Multi-transceiver system with selective transmit branch coupling for optical wireless communication

Abstract

A LiFi system having a plurality of transceivers (11) and a single multiple-input multiple-output (MIMO) modem (41) having at least M outputs, the M transmit outputs of the MIMO modem (41) being fed to a linear combiner (42) that creates M distinct linear combinations based on the N MIMO transmit branch signals of the MIMO modem, the linear combinations being selected to enable decoding of each of the N MIMO transmit branch signals when N of the M distinct output signals are received.

Description

Technical Field

[0001] The present invention relates to the field of communication in optical wireless networks, such as LiFi networks, for use in a variety of different applications for home, office, retail, hospitality, and industrial purposes, among others.

Background Art

[0002] (Similar in name to a WiFi (registered trademark) network) Wireless optical networks such as LiFi networks enable mobile user devices (hereinafter referred to as end points (EPs)), such as laptops, tablets, and smartphones, to wirelessly connect to the Internet. WiFi (registered trademark) achieves this using radio frequencies, while LiFi achieves this using the optical spectrum, which can enable unprecedented data transfer speeds and bandwidths. Additionally, Li-Fi can be used in areas susceptible to electromagnetic interference. It is important to consider that wireless data is needed not only for traditional connected devices but also more. Today, TVs, speakers, headphones, printers, virtual reality (VR) goggles, and even refrigerators connect and perform essential communication using wireless data. Wireless frequency (RF: radio frequency) technologies such as WiFi (registered trademark) have exhausted the spectrum for supporting this digital revolution, and LiFi can help power the next generation of immersive connectivity.

[0003] Based on modulation, the information of the encoded light can be detected using any suitable optical sensor. This can be a dedicated photocell (point detector), an array of photocells optionally with a lens, a reflector, a diffuser or a phosphor converter, or a camera including an array of photocells (pixels) and a lens for forming an image on the array. For example, the optical sensor may be a dedicated photocell included in a dongle that plugs into an endpoint, or the sensor may be a general-purpose (visible or infrared light) camera of the endpoint or an infrared detector originally designed for, e.g., 3D face recognition. In either case, this enables an application operating on the endpoint to receive data via light.

[0004] In a wireless optical network, a physical access device (e.g., a transceiver) may typically be located in a luminaire, and a logical access point may be connected to one or more physical access devices each located in one or more luminaires. A communication signal can be embedded in an optical signal emitted by a lighting source of a physical access device in an everyday luminaire, such as indoor lighting or outdoor lighting, thus enabling the use of the lighting from the luminaire as a carrier for information. Thus, light includes both a visible lighting contribution (typically the primary purpose of the light) for illuminating a target environment such as a room and an embedded signal (typically considered a secondary function of the light) for providing information to the environment. In such a case, modulation is typically performed at a frequency high enough to be beyond human perception or at least weak enough so that any visible temporary light artifacts (e.g., flicker and / or strobe artifacts) are not noticed by humans or are at least tolerable by humans. Thus, the embedded signal does not affect the primary lighting function. That is, the user only perceives the overall lighting and does not perceive the effect of the data modulated in the lighting. A physical access device (e.g., a transceiver) may typically be located in a luminaire, and a logical access point may be connected to one or more physical access devices each located in one or more luminaires. In many illumination systems, a continuous and uniform light level is achieved by the involvement of many luminaires and light sources in the same room that all emit light. This results in the entire area being evenly illuminated and prevents distinct shadows of obstacles blocking the light rays. Similarly, a LiFi system will experience an immediate link outage when the line of sight (LOS) is blocked. This can frequently occur when a user bends forward towards their communication device and gets in between an access device (e.g., a transceiver) mounted on the ceiling and their communication device.

[0005] Here, the idea of using multiple optical transmitters, which may have multiple overlapping coverage areas from multiple emitters, is used for data transmission. This is not limited to visible light and may also be used, for example, for infrared (IR) light or other radiation. Therefore, the criteria for deployment, especially regarding the degree of transmitter overlap, may differ from those required for uniform illumination and can be based, for example, on the goal of achieving a sufficiently homogeneous achievable bitrate across the entire coverage area, even if a dominant light beam is accidentally blocked.

[0006] MIMO (Multiple-Input Multiple-Output) communication can improve this situation because a transceiver attached to an alternative ceiling with still LOS can instantaneously take over the link. This can also be realized at the physical (PHY) layer, the lowest layer of the OSI (Open Systems Interconnection) model, without protocol-level intervention. In fact, this also enables transmission via multiple paths simultaneously and allows for an immediate fallback by utilizing another link if a certain LOS is blocked.

[0007] However, the problem with high-speed communication is that the difference in travel time from different access devices (i.e., light points) can be substantial enough that multipath extinction (null) occurs due to fading of the communication channel. Differences in cable lengths leading to the transceiver can also contribute to these delays. When a client receives a signal in the middle directly below the emitters of access devices mounted on two ceilings, the free space optical paths are of approximately equal length, so the signals arrive with approximately equal intensity. The phase difference due to free space propagation is small, but a cable length difference of 2m corresponds to a phase difference of half a wavelength at 50MHz (assuming a cable speed of 2 / 3 the speed of light in free space), which can cause a null. Larger differences in cable length can lead to a first null in transfers at even lower frequencies. This problem may worsen when future systems use higher bitrates, utilize better emitters (e.g., vertical-cavity surface-emitting lasers (VCSELs) instead of LEDs), and better amplifiers at the receiver. Summary of the Invention Problems to be Solved by the Invention

[0008] The optical wireless communication envisioned in this specification is not limited to the visible light spectrum. Systems that can utilize the infrared spectrum and / or the ultraviolet spectrum additionally or alternatively are envisioned. Here, generally, the infrared spectrum is preferred because the energy quanta of infrared rays have lower energy compared to ultraviolet rays and are thus more suitable for the area where users are present. The substantial advantage of using light outside the visible spectrum is that problems related to the illumination function, such as flicker, dimming, and the need to turn on illumination only to enable communication, no longer exist.

[0009] Even when using light outside the visible light, it can still be beneficial to incorporate such an OWC system into an illumination device. One of the main reasons is location. That is, illumination devices are generally installed in locations that have a direct line-of-sight to where users who need communication are. Additionally, it may be possible to tag existing infrastructure provided by, for example, illumination lights (such as connectivity in the case of power or Power over Ethernet applications).

[0010] An object of the present invention is to provide an optical wireless communication system with improved robustness against multi-path induced fading.

Means for Solving the Problem

[0011] This object is achieved by the device according to claim 1 or 7, the modem according to claim 9, the transceiver according to claim 10, the system according to claim 12, the commissioning device according to claim 13, the method according to claim 14 or 16, and the computer program according to claim 17.

[0012] According to a first aspect directed to a modem or transceiver, an apparatus for controlling an optical wireless communication system, the apparatus comprising: an input unit for receiving at least two transmit branch signals for multi-input multi-output (MIMO) communication; a combiner for combining at least two transmit branch signals via a plurality of linear combinations to generate a plurality of output signals to be supplied to respective spatially separated transmitters, the number of output signals being greater than the number of transmit branch signals; comprising; The combiner is configured to set the linear combinations such that output signals generated from different linear combinations are received in overlapping reception areas of the spatially separated transmitters, and an apparatus is provided.

[0013] Thus, the linear combinations used to combine MIMO channel signals to generate transmitter output signals can be selected and updated such that transmitter output signals generated from different linear combinations are received in overlapping reception areas of the spatially separated transmitters. Thereby, a MIMO system including a plurality of transceivers with enhanced robustness against multipath fading can be provided. Signals reaching an endpoint device from different transmitters having different delays (e.g., due to different cable lengths) no longer cancel each other out and / or significantly reduce the bit rate because the signals are different from each other.

[0014] In this way, a simple MIMO system that supports, for example, two branches can be extended to support more transceivers coupled to the same modem by transmitting different output signals. Installation problems can be reduced, and options for automatic configuration of the MIMO system (e.g., via a commissioning or learning / training procedure) can be provided.

[0015] According to a first option of the first aspect, the combiner may be configured to apply a linear combination by combining at least two transmit branch signals using selected mixing coefficients. Thereby, the linear combination can be flexibly set and modified by changing the respective mixing parameters via the software parameters of the digital signal processor or via analog or digital circuits that reflect the mixing parameters.

[0016] In this way, the combiner may be provided to combine N transmit branch signals via a plurality of linear combinations by combining the N transmit branch signals using the selected mixing coefficients to generate a plurality of M different output signals to be supplied to each of the M spatially separated transmitters. Here, M > N. Such a combiner may be configured to set the linear combination such that the output signals generated from different linear combinations are received in the overlapping reception areas of the spatially separated transmitters.

[0017] Preferably, the combiner is configured to form each of the M output signals by mixing N transmit branch signals using N mixing coefficients, and the N mixing coefficients of each output signal represent points in an N-dimensional space, such that the square of the distance from each of the M points to the origin is the same so that the M output signals are output by M transmitters using the same output signal power. As a result, the likelihood of nulling is reduced, and the likelihood of receiving different linear combinations in the overlapping reception areas of spatially separated transmitters is increased, thereby increasing the likelihood that the N branch signals can be recovered from the received signal.

[0018] According to a second option of the first aspect, which may be combined with the first option or the first aspect, the combiner may be configured to combine two transmit branch signals by using the following matrix of mixing coefficients for the M output signals. TIFF0007702964000001.tif3658

[0019] Thus, a general approach for generating mixing parameters for any number of transceivers can be provided to improve the robustness against multipath fading in a dense optical wireless communication network.

[0020] According to a third option of the first aspect, which may be combined with the first or second option or the first aspect, the combiner may be configured to set the linear combination by associating the ratio of the feedback resistance to the input resistance of the operational amplifier. This approach provides a simple analog implementation of the proposed coefficient-based combination, and the mixing coefficients can be easily adjusted by selecting appropriate resistance values and ratios or by using variable resistors.

[0021] According to a fourth option of the first aspect, which may be combined with any of the first to third options or the first aspect, the combiner may be configured to perform an adaptive setting of linear combination by enabling control of the switching state of a switching element used to supply a transmission branch signal to the combiner or to supply an output signal to spatially separated transceivers. By using the switching element, a flexible and easily implemented solution for introducing adaptive control into a combiner implemented by an analog or digital circuit is provided.

[0022] According to a fifth option of the first aspect, which may be combined with any of the first to fourth options or the first aspect, the apparatus may be configured to enable control of the switching state of the switching element based on a learning or training algorithm or based on a commissioning procedure. Thus, adaptive control of linear combination via a feedback mechanism that may be based on a learning or training or commissioning procedure can be provided.

[0023] According to a second aspect directed to a control device (e.g., a commissioning device), an apparatus for controlling an optical wireless communication system, the apparatus comprising a receiver for receiving from spatially separated transmitters of the optical wireless communication system information indicative of a selected linear combination of each of at least two transmission branch signals for multi-input multi-output (MIMO) communication, each selected linear combination being used, based on the at least two transmission branch signals, to generate a communication signal transmitted by each of the spatially separated transmitters, Comparing selected linear combinations received in the overlapping reception areas of spatially separated transmitters, and determining a linear combination that needs to be changed so that output signals generated from different linear combinations are received in the overlapping reception areas of spatially separated transmitters, a comparator for An apparatus is provided that includes

[0024] Thus, an apparatus for controlling an optical wireless communication system, the apparatus comprising: a receiver for receiving information indicating a selected linear combination of each of N transmitted branch signals output by an N-output multiple input multiple output (MIMO) modem for communication from N spatially separated transmitters of the optical wireless communication system, where N≧2, and each selected linear combination is used to generate a communication signal transmitted by each of the spatially separated transmitters based on the N transmitted branch signals; a comparator for comparing selected linear combinations of the N transmitted branch signals in signals received in an overlapping reception area of the spatially separated transmitters, and determining a linear combination that needs to be changed so that the N transmitted branch signals can be generated from different linear combinations received in the overlapping reception area of the spatially separated transmitters. An apparatus is provided that includes

[0025] According to a first option of the second aspect, the apparatus may be configured to signal a feedback signal having a list of combination states to be updated for spatially separated transmitters. Thereby, a feedback mechanism can be provided for adapting the linear combination selected based on the actual reception situation, which also enables decentralized control of the linear combination for each of the spatially separated transmitters.

[0026] According to a third aspect, a modem for generating an output signal for multi-input multi-output (MIMO) communication in an optical wireless communication system is provided, the modem including an apparatus according to the first aspect.

[0027] According to a fourth aspect, a transceiver for transmitting an output signal for multi-input multi-output (MIMO) communication is provided, the transceiver including an apparatus according to the first aspect.

[0028] According to a first option of the fourth aspect, the transceiver may be configured to transmit information indicating a linear combination used to generate the output signal to a control device for controlling the setting of the linear combination. Thereby, the linear combination can be checked for each individual transceiver by detecting these transmitted linear combinations and confirming that different linear combinations are assigned to adjacent transceiver pairs or transceivers having overlapping coverage or illumination areas.

[0029] According to a fifth aspect, an optical wireless communication system is provided that includes a modem according to the third aspect and a plurality of spatially separated transceivers according to the fourth aspect.

[0030] According to a sixth aspect, there is provided a commissioning device for commissioning an access point of a wireless optical communication system, the commissioning device including a device according to the second aspect.

[0031] According to a seventh aspect directed to a modem or transceiver, there is provided a method for controlling an optical wireless communication system, the method including: receiving at least two transmit branch signals for multi-input multi-output (MIMO) communication; combining the at least two transmit branch signals via a plurality of linear combinations to generate a plurality of output signals to be supplied to respective spatially separated transmitters, the number of output signals being greater than the number of transmit branch signals; setting the linear combinations such that output signals generated from different linear combinations are received in overlapping reception areas of the spatially separated transmitters; and the method includes the above steps.

[0032] Thus, there is provided a method for controlling an optical wireless communication system, the method including receiving N transmit branch signals output by an N-output multi-input multi-output (MIMO) modem for communication, where N≧2; combining the N transmit branch signals via a plurality of linear combinations by using selected mixing coefficients to generate a plurality of M different output signals to be supplied to respective ones of M spatially separated transmitters, where M>N; setting the linear combinations such that output signals generated from different linear combinations are received in overlapping reception areas of the spatially separated transmitters; and supplying the M different output signals to the M spatially separated transmitters for transmission.

[0033] More preferably, combining is forming each of the M output signals by mixing N transmitted branch signals using N mixing coefficients, wherein the N mixing coefficients of each output signal represent points in an N-dimensional space, and the square of the distance from each of the M points to the origin is the same such that the M output signals are output by M transmitters using the same output signal power.

[0034] According to an eighth aspect directed to a control or commissioning device, a method of controlling an optical wireless communication system, the method comprising: receiving, from spatially separated transmitters of an optical wireless communication system, information indicative of a selected linear combination of at least two transmitted branch signals for multi-input multi-output (MIMO) communication, each selected linear combination being used to generate a communication signal transmitted by each of the spatially separated transmitters based on the at least two transmitted branch signals; comparing the selected linear combinations received in overlapping reception areas of the spatially separated transmitters; determining a linear combination that needs to be changed such that output signals generated from different linear combinations are received in overlapping reception areas of the spatially separated transmitters; A method is provided that includes:

[0035] A method for controlling an optical wireless communication system, the method comprising receiving, from N spatially separated transmitters of the optical wireless communication system, information indicative of a selected linear combination of each of N transmit branch signals output by a communication N-output multi-input multi-output (MIMO) modem for communication, where N≧2, and each selected linear combination is used to generate a communication signal transmitted by each of the N spatially separated transmitters based on the N transmit branch signals, and comparing the selected linear combinations of the N transmit branch signals in a signal received in an overlapping reception area of the spatially separated transmitters, and determining a linear combination that needs to be changed so that the N transmit branch signals can be generated from different linear combinations received in the overlapping reception area of the spatially separated transmitters.

[0036] According to a ninth aspect, a computer program product may be provided that includes code means for performing the steps of the above method according to the seventh or eighth aspect when executed on a computer device.

[0037] It should be noted that the above device may be implemented based on discrete hardware components, an array of embedded chips or chip modules, or a discrete hardware circuit, or based on a signal processing device or chip controlled by software routines or programs stored in a memory, written to a computer-readable medium, or downloaded from a network such as the Internet.

[0038] The apparatus according to claim 1 or 7, the modem according to claim 9, the transceiver according to claim 10, the system according to claim 12, the commissioning device according to claim 13, the method according to claim 14 or 16, and the computer program according to claim 17 may have similar and / or identical preferred embodiments, in particular embodiments as described in the dependent claims.

[0039] It should also be understood that the preferred embodiments of the present invention can be any combination of the dependent claims or the above embodiments and their respective independent claims.

[0040] These and other aspects of the present invention will become apparent and be elucidated with reference to the embodiments described below.

Brief Description of the Drawings

[0041]

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[0042] Here, various embodiments of the present invention are described based on an optical wireless lighting and communication (LiFi) system having a multi-transceiver access device.

[0043] Hereinafter, a luminaire as an access device shall be understood as any type of lighting unit or lighting fixture that includes one or more light sources (including visible or invisible (infrared (IR) or ultraviolet (UV)) light sources) for lighting and / or communication purposes and optionally other internal and / or external components necessary for proper operation of the lighting, such as for positioning and protecting the light source and ballast (if applicable), and for connecting the luminaire to a power source. The luminaire can be of a conventional type, such as an embedded or surface-mounted incandescent, fluorescent, or other electric discharge luminaire. Also, the luminaire can be of a non-conventional type, such as an optical fiber with a light source at one location and a fiber core or “light pipe” at the other.

[0044] Furthermore, it should be noted that when using optical wireless communication based on the invisible part of the light spectrum, such as infrared and / or ultraviolet, the device according to the invention described in the claims may be incorporated into a separate transceiver node of an optical wireless communication system.

[0045] FIG. 1 schematically shows a block diagram of a LiFi network in which various embodiments can be implemented.

[0046] The LiFi network includes a plurality of access points (APs) AP1 - AP3 12, such as luminaires of a lighting system, connected via a switch (e.g., an Ethernet (registered trademark) switch) 14, whereby each AP 12 controls one or more transceivers (TRXs) 11 (i.e., combined transmitters (optical emitters) and receivers (optical sensors)) for optical communication towards end points (EPs) EP1 - EP4 10, such as mobile user devices or other user devices. Each optical beam generated by the TRX 11 that defines a coverage area on the plane of the EP 10 is shown as a dashed trapezoid in FIG. 1.

[0047] AP12 may apply a time slot schedule for communicating with one or more EP10s within its coverage area. If the coverage areas of the TRX11 overlap (as shown for EP1 in FIG. 1), coordination of the AP12 is required to reduce cross-AP interference if the associated TRX11s belong to different AP12s.

[0048] The LiFi controller 13 configured to manage the LiFi network is connected to the switch 14 and can provide such coordination to support interference handling and handover when one of the EP10s moves into and out of the overlapping coverage area of the AP12. The controller 13 is connected to one or more AP12s via the switch 14. The switch 13 may be connected to a synchronization server 16 for synchronization management and a router 15 for connecting to a backplane or backhaul network (e.g., Ethernet) 100.

[0049] FIG. 2 schematically shows two options for MIMO communication in the LiFi infrastructure.

[0050] The LiFi infrastructure includes a plurality of TRX11s of each AP (e.g., a luminaire of an illumination system) located in a planar area 19 (e.g., the ceiling wall of a building). Each TRX11 has an optical coverage area for transmitting and receiving a LiFi signal projected onto a second planar area (not shown, e.g., the first floor of a building) where an EP10 having an optical front end 101 for receiving the optical LiFi signal emitted by the TRX is located.

[0051] The system on the left in FIG. 1 is a MIMO system in which the same LiFi signal is exchanged between at least two different TRX11s of each ceiling AP and at least two different optical front ends 101 of the EP10.

[0052] The simple version shown in the right part of FIG. 2 is a multiple input single output (MISO) system in which a plurality (at least two) of TRX11s of each ceiling AP communicate with a single optical front end 101 in an EP10 that receives a plurality of signals from the plurality of TRX11s.

[0053] MISO operation enables the standard optical front end 101 (e.g., dongle) of the EP10 to benefit from the processing in the ceiling TRX11s. Each transmission branch of two or more TRX11s can be used by a single EP10 to create a single common signal, and the present invention may be used to compensate for delays and phase differences. The MIMO-capable modem of the AP in the ceiling can separately and actively estimate both downlink channels. This can ensure that the phase is adjusted (e.g., for each frequency bin of orthogonal frequency-division multiplexing (OFDM)), guarantee coherent constructive addition, and remove multipath fading (e.g., nulling).

[0054] In many cases, it may be sufficient if only one or two light rays of the LiFi signal actually significantly contribute to reception. This means that it may be sufficient for a MIMO-capable modem to be used for MISO transmission, with two optical emitters of each TRX transmitting to one optical receiver in the EP.

[0055] This can be quite effective when the optical receiver is at the crosspoint of the two optical emitters such that the optical path lengths of both transmission links are approximately the same and the amplitudes are the same. However, a remaining phase difference of exp{-2πjτf} can still be introduced due to the difference in the feeding cable lengths of two (or more) TRXs. Here, τ represents the time delay caused by the difference in cable lengths, and f represents the frequency of the LiFi signal. Since the optical receiver in the EP adds the two signals with equal intensities, full extinction or cancellation (null) occurs at frequencies where τf = 1 / 2 or an odd multiple thereof.

[0056] For example, if the cable lengths between the modem and the TRXs differ by, for example, more than 2 m, a receiver located in the middle between the TRXs receives the two signals with a delay that leads to cancellation over a wide portion of the spectrum. In a MISO system, the modem can be operated such that the phase of one of the TRXs is automatically inverted in that frequency range to prevent cancellation. Outside of that frequency range, an optimal phase is selected, and the bandwidth near the notch due to cancellation can still be used.

[0057] Figure 3 schematically shows the frequency diagrams of the signal and noise power spectral density at the input amplifier of a two-ray receiver.

[0058] More specifically, the frequency diagram of FIG. 3 shows the characteristics of the power spectral density (PSD) of the shot noise signal (SN), amplifier noise signal (AN-PSD), and MISO LiFi signal (S-PSD) that occur as the cause of the direct current (DC) of the photocurrent and dark current in the photodetector element in the EP receiver. As can be seen from the frequency diagram, notches due to the multipath cancellation effect (fading) are observed at approximately 25 MHz and 75 MHz. Due to the 25 MHz notch, the channel becomes unsuitable for data communication between 15 and 35 MHz.

[0059] According to various embodiments, a plurality of transmit branch signals at the output of the modulator are combined using selected mixing coefficients to enhance the robustness against the multipath fading effect.

[0060] FIG. 4 schematically shows a block diagram of an optical transmission system having transmit branch coupling according to various embodiments.

[0061] The proposed system enables the use of two or more optical outlets (i.e., TRX), and a combiner function or circuit (CB) 42 (hereinafter referred to as the "combiner") is added to provide a fixed or adaptive linear combination of transmit branch signals using selected mixing coefficients.

[0062] Therefore, the proposed optical wireless communication system (e.g., LiFi system) includes a physical layer (PHY) signal processing communication unit (e.g., MIMO modem) 41 (hereinafter referred to as the "MIMO modem") adapted to generate an MIMO signal having N transmit branches (N is 2 or more). The N transmit branch terminals of the signal processing communication unit 41 are connected to the combiner 42, and the combiner 42 combines the N transmit branches using a plurality of mixing coefficients to generate M output signals (M is N or more) that are supplied to the M optical transmitter units (emitters) of each TRX11.

[0063] Combiner 42 may connect N transmission branches to M transmitters of TRX11 and may be composed of a linear combination of the N transmission branches. The mixing coefficients of the linear combination are transmitted through the wireless space defined by channel matrix 43 and are received by the receiver through analog front end (AFE) 44 (including, for example, a channel filter, an amplifier, an attenuator, a mixer, etc.) followed by physical layer (PHY) unit 10 (which handles, for example, addressing, collision avoidance, data acknowledgment response protocol, etc.). They are selected to reduce the likelihood of cancellation (nulling) of the optical signal.

[0064] As described in the examples of FIGS. 6 to 8 below, the parameter-based combining operation of combiner 42 can be fixed or adaptive (e.g., based on (historical) input from the user to find out which TRX is a neighbor or based on a learning algorithm).

[0065] A simple first example of the mixing coefficients for two MIMO transmission branches or channels or paths (N = 2) and two TRXs (M = 2) may be just to connect the first MIMO transmission branch to the first TRX and the second MIMO transmission branch to the second TRX. In this case, the parameter matrix for combining the two MIMO transmission branches to obtain the input signals of the two TRXs can be expressed as follows. TIFF0007702964000002.tif1119 However, the following second example can also be a good choice. TIFF0007702964000003.tif1127 This means that the input signal of the first TRX corresponds to the sum of two transmission branch signals each multiplied by the mixing coefficient TIFF0007702964000004.tif118, and the input signal of the second TRX corresponds to the difference between two transmission branch signals each multiplied by the mixing coefficient TIFF0007702964000005.tif118.

[0066] According to a third example of two MIMO transmission branches and four TRXs, the mixing coefficients can be selected, for example, as follows. TIFF0007702964000006.tif2521This means that the input signal of the first TRX corresponds to the first transmission branch signal, the input signal of the second TRX corresponds to the second transmission branch signal, and the input signal of the third TRX corresponds to the sum of the first and second transmission branch signals multiplied by the respective mixing coefficients TIFF0007702964000007.tif95, and the input signal of the fourth TRX corresponds to the difference between the first and second transmission branch signals multiplied by the respective mixing coefficient parameters TIFF0007702964000008.tif95.

[0067] More generally, for N MIMO transmission branches and M transceivers, and an N-dimensional space. The coordinates of each point in the N-dimensional space represent the N mixing coefficients of the respective signal combinations, and the square of the distance to the origin represents the signal power.

[0068] In this aspect, the combiner may be configured to form each of the M transmitter / transceiver output signals by mixing the N transmission branch signals of the modem using the N mixing coefficients. The N mixing coefficients of each output signal can be considered to represent a point in the N-dimensional space, where the square of the distance to the origin for each of the M points in the N-dimensional space is the same. As a result, each of the N transmission branch signals (having the same power) can contribute a fraction of the power to the signal power of the M output signals based on the applied coefficients. By setting the distances to the origin equal for all M output signals, the M transmitters will transmit the same output signal power.

[0069] Each of the M points in the N-dimensional space represents a point on the real part of the N-sphere. In the case of N = 2, this translates to the real circle half of the unity circle. In the absence of prior knowledge of the receiver, the points on the real circle half are preferably spaced as far apart as possible to facilitate reconstruction at the receiver.

[0070] Using this approach, it is possible to decode the N transmit branch signals in the area where N of the M output signals are received. Since the possibility of the transmit branch signals canceling each other out is reduced, this improves the robustness of the transmitter system against delays due to, for example, the cable lengths of different optical transmitters.

[0071] FIG. 5 schematically shows a two-dimensional coordinate system having coordinates x and y representing one of two transmit branch signals output by a MIMO modem having mixing coefficients of different transmit branch combinations according to various embodiments.

[0072] Two coordinate points 50-1 and 50-2 on the x-axis and y-axis respectively correspond to the first example having two TRXs.

[0073] Furthermore, four coordinate points 50-1, 50-2, 51-1 and 51-2 correspond to the third example having four TRXs.

[0074] Furthermore, eight coordinate points 50-1, 50-2, 51-1, 51-2, 52-1, 52-2, 52-3 and 52-4 correspond to the case of having eight TRXs.

[0075] As a general example of two transmit branches and M output signals (i.e., M TRXs), an M-dimensional sphere can be drawn centered at the origin of the coordinate system. And considering only the positive parameters of the x-axis, the following matrix of mixing coefficients can be obtained. TIFF0007702964000009.tif2231

[0076] The above-described parameter-based combination can be realized by providing a resistance proportional to the reciprocal of the matrix parameter value in the forward path of the combiner 42. When the value is negative, the resistance is connected to the inverted signal that can be obtained by adding an inverter circuit.

[0077] FIG. 6 schematically shows a block diagram of a multi-transceiver access device having fixed settings for the combination according to various embodiments.

[0078] Note that throughout this disclosure, the structure and / or function of blocks having the same reference numbers as previously described will not be described again unless additional specific functionality is involved.

[0079] The multi-transceiver access device of FIG. 6 includes a MIMO modem 41 having two transmit branches or channels X and Y, and a combiner 42 for combining the two transmit branches using fixed mixing coefficients to generate three output signals supplied to three TRX11s.

[0080] The mixing coefficients are defined by the ratios of the respective feedback resistors R3a, R3b, R3c and the respective input resistors R1a, R2a, R1b, R2b, R1c, R2c of the respective operational amplifiers OPa, OPb, OPc that define the respective amplification factors or gains g1a, g2a, g1b, g2b, g1c, g2c as the mixing coefficients, as follows. g1a = -R3a / R1a g2a = -R3a / R2a g1b = -R3b / R1b g2b = -R3b / R2b g1c = -R3c / R1c g2c = -R3c / R2c

[0081] Due to the first transmission branch signal X of the MIMO modem 41 (the left output in FIG. 6) being guided to the input of the upper operational amplifier OPa via the inverter circuit INV, the sign of the mixing coefficient of this transmission branch with respect to the upper TRX11 in FIG. 6 is inverted to g1a = +R3a / R1a.

[0082] Therefore, the three output signals Oa to Oc of the TRX are obtained by the following combination of the transmission branches X and Y with their respective mixing parameters. Upper TRX11: Oa = (R3a / R1a)X - (R3a / R2a)Y Middle TRX11: Ob = -(R3b / R1b)X - (R3b / R2b)Y Lower TRX11: Oc = -(R3c / R1c)X - (R3c / R2c)Y

[0083] Hereinafter, examples of the adaptive and / or self - learning approach of the combiner 42 will be described based on FIGS. 7 and 8.

[0084] FIG. 7 schematically shows a block diagram of a first example of a multi - transceiver access device having an adaptive setting of the combination according to various embodiments.

[0085] The configuration of the adaptive combiner 42 in FIG. 7 corresponds basically to the example in FIG. 6, except that the inverter circuit INV is omitted and each of the transmission branch signals X and Y is connected to the respective input resistors R1a, R2a, R1b, R2b, R1c, and R2c via the respective switching elements S1a, S2a, S1b, S2b, S1c, and S2c of switching elements that may be implemented as semiconductor switches (e.g., transistors) or mechanical (micro) switches, etc.

[0086] The switch states of switches S1a, S2a, S1b, S2b, S1c, and S2c may be controlled through respective control signals that may be generated, for example, by an adaptive setting mechanism or algorithm based on feedback from a commissioning device or at least one receiver of at least one respective EP. The modem 41 generates two transmit branch signals X and Y, and an appropriate combination of the two transmit branch signals X and Y is selected by each of the TRX11s or for each of the TRX11s.

[0087] FIG. 8 schematically shows a block diagram of a second example of a multi-transceiver access device having an adaptive setting of the combination according to various embodiments. In the second example, the two transmit branch signals X and Y of the modem 41 are first combined in a combiner 42 by an amplifier-resistor circuit 81 having respective mixing coefficients defined by a resistance network to generate three output signals for the three TRX11s, and these output signals are selectively applied to the respective input terminals of the TRX11s via respective switching elements Sa, Sb, and Sc that may be implemented as semiconductor switches (e.g., transistors) or mechanical (micro) switches, etc.

[0088] According to FIG. 8, the first transmit branch signal X (the left output of the modem 41) can be directly applied to each of the three TRX11s when the switching elements Sa to Sc are controlled to select an input terminal among the three input terminals. The upper and lower input terminals of those input terminals are connected to respective output signals generated by a specific combination of the transmit branch signals X and Y as defined by the amplifier-resistor circuit 81.

[0089] In the above first and second examples of adaptive combination in the combiner 42, the connection state of the switching element can be controlled based on a learning or training algorithm so that the TRX of the adjacent lamp or luminaire of the access point receives different communication signals (i.e., output signals Oa to Oc). In a regular rectangular pattern of TRXs on a ceiling or the like, two different MIMO signals are sufficient to ensure that adjacent TRXs never receive the same signal.

[0090] To train the system, all TRXs 11 may be notified of the proper combination of the transmission branches for their emission, or may determine the proper combination. The goal may be to ensure that adjacent TRXs (in each lamp or luminaire) receive different signals (e.g., different phases and / or amplitudes).

[0091] If the adaptive feedback function is implemented without a central controller, the proposed extended network infrastructure can be kept simple. That is, the adaptive combination of the transmission branch signals X and Y for the TRX 11 should be locally controlled.

[0092] The adaptation and / or initial configuration of the combination in the combiner 42 can be achieved by a commissioning procedure. For the communication required in relation to the commissioning procedure, out-of-band (OOB) signaling can be used as it can be implemented in the TRX 11 without adding additional light-emitting diodes (LEDs) and photodetectors. OOB signaling is transmitted outside the defined frequency band of the LiFi communication signal, or, implicitly, outside any other kind of signaling activity of the LiFi network.

[0093] Figure 9 shows a flowchart of a commissioning-based coupling setting procedure in a multi-transceiver access device according to various embodiments.

[0094] In an initial step S901, N MIMO transmit branch signals (e.g., X and Y) are received from the modem 41. Thereafter, in step S902, the coupling of the combiner 42 and the initial setting or default setting of each mixing coefficient are selected, and M corresponding output signals are transferred to the TRX11. In a subsequent step S903, information regarding the selected individual coupling (combination state) is transmitted to a commissioning device, which may be a mobile device that scans the LiFi communication range based on an automatic or manual operation, together with the identifier (ID) of each TRX11 of the access device (e.g., via OOB signaling).

[0095] Thereafter, in step S904, the access device waits until it receives a feedback message from the commissioning device. The feedback message indicates whether the selected coupling of the identified TRX can be maintained or, for example, needs to be updated due to a collision with an adjacent TRX using the same or not sufficiently distinguishable coupling of the transmit branch.

[0096] In a subsequent step S905, the access device checks whether the selected coupling needs to be updated. If not, the procedure jumps back to step S903 and continues to transmit the ID of each TRX and the current coupling, for example, in response to a trigger received from the commissioning device. If the required update of the selected coupling is received from the commissioning device, the procedure continues to step S906, and a new coupling is set up based on each information received from the commissioning device or based on an arbitrary or predetermined self-selection of the new coupling (e.g., by corresponding control signals applied to the switching elements of the combiner 42).

[0097] Figure 10 schematically shows a block diagram of a commissioning device for adaptation settings of couplings according to various embodiments.

[0098] The commissioning device may be a mobile user device having a TRX101 for commissioning signaling (e.g., OOB signaling). The OOB signaling may be in the optical frequency range or the RF range.

[0099] The detector circuit (DET) 102 is configured to detect the coupling state and ID signaled via OOB signaling from each access device disposed, for example, on the ceiling of a building, and transfer the received ID(s) and coupling state(s) to a comparator (CP) 103. The comparator stores a list of the received ID and each coupling state in a memory or database (MEM) 104 together with geographical information regarding the geographical relationship between each TRX identified by the received ID. This geographical information may simply indicate which coupling state belongs to an adjacent TRX (e.g., by a neighbor flag or the like).

[0100] When the comparator 103 determines that two adjacent TRXs have the same or insufficiently distinguishable coupling states, it starts transmitting a feedback signal to the access device of each TRX via its own TRX101, together with an indication that the coupling state of each TRX needs to be updated. As an additional option, the comparator 103 may be configured to select a sufficiently discriminative combination state for the TRX and start transmitting the selected combination state to the TRX.

[0101] Figure 11 shows a flowchart of a commissioning-based coupling setting procedure in a commissioning device according to various embodiments.

[0102] In an initial step S1101, the commissioning device receives information regarding (a plurality of) selected coupling states along with (a plurality of) respective IDs of each TRX11 within the scanned communication range from each access device located within the scanning range of the commissioning device (which may be a mobile device that scans the LiFi communication range based on automatic or manual operation) (e.g., via OOB signaling).

[0103] Thereafter, in step S1102, the commissioning device stores the (plurality of) received coupling states (e.g., in a database or memory of the commissioning device) along with (a plurality of) respective IDs and optional geographical information that may indicate the neighboring state of each TRX. Based on this, a log or list of all (adjacent) TRX pairs that can be used for MIMO or MISO reception in the EP may be generated. In the manual scanning mode, the log may be generated while the user of the commissioning device walks around the room until messages from all TRXs are received. In the automatic scanning mode, the reception range of the receiver in the commissioning device (e.g., antenna characteristics) may be mechanically or electronically controlled to scan the entire area where all TRXs are located.

[0104] Thereafter, in step S1103, it is determined which of the received coupling states need to be updated (e.g., due to collisions in the associated TRX pairs), and a corresponding list may be generated. As an optional measure, the updated coupling state may be selected for each TRX on the generated list.

[0105] Finally, in step S1104, a feedback signal having the binding states to be updated and the corresponding lists of their respective IDs may be signaled to the access device (e.g., via OOB signaling) along with an optional proposal of a new binding state.

[0106] FIG. 12 schematically shows the architecture of a commissioning system for adaptively setting mixing coefficients in a multi-transceiver access device according to various embodiments.

[0107] In the system of FIG. 12, the MIMO modem 41 generates two transmit branch signals X and Y, and the two transmit branch signals X and Y are supplied to four TRX11s (TRX1 to TRX4) each including a respective combiner (not shown) that combines the two transmit branch signals X and Y as follows. TRX1: a1X + b1Y (identified by ID1) TRX2: a2X + b2Y (identified by ID2) TRX3: a3X + b3Y (identified by ID3), and TRX4: a4X + b4Y (identified by ID4)

[0108] As an example, the selection in each combiner of the TRX can be realized by at least one controllable wavelength selective filter inserted into the signal path of the transmit branch signal.

[0109] Via OOB signaling, each TRX11 communicates (e.g., broadcasts) its ID and the selected X-Y combination to the commissioning device 120 having the receiver 121. A random back-off time may be used for transmission to ensure that the commissioning device 120 receives messages from all TRX11s within its communication range.

[0110] FIG. 13 schematically shows an exemplary arrangement of transceivers and their respective selected coupling states.

[0111] In the example of FIG. 13, four TRX11s (TRX1 to TRX4) are arranged in a square pattern on the ceiling of a room or the like. Thus, each TRX pair of the logs may be TRX1 / TRX2, TRX1 / TRX3, TRX2 / TRX4, and TRX3 / TRX4, as indicated by double arrows between the respective TRX11s. As shown in FIG. 13, TRX1, TRX2, and TRX4 are signaling a coupling state "X" (e.g., indicating the first transmission branch of a MIMO modem), and TRX4 is signaling a coupling state "Y" (e.g., indicating the second transmission branch of a MIMO modem). When the log is complete, the commissioning device can define a list of transceivers whose selection should be updated.

[0112] In the example shown in FIG. 13, TRX2 should switch to the coupling state "Y" such that adjacent TRXs of all identified pairs transmit respective optical communication signals having different coupling states "X" and "Y" from each other. This result is communicated back in the feedback signal transmitted via the OOB channel by the commissioning device.

[0113] FIG. 14 schematically shows an exemplary implementation of a multi-transceiver access device having transmit branch coupling according to various embodiments.

[0114] In the exemplary implementation of FIG. 14, the MIMO modem 140 is connected to the mains power grid via the plug 142 and to a communication network (such as a local area network (LAN), etc.) via the network cable 143. The MIMO modem 140 includes a combiner (not shown) as described in the above embodiments, and generates six output signals having different coupling states respectively based on, for example, two transmit branch signals generated by the MIMO modem. The six output signals are connected via respective cables 141 to corresponding sockets 146 provided in respective ceiling units (panels) 145 including one or more lamps or luminaires for optical transmission of the output signals generated by respective TRX11, and are supplied to six TRX11 (such as infrared (IR) transceivers) incorporating plugs or interfaces that can be connected to the sockets 146. The ceiling unit 145 is driven by a driver circuit 144 (such as an LED driver) connected to the mains power grid via another plug 142.

[0115] In this way, the optical wireless TRX unit is incorporated into the ceiling unit and does not require a separate transmitter unit. Alternatively, for example, if the spacing between the illumination ceiling units is too far for the TRX incorporated in the ceiling unit to provide adequate coverage, the TRX may be provided in a separate "stand-alone" unit (not shown).

[0116] The cable lengths between the access device 140 and all the ceiling units 145 should preferably be the same in order to minimize the phase delay between the output signals of different TRX11.

[0117] The exemplary implementation of FIG. 14 can be used to enhance the MIMO capability by increasing the number of transmit branch signals (such as two outlets) generated by the MIMO modem to more output signals having different couplings for a plurality of ceiling units 145 (such as six or twelve units).

[0118] According to various embodiments, the reverse communication direction (i.e., the uplink direction) may also be considered. A communication system that uses any of the aspects of the above embodiments in the downlink direction from an infrastructure device (typically, the TRX 11 attached to the ceiling or wall of the access point 12) to the distributed endpoint 10 may complement this with an uplink communication link from the endpoint 10 back to the infrastructure device. This may be achieved by at least some of the endpoints 10 emitting at least one uplink radiation beam carrying a data signal that can be received by an uplink signal detector or receiver in at least some of the infrastructure devices located on the ceiling or wall of the building, etc. The infrastructure device may then combine the data signals received by the uplink signal receiver to optimize the combining procedure for the downlink MIMO or MISO signals, according to the following non-limiting examples.

[0119] In a first example, two analog signal buses (or the same number as used in downlink distribution) from the uplink signal receiver to the MIMO-capable TRX 11 can be provided. The signals received from the signal buses are combined using weight coefficients (e.g., sharing weights) that are at least substantially the same as the linear combination (e.g., mixing weights) used in the downlink direction. As an example, the weight coefficients may be determined in the same manner as the metrics described in the above embodiments.

[0120] In a second example, all received uplink data signals may be added. However, this is risky because phase cancellation may affect the (higher) portions of the uplink data signals, resulting in an accumulation of noise. This approach is particularly attractive when the required downlink data rate (e.g., including a large download file or streaming a video) is higher than the data rate in the uplink direction.

[0121] In a third example, noise gating may be applied to the received uplink data signals. For example, only a strong copy of the received uplink data signal (e.g., a signal-to-noise ratio or error rate exceeding a predetermined threshold) or the strongest copy may be processed. This enables a signal-adaptive operation in the uplink direction. This avoids the accumulation of noise in a large-scale network.

[0122] In summary, a LiFi system is described having a plurality of M TRXs and a single multi-input multi-output (MIMO) modem having at least N transmit branch outputs, the N transmit branch outputs of the MIMO modem being supplied to a linear combiner. The linear combiner creates M distinct linear combinations based on the N MIMO outputs of the MIMO modem, the linear combinations being selected to enable the decoding of each of the N MIMO signals when N distinct signals of the M transmit signals are received.

[0123] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustrations and descriptions should be regarded as diagrammatic or exemplary and not restrictive. The present invention is not limited to the disclosed embodiments. The proposed MIMO or MISO signal combining procedures can be applied to other types of wireless networks, as well as with other types of access devices and transceivers, and can be standardized if possible. In particular, the present invention is not limited to LiFi-related environments such as ITU-T G.9961, ITU-T G.9960, and ITU-T G.9991 network environments.

[0124] Other variations to the disclosed embodiments can be understood by those skilled in the art and can be implemented when practicing the claimed invention, upon consideration of the drawings, the present disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may perform the functions of several items recited in the claims. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be used advantageously. The foregoing description details particular embodiments of the present invention. However, it should be understood that the present invention can be practiced in many ways, and thus is not limited to the disclosed embodiments, no matter how detailed the foregoing may appear in the text. The use of a certain terminology in describing a certain feature or aspect of the present invention should not be construed as meaning that the terminology is redefined herein so as to be limited to including the particular characteristics of the feature or aspect of the present invention to which the terminology is related.

[0125] A single unit or device may perform the functions of several items recited in the claims. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be used advantageously.

[0126] The described procedures, such as those shown in Figures 9 and 11, etc., can be implemented as program code means of a computer program and / or as dedicated hardware of a receiver device or a transceiver device. The computer program may be stored / distributed in a suitable medium, such as an optical storage medium or a solid state medium, which is supplied together with or as part of other hardware, but may also be distributed in other forms via the Internet or other wired or wireless telecommunications systems, etc.

Claims

1. An apparatus for generating M output signals for use in an optical wireless communication system, the optical wireless communication system comprising: An N-output multi-input multi-output modem, where N ≥ 2, for modulating a modem input signal and outputting N transmission branch signals; an N-output multi-input multi-output modem; M spatially separated optical transmitters, each transmitter configured to emit light based on each of the M output signals, the M spatially separated optical transmitters being arranged to have a plurality of overlapping reception areas capable of receiving light emitted by a plurality of the M spatially separated optical transmitters; M spatially separated optical transmitters; Including The apparatus An input unit for receiving the N transmission branch signals; A combiner for combining the N transmission branch signals via a plurality of linear combinations by combining the N transmission branch signals using a mixing coefficient selected to generate the M output signals, each output signal being a different linear combination and M > N; a combiner; Including The combiner Selecting a mixing coefficient for setting a linear combination so as to reduce the possibility of signal cancellation in an overlapping reception area that receives light from a plurality of the M spatially separated transmitters, and Supplying the M output signals to the M spatially separated optical transmitters such that each transmitter receives a different output signal of the M output signals; An apparatus configured as such.

2. The combiner is configured to form each output signal of the M output signals by mixing the N transmission branch signals using N mixing coefficients, the N mixing coefficients of each output signal representing points in an N-dimensional space, and the square of the distance from the origin to each of the M points being the same such that the M output signals are output by the M spatially separated optical transmitters using the same output signal power. The apparatus according to claim 1.

3. The combiner is configured to combine two transmission branch signals by using the following matrix of mixing coefficients for the M output signals. The apparatus according to claim 2.

4. The apparatus according to claim 1, wherein the combiner is configured to set a linear combination by associating a ratio of a feedback resistance to an input resistance of an operational amplifier.

5. The apparatus according to claim 1, wherein the combiner is configured to perform an adaptive setting of a linear combination by enabling control of a switch state of a switching element used for supplying the transmission branch signal to the combiner or for supplying the output signal to the spatially separated transceiver.

6. The apparatus according to claim 5, wherein the apparatus is configured to enable control of a switch state of a switching element based on a learning or training algorithm or based on a commissioning procedure.

7. An N - output multi - input multi - output modem, where N≥2, an N - output multi - input multi - output modem for modulating a modem input signal and outputting N transmission branch signals, M spatially separated optical transmitters, each transmitter being configured to emit light based on each of the M output signals, the M spatially separated optical transmitters being arranged to have a plurality of overlapping reception areas capable of receiving light emitted by a plurality of the M spatially separated optical transmitters, M spatially separated optical transmitters, An apparatus for generating the M output signals, the apparatus comprising: An input unit for receiving the N transmission branch signals; A combiner for combining the N transmission branch signals via a plurality of linear combinations by combining the N transmission branch signals using a mixing coefficient selected for generating the M output signals, each output signal being a different linear combination and M > N, a combiner; An apparatus including; An optical wireless communication system including; The combiner is configured to: Select a mixing coefficient for setting a linear combination such that the possibility of signal cancellation in an overlapping reception area receiving light from a plurality of the M spatially separated transmitters is reduced, and Supply the M output signals to the M spatially separated optical transmitters such that each transmitter receives a different one of the M output signals. An optical wireless communication system configured as such.

8. An apparatus for providing feedback to the optical wireless communication system according to claim 7, the apparatus comprising: a receiver for receiving, from M spatially separated optical transmitters of the optical wireless communication system, information indicating each identifier and a selected linear combination of each of the N transmission branch signals; a memory for storing geographical information regarding the identifier, the selected linear combination, and the geographical relationship between the respective transmitters; comparing the selected linear combination of the N transmission branch signals in a signal received in an overlapping reception area of the spatially separated transmitters, and determining a linear combination that needs to be changed so that the N transmission branch signals can be generated from different linear combinations received in the overlapping reception area of the spatially separated transmitters. A comparator for; comprising The apparatus is configured to signal a feedback signal having a list of coupling states to be updated for the M spatially separated transmitters.

9. A commissioning device for commissioning a wireless optical communication system, the commissioning device comprising the device according to claim 8.

10. An apparatus for generating M output signals for use in an optical wireless communication system, the optical wireless communication system comprising: M spatially separated optical transmitters, each transmitter being configured to emit light based on each of the M output signals, the M spatially separated optical transmitters being arranged to have a plurality of overlapping reception areas capable of receiving light emitted by a plurality of the M spatially separated optical transmitters. M spatially separated optical transmitters; comprising The apparatus an input unit for receiving a modem input signal; an N-output multi-input multi-output modem, where N≧2, for modulating the modem input signal and outputting N transmission branch signals. A combiner for combining the N transmit branch signals through a plurality of linear combinations by combining the N transmit branch signals using the mixing coefficients selected to generate the M output signals, wherein each output signal is a different linear combination and M > N, a combiner, comprising, the combiner is, selecting mixing coefficients that set the linear combination so as to reduce the possibility of signal cancellation in the overlapping reception areas that receive light from a plurality of the M spatially separated transmitters, and supplying the M output signals to the M spatially separated optical transmitters such that each transmitter receives a different output signal of the M output signals, an apparatus configured as such.

11. A method for controlling an optical wireless communication system, wherein the optical wireless communication system includes, M spatially separated optical transmitters, each transmitter being configured to emit light based on each of the M output signals, the M spatially separated optical transmitters being arranged to have a plurality of overlapping reception areas capable of receiving light emitted by a plurality of the M spatially separated optical transmitters, M spatially separated optical transmitters, comprising, the method is, receiving N transmit branch signals output by an N-output multi-input multi-output modem, where N ≥ 2, combining the N transmit branch signals through a plurality of linear combinations by combining the N transmit branch signals using the mixing coefficients selected to generate the M output signals, wherein each output signal is a different linear combination and M > N, setting the linear combination by selecting the mixing coefficients so as to reduce the possibility of signal cancellation in the overlapping reception areas that receive light from a plurality of the M spatially separated transmitters, supplying the M output signals to the M spatially separated transmitters, a method comprising.

12. The combining is, Forming each of the M output signals by mixing the N transmission branch signals using N mixing coefficients, wherein the N mixing coefficients for each output signal represent points in an N-dimensional space, and the square of the distance from each of the M points to the origin is the same so that the M output signals are output by the M spatially separated optical transmitters using the same output signal power. The method according to claim 11, including this.

13. A method for controlling an optical wireless communication system, the optical wireless communication system comprising: M spatially separated optical transmitters, each transmitter being configured to emit light based on each of the M output signals, and the M spatially separated optical transmitters being arranged to have a plurality of overlapping reception areas capable of receiving light emitted by a plurality of the M spatially separated optical transmitters. Including The method is Receiving, from the M spatially separated transmitters of the optical wireless communication system, information indicating a selected linear combination of each of the N transmission branch signals output by each identifier and an N-output multi-input multi-output modem, where N≧2, and each of the selected linear combinations is used to generate the M output signals based on the N transmission branch signals, and the M output signals are transmitted by the M spatially separated transmitters such that each transmitter optically transmits a different output signal of the M output signals. Comparing the selected linear combinations of the N transmission branch signals in the signals received in the overlapping reception areas of the spatially separated transmitters. Determining a linear combination that needs to be changed so that the N transmission branch signals can be generated from different linear combinations received in the overlapping reception areas of the spatially separated transmitters. A method including this.

14. A computer program including code means for performing the steps of the method according to claim 11 or 12 when executed on a computer device.

15. A computer program comprising code means for performing the steps of the method according to claim 13 when executed on a computer device.

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