A calibration device for an optical wireless communication system

Abstract

A calibration device (200) for a first optical wireless communication (OWC) system (202), the first OWC system (202) being configured to receive a first test signal (T1) from a second OWC system (208), wherein the calibration device (200) is configured to calibrate the first OWC system (202) based on a frequency error determined using the first test signal (T1).

Description

[0001] A CALIBRATION DEVICE FOR AN OPTICAL WIRELESS COMMUNICATION SYSTEM

[0002] FIELD

[0003] The present disclosure relates to a calibration device for an optical wireless communication (OWC) system and a method of calibrating such an OWC system. Also disclosed is an OWC apparatus including the calibration device and an OWC system, and an OWC network comprising the OWC apparatus and at least one other interconnected OWC system.

[0004] BACKGROUND

[0005] It is known to provide wireless data communications by using light instead of radio frequencies to transmit and receive data wirelessly between devices. Data may be transmitted using light by modulating at least one property of the light, for example an intensity of the light. Methods that use light to transmit data wirelessly may be referred to as optical wireless communications (OWC), or light communications (LC). One method that uses light to transmit data wirelessly is LiFi. LiFi technology is fast, convenient, safe, environmentally friendly and thus has broad application prospects.

[0006] Wireless networks using visible light may in some circumstances allow a higher data capacity, greater energy efficiency and greater security than radio frequency wireless networks, and may also be used to replace point-to-point infrastructure in locations where conventional infrastructure does not exist or is too expensive to build.

[0007] Figure 1 (a) is a schematic of an example first OWC system 100 communicating with a second OWC system 102 via a communication signal 104, where the communication signal 104 has a wavelength within the visible light and / or infrared wavebands.

[0008] Figure 1 (b) is a schematic of a specific example of an OWC system 106 comprising a data encoding / decoding block 108 including a data processor, and an electrical-to-optical / optical-to-electrical conversion block 110 including optical transmitter and receiver components. In some systems, the conversion block 1 10 may be referred to as a light antenna module (LAM).

[0009] In known free space LiFi communication apparatus, the data encoding / decoding functions of existing mobile communication standards such as WiFi / 3G / 4G / 5G have been considered in the design of LiFi systems. The WiFi / 3G / 4G / 5G standards may be referred to as RF communication standards. In such approaches, existing wireless communications protocol stacks that enable multiuser access and handover can be reused, which can be beneficial. Such systems use RF down-conversion circuitry to convert the RF signal into a signal which is fed into the LAM or other electrical-optical conversion components. Such systems enable WiFi basebands to be exploited, to send and receive data through optical front ends (OFEs).

[0010] Interoperability issues can arise between optical communication systems. It is desirable to mitigate or overcome one or more of these issues.

[0011] SUMMARY

[0012] Various aspects of the present invention are defined in the independent claims. Some additional features are defined in the dependent claims.

[0013] According to a first aspect of the disclosure there is provided a calibration device for a first optical wireless communication (OWC) system, the first OWC system being configured to receive a first test signal from a second OWC system, wherein the calibration device is configured to calibrate the first OWC system based on a frequency error determined using the first test signal.

[0014] Optionally, the first OWC system is configured to receive a first OWC signal from the second OWC system.

[0015] Optionally, the first test signal is at least one of an optical signal, an RF signal and an acoustic signal.

[0016] Optionally, the calibration device comprises a test signal generator configured to generate a second test signal.

[0017] Optionally, the calibration device comprises a comparison unit configured to receive the first and second test signals, and determine the frequency error by comparing the first and second test signals.

[0018] Optionally, the comparison unit comprises a mixer. Optionally, the mixer may be configured to receive the first and second test signals and provide a frequency error signal that includes information on the frequency error as an output. The frequency of the frequency error signal may be based on, an estimate of, and / or representative of the frequency error.

[0019] Optionally, the mixer may comprise at least one of: an unbalanced mixer, a single balanced mixer, a double balanced mixer and / or a switching mixer. Advantageously, the use of a switching mixer may provide a calibration device requiring fewer, less complex and / or lower cost components. Optionally, the frequency error is the difference between the first and second test signals.

[0020] Optionally, the first OWC signal is, comprises, or is comprised in the first test signal.

[0021] Optionally, the first OWC system comprises a first optical front end module comprising a first receiver configured to receive the first OWC signal and / or the first test signal.

[0022] Optionally, the first receiver comprises at least one of a photodiode, a positive- intrinsic-negative (PIN) device, a phototransistor, a photoresistor, or a light activated silicon controlled rectifier.

[0023] Optionally, the first receiver comprises one or more of a photodiode, an array or an arrangement of photodiodes, a silicon PIN photodiode, a silicon photomultiplier (SiPM), a single photon avalanche diode (SPAD), a Graphene-CMOS high-resolution sensor, an avalanche photodiode (APD), a positive-intrinsic-negative (PIN) device, a phototransistor, a photoresistor, or a light activated silicon controlled rectifier.

[0024] Optionally, the SPAD is as a Geiger-mode APD (G-APD).

[0025] Optionally, the first receiver is configured to detect optical signals having an ultraviolet and / or visible and / or infrared wavelength.

[0026] Optionally, the infrared wavelength is greater than 1 ,000nm.

[0027] Optionally, the first receiver is configured to detect optical signals having a near infrared wavelength.

[0028] Optionally, the near infrared wavelength is in a range of 700 nm to 2500 nm.

[0029] Optionally, the first receiver is configured to convert the first OWC signal into a first OWC electrical signal, and the first OWC system comprises a first radio frequency (RF) communication module configured to receive the first OWC electrical signal, and a first interface module configured to adjust the frequency of the first OWC electrical signal for the first RF communication module, prior to the first OWC electrical signal being received by the first RF communication module.

[0030] Optionally, the first interface module comprises a first RF converter.

[0031] Optionally, the first RF communication module is configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz.

[0032] Optionally, the first RF converter comprises a first mixer configured to receive the first OWC electrical signal, a first clock configured to generate a first clock signal, a first local oscillator configured to provide a first local oscillator signal that is dependent on the first clock signal, wherein the first mixer is configured to adjust the frequency of the first OWC electrical signal using the first local oscillator signal.

[0033] Optionally, the calibration device comprises a first test signal generator configured to generate a second test signal, and a comparison unit configured to receive the first and second test signals, and determine the frequency error by comparing the first and second test signals.

[0034] Optionally, the frequency error is the difference between the first and second test signals

[0035] Optionally, the first test signal generator is configured to receive the first clock signal, and the second test signal is generated using the first clock signal.

[0036] Optionally, the second OWC system comprises a second optical front end module comprising a second transmitter configured to transmit the first OWC signal and / or the first test signal.

[0037] Optionally, the first test signal and / or the second test signal is transmitted periodically or continuously.

[0038] Optionally, the second transmitter comprises one or more of a light emitting diode (LED), an array or an arrangement of LEDs, a laser, or a light-emitting plasma.

[0039] Optionally, the one or more LEDs is one or more of an OLED or a micro LED.

[0040] Optionally the laser is a vertical-cavity surface-emitting laser (VCSEL).

[0041] Optionally, the second transmitter is configured to transmit optical signals having an ultraviolet and / or visible and / or infrared wavelength.

[0042] Optionally, the infrared wavelength is greater than 1 ,000nm.

[0043] Optionally, the second transmitter is configured to transmit optical signals having a near infrared wavelength.

[0044] Optionally, the near infrared wavelength is in a range of 700 nm to 2500 nm.

[0045] Optionally, the second transmitter is configured to convert a first OWC electrical signal into the first OWC signal, and the second OWC system comprises a second radio frequency (RF) communication module configured to provide the first OWC electrical signal, and a second interface module configured to adjust the frequency of the first OWC electrical signal for the second OWC front end module, prior to the first OWC electrical signal being received by the second transmitter.

[0046] Optionally, the second interface module comprises a second RF converter.

[0047] Optionally, the second RF communication module is configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz. Optionally, the second RF converter comprises a second mixer configured to receive the first OWC electrical signal, a second clock configured to generate a second clock signal, a second local oscillator configured to provide a second local oscillator signal that is dependent on the second clock signal, wherein the second mixer is configured to adjust the frequency of the first OWC electrical signal using the second local oscillator signal.

[0048] Optionally, the first test signal is generated by a second test signal generator.

[0049] Optionally, the second test signal generator is configured to receive the second clock signal, and the first test signal is generated using the second clock signal.

[0050] Optionally, the calibration device comprises an adjustment signal generator configured to receive the frequency error, determine an adjustment to apply to the first local oscillator using the frequency error, and provide an adjustment signal for adjusting the first local oscillator.

[0051] Optionally, the frequency error is the difference between the first and second test signals.

[0052] Optionally, the adjustment signal generator comprises an anti-aliasing filter configured to provide a feedback signal having a frequency that is an estimate of the frequency of the first test signal.

[0053] Optionally, the anti-aliasing filter comprises a low pass filter.

[0054] Optionally, the calibration device comprises an analog to digital converter (ADC) for receiving the feedback signal and converting the feedback signal to a digital signal.

[0055] Optionally, the calibration device comprises a frequency counter circuit (comprising a frequency counter) for receiving the feedback signal and / or the frequency error signal and providing a / the digital signal. The digital signal may be based on, and / or representative of, the frequency error and / or the frequency of the feedback signal. Advantageously, the use of a frequency counter (instead of, for example, an ADC) in the calibration device, may reduce the digital signal processing (DSP) required by a / the host processor to determine the adjustment and may require fewer, less complex, and / or lower cost components.

[0056] Optionally, the calibration device comprises a determination unit configured to receive the digital signal, and to determine the adjustment using the digital signal.

[0057] Optionally, the first interface module comprises a phase locked-loop (PLL). The PLL may comprise a fractional-N block. As will be appreciated by those skilled in the art a PLL comprising a fractional N block may be referred to as a frac-N PLL, frac PLL or fPLL. The PLL may comprise a register configured to update based on the received adjustment signal and to adjust the first local oscillator based on the update of the register.

[0058] Optionally, the first OWC system is configured to transmit a second OWC signal.

[0059] Optionally, the first OWC system comprises a first optical front end module comprising at least one of:a first receiver configured to receive the first OWC signal and / or the first test signal; and a first transmitter configure to transmit the second OWC signal.

[0060] Optionally, the first receiver comprises at least one of a photodiode, a positive- intrinsic-negative (PIN) device, a phototransistors, a photoresistor, or a light activated silicon controlled rectifier, and / or the first transmitter comprises one or more of of a light emitting diode (LED), an array or an arrangement of LEDs, a laser, and a light-emitting plasma.

[0061] Optionally, the first receiver comprises one or more of a photodiode, an array or an arrangement of photodiodes, a silicon PIN photodiode, a silicon photomultiplier (SiPM), a single photon avalanche diode (SPAD), a Graphene-CMOS high-resolution sensor, an avalanche photodiode (APD), a positive-intrinsic-negative (PIN) device, a phototransistor, a photoresistor, or a light activated silicon controlled rectifier.

[0062] Optionally, the SPAD is as a Geiger-mode APD (G-APD).

[0063] Optionally, the first receiver is configured to detect optical signals having an ultraviolet and / or visible and / or infrared wavelength.

[0064] Optionally, the infrared wavelength is greater than 1 ,000nm.

[0065] Optionally, the one or more LEDs is one or more of an OLED or a micro LED.

[0066] Optionally the laser is a vertical-cavity surface-emitting laser (VCSEL).

[0067] Optionally, the first transmitter is configured to transmit optical signals having an ultraviolet and / or visible and / or infrared wavelength.

[0068] Optionally, the infrared wavelength is greater than 1 ,000nm.

[0069] Optionally, the first receiver is configured to detect optical signals having a near infrared wavelength, and / or the first transmitter is configured to transmit optical signals having a near infrared wavelength.

[0070] Optionally, the near infrared wavelength is in a range of 700 nm to 2500 nm.

[0071] Optionally, the first receiver is configured to convert the first OWC signal into a first OWC electrical signal, the first transmitter is configured to convert a second OWC electrical signal into the second OWC signal, and the first OWC system comprises a first radio frequency (RF) communication module configured to i) receive the first OWC electrical signal, and ii) provide the second OWC electrical signal, and a first interface module configured to i) adjust the frequency of the first OWC electrical signal for the first RF communication module, prior to the first OWC electrical signal being received by the first RF communication module, and ii) adjust the frequency of the second OWC electrical signal for the first optical front end module, prior to the second OWC electrical signal being received by the first transmitter.

[0072] Optionally, the first interface module comprises a first RF converter.

[0073] Optionally, the first RF communication module is configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz.

[0074] Optionally, the first RF converter comprises a first mixer configured to receive the first OWC electrical signal, a first clock configured to generate a first clock signal, a first local oscillator configured to provide a first local oscillator signal that is dependent on the first clock signal, wherein the first mixer is configured to adjust the frequency of the first OWC electrical signal using the first local oscillator signal, and the first RF converter further comprises a third mixer configured to receive the second OWC electrical signal, a third local oscillator configured to provide a third local oscillator signal that is dependent on the first clock signal, wherein the third mixer is configured to adjust the frequency of the second OWC electrical signal using the third local oscillator.

[0075] Optionally, the calibration device comprises a first test signal generator configured to generate a second test signal, and a comparison unit configured to receive the first and second test signals, and determine the frequency error by comparing the first and second test signals.

[0076] Optionally, the frequency error is the difference between the first and second test signals.

[0077] Optionally, the first test signal generator is configured to receive the first clock signal, and the second test signal is generated using the first clock signal.

[0078] Optionally, the second OWC system comprises a second optical front end module comprising a second transmitter configured to transmit the first OWC signal and / or the first test signal.

[0079] Optionally, the second transmitter comprises one or more of of a light emitting diode (LED), an array or an arrangement of LEDs, a laser, and a light-emitting plasma.

[0080] Optionally, the one or more LEDs is one or more of an OLED or a micro LED.

[0081] Optionally the laser is a vertical-cavity surface-emitting laser (VCSEL).

[0082] Optionally, the second transmitter is configured to transmit optical signals having an ultraviolet and / or visible and / or infrared wavelength.

[0083] Optionally, the infrared wavelength is greater than 1 ,000nm. Optionally, the second transmitter is configured to transmit optical signals having a near infrared wavelength.

[0084] Optionally, the near infrared wavelength is in a range of 700 nm to 2500 nm.

[0085] Optionally, the second transmitter is configured to convert a first OWC electrical signal into the first OWC signal, and the second OWC system comprises a second radio frequency (RF) communication module configured to provide the first OWC electrical signal, and a second interface module configured to adjust the frequency of the first OWC electrical signal for the second OWC front end module, prior to the first OWC electrical signal being received by the second transmitter.

[0086] Optionally, the second interface module comprises a second RF converter.

[0087] Optionally, the second RF communication module is configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz.

[0088] Optionally, the second RF converter comprises a second mixer configured to receive the first OWC electrical signal, a second clock configured to generate a second clock signal, a second local oscillator configured to provide a second local oscillator signal that is dependent on the second clock signal, wherein the second mixer is configured to adjust the frequency of the first OWC electrical signal using the second local oscillator signal.

[0089] Optionally, the first test signal is generated by a second test signal generator.

[0090] Optionally, the second test signal generator is configured to receive the second clock signal, and the first test signal is generated using the second clock signal.

[0091] Optionally, the calibration device comprises an adjustment signal generator configured to receive the frequency error, determine an adjustment to apply to the first local oscillator using the frequency error, determine an adjustment to apply to the third local oscillator using the frequency error, provide a first adjustment signal for adjusting the first local oscillator, and provide a second adjustment signal for adjusting the third local oscillator.

[0092] Optionally, the adjustment signal generator comprises an anti-aliasing filter configured to provide a feedback signal having a frequency that is an estimate of the frequency of the first test signal.

[0093] Optionally, the anti-aliasing filter comprises a low pass filter.

[0094] Optionally, the calibration device comprises an analog to digital converter (ADC) for receiving the feedback signal and converting the feedback signal to a digital signal.

[0095] Optionally, the calibration device comprises a determination unit configured to receive the digital signal, and to determine the adjustments using the digital signal. Optionally, the first interface module comprises a first FRAC N register configured to update based on the received first adjustment signal and to adjust the first local oscillator based on the update of the second FRAC N register, and / or the second interface module comprises a second FRAC N register configured to update based on the received second adjustment signal and to adjust the third local oscillator based on the update of the second FRAC N register.

[0096] Optionally, the calibration device is configured to calibrate the first OWC system in response to a user or as part of an automated process.

[0097] Optionally, the automated process uses software, artificial intelligence or machine learning.

[0098] Optionally, the calibration device comprises a detector configured to detect a parameter that is indicative of calibration being necessary, wherein the calibration device is configured to calibrate the first OWC system in response to the detection of the parameter.

[0099] Optionally, the calibration device is configured to calibrate the first OWC system at start up of the first OWC system.

[0100] The OWC signals described herein may comprise LiFi signals. The OWC signals may comprise modulated visible, infra-red, ultraviolet or terahertz signals. The OWC signals may be in accordance with one or more of IEEE 802.15.7, 802.15.13, 802.1 1 or extensions or developments thereof; ITU-T G.9960 or extensions or developments thereof; or ITU-T G.9991 or extensions or developments thereof.

[0101] According to a second aspect of the disclosure there is provided an optical wireless communication apparatus comprising the calibration device of the first aspect, and the first OWC system.

[0102] Optionally, the first OWC system comprises the calibration device.

[0103] Optionally, the optical wireless communication apparatus is a router, a mobile device, a computer, a laptop, a medical device, a robotic device, an aeronautical vehicle, a drone or a wearable device.

[0104] According to a third aspect of the disclosure there is provided an optical wireless communication network comprising the optical wireless communication apparatus of the second aspect and the second OWC system.

[0105] According to a fourth aspect of the disclosure there is provided a method of calibrating the first OWC system using the calibration device of the first aspect. It will be appreciated that the method of the fourth aspect may include features set out in the first aspect and / or the second aspect and / or the third aspect, and can incorporate other features as described herein.

[0106] According to a fifth aspect of the present disclosure there is provided a calibration device for an optical wireless communication (OWC) system comprising a calibration module. The calibration module may comprise a heuristic calibration module.

[0107] The calibration device may calibrate a plurality of OWC systems, for example a first and second OWC system. The plurality of OWC systems may be calibrated sequentially, in parallel, repeatedly, or in a heuristic manner.

[0108] In order to calibrate an OWC system (e.g. a first OWC system) the heuristic calibration module may collect data from a further OWC system (e.g. a second OWC system) to identify, for example, an optimal data transfer rate heuristic and / or an optimal signal frequency heuristic. The data may be collected iteratively e.g. periodically and the calibration may be achieved after one or more iterations.

[0109] The heuristic calibration module may comprise one or more of software, algorithms, artificial intelligence, and machine learning processes. Software stored on non-transitory computer readable storage media (which may be separate to or comprised in the calibration device) may be executed by processing circuitry of a receiver of the calibration device and may include microprocessors, central processing units (CPUs), application-specific integrated circuits with processing circuitry, or other processing circuits.

[0110] The calibration module may be configured to adjust a frequency of the OWC system (e.g. an LO frequency) and measure a performance metric. This step may then be repeated, for example until the performance metric is optimised. The performance metric may comprise at least one of: a data transfer rate, a packet error rate, latency, however other suitable performance metrics will be apparent to those skilled in the art. The frequency may be adjusted by changing a reference voltage of a PLL comprised in the OWC system and / or calibration module.

[0111] Advantageously a heuristic calibration module may requires no additional hardware as, for example, no test signal generation circuitry is required. This may allow for a smaller OWC device and may provide a simpler and more cost-effective method of manufacture.

[0112] According to a sixth aspect of the present disclosure there is provided the use of repeating structures on the signal itself, like a permanent profile of the signal, for example a G.hn signal has a well defined header structure. According to a seventh aspect of the present disclosure there is provided a method of calibrating a first optical wireless communication (OWC) system. The method comprising: receiving, by the first OWC system, a first test signal from a second OWC system; and calibrating the first OWC system based on a frequency error determined using the first test signal.

[0113] The first OWC system may comprise the calibration device of the first aspect.

[0114] The method may comprise receiving, by the first OWC system, a first OWC signal from the second OWC system. The first OWC signal may be, comprise or be comprised in the first test signal.

[0115] The first OWC system may comprise a test signal generator and the method may comprise generating, by the test signal generator, a second test signal. The first OWC system may comprise a comparison unit, and the method may comprise: receiving, by the comparison unit, the first and second test signals; and determining, by the comparison unit the frequency error by comparing the first and second test signals. The frequency error may be the difference between the first and second test signals.

[0116] The first OWC system may comprise a first optical front end (OFE) module. The first OFE module may comprise a first optical receiver. The method may comprise receiving, by the first optical receiver the first OWC signal and / or the first test signal. The method may comprise converting, by the first optical receiver, the first OWC signal into a first OWC electrical signal.

[0117] The first OWC system may comprise a first radio frequency (RF) communication module. The method may comprise receiving, by the first RF communication module, the first OWC signal. The method may comprise adjusting, by a first interface module, the frequency of the first OWC electrical signal prior to the first OWC electrical signal being received by the first RF communication module.

[0118] The above summary is intended to be merely exemplary and non-limiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilized, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure.

[0119] BRIEF DESCRIPTION OF THE DRAWINGS The disclosure is described in further detail below by way of example and with reference to the accompanying drawings in which:

[0120] Figure 1 (a) is a schematic of a first OWC system communicating with a second OWC system via a communication signal

[0121] Figure 1(b) is a schematic of a specific example of an OWC system;

[0122] Figure 2(a) is a schematic of a calibration device for a first OWC system in accordance with a first embodiment of the present disclosure;

[0123] Figure 2(b) is a schematic of a specific embodiment of the first OWC system, in accordance with a second embodiment of the present disclosure;

[0124] Figure 2(c) is a schematic of a specific implementation of the RF converter of the first OWC system;

[0125] Figure 2(d) is a schematic of a specific implementation of the calibration device in accordance with a third embodiment of the disclosure;

[0126] Figure 2(e) is a schematic of a specific embodiment of a second OWC system, in accordance with a fourth embodiment of the present disclosure;

[0127] Figure 2(f) is a schematic of a specific implementation of the RF converter of the second OWC system;

[0128] Figure 2(g) is a schematic of a specific implementation of the calibration device in accordance with a fifth embodiment of the present disclosure;

[0129] Figure 3(a) is a schematic of a calibration device for calibrating a first OWC system that is in communication with a second OWC system, in accordance with a sixth embodiment of the present disclosure;

[0130] Figure 3(b) is a schematic of a specific embodiment of the first OWC system; Figure 3(c) is a schematic of a specific embodiment of the RF converter for the first OWC system;

[0131] Figure 3(d) is a schematic of a specific embodiment of the calibration device;

[0132] Figure 4(a) is a schematic of a calibration device, an OWC system and an OWC system in accordance with a seventh embodiment of the present disclosure;

[0133] Figure 4(b) is a schematic of a specific embodiment of the mixer of the calibration device of Figure 4(a);

[0134] Figure 4(c) is a schematic of a specific embodiment of the adjustment signal generator of the calibration device of Figure 4(a);

[0135] Figure 5(a) is a schematic of a specific embodiment of the second OWC system;

[0136] Figure 5(b) is a schematic of a specific embodiment of the first OWC system; and

[0137] Figure 6 is a flowchart of a heuristic method of calibrating an OWC system in accordance with an embodiment of the present disclosure.

[0138] In the Figures, like parts are denoted by like reference numerals.

[0139] It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale.

[0140] DETAILED DESCRIPTION

[0141] Unless otherwise specified the terms “light” and “optical signal” may be used, for example, to refer to electromagnetic waves with wavelengths in a range 1 nm to 2500 nm, which includes ultraviolet, visible light and near-infrared wavelengths. Light may be used to refer to both visible light and non-visible light of any suitable wavelengths.

[0142] IEEE 802 refers to a family of IEEE standards dealing with Local Area Networks and Metropolitan Area Network. The IEEE 802 family of standards is maintained by the IEEE 802 LAN / MAN Standards Committee (LMSC). An individual Working Group provides the focus for each area.

[0143] The 802.11 family is a series of over-the-air modulation techniques that share the same basic protocol. These standards provide the basis for wireless network products using the Wi-Fi brand. The segment of the radio frequency spectrum used by 802.1 1 varies between countries.

[0144] IEEE 802.11 bb is a line-of-sight light-based wireless networking standard that is part of the 802.11 suite of standards, which defines an interoperable communications protocol for Li-Fi devices in the near-infrared 800 to 1000 nm waveband to implement data rates between 10 Mbit / s and 9.6 Gbit / s, with interoperability between devices with different capabilities.

[0145] A LiFi device, such as the OWC system 106 may comprise a Radio Frequency (Down and Up) Conversion Module with a reference clock, a Local Oscillator and a mixer, which generates a LiFi signal at a frequency derived from the local oscillator. A local oscillator may alternatively be known as a “synthesizer”.

[0146] The OWC system 106 of Figure 1 (b) may use an OFE for the electro-optical conversion 110 conforming to the IEEE 802.1 1 bb standard. The data encoding / decoding 108 may be provided by a WiFi chipset that may conform to an IEEE 802.11 standard.

[0147] The OWC signals transmitted and / or received by the OWC system 106 may include data received from an external network and / or the OWC signals may establish and / or maintain a communication session via the network with a remote device. The remote device may comprise, without limitation, personal computers, desktops, laptops and smart devices, including mobile devices (for example, mobile phones, tablets or digital book readers). Remote devices may be powered by their own battery resource.

[0148] The bandwidth of the OFEs is larger than the bandwidth of the WiFi signals. However, WiFi chipsets modulate their signals to higher frequencies to communicate through antennas.

[0149] To overcome the difference in frequency between the OFE and the WiFi chipset mixers and synthesisers are used to translate the signals to lower frequencies, suitable for the OFEs. However, in doing so, a frequency error may be added to the communication link that cannot be detected by the WiFi Protocol.

[0150] The frequency of the signals generated by the WiFi chipsets are referenced to a local oscillator, for example a temperature compensated crystal oscillator (TCXO). A TCXO has an inherent tolerance on the accuracy of its frequency, as well as a frequency drift as a function of temperature, input voltage, and / or the like. The tolerance is typically in the 1 -20 part per million (ppm) range.

[0151] Crystals (such as those comprised in a TCXO) generate a reference frequency which is fed to a synthesiser. The synthesiser carries out mathematical operations on this frequency and converts it to another frequency which is used to carry out conversions with a mixer.

[0152] There are a plurality of oscillators involved in a multiple-input multiple-output (MIMO) system: for example, four oscillators being two oscillators on the transmit side, and two oscillators on the receive side (commonly referred to as a 2x2 configuration). As will be appreciated by those skilled in the art other MIMO configurations may be also be used e.g. 3x3, 4x4 or the like. All oscillators generate an error relative to the desired frequency. The system may be an RF (for example WiFi) and Li Fi System.

[0153] In summary, there can be any combination or ratio of oscillators, for example n oscillators on receive and n oscillators on transmit side for a nxn system.

[0154] It will be appreciated that a single-input single-output (SISO) system will comprise two oscillators: one on the transmit side, and one on the receive side.

[0155] The symbol clock frequency error tolerance may be ± 20 ppm maximum for 5 GHz bands and ± 25 ppm for 2.4 GHz bands. An error rate between a transmit and receive oscillator for standard WiFi can be corrected enabling optimal operation at high speeds. If there is an uncorrected frequency error entering the synthesiser it may cascade up through the system to create an unworkable WiFi signal.

[0156] Temperature also has an influence on the clocks, making them drift and introduces further error, for example if the transmit and receive unit do not warm up at the same rate, frequency errors can arise.

[0157] For example, in IEEE 802.11 ac channel 52, we have an orthogonal frequency division multiplexing (OFDM) signal comprising 256 carriers with 312.5 kHz each, totalling 80MHz Bandwidth (BW). This OFDM signal is then mixed up to 5210 MHz centre frequency.

[0158] As an example, we will consider a WiFi link where two chipsets are communicating using channel 52. One chipset has a TCXO with a -10 ppm error on its frequency. The other chipset has a TCXO with a +10 ppm error on its frequency.

[0159] As a consequence of the faster / slower clocks, the center frequencies and bandwidths of the carriers will be affected accordingly.

[0160] For the slower clock WiFi the center frequency Fc1 will be: Fcl 5210

[0161] For the slower clock WiFi the carrier bandwidths will be:

[0162] Carrier BW = 312.5

[0163] For the slower clock WiFi the signal bandwidth will be:

[0164] For the faster clock WiFi the center frequency will be:

[0165] Fcl = 5210

[0166] For the faster clock WiFi the carrier bandwidths will be:

[0167] Carrier BW = 312.5

[0168] For the faster clock WiFi the signal bandwidth will be:

[0169] The errors introduced by the communication link between OWC systems may be a constant shift.

[0170] The errors introduced by the WiFi clocks ppm may be proportional to the frequency of the carrier.

[0171] It is difficult to predict effects on the demodulation as a result of these sources of error, and if the extra error is not compensated for loss of orthogonality is caused, which can negatively impact the operation of OFDM systems.

[0172] Figure 2(a) is a schematic of a calibration device 200 for a first optical wireless communication (OWC) system 202 in accordance with a first embodiment of the present disclosure. The first OWC system 202 is configured to receive a test signal T1 from a second OWC system 208. The first OWC system 202 may be configured to receive an OWC signal 204 from the second OWC system 208.

[0173] The first and / or second OWC system 202, 208 may be, comprise or be comprised in an Access Point device. An access point may provide data transmission to and / or from a wired network or a Wi-Fi™ or other wireless network and / or other optical wireless communications network, optionally a Li Fi network. Data connectivity may be via network connection, wireless connection, PLC connection, PoE connection, OWC connection or other known data connections.

[0174] The test signal T1 may comprise an optical signal, an RF signal or an acoustic signal.

[0175] The calibration device 200 is configured to calibrate the first OWC system 202 based on a frequency error determined using the test signal T 1 .

[0176] During operation, the calibration device 200 uses the first test signal T1 to determine the frequency error, which may then be used to calibrate the first OWC system 202, thereby correcting for the frequency error.

[0177] The first OWC signal 204 may be used to transfer data from the second OWC system 208 to the first OWC system 202. The first OWC signal 204 may comprise the first test signal T1 . For example, the modulation techniques may be used to encode the first test signal T1 on to the first OWC signal 204, with a decoding process being used to extract the first test signal T1 from the first OWC signal 204. In summary, in a specific embodiment, the test signal T1 can be extracted from the OWC signal 204 itself.

[0178] The frequency error may be a result of the communication link between OWC systems 202, 208 as described previously. Alternatively, or additionally, the frequency error may be a result of a mismatch in clock frequencies, between OWC systems 202, 208. The calibration device 200 may be used to prevent a loss of orthogonality thereby improving OFDM systems. In summary, the calibration device 200 acts to improve interoperability between OWC systems 202, 208.

[0179] The OWC systems 202, 208 may be visible and / or infrared light communication systems for 802.1 1 bb standard devices using RF chipsets, such as a WiFi chipset. The calibration device 200 can enable stable data transfer between a WiFi chipset with baseband having a range of error tolerances and optical communication devices within the 802.1 1 bb standard, by correcting for frequency errors. The WiFi chipset may be referred to as a WiFi board, or a WiFi board with baseband. Embodiments of the present disclosure may be used to provide optimal interoperability between a WiFi chipset and a Li Fi IEEE 802.11 bb antenna module.

[0180] The calibration device 200 may be configured to calibrate the first OWC system 202 in response to a user or as part of an automated process. The automated process may, for example, use software, artificial intelligence or machine learning. The automated process may, for example, be initiated using software, artificial intelligence or machine learning. In a further embodiment, the calibration device 200 may be configured to continuously run the calibration.

[0181] In a further embodiment, the calibration device 200 may calibrate the first OWC system 202 at start up of the first OWC system 202, and may continue operating periodically or constantly.

[0182] Some or all of the calibration device 200 may be included within the first OWC system 202, or may be external to the first OWC system 202.

[0183] One or both of the OWC systems 202, 208 and / or the calibration device may be part of a router, a mobile device, a computer, a laptop, a medical device, a robotic device, an aeronautical vehicle, a drone or a wearable device.

[0184] Figure 2(b) is a schematic of a specific embodiment of the first OWC system 202, in accordance with a second embodiment of the present disclosure. The first OWC system 202 comprises an optical front end (OFE) module 210 comprising an receiver 212 that receives the OWC signal 204 and / or the test signal T1 during operation. The receiver 212 may convert the OWC signal 204 into an OWC electrical signal 214. The receiver 212 may be an optical receiver.

[0185] It will be appreciated that the test signal T1 and the OWC signal 204 may have different wavelengths such that the receiver 212 may be configured to be sensitive to more than one wavelength range. For example, the test signal T1 may be an RF signal, and the OWC signal 204 may be an optical signal - in such an example, the receiver 212 may be configured to detect both optical and RF wavelengths. For example, the receiver 212 may comprise an optical receiver component and an RF receiver component. In a further embodiment, the communication may occur through a different medium or channel. In a specific embodiment, the test signal T1 may comprise an acoustic signal.

[0186] The OFE module 210 may function as the electro-optical conversion block 110, as previously discussed in relation to Figure 1 (b) and may be referred to as a LiFi antenna module (LAM). The receiver 212 may comprise a light sensitive component or an array of light sensitive components. For example, the receiver 212 may comprise one or more of: a photodiode, a positive-intrinsic-negative (PIN) device, a phototransistors, a photoresistor, or a light activated silicon controlled rectifier.

[0187] The receiver 212 may comprise one or more of a photodiode, an array or an arrangement of photodiodes, a silicon PIN photodiode, a silicon photomultiplier (SiPM), a single photon avalanche diode (SPAD), a Graphene-CMOS high-resolution sensor, an avalanche photodiode (APD), a positive-intrinsic-negative (PIN) device, a phototransistor, a photoresistor, or a light activated silicon controlled rectifier. The SPAD may be a Geiger-mode APD (G-APD).

[0188] The receiver 212 may be configured to detect optical signals having an ultraviolet and / or visible and / or infrared wavelength. For example, the infrared wavelength may be greater than 1 ,000nm.

[0189] The receiver 212 may be configured to detect optical signals having a wavelength within a near infrared wavelength range, for example, within the range of 700 nm to 2500 nm. The OFE module 210 may be configured to function with data rates in a range of 10 Mbit / s to 9.6 Gbit / s, in accordance with the IEEE 802.11 bb standard. The IEEE 802.1 1 bb OFE module 210 may comprise a commercially available LAM1 ™, Purelifi Ltd (UK), product code PL10U01. The concept has applicability beyond 802.1 1 bb i.e. it may be applicable to any standard which is similar to or derived from 802.1 1 bb .

[0190] The receiver 212 may form part of a remote device that may provide access to a further network, for example a local area network (LAN) and / or the internet or other remote network.

[0191] The first OWC system 202 further comprises an RF communication module 216 configured to receive the OWC electrical signal 214. The RF communication module 216 may be configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz. The RF communication module 216 may be a WiFi chipset, as discussed previously, and may operate in accordance with the WiFi communication protocol. The WiFi chipset with baseband may be part of any WiFi enabled device.

[0192] The RF communication module 216 may be part of, for example: a router, a mobile device, a computer, a laptop, a medical device, a robotic device, an aeronautical vehicle, a drone or a wearable device.

[0193] Some, or all, of the calibration device 200 may be included as part of the RF communication module 216 or the OFE module 210, or may be provided as an external component. The RF communication module 216 may have an input frequency error tolerance of less than 25ppm. For example, between 0 and 5ppm. For example, between 0 and 1 ppm. For example, between 0 and 0.5ppm.

[0194] The first OWC system 202 may further comprise an interface module 218 that functions as an interface between the optical and RF components. Specifically, the interface module 218 is configured to adjust the frequency of the OWC electrical signal 214 in accordance with the requirements of the RF communication module 216.

[0195] The interface module 218 and the RF communication module 216 may function as the data encoding / decoding block 108 as described previously in relation to Figure 1 (b).

[0196] The interface module 218 may comprise an RF converter 220, such as an RF up / down converter. In a specific embodiment, the RF converter 220 may increase the frequency of the OWC electrical signal 214 prior to the OWC electrical signal 214 being received by the RF communication module 216.

[0197] Figure 2(c) is a schematic of a specific implementation of the RF converter 220 comprising a mixer 221 configured to receive the OWC electrical signal 214, a clock 222 configured to generate a clock signal CLK1 , and a first local oscillator (LO) 226 configured to provide a LO signal 228 that is dependent on the clock signal CLK1 . The mixer 221 is configured to adjust the frequency of the OWC electrical signal 214 using the LO signal 228.

[0198] In a specific embodiment, the calibration device 200 may be configured to apply a testing step, to determine whether calibration is required and / or if required, the calibration to apply to the first OWC system 202 that provides optimal interoperability, by plotting the speed of data transfer of one or both of the first OWC signal 204 and the first test signal T1 , against frequency. The testing step may be carried out at one or more time points. The first testing step time point may be at start up of the RF communication module 216 or initial connection to the OFE module 210. A software or algorithm may identify the calibration to apply at various temperatures for optimal operability every 1 , 2, 5, 10 or 20 minutes during use of the OWC system 202. A second testing step time point may be within between 1 minutes and 60 minutes of the RF communication module 216 start up, and may be between 5 minutes and 30 minutes from RF communication module 216 start up or initial connection to the OFE module 210. Additional time points can be every 1 to 30 minutes from the second time point while the RF communication module 216 is communicating with the OFE module 210. In a specific embodiment, the testing step and / or calibration process may be triggered by one or more sensors detecting that the RF communication module 216 is no longer operating within a preferred operational range. The one or more sensors may be part of the calibration device 200.

[0199] The one or more sensors may be one or more of a temperature sensor, a pressure sensor, a gyroscope sensor or a gravity sensor; an indicator is likely to be one or more of a change in the packet error statistics reported by the RF communication module.

[0200] The RF communication module operating temperature may vary during use due to its location, sun light exposure or any other exposure or changes in exposure to sources of heat and / or cold. Temperature affects error rates and may change the necessary calibration steps to provide optimal operation.

[0201] The RF communication module pressure may change with altitude, for example while traveling by road, rail or air. The user may intentionally or unintentionally shake or vibrate the RF communication module containing device and the RF communication module containing device may be used in low or zero gravity conditions (for example space stations or vehicles). As such, the calibration process may need to be adjusted accordingly.

[0202] Figure 2(d) is a schematic of a specific implementation of the calibration device 200 in accordance with a third embodiment of the disclosure. The calibration device 200 comprises a test signal generator 232 configured to generate a test signal T2. The calibration device 200 further comprises a comparison unit 234 that is configured to receive the test signals T1 , T2 and to determine the frequency error by comparing the test signals T1 , T2. The frequency error may be, for example, the difference in frequency between the test signals T1 , T2. The comparison unit 234 may provide a frequency error signal 235 that includes information on the frequency error as an output.

[0203] During operation the test signal generator 232 may receive the clock signal CLK1 , with the second test signal T2 being generated using the clock signal CLK1 . As both the LO signal 228 and the test signal T2 are generated using the same clock signal CLK1 , any error resulting from the clock 222 will be present on both signals 228, T2.

[0204] Figure 2(e) is a schematic of a specific embodiment of the second OWC system 208, in accordance with a fourth embodiment of the present disclosure. The second OWC system 208 may comprise an OFE module 231 , an interface module 233 comprising an RF converter 237, and an RF communication module 236. The OFE module 231 comprises a transmitter 238 that is configured to transmit the OWC signal 204 and / or the test signal T1. The test signal T1 may, for example, be transmitted periodically or continuously to maintain the link even though there may not be any active data communication between the OWC systems 202, 208. In a specific embodiment, the test signal T1 can be periodic by transmitting the test signal T1 at a different time period to the OWC signal 204.

[0205] The transmitter 238 may be an optical transmitter. The transmitter 238 may comprise one or more light transmitting elements. For example, the transmitter 238 may comprise one or more of a light emitting diode (LED), an array or an arrangement of LEDs, a laser, or a light-emitting plasma.

[0206] The one or more LEDs may be one or more of an OLED or a micro LED. The laser may be a vertical-cavity surface-emitting laser (VCSEL). The transmitter 238 may be configured to transmit optical signals having an ultraviolet and / or visible and / or infrared wavelength. For example, the infrared wavelength may be greater than 1 ,000nm.

[0207] The transmitter 238 may be configured to transmit optical signals having a wavelength within a near infrared wavelength range, for example, within the range of 700 nm to 2500 nm. The OFE module 231 may be configured to function with data rates in a range of 10 Mbit / s to 9.6 Gbit / s, in accordance with the IEEE 802.1 1 bb standard or any standard which is related to or derived from 802.11 bb . The IEEE 802.1 1 bb OFE module 231 may comprise a commercially available LAM1 ™, Purelifi Ltd (UK), product code PL10U01.

[0208] The transmitter 238 may be configured to convert an OWC electrical signal 240 into the OWC signal 204 for transmission. In operation, the RF communication module 236 provides the electrical signal 240 and the interface module 233 adjusts the frequency of the OWC electrical signal 240 for use by the OWC front end module 231 .

[0209] The RF communication module 236 may be configured to process electrical signals having a frequency within a range of 20 kHz to 300 GHz. The RF communication module 236 may be configured to process electrical signals within a frequency band, the frequency band being one of 860 / 900 MHz, 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, 5.9 GHz, 6 GHz, 45 GHz and 60 GHz. The RF communication module 236 may be a WiFi chipset, as discussed previously, and may operate in accordance with the WiFi communication protocol. The WiFi chipset with baseband may be part of any WiFi enabled device. The WiFi chipset with baseband may be referred to as a baseband chipset. The RF communication module 236 may be part of, for example: a router, a mobile device, a computer, a laptop, a medical device, a robotic device, an aeronautical vehicle, a drone or a wearable device.

[0210] The RF communication module 236 may have an input frequency error tolerance of less than 25ppm. For example, between 0 and 5ppm. For example, between 0 and 1 ppm. For example, between 0 and 0.5ppm.

[0211] It will be appreciated that the test signal T1 and the OWC signal 204 may have different wavelengths such that the transmitter 238 may be configured to transmit more than one wavelength range. For example, the test signal T1 may be an RF signal, and the OWC signal 204 may be an optical signal - in such an example, the transmitter 238 may be configured to transmit both optical and RF wavelengths. For example, the transmitter 238 may comprise an optical transmitter component and an RF transmitter component. In a further embodiment, the communication may occur through a different medium or channel. In a specific embodiment, the test signal T1 may comprise an acoustic signal.

[0212] In a further embodiment, the test signal T1 and the OWC signal 204 may be transmitted at different wavelengths and received at the same wavelength. In a further embodiment, the test signal T1 and the OWC signal 204 may be received at different wavelengths and transmitted at the same wavelength.

[0213] Figure 2(f) is a schematic of a specific implementation of the RF converter 237 comprising a mixer 242 configured to receive the OWC electrical signal 240, a clock 244 configured to generate a clock signal CLK2, and a second local oscillator (LO) 246 configured to provide a LO signal 248 that is dependent on the clock signal CLK2. The mixer 242 is configured to adjust the frequency of the OWC electrical signal 240 using the LO signal 248.

[0214] Figure 2(g) is a schematic of a specific implementation of the calibration device 200 in accordance with a fifth embodiment of the present disclosure. In the present embodiment the test signal T1 is generated by a test signal generator 252. The test signal generator 252 is configured to receive the clock signal CLK2 from the RF converter 237 and the test signal T1 is generated using the clock signal CLK2.

[0215] The calibration device comprises an adjustment signal generator 254 configured to receive the frequency error from the frequency error signal 235. The adjustment signal generator 254 is further configured to determine a suitable adjustment to apply to the first LO 226 of the RF converter 220 using the frequency error, and then provide an adjustment signal 256 for adjusting the LO 226. The LO 226 may be adjusted directly, or by a different component, such as by adjusting the clock 222. The frequency error may be the difference in frequency between the test signals T1 , T2.

[0216] As both test signals T1 , T2 are dependent on the clocks of their respective OWC systems 202, 208, the frequency error extracted by comparing the test signals T1 , T2 can be used to infer the error between the clocks, which can then be used to adjust the LO 226, and synchronise the LO 226 of the RF converter 220 to the LO 246 of the RF converter 237, thereby reducing the frequency error, and improving interoperability between the OWC systems 202, 208. The calibration device 200 may be referred to as a “synchroniser” when it is used to synchronise all, or part, of the operation of the OWC systems 202, 208.

[0217] The calibration device 200 may identify, by the frequency error, that the mismatch between the RF converters 220, 237 means that there is not an optimal interoperability between the OWC systems 202, 208. The calibration device 200 then corrects for this mismatch by generating the adjustment signal 256 and using it to adjust the performance of the LO 226.

[0218] In a specific embodiment, the calibration device 200 may comprise a detector (not shown) that, during operation of the OWC systems 202, 208, detects a parameter that is indicative of calibration being necessary, and then triggers the calibration procedure in response to the detection of the parameter. The detector may comprise a sensor, such as a temperature sensor, an optical sensor, a pressure sensor, a gyroscope or a gravity sensor, with the calibration process being in response to an appropriate sensed signal. The detector may additionally / alternatively comprise a data rate or signal error sensor, with the calibration process being in response to a change in the sensed data rate (e.g. a drop in data rate) or quantity or rate of sensed signal errors in a specified time period (e.g. a rise in rate of sensed errors).

[0219] In a specific embodiment, the calibration device 200 may comprise a receiver (not shown) that maintains a lock onto a frequency for optimal interoperability until a further calibration procedure is carried out. The calibration device 200 may be configured to use a clock that the transmitter and the receiver share to manage the local oscillator frequency output. The calibration method may synchronise the reference frequency from the WiFi chipset and LiFi antenna module to reduce any difference between the local oscillator frequencies.

[0220] Specific embodiments of the present disclosure may be used to achieve interoperability between WiFi boards that have significantly lower error tolerance when working with IEEE 802.11 bb LiFi devices. In specific embodiments, the calibration device 200 may use algorithms or software or artificial intelligence for automated calibration or more efficient calibration. In a specific embodiment, the calibration device 200 may be configured to run software to confirm optimal operability has been achieved after the calibration process has finished.

[0221] In a specific embodiment, the calibration device 200 may be configured to run algorithms that anticipate when a calibration procedure is required. The calibration device 200 may be configured to use machine learning to learn from each step of the calibration process in order to stabilise and maintain the optimal interoperability under a variety of conditions including environmental conditions, for example temperature, pressure, orientation, vibration and gravity.

[0222] In specific embodiments, the calibration device 200 may have feedback systems for failure prediction and improved working life periods, for example a digital twin and / or closed-loop system may be used separately or in combination.

[0223] Figure 3(a) is a schematic of a calibration device 300 for calibrating a first OWC system 302 that is in communication with a second OWC system 308, in accordance with a sixth embodiment of the present disclosure.

[0224] The calibration device 300 and the OWC systems 302, 308 may function substantially as described for the calibration device 200 and the OWC systems 202, 208, respectively, in accordance with the understanding of the skilled person. In the present embodiment, the first OWC system 302 is a specific embodiment of the first OWC system 202 that is further configured to transmit a second OWC signal 304 that may be received by the second OWC system 308.

[0225] Figure 3(b) is a schematic of a specific embodiment of the first OWC system 302, where the OFE module 210 comprises a transmitter 306 that is configured to transmit the second OWC signal 304. The transmitter 306 may be an optical transmitter. The transmitter 306 may comprise one or more of a light emitting diode (LED), an array or an arrangement of LEDs, a laser, or a light-emitting plasma.

[0226] The one or more LEDs may be one or more of an OLED or a micro LED. The laser may be a vertical-cavity surface-emitting laser (VCSEL). The transmitter 306 may be configured to transmit optical signals having an ultraviolet and / or visible and / or infrared wavelength. For example, the infrared wavelength may be greater than 1 ,000nm.

[0227] The transmitter 306 may be configured to transmit signals having a near infrared wavelength, for example, within a range of 700 nm to 2,500 nm. The transmitter 306 is configured to convert an OWC electrical signal 307 in the OWC signal 304 for transmission. In operation, the OWC electrical signal 307 is provided by the RF communication module 216 via the RF converter 220 of the interface module 218. The OWC signal 304 may be used to transmit data from the OWC system 302 to the OWC system 308.

[0228] Figure 3(c) is a schematic of a specific example embodiment of the RF converter 220 for the OWC system 302. In the present example embodiment, the RF converter 220 comprises a mixer 310, and a local oscillator 312 for generating a local oscillator signal 314.

[0229] In operation the local oscillator 312 provides the local oscillator signal 314 that is dependent on the clock signal CLK1 , and the mixer 310 is configured to adjust the frequency of the OWC electrical signal 307 using the local oscillator 312.

[0230] It will be appreciated that in a further embodiment, the RF converter 220 may comprise a single bi-directional mixer implementing the functionality of the mixers 221 , 310 and a single local oscillator implementing the functionality of the local oscillators 226, 312, as the transmission and receiving processes can occur at different times and therefore share common circuit elements.

[0231] Figure 3(d) is a schematic of a specific embodiment of the calibration device 300, being a specific implementation of the calibration device 200. In the present embodiment, the adjustment signal generator 254 is configured to determine an adjustment to be applied to the local oscillator 226 (being part of the receiving portion of the OWC system 302) and the local oscillator 312 (being part of the transmission portion of the OWC system 302). An adjustment signal 320 is provided to the local oscillator 312 to adjust the local oscillator 312.

[0232] As both local oscillators 226, 312 are dependent on the same clock 222, it is possible to determine the appropriate adjustment to apply to both local oscillators 226, 312 using the test signal T1 as provided by the OWC system 308 and the test signal T2 as generated by the clock 222 of the OWC system 302.

[0233] Figure 4(a) is a schematic of a calibration device 400, an OWC system 402 and an OWC system 408 in accordance with a seventh embodiment of the present disclosure.

[0234] The calibration device 400 is a specific implementation of the calibration device 200 of Figure 2(a) and may comprise any additional features relating to a calibration device as described herein, and in accordance with the understanding of the skilled person. The OWC system 402 is a specific implementation of the OWC system 202 of Figure 2(a) and may comprise any additional features relating to an OWC system as described herein, and in accordance with the understanding of the skilled person.

[0235] The OWC system 408 is a specific implementation of the OWC system 208 of Figure 2(a) and may comprise any additional features relating to an OWC system as described herein, and in accordance with the understanding of the skilled person.

[0236] In the present embodiment, the comparison unit 234 comprises a mixer 404.

[0237] In non-limiting examples, the mixer 404 may comprise at least one of an unbalanced mixer, a single balanced mixer, a double balanced mixer and / or a switching mixer. Other suitable types of mixers and mixer topologies will be apparent to those skilled in the art.

[0238] Figure 4(b) is a schematic of a switching mixer 404’ which may be used as the mixer 404 in the comparison unit 234 of Figure 4(a). The switching mixer 404’ comprises a switch 405 configured to be selectively ‘switched’ by the generated test signal (in this example the first test signal T1 ) so as to ‘switch’ or sample the received test signal (in this example the second test signal T2) to produce the error signal 235.

[0239] In this example, the error signal 235 may comprise a ‘spectral component’ with a frequency that is an estimate of the difference between the frequency of the first test signal T 1 and the second test signal T2, and a spectral component with a frequency that is an estimate of the sum of the frequency of the first test signal T 1 and the second test signal T2.

[0240] Returning now to Figure 4(a), the adjustment signal generator 254 comprises an anti-aliasing filter 406 configured to provide a feedback signal 410 having a frequency that is an estimate of the difference between the frequency of the test signals T 1 and T2. The anti-aliasing filter 406 may comprise a low pass filter. The low pass filter may be configured to filter out a / the spectral component of the error signal 235 having a frequency that is an estimate of the sum of the frequency of the first test signal T1 and the second test signal T2. The low pass filter may be configured to pass a / the spectral component of the error signal 235 having a frequency that is an estimate of the difference in frequency between the frequency of the first test signal T 1 and the second test signal T2 (e.g. the frequency error signal).

[0241] The adjustment signal generator 254 further comprises an analog to digital converter (ADC) 412 for receiving the feedback signal 410 and converting the feedback signal to a digital signal 414. Figure 4(c) is a schematic of a frequency counter circuit 412’ which may be used in place of the ADC 412 of the adjustment signal generator 254 of Figure 4(a). In this example, the frequency counter circuit 412’ comprises a comparator 415 configured to receive the feedback signal 410 (of Figure 4), compare the feedback signal 410 to a constant reference voltage 413, and thereby provide a square wave signal 417 at its output having a frequency that is an estimate of the frequency of the feedback signal 410.

[0242] The frequency counter circuit 412’ further comprises a frequency counter 420 configured to receive the square wave signal 417 and a comparatively higher frequency reference clock signal 419, and thereby provide a measurement of the frequency of the square wave signal 417, and by extension also an estimate of the frequency of the feedback signal 410 for each period of the feedback signal 410.

[0243] As will be appreciated by those skilled in the art, the resolution of the frequency estimate will depend, at least in part, on the frequency of the reference clock signal 419. Similarly, the minimum frequency will depend, at least in part, on the binary width of the hardware implementation of the frequency counter 420.

[0244] Any other suitable circuit may be used to produce the digital signal 414 from the feedback signal 410. For example, in the frequency counter circuit 412’ the comparator 415 may be replaced by a ‘limiting amplifier’ circuit.

[0245] Returning again to Figure 4(a), the adjustment signal generator 254 further comprises a determination unit 416 configured to receive the digital signal 414, and to determine the adjustment using the digital signal 414.

[0246] The interface module 218 may comprise a FRAC N register 418 configured to update based on the received adjustment signal and to adjust the local oscillator 226 based on the update of the FRAC N register 418. The FRAC N register 418 is used for fractional N frequency synthesis alongside the local oscillator 226, the functioning of which will be clear to the skilled person.

[0247] By comparing the “tones” (being the test signals T1 , T2 as previously discussed) generated on either side of the communication link between the OWC systems 402, 408, we can measure the difference between them. Once the frequency delta between the tones is known, we can infer the delta between the local oscillators 226, 246 and align their frequencies by fine tuning the FRAC N register 418. The tones may be referred to as “pilot tones”, which may be referred to as “reference frequencies”.

[0248] The concept relating to the pilot tones is to have the TX and the RX tones having a controlled frequency distance and by mixing the TX and RX tones, we get a low frequency component with frequency equal to their difference. Monitoring the drift on this frequency provides a means to estimate how far apart TX and RX are.

[0249] In further specific embodiments, the calibration device 200 may comprise at least one of a Fast Fourier Transform (FFT) processor, a time-domain processing module, a Digital to Analog Converter (DAC), an Analog to Digital Converter (ADC), and a frequency counter.

[0250] A wideband transformer coupler is used to couple the pilot tone in. However, the coupler is inefficient as it adds 3 dB loss on both paths, but it allows for a flat bandwidth on the main channel.

[0251] Another option would be to add the pilot through a resistive coupling. It would have high loss on the pilot itself, but very low loss for the main signal. The OpAmp predriver would help adding isolation between the pilot and the WiFi card. Another option would be to replace the 3dB coupler by a directional coupler.

[0252] Figure 5(a) is a schematic of a specific embodiment of the OWC system 408. In the present the OWC system 408 is configured to transmit and receive optical signals, and also has a calibration device 600 that enables calibration of the OWC system 408, the operation of which will be clear to the skilled person based on the description provided herein.

[0253] In the present example, the calibration device 600 comprises a mixer 602 and a low pass filter 604. The interface module 233 comprises a diplexer 612, a switch 614 and directional couplers 618, 620. The RF converter 237 of the interface module 233 comprises a mixer 606 and a local oscillator 608.

[0254] Figure 5(b) is a schematic of a specific embodiment of the OWC system 402. The interface module 218 comprises a diplexer 622, a switch 624 and directional couplers 628, 630.

[0255] A specific embodiment of the correction algorithm to determine the necessary adjustment to be performed may be summarised as follows.

[0256] Step 1 :

[0257] The frequency of the pilot tone is estimated, labelled “Feedback” in Figure 5(b) and in the embodiment illustrated in Figure 5(b) is provided by the feedback signal 410. “Feeback” is calculated using the following equation:

[0258] Feedback = Delta Pilot detect x (Ref CLK local / Ref CLK design) (7) where Delta Pilot detect is the difference between T 1 and T2 and provided by the frequency error signal 235. Ref CLK local is the frequency of the clock 222 including the error and therefore representative of the true value during operation. The error in the present example is -1 Oppm. Ref CLK design is the designed frequency of the clock 222 without the inclusion of an error. Feedback may represent the difference between the pilot tones (Delta Pilot detect) with an error.

[0259] Step 2:

[0260] The frequency error compared to the expected pilot tone frequency (the “Design Delta”) is calculated as follows: error = Feedback - Design Delta (8)

[0261] Step 3:

[0262] Corrected N1 and N2 values for the receiver FRAC N phase locked loop in order to compensate the estimated error are calculated as follows:

[0263] N1 corrected = N1 x(1 +ERROR / (Ref design x N3) (9)

[0264] N2corrected = N2 x (1 +error / (Ref design x N3) (10)

[0265] Where N1 is equal to the designed local oscillator 312 frequency divided by the designed clock 222 frequency, N2 is equal to the designed local oscillator 226 frequency divided by the designed clock 222 frequency, Ref design is the designed clock 222 frequency and N3 is the designed pilot tone frequency divided by the design clock 222 frequency. The designed pilot tone frequency is the frequency of the test signal T 1 .

[0266] Once N1 corrected and N2corrected are known, the local oscillators 226, 312 may be adjusted, for example, by adjusting the FRAC N registers.

[0267] Figure 6 is a flowchart of a heuristic method 1000 of calibrating an OWC system in accordance with an example embodiment of the present disclosure. It should be appreciate that the method may be applied to an OWC system as described throughout this disclosure or any other suitable OWC resource.

[0268] At step 1 100 the calibration is initiated. The initiation of the calibration may be at start up of the OWC system or initial connection to the OWC system by a further OWC system. The initiation of the calibration may be periodic during operation of the OWC system and / or may be triggered by one or more sensors detecting a change in a performance metric.

[0269] At step 1200 a performance metric is measured which may be the performance metric which triggered the calibration or may be an alternative performance metric. The performance metric may comprise at least one of: a data transfer rate, a packet error rate, latency, however other suitable performance metrics will be apparent to those skilled in the art.

[0270] At step 1300 the performance metric is compared to an optimised (or target) value to determine whether the performance metric is optimised e.g. whether the value of the performance metric is within a threshold of an optimal value. If the performance metric is not determined to be optimised, at step 1400 an adjustment is made to the OWC system. In this example a frequency e.g. an LO frequency of the OWC system is adjusted, however other suitable adjustments will be apparent to those skilled in the art. Steps 1200 and 1300 (and if required step 1400) are then repeated until the performance metric is determined to be optimised.

[0271] At step 1500 the normal operation of the OWC system continues until a further calibration is initiated. In this example, normal operation of the OWC system is interrupted to perform the calibration, however in other examples the calibration may be performed in parallel with normal operation to minimise any effect on the OWC system from the calibration.

[0272] In the above-described embodiments, it will be understood that for optical wireless communication, any suitable modulation scheme may be used. In embodiments, the optical signals transmitted between systems comprise optical signals modulated in accordance with a suitable scheme. For example, orthogonal frequency division multiplexing (OFDM) modulation schemes are used in some embodiments, and the demodulation is from the OFDM modulation scheme. In further embodiments and without limitation, other modulation schemes may be used, for example on-off keying (OOK), phase shift keying (PSK), M-ary pulse amplitude modulation (M-PAM), M-ary quadrature amplitude modulation (M-QAM), Discrete Hartley transformation, Wavelet packet division multiplexing (WPDM), Hadamard coded modulation (HCM), pulseposition modulation (PPM), Colour shift keying (CSK), carrier-less amplitude and phase (CAP), or discrete multi-tone (DMT). The light may be modulated at a modulation rate between 1 kHz and 1 PHz, for example at a modulation rate between 1 MHz and 100 GHz in some embodiments. The modulation scheme may form part of an OWC communication protocol, such that the optical signal is produced according to the OWC communication protocol. The OWC communication protocol may be packet-based. In embodiments, one or more of the modules may be configured to connect to a network, for example, via an OWC enabled access point. According to various embodiments, the OWC signals may be in accordance with one or more of IEEE 802.15.7, 802.15.13, 802.11 or extensions or developments thereof; ITU-T G.9960 or extensions or developments thereof; or ITU-T G.vlc or extensions or developments thereof.

[0273] The modulation scheme may form part of an OWC communication protocol, such that the optical signal is produced according to the OWC communication protocol. The OWC communication protocol may be packet-based.

[0274] The terms “having”, “containing”, “including”, “comprising” and the like are open and the terms indicate the presence of stated structures, elements or features but do not preclude the presence of additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

[0275] Although the disclosure has been described in terms of specific embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS1 . A calibration device for a first optical wireless communication (OWC) system, the first OWC system being configured to receive a first test signal from a second OWC system; wherein: the calibration device is configured to calibrate the first OWC system based on a frequency error determined using the first test signal.

2. The calibration device of claim 1 , wherein the first OWC system is configured to receive a first OWC signal from the second OWC system.

3. The calibration device of claim 2 comprising: a test signal generator configured to generate a second test signal; and a comparison unit configured to: receive the first and second test signals; and determine the frequency error by comparing the first and second test signals.

4. The calibration device of claim 3, wherein the frequency error is the difference between the first and second test signals.

5. The calibration circuit of any preceding claim, wherein the first OWC signal is, comprises, or is comprised in the first test signal.

6. The calibration device of any preceding claim, wherein the first OWC system comprises a first optical front end module comprising a first receiver configured to receive the first OWC signal and / or the first test signal.

7. The calibration device of claim 6, wherein: the first receiver is configured to convert the first OWC signal into a first OWC electrical signal; and the first OWC system comprises: a first radio frequency (RF) communication module configured to receive the first OWC electrical signal; anda first interface module configured to adjust the frequency of the first OWC electrical signal for the first RF communication module, prior to the first OWC electrical signal being received by the first RF communication module.

8. The calibration device of claim 7, wherein the first interface module comprises a first RF converter.

9. The calibration device of claim 8, wherein: the first RF converter comprises: a first mixer configured to receive the first OWC electrical signal; a first clock configured to generate a first clock signal; a first local oscillator configured to provide a first local oscillator signal that is dependent on the first clock signal; wherein: the first mixer is configured to adjust the frequency of the first OWC electrical signal using the first local oscillator signal.

10. The calibration device of claim 9 comprising: a first test signal generator configured to generate a second test signal; and a comparison unit configured to: receive the first and second test signals; and determine the frequency error by comparing the first and second test signals.1 1. The calibration device of claim 10, wherein the frequency error is the difference between the first and second test signals12. The calibration device of claim 10 or 11 , wherein the first test signal generator is configured to receive the first clock signal, and the second test signal is generated using the first clock signal.

13. The calibration device of claim 12, wherein the second OWC system comprises a second optical front end module comprising a second transmitter configured to transmit the first OWC signal and / or the first test signal.

14. The calibration device of claim 13, wherein:the second transmitter is configured to convert a first OWC electrical signal into the first OWC signal; and the second OWC system comprises: a second radio frequency (RF) communication module configured to provide the first OWC electrical signal; and a second interface module configured to adjust the frequency of the first OWC electrical signal for the second OWC front end module, prior to the first OWC electrical signal being received by the second transmitter.15 The calibration device of claim 14, wherein the second interface module comprises a second RF converter.

16. The calibration device of claim 15, wherein: the second RF converter comprises: a second mixer configured to receive the first OWC electrical signal; a second clock configured to generate a second clock signal; a second local oscillator configured to provide a second local oscillator signal that is dependent on the second clock signal; wherein: the second mixer is configured to adjust the frequency of the first OWC electrical signal using the second local oscillator signal.

17. The calibration device of claim 16, wherein the first test signal is generated by a second test signal generator.

18. The calibration device of claim 17, wherein the second test signal generator is configured to receive the second clock signal, and the first test signal is generated using the second clock signal.

19. The calibration device of claim 18, comprising an adjustment signal generator configured to: receive the frequency error; determine an adjustment to apply to the first local oscillator using the frequency error; and provide an adjustment signal for adjusting the first local oscillator.

20. The calibration device of claim 19, wherein, the frequency error is the difference between the first and second test signals21. The calibration device of any of claims 2 to 5, wherein the first OWC system is configured to transmit a second OWC signal.

22. The calibration device of claim 21 , wherein the first OWC system comprises: a first optical front end module comprising: i) a first receiver configured to receive the first OWC signal and / or the first test signal; and ii) a first transmitter configure to transmit the second OWC signal.

23. An optical wireless communication apparatus comprising: the calibration device of any preceding claim; and the first OWC system.

24. An optical wireless communication network comprising the optical wireless communication apparatus of claim 23 and the second OWC system.

25. A method of calibrating the first OWC system using the calibration device of any of claims 1 to 22.

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