WO2019007478A1

WO2019007478A1 - Wireless communication using multiple transmission points

Info

Publication number: WO2019007478A1
Application number: PCT/EP2017/066492
Authority: WO
Inventors: Stepan Kucera; Traian ABRUDAN
Original assignee: Nokia Technologies Oy
Priority date: 2017-07-03
Filing date: 2017-07-03
Publication date: 2019-01-10

Abstract

A network and method of broadcasting are disclosed, the network comprising: a data port operable to receive a signal comprising a stream of data to be transmitted to users; a plurality of wireless communication transmission points; and communication paths for transmitting the signal from the data port to the plurality of wireless communication transmission points; wherein the network is configured to apply cyclic prefixes to a time domain waveform of the signal prior to the plurality of wireless communication points outputting the signal, a length of the cyclic prefixes being set in dependence upon a difference in propagation delays between signals transmitted from different wireless communication transmission points, such that users receiving the signals from multiple points can compensate for the differences in the propagation delays.

Description

WIRELES S COMMUNICATION USIN G MULTIPLE TRAN SMIS SION

POINTS

FIELD OF THE INVENTION

The field of the invention relates to wireless communication networks and in particular to coordinated multiple point transmission techniques.

BACKGROUND

Ultra-dense networks consisting of extremely small cells, attocells, with coverage limited to a few square meters are theoretically the best way to enable ubiquitous

Gigabit connectivity. Yet due to the high number of wireless access units involved, a key challenge consists in low-cost design (e.g. cost of material) and low- cost deployment (e.g., site location) as well as visual appeal. Furthermore, where many wireless access units are used, then reducing interference between neighboring transmission points becomes in itself a complex problem .

Conventionally such problems have been addressed with synchronizing and

multiplexing schemes which significantly increase the costs and complexity of the system.

It would be desirable to provide a network having multiple transmission points without the need for complex and expensive synchronizing or interference mitigation techniques. SUMMARY

A first aspect provides a network comprising: a data port operable to receive a data signal comprising a stream of data to be transmitted to users; a plurality of wireless communication transmission points; communication paths for transmitting said signal from said data port to said plurality of wireless communication transmission points; wherein said network is configured to apply cyclic prefixes to a time domain waveform of said signal prior to said plurality of wireless communication points outputting said signal, a length of said cyclic prefixes being set in dependence upon a difference in propagation delays between signals transmitted from different wireless communication transmission points, such that different wireless communication transmission points jointly deliver said signal to users. The inventors of the present invention recognized that in many situations data is received for distribution to users as a data stream via for example, a backhaul link such as a wired Internet connection to a building. Such a data stream is made up of a broadcast stream of data segments, which may be data packets or data frames. Where many wireless communication transmission points are available for outputting data to users, then a simple yet effective way of outputting the data may be to send the data stream to a plurality of wireless communication transmission points such that each wireless communication point receives and broadcasts the same data signal, the scheduling of the output of the signal being simply based on the scheduling of the received signal. Not only does this mean that expensive scheduling and interference mitigation techniques are avoided, but also user equipment can roam within the region served by the plurality of wireless access transmission points and seamlessly receive the same data stream. Although each transmission point may transmit the same signal, different propagation delays occurring between the data source and users receiving the signal both in any wired link and in the wireless link, will result in the signals received by the user from the different points not being synchronized with each other. In order to address this, a cyclic prefix is added to the analogue form of the data signal the length of which is selected to be long enough such that users can constructively combine the data received from different points with different delays allowing the data to be delivered jointly by multiple transmission points. In this way the transmission of the same signal from a different point improves rather than compromises the signal received. In effect multiple transmission points are used to transmit the same signal in a similar way to conventional coordinated multi-point (CoMP) transmissions. However, these conventional techniques have generally used extremely tight synchronization of all the participating transmission points. This synchronisation required a central reference clock which is a technically difficult and operationally expensive task. For example, in 4G LTE networks, CoMP-capable remote radio heads are synchronized by using the CPRI protocol (tight bit-level synchronization, configurable static timing offsets, master-slave architecture for clock extraction by daisy-chained remote radio heads) while independent base stations rely on the nanosecond-accurate clock of the Global Positioning System (GPS). In indoor spaces, in which GPS is inapplicable, one can use auxiliary synchronization solutions such as provided by Nanotron Technologies GmbH, requiring the deployment of a parallel synchronization infrastructure which is costly and impractical. The present technique has addressed this problem by using cyclic prefixes which if correctly applied allow the signals to be combined and in effect turns interference into useful signal.

Although the wireless transmission points may be formed in a number of ways and transmit signals in a number of different bandwidths, in some embodiments, said plurality of wireless transmission points are operable to output wireless signals in the optical spectrum .

By using the optical spectrum which may encompass visible light, infrared and ultra- violet as the bandwidth for transmitting the signals, legacy infrastructure can be used to implement the network, leading to what can be viewed as zero or at least low-cost coordinated multi-point transmissions.

A mobile network based on visible light communications (VLC) efficiently reuses existing infrastructure such as ceiling lighting fixtures and Ethernet local area networks.

In this context, it is to be noted that the lighting industry is currently undergoing a transformation from simple bulbs and fluorescent tubes to intelligent solid state LED lighting systems (luminaires). To enable remote lighting control, the luminaires are connected and even powered over Ethernet. Thus, an ultra-dense radio access deployment can be realized at very low cost through direct modulation of the LED light sources of at least some of the luminaires to wirelessly deliver data to nearby user devices, equipped with an optical receiver. Unlike conventional radio frequency systems, no expensive beamforming solutions and frequency band licensing is needed.

In effect embodiments reuse the visible light spectrum in which luminaires operate for wireless data transfers. Herein, the drivers of the LED light sources are directly modulated with payload data to deliver wireless data to nearby user devices equipped with an optical receiver. Visible light communications (VLC) have been demonstrated to achieve wireless data rates in the order of Gigabits per second by modulating the intensity of a light source.

Although it is known to use visible light communication VLC, conventional VLC networking architecture to date does not provide contiguous high-capacity coverage or enable user roaming based on seamless handovers. State of the art in visible light communications (VLC) consists in point-to-point connections achieving around 40 Gbps over the range of 2.5m (see for example http:/ / www.sciencealert.com/ a-new- type-of-li-fi-has-reportedly-cracked-40 -gigabits-per-second- 100-times-faster-than- regular-wi-fi). Initial technology for VLC cells provide coverage of few meters square (see Fig. 1). However, these cells are meant for stand-alone deployment and if placed too close to each other, they interfere directly which causes serious data rate degradation at both cell edges, even call drops.

Moreover, existing VLC cells do not support seamless handovers. If out of coverage, mobile users are required to disconnect from the source cell and re-connect to the target cell. Yet as the target cell does not coordinate IP address assignment with the source cell, a handover practically always terminates any active or inactive user connection. Thus, a new coverage and roaming solution is needed for ultra-dense VLC networks. Embodiments of the present technique address the above drawbacks found with conventional VLC techniques by outputting the same data signal from multiple luminaires and using cyclic prefixes of appropriate length to address potential interference issues and allow the signals to be constructively combined, such that data is jointly delivered to users from different transmission points.

Although prior to being output the signals may be encoded in a number of ways in some embodiments they are encoded using orthogonal frequency division multiplexing or OFDM. This is an effective way of encoding signals which is particularly applicable to signals in the optical spectrum. Furthermore, this form of encoding is very tolerant to differences in symbol timing, allowing the signals to be effectively combined simply with the use of appropriate length cyclic prefixes. In order for this to work effectively, in some embodiments the cyclic prefix are selected to be as long as, or longer than the maximum difference in time propagation delays between signals received at a user from the different transmission points. In this way the differences in propagation time between signals from the different transmission points is encompassed within the transmission time of the cyclic prefix. This allows signals received by a user from different transmission points to be constructively combined.

In some embodiments, a length of said communication paths from said data port to each of said plurality of wireless transmission points are set to be substantially equal, that is they differ in length from each other by less than 10 %. Although differences in transmission times can be compensated for with cyclic prefixes, the length of the cyclic prefix that is required to enable the users to constructively combine the different signals depends on the differences in propagation time delay of the signals from the different transmission points. Thus, in some embodiment it is advantageous if the propagation delay due to the communication paths from the data port to the multiple transmission points, generally a wired link, is set to be

approximately equal for each transmission point. By reducing differences in

propagation delays along this portion of the transmission path the overall differences are reduced and the length required for the cyclic prefix is reduced. Reducing the amount of bandwidth required to transmit the cyclic prefix provides more bandwidth for data transmissions and improves spectral efficiency.

Alternatively, said wireless communication transmission points are operable to apply a delay to outputting data signal to compensate for an estimated propagation delay from said data port due to said communication path.

As an alternative to equalising propagation delays in the communication paths, these may rather be determined and differences compensated for with the use of delays. In this regard the data may be transmitted in segments each segment having a

synchronising signal at the start of the segment. Where, for example, the data is transmitted according to the Ethernet protocol as frames then there is a synchronising signal at a start of each frame and this is used as a trigger to output the data by the wireless transmission point. A delay can be added from detecting the synchronising signal to outputting it, to equalise communication path delays, reduce cyclic prefix lengths and thereby improve spectral efficiency.

In some embodiments, each of said wireless communication transmission points are configured to output pilot signals, said network determining communication path propagation delays for each wireless communication transmission point from analysis of pilot signal responses.

In order to apply a suitable delay, the differences in communication path propagation delays need to be determined. This can be done simply by determining the difference in path length at network configuration, or it can be done during use with the help of pilot signals. Delays in the receipt of these pilot signals at users will depend on both the wired and wireless delays. However, the wired delays will be constant while the wireless delays will change. Thus, analysis of the delays to find the systematic delay will provide information on the communication path lengths between data port and wireless transmission points.

In some embodiment said data port is operable to receive data compliant to Ethernet protocols. In some cases it may be an Ethernet device such as an Ethernet switch.

In some embodiments, said network comprises a further plurality of wireless communication transmission points operable to receive a further signal, and each operable to transmit said further signal to users, such that a first signal is broadcast in a first region by a first set of wireless communication transmission points and a further data signal is broadcast in a further region by a further set of wireless communication transmission points; said network comprising control circuitry operable to control signals sent to said first and second set of wireless communication transmission points. In some embodiments, there are multiple regions or cells formed by different sets of multiple wireless communication transmission points each set of multiple transmission points broadcasting the same data stream , that data stream being different to the stream broadcast by other sets of wireless transmission points. Control circuitry within the network controls the sending of the data streams to the different sets of

transmission points. Generally any further set of wireless communication transmission points will comprise multiple transmission points, but in some embodiments one or more further sets may comprise a single wireless communication transmission point.

In some embodiments, said further signal comprises a further stream of data received at a further data port.

In some cases the different cells or regions may each be independently backhauled each receiving data from a respective data port. In such cases said communication paths connecting said first data port and said first set of wireless communication transmission paths may comprise a first local area network LAN and said communication paths connecting said further data port and said further set of wireless communication transmission paths may comprise a further local area network LAN. In other embodiments, said further signal comprises a subset of said signal received from said data port, said control circuitry being operable to segment said signal received from said data port into said first signal and said further signal. In some embodiments, the data stream received may be split and sent to different sets of wireless communication transmission points which serve different regions. In this way traffic from for example, a single 10 Gbps connection can be divided into ten 1 Gbps connections, each associated with different cells. In some cases, such as where a user is deemed to be static and is perhaps the only user served by one wireless transmission point, then that wireless transmission point may be fed with data extracted from the data stream and directed to that user.

In some embodiments, each set of said wireless communication transmission points are operable to output said signals in different time slots, said control circuitry being operable to control wireless communication transmission points in a same region to output signals in a same set of time slots and to control wireless communication transmission points in at least one different neighbouring region to output signals in a different set of non-overlapping time slots. One way of mitigating interference between adjacent regions transmitting different signals is with the use of time division multiplexing, such that data is output in different time slots at the different sets of wireless transmission points.

In some embodiments, said control circuitry is operable to determine a distribution of users within said first and said second regions and is operable to impede at least one wireless communication transmission point from outputting a data signal in dependence upon said determined distribution of users.

In some embodiments, the user distribution is determined and used to activate or impede individual wireless transmission points from transmitting data, such that wireless transmission points not currently serving a user may be impeded from outputting data perhaps to reduce interference and/ or reduce power consumption. In some cases the control circuitry may also take account of the mobility of users and their location close to the wireless transmission point when determining which transmission points to impeded data output from , such that their roaming ability is not unduly impeded. In some embodiments, said at least one wireless communication transmission point set impeded from outputting said data signal comprises at least one wireless

communication transmission point located towards an edge of said first or said second regions.

In order to reduce interference between neighbouring regions outputting different signals it may be advantageous to impede output of data from region or cell edge transmission points where possible. This will help reduce interference with signals in neighbouring regions.

In some embodiments, said control circuitry is operable to impede said at least one wireless communication transmission point from outputting said data signal by setting said at least one wireless communication transmission point to idle mode. The control of the switching of the transmission points between idle and operational mode may be done by using an Ethernet Wake-on-LAN "magic" packets (IEEE 802.3 Ethernet standard).

In some embodiments, each of said wireless communication transmission points are operable to output said signals in at least one of at least two different bandwidths, said control circuitry being operable to control wireless communication transmission points in a same region to output signals in a same bandwidth and to control wireless communication transmission points in at least one neighbouring regions to output signals in a different bandwidth.

An alternative way of impeding interference between neighbouring cells is to use different bandwidths for the signals in neighbouring cells. Thus, where for example the system is a VLC system and the lights comprises LEDs, then one cell might modulate signals output by a red LED, while a neighbouring cell might modulate a blue LED.

In some embodiments, wherein said control circuitry is operable to assign wireless transmission points to said first set or said further set and to transmit said first signal to said wireless transmission points assigned to said first set and said further signal to said wireless transmission points assigned to said further set.

In some embodiments, the network may be dynamically configurable such that the wireless transmission points serving a particular cell may be changed by control circuitry in response to changes in user distribution for example. Thus, loading can be approximately equalised between the regions and regions can also be selected such that the region edges occur in places where user density is low. Furthermore, where neighbouring transmission points transmit mutually interfering signals, then they may be assigned to the same set such that they transmit the same signals and users in the area can use signals received from both transmission points.

A second aspect provides, a method of controlling the broadcast of signals from multiple wireless communication transmission points said method comprising:

receiving a digital signal comprising a stream of data; applying cyclic prefixes to a time domain waveform of said signal, a length of said cyclic prefixes being set to allow users receiving said signal to compensate for propagation delays between signals transmitted from different wireless communication transmission points; forwarding said signals for broadcast to said plurality of wireless communication transmission points along communication paths linking said data port and said plurality of wireless

communication transmission points.

In some embodiments, the method further comprises OFDM encoding said signal prior to applying said cyclic prefixes.

In some embodiments, said cyclic prefixes are set to be equal to or exceed the maximum time difference in signals received by a user from said wireless transmission point and from other wireless transmission points.

In some embodiments, said received digital signal comprises a signal compliant to Ethernet protocols.

In some embodiments, the method further comprises forwarding a first signal for broadcast in a first region from a first set of wireless communication transmission points and forwarding a further data signal in a further region for broadcast from a further set of wireless communication transmission points.

In some embodiments, the method further comprises a further step of receiving a further signal as a further stream of data. In some embodiments, said first and further signal are received at a same data port, said method comprising segmenting said signal received at said data port into said first signal and said further signal. In some embodiments, the method further comprises determining a distribution of users within said first and said further regions and transmitting a signal impeding at least one wireless communication transmission point from outputting a data signal in dependence upon said determined distribution of users.

In some embodiments, said at least one wireless communication transmission point set impeded from outputting said data signal, comprises at least one wireless

In some embodiments, said step of impeding comprising transmitting a signal setting said at least one wireless communication transmission point to idle mode. In some embodiments, the method further comprises assigning wireless transmission points to said first set or said further set and transmitting said first signal to said wireless transmission points assigned to said first set and said further signal to said wireless transmission points assigned to said further set. In some embodiments, the method further comprises each of said sets of said wireless communication transmission points outputting said signals in different time slots, such that wireless communication transmission points in a same region output signals in a same set of time slots and wireless communication transmission points in at least one different neighbouring region output signals in a different set of non-overlapping time slots.

In some embodiments, the method further comprises each of said sets of said wireless communication transmission points outputting said signals in at least one of at least two different bandwidths, wireless communication transmission points in a same region outputting signals in a same at least one bandwidth and wireless communication transmission points in at least one neighbouring regions outputting signals in a different bandwidth.

In some embodiments, the method further comprises outputting wireless signals in the optical spectrum from said plurality of wireless communication transmission points. In some embodiments, the method further comprises setting a length of said communication paths from said data input to each of said plurality of wireless transmission points set to be substantially equal, that is they differ in length from each other by less than 10 %.

In some embodiments, the method further comprises applying a delay to outputting said data signal to compensate for an estimated propagation delay from said data input due to said communication path differences. In some embodiments, the method further comprises determining communication path propagation delays for each wireless communication transmission point by outputting pilot signals from each wireless communication transmission point and analysing pilot signal responses received from said users. A third aspect provides a network node controller or router configured to perform a method according to a second aspect of the present invention.

The network may comprise a network node controller or router comprising circuitry and/ or logic operable to route the signals and apply cyclic prefixes to the signals. The circuitry may also encode the signals, determine any delays to be applied and send control signals to the wireless transmission points setting them to idle or operational mode. The controller or router may also be operable to divide the wireless

transmission points into different sets serving different cells and transmit the relevant data signals to the relevant sets of wireless communication transmission points. In other embodiments, some of this circuitry such as the encoder and cyclic prefix applier may be present on the wireless communication transmission points themselves rather than in a central controller or router.

A fourth aspect comprises a network comprising: an input means for receiving a signal comprising a data signal to be transmitted to users; a plurality of wireless

communication means for transmitting data; a communication means for transmitting said signal from said data input to said plurality of wireless communication

transmission points; and a cyclic prefix applying means for applying cyclic prefixes to a time domain waveform of said signal prior to said plurality of wireless communication points outputting said signal, said cyclic prefix applying means setting a length of said cyclic prefixes in dependence upon a difference in propagation delays between signals transmitted from different wireless communication transmission points, such that users receiving said signals from multiple points can compensate for said differences in said propagation delays.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function , it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which :

Figure 1 illustrates multiple visible light communication transmission points according to the prior art;

Figure 2 illustrates a network comprising multiple visible light communication points according to an embodiment;

Figure 3 schematically shows multiple path signal propagation and time delays in both wired and wireless communication paths;

Figure 4 shows an interference mitigation technique using idle mode transmission points in a multiple cell network according to an embodiment;

Figure 5 shows an interference mitigation technique using wavelength domain multiplexing in a multiple cell network according to an embodiment;

Figure 6 shows different techniques for reducing the differences in signal path propagation delays in the wired communication paths;

Figure 7 shows a flow diagram illustrating the main points of the technique; and Figure 8 shows steps in a method according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments provide a multiple transmission point network that makes use of the fact that in many cases the data to be transmitted within a space such as a building is received via a wired (for example, optical or copper) link such that data arrives as a broadcast data stream . This data stream can be transmitted to users within the space, in a simple manner by using multiple transmission points to transmit the same received data stream. The wireless transmission points may in some embodiments simply act as repeating stations, in effect making use of the scheduling present in the data stream of the wired link to schedule when to output the data symbols.

In some embodiments the multiple transmission point network may use visible light communications (VLC) techniques and be formed by efficiently reusing existing infrastructure such as ceiling lighting fixtures and Ethernet local area networks.

In some embodiments OFDM encoding is used, this encoding being selected as virtually all indoor backhaul is based on the Ethernet broadcast protocol, and OFDM is the most suitable transmission format for encoding such signals for optical

transmissions:

a VLC cell is created by inter-connecting multiple transmission points (TP) by using a standard physical or virtual local area network (LAN) (e.g., Gigabit Ethernet over copper or fiber).

LAN data is then transmitted to mobile users simultaneously from all TPs using an OFDM format with a cyclic prefix for each time-domain symbol.

Unlike in legacy CoMP systems, TP transmissions according to the proposed scheme do not need to be tightly synchronized. Instead, a novel approach to zero or low-cost transmission coordination is proposed in which

TP transmissions are triggered distributively and asynchronously by only using broadcast signals of the shared backhaul (e.g., Ethernet frame preambles in the simplest case), and

the OFDM cyclic prefix duration is set to exceed the maximum delay difference among trigger signals in the LAN stratum and the OFDM signals in the wireless access stratum (see Fig. 3).

In this way, interference is converted into basic multi-path signal propagation characterized by "synchronous" constructive recombination of OFDM signals from all TPs at the UE receiver despite the lack of any tight synchronization. To minimize or at least reduce the cyclic prefix size, i.e. to maximize or at least increase spectral efficiency of the VLC CoMP system, two optional measures are proposed:

connecting individual TPs to LAN signal source such as an Ethernet switch with

LAN cables of equal length (manual configuration), or delaying trigger signals based on measured timing offset of TP pilot signals reduce by a constant factor accounting for user mobility (automated distribu scheme). As shown in Figure 2, the VLC network architecture is based on a coordinated multipoint (CoMP) transmission paradigm that uses cyclic prefixes to turn interference into useful signal. VLC cells are formed by multiple spatially distributed transmission points (TP) in the lighting fixtures that synchronously broadcast user data delivered by the Ethernet backhaul. This enables contiguous network coverage, dramatically improves network performance, and offers unrestricted user roaming as known from soft handover capable networks.

Regarding a further aspect, interference among different VLC cells (i.e., disjoint CoMP sets having independent backhaul) is mitigated by

- time-division multiplexing of user data - Mutually interfering VLC TPs are assigned to the same physical or virtual LAN to form a single CoMP set with inherent time-division multiplexing of user data flows based on LAN sharing.

Preferably, the IEEE 802.1Q standard for virtual LAN tagging is employed, e.g. to dynamically adjust VLC cell size and location according to user capacity demand (load balancing between independent backhaul connections) and mobility (single TP unicasting to static UEs, CoMP subset multicasting to nomadic UEs, CoMP set broadcasting to mobile UEs).

TP idle modes (see Fig. 4) - In addition to active transmission mode, interfering (cell-edge) VLC TPs are additionally capable of operating in a stand-by mode (only pilot signals are broadcast) and idle mode (no transmission at all).

Preferably, the Wake-on-LAN "magic" packets (IEEE 802.3 Ethernet standard) are used for an efficient configuration of the operational mode. - OFDM wavelength/ bandwidth-division multiplexing (see Fig. 5) - Orthogonal carrier wavelengths are configured for interfering VLC cells that use a combination of LEDs of different wavelengths (eg, red, green, blue) as transmission front-ends, as well as a source of white light for illumination purposes. Alternatively, or in addition to the carrier wavelength approach, OFDM sub-bands can be also multiplexed. WakeOnLAN standard is again the preferred signaling method. If physical layer resources are insufficient to accommodate the backhaul data rate, data rate of ingress traffic is reduced using standard L1-L3 techniques (see Examples for more details).

Although the above is explained with respect to VLC, as this has particular advantages, with existing infrastructure, it would be clear to a skilled person that the same technique could be used with other bandwidths.

Netw o rk archite cture The following example assumes an indoor Visible light communication (VLC) network whose transmission points (TP) are mounted in the ceiling of an enterprise. As part of conventional off-the-shelf components, one can also use for example the Wifi-by- Luminaire lighting fixtures developed by Nokia Bell Labs under the Future Indoor Network project in collaboration with Osram, leading lighting infrastructure manufacturer.

As for backhaul, existing copper-based and/ or fiber-based Ethernet infrastructure for data distribution between a gateway node and VLC TPs are reused entirely to minimize or at least reduce the deployment costs. Fig. 6 shows four examples of TP deployment inter-connection with an Ethernet hub. Physical and virtual Ethernet switches can be used as well. Control circuitry for encoding, controlling the transmissions by the different transmission points, and segmenting and transmitting the data signals may be present in the Ethernet hub, alternatively such control circuitry may be distributed within the network some encoding and control occurring at the individual TPs and other more central control occurring at the Ethernet hub.

According to embodiments, each Ethernet local area network (LAN) defined by independent shared broadcast region defines one VLC CoMP set (i.e., a VLC cell) that broadcasts the downlink Ethernet frames to mobile users in the area. If virtual LAN technology is used, CoMP sets can be defined dynamically and in software as discussed subsequently in more detail. Multiple such LANs can share disjoint portions of the same IP subnet.

In a network with backhaul shared by multiple users, individual per-user peak rates are generally limited only by the physical-layer bit rate of the Ethernet/ backhaul technology but all served data flows share the available bandwidth (typically via the TCP congestion control mechanism). Since uplink-downlink contention in a share medium results in a significant loss of capacity due to broadcast storms, especially in networks with large broadcast scope, full-duplex Gigabit Ethernet technology is preferably used to implement the distribution and backhaul network (e.g., 1 Gbps for copper-based 1000BASE-T Ethernet, or optic fiber-based 1000BASE-LX Ethernet). For sub-Gigabit Ethernet, uplink traffic of mobile users can be seamlessly offloaded from the VLC stratum to the Wifi/ LTE stratum (e.g., using the techniques disclosed in co-pending Nokia European patent application 16306173.2). The advantage of the latter solution is that it also efficiently resolves the hidden node problem. Netw o rk o peratio n

Transmission coordination

All TPs broadcast the ingress Ethernet frames with a given delay after having received the synchronization preamble of the Ethernet frame (64 bits of repeated atomic pattern "01"). According to embodiments, the transmission is carried out in an OFDM format with a cyclic prefix sufficiently long to compensate not only for delay spread due to multi-path propagation in the VLC access stratum but also for any differences in propagation delay in the LAN stratum. To minimize or at least reduce the cyclic prefix length, i.e. to maximize or increase the VLC spectral efficiency, the differences in LAN propagation delays may be reduced. One method consists in deploying TPs by using even-length cabling (see Fig. 6 (b) and (d)). Alternatively (see Fig. 6 (a) and (c)), additive delay of the synchronization trigger signals can be used to compensate for different cable lengths. The timing offset can be configured either automatically during network operation, or manually during network deployment. In the former case, timing advances can be established by correlating specially designed TP pilot signals (e.g., Zadoff-Chou sequences used in LTE). In the latter case, the deploying technician can set the additive delay in each TP proportionally to the difference between the maximum length of a standard Ethernet cable (100m for 10/ 100/ 1000 Megabit Ethernet) and the actual cable length to the LAN signal source to roughly compensate for LAN signal delay. Note that both schemes are fully distributed.

Interference mitigation

Each TP broadcasts a beacon indicating its physical cell identity such as the globally unique Medium Access Control (MAC) Address of the Ethernet device. Similarly to 3G W-CDMA and 4G LTE networks, automatic neighbor relationships are collected based on user measurement reports to define interfering or adjacent TPs.

To mitigate interference, virtual LANs are dynamically reconfigured such that all or at least most interfering TPs belong to the same VLC cell, i.e. form the same CoMP set. By sending Wake-on-LAN packets, cell-edge TPs that do not actively serve any network user can be forced to enter idle state to avoid interference to neighboring cells. The idle mode concept is visualized in Fig. 4. In general, static users can be served by a single TP, possibly including the immediate neighbors to create soft-handover conditions. Selective TP multicast is to be used for nomadic users while LAN-wide broadcast engaging all LAN TPs is to be used to serve mobile UEs. User mobility can be detected by using known methods exploiting information on UE handovers, signal strength measurements, gyroscope reading, etc.

Alternatively, interference can be controlled by using wavelength and frequency multiplexing (see Fig. 5). Standard Wake-on-LAN packets can again serve as control messages. If OFDM resources are not sufficient, the ingress data rate can be reduced by using

- LI techniques: Ethernet bit rate adaptation (e.g., by auto-negotiation) with normal frame spacing,

L2 techniques: Ethernet inter-frame spacing adaptation for given bit rate, and/ or addition of "spacing" packets (extra to-be-dropped frames to invalid MAC address),

- L3 techniques: IP segmentation combined with L2 gap and padding measures.

Virtual Ethernet devices can be used to simplify any type of multiplexing, rate reduction, CoMP set control as well as data frame filtering.

Figure 7 shows a flow diagram schematically setting out the main points of

embodiments.

Two major innovations are summarized in Fig. 7 and described in more detail below:

1. a cost-efficient method for creating VLC cells from multiple coordinated

transmission points (CoMP) without the need for their tight synchronization with central reference clock using orthogonal infrastructure, and

2. robust low-complexity methods for interference mitigation among VLC CoMP cells that natively exploit unique features of the proposed design such as the usage of OFDM cyclic prefix for turning interference into useful signal. Multiple transmission points are used for broadcasting data across a region covered by the transmission points, the data being backhauled using a standard LAN. Cyclic prefixes are added to the data prior to transmission by the multiple transmission points and these allow users to combine transmission from different points and in effect use constructive interference of the signals without and the need for tight transmission point synchronization. The cyclic prefixes are set to have a length such that the transmission time for a cyclic prefix exceeds the maximum delay difference between transmission from different transmission points due to the combined wireline and wireless signal propagation times.

The transmissions from each transmission point are triggered asynchronously and distributively by the frame preambles of the data received on the backhaul link. Thus, output of the data is in effect are simply scheduled by the scheduling of the original data using the synchronising signals associated with the data.

In some cases different sets of wireless transmission points transmit different data signals. In such cases some wireless transmission points may be set not to transmit to reduce interference between the different regions covered by the different sets of wireless transmission points.

Figure 8 shows a flow diagram illustrating steps in a method according to an embodiment. All but the final step shown may be performed by a network node controller. In this method user distribution and mobility in a region served by a plurality of wireless transmission points is determined and the wireless transmission points are assigned to different sets such that each set serves a similar number of users. The different sets are then linked with an Ethernet switch and data streams received at each Ethernet switch are fed to the wireless transmission points associated with that switch.

In some embodiments in order to reduce interference it is determined if any of the wireless transmission points towards an edge of a region are not serving users and if they are not they are set to idle mode, those that are serving users are set to or maintained in operational mode. In some cases the allocation of the wireless transmission points to a particular set will be done to maximize the number of edge transmission points that do not serve users. Prior to being output by the transmission points of the respective sets, the data streams will be encoded and cyclic prefixes affixed prior to each time domain symbol. The length of the cyclic prefix will be selected in dependence upon the difference in propagation delay between signals reaching a user from different transmission points in a set of transmission points. The length will be selected so that it is longer than the maximum difference in propagation delays.

The determination of the distribution of users is performed periodically and the assigning of the wireless transmission points to the different sets may be updated as user densities change. Similarly the wireless transmission points towards an edge of a region are switched between idle and operational mode as users enter and leave the area served by the wireless transmission points.

In summary embodiments provide an extremely cost-efficient networking architecture for visible light communications that implements the coordinated multi-point transmission paradigm. By turning inter-cell interference into useful signal, contiguous network coverage, dramatically improved network performance, and relatively unrestricted user roaming is provided. Embodiments offer the first commercially viable solution for high-performance networking based on multi-cell visible light communications (VLC). Existing startups currently offer only VLC cell that are designed for stand-alone deployment and which if placed too close to each other, they interfere directly which causes serious data rate degradation at both cell edges, even call drops. Moreover, seamless handovers are not supported, resulting in session disruption due to IP address change.

The present technology offers for some embodiments, an interference-free, zero-cost solution to indoor Gigabit wireless networking. A person of skill in the art would readily recognize that steps of various above- described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine- executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods.

The functions of the various elements shown in the Figures, including any functional blocks labelled as "processors" or "logic", may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term

"processor" or "controller" or "logic" should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/ or custom, may also be included. Similarly, any switches shown in the Figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.

It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

1. A network comprising:

a data port operable to receive a signal comprising a stream of data to be transmitted to users;

a plurality of wireless communication transmission points;

communication paths for transmitting said signal from said data port to said plurality of wireless communication transmission points; wherein

said network is configured to apply cyclic prefixes to a time domain waveform of said signal prior to said plurality of wireless communication points outputting said signal, a length of said cyclic prefixes being set in dependence upon a difference in propagation delays between signals transmitted from different wireless communication transmission points, such that different wireless communication transmission points jointly deliver said signal to users.

2. A network according to claim 1, wherein said plurality of wireless transmission points are operable to output wireless signals in the optical spectrum .

3. A network according to any preceding claim, wherein said network comprises an orthogonal frequency division multiplexing encoder operable to encode said signal prior to said signal being output.

4. A network according to any preceding claim, wherein said cyclic prefixes are set to be equal to or exceed the maximum time difference in signals received by a user from said wireless transmission point and from other wireless transmission points.

5. A network according to any preceding claim , wherein a length of said communication paths from said data port to each of said plurality of wireless transmission points are set to be substantially equal, that is they differ in length from each other by less than 10 %.

6. A network according to any one of claims 1 to 4, wherein said wireless communication transmission points are operable to apply a delay to outputting said data signal to compensate for an estimated propagation delay from said data port due to said communication path.

7. A network according to claim 6 , wherein each of said wireless communication transmission points are configured to output pilot signals, said network determining communication path propagation delays for each wireless communication transmission point from analysis of pilot signal responses.

8. A network according to any preceding claim , wherein said data port comprises an Ethernet device.

9. A network according to any preceding claim , said network comprising a further plurality of wireless communication transmission points operable to receive a further signal, and each operable to transmit said further signal to users, such that a first signal is broadcast in a first region by a first set of wireless communication transmission points and a further data signal is broadcast in a further region by a further set of wireless communication transmission points;

said network comprising control circuitry operable to control signals sent to said first and second set of wireless communication transmission points.

10. A network according to claim 9, wherein said further signal comprises a further stream of data received at a further data port.

11. A network according to claim 10 ,wherein said communication paths connecting said first data port and said first set of wireless communication transmission paths comprise a first local area network and said communication paths connecting said further data port and said further set of wireless communication transmission paths comprise a further local area network.

12. A network according to claim 9 , wherein said further signal comprises a subset of said signal received from said data port, said control circuitry being operable to segment said signal received from said data port into said first signal and said further signal.

13. A network according to any one of claims 9 to 12, wherein said control circuitry is operable to determine a distribution of users within said first and said second regions and is operable to impede at least one wireless communication transmission point from outputting a data signal in dependence upon said determined distribution of users.

14. A network according to claim 13 , wherein said at least one wireless

communication transmission point set impeded from outputting said data signal, comprises at least one wireless communication transmission point located towards an edge of said first or said second regions.

15. A network according to claim 13 or 14, wherein said control circuitry is operable to impede said at least one wireless communication transmission point from outputting said data signal by setting said at least one wireless communication transmission point to idle mode.

16. A network according to any one of claims 9 to 13 , wherein each of said wireless communication transmission points are operable to output said signals in at least one of at least two different bandwidths, said control circuitry being operable to control wireless communication transmission points in a same region to output signals in a same bandwidth and to control wireless communication transmission points in at least one neighbouring regions to output signals in a different bandwidth.

17. A network according to any one of claims 9 to 16, wherein said control circuitry is operable to assign wireless transmission points to said first set or said further set and to transmit said first signal to said wireless transmission points assigned to said first set and said further signal to said wireless transmission points assigned to said further set.

18. A method of controlling the broadcast of signals from multiple wireless communication transmission points said method comprising:

receiving a digital signal comprising a stream of data;

applying cyclic prefixes to a time domain waveform of said signal, a length of said cyclic prefixes being set to allow users receiving said signal to compensate for propagation delays between signals transmitted from different wireless communication transmission points;

forwarding said signals for broadcast to said plurality of wireless

communication transmission points along communication paths linking a data input and said plurality of wireless communication transmission points.

19. A method according to claim 18 , further comprising orthogonal division frequency multiplex encoding said signal prior to applying said cyclic prefixes.

20. A method according to claim 18 or 19 , wherein said cyclic prefixes are set to be equal to or exceed the maximum time difference in signals received by a user from said wireless transmission point and from other wireless transmission points.

21. A method according to any one of claims 18 to 20 , wherein said received digital signal comprises a signal compliant to Ethernet protocols.

22. A method according to any one of claims 18 to 21, said method comprising forwarding a first signal for broadcast in a first region from a first set of wireless communication transmission points and forwarding a further data signal in a further region for broadcast from a further set of wireless communication transmission points.

23. A method according to claim 22, comprising a further step of receiving a further signal as a further stream of data.

24. A method according to claim 23 , wherein said first and further signal are received at a same data port, said method comprising segmenting said signal received at said data port into said first signal and said further signal.

25. A method according to any one of claims 22 to 24, comprising determining a distribution of users within said first and said further regions and transmitting a signal impeding at least one wireless communication transmission point from outputting a data signal in dependence upon said determined distribution of users.

26. A method according to claim 25, wherein said at least one wireless

27. A method according to claim 25 or 26, said step of impeding comprising transmitting a signal setting said at least one wireless communication transmission point to idle mode.

28. A method according to any one of claims 22 to 27, comprising assigning wireless transmission points to said first set or said further set and transmitting said first signal to said wireless transmission points assigned to said first set and said further signal to said wireless transmission points assigned to said further set.

29. A method according to any one of claims 22 to 28 , further comprising each of said sets of said wireless communication transmission points outputting said signals in different time slots, such that wireless communication transmission points in a same region output signals in a same set of time slots and wireless communication transmission points in at least one different neighbouring region output signals in a different set of non-overlapping time slots.

30. A method according to any one of claims 22 to 28 , further comprising each of said sets of said wireless communication transmission points outputting said signals in at least one of at least two different bandwidths, wireless communication transmission points in a same region outputting signals in a same at least one bandwidth and wireless communication transmission points in at least one neighbouring regions outputting signals in a different bandwidth.

31. A method according to any one of claims 18 to 30 , further comprising outputting wireless signals in the optical spectrum from said plurality of wireless communication transmission points.

32. A method according to any one of claims 18 to 31, further comprising setting a length of said communication paths from said data input to each of said plurality of wireless transmission points set to be substantially equal, that is they differ in length from each other by less than 10 %.

33. A method according to any one of claims 18 to 32, further comprising applying a delay to outputting said data signal to compensate for an estimated propagation delay from said data input due to said communication path differences.

34. A method according to claim 33 , further comprising determining

communication path propagation delays for each wireless communication transmission point by outputting pilot signals from each wireless communication transmission point and analysing pilot signal responses received from said users.

35. A network node controller or router configured to perform a method according to any one of claims 18 to 28.

36. A network comprising: an input means for receiving a signal comprising a stream of data to be transmitted to users;

a plurality of wireless communication means for transmitting data;

a communication means for transmitting said signal from said data input to said plurality of wireless communication transmission points; and

a cyclic prefix applying means for applying cyclic prefixes to a time domain waveform of said signal prior to said plurality of wireless communication points outputting said signal, said cyclic prefix applying means setting a length of said cyclic prefixes in dependence upon a difference in propagation delays between signals transmitted from different wireless communication transmission points, such that users receiving said signals from multiple points can compensate for said differences in said propagation delays.