Patent ID: 12192834

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the present invention will now be described based on an optical wireless communication (OWC) network system100, or more specifically a Li-Fi network system, as shown inFIG.1. For illustration purposes, the Li-Fi network100is connected to a backbone network20via an IP router15and an Ethernet switch14, while in a practical system more routers and switches may be connected between the Li-Fi network and the backbone network. In this example, the Li-Fi network is connected to the backbone network via a backbone connection21. The backbone connection is a stable and high-speed link, which can be a wired connection, such as Ethernet, or a wireless connection based on radio frequency (RF) or millimeter-wave. The backbone connection can also be another kind of optical wireless link that is different from the one that an end point is performing in the optical multi-cell wireless network. One example of the other kind of optical wireless link can be free space point-to-point optical links.

Li-Fi System Overview and Network Architecture

As a wireless communication technology for local area networking, Li-Fi plays a similar role as Wi-Fi to provide the last tens of meters connectivity. A Li-Fi network100may comprise a plurality of optical access points (APs)120and network devices or end points (EPs)110. Each end point110is selectively associated to and synchronized with a respective one of the access points120. A Li-Fi AP120may be connected to one or multiple optical front ends or Li-Fi transceivers (TRX)121, for providing access to Li-Fi devices or Li-Fi end points (EPs)110. The trapezoids shown in dash lines illustrate field-of-views (FoVs) or coverage of individual Li-Fi transceivers121. Only when an EP110is located in the coverage of a Li-Fi AP120, will it be able to receive a downlink communication from that AP120. By assuming symmetrical up and down links of the optical communication, a bidirectional optical link can be built up under the same condition. Because of the line-of-sight character of the optical communication link, adjacent access points120do not have a direct optical link amongst each other, whereas an end point110located in the overlapping area of the coverage of adjacent access points120is able to detect optical signals from both access points.

In one example, a Li-Fi AP120may also operate as a domain master with additional functionalities according to G. hn, ITU G.9960 and G.9961, to manage several Li-Fi EPs110. In one implementation, handover happens when an EP roams from one domain to another. In another implementation, each Li-Fi AP120is operated as a domain master managing an individual domain hosting multiple Li-Fi EPs, which can be up to 255 Li-Fi EPs. Such Li-Fi APs120are typically located on the ceiling. They may, but not necessarily, be collocated with luminaires, especially when the communication is not based on visible light. The main functions of a Li-Fi AP120may include to advertise the presence of an AP120to Li-Fi EPs110in the surroundings, to register and deregister Li-Fi EPs110, to provide medium access control (MAC) scheduling among associated Li-Fi EPs110, to collect interference reports from EPs110, to adjust local schedule in response to interference reports, and/or to report neighboring relations to the Li-Fi controller13. Some of the functions of the Li-Fi AP120, such as MAC scheduling for interference avoidance, may be implemented by the Li-Fi controller13in a centralized manner.

Li-Fi EPs or Li-Fi devices110are end user modems that facilitates end devices to connect to the Li-Fi network100. Nowadays, a Li-Fi EP110is typically a dedicated entity connected to a laptop or other end devices. In the future, a Li-Fi EP110may be partially or fully integrated to a smart phone, a tablet, a computer, a remote controller, a smart TV, a display device, a storage device, a home appliance, or another smart electronic device.

There may be a L-Fi controller or central controller13connected to the plurality of access points120in the Li-Fi network100. The Li-Fi controller or central controller13is in charge of controlling the Li-Fi system in a centralized manner when necessary, such as deriving information about the topology and neighboring relationship, deciding scheduling among different Li-Fi access points (APs) for interference suppression. Furthermore, Li-Fi controller13may also be employed to provide a user interface that allows a user or admin, such as an IT manager, to configure schedules among multiple Li-Fi APs, monitor reports from these Li-Fi APs, and/or to derive further statistic information about the system performance. It is typically ensured that there is only one Li-Fi controller13is visible to an individual AP, which is achieved by means of network configuration so that traffic to and from a Li-Fi controller13is isolated inside its own network segment, via virtual LANs (VLANs) or similar. Furthermore, a protocol, such as a Control and Provisioning of Wireless Access Points (CAPWAP) protocol, can be used to discover multiple controllers and to select one controller that has free resources to host/manage an access point joining an infrastructure.

In one exemplary implementation the Li-Fi system may make use of G.vlc based technology, a Li-Fi synchronization server16is connected to the system, which is in charge of synchronizing (or aligning) the G.vlc medium access control (MAC) cycles of the different G.vlc domains. This is needed to align some common time slots for detecting neighboring APs120and avoiding interference to an EP110located in the overlapping area of neighboring APs120. Because of the line-of-sight characteristic of an optical link, neighboring APs120typically cannot detect the signals from one another directly. However, an EP110located in the overlapping area of two neighboring APs120may experience interference if the neighboring APs120are transmitting simultaneously. To avoid such situation, it may be necessary to keep adjacent APs120synchronized to a common time base, and to prevent them to transmit at the same moment. One preferred option for network synchronization is to employ the Precision Time Protocol (PTP), IEEE 1588v2. The PTP provides a sub-microsecond accuracy, which is fair enough for inter G.vlc domain MAC alignment. To keep the PTP accuracy, support from Ethernet switch is necessary, which should also be PTP capable. To keep the PTP accuracy, any element in the Ethernet network must handle PTP so the switch selected for any deployment must support and be configured to operate in the PTP mode accordingly.

It may also happen that a Li-Fi system is to be deployed in a legacy system where PTP is not supported by the existing infrastructure. And hence, additional measures should be taken to synchronize neighboring APs120in a different and maybe sub-optimal manner, and accordingly a solution should be found for an EP110to deal with the non-ideal synchronization among neighboring APs120.

Detailed System Description

Li-Fi AP

A Li-Fi AP120is a key unit to establish a Li-Fi network100. In some scenarios, a Li-Fi AP120also forms the interface between an existing IT infrastructure and a Li-Fi network100. A high-level block diagram of a Li-Fi AP120is shown inFIG.2. On one side, the Li-Fi AP120has an interface124to a backbone network, which can be a wired connection (Ethernet), or a wireless connection (RF, millimeter-wave, or another kind of optical wireless that is different from the one a Li-Fi EP is performing). And on the other side, the Li-Fi AP120has an optical front end121to enable the optical link with one or more Li-Fi EPs110. Furthermore, the Li-Fi AP120also carries out the function to implement bi-directional translation or conversion between the data on the backbone network20and data on an optical link, in terms of conversion between different modulation schemes and conditioning of the analog signals. Therefore, a Li-Fi AP120comprises at least also a digital modulator and demodulator component123and an analog front end122. In the transmission path, the analog front end (AFE)122may comprise a programmable amplifier, a filter, and a driver to condition and amplify the baseband signal to drive the optical front end. For the receiving path, the AFE122may comprise an attenuator, a low noise amplifier, a filter and a programmable gain amplifier to accommodate the received signals for the further digital processing.

The optical front end121comprising at least a light source and a light sensor implements the conversion between electrical signals and optical signals. In the transmitter chain, the optical front end121is used to convert the electrical transmitting signals to output optical signals via the light source. In the receiver chain, the optical front end121is used to convert the received optical signals to output electrical signals via the light sensor for further signal processing. The optical front end121is also called Li-Fi transceiver (TRX), such that: Li-Fi transmitter (Tx): transforms an electrical signal obtained from the AFE to an optical signal (e.g. to be emitted by an LED), and

Li-Fi receiver (Rx): transforms a received optical signal (e.g. from a photodiode) to an electrical signal for the AFE.

A Li-Fi AP120may be connected to a single Li-Fi TRX121, or multiple Li-Fi TRXs121, which allows to transmit the optical signals over different optical paths. In case a Li-Fi AP120is connected to multiple Li-Fi TRXs121, the Li-Fi AP may handle them as one coherent signal, or as (partially) separate incoherent signals for establishing a communication link.FIG.3shows an example of a Li-Fi AP120with multiple Li-Fi TRXs121. A Li-Fi interface component125is adopted to split or combine the data sent to or received from the multiple Li-Fi TRXs121.

Li-Fi EP

A high-level overview of a Li-Fi EP or a Li-Fi device110is shown inFIG.4. Similar to a Li-Fi AP120, a Li-Fi EP110comprises at least an optical front end111, an analog front end112, a digital modulator/demodulator113, and an interface114to the end device or a processor.

A Li-Fi EP110may be connected to an end device as a separate entity via a cable or be partially or entirely integrated in the end device. For many end devices, such as laptop, smart phone, remote controller, Ethernet is a well-established interface in the operating system of the end devices. Li-Fi may also be used to provide communication interface to the end device in addition or instead. To simplify the system integration of a Li-Fi EP or Li-Fi device to the operating system of an end device, it is advantageous to employ Ethernet over USB. Therefore, in one option, the Li-Fi EP or Li-Fi device110can be connected to the end device via a standard USB cable. With the example of using Ethernet over USB, a Li-Fi EP110may comprise the Ethernet over USB interface114and connect to the end device via a USB cable115. A Li-Fi EP110may also be connected to one or more client optical TRXs111, same as in a Li-Fi AP120. Alternatively, a single optical frontend that has segmented transmitters/receivers where each transceiver/receiver is directed in a different respective direction is also envisaged.

In another example, a different interface114may be used to connect the Li-Fi EP to the operation system of the end device, and the corresponding interface114(Ethernet over USB) and/or the cable115should be replaced accordingly.

FIG.5provides exemplary components of an optical front end or optical TRX111,121comprised in or connected to a Li-Fi AP120and a Li-Fi EP110. An optical TRX111,121comprises at least a light source1211, a light sensor1212, a driver1213, and an amplifier1214. The light source1211is used to convert the electrical transmitting signals to output optical signals, which can be a Light-emitting diode (LED), a Laser diodes (LD), or Vertical Cavity Surface Emitting Laser (VCSEL). The light sensor1212is used to convert the received optical signals to output electrical signals, which can be a photodiode, an avalanche diode, or another type of light sensor. The driver1213is mainly used for regulating the power required for the light source1211. The amplifier1214is mainly used to condition the received signals by the light sensor1212to make the signals suitable for further processing in the electrical circuits. In one example, the amplifier1214can be a transimpedance amplifier (TIA), which is a current to voltage converter implemented with one or more operational amplifiers. TIA may be located close to the receiving light sensor or photodiode1212to amplify the signal with the least amount of noise.

Inter-Connection in a Li-Fi System

Typically, Li-Fi APs120are deployed on the ceiling. And such APs120need to be powered first in order to carry out communication activities. Therefore, the connections to the APs120are meant for both power and data. An AP120sets up bidirectional link with the cloud, or the backbone network20at one side, and at the other side the AP120communicates with one or more associated EPs110via optical links. An EP110typically obtain power from the end device that the EP is coupled to or integrated in and communicates with an associated AP120via an optical link.

Connecting a Li-Fi AP to the Backbone Network

Different options can be taken for a Li-Fi AP120to get connected to the backbone network20.

In one aspect, data and power may be jointly delivered to a Li-Fi AP, which can be implemented via a single power cable with power line communication (PLC) or a single Ethernet cable with power over Ethernet (POE).

PLC makes use of the existing power line cables, i.e. for providing a device with mains power, also for data communication. Popular PLC communication standards, such as HomePlug® or G.hn, utilize Orthogonal Frequency Division Multiplexing (OFDM) technology, which is also widely adopted in a Li-Fi system. Hence, the physical layers (PHY) of a PLC system and a Li-Fi system may be quite similar, such as the modulation methods and the synchronization methods used in both systems. However, transmission in the optical domain are unipolar whereas in general OFDM uses bipolar signals. As a result, some adaptation may be required for transmission in the optical network. A simple solution is the use of a DC-offset which does not require demodulation and subsequent remodulation of an OFDM based PLC signal prior to optical transmission, or alternatively demodulation and subsequent remodulation using unipolar OFDM modulation techniques such as ACO-OFDM, DCO-OFDM, ADO-OFDM and/or Flip OFDM. Therefore, it may be quite convenient for a Li-Fi AP120, which is typically collocated with the luminaire on the ceiling, to make use of the existing power cable to obtain also the data connection to the backbone network20.

However, it is also recognized that the channel of a PLC system is quite noisy, given that the mains power line may act as an antenna to pick up all kinds of undesired signals that may interfere with communication signals that are also present on the mains power line. It is thus important for the Li-Fi over PLC enabled devices to cope with such external interference. Furthermore, a communication signal over a mains power line experiences an amount of attenuation that cannot be predicted during manufacturing and may vary over the day. The impact factors include the length of the cable that varies from building to building, that power loads that form more or less a short circuit for high frequencies and be switched on or off, etc.

A known solution to address the problems introduced by a PLC system on signal integrity is to equip an Li-Fi over PLC enabled device with a PLC decoder for decoding a PLC communication signal received over the mains power line. Impairments on communication signals are handled digitally. For instance, a narrowband interferer causes error on just single subcarrier of an OFDM modulated signal. The reconstructed data may be corrected using error correction algorithms. Subsequently, the reconstructed data is then transformed back to the analog domain for modulating the LED current flowing to the at least one LED. In such a way, a more robust operating device can be provided wherein the loss of data is reduced, although one of the drawbacks of this solution is that the device gets large in size, complex, costly.

On the other hand, if power can be delivered via an Ethernet cable, it may also be convenient for a Li-Fi AP to make use of existing IT infrastructure to obtain both power and connection to the backbone network20. Power over Ethernet (POE) is described in the IEEE802.3af/at standard and is currently being extended towards 4-pair power in the IEEE Task Force P802.3bt. PoE is intended to supply power voltage levels of 40 V to 48 V from Power Sourcing Equipment (PSE) to Powered Devices (PDs), alongside data lines for control and communication purposes. A PSE device is also referred to as POE switch. In PoE lighting systems PDs may be light sources, user interface devices and sensors. The PSE is typically powered from a mains power source, such as according to the IEC/TR 60083 standard. Traditional PoE systems will transport data and power over a network and its end points, hence among PSEs and PDs.

Data can therefore be received by the control device, e.g., via Ethernet connection using the Ethernet Protocol. Data is communicated via the Ethernet Protocol between devices in power over Ethernet systems. Therefore, a microchip in form of an Ethernet controller can be used to establish a communication link between the devices, which supports Media Access Control (MAC) and physical layer (PHY) of the Open Systems Interconnection model (OSI model).

An Ethernet connection can for example be an optical fiber, an electric wire or a twisted pair cable, such as a Cat 3 cable, Cat 4 cable, Cat 5 cable, Cat 5e cable, Cat 6 cable, Cat 6A cable, Cat 7 cable, Cat 7A cable, Cat 8 cable, Cat 8.1 cable, or Cat 8.2 cable. The Ethernet connection can have several pairs of cables, e.g., 2, 3, 4, or more pairs of cables. The cables can be unshielded or shielded, in particular individually or overall shielded. The power and data can be transmitted via the same fiber, wire, or cable of the Ethernet connection or via different fibers, wires, or cables of the Ethernet connection. In case of transmission of power via an optical fiber the power can be transmitted in the form of photons that can be received by a solar cell unit of the data receiving device.

The data receiving device in a PoE system can comprise one or more ports. Each of the ports can comprise one or more pins. A pin can be configured for receiving power, data or power and data. Additionally, or alternatively, the port can also comprise one or more solar cell units for receiving power in the form of photons. As the ports can receive power and data via the Ethernet connection some of the pins can be supplied with power, while other pins are supplied with data via the Ethernet connection. Alternatively, or additionally, a pin can also be supplied with power and data via the Ethernet connection.

In another aspect, data and power may be separately delivered to a Li-Fi AP, and the options can be either via both a power cable and an Ethernet cable (wired connection to a backbone network), or a combination of a power cable and a wireless link to the backbone20(optical wireless links or free space optical link).

Preferably, a Li-Fi system may be integrated to an existing wireless communication system, such as a Wi-Fi system or a cellular system. And hence, a Li-Fi AP120may be integrated to or directly-connected a Wi-Fi access point or a cellular base station. By having a conversion or translation of signals between the Li-Fi AP120and the Wi-Fi access point or the cellular base station, the existing infrastructure of a Wi-Fi system or cellular system can be employed to provide the connection to the backbone network20for the Li-Fi AP120.

Connecting a Li-Fi EP to a Li-Fi AP

A Li-Fi EP110gets access to a Li-Fi system via a Li-Fi AP120, and the associated Li-Fi AP120is often called a local AP. There are several aspects to be considered for the connection between a Li-Fi EP120and a Li-Fi AP110:Coverage: a Li-Fi EP may not always be able to see a Li-Fi AP depending on its location, its orientation, the positioning of the Li-Fi APs, and the size of the Li-Fi EP's transducer/sensor coverage area.Downlink interference: a Li-Fi EP that is in the overlapping coverage area of multiple optical downlinks experiences interference if these Li-Fi APs transmit at the same time.Uplink interference: A Li-Fi EP that transmits a signal to an associated Li-Fi AP while another Li-Fi EP is transmitting to this same Li-Fi AP results in uplink interference at the Li-Fi AP.

Handover: Because of the mobility of a Li-Fi EP, a handover is needed when a Li-Fi EP moves from the coverage area of one Li-Fi AP to a neighboring Li-Fi AP. That is to say, when a Li-Fi EP (such as connected to or comprised in a user device, a client device, a mobile phone, etc.), moves from the current cell to the neighboring cell, then any active communication must be handed over to the node or access point of that neighboring cell. Handovers are intended to be made as quickly as possible in order to reduce disruption to any ongoing communication or data transfers and may include a preparation period in order to facilitate this. When insufficient time is available to prepare and establish a link to the new Li-Fi AP before the link with the existing Li-Fi AP is broken, the Li-Fi EP may experience a period in which it has no connection. Considering the relatively small size of a Li-Fi cell due to the line-of-sight character of the optical link, seamless handover is important to guarantee the link quality and the user experience.

Basically, a Li-Fi EP110can be connected to a Li-Fi AP120via bidirectional optical link, or a hybrid downlink and uplink. Note that here the downlink stands for the communication link from the Li-Fi AP120to the Li-Fi EP110, and the uplink stands for the communication link from the Li-Fi EP110to the Li-Fi AP120. A bidirectional optical link enables a relatively symmetrical connection between the Li-Fi EP110and the Li-Fi AP120. Hence, both downlink and uplink enjoy the same advantages of Li-Fi communication as addressed above. However, in some application scenarios, such as for web-surfing or video streaming, the link between a Li-Fi AP and a Li-Fi EP can also be a hybrid link, which is a combination of an optical downlink from the Li-Fi AP120to the Li-Fi EP110and a radio frequency (RF) uplink from the Li-Fi EP120to the Li-Fi AP110. The RF link may be in accordance with a popular short-range wireless communication protocol, such as Wi-Fi, BLE, or Zigbee, or be in accordance with a cellular communication protocol, such as 4G or 5G cellular.

Referring back to the options that the Li-Fi AP120may be built via a combo device supporting both Li-Fi AP function and Wi-Fi access point or cellular base station function, such hybrid link can be handled seamlessly by a controller at the Li-Fi AP side. Since a Li-Fi EP110is typically connected or integrated to an end device, which can be a smart phone, a tablet, a computer, or another smart device, the end device may already have the hardware support for the short range wireless communication protocol or cellular protocol used in the hybrid link. Therefore, such hybrid link also leverages the existing resource of the end device, and provide a simplified solution for the Li-Fi EP, which only requires a receiving path, but not a transmitting path. The cost, power consumption, and form factor of the EP110may be further reduced in such a manner. Correspondingly, the Li-Fi AP120is also simplified by comprising mainly an optical transmitter to send data to the Li-Fi EP110via an optical downlink, whereas the RF-based uplink from the Li-Fi EP110to the AP120may be received by leveraging the RF receiver in the combo device or co-located Wi-Fi access point/cellular base station, or via a dedicated RF receiver comprised in the Li-Fi AP120itself.

Scheduling and Interference Suppression within an Optical Multi-Cell Wireless Network

When there are multiple Li-Fi APs120deployed next to each other or when there are multiple EPs110associated to the same local AP120or to adjacent APs120, medium access control (MAC) become necessary for an interference free optical communication. Different MAC mechanisms are possible to be employed in the optical multi-cell wireless network, such as time-division multiple access (TDMA), frequency-division multiple access (FDMA), carrier-sense multiple access (CSMA), code division multiple access (CDMA), space-division multiple access, or a combination of one or more aforementioned mechanisms. TDMA is based on time-division multiplexing scheme, where radio resource is scheduled in time domain and different time slots are assigned to different transmitters in a typically cyclically repetitive frame structure or MAC cycles. FDMA is based on frequency-division multiplexing, where different frequency bands are allocated to different devices for simultaneous transmission. And in optical communication, FDMA can also be evolved into wavelength division multiple access (WDMA), which is based on wavelength-division multiplexing. Another advanced version of FDMA is orthogonal frequency-division multiple access (OFDMA), where each device may use one or more subcarriers out of the entire band. OFDMA has more flexibility in providing different data rates or quality of service to different users, and in the meanwhile a high resource efficiency can be maintained despite of such diversity. CSMA typically employs “listen-before-talk” approach, where a device verifies the absence of any other traffic before transmitting on a shared medium. CSMA is widely used in a sparse network, and when the density of nodes scales, further collision-avoidance techniques come into place. CDMA is typically built on top of spread spectrum, and a common form is direct-sequence CDMA that is based on direct-sequence spread spectrum, where different devices send messages simultaneously with different spreading codes that are orthogonal to each other. Given the typically smaller FoV of an optical link as compared to a radio link, space-division multiple access may also be a very attractive solution here.

In a TDMA-based multi-cell network with multiple APs120, due to the lack of direct communication, adjacent APs120sometimes may not have synchronous MAC cycles. Although the durations of one MAC cycle or super frame is typically the same for all the APs120in the network, the start times of MAC cycles can be different for individual APs120. Note that the start time of a MAC cycle is used by an AP as a local time reference to divide the wireless medium into consecutive time slots. Such an offset of MAC cycles among two adjacent APs120may cause interference to an EP110located in the overlapping coverage areas of these two adjacent APs120, even when a time slot is allocated exclusively to one AP120for communication with the EP110in the overlapping area. Therefore, it may be necessary for the APs120to synchronize to a common time base. The common time base may be obtained via synchronization handshake, via a reference clock distributed over the network (such as synchronous Ethernet clocks), or via a dedicated synchronization server in the network, or derived from a common signal, such as the zero crossing of the mains power. However, due to an uncertain delay in the network or an interference, there may still be timing synchronization uncertainty of the APs against the timing reference. It may still be necessary for an EP110located in the overlapping area of at least two adjacent APs120to derive timing information related to MAC cycles of the at least two APs120based on downlink communication from these APs, which can be either a normal data communication link or an out-of-band signaling message. Then, based on the derived timing information related to MAC cycles of the at least two APs120, the EP110may further assist at least one out of the two adjacent APs120to adjust its MAC cycles to get aligned with the other.

Fast Secure Handover

For a Wi-Fi system, IEEE 802.11 defines that the communication for a handover or transition may be conducted directly with the neighboring access point, e.g., on a direct path (i.e. “over-the-air”) or via the local access point of the distribution system (DS) (i.e. “over-the-DS”). In addition, the EP may want the neighboring access point to reserve resources prior to the transition, e.g., based on a fast transition (FT) resource request protocol according to section 13 of the IEEE 802.11 (2016) specification (Fast BSS transition). To this end, two FT protocols are defined. These are an FT protocol which is executed when a transition to a target access point is made and a resource request is not required prior to the transition, and an FT resource request protocol which is executed when a resource request is required prior to the transition. For a fast transition/handover of an EP from its currently associated access point to a target access point utilizing the FT protocols, message exchanges may be performed using the over-the-air approach (where the EP communicates directly with the target AP using an IEEE 802.11 authentication with an FT authentication algorithm) or the over-the-DS approach (where the EP communicates with the target AP via its current local AP). The communication between the EP and the target AP may be carried in FT action frames between the EP and its current local AP. Between the current AP and the target AP, the communication may be achieved via an encapsulation method, e.g., such as described in section 13.10.3 of the IEEE 802.11 (2016) specification. The current local AP may convert between the two encapsulations.

A fast and secure roaming technique based on the 802.11r amendment (officially known as fast BSS transition) is the first method to be officially ratified by the IEEE to perform fast secure transitions between Wi-Fi access points. It works by having the client complete an initial successful 802.1X Extensible authentication protocol (EAP) authentication with the authentication server. The resultant master session key (MSK) is, then, transferred to the Wireless LAN controller (WLC) like in other methods. The method, however, differs by deriving a slightly different key hierarchy. A pairwise master key (PMK)-R0is derived from the MSK is known only to the client and the WLC. A PMK-R1is derived from PMK-R0and is known to the client and APs managed by the WLC that holds PMK-R0. The final level is the pairwise transient key (PTK), derived from PMK-R1and is known to the client and the APs managed by the WLC. Typically, the APs managed by the WLC form a group referred to as a FT mobility domain, which is essentially all APs that have the same SSID. How PMK-R1is made known to other APs is not defined by the IEEE 802.11r amendment.

During the initial authentication, the client performs full 802.1X authentication, completes the 4-Way Handshake to derive a Pairwise Transient Key Security Association (PTKSA) with the AP (using PMK-R1key material), and then is allowed access to the network. When the client begins to roam, the client and the target AP derive a new key based on PMK-R1. The method is even more efficient since the four-way handshake takes place within the Open System Authentication from the client, Open System Authentication from the AP, Reassociation Request, and Reassociation Response. This substitutes the four-way handshake, which occurs after these frames in other methods.

There is a lesser deployed variant of this technique known as Fast BSS transition over the Distribution System (DS). With this technique, once the client decides it might roam to another AP, it sends a FT Action Request frame to the original AP. The client indicates the MAC address of the target AP where it wants to roam. The original AP forwards this FT Action Request frame to the target AP over the DS and the target AP responds to the client with an FT Action Response frame (also over the DS). Once this FT Action frame exchange is successful, the client finishes the FT roaming. The client sends the Reassociation Request to the target AP over-the-air and receives a Reassociation Response from the new AP in order to confirm the roaming and final key derivation. These last two messages are exchanged when the client finally roams to the target AP. Therefore, Fast Transition allows roaming faster than static PMK caching.

It is clear that a fast handover is crucial to guarantee the quality of service when an end point is roaming in a multi-cell network. As compared to a RF system, such as a Wi-Fi system, the design challenge is even bigger in a Li-Fi system, considering the smaller optical cell and the smaller overlapping area in an optical communication system.

FIG.6illustrates an end point110roams in an optical multi-cell wireless communication network100and the corresponding coverage areas of the end point110, an associated access point120, and a neighbor access point120. The plurality of access points, comprising at least the associated access point and a candidate access point, are located on a first planar surface410. In a typical application scenario, the first planar surface410is the ceiling. On the first planar surface410, the coverage area412of an end point is illustrated by a dash circle, which covers both the associated access point and a neighbor access point. The end point is located on a second planar surface420, which can be the planar surface of the floor, the table, another horizontal surface that the end point is located, or any arbitrary planar area the end point is roaming with a user. On the second surface420, the coverage areas422of the associated access point and the neighbor access point are illustrated with shadowed circles, and the end point110is located in an overlapping area of the two coverage areas. The arrow indicates the moving direction of the end point, which is heading for the neighbor access point and suggests a potential handover. InFIG.6the identical coverage areas of an end point and an access point are merely for an exemplary purpose. Depending on the optical components used by the plurality of access points and the end point, the coverage area422of an access point120and the coverage area412of an end point110may be different. Furthermore, even if the optical components remain the same, the actual coverage area will also change with the distance between the first and the second planar surfaces.

For the ease of explanation, it is assumed here that each access point120comprises a single optical front end, and each dot on the first planar surface410represents a different access point120. Therefore, a fast handover is always necessary when the end point roams to the coverage of an adjacent access point120. In another example, if an access point comprises more than one optical front ends, a handover may not be necessary when the end point is roaming within the coverage areas of multiple optical front ends belonging to the same access point120that is sending identical information via the multiple optical front ends.

FIG.7provides an overlay top view of the first planar surface410and the second planar surface420, when the Li-Fi end point110is roaming in the optical multi-cell wireless communication network100. It can be seen that depending on the moving trajectory of the end point different adjacent access points may be the candidate access point for a potential handover. In order to derive the information about the candidate access point, it is necessary for the subsystem500,510,510′ to first obtain the one or more neighbor relationships among the plurality of access points120in the optical multi-cell wireless communication network100.

FIG.8illustrates the exchange of messages for building up the one or more neighbor relationships in a centralized subsystem500via a downlink advertisement from the plurality of access points120. Given that adjacent access points AP1and AP2are typically located on the same planar surface, the first planar surface410shown in the figure, the field of view of the AP1and AP2are projected to the same second planar surface420where the end point110is located. Therefore, there is no direct line-of-sight optical link between adjacent access points120. Thus, the information related to the neighbor relationships among the plurality of access points are not directly available. However, the end point EP1located in the overlapping area of two adjacent access points or two adjacent optical cells is able to detect signals from both. In a preferred setup, an access point periodically sends out downlink advertisements DL-A, which may comprise a unique identifier of the access point, to announce its presence. By detecting such a downlink advertisement DL-A from a neighbor access point AP2rather than the currently associated access point AP1, the end point EP1can send a report REP to the currently associated access point AP1about the presence of the neighbor access point AP2. The associated access point AP1will forward the report REP to the subsystem500. With the end point and/or a further end point roaming through the area, the subsystem500can build up over time a good overview of the neighbor relationships among the plurality of access points. The subsystem500may be comprised in a standalone controller, such as a Li-Fi controller or a central controller13in the system. The connection between the subsystem and the plurality of access points is a backbone connection21, which is a stable and high-speed link and in certain scenarios may even be an always-connected link. The backbone connection can be a wired connection, such as Ethernet, or a wireless connection based on radio frequency (RF) or millimeter-wave. The backbone connection can also be another kind of optical wireless link that is different from the one that an end point is performing in the optical multi-cell wireless network. Such an example can be free space optical communication.

FIG.9illustrates the exchange of messages for building up the one or more neighbor relationships in a centralized subsystem500via an uplink advertisement from the end point. In this setup, the end point EP1is configured to send out an uplink advertisement UL-A, which may comprise a unique identifier of the end point, to announce its presence. Such an uplink advertisement UL-A is detected by both the currently associated access point AP1and a neighbor access point AP2that is also located in the coverage area of the optical uplink from the end point EP1. Thus, the neighbor access point AP2will recognize from the received uplink advertisement UL-A that the end point is not associated to itself and will send a report REP to the subsystem500directly related to this detection. With the report REP from the neighbor access point AP2and the knowledge on the association between that end point EP1and its currently associated access point AP1, a neighbor relationship can be derived by the subsystem500with regard to the neighbor access point AP2and the currently associated access point AP1of that end point EP1. With one or more end points roaming through the area, the subsystem500can build over time a good overview of the neighbor relationships based on the uplink advertisements of the one or more end points.

In another setup, a distributed subsystem510,510′ is adopted for a relatively small-scale optical multi-cell wireless network100.FIG.10andFIG.11illustrate the exchange of messages for building up neighbor relationships in the distributed subsystem510,510′ via a downlink advertisement DL-A from the plurality of access points or an uplink advertisement UL-A from the end point respectively. The distributed subsystem510,510′ may be comprised in more than one access points120. For the ease of explanation,FIG.10andFIG.11show the examples that parts of the distributed subsystem510,510′ are comprised in the currently associated access point AP1and the neighbor access point AP2respectively. It can also be the case that the distributed subsystem is comprised in more than one access points out of the plurality of access points, other than AP1and AP2. The plurality of access points may be connected to one another via a backbone connection21, which can be either wired (Ethernet) or wireless.

InFIG.10, the currently associated access point AP1receives the report REP from the associated end point EP1about the presence of a neighbor access point AP2, and then it will forward the report REP to the distributed subsystem510and510′. In this example, part of the distributed subsystem510is co-located with AP1, and the other part of the distributed subsystem510′ is reachable via e.g. a backbone connection21. Neighboring APs may also be connected via free space optical communication.

Similarly, inFIG.11, the neighbor access point AP2receives the uplink advertisement UL-A from an end point EP1that is not associated to itself. AP2will send a report REP to the distributed subsystem510,510′. Here, part of the distributed subsystem510′ is co-located with AP2, and the other part of the distributed subsystem510is reachable via a backbone connection21.

With the overview of the one or more neighbor relationships among the plurality of access points120in the network, the subsystem510,510,510′ may select one or more adjacent access points as the candidate access point for the end point. However, the selection can be further improved by considering additional information, which can be a floor plan of an area and the locations of the access points in that area, statistics on a handover history of the currently associated access point, or a combination of both.

FIG.12demonstrates a floor plan of a room with multiple access points deployed in the area and the potential moving trajectories650of a roaming end point. The strips with pattern fill represent the boundaries of the room, with the opening indicating the entrance to the room. The rectangular blocks651represent a few pieces of furniture in the room, which make certain moving trajectories less likely to happen. Sometimes two access points may be located next to each other on the ceiling, a potential handover from one access point to the other may be prevented due to a separation from a wall or a piece of furniture, such as a desk or a cupboard, which blocks the underneath moving path on the second planar surface420. In the example ofFIG.12, it can be seen that the potential moving trajectories650represented with dash lines are rooted at the entrance to the room. The middle part of the room extending from the entrance is a kind of corridor inside the room, which is also the most visited parts of the room. The access points located on the ceiling of the corridor may also indicate several potential handovers. Therefore, using additional information according to a floor plan may help to make the selection of one or more candidate access points more intelligent.

Preferably, the selection of the one or more candidate access points can be further improved by considering statistics on a handover history of the currently associated access point, which may represent a probability distribution of previous handovers from the access point of interest to any one of the adjacent access points. Because of the layout of the room, or moving behavior of the user, or some other factors, handovers between some of the two adjacent access points may happen more often than others. A higher probability of occurrence in the past may be indicative of a larger chance of a future handover event. Therefore, such statistics derived from handover history can be used to improve the accuracy of the selection on the one or more candidate access points by the subsystem. Furthermore, as the statistics may be updated over time, this enables the subsystem to have a self-learning capability and to adapt to changes in user behavior, in the system, in the environment, or in the network.

Considering the relatively small coverage of a single optical cell, it could be even more beneficial to anticipate several subsequent handovers, which comprise a potential handover from the currently associated access point to a direct neighbor, and also one or more subsequent (potential) handovers from the direct neighbor to a non-adjacent further neighbor and/or from the non-adjacent further neighbor to an even further access point. Therefore, by knowing the candidate access points of several potential subsequent handovers, the end point can pre-establish a couple of pairwise security keys or pairwise transient keys, each dedicated to a different potential access point, well ahead of time resulting in a more seamless handover experience without delays incurred by the security key provisioning.

Referring back toFIG.12, when the end point just enters the room and is associated to the middle left access point that is most close to the entrance, three direct neighbors of the associated access point can be selected as candidate access points, by assuming that the end point will move left or right, and the user of the mobile end point may subsequently sit down at the desk, or to move forward in the corridor area. Typically, the moving speed of the end point is higher when it is in the corridor than when it dwells along the furniture. Therefore, it may also be beneficial to prepare the subsequent handovers to the two further access points in the corridor area, which are not the direct neighbor of the associated access point but are further non-adjacent neighbor of the associated access point. Such information may be derived according to a floor plan, and it may also be derived according to statistics on a handover history. For example, if the subsystem has a probability distribution of previous handovers from each one of the plurality of access points to any one of its adjacent access points, each time when a new subsequent access point is added, the probabilities of handovers from the new subsequent access point to its adjacent access points should be considered to make the decision on another further handover.

A statistical data model could be maintained in the form of a directed adjacency graph wherein the edge weights represent the transition-probabilities among APs over time. Considering that end devices that the end points are connected to or comprised in can be personal devices, such as mobile phones, a preferred solution determines such a statistical data model aggregated over all end points, as well as for each individual end point. The aggregated data may then be used for new or unknown devices in the system as this data already captures the limitations of the physical world; whereas the individual end device data, when sufficient data has been collected, may capture a specific end user's routing/behavior.

FIG.13schematically depicts basic components of an end point of the present invention. The end point110comprises at least an optical transceiver117and a controller118. The optical transceiver117should be understood as a complete Li-Fi transceiver that comprises at least an optical front end111, an analog front end112, a digital modulator/demodulator113, and an interface114to an end device that the Li-Fi transceiver is connected to or comprised in. The controller118may be a dedicated controller or a controller shared with the end device. The end device may optionally comprise a user interface119, which can provide users with added convenience of status inquiry or operation.

FIG.14shows a flow diagram of a method700carried out by a subsystem500,510,510′ for supporting an end point110to carry out a secure handover from an access point currently associated with the end point110to another access point out of a plurality of access points120in an optical multi-cell wireless communication network100. The method700comprises the following steps of the subsystem500,510,510′: in step S701, the subsystem500,510,510′ obtains neighbor relationship among the plurality of access points120; the subsystem500,510,510′ selects, in step S702for the end point110a candidate access point out of the plurality of access points120, other than the currently associated access point, for a secure handover of the end point110, in view of the obtained neighbor relationship; and then in step S703, the subsystem500,510,510′ informs the end point110via the currently associated access point about the candidate access point to trigger the end point110to start a procedure for pre-establishing a new pairwise transient key between the end point110and the candidate access point for the secure handover.

FIG.15shows a flow diagram of a method800carried out by an end point110for performing a secure handover from an access point120currently associated with the end point110to another access point out of a plurality of access points120in the optical multi-cell wireless communication network100. The method800comprises the following steps of the end point110: in step S801, the end point110performs optical wireless communication with the currently associated access point; in step S802, the end point secures an optical wireless communication link with the currently associated access point by using a pairwise transient key to encrypt or decrypt data communicated on the link; and then, upon detection of information related to the candidate access point in step S803, the end point110, in step S804, triggers a procedure for pre-establishing a new pairwise transient key between the end point110and a candidate access point for a secure handover, and wherein the procedure is triggered before the secure handover to the candidate access point actually takes place.

The methods according to the invention may be implemented on a computer as a computer implemented method, or in dedicated hardware, or in a combination of both.

Executable code for a method according to the invention may be stored on computer/machine readable storage means. Examples of computer/machine readable storage means include non-volatile memory devices, optical storage medium/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 a method according to the invention when said program product is executed on a computer.

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

The term “controller” is used herein generally to describe various apparatus relating to, among other functions, the operation of one or more network devices or coordinators. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A “processor” is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform 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).

In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as “memory,” e.g., volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM, compact disks, optical disks, etc.). 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. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein. The terms “program” or “computer program” are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program one or more processors or controllers.

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