Abstract:
In order to provide a communication system as well as a vehicle comprising such a communication system for communication between and among vehicles and a method for communication between and among vehicles moving in any different directions within the same area by means of at least one channel designed for transmitting at least one message, the channel comprising at least one code for communication of the vehicles within at least one cluster in which at least one group of vehicles are clustered, wherein interference is to be eliminated, the vehicle comprises directional antennas oriented in different directions in relation to the moving direction of the vehicle to enable the sending direction of the message to be allocatable to directions in relation to the moving direction of the vehicle and the channel is assigned to direction areas, north, east, south and west.

Description:
The present invention relates to a communication system for as well as to a method of communication between and among vehicles by means of at least one channel designed for transmitting at least one message, the channel comprising at least one code for communication of the vehicles within at least one cluster in which at least one group of vehicles are clustered. The present invention further relates to a vehicle comprising such a communication system for communication between and among vehicles. 
     BACKGROUND 
     The use of inter-vehicular communication is an essential part of future smart cars and roads. For example, prior art document WO 01/01587 A2 discloses a dynamic wireless networking between vehicles, wherein each vehicle is capable of transmitting information and receiving information. 
     Generally, vehicle-to-vehicle messages can be exchanged in an ad hoc network by using a M[edium]AC[cess] protocol like IEEE802.11 that regulates the access to the shared communication medium without the usage of a central controller (cf. Ysuhiko Inoue, Masao Nagakawa, “MAC protocol for inter-vehicle communication network using spread spectrum technique”, Vehicle navigation &amp; information conference proceedings (IEEE), 1994). 
     Furthermore, in inter-vehicular communication, groups of vehicles can be clustered, for example by choosing a multicast ID in the cluster. In this context, prior art document U.S. Pat. No. 6,397,149 B1 discloses a processional travel control apparatus by which respective vehicle groups travelling in processions can be distinguished. 
     As long as the relative speed between the cars is low, the relative positional change of the cars is low and clustering is a not too difficult task. To establish and maintain a cluster, cars must not have a too large distance and must be also within this maximum distance during a longer period; otherwise, too much communication overhead will be required to keep the cluster up-to-date of the latest cluster configuration. 
     One of the main problems is that if two clusters come into contact they can interfere with each other. For this reason, the clusters have to be separated using different channels. To this aim, prior art document U.S. Pat. No. 6,397,149 B1 proposes to assign the frequency band according to the vehicle group ID. To achieve communication between vehicles in the same lane without interfering with communication between vehicles in other lanes, one of the frequencies is switched to a different frequency when the vehicle groups approach each other and when the frequency bands of the vehicle groups have been the same. 
     Since the number of available channels is directly bound to the cost of the communication system, this number of available channels has to be kept as low as possible, so a re-use of the channels is required. In this context, the problem arises that clusters travelling in opposite directions will come into contact with each other quite often whereby the probability of the clusters using the same channel can be quite high (depending on the number of channels available). 
     Another common problem occurring when IEEE802.11 compliant equipment is used outdoors is the so-called “hidden node problem”. This problem occurs when a wireless node cannot hear one or more of the other wireless nodes, as a consequence of which the media access protocol cannot function properly. When this happens, multiple nodes will attempt to transmit their data over the shared medium simultaneously, causing signal interference with one another and consequently collisions on data transmission. 
     The hidden node problem gets an extra dimension when a C[lear]T[o]S[end] signal from a receiver might be valid at the moment the CTS is given but a short time later a car approaching the cluster might interfere the data transfer without being aware of it. 
     One of the main difficulties in vehicle-to-vehicle networks, in particular in car-to-car networks, is that the configuration of the network changes rapidly due to the velocity of the individual nodes especially if the vehicles have different directions. In connection therewith or independently thereof, another problem is the rapidly changing density of the cars. 
     In this context, it has to be taken into consideration that power control mechanisms, collision avoidance techniques and synchronization normally need some time to adapt to a new situation. If the available time is not sufficient, these techniques will fail and the network performance can collapse. 
     A potential solution for these problems might be found by setting up a separate channel for each peer-to-peer connection and for each cluster communication. However, this would make the receiver of a node quite expensive to support all its active channels in parallel. Moreover, the number of frequencies would have to be very high, in case the channels are separated by frequency. 
     Concerning the above-mentioned M[edium]AC[cess] layer of a protocol for car-to-car communication purpose, a lot of work has already been done. The problem of medium accessing becomes of primary importance because the number of nodes and their mobility can vary within a very large range as well as on a very large scale. 
     Some conventional solutions have been proposed that consider
         T[ime]D[ivision]M[ultiple]A[ccess] (cf. Lechlan B. Michael, Masao Nakagawa, “Non-platoon inter-vehicle communication using multiple hops”, IEEE Trans. Commun. Vol. E82-B No. 10, October 1999) or   C[ode]D[ivision]M[ultiple]A[ccess] (cf. Ysuhiko Inoue, Masao Nagakawa, “MAC protocol for inter-vehicle communication network using spread spectrum technique”, Vehicle navigation &amp; information conference proceedings (IEEE), 1994).       

     In this context, CDMA seems to be the preferable solution because CDMA does not require any synchronization, which could be quite difficult to achieve in a decentralized and highly variable environment. However, in case the channels are separated by CDMA for setting up a separate channel for each peer-to-peer connection and each cluster communication the codes would have to be very long for differentiating the channels. 
     By using smart antenna arrays, it is possible to use S[pace]D[ivision]M[ultiple]A[ccess]. In this case, users may use the same frequency, time, or code allocations over the air interface and may only be separated spatially. This enables SDMA to be a complementary scheme to F[requency]D[ivision]M[ultiple]A[ccess], T[ime]D[ivision]M[ultiple]A[ccess], and C[ode]D[ivision]M[ultiple]A[ccess]; thus, S[pace]D[ivision]M[ultiple]A[ccess] provides increased capacity within congested areas (cf. 
     http://www.xilinx.com/publications/xcellonline/partners/xc_pdf/xc_nallatec h45.pdf). In this context, prior art document WO 02/41643 A2 discloses a method for operating a synchronous SDMA and a CDMA. 
     Anyway, cluster organization seems to bring significant advantages with respect to simple peer-to-peer connections. One of the main issues when considering clustering for car-to-car communication purpose is how to separate different clusters to avoid interference. Some work has already been done concerning ways to separate codes in car-to-car environment. 
     In this context, prior art document US 2002/0198632 A1 discloses a method and an arrangement for communicating between vehicles wherein a code depending on the area where the vehicles are located is assigned. This anyway seems to require a real large number of codes, and does not resolve the problem associated with the interference between clusters of cars travelling in opposite directions. 
     In the prior art article “Interference characteristics in inter-vehicle communication from oncoming vehicles” from Lachlan B. Michael and Masao Nakagawa, (Vehicular Technology Conference, Amsterdam, Netherlands, September 1999, volume 2, pages 753 to 757, ISBN 0-7803-5435-4), the use of directional or beam antennas, coupled with separating the frequency band into forward transmit/reverse receive operation, and reverse transmit/forward receive operation is proposed to solve the problems of interference from oncoming vehicles. 
     Since this known system is only based on forward and backward directions, it is feasible only to reduce interference between cars having opposite directions. Thus, the system presented in this article is not suitable when a vehicle is passing a cross-over or a car is entering a highway, because it is not possible to make a difference between cars having an other direction than forward or reverse; for example, this known system does not enable to make a difference between cars having west-to-east direction and cars having southwest-to-northeast direction. 
     Starting from the disadvantages and shortcomings as described above and taking the prior art as discussed into account, an object of the present invention is to provide a communication system as well as a vehicle comprising such a communication system and a communication method for vehicles moving in any different directions within the same area, wherein interference is to be eliminated. 
     The object of the present invention is achieved by a communication system comprising the features of claim  1  as well as by a method comprising the features of claim  5  and by a vehicle comprising the features of claim  11 . Advantageous embodiments and expedient improvements of the present invention are disclosed in the respective dependent claims. 
     The present invention is principally based on the idea of direction dependent channel selection for ad hoc wireless network of vehicles wherein a pre-clustering of at least one network can be defined. 
     BRIEF SUMMARY 
     According to the present invention, the problem associated with the interference between clusters of vehicles moving in different directions is solved by the communication system as described above characterized in that the channel is assigned to at least one direction area, for example north, east, south or west and that the sending direction of the message can be allocated to at least one of any directions in relation to the moving direction of the vehicle, for example forward, backward and/or sideward. 
     In further accordance to the present invention, the vehicle comprises at least one, preferably two or more directional antennas oriented in different directions in relation to the moving direction of the vehicle, for example forward, backward and/or sideward. Although sectorized antennas are a commercial product (see as an example http://www.mrcbroadcast.com/datasheets2/QuadSector_Rx.pdf) such antennas are not used for vehicle-to-vehicle communication yet. The use of directional antennas for vehicle-to-vehicle communication allows a direction selection for each message. 
     As a consequence, the system according to the present invention as well as the method according to the present invention are suitable when a vehicle is passing a cross-over or a car is entering a highway, because the present invention enables to make a difference between cars having an other direction than forward or reverse; for example, the present invention enables to make a difference between cars having west-to-east direction and cars having southwest-to-northeast direction. 
     Thus, in a preferred embodiment of the present invention every vehicle is equipped with directional antennas and favourably in each sent message a flag indicates if it was sent forward, backward, left or right from the car; in particular, a message sent by different antennas in different directions will contain flags to indicate its respective sending direction. 
     In an advantageous embodiment of the present invention, the code for communication of the vehicles within one cluster can be assigned depending on the vehicle direction, which leads to the additional advantage that the number of required codes is low. 
     To avoid too many channels, the communication system can be restricted to a couple of common channels. These common channels can be organized depending on the type of communication, for example
         awareness messages being sent by at least one broadcast channel,   emergency warning messages being sent by at least one broadcast channel,   messages within clusters being sent by at least one multicast channel, and   messages between pairs of cars being sent by at least one peer-to-peer communication channel.       

     Moreover, the vehicle optionally comprises at least one direction determining unit for determining the moving direction of the vehicle; by providing this optional feature, favourably every vehicle in the network is able to find out its direction. 
     Additionally the vehicle optionally comprises at least one selection unit for selecting the channel for communication, wherein the selection of the channel is optionally dependent on the moving direction of the vehicle. Thus, a preferable embodiment of the present invention is for example a vehicle-to-vehicle ad hoc wireless network in which the direction of the vehicle is taken into account for selection of the channel and preferably every vehicle can select one or more channels for communication. 
     Furthermore, the vehicle optionally comprises at least one transmitter unit for sending the message, in particular with identification means, with position, with speed, with time stamp and/or with direction of the vehicle, to other vehicles, wherein the transmitter unit can be part of the directional antenna. Thus, in a preferred embodiment of the present invention every vehicle in the network makes itself aware to the other vehicles in the neighbourhood by sending repetitive broadcast (alive) message with ID, with position, with speed, with time stamp and/or with direction. 
     In a further preferred embodiment of the present invention, a message to be transmitted from at least one first vehicle to at least one second vehicle being out of the communication range of the first vehicle is carried and/or forwarded by at least one vehicle (=so-called intermediate vehicle) being between the first vehicle and the second vehicle. 
     As already stated above, the present invention further relates to the method of communicating between and among vehicles by means of at least one channel. The sending direction of the message is allocated to at least one of any directions in relation to the moving direction of the vehicle, for example forward, backward and/or sideward, and the message is sent by at least one, preferably two or more directional antennas. 
     According to a preferred embodiment of the present invention, the available channel is assigned to a direction area for each type of communication. Furthermore, the moving direction of the vehicle can preferably be determined and the direction of the vehicle can be classified in for example north, north-west, west, south-west, etc. 
     Moreover, the channel can be selected wherein the selection of the channel is optionally dependent on the moving direction of the vehicle and the message is sent to other vehicles via the channel. 
     The channel separation is optionally accomplished by use of C[ode]D[ivision]M[ultiple]A[ccess], while the access within a channel is optionally controlled by C[arrier]S[ense]M[ultiple]A[ccess]-C[ollision]A[voidance]. 
     In order to avoid too many codes the number of codes being used optionally depends on the environment of the cluster, in particular on the road topology. For example if vehicles are moving on a lane, in particular on a road, without intersections only two codes will be used, for instance for the direction north and for the direction south. 
     In case vehicles are moving on a road with an X-Cross, at least four codes are optionally used; in case vehicles are moving on more complex topologies with several roads next to each other having different directions, exemplarily up to sixteen codes can be used. 
     Thus, in a preferred embodiment of the present invention depending on its direction the vehicle, in particular the selection unit of the vehicle, chooses only one channel or two channels within a communication type; for example, a vehicle having west-northwest as a direction in an area where eight codes (namely north, northeast, east, southeast, south, southwest, west, northwest) are used will use at the same time codes for two different directions: west and northwest. 
     In case of necessity, for example in a highly congested area, the code assignment can be re-adapted: if there are N codes available and the road mainly goes in one direction without many intersections with secondary streets, preferably only two directions, exemplarily the north direction versus the south direction, are differentiated, and preferably N/2 codes are assigned to each direction. In this way, also clusters that travel in the same direction and are very close to each other can be physically separated in an inventive manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       As already discussed above, there are several options to embody as well as to improve the teaching of the present invention in an advantageous manner. To this aim, reference is made to the claims dependent on claim  1  and on claim  5 ; further improvements, features and advantages of the present invention are explained below in more detail with reference to a preferred embodiment by way of example and to the accompanying drawings where 
         FIG. 1  shows schematically an embodiment of a communication system according to the present invention using the method according to the present invention; 
         FIG. 2  shows schematically an embodiment of an allocation of messages according to the communication system of  FIG. 1 ; 
         FIG. 3  shows schematically a first example of communication in the communication system of  FIG. 1 ; 
         FIG. 4  shows schematically a second example of communication in the communication system of  FIG. 1 ; 
         FIG. 5  shows schematically a third example of communication in the communication system of  FIG. 1 ; 
         FIG. 6  shows schematically a fourth example of communication in the communication system of  FIG. 1 ; 
         FIG. 7  shows schematically a fifth example of communication in the communication system of  FIG. 1 ; 
         FIG. 8  shows schematically a sixth example of communication in the communication system of  FIG. 1 ; 
         FIG. 9A  shows schematically a first example of application of inter-vehicular communication in the case of lane change or merge manoeuvre (Source: CarTalk project); 
         FIG. 9B  shows schematically a second example of application of inter-vehicular communication in the case of an accident ahead (Source: CarTalk project); 
         FIG. 9C  shows schematically a third example of application of inter-vehicular communication in the case of an invisible obstacle (Source: CarTalk project); and 
         FIG. 10  shows perspectively a fourth example of application of inter-vehicular communication in the case of a crossing or of an intersection (Source: US DoT: intelligent vehicle initiative). 
     
    
    
     The same reference numerals are used for corresponding parts in  FIG. 1  to  FIG. 10 . 
     DETAILED DESCRIPTION 
     In  FIG. 1 , an example of an arrangement for inter-vehicular communication or a communication system  100 , namely a car-to-car communication system, according to the present invention is depicted. 
     According to this communication system  100 , each car comprises
         a direction determining unit  50  for determining the moving direction M of the car and   a selection unit  60  for selecting a communication channel.       

     This selection unit  60  is connected
         with the direction determining unit  50 ,   with a first directional antenna  40 , a second directional antenna  42 , a third directional antenna  44  and a fourth directional antenna  46 , as well as   with a first transmitter unit  70 , a second transmitter unit  72 , a third transmitter unit  74  and a fourth transmitter unit  76 .       

     Moreover, each transmitter unit  70 ,  72 ,  74 ,  76  is in contact with the respective directional antenna  40 ,  42 ,  44 ,  46 , wherein
         the first directional antenna  40  is part of the first transmitter unit  70 ,   the second directional antenna  42  is part of the second transmitter unit  72 ,   the third directional antenna  44  is part of the third transmitter unit  74 , and   the fourth directional antenna  46  is part of the fourth transmitter unit  76 .       

     These directional antennas  40 ,  42 ,  44 ,  46  are designed for a direction selection of each message  30 ,  32 ,  34 ,  36  as depicted in  FIG. 2 . By means of the directional antennas  40 ,  42 ,  44 ,  46 , messages can be separated for example into
         first messages  30  being sent forward with respect to the car  10 ,   second messages  32  being sent to the right side of the car  10 ,   third messages  34  being sent backward with respect to the car  10  as well as   fourth messages  36  being sent to the left side of the car  10 .       

     In  FIG. 3  to  FIG. 8 , some example scenarios for communication according to the communication system  100  are shown, wherein cars are communicating on a limited number of channels to other cars. In this context, it may be noted that the system according to the article “Interference characteristics in inter-vehicle communication from oncoming vehicles” (Vehicular Technology Conference, Amsterdam, Netherlands, September 1999, volume 2, pages 753 to 757, ISBN 0-7803-5435-4) is not suitable for the applications as illustrated in  FIG. 3  to  FIG. 8 . 
     To reduce interference, a code for communication of the cars  10 ,  12 ,  14 ,  16 ,  18  is assigned in dependence on the respective moving direction of the cars  10 ,  12 ,  14 ,  16 ,  18  (cf.  FIG. 3 ). For example, on a bidirectional highway section having two lanes in west-to-east direction and two lanes in east-to-west direction, three cars, namely a first car  12 , a second car  14  and a third car  16 , having west-to-east direction are communicating in a cluster  20  using the specific code assigned to their west-to-east direction. 
     In the example depicted in  FIG. 3 , the car  12  belonging to the cluster  20  receives a first message  30  with west-to-east direction code from a car  10  which is entering the communication range of the cluster  20 . Since the car  10  is using the same code and is having the same direction as the cars  12 ,  14 ,  16  being part of the cluster  20 , cluster topology update algorithms can start. 
     At the same time, alive messages as well as warning messages  38  using east-to-west direction code will be ignored by the cluster  20 ; in other words, communication between cars having opposite directions will not interfere with each other; in particular, the message  30  having west-to-east direction code does not interfere with the message  38  having east-to-west direction code (cf.  FIG. 3 ). 
     Apart from that, the present invention also enables to distinguish between messages comprising a respective code for any sending direction in relation to the moving direction M (cf.  FIG. 1 ) of the respective vehicle. 
     To achieve this technical aim, according to the communication system  100  every car uses a frequency or a code which is depending on car direction; thus, messages sent by cars having for example west-to-east direction can be distinguished from messages sent by cars having any other direction, for example southwest-to-northeast direction. To avoid collisions, it is in particular in situations as shown in  FIG. 4  crucial to differentiate between messages sent into different directions. 
     In  FIG. 4 , a car  10  is entering a highway in southwest-to-northeast direction. The cars  12 ,  14  on the highway in west-to-east direction communicate with the entering car  10  (cf. for example message  30  from the car  10  to the next car  12 ), wherein the communication system  100  is able to differentiate
         the cars  12 ,  14  having west-to-east direction and communicating by way of west-to-east messages  38  from cars having southwest-to-northeast direction as well as   the cars  12  having already passed the entrance from the cars  14  having not yet passed the entrance.       

       FIG. 5  explains the behaviour of the communication system  100  in case of change in highway direction, for instance in case of a curve or of a turn. A car  10  is driving in west-to-east direction on the highway, but a little further the highway turns to a different direction, namely to south-to-north direction. As a consequence, the car  10  selects two different channels  90 ,  92 , for example a south-to-north channel  90  and a west-to-east channel  92 , in order to be able to communicate in parallel
         via the south-to-north code with cars  12  being clustered (=reference numeral  20 ) in front of the car  10  as well as   via the west-to-east code with cars  14  being clustered (=reference numeral  22 ) behind the car  10  (cf.  FIG. 5 ).       
     In  FIG. 6 , an example for enlarging the admissible communication range of the arrangement  100  for communicating is given. In this example, a warning message  34  is to be sent backwards by a car  10  having west-to-east direction but the distance between the car  10  and two following cars  12  is greater than the admissible communication range of the communication system  100 . 
     In this case, a car  18 
         being between the emitting car  10  and the potentially receiving cars  12  and   running on the opposite east-to-west direction   receives this west-to-east message  34  and is able to understand by means of the used code that the message  34  is valid and important for the cars  12 ; thus, the intermediate car  18  being close to the sending car  10  immediately forwards the message  34  to the receiving cars  12  via a further intermediate car  18  being close to these receiving cars  12 .       

     Alternatively, the message  34  to be transmitted can be carried by the (first) intermediate car  18  and can be forwarded to the cars  12  later in time. 
     By both of these ways, the warning message  34  can also reach the cars  12  which are out of the actual communication range of the communication arrangement  100  assigned to the car  10  (cf.  FIG. 6 ). 
     In  FIG. 7 , an application of the communication system  100  is shown where a car  10  is entering a highway in west-to-east direction: The driver of the car  10  wants to know if he can enter or if he has to wait until the road is free. In this case, the entering car  10  only accepts (, i.e. listens to) messages  30  transmitted by cars  12 ,  14  (corresponding to a first cluster  20 ) with west-to-east direction code, whereas messages  38  transmitted by cars  18  (corresponding to a second cluster  22 ) with east-to-west direction code are not important and therefore are not received and accepted by the entering car  10 . 
     The entering car  10  will receive and accept two different kinds of messages  30 ,  34  using west-to-east direction code, namely
         messages  30  from approaching cars  14 : if the entering car  10  receives this kind of message  30 , the car  10  will wait because the road is not free; and   messages  34  from cars  12  being already ahead: if the entering car  10  receives this kind of message  34 , the car  10  will ignore it because this message  34  contains flags indicating that this message  34  was sent backward.       

       FIG. 8  shows a traffic situation next to a cross-over where a number of cars are communicating. Because of the cross-over constellation, communication is only required
         between the vehicles  10  having west-to-east direction and the vehicles  12  having east-to-west direction and   between the vehicles  14  having south-to-north direction and the vehicles  16  having north-to-south direction, but   neither between the vehicles  10  and the vehicles  14 ,  16     nor between the vehicles  12  and the vehicles  14 ,  16 .       
     In  FIG. 8 , the reference numerals  80 ,  82 ,  84 ,  86 ,  88  indicate the codes (as used for communication) instead of the messages. To reduce the interference in the vicinity of the cross-over, the use of different codes for each direction, namely
         a first code  80  for west-to-east direction,   a second code  82  for east-to-west direction,   a third code  84  for south-to-north direction and   a fourth code  86  for north-to-south direction, combined with information provided by G[lobal]P[ositioning]S[ystem] and maps giving information on the road topology, enables the cars  10 ,  12 ,  14 ,  16  to select and filter the respectively relevant messages.       

     Since a message with a non-acceptable code  88  sent by the car  14  on the south-to-north road is neither important for the cars  10  on the west-to-east road nor for the cars  12  on the east-to-west road, such message is ignored by the cars  10 ,  12 . This results
         in a reduced interference from the cars  14 ,  16  in relation to the cars  10 ,  12  as well as   in a reduced interference from the cars  10 ,  12  in relation to the cars  14 ,  16         

     being in the same area but not on the same road. 
     The disclosure of the present invention relates in general to the field of car-to-car communication, in particular with the aim of accident-free driving. Thus, the communication system  100  according to the present invention can be used for cooperative interaction of cars and for distributing in particular warning messages, especially
         in order to avoid collisions during lane change or merge manoeuvres (cf.  FIG. 9A ),   for reporting an accident on the lanes used (cf.  FIG. 9B ), and   for reporting an “invisible” obstacle, for example an obscured or shadowed object (cf.  FIG. 9C ),       

     when vehicles are moving in different directions within the same area. 
     Likewise, car-to-car communication is considered crucial for intersection collision avoidance, in particular to avoid collisions when cars are entering an intersection that should be kept free for a fire truck (cf.  FIG. 10 ) or the like. 
     LIST OF REFERENCE NUMERALS 
     
         
           100  communication system or arrangement for inter-vehicular communicating 
           10  first vehicle, in particular first car 
           12  second vehicle, in particular second car 
           14  third vehicle, in particular third car 
           16  fourth vehicle, in particular fourth car 
           18  fifth vehicle,
       in particular car having opposite direction to the vehicles  10 ,  12 ,  14 ,  16  and/or   in particular intermediate car being between the first vehicle  10  and the second vehicle  12       
           20  cluster, in particular first cluster 
           22  second cluster 
           30  first message,
       in particular into forward direction with respect to the moving direction M     
           32  second message,
       in particular into right direction with respect to the moving direction M     
           34  third message,
       in particular into backward direction with respect to the moving direction M     
           36  fourth message,
       in particular into left direction with respect to the moving direction M     
           38  further message,
       in particular into diagonal direction with respect to the moving direction M     
           40  first directional antenna 
           42  second directional antenna 
           44  third directional antenna 
           46  fourth directional antenna 
           50  direction determining unit 
           60  selection unit 
           70  first transmitter unit 
           72  second transmitter unit 
           74  third transmitter unit 
           76  fourth transmitter unit 
           80  first code, in particular west-to-east code 
           82  second code, in particular east-to-west code 
           84  third code, in particular south-to-north code 
           86  fourth code, in particular north-to-south code 
           88  non-acceptable code 
           90  first channel, in particular south-to-north channel 
           92  second channel, in particular west-to-east channel 
         M moving direction of the vehicle  10