US-12621053-B2 - Synchronization in optical wireless networks for interference suppressing

US12621053B2US 12621053 B2US12621053 B2US 12621053B2US-12621053-B2

Abstract

In an optical wireless communication, OWC, network ( 100 ), time division multiple access, TDMA, is commonly used for interference suppression among multiple network devices (D 1 -Dn) associated to a single coordinator, or among adjacent coordinators (C 1 -Cn). However, an offset of MAC cycles among two adjacent coordinators (C 1 -C 2 ) may still result in interference to a network device (D 1 ) located in the overlapping area of the coverage areas of these two adjacent coordinators (C 1 -Cn). This invention is directed to various methods, apparatus, systems, computer program and computer-readable media for providing a mechanism to make use of an out-of-band channel to send a signal from a network device (D 1 ) to the two adjacent coordinators (C 1 -C 2 ), for assisting the alignment of MAC cycles among the two adjacent coordinators (C 1 -C 2 ), and the out-of-band channel has a line-of-sight character, and is out-of-band as compared to the OWC network ( 100 ).

Inventors

Andries Van Wageningen
KARNEKUMAR ARULANDU

Assignees

SIGNIFY HOLDING B.V.

Dates

Publication Date: 20260505
Application Date: 20200929
Priority Date: 20191008

Claims (20)

1 . A network device out of a plurality of network devices for reducing interference in an optical wireless communication (OWC) network having a line-of-sight character, the OWC network comprising at least two coordinators and the plurality of network devices selectively associated to and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share a same optical wireless medium of the OWC network covering a first pre-defined spectrum range via a time-division multiple access (TDMA) approach by dividing the wireless medium into consecutive time slots, the network device comprising: a first receiver configured to detect downlink communication over the OWC network within the first pre-defined spectrum range; a controller configured to decide on assisting with alignment of Medium Access Control (MAC) cycles between at least two adjacent coordinators, based on the downlink communication from the at least two adjacent coordinators detected by the first receiver; a second transmitter configured to send a signal comprising timing information related to MAC cycles of the at least two adjacent coordinators, via an out-of-band channel outside the first pre-defined spectrum range, upon the decision on assisting with the alignment of MAC cycles between the at least two adjacent coordinators for interference suppression; wherein the timing information is one of a signal to indicate the start time of individual MAC cycles of the two adjacent coordinators, an offset among individual MAC cycles of the adjacent coordinators, and a start time of MAC cycles of one coordinator out of the adjacent coordinators; and wherein the out-of-band channel has a line-of-sight character.
2 . The network device of claim 1 , the network device further comprising: a second receiver configured to detect downlink data on the out-of-band channel; and wherein the decision on assisting with the alignment of MAC cycles between at least two adjacent coordinators, is based on either the downlink communication from the at least two adjacent coordinators detected by the first receiver or the downlink data from the at least two adjacent coordinators detected by the second receiver.
3 . The network device of claim 1 , wherein the second transmitter is configured to send periodic signals to the at least two adjacent coordinators for use by the at least two adjacent coordinators as timing reference in adjusting their MAC cycles, and wherein the periodic signals indicate a new start-time of MAC cycles according to the timing information related to MAC cycles of the at least two adjacent coordinators.
4 . The network device of claim 1 , the network device configured to: detect periodic beacons from the at least two adjacent coordinators by the first receiver over the OWC network, and send signals to the at least two adjacent coordinators by the second transmitter on the out-of-band channel, by reflecting each of the periodic beacons to the at least two adjacent coordinators with a predefined delay following receipt of each of the periodic beacons.
5 . The network device according to claim 2 , the network device configured to: detect periodic beacons from the at least two adjacent coordinators by the second receiver on the out-of-band channel, and send signals to the at least two adjacent coordinators by the second transmitter on the out-of-band channel, by reflecting each of the periodic beacons to the at least two adjacent coordinators with a predefined delay following receipt of each of the periodic beacons.
6 . The network device of claim 1 , the network device associated to a local coordinator out of the at least two adjacent coordinators, and another one out of the at least two adjacent coordinators being a neighboring coordinator of the network device, the network device configured to: detect synchronization messages from the local coordinator and the neighboring coordinator by the first receiver over the OWC network; and wherein a synchronization message comprises identification information of the individual coordinator; derive a relative offset by the controller, following receipt of the synchronization message from the neighboring coordinator, and wherein the relative offset is the time interval between the time of reception of the synchronization message from the neighboring coordinator as compared to the start-time of the current MAC cycle of the local coordinator; and send a signal to the local coordinator by the second transmitter on the out-of-band channel, the signal comprising identification information of the neighboring coordinator and the relative offset derived by the network device.
7 . The network device according to claim 2 , the network device associated to a local coordinator out of the at least two adjacent coordinators, and another one out of the at least two adjacent coordinators being a neighboring coordinator of the network device, the network device configured to: detect synchronization messages from the local coordinator and the neighboring coordinator by the second receiver on the out-of-band channel; and wherein a synchronization message comprises identification information of the individual coordinator; derive a relative offset by the controller, following receipt of the synchronization message from the neighboring coordinator, and wherein the relative offset is the time interval between the time of reception of the synchronization message from the neighboring coordinator as compared to the start-time of the current MAC cycle of the local coordinator; and send a signal to the local coordinator by the second transmitter on the out-of-band channel, the signal comprising identification information of the neighboring coordinator and the relative offset derived by the network device.
8 . The network device of claim 1 , the network device associated to a local coordinator out of the at least two adjacent coordinators, and another one out of the at least two adjacent coordinators being a neighboring coordinator of the network device, the network device configured to: detect synchronization messages of a second type from the local coordinator and the neighboring coordinator by the first receiver over the OWC network; and wherein a synchronization message of the second type comprises identification information of the respective coordinator transmitting the synchronization message and a time interval between the transmission of the synchronization message as compared to the start-time of a MAC cycle of the respective coordinator; derive a second time interval by the controller, following receipt of a synchronization message of the second type from the neighboring coordinator, and wherein the second time interval is the time interval between the reception of the synchronization message from the neighboring coordinator as compared to the start-time of the current MAC cycle of the local coordinator; and send a signal to the local coordinator by the second transmitter on the out-of-band channel, and wherein the signal comprises identification information of the neighboring coordinator, the time interval comprised in the synchronization message from the neighboring coordinator, and the second time interval derived by the network device.
9 . The network device according to claim 2 , the network device associated to a local coordinator out of the at least two adjacent coordinators, and another one out of the at least two adjacent coordinators being a neighboring coordinator of the network device, the network device configured to: detect synchronization messages of a second type from the local coordinator and the neighboring coordinator by the second receiver on the out-of-band channel; and wherein a synchronization message of the second type comprises identification information of the respective coordinator transmitting the synchronization message and a time interval between the transmission of the synchronization message as compared to the start-time of a MAC cycle of the respective coordinator; derive a second time interval by the controller, following receipt of a synchronization message of the second type from the neighboring coordinator, and wherein the second time interval is the time interval between the reception of the synchronization message from the neighboring coordinator as compared to the start-time of the current MAC cycle of the local coordinator; and send a signal to the local coordinator by the second transmitter on the out-of-band channel, and wherein the signal comprises identification information of the neighboring coordinator, the time interval comprised in the synchronization message from the neighboring coordinator, and the second time interval derived by the network device.
10 . A coordinator out of at least two coordinators for reducing interference in an optical wireless communication (OWC) network having a line-of-sight character, the OWC network comprising the at least two coordinators and a plurality of network devices selectively associated to and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share a same optical wireless medium of the OWC network covering a first pre-defined spectrum range via a time-division multiple access (TDMA) approach by dividing the wireless medium into consecutive time slots, the coordinator comprising: a first transmitter configured to send out data via downlink communication over the OWC network within the first pre-defined spectrum range; a second receiver configured to receive a signal from a network device via an out-of-band channel outside the first pre-defined spectrum range, and wherein the signal comprising timing information related to MAC cycles of at least two adjacent coordinators is intended to assist with alignment of MAC cycles between the at least two adjacent coordinators, from which the network device detects downlink communication; wherein the out-of-band channel has a line-of-sight character.
11 . The coordinator of claim 10 , the coordinator further comprising: a second transmitter configured to send downlink data on the out-of-band channel.
12 . The coordinator of claim 10 , the coordinator further comprising: a controller configured to adjust a start-time of its MAC cycles according to the signal received from the network device, and wherein a time duration of a MAC cycle is defined by the OWC network, and the start-time of a MAC cycle is a local time reference used by the coordinator to divide the wireless medium into consecutive time slots.
13 . The coordinator of claim 10 , the coordinator further comprising: a third transmitter configured to forward the signal received from the network device to a network controller via a wired connection; a third receiver configured to receive an instruction from the network controller via the wired connection; and a controller configured to adjust a start-time of its MAC cycles according to the instruction received from the network controller, and wherein a time duration of a MAC cycle is defined by the OWC network, and the start-time of a MAC cycle is a local time reference used by the coordinator to divide the wireless medium into consecutive time slots.
14 . A method for reducing interference at a network device out of the plurality of network devices in an optical wireless communication (OWC) network having a line-of-sight character, the OWC network comprising at least two coordinators and the plurality of network devices selectively associated to and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share a same optical wireless medium of the OWC network covering a first pre-defined spectrum range via a time-division multiple access (TDMA) approach by dividing the wireless medium into consecutive time slots, the method comprising the network device out of the plurality of network devices: detecting a downlink communication over the OWC network within the first pre-defined spectrum range; deciding on assisting with alignment of MAC cycles between at least two adjacent coordinators, based on the detected downlink communication from the at least two adjacent coordinators; and sending a signal comprising timing information related to MAC cycles of the at least two adjacent coordinators via an out-of-band channel outside the first pre-defined spectrum range, upon the decision made by the controller on assisting with the alignment of MAC cycles between the at least two adjacent coordinators for interference suppression; wherein the timing information is one of a signal to indicate the start time of individual MAC cycles of the two adjacent coordinators, an offset among individual MAC cycles of the adjacent coordinators, and start time of MAC cycles of one coordinator out of the adjacent coordinators; and wherein the out-of-band channel has a line-of-sight character.
15 . A method for reducing interference at a coordinator out of at least two coordinators in an optical wireless communication (OWC) network having a line-of-sight character, the OWC network comprising the at least two coordinators and a plurality of network devices selectively associated to and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share a same optical wireless medium of the OWC network covering a first pre-defined spectrum range via a time-division multiple access (TDMA) approach by dividing the wireless medium into consecutive time slots, the method comprising the coordinator out of the at least two coordinators: sending out data via downlink communication over the OWC network within the first pre-defined spectrum range; and receiving a signal from a network device comprising timing information related to MAC cycles of at least two adjacent coordinators via an out-of-band channel outside the first pre-defined spectrum range, and wherein the signal is intended to assist with alignment of MAC cycles between the at least two adjacent coordinators, from which the network device detects downlink communication; wherein the out-of-band channel has a line-of-sight character.
16 . The method of claim 15 , further comprising a coordinator out of the at least two coordinators: sending periodic beacons at a predefined position in each of its MAC cycles; and receiving signals via the out-of-band channel from the network device, and wherein the signals are reflected periodic beacons received by the network device from the at least two adjacent coordinators; and adjusting the start-time of its MAC cycles, following receipt of more than one reflected beacon within a single MAC cycle from the network device, for reducing a gap among the more than one reflected beacon.
17 . The method of claim 15 , further comprising a coordinator out of the at least two coordinators: sending a synchronization message, and wherein the synchronization message comprises identification information of the coordinator; and receiving a signal via the out-of-band channel from the network device that is associated to the coordinator and wherein the signal comprises identification information of a neighboring coordinator and a timing offset, and wherein the timing offset is derived by the network device and represents a time interval between the time of reception of the synchronization message from a neighboring coordinator as compared to the start-time of the current MAC cycle of the coordinator; forwarding the signal received from the network device to a network controller via a wired connection; receiving an instruction from the network controller via the wired connection; and adjusting the start-time of a MAC cycle according to the instruction received from the network controller.
18 . The method of claim 15 , further comprising a coordinator out of the at least two coordinators: sending a synchronization message, and wherein the synchronization message comprises identification information of the coordinator and a time interval between the transmission of the synchronization message as compared to the start-time of a current MAC cycle; and receiving a signal via the out-of-band channel from the network device that is associated to the coordinator, and wherein the signal comprises identification information of a neighboring coordinator, the time interval comprised in another synchronization message from the neighboring coordinator, and a second time interval derived by the network device representing a time interval between the time of reception of the other synchronization message from a neighboring coordinator as compared to the start time of the current MAC cycle of the coordinator; and the method further comprising either: adjusting the start time of a MAC cycle according to the signal received from the network device; or: forwarding the signal received from the network device to a network controller via a wired connection; receiving an instruction from the network controller via the wired connection; and adjusting the start time of a MAC cycle according to the instruction received from the network controller.
19 . A non-transitory computer readable medium comprising instructions which, when the instructions are executed by a computer, cause the computer to carry out the method of claim 14 .
20 . A system for reducing interference, the system comprising: an optical wireless communication (OWC) network with a line-of-sight character comprising at least two coordinators according to claim 13 and a plurality of network devices selectively associated to and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share a same wireless medium of the OWC network via a TDMA approach by dividing the wireless medium into consecutive time slots; and a network device out of the plurality of network devices; and a first coordinator, which the network device is associated to, being one out of the at least two coordinators; and a second coordinator, which is next to the local coordinator, being another one out of the at least two coordinators; and a network controller, which is connected to the first coordinator and the second coordinator via a wired connection; and wherein the network controller is configured to: receive the signal originated from the network device and forwarded by the first coordinator; determine an adjustment needed to align the MAC cycles of the first coordinator and the second coordinator, following receipt of the signal; and instruct at least one of the first coordinator or the second coordinator to implement the determined adjustment of the MAC cycles.

Description

CROSS-REFERENCE TO PRIOR APPLICATIONS This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2020/077174, filed on Sep. 29, 2020, which claims the benefit of European Patent Application No. 19201936.2, filed on Oct. 8, 2019. These applications are hereby incorporated by reference herein. FIELD OF THE INVENTION The invention relates to the field of scheduling transmissions in optical wireless networks, such as Li-Fi networks. More particularly, various methods, apparatus, systems and computer-readable media are disclosed herein related to synchronization among adjacent access points or coordinators. BACKGROUND OF THE INVENTION To enable more and more electronic devices like laptops, tablets, and smartphones to connect wirelessly to the Internet, wireless communication confronts unprecedented requirements on data rates and also link qualities, and such requirements keep on growing year over year, considering the emerging digital revolution related to Internet-of-Things (IoT). Radio frequency technology like Wi-Fi is running out of spectrum to support this revolution. In the meanwhile, Li-Fi is drawing more and more attention with its capability to support higher data rates over the available bandwidth in visible light, ultraviolet, and infrared spectra. Other benefits of Li-Fi include data security, and the ability to function safely in areas otherwise susceptible to electromagnetic interference. Therefore, Li-Fi is a very promising new technology to enable the next generation of immersive connectivity. Visible-light communication (VLC) transmits data by intensity modulating optical sources, such as light emitting diodes (LEDs) and laser diodes (LDs), faster than the persistence of the human eye. VLC merges lighting and data communications in applications such as illumination, signage, streetlights, vehicle lighting, and traffic signals. The IEEE 802.15.7 visible-light communication personal area network (VPAN) standard maps the intended applications to four topologies: peer-to-peer, star, broadcast and coordinated. Optical Wireless PAN (OWPAN) is a more generic term than VPAN also allowing invisible light for communication. In the star topology, the communication is established between devices and a single central controller, called the coordinator. In the peer-to-peer topology, one of the two devices in an association takes on the role of the coordinator. In the coordinated topology, multiple devices communicate with multiple coordinators, supervised by a global network controller. The global network controller has a fixed network link to each coordinator. For handling interference in an optical wireless communication system, or a Li-Fi system, with multiple access points (APs) or coordinators having overlapping coverage area based on time-division, the coordinators must be synchronized to a common time base. IEEE Std 1588-2008 describes a protocol to synchronize distributed clocks across a network, whereby a high accuracy can be achieved by dedicate support from routers/switches in the network. However, with an existing infrastructure, the routers/switches may not yet support this protocol, and hence, the uncertainty of the synchronization increases, which also results in interference to the end points (EPs) or network devices located in the overlapping area of adjacent APs or coordinators. US2006007907A1 relates to beacon scheduling in a wireless personal area network including a device and multiple coordinators within transmission range of the device. A superframe is defined to include a beacon period, a contention access period, and a contention free period. The beacon period includes multiple slots. In each coordinator, a particular beacon slot is selected to be non-conflicting with beacon slots selected by other coordinators. Beacons are then transmitted to the device by the coordinators at time periods associated with the selected slots. US2019020414A1 relates to a beacon sending method and a network access method. The method is: sending, by a coordinator, a beacon by occupying a first beacon timeslot in a current super frame, and after sending the beacon, receiving a first beacon request frame sent by a device, where the first beacon request frame is used to indicate that the device is located in an interference area between an area of a first network in which the coordinator is located and an area of at least one second network; and after receiving the first beacon request frame, sending, by the coordinator, a beacon by separately occupying a first beacon timeslot and a second beacon timeslot in each of N super frames starting from a next super frame, where N≥1, and N is a positive integer. SUMMARY OF THE INVENTION Because of the line-of-sight character of an optical wireless communication network, neighboring coordinators may not be able to communicate with each other directly. Hence, the information exchange among neighboring coordinat