Time alignment subsystem and method for use with an optical transceiver

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. In particular, various methods, apparatuses, systems, and computer-readable media are disclosed herein related to time alignment subsystems and methods for use with an optical transceiver having two or more optical front ends.

Background Art

[0002] In recent years, light fidelity (Li-Fi) has been attracting increasing attention due to its intrinsic security enhancement and its capability to support higher data rates with the available bandwidth in the visible light, ultraviolet (UV), and infrared (IR) spectra. Further, Li-Fi is directional and shielded by light-blocking materials, thus having the potential to deploy more access points by spatially reusing the same bandwidth. These important advantages over wireless radio frequency communication make Li-Fi a promising secure solution to relieve the pressure on the crowded radio spectrum for IoT applications and indoor wireless access. For industrial applications, further advantages of Li-Fi can include guaranteed bandwidth for a user and the ability to function robustly in areas susceptible to electromagnetic interference. Therefore, Li-Fi is a very promising technology enabling next-generation immersive connectivity.

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

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

[0005] WO9735387A1 relates to a method and apparatus for communication from a base unit using an established communication protocol to a plurality of portable infrared devices, wherein signals are exchanged with a portable device in a standard transmit / receive slot via an infrared carrier from the base unit along a cable through a fixed infrared receiver / transmitter module. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] In the case of an indoor application such as an office space, a classroom, a conference hall, etc., the same information needs to be distributed to a large number of audiences in a wide area. Considering the line-of-site requirements of optical wireless communication, the coverage area of a single OFE is relatively limited, especially when there are also requirements for the data rate to be supported. When an optical transceiver includes a single OFE, optical access points (APs) need to be deployed at high density to provide complete coverage. To reduce the cost of deploying such an OWC network, it is desirable to provide multiple OFEs in the optical transceiver and share the baseband module of the optical transceiver among multiple OFEs. Multiple cables are used to connect multiple OFEs to the shared baseband module. If all the cables are not of the same length, the signal paths on different cables may add different time delays or phase shifts to the signal. When there are end points (EPs) located in the overlapping area of the fields 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 cause canceling interference with each other. Similarly, in the case of downlink communication, the same problem occurs on the EP side because the EPs receive different copies of the same signal transmitted from the shared baseband module. Therefore, it is beneficial to provide a solution that automatically calibrates or compensates for such phase shifts so that the performance of the system is not affected by different cable lengths.

[0007] In view of the above, the present disclosure is directed to a method, an apparatus, and a system for providing a solution that selectively adds different delays to different signal paths related to different cables to improve the time-alignment between these signal paths. In particular, the object of the present invention is achieved by the time-alignment subsystem according to claim 1, the time-alignment method according to claim 12, and the optical transceiver according to claim 13.

Means for Solving the Problem

[0008] According to a first aspect of the present invention, a time - alignment subsystem is provided. A time - alignment subsystem for use with an optical transceiver including a plurality of optical front ends (OFE) connected to a common baseband module via a plurality of cables, the subsystem comprising: a controller; a plurality of signal detectors each connected to an individual cable among 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 add an individual delay to an individual signal path of each individual cable to improve time - alignment between a plurality of signal paths, the controller being configured to determine an individual delay for an individual signal path based on the detection information provided by each signal detector.

[0009] An OFE including at least a light source and a photosensor performs conversion between an electrical signal and an optical signal. In a transmitter chain, the OFE is used to convert an electrical transmission signal into an optical signal output via the light source. In a receiver chain, the OFE is used to convert the received optical signal into an electrical signal output via the photosensor 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 VCSELs. The photosensor can be a photodiode, an avalanche diode, or another type of photosensor. In some cases, the photosensor is also referred to as a photo detector, a light detector, or a photo sensor.

[0010] Multiple OFEs are connected to the same baseband module of an optical transceiver via multiple cables. These cables may not be of the same length, and as a result, a phase difference occurs between different copies of the signals received by the multiple OFEs. A time alignment subsystem is used to provide a convenient approach for measuring and compensating for such a phase difference. When there is a signal sent by a device located in the 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 the 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 determines to apply a correction to this difference in phase (or arrival time / delay) between the different copies by selectively activating the individual delay elements of the corresponding signal paths. The adjustment is applied to the individual signal paths via a delay network.

[0011] Advantageously, 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 the corresponding signal path of each individual cable, and the delay network is configured to selectively turn on one delay element according to the individual delay determined by the controller.

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

[0013] The lumped-element delay line may include a number of inductors and capacitors having similar parameters. The inductors are typically connected in series, and the capacitors are connected to ground via junctions between the inductors. The lumped-element delay line may be tapped at several points to provide a series of delay values.

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

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

[0016] Advantageously, each 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, the RF detector monitors or samples the output of an RF circuit and generates a DC output voltage proportional to the RF power at that point.

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

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

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

[0021] The signal detector may be able to compare two analog signal inputs, or one analog input signal with a threshold, and generate a digital output representing the comparison result.

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

[0023] If the result obtained from the signal detector is a digital signal, the controller may be a programmable logic device, which can 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 delay by measuring the difference in the arrival times of different individual copies of the incoming signal received by two or more OFEs among the plurality of OFEs.

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

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

[0027] In a preferred example, the controller is configured to approximate the determined individual delay to one of the different delay values available for 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, such as determined by the delay elements of the delay network. Therefore, the controller may 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, a time - alignment method is provided. A time - alignment method executed 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 including the following steps of the time - alignment subsystem: detecting an individual copy of an incoming signal received by a corresponding OFE on an individual cable; determining an individual delay for an individual signal path of each individual cable based on the detection information; selectively adding an individual delay to the individual signal path of each individual cable by a delay network to improve time - alignment between a plurality of signal paths.

[0030] According to a third aspect of the present invention, an optical transceiver is provided. The optical transceiver includes 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 communication access point.

[0032] An optical wireless communication (OWC) access point, such as a Li-Fi access point, provides access to an external network via an optical wireless link to an electronic device or end device within a corresponding optical cell. The OWC access point can simultaneously support bi-directional optical links with two or more end devices and can also form a point-to-multipoint (P2MP) system.

Brief Description of the Drawings

[0033] In the drawings, like reference numerals generally refer to the same parts throughout different figures. Also, these drawings are not necessarily to scale; instead, generally, emphasis is placed on illustrating the principles of the present invention.

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Modes for Carrying Out the Invention

[0034] The embodiments described below represent information to enable those skilled in the art to practice the embodiments. By reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and will recognize applications of these concepts that are 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 communication (OWC) system operating in an optical band, such as in the visible light, ultraviolet (UV), or infrared (IR) spectrum, in which a high-speed communication link can be established between two remote devices via a direct and unobstructed path, i.e., a line-of-sight path.

[0036] To accommodate large coverage areas such as office spaces, classrooms, conference rooms, etc., wide-angle LiFi point-to-multi-point (P2MP) systems are typically cost-effectively implemented by deploying a single analog front-end (AFE) and multiple optical front-ends (OFEs) connected to a baseband module at an optical access point (AP). In such a system, long cables are required to connect two or more OFEs to a shared AFE and baseband module. The lengths of these cables can be on the order of 1 - 10 m depending on the actual area to be covered. The signal path on such long cables adds a cable-length dependent phase shift to the signal. Problems can occur if not all cables are of the same length. For example, when different copies of the same optical signal from an end point (EP) detected by different OFEs are summed at the baseband module, these different copies can cause destructive interference with each other due to the cable-length dependent phase shift, in cases where there are different fields of view (FoV) of different OFEs or end points (EPs) located in the overlapping areas of the coverage area. Similarly, in the case of downlink communication, the same problem occurs at the EP side because the EP receives different copies of the same signal transmitted from the shared baseband module. Such destructive interference can cause significant throughput degradation and ultimately lead to disconnection.

[0037] On the one hand, ensuring that all cables are of the same length would lead to such additional restrictions that would result in a great deal of installation and maintenance effort and may not actually be easy. In view of this, the present invention discloses a time alignment subsystem that can be used to measure and adjust delays or phase shifts caused by different cable lengths so that the above-mentioned cancellation interference can be reduced.

[0038] FIG. 1 shows an optical wireless communication (OWC) system according to the present invention. In this exemplary system setup, the optical transceiver 200 includes a central module 250 connected to a plurality of optical front ends (OFEs) 210 via a plurality of cables 211. Similar to the conventional system, the central module 250 includes a common baseband module 220, an analog front end (AFE) module 230, and a summing amplifier 240. Further, the central module 250 includes a time alignment subsystem 100 disposed between the transceiver line and the summing amplifier 240. The time alignment subsystem 100 may perform an initial or periodic measurement of the phase difference between different copies of the incoming signals received by different OFEs 210. After the measurement, the phase difference between the signals is compensated by selectively adding a delay to the individual signal paths as needed before reaching the summing amplifier.

[0039] The central module 250 may be placed in a single housing. The optical transceiver 200 may operate as an access point (AP) 250 that is deployed on the ceiling and provides connectivity to an endpoint (EP) 300 within the area covered by the plurality of OFEs 210. The EP may be a mobile electronic device having optical wireless communication capabilities.

[0040] A basic block diagram of the optical transceiver 200 is shown in FIG. 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 plurality of cables 211, and a plurality of OFEs 210.

[0041] FIG. 3 shows one possible implementation of the time alignment subsystem 100. In this exemplary drawing, two OFEs 210 are shown as an example, but in an actual system, more OFEs 210 may be deployed to expand the coverage area of a single OWC transceiver. For example, more than 8 OFEs are often used in commercial products. If there is an EP located in the overlapping area of the FoV of two or more OFEs 210, a test frame, data packet, or another 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 an individual cable or signal path connected to 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 times of different copies received by different OFEs 210.

[0042] Based on this time difference, the controller 110 determines to apply a correction to this difference in phase (or arrival time / delay) between different copies by selectively activating the individual delay elements of the corresponding signal paths. The signal copy from OFE210 with the longest cable is likely to be detected the latest, and thus the direct path may be selected without adding additional delay. For other signal paths, the corresponding switch may be controlled to select the corresponding delay element that is inversely correlated with the delay of the arrival time (and thus also inversely proportional to the corresponding cable length). After delay compensation, different copies of the same incoming signal will arrive at the summing amplifier with substantially synchronized phases, such as within a certain tolerance range. The number of delay elements 121 incorporated in the delay network 120 may depend on the possible variation in cable lengths, the granularity requirement for the phase tuning of the system, and / or the tolerance range of the system to phase fluctuations.

[0043] The delay network may be implemented in different topologies. In FIG. 3, parallel delay lines or delay elements are illustrated, and each branch is 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 one or more ADCs included in the CPU 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 serves as an indication that a frame or packet has arrived, which is the arrival time of a certain copy. Subsequently, the difference between the arrival times of different copies is measured and used to control the delay network.

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

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

[0047] FIG. 4 schematically shows 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 copies of signals on individual cables and provide detection information to the controller. Based on the detection information provided by each signal detector 111, the controller 110 is configured to determine an individual delay for an individual signal path. The delay network 120 is configured to selectively add individual delays to the individual signal paths of each individual cable 211 in accordance with instructions from the controller 110 to improve the time alignment between the plurality of signal paths.

[0048] FIG. 5 shows a flowchart of a time alignment method 500 executed by a time alignment subsystem 100 for use with an optical transceiver 200 that includes a plurality of optical front ends (OFEs) 210 connected to a common baseband module 220 via a plurality of cables 211. The method includes the following steps of the time alignment subsystem 100: In step S501, detecting an individual copy of an incoming signal received by a corresponding OFE 210 on an individual cable 211; In step S502, determining an individual delay for an individual signal path of each individual cable 211 based on the detection information; and In step S503, selectively adding an individual delay to the individual signal path of each individual cable 211 to improve time alignment between the plurality of signal paths.

Claims

1. 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 time alignment subsystem comprising: a controller; each connected to an individual cable of the plurality of cables, detecting an individual copy of an incoming signal received by a corresponding OFE on the individual cable, and providing detection information to the controller, a plurality of signal detectors configured as such; a delay network configured to selectively apply an individual delay to an individual signal path of each individual cable to improve time alignment between a plurality of signal paths; including; The controller is configured to determine an individual delay for the individual signal path based on the detection information provided by each signal detector, and the individual delay depends on the length of each individual cable. The time alignment subsystem.

2. 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 the individual cable, and the delay network is configured to selectively turn on one delay element according to the individual delay determined by the controller. The time alignment subsystem according to claim 1.

3. The time alignment subsystem according to claim 2, wherein each delay element is a lumped element delay line implemented by an LC component.

4. The time alignment subsystem according to claim 2, wherein each delay element is a printed circuit board trace having a layout introducing different delay values.

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

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

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

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

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

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

11. The time alignment subsystem according to claim 9, wherein the controller is configured to approximate the determined individual delay to one of different delay values available for each individual cable in the delay network.

12. A time alignment method executed 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 including the following steps of the time alignment subsystem Detecting an individual copy of an incoming signal received by a corresponding OFE on an individual cable; Determining an individual delay for an individual signal path of each individual cable based on the detection information, wherein the individual delay depends on the length of each individual cable; Selectively adding the individual delay to the individual signal path of each individual cable by a delay network to improve time alignment between a plurality of signal paths.

13. A baseband module; An analog front end (AFE) module connected to the baseband module; A sampling amplifier connected to the AFE module; A plurality of optical front ends connected to the sampling amplifier via a plurality of cables; The time alignment subsystem according to claim 1 coupled between the sampling amplifier and the plurality of cables; An optical transceiver including the same.

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

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