WO2016029434A1

WO2016029434A1 - Method and apparatus for antenna calibration in tdd systems

Info

Publication number: WO2016029434A1
Application number: PCT/CN2014/085524
Authority: WO
Inventors: Gang Wang; Chuangxin JIANG; Hongmei Liu; Zhennian SUN
Original assignee: Nec Corporation
Priority date: 2014-08-29
Filing date: 2014-08-29
Publication date: 2016-03-03

Abstract

Embodiments of the present invention provide methods and apparatus for antenna calibration in TDD systems. One aspect of the invention provides a method in a device equipped with multiple antennas, the method comprising self-calibrating the multiple antennas, such that transmitting gain to receiving gain ratio for each of the multiple antennas is the same; generating a first precoding matrix; applying the first precoding matrix to the multiple antennas to form a virtual antenna, and transmitting a reference signal to a second device via the virtual antenna, wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device. Corresponding methods and apparatus to assist the above method are also provided. By implementing some embodiments of the invention, more accurate antenna calibration can be provided and thus improve the performance of the TDD system.

Description

METHOD AND APPARATUS FOR ANTENNA CALIBRATION IN TDD SYSTEMS

FIELD OF THE INVENTION

Exemplary embodiments of the present invention relate generally to the wireless communication and， more specifically， to a method and apparatus for antenna calibration in TDD system.

BACKGROUND OF THE INVENTION

Long Term Evolution (LTE) developed by the 3rd Generation Partnership Project (3GPP) is a specification aiming to increase data transmission rates and decrease delays， among other things. LTE Advanced is an evolved version of LTE， also known as LTE-A， fulfilling the 4th generation wireless communication system requirements.

Both LTE and LTE-A may utilize a technique called time division duplex (TDD) for separating the transmission directions from the user to the base station and back. In TDD mode， the downlink (DL) and the uplink (UL) are on the same frequency and the separation between DL and UL occurs in the time domain. This characteristic enables to exploit the reciprocity of downlink and uplink channels， for example， DL channel state information (CSI) can be inherently obtained through uplink channel estimation. This however requires calibration of the radio frequency (RF) chains of the receiver and the transmitter， which are in general not reciprocal， otherwise significant deviations between DL and UL channel state information may occur and result in degradation of system performance， as precoding and/or scheduling at the evolved NodeB (eNB) side will be based on erroneous channel state information.

In Fig. 1， it illustrates an schematic diagram of the DL and UL channels of a TDD system. In this example， the DL channel between the nth antenna of a NB to the mth antenna of a UE can be represented by

whereT_NB，n denotes the transmitting gain at the nth antenna of the NB， R_UE，m denotes the receiving gain at the mth antenna of the UE， and

denotes the radio propagation channel parameter in DL； similarly， the UL channel between the nth antenna of a NB to the mth antenna of a UE can be represented by

where R_NB，n denotes the receiving gain at the nth antenna of the NB， T_UE，m denotes the transmitting gain at the mth antenna of the UE， and

denotes the radio propagation channel parameter in UL， respectively. The radio propagation channel parameter can be considered as ideal reciprocal， i.e.，

however， the transmitting gain and the receiving gain of RF are usually not reciprocal， i.e.， T_NB，n≠R_NB，n， and T_UE，m≠R_UE，m， which results in non-reciprocity of the DL and UL channels， i.e.，

To recover the reciprocity， antenna calibration has been proposed， which is to apply a calibrating factor at each transmitter and/or receiver， such that after calibration， for each antenna， the ratio between the transmitting gain and the receiving gain is a same constant， i.e. ，

where c_NB，i is the calibrating factor applied to the ith transmitting antenna of the NB. Likewise， the target of the calibration at UE side is to achieve

where c_UE，i is the calibrating factor applied to the ith transmitting antenna of the UE. Thus， determining a proper calibrating factor is the most important step in antenna calibration.

There are at least two alternatives to determine the proper calibrating factor. The first alternative is called self-calibration， where the calibration is implemented by a device itself without assistance from other devices， and an example of which is shown in Fig. 2. In this example， the calibration is implemented at the eNB side through the following steps：

Step 1： Sets antenna 1 as a reference supporter and transmits a reference signal (RS) to the other antennas； generally， antenna 1 transmits RS in all frequency bandwidth；

Step 2： The other antennas transmit orthogonal RS to the reference supporter， i.e.， antenna 1；

Step 3： Calculates the calibrating factor ci for each antenna i according to the estimated transmitting and receiving channels， i.e，

c₁＝1

where h_m，n stands for the estimated channel between the mth receiving antenna and the nth transmitting antenna.

The second alternative for calibration can be called network-assisted calibration， or， over-the-air calibration， where the calibration is implemented with assistance from a network， and one example of which is shown in Fig. 3. Fig. 3 illustrates a flow chart of an exemplary network-assisted calibration method implemented at a UE side. Firstly， to calibrate the UE antennas， one antenna of the eNB is set as reference supporter， and it is assumed to be antenna 1 in this example； then eNB measures the UL channel based on a UL RS transmitted by the UE and reports the quantified UL CSI to the UE； at the UE side， the DL channel is estimated based on DL RS from the eNB； finally， based on the estimated DL channel and the quantified UL channel informed by the eNB， the UE calculates the calibrating factors by， e.g. ，

where c_UE，m is the obtained calibrating factor for the mth antenna，

and

denotes the estimated DL channel and the reported quantified UL channel between the mth antenna at the UE side and the 1 st antenna at the eNB side， respectively. It is to be noted that， the UL RS and DL RS should be transmitted within a time interval no larger than channel coherent time to guarantee the accuracy of the calibrating factor calculation.

Though in the example of Fig. 2 and 3， the calibration is done to achieve reciprocity between the transmitting (Tx) chain and receiving (Rx) chain of a same device (e.g. ， eNB or UE) ， it has been observed that calibration between antennas of different devices may also be necessary. One scenario of which is TDD CoMP， where multiple BSs serve as a single super BS to mitigate the inter-cell interference. In this scenario， inter-cell calibration maybe necessary to remove the gain ambiguity among all the cooperative BSs and to obtain maximum performance gain from COMP. Assume for eNB k， after intra-eNB calibration，

then the relation of DL/UL channel between eNB k and UE m satisfies

where it is assumed that there is a single antenna at UE side，

and T_NBk，n and R_NBk，n denote transmitting gain and receiving gain at the nth antenna of the NB k， respectively. In such a case， by applying a precoding matrix

the received signal at the mth UE side can be written as：

where s_m means transmitted signal for UE m， n_m stands for the noise observed at the mth UE side. So to maximize the SINR， the phase of γ_mk should be aligned between the CoMP transmission points， i.e. ， inter cell antenna calibration is necessary for TDD CoMP. In Fig. 4， a schematic diagram for inter-cell calibration is illustrated， and the inter-cell calibration can be performed via following steps：

Step 1： Choose one reference antenna from each cooperate eNB， and choose an antenna of a UE as calibration supporter， such that there are totally L antennas from L cooperate base station；

Step 2： eNBs measure lxL UL channels based on received signal from the L reference antennas；

Step 3： eNB transmits RS to UE by each of the L reference antennas；

Step 4： UE measures and feed back the estimated Lxl DL channels；

Step 5： eNB performs inter-cell calibration by calculating the gain ambiguities among the reference antennas of cooperate cells based on the estimated DL channels feedback by UE and the measured UL channels；

Step 6： eNB performs self-calibration for each of intra cell antennas based on inter cell calibration results.

It is observed that in both UE calibration method as depicted in Fig. 3 and inter-cell calibration method depicted in Fig. 4， only one antenna from a transmission point (e.g. ， eNB in Fig. 3， and eNB 1 in Fig. 4) is used as reference antenna which transmits and receives RS to/from UE for DL/UL channel estimation， and then if the channel condition between the reference antenna of a transmission point and UE antennas is not good， i. e. ， SINR is low， the channel estimation performance will be degraded， resulting in large calibration error.

One object of embodiments of the invention is to solve at least some of the problems.

SUMMARY OF THE INVENTION

Various embodiments of the invention aim at addressing at least part of the above problems and disadvantages. Other features and advantages of embodiments of the invention will also be understood from the following description of specific embodiments when read in conjunction with the accompanying drawings， which illustrate， by way of example， the principles of embodiments of the invention.

It should be noted that though the problems to be solved by the embodiments of the invention have been described in a cellular scenario， the invention is also applicable to other wireless communication systems with similar problems， e.g. ， ad hoc network， or device to device (D2D) communication， where a station， an access point (AP) ， or a device involved in D2D may need antenna calibration.

Various aspects of embodiments of the invention are set forth in the appended claims and summarized in this section. It shall be noted that the protection scope of the invention is only limited by the appended claims.

According to a first aspect, embodiments of the present invention provide a method in a first device for antenna calibration, wherein the first device is equipped with multiple antennas, the method comprises： self-calibrating the multiple antennas, such that transmitting gain to receiving gain ratio for each of the multiple antennas is the same； generating a first precoding matrix； applying the first precoding matrix to the multiple antennas to form a virtual antenna； and transmitting a reference signal to a second device via the virtual antenna, wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device.

In accordance with an embodiment of the invention, the method further comprises： estimating virtual channels between each antenna of the second device and the virtual antenna； generating a second precoding matrix based on the estimated virtual channels； transmitting multiple reference signals to the second device via the virtual antenna, wherein each of the multiple reference signals is weighted by a weighting factor from the second precoding matrix.

In accordance with another embodiment of the invention, estimating virtual channels comprises estimating virtual channels based on a reference signal from each antenna of the second device.

According to still another embodiment of the invention, generating a second precoding matrix based on the estimated virtual channels comprises： calculating a ratio between each of the virtual channels and a reference virtual channel, wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix, and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna of the first device； and generating the second precoding matrix based on the calculated ratio.

In one embodiment of the invention, the first precoding matrix includes only one non-zero element.

In another embodiment of the invention, the method further comprises： receiving, from the second device, information on a channel estimation based on the reference signal from the virtual antenna of the first device and/or another reference signal from a third device which transmits the another reference signal to the second device in the same way as the first device； calculating calibration parameters to be applied at least partly based on the received information on the channel estimation from the second device.

According to a second aspect, embodiments of the present invention provide a method in a second device for antenna calibration, wherein the second device is equipped with multiple antennas, the method comprises： receiving, from a first device, a reference signal to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device； wherein the reference signal is transmitted by the first device via a virtual antenna which is formed by applying a first precoding matrix to multiple antennas of the first device, and wherein the multiple antennas of the first device are self-calibrated before transmitting the reference signal.

In accordance with an embodiment of the invention, receiving, from a first device, a reference signal comprises： receiving multiple reference signals from the first device via the virtual antenna, wherein each of the multiple reference signals is weighted by a weighting factor from a second precoding matrix which is generated by the first device based on estimated virtual channels between each antenna of the second device and the virtual antenna of the first device； and the method further comprises： calculating calibration parameters to be applied based on a channel estimation obtained from the received multiple reference signals.

In accordance with another embodiment of the invention, the method further comprises： transmitting a reference signal from each antenna of the second device, to facilitate estimating the virtual channels. In a further embodiment of the invention, the second precoding matrix is generated by the first device through the following steps： calculating a ratio between each of the virtual channels and a reference virtual channel, wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix, and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna of the first device； and generating the second precoding matrix based on the calculated ratio.

In accordance with another embodiment of the invention, the method further comprises transmitting, to the first device and/or a third device, quantized channel estimation results obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from the third device which transmits the another reference signal to the second device in the same way as the first device.

According to a third aspect, embodiments of the present invention provide an apparatus for antenna calibration, which is configurable to perform any of the methods according to the first aspect of the invention.

According to a fourth aspect, embodiments of the present invention provide an apparatus for antenna calibration, which is configurable to perform any of the methods according to the second aspect of the invention.

These and other optional embodiments of the present invention can be implemented to realize one or more of the following advantages. In accordance with some embodiments of the present invention， by transmitting a precoded reference signals via a virtual antenna， the received signal to interference plus noise ratio (SINR) can be increased， and then channel estimation and antenna calibration accuracy can be greatly improved. In accordance with some further embodiments of the present invention， by precoding the reference signals using weighting factors obtained based on a ratio between channels from a reference antenna at base station side to different antennas at the UE side， feedback of quantized channel estimation results can be avoided， and then reduces the loss from quantization， resulting in further improved calibration accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects， features， and benefits of various embodiments of the invention will become more fully apparent， by way of example， from the following detailed description and the accompanying drawings， in which like reference numerals refer to the same or similar elements：

Fig. 1 is illustrates an schematic diagram of the DL and UL channels of a TDD system；

Fig. 2 illustrates a schematic diagram of self-calibration implemented at an eNB side；

Fig. 3 illustrates a flow chart of network-assisted calibration method implemented at a UE side；

Fig. 4 illustrates a schematic diagram for inter-cell calibration；

Fig. 5 illustrates a schematic wireless communication system where an embodiment of the invention can be implemented；

Fig. 6a is a flow chart of a method for antenna calibration implemented at an eNB side according to an embodiment of the invention；

Fig. 6b is a flow chart of a method for antenna calibration implemented at an eNB side according to another embodiment of the invention；

Fig. 6c is a flow chart of a method for antenna calibration implemented at an eNB side according to a further embodiment of the invention；

Fig. 6d is a flow chart of a method for antenna calibration implemented at an eNB side according to still another embodiment of the invention；

Fig. 6e is a schematic diagram illustrating a method for antenna calibration according to an embodiment of the invention

Fig. 7a is a flow chart of a method for antenna calibration implemented at a UE side according to an embodiment of the invention；

Fig. 7b is a flow chart of a method for antenna calibration implemented at a UE side according to another embodiment of the invention；

Fig. 8a is a block diagram of an apparatus for antenna calibration according to an embodiment of the invention；

Fig. 8b is a block diagram of an apparatus for antenna calibration according to another embodiment of the invention； and

Fig. 9 is a block diagram of an apparatus for antenna calibration according to still another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Some preferred embodiments will be described in more detail with reference to the accompanying drawings， in which the preferred embodiments of the present disclosure have been illustrated. However， the present disclosure can be implemented in various manners， and thus should not be construed to be limited to the embodiments disclosed herein. On the contrary， those embodiments are provided for thorough and complete understanding of the present disclosure， and completely conveying the scope of the present disclosure to those skilled in the art.

In the following description， numerous specific details of embodiments of the present invention are set forth. However， it is understood that embodiments of the invention may be practiced without these specific details. In other instances， well-known circuits， structures and techniques have not been shown in detail in order not to obscure the understanding of this description. Those of ordinary skills in the art， with the included descriptions， will be able to implement appropriate functionality without undue experimentation.

References in the specification to “one embodiment， ” “an embodiment， ” “an example embodiment， ” etc.， indicate that the embodiment described may include a particular feature， structure， or characteristic， but every embodiment may not necessarily include the particular feature， structure， or characteristic. Moreover， such phrases are not necessarily referring to the same embodiment. Further， when a particular feature， structure， or characteristic is described in connection with an embodiment， it is submitted that it is within the knowledge of one skilled in the art to affect such feature， structure， or characteristic in connection with other embodiments whether or not explicitly described. It shall be understood that the singular forms “a” , “an” and “the” include plural referents unless the context explicitly indicates otherwise.

Reference is now made to Fig. 5 which is a schematic diagram of wireless communication system 500 where an embodiment of the invention can be implemented. For illustrative purposes, the wireless communication network 500 is shown to be in a cellular structure. Those skilled in the art will appreciate, however, that embodiments of the invention also apply to non-cellular wireless communication networks, such as ad hoc network, or D2D communication network, as long as similar problems exist. The wireless communication network comprises one or more macro cells each covered by a network node 501, here for illustrative purpose, in the form of 3GPP LTE evolved Node B, also known as eNodeB or eNB. The network nodes 501 could also take the form of Node Bs, Base Transceiver Stations (BTSs) , Base Station (BS) and/or Base Station Subsystems (BSSs) , etc. The network nodes 501 provide radio connectivity to a plurality of user equipments (UEs) 502. The term user equipment is also known as mobile communication terminal, wireless terminal, mobile terminal, user terminal, user agent, machine-to-machine devices etc. , and can be, for example, what today is commonly known as a mobile phone or a tablet/laptop with wireless connectivity or fixed mounted terminal. Moreover, the UEs may, but do not need to, be associated with a particular end user. The downlink from an eNB 501 to a UE 501, and the uplink from the UE 502 to the eNB 501 are on the same frequency and are time division duplexed (TDD) . This characteristic enables to exploit the reciprocity of downlink and uplink channels. However, due to un-ideal RF chains of the receiver and the transmitter, the reciprocity can only be employed when transmitting and receiving antennas of the eNB and UE are calibrated. Furthermore, the network 500 depicted in Fig. 1 may comprises a number of eNBs which can jointly transmit/receive to/from a UE 502 through COMP. As already described above, in such a scenario, inter-cell antenna calibration is necessary, in order to maximize the gain from COMP.

Some antenna calibration methods already known in the art are depicted in Fig. s 2-4, however, it is observed that in both UE calibration method as depicted in Fig. 3 and inter-cell calibration method depicted in Figure 4, only one antenna from a transmission point (e.g. , eNB in Fig. 3, and eNB 1 in Fig. 4) is used as reference antenna for transmitting and receiving reference signals (RS) to/from UE to facilitate DL/UL channel estimation and antenna calibration. It makes the channel estimation and antenna calibration sensitive to channel status, and if the channel condition between the reference antenna of a transmission point and UE antennas is not good, i.e. , SINR is low, the channel estimation performance will be degraded, resulting in large calibration error.

One object of embodiments of the invention is to provide improved method and apparatus for antenna calibration and solve at least part of the problems.

Reference is now made to Fig. 6a, which illustrates a flow chart of a method 600 for antenna calibration implemented by a device according to an embodiment of the invention. The device is equipped with multiple antennas and can be, for example, an eNB 501 as shown in Fig. 5 and can be denoted as eNB 1, but is not limited to this.

The method 600 comprises a step 601 for self-calibrating the multiple antennas, such that transmitting gain to receiving gain ratio for each of the multiple antennas is the same； a step 602 for generating a first precoding matrix； a step 603 for applying the first precoding matrix to the multiple antennas to form a virtual antenna； and a step 604 for transmitting a reference signal to a second device via the virtual antenna, wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device.

In an embodiment of the invention, the step 601 can be implemented in a similar way at an eNB side as described with reference to Fig. 2, such that after calibration, the ratio between the transmitting gain and receiving gain satisfies:

where c_NB,k denotes calibrating factor for the kth antenna, and c_NB,k T_NB,k denotes the calibrated transmitting gain of the kth antenna of the eNB, R_NB,n is the receiving gain of the nth antenna of the eNB； k is an integer less than N which is the number of antennas of the eNB, and λ_NB is a constant value. For simplicity purpose, the formula can also be written as:

In accordance with some embodiments of the invention, in step 602, the first precoding matrix W＝[w₁ … w_N]^Tis generated based on channel estimation based on UL RS from a UE. In another embodiment of the invention, the first precoding matrix is generated based on channel state information (CSI) feedback by a UE. The first precoding matrix generated in step 602 is used in step 603 to form a virtual antenna by multiplying each of the multiple antennas with an element w_i of the first precoding matrix. That is, elements of the first precoding matrix are used as weighting factors for corresponding antennas.

In step 604, a RS is transmitted to a second device via the virtual antenna, i.e. , the RS is transmitted from multiple antennas with corresponding weighting factors, such that at the second device side， it will be considered as a RS transmitted from a single antenna and at the same time transmit diversity gain is achieved. It provides higher receiving SINR which means better antenna calibration performance.

In accordance with some embodiments of the invention， the method 600 can be used for antenna calibration at a UE side. In these embodiments， as shown in Fig. 6b as an example， the method 600 may further comprise a step 605 for estimating virtual channels， wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix； a step 606 for generating a second precoding matrix based on the estimated virtual channels in step 605； and in step 604， multiple reference signals are transmitted to the second device via the virtual antenna， wherein each of the multiple reference signals is further weighted by a weighting factor from the second precoding matrix generated in step 606.

In accordance with an embodiment of the invention， in step 605， the virtual channels are estimated based on a reference signal from each antenna of the second device， which means for a UE with M antennas， M virtual channels will be estimated in step 605， each estimation is based on a RS from one antenna of the UE. Then the step 605 can also include the operation of receiving， via the virtual antenna， a RS from the UE.

In accordance with another embodiment of the invention， in step 606， the eNB may calculate a ratio between each of the virtual channels and a reference virtual channel， wherein a virtual channel stands for a channel between an antenna ofthe UE and the virtual antenna of the eNB， while the reference virtual channel corresponds to a channel between a reference antenna of the second device， e.g. ， first antenna of the UE， and the virtual antenna of the eNB； and generate the second precoding matrix based on the calculated ratio. For example， the ratio can be calculated as：

where

stands for a channel from the mth antenna of the UE to the virtual antenna of the eNB，

stands for a channel from the mth antenna of the UE to the nth antenna of the eNB， T_UE， m is a transmitting gain of the mth antenna of the UE， R_NB， n is the receiving gain of the nth antenna of the eNB，

stands for the radio propagation channel parameter for the UL from the mth antenna of the UE to the nth antenna of the eNB， and w_n denotes the weithting factor applied to the nth antenna of the eNB to form the virtual antenna in step 603. Then the r_m is used to generate the second precoding matrix. In step 604， r_m is used as a weighting factor， to further precode one ofthe multiple RS， e.g. ， the mth RS， and wherein the multiple RS are transmitted to the UE in orthogonal resources.

With such a method 600 as illustrated in Fig. 6b， at the UE side， the estimated DL channel at each antenna of the UE can be：

where

Then by calculating as below a ratio between each of estimated DL channels and a reference DL channel， e.g. ， the first estimated DL channel， the calibrating factor to be applied to each antenna can be obtained as：

In accordance with still another embodiment of the invention， when the method 600 as illustrated in Fig. 6b is utilized to facilitate antenna calibration at a UE side， the first precoding matrix generated in step 602 may include only one non-zero element， which means only one antenna of the multiple antennas is used for transmission and in such case， the step 601 for self-calibrating the multiple antennas can be omitted.

In accordance with some embodiments of the invention， the method 600 can be used for inter-cell antenna calibration， e.g. ， in a COMP scenario. In these embodiments， as shown in Fig. 6c as an example， the method 600 may further include a step 607 for receiving information on calibration from a third device (e.g. ， another eNB 501 as shown in Fig. 5， and can be denoted as eNB 2) which also transmits another reference signal to the second device in the same way as the first device； a step 608 for receiving， from the second device， information on a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or the another reference signal from the third device； a step 609 for calculating calibration parameters to be applied at least partly based on the received information on the channel estimation from the second device and/or the information on calibration from the third device； and a step 610 for transmitting at least one of the calculated calibration parameters to the third device.

In accordance with an embodiment of the invention， eNB 1 and eNB 2 cooperate to form a COMP scenario. For the inter-cell antenna calibration， both the eNB 1 and the eNB 2 perform the steps 601 to 604， i.e. ， both eNBs send precoded RS to the UE. Assume that after the self-calibration in step 601 at eNB 1 and eNB 2 sides， the transmitting gain to receiving gain ratio can be presented as

and

for eNB 1 and eNB 2， respectively， where C_NB1，k denotes the calibrating factor applied to the kth antenna of the eNB 1， and T_NB1，k denotes the transmitting gain of the kth antenna of eNB 1； similarly， C_NB2，k denotes the calibrating factor applied to the kth antenna of the eNB 2， and T_NB2，k denotes the transmitting gain of the kth antenna of eNB 2； α₁， α₂ are both constant. In step 602， assume that the generated precoding matrix at the eNB 1 and the eNB 2 sides can be presented as

respectively， then the estimated UL channel between the virtual antenna formed in step 603 and an antenna of the UE can be written as

and

for eNB 1 and eNB 2， respectively， where

stands for the estimated UL virtual channel for eNB 1 and

stands for the estimated UL virtual channel for eNB 2；

and

stand for the UL channel and the UL radio propagation channel parameter from the 1 st antenna of the UE to the nth antenna of the eNBl， respectively， and similarly，

stand for the UL channel and the UL radio propagation channel parameter from the 1 st antenna of the UE to the nth antenna of the eNB 2； T_UE，1 stands for the transmitting gain of the 1st antenna of the UE. The UL virtual channel estimation can be implemented based on received UL RS from the UE， both the UL RS receiving and the UL virtual channel estimation can be performed， e.g. ， in an additional step 618 using any of the known estimation technics in the art and will not be detailed here. However， it can be appreciated that the receiving and estimation can also be implemented in separate steps.

In accordance with an embodiment of the invention， eNB 1 can receive the estimated UL virtual channel of the eNB 2 from the eNB 2 in step 607， to facilitate the inter-cell antenna calibration. In an embodiment of the invention， the eNB2 and eNB lcan be controlled by a same central control unit， e.g. ， when both eNB 1 and eNB 2 serve different sectors of a same cell. In such case， it can be assumed that there is ideal backhaul， e.g. ， fiber， between eNB 1 and eNB 2 and between eNB 1， eNB 2， and the central unit. In one embodiment of the invention， the central unit can be embodied in one of eNB 1 and eNB 2； in another embodiment of the invention， the central unit can be a separate device located outside of eNB 1 or eNB 2， e.g. ， located at the network side. In another embodiment of the invention， eNB 2 can be a remote radio head of the eNB 1， and there is also ideal backhaul between them. However， the embodiments of the invention are not limited to this. Actually， the embodiments of the invention can be implemented with any kind of links between eNB 1 and eNB 2， e.g. ， radio interface， or fiber. Thus， it can be appreciated that in accordance with an embodiment of the invention， the UL virtual channel of the eNB 2 can also be estimated at the eNB 1 side， and in such case， information exchange on calibration information is not necessary. Then in step 608， eNB 1 further receives information on the estimated DL virtual channel from UE. By calculating a ratio between the virtual UL and DL channels and comparing the ratio obtained for each eNB， calibration parameters can be obtained in step 609.

In an embodiment of the invention， in step 608， eNB 1 receives quantized channel estimation results for the DL virtual channel for both eNB 1 and eNB 2 from the UE， and the DL virtual channels can be written as：

and

The DL virtual channel is estimated by the UE based on the reference signal from the virtual antenna of both eNBs. In another embodiment， the eNB 1 may receive quantized channel estimation results for the DL virtual channel of eNB 1 only， or， receive channel estimation results for the DL virtual channel of eNB 1 and eNB 2 separately in multiple transmissions from the UE.

In an embodiment of the invention， in step 609， the calibration parameters can be obtained as following：

Firstly， calculating a ratio between DL and UL virtual channels for each eNB， i.e. ，

and

Then taking the ratio obtained for eNB 1 as a reference， the inter-cell calibration parameters for eNB 1 and eNB 2 can be obtained as:

and

It can be noted that to get the value of α₁/α₂， information on calibration factors ofα₁/β and α₂/β is required. Regarding α₁/β， it can be obtained by eNB 1 based on the estimated UL virtual channel and the feedback from UE on the DL virtual channel. Regarding α₂/β， there are several alternatives to get the information. In an embodiment of the invention， the calibration factor α₂/β can be obtained by eNB 1 in step 607 by receiving from eNB 2， e.g. ， via radio interface or fiber； and in another embodiment of the invention， in step 607， the eNB 1 only receives from the eNB 2 channel estimation results for a channel between a virtual antenna of the eNB 2 and an antenna of the UE， e.g. ， eNB 1 only receives information on the estimated UL virtual channel of eNB 2， e.g. ， via radio interface or via fiber， then eNB 1 calculate the value of α₂/β itself based on the received information from the eNB 2 and the received DL virtual channels for both eNB 1 and eNB 2 from the UE in step 608. Then in step 610， the eNB 1 can send the calculated value of α₁/α₂ to eNB 2. After applying the inter-cell calibration parameter at both eNB sides， we can get

which means same ratio of transmitting gain to receiving gain is obtained at eNB 1 and eNB 2.

In accordance with another embodiment of the invention， the calculation of α₁/α₂ can also be implemented in a central unit which controls both eNB 1 and eNB 2. For example， both eNB 1 and eNB 2 can send calibration related information， such as estimated UL/DL virtual channel， calculated α₁/β， calculated α₂/β， etc. ， to the central unit， and then the central unit derives the value of α₁/α₂ based on the calibration related information which has been collected from eNB 1 and eNB 2. In one embodiment of the invention， the central unit can be embodied in one of eNB 1 and eNB 2； in another embodiment of the invention， the central unit can be a separate device located outside of eNB 1 or eNB 2， e.g. ， located at the network side.

In Fig. 6d， it illustrates a flow chart of the method 600 according to another embodiment of the invention. The method can be implemented， e.g. ， by the eNB 2 which is involved in COMP scenario with the eNB 1 whose operation has been described with reference to Fig. 6c. In this example embodiment， as shown in Fig. 6d， besides the steps of 601 to 604， the method may further include a step 611 for transmitting information on calibration to a third device which transmits the another reference signal to the second device in the same way as the first device； and a step 612 for receiving at least one calibration parameter from the third device.

In accordance with an embodiment of the invention， the step 611 is a corresponding step of the step 607 described with reference to Fig 6c， i.e. ， the eNB 1 described with reference to Fig. 6c can be the third device in Fig. 6d， and the eNB 2 described with reference to Fig. 6c can be the first device in Fig. 6d， and the information on calibration may include channel estimation results for a channel between the virtual antenna of the first device and an antenna of the second device. Similarly， the step 612 is a corresponding step of the step 610 described with reference to Fig. 6c.

In another embodiment of the invention， the method further comprises a step 613 for receiving， from the second device (e.g. ， a UE) ， information on channel estimation which includes e.g. ， channel estimation results obtained based on the reference signal from the virtual antenna of the fnst device； and such information from the UE enables the device (e.g. ， eNB 2) to calculate the calibration factor for own cell and inform the calibration factor to another eNB，e.g. ， eNB 1， to facilitate inter-cell calibration. Then in such case， in the step 611， the operation of transmitting information on calibration to a third device may comprise transmitting the calculated calibration factor to the third device， e.g. ， eNB 1.

In an embodiment of the invention， the eNB2 and eNB 1 can be controlled by a same central control unit， e.g. ， when both eNB 1 and eNB 2 serve different sectors of a same cell. In such case， it can be assumed that there is ideal backhaul， e.g. ， fiber， between eNB 1 and eNB 2 and between eNB 1， eNB 2， and the central unit. In one embodiment of the invention， the central unit can be embodied in one of eNB 1 and eNB 2； in another embodiment of the invention， the central unit can be a separate device located outside of eNB 1 or eNB 2， e.g. ， located at the network side. In another embodiment of the invention， eNB 2 can be a remote radio head of the eNB 1， and there is also ideal backhaul between them. However， the embodiments of the invention are not limited to this. Actually， the embodiments of the invention can be implemented with any kind of links between eNB 1 and eNB 2， e.g. ， radio interface， or fiber.

In Fig. 6e， it illustrates a schematic diagram of a method for over the air inter-cell antenna calibration according to an embodiment of the invention. Different from the method depicted in Fig. 4， multiple antennas at both eNB 1 and eNB 2 sides are utilized as a reference supporter (i.e. ， reference antenna) by applying precoding to form a virtual antenna. By this way， the SINR can be greatly improved， and correspondingly resulting in more accurate channel estimation and antenna calibration.

Reference is now made to Fig. 7a， which illustrates a flow chart of a method 700 for antenna calibration implemented by a second device according to an embodiment of the invention. The device is equipped with multiple antennas and can be， for example， an EU 502 as shown in Fig. 5， but is not limited to this.

The method 700 comprises a step 701 for receiving， from a first device， a reference signal to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device； wherein the reference signal is transmitted by the first device via a virtual antenna which is formed by applying a first precoding matrix to multiple antennas of the first device， and wherein the multiple antennas of the first device are self-calibrated before transmitting the reference signal.

In accordance with an embodiment of the invention， the step 701 may correspond to the step 604 in Fig 6a； in another embodiment of the invention， the step 701 further comprises receiving multiple reference signals from the first device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from a second precoding matrix which is generated by the first device based on estimated virtual channels between each antenna of the second device and the virtual antenna of the first device. In such case， the step 701 may correspond to the step 604 in Fig. 6b， and then the multiple reference signals received in step 701 are weighted in a similar way as described with reference to Fig. 6b， e.g. ， weighted using a factor rm. The receiving of the multiple precoded reference signals enables antenna calibration at the UE side， and also provides higher receiving SINR to guarantee satisfying calibration accuracy， compared with the method illustrated in Fig. 3. Then in an embodiment of the invention， the method further comprises a step 702 for calculating calibration parameters to be applied based on a channel estimation obtained from the received multiple reference signals， e.g. ， the calibration factor can be calculated using equation (4) described with reference to Fig. 6b.

In an embodiment of the invention， the method 700 further comprises a step 703 for transmitting a reference signal from each antenna of the second device， to facilitate estimating the virtual channels. The transmitted reference signal in step 703 can be used by， e.g. ， eNB 1 which implements the method in Figu 6b， for estimating the virtual UL channels between the antenna of the UE and the virtual antenna at the eNB side， then generating the second precoding matrix based on the estimation in step 606. The second precoding matrix can be generated in similar as described with reference to Fig. 6b， and will not be repeat here.

According to some embodiments of the invention， the method 700 can be employed to facilitate inter-cell antenna calibration， e.g. ， for inter-cell antenna calibration between eNB 1 and eNB 2. In an embodiment of the invention， the method 700 can further comprise a step 704 for transmitting a RS to the eNB 1 and/or the eNB 2 to facilliate estimating UL virtual channel， and， also for transmitting channel estimation results obtained based on the reference signal from the virtual antenna of the eNB 1 and/or eNB2， i.e. ， the estimated DL virtual channels (e.g. ， quantized DL virtual channel) to the eNB 1 and/or the eNB 2. For example， in step 704， UE can transmit the quantized

and

to eNB 1 and/or eNB2. Then the information on

and

can be utilized by eNB 1 and/or eNB2 for calculating calibration factors， e.g. ， using equation (5) and (6) .

Though

methods

600 and 700 have been described with reference to Fig. 6a-6d and Fig. 7a-7b， in the context of a cellular network where eNB and UEs are involved， it can be appreciated that these methods can also apply to other scenarios， e.g. ， ad hoc network， or D2D communication network， if similar requirements for antenna calibration exist.

Reference is now made to Fig. 8a， which illustrates a block diagram of an apparatus 800 for antenna calibration according to an embodiment of the invention. The apparatus 800 according to Fig. 8a may perform a method described with reference to Figs. 6a-6d but is not limited to these methods. The methods of Fig. 6a-6d may be performed by the apparatus of Fig. 8a but is not limited to being performed by this apparatus 800. The apparatus 800 may be a eNB 501 shown in Fig 5， or a part of the eNB 501， or any other suitable devices.

As shown in Fig. 8a， the apparatus 800 comprises a self-calibration unit 801， configured for self-calibrating of multiple antennas of the apparatus 800， such that a ratio of transmitting gain to receiving gain for each of the multiple antennas is the same； a first precoding matrix generator 802， configured for generating a first precoding matrix； a virtual antenna forming unit 803， configured for applying the first precoding matrix to the multiple antennas to form a virtual antenna； and a first transmitting unit 804， configured for transmitting a reference signal to a second device via the virtual antenna， wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device.

In an embodiment of the invention， the units 801 to 804 can be configured to perform the actions in steps 601 to 604， respectively， which are described with reference to Fig. 6a. Then it can be appreciated that the operations of self-calibration， generation of the first precoding matrix and the forming of the virtual antenna described with reference to Fig. 6a also apply here， and then will not be repeated.

In another embodiment of the invention， the apparatus 800 further comprises a virtual channel estimation unit 805， configured for estimating virtual channels between each antenna of the second device and the virtual antenna； a second precoding matrix generator 806， configured for generating a second precoding matrix based on the estimated virtual channels； and the first transmitting unit 804 is further configured for transmitting multiple reference signals to the second device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from the second precoding matrix. In accordance with an embodiment of the invention， the units 804 to 806 are configured to perform the actions in steps 604 to 606 described with reference to Fig 6b. Then it can be appreciated that the operations of virtual channel estiamtion， generation of the second precoding matrix and the transmission of the mnltiple RS described with reference to Fig. 6b also apply here， and then will not be repeated here

In accordance with an embodiment of the invention， the first precoding matrix generated by the first precoding matrix generator 802 may include only one non-zero element， which means only one antenna of the multiple antennas of apparatus 800 is used for transmitting the RS and in such case， the self-calibrating unit 801 can be omitted.

According to some embodiments of the invention， the apparatus 800 may further comprise a first receiving unit 807， configured for receiving information on calibration from a third device which also transmits another reference signal to the second device in the same way as the first device； and a second receiving unit 808， configured for receiving， from the second device， information on the a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or the another refefence signal from the third device； a channel estimation unit 818 for receiving RS from the second device and then estimating the corresponding channel based on it； a first calibration parameters calculation unit 809， configured for calculating calibration parameters to be applied at least partly based on the received information on the channel estimation from the second device and/or the information on calibration from the third device； and a second transmitting unit 810， configured for transmitting at least one of the calculated calibration parameters to the third device. In accordance with an embodiment of the invention， the units 807 to 810 can be configured to perform the actions in step 607 to 610， described with reference to Fig. 6c. Then it can be appreciated that the operations described for steps 607 to 610 with reference to Fig. 6c also apply here， and then will not be repeated here.

In an embodiment， the second receiving unit 808 may configured to channel estimation results for the DL virtual channel of the apparatus 800 only， or， receive channel estimation results for the DL virtual channel of the apparatus 800 and the third device separately in multiple transmissions from the UE.

In an embodiment of the invention， the eNB2 and eNB 1 involved in inter-cell antenna calibration can be controlled by a same central control unit， e.g. ， when both eNB 1 and eNB 2 serve different sectors of a same cell. In such case， it can be assumed that there is ideal backhaul， e.g. ， fiber， between eNB 1 and eNB 2 and between eNB 1， eNB 2， and the central unit. In one embodiment of the invention， the central unit can be embodied in one of eNB 1 and eNB 2； in another embodiment of the invention， the central unit can be a separate device located outside of eNB 1 or eNB 2， e.g. ， located at the network side. In another embodiment of the invention， eNB 2 can be a remote radio head of the eNB 1， and there is also ideal backhaul between them. However， the embodiments of the invention are not limited to this. Actually， the embodiments of the invention can be implemented with any kind of links between eNB 1 and eNB 2， e.g. ， radio interface， or fiber. Then it can be appreciated that in some embodiments， the first receiving unit 807 can be omitted since the information on calibration of eNB 2 is also directly available at eNB 1. Similarly， the second transmitting unit 810 can also be omitted. Regarding the first calibration parameters calculation unit 809， in an scenario where there is a central unit responsible for the calibration calculation， the unit 809 can also be omitted from the apparatus 800.

In some embodiments of the invention， the apparatus 800 may be a transmission point (TP) involved in COMP. In an embodiment， the apparatus 800 may be configured to perform the actions described with reference to Fig. 6d to facilitate inter-cell antenna calibration. As shown in Fig. 8b， in one embodiment， to perform these actions， the apparatus 800 may further comprise a third transmission unit 811， configured for transmitting information on calibration to the third device which transmits the another reference signal to the second device in the same way as the first device； and a fourth receiving unit 812， configured for receiving at least one calibration parameter from the third device.

In another embodiment of the invention， the apparatus 800 may further comprise a third receiving unit 813， configured for receiving， from the second device， information on a channel estimation， which includes， e.g. ， quantized channel estimation results obtained based on the reference signal from the virtual antenna of the first device； and the third transmission unit 811 is further configured for transmitting to the third device a calibration factor calculated based on the information on the channel estimation from the second device.

In an embodiment of the invention， the

units

811 and 813 are configured to perform the actions of

steps

611 and 613 which are described with reference to Fig. 6d. Then it can be appreciated that the operations described for

steps

611 and 613 with reference to Fig. 6d also apply here， and then will not be repeated.

Reference is now made to Fig. 9， which illustrates a block diagram of an apparatus 900 for antenna calibration according to an embodiment of the invention. The apparatus 900 according to Fig. 9 may perform a method described with reference to Figs. 7a-7b but is not limited to these methods. The methods of Fig. 7a-7b may be performed by the apparatus of Fig. 9 but is not limited to being performed by this apparatus 900. The apparatus 900 may be a UE 502 shown in Fig 5， or a part of the UE， or any other suitable devices.

In accordance with an embodiment of the invention， the apparatus 900 can comprise a receiving unit 901， configured for receiving from a first device a reference signal to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device； wherein the reference signal is transmitted by the first device via a virtual antenna which is formed by applying a first precoding matrix to multiple antennas of the first device， and wherein the multiple antennas of the first device are self-calibrated before transmitting the reference signal.

In another embodiment of the invention， the receiving unit 901 can be further configured for receiving multiple reference signals from the first device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from a second precoding matrix， and the second precoding matrix is generated by the first device based on estimated virtual channels between each antenna of the second device and the virtual antenna of the first device； and the apparatus 900 can further comprising a second calibration parameters calculating unit 902， configured for calculating calibration parameters to be applied based on a channel estimation obtained from the received multiple reference signals.

In an example embodiment of the invention， the apparatus 900 can further comprise a first transmitting unit 903， configured for transmitting a reference signal from each antenna of the second device， to facilitate estimating the virtual channels.

In an embodiment of the invention， the units 901 to 903 are configured to perform the actions in 701 to 703 described with reference to Fig. 7a， for antenna calibration at a UE side. Thus it can be appreciated that the operations described with reference to steps 701 to 703 also apply here.

In a further embodiment of the invention， the apparatus 900 can further comprise a second transmitting unit 904， configured for transmitting a reference signal from an antenna of the second device to the first device and/or a third device， and for transmitting， to the first device and/or the third device， information on a channel estimation， e.g. ， channel estimation results obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from the third device which transmits the another reference signal to the apparatus 900 in the same way as the first device

In an embodiment of the invention， the operation performed by unit 904 corresponds to the step 704 described with reference to Fig. 7b， for the purpose of inter-cell antenna calibration to enable efficient utilization of TDD COMP.

By implementing some of the above embodiments of the invention， more accurate antenna calibration performance is made available for UE antenna calibration， and/or for inter-cell antenna calibration. It enables more reliable channel reciprocity， and thereby enables improvement for TDD system performance.

The flow charts and block diagrams in the figures illustrate the likely implemented architecture， functions， and operations of the system， method， and apparatus according to various embodiments of the present invention. In this point， each block in the flow charts or block diagrams could represent a part of a module， a program segment， or code， where the part of the module， program segment， or code comprises one or more executable instructions for implementing a prescribed logic function. It should also be noted that each block in a block diagram and/or a flow chart， and a combination of the blocks in the block diagram and/or flow chart could be implemented by software， hardware， firmware， or any of their combinations. Furthermore， it will be appreciated that in some embodiments， function of a block can also be implemented by multiple blocks， and functions of multiple blocks shown in Fig. 6a-9 may also be implemented by a single block in other embodiments.

The example embodiments can store information relating to various processes described herein， e.g. ， store the channel estimation results， the precoding matrix etc. The components of the example embodiments can include computer readable storage medium or memories according to the teachings of the present inventions and for holding data structures， tables， records， and/or other data described herein， or the program codes for implementing any of the methods according to the embodiments of the invention.

While the present inventions have been described in connection with a number of example embodiments， and implementations， the present inventions are not so limited， but rather cover various modifications， and equivalent arrangements， which fall within the purview of prospective claims. It is also obvious to a person skilled in the art that with the advancement of technology， the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above； instead they may vary within the scope ofthe claims.

Claims

A method in a first device for antenna calibration， wherein the first device is equipped with multiple antennas， the method comprising:

self-calibrating the multiple antennas， such that a ratio of transmitting gain to receiving gain for each of the multiple antennas is the same；

generating a first precoding matrix；

applying the first precoding matrix to the multiple antennas to form a virtual antenna； and

transmitting a reference signal to a second device via the virtual antenna， wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device.
The method according to Claim 1， further comprising:

estimating virtual channels between each antenna of the second device and the virtual antenna；

generating a second precoding matrix based on the estimated virtual channels；

transmitting multiple reference signals to the second device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from the second precoding matrix.
The method according to Claim 2， wherein estimating virtual channels comprising estimating virtual channels based on a reference signal from each antenna of the second device.
The method according to Claim 2， wherein generating a second precoding matrix based on the estimated virtual channels comprising:

calculating a ratio between each of the virtual channels and a reference virtual channel， wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix， and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna； and

generating the second precoding matrix based on the calculated ratio.
The method according to Claim 4， wherein the first precoding matrix includes only one non-zero element.
The method according to Claim 1， further comprising:

receiving a reference signal from an antenna of the second device， and estimating a channel between the antenna of the second device and the virtual antenna；

receiving， from the second device， information on a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from a third device which transmits the another reference signal to the second device in the same way as the first device； and

calculating calibration parameters to be applied at least partly based on the received information on the channel estimation from the second device and the received reference signal from the second device.
A method in a second device for antenna calibration， wherein the second device is equipped with multiple antennas， the method comprising:

receiving， from a first device， a reference signal to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device；

wherein the reference signal is transmitted by the first device via a virtual antenna which is formed by applying a first precoding matrix to multiple antennas of the first device， and

wherein the multiple antennas of the first device are self-calibrated before transmitting the reference signal.
The method according to Claim 7， wherein receiving， from a first device， a reference signal comprising:

receiving multiple reference signals from the first device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from a second precoding matrix which is generated by the first device based on estimated virtual channels between each antenna of the second device and the virtual antenna of the first device； and

wherein the method further comprising:

calculating calibration parameters to be applied based on a channel estimation obtained from the received multiple reference signals.
The method according to Claim 8， further comprising:

transmitting a reference signal from each antenna of the second device， to facilitate estimating the virtual channels.
The method according to Claim 8， wherein the second precoding matrix is generated by the first device through the following steps：

calculating a ratio between each of the virtual channels and a reference virtual channel， wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix， and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna of the first device； and

generating the second precoding matrix based on the calculated ratio.
The method according to Claim 7， further comprising：

transmitting a RS to the first device and/or a third device； and

transmitting， to the first device and/or the third device， information on a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from the third device which transmits the another reference signal to the second device in the same way as the first device.
An apparatus for antenna calibration， wherein the apparatus is equipped with multiple antennas， the apparatus comprising：

self-calibration unit， configured for self-calibrating of the multiple antennas， such that a ratio of transmitting gain to receiving gain ratio for each of the multiple antennas is the same；

first precoding matrix generator， configured for generating a first precoding matrix；

virtual antenna forming unit， configured for applying the first precoding matrix to the multiple antennas to form a virtual antenna； and

first transmitting unit， configured for transmitting a reference signal to a second device via the virtual antenna， wherein the reference signal is to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device.
The apparatus according to Claim 12， further comprising：

virtual channel estimation unit， configured for estimating virtual channels between each antenna of the second device and the virtual antenna；

second precoding matrix generator， configured for generating a second precoding matrix based on the estimated virtual channels； and

the first transmitting unit is further configured for transmitting multiple reference signals to the second device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from the second precoding matrix.
The apparatus according to Claim 13， wherein the virtual channel estimation unit is further configured for estimating virtual channels based on a reference signal from each antenna of the second device.
The apparatus according to Claim 13， wherein second precoding matrix generator is further configured for：

calculating a ratio between each of the virtual channels and a reference virtual channel， wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix， and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna； and

generating a second precoding matrix based on the calculated ratio.
The apparatus according to Claim 15， wherein the first precoding matrix generated by the first precoding matrix generator includes only one non-zero element.
The apparatus according to Claim 12， further comprising：

channel estimation unit， configured for receiving a reference signal from an antenna of the second device， and estimating a channel between the antenna of the second device and the virtual antenna；

second receiving unit， configured for receiving， from the second device， information on a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from a third device which transmits the another reference signal to the second device in the same way as the first device； and

first calibration parameters calculation unit， configured for calculating calibration parameters to be applied at least partly based on the received information on the channel estimation from the second device and the information on calibration from the third device.
An apparatus for antenna calibration， wherein the apparatus is equipped with multiple antennas， the apparatus comprising：

receiving unit， configured for receiving from a first device a reference signal to be used for channel estimation to facilitate antenna calibration at the first device and/or the second device；

wherein the reference signal is transmitted by the first device via a virtual antenna which is formed by applying a first precoding matrix to multiple antennas of the first device， and

wherein the multiple antennas of the first device are self-calibrated before transmitting the reference signal.
The apparatus according to Claim 18， wherein the receiving unit is further configured for receiving multiple reference signals from the first device via the virtual antenna， wherein each of the multiple reference signals is weighted by a weighting factor from a second precoding matrix， and the second precoding matrix is generated by the first device based on estimated virtual channels between each antenna of the second device and the virtual antenna of the first device； and

the apparatus further comprising：

second calibration parameters calculating unit， configured for calculating calibration parameters to be applied based on a channel estimation obtained from the received multiple reference signals.
The apparatus according to Claim 19， further comprising：

first transmitting unit， configured for transmitting a reference signal from each antenna of the second device， to facilitate estimating the virtual chanels.
The apparatus according to Claim 19， wherein the second precoding matrix is generated by the first device through the following steps：

calculating a ratio between each of the virtual channels and a reference virtual channel， wherein each of the virtual channels corresponds to a channel between an antenna of the second device and the virtual antenna formed by using the first precoding matrix， and the reference virtual channel corresponds to a channel between a reference antenna of the second device and the virtual antenna of the first device； and

generating the second precoding matrix based on the calculated ratio.
The apparatus according to Claim 18， further comprising a second transmitting unit， configured for：

transmitting a RS to the first device and/or a third device； and

transmitting， to the first device and/or a third device， information on a channel estimation obtained based on the reference signal from the virtual antenna of the first device and/or another reference signal from the third device which transmits the another reference signal to the second device in the same way as the first device.