WO2021233690A1

WO2021233690A1 - Power density adjustment for multicarrier transmissions

Info

Publication number: WO2021233690A1
Application number: PCT/EP2021/061994
Authority: WO
Inventors: Christian Jordan; Andreas Felix Alfred BLUSCHKE; Philippe Henri André SIMON
Original assignee: Signify Holding B.V.
Priority date: 2020-05-18
Filing date: 2021-05-06
Publication date: 2021-11-25

Abstract

This invention relates to power density shaping for an adaptive or non-adaptive multicarrier modulation system (e.g. OFDM), wherein the multicarrier modulation system comprises a digital signal generating component (21) that generates a multicarrier modulation signal having a fixed power spectral density in a signal processing chain. This signal is supplied to an analog domain filter arrangement (22) for setting different frequency ranges, that can be adaptively configured based on measured channel quality parameters.

Description

Power density adjustment for multicarrier transmissions

FIELD OF THE INVENTION

The invention relates to the field of multicarrier communication in wireless networks, such as - but not limited to - optical communication in e.g. LiFi networks, for use in various different applications for home, office, retail, hospitality and industry.

BACKGROUND OF THE INVENTION

Wireless optical networks, such as LiFi networks (named like Wi-Fi networks), enable mobile user devices (called end points (EP) in the following) like laptops, tablets, smartphones or the like to connect wirelessly to the internet. Wi-Fi achieves this using radio frequencies, but LiFi achieves this using the light spectrum which can enable unprecedented data transfer speed and bandwidth. Furthermore, it can be used in areas susceptible to electromagnetic interference. It’s important to consider that wireless data is required for more than just our traditional connected devices - today televisions, speakers, headphones, printer’s, virtual reality (VR) goggles and even refrigerators use wireless data to connect and perform essential communications. Radio frequency (RF) technology like Wi-Fi is running out of spectrum to support this digital revolution and LiFi can help power the next generation of immersive connectivity.

Based on multicarrier modulation (such as orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT)), the information embedded in the light signal can be detected using any suitable light sensor. This can be a dedicated photocell (photo detector such as PIN photo diode), an array of photocells possibly with a lens, reflector, diffuser of phosphor converter, or a camera comprising an array of photocells (pixels) and a lens for forming an image on the array. E.g., the light sensor may be a dedicated photocell included in a dongle which plugs into the end point, or the sensor may be a general purpose (visible or infrared light) camera of the end point or an infrared detector initially designed for instance for 3D face recognition. Either way this may enable an application running on the end point to receive data via the light.

The multicarrier-modulated communication signal can be embedded in a visible or invisible optical signal emitted by an illumination source of an access device, such as an everyday luminaire, e.g. room lighting or outdoor lighting, thus allowing use of the illumination from the luminaires as a carrier of information. The light thus comprises both a visible illumination contribution for illuminating a target environment such as a room (typically the primary purpose of the light), and an embedded signal for providing information into the environment (typically considered a secondary function of the light). In such cases, the modulation may typically be performed at a high enough frequency to be beyond human perception, or at least such that any visible temporal light artefacts (e.g. flicker and/or strobe artefacts) are weak enough and at sufficiently high frequencies not to be noticeable or at least to be tolerable to humans. Thus, the embedded signal does not affect the primary illumination function, i.e., so the user only perceives the overall illumination and not the effect of the data being modulated into that illumination.

In multicarrier modulation (MCM) systems (like OFDM systems) the bandwidth of the digital transmission signal may be decomposed into a set of orthogonal narrowband subcarriers of equal bandwidth. Each subcarrier transmits a number of bits using a portion of the total available power. A delay- and error-free feedback channel is assumed to exist between the transmitter and receiver for reporting channel state information.

The performance of LiFi-systems based on adaptive MCM in terms of data rate, coverage and range may be limited by space, cost, power consumption (e.g. number of light emitting diodes (LEDs), type and number of amplifiers etc.), quality and characteristics of the optical system (e.g. transmission, field of view (FoV) etc.) and the choice of the optoelectronic components including their and other physical limitations (e.g. frequency response, noise contribution, modulation index vs. nonlinear distortion etc.). Especially, devices which are integrated in user equipment or small handhelds suffer from that many constraints.

United States patent US 9,853,728 B2 discloses a bit allocation method is used in an optical transmission system that transmits multicarrier signals of different wavelengths in wavelength division multiplexing. The method includes measuring transmission characteristics of the subcarriers included in the multicarrier signals and determining a number of bits to be allocated to each of the subcarriers.

The paper ‘Theoretical and experimental optimization of DMT -based visible light communication under lighting constraints”, by Ahmad Jabban et al, published in EURASIP Journal on Wireless Communication and Networking (2020), discloses various optimization techniques for a combined illumination and communication system, that utilizes an equal power allocation scheme. In "Efficiency of Power Loading Strategied for Visible Light Communication", published at 2018 IEEE Globecom Workshops, Shokoufeh Mardani, et al, discuss theoretical capacity of an OFDM transmitter using LEDs. The paper discusses performance of waterfilling and uniform power loading strategies. The paper concludes that for LEDs, waterfilling does not yield any significantly larger rate than the simpler uniform loading.

LiFi devices are usually designed under the mentioned constraints with simple fixed hardware (optical, electrical and optoelectrical), having a specific target data rate at a specific target distance but must also perform well for higher and lower distances. Appropriately, designers choose/design carefully a set of components (photodiode, transimpedance amplifier (TIA), LED, optics etc.), circuits and settings (modulation index, LED DC-Bias, pre-equalization, FoV etc.), trying to get best performance for all circumstances, making various trade-offs because of the impossibility of this task. E.g., by simply raising the FoV one may improve coverage, but this will also lead to an overall loss in data rate because the transmitted power is spread over a wider area. Compensating that by raising the transmitted power will lead to a higher power consumption or nonlinear distortion and so on. Unfortunately, with tough design constraints and a simple fixed hardware as mentioned before, it is not possible to perform best under all possible variations of the optical channel when the distance and/or the alignment between communicating devices changes during runtime.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multicarrier communication system with enhanced performance under various channel conditions.

This object is achieved by an apparatus as claimed in claim 1, a system as claimed in claim 11, a method as claimed in claim 14, and a computer program product as claimed in claim 15.

Accordingly, transmission power of a multicarrier signal is controlled by modulating the multicarrier signal, determining at least one channel quality parameter of the transmission path of the multicarrier signal and dynamically adjusting the PSD of the multicarrier signal by attenuating or amplifying portions of the total bandwidth of the multicarrier signal in response to the determined at least one channel parameter to improve channel efficiency of the multicarrier signal at a given optical transmission power. Preferably, the optical communication signal consists of a DC bias component and the multicarrier signal. The average current through the optical emitter (e.g. LED) is almost proportional to the optical power and determines the power consumption of the optical transmission system. The arithmetic mean of the multicarrier signal can be zero and may therefore add no power consumption. If the multicarrier signal is adapted to the channel to optimize the PSD in accordance with the channel characteristic, the data rate per power (e.g. Mbps/W) can be increased. As a result, channel efficiency can be improved by modifying the multicarrier signal to achieve higher data rates (i.e. higher data throughput) at a given optical transmit power.

Thus provided is an optical wireless transmission apparatus for controlling transmission power of a multicarrier signal, the apparatus comprising: a digital signal processor configured to modulate the multi carrier signal having a fixed power spectral density over a bandwidth of the multicarrier signal, with or without pre-equalization; an optical front end comprising an optical emitter for emitting an optical output signal; a controller configured to determine at least one channel quality parameter of the transmission path of the multicarrier signal and determine at least one filter configuration or characteristic of an analog domain filter system based on the at least one channel quality parameter, and to control the filter system to set the determined at least one filter configuration or characteristic; and the analog domain filter system configured to shape a power spectral density of the multicarrier signal by amplifying or attenuating portions of the total bandwidth of the multicarrier signal in response to the determined at least one filter configuration or characteristic; and an output for outputing the power-density adjusted multi carrier signal to an optical frontend.

Preferably, improving channel efficiency comprises increasing transmit power in regions where the signal to noise ratio is above the minimum signal to noise ratio for bit loading for the modulation used.

The proposed solution can be implemented in any multicarrier transmission system with little development and/or modification effort and can handle fast channel changes because it does not require iterative control.

According to a first option, a bit loading technique may be applied by using different modulation schemes involving different numbers of bits for different sub-carriers of the multicarrier signal based on the determined at least one channel quality parameter. Thereby, different levels of attenuation faced by different subcarriers of the multicarrier signal can be compensated. Preferably the digital signal processor is configured to apply the bit loading technique.

According to a second option which may be combined with the first option, the power spectral density of the multicarrier signal may be adjusted by amplifying and/or attenuating portions of a total signal bandwidth of the multicarrier signal when the at least one channel quality parameter indicates that the channel quality changes. Thereby, the distribution of the power across the subcarriers of the multicarrier signal can be adapted to the channel situation to prevent waste of power and/or spread power to additionally available bandwidth portions.

According to a third option which may be combined with the first or second option, a control signal indicating the at least one channel quality parameter may be received by the controller. Thus, the proposed dynamic PSD adjustment can be achieved by simply inserting a controllable filter system into the processing path of the multicarrier signal.

According to a fourth option which may be combined with any one of the first to third options, the filter system may be configured to reduce the signal power of the multicarrier signal for high frequencies and to increase signal power of the multicarrier signal for the remaining signal bandwidth of the multicarrier signal when the at least one channel quality parameter indicates a low channel quality, and to reduce the signal power of the multicarrier signal for low frequencies and increase the signal power of the multicarrier signal for high frequencies when the at least one channel quality parameter indicates a high channel quality. These measures ensure that the total signal power available for the subcarriers of the multicarrier signal is more effectively spread over the total signal bandwidth of the multicarrier signal.

According to a fifth option which may be combined with any one of the first to fourth options, variable or fixed gain stages may be controlled for achieving pre- and/or post amplification. Thereby, filter circuits of the filter system can be kept simple and do not need to amplify the multi carrier signal during the filtering process.

According to a sixth option which may be combined with any one of the first to fifth options, a pre-equalization or pre-emphasis may be applied to the multicarrier signal. Thereby, the transmission performance of the multi-carrier signal can be improved e.g. through compensation of the transfer function of the transmission system.

According to a seventh option which may be combined with any one of the first to sixth options, one of a plurality of filter circuits of a filter bank may be selected according to a value of the control signal via a demultiplexer, and a power-density adjusted multicarrier signal at the output of the selected one of the filter circuits may be guided to an optical frontend via a multiplexer. With such an implementation, the proposed PSD adjustment can be achieved by simply inserting a filter bank with selectable filter circuits between a signal generation stage and the optical frontend of the transmission system.

Preferably the filter system comprises the plurality of filter circuits and the filter is selected according to the value of the determined at least one filter configuration or characteristic

According to an eighth option which may be combined with any one of the first to seventh options, the power spectral density of the multicarrier signal may be adjusted by controlling at least one of an amplification and a bandwidth of an operational amplifier of an active filter circuit. Thus, a simple implementation with an adjustable active filter circuit can be provided. The adjustment of the power spectral density is performed by the analog domain filter system.

According to a ninth option which may be combined with any one of the first to eighth options, the at least one channel quality parameter may be derived from an amplitude response, a measured noise, a measured received signal strength, a signal-to-noise ratio, a direct current attenuation, a data rate or another value related to a transmission quality of a transmission channel of the multicarrier signal. Thereby, the PSD adjustment can be implemented in a wide variety of transmission systems where one of the above parameters is available at the transmitter end.

According to a second aspect, which may be combined with the first aspect or any of the above first to seventh options, a multicarrier signal transmission system comprises at least one a transmitter device comprising the apparatus of the first aspect and a receiver device, wherein the receiver device is configured to transmits a feedback information indicating the at least one channel quality parameter to the transmitter device.

According to a first option of the second aspect, which may be combined with the first aspect or any of the above first to seventh options, the transmitter device may be configured to transmit a control information that indicates at least one filter parameter or characteristic selected for power spectral density to the receiver device to allow control of a receiver bandwidth based on the selected at least one filter parameter or characteristic.

According to a second option of the second aspect, which may be combined with the first aspect or any of the above first to seventh options, the multicarrier transmission system may be an optical communication system, in particular a LiFi network, wherein the multicarrier signal may be an orthogonal frequency division multiplexing signal or a discrete multi-tone signal, and wherein the multicarrier signal may be embedded in a light signal emitted by an illumination source of a room or outdoor lighting device. The light signal in which the multicarrier signal is embedded may be a visible light signal or may be an infrared signal. In the latter case the lighting devices comprises separate light emitters for the illumination light and for the infrared light. As a result, the lighting and communication functions may be fully decoupled. Furthermore, when no illumination is required, for example during the day, usage of the infrared light emitters will enable communication without having to switch on the illumination light.

According to third aspect, a method of controlling transmission power of a multicarrier signal is provided, wherein the method comprises: modulating the multicarrier signal; determining a channel quality parameter of a transmission path of the multicarrier signal; and shaping a power spectral density of the multi carrier signal by attenuating or amplifying portions of the total bandwidth of the multicarrier signal in response to the determined at least one channel quality parameter to improve channel efficiency of the multicarrier signal.

Preferably, the method of controlling optical wireless transmission power of a multicarrier signal comprises: modulating the multicarrier signal having a fixed power spectral density over a bandwidth of the multicarrier signal, with or without pre-equalization; determining a channel quality parameter of a transmission path of the multicarrier signal; determining at least one filter configuration or characteristic of an analog domain filter system based on the at least one channel quality parameter; controlling the filter system to set the determined at least one filter configuration or characteristic; and using the analog domain filter system for shaping a power spectral density of the multicarrier signal by amplifying or attenuating portions of the total bandwidth of the multicarrier signal in response to the at least one filter configuration or characteristic; and outputting the power-density adjusted multicarrier signal to an optical frontend.

According to a fourth aspect, a computer program product may be provided, which comprises code means for producing the steps of the above methods of the third aspect when run on a computer device or an optical wireless transmission apparatus as claimed. It is noted that the above apparatuses may be implemented based on discrete hardware circuitries with discrete hardware components, integrated chips, or arrangements of chip modules, or based on signal processing devices or chips controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet.

It shall be understood that the apparatus of claim 1, the system of claim 11, the method of claim 14, and the computer program product of claim 15 may have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 shows schematically a block diagram of a LiFi network in which various embodiments can be implemented;

Fig. 2 shows schematically a block diagram of a first example of the power density adjustment system according to various embodiments;

Fig. 3 shows schematically a block diagram of a second example of the power density adjustment system according to various embodiments;

Fig. 4 shows schematically a block diagram of a power density adjustment system according to various embodiments;

Figs. 5A and 5B show schematically exemplary circuit diagrams of active filters with operational amplifier that can be used in various embodiments;

Fig. 6 shows a flow diagram of a power density adjustment procedure according to various embodiments;

Fig. 7 shows a frequency diagram with different power spectral density characteristics and a diagram with characteristics of data rate vs. path loss for different PSD settings;

Fig. 8 shows a frequency diagram indicating quality gain achieved with PSD adjustment at a high path loss; and Fig. 9 shows a frequency diagram indicating quality gain achieved with PSD adjustment at a low path loss.

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the present invention are now described based on an optical illumination and communication system (i.e. LiFi system) with MCM-based transmission.

Fig. 1 shows schematically a block diagram of a LiFi network in which various embodiments can be implemented.

It is noted that - throughout the present disclosure - the structure and/or function of blocks with identical reference numbers that have been described before are not described again, unless an additional specific functionality is involved. Moreover, only those structural elements and functions are shown, which are useful to understand the embodiments. Other structural elements and functions are omitted for brevity reasons.

The LiFi network comprises multiple access-points (APs) API to AP3 12, e.g. luminaires of a lighting system, connected via a switch (e.g. an Ethernet switch) 14, whereby each AP 12 controls one, or multiple transceivers (TRX) 11 (i.e. combined transmitters (optical emitters) and receivers (light sensors)) for optical communication towards end points (EP) EP1 to EP4 10, e.g., mobile user devices. Respective light beams generated by the TRXs 11 and defining coverage areas on the plane(s) of the EPs 10 are indicated by the dashed trapezoids in Fig. 1.

The luminaires can be any type of lighting unit or lighting fixture which comprises one or more light sources (including visible or non-visible (infrared (IR) or ultraviolet (UV)) light sources) for illumination and/or communication purposes and optionally other internal and/or external parts necessary for proper operation of the lighting, e.g., to distribute the light, to position and protect the light sources and ballast (where applicable), and to connect the luminaires to a power supply. Luminaires can be of the traditional type, such as a recessed or surface-mounted incandescent, fluorescent or other electric-discharge luminaires. Luminaires can also be of the non-traditional type, such as fiber optics with the light source at one location and the fiber core or “light pipe” at another.

An AP 12 may apply a time-slot schedule for communicating with EP(s) 10 in its coverage area. Where the coverage areas of the TRXs 11 overlap (as shown for EP1 in Fig. 1), coordination of APs 12 is needed if the related TRXs 11 belong to different APs 12. A LiFi controller 13 configured to manage the LiFi network is connected to the switch 14 and can provide such coordination for supporting interference handling and handover when one of the EPs 10 moves into and out of overlapping coverage areas of the APs 12. The controller 13 is connected via the switch 14 to the APs 12. The switch 14 may be connected to a synchronization server 16 for synchronization management and to a router 15 for connecting to a backplane or backhaul network (e.g. Ethernet) 100.

Since luminaires are used in the LiFi network as light transmitters, intensity- modulated direct-detected modulation techniques are used. Although single carrier modulation techniques are straightforward to be used in LiFi networks, computationally complex equalization processes are required in frequency selective LiFi channels. On the other hand, MCM techniques offer a viable solution for LiFi networks in terms of power, spectral and computational efficiency. In particular, OFDM based modulation techniques offer a practical solution for LiFi networks, especially when direct current (DC) wander, and adaptive bit and power loading techniques are considered. As an example, an OFDM modulator may be implemented by an inverse discrete Fourier transform block, which can be efficiently realized using the inverse fast Fourier transform (IFFT), followed by a digital -to- analogue converter (DAC). As a result, the OFDM generated signal is complex and bipolar by nature.

OFDM or DMT based (MCM-) systems for wired and RF communication normally use a fixed power spectral density (PSD) because of (national) regulations. In LiFi, the PSD of the channel can be better adjusted, which can lead to an increase of the data rate, coverage and/or range.

Different sub-carriers within a transmitted multicarrier (e.g. OFDM, etc.) signal may experience different levels of attenuation within the wireless channel. To compensate for such effects, bit loading techniques may be implemented within the multicarrier system. "Bit loading" refers to techniques that use different modulation schemes (involving different numbers of bits) for different sub-carriers based on corresponding channel information (e.g., channel gains). Thus, a sub-carrier having a higher channel gain may use a higher order modulation scheme to carry more bits (e.g., quadrature amplitude modulation with e.g. 64 symbols (64-QAM)) while a sub-carrier having a lower channel gain may use a lower order modulation scheme to carry less bits (e.g., binary phase shift keying (BPSK)). In adaptive schemes, the power allocation and bit loading settings may continually change in a system based upon changing channel conditions. To implement adaptive power allocation and bit loading within a multicarrier system, instantaneous channel related feedback information may continuously be delivered from a receiving device to a transmitting device.

However, in cases where channel attenuation is higher or lower than a typical attenuation for which the LiFi system is designed, power may be wasted in MCM systems with fixed PSD transmit schemes, as explained later with reference to Figs. 8 and 9. On one hand, if in the MCM system a carrier or a carrier group reaches its maximum bit loading (especially when the carrier frequency and the channel attenuation is low), any additional transmission power is wasted. On the other hand, if in the MCM system a carrier or a carrier group cannot reach a minimum bit loading (especially when the carrier frequency and the channel attenuation is high) any used transmission power is wasted.

Furthermore, adjusting the PSD automatically according to the channel quality during runtime in MCM-Systems with adaptive bit loading is called “power loading”. This approach may be implemented in the digital domain of a digital signal processor (DSP), e.g., with some special algorithms. However, implementing the digital approach in standardized MCM-LiFi systems with adaptive bit loading can lead to an impractical effort. So called “Power Loading” for MCM systems may perform well, but complex algorithms must be implemented in the signal processing chain. Iterative algorithms in software may perform bad, when the channel attenuation changes fast. Moreover, in an off-the-shelf LiFi DSP, it may often not be possible to implement such algorithms.

According to various embodiments, a PSD adjustment system is proposed, that can be implemented in hardware and in the analog domain.

Fig. 2 shows schematically a block diagram of a power density adjustment system according to various embodiments. A DSP 21 or another signal generator generates a digital multichannel signal and supplies it to an adjustable filter system (FS) 22. Furthermore, variable or fixed gain stages (not shown) for pre- and/or post-amplification may be provided. The filter system 22 may comprise a selector to select pre-defmed analog filter or switches to adjust filter values of passive or active filters. The filtered multichannel signal is supplied to an optical frontend (OFE) 24 for optical transmission via a transmission beam of at least one luminaire. Additionally, a control system (CTRL) 23 is configured to receive a control signal 200 comprising at least one measured channel quality parameter and to determine a filter configuration(s) or characteristic(s) of the filter system 22. Based on the determination result, the control system 23 applies control signal(s) to the filter system 22 to set the determined filter configuration(s) or characteristic(s). According to various embodiments, the PSD of the transmitted multicarrier signal is adjusted e.g. by the filter system 22 in runtime, discrete, digital or analog after the generation of the signal at the DSP 21 in dependence on the at least one measured channel quality parameter (e.g. channel attenuation) to improve data rate, coverage and range. In an example, the general set of chosen hardware of the multicarrier signal transmission system may otherwise remain the same.

The PSD adjustment may be achieved by amplifying and/or attenuating portions of the total signal bandwidth of the multicarrier signal when the measured channel quality parameter(s) indicate that the channel quality (e.g. channel attenuation) changes. Thereby, at least two different PSDs can be achieved by filtering or gain adjustment or attenuation or a combination of these.

As a further option, pre-equalization may be added point by point if necessary. In an example, in case of a measured or otherwise determined (e.g. though feedback from the receiver) high channel attenuation, the signal bandwidth is attenuated for high frequencies and the freed power may be transferred to the remaining signal bandwidth thus increasing the PSD, e.g., to achieve a minimum carrier modulation. In the opposite case when the determined channel attenuation is low, power may be taken from the signal bandwidth of low frequencies and transferred to the high frequency signal bandwidth thus increasing the number of data carrying carriers. This dynamic system behavior optimizes the signal-to-noise ratio (SNR) when the channel quality (e.g. attenuation) is either becoming very high or low.

A multicarrier transmission system (e.g. a LiFi system) may therefore reach higher transmission distances until the communication stops due to low transmission quality and/or can be used for higher transmission data rates when the channel quality is low. As a result, previously wasted power can be minimized and/or transformed into better performance.

According to various embodiments, the filter system 22 may consist of a special active filter block which may simply be added between conventional system blocks, such as a signal processor unit (e.g. DSP 21) and a transceiver unit (e.g. optical frontend 24). Therefore, the proposed PSD adjustment system can be configured in a manner so that already designed multicarrier transmission system parts do not have to be changed. The filter system 22 may controlled by control information derived from quality parameters (e.g. channel attenuation or a sufficient proportional value) which may be received e.g. from the signal processor unit or fed back from the receiver side (e.g. EP 10 in Fig. 1). As this type of control information is usually available at conventional signal processor units with adaptive MCM, it is usually easy to obtain or derive the desired control values for the filter system 22. Many variants for implementation exist and will be shown in the following exemplary embodiments.

Fig. 3 shows schematically a block diagram of a first example of the power density adjustment system according to various embodiments.

In the exemplary technical implementation of the PSD adjustment system of Fig. 3, signals coming from the DSP 21 are amplified by a pre-amplifier 220 with fixed or variable amplification, analog or digitally controlled by the control system 23. The control system 23 is further configured to choose with or without hysteresis one of a plurality of filter circuits 222-0 to 222-3 of a filter bank according to a value of the control signal 200 via a demultiplexer (DE-MUX) 221. The filter circuits 222-0 to 222-3 may be isolated electrically and/or electromagnetically from each other to prevent mutual interference.

It is however noted that more or less filter circuits may be provided in the filter bank depending on the desired bandwidth granularity of the PSD adjustment.

The value of the control signal 200 may be derived from an amplitude response, a measured noise, a measured received signal strength, a SNR, a DC-attenuation, a data rate or other values which are related to the transmission quality (e.g. attenuation) of the current transmission channel(s). The control signal 200 may be input to the control system 23 by the DSP 21 based on received channel estimation values for PSD adjustment. Such feedback from the receiver may be signaled to the DSP 21 via a low-speed out-of-band channel or a high-speed MCM channel.

The filter circuits 222-0 to 222-3 of the filter bank may comprise analog active or analog passive filters. In case the pre-amplifier 220 is configured with a fixed gain and analog passive filtering also an attenuator, the filter circuits 222-0 to 222-3 may comprise attenuators as well.

The demultiplexer 221 comprises one data or signal input, a selection input (i.e. for the control signal 200) and several outputs (i.e. for outputs in the present example). It is configured to forward the input signal to one of the outputs depending on the value of the selection input. The demultiplexer 221 may be designed as an analog switch circuit (e.g. a single-input, multiple-output switch) or as a digital logic circuit (e.g. a binary decoder).

Furthermore, a multiplexer (MUX) 223 is provided to guide the adjusted (i.e. filtered and/or attenuated) signal at the output of the selected one of the filter circuits 222-0 to 222-3 to the optical frontend 24. The multiplexer 223 may be configured as a data selector that selects between several analog or digital input signals and forwards it to a single output line. It can be considered as a multiple-input, single-output switch and may be implemented as an analog switch circuit or a digital logic circuit.

The demultiplexer 221 and the multiplexer 223 may be implemented as programmable logic devices (PLDs), e.g. to implement Boolean functions underlying their selection functions.

Fig. 4 shows schematically a block diagram of a second example of a power density adjustment system according to various embodiments.

In the exemplary technical implementation of the PSD adjustment system of Fig. 4, a post-amplifier 224 with fixed or variable amplification, analog or digitally controlled by the control system 23, is provided after the filter bank. In case that active filter circuits 222-0 to 222-3 (i.e. filter circuits with amplification) are provided in the filter bank, a fixed- gain post-amplifier 224 can be used or the post-amplifier 224 could be completely omitted.

In both exemplary technical implementations of Figs. 3 and 4, the selection process of at least one of the filter circuits 222-0 to 222-3 in response to the control signal 200 may be implemented as described above in connection with Fig. 2. I.e., the PSD adjustment may be achieved by amplifying and/or attenuating portions of the total signal bandwidth of the multicarrier signal at the output of the DSP 21 when the measured channel quality parameter(s) indicate that the channel quality (e.g. channel attenuation) changes. In an example, in case of a low channel quality (e.g. high channel attenuation), a filter circuit which attenuates the signal bandwidth for high frequencies is selected by the control system 23 and the pre-amplifier 220 or the post-amplifier 224 is controlled by the control system 23 to increase power of the remaining signal bandwidth thus increasing the PSD. On the other hand, when the determined channel quality is high (e.g. low channel attenuation), the selected filter circuit and amplification may be controlled by the control system 23 to reduce the power of the signal bandwidth of low frequencies and transfer it to high frequency signal bandwidth. This dynamic system control optimizes the signal-to-noise ratio (SNR) when the channel quality (e.g. attenuation) is either becoming very high or low.

Generally, the analog filters circuits 222-0 to 222-3 of the filter bank may be buffered (e.g. by an amplifier) to improve their performance (e.g. stopband attenuation and roll-off).

Furthermore, the filter circuits 222-0 to 222-3 may be fixed filters with a fixed filter characteristic determined by passive circuit components or may be adjustable active filters (e.g. Sallen Key filters) or digital filters (e.g. field programmable gate arrays (FPGA)) with a variable filter characteristic. In examples, they can be combined with a pre-emphasis function (e.g. high pass characteristic) or with a bandwidth limiting function (e.g. low pass characteristic) or a combination of both. In an example, active pre-emphasis can be achieved by using an active high-pass filter. De-emphasis can be handled at the receiver side, e.g., in the digital domain via channel estimation information. Figs. 5A and 5B show schematically exemplary circuit diagrams of active filters with operational amplifier that can be used instead of the filter bank of Fig. 3 and 4 or as one of the filter circuits 222-0 to 222-3 of the filter bank in various embodiments.

According to Fig. 5A, an operational amplifier is provided, which may be connected in a “Sallen-Key” active amplifier low-pass configuration. This approach may even completely waive the need for signal multiplexing, since the active filter circuit of Fig. 5A can be directly inserted between the DSP 21 and the optical frontend 24. The gain or amplification can be adjusted (e.g. manually or by an input of the control system 23 in case of an electronically controlled resistor (e.g. digital variable resistor)) independently from the bandwidth by a first resistor R1 in the feedback path. The bandwidth can be adjusted (e.g. manually or by an input of the control system 23 in case of electronically controlled resistors or capacitors (e.g. varactor diode or the like)) by changing a third and/or fourth resistor R3, R4 or a first and/or second capacitor Cl, C2 or both.

Similarly, in Fig. 5B, the operational amplifier is connected in a “Sallen-Key” active amplifier high-pass configuration. This approach may as well completely waive the need for signal multiplexing, since the active filter circuit of Fig. 5B can be directly inserted between the DSP 21 and the optical frontend 24. The gain or amplification can be adjusted (e.g. manually or by an input of the control system 23 in case of an electronically controlled resistor (e.g. digital variable resistor)) independently from the bandwidth by a first resistor R1 in the feedback path. Thereby, a pre-emphasis can be achieved by amplifying a high frequency range. The bandwidth can be adjusted (e.g. manually or by an input of the control system 23 in case of electronically controlled resistors or capacitors (e.g. varactor diode or the like)) by changing a third and/or fourth resistor R3, R4 or a first and/or second capacitor Cl, C2 or both.

Adjusting either the capacitors Cl, C2 or the resistor R3, R4 has specific advantages and disadvantages (such as changed input impedance, tolerances etc.). An advantage of using the active filter circuit of Fig. 5 A or Fig. 5B is a lightweight implementation saving cost, space and power consumption.

A lower performance in terms of high-speed implementation, accuracy of bandwidth adjustment, limited filter order, limited stopband attenuation, gain flatness and decay can be improved by cascading two or more of the active filter circuits of Fig. 5 A or 5B, respectively.

Fig. 6 shows a flow diagram of a power density adjustment procedure according to various embodiments.

In step S 601, at least one of the channel quality parameters mentioned earlier is measured or obtained from the receiving end (e.g. an EP 10 of Fig. 1) via a feedback channel of the multicarrier transmission link. Then, in step S602, filter parameters or characteristics (e.g. for the adjustable filter system 22) are determined based on the obtained channel quality parameter(s), which are set or selected for filtering the multi carrier signal prior to transmission. Optionally, an amplification factor may be determined based on the obtained channel quality parameter(s) to increase the power of selected bandwidth portions (i.e. subcarriers located within the selected bandwidth portions). Thereafter, in step S603, a filter function (e.g. as achieved by the filter circuit(s) of the filter system 22) is controlled by a selection or adjusting function to modify the frequency characteristic of the multicarrier signal. Finally, in step S604, the modified multicarrier signal is transmitted through the transmission link.

In an example, a simple way of implementing the determination and selection or adjusting function of steps S602 and S603 could be to tune filters of the filter system 22 separately and measure e.g. data rate vs. pathloss for each of the tuned filters. Then, a breakeven point in data rate vs. pathloss may be determined for both. The quality parameter received in step S601 via the feedback channel can be used to indicate when the breakeven point has been reached, e.g. based on a control mechanism.

In an automatic system, active filters or digital implementation may be tuned based on a feedback from the receiver to provide a full control loop. In an example, fixed factors in that control loop may be settled in an empiric way.

According to various embodiments, the filter system 22 of Fig. 2 and/or the filter bank of Figs. 3 and 4 could be implemented by fixed or adjustable digital filters with digital control, e.g. as field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICs) and with analog-to-digital converters (ADCs) and/or digital-to- analog converters (DAC).

Thus, passive filters or active filters (e.g. Sallen Key low pass or high pass filters) or digital implementations can be provided, which may be adjustable in at least one of their bandwidth, attenuation and filter shape. According to various other embodiments, a control information (e.g. digital value or flag) that indicates the selected filter parameters or characteristics is transmitted e.g. via a control channel or within the multicarrier signal to the receiver end (e.g. an EP 10 of Fig. 1) to allow control of a receiver bandwidth based on the selected filter parameters or characteristics at the transmitter end. Thereby, the signal quality (e.g. SNR, error rate etc.) of the modified multicarrier signal at the receiver end may be increased e.g. by reducing the amount of noise received in case of a reduced bandwidth of the modified multicarrier signal.

According to further embodiments, the proposed adaptive PSD adjustment may also be implemented in non-adaptive systems (e.g. non adaptive bit loading). This also covers LiFi systems which use a baseband multi-carrier modulation as currently applied in 802.11 based protocols or 3G, 4G or 5G signaling/modulation, but then applied for transmission over LiFi channels (As indicated herein above; an additional DC offset will need to be added to accommodate for the fact that optical data transmission requires a unipolar multi-carrier modulation signal).

In the following, practical examples of PSD characteristics and quality gain achievements at the receiver end with PSD adjustment are explained with reference to Figs. 7 to 9.

Fig. 7 shows a PSD diagram (upper diagram) with different power spectral density characteristics and a diagram (lower diagram) with characteristics of data rate vs. path loss of an optical transmission channel for different PSD settings.

More specifically, the upper diagram of Fig. 7 indicates three different PSD frequency characteristics 701 to 703 for a generic system and the performance in terms of data rate (DR) vs. path loss (PL) is shown. The inclined dotted part of the three PSD characteristics corresponds to an additional pre-equalization where power increases with increased frequency. A first PSD characteristic 701 is configured with a high power density of the multicarrier signal over a small bandwidth. Furthermore, a second PSD characteristic 702 is configured with a medium power density of the multicarrier signal over a medium bandwidth. Additionally, a third PSD characteristic 703 is configured with a low power density of the multicarrier signal over a wide bandwidth. In an example, the amount of power which corresponds to the integrated value of the PSD over the related bandwidth may be the same for all three characteristics. These characteristics may be achieved by corresponding filter characteristics of low-pass filters. The use of pre-equalization provides the advantage of achieving higher data rates due to lower cyclic prefix lengths and flatter SNR, which may be preferred for non- adaptive MCM systems and MCM systems with high subcarrier spacing.

When choosing a fixed PSD with or without pre-equalization, the data rate is usually high either when path loss is low or high in comparison to another PSD. With the proposed PSD adjustment this drawback of the fixed PSD is eliminated, because the system chooses the best available PSD characteristic for a specific path loss range.

In the example of the lower diagram of Fig. 7, the control system switches at predetermined path loss levels al and a2 (e.g. as indicated by corresponding signal quality parameters) between respective two of the three available PSDs 701 to 703 to obtain the modified characteristic 704. When choosing optimized PSDs with pre-equalization (i.e. dotted lines) the points of switching might be changed to path loss level points bl and b2, but the performance (indicated by the achieved data rate) of the modified characteristic 704’ with pre-equalization is higher, except high path losses. Therefore, the proposed PSD adjustment by combining the three different PSD characteristic 701 to 703 (e.g. by selecting different filter characteristics of the filter system 22) with and without pre-equalization provides improved data rates.

Fig. 8 shows a PSD diagram indicating quality gain achieved at the receiver end with PSD adjustment at a high path loss.

In the diagram of Fig. 8, a characteristic of a PSD_RX VS. frequency at the receiver is shown. A first line 803 indicates the minimum PSD for minimum bit loading, 808 indicates the maximum PSD for maximum bit loading and a third line 804 indicates a receiver noise floor. The minimum PSD for bit loading can be obtained based on a known minimum signal -to-noise ratio (SNR) for the selected modulation (e.g. 6.8dB for 2-QAM at a Bit error rate of BER=le-3) and a measured receiver noise floor. Similarly, the maximum PSD for bit loading can be obtained based on the maximum SNR for a maximum possible subcarrier modulation (e.g. >40.1dB for 4096-QAM at BER=le-3). In bidirectional MCM systems, these values can be obtained from channel estimations. E.g., the noise floor can be derived from a measured error vector magnitude (EVM) for known OFDM symbols via the channel estimation.

A first PSD characteristic 801 corresponds to an unadjusted or non-modified multicarrier signal of a conventional system without the proposed PSD adjustment. The obtained SNR 807 with respect to the receiver noise floor 804 of the conventional system at the receiver is indicated by a two-sided arrow. Furthermore, a second PSD characteristic 802 corresponds to an adjusted or modified multicarrier signal of a system with the proposed PSD adjustment. The resulting increased SNR 805 of the proposed system with respect to the receiver noise floor 804 at the receiver is indicated by another two-sided arrow. Thus, an SNR gain 806 is achieved by the proposed PSD adjustment.

In this example, the PSD characteristic 801 of the unadjusted multicarrier signal has a bandwidth fl and it can clearly be seen that the SNR beyond a higher 13 is not high enough to load bits via modulation on these carriers, as it crosses the minimum PSD line 803 for minimum bit loading (where the minimum PSD line as indicated above is generally dependent on the modulation used). Therefore, the power of the sub-carriers located within the upper frequency range between f3 and fl is wasted for that specific high amount of path loss, because no data is transmitted on them. In comparison, the PSD characteristic 802 of the adjusted multicarrier signal shows a lower bandwidth f2, but a higher power density. For example, assuming fl/f2=0.25, the density can be increased by a factor 2 or 6dB electrical, which is directly transformed into a SNR gain, when receiver noise dominates, maintaining still the same transmit power.

Fig. 9 shows a PSD diagram indicating quality gain achieved with PSD adjustment at a low path loss. Here, in a generic way, the benefit of the proposed PSD adjustment approach in comparison to Fig. 8 is shown in case of a low path loss.

A first line 903 indicates the maximum PSD for maximum bit loading 908 indicates the minimum PSD for minimum bit loading and a third line 904 indicates the receiver noise floor.

A first PSD characteristic 901 corresponds to an unadjusted or non-modified multicarrier signal of a conventional system without the proposed PSD adjustment. The obtained SNR 907 with respect to the receiver noise floor 904 of the conventional system at the receiver is indicated by a two-sided arrow. Furthermore, a second PSD characteristic 902 corresponds to an adjusted or modified multi carrier signal of a system with the proposed PSD adjustment. The resulting increased SNR 905 of the proposed system with respect to the receiver noise floor 904 at the receiver is indicated by another two-sided arrow. Thus, an SNR gain 906 is achieved by the proposed PSD adjustment.

As can be gathered from the diagram of Fig. 9, a signal power density of the conventional PSD characteristic 901 below a frequency 13 would result in a SNR that is higher than the maximum achievable SNR due to noise through nonlinearity or maybe another bottleneck in the system and is therefore generally wasted. Furthermore, as can be seen at frequency fl, the multicarrier signal with PSD adjustment (i.e. PSD characteristic 902) can achieve a higher useful SNR, when it uses the wasted power to perform e.g. pre- equalization. Additionally, the wasted power of the conventional system could be used to extend the bandwidth of the signal to frequency f2 to improve the system performance.

To summarize, power density shaping for an adaptive or non-adaptive multicarrier modulation system (e.g. OFDM) has been described, wherein the multicarrier modulation system comprises a digital signal generating component that generates a multicarrier modulation signal in a signal processing chain. This signal is supplied to a filter arrangement for setting different frequency ranges, that can be adaptively configured based on measured channel quality parameters.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. The proposed detection and/or selection procedures can be applied to other types of wireless networks. In particular, the invention is not limited to LiFi systems and OFDM modulation. It can be applied to all kinds of optical wireless multicarrier transmission systems, more particular to all kinds of LiFi devices which are MCM-based with adaptive bit loading. The proposed PSD adjustment could also be used in connection with a pulse amplitude modulation (PAM) based signalling with different speed modes (e.g. 10Mbps and 100Mbps) and auto negotiation.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in the text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated. A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The described operations or procedures like those indicated in Fig. 6 can be implemented as program code means of a computer program and/or as dedicated hardware of the receiver devices or transceiver devices, respectively. The computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid- state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:

1. An optical wireless transmission apparatus for controlling transmission power of a multicarrier signal, the apparatus comprising: a digital signal processor (21) configured to modulate the multi carrier signal having a fixed power spectral density over a bandwidth of the multicarrier signal, with or without pre-equalization; a controller (23) configured to determine at least one channel quality parameter of the transmission path of the multicarrier signal and determine at least one filter configuration or characteristic of an analog domain filter system (22) based on the at least one channel quality parameter, and to control the filter system (22) to set the determined at least one filter configuration or characteristic; the analog domain filter system (22) configured to shape a power spectral density of the multicarrier signal by amplifying or attenuating portions of the total bandwidth of the multicarrier signal in response to the determined at least one filter configuration or characteristic parameter to improve channel efficiency of the multicarrier signal; and an output for outputting the power-density adjusted multicarrier signal to an optical frontend (24).

2. The apparatus of claim 1, wherein the digital signal processor (21) is configured to apply a bit loading technique by using different modulation schemes involving different numbers of bits for different sub-carriers of the multicarrier signal based on the determined at least one channel quality parameter.

3. The apparatus of claim 1, wherein the apparatus is configured to adjust the power spectral density by amplifying and/or attenuating portions of the total bandwidth of the multicarrier signal when the at least one channel quality parameter indicates that the channel quality changes.

4. The apparatus of claim 3, wherein the controller determines the at least one channel quality parameter of the transmission path by receiving a control signal (200) indicating the at least one channel quality parameter.

5. The apparatus of claim 4, wherein the apparatus is configured to control the filter system (22) to reduce the signal power of the multicarrier signal for high frequencies and to increase signal power of the multicarrier signal for the remaining signal bandwidth of the multicarrier signal when the at least one channel quality parameter indicates a low channel quality, and to reduce the signal power of the multicarrier signal for low frequencies and increase the signal power of the multicarrier signal for high frequencies when the at least one channel quality parameter indicates a high channel quality.

6. The apparatus of claim 1, wherein improving channel efficiency comprises increasing transmit power in regions where the signal to noise ratio is above the minimum power spectral density for bit loading for the modulation used.

7. The apparatus of claim 1, wherein the apparatus is configured to apply a pre- equalization or pre-emphasis of the multicarrier signal.

8. The apparatus of claim 4, wherein the filter system (22) comprises a plurality of filter circuits (222-0 to 222-3) and the apparatus is configured to select one of the plurality of filter circuits (222-0 to 222-3) of the filter system (22) according to a value of the determined at least one filter configuration or characteristic via a demultiplexer (221), and to guide a power-density adjusted multicarrier signal at the output of the selected one of the filter circuits (222-0 to 222-3) to the optical frontend (24) via a multiplexer (223).

9. The apparatus of claim 1, wherein the apparatus is configured to adjust the power spectral density of the multicarrier signal using the filter system (22) by controlling at least one of an amplification and a bandwidth of an operational amplifier of an active filter circuit of the filter system (22).

10. The apparatus of claim 1, wherein the at least one channel quality parameter is derived from an amplitude response, a measured noise, a measured received signal strength, a signal-to-noise ratio, a direct current attenuation, a data rate or another value related to a transmission quality of a transmission channel of the multi carrier signal.

11. An optical wireless multicarrier signal transmission system comprising at least one a transmitter device (11) comprising the apparatus of claim 1 and a receiver device (10), wherein the receiver device (10) is configured to transmit a feedback information indicating the at least one channel quality parameter to the transmitter device (11).

12. The system of claim 11, wherein the transmitter device (11) is configured to transmit a control information that indicates at least one filter parameter or characteristic selected for power spectral density to the receiver device (10) to allow control of a receiver bandwidth based on the selected at least one filter parameter or characteristic.

13. The system of claim 11, wherein the optical wireless multicarrier transmission system is a LiFi network, wherein the multicarrier signal is an orthogonal frequency division multiplexing signal or a discrete multi-tone signal, and wherein the multicarrier signal is embedded in a visible or infrared light signal emitted by an illumination source of a room or outdoor lighting device.

14. A method of controlling optical wireless transmission power of a multicarrier signal, the method comprising: modulating the multicarrier signal having a fixed power spectral density over a bandwidth of the multicarrier signal, with or without pre-equalization; determining a channel quality parameter of a transmission path of the multicarrier signal; determining at least one filter configuration or characteristic of an analog domain filter system (22) based on the at least one channel quality parameter; controlling the filter system (22) to set the determined at least one filter configuration or characteristic; and using the analog domain filter system (22) for shaping a power spectral density of the multicarrier signal by amplifying or attenuating portions of the total bandwidth of the multi carrier signal in response to the at least one filter configuration or characteristic; and outputting the power-density adjusted multicarrier signal to an optical frontend

(24).

15. A computer program product comprising code means for producing the steps of claim 14 when run on an optical wireless transmission apparatus according to claim 1.