WO2017063696A1

WO2017063696A1 - A full-duplex communication apparatus and a method of operating a full-duplex communication apparatus

Info

Publication number: WO2017063696A1
Application number: PCT/EP2015/073840
Authority: WO
Inventors: Italo ATZENI; Marco MASO; Marios Kountouris
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2015-10-15
Filing date: 2015-10-15
Publication date: 2017-04-20
Also published as: CN108141343A

Abstract

The invention relates to a communication apparatus (100) capable of full-duplex communication with another communication apparatus (110a-c) via a communication channel. The communication apparatus (100) comprises at least two transmitter antennas (101) and at least one receiver antenna (103), wherein the at least two transmitter antennas (101) are associated with a transmitter filter and the at least one receiver antenna (103) is associated with a receiver filter and wherein the at least two transmitter antennas (101) and the at least one receiver antenna (103) define a self-interference channel, and a determiner (105) configured to determine the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the determiner (105) is configured to determine the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self- interference threshold value.

Description

A FULL-DUPLEX COMMUNICATION APPARATUS AND A METHOD OF OPERATING A FULL-DUPLEX COMMUNICATION APPARATUS

TECHNICAL FIELD

Generally, the present invention relates to wireless communications. More specifically, the present invention relates to a multi-antenna communication apparatus capable of full- duplex communication as well as a method of operating such a communication apparatus. BACKGROUND ln-band full-duplex (FD) radio devices, i.e., devices that are able to simultaneously transmit and receive signals on the same frequency band, can theoretically double the throughput of the transmission with respect to their half-duplex (HD) counterparts. In practice, both the bandwidth footprint, e.g., with respect to a frequency-division duplex (FDD) approach, and the number of channel uses, e.g., with respect to a time-division duplex approach (TDD), would be significantly reduced as compared to HD operations.

A critical issue generally hinders the effective implementation and adoption of FD radio devices in practical systems: physical limitations do not allow a perfect isolation between the transmit (TX) and receive (RX) antennas of the FD radio. As a result, strong self- interference (SI) appears in the RX chain, with a consequent reduction of the signal to interference plus noise ratio (SINR) of the received signals. As a consequence the transmitter (TX) is strongly limited in terms of transmission (TX) power, which cannot exceed a certain threshold, in order to protect the incoming signals.

State-of-the-art solutions enable SI cancellation (SIC) techniques to be employed at the RX chain of the FD device. These SIC techniques are meant to reduce the intensity of the SI at least to the noise floor level and increase the maximum allowable TX power for the FD device. However, if one considers the very low noise floor of current mobile devices, and the limited SIC capabilities of state-of-the-art FD devices, one will appreciate that an implicit upper bound on TX power still exists in the state-of-the-art devices. As a consequence, the adoption of FD radios for outdoor scenarios, where the TX power can range from few to tens of Watts, does not seem possible without sacrificing their performance. Many solutions have been proposed in the literature to cancel the SI appearing in the RX chain and approach the theoretical throughput of FD communications. These SIC techniques for FD radios can be divided into two main categories: analog and digital cancellation. Perfect SIC is achieved whenever the SI is reduced to the same level as the noise floor, and thus is feasible only if the gap between the TX power and the noise floor of the device is below a certain value. A remarkable result in this respect (i.e., up to 1 10 dB of SIC over a bandwidth of 80 MHz) is demonstrated in D. Wu and R. Negi, "Downlink scheduling in a cellular network for quality-of-service assurance," IEEE_J_VT, vol. 53, no. 5, pp. 1547-1557, Sept. 2004 and in D. Bharadia, E. McMilin and S. Katti, "Full Duplex Radios," in Proc. ACM Special Interest Group on Data Commun. (SIGCOMM), Hong Kong, China, 2013, where a hybrid analog/digital cancellation algorithm is proposed for both single- and multiple-antenna settings. However, despite its impressive performance, the full effectiveness of this solution can be guaranteed only if the TX power of the FD radio is smaller than a given threshold. This can be guaranteed by employing spatial SIC.

In this regard, a possible approach is to design the TX filter in order to apply full zero- forcing processing to the SI channel, which allows to null the self-interference entirely in case of perfect CSI, as has been proposed in T. Riihonen, S. Werner and R. Wichman, "Spatial loop interference suppression in full-duplex MIMO relays," in

I E E E_C_AS I LO M A R, 2009, T. Riihonen, S. Werner and R. Wichman, "Residual self- interference in full-duplex MIMO relays after null-space projection and cancellation," in I E E E_C_AS I LO M A R, 2010 and T. Riihonen, S. Werner and R. Wichman, "Mitigation of Loopback Self-Interference in Full-Duplex MIMO Relays," IEEE_J_SP, vol. 59, no. 12, pp. 5983-5993, Dec. 201 1 . However, such a solution may prove excessively aggressive since the RX chain can tolerate the SI to be below a certain (positive) threshold. Therefore, it is not necessary to null the SI completely; instead, by allowing the SI to be non-zero, it is possible to convey more power towards the desired link. This problem is formulated in J. Zhang, O. T. Motlagh, J. Luo and M. Haardt, "Full duplex wireless communications with partial interference cancellation," in IEEE_C_ASILOMAR, 2012, which also proposes a search algorithm that leverages on well-known convex optimization techniques. However, such scheme may not be adequately efficient to compute the optimal TX filter within the coherence time allowed by the fluctuations of the wireless channel.

Thus, there is still a need for an improved communication apparatus capable of full-duplex communication as well as an improved method of operating such a full-duplex

communication apparatus. SUMMARY

It is an object of the invention to provide an improved communication apparatus capable of full-duplex communication as well as an improved method of operating such a fu II- duplex communication apparatus.

The foregoing and other objects are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, the invention relates to a communication apparatus capable of full-duplex communication with another communication apparatus via a communication channel. The communication apparatus comprises at least two transmitter antennas and at least one receiver antenna, wherein the at least two transmitter antennas are associated with a transmitter filter and the at least one receiver antenna is associated with a receiver filter and wherein the at least two transmitter antennas and the at least one receiver antenna define a self-interference channel, and a determiner configured to determine the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the determiner is configured to determine the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self- interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value.

In a first possible implementation form of the communication apparatus according to the first aspect of the invention as such, the determiner is configured to determine the transmitter filter on the basis of the weight factor a on the basis of the following equation:

wherein w denotes the transmitter filter, / denotes the identity matrix, h denotes the channel state information of the self-interference channel, v denotes the receiver filter, denotes the channel state information of the communication channel, x^H denotes the Hermitian transpose of x and x^# denotes the Moore-Penrose pseudoinverse of x. In a second possible implementation form of the communication apparatus according to the first implementation form of the first aspect, the determiner is configured to determine a preferred value a^* of the weight factor a by determining the transmitter filter w such that the following equation is satisfied: ε = \v^Hhw\², wherein ε denotes the predefined self-interference threshold value, v denotes the receiver filter, h denotes the channel state information of the self-interference channel and x^H denotes the Hermitian transpose of x.

In a third possible implementation form of the communication apparatus according to the second implementation form of the second aspect of the invention, the determiner is configured to determine the preferred value a^* of the weight factor a on the basis of the following equation: a^* = 1 - mm (1, J j-— ), wherein the determiner is configured to determine β and γ on the basis of the following equations: β (1 - s(v^Hhh^Hv)-¹)h^h^Hvv^Hhh_i, and 7 h^(h^Hvv^Hh - sl)hi, wherein ε denotes the predefined self-interference threshold value.

In a fourth possible implementation form of the communication apparatus according to the first aspect as such or any one of the first to third implementation form thereof, the predefined self-interference threshold value imposes an upper bound on the transmission power of the communication apparatus. In an implementation form, the predefined self- interference threshold value can be determined by the self-interference cancellation capability of the communication apparatus. In a fifth possible implementation form of the communication apparatus according to the fourth implementation form of the first aspect of the invention, the predefined self- interference threshold value imposes an upper bound on the transmission power of the communication apparatus on the basis of the following equation: ε≥ \v^Hhw\², wherein ε denotes the predefined self-interference threshold value, v denotes the receiver filter, h denotes the channel state information of the self-interference channel, w denotes the transmitter filter and x^H denotes the Hermitian transpose of x.

In a sixth possible implementation form of the communication apparatus according to the first aspect as such or any one of the first to fifth implementation form thereof, the communication apparatus is further configured to communicate with a plurality of other communication apparatuses, wherein each other communication apparatus defines a respective communication channel between the communication apparatus and the other communication apparatus associated with a respective channel state information h_t and wherein the communication apparatus further comprises a selector configured to select one of the plurality of other communication apparatuses for communicating with the communication apparatus on the basis of a measure of the transmission power associated with a respective communication channel depending on the respective channel state information h_t and the transmitter filter w determined by the determiner.

In a seventh possible implementation form of the communication apparatus according to the sixth implementation form of the first aspect of the invention, the selector is configured to select one of the plurality of other communication apparatuses for communicating with the communication apparatus on the basis of a measure of the transmission power associated with a respective communication channel on the basis of the following equation:

wherein δ denotes the measure for the transmission power associated with a respective communication channel h^. In an eighth possible implementation form of the communication apparatus according to the seventh implementation form of the first aspect, the selector is configured to select another one of the plurality of other communication apparatuses, in case the measure of the transmission power associated with a respective communication channel is smaller than a transmission power threshold value.

In a ninth possible implementation form of the communication apparatus according to the eighth implementation form of the first aspect, the selector is configured to select another one of the plurality of other communication apparatuses randomly, in case none of the communication channels of the plurality of other communication apparatuses is associated with a measure of the transmission power larger than the transmission power threshold value.

In a tenth possible implementation form of the communication apparatus according to the eighth implementation form of the first aspect, the selector is configured to select the communication apparatus of the plurality of other communication apparatuses, which is associated with the largest measure of the transmission power, in case none of the communication channels of the plurality of other communication apparatuses is associated with a measure of the transmission power larger than the transmission power threshold value.

In an eleventh possible implementation form of the communication apparatus according to the eighth implementation form of the first aspect, the selector is configured to select the communication apparatus of the plurality of other communication apparatuses, which is associated with the smallest measure of the transmission power, in case none of the communication channels of the plurality of other communication apparatuses is associated with a measure of the transmission power larger than the transmission power threshold value. In a twelfth possible implementation form of the communication apparatus according to the first aspect as such or any one of the first to eleventh implementation form thereof, the communication apparatus is a base station and the other communication apparatus is a user equipment. According to a second aspect the invention relates to a method of operating a

communication apparatus capable of full-duplex communication with another communication apparatus via a communication channel, wherein the communication apparatus comprises at least two transmitter antennas and at least one receiver antenna, wherein the at least two transmitter antennas are associated with a transmitter filter and the at least one receiver antenna is associated with a receiver filter and wherein the at least two transmitter antennas and the at least one receiver antenna define a self- interference channel. The method comprises the step of determining the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter is determined on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value. The method according to the second aspect of the invention can be performed by the base station according to the first aspect of the invention. Further features of the method according to the second aspect of the invention result directly from the functionality of the communication apparatus according to the first aspect of the invention and its different implementation forms described above.

According to a third aspect the invention relates to a computer program comprising program code for performing the method according to the second aspect of the invention when executed on a computer. The invention can be implemented in hardware and/or software.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a schematic diagram illustrating a communication apparatus according to an embodiment in the context of a communication network; Fig. 2 shows a schematic diagram illustrating steps of a method of operating a communication apparatus according to an embodiment; Fig. 3 shows a schematic diagram illustrating a communication apparatus according to an embodiment; and

Fig. 4 shows a table illustrating objective gains and corresponding power savings associated with a communication apparatus according to an embodiment as a function of the number of transmitter and receiver antennas.

DETAILED DESCRIPTION OF EMBODIMENTS In the following detailed description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present invention may be practiced. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

Figure 1 shows a schematic diagram illustrating a communication apparatus 100 according to an embodiment in the context of a wireless communication network 120. The communication apparatus 100 is capable of a full-duplex communication with another communication apparatus, for instance with one or more of the other communication apparatuses 1 10a-c, over a respective communication channel. In an embodiment, the communication apparatus 100 can be implemented in form of a base station and the other communication apparatuses 1 10a-c can be implemented in the form of user equipments or mobile stations. The person skilled in the art will appreciate that, although for the sake of simplicity figure 1 shows by way of example only three further communication apparatuses 1 10a-c, embodiments of the present invention can be advantageously employed also in scenarios with a different, in particular a larger number of further communication apparatuses 1 10a-c.

The communication apparatus 100 comprises at least two transmitter antennas 101 and at least one receiver antenna 103, wherein the at least two transmitter antennas 101 are associated with a transmitter filter and the at least one receiver antenna 103 is associated with a receiver filter and wherein the at least two transmitter antennas 101 and the at least one receiver antenna 103 define a self-interference channel. By way of example, for the following discussion it is assumed that the communication apparatus 100 is equipped with N_R receiver antennas and N_T transmitter antennas. The channel state information of the self-interference channel is denoted with h e C^{NR NT} . The receiver filter is denoted with v e C^NR , the transfer filter is denoted with w e C^NT and hi e C^NT denotes the channel state information of the communication channel towards the other communication apparatus, for instance, one of the other communication apparatuses 1 10a-c shown in figure 1 .

The power conveyed by the at least two transmitter antennas 101 of the communication apparatus 100 towards the other communication apparatus, for instance, one of the other communication apparatuses 1 10a-c shown in figure 1 , can be expressed as \h"w\ , wherein x^H denotes the Hermitian transpose of x. The power in the self-interference channel can be expressed as \v^Hhw\².

Conventionally, the optimal filter problem is formulated as follows:

(Pw) : s. t. \v^Hhw\²≤ ε (1 )

wherein ε > 0 denotes a predefined self-interference threshold value. Surprisingly, it has been found out that the equation and constraints defined in complex 1 above, can be reformulated in the following way:

wherein a is a weight factor. A transmitter filter w solving the optimization problem defined in complex 2 can be expressed as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter is determined by the weight factor a, which depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value.

Thus, the communication apparatus 100 comprises, moreover, a determiner 105 configured to determine the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the determiner 105 is configured to determine the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value.

In an embodiment, the determiner 105 is configured to determine the transmitter filter w on the basis of the weight factor a on the basis of the following equation:

wherein / denotes the identity matrix and x^# denotes the Moore-Pen rose pseudoinverse of x.

In an embodiment, the determiner 105 is configured to determine a preferred or optimal value a^* of the weight factor a by determining the transmitter filter w such that the following equation is satisfied: ε = \v^Hhw\² (4) In an embodiment, the determiner 105 is configured to determine the preferred or optimal value a^* of the weight factor a on the basis of the following equation: a = 1 - mm (1, J ), (5) wherein the determiner 105 is configured to determine β and γ on the basis of the following equations: β (1 - 8(v^Hhh^Hv)-¹)h^h^Hvv^Hhh_i (6), and

7 =_= h (h^Hvv^Hh - 8l)hi (7)

In an embodiment, the predefined self-interference threshold value ε, which in an embodiment is determined by the self-interference cancellation capability of the communication apparatusi 00, imposes an upper bound on the transmission power of the communication apparatus 100.

In an embodiment, the predefined self-interference threshold value ε imposes an upper bound on the transmission power of the communication apparatus on the basis of the following equation: ε > \v^Hhw\² (8)

As already mentioned above, the communication apparatus 100 can be further configured to communicate with a plurality of other communication apparatuses, such as the other communication apparatuses 1 10a-c shown in figure 1. Each one of the other

communication apparatus 1 10a-c defines a respective communication channel between the communication apparatus 100 and the respective other communication apparatus 1 10a-c associated with a respective channel state information h^.

In an embodiment, the communication apparatus 100 further comprises a selector 107 configured to select one of the plurality of other communication apparatuses 101 a-c on the basis of a measure of the transmission power associated with a respective communication channel depending on the respective channel state information h_t and the transmitter filter w determined by the determiner 105. In an embodiment, the determiner 105 and/or the selector 107 can be implemented in form of hardware elements and/or software elements, running, for instance, on a processor of the communication apparatus 100. In an embodiment, the selector 107 is configured to select one of the plurality of other communication apparatuses 1 10a-c on the basis of a measure of the transmission power associated with a respective communication channel on the basis of the following equation:

wherein δ denotes the measure for the transmission power associated with a respective communication channel . In an embodiment, the selector 107 is configured to select another one of the plurality of other communication apparatuses 1 10a-c, in case the measure of the transmission power δ associated with a respective communication channel is smaller than a transmission power threshold value. In an embodiment, the selector 107 is configured to select another one of the plurality of other communication apparatuses 1 10a-c randomly, in case none of the communication channels of the plurality of other communication apparatuses 1 10a-c is associated with a measure of the transmission power δ larger than the transmission power threshold value. In an embodiment, the selector 107 is configured to select the communication apparatus of the plurality of other communication apparatuses 1 10a-c, which is associated with the largest measure of the transmission power 5, in case none of the communication channels of the plurality of other communication apparatuses 1 10a-c is associated with a measure of the transmission power δ larger than the transmission power threshold value.

In an embodiment, the selector 107 is configured to select the communication apparatus of the plurality of other communication apparatuses 1 10a-c, which is associated with the smallest measure of the transmission power 5, in case none of the communication channels of the plurality of other communication apparatuses 1 10a-c is associated with a measure of the transmission power δ larger than the transmission power threshold value. Figure 2 shows a schematic diagram illustrating a method 200 of operating a

communication apparatus, for instance the communication apparatus 100 shown in figure 1 , capable of full-duplex communication with another communication apparatus, such as one or more of the other communication apparatuses 1 10a-c shown in figure 1 , via a communication channel, wherein the communication apparatus 100 comprises at least two transmitter antennas 101 and at least one receiver antenna 103, wherein the at least two transmitter antennas 101 are associated with a transmitter filter and the at least one receiver antenna 103 is associated with a receiver filter and wherein the at least two transmitter antennas 101 and the at least one receiver antenna 103 define a self- interference channel. The method 200 comprises the step 201 of determining (201 ) the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter is determined on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self- interference threshold value.

In the following, further implementation forms, embodiments and aspects of the communication apparatus 100 and the method 200 will be described.

Figure 3 shows a schematic diagram illustrating the DSP block 301 of a communication apparatus 100 according to an embodiment further comprising a scheduler 303 and a self- interference cancellation (SIC) controller 305.

In figure 3, the scheduler 303 selects a further communication apparatus i to be served in an uplink direction, which is characterized by some CSI h_; and a QoS requirement in the form of a threshold value S_t . Then, the SIC controller 305 provides the CSI of the SI channel h and the predefined self-interference threshold value ε , together with the receiver filter v adopted by the communication apparatus 100 to the determiner 105 implemented in the form of an optimal filter block 105. As described above, the determiner 105 implemented in the form of an optimal filter block 105 computes the optimal TX filter w(a*) in one shot (i.e. no search algorithm is required) on the basis of the optimal weight a . In this regard, the low complexity of said computation, whose speed is significantly higher than what can be achieved by means of state-of-the-art solutions, allows additional operations to be performed by the DSP block 301 of the communication apparatus 100. More precisely, this allows the communication apparatus to check if a certain QoS can be guaranteed to the considered further communication apparatus 1 10a-c while delivering an adequate SIC. Remarkably, the feasibility of this condition can be easily verified from w(a^*), and this operation comes at negligible computational cost for the communication apparatus. This functionality can be provided by the selector 107 in form of a 5 feasibility checker 107, which receives as inputs from the determiner 105 the optimal transmitter filter and from the scheduler 303 the CSI h_; and the QoS requirement 5_t associated with the further communication apparatus 1 10a-c.

In practice, after a preliminary low-complexity transmitter filter computation, the selector 107 in form of the δ feasibility checker 107 can yield two possible outputs, namely the QoS requirement can be guaranteed or the QoS requirement cannot be guaranteed.

At this stage, if the QoS requirement S_t is feasible, then the other communication apparatus 1 10a-c is selected and the optimal transmitter filter w(a^*) is used to transmit. Alternatively, if the threshold S_t is unfeasible, the scheduler 303 is triggered by the selector 107 to propose a new communication apparatus 1 10a-c to be served. This process can be repeated multiple times within the coherence time of the channel, thanks to the one-shot computation of the proposed transmitter filter design for a particular further communication apparatus, until a further communication apparatus with a feasible QoS requirement is served and the optimal transmitter filter is computed by the determiner 105 accordingly. In an embodiment, the QoS requirements 5_t are designed in order to be feasible with reasonable probability. In particular, this could imply the impossibility of satisfying any of the QoS constraints. In this case, i.e., if no further communication apparatus 1 10a-c is found to be feasible at the end of the operations performed by the selector 107, the communication apparatus 100 can decide to serve a further

communication apparatus 1 10a-c even if its QoS are not satisfied according to some selection policy, as already described above.

Figure 4 shows a table illustrating objective gains and corresponding power savings associated with a communication apparatus according to an embodiment as a function of the number of transmitter and receiver antennas. More specifically, considering the full zero-forcing TX filter as a basis for comparison, the table shown in figure 4 illustrates the average values of the objective gain, i.e., the additional power that is conveyed towards the desired link for a fixed TX power, and the power saving, i.e. the TX power that the communication apparatus 100 would save if the same target power were to be conveyed towards the desired link.

As has been described above, embodiments of the invention target scenarios in which a communication apparatus with multiple TX/RX antennas can profit from the simultaneous TX/RX operations over the same frequency band. More specifically, embodiments of the invention focus on settings in which, at each timeslot, the communication apparatus with multiple antennas can serves at least one user terminal in the uplink and one user terminal (the same or a different one) in the downlink.

Embodiments of the invention allow maximizing the power throughput of transmitter antennas in a full duplex communication apparatus by decreasing the self-interference level. Embodiments of the present allow to increase the transmitter signal power and to better comply with the quality of service requirements. Embodiments of the invention significantly outperform existing algorithms for maximizing the transmitter antenna power with respect to the performance and algorithm complexity. Embodiments of the invention can be efficiently implemented for various networks and communication systems.

Embodiments of the invention are fully scalable - in terms of computational complexity - with the number of antennas of the communication apparatus, since it only requires the computation of the one-dimensional weight vector, whose solution is in closed-form. While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms "coupled" and "connected", along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other. Although specific aspects have been illustrated and described herein, it will be

appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.

Claims

1. A communication apparatus (100) capable of full-duplex communication with another communication apparatus (1 10a-c) via a communication channel, the

communication apparatus (100) comprising: at least two transmitter antennas (101 ) and at least one receiver antenna (103), wherein the at least two transmitter antennas (101 ) are associated with a transmitter filter and the at least one receiver antenna (103) is associated with a receiver filter and wherein the at least two transmitter antennas (101 ) and the at least one receiver antenna (103) define a self-interference channel; and a determiner (105) configured to determine the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the determiner (105) is configured to determine the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value.

2. The communication apparatus (100) of claim 1 , wherein the determiner (105) is configured to determine the transmitter filter on the basis of the weight factor a on the basis of the following equation:

(l- h^Hv(h^Hv)^#)hi

W^ > ~ ||(7-αΛ (7ι )^#)Λί||' wherein w denotes the transmitter filter, / denotes the identity matrix, h denotes the channel state information of the self-interference channel, v denotes the receiver filter, denotes the channel state information of the communication channel, x^H denotes the Hermitian transpose of x and x^# denotes the Moore-Penrose pseudoinverse of x.

3. The communication apparatus (100) of claim 2, wherein the determiner (105) is configured to determine a preferred value a^* of the weight factor a by determining the transmitter filter w such that the following equation is satisfied: ε = \v^Hhw\², wherein ε denotes the predefined self-interference threshold value, v denotes the receiver filter, h denotes the channel state information of the self-interference channel and x^H denotes the Hermitian transpose of x.

4. The communication apparatus (100) of claim 3, wherein the determiner (105) is configured to determine the preferred value a^* of the weight factor a on the basis of the following equation:

a^* = 1 - mm (1, / -f-^), wherein the determiner (105) is configured to determine β and γ on the basis of the following equations: β (1 - ε(ν^ΗΗΗ^Ην)-¹)^Η^Ηνν^ΗΗΗ_ί, and γ h^(h^Hvv^Hh - el)hi, wherein ε denotes the predefined self-interference threshold value.

5. The communication apparatus (100) of any one of the preceding claims, wherein the predefined self-interference threshold value [determined by the self-interference cancellation capability of the communication apparatus] imposes an upper bound on the transmission power of the communication apparatus (100).

6. The communication apparatus (100) of claim 5, wherein the predefined self- interference threshold value imposes an upper bound on the transmission power of the communication apparatus (100) on the basis of the following equation: ε≥ \v^Hhw\², wherein ε denotes the predefined self-interference threshold value, v denotes the receiver filter, h denotes the channel state information of the self-interference channel, w denotes the transmitter filter and x^H denotes the Hermitian transpose of x.

7. The communication apparatus (100) of any one of the preceding claims, wherein the communication apparatus (100) is further configured to communicate with a plurality of other communication apparatuses (1 10a-c), wherein each other communication apparatus (1 10a-c) defines a respective communication channel between the communication apparatus (100) and the other communication apparatus (1 10a-c) associated with a respective channel state information h_t and wherein the communication apparatus (100) further comprises a selector (107) configured to select one of the plurality of other communication apparatuses (101 a-c) for communicating with the communication apparatus (100) on the basis of a measure of the transmission power associated with a respective communication channel depending on the respective channel state information hi and the transmitter filter w determined by the determiner (105).

8. The communication apparatus (100) of claim 7, wherein the selector (107) is configured to select one of the plurality of other communication apparatuses (1 10a-c) for communicating with the communication apparatus (100) on the basis of a measure of the transmission power associated with a respective communication channel on the basis of the following equation:

wherein δ denotes the measure for the transmission power associated with a respective communication channel h^.

9. The communication apparatus (100) of claim 7 or 8, wherein the selector (107) is configured to select another one of the plurality of other communication apparatuses

(1 10a-c), in case the measure of the transmission power associated with a respective communication channel is smaller than a transmission power threshold value.

10. The communication apparatus (100) of claim 9, wherein the selector (107) is configured to select another one of the plurality of other communication apparatuses

(1 10a-c) randomly, in case none of the communication channels of the plurality of other communication apparatuses (1 10a-c) is associated with a measure of the transmission power larger than the transmission power threshold value.

1 1. The communication apparatus (100) of claim 9, wherein the selector (107) is configured to select the communication apparatus of the plurality of other communication apparatuses (1 10a-c), which is associated with the largest measure of the transmission power, in case none of the communication channels of the plurality of other

communication apparatuses (1 10a-c) is associated with a measure of the transmission power larger than the transmission power threshold value.

12. The communication apparatus (100) of claim 9, wherein the selector (107) is configured to select the communication apparatus of the plurality of other communication apparatuses (1 10a-c), which is associated with the smallest measure of the transmission power, in case none of the communication channels of the plurality of other

13. The communication apparatus (100) of any one of the preceding claims, wherein the communication apparatus is a base station (100) and the other communication apparatus is a user equipment (1 10a-c).

14. A method (200) of operating a communication apparatus (100) capable of full- duplex communication with another communication apparatus (1 10a-c) via a

communication channel, wherein the communication apparatus (100) comprises at least two transmitter antennas (101 ) and at least one receiver antenna (103), wherein the at least two transmitter antennas (101 ) are associated with a transmitter filter and the at least one receiver antenna (103) is associated with a receiver filter and wherein the at least two transmitter antennas (101 ) and the at least one receiver antenna (103) define a self- interference channel, wherein the method (200) comprises: determining (201 ) the transmitter filter as a linear combination of a matched filter towards the communication channel and a zero-forcing filter towards the self-interference channel, wherein the contribution of the zero-forcing filter towards the self-interference channel to the transmitter filter is determined on the basis of a weight factor a, wherein the weight factor a depends on a channel state information of the self-interference channel, a channel state information of the communication channel, the receiver filter and a predefined self-interference threshold value.

15. A computer program comprising program code for performing the method of claim 14 when executed on a computer.