Optical wireless device for transmitting an optical wireless signal

Description

[0001] The present invention relates to an optical wireless device, an optical wireless network, a method for creating an information memory for an optical wireless device, and a method for creating an information memory for an optical wireless device. The present invention further relates to an adaptive transmitter for optical wireless communication for analog modulated signals.

[0002] Modern optical wireless transceivers use orthogonal frequency division multiplexing (OFDM) to achieve high data rates and ensure high robustness against multipath propagation. OFDM is often combined with adaptive bit loading. If the received signal and thus the carrier-to-noise ratio (CNR) is high, for example, because the transmission distance is short, more bits can be encoded per carrier. The product of the symbol rate and the sum of the encoded bits of all carriers corresponds to the data rate. The data rate can be maximized by increasing the symbol rate and / or CNR. A higher symbol rate requires a higher bandwidth. However, this bandwidth is limited by the optical emitters, such as light-emitting diodes (LEDs), lasers, or laser diodes. The CNR, in turn, is reduced by noise resulting from nonlinear distortion.Typically, the component with the most nonlinear distortion is the optical emitter (e.g., LED). To reduce this distortion and achieve the highest possible peak data rates, the signal amplitude at the transmitter is reduced. However, this reduces the signal power and thus the range or coverage of the optical wireless data connection.

[0003] In current literature, a control level is chosen that represents a compromise between data rate and range. Since nonlinear distortion and range depend nonlinearly on the control level, there is a control level at which the range is maximized for a defined data rate. [a][b][c]

[0004] Other approaches include, for example, nonlinear pre- or post-distortion of the signal to compensate for the nonlinearity [d]. However, these approaches are complex in dimensioning and manufacturing.

[0005] WO 95 / 28777 relates to an optical communication system comprising a transmitter and a receiver for dynamically adjusting the data rate and / or the optical power of the transmitter.

[0006] "Modelling the Point-to-Point Wireless Communication Channel under the Adverse Weather Conditions", Jaruwatanadilok et al in IEICE Transactions on Electronics, Vol. E87-C, No. 9, pages 1455-1462, XP 001209713 refers to a weather dependence of optical transmission links.

[0007] US 2020 / 0136661 A1 relates to an optimization of the transmission power of an RF signal.

[0008] There is a need for higher data rates in optical wireless transmissions that enable reliable data transmission.

[0009] Transmitters for optical wireless communication that can provide both a high data rate and a long range would therefore be desirable.

[0010] The object of the present invention is therefore to provide an optical wireless device, an optical wireless network, methods for creating information storage devices for use according to the invention and a corresponding computer program product which enable optical wireless communication with a high data rate and a long range.

[0011] This problem is solved by the subject matter of the independent patent claims.

[0012] A core idea of the present invention is to effect an adaptation, for example a compensation or reduction of a non-linear channel distortion, by adjusting an operating point of an optical signal source and / or by changing an operating state of an electronic signal source of an optical-wireless device in order to always obtain an optimal compromise between data rate and range, so that both parameters can always be optimized.

[0013] According to one embodiment, an optical wireless device configured to transmit an optical wireless signal over an optical wireless channel comprises an electronic signal source configured to provide a data signal. Furthermore, the optical wireless device comprises an optical signal source configured to convert the data signal into the optical wireless signal and transmit it.The optical wireless device is designed to receive channel information comprising information associated with nonlinear channel distortion of the optical wireless channel and is configured to perform an adjustment of a control of the optical signal source describing an amplitude variation around a mean value by changing an operating state of the electronic signal source, by changing a control of a digital signal processor (DSP), and / or by adjusting a gain of a driver of the electronic signal source to adjust the nonlinear channel distortion, and / or to perform an adjustment of an operating point of the optical signal source to adjust the nonlinear channel distortion. The adjustment of the nonlinear channel distortion comprises a compensation or reduction of the nonlinear channel distortion.

[0014] According to one embodiment, the adaptation of the nonlinear channel distortion depends on the current transmitter-receiver arrangement or the current channel attenuation.

[0015] According to one embodiment, an optical wireless network comprises at least one such optical wireless device and furthermore a receiver for receiving the optical wireless signal.

[0016] According to an embodiment not falling within the scope of the invention, a method for adjusting an information memory for an optical wireless device to enable adjustment of a control level of an optical signal source comprises arranging a transmitter and a receiver for transmitting an optical wireless signal over an optical wireless channel. The method comprises repeatedly transmitting an optical wireless signal for different distances between the transmitter and the receiver, such that at each distance a plurality of control levels are used for transmission, which makes it possible to find the ideal control level for the existing channel attenuation.The method comprises determining at least one achievable data rate and / or signal-to-noise ratio at the receiver location for each of the transmitted signals, as well as determining a control level to be selected for each of the distances at which a data rate or signal-to-noise ratio is maximized. The method comprises compiling the control levels to be selected for the different distances. Knowing the control settings for obtaining the maximum data rate or signal-to-noise ratio for a given channel attenuation, the control level can be adjusted when the corresponding event occurs during operation so that the data rate or signal-to-noise ratio is still maximized.

[0017] According to an embodiment not falling within the scope of the invention, a method for creating an information storage device for an optical wireless device to enable adjustment of an operating point of an optical signal source comprises arranging a transmitter and a receiver for transmitting an optical wireless signal over an optical wireless channel. The method comprises repeatedly transmitting an optical wireless signal for different distances between the transmitter and the receiver, such that a plurality of operating points are used for transmission at each distance.

[0018] The method comprises determining an achievable data rate and / or signal-to-noise ratio at the receiver location for each of the transmitted signals, as well as determining an operating point to be selected for each of the distances at which a data rate or signal-to-noise ratio is maximized. The method further comprises compiling the operating points to be selected for the different distances. The method for collecting data for adjusting the operating point can thus be carried out in a similar manner to the method for collecting information for controlling an optical signal source.

[0019] Further embodiments relate to a computer program or to a data memory that has stored such a computer program for carrying out the methods described herein.

[0020] Further advantageous embodiments are the subject of dependent patent claims.

[0021] Particularly preferred embodiments of the present invention are explained below with reference to the accompanying drawings. They show: Fig. 1a shows a schematic block diagram of an optical wireless network with an optical wireless device according to an embodiment; Fig. 1b shows an exemplary schematic representation of a graph for explaining the control of an optical signal source in accordance with embodiments; Fig. 2a shows a schematic representation of exemplary graphs for discussing embodiments described herein, for which an achievable data rate is plotted over different distances; Fig. 2b shows an exemplary representation of different curves of achievable data rates DR / Mbps for different transmitter control levels and with regard to the DC component of the received power, according to an embodiment; Fig. 3 shows a schematic block diagram of an optical wireless network according to an embodiment, in which the optical wireless device has a transmitter; Fig.4 a schematic block diagram of a part of a device according to an embodiment, with a opposite . Fig. 3 modified transmitter; Fig. 5 is a schematic block diagram of a part of a device according to an embodiment, in which a driver circuit is designed to implement a variable gain set based on a control signal; Fig. 6 is a schematic block diagram of an optical wireless network according to an embodiment, which, compared to the optical wireless network of Fig. 3 a feedback through a receiver; Fig. 7 a schematic block diagram of an optical wireless network similar to Fig. 6 and according to an embodiment, wherein a measuring device is configured to determine an alternating component of the amplifier signal; Fig. 8 shows an exemplary comparison of achievable data rates, DR, against a distance plotted on the abscissa and for different operating points of the optical signal source according to an embodiment; Fig. 9 shows a schematic block diagram of an optical wireless network according to an embodiment, in which a processor device is configured on the receiver side to provide the control signal for setting the driver circuit; Fig. 10a shows a schematic block diagram of a part of an optical wireless device according to an embodiment; Fig. 10b shows a schematic block diagram of another optical wireless device according to an embodiment, which has a control element for receiving the control signal; Fig. 11 again shows some of the curves from Fig. 2a ; Fig. 12a-c each show an exemplary table for explaining the obtaining of values to be set on an optical wireless device according to an embodiment, wherein the tables are linked to each other by the respective properties; Fig. 13 shows a schematic flow diagram of a known method in which a result of the channel estimation leads to a changed bit rate of the data signal; Fig. 14 shows a schematic flow diagram of a method according to an embodiment that takes into account a change in the channel attenuation; Fig. 15 shows a schematic block diagram of a further method according to an embodiment that uses the method from Fig. 14 modified; Fig. 16 shows a schematic flow diagram of a method for determining a control level to be selected for an optical signal source according to an embodiment; and Fig. 17 shows a schematic flow diagram of a method for determining operating points to be selected for an optical signal source according to an embodiment.

[0022] Before exemplary embodiments of the present invention are explained in more detail below with reference to the drawings, it is pointed out that identical, functionally equivalent or equivalent elements, objects and / or structures in the different figures are provided with the same reference numerals, so that the description of these elements shown in different exemplary embodiments is interchangeable or can be applied to one another.

[0023] The embodiments described below are described in conjunction with numerous details. However, embodiments may also be implemented without these detailed features. Furthermore, for clarity, embodiments are described using block diagrams instead of detailed illustrations. Furthermore, details and / or features of individual embodiments may be readily combined with one another, unless explicitly described otherwise.

[0024] Some of the embodiments explained herein are explained in connection with optical wireless communication, for which the term LiFi (Light Fidelity) is also used. According to embodiments, this is combined with the generation of quantum keys, i.e., cryptographic keys created using quantum-based approaches. The advantages obtained thereby can also be used in other areas of wireless communication, so that the described embodiments are not limited to optical wireless communication, but can also be implemented in other areas of wireless communication, for example, in the field of radio wave communication in the frequency range from approximately 1000 MHz.800 MHz, with frequencies and techniques for beamforming particularly suitable, as is possible, for example, in the frequency band from 4 GHz onwards, but also at higher frequencies of at least 40 GHz, for example, in the 50 GHz or 60 GHz range. Although devices for active radio transmission, i.e., beamforming, are preferred, mechanical devices, such as antenna housings, can also be used for beamforming.

[0025] According to embodiments of the present invention, radio wavelengths can also be used as an alternative or in addition to optical wireless communication, in particular those that are particularly well suited for point-to-point radio transmission, for example, using so-called beamforming techniques. This is particularly advantageous for transmitters that lead to essentially nonlinear distortions or contribute significantly to the overall nonlinearity, as is the case with the optical emitter, but can also occur with other nonlinear transmitters, e.g., with nonlinear antenna characteristics of radio wave transmitters.

[0026] The following embodiments relate to optical wireless signal transmission or data transmission. In the context of the embodiments described herein, this is also referred to as LiFi (Light Fidelity). The term "LiFi" refers to terms such as IrDA (Infrared Data Association) or OWC (Optical Wireless Communication). This means that the terms "optical wireless data transmission" and "LiFi" are used synonymously. Optical wireless data transmission is understood to mean the transmission of an electromagnetic signal through a free transmission medium, such as air or another gas or fluid.For this purpose, wavelengths in the ultraviolet (UV) range of at least 53 nm and the infrared range, for example, up to 1550 nm, can be used. Other wavelengths that differ from those used for radio standards are also possible. Optical wireless data transmission must also be distinguished from fiber-based optical data transmission, which is implemented, for example, using fiber optic cables or optical fibers.

[0027] Fig. 1a shows a schematic block diagram of an optical wireless network 100 with an optical wireless device 10 according to an embodiment and a receiver or receiver device 12 configured to receive a signal 14 emitted by the optical wireless device 10. The optical wireless device 10 can be configured to transmit the optical wireless signal 14 and operate as an optical wireless transmitter. However, the optical wireless device can also be configured, without limitation, to receive optical wireless signals and, for example, be an optical wireless transceiver. Likewise, the receiver device 12 can be configured to send an optical wireless signal back to the optical wireless device 10, i.e., can be configured as a transceiver. A channel 16 between the optical wireless device 10 and the receiver 12 can thus be used unidirectionally or bidirectionally.Channel 16 can lead to channel-induced distortion of the optical wireless signal 14, i.e., channel distortion. This can be nonlinear, for example, and can have different effects for different frequency ranges or wavelength ranges.

[0028] The optical wireless device 10 comprises an electronic signal source 18 configured to provide a data signal 22. Furthermore, the optical wireless device 10 comprises an optical signal source 24 configured to convert the data signal 22 into the optical wireless signal 14 and transmit it. The optical signal source 24 may comprise an optical emitter, for example a laser source or a light-emitting diode, with a light-emitting diode being preferred. An operating point of the optical emitter may correspond to a DC component of the current consumption of the optical emitter 24. The electronic signal source 18 and the optical signal source 24 may be directly coupled to one another or even form an integrated component. However, this does not preclude intermediate processing of the electronic data signal 22, for example, through the use of a filter, amplifier, or the like.

[0029] The optical wireless device 10 is configured to receive channel information 26 comprising information associated with a nonlinear distortion of the signal 14 in the optical wireless channel 16. The channel information 26 may, for example, indicate information associated with a distance or spacing 28 between the receiver 12 and the optical wireless device 10 or directly indicate this, wherein, for example, a continuous or discrete value range is possible for this purpose. Alternatively or additionally, the channel information 26 may also correlate with or indicate a signal-to-noise ratio, wherein the signal-to-noise ratio may relate to a reception quality of the optical wireless signal 14 at the location of the receiver 12.Such information may be obtained or received by the receiver 12, but may also be estimated by the receiver 12, for example, by exploiting the assumption of channel reciprocity, by receiving an optical wireless signal from the receiver 12, wherein it is possible, but not necessary, to use the same wavelength range as for the optical wireless signal 14.

[0030] The optical-wireless device 10 is configured to adjust the modulation of the optical signal source 24 based on the channel information in order to at least partially adjust the nonlinear channel distortion. To this end, the optical-wireless device 10 can change the operating state of the electronic signal source, for example by changing the modulation of a digital signal processor (DSP) and / or by adjusting the gain of a driver of the electronic signal source 18. Other means are also readily possible. Alternatively or in addition to adjusting the operating state of the electronic signal source 18, an adjustment of an operating point of the optical signal source 24 can be performed by the optical-wireless device in order to adjust the modulation of the optical signal source.

[0031] The adaptation can, for example, be carried out as at least partial compensation of the nonlinear channel distortion, i.e., to counteract the momentary or detected or current nonlinear channel distortion. Nonlinear predistortions and / or postdistortions can be used, which can improve the linearity, which can be understood as compensation. Embodiments relate to selecting the operating state of the transmitter such that the existing nonlinear distortions no longer cause interference, which, however, can also mean accepting severe distortions over long distances, for example. An adaptation in connection with embodiments can serve the goal of improving the signal quality / data rate. In cases where the modulation is increased or the operating point is reduced, this can also be understood as a reduction.

[0032] Changing the operating state of the electronic signal source 18 and / or adjusting the operating point of the optical signal source 24 can be performed by a respective control device (not shown), which evaluates the channel information 26 and derives the parameters to be set for the electronic signal source 18 or the optical signal source 24. For this purpose, for example, an information memory can be provided that directly specifies the parameters to be set or that has a calculation rule stored from which the parameters to be set can be derived using the channel information 26.

[0033] According to one embodiment, the channel information 26 can be based on a signal-to-noise ratio, such as a signal-to-noise ratio (SNR) and / or a carrier-to-noise ratio (CNR). The signal-to-noise ratio can relate to the optical wireless signal 14, and the channel information 26 can further be based on a signal power of the optical wireless signal at the receiver 12, such as a power spectral density (PSD) or a DC component of the photocurrent at the anode or cathode of a photodiode of the receiver 12. These values also depend on the distance 28, so that the signal-to-noise ratio and / or the signal power at the location of the receiver 12 can be deduced from the distance 28, at least taking into account the values set in the optical wireless device 10.

[0034] The optical wireless device 10 can be configured to adjust the control of the optical signal source 24 during operation, meaning to set different settings for the control of the optical signal source at different times, for example, upon detecting a change in the optical wireless channel 16. According to one embodiment, the optical wireless signal 14 is a first optical wireless signal that is transmitted in a first transmission interval. At a later second transmission interval, a later second optical wireless signal is transmitted by the device 10, for example, using the optical signal source 24.The optical wireless device 10 is configured to determine, for transmission of this later optical wireless signal in the later second transmission interval, that a channel attenuation has increased compared to the first transmission interval, and to increase the modulation and / or to decrease the operating point in order to benefit from a greater signal modulation / signal power or to reduce the power loss, whereby greater nonlinear distortions are accepted, which, however, are not significant with regard to the data rate / error rate. Alternatively or additionally, the optical wireless device can be configured to determine, for transmission of the later signal, that a channel attenuation has decreased compared to the first transmission interval, and to decrease the modulation and / or to increase the operating point in order to reduce the nonlinear distortions, for example of the electronic signal source.A reduction in the modulation level can be achieved, for example, by lowering the power of the electronic signal source 18. The adjustment of the operating point is discussed in detail in connection with the embodiments described herein.

[0035] The improvement or deterioration of the channel, i.e., the detection of reduced or increased channel attenuation, can be communicated explicitly or implicitly to the optical wireless device 10, for example, by communicating corresponding values for the signal-to-noise ratio or the signal power at the receiver 12. Alternatively or additionally, the optical wireless device 10 can estimate corresponding values from self-received optical wireless signals, for example, by receiving a signal 32. According to one embodiment, the optical wireless device 10 is configured to estimate channel attenuation and is configured to estimate the nonlinear distortion based on reference information indicating noise at the receiver 12 of the optical wireless channel 16.

[0036] According to one embodiment, the optical wireless device 10 is configured to estimate the channel attenuation based on a signal power at the receiver 12 and a signal power at the transmitter of the optical wireless channel 16, i.e., the optical channel source 24.

[0037] According to one embodiment, the optical wireless device 10 comprises an information memory (not shown) and / or a channel estimator (not shown) configured to output information indicating a relationship between a signal transmitted in the optical wireless channel 16 and the nonlinear distortion.

[0038] According to one embodiment, the channel information 26 is based at least partially on feedback from the receiver 12 regarding a signal power of the optical wireless signal 14 at the receiver 12, on a signal-to-noise ratio of the optical wireless signal 14 at the receiver 12, and / or an error rate. The signal-to-noise ratio and the error rate are directly related to one another, which is why a reference to the signal-to-noise ratio also simultaneously establishes a reference to the error rate. This information can be specified explicitly for the feedback or specified using other information, such as an index of a look-up table or the like. According to one embodiment, the optical wireless device 10 is configured to transmit the optical wireless signal 14 via the optical wireless channel 16.Optionally, the device 10 may be configured to receive an optical wireless signal 32, which may be received via the optical wireless channel 16, such as from the receiver 12 or another device. The optical wireless device 10 may be configured to perform channel estimation of the optical wireless channel 16 based on the optical wireless signal 32 to determine a signal-to-noise ratio of the signal 32.

[0039] Alternatively or additionally, the optical wireless device 10 can be configured to receive the channel information 26 from the receiver 12 of the optical wireless signal 14. The optical wireless device 10 can be configured to adapt the modulation of the optical signal source 24 based on channel information such that it indicates a reduced received power compared to a previous transmission interval and / or a reduced signal-to-noise ratio at the receiver of the optical wireless signal, by increasing an alternating component of the electronic signal 22 for a later optical wireless signal toward a second alternating component. This adjusts the operating point of the optical signal source 24.

[0040] Alternatively or additionally, the optical wireless device 10 can be configured to adapt an operating state of a driver circuit of the electronic signal source 18 to adapt the modulation of the optical signal source 24, thereby adapting the modulation of the optical signal source 24. Such a driver circuit can have an amplifier element configured to amplify a received processor signal or convert it into a current. The optical wireless device 10 can be configured to adjust a gain of the amplifier element to change the operating state of the driver circuit, thereby adapting the nonlinear distortion at the optical signal source. For example, the driver circuit can comprise an adjustable attenuator to adapt the gain of the amplifier element.Alternatively or additionally, an active adjustment of a gain factor of the amplifier element can be set by the driver circuit.

[0041] The amplifier element can be coupled to a resistance circuit that adjusts the gain. The optical wireless device 10 can be configured to adjust the gain at least approximately continuously and / or to adjust resistance values discretely. A continuous change can be achieved, for example, using a potentiometer. Alternatively or additionally, the resistance circuit can have a plurality of circuit states in which mutually different resistance values are coupled to the amplifier element for adjusting the gain. The optical wireless device 10 can be configured to select and adjust one of this plurality of circuit states to change the control.

[0042] According to one embodiment, the electronic signal source 18 can comprise a signal processor, in particular a digital signal processor (DSP), configured to provide a processor signal to the driver circuit. The driver circuit can be configured to adjust its gain according to the processor signal. The signal processor can be configured to adjust an alternating component of the processor signal using a digital-to-analog converter in order to feed a signal of varying strength into the driver circuit.

[0043] In other words, embodiments of the present invention describe an optical wireless transmitter that addresses the problem of emitter nonlinearity through adaptive AC control. The AC control is based on the current channel attenuation, which arises from geometric channel losses or absorption in the channel. At short distances, the channel losses are low, so the received signal is strong. The control is then reduced so that the nonlinear distortions at the emitter and in the other transmitter components are reduced. As a result, a higher CNR and thus higher peak data rates can be achieved. If the communication distance and thus the channel attenuation are high, the control is increased. Higher transmission power is associated with a longer connection range. If the control is designed correctly, the data rate over the entire distance also increases.Bit loading means that reducing the modulation depth is advantageous over longer communication distances. While the higher modulation level increases nonlinear distortion, the lower modulation depth may mean that a particularly high CNR is not even necessary. For example, if the noise from the nonlinear distortion is exceeded by another noise factor (e.g., internal receiver noise), a higher signal level increases the signal power more than the noise power. Consequently, the CNR and thus the data rate increase.

[0044] Fig. 1b shows an exemplary schematic representation of a graph with a curve G, which, for example, represents a signal amplitude S of an optical emitter of an optical signal source, for example the signal source 24, over time t. In order to possibly set a mean value MW, which can describe a DC component, by a provided current, which describes the operating point, for example, the signal can vary with an AC component with a value range H, which can be described by an amplitude A. The operating state of the signal source 24 or the value range H can be changed by varying the value of the mean value, which can be done, for example, via an operating point of the optical signal source and / or via a DC component of the electrical signal 18 fed into the optical signal source. A change in the value range H can also lead to a changed modulation, which, for example,This can be achieved by changing the amplitude of the electronic signal, for example, through variable gain factors. In this context, control can be understood as an amplitude variation that differs from an adaptive operating point. Both can be summarized under the term "operating state."

[0045] Fig. 2a shows exemplary graphs 34 1 to 34 5 for discussing embodiments described herein. The abscissa shows, by way of example, the distance 28 from Fig. 1a and with the parameter z in the unit meter (m). The ordinate represents the data rate, DR, in units of Mbps, which was achieved in the non-limiting experiment represented by the different curves 34 1 to 34 5.

[0046] The dashed curves show measured values for various AC transmitter modulations. The values correspond to the effective value of the AC component of the optical transmission power. The solid line illustrates that an adaptive transmitter achieves the envelope performance of all curves. The change in modulation initially refers to the AC signal component.

[0047] Different curves 34 1 to 34 4 differ with regard to the set transmitter modulation, which refers to the optical signal source 24. For different modulations, which are determined by the effective value (rms) of the alternating component (AC) of the optical transmitter (TX) power Φ TX AC eff of 9.3 dBm, 13.7 dBm, 17.2 dBm and 22.0 dBm respectively, different data rates result for different distances z. A direct comparison of curves 34 1 to 34 4 with each other shows, for example, that a low modulation at a short distance, which corresponds to a comparatively low channel attenuation, can lead to higher data rates than larger modulations due to the reduction of non-linear distortions in the transmitter and receiver. For greater distances z, however, higher modulations offer advantages, as can be seen, for example, in curves 34 3 for a distance of 6 meters and curve 34 4 for distances greater than 7 meters.

[0048] A variable control according to the embodiments described herein, which is represented by way of example by the curve 34 5, thus enables a consistently high achievable data rate for different distances or different channel attenuations, taking into account the non-linear distortions, which is advantageous.

[0049] In other words, Fig. 2a The data rate of an optical wireless transceiver over the range. The transceiver transmits OFDM signals with adaptive bit loading, thus adapting the data rate to the channel attenuation. Curves for various transmitter drive levels are shown. Φ TX AC eff encodes the effective value of the AC component of the optical power. The dashed curves correspond to the configurations, one of which is selected in a conventional solution. A high transmitter power Φ TX AC eff (high modulation) is accompanied by a low maximum data rate (due to non-linear distortion), but a high range. At the same time, a lower transmitter power Φ TX AC eff (lower modulation) with high maximum data rate, but shorter range. The adaptive modulation described here increases with increasing communication distance z the control to adjust the data rate DR in distance. If this is successful, the DR-z graph corresponds to the envelope of the curves of the individual configurations. It is clearly visible how the range and data rate increase compared to conventional approaches.

[0050] Fig. 2b shows an example of different curves of achievable data rates DR / Mbps for different transmitter control levels Φ TX AC eff and with regard to the DC component of the received power ( Φ RX DC / dBm). For comparison, the DR in Fig. 2a over the communication distance z.

[0051] The different curves 68 1 to 68 4 indicate the different effective values of the alternating component of the optical transmitter power Φ TX AC eff .

[0052] Fig. 3 shows a schematic block diagram of an optical wireless network 300 according to an embodiment. The optical wireless network includes an optical wireless device 30 according to an embodiment and the receiver 12, which, compared to the Fig. 1a is shown in detail. The receiver 12 may, for example, comprise a receiver 17 configured to receive the optical wireless signal 14'.

[0053] The device 30 can comprise the same functions as the device 10. The device 30 can comprise a transmitter 35, which can comprise, for example, an electronic signal source 18' and the optical signal source 24. An electronic signal source 18' of the device 30 comprises, for example, a signal processor 36, which is configured to output a processor signal 38 and apply it to a driver circuit 42 with a possibly variable gain factor in order to obtain the data signal 22. The optical signal source 24 is configured to receive the data signal 22 and to transmit the optical wireless signal 14. After passing through the channel 16, a distorted signal 14' is received by a photodetector 44 of the receiver 12, which can provide a received signal based thereon.This received signal 46 can be fed to an amplifier 48 of the receiver 12, which can be configured to provide an amplified signal 52, which can be provided to a signal processor, for example a DSP, 54 to process the information transmitted by the device 30.

[0054] In other words, Fig. 3 an optical wireless data transmission link comprising the transmitter 30 and the receiver 17. The system 300 is shown in a simplified unidirectional manner, but can also be bidirectional in reality. A digital signal processor 36 (DSP) feeds a modulated data signal 38 into the driver 42 of the optical wireless transmitter 30. In the system 300, the driver is the component with variable gain. In an alternative embodiment, the DSP 36 can alternatively or additionally provide a variable output level by means of an analog-to-digital converter. The crucial point is that a signal 22 is fed into the optical wireless emitter 24 (e.g., a light-emitting diode or laser diode) at the output of the driver 42, the control of which is variably adjustable. The emitted optical signal is designated 14 when it is transmitted and 14' when it reaches a receiver. The photodetector 44 (e.g.,A photodiode detects signal 14' and converts it into received signal 46. Signal 46 is amplified by an amplifier 48 (e.g., a transimpedance amplifier). The amplified signal 52 is fed into a DSP 54, where it is demodulated and further processed. In transceivers for bidirectional data connections, the transmitter 35 and the receiver 17 form a transceiver. In this case, a DSP can combine the functions of blocks 36 and 54.

[0055] Fig. 4 shows a schematic block diagram of a part of a device 40, in particular a transmitter 35' thereof. In the transmitter 35', for example, the concept of changing the operating state of the electronic signal source to adapt the non-linear channel distortion is shown. In the embodiment of the Fig. 4 An analog implementation of a variable gain of the amplifier or driver circuit 42 is shown. The gain control can be continuous or stepwise. The driver circuit 42 includes an adjustable attenuator 56 or an attenuation circuit configured to adjust the gain of the amplifier element. Fig. 4 The variable driver gain is realized by means of the variable attenuator 56 in the driver 42, which can be controlled, for example, via a control signal 58. An output of an amplifier 62, i.e., the data signal 22, can be adjusted thereby.

[0056] The control signal 58 can be based on at least one of several possible information sources and can transport the corresponding information as an analog signal, for example by means of a signal amplitude, a frequency, an on-off level, or the like. Alternatively, the control signal 58 can comprise a message within the framework of a communication protocol that carries corresponding information for controlling the respective element on the device 40 and is, for example, decoded or interpreted. The information source can refer directly or indirectly, i.e., explicitly or implicitly, to a measured variable related to the channel or the channel attenuation, e.g., a distance between transmitter / receiver, a resulting change in a DC component or AC component of a received signal, or the like. Alternatively, the message can implicitly or explicitly specify which setting is to be made on the transmitting device.

[0057] To adjust the modulation of the optical signal source 24, the channel information can be based on a DC component of an optical wireless signal received by a receiver of the optical wireless channel 16, such as signal 14, and can be associated with an adjustment of an attenuator of the electronic signal source at the transmitter, such as device 40. Alternatively or additionally, to adjust the modulation of the optical signal source 24, the channel information transported with the control signal 58 can be based on an AC component of an optical wireless signal received by a receiver of the optical wireless channel 16, such as signal 14, and can be associated with an adjustment of an amplification factor of the electronic signal source.

[0058] According to embodiments, it is possible, but not necessary, to provide an information memory, for example in the form of a look-up table or the like, in the transmitting device. Different predefined values for the modulation associated with different values of the channel distortion can be stored in the information memory. The optical wireless device can be configured to obtain and apply a value to be set for the modulation from the information memory using the channel information 26, for example, a value for the signal 58.

[0059] Fig. 5 shows a schematic block diagram of a part of a device 50 in which a driver circuit 42" is designed to provide a variable gain of the driver circuit 42" based on the control signal 58. For this purpose, an interaction with the amplifier element 62 can be varied in a circuit 56', and thus the gain of the driver circuit 42" can be adjusted in order to adapt the control of the optical signal source 24.

[0060] The control signal 58 of the Fig. 4 and / or the Fig. 5 Depending on the channel information 26 can be Fig. 1a be selected so that the adjustment of the driver circuit 42' and / or 42" depends on the channel information in order to at least partially compensate for the non-linear distortion.

[0061] An implementation of embodiments in accordance with the invention described herein may thus include an implementation in the transmitter. In other words, the adaptive control may be implemented analogically in the transmitter 35 of the system 300. According to embodiments, the adaptive control may be implemented entirely or partially digitally. In the analog implementation, the driver circuit 42 has a more variable gain. The gain control is, for example, continuous or stepwise. The gain is varied, for example, by using an adjustable attenuator ( Fig. 4 ) or the active gain factor is controlled ( Fig. 5 ). Both implementations are controlled by control signal 58.

[0062] Fig. 4 shows a realization of the variable driver gain by means of a variable attenuator 56 in the driver and Fig. 5 a realization of the variable driver gain by means of variable gain through the resistor network 56'in the driver.

[0063] Reference numeral 62 designates an amplifier that, in conjunction with the variable circuitry and / or due to variable control, can result in a variable gain. For example, a variable circuitry can be adjusted using an attenuation circuit 56. This means that the adjustment of the output of the emitter 24 can be achieved by adjusting a control of the amplifier element 42' / 42", for example, by a signal amplitude and / or an effective value of an alternating component of the signal 38, and / or by changing an operating state of the amplifier element, for example, to adjust the gain factor.

[0064] The implementations presented are design examples in connection with embodiments of the present invention. It is of course conceivable to vary the respective other resistor and to implement the variable amplifier in other known architectures. The amplifier can also be operated in an inverting configuration or be designed differentially. As already mentioned, the control can be implemented stepwise or continuously. A stepwise implementation can, for example, be realized with different resistors and switches, whereby the impedance and thus the gain is varied by opening and closing the switches. The switches can be transistors in this case. Alternatively, it is also conceivable to connect one or more transistors in parallel with a resistor.The voltage at the control input of the transistor is used to control the channel resistance of the transistor in order to vary the gain of the VGA.

[0065] Changing the gain of a system, i.e., the change process itself, can lead to nonlinear distortions, which in turn could lead to a reduction in CNR. There are several implementation options to address this problem: The system knows when a data signal (data packet, data frame) is transmitted. For this purpose, the DSP 36, for example, provides a logical control signal. While the signal is being transmitted, the gain cannot be adjusted, or the gain is not changed, or at least remains approximately unchanged. The gain is then adjusted accordingly between packets / frames. The control is so weak or in small steps / change rates that the noise from the nonlinear distortions resulting from the gain adjustment is small compared to other effective noise sources (e.g. receiver noise). With step-by-step adjustment, the steps must be very small, which may be impractical in most systems. With continuous gain adjustment, on the other hand, the gain changes very slowly, i.e., over a long time interval, i.e., over a large number of data packets / frames.During the duration of a packet, the gain can thus be approximated as constant.

[0066] As mentioned, the level of signal 22 can alternatively or additionally be controlled entirely or partially by DSP 36. During OFDM signal generation, the DSP can apply a higher power to the entire carrier set if the attenuation in the optical channel is particularly high. Accordingly, it can reduce the power per carrier if the channel attenuation is particularly low. In this case, not only the nonlinear distortions at the optical emitter 24 are amplified, but also at the digital-to-analog converter (DAC) of DSP 36. It remains crucial that the total noise from nonlinear distortions is appropriately controlled relative to the other noise components.

[0067] This functionality should not be confused with conventional, adaptive power loading, as is also occasionally used in OFDM. Here, the power per carrier is varied across the signal spectrum in such a way that the non-ideal transfer function of the connection, for example, low-pass attenuation, is compensated [e,f], whereby such distortions are referred to as linear distortions. In contrast, in the present invention, the effect of channel attenuation is compensated with regard to the influence of non-linear distortions. The control does not only take place when the signal is very weak, but also at very strong signals, since, as in Fig. 2a shown, for example, even from a distance of ~1 meter you can benefit from a stronger or weaker control.

[0068] As an alternative to determining the control signal 58 on the optical-wireless device itself, corresponding information can also be received by a receiver 12' set up for this purpose, as is the case, for example, in Fig. 6 which shows a schematic block diagram of an optical wireless network 300' which, in comparison to the optical wireless network 300 of Fig. 3 enables feedback from the receiver 12' to the device 30 via a receiver 17'. For this purpose, the receiver 12' can comprise a measuring device 64, which is designed, for example, to receive and evaluate the received signal 46, for example with regard to the received power. The measuring device 64 can be designed to detect a DC component of the photocurrent of the received signal 46 and to transmit this information or information derived therefrom, for example, a quantized value or the like, to the optical wireless device 30.

[0069] In Fig. 7 is an optical wireless network 300" similar to the Fig. 6 , in which a measuring device 64' is designed to detect an AC signal power, i.e., an alternating component, of the amplifier signal 52. This measurement result can, alternatively or in addition to the result of the measuring device 64, form at least part of the control signal 58 for the optical wireless device 30. While in the optical wireless network 300' the channel information is based on a DC component of an optical wireless signal 14' received by the receiver 12" of the optical wireless channel 16, and this can be associated, for example, for adapting an attenuator of the electronic signal source 42, Fig. 7 an embodiment in which the channel information is based on an alternating component of an optical wireless signal 14' received by the receiver 12" of the optical wireless channel 16 and is associated with an adjustment of a gain factor of the electronic signal source. For example, the receiver 12' can be used to adjust the receiver 35' and the receiver 12" can be used to adjust the receiver 35".

[0070] In both cases, a control signal for the variable modulation is provided. Some of the embodiments described herein provide for the use of a calculation rule for generating the control signal 58. This can be used to predetermine the control signal 58, for example, for storage in an information or data memory, alternatively or additionally, but also during operation. As formula (1) illustrates, the CNR can be derived from the signal power P sig to noise power Pn. Signal and noise power could be replaced by the corresponding effective values of the voltages. In the following considerations, the voltages are accordingly to be understood as effective values. In the model described here as an example, the noise consists of the independent noise quantities of the internal receiver noise. u n RX and the noise from non-linear distortions at the transmitter and nl TX or receiver u n nl RX together. u n nl Rx is usually opposite to u n RX is negligible until overloading occurs in the receiver. A lower transmitter level prevents overloading or allows it to occur only at higher reception levels, thus increasing the dynamic range of the link or data connection even over short distances. The relationship between the signal voltage usig and the transmitter control is approximately linear, but the dependence of u sign to u n nl Tx is highly nonlinear. An example of this is triple-beat distortion, which increases with the square of the drive. This explains why the CNR can be improved by reducing the drive. CNR = P sig P n = u sig 2 u n RX 2 + u n nl Tx 2 + u n nl Rx 2

[0071] According to one embodiment, the optical wireless device is configured to adjust the modulation of the optical signal source 14 / 24. For this purpose, the adjustment can achieve a noise component due to nonlinear distortion in the total noise at the receiver location that is of the same order of magnitude as the internal receiver noise. In this case, the highest data rates are typically achieved. "Ordinance" is understood here to mean that the value corresponds to at least 1 / 3 and at most 10 times the other value; this means that the noise component of the nonlinear distortion in the total noise at the receiver location is at least 33% and at most 1000% of the internal receiver noise.

[0072] In other words, according to one embodiment, a control device of the optical wireless device and / or the processor 36 can be implemented in such a way that the modulation of the signal 24 is controlled in such a way that the noise from the non-linear distortions u n nl Tx does not reduce the CNR and therefore also the data rate. In practice, this means that u n nl Tx in the order of magnitude of the receiver noise u n Rx, which can represent a control variable, for example. At the same time, some designs provide for the control level not to be too low in order to avoid a signal voltage that is too low u sig also limits the CNR, which in this case consists only of u sig and u n RX forms. In practice, u n nl Tx is approximately as large as the dominant noise source, i.e. u n RX or un nl Rx. In this case, u n nl Tx for example, the noise power by 2 A further reduction reduces the control u However, the signal-to-noise ratio is higher than the signal-to-noise ratio. The fundamental goal of variable transmitter gain control is to maximize the CNR at a given reception level at a given distance.

[0073] The control signal 58, which is the control for the transmitter control, can be determined, for example, as follows: It can be generated at your own receiver, assuming that the signal at the receiver of the communication partner is similar. ∘ Fig. 6 : Measurement of the DC received power at block 64 before or after the photodetector 44. In the case of a photodiode, this means measuring the DC component of the photocurrent at the anode or cathode of the photodiode. This signal is amplified and fed to the driver 42. ∘ Fig. 7 : By measuring the AC signal power at block 64' after the transimpedance amplifier 48, the signal 52 can be tapped and the AC component determined (for example, determining the RMS value). This voltage is amplified accordingly and fed to the driver 42. At the receiver 12' / 12" of the communication partner: Systems with bit loading have a function for evaluating the system's error rate or CNR in order to adjust the bit loading accordingly. This information and the current bit loading can be communicated to the communication partner via the protocol by transmitting a corresponding message in the same or a different wavelength or frequency range. Alternatively, the protocol can also directly communicate whether the level should be increased or decreased. The optical wireless device can have another optical detector that also detects the signal.This receiver can utilize a large photodetector. The signal can be amplified with particularly low noise at narrow bandwidths to generate a measure of the DC component of the signal 14'. The optical wireless device could incorporate a rangefinder, such as a time-of-flight sensor. From this distance, the channel attenuation can be determined. In a bidirectional optical wireless device, the control signal can be generated in the DSP 54 using digital data processing from the signal 52. By determining the harmonic distortion, intermodulation products, or other information about nonlinear distortions extracted from the signal or a test signal, the transmitter drive of the opposing device can be determined. In this way, the channel attenuation and channel distortion can be calculated by associating the channel distortion with a system-specific transmit power.It goes without saying that the described DSP functionality could also be implemented analogously in the receiver.

[0074] The relationship between the measured variable in the receiver and the change in gain in the transmitter can be linear or non-linear, whereby non-linear control allows a larger dynamic range of the control.

[0075] Fig. 6 shows an optical wireless transceiver with variable driver gain, where the gain is controlled by a control signal from the receiver, which is generated, for example, by measuring the DC component of the photocurrent.

[0076] Fig. 7 shows an optical wireless transceiver with variable driver gain, wherein the gain is controlled by a control signal from the receiver, which is generated from the signal 52 after the amplifier 48.

[0077] Alternatively or in addition to changing the operating state of the electronic signal source, for example by adjusting the signal amplitude in the signal processor 36 and / or by adjusting the gain in the driver circuit 42, an operating point of the optical signal source 24 can also be adjusted to adjust the non-linear channel distortion.

[0078] Fig. 8 shows an example comparison of achievable data rates, DR, compared to a distance z plotted on the abscissa. Curves 66 1 , 66 2 and 66 3 show different working currents ILED DC for the optical signal source 24. It can be seen that at short distances, for example up to one meter, a higher current is associated with a higher data rate, but that this effect already reaches a kind of saturation as soon as the distance exceeds one meter, with curves 66 2 and 66 3 approaching each other and almost overlapping at a distance of more than 1.5 meters. In contrast, at larger distances, for example more than 5 meters, a higher data rate can be achieved with a high operating current (curve 66 3 ) or a low operating current (curve 66 1 ) than with a medium operating current (curve 66 2 ).

[0079] In other words, Fig. 8 a data rate over distances for different LED operating points.

[0080] For example, the applied time-averaged operating current or working current can be set as the operating point. Alternatively or in addition to setting up an information memory with different values for the modulation, an information memory or the same information memory can be set up with different predefined values for the operating point, which are associated with different values of the channel distortion. Such an optical wireless device can be configured to obtain and apply a value to be set for the operating point from the information memory using the channel information.

[0081] An optical wireless device configured to adjust the operating point of the optical signal source 24 may be configured to increase the operating point when there is a dominant or relevant portion of the nonlinear distortion in the overall noise and at the same time a high signal-to-noise ratio of the optical wireless signal at a receiver of the optical wireless signal and at the same time a high data rate in the optical wireless signal. With reference to Fig. 8 This is the case, for example, at short distances of less than one meter. Alternatively or additionally, such a device can be configured to reduce the operating point when the proportion of nonlinear distortion in the total noise is insignificant and the signal power of the optical wireless signal at a receiver is weak. Such an approach allows a significant reduction in the power consumption of the transmitter. Depending on the transceiver arrangement, this can be up to 10%, 30%, or even more than 50%. The proportion of nonlinear distortion is not relevant, for example, if the data rate is reduced by less than 5% as a result of the adjustment of the operating point due to the nonlinear distortion.

[0082] As explained in connection with the adjustment of the control, such a determination can be made by receiving appropriate information from the receiver or by making an appropriate determination at the location of the transmitter, i.e. the optical wireless device.

[0083] In other words, alternatively or in addition to adjusting the emitter's drive, the transmitter can benefit from a variable operating point setting of the optical emitter (i.e., with and without variable transmitter drive). The emitter's operating point influences the efficiency, bandwidth, and linearity of the emitter. A relevant or even crucial aspect for OFDM data transmission is that linearity has a significant influence on the maximum data rate, but less influence once the receiver noise un RX dominates. In this range, the operating point influences the data rate only through the dependence of bandwidth and efficiency. This influence is usually much weaker. The measurement in Fig. 8 shows these relationships using data rate curves over distance for different LED operating point currents with the same transmitter control.

[0084] As with variable transmitter gain, this behavior can be exploited by an adaptive transmitter. If the CNR (and the data rate) is high, it is usually limited by the nonlinear distortion. In this case, the operating point is increased, for example, to reduce the distortion. This achieves a higher maximum data rate. However, if the received signal is weaker due to a greater communication distance, the operating point is reduced. This reduces Fig. 8 For the three working streams, the data rate at z=3m is not affected. However, in this example, this reduces the power consumption by approximately 25%. Since the transmitter is a significant consumer in the transceiver, this not only reduces the power consumption of the entire system, but also reduces the thermal load, thus increasing the service life.

[0085] The control signal for such a control can be generated analogously to the variable transmitter control in the receiver of the own transceiver or in the DSP of the communication partner. The operating point can be set directly by the amplifier 42 or, for example, impressed via a bias tee or AC coupling. The operating point is then varied accordingly by adjusting the components. With a discrete amplifier 42, this can be achieved by adjusting the common-mode voltage via the control voltage 58. With the bias tee, the control voltage 58 is also applied directly according to embodiments. With AC coupling, the DC component can be adjusted, for example, via a divider ratio of a voltage divider by designing at least one of the impedances to be adjustable. Digital programming of the amplifier 42 for a variable output level is also conceivable.

[0086] Fig. 9 shows a schematic block diagram of an optical wireless network 300‴, in which the processor device 54 is configured on the receiver side to provide the control signal 58 for setting the driver circuit 42. Alternatively or additionally, the signal 58 can also be used to set the operating point of the optical signal source 24. This means, unlike in connection with the Fig. 6 and 7 described, the control signal 58 can also be provided by the processor device 54. Deviating from the representation of the Fig. 9 the control signal 58 can also be provided directly to the processing device 36, which implements the corresponding instructions or derives the control commands to be generated therefrom.

[0087] Fig. 9 further shows an information memory 72, in which, alternatively or in addition to the control signal 58, specifications for controlling the driver circuit 42 and / or the optical signal source 24 can be stored. This information can be used, for example, to interpret the control signal 58, for example, when it is received by the signal processor 36.

[0088] The information storage 72 can contain different predefined values for the operating point and / or the modulation level, which are associated with different values of the channel distortion and are stored therein. The optical wireless device can be configured to obtain and apply a value to be set for the operating point from the information storage 72 using the channel information, such as the control signal 58.

[0089] In other words, Fig. 9 a schematic block diagram of an optical wireless network 300‴ in which the control signal 58 is generated by a block 54 from a receiver chain of the optical wireless signal 14. Alternatively, the control signal can also be generated based on reception at the transceiver, i.e., along the opposite direction and / or for other signals, for example, reference signals or pilot signals.

[0090] Fig. 10a shows a schematic block diagram of a part of an optical-wireless device 110 according to an embodiment. This is configured, for example, to adjust the control of the optical signal source by changing the operating state of the electronic signal source and, for this purpose, receives, for example, the control signal 58, as is used, for example, in connection with the Fig. 4 , 5 , 6 , 7 or 10The optical wireless device 10 may include a control element 74, which may be used, for example, alternatively or in addition to the attenuation circuit 56 or 56' in the Fig. 4 or 5 can be used. A control for the driver circuit 42 can thus be derived from the control signal 58. In other words, Fig. 10a a schematic block diagram of an optical wireless device for adjusting a gain of the driver 42 in order to influence the control of the optical signal source.

[0091] Fig. 10b shows a schematic block diagram of another optical wireless device 110' according to an embodiment, which has a control element 76 for receiving the control signal 58. A control signal 78 obtained therefrom can be used by the optical wireless device 110 to control the optical signal source 24 and / or the electronic signal source, i.e., the signal processor 36 and / or the driver circuit 42.

[0092] In other words, Fig. 10b A schematic block diagram of an optical-wireless device for adjusting an operating point of the optical signal source 24. The emitter can be adjusted via a DC component of the amplifier 42 (signal 78a) and / or via a control signal to the emitter 24 itself, control signal 78b. The signals 78a and 78b can be identical in terms of amplitude, frequency, or the like, but can also differ from each other.

[0093] The behavior of the control devices 74 and / or 76 can be linear or nonlinear, but is preferably nonlinear. Optionally, the control devices 74 and / or 76 can be coupled to the information memory 72. This can define at which control signal 58 or which amplitude, frequency, or other property, which gain or property is set in the electronic signal source or the optical signal source. In other words, some properties of the embodiments described herein can also be described as: Control signal 58: has a known dependence on the channel attenuation and can therefore be used to infer the channel attenuation. It comes, for example, from the receiver (DC / AC component, before or after the receiver amplifier; or from the receiver DSP as PSD (Power Spectral Density); or as a data packet from the opposite transceiver), or from a distance measurement (e.g. beacon, or similar). Control element 74 (this is, for example, device 56 or 56'): defines the extent to which a change in the control signal 58 leads to a change in the gain; the transfer function is non-linear since the attempt is made to maximize the CNR by changing the modulation and to enable the greatest possible dynamic range of the control. Behavior: A look-up table can be stored which defines which gain is set for which control signal.Alternatively, the gain can be optimized iteratively: if the control signal changes by a certain value, for example, the gain is increased; if this has resulted in a deterioration in the CNR / data rate in the DSP, the opposite adjustment is made; this is repeated until no further improvement can be achieved. Alternatively or additionally: the control element 76 for changing the operating point current is controlled by the same control signal 58, but unlike the control element 74, it controls the operating point of the optical emitter. The signal 58 of the . Fig. 10b can be the identical signal as in Fig. 10a , for example, if elements 74 and 76 derive different actions from this signal (or they may be two individual signals, possibly adapted to elements 74 and 76, respectively). Adjusting the level in the DSP and / or the amplifier element, on the one hand, and adjusting the emitter's operating point in the DSP and / or by element 76, on the other, can be done jointly or independently of each other.

[0094] The functions in the elements 74 and / or 76 can be linear or, preferably and as in Fig. 10a and Fig. 10b shown, be nonlinear.

[0095] Fig. 11 shows again the curves 34 1 , 34 2 , 34 3 and 34 4 from Fig. 2a .

[0096] The Fig. 12a, 12b und 12c each show an example table, whereby the tables of the Fig. 12a bis 12c are linked to each other by their respective properties. The tables of the Fig. 12a und 12b matching distances between an optical wireless transmitter and corresponding receiver, where the table of Fig. 12a in the lines the different levels of the Fig. 2a and Fig. 11 shows. In the table of Fig. 12b For this purpose, a measured value is shown for the respective distance or the corresponding distance, as well as a control value amplified from this measured value, which can serve, for example, as a control signal 58, or as the basis thereof. In the Fig. 12c An example is shown of the control of the electronic signal source with the respective measured value from the table Fig. 12b can be associated, whereby the values can be divided into the different outputs of the table in the Fig. 12a are quantized. This means that Fig. 12b can be obtained for different distances, at which measurement value the respective data rate can be maximized, which is shown in the fields of the table of Fig. 12a described.

[0097] In other words, the table shows the Fig. 12a in connection with the exemplary control for Fig. 2a or Fig. 11 and the data rates shown therein, for which, according to Fig. 2a the envelope is created, an example measurement result of a value for a data rate DR in Mbps. Different optical emitter output levels were set for each distance between transmitter and receiver. The maximum values in a column can be understood as the selected output level for the respective distance associated with channel attenuation. As can be seen from the highlighted numbers, a level of 9.3 dBm results in the highest data rate for a distance of 0.15 meters, as well as for a distance of 0.4 meters. A level of 13.7 dBm delivers the best results at a distance of one meter, while a level of 17.2 dBm delivers the best data rate results for distances of 2 meters, 4 meters, and 6 meters. A level of 22 dBm is effective for longer distances of 8 meters and 10 meters.

[0098] The table of Fig. 12b shows a connection with the exemplary control from Fig. 2a or Fig. 11 and the data rates shown therein as well as values of a measured quantity, such as a received power of the Fig. 12a underlying optical-wireless signals. The different measured variables can be converted into a control variable, the step size of which can be adapted to the hardware of the transmitter and / or receiver. This is shown in the line "Control variable amplified from measured variable" and can, for example, form the basis for control signal 58. A resulting functional relationship between the measured variables and the control variable is, for example, nonlinear, just as it is between the control variable and the modulation and / or operating point.

[0099] The table from Fig. 12c shows a connection with the exemplary control from Fig. 2a or Fig. 11 and the data rates shown therein as well as a result of the correlation from the tables of the Fig. 12a und 12b . A value that, for example, represents a control variable from Fig. 12b can be adjusted with a level of Fig. 2a and / or Fig. 11 or the table from Fig. 12a be associated with a component, where signal 58 is, for example, a voltage level. Alternatively or additionally, the displayed value can be transmitted to a component as part of a code or a bit pattern as a quantity to be applied so that it changes its state.

[0100] The doctrine of Fig. 12a, 12b und 12c are equally valid for changing the operating state of the electronic data source, for adjusting the control of the optical signal source and for adjusting the operating point of the optical signal source.

[0101] Fig. 2a , Fig. 11 as well as the tables of Fig. 12a-c show: Measure DR or CNR over distance for different transmitter modulations, determine the maximum of all curves for each distance, and establish the relationship between control variable 58 and distance. Adjust the modulation or operating point according to control variable 58.

[0102] The control signal can, for example, be used as a basis for controlling the element 56, 56' from Fig. 4 and / or Fig. 5 and / or as a basis for setting the control element 74 from Fig. 10a be used.

[0103] Similarly, the respective maximum values of the data rate from Fig. 8 a control current can be determined which is to be set for a respective channel attenuation, for example by determining the distance between transmitter and receiver.

[0104] Fig. 13 shows a schematic flow diagram of a known method 1400, in which a result of the channel estimation 1420 leads to a changed bit rate of the data signal, the bit loading, if the carrier-to-noise ratio is changed (step 1410). The change is implemented in a step 1430 and leads to a transmission 1440 before a further check is performed in a further iteration. Here, t encodes the current time period or the current clock pulse, and t +1 the following time period or bar.

[0105] Fig. 14 shows a schematic flow diagram of a method 1500 according to an embodiment. In an iteration run, for example, the carrier-to-noise ratio changes, for example due to a change in the channel attenuation or the drive (step 1510). This is accompanied by a data rate at time t and a first error rate 1. As a result, for the same data rate at t, a second error rate 2 is present, which is greater than the error rate 1. A channel estimation 1520 can provide the CNR and cause a changed data rate for a time interval p+1. The method 1500 can include a decision as to whether a control signal 58 has changed between the previous run t and the current run t +1 has changed. If this is not the case, the signal can be transmitted (step 1540), which, apart from decision 1535, can lead to the conventional method 1400. However, if the control signal 58 has changed (path "yes" at decision 1535), a decision 1545 can determine whether the channel attenuation has increased. If the result is "yes," the transmitter modulation can be increased and / or the operating point of the optical signal source can be reduced (step 1550). If decision 1545 returns the result "no," the transmitter modulation can be reduced and / or the operating point of the optical signal source can be increased (step 1560).

[0106] In other words, Fig. 14 a schematic flow diagram of a method according to an embodiment in which it is checked whether a control signal indicating channel attenuation (indicated by the control signal 58) occurs between two transmission intervals t; t+1 has changed. If so, an evaluation is carried out to determine whether the channel attenuation has increased or not, i.e. whether it has decreased. If the attenuation has increased, the transmitter control can be increased and / or the operating point reduced, i.e., for example, by using a low operating point current. Otherwise, the transmitter control can be reduced and / or the operating point increased. This method can be implemented in such a way that, for example, the DSP 36 can implement an adjustment of the signal transmission depending on the results obtained, for example by adjusting the digital signal processing. For example, this can be done by changing the control at the analog-to-digital converter of the DSP itself, as described, and / or the DSP can adjust the operating point of the emitter.

[0107] Fig. 15 shows a schematic block diagram of a further method 1600 according to an embodiment. The steps 1510, 1520 and 1530 can be implemented in the same order as the method 1500. Alternatively, however, at the beginning of a new iteration loop, parallel to these steps, a decision 1635 can be executed corresponding to the decision 1535 as to whether the control signal 58 has changed between the previous time interval t and the current time interval t +1. If this is the case, it can be checked whether the channel attenuation has increased, which is discussed in decision 1645. Analogous to method 1500, steps 1550 and 1560 can be implemented depending on the response of decision 1645 and culminate in step 1670, which results in transmission with the adjusted transmitter gain.

[0108] Here, too, the transmitter control and / or the operating point are adjusted due to a change in control by the control signal 58.

[0109] In other words, Fig. 15 a schematic flow diagram of a method according to an embodiment, in which it is also checked whether a control signal indicating channel attenuation (indicated by the control signal 58) occurs between two transmission intervals t; t +1. If so, an evaluation is performed to determine whether the channel attenuation has increased or not, i.e., whether it has decreased. If the attenuation has increased, the transmitter control can be increased and / or the operating point reduced, for example, by using a lower operating point current. Otherwise, the transmitter control can be reduced and / or the operating point increased. This process can be independent of the DSP's bit loading. For example, it can be done by controlling the gain and / or the emitter operating point.

[0110] Embodiments described herein also describe optical wireless devices configured to adapt the optical wireless signal to perform adjustment of the drive level of the optical signal source and / or adjustment of the operating point of the optical signal source to vary the nonlinear distortions generated by the optical wireless device.

[0111] According to one embodiment, an amplitude and / or an effective value of an alternating component of the data signal can be adjusted to adjust the modulation, for example by a signal processor and / or a driver circuit or amplifier circuit.

[0112] According to one embodiment, an optical-wireless device can be configured to adjust the operating point of the optical signal source in order to adjust a DC component of the data signal and / or to adjust a current consumption of the optical emitter. An impressed operating point current or the impressing of the operating point current can be achieved, for example, using and adjusting a bias tee, a current source, and / or by changing the DC component of signal 22. The driver can make the change. According to one embodiment, the driver circuit 42 can be configured for such an adjustment.

[0113] According to one embodiment, an optical wireless device configured to adjust the modulation of the optical signal source can be designed to increase the signal-to-noise ratio of a subsequent optical wireless signal transmitted over the optical wireless channel in a subsequent transmission interval at the location of a receiver of the optical wireless signal by adjusting the modulation. By reducing the nonlinear distortions or increasing the signal power, the reception quality can thus be improved.

[0114] According to one embodiment, the electronic data source can be configured to increase the data rate for the subsequent wireless optical signal based on an increased signal-to-noise ratio. This means that, in anticipation of improved reception quality at the receiver, the bit loading can be adjusted accordingly using a DSP control.

[0115] According to one embodiment, an optical wireless device for adjusting the control of the optical signal source is configured and designed to transmit the optical wireless signal based on a first power value of the electronic signal.The optical wireless device can be designed to adapt the modulation of the optical signal source based on channel information such that it indicates an increased received power compared to a previous transmission interval and a reduced signal-to-noise ratio at the receiver of the optical signal, by reducing an alternating component of an electronic signal for a later optical wireless signal towards a second alternating component. The optical wireless device can additionally be designed such that it adapts the modulation of the optical signal source based on channel information such that it indicates a reduced received power compared to a previous transmission interval and a reduced signal-to-noise ratio at the receiver of the optical wireless signal, by increasing an alternating component of an electronic signal for a later optical wireless signal towards a second alternating component.This is indicated, for example, in step 1550 and the reduction of the alternating component in step 1560.

[0116] Embodiments described herein provide optical wireless devices configured to adjust the modulation of the optical signal source and configured to receive the channel information as a control signal 58 having a dependence on a channel attenuation and on the nonlinear distortion of the optical wireless channel, for example the CNR.

[0117] Optical wireless devices described herein may be configured such that the electronic signal source is configured to adapt a data rate of the data signal by means of bit loading based on a signal-to-noise ratio of optical signals transmitted and / or received in previous transmission intervals, as is described, for example, in connection with the Fig. 14 and 15 is explained.

[0118] Embodiments described herein further provide an optical wireless device configured as a transceiver and configured to transmit and receive optical wireless signals.

[0119] According to one embodiment, an optical wireless device can be configured to perform the adjustment of the control of the optical signal source partially or exclusively outside of a transmission interval in which an optical wireless signal is transmitted, for example, to avoid the nonlinearities caused thereby. Alternatively or additionally, the device can be configured to perform the adjustment of the operating point of the optical signal source partially or exclusively outside of a transmission interval in which an optical wireless signal is transmitted, again in this case to avoid the nonlinearities.

[0120] According to one embodiment, an optical wireless device can be designed to adjust the control of the optical signal source step by step, wherein a step size is adapted to the optical wireless channel such that non-linear distortions in the optical wireless channel occurring as a result of the change are negligible. This also enables adjustment during transmission, although the use of several smaller steps can result in a longer time requirement. Alternatively or additionally, the adjustment of the operating point of the optical signal source can be carried out step by step and a step size can be adapted to the optical wireless channel such that non-linear distortions in the optical wireless channel occurring as a result of the change are negligible. For this purpose, the step size can be, for example,be chosen so that the resulting noise due to non-linear distortion increases the noise at the receiver output by less than 20%, 15% or 10%, which means that this noise component is significantly smaller than the intrinsic receiver noise.

[0121] According to one embodiment, the optical wireless device is configured to transmit the optical wireless signal as an amplitude-modeled signal or as a multi-carrier modulated signal such as an orthogonal frequency division multiplexing (OFDM) signal.

[0122] According to one embodiment, the optical wireless device is configured to receive the channel information as an instruction to adjust the modulation or set the operating point, and to implement the instruction. For this purpose, control signal 58 can be used, for example.

[0123] Embodiments described herein further describe an optical wireless network comprising an optical wireless device according to one of the embodiments described herein and a receiver for receiving the optical wireless signal. This receiver can optionally be configured to output appropriate feedback for estimating the channel.

[0124] Fig. 16 shows a schematic flow diagram of a method 1700 according to an embodiment. A step 1710 comprises arranging a transmitter and a receiver for transmitting an optical wireless signal over an optical wireless channel. A step 1720 comprises repeatedly transmitting an optical wireless signal for different distances between the transmitter and the receiver, such that at each distance a plurality of transmission levels are used. With reference to the tables of Fig. 12a, 12b und 12c Thus, several of the output levels can be tested for different distances, see the table of Fig. 12a .

[0125] A step 1730 comprises determining at least one achievable data rate and / or a signal-to-noise ratio at the location of the receiver for each of the transmitted signals, see for example the measurement of the Fig. 12b and / or the data rate in the Fig. 12a .

[0126] A step 1740 comprises determining a control level to be selected for each of the distances using which a data rate or the signal-to-noise ratio is maximum, see the contents of the table of Fig. 12c .

[0127] A step 1750 comprises compiling the control to be selected for the different distances, which can be stored, for example, in the information memory 72.

[0128] Fig. 17 shows a schematic flow diagram of a method 1800 according to an embodiment. Compared to method 1700, it can be used to determine the setting of the operating points of the optical signal source. In a step 1810, a transmitter and a receiver are arranged for transmitting an optical wireless signal over an optical wireless channel. In a step 1820, an optical wireless signal is repeatedly transmitted for different distances between the transmitter and the receiver, such that a plurality of operating points are used for transmission at each distance.

[0129] A step 1830 includes determining at least one achievable data rate and / or signal-to-noise ratio at the location of the receiver for each of the transmitted signals.

[0130] In a step 1840, an operating point to be selected is determined for each of the distances using which a data rate or the signal-to-noise ratio is maximum.

[0131] In a step 1850, the working points to be selected for the different distances are compiled.

[0132] According to embodiments, the variations in the control can be carried out both for the adaptation of the operating states of the electronic signal source and for the adaptation of the operating points of the optical signal source, wherein only one of these two concepts is possible, depending on the later setting option in the optical-wireless device.

[0133] Embodiments have the positive side effect of preventing or reducing receiver overload, similar to an automatic gain control (AGC) on the receiver. Since the AC component of the transmitted signal is reduced with low channel attenuation, it does not lead to overload as quickly. If the DC component of the optical signal leads to receiver saturation, the invention described here does not lead to any improvement.

[0134] The control range can be a few dB, but also, as Fig. 2a shows, more than 10 dB. The actual usable range depends on the maximum modulation depth of the OFDM signal and the specific technical implementation of the control.

[0135] Embodiments are not limited to OFDM signals, but also relate to other types of signal modulation in which non-linear distortions can have a negative influence, for example, in amplitude modulation. Although some aspects have been described in connection with a device, it is understood that these aspects also represent a description of the corresponding method, so that a block or component of a device can also be understood as a corresponding method step or as a feature of a method step. Analogously, aspects that have been described in connection with or as a method step also represent a description of a corresponding block or detail or feature of a corresponding device.

[0136] Depending on specific implementation requirements, embodiments of the invention may be implemented in hardware or software. The implementation may be performed using a digital storage medium, such as a floppy disk, a DVD, a Blu-ray Disc, a CD, a ROM, a PROM, an EPROM, an EEPROM, or a FLASH memory, a hard disk, or other magnetic or optical storage device storing electronically readable control signals that can interact or cooperate with a programmable computer system to perform the respective method. Therefore, the digital storage medium may be computer-readable.Some embodiments according to the invention thus comprise a data carrier having electronically readable control signals capable of interacting with a programmable computer system such that one of the methods described herein is carried out.

[0137] In general, embodiments of the present invention can be implemented as a computer program product with a program code, wherein the program code is effective to perform one of the methods when the computer program product is run on a computer. The program code can also be stored, for example, on a machine-readable medium.

[0138] Other embodiments include the computer program for performing one of the methods described herein, wherein the computer program is stored on a machine-readable carrier.

[0139] In other words, one embodiment of the method according to the invention is thus a computer program comprising program code for performing one of the methods described herein when the computer program is run on a computer. Another embodiment of the method according to the invention is thus a data carrier (or a digital storage medium or a computer-readable medium) on which the computer program for performing one of the methods described herein is recorded.

[0140] A further embodiment of the method according to the invention is thus a data stream or a sequence of signals that represents the computer program for carrying out one of the methods described herein. The data stream or the sequence of signals can be configured, for example, to be transferred via a data communication connection, for example, via the Internet.

[0141] A further embodiment comprises a processing device, for example a computer or a programmable logic device, which is configured or adapted to carry out one of the methods described herein.

[0142] A further embodiment comprises a computer on which the computer program for performing one of the methods described herein is installed.

[0143] In some embodiments, a programmable logic device (e.g., a field-programmable gate array, an FPGA) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field-programmable gate array may interact with a microprocessor to perform any of the methods described herein. In general, in some embodiments, the methods are performed by any hardware device. This may be general-purpose hardware such as a computer processor (CPU) or method-specific hardware such as an ASIC.

[0144] The above-described embodiments are merely illustrative of the principles of the present invention. It is understood that modifications and variations of the arrangements and details described herein will be apparent to others skilled in the art. Therefore, it is intended that the invention be limited only by the scope of the following claims and not by the specific details presented in the description and explanation of the embodiments herein. Literatur

[0145] [a] Elgala, H. ; Mesleh, R. ; Haas, H.: A study of LED nonlinearity effects on optical wireless transmission using OFDM. In: 2009 IFIP International Conference on Wireless and Optical Communications Networks, IEEE, 2009, S. 1-5 [b] Huang, Xingxing ; Wang, Zhixin ; Shi, Jianyang ; Wang, Yiguang ; Chi, Nan: 1.6 Gbit / s phosphorescent white LED based VLC transmission using a cascaded preequalization circuit and a differential outputs PIN receiver. In: Opt. Express 23 (2015), Aug., Nr. 17, S. 22034-22042. http: / / dx.doi.org / 10.1364 / OE.23.022034 [c] Chun, H. ; Rajbhandari, S. ; Tsonev, D. ; Faulkner, G. ; Haas, H. ; O'Brien, D.: Visible light communication using laser diode based remote phosphor technique. In: 2015 IEEE International Conference on Communication Workshop (ICCW), IEEE, 2015, S. 1392-1397. http: / / dx.doi.org / 10.1109 / ICCW.2015.7247373 [d] Ying, K. ; Yu, Z. ; Baxley, R. J. ; Qian, H. ; Chang, G. ; Zhou, G. T.: Nonlinear distortion mitigation in visible light communications.In: IEEE Wireless Communications 22 (2015), Nr. 2, S. 36-45 [e] Khalid, A. M. ; Cossu, G. ; Corsini, R. ; Choudhury, P. ; Ciaramella, E.: 1-Gb / s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation. In: IEEE Photonics Journal 4 (2012), Nr. 5, S. 1465-1473 [f] Hong, Y. ; Wu, T. ; Chean, L.: On the Performance of Adaptive MIMO-OFDM Indoor Visible Light Communications. In: IEEE Photonics Technology Letters 28 (2016), Nr. 8, S. 907-910. http: / / dx.doi.org / 10.1109 / LPT.2016.2517192.

Claims

1. An optical wireless apparatus implemented for transmitting an optical wireless signal (14) via an optical wireless channel (16), comprising: an electronic signal source (18) configured to provide a data signal (22); and an optical signal source (24) configured to convert the data signal (22) into the optical wireless signal (14) and to emit the same; wherein the optical wireless apparatus is configured to obtain channel information (26) comprising information associated with a non-linear channel distortion of the optical wireless signal (14) in the optical wireless channel (16); and characterized in that the optical wireless apparatus is configured to perform adaptation of a modulation of the optical signal source (24), describing an amplitude variation by a mean value, by changing an operating state of the electronic signal source (18) by changing a modulation of a digital signal processor, DSP, and / or by adapting an amplification of a driver of the electronic signal source (18) for adapting the non-linear channel distortion and / or to perform adaptation of an operating point of the optical signal source (24) for adapting the non-linear channel distortion; wherein this adaptation of the non-linear channel distortion comprises compensation or reduction of the non-linear channel distortion.

2. The optical wireless apparatus according to claim 1, wherein the adaptation of the non-linear channel distortion depends on the current transmitter-receiver arrangement or the current channel attenuation.

3. The optical wireless apparatus according to claim 1 or 2, wherein the channel information (26) is based on a noise ratio of the optical wireless signal (14) and on a signal power of the optical wireless signal (14) at a receiver (12).

4. The optical wireless apparatus according to any of the preceding claims, wherein the optical wireless signal (14) is a first optical wireless signal that is emitted in a first transmission interval, wherein the optical wireless apparatus is configured to determine, for transmitting a second optical wireless signal in a later second transmission interval, that a channel attenuation has increased compared to the first transmission interval, and to increase the modulation and / or to reduce the operating point; and / or wherein the optical wireless apparatus is configured to determine, for transmitting a second optical wireless signal in a later second transmission interval, that a channel attenuation is reduced compared to the first transmission interval, and to reduce the modulation and / or to increase the operating point.

5. The optical wireless apparatus according to any of the preceding claims, wherein the channel information (26) is based at least partly on a feedback of a receiver (12) of the optical wireless signal (14) with respect to a signal power of the optical wireless signal (14) at the receiver (12) and a noise ratio of the optical wireless signal (14) and the receiver (12).

6. The optical wireless apparatus according to any of the preceding claims, configured to transmit the optical wireless signal (14) as a first optical wireless signal via the optical wireless channel (16) and is implemented to receive a second optical wireless signal (14) via the optical wireless channel (16), and is configured to perform channel estimation of the optical wireless channel (16) based on the second optical wireless signal to determine a noise ratio of the second signal.

7. The optical wireless apparatus according to any of the preceding claims, configured to adapt the modulation to adapt an amplitude and / or an effective value of an alternating component of the data signal (22); or configured to adapt the operating point to adapt a direct component of the data signal (22).

8. The optical wireless apparatus according to any of the preceding claims, implemented for the adaptation of the modulation of the optical signal source (24) and configured to increase, by means of adapting the modulation, a noise ratio of a later optical wireless signal that is transmitted in a subsequent transmission interval via the optical wireless channel (16) at the location of a receiver (12) of the optical wireless signal (14); and / or wherein the electronic signal source (18) is configured to adapt a data rate of the data signal (22) based on a noise ratio in the previous transmission interval of transmitted and / or received optical signals.

9. The optical wireless apparatus according to any of the preceding claims, implemented for the adaptation of the modulation of the optical signal source (24) and configured to emit the optical wireless signal based on a first power value of the electronic signal, wherein the optical wireless apparatus is configured to adapt the modulation of the optical signal source (24) based on channel information (26), such that the same indicates an increased received power and a reduced noise ratio at the receiver (12) of the optical wireless signal compared to a previous transmission interval, by reducing an alternating component of an electronic signal for a later optical wireless signal towards a second alternating component; and / or wherein the optical wireless apparatus is configured to adapt the modulation of the optical signal source (24) based on channel information (26), such that the same indicates a reduced received power and a reduced noise ratio at the receiver (12) of the optical wireless signal (14) compared to a previous transmission interval, by increasing an alternating component of an electronic signal for a later optical wireless signal towards a second alternating component.

10. The optical wireless apparatus according to any of the preceding claims, implemented for the adaptation of the modulation of the optical signal source (24) and configured to obtain the channel information (26) as a control signal (58) that shows a dependence on a channel attenuation and on the non-linear channel distortion of the optical wireless channel (16).

11. The optical wireless apparatus according to any of the preceding claims implemented for the adaptation of the modulation of the optical signal source (24), wherein the channel information (26) is based on an alternating component of an optical wireless signal (14) received by a receiver (12) of the optical wireless channel (16) and is associated with an adaptation of a gain factor of the electronic signal source (18).

12. The optical wireless apparatus according to any of the preceding claims, implemented for the adaptation of the modulation of the optical signal source (24) and comprising an information memory (72) in which different predefined values for the modulation that are associated with different values of the channel distortion are stored, and configured to obtain and apply a value to be adjusted for the modulation from the information memory (72) by using the channel information (26); and / or implemented for the adaptation of the operating point of the optical signal source (24), and comprising an information memory (72) in which different predefined values for the operating point that are associated with different values of the channel distortion are stored, and configured to obtain and apply a value to be adjusted for the operating point from the information memory (72) by using the channel information (26).

13. The optical wireless apparatus according to any of the preceding claims, implemented for the adaptation of the modulation of the optical signal source (24), wherein the optical wireless apparatus is configured to obtain, by the adaptation, a noise component of the non-linear channel distortion of an overall noise at the location of the receiver (12) in an order of an internal receiver noise (un Rx).

14. The optical wireless apparatus according to any of the preceding claims, implemented for adapting the operating point, wherein the optical wireless apparatus is configured to reduce the operating point in a non-dominant or less relevant component of the non-linear channel distortions of an overall noise and low noise ratio of the optical wireless signal (14) at a receiver (12) of the same and a high data rate in the optical wireless signal.

15. The optical wireless apparatus according to any of the preceding claims, configured to perform the adaptation of the modulation of the optical signal source (24) partly or exclusively outside a transmission interval in which an optical wireless signal is transmitted; and / or to perform the adaptation of the operating point of the optical signal source (24) partly or exclusively outside a transmission interval in which an optical wireless signal is transmitted; or configured to perform the adaptation of the modulation of the optical signal source (24) gradually, wherein a step size is adapted to the optical wireless channel (16) such that non-linear channel distortions in the optical wireless channel (16) occurring due to the change can be neglected; and / or to perform the adaptation of the operating point of the optical signal source (24) gradually, wherein a step size is adapted to the optical wireless channel (16) such that non-linear channel distortions in the optical wireless channel (16) occurring due to the change can be neglected.