TECHNICAL FIELD

The invention relates to a communication system and a communication method, especially for performing a downlink communication from a base station to a mobile station.

BACKGROUND

In a world that is increasingly relying on wireless technologies, the need for secure communication through this medium is becoming paramount. Secure transmission is, however, significantly more challenging in wireless communications, where information is broadcast over the air and can be, in principle, received and recorded by everyone with a sufficiently good signal to noise plus interference ratio (SINR). Sometimes a malicious user (MU) might even have a better SINR than the intended receiver (IR) himself.

Message privacy is a fundamental aspect of secure communication and it is typically achieved by means of encryption, which ensures that no information about the encrypted message can be obtained without the appropriate decryption key. Although in theory there are many secure encryption schemes proposed in the cryptographic literature, practice shows that unexpected vulnerabilities, such as side-channels and implementation errors, lead to a breach of privacy. While researchers are trying to bridge the gap between theory and practice, other methods can be envisaged to enhance the security level of wireless communication.

Modern communication systems, such as LTE, LTE-A and 5G, adopt multi source and/or multiple antenna systems to focus the signal power in region closely located to the intended receiver (IR); this is usually called spatial diversity multiplexing (SDM[A]). This is usually achieved using a method known as beam forming, beam steering, or spatial precoding. Such features automatically protect the transmitted messages from being intercepted by MUs that are located far from the IR. However, there exists a dangerous zone (known as critical area) in which a MU can physically decode the message with high probability. Within this dangerous zone, a malicious user can receive messages intended for the intended receiver. Only cryptographic measures remain for protecting the messages. This leads to a sub-optimal security of the communication.

SUMMARY

Accordingly, an object of the present invention is to provide a communication system and a communication method, which allow for a very high communication security, especially in the downstream. The object is solved by the features of claim 1 for the communication system and by the features of claim 14 for the communication method. The features of claim 15 solve the problem for the associated computer program. The dependent claims contain further developments. According to a first aspect of the invention, a communication system comprising an interference unit, at least a first antenna unit and at least a second antenna unit, is provided. The communication system is adapted to transmit a payload signal to an intended receiver. The interference unit comprises an interference generator, adapted to generate at least one interference signal, and supply the at least one interference signal to the at least one first antenna unit. Moreover, the interference generator is adapted to generate the at least one interference signal so that interference is generated in an interference area around the intended receiver. The interference unit moreover comprises an interference remover, adapted to generate at least one interference removal signal, and supply the at least one interference removal signal to the at least one second antenna unit. The interference remover is adapted to generate the at least one interference removal signal, so that the interference generated by the at least one interference signal is cancelled out at the location of the intended receiver. Thereby, the signal-to-noise ratio for a malicious user within the interference area is significantly decreased, while it is not decreased for the intended receiver. This allows for physically preventing the malicious user from receiving the payload signal.

According to a first implementation form of the first aspect, the interference unit comprises a long-term channel quality indicator determiner, which is adapted to determine or receive a long-term channel quality indicator of a communication channel to users within the interference area. The interference generator is then adapted to generate at least one interference signal based upon the long-term channel quality indicator. It is thereby possible to accurately generate the interference signal and make sure that the interference is significantly increased within the interference area.

According to a further implementation form of the first aspect, the long-term channel quality indicator determiner comprises a look-up-table adapted to store long-term channel quality indicators of a plurality of locations. The long-term channel quality indicator determiner is then adapted to determine the long-term channel quality indicator of the interference area from the stored look-up-table. Alternatively, the communication system comprises a long- term channel quality indicator storage unit which is adapted to store long-term channel quality indicators of a plurality of locations. The long-term channel quality indicator determiner is then adapted to receive the long-term channel quality indicator of the interference area from the long-term channel quality indicator storage unit. It is thereby possible to accurately generate the interference signal based upon the long-term channel quality indicator.

According to a further implementation form of the first aspect, the interference unit comprises a short-term channel quality indicator determiner, which is adapted to determine or receive a short-term channel quality indicator of a communication channel to the intended receiver. The interference remover is adapted to generate the at least one interference removal signal based upon the short-term channel quality indicator. It is thereby possible to very accurately generate the interference removal signal and thereby very effectively remove the interference. This allows for the intended receiver to receive the payload signal without problems.

According to a further implementation form of the first aspect, the short-term channel quality indicator determiner is adapted to receive from the intended receiver, at least one pilot symbol, and determine the short-term channel quality indicator from the pilot symbol transmitted by the intended receiver, or wherein the short-term channel quality indicator determiner is adapted to transmit at least one pilot symbol to the intended receiver, and receive the short-term channel quality indicator from the intended receiver. It is thereby possible to very accurately determine the short-term channel quality indicator. According to a further implementation form of the first aspect, the communication system is adapted to determine or receive a location of the intended receiver. The interference generator is then adapted to generate the at least one interference signal based on the location of the intended receiver. The interference remover is then adapted to generate the at least one interference removal signal based on the location of the intended receiver. A further increase in accuracy of generating the interference signal and the interference removal signal is thereby achieved.

According to a further implementation form of the first aspect, the at least one first antenna unit is at least one single antenna or at least one antenna array. If the at least one first antenna is at least one antenna array, an individual interference signal is provided by the interference generator for each antenna of the antenna array. If the at least one first antenna unit is at least one single antenna, only a single interference signal is provided by the interference generator for the at least one antenna. Additionally or alternatively, the at least one second antenna unit is at least one single antenna or at least one antenna array. If the at least one second antenna unit is at least one antenna array, an individual interference removal signal is provided by the interference remover for each antenna of the antenna array. If the at least one second antenna unit is at least one single antenna, only a single interference removal signal is provided by the interference remover for the at least one antenna. It is thereby possible to accurately generate the interference in the interference area and remove it at the exact location of the intended receiver.

According to a further implementation form of the first aspect, the communication system comprises a first base station. The communication system moreover comprises a single first antenna unit and a single second antenna unit. The first antenna unit and the second antenna unit are both arranged within the first base station. This allows for a very simple construction of the communication system.

According to a further implementation form of the first aspect, the communication system comprises a first base station and a second base station. The at least one first antenna unit is arranged in the first base station, and the at least one second antenna unit is arranged in the second base station. It is thereby possible to increase the interference generated in the interference area while at the same time keeping the interference at the exact location of the intended receiver minimal. According to a further implementation form of the first aspect, the communication system comprises a first base station, a second base station, and a third base station. At least one of the at least one first antenna units is arranged in the first base station. At least one of the at least one second antenna units is arranged in the second base station. At least one of the at least one second antenna units is arranged in the third base station. This further increases the effectiveness of interference remover at the exact location of the intended receiver.

According to a further implementation form of the first aspect, the communication system comprises a first base station and a first relay unit. At least one of the at least one first antenna units is arranged in the first base station or in the first relay unit. At least one of the at least one second antenna units is arranged in the first relay unit or in the first base station. A relay unit requires a significantly reduced hardware effort with regard to a base station. This approach therefore allows for a significant reduction in system complexity with regard to using two separate base stations.

According to a further implementation form of the first aspect, the communication system is adapted to dynamically switch antennas and/or antenna arrays of different base stations or relay units to form the at least one first antenna unit and/or the at least one second antenna unit, based on a security need of the payload transmission and/or based on a short-term channel quality indicator and/or based on a long-term channel quality indicator. An overall increase in efficiency and security can thereby be achieved.

According to a further implementation form of the first aspect, the communication system comprises a payload transmitter, which is adapted to transmit the payload signal to the intended receiver. The payload transmitter is arranged in a base station or in a relay unit. Additionally or alternatively, the interference unit is arranged in a base station. It is thereby assured that with low hardware effort, the payload signal as well as the interference signal and also the interference removal signal are generated. According to a second aspect of the invention, a communication method for transmitting a payload signal to an intended receiver is provided. The method comprises generating, by an interference generator, at least one interference signal and supplying it to at least one first antenna unit for transmission. The at least one interference signal is generated so that interference is generated in an interference area around the intended receiver. Moreover, at least one interference removal signal is generated by an interference remover, and supplied to at least one second antenna unit for transmission. The at least one interference removal signal is generated so that the interference generated by the at least one interference signal is cancelled out at the location of the intended receiver. Thereby, the signal-to-noise ratio for a malicious user within the interference area is significantly decreased, while it is not decreased for the intended receiver. This allows for physically preventing the malicious user from receiving the payload signal.

According to a first implementation form of the second aspect, a long-term channel quality indicator is determined or received. The at least one interference signal is determined based upon the long-term channel quality indicator. It is thereby possible to very accurately generate the interference signal and make sure that the interference is significantly increased within the interference area.

According to a further implementation form of the second aspect, the long-term channel quality indicator is determined from a look-up-table or received from a storage. It is thereby possible to accurately generate the interference signal based upon the long-term channel quality indicator.

According to a further implementation form of the second aspect, a short-term channel quality indicator is determined or received. The interference removal signal is then determined based upon the short-term channel quality indicator. It is thereby possible to very accurately generate the interference removal signal and thereby very effectively remove the interference. This allows for the intended receiver to receive the payload signal without problems.

According to a further implementation form of the second implementation form of the second aspect, at least one pilot symbol is received from the intended receiver. The short- term channel quality indicator is determined from this pilot symbol. Alternatively, a pilot symbol is transmitted to the intended receiver. The short-term channel quality indicator is then received from the intended receiver. It is thereby possible to very accurately determine the short-term channel quality indicator.

According to a further implementation form of the second aspect, a location of the intended receiver is received. The at least one interference signal and the at least one interference removal signal are generated based upon the received location. A further increase in accuracy of generating the interference signal and the interference removal signal is thereby achieved.

According to a further implementation form of the second aspect, an individual interference signal is provided for each antenna of the first antenna unit, if the first antenna unit is an antenna array. Alternatively, only a single interference signal is provided to at least one antenna if the at least one first antenna unit is only at least one antenna. Additionally or alternatively, an individual interference removal signal is provided to each antenna of an antenna array, if the at least one second antenna unit is at least one antenna array. Alternatively, only a single interference removal signal is provided by the interference remover for at least one antenna, if the at least one second antenna unit is at least one antenna. It is thereby possible to very accurately generate the interference in the interference area and remove it at the exact location of the intended receiver. According to a further implementation form of the second aspect, a dynamic switching of antennas and/or antenna arrays of different base stations or relay units to form the at least one first antenna unit and/or the at least one second antenna unit is performed, based upon a security need of the payload transmission and/or based on a short-term channel quality indicator and/or based on a long-term channel quality indicator. An overall increase in efficiency and security can thereby be achieved.

Generally, it has to be noted that all arrangements, devices, elements, units and means and so forth described in the present application could be implemented by software or hardware elements or any kind of combination thereof. Furthermore, the devices may be processors or may comprise processors, wherein the functions of the elements, units and means described in the present applications may be implemented in one or more processors. All steps which are performed by the various entities described in the present application as well as the functionality described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if in the following description or specific embodiments, a specific functionality or step to be performed by a general entity is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respect of software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is in the following explained in detail in relation to embodiments of the invention in reference to the enclosed drawings, in which:

Fig. 1 shows an exemplary communication setup;

Fig. 2 shows a first embodiment of the communication system according to the first aspect of the invention;

Fig. 3 shows a second embodiment of the communication system according to the first aspect of the invention;

Fig. 4 shows a detail of a fourth embodiment of the communication system according to the first aspect of the invention;

Fig. 5 shows a detail of a fifth embodiment of the communication system according to the first aspect of the invention;

Fig. 6 shows a detail of a sixth embodiment of the communication system according to the first aspect of the invention;

Fig. 7 shows a detail of a seventh embodiment of the communication system according to the first aspect of the invention;

Fig. 8 shows a detail of an eighth embodiment of the communication system according to the first aspect of the invention;

Fig. 9 shows a detail of a ninth embodiment of the communication system according to the first aspect of the invention;

Fig. 10 shows a detail of a tenth embodiment of the communication system according to the first aspect of the invention; Fig. 11 shows an embodiment of the communication method according to the second aspect of the invention in a flow diagram;

Fig. 12 shows a signal strength within a critical area in an exemplary communication system;

Fig. 13 shows reception conditions within an interference area and at the location of the intended receiver in an eleventh embodiment of the communication system according to the first aspect of the invention, and

Fig. 14 shows reception conditions within an interference area and at a location of the intended receiver in a twelfth embodiment of the communication system according to the first aspect of the invention.

DESCRIPTION OF THE EMBODIMENTS

First, we demonstrate the general communication setup along Fig. 1. Along Fig. 2 - 10, different embodiments of the communication system according to the first aspect of the invention are described. With regard to Fig. 11, an embodiment of the communication method according to the second aspect of the invention is shown. With regard to Fig. 12 - 14, achievable results by use of the communication system according to the first aspect of the invention or the communication method according to the second aspect of the invention are shown. Similar entities and reference numbers in different figures have been partially omitted.

In Fig. 1, a general communication setup is shown. A base station (BS) 4 intends to transmit a payload signal to an intended receiver (IR) 2. A malicious user (MU) 3 is within the above- described critical zone and can physically receive the signal. The system can contain a plurality of each and all of these entities. The BS 4 wishes to communicate a private/secret message m to the IR 2 over a wireless channel, possibly using a subset of available relays, and MU 3 is an attacker that wants to eavesdrop on the communication. A MU 3 can be located anywhere, its location is unknown, it can be equipped with multiple antennas, and, as a worst case scenario, we assume that its received noise level is equal to zero. The approach involves two entities:

Entity 1 , also referred to as interference generator: It takes as an input, long term statistics of the channel towards the IR 2 and/or an approximate position of the IR 2. It generates (pseudo) random noise, also referred to as interference towards the IR 2. This will create a floor of noise around the zone close to the IR 2. Therefore, interference is generated in an interference area around the IR 2. This interference area advantageously overlaps the entire critical zone.

Entity 2, also referred to as interference remover: It takes as an input, the instantaneous channel gains, also referred to as short term channel quality indicators, and it uses it to cancel the effect of the Entity 1 exclusively at the location of the IR 2.

Consider a wireless system in which a BS 4 wishes to transmit a message m to an IR 2 avoiding one or more eavesdroppers or malicious users (MUs) 3, whose position is unknown to the BS 4. Denote as p = (x, y, z) an arbitrary position on the map containing the BS 4, the IR 2 and the MUs 3. Assume that a message is transmitted by a source with power ?T , bandwidth W and a symbol rate R, Denote also by ¾(P' ^r ) the power received by a receiver r at position V. A receiver in position p is, thus able to decode the message if and only if the received signal power is above a detection threshold, that is if

where °r represents the noise (thermal and otherwise) at the receiver r. The minimum value of «*r ² for which ( 1 ) is verified is called SINR threshold and it is referred to as Γ.

The SINR threshold grows exponentially with the symbol rate and decreases exponentially with the bandwidth, as follows:

R

Γ = 2w— l. ( 2 )

According to the Shannon-Hartley theorem, if a receiver has an SINR below the threshold it cannot decode the message with zero probability of error; no matter what decoding technique it adopts. Hence, Γ defines a decoding threshold. The value of ¾(P* ^r ) is known to depend on many factors such as distance from the transmitter, presence of objects between the transmitter and the receiver, fading, antenna gain. In general, the ratio between the transmit power ?τ and the received power is the so called link budget

Ργ

( 3 )

If the transmitter had beforehand the value of for each point in space and each receiver, it can establish where, and which receiver, is able to decode the message. Any receiver that receives the signal coming from the BS 4 at a level above the detection threshold, can detect and decode the symbols intended for the IR 2. This is true for the legitimate receiver 2, as well as an illegitimate receiver 3. Even worse, an illegitimate receiver 3 might invest considerable resources to improve its reception level. In order to make the communication between BS 4 and IR 2 (more) secure, we want make it as hard as possible for the MU 3 to achieve a reception level above the detection threshold.

An approach to increase the detection threshold for MUs 3 is shown here. In the most vulnerable spatial area (critical area; just around the IR 2), thus making it harder for the MUs 3 to detect the signal.

A communication system and communication method to protect message delivery in wireless networks by means of physically reducing the ability of a malicious user 3 to eavesdrop on the message, are provided. The approach consists of a transmission scheme that:

1) creates a shadow zone of artificial noise/interference around the intended receiver 2, and

2) cancels this disturbance only for the particular position of the intended receiver (IR) 2.

In Fig. 2, a first embodiment of the communication system 1 of the first aspect of the invention is shown. The communication system 1 comprises an interference unit 10, which is connected to a first antenna unit 12 and to a second antenna unit 13. The interference unit 10 comprises an interference generator, which generates at least one interference signal 15 and supplies it to the at least one first antenna unit 12. The interference signal is generated so that interference is generated in an interference area around an intended receiver 2. The interference unit 10 moreover comprises an interference remover, which generates at least one interference removal signal 16 and supplies it to the at least one second antenna unit 13. The interference remover generates the at least one interference removal signal, so that the interference generated by the at least one interference signal is cancelled out at the location of the intended receiver 2.

In Fig. 3, a more detailed embodiment of the first aspect of the invention is shown. Here, additionally a payload transmitter 11 and at least one third antenna unit 19 are depicted. The payload transmitter 11 generates the payload signal 17 and hands it to the at least one third antenna unit 19 for transmission towards the intended receiver 2. Moreover, in this embodiment, a malicious user 3 is depicted. In general, we want to:

1. Use long term channel properties and realizations along with (radio) wave and/or energy direction and/or focusing devices at one or more cooperative transmission points to promote additional noise in the proximity of one or more target devices.

2. Use short term channel properties and realizations to exempt target device from enhancement via SDMA techniques.

The transmission side is assumed to have the following information:

1. An approximate position of the IR 2.

In general, the second order statistics of the CSI directly relates to the spatial distribution of the direction(s) of arrival of the signal energy. This information can be easily extracted by knowing an array steering vector(s).

For the proposed embodiment, only the directions of arrival are actually sufficient, but other approaches might want to work with more precise location information.

Furthermore, the location of the IR 2 can be obtained in several different ways; here we provide a few examples:

a. Explicit signaling through the communication protocol: The IR 2 activates its GPS or other geo-location device and provides the BS with sufficiently precise localization information via a feedback protocol.

b. Implicit signaling: The BS 4 determines the IR 2 position by combining the signals received by different BSs and originated by the IR 2. Notice that since only an approximate position of the IR 2 is necessary this information can be slightly outdated of a few seconds. Implicit signaling also contains the direction estimation approach from before.

2. The instantaneous value of the channel gain between BS 4 and IR 2 (channel state information; CSI).

a. Explicit signaling through the communication protocol: CSI is obtained via pilot schemes or other methods.

Note that both 1) and 2) are standard information in 4G and 5G networks. In Fig. 4, a detail of a further embodiment of the communication system 1 of the first aspect of the invention is shown. Here, details of the interference unit 10 are shown. The interference unit comprises a long-term channel quality determiner 30, connected to an interference generator 31. Moreover, the interference unit 10 comprises a short-term channel quality determiner 33, connected to an interference remover 32.

In order to generate the at least one interference signal, a long-term channel quality indicator is determined or received by the long-term channel quality determiner 30. This long-term channel quality indicator is handed to the interference generator 31, which generates the interference signal based thereupon. Additionally, the interference generator 31 can generate the interference signal based upon the location of the intended receiver 2.

In order to generate the at least one interference removal signal, the short-term channel quality determiner 33 determines or receives a short-term channel quality indicator and hands it to the interference remover 32. The interference remover 32 generates the at least one interference removal signal based upon the short-term channel quality indicator. Additionally, the interference remover 32 can generate the interference removal signal based upon the exact location of the intended receiver 2.

In Fig. 5, a further detail of an embodiment of the communication system is shown. Here, the construction of the first antenna unit 11 is shown. The antenna unit 11 comprises a first antenna array 40, a second antenna array 41, a third antenna array 42 and a fourth antenna array 43. The antenna arrays 40 - 43 are each supplied with individual signals for each antenna of each antenna array. In an alternative embodiment, shown in Fig. 6, the first antenna unit 11 comprises individual antennas 50 - 53, which are each supplied with an individual signal.

It is important to note that each antenna unit can represent only a single antenna, or a number of antennas, as shown in Fig. 6. Also it is possible that one antenna unit represents one antenna array or a plurality of antenna arrays as shown in Fig. 5.

Especially, the interference generator 31 focuses noise/interference power to the IR 2 (directly on its position or simply in its direction), through any digital and analog methods (e.g., antenna beamsteering, SDMA, precoding, digital beamsteering, hybrid precoding).

Possible embodiments are: MIMO BS with precoding, LOS multi antenna transmitter with beam- steering, multi point transmission through relays, distributed precoding through coordinated multi point transmitters (D-MIMO). In a preferred embodiment, the interference generator 31 is implemented in a MIMO BS, adopting classic precoding strategies (MF, ZF, multi point coordination, etc.). In any case, the power of the transmission increases in the surrounding area 130 of the IR 2, as depicted in Figure 12. There the SNR and the respective threshold for the critical area are shown over a normal receiver' s noise floor. The interference remover 32 can be implemented in a multi antenna transmitter built from several realizations of Entity 1, e.g., a set of relays or a distributed set of transmitters. In order to cancel the interference at the IR 2, the interference remover 32 must be able to estimate the CSI of the IR 2. This is simple in MaMIMO networks, where the IR 2 transmits pilot symbols that can be received and then exploited to compute the channel quality indicator.

The two entities, the interference generator 31 and the interference remover 32 are not necessarily independent and can be realized in a communication system as follows:

- The available antennas of a communication system are divided into groups, where each antenna belongs to one and only one group. Each group represents one "Entity 1". The "one and only one" requirement might be abandoned for more advanced implementations, where additional signal processing becomes necessary. At least two groups are required.

The number of groups and antennas per group are dependent on the security requirements: More antennas per group generally allow for better artificial noise/interference cancelation at the IR 2. Better meaning that less residual interference is observed at the IR 2.

More groups, generally allow for better artificial noise/interference generation around the IR 2. Better meaning that less transmit power is required at the BSs and the interference area can be defined more accurately.

Having more antennas per group is generally more desirable than having many groups, i.e., it is often not necessary to have more than the minimum required two groups. Exceptions might be, that one group cannot create shadowing at the IR 2, due to shadowing (no physical transmission path because of obstruction) or other physical phenomena.

Each group creates a beam of interference in the direction of the IR 2, increasing the level of interference around it.

The width of such a beam and thus the dimension of the interference covered area is an important design parameter.

A larger area means less interference power (given constant transmit power).

The possible shapes of the beam generally depends on the number of antennas in the respective group. More antennas allow for more control over the beam shape.

Each group is now considered as a single antenna. The combination of which are the "Entity 2".

Entity 2 measures or calculates the short term channel properties between these "virtual antennas" and the IR 2.

In one embodiment, entity 2 calculates global projection weights for its "virtual antennas" that form a null space for the IR 2, given the short term channel properties. Such null space projection methods and how to calculate the associated weights are common knowledge (e.g. ZF, orthogonal projection matrix).

Other projections are possible, but will not keep the IR 2 completely interference free, while allowing for more artificial noise/interference. Hence, a tradeoff between security and performance can be achieved.

If noise/interference is now transmitted by entity 2 via the "virtual antennas", the IR 2 will remain interference free, while the critical area is improved.

All these steps are applicable using many types of multi-antenna arrays. The invention can be adopted in a number of multi-point transmission scenarios:

Single MIMO BS adopting any "signal focusing" strategy (MRT, MSE, etc.)

The BS 4 (or a set of BSs) takes care of creating both the interference in the surrounding of the IR 2 and to cancel the negative effect to the IR 2. Multiple Input multiple output (MIMO) refers to the fact that the BS 4 has multiple antennas, and it serves one or more IRs 2 with one or more antennas each. Note that this will happen with no extra cost from standard communication since the CSI acquisition is a standard feature of MIMO BSs. Such an approach is shown in Fig. 7. Here, within a single base station 60, the interference unit 10, a first antenna unit 12, a second antenna unit 13 and a third antenna unit 14 are located. For example, the first antenna unit 12 is used for transmitting the at least one interference signal, while the second antenna unit 13 and the third antenna unit 14 are used for transmitting the interference removal signal. The payload transmitter and an accordingly connected antenna have been omitted here for reasons of clarity.

In Fig. 8, an alternative embodiment of the communication system 1 is shown. Here, the interference unit 10 is not integrated into a base station. Here, the interference unit 10 is connected to a separate base station 70, which comprises a first antenna unit 12, a second antenna unit 13 and a third antenna unit 14.

The interference unit 10 generates the interference signal and the interference removal signal and hands them to the base station 70. The base station 70 then takes care of assigning its antenna units 12 - 14 independently to the different signals to be transmitted.

Multiple coordinated transmitter: D-MIMO

Several different transmitters are coordinated and behave as a single multi antenna transmitter. The CSI acquisition is a standard feature of such systems and can is used in the invention to cancel the noise at the IR.

Such an approach is shown in Fig. 9. Here, the communication system 1 comprises an interference unit 10, which is connected to a first base station 80, a second base station 81, and a third base station 82. Base station 80 comprises a first antenna unit 12, base station 81 comprises a second antenna unit 13, and base station 82 comprises a third antenna unit 14. The interference unit 10 generates the at least one interference signal and the at least one interference removal signal and assigns them to the individual base stations 80 - 82. For example, the interference unit 10 assigns the base station 80 to transmit the interference signal. The interference unit 10 then sends the interference signal to the base station 80, which transmits it using the first antenna unit 12. In this example, the interference unit 10 assigns the second base station 81 and the third base station 82 for transmitting the interference removal signal. The interference unit 10 then generates the at least one interference removal signal and hands the at least one interference removal signal to the base station 81 and 82, which transmit the signals using the first antenna unit 13 and the second antenna unit 14. It is important to note that each antenna unit/base station is supplied with signals dependent upon the specific location/orientation of the respective antenna units 12 - 14.

Multiple uncoordinated transmitters (Relay)

A set of relays transmit in a coordinated manner a message or a set of messages to the IR 2. In this case, other relays or other multi antenna BS needs to be coordinated to create the interference area.

Such an approach is shown in Fig. 10. Here, an interference unit 10 is connected to a base station 91, which in turn is connected to a first relay 90 and to a second relay 92. Within the first relay 90, a first antenna unit 12 is located. Within the base station 91, a second antenna unit 13 is located. Within the second relay 92, a third antenna unit 13 is located.

For example, the interference unit 10 generates the at least one interference signal and the at least one interference removal signal and hands them all to the base station 91. The base station 91 is in charge of dividing the signals between itself and the relays 90 and 92. For example, the base station 91 decides that the relays 90, 92 are in charge of generating the interference and therefore hands the interference signal to the relays 90, 92, which transmits these signals using the antenna units 12, 14. In this example, the base station 91 generates the interference removal signal itself and transmits it using its antenna unit 13.

It is important to note that within a communication system comprising a number of base stations and/or relays, it is also possible to dynamically assign different antennas or antenna arrays to transmit the different signals (interference signal, interference removal signal, payload signal) dependent upon different factors. On the one hand, the security need may be such a factor. The higher the security need, the higher the number of antennas used for transmitting the interference signal. This on the other hand reduces the reception quality of the intended receiver 2. Another factor can be the actual transmission conditions. If the intended receiver 2 even without the interference signal is already faced with harsh reception conditions and therefore a low signal-to-noise ratio, it might be prudent to assign especially many resources to generating the interference removal signal so as to not further hinder the reception by the intended receiver 2.

Therefore, also the long-term channel quality indicator and the short-term channel quality indicator can be used as factors influencing the dynamic allocation of resources.

Line of sight MIMO (LOS-MIMO)

In this scenario a multi antenna transmitter is in line-of-sight (LOS) with the IR 2. The transmitter consist of electronically steerable antenna arrays (e.g., uniform linear arrays; ULAs) and the arrays are tuned with the geometric information of the IR 2. All available antennas (i.e., all except the ones needed for signal transmission / entity 1) are used to create the interference in the interference area. This is done by using the ULAs to beam the output of a (single) interference source in the direction of the IR 2. By adjusting the weighting of the input of each ULA, it is possible to cancel the noise at one (or more) specific spatial points. In more mathematical detail:

The global amount of available antennas is divided into N different groups. Each group is used as an "array", i.e., as if it was a single antenna. Their relative channel gain 9n is henceforth estimated via pilot schemes. For example, the IR 2 transmit pilots that allow the multi antenna transmitter to estimate the channel gains. The antenna-arrays/groups transmit the output of a (single) noise source (possibly pseudo random sequences), focusing the power in the direction of the IR 2; this can be done by following standard techniques borrowed from phased arrays. Each antenna- array/group however, is also controlled by a parameter ^c k. These parameters are set in such a way that:

N

/ _t ^c n9n ⁼ 0- £=i That is, the signals transmitted by the antenna-arrays generating noise cancel in the exact position of the IR 2. If it is not possible to solve this equation system, then a solution that gets sufficiently close to zero, should be adopted. In Fig. 11, an embodiment of the communication method according to the second aspect of the invention is shown. In a first step 100, at least one interference signal is generated. It is supplied to at least one first antenna unit in a second step 101. In a third step 102 the at least one interference signal is transmitted by the at least one antenna unit, in order to generate interference in an interference area around an intended receiver. In a fourth step 103, at least one interference removal signal is generated. It is supplied to at least one second antenna unit in a fifth step 104. In a sixth step 105, the at least one interference removal signal is transmitted by the at least one second antenna unit in order to cancel out interference generated by the interference signal at the location of the intended receiver. In Fig. 12, a critical area 130 around a location of an intended receiver 2 is depicted. In the diagram, the reception conditions are shown. When the reception conditions are better than the detection threshold, it is possible to detect the signal. This is indicated by the critical area 130. In Fig. 13, the interference level at a location of an intended receiver 2, when using a communication system according to the first aspect of the invention, is shown. Within an interference area 9, a high interference level is achieved, in order to prevent malicious users 3 from receiving the payload signal. At the exact location 140 of the intended receiver, a very low interference level is generated by cancelling out the interference signal using the interference removal signal. Therefore, only at the exact location of the intended receiver 2, it is possible to detect the payload signal, since only there the interference level is low enough, so that the payload signal is stronger than the detection threshold.

In Fig. 14, a further diagram showing the reception conditions at different locations is shown. Here, the interference signal is generated from two different locations, visible as bright spots in the diagram. Around a location 140 of an intended receiver, an interference area 9 is depicted. Within the interference area 9, the reception conditions are artificially bad so that the payload signal is lower than the detection threshold. Only at the exact location 140 of the intended receiver, the interference removal signal cancels out the interference and enables the payload signal to emerge over the detection threshold. The invention is not limited to the examples and especially not to a specific number of antennas or antenna arrays or base stations or relays. Also, there is no limitation to specific communication standards, by which the communication system communicates with the intended receiver. The characteristics of the exemplary embodiments can be used in any combination.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising " does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in usually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless communication systems.