The present invention relates to visible light communication and relaying data in a visible light communication network.

Description of the Invention Background Visible Light Communication (VLC) is a wireless communication technology that uses visible light to transmit data across distances. Varying the intensity of a beam of light can be used to encode information, and the encoded information transmitted using the light to a destination device or terminal. Visible light communication may involve the use of Light Emitting Diodes (LED) for wireless data transmission. LEDs modulated at very high data speeds are not noticeable to the human eye. The use of LEDs, in many environments, enables a dual use for both illumination and communication purposes.

Current VLC systems typically assume the use of ambient light sources such as luminaires installed at ceilings in a room. In addition to these ambient light sources, there may exist a number of secondary light sources and may include, but are not limited to, task lights (e.g., table lamp fixtures, standard lamp fixtures and office task light luminaires) and accent lights (e.g., used to highlight art or other artefacts). Secondary light sources may act as potential relay terminals that can receive and transmit data in a VLC network. Using the secondary light sources as relays extends the range of the VLC system by relaying data from a primary light source via secondary light sources, which act as potential relay terminals, to a destination, such as a mobile device or laptop. VLC has the advantages of potentially: very high bandwidth; visible light is in an unlicensed spectrum, so no licence is needed; cheap; uses simple transmitters (LEDs) and receivers (photodiodes); no Radio frequency (RF) interference issues; safe; secure; high degree of spatial reuse; and fault finding is easy for visible light.

The potential applications are at least the following:

RF Spectrum Relief

Smart Lighting

Mobile Connectivity

Hazardous Environments

Hospital & Healthcare

Aviation

Underwater Communications

Vehicles & Transportation

RF Avoidance

Location Based Services

High Data Rate Communication:

Li-Fi Technology similar to WiFi

• Low Data Rate Communication:

Optical camera communications.

Use of camera of the mobile as a receiver

Further applications of VLC include environmental monitoring, underwater exploration, scientific data collection, maritime archaeology, offshore oil field exploration/monitoring, port security and tactical surveillance. The use of VLC networks can reduce energy consumption, realise environmental benefits and be a benefit for health care applications.

Energy context: The increasing demand for mobile data services, including video streaming and data applications, has raised the energy consumption of wireless networks. The annual energy consumption of a mobile service operator is around 500-1 OOGWh, which represents 50% of the operator's annual expenses. From an environmental viewpoint, the C02 emissions by the mobile networks are 2% of the total emissions worldwide, and the percentage is expected to reach 4% by 2020. Hence, employing energy-efficient VLC in parallel to the existing RF networks is likely to have an environmental impact on energy consumption.

Some of the aforementioned applications, particularly environmental monitoring and offshore oil field exploration/monitoring are directly related with oil producing countries such as Qatar. Oil and gas industry is the key sector in Qatar's stunning economic growth and revenues from this industry amount to 50% of the country's gross domestic product. With significant infrastructure investments in the North Field of Qatar, the need for advanced supervisory control and data acquisition (SCADA) and telecommunication technologies is more than ever. Qatar hosts Dolphin Gas Pipeline, one of the world's longest underwater pipelines, to transfer processed gas from the offshore North Field to the UAE. Security of such critical offshore infrastructures against threats along with environmental monitoring (e.g., pollution, leakage) are therefore of utmost importance.

Healthcare context: One of the great advantages of VLC is being safe to human beings. Conventional RF systems exploit electromagnetic wave propagation which can be hazardous and can cause serious health risks at certain power levels. In VLC, light intensity signals are exploited which are generally safe to human beings and other life forms and can be employed in places and scenarios where RF signals are forbidden, such as hospitals and, oil and gas farms. Particularly, we envisage VLC technology as an enabling technology for implementing an infrastructure for healthcare systems such as telemedicine and m-health systems due to its interference-free feature.

VLC networks and conventional RF networks

We review the differences between Radio Frequency and VLC networks to provide the skilled person with an appreciation of the challenges in VLC networks.

There is keen interest VLC and VLC networks and Li-Fi ® has tested VLC in the real world, and it's 100 times faster than Wi-Fi. Deepak Solanki, CEO of Estonian tech company Velmenni is cited as saying "Expect to hear a whole lot more about Li-Fi - a wireless technology that transmits high-speed data using visible light communication (VLC) - in the coming months. With scientists achieving speeds of 224 gigabits per second in the lab using Li-Fi, the potential for this technology to change everything about the way we use the Internet is huge." Source: http://www.scienceaiert.com/ii-fi-tested-in-the-reai- worid-for~the~first~time~is- 110~times~faster-than~wi-fi.

RF systems do not operate in the visual portion of the electromagnetic spectrum. Method and techniques designed for RF frequencies cannot be applied to VLC systems. We discuss some of the salient points below to provide a background of the challenges faced in VLC communications.

VLC relies on optical signals, these optical signals must be real and non- negative and are completely different than RF signals (Optical Wireless Communications is based on using light wavelengths from 10,000 nm to 190 nm in optically transparent media, as discussed in "IEEE P802. 15.7r1 Short- Range Optical Wireless Communications Task Group Project Authorization Request (PAR)").

The skilled person will appreciate that when using a particular type of LED for VLC the wavelengths of the light output may not extend to the extremes of the above defined optical wavelengths. The type of LED dictates the range of optical wavelengths used in the VLC and for illumination.

VLC communications do not utilise conventional RF network designs methods. In particular, for VLC network design, the data processing to compute the signal power and the noise power does not follow established RF practices. As a consequence, the design of the signal waveforms and data processing methods to compute the signal power must take into account the illumination performance of the secondary light sources (relay terminals) when establishing a relay path. If the illumination performance of the secondary light source (relay terminal) the determined relay path will not be accurate.

The illumination performance may be in one aspect considered as an additional constraint arising due to the dual function of the light sources (terminals), acting as both as VLC terminals and luminaires. The illumination constraints limit the power that can be used for transmitting the signal. For example, in a VLC network comprising a primary light source (source terminal) and several secondary light sources (possible relay terminals), the power level of a particular secondary light source (a relay terminal) may exclude this particular secondary light from being selected as a relay terminal. This exclusion may still apply even if the VLC channel conditions favour the selection of this relay terminal to achieve the required performance.

Such illumination constraints do not exist in RF technologies and incorporating the illumination constraints when considering network performance is important and requires knowledge of the properties of the VLC terminals. For example, if secondary light sources (relay terminals) are 'dimmed' (level of luminosity or another metric for determining the visual light performance) at different levels, then we propose using a visual-light parameter to denote the level of 'dimming'. The visual-light parameter may be incorporated when determining the performance of the VLC network, particularity when secondary light sources, acting as relay terminals, are involved.

VLC channels used in VLC networks do not suffer from fading behaviour which is an advantage of RF networks, where the RF signals experience fading. The channel models are being developed and the skilled person will be familiar with IEEE standard 802.15.7. .

Prior art We review the prior art for VLC systems, briefly comment on the state of the art and challenges faced.

In [1 ], a single secondary light source is used as a repeater in order to extend the coverage area of the VLC system. In [2], another work on repeater based system is discussed and performance improvement is demonstrated when the direct link is blocked. An LED-to-LED multi-hop VLC system for toys is demonstrated in [3]. In [4], a multi-user VLC system is proposed where the message is forwarded through other users (who act as relays) to the destination when the source-to destination link is shadowed or blocked. In an experimental VLC study [5], an audio signal is successfully delivered to the destination over two intermediate relay terminals. In [6], multi-hop inter- vehicular message forwarding scheme is considered for outdoor VLC systems. In [7] and [8], a loop interference cancelation method is investigated for full- duplex cooperative VLC systems. The work in [9] investigates a scenario in which user terminals can act as relays in order to extend the coverage of VLC- based downlink. In [10] and [1 1 ], a single secondary light source is used as a relay in order to improve the performance of the transmission. Similarly, the references [12-18] describe different applications for relays in visible light communication systems. In [12], a VLC relay is used to extend the coverage area of the system. [13] describes a similar setting for coverage extension using relay nodes. In [14], a hybrid power line and visible light access points are defined in a network that also supports relaying for expanding the coverage. In [15], a multi-hop system is introduced where the source message is addressed to destination via multiple relay. [16] is another coverage extender with a single VLC relay. In [17], a hybrid VLC and RF communication system is described with multiple relays. In [18], a VLC system is defined that includes a controller apparatus that acts similar to a relay.

We highlight some of the challenges experienced in VLC networks:

o High path loss

o Large free space path loss

o Short communication coverage

o High penetration loss by human

o High penetration loss (e.g., human body ~25dB)

o Resulting in no or lower-rate communication between source and destination

Using a secondary light source to relay information can overcome may of the above challenges.

Providing a reliable data transmission path through a VLC network while, considering the dual function of luminaires, for illumination and VLC is a challenge addressed by the embodiments of the present invention. In particular, the embodiments provide method and system for a reliable relay path, where both functions of the luminaire are considered, using N secondary light sources (relay terminals).

The present invention and embodiments thereof seek to overcome or ameliorate difficulties faced in the prior art and provide alternate mechanisms for data communication using VLC terminals, relays and networks. Aspects of the present invention and embodiments also consider the effect of illumination of the VLC terminals and relays (light sources) when determining a relay path from a source terminal to a destination via relay terminal/s.

When considering the term light source or variations thereof, the skilled person will appreciate that in context the light source is a light source capable of VLC communication. One aspect of the invention provides a method for transmitting data from a source terminal to a destination terminal via a plurality of potential relay paths, each potential relay path having at least one relay terminal between the source terminal and the destination terminal, wherein the or each relay terminal is operable to transmit visual light including terminal state information for that relay terminal, the method comprising:

at the source terminal, collecting the terminal state information for one or more of the relay terminals;

each terminal state information collected from the one or more relay terminals identifying a potential relay path and providing a level of a visual-light parameter for that potential relay path;

determining a performance value for each potential relay path from the level of the visual-light parameter for that potential relay path;

selecting a relay path based on the performance value of the one or more potential relay paths; and

transmitting the data from the source terminal via the selected relay path.

Another aspect of the invention provides, wherein the potential relay path comprises two or more relay terminals.

In a further aspect of the invention, wherein the data is transmitted onward from the relay terminal to the destination receiver. Another aspect of the invention, wherein each terminal state information collected from the one or more relay terminals identifying a potential relay path and providing a level of a visual-light parameter for that potential relay path, wherein the visual-light parameter is the parameter being modulated in an optical modulation scheme.

Another aspect of the invention, wherein the optical modulation scheme is DCO-OFDM and the visual-light parameter is a DC-bias level of the one or more relay terminals.

Another aspect of the invention, wherein the optical modulation scheme is ACO-OFDM and the visual-light parameter is the information signal power of the one or more relay terminals.

Another aspect of the invention, wherein the optical modulation scheme is OOK and the visual-light parameter is the amplitude level of the pulse that controls luminosity of the one or more relay terminals. Another aspect of the invention, wherein the optical modulation scheme is LI- OFDM.

Another aspect of the invention, wherein the optical modulation scheme is ell- OFDM.

A further aspect of the invention, wherein determining the performance value for each potential relay path from the level of the visual-light parameter for that potential relay path comprises determining the performance value using a Signal-to-Noise-Ratio (SNR) based on the visual-light parameter and channel information. Another aspect of the invention, wherein at the source terminal, collecting the terminal state information for one or more of the relay terminals comprises transmitting a pilot signal to the one or more of the relay terminals. A further aspect of the invention, wherein at least one of the terminals is at least one of: an LED transceiver; a photodiode; or a digital camera.

Another aspect of the invention, wherein selecting a relay path based on the performance value of the one or more potential relay paths comprises comparing the performance value of the one or more potential relay paths with a target performance value; and selecting the relay path with the closest performance value to the target performance value.

A further aspect of the invention, wherein the terminal state information comprises at least one of: channel information; LED parameters; and luminosity values.

Another aspect of the invention, wherein one or more of the terminals comprise one or more LEDs that illuminate in the visible portion of the electromagnetic spectrum.

Another aspect of the invention provides a communication system for transmitting data from a source terminal to a destination terminal via a plurality of potential relay paths, each potential relay path having at least one relay terminal between the source terminal and the destination terminal, wherein the or each relay terminal is operable to transmit visual-light including terminal state information for that relay terminal, the system comprising:

a source terminal operable to collect the terminal state information for one or more of the relay terminals;

a processor operable to:

process each collected terminal state information from the one or more relay terminals to identify a potential relay path and provide a level of a visual-light parameter for that potential relay path;

determine a performance value for each potential relay path from the level of the visual-light parameter for that potential relay path; and

select a relay path based on the performance value of the one or more potential relay paths; and

the source terminal is further operable to transmit the data via the selected relay path.

Another aspect of the invention provides, wherein the communication system is operable to perform the aspects of the invention discussed in the method steps above. Another aspect of the invention provides, a relay terminal for relaying data from a source terminal by receiving and transmitting visual light signals, the relay terminal comprising:

a transceiver operable to transmit and receive visual light signals and relay data; and

a processor operable to determine terminal state information and a visual-light parameter for the relay terminal,

wherein the transceiver is operable to deliver the terminal state information to the source terminal for evaluation of the terminal state information at the source terminal to determine a level of the visual-light parameter for the potential relay path to the relay terminal.

A further aspect of the invention provides, a source terminal for use in a visual light communications network comprising:

a transceiver operable to transmit and receive visual light signals, and a processor operable:

to collect terminal state information for one or more relay terminals; to process each collected terminal state information from the one or more relay terminals to identify a potential relay path, the terminal state information providing a level of a visual-light parameter for that potential relay path;

to determine a performance value for each potential relay path from the level of the visual-light parameter for that potential relay path; and

to select a relay path based on the performance value of the one or more potential relay paths,

wherein the transceiver is operable to transmit data via the selected relay path.

In order that the present invention may be more readily understood, embodiments of the present invention are now described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 . is an illustration of an indoor space with a primary light source and multiple secondary light sources. The primary light source is the luminary installed at the ceiling and the source terminal. The destination terminal is the laptop located on the desk.

Figure 2. shows the VLC waveform for a DCO-OFDM. Fig. 2(a), shows a first level of brightness. Fig.2 (b) shows a higher brightness level than in Fig 2(a). Figure 3. shows a flow chart for selecting the N best relay terminals for a VLC network.

Figure 4. shows simulation results for a VLC system using an embodiment of the invention.

Figure 5. shows simulation results for a VLC system using an embodiment of the invention. Figure 6. shows the breaking of a direct link between a source terminal and destination terminal by intermediate object, such as a person, and a relay path using a relay terminal to re-route the link.

Figure 7. shows core components of a source terminal and destination terminal connected using a VLC signal path and possible effects of object on channel coefficients. Figure 8. shows how VLC networks can be integrated into existing wireless networks.

Detailed Description The embodiments of the present invention for a VLC network using a selection method to select a relay terminal/s are exemplified using indoor use, for example in a room. The present invention is not limited for use indoors, and the skilled person may easily envisage embodiments of the present invention for use in other environments, including in or on vehicles and outdoors.

In many indoor environments, there is a primary light source (source terminal) and typically secondary light sources (such as a desk light and/or floor lights) in order to improve the indoor illumination. These secondary light sources may each be used as a relay(act as relay terminals) as part of a VLC network to relay data from the primary light source (source terminal) to a destination terminal.

In an embodiment of the present invention, the optimum performance relay path is determined using the N best relay terminals to relay data from the source terminal to the destination terminal using the N relay terminals.

Fig. 1 illustrates part of a VLC network in an indoor space with multiple secondary luminaries. The primary light source (source terminal) 101 is installed at the ceiling and may be regarded as the main information source. The source terminal is not limited to any particular orientation or position indoors. The primary light source may also perform its function as a luminaire, light source.

The secondary light sources 102 may be equipped with VLC transceivers and act as relay terminals, in addition to their function as luminaries.

The destination terminal may be placed on a desk 103, or may be a mobile device terminal and comprise a suitable receiver such as a photodiode to receive VLC signals. The destination terminal may also in another aspect of the embodiment comprise a transceiver, for transmitting and receiving VLC signals. The skilled person will appreciate that the source, relay and destination terminals are not limited to certain fixtures or locations. References to the 'terminal/s', may include the source terminal, relay terminal or destination terminal, or any combination thereof. The source terminal 101 collects terminal state information from the relay terminals and optionally from the destination terminal. The terminal state information may include channel state information, LED parameters, a visual- light parameter and other information about the lighting properties of the light source. In one aspect of the embodiment the terminal state information may be stored in a machine readable storage medium, in another aspect of the embodiment the terminal state information may be provided to the source terminal using a data link, wired or wireless, from a storage medium. In a further aspect of the embodiment, the source terminal may actively collect the terminal state information directly from the relay and destination terminals.

Channel state information may comprise channel coefficients between source- destination terminals, source-relay terminals, relay-destination terminals. LED parameters may comprise turn-on-voltage level, maximum-allowed voltage level and minimum operating voltage. Other parameters specific to the particular LED may also be collected by or provided to the source terminal. The visual-light parameter may be collected separately or derived from the LED parameters where possible, depending on available LED information.

The visual-light parameter, in one aspect of the embodiment, may be a level of dimming of the LED of the terminal. In another aspect, the visual-light parameter may directly represent the level of luminosity of the light source, i.e. the level of illumination of the light source (as a source, relay or destination terminal). In another aspect, the visual light parameter may be defined by a voltage, current, or another defining parameter of the light source which affects the level of the luminosity of the light source.

The information waveform is responsible for carrying the data between terminals in the VLC network. The power of the information waveform is related to the power level of the light source, in particular, the level of the visual-light parameter. For example, the power of the information waveform may be directly related to the dimming level of the light source.

A relay path between the source terminal, via a relay terminal/s, to the destination terminal, is based on a consideration of the visual-light parameter for at least some of the terminals. For example, the dimming level of the relay terminals in the network is taken into consideration when determining a performance metric for the relay path.

In one aspect of the embodiment, the source terminal may estimate channel state information and channel state coefficients between source-destination, source-relay terminals, relay-destination terminals using pilot signals sent to the relay terminal/s and destination terminal. The VLC network uses a modulation scheme to modulate the data onto a VLC signal. Any suitable modulation scheme may be used to modulate the data onto the VLC signal, including and not limited to Orthogonal Frequency Division Multiplexing (DCO)-OFDM, Asymmetrically Clipped Optical (ACO)- OFDM, Uni-polar (U)-OFDM, and enhanced Unipolar (eU)-OFDM. Standard waveforms may be used as defined in IEEE 802.1 5.7 and IEEE 802.1 5.7r1 .

Fig. 2 shows the VLC waveform for a DCO-OFDM modulation scheme. Fig. 2(a), shows a first level of brightness (luminosity). Fig.2 (b) shows a higher brightness (luminosity) level than in Fig 2(a). V| _0W and Vhigh are the turn on voltage, and the maximum allowed voltage levels by the LED, respectively. The amplitude levels beyond these limits are clipped. As seen in Figures 2(a) and 2(b), the power of the information signal changes as the visual-light parameter level ('dimming' level for DCO-OFDM) changes from 2(a) to 2(b).

The DC bias value in 2(a) is chosen at the middle point of the dynamic range, whereas in 2(b) the DC bias value is chosen close to upper clip level for the same LED characteristic. The operating region is [V| _0W , Vhigh] . For DCO-OFDM, the visual-light parameter is derived from the collected (or provided) LED parameter, DC bias. The visual-light parameter level corresponds with the DC bias level.

The terminal state information, including the channel state information, LED parameters (V| _0W , V _h i _g h,DC bias level), and derived visual-light parameter level are used to determine a maximum signal-to-noise ratio (SNR) for links between the source terminal and relay terminals in the VLC network. The maximum SNR is determined for terminals which are able to provide terminal state information.

The signal-to-noise ratio (SNR) is the ratio of information signal power σ _χ ² to the clipping distortion power σ] plus effective noise power .

SUBSTITUTE SHEET RULE 26 SNR =

σ ² + σ ^{2 ■} (1 )

c n as an example, for DCO-OFDM is written as

∞ Vtov

< l = J (* - v _max ) ² Λ Wife + J (x - y,„ _v ) ² Λ w<& (2)

V max —∞

A relay path is determined using the SNR values. In a VLC network with a plurality of relay terminals, the relay path may include using N relay terminals to communicate between the source terminal and the destination terminal.

Table 1

Table 1 gives the luminous flux achieved for a given average forward voltage for an actual LED - OS RAM LW W5SM. Maximum SNR is calculated for DCO-OFDM using equation (1 ). Then the best relay is selected based on the SNRs for each link.

SUBSTITUTE SHEET RULE 26 For DCO-OFDM the 'dimming' level is set by the DC bias level. Different optical modulation schemes may use different LED characteristics or channel state information to derive the visual-light parameter, such as the dimming level in DCO-OFDM.

For example, one aspect of the embodiment may use Asymmetrically Clipped Optical (ACO)-OFDM and the visual-light parameter level corresponds with the information signal power itself. Another aspect of the embodiment may use On-Off Keying (OOK) OFDM where the amplitude levels of the signal pulses correspond with the visual-light parameter level.

We describe an embodiment of the invention using an example. The example comprises four phases to select an optimum relay path via relay terminals, and transmit the data to a destination terminal.

Figure 3 shows a flow diagram of a selection method to determine the optimum performance relay path using N relay terminals between the source terminal and destination terminal. Initialization (Phase I)

The start of the method (301 ). In the initialization stage (302) the terminal state information is collected (or provided) and from the terminal state information at least the following are determined: the number of available relay terminals (RMAX); and a performance metric. Optionally, the relay-assisted transmission mode is also determined (i.e., half-duplex amplify-and-forward, half-duplex decode-and-forward, full-duplex amplify-and-forward, full-duplex decode-and- forward) and a constraint metric for the VLC link to the respective relay terminal.

The number of relay terminals to be selected (r) out of RMAX available relay terminals is set from zero to RMAX. Table 2 illustrates the candidate terminal relay (or relays) for different r. ID field is placed to identify each selection with a numeric value.

Table 2

Estimation Process (Phase II) 303

Phase II acquires the relevant information to determine the optimum relay path. The collected channel state information (i.e., channel coefficients between source-destination, source-relays, relays-destination etc.) and LED parameters (i.e., turn-on-voltage level, maximum-allowed voltage level) are used to determine the visual-light parameter level before passing visual light parameter level to phase III. Initially r is set to zero to denote a direct transmission from the source terminal to the destination terminal. The parameter r denotes the number of relay terminals used in the relay path between the source terminal and destination terminal.

Finding the Best r Relay terminals (Phase III)

A performance value based on the performance metric and the visual light parameter is determined. For example the performance metric may be a bit- error rate or packet-error-rate, etc. The skilled person in the art would appreciate that any performance metric may be chosen depending on the requirements of the VLC network design. The relay path with the highest performance value is selected as the optimum relay path. In general terms: terminal state information for one or more of the relay terminals is collected at the source terminal and each terminal state information collected from the one or more relay terminals identifies a potential relay path and provides a level of a visual-light parameter for that potential relay path, the performance value for each potential relay path from the level of the visual-light parameter for that potential relay path is determined and a relay path is selected based on the performance value of the one or more potential relay paths.

Optionally, the source terminal may determine (or is provided with) a constraint metric (304) (i.e. bit-error-rate, energy efficiency, or illumination) for candidate r relay terminals for all transmission modes and compares them with a target value. The set of r relay terminals (305) which give the best performance value while satisfying the constraint metric, are listed. In the first iteration when r is zero, if the target value is satisfied, no relay is selected (309) and direct transmission is used. If none of the r relays satisfies the target value, r is increased (307) and the same process is performed (back to 303) with respect to a new r value. If r exceeds maximum value (RMAX) (308), the setting with the best performance metric (309) value is selected and all respective relay terminals are informed. For example using table 2, where the optimum relay path is when r = 3 using relay terminals R1 , R2 and R3, the selected route will use relay terminals R1 , R2 and R3. Optionally, if a constraint metric is incorporated when determining the optimum relay path then the selection may vary and use a different number of relay terminals, and selection of relay terminals.

Data Transmission (Phase IV) There is an indicator to measure instantaneous error rate (i.e., packet error rate) against blocking, high attenuation and/or noise. At the beginning of transmission time interval (TTI) or if the indicator gives an error, the method in the above phases is repeated.

Figures 4 and 5 illustrate simulation results for a VLC system using an embodiment of the invention. There is a 300% gain in link performance. A VLC system using relay terminals according to an embodiment of the invention provides data rates higher (300%) than a direct link between a source terminal and a destination terminal. We can also determine the best set of relay terminals as well as the optimal transmission power to maximise the performance of the system under illumination constraints using a visual light parameter. The existing links or pathways from a source terminal to a destination terminal may suffer poor communication or even a break. The VLC network may adapt to such situations my determining a new relay path to overcome the situation. For example, Fig 6, the new relay path may use relay terminals selected using an embodiment of the present invention if:

· Source-Destination Link Failure to initiate relay communication;

• If the S-D PHY link (direct link) is disrupted, transmission of frames addressed to the destination via a relay path; and

• Direct link can resume after it is recovered. Embodiments of this invention have a direct application in the wireless industry. The dual use of LEDs for both illumination and communication purposes is a breakthrough technology. Since VLC takes advantage of the existing illumination infrastructure for wireless access, it is a cost-effective and energy-efficient solution. There are already a number of companies such as PureLiFi, Oledcomm, LVX and Huawei which aim to commercialize this emerging technology. Using embodiments of the present invention in VLC networks would significantly enhance the performance of VLC networks providing a better user experience.

Embodiments of this invention will allow for the wide deployment of VLC systems enabling superior quality wireless services for end users. One of the problems of with current VLC systems is the need for a line-of-sign communication between the source and destination to achieve high quality of service. Embodiments of the invention will overcome this problem by including relay terminals for VLC systems and hence increase the communication range and the coverage area.

Fig 8 shows how VLC networks can be integrated into existing wireless networks. A destination device, may be a mobile terminal such as a phone, where a user may be using 3G/4G or LTE network, or iterations of such standards, in a first instance. On entry to a building, the signal of the 3G/4G or LTE network may significantly reduce and the users' mobile terminal switches to Wi-Fi, again the Wi-Fi signal may weaken and the users' mobile terminal switches to using a VLC network. The VLC network can be considered as atto- cells in the wireless communications landscape based on the coverage of the VLC terminals (light sources).

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. [1 ] S. Vikramaditya T. A. Sewaiwar and Y.H. Chung, "An Efficient Repeater Assisted Visible Light Communication", 21th European Wireless Conference; Proceedings of, 22 May 2015.

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