Abstract:
A video projector, a cluster of video projectors and a method for wirelessly transmitting image data within the cluster of video projectors. The video projector includes a first antenna located at a first side of the projector and a second antenna located at a second side of the projector, opposite to the first side. Image data is divided into sub-parts, and distributed by assigning each image data sub-part to a video projector. A video projector receiving image data sub-parts extracts and stores the image data sub-part assigned to it, and retransmits image data sub-parts assigned to respectively a second and third video projectors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a)-(d) of United Kingdom Patent Application No. 1216557.7, filed on Sep. 17, 2012 and entitled “A method and device for encoding and decoding a video signal”. 
         [0002]    The above cited patent application is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0003]    The invention relates to the field of multiple projectors system, and in particular to the field of video distribution to a plurality of video projectors forming a cluster of video projectors. 
         [0004]    The invention relates to a device, a system and a method for distributing an image to the video projectors (VP) of a multi projection (MP) system. 
         [0005]    The abbreviations VP, standing for Video Projector, and MP, standing for Multi Projection system, will be employed in the following description. 
       BACKGROUND OF THE INVENTION 
       [0006]    Multi projection (MP) systems are attracting attention to provide very high precision and high quality image on large display area. Such MP system might be employed for large projection areas like dome, stadium and concert hall or for projection on building. 
         [0007]    Each single video projector (VP) of a MP system generates an image with a definition and a size determined by the VP lens focal length, the size of the VP&#39;s light modulation device (e.g. a LCD panel) and the distance between the VP and the screen or display zone. Covering a large projection screen with a sufficient definition and brightness usually requires aggregating several VPs in a manner that they cover adjacent, partially overlapping zones of the total screen area. In the overlapping zones, blending ensures a smooth transition among different VPs so as to be tolerant against small displacements introduced e.g. by vibrations or thermal expansion. VPs are commonly equipped with zoom lenses (i.e. lenses with variable focal length) to provide the user some freedom when installing the VP, as for example for adapting to the distance between the VP and the screen. 
         [0008]    Usually a MP system includes a wired communication network to distribute video image data from a video source device to the VPs. The video image is distributed to the VPs through the wired communication network. Such a MP system is illustrated in US2008100805 which discloses an asynchronous, distributed, and calibrated apparatus providing a composite display from a plurality of plug-and-play projectors. The apparatus comprises a plurality of self-sufficient modules. Each module comprises a plug-and-play projector. A camera is coupled to each projector. A software or firmware controlled, computation and communication circuit is coupled to the projector and executes a single-program-multiple-data (SPMD) calibration algorithm that simultaneously runs on each self-sufficient module to generate a scalable and reconfigurable composite display without any need for user input or a central server. 
         [0009]    The drawback of such system is that it sometimes requires long and costly cables to connect the video source to each VP. For very long distances, some repeaters shall be added to guarantee correct signals shape at VP input connector. Also especially for outdoor installation, it may be difficult even impossible to install several cables from the video source to each projector. To mitigate the burden of cabling operation, one solution is to connect the video source to only one video projector and then to interconnect each VP through a daisy chain wired structure. This is illustrated in the U.S. Pat. No. 7,061,476 where an image to be displayed is transferred from the video source to a first VP, and then from first VP to a second VP and so on. Here again specific cabling is required, with potentially long cables if VPs are far from each other. 
         [0010]    From the above examples, it appears that wireless connectivity can solve some cabling issue in MP system. As the performance of wireless technology is improving in terms of throughput, it becomes conceivable to wirelessly interconnect VPs within a MP system, even for the transmission of uncompressed video. The advantage of transferring uncompressed video is to benefit from the highest quality as there is no compression, and to provide a very low latency system allowing interactivity with the user (for instance for simulation tool). Transferring uncompressed video requires large bandwidth, in the order of several Gbps (Giga bits per second), but it becomes achievable with most recent wireless technologies like 60 GHz millimeter wave operating in the 57-66 GHz unlicensed spectrum. 
         [0011]    60 GHz-based communication systems are widely studied (e.g. IEEE 802.11ad Task Group; IEEE 802.15.3c standard; Wireless HD; WiGig) and the research community proposes several solutions and methods to transport the audio and video applications with a desired quality of service. 
         [0012]    In a wireless communication system, connection setup and communication link bandwidth reservation are conducted before transmitting a video stream. Ideally, sufficient link bandwidth can be allocated and the video stream can be transmitted smoothly after stream set-up control. 
         [0013]    An approach of wireless VP is illustrated in the publication US20040217948, depicting a 60 GHz millimeter wave connection between a laptop and a single VP. Also the publication US20100045594 describes a system made of several displays connected to a video source and a control device through wireless connections. In this latter system, there are no wireless transmissions of video data between displays; video data are only transmitted by the video source. This solution is not adapted in the case of MP system, as some VPs may be not visible from the video source, or the quality of communications may be poor or subject to masking conditions. Indeed, video source is usually located at the ground level while VPs are typically hanging overhead or to a metallic structure so that there are no obstacles between VPs and the display screen. As a consequence the conditions of wireless communications between VPs are likely to be safe, at least with better robustness than the wireless communication between a VP and a video source. For this reason, it is advantageous to consider transmitting all video data from the video source to one of the VP of the MP system, and then to wirelessly distribute video data from this particular VP to the other VPs. 
         [0014]    There are various ways to implement the wireless function according to some dimensioning parameters or constraints. First of all, the video resolution defines the required bandwidth for data transmission. Let&#39;s consider a cluster of 4 VPs to display a 3840×2160 pixel uncompressed video source with 60 frames per second and 4:2:0 chrominance sub-sampling (i.e. average of 12 bits per pixel). Taking into account that a video image is split into 4 image sub-parts with, for instance, 20% of overlapping zones for blending function, the VP connected to the video source, also called the master VP, receives all video image sub-parts at a bit rate of 7.16 Gbps. Then, the master VP shall keep one image sub-part for display and it shall transmit the 3 other image sub-parts to the other projectors. Therefore, the minimum bandwidth required on the wireless medium is 5.37 Gbps (3×1.79 Gbps). Based on specifications of current emerging standards for 60 GHz millimeter wave domain, up to 4 radio channels can be simultaneously used with typical useful throughput of around 3.8 Gbps per radio channel (after demodulation and error correction code). For instance, such figure corresponds to the HRP mode 2 of IEEE 802.15.3c AV mode standard specifications. However this figure shall be lower to around 3.5 Gbps to take into account the overhead due to inter-frame gaps, transmission of preambles and MAC (Medium Access Control) headers. According to the above example, a way to transmit video image sub-parts from one master VP to 3 VPs would be to install three radio modules on master VP operating on 3 different radio channels, and to install one radio module on each other VP which supposes providing three independent point-to-point transmissions. However such a solution would appear costly. Indeed, it would be advantageous to deliver all projectors with identical hardware configuration, letting the user free to install VPs in a MP system or just to benefit from wireless connectivity in a single VP configuration. 
         [0015]    A further question is how to place the antenna of each radio module so that VPs can efficiently communicate in different MP system configuration (generally square or rectangular shape). Here, it may be proposed to use smart antennas that allow controlling the direction of antenna radiation pattern. The smart antennas are made of a network of radiating elements distributed in a matrix form on a support. These types of antennas allow the implementation of the technique known as “Beam Forming”. According to this technique, each radiating element of the antenna is electronically controlled in phase and power to obtain a directional transmission and/or directional reception beam. As this technique involves additional complexity for control and additional cost for the antenna itself, it is preferable to consider static antenna with a quasi omni-directional radiation pattern. In case of long distance between VPs, then smart antennas providing beam forming technique would be chosen but here again it is advantageous to limit their numbers for cost reason. 
         [0016]    A goal of certain aspects of the present invention is to provide a VP with limited hardware cost for the wireless communication means allowing efficient wireless video data distribution within a cluster of VPs forming a MP system. 
         [0017]    Another goal of aspects of the invention is to facilitate the set-up of MP system and make it more flexible. 
         [0018]    It is another goal of aspects of the invention to restrict the number of occupied radio channels as the number of available radio channels can be limited due to regulations restrictions or to the presence of interference. 
         [0019]    A still further goal aspects of is to efficiently and reliably control the image distribution and the image projection timing in a MP system. 
       SUMMARY OF THE INVENTION 
       [0020]    According to an aspect, the present invention provides a video projector including a first antenna located at a first side of the projector and a second antenna located at a second side of the projector, opposite to the first side, wherein the first antenna is configured to form a first radiation pattern that extends outwardly from the first side, the second antenna is configured to form a second radiation pattern that extends outwardly from the second side, and wherein the first and second radiation pattern do not interfere. 
         [0021]    Thanks to this position of antenna, a video projector in a multi-projection system can potentially communicate with all other adjacent video projectors. Only two antennas are used to provide this communications capability. For a single wireless video projector usage scenario, this arrangement provides more flexibility for the positioning of the video projector relative to a wireless video source (e.g. a laptop). 
         [0022]    In one embodiment each antenna defines a central beam axis and is configured to form a substantially hemi-elliptical radiation pattern centered about this axis. In another embodiment the first and second antennas are arranged with their central beam axes substantially anti-parallel. These embodiments allow limiting interferences between the patterns while enabling communication with adjacent video projectors. 
         [0023]    In a particular embodiment the central beam axes are arranged to be substantially perpendicular to an optical axis of the projector. This allows simplifying the installation of a MP system in particular in cases where the VP are located on a grid situated in front of a display zone. 
         [0024]    In one other embodiment, the central beam axes are arranged to be inclined with respect to a reference direction of the VP when in use. Advantageously, the reference direction is a reference line joining the antennas and extending from the first side to the second side. These particular features further limit interference risks between the patterns while still enabling communication with adjacent video projectors. 
         [0025]    In one further embodiment, the video projector includes a third antenna located at the first side adjacent to the first antenna and a fourth antenna located at the second side adjacent to the second antenna, each third and fourth antenna defining a central beam axis and being configured to form a substantially hemi-shaped radiation pattern centered about this axis, wherein the central beam axes of the third and fourth antennas are arranged to be inclined with respect to the reference direction of the VP when in use. These particular features confer to the VP more communication alternatives while avoiding interference with certain adjacent video projectors. 
         [0026]    Further, the inclination of the beam axis of the third and fourth antennas is opposite to the inclination of the beam axis of the first and second antennas with respect to the reference direction allowing to favor communications with certain adjacent video projectors while avoiding interference with others. 
         [0027]    Advantageously, the video projector further includes switching means enabling to switch from one antenna to another antenna for selecting the inclination of the beam axes. This feature simplifies setting a reliable communication network within a MP system by selecting the antennas communicating together. 
         [0028]    Alternatively, the antennas are steerable in order to control the inclination of the central beam axes with respect to the reference direction. 
         [0029]    In a particular embodiment, the video projector includes an electro-magnetic shield disposed inside the video projector such that the first and second radiation patterns do not interfere. The same radio channels can therefore be used to transmit data with both antennas. 
         [0030]    The shield can be disposed between the antennas. Advantageously, the shield is constituted by at least one component of the VP which can be constituted by existing electronics parts inside a video projector. 
         [0031]    The shield can be disposed adjacent to each antenna. The shield may contain electro-magnetic reflecting and/or absorbing materials. 
         [0032]    According to another aspect, the invention provides a wireless cluster of video projectors for the transmission of image data from video projector to video projector, wherein each video projector of the cluster is a video projector previously disclosed, the video projectors being arranged so that the first and second radiation pattern of a first video projector is configured to communicate with, respectively, at least one antenna of a second and one antenna of a third video projector. 
         [0033]    Thanks to this arrangement, the first video projector which receives video image sub-parts from the video source is able to simultaneously transmit the image data sub-parts to at least two other video projectors. This enables to aggregate bandwidth provided by two radio modules in order to support high video resolution as 4K2K. 
         [0034]    Advantageously, the video projectors are further arranged so that the radiation pattern of an antenna of the second video projector and the radiation pattern of an antenna of the third video projector reaches respectively first and second antennas of a fourth video projector. The fourth VP can than reconstruct the image to be displayed. Thanks to this arrangement, certain video projectors are able to relay image data sub-parts to other video projectors not directly reachable by the first video projector. 
         [0035]    In an embodiment, the video projectors are arranged on a grid and the central beam axes defined by the antennas are inclined relative to the grid, which limit interference risks between the patterns while still enabling communication with adjacent video projectors. The grid may be a regular rectangular array for example with VPs located at grid intersections. In this way, the beam axes may be inclined with respect to straight lines joining adjacent projectors of the cluster. 
         [0036]    In still another aspect, the invention provides a wireless transmission method for distributing image data within a cluster of video projectors from a first video projector to other video projectors of the cluster, the method including: 
         [0037]    dividing the image data into several image data sub-parts; 
         [0038]    assigning each image data sub-part to a video projector in accordance to a composite image to be displayed by the cluster; 
         [0039]    the method further including at the first video projector:
       receiving image data sub-parts;   extracting and storing the image data sub-part assigned to the first video projector; and   retransmitting through the first and second antenna image data sub-parts assigned to respectively the second and third video projectors.       
 
         [0043]    A video projector can thus act as a relay, receiving image data from one antenna associated to a radio module, keeping certain image data to display, and forwarding other image data for other video projectors through its antennas. 
         [0044]    In an embodiment the method further includes second and third video projectors:
       receiving through the first antenna the image data sub-parts transmitted by the first video projector,   extracting and storing the image data sub-part assigned to the respective video projector; and   retransmitting through the second antenna the image data sub-parts assigned to a fourth video projector.       
 
         [0048]    Thanks to this arrangement, video projectors are able to relay image data sub-parts to other video projectors not directly reachable by the first video projector. 
         [0049]    In a particular embodiment the second and a third video projectors receive image data subparts through a first radio channel and retransmit image data subparts on a second radio channel different from the first channel. This avoids interference between the concurrent wireless communications involving two radio modules of the video projector. 
         [0050]    In an embodiment the fourth video projector assemble the received image data sub-parts. The last video projector in the transmission chain may thus receive its image data sub-parts from different video projectors and merges data received from the different video projectors to recreate the image data to display. 
         [0051]    In an embodiment the method includes, before the step of receiving image data sub-parts, exchanging through the antennas control data relative to the image data sub-parts to be received. This allows each VP to efficiently and reliably control the image distribution and processing within the MP system. 
         [0052]    In a particular embodiment the image data are transmitted within frames according to a TDMA sequence, the first VP transmitting control data marking the beginning of the TDMA sequence (also called surperframe), the other VPs determining the beginning of each TDMA sequence according to the reception time of the frames. These features allow to efficiently control the image projection timing in the MP system. 
         [0053]    Advantageously, the first VP marks the beginning of the TDMA sequence by transmitting the first frame at the beginning of the TDMA sequence. To do so, the first frame may include a beacon signal. 
         [0054]    The invention is also directed to a non-transitory computer-readable medium storing a program and an information storage means readable by a computer or a microprocessor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]      FIG. 1  represents a MP system for displaying video according to an embodiment of the invention. 
           [0056]      FIG. 2  is a functional block diagram of a video projector according to an embodiment the invention. 
           [0057]      FIG. 3   a  shows an example of hardware architecture of a video projector according to an embodiment of the invention. 
           [0058]      FIG. 3   b  is a front view of the projector of  FIG. 3   a  representing the radiation patterns of antennas. 
           [0059]      FIG. 3   c  is a side view of the projector of  FIG. 3   a  representing the radiation pattern of one antenna. 
           [0060]      FIG. 4  shows an example of radiation patterns obtained when video projectors activate their wireless transmission means for video data transmission. 
           [0061]      FIG. 5  shows an example of the functional block diagram of a physical layer unit. 
           [0062]      FIG. 6  shows an example of the TDMA sequence for video data transmission. 
           [0063]      FIG. 7  shows an example of radiation patterns obtained when video projectors activate their wireless transmission means for control data transmission. 
           [0064]      FIG. 8  shows an example of the TDMA sequence for control data transmission in addition to video data transmission. 
           [0065]      FIG. 9  is a flow chart of the algorithm executed at the initialization of a video projector. 
           [0066]      FIG. 10  is a flow chart of the algorithm executed by MAC  238  for transmission and reception of MAC frames. 
           [0067]      FIG. 11  is a flow chart of the algorithm executed by the video processing unit  235  to manage transmission and reception of video data. 
           [0068]      FIG. 12  is a flow chart of the algorithm executed by the video processing unit  235  to manage video image display. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0069]      FIG. 1  shows an example of a MP system according to an example embodiment of the invention, composed of four VPs,  111 ,  112 ,  113  and  114  which are preferably identical regarding the hardware architecture. When the optical zoom lens of each VP is adjusted to a given focal length, each of these VPs illuminates a quadrilateral area, respectively  141 ,  142 ,  143  and  144  to form an entire image  140 , also called composite image, to be displayed. The four areas are arranged in two horizontal rows and two vertical columns and their border zones overlap. In a more general case, the number of VPs in the system can be less or more than four and the number of horizontal and vertical columns can be less or more than two. Preferably, the VP&#39;s are arranged in a regular grid. 
         [0070]    A video source device  105  generates the whole video source data to be displayed by the MP system. The video source device  105  may be a digital video camera, a hard-disk or solid-state drive, a digital video recorder, a personal computer, a set-top box, a video game console or similar. The video source device  105  is connected to or embeds control device  110  which is responsible for splitting the whole video image generated by the video source device  105  into sub-images  141 ,  142 ,  143  and  144 , applying blending process to the split sub-images, and transmitting them to at least one predefined VP. The control device  110  may be connected to a digital camera device (not represented) to capture the whole display with the four sub-images  141 ,  142 ,  143  and  144  for blending processing. Here, the predefined VP receiving all sub-images is VP  111 , called the master VP, through a link  121  between the master VP  111  and the control device. The link can be wired or wireless. The master VP  111  distributes the sub-images to others VP through wireless communication links described with respect to the following figures. 
         [0071]      FIG. 2  shows the functional block diagram of any VP represented in  FIG. 1 . A VP comprises: 
         [0072]    a main controller  201 , 
         [0073]    two physical layer units (denoted PHY A and PHY B)  211  and  212 , 
         [0074]    an optical system  239  including a projection lamp, 
         [0075]    a projection lens  240 . 
         [0076]    The main controller  201  is itself composed of: 
         [0077]    a Random Access Memory (denoted RAM)  233 , 
         [0078]    a Read-Only Memory (denoted ROM)  232 , 
         [0079]    a micro-controller or Central Processing Unit (denoted CPU)  231 , 
         [0080]    a user interface controller  234 , 
         [0081]    a medium access controller (denoted MAC)  238 , 
         [0082]    a video processing controller  235 , 
         [0083]    a video interface controller  236 , 
         [0084]    a video Random Access Memory (denoted video RAM)  237 . 
         [0085]    CPU  231 , MAC  238 , video processing controller  235 , user interface controller  234  exchange control information via a communication bus  244 , on which is also connected RAM  233 , and ROM  232 . CPU  231  controls the overall operation of the VP as it is capable of executing, from the memory RAM  233 , instructions pertaining to a computer program, once these instructions have been loaded from the memory ROM  232 . 
         [0086]    Thanks to the user interface controller  234 , the installer of the MP system can configure each VP. This interface can be a wired interface (like Ethernet, Universal Serial Bus USB) or a wireless interface (like infrared, WiFi). Through this interface, the installer can define the MP system configuration (number of VP per rows and columns) and it can assign the role of each VP (master/non master VP, image sub-part to display). The user&#39;s settings are stored in RAM memory  233 . 
         [0087]    The video processing controller  235  performs all necessary transformations of video data which are temporary stored in video RAM  237 . The operations performed by video processing controller  235  depend on the role of the VP. For a master VP connected to the source device  105  by wire, all video image sub-parts are received by the video processing controller  235  through the video interface controller  236 . For instance, it can be a HDMI receiver or a DisplayPort receiver. For a master VP wirelessly connected to the video source device  105 , the video processing controller  235  receives all video image sub-parts from the MAC  238 . For non-master VPs, video image sub-parts to display locally or to forward to another VP are received from the MAC  238 . For any VP, video processing controller  235  has to deliver the image sub-part to the optical system  239  in synchronization with all other VPs. Prior to this transfer, video processing controller  235  may have to apply some digital adaptation like a digital zoom, or an upscale to a higher video resolution. The optical system  239  will typically operate in the analogue domain, and will transmit the analogue signal to the projection lens  240 . Also, video processing controller  235  has to transmit the image sub-parts to be forwarded to other VPs. This forwarding operation is controlled by MAC  238  that will request new data to video processing controller  235  at transmission times on the wireless medium. 
         [0088]    MAC  238  controls the emission and reception of MAC frames conveying control data and video data. For data communications between VPs, the MAC  238  can use two physical layer units  211  and  212 . In case of a master VP wirelessly connected to the video source device  105 , then MAC  238  is connected to additional physical layer units (not represented on the  FIG. 2 ) and accessible through the specific interface  241 . Preferably, all the physical layer units are operating in the 60 GHz band. Useful throughput between MAC  238  and each physical layer unit is in the order of 3.5 Gbps. 
         [0089]    Each physical layer unit  211  and  212  comprises a modem, a radio module and antennas. The radio module is responsible for processing a signal output by the modem before it is sent out by means of the antenna. For example, the processing can be done by frequency transposition and power amplification processes. Conversely, the radio module is also responsible for processing a signal received by the antenna before it is provided to the modem. The modem is responsible for modulating and demodulating the digital data exchanged with the radio module. For instance, the modulation and demodulation scheme applied is of Orthogonal Frequency-Division Multiplexing (OFDM) type. In the preferred embodiment, antennas are quasi omni-directional antenna with static radiation pattern but the invention is not limited to this type of antenna. Smart antennas with configurable directional radiation pattern could be used instead, the antennas being steerable to a given direction. Typically, each physical layer unit embeds antenna for transmission and antenna for reception. 
         [0090]    MAC  238  acts as a synchronization control unit, which controls scheduling of transmissions via the network. It means that MAC  238  schedules the beginning and the end of an emission of radio frames over the medium, as well as the beginning and the end of a reception of frames from the medium. In the preferred embodiment, access to the medium is scheduled according to a TDMA (Time Division Multiple Access) scheme, where each transmission time slot is associated to only one VP. A single MAC frame is transmitted during each transmission slot. The set of MAC frames transmitted during one TDMA sequence is called a superframe. Typically, superframe duration is 20 ms and time slot duration is in the order of 200 μs. 
         [0091]    Among VPs, one is in charge of defining the beginning of each superframe cycle. For instance it can be the master VP  111  transmitting a first MAC frame at fixed periodic interval. This MAC frame is generally called a beacon frame marking the beginning of the superframe. Then, others VPs can determine the beginning of each superframe cycle according to the reception time of the beacon frame from the master VP. 
         [0092]    To synchronize video image display between each VP, a timestamping technique can be used. In the beacon frame transmitted at the beginning of each superframe cycle, the master VP  111  inserts its local time value. Therefore a VP receiving the beacon frame can adjust its local time in phase with the master VP  111 . Every non master VP can also forward beacon information to other VPs. Then each MAC frame conveying video data corresponding to the beginning of new video image, will include a timestamp value indicating at what time shall be displayed this new video image. Each non master VP will store image sub-part data it has to display until their local time reaches the timestamp value associated to the image sub-part. 
         [0093]      FIG. 3   a  shows an example of hardware architecture of a video projector according to an embodiment of the invention. This architecture is identical for VPs  111 ,  112 , 113  and  114 . Inside the casing  320  are represented the main elements of  FIG. 2 : the main controller  201 , the optical system  239  having a projection axis  340 , the projection lens  240  and the two physical layer units  211  and  212 . Each physical layer units embeds independent antennas. Due to the strong attenuation of 60 GHz millimeter wave signals crossing materials, it is necessary to locate the antennas close to the casing of the VP, in a place free from obstacle (e-g metallic parts) for the radio waves at the frequency of operation. The antennas are located close to the edge of the VP casing. Ideally they are placed on two opposite sides of the VP casing: top/bottom sides, or right/left sides  322 / 321  as in the example of  FIG. 3   a . This arrangement will enable communications with others VPs and it will help to avoid interference between antenna from PHY A  211  and PHY B  212 . Front/rear sides of the casing may not be convenient for locating antennas in a MP system where video projectors are arranged in a regular grid in square or rectangular manner. As shown in  FIG. 3   a , the optical system  239  acts as a shield isolating both PHY units electromagnetically from each other and thus avoiding interference. Alternatively or additionally, a dedicated shielding material can be installed inside the casing. The shield can be located between the PHY units or between the antennas. The shield can be disposed adjacent to each antenna. The shielding may contain electro-magnetic reflecting and/or absorbing materials. For signals such as 60 GHz, such material would include metal or carbon preferably in sheet or foam form. 
         [0094]      FIG. 3   b  is a front view of a projector showing the radiation patterns of the antennas. The projector  111  has two physical layer units  211  and  212 , which antennas are located near respective opposite sides  321  and  322 . The radiation pattern obtained with the antennas of physical layer unit  211  and  212  are respectively represented by the shapes  301  and  302 . The antennas radiate in the lateral directions outwardly from the casing on respective sides about central beam axes  311  and  312 . The antennas are arranged with their central beam axis anti-parallel, that is having substantially the same directions but opposite magnitudes. The antennas radiate substantially symmetrically about the central beam axis, resulting in a substantially hemi-spherical or hemi-elliptical radiation pattern. The antennas radiate essentially from the casing sides  321 , 322  in the direction of the central beams axes  311 , 312  and also up and down at a base  331 , 332  of the hemi-shaped pattern. The base of the pattern is substantially planar while its body has substantially an elliptical shape  341 , 342  so that the radiation pattern shape is close to hemi-elliptic. Each antenna can thus reach antennas from other VP situated on the grid of VP even if the VPs are not perfectly aligned on the grid. The radiations patterns are preferably symmetrical on both sides of the projector  111 . The central beam axes  311 , 312  are arranged to be perpendicular to the projection direction  340 . 
         [0095]      FIG. 3   c  is a side view of the projector showing the radiation pattern  302  of one antenna belonging to the physical layer unit  212 . From the point of view of  FIG. 3   c , the radiation shape is omni directional and centered about the central beam axis  312 . 
         [0096]      FIG. 4  shows an example of radiation patterns obtained when VPs  111 ,  112  and  113  activate their 60 GHz transmission means for video data transmission. The radiation pattern  401  is obtained when VP  111  is transmitting through its physical layer unit  211 . It enables communication with VP  113  receiving millimeter wave signals through its corresponding physical layer unit  211 . The radiation pattern  402  is obtained when VP  111  is transmitting through its physical layer unit  212 . It enables communication with VP  112  receiving through its physical layer unit  211 . It also enables communication with VP  114  receiving through its physical layer unit  211 . In the same way, the radiation pattern  403  is obtained when VP  113  is transmitting through its physical layer unit  212 , enabling communications with VPs  112  and  114 . The radiation pattern  404  is obtained when VP  112  is transmitting through its physical layer unit  212 , enabling communications with VP  114 . 
         [0097]    In the present embodiment one can observe that the central beam axes  311  and  312  are inclined with respect to horizontal line  410  when the VP is in use. Horizontal line is taken as a reference for the VP but any given direction of the VP could serve as reference line provided the reference line is taken in the plane containing the VP grid: for example a line joining the two physical layer units/antennas or a line crossing the two opposite sides of the casing would constitute suitable references lines. For VP  111 , the central beam axes  311  is inclined above horizontal line  410  enabling VP  111  to communicate with the physical layer unit  211  of VP  113 . The central beam axes  312  is inclined below horizontal line  410  enabling VP  111  to communicate with the physical layer unit  211  of VP  112  and  214  without interfering with VP  113 . 
         [0098]      FIG. 5  shows an example of the functional block diagram of a physical layer unit like  211 . As mentioned in the description of  FIG. 2 , it embeds a modem  501 , a radio module  502 , and in this particular example, two transmitting antenna  511  and  512  located at side  321  and arranged close to each other. The antennas produce identical radiation patterns but with different directions  311 , 312  with respect to a horizontal reference direction  410 . The physical layer also embed two receiving antenna  521  and  522  arranged in the same way and adjacent to antennas  511  and  512 . The inclination angle between the beam axis of the antennas and the reference direction may be adapted according to the system configuration (distance between VP, relative position of the VP). Angles comprised between 30° aid 60° would be well suited values. The inclination of the beam axis of the antennas  511  and  521  is opposite to the inclination of the beam axis of the antennas  512  and  522  with respect to the reference direction. 
         [0099]    The radio module is mainly in charge of up-converting and down-converting signals frequency. For transmission, it provides up-conversion from the low frequency of the modem to the high frequency (e.g. 60 GHz) of radio signals. For reception, it provides the reverse operation (down-conversion from high to low frequency). Inside radio module, both transmitting antenna  511 , 512  are fed from the same source signal through a power splitter (not represented). After the power splitter, a power amplifier is placed before each antenna: power amplifier  531  for antenna  511  and power amplifier  532  for antenna  512 . An on/off command constitutes a switching means on the power amplifier which enables to activate/disable the transmission on each antenna. The switching means enable to switch from one antenna to the other and thus to select the inclination angle of the beam axes. Command signal for antenna  511  is represented by the arrow  541 ; it is driven by the CPU  231  through the MAC  238  and the modem  501 . Similarly the arrow  542  is the command signal for the antenna  512 . The radiation patterns described in  FIG. 4  can be obtained with this circuit. For instance, the radiation pattern  401  is obtained by activating transmission antenna  511  of physical layer unit  211  and by disabling all others transmission antennas. Also, a switch  540  controlled by the signal  550  from CPU  231  enables to select receiving antenna  521  or receiving antenna  522 . Before going through the switch  540 , the radio signals are amplified by a low noise amplifier (LNA) referred  541  for the signal received on antenna  521 , and referred  542  for the signal received on antenna  522 . 
         [0100]    In another embodiment, one may use a single omni-directional transmitting antenna and a single omni-directional receiving antenna (as in  FIG. 3 ). In still another embodiment, one may use smart antennas with configurable directional radiation pattern, the antennas being steerable such that the central beam axis can be oriented to be inclined with respect to the reference direction of the VP. 
         [0101]      FIG. 6  shows an example of the TDMA sequence for video data transmission corresponding to the example of  FIG. 4 . This figure represents one superframe timeline (superframe cycle N), with all MAC frames transmitted by VPs  111 ,  112 ,  113 ,  114  (respectively VP1, VP2, VP3, VP4). For instance, duration of superframe cycle is around 20 ms while duration of MAC frame like  602  is around 200 μs and duration of MAC frame like  601  is around 100 μs. The master VP  111  transmits MAC frames like  602  conveying video data of image sub-parts  143 . This transmission is performed with the physical layer unit PHY A  211  and its antenna  511 , using a first 60 GHz radio channel. These MAC frames are received by VP  113  through its physical layer unit PHY A  211  and its antenna  522 . Such MAC frame is repeated 64 times (for example) within one superframe. Also the master VP  111  transmits MAC frames like  601  conveying video data of first half of image sub-part  144 . These transmissions are performed prior to MAC frames  602  with the same radio configuration. These MAC frames are also received by VP  113 , and they are also repeated 64 times (for example) within one superframe. 
         [0102]    In the same way the master VP  111  transmits MAC frames like  604  conveying video data of image sub-part  142 . This transmission is performed with its physical layer unit PHY B  212  and its antenna  512 , using the first 60 GHz radio channel. These MAC frames are received by VP  112  through its physical layer unit PHY A  211  and its antenna  521 . Such MAC frame is repeated 64 times (for example) within one superframe. Also the master VP  111  transmits MAC frames like  603  conveying video data of second half of image sub-part  144 . These transmissions are performed prior to MAC frames  602  with the same radio configuration. These MAC frames are also received by VP  112 , and they are also repeated 64 times (for example) within one superframe. 
         [0103]    Still in  FIG. 6 , it is shown that VP  113  relays first half of image sub-part  144  through MAC frames like  605  (forwarding of MAC frames like  603 ), and VP  112  relays second half of image sub-part  144  through MAC frames like  606  (forwarding of MAC frames like  601 ). For transmissions, VP  113  and VP  112  use their physical layer unit PHY B  212  with antenna  511 . VP  114  selects its physical layer unit PHY A  211  for reception of MAC frames from VP  113 , and selects its physical layer unit PHY B  212  for reception of MAC frames from VP  112 . Transmission of MAC frames from VP  113  or VP  112  are concurrent with transmission of MAC frames from VP1  111 . To avoid radio interference, VP  112  and VP  113  selects a radio channel different from the radio channel used by VP  111 . VP  112  and VP  113  can select the same radio channel as their transmission time slots are not concurrent. After reception of MAC frames from VP  112  and VP  113 , the video projector VP  114  can reassemble the image sub-part  144  to display. 
         [0104]      FIG. 7  shows an example of radiation patterns obtained when VPs  111 ,  112  and  113  activate their transmission means for transmission of control data. The radiation pattern  701  is obtained when VP  111  is transmitting through its physical layer unit  211 . It enables communication with VP  113  receiving millimeter wave signals through its physical layer unit  211 . The radiation pattern  703  is obtained when VP  113  is transmitting through its physical layer unit  212 . It enables communication with VP  114  receiving through its physical layer unit  211 . It also enables communication with VP  112  receiving through its physical layer unit  211 . In the same way, the radiation pattern  704  is obtained when VP  114  is transmitting through its physical layer unit  212 , enabling communications with VP  112 . The radiation pattern  702  is obtained when VP  112  is transmitting through its physical layer unit  212 , enabling communications with VP  111  and VP  113 . Similarly to  FIG. 4 , one can observe that the central beam axes  311  and  312  are inclined with respect to horizontal line  410  when the VP is in use. For VP  111 , the central beam axis  311  is inclined above horizontal line  410  while for VP  112  the central beam axes  311  is inclined below horizontal line, for VP  113  the central beam axis  312  is inclined above horizontal line and for VP  114  the central beam axis  312  is inclined below horizontal line. 
         [0105]      FIG. 8  shows an example of the TDMA sequence for control data transmission in addition to video data transmission corresponding to the example of  FIGS. 4 ,  6  and  7 . This figure includes the MAC frames for video transmission like  601 ,  602 ,  603 ,  604 ,  605 ,  606  described in  FIG. 6 . In addition, very short MAC frames  801  to  804  (in the order of 10 μs duration) convey control data transmitted by each VP. 
         [0106]    Master VP  111  transmits MAC frame  801  including control information like the current local time in master VP and like the TDMA sequence description to be followed by each VP. This transmission is performed with its physical layer unit PHY A  211  and its antenna  511 , using a first 60 GHz radio channel. This MAC frame is received by VP  113  through its physical layer unit PHY A  211  and its antenna  522 . At the same time (in this example), VP  113  transmits MAC frame  803  including its own control information and relaying control information form master VP received during the previous superframe. This transmission is performed with its physical layer unit PHY B  212  and its antenna  511 , using a second 60 GHz radio channel. This MAC frame is received by VP  114  through its physical layer unit PHY A  211  and its antenna  521 . 
         [0107]    In the same way, VP  114  transmits MAC frame  804  including its own control information and relaying control information form master VP  111  and VP  113  received during the previous superframe (through VP  113 ). This transmission is performed with its physical layer unit PHY B  212  and its antenna  512 , using the second 60 GHz radio channel. This MAC frame is received by VP  112  through its physical layer unit PHY B  212  and its antenna  521 . At the same time (in this example), VP  112  transmits MAC frame  802  including its own control information and relaying control information form master VP  111 , VP  113  and VP  114  received during the previous superframe (through VP  114 ). This transmission is performed with its physical layer unit PHY A  211  and its antenna  512 , using the first 60 GHz radio channel. This MAC frame is received by master VP  111  through its physical layer unit PHY B  212  and its antenna  522 . 
         [0108]      FIG. 9  is a flow chart of the algorithm executed by CPU  231  at the initialization of a video projector. 
         [0109]    In step  900 , the CPU  231  is receiving a new configuration from the user interface  234 . This configuration includes the description of the MP system to setup and the role of the video projector in this MP system (image data sub-part to display or to forward). 
         [0110]    In step  901 , the CPU  231  proceeds to the wireless network synchronization. For the master VP  111 , it consists in starting emitting a beacon frame at the beginning of each superframe cycle (on a first radio channel), and then waiting for the reception of MAC frame from a non-master VP. The beacon information includes the local time in master VP  111  at the beginning of the superframe cycle, the duration of time slots in the TDMA sequence, the allocation of time slots to each VP. In the example of  FIG. 1 , the master VP  111  is on the left bottom corner of the MP system, therefore the master VP transmits beacon frame as described in  FIGS. 7 and 8 . In the same way it waits from reception of MAC frame from VP  112  as described in the same figures. For a non-master VP, the network synchronization consists in waiting for the reception of MAC frame including beacon information (directly from the master VP or indirectly relayed by a non-master VP) in order to adjust local time. Once this is done, a non-master VP can transmit its own MAC frame during the time slot allocated by the master VP  111 . When transmitting a MAC frame with control information, a VP can confirm its current synchronization status. It can also relay similar information received from other VPs 
         [0111]    In case of failure to synchronize after a predefined time, a non-master VP may decide to change the radio channel used to receive MAC frames. 
         [0112]    When the master VP detects that all VPs are synchronized, it can move to the step  902  launching the video data transmission. It first checks if video data are received from the video source  105 . In the presence of video data, CPU  231  updates the beacon information to transmit indicating that video transmission is active. 
         [0113]    In step  903 , CPU  231  checks for detection of configuration change received from the user interface  234 . If change is confirmed, CPU  231  updates beacon information to notify the other VPs the deactivation of video transmission and it returns to step  900  for initialization. 
         [0114]      FIG. 10  is a flow chart of the algorithm executed by MAC  238  for transmission and reception of MAC frames. MAC  238  is configured by CPU  231  after initialization (step  900 ), after network synchronization (step  901 ) or after change request detection (step  903 ). 
         [0115]    In step  1000 , MAC  238  checks for the beginning of a new superframe. For a non-master VP not yet synchronized, a superframe cycle start is generated to enable the reception of a first MAC frame for synchronization. Otherwise the VP waits for the start of new superframe cycle indicated by its local time (the duration of a superframe cycle is fixed and predefined). 
         [0116]    In step  1005  the MAC  238  transmits settings to the physical layer units to configure them to perform the desired operation during the next time slot in the superframe (enable/disable, transmission/reception, selection of antenna, selection of radio channel). After the test of step  1015 , in case of transmission slot, the MAC  238  checks if the slot is for control data transmission. If yes, then in step  1025 , MAC  238  requests control data to transmit. It means that MAC  238  gets control data stored by CPU  238  in RAM  233 . Then MAC  238  triggers the emission of preamble in the selected physical layer unit and forwards these control data to the selected physical layer unit. These latter operations are performed at step  1035 . 
         [0117]    After the test of step  1020 , in case of video data to transmit, MAC  238  requests video data to the video processing unit  235  in step  1030 . The identification of image sub-part concerned and the amount of data to be transmitted during the time slot are included in the request. In response, the video processing unit  235  delivers data that are forwarded to the selected physical layer unit by MAC  238  (step  1035 ). For a master VP that simultaneously transmits on both physical layer units, two requests shall be transmitted to the video processing unit  235  and two concurrent transmissions will occur. 
         [0118]    During data transmission, MAC  238  can insert CRC (Cyclic Redundancy Check) at regular period delimiting blocks of data. This will be used at reception side to detect transmission errors at the granularity of block of data. 
         [0119]    After launching a transmission, MAC  238  checks if the time slot is also a reception slot by moving to step  1040 . Step  1040  is also executed in case of negative answer to the step  1015  (check for transmission slot). If a reception is not planned (check  1040  negative), it is checked at step  1060  if the time slot is the last time slot of the superframe. If not, the MAC  238  waits for the beginning of next time slot and goes back to step  1005  to configure the physical layer units according to the TDMA sequence description. In case of last time slot (check  1060  positive) the MAC  238  returns to step  1000  waiting for the next superframe cycle start. 
         [0120]    In case the time slot is a reception slot (positive check at step  1040 ), the MAC  238  launches the data reception at step  1045 . For control data, MAC  238  writes received data to RAM  233  (and notifies CPU  231 ), while for video data, MAC  238  sends received data to the video processing unit  235 . During reception MAC  238  can check CRC on each block of data. In case of errors on control data, MAC  238  notifies CPU  231 . In case of errors on video data, MAC  231  notifies the video processing unit  235 . A notification is also sent in case of missing data due to the absence of correct signals at the input of physical layer units. 
         [0121]    In case of reception time slot for control data, checked during step  1050  for a non-master VP, the MAC  238  updates the network synchronization status by adjusting its local time in order to keep in phase with the master VP. 
         [0122]      FIG. 11  is a flow chart of the algorithm executed by the video processing unit  235  to manage transmission and reception of video data. Video processing unit  235  is configured by CPU  231  after initialization (step  900 ) 
         [0123]    In step  1100  the video processing unit  235  checks if a transmission request is received from MAC  238 . If yes, then in step  1105 , the video processing unit  235  starts reading video RAM  233  according to the information contained in the request in particular the image data sub-part identification and the amount of data to transmit. For instance, video RAM  233  is divided into 4 zones, each zone being dedicated to one image sub-part. The fourth zone for image sub-part  144  is itself divided into 2 sub-zones for identifying the two halves of image sub-part  144 . The video processing unit  235  also reads its internal registers to know where the current read pointer is positioned to access the desired video RAM zone. It also reads its internal registers indicating the position in video RAM  233  of the beginning of next image. This will allow video processing unit  235  to detect if the beginning of a new image will be transmitted within the MAC frame. If yes, then the video processing unit shall read timestamp value associated to this new image. In step  1110  the timestamp value is transmitted to the MAC together with the position of the start of new image in the MAC frame. These two pieces of information are transmitted to the destination VP at the beginning of the MAC frame. In case no new image is present, the timestamp field of the MAC frame is left empty. For transmission to the MAC  238 , video data are read from video RAM  233  according to the current read pointer incremented after each word read. 
         [0124]    After launching the transmission, the video processing unit  235  checks if valid data are received in step  1115 . If not it returns to the step  1100  waiting for a new transmission request. If yes, the video processing unit  235  checks in step  1120  if video data are received from the wired video interface  236  or from the wireless interface through MAC  238 . Reception from wired video interface  236  only concerns the master video projector  111  connected by wire to the video source control device  110 . For reception from wireless interface, in step  1125 , the video processing unit  235  receives the timestamp information and the position of new image data in the MAC frame prior to the video data. In case of non-empty value, the timestamp is stored in internal register together with the pointer value of video RAM  233  where the beginning of new image will be stored. In next step  1135 , the video processing unit starts writing video data at the location indicated by the current write pointer (incremented at each word written in video RAM  233 ). At the beginning of the process, the VP waits for the reception of a first video image to store video data. 
         [0125]    In case of missing or erroneous received data notified by the MAC  238 , the video processing unit  235  may apply some concealment mechanism like replacing missing data by previous image data. 
         [0126]    After launching the reception operation, the video processing unit returns to step  1100  checking for new transmission request. 
         [0127]    In step  1120 , in case of master VP  111  receiving video data from the video interface  236 , the video processing unit  235  shall receive video data in step  1130  and it shall check for the beginning of a new video image. When it is the case, the video processing unit  235  computes the timestamp value corresponding to the time when the new image must be displayed. This timestamp corresponds to the local time plus a fixed predefined value covering the latency L for video transmission to all VPs within the MP system. Considering that the video source control device  110  delivers first image sub-parts  143  and  144  and then image sub-parts  141  and  142 , the master VP  111  stores full image sub-parts  143  and  144  and starts wireless transmission after receiving first lines of image sub-parts  141  and  142 . The timestamp value is computed when starting reception of image sub-part  143 , and this timestamp value will be associated to each image sub-part (for synchronized display by all VPs). Considering frame rate of 60 frames per second (16.66 ms between two images), and according to the transmission scheme described in  FIG. 8  (about 300 μs for wireless transmission), one can choose L=20 ms. 
         [0128]    In case the master VP  111  wirelessly receives the video image sub-parts, above timestamp computation operation is computed by video source control device  110 . The value L may thus be higher in order to take into account the latency for transmission from video source control device  110  to the master VP  111 . 
         [0129]      FIG. 12  is a flow chart of the algorithm executed by the video processing unit  235  to manage video image display. When a VP has received video data and one timestamp value is stored in its internal register, the video processing unit  235  checks in step  1205  if the local time is equal to the timestamp value. When it is the case, the video processing unit  235  reads video RAM  233  at the location indicated by the read pointer stored with the timestamp value. This operation is done at step  1210  where one data word is read and the read pointer is incremented. In step  1215 , video data is transmitted to the optical system  239 . This is repeated until the end of the image, condition checked at step  1220 . Then the video processing unit  235  can wait again in step  1205  until next timestamp value reaches by the local timer.