Wireless optical communication device, system and communication method

Abstract

The LiFi transmitter has a first light emitting element and a second light emitting element with different optical characteristics (wavelength or polarization). A bias circuit sets an operating point and a bias condition (e.g., a bias current) for the first and second light emitting elements. A first polarity (e.g., positive) of the modulation current is driven through the first light emitting element, and a second polarity (e.g., negative) of the modulation current is driven through the second light emitting element, thereby implementing bipolar OFDM modulation. A corresponding receiver has a first light sensing element and a second light sensing element with different sensing characteristics and a bias circuit. The sensed differential current signal is amplified and it is generated by the first or second light sensing element depending on the polarity of the OFDM modulation, thereby enabling bipolar OFDM reception.

Description

Technical Field

[0001] The present invention relates to an optical wireless communication system. Background Art

[0002] Li-Fi (Light Fidelity) is a new type of optical wireless communication (OWC), which also includes visible light communication (VLC). OWC (and thus Li-Fi and VLC) uses light as a communication medium to replace cable (wired) communication.

[0003] Communication based on light provides the ability for high data rate communication (e.g., even exceeding 10 Gbit / s) for these devices that have a line of sight between the devices. This is applicable, for example, to a group of communication devices within an office environment.

[0004] Known Li-Fi products rely on a grid of optical access points installed on the ceiling. The beams of these access points are wide enough (and thus have a large field of view and / or coverage area) to create an overlap with adjacent access points on the horizontal plane under the table. Receiving devices in such a system are typically located at the table or are being held at a height close to the table.

[0005] For ease of installation, the access point grid is aligned, for example, with the grid of luminaires in the ceiling. Each access point in such an installation must reach (in the case of visible light, illuminate) several square meters and thus illuminate a rather large conical area. Such an installation can utilize the illumination light for the downlink (to the terminal device), and can use invisible light (such as infrared or ultraviolet) for the uplink (towards the access point) so as not to disturb mobile device users. Alternatively, both the downlink and the uplink can utilize invisible light, thereby at least partially or completely disentangling the illumination and communication infrastructure.

[0006] To communicate with the access point, currently a dongle is connected to a user device (such as a laptop or a tablet). These dongles also emit similar wide beams to ensure that at least one access point will receive the signal from the dongle. The beams of the access point and the dongle are fixed in direction, so no adjustment of the beam direction is required.

[0007] Each access point includes a modem connected to one or more optical transceivers. Terminal devices (e.g., a laptop with a dongle) are connected to the access point via an optical link, and they also include a modem connected to one or more transceivers.

[0008] The function of a modem is to handle the protocols (modulation and demodulation) for transmitting and receiving data over a visible or invisible light connection. The modem transmitter includes an optical front end that converts the electrical signal of the transmitted data into an optical signal / light signal (e.g., using an LED), and the modem receiver converts the optical signal / light signal into an electrical received data signal (using a photodiode).

[0009] (In the THz range) A large unlicensed bandwidth can be used for Li-Fi communication. Li-Fi typically uses orthogonal frequency division multiplexing (OFDM) signaling to achieve high-speed communication with bandwidth-constrained electro-optical transmitters such as LEDs or vertical cavity surface emitting lasers (VCSELs).

[0010] Since OFDM signaling utilizes bipolar signals, a typical approach for Li-Fi is to use a DC-biased optical OFDM (DCO-OFDM) transmitter that couples the OFDM signal onto a DC bias. Due to the high peak-to-average power ratio (PARR) of the OFDM signal, a high DC bias is required to achieve a low-distortion optical OFDM signal.

[0011] Several methods have been proposed to create unipolar OFDM signals, including asymmetrically clipped optical OFDM, ACO-OFDM, Flip-OFDM, etc. These all aim to reduce power consumption by eliminating (or reducing) the high DC bias of the electro-optical components.

[0012] One proposed solution is described in the article "Color Multiplexing Based Unipolar OFDM for Indoor RGB LED Visible Light Communication" by Chen Chen et al., Procedia Engineering Vol.140, 2016, pp.159-165. This method is called CMO-OFDM and involves transmitting the positive and negative parts of the OFDM signal at two different wavelengths, using two optical receivers with corresponding optical filters, and combining the OFDM signals in the electrical domain or in the logical domain (after digitization and before demodulation).

[0013] However, it is not possible to use existing OFDM chip sets to transmit or receive using the CMO-OFDM method. In particular, it provides the separation of the modulation signal into components of different polarities before A / D conversion, thus requiring two A / D converters. Summary of the Invention

[0014] The present invention is defined by the claims.

[0015] According to an example of one aspect of the present invention, there is provided a transmitter circuit for transmitting data using optical wireless communication, comprising:

[0016] A first light-emitting element arranged to emit light having a first optical characteristic;

[0017] A second light-emitting element, which is topologically in series with the first light-emitting element and is arranged to emit light having a second optical characteristic different from the first optical characteristic;

[0018] A biasing circuit for setting the biasing conditions of the light-emitting elements;

[0019] A driving circuit for driving the first light-emitting element and the second light-emitting element using a bipolar modulation signal such that a first polarity of the bipolar modulation signal is used to drive through the first light-emitting element to a reference terminal, and an opposite second polarity of the bipolar modulation signal is used to drive through the second light-emitting element to the reference terminal, thereby achieving bipolar OFDM modulation. The OFDM signaling is bipolar and both polarities are used for transmission such that capacity is not sacrificed. The light-emitting elements are unidirectional components such as LEDs, VCSELs, edge-emitting laser diodes or other diode laser components, and they are connected such that one is in the forward current direction (with respect to the connection from the driving circuit to the light-emitting element) and the other is in the negative current direction. The term "topologically in series" refers to the physical electrical layout of the light-emitting elements, for example where the series connection is anode 1 - cathode 1 - anode 2 - cathode 2. Electrically, this provides a series connection for DC signals, but for AC signals, this connection can be considered parallel.

[0020] One light-emitting element is used for light generation during one polarity of the OFDM modulation signal, and the other is used for light generation during the opposite polarity of the OFDM modulation signal. The bias generated by the biasing circuit shifts the operating points of the first light-emitting element and the second light-emitting element such that the separation of the opposite polarity components occurs in the optical domain, and a single bipolar modulation signal drives the two light-emitting elements in a time-interleaved manner. Thus, there is a single bipolar modulation signal that requires a single A / D conversion. This means that existing OFDM chip sets can be used.

[0021] The first polarity of the bipolar modulation signal is not intended to generate any light output from the second light-emitting element, and the second polarity of the bipolar modulation signal is not intended to generate any light output from the first light-emitting element.

[0022] The first and second optical characteristics allow the separation of the two polarities of the modulation at the receiver side.

[0023] A known method of transmitting a bipolar OFDM signal over a unipolar channel is to add a DC bias of half the total signal amplitude. In contrast, the present invention enables the use of different light emitting elements, each with a minimal DC bias such that only the relevant half of the voltage swing is used in the forward mode. Thus, separation is achieved in the optical (wavelength or polarization) domain, and this saves power by avoiding the need for a large bias voltage or current. In particular, the proportion of the total power consumption of the output stage required by the DC bias function is reduced (e.g., from approximately 50% of the total power consumption to approximately 20%, but this depends on the application).

[0024] The first and second optical characteristics may be different wavelength characteristics and / or different polarization characteristics.

[0025] Different wavelengths include, for example, wavelength ranges having different intensity versus wavelength distributions, e.g., having different wavelengths at the peak intensity, i.e., different dominant wavelengths. However, the broad spectra of different LEDs may overlap. This will have an impact on the signal-to-noise ratio, but the system does not require perfect separation in order to achieve a suitable signal-to-noise ratio.

[0026] For example, for a fixed transmitter-receiver pair, circular polarization modulation or linear polarization is used to achieve different polarization characteristics, where the polarization direction can be correctly aligned during setup / network initialization.

[0027] In all cases, the opposite polarities of the modulation signal can be separated in the optical domain rather than in the electrical domain.

[0028] The drive circuit includes, for example, an OFDM modulator circuit.

[0029] The drive circuit may include an OPAMP circuit. For example, the drive circuit includes an OPAMP that is controllable in inverting and non-inverting operating modes. Thus, both polarities of bipolar OFDM modulation can be used.

[0030] The drive circuit includes, for example, a negative feedback circuit connected to the inverting input terminal of the OPAMP. This is used to regulate the output of the OPAMP.

[0031] The transmitter circuit includes, for example, a current sensor for sensing the current flowing to a reference terminal, and the feedback circuit includes a coupling capacitor for coupling the current sensing voltage to the inverting input terminal of the OPAMP.

[0032] This current feedback causes the circuit to operate in current control mode rather than in voltage control mode. At least for LEDs, the light output intensity is (more) related to the drive current, so operating the circuit in current control mode avoids the problems of the non-linearity of the I-V characteristic and the non-linearity of the light output with respect to voltage.

[0033] The transmitter circuit includes, for example, a first AC coupling circuit between the output of the drive circuit and the first and second light-emitting elements. A second AC coupling circuit may also be provided between the first and second light-emitting elements and the reference terminal.

[0034] The light-emitting element includes an LED or a VCSEL.

[0035] The present invention also provides a receiver circuit for receiving data using optical wireless communication, comprising:

[0036] A first photosensing element having a first photosensing characteristic for sensing an incident OFDM-modulated optical signal and having a first optical characteristic;

[0037] A second photosensing element having a second photosensing characteristic, which is connected in series with the first light-emitting element having the same polarity in terms of topology, for sensing an incident OFDM-modulated optical signal having a second optical characteristic, the second photosensing characteristic being different from the first photosensing characteristic, wherein the first and second photosensing elements are connected at a junction node;

[0038] A bias circuit for setting the bias conditions of the first and second photosensing elements; and

[0039] An amplifier for receiving the sensed current signal from the junction node, the sensed current signal being generated by the first or second photosensing element depending on the polarity of the OFDM modulation, thereby enabling bipolar OFDM reception.

[0040] The photosensing elements operate in a differential manner (since the amplifier is connected to the node between them), such that the signal passed to the amplifier is the difference between the currents of the two photosensing elements. This, in combination with an optical filter, enables the receiver to receive OFDM signals generated using the transmitter circuit as claimed. The photosensing elements are, for example, photodiodes or phototransistors.

[0041] The two received polarities are received as a single modulation signal.

[0042] The receiver circuit includes, for example, an OFDM demodulator circuit for receiving the amplified sensed current signal.

[0043] The photosensing characteristics include, for example, different wavelength reception characteristics. Corresponding bandpass optical filters may be provided for the first and second photosensing elements. This provides a narrow reception bandwidth for each photosensing element.

[0044] For example, an RF blocking circuit is provided for receiving any DC imbalance between the sensed current signals of the first and second photosensing elements.

[0045] The light sensing element includes, for example, a photodiode.

[0046] The present invention also provides a wireless optical communication system, which includes:

[0047] One or more transmitter circuits, each transmitter circuit being defined as above; and

[0048] One or more receiver circuits, each receiver circuit being defined as above.

[0049] The present invention also provides a method for transmitting data using optical wireless communication, which includes:

[0050] Setting the bias conditions of a first light emitting element and a second light emitting element, wherein the first light emitting element is arranged to emit light with a first optical characteristic, and the second light emitting element is topologically in series with the first light emitting element and is arranged to emit light with a second optical characteristic different from the first optical characteristic;

[0051] Driving a first polarity of a bipolar modulation signal to a reference terminal through the first light emitting element; and

[0052] Driving a second polarity of the bipolar modulation signal opposite to the first polarity to the reference terminal through the second light emitting element,

[0053] Thereby realizing bipolar OFDM modulation.

[0054] The present invention also provides a method for receiving data using optical wireless communication, which includes:

[0055] Sensing an incident OFDM modulated optical signal with a first optical characteristic using a first light sensing element having a first light sensing characteristic;

[0056] Sensing an incident OFDM modulated optical signal with a second optical characteristic using a second light sensing element, the second light sensing element being topologically in series with the first light emitting element having the same polarity, the second sensing element having a second light sensing characteristic different from the first light sensing characteristic; the first light sensing element and the second light sensing element are connected at a junction node;

[0057] Setting the bias conditions of the first light sensing element and the second light sensing element; and

[0058] Receiving a sensed current signal from a junction node between the first light sensing element and the second light sensing element, the sensed current signal being generated by the first or second light sensing element depending on the polarity of the OFDM modulation,

[0059] Thereby realizing bipolar OFDM reception.

[0060] The present invention provides options for designing and building compact, power-efficient OFDM transmitters and receivers using commonly available components.

[0061] These and other aspects of the present invention will be apparent and elucidated with reference to the (one or more) embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] For a better understanding of the present invention, and to more clearly show how the present invention may be implemented, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0063] Figure 1 A typical LiFi system is shown;

[0064] Figure 2 The architecture of an OFDM transmitter is shown;

[0065] Figure 3 Shows the current path through Figure 2 the circuit;

[0066] Figure 4 An example of the relationship between the LED forward current and the forward voltage characteristic is shown, and how to achieve the bias point and modulation is shown; and

[0067] Figure 5 The architecture of an OFDM receiver is shown. DETAILED DESCRIPTION

[0068] The present invention will be described with reference to the drawings.

[0069] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems, and methods, are intended for illustrative purposes only and are not intended to limit the scope of the present invention. These and other features, aspects, and advantages of the apparatus, systems, and methods of the present invention will become better understood from the following description, the appended claims, and the drawings. It should be understood that the drawings are merely schematic and are not drawn to scale. It should also be understood that in all the drawings, the same reference numerals are used to indicate the same or similar parts.

[0070] The present invention provides a LiFi transmitter having a first light-emitting element and a second light-emitting element with different optical characteristics (wavelength or polarization). A biasing circuit sets an operating point and sets biasing conditions for the first light-emitting element and the second light-emitting element. A first polarity (e.g., positive) of a modulation current is driven through the first light-emitting element, and a second polarity (e.g., negative) of the modulation current is driven through the second light-emitting element, thereby implementing bipolar OFDM modulation. A corresponding receiver has a first photosensing element and a second photosensing element with different sensing characteristics and a biasing circuit. A sensed differential current signal is amplified and is generated by the first or second photosensing element depending on the polarity of the OFDM modulation, thereby implementing bipolar OFDM reception.

[0071] Figure 1 A typical LiFi system is shown, where a set of access points (APs) 10 form a ceiling-mounted infrastructure, and LiFi terminal devices (also called stations, STAs) are implemented by dongles 14 attached to mobile devices such as laptops 12. The combination of units 12 and 14 is the terminal device (also called an endpoint). The access points are preferably linked to a backbone network, for example, by means of a wired link such as an Ethernet link using twisted pair cables or a fiber optic network, allowing the access points and / or a global system controller to align, for example, during handover.

[0072] Each AP includes a modem connected to one or more LiFi transceivers. The terminal device can be connected to the AP via an optical link. Each terminal device also includes a modem connected to one or more LiFi transceivers. The function of the LiFi-modem is to handle the physical layer (PHY) and medium access control layer (MAC) protocols to transmit and receive data over a visible or invisible light connection.

[0073] The LiFi transceiver includes a transmitter for converting an electrical signal of the transmitted data of the modem into an optical signal / light signal (e.g., via an LED, VCSEL, or laser diode), and provides a receiver for converting the optical signal / light signal into an electrical signal of the received data of the modem (e.g., via a photodiode). The terminal device is implemented, for example, by a dongle 14 attached to a mobile device such as a laptop. Instead of a retrofit, it is envisioned that the receiver functionality is ideally integrated with the user receiving device itself, in such a way that laptops, tablets, mobile phones, and / or other devices can use optical communication without the need for a dongle.

[0074] The present invention can be explained from different perspectives. On the one hand, it provides a power - efficient alternative for DCO - OFDM transmission. On the other hand, it also provides a solution that allows a typical bipolar OFDM modulator to be used to implement the CMO - OFDM signaling method outlined above. The prior - art CMO - OFDM method uses a special base - band chip - set that divides the generated OFDM data into a positive part and a negative part in the digital domain (before digital - to - analog conversion). It is a theoretical solution that cannot be used with existing OFDM chip - sets (e.g., G.hn, G.vlc).

[0075] The present invention provides a solution that enables existing OFDM chip - sets to implement CMO - OFDM signaling, i.e., transmission and reception.

[0076] In particular, the present invention provides transmitter and receiver topologies that allow the use of an OFDM modulator that outputs a bipolar electrical signal for use in an optically intensity - modulated (IM) OFDM system, which means they can use existing OFDM chip - sets with single A / D and D / A conversion capabilities.

[0077] Figure 2 The architecture of a (CMO - )OFDM transmitter 100 is shown.

[0078] The transmitter 100 includes a high - bandwidth, high - output - current OPAMP 101 that can be used in both inverting and non - inverting configurations depending on the input circuit 102 and the feedback circuit 110. The OPAMP receives a drive signal from the input circuit 102, which includes a D / A converter and an OFDM modulator circuit for the modulation signal. The OPAMP 101, the output circuit 102, and the feedback circuit 110 together can be considered to form a drive circuit for driving analog current modulation.

[0079] The input circuit includes an OFDM modulator circuit. The input circuit provides signals to the inverting and non - inverting input terminals of the OPAMP 101 such that the OPAMP can operate in inverting and non - inverting modes. The feedback circuit 110 is connected to the inverting input terminal and the output terminal of the OPAMP to provide a negative - feedback loop. The feedback circuit is also optionally controlled by the output from a current - sensing circuit, as discussed further below.

[0080] Two light-emitting elements with different optical characteristics are provided, such as LEDs or VCSELs 104, 105. These different optical characteristics enable the separation of the generated optical signals / light signals on the receiver side in the optical domain. The following description is for a preferred example of different wavelengths, i.e., different wavelengths at which the output intensity is at a peak. To separately receive signals in the optical domain, different optical bandpass filters can be used.

[0081] However, the different optical characteristics can alternatively include different polarizations, thus enabling polarization modulation.

[0082] The light-emitting elements are each DC-biased using respective DC bias circuits 106, 107. Each DC bias circuit also provides a high-impedance RF blocking function. For example, the bias circuit includes a linear current source circuit or a switched current source circuit, or a voltage source can be used, assuming that the I-V characteristics of the light source are known and there are no large differences between components.

[0083] The two light-emitting elements are connected between a first output node N1 of the first DC bias circuit 106 and a second output node N2 of the second DC bias circuit 107. They are connected in series in the same polarity topologically (i.e., the cathode of the first is connected to the anode of the second), with a junction N3 between them. Thus, for DC signals, they are electrically in series (but at AC frequencies, they can be considered in parallel).

[0084] The RF current from the OPAMP flows through the first AC coupling circuit 103 to the light-emitting elements 104, 105, and then to the second AC coupling circuit 108 and to the current sensing and termination circuit 109. The current sensing circuit includes a current sensing resistor and a reference terminal (such as ground which acts as a sink for the current). However, this current sensing and termination circuit is optional.

[0085] When the current sensing option is used, the signal from the sensing resistor is used in the OPAMP feedback circuit 110. This topology splits the RF current in such a way that the positive RF current and the negative RF current flow through different light-emitting elements.

[0086] The current paths are shown in Figure 3 as follows.

[0087] The first current path 120 goes from the OPAMP 101 through the first AC coupling shown as capacitor C1 through the first light-emitting element 104, and then through the second AC coupling shown as capacitor C3 through the sensing resistor R3 to ground. This is the positive OPAMP output RF current and thus the positive polarity of the bipolar OFDM modulation current.

[0088] The second current path 122 starts from ground, passes through the current sensing resistor R3 and a second AC coupling, again a capacitor C3, passes through the second light emitting element 105, and flows to the OPAMP 101 via a first AC coupling shown as capacitor C2. This is the negative OPAMP output RF current and thus the negative polarity of the bipolar OFDM modulation current.

[0089] During modulation, due to the voltage drops across the sensing resistor and the AC coupling capacitor C4, the voltage at node N3 will change.

[0090] In this example, the first AC coupling circuit 103 thus includes a first capacitor C1 between the OPAMP output and the first node N1 and a second capacitor C2 between the OPAMP output and the second node N2, and the second AC coupling circuit 104 includes a third capacitor between the central node N3 and ground.

[0091] The AC coupling circuits can be designed in various ways and one of them can be omitted in a specific case.

[0092] The feedback from the current sensing resistor R3 is AC coupled to the inverting input of the OPAMP using the coupling capacitor C4. The resistor R4 provides DC feedback to define the DC output voltage of the OPAMP. The current feedback means that the circuit is current controlled. This avoids problems caused by the non-linearity of the I-V characteristics of the light emitting element.

[0093] Thus, the capacitor C4 and the resistor R4 together give an example of a possible feedback circuit 110.

[0094] In the IDLE state (no modulation), the bias current generated by the bias circuit only flows through the light emitting element (since the capacitors of the coupling circuit 103 are charged to the forward voltage of the LED). During modulation, each of the two AC coupling capacitors C1, C2 only passes one polarity of the modulation current, which means they are effectively discharged. This reduces the forward voltage of the light emitting element and thus causes part of the bias current to charge back the capacitors.

[0095] During the IDLE state, the voltages at nodes N1 and N2 are such that there is a desired bias voltage with respect to node N3. For example, N3 is at a voltage around half of the supply voltage (which depends on the difference in the forward voltages of the two light emitting elements).

[0096] During this IDLE time, for simplicity, the bias current only flows through the light emitting element. The bias current is, for example, in the range of 15 mA - 20 mA. Thus, the bias current is low and the non-linear part of the I-V characteristics of the light emitting element is used. For example, the forward voltage of the LED can be around 2.2 V.

[0097] The bias circuit serves two purposes:

[0098] (i) At the zero-crossing of the modulation signal, there will be some forward current flowing through the light-emitting element - if this is too low, the light-emitting element loses its electro-optical bandwidth, so the bias current ensures that the light-emitting element has sufficient response speed.

[0099] (ii) During modulation, the forward current is the drive current through one of the light-emitting elements. Since this circuit acts as a current source for driving the light-emitting element, the voltage is not directly controlled. However, the voltage increase at the OPAMP output corresponding to the forward current of the first light-emitting element is also applied to the second light-emitting element (and vice versa). The bias current is set in such a way that the currently undriven light-emitting element does not enter the reverse-polarity state. This requires that the forward voltage of the driven light-emitting element is greater than the threshold. It is desirable to prevent the reverse-polarity state because the reverse-polarity recovery of the LED (at the OFDM modulation frequency) is very slow. For an LED, a very low bias current can achieve the desired relatively high forward voltage, and thus the desired operating point can be achieved with low power consumption.

[0100] Therefore, the bias circuit sets the bias conditions of the light-emitting element, especially the operating voltage. However, in practice, the bias circuit can be implemented as a current source circuit, which is used to set the forward voltage of the light-emitting element and thus indirectly set the operating voltage.

[0101] During modulation, current flows through C1 in one direction and through C2 in the other direction. Considering only C1, in the IDLE state, the capacitor C1 is charged to a specific voltage, which depends on the IDLE state output voltage, the bias point of the light-emitting element, and its characteristics. During modulation, only one polarity of the modulation current flows through C1 and thus discharges the capacitor. At the moment when the bipolar signal crosses zero in the modulation period, no AC current flows through C1 or C2. The modulation current discharges C1 and C2, which means that the voltage across C1 and C2 is lower compared to the IDLE state.

[0102] The sum of the voltages across C1 and C2 is equal to the sum of the forward voltages of the two light-emitting elements. If this voltage has decreased, the bias current through the light-emitting element has also decreased. Therefore, the bias current through the light-emitting element changes during modulation.

[0103] The difference in the LED bias currents will flow through C1 and C2 in a direction opposite to the modulation current, thereby preventing (or slowing down) further discharge of C1 and C2. Thus, there is a balancing current, as shown in 124, which is equal to the average current through the light-emitting element. The average current of the light-emitting element is thus the same. The DC balancing current flows from the DC positive voltage VCC through the first capacitor C1 and the second capacitor C2 to ground, and it is generated by the DC bias and RF blocking circuits 106, 107.

[0104] The DC bias circuit thus provides a bias for the LED and a balancing current for the capacitors C1 and C2. The circuit can be designed, for example, in such a way that the balancing current is only used during modulation to reduce power consumption.

[0105] The G.hn / G.vlc protocol used in LiFi is a TDMA (Time Division Multiple Access) protocol, which means that data is not transmitted continuously. Therefore, some imbalance in the current of the AC coupling capacitor is acceptable and will reach equilibrium again during the reception time frame.

[0106] Figure 4 An example of the LED forward current (I FW ) relative to the forward voltage (V FW ) is shown, which is characterized by the target DC bias point (V DC ) and modulation (from V MOD_MIN to V MOD_MAX ). As can be seen, the DC bias corresponds to a very low bias current I DC . Experimental values show that a DC bias current of approximately 15 mA is sufficient.

[0107] Compared with traditional DCO-OFDM transmitters, this transmitter circuit achieves lower power consumption.

[0108] Figure 5 The architecture of the (CMO-)OFDM receiver 200 is shown.

[0109] The receiver includes two photodiodes 201, 202, each equipped with a corresponding bandpass optical filter 203, 204 that matches the wavelength of the LED or VCSEL transmitter. The optical filters can be separate components, or they can be built into the photodiode structure.

[0110] The first and second photodiodes are topologically connected in series between the first and second DC bias and decoupling circuits 205, 206. They are connected at the junction node N4. The circuit 207 for RF blocking and DC termination provides a DC path for any optical current imbalance.

[0111] The difference in RF photocurrents between two photodiodes 201, 202 is AC-coupled from the bonding node N4 through an AC coupling capacitor 208 to a transimpedance amplifier (TIA) 209. A DC-coupled TIA may also be used without the need for an AC coupling capacitor.

[0112] A sensed current signal is received from the bonding node N4 and a current is generated by the first or second photosensing element depending on the polarity of the OFDM modulation of the received optical wireless signal.

[0113] The TIA passes the received signal to an OFDM demodulator circuit 212 which includes an A / D conversion of the combined received signals (i.e., two polarities are reconstructed and a single modulated signal is created before a single A / D conversion).

[0114] From a study of the drawings, the disclosure, and the appended claims, those skilled in the art will appreciate and realize variations of the disclosed embodiments in practicing the claimed invention. 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.

[0115] 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.

[0116] If the term "adapted to" is used in a claim or specification, it should be noted that the term "adapted to" is intended to be equivalent to the term "configured to". If the term "arranged" is used in a claim or specification, note that the term "arranged" is intended to be equivalent to the term "system", and vice versa.

[0117] Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. A transmitter circuit (100) for transmitting data using optical wireless communication, comprising: A first light-emitting element (104) arranged to emit light having a first light characteristic; A second light-emitting element (105) arranged to emit light having a second light characteristic different from the first light characteristic; and Drive circuits (101, 102, 110) for driving the first and second light-emitting elements using a bipolar modulation signal, wherein the second light-emitting element is topologically in series with the first light-emitting element, and the transmitter circuit includes: Bias circuits (106, 107) for setting the bias conditions of the light-emitting elements; and Drive circuits (101, 102, 110) for driving the first and second light-emitting elements using a bipolar modulation signal such that a first polarity of the bipolar modulation signal is used to drive through the first light-emitting element to a reference terminal, and an opposite second polarity of the bipolar modulation signal is used to drive through the second light-emitting element to the reference terminal, thereby implementing bipolar OFDM modulation.

2. The transmitter circuit according to claim 1, wherein the first optical characteristic and the second optical characteristic include different wavelength characteristics or different polarization characteristics.

3. The transmitter circuit according to claim 1 or 2, wherein the drive circuit (101, 102, 110) includes an OFDM modulator circuit (102).

4. The transmitter circuit according to claim 1 or 2, wherein the drive circuit includes an OPAMP circuit, and the OPAMP circuit includes an OPAMP (101) controllable in inverting and non-inverting operation modes.

5. The transmitter circuit according to claim 4, wherein the drive circuit (101, 102, 110) includes a negative feedback circuit (110) connected to the inverting input terminal of the OPAMP.

6. The transmitter circuit according to claim 1 or 2, including a current sensor (109) for sensing the current flowing to the reference terminal, and the feedback circuit (110) includes a coupling capacitor (C4) for coupling the current sensing voltage to the inverting input terminal of the OPAMP.

7. The transmitter circuit according to claim 1 or 2, including a first AC coupling circuit (103) between the output terminal of the drive circuit and the first light emitting element and the second light emitting element.

8. The transmitter circuit according to claim 7, including a second AC coupling circuit (106) between the first light emitting element and the second light emitting element and the reference terminal.

9. A receiver circuit (200) for receiving data using optical wireless communication, comprising: A first light-sensing element (201) having a first light-sensing characteristic for sensing an incident OFDM-modulated optical signal having the first light characteristic; A second light-sensing element (202) having a second light-sensing characteristic for sensing an incident OFDM-modulated optical signal having the second light characteristic, the second light-sensing characteristic being different from the first light-sensing characteristic, wherein the second light-sensing element is topologically in series with the first light-emitting element having the same polarity, and the first and second light-sensing elements are connected at a junction node; and the receiver circuit includes: Bias circuits (205, 206) for setting the bias conditions of the first and second light-sensing elements; and An amplifier (209) for receiving and amplifying a resulting sensed current signal from the junction node between the first and second light-sensing elements.

10. The receiver circuit according to claim 9, comprising an OFDM demodulator circuit (212) for receiving the amplified resulting sensed current signal.

11. The receiver circuit according to claim 9 or 10, wherein the different optical sensing characteristics include different wavelength sensing responses, and the receiver circuit further comprises corresponding band - pass optical filters for the first and second optical sensing elements.

12. The receiver circuit according to claim 9 or 10, comprising an RF blocking circuit for providing a current path for the current generated by any DC imbalance between the sensed current signals of the first and second optical sensing elements.

13. A wireless optical communication system, comprising: One or more transmitter circuits, each transmitter circuit as claimed in claim 1; and One or more receiver circuits, each receiver circuit as claimed in claim 9.

14. A method for transmitting data using optical wireless communication, comprising: Emitting light having a first light characteristic using the first light-emitting element; Emitting light having a second light characteristic different from the first light characteristic using the second light-emitting element; Driving the first and second light-emitting elements using a bipolar modulation signal, wherein the second light-emitting element is topologically in series with the first light-emitting element; and the method includes: Setting the bias conditions of the first and second light-emitting elements, and wherein driving the first light-emitting element includes driving a first polarity of the bipolar modulation signal through the first light-emitting element to a reference terminal; and driving the second light-emitting element includes driving a second polarity of the bipolar modulation signal opposite to the first polarity through the second light-emitting element to the reference terminal, thereby implementing bipolar OFDM modulation.

15. A method for receiving data using optical wireless communication, comprising: Sensing an incident OFDM-modulated optical signal having the first light characteristic using a first light-sensing element having a first light-sensing characteristic; Sensing an incident OFDM-modulated optical signal having the second light characteristic using a second light-sensing element, the second sensing element having a second light-sensing characteristic different from the first light-sensing characteristic; wherein The second photosensing element is topologically connected in series with the first light-emitting element having the same polarity, and the first photosensing element and the second photosensing element are connected at the bonding node; and the method includes: setting the bias conditions of the first photosensing element and the second photosensing element; and receiving and amplifying the resulting sensed current signal from the bonding node between the first photosensing element and the second photosensing element.

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