Synchronization in optical wireless networks for interference suppression

Abstract

In an optical wireless communication (OWC) network (100), time division multiple access (TDMA) is typically used to mitigate interference between multiple network devices (D1-Dn) associated with an individual coordinator or between adjacent coordinators (C1-Cn). However, the MAC cycle offset between two adjacent coordinators (C1-C2) may still cause interference to a network device (D1) located in an overlapping area of coverage of the two adjacent coordinators (C1-Cn). The present invention relates to various methods, apparatus, systems, computer programs, and computer-readable media for providing a mechanism for transmitting a signal from a network device (D1) to two neighboring coordinators (C1-C2) using an out-of-band channel to assist in aligning the MAC cycles between the two adjacent coordinators (C1-C2), wherein the out-of-band channel has a line-of-sight characteristic and is out-of-band compared to the OWC network (100).

Description

Technical Field

[0001] The present invention relates to the field of scheduling transmissions in optical wireless networks, such as Li-Fi networks. More particularly, various methods, apparatuses, systems, and computer-readable media related to synchronization between neighboring access points or coordinators are disclosed herein. Background Art

[0002] To enable more and more electronic devices (such as laptops, tablets and smartphones) to connect wirelessly to the Internet, wireless communications face unprecedented demands for data rates and link quality, and given the emerging digital revolution associated with the Internet of Things (IoT), such demands continue to grow year by year. Radio frequency technologies such as Wi-Fi are exhausting the spectrum to support this revolution. At the same time, Li-Fi is attracting increasing attention for its ability to support higher data rates over available bandwidth in the visible, ultraviolet and infrared spectrums. Other benefits of Li-Fi include data security and the ability to operate safely in areas that are otherwise susceptible to electromagnetic interference. Therefore, Li-Fi is a very promising new technology that enables the next generation of immersive connectivity.

[0003] Visible light communication (VLC) transmits data faster than the human eye's persistence by using intensity-modulated light sources, such as light-emitting diodes (LEDs) and laser diodes (LDs). VLC merges lighting and data communications into applications such as lighting, signage, streetlights, vehicle lighting, and traffic signals. The IEEE 802.15.7 Visible Light Communication Personal Area Network (VPAN) standard maps intended applications to four topologies: peer-to-peer, star, broadcast, and coordinated. Optical Wireless PAN (OWPAN) is a more general term than VPAN that also allows for invisible light to be used for communications.

[0004] In a star topology, communication is established between devices and a single central controller called a coordinator. In a peer-to-peer topology, one of the two devices in the association assumes the role of coordinator. In a coordinated topology, multiple devices communicate with multiple coordinators, overseen by a global network controller. The global network controller has a fixed network link to each coordinator.

[0005] To handle interference in optical wireless communication systems, or Li-Fi systems, where multiple access points (APs) or coordinators have overlapping coverage areas based on time division, the coordinators must be synchronized to a common time base. IEEE Std 1588-2008 describes a protocol for synchronizing distributed clocks across a network, which can achieve high accuracy with dedicated support from routers / switches in the network. However, in the case of existing infrastructure, routers / switches may not yet support this protocol, and as a result, synchronization uncertainty increases, which also leads to interference to endpoints (EPs) or network devices located in the overlapping areas of adjacent APs or coordinators.

[0006] US2006007907A1 relates to beacon scheduling in a wireless personal area network (WAN) comprising a device and multiple coordinators within the device's transmission range. A superframe is defined as consisting of a beacon period, a contention access period, and a contention-free period. The beacon period consists of multiple time slots. Each coordinator selects a specific beacon slot that does not conflict with beacon slots selected by other coordinators. The coordinator then transmits a beacon to the device during the time period associated with the selected slot.

[0007] US2019020414A1 relates to a beacon sending method and a network access method. The method comprises: sending a beacon by a coordinator occupying the first beacon time slot in the current superframe, and after sending the beacon, receiving a first beacon request frame sent by a device, wherein 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 where the coordinator is located and an area of at least one second network; and after receiving the first beacon request frame, the coordinator sends a beacon by separately occupying the first beacon time slot and the second beacon time slot in each superframe of N superframes starting from the next superframe, where N≥1 and N is a positive integer. Summary of the Invention

[0008] Due to the line-of-sight nature of optical wireless communication networks, neighboring coordinators may not be able to communicate directly with each other. Therefore, information exchange between neighboring coordinators may rely on network devices located in the overlapping area of the coverage areas of these neighboring coordinators, or on another device in the backbone network, such as a network controller. Due to the lack of direct communication, such neighboring coordinators may sometimes not have synchronized MAC cycles. Although the duration of a MAC cycle is generally the same for all coordinators in the network, the start time of the MAC cycle may be different for individual coordinators. Note that the start time of the MAC cycle is used by the coordinator as a local time reference to divide the wireless medium into consecutive time slots. Such a MAC cycle offset between two neighboring coordinators may cause interference to network devices located in the overlapping coverage area of the two neighboring coordinators, even when the time slot is specifically allocated to one coordinator for communicating with network devices in the overlapping area.

[0009] In view of the above, the present disclosure is directed to methods, apparatuses, systems, computer programs, and computer-readable media for providing a mechanism for handling MAC cycle synchronization between neighboring APs or coordinators. More particularly, the present invention is achieved by a network device as claimed in claim 1, a coordinator as claimed in claim 10, methods for a network device and a coordinator as claimed in claims 14 and 15, respectively, a computer program as claimed in claim 19, and a system as claimed in claim 20.

[0010] Thus, a network device located in an overlapping area of at least two adjacent coordinators can derive timing information related to the MAC cycles of the at least two coordinators based on downlink communications from these coordinators. The network device is configured to subsequently send such timing information to at least one of the two adjacent coordinators via an out-of-band channel when the network device determines that it is necessary to assist in aligning the MAC cycles between the at least two adjacent coordinators. The timing information can be derived from conventional downlink data communications over the OWC network or downlink data on the out-of-band channel. To facilitate the network device in deriving the timing information, the coordinator can send periodic beacons or synchronization messages in downlink communications over the OWC network or the out-of-band channel.

[0011] According to a first aspect of the present invention, a network device is provided. A network device among a plurality of network devices is configured to reduce interference in an optical wireless communication (OWC) network having a line-of-sight characteristic, the OWC network comprising at least two coordinators, the plurality of network devices selectively associated with and synchronized with a respective one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share the same optical wireless medium of the OWC network covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method. The network device includes: a first receiver configured to detect downlink communications over the OWC network within a first predefined spectrum range; a controller configured to decide to assist in aligning the media access control (MAC) cycles between the at least two adjacent coordinators based on the downlink communications from the at least two adjacent coordinators detected by the first receiver; a second transmitter configured to send a signal including timing information related to the MAC cycles of the at least two adjacent coordinators via an out-of-band channel outside the first predefined spectrum range when deciding to assist in aligning the MAC cycles between the at least two adjacent coordinators for interference suppression; wherein the timing information is one of a signal indicating the start time of the individual MAC cycles of the two adjacent coordinators, an offset between the individual MAC cycles of the adjacent coordinators, and a start time of the MAC cycle of one of the adjacent coordinators; and wherein the out-of-band channel has a line-of-sight characteristic.

[0012] Since neighboring coordinators cannot communicate directly, they may not be aware of the misalignment of MAC cycles with respect to each other and may not be able to resolve the issue. Beneficially, the network device may decide to act as a middleman to assist neighboring coordinators in aligning MAC cycles for interference suppression. The network device is configured to monitor downlink communications over the OWC network to derive timing information related to the MAC cycle so that it can provide the timing information to the coordinator via an out-of-band channel. Assuming that both the OWC network and the out-of-band channel have line-of-sight characteristics, as long as the network device can detect the downlink data from the neighboring coordinator, the uplink signal from the network device can also reach the corresponding neighboring coordinator.

[0013] The timing information may be a signal indicating the start time of the individual MAC cycles of neighboring coordinators, the offset between the individual MAC cycles of neighboring coordinators, or the start time of the MAC cycle of one of the neighboring coordinators. The main purpose of such timing information is to provide a common timing reference for neighboring coordinators that cannot reach each other so that their MAC cycles can be adjusted.

[0014] The benefit of utilizing an out-of-band channel is to reduce the overhead for MAC cycle alignment in the OWC network. In one example, network devices can derive timing information via regular data communications with the coordinator and thus reduce the additional overhead of supporting such alignment over the OWC network.

[0015] The out-of-band channel can differ from the channel of the OWC network in terms of spectral range, wavelength, or another property that allows for concurrent communication over different spectral ranges. In another example, the out-of-band channel is another optical channel having another spectral range that is lower than the first predefined spectral range. Preferably, the signals sent over the out-of-band channel can propagate over the same or a larger coverage area than the signals sent over the OWC network.

[0016] Advantageously, the network device further includes a second receiver configured to detect downlink data on an out-of-band channel; and wherein the decision about assisting MAC cycle alignment between at least two adjacent coordinators is based on: either downlink communications from the at least two adjacent coordinators detected by the first receiver, or downlink data from the at least two adjacent coordinators detected by the second receiver.

[0017] It is also possible to utilize an out-of-band channel as a bidirectional signaling channel. The network device can also detect downlink data from neighboring coordinators on the out-of-band channel and derive timing information from it. In this sense, the network device can have two instruments to obtain timing information related to the MAC cycle of the neighboring coordinator, either via in-band downlink communication over the OWC network or via out-of-band downlink data on the out-of-band channel.

[0018] In one embodiment, the second transmitter is configured to send a periodic signal to the at least two neighboring coordinators for use as a timing reference by the at least two neighboring coordinators when adjusting their MAC cycles, and wherein the periodic signal indicates a new start time of the MAC cycle based on timing information related to the MAC cycles of the at least two neighboring coordinators.

[0019] One example of an uplink signal from a network device to assist in MAC alignment is to utilize a periodic signal from the network device. The periodic signal indicates a new start time for the MAC cycle that the neighboring coordinator can follow, which can be a compromise of the start time of the MAC cycle of two neighboring coordinators, a compromise of the start time of the MAC cycle of one of the neighboring coordinators, or simply a new start time derived by the network device after knowing the start time of the MAC cycle of the neighboring coordinator. It is relevant that the network device and the coordinator operate under the same assumptions about the reported information and the subsequent correction(s) made going forward.

[0020] Further disclosed is that the network device is configured to detect, by a first receiver, periodic beacons from the at least two neighboring coordinators over the OWC network, and to send, by a second transmitter, a signal to the at least two neighboring coordinators on an out-of-band channel by reflecting each periodic beacon to the at least two neighboring coordinators with a predefined delay after receiving each periodic beacon.

[0021] In another example, downlink communications over the OWC network include downlink periodic beacons from neighboring coordinators. It is not necessary to send periodic beacons at the start time of each MAC cycle. Periodic beacons can be sent at any predefined offset from the start time of the MAC cycle. It is necessary to keep the interval between adjacent periodic beacons the same as the duration of the MAC cycle, which is agreed upon by the entire OWC network. Such periodic beacons can be, but need not be, dedicated beacons used to assist in the alignment of neighboring coordinators on the MAC cycle. They can simply be reused from regular beacons in a TDMA system for synchronization between multiple network devices associated with the coordinator.

[0022] The network device can be configured to reflect each periodic beacon back to the adjacent coordinator as a relay node to enable an indirect link between adjacent coordinators. In order to tolerate certain transmission delays, processing delays, or even transmitter and receiver switching delays in the case of transceiver settings, the network device will reflect each beacon with a predefined delay after receiving the beacon. The predefined delay can be a fixed time offset defined according to those delays. The predefined delay can also be a fixed delay agreed upon by the coordinator, such as a MAC cycle duration. And then, the network device will send back the reflected beacon after receiving a complete MAC cycle.

[0023] If there are multiple beacons reflected by a network device, it is clear to the coordinator that there is an offset in the MAC period between neighboring coordinators, indicating that further adjustments to the MAC period will be needed. In this way, the problem at hand is identified as a first step towards resolving the problem.

[0024] Preferably, the network device is configured to detect periodic beacons from the at least two neighboring coordinators on the out-of-band channel by the second receiver, and to send a signal to the at least two neighboring coordinators on the out-of-band channel by the second transmitter by reflecting each periodic beacon to the at least two neighboring coordinators with a predefined delay after receiving each periodic beacon.

[0025] Periodic beacons may also be sent from the coordinator to the network devices on an out-of-band channel. This may be more efficient when there are no existing periodic beacons in the OWC network that can be shared to support the present invention.

[0026] Advantageously, the network device is associated with a local coordinator among the at least two neighboring coordinators, and another one of the at least two neighboring coordinators is a neighboring coordinator of the network device. Preferably, the network device is configured to detect synchronization messages from the local coordinator and the neighboring coordinators over the OWC network by a first receiver; and wherein the synchronization messages include identification information of the individual coordinators;

[0027] deriving, by the controller, a relative offset after receiving a synchronization message from a neighbor coordinator, and wherein the relative offset is a time interval between a time when the synchronization message is received from the neighbor coordinator and a start time of a current MAC cycle of the local coordinator; and

[0028] A signal is sent by the second transmitter to the local coordinator on an out-of-band channel, the signal including identification information of the neighbor coordinator and the relative offset derived by the network device.

[0029] The network device may also associate with a local coordinator among the two neighboring coordinators so that the network device is synchronized with the local coordinator. Downlink communications from the local coordinator and neighboring coordinators may be a single synchronization message or several recursive synchronization messages each including identification information of an individual coordinator.

[0030] The synchronization message can be sent by an individual coordinator with a predetermined, possibly arbitrary, offset from the start of the MAC cycle of the individual coordinator. Although the network device may not have knowledge of such arbitrary offsets of the neighboring coordinators, it can derive the relative offset after receiving the synchronization message from the neighboring coordinator. The relative offset is the time interval between the time the synchronization message is received from the neighboring coordinator and the start time of the current MAC cycle of the local coordinator. The network device will then forward the identification information and the derived relative offset back to the local coordinator via an out-of-band channel. When sending the synchronization message, the local coordinator also needs to know the arbitrary offset of the neighboring coordinator in order to derive the actual offset on the MAC cycle and implement any adjustments accordingly. If the local coordinator does not have this knowledge available, it can forward the signal received from the network device to a centralized device that has such knowledge, such as a network controller. And then a decision can be made on the network controller side about how to adjust the MAC cycle of at least one of the neighboring coordinators.

[0031] Alternatively, the local coordinator can have a direct wired connection to the neighbor coordinator. Given the identification information received from the network device, the local coordinator can use it to address the neighbor coordinator to request information about the predetermined offset used by the neighbor coordinator when sending synchronization messages. The local coordinator then also obtains enough information to implement local adjustments to the MAC cycle to achieve synchronization with the neighbor coordinator.

[0032] Optionally, the neighbor coordinator can also eavesdrop on the uplink signal from the network device. The neighbor coordinator then knows both the relative offset derived by the network device (relative to the start time of the local coordinator) and its own arbitrary offset to send synchronization messages. Thus, the neighbor coordinator has sufficient knowledge to achieve MAC cycle alignment locally with respect to the local coordinator.

[0033] Preferably, the network device is configured to detect synchronization messages from the local coordinator and neighbor coordinators via a second receiver over an out-of-band channel; wherein the synchronization messages include identification information of the individual coordinators. The network device is then configured to derive a relative offset by a controller after receiving the synchronization message from the neighbor coordinator, wherein the relative offset is a time interval between the time when the synchronization message is received from the neighbor coordinator and the start time of the current MAC cycle of the local coordinator. The network device is further configured to send a signal to the local coordinator via a second transmitter over the out-of-band channel, wherein the signal includes the identification information of the neighbor coordinator and the relative offset derived by the network device.

[0034] Advantageously, the network device is configured to detect, by a first receiver, a second type of synchronization message from a local coordinator and a neighboring coordinator over the OWC network; wherein the second type of synchronization message includes identification information of the corresponding coordinator transmitting the synchronization message and a time interval between the transmission of the synchronization message and the start time of the MAC cycle of the corresponding coordinator. The controller is configured to derive a second time interval after receiving the second type of synchronization message from the neighboring coordinator, wherein the second time interval is a time interval between the reception of the synchronization message from the neighboring coordinator and the start time of the current MAC cycle of the local coordinator. The second transmitter is configured to send a signal to the local coordinator on an out-of-band channel, wherein the signal includes the identification information of the neighboring coordinator, the time interval contained in the synchronization message from the neighboring coordinator, and the second time interval derived by the network device.

[0035] The synchronization message from the coordinator may also be of a second type, with embedded timing information indicating an arbitrary offset, which is the time interval between the transmission of the synchronization message and the start time of the corresponding coordinator's MAC cycle. Upon receiving the second type of synchronization message, the network device derives a second time interval, which indicates the timing gap between the reception of the synchronization message from the neighboring coordinator and the start time of the local coordinator's current MAC cycle. The network device then sends a signal to the local coordinator over an out-of-band channel. The signal includes the neighboring coordinator's identification information, the time interval contained in the synchronization message from the neighboring coordinator, and the second time interval derived by the network device. This provides the local coordinator with sufficient information to achieve MAC cycle alignment and synchronize with the neighboring coordinator. Similarly, if a neighboring coordinator overhears the signal, the neighboring coordinator can synchronize. If both coordinators overhear, preferably only one of the two neighboring coordinators will achieve alignment, or the two coordinators will coordinate the alignment, such as by adjusting only half of the locally derived offset. Otherwise, a ping-pong effect may occur. To avoid this, involvement of the network controller may be beneficial.

[0036] Advantageously, the network device is configured to detect, via a second receiver, a second type of synchronization message from a local coordinator and a neighboring coordinator on an out-of-band channel; wherein the second type of synchronization message includes identification information of the corresponding coordinator transmitting the synchronization message and a time interval between the transmission of the synchronization message and the start time of the MAC cycle of the corresponding coordinator. The controller is configured to derive a second time interval after receiving the second type of synchronization message from the neighboring coordinator, wherein the second time interval is the time interval between the reception of the synchronization message from the neighboring coordinator and the start time of the current MAC cycle of the local coordinator. The second transmitter then sends a signal to the local coordinator on the out-of-band channel, wherein the signal includes the identification information of the neighboring coordinator, the time interval contained in the synchronization message from the neighboring coordinator, and the second time interval derived by the network device.

[0037] Using the signals received from the network devices, which include two types of timing information related to the local and neighboring coordinators respectively, the local coordinator can derive the offset of the MAC cycle relative to the neighboring coordinators and implement adjustments.

[0038] According to a second aspect of the present invention, a coordinator device is provided. One of at least two coordinators is used to reduce interference in an optical wireless communication (OWC) network with line-of-sight characteristics, the OWC network comprising at least two coordinators and a plurality of network devices selectively associated with and synchronized with a corresponding one of the coordinators, wherein the at least two coordinators and the plurality of network devices share the same optical wireless medium of the OWC network, the optical wireless medium covering a first predefined spectrum range via a time division multiple access (TDMA) method 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 predefined spectrum range; a second receiver configured to receive a signal from the network device via an out-of-band channel outside the first predefined spectrum range, wherein the signal including timing information related to the MAC cycle of at least two adjacent coordinators is intended to assist in aligning the MAC cycle between the at least two adjacent coordinators, the network device detecting the downlink communication from the at least two adjacent coordinators;

[0039] The out-of-band channel has a line-of-sight characteristic.

[0040] Advantageously, a coordinator device is also provided to cooperate with the aforementioned (one or more) network devices. As a basic setting, the coordinator will carry out two downlink data communications as usual, and in addition, the coordinator will also monitor the uplink signal from the network device on the out-of-band channel, said uplink signal including timing information related to the MAC cycle of the neighboring coordinator, which is required for aligning the MAC cycle.

[0041] Preferably, the coordinator further comprises a second transmitter configured to send the downlink data on the out-of-band channel.

[0042] Corresponding to the option that the network device detects downlink data on an out-of-band channel to derive timing information to assist in MAC alignment, the coordinator may include a second transmitter to support downlink signaling to convey information related to the start time of the MAC cycle.

[0043] In another embodiment, the coordinator further includes a controller configured to adjust a start time of its MAC cycle based on a signal received from a network device, and wherein the duration of the MAC cycle is defined by the OWC network and the start time of the MAC cycle is a local time reference used by the coordinator to divide the wireless medium into consecutive time slots.

[0044] In the case where the coordinator obtains sufficient information via an out-of-band channel to adjust its MAC cycle, the adjustment can be performed locally at the coordinator.The timing information contained in the signal received from the network device will be used as a reference for the coordinator to adjust the start time of its MAC cycle.

[0045] In another embodiment, the coordinator further comprises: a third transmitter configured to forward a signal received from a network device to a network controller via a wired connection; and a third receiver configured to receive an instruction from the network controller via the wired connection. The coordinator further comprises a controller configured to adjust a start time of its MAC cycle according to the instruction received from the network controller, wherein the duration of the MAC cycle is defined by the OWC network, and the start time of the MAC cycle is a local time reference used by the coordinator to divide the wireless medium into consecutive time slots.

[0046] When the timing information contained in the signal received from the network device is insufficient for the coordinator to adjust its MAC cycle to synchronize with the neighboring coordinators, the coordinator may further seek assistance from the network controller by forwarding the signal received from the network device to the network controller. It is possible that the network controller may have an overview of the timing information, such as any time offset when sending synchronization messages for neighboring coordinators on the network.

[0047] Alternatively, when the network controller does not have all the timing information in advance, it can query such timing information on the fly - assuming that the network controller will have a wired connection to all coordinators in the OWC network. In addition, based on the signal from the network device forwarded by the coordinator, including the neighbor coordinator identification information, the network controller has a good understanding of the neighbor relationship of the at least two neighbor coordinators - assuming that the network controller will also know which coordinator is forwarding the signal. And then the network controller can decide on the adjustment and send instructions to at least one coordinator or both coordinators for implementation. Thus, the timing information required for auxiliary MAC alignment is collected in a distributed manner, and the MAC cycle alignment is controlled in a centralized manner.

[0048] According to a third aspect of the present invention, a method for a network device is provided. A method for reducing interference at a network device among a plurality of network devices in an optical wireless communication (OWC) network having a line-of-sight characteristic, the OWC network comprising at least two coordinators and a plurality of network devices selectively associated with and synchronized with a corresponding one of the coordinators, wherein the at least two coordinators and the plurality of network devices share the same optical wireless medium of the OWC network covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method. The method includes a network device among the plurality of network devices: detecting downlink communication over the OWC network within the first predefined spectrum range; deciding, based on the detected downlink communication from at least two adjacent coordinators, to assist in alignment of MAC cycles between the at least two adjacent coordinators; and, when a controller makes a decision regarding assisting in alignment of MAC cycles between the at least two adjacent coordinators for interference suppression, transmitting a signal including timing information related to the MAC cycles of the at least two adjacent coordinators via an out-of-band channel outside the first predefined spectrum range; and wherein the out-of-band channel has a line-of-sight characteristic.

[0049] According to another aspect of the present invention, a method for a coordinator device is provided. A method for reducing interference at one of at least two coordinators in an optical wireless communication (OWC) network having a line-of-sight characteristic, the OWC network comprising the at least two coordinators and a plurality of network devices selectively associated with and synchronized with a corresponding one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share the same optical wireless medium of the OWC network covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method, the method comprising a coordinator of the at least two coordinators: transmitting data via downlink communication over the OWC network within the first predefined spectrum range; receiving a signal from a network device via an out-of-band channel outside the first predefined spectrum range, the signal comprising timing information related to MAC cycles of at least two adjacent coordinators, and wherein the signal is intended to assist in aligning the MAC cycles between the at least two adjacent coordinators, the network device detecting the downlink communication from the at least two adjacent coordinators; wherein the out-of-band channel has a line-of-sight characteristic.

[0050] Preferably, the method comprises a coordinator among the at least two coordinators transmitting downlink data on an out-of-band channel. The downlink data comprises information related to a MAC cycle of the individual coordinator on the OWC network.

[0051] More preferably, the method further comprises a coordinator among the at least two coordinators adjusting a start time of its MAC cycle based on a signal received from a network device, and wherein the duration of the MAC cycle is defined by the OWC network, and the start time of the MAC cycle is a local timing reference used by the coordinator to divide the wireless medium into consecutive time slots.

[0052] Advantageously, the method further includes a coordinator among the at least two coordinators: sending a periodic beacon at a predefined position in each of its MAC cycles; and receiving a signal from a network device via an out-of-band channel, and wherein the signal is a reflected periodic beacon received by the network device from the at least two adjacent coordinators; and after receiving more than one reflected beacon from the network device within a single MAC cycle, adjusting the start time of its MAC cycle to reduce the gap between more than one reflected beacons.

[0053] In another embodiment, the method further includes a coordinator among the at least two coordinators: sending a synchronization message, the synchronization message including identification information of the coordinator; and receiving a signal from a network device associated with the coordinator via an out-of-band channel, the signal including identification information of a neighboring coordinator and a timing offset, the timing offset being derived by the network device and representing the time interval between the time when the synchronization message is received from the neighboring coordinator and 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 a wired connection; and adjusting the start time of the MAC cycle according to the instruction received from the network controller.

[0054] In another preferred embodiment, the method further includes a coordinator among the at least two coordinators: sending a synchronization message, and wherein the synchronization message includes identification information of the coordinator and a time interval between the transmission of the synchronization message and the start time of the current MAC cycle; and receiving a signal from a network device associated with the coordinator via an out-of-band channel, and wherein the signal includes identification information of the neighboring coordinator, a time interval contained in another synchronization message from the neighboring coordinator, and a second time interval derived by the network device, the second time interval representing the time interval between the time of receiving another synchronization message from the neighboring coordinator and the start time of the current MAC cycle of the coordinator; and the method further includes: adjusting the start time of the MAC cycle according to the signal received from the network device; or: forwarding the signal received from the network device to the network controller via a wired connection; receiving an instruction from the network controller via a wired connection; and adjusting the start time of the MAC cycle according to the instruction received from the network controller.

[0055] The invention may further be embodied in a computer program comprising code means for causing a computer to carry out the method of the network device and the coordinator when the computer executes the program.

[0056] In a particularly advantageous system for reducing interference, the system includes an optical wireless communication (OWC) network with line-of-sight characteristics, the OWC network including at least two coordinators according to the present invention and a plurality of network devices selectively associated to and synchronized with a corresponding one of the coordinators, and wherein the at least two coordinators and the plurality of network devices share the same optical wireless medium of the OWC network via a TDMA method by dividing the wireless medium into consecutive time slots; a network device among the plurality of network devices according to the present invention; and a first coordinator associated with the network device, which is one of the at least two coordinators; and a second coordinator, which is next to the local coordinator and is the other 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 a signal originating from the network device and forwarded by the first coordinator; after receiving the signal, determine the adjustment required to align the MAC cycles of the first coordinator and the second coordinator; and instruct at least one of the first coordinator or the second coordinator to implement the determined MAC cycle adjustment.

[0057] In another example, centralized control of MAC cycle alignment is performed by a control function unit, which can be located at the coordinator or the network controller. When the control function unit is located on the coordinator side, when there is no wired connection between the coordinators, it can query timing information related to the neighboring coordinators or send (one or more) instructions to the neighboring coordinators via the network controller. When there is a wired connection between the local coordinator and the neighboring coordinators, the control function unit can query timing information or send (one or more) instructions directly to the neighboring coordinators. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In the drawings, like reference numerals generally refer to the same parts throughout the different views. Furthermore, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[0059] Figure 1 An overview of the OWC network and the backbone network connected to it is shown;

[0060] Figure 2 illustrates the line-of-sight characteristics of both downlink communications and uplink communications on out-of-band channels of an OWC network;

[0061] Figure 3 Schematically depicts the basic components of a network device;

[0062] Figure 4 It is shown that the coordinator adjusts its MAC period based on a mechanism whereby neighboring coordinators send periodic beacons to network devices and the network devices reflect the periodic beacons back to the coordinator after a predefined delay;

[0063] Figure 5 A mechanism is presented in which a coordinator adjusts its MAC period based on a neighboring coordinator sending a synchronization message to a network device and the network device sending back a signal including both information contained in the synchronization message and information locally derived by the network device from the synchronization message;

[0064] Figure 6 The basic components of the coordinator are schematically depicted;

[0065] Figure 7 A flow chart illustrating a method performed by a network device;

[0066] Figure 8 A flow chart illustrating a method performed by a coordinator;

[0067] Figure 9 A flow chart illustrating a method performed by a coordinator in transmitting periodic beacons and receiving signals from network devices for assisting in MAC cycle alignment;

[0068] Figure 10 A flow chart illustrating a method performed by a coordinator in sending synchronization messages, receiving a signal from a network device for assisting MAC cycle alignment, and forwarding the signal to a network controller for centralized control of MAC cycle alignment;

[0069] Figure 11 A flow chart illustrating a method performed by a coordinator when sending a second type synchronization message, receiving a signal from a network device for assisting MAC cycle alignment, and determining whether to implement MAC cycle alignment locally or forward the signal to a network controller for centralized control of MAC cycle alignment. DETAILED DESCRIPTION

[0070] Now based on Figure 1Various embodiments of the present invention are described using an optical wireless communication (OWC) network system 100 shown in FIG. For illustrative purposes, the OWC network is connected to a backbone network BN via an IP router IR and an Ethernet switch ES. In a practical system, more routers and switches may be connected between the OWC network and the backbone network. For centralized control of the OWC network system 100, a network controller NC is provided, connected to a coordinator in the OWC network. The OWC network includes at least two coordinators C1-Cn and a plurality of network devices D1-Dn that selectively associate with and synchronize with a corresponding one of the coordinators C1-Cn. Due to the line-of-sight nature of optical communication links, adjacent coordinators C1-Cn do not have direct links to each other. Network devices D1-Dn located in the overlapping coverage area of adjacent coordinators can detect optical signals from both coordinators. The system uses a time division multiple access (TDMA) method to divide the wireless medium into consecutive time slots, which can mitigate interference issues for network devices in the overlapping area if the adjacent coordinators do not use the same time slots for downlink communication. Of course, this is based on the assumption that neighboring coordinators are synchronized to a common timing reference, or their MAC cycles are aligned.

[0071] For OWC networks, there are different ways to achieve such synchronization. This can be achieved locally by detecting the zero crossings of the AC mains. Given the relatively low frequency of the AC mains and the potential high spurious levels, zero-crossing-based methods are not very reliable for achieving high-accuracy synchronization. An alternative approach is to achieve synchronization based on a common remote timing reference, such as a clock at a remote synchronization server.

[0072] Over the past few decades, the problem of clock synchronization has been thoroughly studied in the fields of the Internet and local area networks (LANs). Many well-known protocols exist for maintaining synchronization in computer networks. As explained in the Summary of the Invention section, a major issue to be addressed in network-based clock synchronization is time transmission delay, which includes both processing delays of the operating system and network delays caused by hubs, switches, cables, and other hardware components between the remote reference clock and the local clock. Processing delays are more deterministic, while network delays are much more uncertain. Network delay depends on the length of the path between the remote timing reference and the local clock, the number of hardware components located along the path, and the dynamic traffic conditions in the network. If the network is heavily loaded, more buffering time will be required at the routers or switches, which will all add to the time transmission delay from the remote timing reference to the local clock. Conversely, if the network experiences little traffic, the time transmission delay will be much shorter than in the previous scenario due to the reduced buffering time at the routers and switches.

[0073] Another issue is that for individual coordinators, the local clock may drift, and the drift speed will depend on the accuracy of the clock generation system of the individual coordinator. On the other hand, if no network device is located in the overlapping area of neighboring coordinators, then there is no urgent need to ensure the MAC cycle alignment between neighboring coordinators.

[0074] Generally speaking, synchronization between neighboring coordinators using optical or Li-Fi media is hampered by their lack of line of sight—given that the coordinators are typically deployed on ceilings, out of sight of each other. Neighboring coordinators require network devices to be present in overlapping areas to relay the necessary communications for synchronization. When neighboring coordinators are out of sync, scheduled, interference-free communication with network devices is difficult to achieve. This means that collisions due to misaligned MAC cycles are expected. To reduce the probability of collisions to an acceptable level, coordinators must reserve significant time for neighbor detection and reporting tasks, resulting in inefficient use of radio resources.

[0075] Therefore, the inventors have recognized and appreciated that it would be beneficial to utilize out-of-band signaling to control the alignment of the MAC cycle. Network devices located in the overlapped area would utilize out-of-band signaling to relay timing information to the coordinator. Out-of-band signaling should not interfere with normal OWC network or Li-Fi communications. Consequently, control of MAC cycle adjustments can remain in the background, improving the efficiency of data communications using the OWC network.

[0076] like Figure 1 As shown in the figure, each coordinator controls one or more transceivers (TRX) for optical communication toward network devices D1-Dn. The coordinator applies time slot scheduling to communicate with the network devices associated with it. Note that the transceiver can also be implemented using two separate devices, such as an independent transmitter and an independent receiver, and the TRX shown in the figure is for illustrative purposes only. The shaded area next to each transceiver indicates the communication coverage of each coordinator. For network devices located in the overlapping coverage area belonging to different coordinators, alignment on the MAC cycle will be required. In the example shown in the figure, if there is a misalignment between the MAC cycles of TRX1 and TRX2, the transceivers TRX1 and TRX2 controlled by the neighboring coordinators C1 and C2 may cause interference to network device D1. Network device D4 is within the coverage area of both TRX3 and TRX4, but is not interfered with because TRX3 and TRX4 are controlled by the same coordinator C3. Therefore, no alignment is required between TRX3 and TRX4.

[0077] Figure 2The figure shows the line-of-sight characteristics of both the downlink communication of the OWC network and the uplink communication on the out-of-band channel from network device D1. Since both the OWC network and the out-of-band channel carry the same line-of-sight characteristics, as long as network device D1 can receive downlink communications from both, the uplink signal from D1 to neighboring coordinators C1 and C2 will be received by these neighboring coordinators.

[0078] refer to Figure 3 , we take the network device D1 as an example to explain the basic components of the network device. The blocks with solid lines 210, 220, and 230 are the most basic building blocks that enable the implementation of the present invention. The first receiver 210 is configured to detect downlink communications over the OWC network within a first predefined spectrum range. The controller 220 is configured to decide to assist in the alignment of the media access control (MAC) cycles between at least two adjacent coordinators. What may happen is that the controller performs some preliminary evaluation of whether the network device experiences interference due to MAC cycle misalignment. When interference is encountered, the controller will decide to assist in alignment. The second transmitter 230 is configured to send an uplink signal to the coordinator via an out-of-band channel (which is out-of-band for the OWC network), and the uplink signal includes timing information related to the MAC cycle of the adjacent coordinator.

[0079] Optionally, the network device D1 may further include a second receiver 240 to detect downlink data on an out-of-band channel from a neighboring coordinator. In this example, the coordinator will use the out-of-band channel as a bidirectional signaling channel to send out downlink data on the out-of-band channel, and also receive a signal including timing information related to the MAC cycle from the network device on the same out-of-band channel. And then, the controller 220 can make a decision about auxiliary MAC alignment based on the downlink communication detected by the first receiver 210 or the downlink data detected by the second receiver 240. The benefit of using a bidirectional signaling channel is that the downlink data on the out-of-band channel can convey dedicated information related to the MAC cycle, and it is more straightforward for the network device to derive timing information related to the MAC cycle of the neighboring coordinator.

[0080] Figure 4 The coordinator is shown to adjust its MAC cycle based on a mechanism whereby neighboring coordinators send periodic beacons to network devices and the network devices reflect the periodic beacons back to the coordinator after a predefined delay. Note that for illustration purposes, the figure only shows beacons sent by transceivers TRX1 and TRX2 and network device D1. Considering Figure 2In the system layout shown in , there are direct links between TRX1-D1 and TRX2-D1 respectively. For two devices with a direct link, they will be able to receive any signal transmitted by the other side. In this example, both TRX1 and TRX2 receive the reflected beacon from network device D1. The larger the MAC cycle offset between adjacent coordinators, the larger the gap between two reflected beacons within the duration of one MAC cycle. For ease of explanation, only one coordinator implements the adjustment in this example. C1 keeps its cycle fixed, while C2 tries to adjust the start time of its MAC cycle to reduce the gap. It can be seen that when the periodic beacons from adjacent coordinators are aligned with each other, the network device will only reflect one beacon in the MAC cycle.

[0081] Note that it is not necessary for the periodic beacons from the coordinator to be sent exactly at the start time of each individual MAC cycle. It is possible that those beacons are sent with any predefined offset from the start time of each MAC cycle, and this predefined offset should be consistent for the coordinators in the OWC network. It is also important to keep the interval between adjacent periodic beacons the same as the duration agreed upon by the entire OWC network, which can be one MAC cycle or several MAC cycles.

[0082] Figure 5 A mechanism is presented in which a coordinator locally adjusts its MAC cycle based on a mechanism in which a neighboring coordinator sends a second type of synchronization message to a network device, and the network device sends back a signal that includes both information contained in the synchronization message and information locally derived by the network device from the synchronization message. The second type of synchronization message includes the identification information of the corresponding coordinator transmitting the synchronization message and the time interval between the transmission of the synchronization message and the start time of the corresponding coordinator's MAC cycle. As shown in the figure, D1 is associated with coordinator C1. Coordinator C2 is therefore a neighbor coordinator of D1. The time interval indication Δm included in the synchronization message from TRX2 is an offset from the start time of the MAC cycle for C2 or TRX2. Since network device D1 is associated with C1, it will derive a second time interval Δl, which indicates the duration of receiving synchronization messages from neighboring coordinator C2 compared to the start time of the current MAC cycle of local coordinator C1. Network device D1 sends a signal to local coordinator C1 that includes information about C2's identification, Δm, and Δl. And therefore, the local coordinator is able to adjust the start time of its MAC cycle based on the information received from network device D1 in order to reduce the gap between Δm and Δl. As can be seen from the figure, after two iterations, the start time of local coordinator C1 is aligned with the start time of neighbor coordinator C2.

[0083] In another example, if the coordinator is configured to send synchronization messages with only identification information and no timing information, and the network device can only derive Δl without knowing Δm, upon receiving a signal from the network device that includes C2's identification information and Δm, the local coordinator C1 will be unable to adjust the MAC cycle—assuming that such a synchronization message can be sent anywhere in the MAC cycle. Therefore, the local coordinator may have to forward the signal received from D1 to the network controller to centrally control the alignment of the MAC cycle.

[0084] Alternatively, when there is a wired connection between C1 and C2, the local coordinator C1 requests information of Δm from the neighbor coordinator C2 via the wired connection by using identifier information of C2 reported by D1 to address C2.

[0085] Figure 6 The figure illustrates the basic setup of a coordinator device. The blocks with solid lines 310 and 330 are the most basic building blocks in coordinators C1-Cn. A first transmitter 310 is configured to transmit data via downlink communication over the OWC network within a first predefined spectrum range. A second receiver 330 is configured to receive a signal from a network device via an out-of-band channel outside the first predefined spectrum range, and the signal includes timing information related to the MAC cycles of at least two neighboring coordinators. Optionally, the coordinator also includes a second transmitter 340 for transmitting downlink data on the out-of-band channel.

[0086] Advantageously, the coordinator further comprises a controller 320 configured to adjust the start time of its MAC cycle according to a signal received from the network device D1 , which enables the coordinator to locally adjust its MAC cycle.

[0087] In some situations, the signals received from the network devices are not sufficient to enable local adjustment of the coordinator's MAC cycle. The coordinator may have to forward the signals received from the network devices to the network controller in order to centrally control the alignment on the MAC cycle. To facilitate this option, a third transmitter 350 is required to forward the signals received from the network devices to the network controller NC via a wired connection. Since the network controller NC has an overview of the coordinators in the OWC network and it can receive signals about MAC cycle alignment from multiple coordinators in the neighborhood, the network controller can make a centralized decision on how to apply adjustments to the MAC cycle of each coordinator or a subset of coordinators. After making the centralized decision, the network controller NC can send instructions to the relevant coordinators with information on how to adjust their MAC cycles. The third receiver 360 is configured to receive such instructions from the network controller NC via a wired connection. In this case, the controller 320 is configured to adjust the start time of its MAC cycle according to the instructions received from the network controller NC.

[0088] Figure 7 A flow chart of a method 400 performed by a network device is shown. In step S401, the network device detects downlink communications on the OWC network 100 within a first predefined spectrum range, and in step S402, based on the detected downlink communications from at least two neighboring coordinators C1-Cn, the network device decides to assist in aligning the MAC cycles between the at least two neighboring coordinators C1-Cn. In step S403, upon the controller 220 making the decision to assist in aligning the MAC cycles between the at least two neighboring coordinators C1-Cn for interference suppression, the network device transmits a signal including timing information related to the MAC cycles of the at least two neighboring coordinators C1-Cn via an out-of-band channel outside the first predefined spectrum range.

[0089] Figure 8 A flow chart of a method 500 performed by a coordinator is shown. In step S501, the coordinator sends data via downlink communication over the OWC network 100 within a first predefined spectrum range. And then in step S502, it receives a signal from a network device D1 via an out-of-band channel outside the first predefined spectrum range, the signal including timing information related to the MAC cycles of at least two neighboring coordinators (C1-Cn).

[0090] Figure 9A flow chart of a method 600 performed by a coordinator when transmitting periodic beacons and receiving signals from network devices to assist in MAC cycle alignment is shown. In step S601, the coordinator transmits a periodic beacon at a predefined position in each of its MAC cycles; and in step S602, it receives a signal from network device D1 via an out-of-band channel, wherein the signal is a reflected periodic beacon received by network device D1 from the at least two neighboring coordinators (C1-Cn). In step S603, after receiving more than one reflected beacon from network device D1 within a single MAC cycle, the coordinator adjusts the start time of its MAC cycle to reduce the gap between the more than one reflected beacon.

[0091] Figure 10 A flowchart illustrates a method 700 performed by a coordinator in transmitting synchronization messages, receiving signals from network devices to assist in MAC cycle alignment, and forwarding the signals to a network controller for centralized control of MAC cycle alignment. In step S701, the coordinator transmits a synchronization message including the coordinator's identification information. In step S702, the coordinator receives a signal from network device D1 associated with coordinator C1 via an out-of-band channel, the signal including the identification information of neighboring coordinator C2 and a timing offset. The timing offset is derived by network device D1 and represents the time interval between the time the synchronization message is received from neighboring coordinator C2 and the start time of the current MAC cycle of coordinator C1. The method further includes step S703, where the coordinator forwards the signal received from the network device to the network controller NC via a wired connection. In step S704, the coordinator receives an instruction from the network controller via the wired connection. In step S705, the coordinator adjusts the start time of the MAC cycle based on the instruction received from the network controller.

[0092] Figure 11A flowchart of a method 800 is shown, performed by a coordinator when sending a second-type synchronization message, receiving a signal from a network device to assist in MAC cycle alignment, and determining (S807) whether to implement MAC cycle alignment locally or forward the signal to a network controller for centralized control of MAC cycle alignment. In step S801, the coordinator sends a synchronization message, and the synchronization message includes identification information of the coordinator and the time interval between the transmission of the synchronization message and the start time of the current MAC cycle. Then, in step S802, the coordinator receives a signal from a network device D1 associated with the coordinator via an out-of-band channel, and the signal includes identification information of a neighboring coordinator C2, the time interval contained in another synchronization message from the neighboring coordinator C2, and a second time interval derived by the network device, the second time interval representing the time interval between the time of receiving the other synchronization message from the neighboring coordinator and the start time of the coordinator's current MAC cycle. Thus, coordinator C1 obtains sufficient information from network device D1 to synchronize with its neighboring coordinator C2. Coordinator C1 can choose to implement MAC cycle alignment locally or submit this information to a central controller, which then centrally controls the alignment of the MAC cycles. If coordinator C1 chooses local adjustment, it adjusts the start time of the MAC cycle based on a signal received from the network device in step S803. If coordinator C1 chooses centralized control, coordinator C1 forwards the signal received from the network device to the network controller NC via a wired connection in step S804, and receives instructions from the network controller NC via a wired connection in step S805. Based on the instructions received from the network controller NC, the coordinator adjusts the start time of the MAC cycle in step S806. It is also possible that the network controller NC decides to have the neighboring coordinator C2 perform the adjustment instead of C1. It is also possible that both C1 and C2 have to make adjustments. This can occur due to another signal received from another coordinator C3, which reports that another alignment is required between C2 and C3, and that C3 is a neighbor of C2, not C1. The benefit of having centralized control over the MAC cycle alignment is that the network controller has a better overview of the overall neighbor information and timing information, and therefore, the alignment can be made more efficient.

[0093] The method according to the present invention can be implemented on a computer as a computer-implemented method, or in dedicated hardware, or in a combination of both.

[0094] The executable code of the method according to the present invention may be stored on a computer / machine-readable storage device. Examples of computer / machine-readable storage devices include non-volatile memory devices, optical storage media / devices, solid-state media, integrated circuits, servers, etc. Preferably, the computer program product comprises non-transitory program code means stored on a computer-readable medium for performing the method according to the present invention when the program product is executed on a computer.

[0095] Methods, systems, and computer-readable media (transitory and non-transitory) may also be provided to implement selected aspects of the above-described embodiments.

[0096] The term "controller" is used generally herein to describe various apparatuses associated with the operation of, among other functions, one or more network devices or coordinators. A controller can be implemented in a variety of ways (e.g., such as with dedicated hardware) to perform the various functions discussed herein. A "processor" is an example of a controller that employs one or more microprocessors that can be programmed using software (e.g., microcode) to perform the various functions discussed herein. A controller can be implemented with or without a processor, and can also be implemented as a combination of dedicated hardware that performs some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) that performs other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field programmable gate arrays (FPGAs).

[0097] In various implementations, a processor or controller may be associated with one or more storage media (collectively referred to herein as "memory," e.g., volatile and non-volatile computer memory, such as RAM, PROM, EPROM, and EEPROM, compact disks, optical disks, and the like). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and / or controllers, perform at least some of the functions discussed herein. The various storage media may be fixed within the processor or controller, or may be transportable such that one or more programs stored thereon may be loaded into the processor or controller in order to implement the various aspects of the invention discussed herein. The terms "program" or "computer program" are used herein in a general sense to refer to any type of computer code (e.g., software or microcode) that can be used to program one or more processors or controllers.

[0098] The term "network" as used herein refers to any interconnection of two or more devices (including controllers or processors) that facilitates the transfer of information (e.g., for device control, data storage, data exchange, etc.) between any two or more devices and / or between multiple devices coupled to the network.

[0099] The indefinite articles "a" and "an" as used herein in the specification and claims, unless explicitly indicated to the contrary, should be understood to mean "at least one".

[0100] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and / or" defined above. For example, when separating the items in a list, "or" or "and / or" should be interpreted as inclusive, i.e., including at least one of a plurality of elements or a list of elements, but also including more than one of a plurality of elements or a list of elements, and optionally, additional unlisted items. Only terms that clearly indicate the opposite, such as "only one of..." or "exactly one of...", or "consisting of..." when used in the claims will refer to including exactly one element in a plurality of elements or a list of elements. In general, the term "or" as used herein should only be interpreted as indicating exclusive alternatives (i.e., "one or the other, but not both") when preceded by exclusive terms such as "any one of...", "only one of..." or "exactly one of...". "Substantially consisting of..." should have the ordinary meaning used in the field of patent law when used in the claims.

[0101] As used in the specification and in the claims, the phrase "at least one" referring to a list of one or more elements should be understood to mean at least one element selected from any one or more elements in the list of elements, but does not necessarily include at least one of every element specifically listed in the list of elements and does not exclude any combination of elements in the list of elements. This definition also allows that elements may optionally be present in addition to the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified.

[0102] It should also be understood that, unless explicitly indicated to the contrary, in any method claimed herein that includes more than one step or action, the order of the steps or actions of the method is not necessarily limited to the order in which the steps or actions of the method are recited. Furthermore, reference numerals appearing between parentheses in the claims, if any, are provided merely for convenience and should not be construed as limiting the claims in any way.

[0103] In the claims and the description above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "maintaining," "including," etc. should be understood as open-ended, i.e., meaning including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of" should be closed or semi-closed transitional phrases.

Claims

1. A network device (D1) among a plurality of network devices (D1-Dn) for reducing interference in an optical wireless communication (OWC) network (100) having a line-of-sight characteristic, the OWC network (100) comprising at least two coordinators (C1-Cn) and a plurality of network devices (D1-Dn), the plurality of network devices (D1-Dn) selectively associated with and synchronized with a respective one of the coordinators (C1-Cn), and wherein: The at least two coordinators (C1-Cn) and the plurality of network devices (D1-Dn) share the same optical wireless medium of the OWC network (100) covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method, The network device (D1) comprises: - a first receiver (210) configured to detect downlink communications over the OWC network within a first predefined spectrum range; - a controller (220) configured to decide to assist in aligning a medium access control (MAC) cycle between the at least two neighboring coordinators based on downlink communications from the at least two neighboring coordinators detected by the first receiver; - a second transmitter (230) configured to, when deciding to assist in aligning the MAC cycles between the at least two neighboring coordinators for interference suppression, transmit a signal including timing information related to the MAC cycles of the at least two neighboring coordinators via an out-of-band channel outside the first predefined spectrum range; wherein the timing information is one of a signal indicating the start time of the individual MAC cycles of the two neighboring coordinators, an offset between the individual MAC cycles of the neighboring coordinators, and a start time of the MAC cycle of one of the neighboring coordinators; and The out-of-band channel has a line-of-sight characteristic.

2. The network device according to claim 1, further comprising: - a second receiver (240) configured to detect downlink data on an out-of-band channel; and The decision to assist in MAC cycle alignment between at least two neighboring coordinators (C1-Cn) is based on downlink communications from the at least two neighboring coordinators (C1-Cn) detected by a first receiver (210) or downlink data from the at least two neighboring coordinators (C1-Cn) detected by a second receiver (240).

3. The network device according to claim 1, wherein: The second transmitter (230) is configured to send a periodic signal to the at least two neighboring coordinators (C1-Cn) for use as a timing reference by the at least two neighboring coordinators (C1-Cn) when adjusting their MAC cycles, and wherein the periodic signal indicates a new start time of the MAC cycle based on timing information related to the MAC cycles of the at least two neighboring coordinators (C1-Cn).

4. The network device according to claim 1 or 2, wherein the network device (D1) is configured as follows: - detecting, by a first receiver (210), periodic beacons from said at least two neighboring coordinators (C1-Cn) over the OWC network (100), and - sending a signal by a second transmitter (230) on an out-of-band channel to the at least two neighboring coordinators (C1-Cn) by reflecting each periodic beacon to the at least two neighboring coordinators (C1-Cn) with a predefined delay after receiving each periodic beacon.

5. The network device according to claim 2, wherein the network device (D1) is configured to: - detecting, by a second receiver (240), periodic beacons from said at least two neighboring coordinators (C1-Cn) on an out-of-band channel, and - sending a signal by a second transmitter (230) on an out-of-band channel to the at least two neighboring coordinators (C1-Cn) by reflecting each periodic beacon to the at least two neighboring coordinators (C1-Cn) with a predefined delay after receiving each periodic beacon.

6. The network device according to claim 1 or 2, wherein the network device (D1) is associated with a local coordinator (C1) among the at least two neighboring coordinators (C1-Cn), and another one of the at least two neighboring coordinators is a neighboring coordinator (C2) of the network device (D1), and the network device (D1) is configured to: - detecting, by a first receiver (210), synchronization messages from a local coordinator (C1) and a neighboring coordinator (C2) over the OWC network (100); and wherein the synchronization messages include identification information of the individual coordinators; - after receiving a synchronization message from a neighboring coordinator (C2), deriving a relative offset by the controller (220), and wherein the relative offset is the time interval between the time of receiving the synchronization message from the neighboring coordinator (C2) and the start time of the current MAC cycle of the local coordinator (C1); as well as - sending a signal to the local coordinator (C1) on an out-of-band channel by a second transmitter (230), said signal including identification information of the neighboring coordinator (C2) and the relative offset derived by the network device (D1).

7. The network device according to claim 2, wherein the network device (D1) is associated with a local coordinator (C1) among the at least two neighboring coordinators (C1-Cn), and another one of the at least two neighboring coordinators is a neighboring coordinator (C2) of the network device, and the network device (D1) is configured to: - detecting, by a second receiver (240), synchronization messages from the local coordinator (C1) and the neighboring coordinators (C2) on an out-of-band channel; and wherein the synchronization messages include identification information of the individual coordinators; - after receiving a synchronization message from a neighboring coordinator (C2), deriving a relative offset by the controller (220), and wherein the relative offset is the time interval between the time of receiving the synchronization message from the neighboring coordinator (C2) and the start time of the current MAC cycle of the local coordinator (C1); as well as - sending a signal to the local coordinator (C1) on an out-of-band channel by a second transmitter (230), said signal including identification information of the neighboring coordinator (C2) and the relative offset derived by the network device (D1).

8. The network device according to claim 1 or 2, wherein the network device (D1) is associated with a local coordinator (C1) among the at least two neighboring coordinators (C1-Cn), and another one of the at least two neighboring coordinators is a neighboring coordinator (C2) of the network device (D1), and the network device (D1) is configured to: - detecting, by a first receiver (210), a second type of synchronization message from a local coordinator (C1) and a neighboring coordinator (C2) over the OWC network (100); and wherein the second type of synchronization message comprises identification information of the respective coordinator transmitting the synchronization message and a time interval between the transmission of the synchronization message and the start time of a MAC cycle of the respective coordinator; - after receiving a synchronization message of the second type from the neighboring coordinator (C2), deriving, by the controller (220), a second time interval, and wherein the second time interval is a time interval between receiving the synchronization message from the neighboring coordinator (C2) and a start time of a current MAC cycle of the local coordinator (C1); as well as - a second transmitter (230) sends a signal to the local coordinator (C1) on an out-of-band channel, wherein the signal includes identification information of the neighbor coordinator (C2), the time interval contained in the synchronization message from the neighbor coordinator (C2), and the second time interval derived by the network device (D1).

9. The network device according to claim 2, wherein the network device (D1) is associated with a local coordinator (C1) among the at least two neighboring coordinators (C1-Cn), and another one of the at least two neighboring coordinators is a neighbor coordinator (C2) of the network device, and the network device (D1) is configured to: - detecting, by a second receiver (240), synchronization messages of a second type from the local coordinator (C1) and the neighboring coordinators (C2) on an out-of-band channel; and wherein the synchronization messages of the second type include identification information of the respective coordinators transmitting the synchronization messages and a time interval between the transmission of the synchronization messages and the start time of the MAC cycles of the respective coordinators; - after receiving a synchronization message of the second type from the neighboring coordinator (C2), deriving, by the controller (220), a second time interval, and wherein the second time interval is a time interval between receiving the synchronization message from the neighboring coordinator (C2) and a start time of a current MAC cycle of the local coordinator (C1); as well as - a second transmitter (230) sends a signal to the local coordinator (C1) on an out-of-band channel, wherein the signal includes identification information of the neighbor coordinator (C2), the time interval contained in the synchronization message from the neighbor coordinator (C2), and the second time interval derived by the network device (D1).

10. A coordinator (C1-Cn) among at least two coordinators (C1-Cn) for reducing interference in an optical wireless communication (OWC) network (100) with line-of-sight characteristics, the OWC network (100) comprising the at least two coordinators (C1-Cn) and a plurality of network devices (D1-Dn), the plurality of network devices (D1-Dn) selectively associated to and synchronized with a respective one of the coordinators (C1-Cn), and wherein the at least two coordinators (C1-Cn) and the plurality of network devices (D1-Dn) share the same optical wireless medium of the OWC network (100) covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method, the coordinator (C1-Cn) comprising: - a first transmitter (310) configured to send out data via downlink communication over the OWC network within a first predefined spectrum range; a second receiver (330) configured to receive a signal from a network device via an out-of-band channel outside the first predefined spectrum range, wherein the signal including timing information related to MAC cycles of at least two neighboring coordinators is intended to assist in aligning the MAC cycles between the at least two neighboring coordinators, the network device detecting downlink communications from the at least two neighboring coordinators, wherein the timing information is one of a signal indicating start times of individual MAC cycles of the two neighboring coordinators, an offset between individual MAC cycles of the neighboring coordinators, and a start time of a MAC cycle of one of the neighboring coordinators; The out-of-band channel has a line-of-sight characteristic.

11. The coordinator according to claim 10, further comprising: - a second transmitter (340) configured to send downlink data on an out-of-band channel.

12. The coordinator according to claim 10 or 11, further comprising: - A controller (320) configured to adjust the start time of its MAC cycle based on a signal received from the network device (D1), and wherein the duration of the MAC cycle is defined by the OWC network (100), and the start time of the MAC cycle is a local time reference used by the coordinator to divide the wireless medium into consecutive time slots.

13. The coordinator according to claim 10 or 11, further comprising: - a third transmitter (350) configured to forward a signal received from the network device to a network controller (NC) via a wired connection; - a third receiver (360) configured to receive instructions from a network controller (NC) via a wired connection; And the coordinator (C1-Cn) further includes: - A controller (320) configured to adjust the start time of its MAC cycle according to instructions received from a network controller (NC), and wherein the duration of the MAC cycle is defined by the OWC network (100) and the start time of the MAC cycle is a local time reference used by the coordinators (C1-Cn) to divide the wireless medium into consecutive time slots.

14. A method (400) for reducing interference at a network device (D1) among a plurality of network devices (D1-Dn) in an optical wireless communication (OWC) network (100) having a line-of-sight characteristic, the OWC network (100) comprising at least two coordinators (C1-Cn) and a plurality of network devices (D1-Dn), the plurality of network devices (D1-Dn) selectively associated to and synchronized with a respective one of the coordinators (C1-Cn), and wherein: The at least two coordinators (C1-Cn) and the plurality of network devices (D1-Dn) share the same optical wireless medium of the OWC network (100) covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method, The method includes a network device (D1) among the plurality of network devices (D1-Dn): - detecting (S401) downlink communications on the OWC network (100) within a first predefined spectrum range; - based on detected downlink communications from the at least two neighboring coordinators (C1-Cn), deciding (S402) to assist in alignment of MAC cycles between the at least two neighboring coordinators (C1-Cn); as well as - when the controller (220) makes a decision to assist in alignment of MAC cycles between the at least two neighboring coordinators (C1-Cn) for interference suppression, transmitting (S403) a signal including timing information related to the MAC cycles of the at least two neighboring coordinators (C1-Cn) via an out-of-band channel outside the first predefined spectrum range; wherein the timing information is one of a signal indicating start times of individual MAC cycles of the two neighboring coordinators, an offset between individual MAC cycles of the neighboring coordinators, and a start time of a MAC cycle of one of the neighboring coordinators; and The out-of-band channel has a line-of-sight characteristic.

15. A method (500) for reducing interference at a coordinator (C1-Cn) among at least two coordinators in an optical wireless communication (OWC) network (100) with line-of-sight characteristics, the OWC network (100) comprising the at least two coordinators (C1-Cn) and a plurality of network devices (D1-Dn), the plurality of network devices (D1-Dn) selectively associated to and synchronized with a respective one of the coordinators (C1-Cn), and wherein the at least two coordinators (C1-Cn) and the plurality of network devices (D1-Dn) share a same optical wireless medium of the OWC network (100) covering a first predefined spectrum range by dividing the wireless medium into consecutive time slots via a time division multiple access (TDMA) method, the method comprising the coordinator among the at least two coordinators: - sending (S501) data via downlink communication over the OWC network (100) within a first predefined spectrum range; - receiving (S502) a signal from a network device (D1) via an out-of-band channel outside a first predefined spectrum range, the signal comprising timing information related to MAC cycles of at least two neighboring coordinators (C1-Cn), and wherein the signal is intended to assist in alignment of the MAC cycles between the at least two neighboring coordinators (C1-Cn), the network device (D1) detecting downlink communications from the at least two neighboring coordinators (C1-Cn), wherein the timing information is one of a signal indicating start times of individual MAC cycles of the two neighboring coordinators, an offset between individual MAC cycles of the neighboring coordinators, and a start time of a MAC cycle of one of the neighboring coordinators; The out-of-band channel has a line-of-sight characteristic.

16. The method according to claim 15, the method (600) further comprising a coordinator (C1-Cn) of the at least two coordinators: - sending (S601) a periodic beacon at a predefined location in each of its MAC cycles; and - receiving (S602) a signal from a network device (D1) via an out-of-band channel, and wherein the signal is a reflected periodic beacon received by the network device (D1) from the at least two neighboring coordinators (C1-Cn); and - After receiving more than one reflected beacon within a single MAC cycle from the network device (D1), adjusting (S603) the start time of its MAC cycle to reduce the gap between the more than one reflected beacons.

17. The method according to claim 15, the method (700) further comprising a coordinator among the at least two coordinators (C1): - sending (S701) a synchronization message, wherein the synchronization message includes identification information of the coordinator; and - receiving (S702) a signal from a network device (D1) associated with the coordinator (C1) via an out-of-band channel, wherein the signal comprises identification information of a neighboring coordinator (C2) and a timing offset, and wherein the timing offset is derived by the network device (D1) and represents a time interval between a time of receiving a synchronization message from the neighboring coordinator (C2) and a start time of a current MAC cycle of the coordinator (C1); - forwarding (S703) the signal received from the network device to the network controller (NC) via a wired connection; - receiving (S704) instructions from a network controller (NC) via a wired connection; and - Adjusting (S705) the start time of the MAC cycle according to the instruction received from the Network Controller (NC).

18. The method according to claim 15, the method (800) further comprising a coordinator (C1) among the at least two coordinators: - sending (S801) a synchronization message, wherein the synchronization message includes identification information of the coordinator and a time interval between transmission of the synchronization message and a start time of a current MAC cycle; and - receiving (S802) a signal from a network device (D1) associated with the coordinator via an out-of-band channel, wherein the signal comprises identification information of a neighboring coordinator (C2), a time interval contained in another synchronization message from the neighboring coordinator (C2), and a second time interval derived by the network device, the second time interval representing a time interval between a time when the another synchronization message is received from the neighboring coordinator and a start time of a current MAC cycle of the coordinator; and The method further comprises: - adjusting (S803) the start time of the MAC cycle according to the signal received from the network device; or: - forwarding (S804) the signal received from the network device to the network controller (NC) via a wired connection; - receiving (S805) instructions from a network controller (NC) via a wired connection; and - Adjusting (S806) the start time of the MAC cycle according to the instruction received from the network controller (NC).

19. A computer program product comprising a computer program which, when executed by a computer, causes the computer to carry out any one of the methods of claims 14 to 18.

20. A system for reducing interference, the system comprising: - an optical wireless communication OWC network (100) with line-of-sight characteristics, comprising at least two coordinators (C1-Cn) and a plurality of network devices (D1-Dn) according to claim 13, the plurality of network devices (D1-Dn) selectively associated to and synchronized with a respective one of the coordinators (C1-Cn), and wherein the at least two coordinators (C1-Cn) and the plurality of network devices (D1-Dn) share the same wireless medium of the OWC network (100) by dividing the wireless medium into consecutive time slots via a TDMA method; and - a network device (D1) among a plurality of network devices (D1-Dn) according to any one of claims 6 to 9; and - a first coordinator (C1) associated with the network device, which is one of the at least two coordinators (C1-Cn); and - a second coordinator (C2) next to the local coordinator, which is another one of the at least two coordinators (C1-Cn); and - a network controller (NC) connected to the first coordinator (C1) and the second coordinator (C2) via a wired connection; and The network controller (NC) is configured as follows: - receiving a signal originating from a network device (D1) and forwarded by a first coordinator (C1); - after receiving said signal, determining the required adjustments to align the MAC cycles of the first coordinator (C1) and the second coordinator (C2); - instructing at least one of the first coordinator (C1) or the second coordinator (C2) to implement the determined adjustment of the MAC cycle.

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