far as data-rate optimizations are concerned. The arrangement of sources and detectors in Fig. 1 results in an IR channel defined between a pair of transmitter/receivers (Tx/Rx respectively) or transceivers indicated as A and B in Fig. lb. This figure shows the positions of these transmitters and receivers/detectors in the horizontal floor of a 4 x 5 meter room. All of the communications and channel responses are defined between the TxRx pairs of the sets A and B. Fig. 2 shows the patterns of IR illumination on the ceiling detected by set B acting as a receiver, when set A is acting as transmitter for two different cases. The directivity patterns of the transmitters and receivers are chosen first arbitrarily for Fig.2a and then optimized for the pattern in Fig. 2b. The pattern in Fig. 2a is only the intensity of the detected IR illumination. Each point in this pattern has time delays associated with the travel of that light from its source to the detector. The impulse response of the IR channel example in Fig. 2a is shown in Fig. 3 a. It is seen that this impulse response shows multi-path behavior due to the arbitrary selection of the source/detector directivity angles. Taking the Fourier transform of the impulse response of Fig. 3a gives us the frequency response and channel bandwidth as shown in Fig. 3b. The + in Figure 3b indicates a point for the 3dB bandwidth which is only about 10 MHz for this case. By narrowing the beam/FOV patterns of the previous example and also by changing the directivity angles, as shown in Fig. 2b, the impulse response of the IR channel can be optimized. The results of this optimization are shown in Fig. 4. The impulse response of the optimized IR channel example is shown in Fig. 4a. It is seen that this impulse response shows single peak due to a special selection of the source/detector directivity angles and widths. The frequency response and channel bandwidth is shown in Fig. 4b. The 3dB bandwidth for this case is about 80 MHz. The data rates for channels in Fig. 3 and 4 can be compared by considering their 3dB points only. As indicated in these figures, the 3dB bandwidth for the arbitrarily configured case (Fig. 3) is at about 10 MHz and in the optimized configuration (Fig. 4) is about 80 MHz. Thus the infrared communication bandwidth for these specific examples can be improved by a factor 8. Furthermore, gains of another factor of nearly 2 from the higher peak signal powers in Fig. 4 can be achieved as compared to Fig. 3. The actual data rates would then depend on the signal-to-noise ratio and the type of modulation. Since with proper modulation and signal-to-noise (SNR) ratio, the efficiency of spectral utilization can reach several bits/sec/Hz, a quasi-diffuse multi-spot IR communications channel can be expected to reach data rates towards a good fraction of Gbps level. However a problem with this known system is that to obtain high data rates careful alignment of source and detectors is required. Some of the problems: of known networks relate to destructive interference at the data signal level coming from multipath environment.

The present invention aims at providing an optical wireless network and method of operating the same which has a high data rate without requiring careful alignment of transmitters or receivers. The present invention provides an optical communications arrangement comprising: a Photon Density Wave (PDW) source producing a Photon Density Wave signal, a reflector for reflecting the Photon Density Wave signal, and a receiver for receiving the reflected Photon Density Wave signal. The arrangement may also provide a source of digital signals, a means for modulating an output of the Photon Density Wave source with the digital signals to generate the Photon Density Wave signal, and the receiver being adapted for extracting the digital signals therefrom. In the optical communications arrangement the Photon Density Wave signal may be an infrared signal. In particular, the frequency of the Photon Density Wave signal may be in the range from about 1 GHz to 10 GHz, preferably 1 GHz to 2.5 GHz. In the optical communications arrangement the Photon Density Wave source may be an array of sources. This provides the advantage that advanced signal processing techniques may be used to improve data capacity and/or received signal quality. Also, in the optical communications arrangement the receiver may comprise an array of detectors. The multiple detectors provide a plurality of received signals. The combination of these results in a suitable combiner can improve data capacity and/or received signal quality. For example, the Photon Density Wave signal may be space-time coded. The reflector may comprise a plurality of reflectors thus providing a multiple number of paths. The arrangement may be part of a shared resource network such as a Local Area Network, especially in indoor LAN. The present invention also provides a transceiver for an optical communications network, the transceiver comprising: a Photon Density Wave source to generate a Photon Density Wave signal, and a receiver for receiving a reflected Photon Density Wave signal. The transceiver may also comprise a means for providing digital signals, and a means for modulating an output of the Photon Density Wave source with the digital signals to generate the Photon Density Wave signal, the receiver being adapted for extracting digital signals therefrom. The present invention may also provide a method of operating an optical communications network comprising: generating a Photon Density Wave and outputting a Photon Density Wave signal, reflecting the Photon Density Wave signal, and receiving the reflected Photon Density Wave signal. The method may also comprise generating digital signals, modulating the Photon Density Wave with the digital signals to generate the Photon Density Wave signal and extracting digital signals from the reflected Photon Density Wave signal. The method may further comprise a step of optimizing the PDW frequency in order to increase data capacity. The optimization may be achieved by reducing the "r" value. In addition the method may comprise use of more than one PDW frequency. The present invention provides a network and method of operating the same as well as network components with an additional physical layer concept for IR communications to decrease the disadvantages of fading due to multipath signals. In one aspect Photon Density Waves on the original IR signal are used. In a second aspect IR sources and detectors in array form with separately addressable components are used. In a third aspect space-time coding to increase the data communication rate by taking advantage of the multipath is used. In a fourth aspect the space-time coding is optimized adaptively for a given indoor environment by engineering the spectral composition of the PDW components. In a fifth aspect a set of tools to optimize the indoor channel utilization for any given environment is provided. These tools make use of moving the central frequency of modulation from the IR frequency to the PDW frequency. In a sixth aspect PDW excited IR arrays are implemented with space-time coding. In accordance with the first aspect of the present invention use is made of Photon-Density- Waves. A detector receives the data symbols modulated over a PDW modulated IR signal after multipath indoor propagation. The frequency of the PDW and the symbol rate is chosen to eliminate the destructive interference caused by multipath at the detector. To reach high data rates more PDW components are used over the IR signal. In accordance with the second and third aspects of the present invention Photon-Density- Waves are used with Source-Detector arrays and spectral Space-Time coding.

Fig. 1: (a) gives the cross-section diagram of a room with an IR communications system. The transmitters (T) and detectors (D) interact via reflections from, the ceiling only. Fig. (b) indicates the positions of two specific sets (A and B) transmitter/detector pairs on the floor of a 4x5 meter room (axes are in dm). Fig. 2 shows a further IR communications system. In Fig.2b the system is defined between the transmitter/receiver pair sets A and B. Figs, (a) and (b) indicate the detected IR light pattern by set B as it appears at a surface, such as the ceiling, when set A is transmitting in two cases corresponding to an arbitrary and optimized beam/FOV angles and widths. Fig. 3: (a) shows the impulse response of the channel corresponding to the illumination pattern example in Fig. 2a corresponding to an arbitrary arrangement of source/detector directivity angles, (b) shows the frequency response for the impulse response in Fig. 3a. The + in Fig. 3b indicates the 3dB point of the channel response. Intensity is in relative units. Fig. 4: (a) shows the impulse response of the channel after optimization of source/detector directivity angles and widths, (b) shows the frequency response and indicates the 3dB point of the channel bandwidth (+ sign). Fig. 5 shows an indoor arrangement in accordance with an embodiment of the present invention. Fig. 6 shows a further indoor arrangement in accordance with a further embodiment of the present invention. Fig. 7 shows an indoor arrangement which can be used with the present invention. Fig. 8 shows schematically how a PDW signal can be scattered. Fig. 9 shows an indoor receiver and transmitter arrangement in accordance with an embodiment of the present invention. Fig. 10 shows an indoor receiver and transmitter arrangement in accordance with a further embodiment of the present invention. Fig. 11 shows schematically a multipath, multi-source multi-detector arrangement in accordance with an embodiment of the present invention together with a wireless RF array for comparison purposes only. Fig. 12 shows a PDW source which can be used with the present invention.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Furthermore, any terms such as first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, any terms such as top, bottom, under, over, ceiling, floor and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. One aspect of the present invention is to use a Shared Resources Network (SRN) although single transceiver pairs are included within the scope of the present invention. In an SRN, hardware resources are shared. An SRN in accordance with the present invention is more-or-less synonymous with a LAN or WAN, but the term SRN is used to indicate that the present invention is not limited to specific aspects of known LAN's e.g. contention method or whether Ethernet, Token Ring or Wireless LAN. Also the topology of the LAN or WAN is not considered a limit on the present invention, e.g. bus physical, star physical, distributed star, ring physical, bus logical, ring logical may all be used as appropriate. The present invention relates to optical communications networks and components thereof as well as to methods of operating these. By "optical" is meant suitable wavelength or wavelengths of light which can be used for communications purposes, e.g. visible, infrared, or ultraviolet. Generally, infrared light can be used and no restriction on wavelength is anticipated, i.e. the near or far infrared wavelengths are suitable. Wavelength choice may mainly be determined by the commercial availability of components for the networks. The wavelength can be selected in a range from 600 nm to 1500 nm, but preferably at around 800 nm to 1300 nm due to the availability of cheaper electronic components. The present invention can make use of source and detector arrays (SA and DA) and/or space-time (ST) coding. SA/DA and especially the ST technologies rely on the fact that the radiation gets scattered from a variety of objects in the environment and provide additional channels in between the elements of the SA or DA. However, if these scatterings are correlated, the communication channel is also correlated and obviously no true additional channels are generated. For scattering the wavelength should be comparable to the size and distances of the objects around the SA. This results in near resonance scattering and it provides a rich set of signatures as a function of angle and size. This helps to de-correlate the channels arriving at the DA, therefore helping to obtain higher data rates at the expense of increased signal processing. The "r" parameter in eq. 2 can be used as a measure of

(de)correlation, its value is between 0 and 1. It is a parameter defining how independent one scattering event is from another in a room. A suitable range for the value of r is 0.0 to 0.4. A value close to 1 is less preferred. In one aspect of the present invention a D-IR-WLAN is modified by the use of SA/DA and/or ST methods. The problem with straightforward D-IR-WLAN is that the carrier frequency is now in the optical spectrum range with a wavelength in the range of about 1 micrometer. In practice, the distances and object dimensions, including the source and detector sizes for the IR radiations are much larger than a micron. Therefore, any micron- sized scattering, at this optical range, has no effect at the source and detector arrays. Under such circumstances the multi-path channel characteristics are highly correlated. The present invention provides a wireless optical network especially for indoor use which has a high data rate without requiring extensive and careful adjustments of position and or alignment of transmitters, receivers, reflectors or diffusers. In one aspect the present invention overcomes multipath interference caused by reflections in an indoor environment by creating in an IR signal a large number of relatively uncorrelated signals which can be processed in accordance with well known principles of multiple- input-multiple- output (MIMO) techniques. In fact the present invention makes use of the multipaths caused by multiple scattering from indoor objects in order to increase channel capacity. In another aspect of the present invention a network is provided especially for indoor use which provides an IR signal which scatters from any of a large number of conventional items found within an indoor environment, especially an indoor office or business environment. This can be achieved by transmitting the IR signal as a Photon Density Wave. Further improvements can be achieved by using a plurality of receivers and transmitters as well as space-time coding. To achieve this, a transmitter is composed of a plurality of spatial arranged independently drive-able IR sources, especially Photon Density Wave IR sources. To provide a multiple of scattered signals the IR signals from a transmitter may be reflected by a multi- spot quasi diffuse arrangement, e.g. located on the ceiling of a room. In accordance with an aspect of the present invention, the problem with the huge differences in the ranges of object and wavelength sizes can be eliminated by using photon density wave (PDW) technology. PDW is an intensity modulated propagating light wave, usually with a modulation frequency in the RF range. This can be made to cover frequencies from 100s of MHz to 10s of GHz giving the freedom to generate PDW wavelengths comparable with almost any size distribution in the objects in the environment, especially indoors. In one aspect of the present invention, several frequencies of PDW may be used in the same system, e.g. as an option. As the scattering of PDW is compatible with typical indoor object sizes, the multi-path IR channels can be uncorrelated, e.g. by changing the modulation frequency of PDW. In other aspects of the present invention, the IR-WLAN data rate can be optimized by changing the wavelength of the PDW in any given specific environment. Embodiments of the present invention include PDW assisted D-IR-WLAN's and their network components utilizing SA/DA and ST methods. Since the PDW can be designed to minimize the correlation between the channels as required for optimal benefits from SA/DA/ST methods, data rates can be improved compared to the simple D-IR-WLAN, while still keeping the stability and reliability of the link. That is, in embodiments of the present invention an optical source array (SA) is provided as well as an optical detector array (DA) for a D-IR-WLAN. The rest of the system components can be kept largely the same. For example, this can include a RF front-end and base-band signal processing. In accordance with an aspect of the present invention a PDW light source/generator is used including coupling to an array of optical fiber tips for SA and the Silicon photo diode technology for DA. As shown schematically in Fig. 5 in an embodiment of the present invention a detector D receives the data symbols modulated at a source S and transmitted over a PDW modulated IR signal after multipath indoor propagation. The frequency of the PDW and the symbol rate are chosen to eliminate the destructive interference caused by multipaths at the detector. In the network of Fig. 5 the wavelength of the IR light is less than the mean scattering length within the indoor environment, in actual cases very much shorter than the scattering length. However, in accordance with an aspect of the present invention the wavelength of the PDW can be made of the same order of magnitude as the scattering length. A preferred range for the frequency of the PDW is from about 1 GHz to 10 GHz, but preferably around 1 GHz or 2.4 GHz due to the existence of RF electronics at those latter frequencies. The effect of this is that normal objects within the room will usually scatter the IR signal and the resulting substantially uncorrelated signals will result in an improved reception at the receiver R. As shown schematically in Fig. 6 in a further embodiment of the present invention a detector array D receives the data symbols modulated at a source array S and transmitted over a number of PDW modulated IR signals after multipath indoor propagation. The source array comprises two or more optical sources. The frequency and spatial location of the PDW transmitted signals and the symbol rate are chosen to eliminate the multipath caused destructive interference at each of the detector array components. As with the network of Fig. 5, the wavelength of the IR light is very much less than the mean scattering length within the indoor environment and the PDW is made of the same order of magnitude as the scattering length. The effect of this is that normal objects within the room will scatter the IR signal and will result in an improved reception at the receiver R. A preferred range for the wavelength of the PDW is from about 1 GHz to 10 GHz, but preferably around 1 GHz or 2.4 GHz due to the existence of RF electronics at those latter frequencies. Due to the rich number of "degrees-of-freedom" coming from the combination of multipaths, the spatial separation of IR sources, and the PDW design parameters, the data rates can be higher than previous known techniques. In accordance with this embodiment of the present invention the space- time coding and spectral components of the PDW IR channel can be adapted specifically for a given indoor environment to achieve high data rates. The spacing between the sources of IR light in the transmitter should preferably be chosen so that correlation between the signals received at the receiver is reduced to a minimum. Similar to the technologies in RF cases, this spacing can preferably be chosen at around half of the wavelength of the PDW, e.g. 40 to 70%, preferably 50 to 60% of the PDW wavelength. From theoretical considerations this can be usually achieved provided the spacing between the sources is more than half the wavelength of the PDW. In an indoor environment this can be achieved easily by separating them accordingly. The data carrying capacity of spectral space-time coded multimode telecommunications channels estimated for an indoor environment such as in Fig. 5 or 6 can be determined by the skilled person. For example, the channel capacity of such a system has been investigated theoretically by Sergey Loyka in "Channel capacity of n- Antenna BLAST Architecture", Electronic Letters, vol. 36, No. 7, p. 660, see also A. Alonso, S. B. Colak,

Proc. LEOS Benelux, p.129 (2001), both of which are incorporated by reference. The skilled person will appreciate that reference to antenna in this article refers, in this invention, to a radiating element and is an IR source or a detector in the present invention. Accordingly, the received signal vector, y, is obtained from the transmitted signal vector, x, by:

where H is the normalized communication channel matrix of the indoor environment. The capacity, C in [(bits/s)/Hz], of the multi-channel is then given by:

S H(w) H(w) * , ,, S „ ₂, _l bit I s

C = log₂ det( 7 + — ) = log₂(l + - + [l -r²(w)] x Eq. 2 N n N Gϊ Hz where, I is identity matrix, S N is signal to noise ratio (dB), and n is the number of transmitters/receivers. The capacity is determined by the rank of the channel matrix, H. The exact gain is also a function of the matrix components, which in turn are dependent on the correlation, r(w), between the different channels of the medium. The input vector has additional degrees of freedom in the spectral domain, in terms of different wavelengths, on top of the space-time domain. This helps the capacity by increasing the rank of the matrix. In accordance with an embodiment of the present invention signals are tailored by choosing spectral components in the IR signal which minimize correlation between adjacent channels. These two coding components provide a design parameter to optimize indoor communications against the inter-channel correlation and intra-channel dispersion. A schematic QDMS or MSDC IR-LAN is shown in Fig. 7. It comprises one or more transmitters T located near the floor for example, a plurality of diffusing spots located on and distributed over the ceiling and one or more receivers R located at some position, e.g. on a desk. In a QDMS or MSDC -IR LAN, the one or more transmitters T illuminate the room surfaces with multiple narrow beams. The reflections of these beams are then imaged into the one or more receivers R. The IR light sources in the transmitter T can emit IR beams in the Lambertian form, for example. The reflectors of the diffusing spots can reflect these beams also in Lambertian form, for example. The detectors of the receiver R can be chosen, for example, to have sensitivities with a Field-of-View given in either sharp-cone or Lambertian form. It is preferable to place the source/detector pairs arbitrarily. In accordance with an embodiment of the present invention multiple-input-multiple- output (MIMO) techniques with space-time (ST) coding for achieving high data rates is used. This MIMO-ST embodiment introduces an electronic/algorithm solution which is much more practical and cheaper that the mechanical alignment optimization required for QDMS alone. For MIMO- ST a large number of independent scatterers in the channel region are provided. If scatter from objects in the room are used, the dimensions of the scatters may not generally be suitable. In such cases, it may be advantageous to incorporate additional scattering objects into the room as shown in Fig. 9 and Fig. 10. Space-time coding exploits spatial diversity of the signal. This spatial diversity allows for a significant increase in the capacity of an IR communication system faced with multipath distortions. The transmitter T may be formed from an array or infrared sources, each of which is individually addressable. Each source is coded. For example convolutional coding techniques in addition to simpler methods such as Solomon coding. A typical specific example is Viterbi (de)coding which can be implemented at either ends of the communication link. In addition, the receiver R can use a maximum likelihood (ML) decoder. In order to apply MIMO operation and ST coding to IR wireless LAN communications, an embodiment of the present invention makes use of Photon Density Waves (PDW). Using the ceiling as a reflector results in dimensions of meters, and therefore that the scattering dimensions of the PDW at the reflector surface is a fraction of a meter. This means that any object in a room with dimensions in the order of tens of cm will provide scattering which can be used for ST coding of the IR- wireless LAN. Thus, as indicated above, in this aspect of the present invention the wavelength of the PDW is chosen so that normal objects in a room will cause scattering. A photon density wave can be produced by a two wavelength laser or by an optical modulation method on a single laser or can be obtained when photons from a light source penetrate a turbid media with a thickness exceeding about 10 times the mean-free path of the photon and are multiply scattered therein. The photons propagating through the medium in the diffuse photon density wave are scattered and absorbed such that any coherence in the individual wavelengths of the photons is lost. However, the photon density wave (i.e. a wave propagating through the medium representing the density of the photons) moves through the medium with a coherent wave front. The amount of information available in the diffuse wave may be increased by the use of intensity (i.e. amplitude) modulation and phase modulation of the source of imaging IR radiation. The IR radiation may be generated by a laser or any other suitable form of IR radiation. The wavelength and attenuation of an intensity-modulated photon-density wave are complex functions of modulation-frequency and absorption. At a given modulation frequency, the photon-density wave travels with constant phase velocity in a homogeneous material, which implies that its phase-front maintains coherence. The wavelength of a diffusive photon density wave increases as the absorption increases, because long photon paths become less likely. Attenuation of the diffusive photon density wave is exponential as it propagates through a turbid medium. The PDW is scattered when it is diffracted around objects having a dimensions similar to its wavelength (see Fig. 8 for a schematic representation thereof). Fig. 9 shows a schematic block diagram of a Quasi-diffuse Multi Spot network

100 of the type shown in Fig. 5 preferably for use indoors, i.e. in a room having a floor, walls and a ceiling. It comprises one or more transmitters 10 comprising sources of data 2, one or more optional source coding units 4, one or more optional channel coding units 6 and one or more drivers 8 for driving one or more IR sources 12 to generate a PDW 16. The PDW is modulated by the driver 8 with digital signals which have been generated by the transmit chain which may comprise the source of data 2 and optionally the source coding unit 4 and the channel coding unit 6. In the present invention, PDW is diffuse or at least it has "multiple scattered" character. The source 2 may be any source of digital signals, e.g. a desktop computer, a laptop computer, a palmtop computer, a personal digital assistant (PDA), a printer, a fax machine, a network card, etc. The source coding unit 4 may carry out conventional source coding of the digital signal, e.g. compression, encryption, etc. The channel coding unit 6 may carry out any conventional channel coding including modulation, interleaving, turbocoding, block or trellis coding, error correction coding, etc. A plurality of reflecting spots 18 are arranged in a spatial array, preferably on the ceiling of the room for reflecting the PDW signal 16. This array is overemphasized in this figure. It does not have to extend much beyond the natural surface of the ceiling and it does not need to be a regular array. Some small, almost "natural" irregularities in some ceilings can provide sufficient de- correlation in the "r" parameter. Besides, one can also use scattering objects on the other surfaces of a room as well if desired. One or more receivers 20 are provided comprising one or more IR detectors 14, one or more drivers 22 for the one or more IR detectors, one or more optional demodulators 24, one or more optional channel decoders 26, one or more optional source decoders 28 to generate an output digital signal 30. The decoders perform the inverse functions of the coders. A combiner such as a RAKE combiner can be used. This can either be a hard wired filter or can be implemented by digital signal processing electronics. An example is a pair of source/detector used in the opposite corners of a ceiling reflector. The IR light sources in the transmitter 10 can emit IR beams in the Lambertian form, for example. The reflectors of the diffusing spots 18 can reflect these beams also in Lambertian form, for example. The detectors 14 of the receiver 20 can be chosen, for example, to have sensitivities with a Field-of-View given in either sharp-cone or Lambertian form. In Fig. 9 separate transmitters and receivers are shown for clarity reasons however it should be understood that generally network components are provided as transceivers for two way communication. Further the transceivers may be part of a shared resource network such as a LAN and may operate a LAN protocol such as Ethernet or similar. In particular a combination of a transmitter and receiver as described above may be in a node of a shared resource network such as a LAN. Fig. 10 shows a schematic block diagram of a Multi-spot Quasi-diffuse network 200 of the type shown in Fig. 6 preferably for use indoors, i.e. in a room having a floor, walls and a ceiling. It comprises one or more transmitters 40 comprising an array of sources of data 42, one or more optional source coding units 44, one or more channel coding units 46 and one or more drivers 48 for driving one or more arrays 50 of IR sources 52. The PDW is modulated by the driver 48 with digital signals which have been generated by the transmit chain which may comprise the source of data 42 and optionally the source coding unit 44 and the channel coding unit 46.The sources 52 generate PDWs 56. The source 42 may be any source of digital signals, e.g. a desktop computer, a laptop computer, a palmtop computer, a personal digital assistant (PDA), a printer, a fax machine, a network card, etc. The source coding unit 44 may carry out conventional source coding of the digital signal, e.g. compression, encryption, etc. The channel coding unit 46 may carry out any conventional channel coding including modulation, interleaving, turbocoding, block or trellis coding, error correction coding, etc. In particular it carries out the coding for the space-time coding of the signals passed to the IR sources 52. A plurality of reflecting spots 58 are arranged in a spatial array, preferably on the ceiling of the room for reflecting the PDW signals 56. One or more arrays of receivers 60 are provided comprising one or more arrays 61 of IR detectors 63, one or more optional demodulators 64, one or more channel decoders 66 including one or more combiners, one or more optional source decoders 68 to generate an output digital signal 70. The decoders perform the inverse functions of the coders. The IR light sources 52 in the transmitter 40 can emit IR beams in the Lambertian form, for example. The reflectors of the diffusing spots 58 can reflect these beams also in Lambertian form, for example. The detectors 61 of the receiver 60 can be chosen, for example, to have sensitivities with a Field-of-View given in either sharp-cone or Lambertian form. In Fig. 10 separate transmitters and receivers are shown for clarity reasons however it should be understood that generally network components are provided as transceivers for two way communication. Further the transceivers may be part of a shared resource network such as a LAN and may operate a LAN protocol such as Ethernet or similar. In particular a combination of a transmitter and receiver as described above may be in a node of a shared resource network such as a LAN. Referring to Fig. 11, the SA on the transmitter side and by DA on the receiver side are shown as "diode" components together with RF wireless antenna arrays for comparison. More specifically, there is an IR source array on the left giving PDW and an IR detector array on the right. For example, the SA can be a set of semiconductor laser diodes that are modulated by RF electronics in the transmitters Tx. The output impedances of the transmitters are preferably matched to the diode lasers. Depending on the separations between the laser diode outputs and their respective phases, an intensity modulated wave is generated, that is a PDW. The DA can be a set of Silicon IR photodiode detectors that are coupled to the RF electronics of receivers Rx impedance matched to the detector diode load. The SA array is modulated, for example at 1GHz and the resultant PDW wave from the SA array propagates in the RF channel. After the PDW propagates in this channel, it will impinge on the DA array creating both a DC (continuous wave) and an AC (RF part at 1GHz) component. Depending on the history of the scatterings in each PDW path from one source to a detector, the ratio of the DC to AC will be different. These differences will fluctuate at the length scale of the PDW, that is, for example, at about 30 cm for 1GHz. Apart from the direct modulation of laser diodes as mentioned above, PDW can be generated using a single dual wavelength laser and then feeding the output thereof via a set of optical fibers with different lengths (to provide phase changes) and different time dependent amplitudes (to have a data signal using the PDW). A suitable arrangement is shown in Fig. 12. This comprises a dual wavelength laser source 72, e.g. with a separation of laser frequencies of 219 MHz whose output is fed to a splitter 74 where the beam is split into two and transmitted along two optical fibers 76, 78 of different lengths. In at least one of these fibers 76 a light power control unit 80 is provided for modulation of the one of the beams, e.g. a modulator-attenuator to modulate the amplitude of the beam. The lengths of the fibers are chosen so that there is a phase difference between the two coherent laser beams, e.g. 180°, with respect to the modulated wave. Hence, this phase separation is a fraction of the modulation wavelength. Although two methods of generating PDW are disclosed, the present invention is not limited to these two methods but includes any suitable method. With respect to detection methods for the received PDW a preferred and economic method is to collect the received optical signal on the tips of fiber optical fibers and to transfer these signals to an array of optical detectors such as a single chip detector array, e.g. a Silicon CMOS image detector array or a CCD array. Each detector receives the signal from one fiber and detects both the DC and AC portions of the PDW. The AC part is then separated by a suitable separating means, e.g. a high pass filter such as a capacitor or capacitor array. The separated signal is then transferred to an RF electronic circuit.

As will be appreciated from the above the present invention improves channel capacity, for example by at least a factor of 2 for the 2x2 source/detector array case, by using uncorrelated PDW scattering as compared to the case of DC illumination which results in highly correlated channel matrix elements. Better capacity improvements are possible with arrays containing larger numbers of sources/detectors. Due to the wave nature of the PDW uncorrelated channels are obtained and further benefits are obtained from SA/DA and ST coding technologies for increasing channel efficiency. The detected PDW fluctuation is qualitatively identical to multi-path effects with the exception that it is modified quantitatively due to the differences in the reflectivity and absorption of the surfaces of the objects. These fluctuations are utilized for defining new SA/DA channels and ST coding for a PDW assisted D-IR-WLAN. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention.

Claims

CLAIMS:

1. An optical communications arrangement (100, 200) comprising: a source of digital signals (2; 42), a Photon Density Wave source (12; 50, 52), a means (8; 48) for modulating an output of the Photon Density Wave source (12; 50, 52) with the digital signals to generate a Photon Density Wave signal (16; 56), a reflector (18; 58) for reflecting the Photon Density Wave signal, and a receiver (20; 60) for receiving the reflected Photon Density Wave signal and for extracting digital signals (30; 70) therefrom.

2. The optical communications arrangement according to claim 1, wherein the

Photon Density Wave signal is an infrared signal.

3. The optical communications arrangement according to claim 1 wherein the frequency of the Photon Density Wave signal is in the range from about 1 GHz to 10 GHz, preferably 1 GHz to 2.5 GHz.

4. The optical communications arrangement according to claim 1, wherein the Photon Density Wave source (50, 52) is an array of sources (52).

5. The optical communications arrangement according to claim 4, wherein the

Photon Density Wave signal is space-time coded.

6. The optical communications arrangement according to claim 1, wherein the receiver (60) comprises an array of detectors (61).

7. The optical communications arrangement according to claim 1, wherein the reflector (18) comprises a plurality of reflectors.

8. The optical communications arrangement according to claim 1, wherein the arrangement comprises part of a shared resource network.

9. The optical communications arrangement of claim 1, wherein the arrangement is located indoors.

10. A transceiver for an optical communications network, the transceiver comprising: a means for providing digital signals (2; 42), a Photon Density Wave source (12; 50, 52), a means (8; 48) for modulating an output of the Photon Density Wave source (2; 50, 52) with the digital signals to generate a Photon Density Wave signal (16; 56), and a receiver (20, 60) for receiving a reflected Photon Density Wave signal and for extracting digital signals therefrom.

11. The transceiver according to claim 10, wherein the Photon Density Wave signal is an infrared signal.

12. The transceiver according to claim 10, wherein the Photon Density Wave source is an array of sources (52).

13. The transceiver according to claim 12, wherein the Photon Density Wave signal is space-time coded.

14. The transceiver according to claim 10, wherein the receiver comprises an array of detectors (61).

15. The transceiver according to claim 10, wherein the transceiver is a node of a shared resource network.

16. A method of operating an optical communications network comprising: generating digital signals, generating a Photon Density Wave, modulating the Photon Density Wave with the digital signals to generate a Photon Density Wave signal, reflecting the Photon Density Wave signal, and receiving the reflected Photon Density Wave signal and extracting the digital signals from the reflected Photon Density Wave signal.

17. The method according to claim 16, wherein the Photon Density Wave signal is an infrared signal.

18. The method according to claim 16, wherein the Photon Density Wave signal is generated from an array of sources.

19. The method according to claim 18, further comprising space-time coding of the Photon Density Wave signal.

20. The method according to claim 16, wherein receiving the reflected Photon Density Wave signal comprises receiving the signal at an array of detectors.

21. The method according to claim 16, further comprising a step of optimizing the Photon Density Wave signal in order to increase data capacity.

22. The method according to claim 16, further comprising use of more than one Photon Density Wave signal.