CONTEXT-AWARE DIRECTIONAL ANTENNA

FIELD OF THE INVENTION

The present invention relates to a context-aware antenna system, and in particular it relates to an antenna system for a vehicle the directional character of which var- ies depending on the context of said vehicle.

BACKGROUND OF THE INVENTION

Typically mobile communications networks which are under use in our days employ cell-like structures where one base station serves multiple mobile terminals and communications between individual mobile terminals is routed through the base stations. However, recently so-called ad-hoc networks have become under investi¬ gation.

In an ad-hoc network, mobile devices communicate with each other in a peer-to- peer fashion; they establish a self-organizing wireless network without the need for base stations or any other pre-existing network infra-structure. An outstanding fea¬ ture of this emerging technology is wireless multihop communication: If two devices cannot establish a direct wireless link (because they are too far away from each other), devices in between act as relays to forward the data from the source to the destination. In other words, each device acts as both a mobile terminal and a node of the network. In this way, ad-hoc networking creates a new paradigm for mobile communications, where networks can be established in a spontaneous manner ("on the fly") without any cost or effort of building up and maintaining a network infra- structure. One particularly interesting application of such ad-hoc networks is their use in communication between vehicles. Wireless ad hoc networking among vehicles gives rise to new telematic applications that increase the passengers' safety and comfort. A promising example is a decentralized accident warning system, which is implemented as a multihop network among cars. Basically, this system works as follows: A car involved in an accident sends automatically warning messages to nearby cars that are approaching the accident. Upon receiving such a message, a car displays in the panel a warning sign to the driver. The message is then for¬ warded, in a hop-by-hop manner, to further away cars. Such a system may help to avoid motorway pileups. The warning message may be triggered by a heavy brak¬ ing manoeuvre, an airbag ignition, or by the driver himself/herself who switches on his/her warning lights. For the transmission of the message, also oncoming cars can be used as relays. This example belongs to a larger set of distributed applica¬ tions for cooperative driver assistance and floating car data. In addition to accident warnings, cars can exchange information about traffic jams, bad physical road con¬ ditions, and gas stations. Other examples are person-to-person applications (e.g., text messaging, game communities) and city-wide communication networks formed by cars in an urban region.

In addition to the general desire of having a communication network between vehi¬ cles which has a high performance and good connectivity, it is furthermore desir¬ able to have a low end-to-end delay in such a multihop communications network to improve overall efficiency.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided an antenna sys¬ tem for a vehicle, said antenna system comprising an array of a plurality of antenna elements which can be controlled independently of each other, and an antenna control unit for controlling said plurality of antenna elements such that the directivity of the antenna beam of the antenna system varies adaptedly depending on one or more context parameters describing the context in which said vehicle and/or said driver are situated.

Adapting the directivity of the antenna beam depending on the context parameters which describe the context in which the vehicle and/or the driver are situated makes it possible to improve the efficiency of the individual mobile terminal connected to such an antenna system and thereby can improve the overall efficiency of a com¬ munications system comprising a plurality of such mobile terminals.

According to one embodiment the control unit comprises a context reception unit, a context interpretation unit, and an antenna driving unit. A context reception unit may receive context information from the outside, either from sensors of the vehicle or from a navigation system, the context interpretation unit interprets this information and the antenna driving unit then outputs the driving signals to the individual an- tenna elements.

According to one embodiment the control unit may further comprise a filter unit. The filter unit may check whether there are changes and/or significant changes com¬ pared to the previously received information. Only if there is a change at all in the received information or if the change is found to be significant the relevant informa¬ tion is forwarded to the context interpretation unit for interpretation and for comput¬ ing then the appropriate antenna control parameters.

According to one embodiment the control unit controls the phases and amplitudes of the individual elements such as to obtain the desired beam shape to thereby maximize the antenna efficiency according to the given circumstances in which the vehicle and/or the driver are situated.

According to one embodiment the antenna system switches between directional and omnidirectional modes depending on the environment. There are environments such as urban areas where the omnidirectional mode is preferable and outside ur- ban areas at the country side or in sparsely populated areas a directional mode can be preferable.

According to one embodiment the selected beam shape depends on the density of street crossings, on the curvature of the street, or on whether the vehicle is in an urban or a countryside area.

According to one embodiment the antenna system comprises a uniform linear array ULA. Such an array is particularly easy to control and has good directivity charac- teristics. However, other patterns of antenna elements can be imagined as well.

According to one embodiment there is provided a plurality of arrays, e.g. one array being mounted on the front side of the car and one being mounted on the rear side. In one embodiment the two arrays share the same control unit.

According to one embodiment reception and/or transmission diversity is employed by using a plurality of antenna elements and a certain diversity technique. This can enhance the signal to noise characteristics and may improve the processing capa¬ bilities of the overall system.

According to one particular embodiment the beam shape is selected according to the steering direction of the vehicle. This makes it possible to adapt the antenna beam according to the curvature of the street in order to obtain a maximum antenna gain at directions where most likely communications partners are located.

According to a further embodiment the shape of the antenna beam is selected and controlled based on the speed of the vehicle. For higher speeds this makes it pos¬ sible to chose a higher antenna gain thereby yielding a more far reach of the indi¬ vidual mobile terminal. This can reduce the end-to-end delay and is particularly suitable at environmental conditions where like at the countryside or on highways due to the speed of the vehicles the distance between individual vehicles is large and therefore a higher antenna gain is preferable. According to one further embodiment the shape of the antenna beam is based on the street width. This makes it possible to take into account that larger street widths mean that there is a higher likelihood of nearby communications partners which are not straight in the direction at which the vehicle is moving, and therefore this is par¬ ticularly suitable for urban or municipal areas.

According to one particular embodiment taperings can be used to improve or chose desired beam shapes. Appropriate taperings may include uniform tapering or Dolph-Tschebyscheff tapering.

According to one embodiment the shape of the beam depends on the curvature of the street, for example a very small curvature like in mountainous regions can mean that it is preferable to have a less directional beam and rather make it preferable to have an omnidirectional beam.

According to one embodiment the antenna system is adapted to receive signals from sensors mounted in the vehicle and/or a navigation system in the vehicle. This makes it possible to take into account the context parameters delivered by the context yielding components of the vehicle.

According to one embodiment there is provided a vehicle comprising an antenna system according to embodiments of the invention. With such a vehicle equipped with an antenna system according to embodiments of the invention a wireless multihop communications system of high efficiency can be obtained.

According to one embodiment there is provided a mobile terminal comprising an antenna system according to the embodiments of the invention. With such a mobile terminal when being located in a vehicle a multihop communications system of high efficiency can be obtained. According to one embodiment such a mobile terminal can be generated by making a contact between a normal terminal unit and an antenna unit according to an em¬ bodiment of the invention. The contact can be established by a docking station pro¬ vided in the vehicle, or it may be established in a wireless manner if the terminal comes close enough to a transceiver mounted in the vehicle being in electrical contact with the antenna unit and being configured to be able to establish a con¬ nection to the terminal.

According to one embodiment there is provided a communications system com- prising a plurality of mobile terminals according to embodiments of the invention.

According to one further embodiment the control unit comprises a filtering unit for filtering out those context parameters which have changed or which have signifi¬ cantly changed and those filtered context parameters are then forwarded to the context interpretation unit. Such a filtering process avoids the need to make an as¬ sessment of the context parameters where actually there has been no change.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a block diagram of an antenna system according to an embodi¬ ment of the present invention.

Fig. 2 illustrates an example of a uniform linear array ULA according to an em¬ bodiment of the present invention.

Fig. 3 shows a beam pattern radiated by an antenna system according to an embodiment of the present invention.

Fig. 4 shows schematically a vehicle having mounted thereupon an antenna system according to an embodiment of the present invention. Fig. 5 schematically illustrates a block diagram according to a further embodi¬ ment of the present invention.

Fig. 6 schematically illustrates a vehicle environment and a corresponding beam pattern according to an embodiment of the present invention.

DETAILED DESCRIPTION

In the following embodiments of the invention will be described by referring to the accompanying drawings.

A first embodiment of the present invention is now explained referring to Fig. 1. Fig. 1 shows a control unit 100 which receives context parameters 110. These context parameters are representative of or describe the present situation of the vehicle and/or its driver. The context parameters may for example be delivered by sensors mounted in the vehicle which measure such parameters as the steering direction of the vehicle, its speed, or its location. The location parameter may for example be delivered by a navigation system. These parameters then are fed into control unit 100. The control unit 100 evaluates and processes these parameters and it gener- ates output signals 120 which are then forwarded to antenna 130 and which drive the antenna 130 comprising individual antenna elements. Each of the individual antenna elements of antenna 130 receives its own individual and distinct antenna driving signal 120. Based on the combination of the thus generated and delivered driving signal 120 the antenna 130 radiates an antenna beam 140 which has a certain directional characteristic which adaptedly changes or varies depending on the context parameters 110. Based on the varying context parameters the control unit 100 adaptedly generates the control signals 120 and thus causes the antenna 130 to radiate the suitably shaped antenna beam 140.

According to one particular embodiment the antenna 130 is a uniform linear array ULA as shown in Fig. 2. As shown in Fig. 2, a ULA is an adjustment of m identical isotropic (or dipole) an¬ tenna elements that are arranged in a line and spaced with equal distance d. The distance d should be chosen to be half of the wavelength λ of the signal or smaller.

By adjusting the amplitudes and phases of the antenna elements of the ULA, the transmitted power is not distributed homogeneously in all directions of space but is concentrated into certain directions. This technique is commonly known as beamforming. Fig. 3 gives an example: it shows the radiation pattern of an ULA with eight antenna elements with d =0.5 λ in the x-y-plane. The beams with the highest gain are denoted as main beams. The direction of a main beam is called boresight direction Φ. In this example, we have a main beam with boresight direction Φ =100° and, as a result of the symmetrical arrangement of the antenna elements, a second main beam is always formed in the direction of Φ1 = - Φ. In fact, the array forms mir¬ ror image beams on each side of the axis of the array.

An introduction to adaptive antennas can be found in C. A. Balanis, Antenna The¬ ory: Analysis and Design. Wiley, 2 ed., 1997 and in P. H. Lehne and M. Pettersen, "An overview of smart antenna technology for mobile communication systems," IEEE Commun. Surveys, vol. 2, pp. 2-13, fourth quarter 1999.

The ULA is positioned on top of a vehicle in a such a way that the axis of the array is orthogonal to the driving direction as shown in Fig. 4.

Alternatively, one can use two antenna arrays of arbitrary type and position one on the front side of the vehicle, another one on the rear side of the vehicle. In one em¬ bodiment both antennas share the same control unit. While in one embodiment both arrays may radiate the same beam pattern in another embodiment the control unit may be configured such that the front and rear arrays radiate different beam patterns which are adapted to the particular needs at the front side and at the rear side of the vehicle. Since the environment and accordingly the context parameters may be different at the front and the rear side of the vehicle the beam patterns radi¬ ated by the front and the rear side arrays may hence be different. Fig. 5 schematically illustrates a further embodiment of the present invention. The basic operational principle of the embodiment of Fig. 5 is to adapt the radiation pattern to the current context of the vehicle and/or the driver. By adjusting the rela¬ tive phases and amplitudes of the antenna elements the control unit is able to con- trol the radiation pattern (beam shape) of the ULA. The phases and amplitudes are, in turn, controlled by the context parameters. The context parameters describe the current situation of the vehicle and/or the driver (e.g. the environment in which the vehicle is currently located, e.g. downtown/urban area or highway/countryside street), the drivers steering direction, and/or the width of the street. As can be seen from Fig. 5 these context parameters are fed to the antenna control unit 500. The antenna control unit then is responsible for interpreting these parameters and for outputting driving signals which then drive the antenna 510 by suitable phases and amplitudes.

Depending on the different driving situations (the "context" of the vehicle and/or the driver), different radiation patterns may be beneficial for the network, and accord¬ ingly the control unit 500 outputs suitable driving signals. By thereby adapting the radiation pattern it is possible to optimize the resulting network parameters, such as connectivity and end-to-end delay.

In the following the antenna system according to this embodiment will be described in somewhat more detail referring to Fig. 5. As mentioned already, the control unit 500 receives the context parameters. This may include location and environment information which may for example be delivered by the navigation system of the vehicle. This location information may then be classified, for example depending on the location of the vehicle the navigation system or the control unit may find out or determine that the present location falls into a certain category, such as urban area, countryside, or the like. Other context parameters may be delivered by sensors mounted in the vehicle, they may include for example the steering direction, the speed of the vehicle, or the street width which itself may for example again be de¬ livered by the navigation system. The control unit shown in Fig. 5 comprises a context reception unit 520 which forms the interface to the outside world of the sensors and the navigation system. It fur¬ ther comprises a filter unit 525 which obtains which a certain periodicity from the reception unit the context parameters. If there are changes and/or significant changes (e.g. beyond a certain threshold) compared to the previously received in¬ formation (e.g. a change in the environment), then the relevant information (e.g. the information which has changed) is forwarded to the context interpretation unit 530. The context interpretation unit then interprets this information and computes the appropriate antenna control parameters based on which a suitable control of the antenna can be made such as to yield an appropriate beam shape.

The antenna parameters are then forwarded to an antenna driving unit 540 which is responsible for generating the driving signals for the individual antenna elements of antenna 510. This may involve signal amplification and digital to analogue conver- sion for driving the individual antenna elements.

It should be mentioned here that the control unit 500 may be implemented for ex¬ ample by a digital signal processor or by any computer system capable of receiving input data to be processed, processing it and yielding appropriate output data.

The context interpretation unit may comprise a look-up-table in which correspon¬ dences between context parameters and corresponding antenna parameters are stored and which is referred to in order to obtain the appropriate antenna parame¬ ters.

Depending on the necessary output power a separate amplification stage may be inserted.

With the thus generated signals which have appropriate amplitudes and phases antenna 510 then is driven to generate the appropriate radiation pattern (antenna beam) 550 which then finally yields an improved network performance. In the following a further embodiment according to the present invention will be de¬ scribed. According to this embodiment the ULA can be operated in directional or omnidirectional mode. In the directional mode, the signal is transmitted on all an¬ tenna elements at the same time to achieve a directional radiation pattern. In the omnidirectional mode, e.g. only one antenna element is active for both sending and receiving, such that an omnidirectional radiation pattern results. According to this embodiment the system switches between directional and omnidirectional mode depending on the vehicle's environment.

If a vehicle is driving on a highway or countryside street, a rather narrow main beamwidth with a large gain in the direction of the street is used. Thus, the antenna array is used in directional mode. Since the ULA is orthogonal to the driving direc¬ tion and we consider boresight directions close to 90°, the main beams of the an¬ tenna are always directed in the direction of other vehicles on the same street in front of and behind the considered vehicle (see Fig. 3 and 6). Due to the fact that less transmission power is wasted in directions where it is unlikely to communicate with other vehicles - namely in directions orthogonal to the driving direction - more transmission power is emitted in the forward and backward direction. As a conse¬ quence, longer transmission ranges are achieved compared to omni-directional antennas (see Fig. 3). Longer transmission ranges in turn lead to better connec¬ tivity and to lower end-to-end delay, since less hops are necessary in average in a communication between two arbitrary vehicles.

If a vehicle is driving in an urban or downtown area, an omnidirectional radiation pattern is more beneficial because of the following reasons:

• The possibility to encounter other vehicles is no more limited to the forward and backward direction. Vehicles can be situated in almost every direction relative to the boresight direction Φ.

• The presence of scatterers such as buildings and other obstacles severely affect the propagation of radio signals in urban areas. It is therefore useful to transmit omni-directionally to allow other vehicles to combine more multipath components in order to "amplify" the received signal. Thus, optionally, one can use diversity tech¬ niques such as the following:

- Two or more antenna elements are used for reception, and then one combines the signals using a well-known reception diversity technique (e.g., maximum ratio combining), and/or

- two or more antenna elements are used for transmission in combination with a well-known sending diversity method (e.g., delay diversity, space time codes).

Ideally, in the last mentioned two techniques, antenna elements with maximum possible physical distance are used to reduce correlation between signals.

Thus, when being located in an urban or downtown context, the vehicle in one em¬ bodiment switches to omni-directional mode.

According to one particular embodiment one of the context parameters is the steering behaviour of the driver or the steering angle of the vehicle which can be measured by a suitable sensor. According to this embodiment the antenna beam is suitably chosen in a manner adapted to the steering behaviour.

By introducing a constant phase shift Δφ between neighboring antenna elements, the main beam can be steered into an arbitrary direction Φ according to

φ = arccos ( —λAφ —λ , K 2πd ) (1) where λ is the wavelength of the signal. The second main beam is then always formed in the direction Φ' = - Φ. Since λ and d are known, the needed phase shift Δφ for a particular boresight direction Φ can be pre-calculated easily and stored in a look-up table. According to this embodiment the control unit dynamically controls the current boresight direction Φ of the ULA according to the current steering direction of the vehicle. This is schematically illustrated in Fig. 6 where can be seen that the main lobe of the antenna beam reflects the curvature of the street with respect to the tangential straight line.

According to one further embodiment the beam width of the antenna beam is varied according to context parameters such as steering behaviour, speed, and street width. The width of the main beam (i.e. the boresight beam width) can be con- trolled by changing the amplitude weights of the individual antenna signals. This method of assigning different weights to individual antenna elements according to a certain pattern is also known in the art as "tapering".

There exist several techniques of amplitude excitations, so-called taperings, that can be chosen adapted to the desired antenna pattern. In the following we just give two examples of amplitude distributions that can be appropriately used in a vehicu¬ lar environment.

One particular example is the so-called uniform tapering which usually possesses the minimum beam width (i.e. the largest directivity and thereby simultaneously the largest gain for farthest reach compared to other amplitude distributions. This ta¬ pering is particularly suitable for environments where a large gain is desirable, for example in areas where the distance between individual vehicles is large such as at the countryside with mostly straight streets with almost no curvature and a large average speed of the vehicles.

Another example is Dolph-Tschebyscheff tapering which yields for a prescribed side beam level a minimum possible beam width and side beams of equal attenua¬ tion. However, this is only true for a so-called broadside array which is steered to Φ = 90°. For boresight directions close to 90° corresponding to a large curvature ra¬ dius one can assume that Dolph-Tschebyscheff tapering will provide appropriate results. Fig. 3 illustrates an example where a Dolph-Tschebyscheff tapering according to a side beam level 15 dB down from maximum gain is used.

The basic idea of the tapering for the purpose of directional control is to control the amplitudes and thus the current boresight beam width of the ULA according to the current steering direction and the speed of the vehicle as well as the estimated street width. In this embodiment the smaller the curvature radius, the lower the speed, and the broader the street, the broader the beam width should be. The in- formation about the street width thereby can be derived from the navigation system of the vehicle, for example by pre-classifying streets into certain categories as was previously already in a similar manner explained with respect to the classification of environmental areas.

The present invention has been described in the foregoing by making reference to exemplary embodiments. The skilled person will readily recognise that modifica¬ tions may be possible to these embodiments. For example, instead of a uniform linear array other arrays of dipoles or antenna elements may be used as well if the control unit controls the driving phases and amplitudes in an appropriate manner to adapt to the context of the vehicle. With the embodiments described hereinbefore an improved communications system can be obtained by adaptive optimisation of the network topology leading to improved network performance, i.e. network con¬ nectivity and lower end-to-end delay.