Time alignment subsystem and method for use with an optical transceiver - Patents.com

Abstract

The present invention discloses a time alignment subsystem (100) for use with an optical transceiver (200) including a plurality of optical front ends (OFEs) (210) connected to a common baseband module (220) via a plurality of cables (211). The time alignment subsystem includes a controller (110), a plurality of signal detectors (111) each connected to an individual cable (211) of the plurality of cables (211), and a delay network (120). Each of the plurality of signal detectors (111) is configured to detect an individual copy of an incoming signal received by a corresponding OFE (210) on the individual cable (211) and provide the detection information to the controller (110). The delay network (120) is configured to selectively apply an individual delay to an individual signal path of each individual cable (211) to improve time alignment between the plurality of signal paths. The controller (110) is configured to determine individual delays for the individual signal paths based on the detection information provided by each signal detector (111).

Description

[Technical field]

[0001] The present invention relates to the field of optical wireless communication networks, such as Li-Fi networks. Disclosed herein are various methods, apparatus, systems, and computer readable media relating to, among other things, time alignment subsystems and methods for use with optical transceivers having two or more optical front ends. [Background technology]

[0002] In recent years, light fidelity (Li-Fi) has been gaining increasing attention for its intrinsic security enhancement and its capability to support higher data rates in the available bandwidth of visible, ultraviolet (UV), and infrared (IR) spectrum. Furthermore, Li-Fi is directional and shielded, which offers the possibility to deploy more access points by spatially reusing the same bandwidth. These significant advantages over wireless radio frequency communication make Li-Fi a promising secure solution to ease the pressure on the crowded radio spectrum for IoT applications and indoor wireless access. For industrial applications, additional advantages of Li-Fi include guaranteed bandwidth for a given user and its ability to function robustly in areas prone to electromagnetic interference. Therefore, Li-Fi is a very promising technology to enable the next generation of immersive connectivity.

[0003] Given the line-of-sight requirement of optical wireless communication, in a point-to-multi-point (P2MP) LiFi system, multiple Optical Front Ends (OFEs) may be connected to a single baseband module via multiple cables to achieve wide coverage at low cost. However, the length of the cables connecting these OFEs to the baseband module must be the same. Otherwise, destructive interference may occur when signals with different delays due to different cable lengths are summed at baseband. Destructive interference may degrade signal quality or even cut off the connection completely.

[0004] WO2021219435A1 relates to a LiFi system having multiple transceivers and a single multiple-input- multiple-output (MIMO) modem having at least M outputs, where the M transmit outputs of the MIMO modem are fed to a linear combiner.

[0005] WO9735387A1 relates to a method and apparatus for communication from a base unit to a number of portable infrared devices using an established communications protocol, whereby signals are exchanged along a cable from the base unit via a fixed infrared receiver / transmitter module on an infrared carrier to the portable devices in a standard transmit-receive slot. Summary of the Invention [Problem to be solved by the invention]

[0006] For some indoor applications, such as office spaces, classrooms, and conference halls, the same information needs to be distributed to many audiences in a wide area. Given the line-of-sight requirements of optical wireless communication, the coverage area of ​​a single OFE is relatively limited, especially when there is also a requirement for data rates to be supported. If an optical transceiver includes a single OFE, optical access points (APs) need to be densely deployed to provide complete coverage. To reduce the cost of deploying such an OWC network, it is desirable to provide multiple OFEs in an optical transceiver, and a baseband module of the optical transceiver is shared by multiple OFEs. Multiple cables are used to connect the multiple OFEs with the shared baseband module. If all cables are not the same length, the signal paths on different cables may add different time delays or phase shifts to the signal. If there are end points (EPs) located in overlapping areas of the Field-of-View (FoV) of different OFEs, when different copies of the same optical signal from the EPs detected by different OFEs are summed in the baseband module, these different copies will cause destructive interference with each other. Similarly, the same problem occurs on the EP side in case of downlink communication, since the EPs receive different copies of the same signal originating from a shared baseband module. Therefore, it is beneficial to provide a solution that automatically calibrates or compensates for such phase shifts so that the system performance is not affected by different lengths of cables.

[0007] In view of the above, the present disclosure is directed to methods, apparatus, and systems for providing a solution for selectively applying different delays to different signal paths on different cables to improve time-alignment between these signal paths. Among other things, the object of the present invention is achieved by a time alignment subsystem as set forth in claim 1, a time alignment method as set forth in claim 12, and an optical transceiver as set forth in claim 13. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided a time-alignment subsystem for use with an optical transceiver including a plurality of Optical Front Ends (OFEs) connected to a common baseband module via a plurality of cables, the subsystem including a controller, a plurality of signal detectors each connected to an individual cable of the plurality of cables and configured to detect an individual copy of an incoming signal received by a corresponding OFE on the individual cable and provide detection information to the controller, and a delay network configured to selectively apply an individual delay to the individual signal path of each individual cable to improve time alignment between the plurality of signal paths, the controller configured to determine an individual delay for the individual signal path based on the detection information provided by each signal detector.

[0009] The OFE, which includes at least a light source and a light sensor, performs conversion between electrical and optical signals. In the transmitter chain, the OFE is used to convert an electrical transmission signal into an optical signal outputted via a light source. In the receiver chain, the OFE is used to convert a received optical signal into an electrical signal outputted via an optical sensor for further signal processing. The light source can be a light-emitting diode (LED), a laser diode (LD), a vertical cavity surface emitting laser (VCSEL), or an array of LEDs, LDs, or VESELs. The optical sensor can be a photodiode, an avalanche diode, or other type of optical sensor. In some cases, the optical sensor is also called a photo detector, a light detector, or a photo sensor.

[0010] The multiple OFEs are connected to the same baseband module of the optical transceiver through multiple cables. These cables may not be of the same length, resulting in phase differences between different copies of a signal received by the multiple OFEs. The time alignment subsystem is used to provide a convenient approach to measure and compensate for such phase differences. When there is a signal sent by a device located in an overlapping area of ​​the coverage areas of two or more of the multiple OFEs, the corresponding signal detector detects different copies of the incoming signal on the individual cables or signal paths connected to the two or more OFEs. The detection information is provided to a controller. The controller measures and determines the difference in arrival times of the different copies received by the different OFEs. Based on this time difference, the controller decides to apply a correction to this difference in phase (or arrival time / delay) between the different copies by selectively activating individual delay elements of the corresponding signal paths. The adjustment is applied to the individual signal paths through the delay network.

[0011] Beneficially, the delay network includes, for each individual cable, one or more delay elements each corresponding to a different delay value to be selectively applied to a corresponding signal path of each individual cable, and the delay network is configured to selectively turn on one delay element in accordance with an individual delay determined by the controller.

[0012] In one example, each delay element is a lumped-element delay line implemented with LC components.

[0013] A lumped element delay line may include multiple inductors and capacitors with similar parameters. The inductors are typically connected in series and the capacitors are connected to ground through junctions between the inductors. Lumped element delay lines may be tapped at several points to provide a range of delay values.

[0014] In another example, each delay element is a printed circuit board trace having a layout that introduces a different delay value.

[0015] In this example, the delay value may be determined by the layout of the delay elements, which may be characterized by a particular shape, pattern, size, or combination thereof.

[0016] Advantageously, each signal detector of the plurality of signal detectors is a Radio Frequency (RF) power detector.

[0017] An RF power detector, or RF detector, is a device used to detect the presence of RF waves. In one example, an RF detector monitors or samples the output of an RF circuit and produces a DC output voltage proportional to the RF power at that point.

[0018] In a preferred implementation, the controller is a microcontroller with an integrated analog to digital converter (ADC).

[0019] If analog samples are obtained from a signal detector, it is beneficial for the microcontroller to include an integrated ADC. The quantized signal is then used for further processing.

[0020] Beneficially, each signal detector of the plurality of signal detectors includes a comparator.

[0021] A signal detector may be capable of comparing two analog signal inputs, or one analog input signal, to a threshold and generating a digital output representative of the comparison result.

[0022] In another preferred implementation, the controller is a programmable logic device.

[0023] If the result from the signal detector is a digital signal, the controller may be a programmable logic device, which may be either a field-programmable gate array (FPGA) or a complex programmable logic device (CPLD).

[0024] Preferably, the controller is configured to determine the individual delays by measuring a difference between the time of arrival of different individual copies of an incoming signal received by two or more OFEs of the plurality of OFEs.

[0025] Advantageously, the controller is configured to determine the individual delays to reduce the difference between the arrival times of the different individual copies of the incoming signal.

[0026] The controller derives individual delays for the different signal paths to compensate for the difference in arrival times. For example, the controller may decide to assign the maximum delay value to an individual signal path corresponding to an OFE that detects the incoming signal first. The controller may decide to assign the shortest delay value or no delay to another individual signal path corresponding to an OFE that detects the incoming signal last. The objective is to have the signal copies from the different paths arrive substantially synchronously at baseband after being compensated with the individual delay values.

[0027] In a preferred example, the controller is configured to approximate the determined individual delay to one of different delay values ​​available to each individual cable in the delay network.

[0028] The granularity of the actual delay compensated by the time alignment subsystem is related to the different delay values ​​available in the delay network, e.g., as determined by the delay elements of the delay network, and the controller may therefore be configured to perform an approximation by mapping the determined individual delays to the delay values ​​available in the delay network for a particular individual cable.

[0029] According to a second aspect of the present invention, there is provided a time-alignment method performed by a time alignment subsystem for use with an optical transceiver including multiple optical front-ends (OFEs) connected to a common baseband module via multiple cables, the method comprising the steps of: detecting, on the individual cables, individual copies of an incoming signal received by a corresponding OFE, determining individual delays for the individual signal paths of each individual cable based on the detected information, and selectively applying the individual delays by a delay network to the individual signal paths of each individual cable to improve time alignment between the multiple signal paths.

[0030] According to a third aspect of the present invention, there is provided an optical transceiver, the optical transceiver including a baseband module, an analog front end (AFE) module connected to the baseband module, a summing amplifier connected to the AFE module, a plurality of optical front ends (OFEs) connected to the summing amplifier via a plurality of cables, and a time alignment subsystem according to the present invention coupled between the summing amplifier and the plurality of cables.

[0031] Preferably, the optical transceiver is included in an optical wireless communications access point.

[0032] An optical wireless communication (OWC) access point, such as a Li-Fi access point, provides electronic devices or end devices in a corresponding optical cell with access to an external network via an optical wireless link. An OWC access point can also simultaneously support bidirectional optical links with two or more end devices, forming a point-to-multipoint (P2MP) system. [Brief description of the drawings]

[0033] In the drawings, like reference characters generally refer to the same parts throughout the different views and the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. [Figure 1] 1 illustrates an optical wireless communication system according to the present invention. [Diagram 2] A basic block diagram of an optical transceiver is shown. [Diagram 3] 1 illustrates one implementation of a time alignment subsystem. [Figure 4] A basic block diagram of a time alignment subsystem is shown. [Diagram 5] 1 shows a flow chart of a method for a time alignment subsystem. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The embodiments described below represent information to enable those skilled in the art to practice the embodiments. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and recognize applications of these concepts not specifically addressed herein. It should be understood that these concepts and applications are within the scope of the present disclosure.

[0035] The present invention relates to an Optical Wireless Communications (OWC) system operating in the optical band, e.g., in the visible, ultraviolet (UV), or infrared (IR) spectrum, in which a high-speed communication link can be established between two remote devices over a direct and unobstructed path, i.e., a line-of-sight path.

[0036] To address large coverage areas, such as office spaces, classrooms, and conference halls, wide-angle LiFi point-to-multipoint (P2MP) systems are typically cost-effectively achieved by deploying multiple optical front-ends (OFEs) connected to a single analog front-end (AFE) and baseband module at an optical access point (AP). In such systems, long cables are required to connect two or more OFEs to a shared AFE and baseband module. The length of these cables can be on the order of 1-10 meters, depending on the actual area to be covered. The signal path over such long cables adds a cable-length dependent phase shift to the signal. Problems can arise if all the cables are not the same length. For example, if there are end points (EPs) located in overlapping areas of the Field of View (FoV) or coverage areas of different OFEs, when different copies of the same optical signal from the EPs detected by the different OFEs are summed at the baseband module, these different copies will cause destructive interference with each other due to the cable-length dependent phase shift. Similarly, the same problem occurs on the EP side in case of downlink communication, as the EPs receive different copies of the same signal emanating from a shared baseband module. Such destructive interference can cause severe throughput degradation and ultimately disconnection.

[0037] On the other hand, ensuring that all cables are the same length may not be easy in practice as such additional restrictions would lead to significant installation and maintenance efforts. In view of this, the present invention discloses a time alignment subsystem that can be used to measure and adjust the delay or phase shift caused by different cable lengths so that the aforementioned destructive interference can be reduced.

[0038] FIG. 1 illustrates an optical wireless communication (OWC) system according to the present invention. In this exemplary system setup, an optical transceiver 200 includes a central module 250 connected to multiple optical front ends (OFEs) 210 via multiple cables 211. As in conventional systems, the central module 250 includes a common baseband module 220, an analog front end (AFE) module 230, and a summing amplifier 240. In addition, the central module 250 includes a time alignment subsystem 100 that is placed between the transceiver lines and the summing amplifier 240. The time alignment subsystem 100 may perform initial or periodic measurements of the phase difference between different copies of the incoming signal received by the different OFEs 210. After the measurement, the phase difference between the signals is compensated for by selectively adding delays to the individual signal paths, as necessary, before reaching the summing amplifier.

[0039] The central module 250 may be located in a single housing. The optical transceiver 200 may be deployed on a ceiling and operate as an access point (AP) 250 providing connectivity to endpoints (EPs) 300 within an area covered by multiple OFEs 210. The EPs may be mobile electronic devices with optical wireless communication capabilities.

[0040] A basic block diagram of an optical transceiver 200 is shown in Figure 2. The optical transceiver 200 includes a common baseband module 220, an analog front-end (AFE) module 230, a summing amplifier 240, a time alignment subsystem 100, a number of cables 211, and a number of OFEs 210.

[0041] FIG. 3 illustrates one possible implementation of the time alignment subsystem 100. In this exemplary diagram, two OFEs 210 are shown as an example, but in a practical system, more OFEs 210 may be deployed to extend the coverage area of ​​a single OWC transceiver. For example, eight or more OFEs are often used in commercial products. If there is an EP located in an overlapping area of ​​the FoV of two or more OFEs 210, a test frame, data packet, or other signal from the EP will be received by two or more OFEs 210. The signal detector 111 detects different copies of the incoming signal on the individual cables or signal paths connected to the two or more OFEs 210. The detection information is provided to the controller 110, which may be a CPU, programmable logic, or microprocessor. The controller 110 measures and determines the difference in arrival time of the different copies received by the different OFEs 210.

[0042] Based on this time difference, the controller 110 decides to apply a correction to this difference in phase (or arrival time / delay) between the different copies by selectively activating individual delay elements of the corresponding signal paths. The signal copy from the OFE 210 with the longest cable is likely to be detected latest, and therefore a direct path may be selected without adding additional delay. For the other signal paths, corresponding switches may be controlled to select corresponding delay elements that are inversely related to the arrival time delay (and therefore also inversely proportional to the corresponding cable length). After delay compensation, the different copies of the same incoming signal will arrive at the summing amplifier substantially synchronized in phase, such as within a certain tolerance. The number of delay elements 121 incorporated in the delay network 120 may depend on the possible length variations of the cables, the granularity requirement for the phase tuning of the system, and / or the tolerance of the system to phase variations.

[0043] The delay network may be implemented with different topologies: in Fig. 3, a parallel delay line or delay element is shown, with each branch associated with an individual delay value to be selected; alternatively, different delay values ​​may be realized in a cascaded manner.

[0044] In one implementation, the signal detector 111 may be an RF power detector and the controller 110 may be a low-cost CPU (e.g., a microcontroller) integrated with one or more ADCs. The CPU includes one or more ADCs to quantize the detected copies obtained from the signal detector. The signal strength level of each individual copy is monitored. The time moment when a certain threshold is reached is an indication that a frame or packet has arrived, which is the arrival time of a certain copy. The difference between the arrival times of the different copies is then measured and used to control the delay network.

[0045] In another implementation, the signal detector may include a comparator (with or without an amplifier) ​​and the controller 110 may be a programmable logic device (e.g., an FPGA or CPLD) to enable faster and more accurate time measurements, at the penalty of less noise immunity.

[0046] The delay network can be implemented as a lumped element delay line using LC components, or as a lower cost alternative by using long PCB traces in different patterns to minimize PCB size.

[0047] 4 shows a schematic of the basic components of the time alignment subsystem 100. The time alignment subsystem 100 includes a controller 110, a plurality of signal detectors 111, and a delay network 120. The plurality of signal detectors 111 are configured to detect signal copies on the individual cables and provide the detection information to the controller. Based on the detection information provided by each signal detector 111, the controller 110 is configured to determine individual delays for the individual signal paths. The delay network 120 is configured to selectively add individual delays, according to instructions from the controller 110, to the individual signal paths of each individual cable 211 to improve time alignment between the multiple signal paths.

[0048] 5 shows a flowchart of a time alignment method 500 performed by a time alignment subsystem 100 for use with an optical transceiver 200 including multiple optical front ends (OFEs) 210 connected to a common baseband module 220 via multiple cables 211. The method includes the following steps of the time alignment subsystem 100: in step S501, detecting individual copies of an incoming signal received by a corresponding OFE 210 on the individual cables 211, in step S502, determining individual delays for the individual signal paths of each individual cable 211 based on the detected information, and in step S503, selectively applying individual delays to the individual signal paths of each individual cable 211 to improve time alignment between the multiple signal paths.

Claims

1. 1. A time alignment subsystem for use with an optical transceiver including a plurality of optical front ends (OFEs) connected via a plurality of cables to a common baseband module, the time alignment subsystem comprising: A controller; each of the plurality of cables is connected to a respective cable, Detecting on said separate cables separate copies of the incoming signal received by corresponding OFEs; and providing the detection information to the controller; a plurality of signal detectors configured to a delay network configured to selectively apply individual delays to individual signal paths of each individual cable to improve time alignment between the plurality of signal paths; Including, The controller is configured to determine individual delays for the individual signal paths based on the detection information provided by each signal detector, a time alignment subsystem.

2. 2. The time alignment subsystem of claim 1, wherein the delay network includes, for each individual cable, one or more delay elements each corresponding to a different delay value to be selectively added to a corresponding signal path of each individual cable, and the delay network is configured to selectively turn on one delay element in accordance with an individual delay determined by the controller.

3. The time alignment subsystem of claim 2 , wherein each delay element is a lumped element delay line implemented with LC components.

4. 3. The time alignment subsystem of claim 2, wherein each delay element is a printed circuit board trace having a layout that introduces a different delay value.

5. 5. The time alignment subsystem of claim 1, wherein each signal detector of the plurality of signal detectors is a radio frequency power detector.

6. The time alignment subsystem of claim 5 , wherein the controller is a microcontroller with an integrated analog-to-digital converter.

7. The time alignment subsystem of claim 1 , wherein each signal detector of the plurality of signal detectors includes a comparator.

8. The time alignment subsystem of claim 7 , wherein the controller is a programmable logic device.

9. 9. The time alignment subsystem of claim 1, wherein the controller is configured to determine the individual delays by measuring a difference between arrival times of different individual copies of an incoming signal received by two or more OFEs of the plurality of OFEs.

10. The time alignment subsystem of claim 9 , wherein the controller is configured to determine individual delays to reduce a difference between arrival times of different individual copies of the incoming signal.

11. 11. A time alignment subsystem as claimed in claim 9 or 10, wherein the controller is configured to approximate the determined individual delay to one of different delay values ​​available to each individual cable in the delay network.

12. 1. A method of time alignment performed by a time alignment subsystem for use with an optical transceiver including a plurality of optical front ends (OFEs) connected to a common baseband module via a plurality of cables, the method comprising the steps of: detecting separate copies of the incoming signal on separate cables as received by corresponding OFEs; determining an individual delay for an individual signal path of each individual cable based on the detected information; Selectively adding individual delays by a delay network to individual signal paths of each individual cable to improve time alignment between the plurality of signal paths.

13. A baseband module; an analog front-end (AFE) module connected to the baseband module; A summing amplifier connected to the AFE module; a plurality of optical front-ends connected to the summing amplifier via a plurality of cables; a time alignment subsystem as claimed in claim 1 coupled between said summing amplifier and said plurality of cables; 2. An optical transceiver comprising:

14. The optical transceiver of claim 13 , wherein the optical transceiver is included in an optical wireless communication access point.

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