WO2014032271A1

WO2014032271A1 - Method and apparatus for antenna calibration

Info

Publication number: WO2014032271A1
Application number: PCT/CN2012/080834
Authority: WO
Inventors: Gang Wang; Ming Lei; Liyan SU; Chenyang Yang
Original assignee: Nec(China) Co., Ltd.
Priority date: 2012-08-31
Filing date: 2012-08-31
Publication date: 2014-03-06
Also published as: CN104025526A; CN104025526B

Abstract

Methods and apparatuses for calibrating antennas among multiple BS (Base Stations) s have been disclosed. A method for calibrating antennas between a first BS and a second BS is provided. The method comprises: obtaining UL (Uplink ) CSI (Channel State Information) of the first BS based on a SRS (Sounding Reference Signal) sent from a UE (User Equipment); obtaining UL equivalent CSI of the second BS via a weighted SRS sent from the UE. The weighted SRS is weighted with a ratio of DL (Downlink) CSI of the first BS to that of the second BS. The method further comprises calculating an ambiguity factor between the first BS and the second BS based on the UL CSI of the first BS and the UL equivalent CSI of the second BS. Then, the ambiguity factor may be used for calibrating the antenna between the two BSs.

Description

METHOD AND APPARATUS FOR ANTENNA CALIBRATION

FIELD OF THE INVENTION

[0001] Embodiments of the present invention generally relates to communication systems, and more particularly to methods, apparatuses, base stations, user equipments, and computer programs for calibrating antenna between multiple base stations.

BACKGROUND OF THE INVENTION

[0002] This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

[0003] The abbreviations and terms appearing in the description and drawings are defined as below.

3 GPP Third Generation Partnership Project

BS Base Station

CoMP Coordinated multipoint

CSI Channel State Information

DL Downlink

FDD Frequency Division Duplex

ICI Inter-Cell Interference

LTE Long Term Evolution

LTE-A Long Term Evolution-Advanced

SPvS Sounding Reference Signal

TDD Time Division Duplex

UE User Equipment

UL Uplink

UTRAN Universal Terrestrial Radio Access Network

[0004] In LTE-Advanced (LTE-A) systems, base station cooperation transmission, which is also known as Coordinated multipoint (CoMP) transmission, is a promising technique to reduce the Inter-cell interference (ICI) and improve system spectrum efficiency in cellular networks, especially for multiple antenna systems.

[0005] There are two duplex modes in LTE and LTE-A systems, frequency division duplex (FDD) and time division duplex (TDD). Channel reciprocity has been regarded as one of the main advantages of a TDD system over an FDD system, because the uplink and downlink in TDD systems share the same frequency band. In other words, a base station (BS) can predict the downlink channel information by estimating the uplink channel information via a training sequence (e.g., Sounding Reference Signal (SRS)) which is transmitted from a user equipment (UE). However, the reciprocity between uplink and downlink channels in TDD systems is guaranteed only for spatial propagation channels, which is invalid in practical systems due to the mismatches of radio frequency (RF) chains used in reception and transmission for each antenna.

[0006] FIG. 1 shows a signal transmission model for illustrating the non-reciprocity between uplink and downlink channels in TDD systems.

[0007] As shown in FIG. 1, for each antenna either in the BS or in the UE, its RF chain includes a high power amplifier (HPA) for transmitting and a low noise amplifier (LNA) for receiving. A transmit-receive switch is arranged to switch between the transmission and the reception. Thus, the equivalent channel between the BS baseband and the UE baseband is consisted of a HPA, a spatial propagation channel, and a LNA. As mentioned above, the spatial propagation channel is reciprocal ideally.

[0008] Take the channel between the i^th antenna in the UE and the j^th antenna in the BS as an example. Let Sl^s and Y^BS represent the gain of the HPA and the gain of the LNA of the j^th antenna in the BS, respectively, S^^E and Υ"^Ε represent the gain of the HPA and the gain of the LNA of the i^th antenna in the UE, and c_i} represents the spatial propagation channel between the i^th antenna in the UE and the j^th antenna in the BS. Thus, the equivalent uplink channel and the equivalent downlink channel may be represented as:

εΐ = $* . ο₉ . γ? (l) gf ^f - ^ (2) [0009] Let γ^υΕ = and y = -~ denote the gain ratio of the transmission chain to the reception chain in the UE and in the BS, respectively. Thus, the relationship between the uplink channel and the downlink channel can be expressed as: = · 8? (3)

Y^BS

where —~ is referred as reciprocity errors.

[0010] Since the properties of RF chains can be varied by temperature, humidity, etc., antenna calibrations are necessary in order to fully exploit channel reciprocity.

[0011] Self calibration is a popular antenna calibration method used in single-cell systems. With such a method, the reciprocity errors of each antenna in a same BS have been calibrated to a same reference value. FIG. 2 illustrates a method for self calibration of two antennas in a BS.

[0012] As shown in FIG. 2, in the block 201 , a calibration signal (e.g.=l) is generated. The calibration signal is transmitted to amplifiers (i.e., LNAs and HPAs) of two antennas to be calibrated as indicted in the block 202. Then, two signals (e.g.=l.l and 1.2) are received and saved in the block 203. It means the amplifiers of these two antennas have different gains. Afterwards, in the block 204, the ratio between the gains of amplifiers of the two antennas is calculated and saved. In the illustrated example, the ratio is 1.1/1.2^0.92. Subsequently, the ratio may be used to calibrate the two antennas. Specifically, in the block 205, the first antenna may transmit data directly, while the second antem a may transmit data by multiplying the ratio, i.e., 0.92.

[0013] The illustrated method may be extended to three or more antennas within a BS, wherein the gains of all the antennas are calibrated to a reference value (e.g., the gain of the first antenna).

[0014] However, in multi-cell cooperative systems, e.g., in CoMP systems, where multiple BSs serve as a single super BS to mitigate the inter-cell interference, self calibration performed at each BS individually will lead to a gain ambiguity among all the cooperative BSs that seriously degrades the system performance.

[0015] An air interface calibration method among multiple BSs has been proposed to recover channel reciprocity. In such a method, user equipments (UEs) feed back estimated downlink channel information via a limited number of bits to multiple BSs to be calibrated. Then, the BSs calibrate the gain ratio of the transmit-receive chain of all their antennas based on the downlink channel information fed back from the UEs and estimated uplink channel information.

[0016] However, due to the channel estimation errors and the feedback errors, it is still hard to recover the channel reciprocity between the uplink and downlink in TDD CoMP systems with the above described air interface calibration method. Further, those errors would greatly impact the system performance.

SUMMARY OF THE INVENTION

[0008] Therefore, it would be desirable in the art to provide solutions for calibrating antennas between at least two base stations. It would also be desirable to provide a method by which BSs in TDD CoMP systems can calibrate their antennas accurately without degrading the system performance.

[0009] To better address one or more of the above concerns, in a first aspect of the invention, a method for calibrating antennas between a first base station (BS) and a second BS is provided. The method may comprise: obtaining uplink channel information of the first BS based on a training sequence sent from a user equipment (UE); obtaining uplink equivalent channel information of the second BS via a weighted training sequence sent from the user equipment, wherein the weighted training sequence is obtained by weighting the training sequence with a ratio of downlink channel information of the first BS to that of the second BS; and calculating an ambiguity factor between the first BS and the second BS based on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS, wherein the ambiguity factor is used for calibrating the antenna in at least one of the first BS and the second BS.

[0010] In some embodiments, at least one of the first BS and the second BS may comprise multiple antennas, and the multiple antemias have been calibrated by a self calibration process.

[0011] In some embodiments, calculating an ambiguity factor may comprise: jointly estimating the ambiguity factor based on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS obtained through multiple user equipments and/or during multiple frames.

[0012] In some embodiments, the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the first BS and the second BS higher than a predetermined threshold.

[0013] In some embodiments, weighting the training sequence may comprise: weighting the training sequence with an amplitude and/or phase of the ratio of downlink channel information of the first BS to the second BS.

[0014] In a second aspect of the invention, a method for a user equipment to assist antenna calibration between two base stations is provided. The method may comprise: transmitting a training sequence to the two base stations; estimating downlink channel information of the two base stations; weighting the training sequence with a ratio of the downlink channel information of the two base stations; and transmitting the weighted training sequence to at least one of the two base stations which is serving the user equipment.

[0015] In some embodiments, the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the two base stations higher than a predetermined threshold.

[0016] In some embodiments, the user equipments repeats the estimating, weighting and transmitting during multiple frames.

[0017] In some embodiments, the estimating is based on training sequences sent from the two base stations.

[0018] In a third aspect of the invention, an apparatus is provided to implement various embodiments of the method of the first aspect of the invention. Specifically, an apparatus for calibrating antennas between a first BS and a second BS is provided. The apparatus may comprise an obtainment unit, configured to obtain uplink channel information of the first BS based on a training sequence sent from a user equipment, and to obtain uplink equivalent channel information of the second BS via a weighted training sequence sent from the user equipment, wherein the weighted training sequence is obtained by weighting the training sequence with a ratio of downlink channel information of the first BS to that of the second BS. The apparatus may further comprise a calculation unit, configured to calculate an ambiguity factor between the first BS and the second BS based on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS, wherein the ambiguity factor is used for calibrating the antenna in at least one of the first BS and the second BS.

[0019] In a fourth aspect of the invention, an apparatus is provided to implement various embodiments of the method of the second aspect of the invention. Specifically, an apparatus for a user equipment to assist antennas calibration between two base stations is provided. The apparatus may comprise a transmitting unit, configured to transmit training sequence to the two base stations; an estimation unit, configured to estimate downlink channel information of the two base station; and a weighting unit, configured to weight the training sequence with a ratio of the downlink channel information of the two base stations; wherein the transmitting unit is further configured to transmit the weighed training sequence to at least one of the two base stations which is serving the user equipment.

[0020] In a fifth aspect of the invention, an apparatus is provided, which comprises at least one processor and at least one memory including computer program code. The memory and the computer program code are configured to cause the apparatus to perfonn embodiments of the method of the first aspect of the invention or embodiments of the method of the second aspect of the invention.

[0021] In a sixth aspect of the invention, a computer program product is provided, which, comprises at least one computer readable storage medium having a computer readable program code portion stored thereon. The computer readable program code portion comprises program code instructions for perfonn embodiments of the method of the first aspect of the invention or embodiments of the method of the second aspect of the invention.

[0022] In a seventh aspect of the invention, an apparatus is provided, which comprises means for implementing each step of the method of the first aspect of the invention or each step of the method of the second aspect of the invention.

[0023] Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages.

[0024] With particular embodiments of the techniques described in this specification, antennas may be calibrated among multiple BSs. Further, by feedback via weighted sounding reference signal (SRS), the feedback errors due to quantization or limited bits may be omitted and the feedback overhead can be reduced. Thus, the ambiguity factor among multiple BSs which is caused by self calibration may be calibrated. The proposed solutions may be applied in TDD Co P systems to recover channel reciprocity without degrading the system performance.

[0025] Other features and advantages of the embodiments of the present 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 present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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:

[0027] FIG. 1 shows a signal transmission model for illustrating the non-reciprocity between uplink and downlink channels in TDD systems;

[0028] FIG. 2 illustrates a method for self calibration of two antennas in a BS; [0029] FIG. 3 illustrates an exemplary signal flow according to embodiments of the present invention;

[0030] FIG. 4 illustrates the comparison of simulation result of various calibration methods;

[0031] FIG. 5 illustrates an exemplary flowchart of a method 500 according to one aspect of the present invention;

[0032] FIG. 6 illustrates an exemplary flowchart of a method 600 according to another aspect of the present invention;

[0033] FIG. 7 is a schematic block diagram of an apparatus 700 that may be configured to practice exemplary embodiments according to one aspect of the present invention;

[0034] FIG. 8 is a schematic block diagram of an apparatus 800 that may be configured to practice exemplary embodiments according to another aspect of the present invention; and

[0035] Fig. 9 is a schematic block diagram of a BS and a UE that are suitable for use in practicing the exemplary embodiments of the present invention.

[0036] Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION OF EMBODIMENTS

[0037] Hereinafter, the principle and spirit of the present invention will be described with reference to the illustrative embodiments. It should be understood, all these embodiments are given merely for the skilled in the art to better understand and further practice the present invention, but not for limiting the scope of the present invention. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield still a further embodiment. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers' specific goals, such as compliance with system-related and business -related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0038] The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the description with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

[0039] As mentioned previously, in multi-cell cooperative systems, self calibration individually performed at each BS leads to a gain ambiguity among multiple BS, which seriously degrades the CoMP system performance.

[0040] In the proposed solutions in this disclosure, one or more UEs may be selected to assist or support antenna calibration among multiple BSs. Each UE can feed back its downlink channel infomiation via a training sequence (e.g., SRS), which would eliminate the feedback errors due to quantization or limited bits as occurring in the previous described air interface calibration method. Specifically, each UE can send a SRS weighted with a ratio of downlink channel information of two BSs to be calibrated. Then, the weighted SRS may be used to calculate the ambiguity factor between the two BSs.

[0041] Consider a TDD CoMP system consisting of B base stations connected to a central processing unit (CU) via low latency backhaul links, which is known as a centralized CoMP system. The skilled in the art should appreciate that the proposed solutions of the disclosure may be applied in a decentralized CoMP system. The above example is merely for illustration.

[0042] In this example, each of the B base stations is equipped with Nt antennas and jointly serves M user equipments with single antenna. Let g^ = [g^" ■■■ g^"]" , b e {1,2,..., B} , denote the downlink channel matrix of the m^lh UE, where = ^ _mbh^ is a M-dimension vector and denotes the downlink channel between the b^xh BS and the m^lh UE, a_mb denotes large-scale attenuation including path loss and shadowing fading, denotes a channel vector consisted of small-scale fading. [0043] Thus, according to the above formula (3), the relationship between the uplink channel and the downlink channel c be expressed as:

where Y_b - rrors, and diag(x) represents a

diagonal matrix with the elements of vector x.

[0044] The aim of antenna calibration among multiple BSs is to obtain the ratio of reciprocity errors between different BSs, which may be defined as λ_Γ = j ,

ij e {1,2,..., 5} , i.e., the ambiguity factor between the i^th BS and the ^h BS. Then, the ambiguity factor may be used to adjust the power of the signal to be transmitted, so as to compensate the imbalance between the reciprocity errors of each antenna in multiple BSs.

[0045] According to the definition of reciprocity errors in formula (3), the ambiguity factor between the j^'th BS and the f^h BS may be represented directly by the downlink and uplink channels between the two BSs to a UE as blow.

where g_{mj k} denotes the channel between the k-th antenna in the i-th BS to the m-th UE.

[0046] From the equation (5), it can be seen that the ambiguity factor between two BSs may be calculated based on the downlink and uplink ratio of the two BSs to a UE.

[0047] FIG. 3 illustrates an exemplary signal flow according to embodiments of the present invention. In the scenario illustrated in FIG. 3, we assume that the antennas in both of the two BSs, i.e., BSi and BSj, have been self calibrated. In other words, the reciprocity error of each antenna in a same BS to a UE is the same. Thus, the calibration between two BSs is actually equivalent to calibration of one antenna in each BS. For simplification of illustrating, consider each BS merely with a single antenna, and the calibration is perfonned between B=2 base stations, BSi and BSj.

[0048] Further, we assume a centralized CoMP system where cooperative BSs (e.g.,

BSi 3 lOi and BSj 31 Oj) should be connected with a central processing unit (CU) 330 by backhaul links. Under such a framework, each UE 320 feeds back its channel state information (CSI) to its local or serving BS, and then the CU collects CSI from all BSs through the backhaul links. With CSI of all users at CU, the centralized CoMP system enables globally optimal cooperation among BSs. However, the skilled in the art should appreciate that the proposed solutions of the disclosure may be applied in a decentralized CoMP system, e.g., distributed CoMP systems, and thus the communication between the BSs and the CU may be adapted accordingly.

[0049] As shown in FIG. 3, one or more UEs 320 are selected to support or assist the antenna calibration between a first BSi 310i and a second BSj 310j. Both of the BSs may communicate with the CU 330. Without loss of generality, suppose the UE 320 is being served by the BSj 310j. Therefore, the link between the UE 320 and the BSj 310j may be called as local link, while the link between the UE 320 and the BSi 340i may be called as cross link,

[0050] At the step S301, the BSi and the BSj transmit training sequences, e.g., downlink reference signal (DL RS) to the UE 320. Upon received the training sequences, at the step S303, the UE 320 can estimate the downlink channel state information (DL CSI) from the received RS. The channel estimation may be implemented by various known techniques, for example least square error (LSE), minimum mean-square error (MMSE), and modified MMSE.

[0051] For example, based on MMSE estimation, the real downlink channel may be expressed as:

where denotes the estimated downlink channel, and denotes the noise.

[0052] Then, the UE 320 can calculate a ratio of the two DL CSI. Specifically, the UE can calculate a ratio of downlink small-scale channel of the two BSs to be calibrated as follows:

ft +ft - ⁽r?/r? + /

h^DL

[0053] Define A_i} exp(z 6_y) = -£~ , where A_g and 9_i} denote the amplitude and h»,j

phase of the ratio of DL CSI, respectively.

[0054] Meanwhile, at the step S302, the UE 320 transmits an uplink sounding reference signal (UL SRS, denoted as s_m) to the BSs 310i and 310j. Upon received the UL SRS, at the step S304, each of the BSs can estimate the uplink channel state infonnation (UL CSI) from the received SRS. Similarly, based on e.g., M SE, the real uplink channel may be expressed as:

S J_mUL_b

where g^ denotes the estimated uplink channel, and n^_b denotes the noise.

[0055] In some embodiments, the estimated UL CSI may be transmitted to the CU 330 via backhaul links at the step S305.

[0056] Having calculated the ratio of DL CSI, at the step S306, the UE 320 may transmit, during the next uplink frame, a weighted SRS to at least one of the two BSs.

[0057] In some embodiments, the SRS may be weighted with an amplitude and/or phase of the ratio of DL CSI. For example, considering that the transmitted signal would be power normalized, the SRS may be weighed with only the phase of the ratio, i.e.,

s_m.

Generally, the weighted SRS may be expressed as A_l} exp(/^'^) s_m. Such weighting would not influence the orthogonality of the SRS among multiple UEs.

[0058] For simplification, the UE 320 may broadcast the weighted SRS to both of the BSs to be calibrated, because the SRS will be used by the BSs for other functions than channel estimation.

[0059] Afterwards, at the step S307, the BS, e.g., the serving BSj, can obtain through channel estimation:

[0060] By simplifying the above formula (9), an observation equation (10) may be obtained:

where '" _* + 2 denotes the observation noise, and

its variance may be expressed as

[0061] As seen from the observation equation (10), the ambiguity factor X_tJ between the two BSs may be obtained based at least on the UL CSI of the i-th BS and the UL equivalent CSI of the j-th BS. In other words, depending on the definition of the X_i} (i.e., the reciprocity error ratio of the cross link to the local link), X.. may be calculated based on the UL

CSI of the cross link and the UL equivalent CSI of the local link for higher precision.

10062] Thus, going on with FIG. 3, at the step S308, the BSs 3 lOi and 310j may transmit the estimated UL equivalent CSI to the CU 330.

[0063] Alternatively, the estimated UL CSI transmitted in the step S305 may be transmitted together with the equivalent CSI in the step S308.

[0064] Then, at the step S309, having obtained those UL CSI and UL equivalent CSI, the CU can calculate the ambiguity factor based on the above observation equation (10). A plurality of observation equations may be used to jointly estimate the ambiguity factor.

[0065] In one embodiment, a plurality of user equipments may be selected to support the calibration, which may be referred as multi-user calibration.

[0066] In another embodiment, a plurality of uplink frames of a single user equipment may be used to support the calibration, which may be referred as multi-frame calibration.

[0067] In yet another embodiment, a plurality of uplink frames of multiple user equipments may be used to support the calibration, which may be referred as multi-user multi- frame calibration.

[0068] No matter which manner as above is employed, a number of (e.g., M) observation equations can be obtained which can be combined as:

where y_Tm

The co variance of w.

Tra

Li/

(13).

„7ra

'M.ij

[0069] By using the parameter estimation based on Weighted Least Square (WLS), the ambiguity factor may be obtained as:

h" R-¹

^■ y_Tra (14),

nTm^I T,a^fiTra

and the estimation error may be expressed as:

[0070] seen from the formula (15), the estimation error relates to the modulus

UL of the small-scale fading channel of the UE. Thus, when the multi-user calibration is employed is statistical independent; when the multiple frame calibration is employed, r.UL has time correlativity. If the UE moves slowly and the channel is in a deep fading, the estimation error of the multiple frame calibration will increase significantly. Apparently, more statistical independent observation equation are obtained, the ambiguity factor can be estimated more accurately.

[0071] Further, from the observation equation (10), its SNR may be expressed as:

1 1 a.. 1 1

+^■ +^■ ^■ +

L UL

SNR, SNR D a "m„„j S ~"N—R mj SN 'R m, j J

where the condition (a) assumes that the edge SNR is far greater than 1. Therefore, it can be seen from the formula (15) that SNR°f is the harmomic mean of the uplink SNRs and the downlink SNRs between the UE and the two BSs. Thus, the minimum uplink SNR of the cross link (i.e.,

) is dominated. In order to reduce the observation errors, the most efficient way is to increase the SNR of the cross link. In other words, when selecting UEs as supporters for calibration, those cell edge UEs with a high SNR of cross link, i.e., nearing the exact cell edge, should be selected for assisting the calibration among multiple BSs. In some embodiments, the selected supporter UEs are edge UEs which have SNR of link with each of the BSs to be calibrated higher than a predetermined threshold.

[0072] The skilled in the art could appreciate that other parameter estimation algorithms may also be employed, such as Least Square (LS), Random Sample Consensus LS (Ransac LS), Least Median of Squares (LMedS), etc.

[0073] Going on with FIG. 3, having obtained the ambiguity factor, the CU 330 can transmit the ambiguity factor to either or both of the BSs 3 lOi and 31 Oj at the step S310. Then at the step S311 , the BSi and/or BSj can use the received ambiguity factor to calibrate its respective antenna.

[0074] In some embodiments, either of the BSs may be selected as a reference, and then the other one can be calibrated based on the ambiguity factor. In some other embodiments, the BS whose gain of RF chain is lower may be selected as the reference. In further embodiments, both of the BSs may be calibrated based on the ambiguity factor, and algorithms may be needed for allocating the gain ratio between the BSs.

[0075] Thus the proposed mechanism of antenna calibration between two BSs has been discussed with respect to FIG. 3. The skilled in the art would appreciate that the calibration between two BSs may be easily extended to more than two BSs. Specifically, when there are B>2 base stations to be calibrated, the i-th BS may be selected as a reference. The calibration process is performed between the i-th BS and one of the remaining (B-1) BSs at each time. After perfonriing the above calibration process (B-1 ) times, all the B BSs have been calibrated. The skilled person could appreciate that the B-1 calibrations may also be done in the same time.

[0076] With embodiments of the present invention, antem as may be calibrated among multiple BSs. Further, by feedback via weighted SRS, the feedback errors due to quantization or limited bits may be eliminated and the feedback overhead can be reduced. Thus, the ambiguity factor among multiple BSs which is caused by self calibration may be calculated accurately and thus the antennas may be calibrated precisely. The proposed solutions may be applied in TDD CoMP systems to recover channel reciprocity without degrading the system performance. [0077] Simulations have been executed to evaluate the performance of the antenna calibration according to embodiments of the present invention. Table I summarizes the general environment simulation parameters.

TABLE ] .: Simulation parameters

Simulation Parameters Assumption/Explanation

Coordinated BSs number 3 BSs

Cell Radius 500 m

Path Loss Model Path Loss(dB) = 15.3 + 37.6 * loglO (d in m), where d is the distance f om the

BS

Multi-path Model Rayleigh fading

Antenna configuration 4Tx/lRx

Precoder ZF

Amplitude of ambiguity among BSs [-3, 3] dB log uniform

Phase of ambiguity among BSs [-π, π] log uniform

UE's distribution 10 dB cell edge

[0078] FIG. 4 illustrates the comparison of simulation result of various calibration methods.

[0079] As shown in FIG. 4, it shows the simulation results of ideal calibration, calibration via training (i.e., calibration via weighted SRS with 30 UEs), only phase ideal calibration, calibration with normalized pilots (i.e., calibration via phase weighted SRS), non CoMP ( C), and intra BS calibration (i.e., self calibration with each BS). The horizontal axis represents cell edge SNR in dB, and the vertical axis represents average per-user rate.

[0080] As seen from FIG. 4, the gap between the top line and the bottom line is caused by the ambiguity among BSs. The calibration via training according to embodiments of the present invention is very close to the ideal calibration; the calibration with normalized pilots is very close to the only phase ideal calibration. Thus, the proposed calibration method can almost completely regenerate the performance.

[0081] In the following description, the proposed mechanism will be detailed with respect to exemplary embodiments illustrated in the drawings.

[0082] FIG. 5 illustrates an exemplary flowchart of a method 500 according to one aspect of the present invention. Depending on the framework of the TDD CoMP system, i.e., centralized or decentralized CoMP, the method 500 may be perfomied by an entity in a base station (e.g., an eNB) cooperating in the CoMP system, or by multiple entities distributed among the BS and the CU in the CoMP system. The method 500 can be performed for calibrating antennas among multiple BSs. For illustration, the method 500 will be discussed with respect to two BSs (a first BS and a second BS) and one supporter UE as an example. The second BS is serving for the UE, i.e., the link between the second BS and the UE is the local link, while the link between the first BS and the UE is the cross link.

[0083] As shown in FIG. 5, the method 500 may begin at the step S501 and proceed to the step S502. At the step S502, if the BS to be calibrated comprises multiple antennas, the multiple antennas may be calibrated by a self calibration process. The self calibration process may be performed according to any known or further developed technique, such as hardware self calibration, feedback self calibration. The present invention has no limitation in this point.

[0084] Then, at the step S503, uplink channel state information (UL CSI) of the two BSs may be obtained based on a training sequence (i.e., S S) sent from the UE. As described previously, the calculation of the ambiguity factor λ~ may be based at least on the UL CSI of the cross link and the UL equivalent CSI of the local link. Therefore, at this step in the example, it is only needed to obtain the UL CSI of the first BS. Nevertheless, the UL CSI of the two BSs may be estimated for other purposes.

[0085] The method 500 may proceed to step S504. At the step S504, uplink equivalent CSI of the two BSs may be obtained based on a weighted training sequence (i.e., weighted SRS) sent from the UE. Likewise, at the step S504 in this example, it is only needed to obtain the UL equivalent CSI of the local link or the second BS.

[0086] As described with the step S303 and S306 in FIG. 3, the weighted SRS may be obtained by weighting the SRS with a ratio of downlink CSI (DL CSI) of the first BS to that of the second BS. In some embodiment, the SRS may be weighted with an amplitude and/or phase of the ratio of DL CSI. For example, considering that the transmitted signal would be power normalized, the SRS may be weighed with only the phase of the ratio.

[0087] Then, at the step S505, having obtained the UL CSI and the UL equivalent CSI, the ambiguity factor between the two BSs may be calculated based at least on the UL CSI of the first BS and the UL equivalent CSI of the second BS. The ambiguity factor may be jointly estimated based on the UL CSI of the first BS and the UL equivalent CSI of the second BS obtained through multiple UEs and/or during multiple frames. The calculation of the ambiguity factor may refer to the above description with respect to the step of S309 in FIG 3, and thus the detailed description is omitted here.

[0088] Thereafter, the calculated ambiguity factor may be used to calibrate the antennas between the two BSs. For example, the first BS may be selected as a reference, and the second BS adjusts the gain of its RF chain based on the ambiguity factor when transmission.

[0089] FIG. 6 illustrates an exemplary flowchart of a method 600 according to another aspect of the present invention. The method 600 may be performed by an entity in a user equipment in a TDD CoMP system. For illustration, the method 600 will be discussed with the same scenario as the method 500, that is, two BSs (a first BS and a second BS) and one supporter UE as an example. The second BS is serving for the UE, i.e., the link between the second BS and the UE is the local link, while the link between the first BS and the UE is the cross link.

[0090] As shown in FIG. 6, the method 600 may begin at the step S601 and proceed to the step S602. At the step S602, the UE may transmit a SRS to the BSs to be calibrated.

On the other hand, at the step S603, the UE can receive a pilot signal or a reference signal (RS) from each of the BSs to be calibrated, respectively. The UE can estimate the DL CSI of the two BSs and calculate the ratio of the two DL CSI.

[0091] Then, at the step S604, the UE can weight its SRS with the calculated ratio of the two DL CSI. In some embodiment, the SRS may be weighted with an amplitude and/or phase of the ratio of DL CSI.

[0092] Finally, at the step S605, the UE may transmit the weighted SRS to at least one of the two BSs which is serving the UE. As mentioned previously, the calculation of the ambiguity factor may be based at least on the UL CSI of the cross link and the UL equivalent CSI of the local link. Therefore, the UE can merely transmit the weighted SRS to the second BS, i.e., its local BS. Preferably, the UE can broadcast the weighted SRS to both of the BSs, because the SRS may be further used for other purposes by the BSs.

[0093] Thus, via a weighted SRS with the ratio of DL CSI, the UE can feed back the DL CSI accurately, which may be used to estimate the ambiguity factor.

[0094] In some embodiments, multiple UEs (e.g., 30 or more) may be selected to support the antenna calibration. Each of these UEs can perform the method 600 to assist the antenna calibration. Those cell edge UEs with a high SNR of cross link, i.e., nearing the absolute edge, may be selected for assisting the calibration among multiple BSs. In other words, the selected supporter UEs are edge UEs which have SNR of link with each of the BSs to be calibrated higher than a predetermined threshold.

[0095] In some other embodiments, a plurality of uplink frames of a single user equipment may be used to support the calibration. If the UE is moving quickly, then the performance of the multi-frame calibration may increase due to the time-variant channel. The UE can repeat the method 600 during multiple frames.

[0096] In further embodiments, a plurality of uplink frames of multiple user equipments may be used to support the calibration.

[0097] FIG. 7 is a schematic block diagram of an apparatus 700 that may be configured to practice exemplary embodiments according to one aspect of the present invention. The apparatus 700 may be incorporated in a BS or eNB and be configured to perform methods of the exemplary embodiments of the present invention. Alternatively, the apparatus 700 may be distributed among various network elements, such as BS and CU, so as to perform the methods of the exemplary embodiments of the present invention.

[0098] As shown in Fig. 7, the apparatus 700 may comprise an obtainment unit 701 and a calculation unit 702.

[0099] The obtainment unit 701 may be configured to obtain UL CSI of a first BS based on a training sequence sent from a UE, and to obtain UL equivalent CSI of a second BS via a weighted training sequence sent from the UE. The weighted training sequence is obtained by weighting the training sequence with amplitude and/or phase of a ratio of DL CSI of the first BS to that of the second BS.

[00100] The calculation unit 702 may be configured to calculate an ambiguity factor between the first BS and the second BS based at least on the UL CSI of the first BS and the UL equivalent CSI of the second BS. Then, the calculated ambiguity factor may be used to calibrate the antennas between the two BSs.

[00101] The calculation unit 702 may be further configured to jointly estimate the ambiguity factor based on the UL CSI of the first BS and the UL equivalent CSI of the second BS obtained through multiple UEs and/or during multiple frames.

[00102] It should be understood, the units 701-702 contained in the apparatus 700 are configured for practicing exemplary embodiments of the present invention. Thus, the operations and features described above with respect to FIGs. 3 and 5 also apply to the apparatus 700 and the units therein, and the detailed description thereof is omitted here.

[00103] FIG. 8 is a schematic block diagram of an apparatus 800 that may be configured to practice exemplary embodiments according to another aspect of the present invention. The apparatus 800 may be incorporated in a UE and be configured to perform methods of the exemplary embodiments of the present invention.

[00104] As shown in FIG. 8, the apparatus 800 may comprise an estimation unit 801 , an weighting unit 802 and a transmitting unit 803.

[00105] The estimation unit 801 may be configured to estimate DL CSI of at least two BSs. The estimation is based on training sequence (RS) sent from the BSs.

[00106] The weighting unit 802 may be configured to weight the SRS with a ratio of the DL CSI of the two BSs. The ratio is calculated based on the estimated DL CSI by the estimation unit 801.

[00107] The transmitting unit 803 may be configured to transmit the SRS or the weighted SRS to the BSs to be calibrated.

[00108] It should be understood, the units 801-803 contained in the apparatus 800 are configured for practicing exemplary embodiments of the present invention. Thus, the operations and features described above with respect to FIGs. 3 and 6 also apply to the apparatus 800 and the units therein, and the detailed description thereof is omitted here.

[0017] Fig. 9 illustrates a simplified block diagram of a BS 901 and a UE 902 that are suitable for use in practicing the exemplary embodiments of the present invention. In Figure 9, a wireless network is adapted for communication with the UE 902, referred to in the above examples as an LTE-LAN UE, via the BS 901, referred to in the above examples as an LTE-LAN BS (or eNB). The UE 902 includes a data processor (DP) 903, a memory (MEM) 904 coupled to the DP 903, and a suitable RF transmitter TX and receiver RX 905 (which need not be implemented in a same component) coupled to the DP 903. The MEM 904 stores a program (PROG) 906. The TX/RX 905 is for bidirectional wireless communications with the BS 901. Note that the TX/RX 905 has at least one antenna to facilitate communication; multiple antennas may be employed for multiple-input multiple-output MIMO communications in which case the UE 902 may have multiple TXs and/or RXs.

[0002] The BS 901 includes a data processor (DP) 907, a memory (MEM) 908 coupled to the DP 907, and a suitable RF transmitter TX and receiver RX 909 coupled to the DP 907. The MEM 608 stores a program (PROG) 910. The TX RX 909 is for bidirectional wireless communications with the UE 902. Note that the TX/RX 909 has at least one antenna to facilitate communication, though in practice a BS will typically have several. The BS 901 may further include a communication interface 911 for interfacing with other network elements. The communication interface 911 may be X2 interface for bidirectional communications with other BSs. Alternatively or additionally, the communication interface 911 may be backhaul links with the CU. The BS 1100 may be coupled via a data path to one or more external networks or systems, such as the internet, for example.

[0001] The BS 901 may be coupled via a data path to one or more external networks or systems, such as the internet, for example.

[0002] At least one of the PROGs 906 and 910 is assumed to include program instructions that, when executed by the associated DPs 903 and 907, enable the UE 902 and BS

901 to operate in accordance with the exemplary embodiments of this invention, as discussed herein with the methods 500 or 600.

[0003] In general, the various embodiments of the UE 902 can include, but are not limited to, cellular phones, personal digital assistants (PDAs) having wireless communication capabilities, portable computers having wireless communication capabilities, image capture devices such as digital cameras having wireless communication capabilities, gaming devices having wireless communication capabilities, music storage and playback appliances having wireless communication capabilities, Internet appliances permitting wireless Internet access and browsing, as well as portable units or terminals that incorporate combinations of such functions.

[0004] The embodiments of the present invention may be implemented by computer software executable by one or more of the DPs 903, 907 of the UE 902 and the BS 901, or by hardware, or by a combination of software and hardware.

[00109] The MEMs 904 and 908 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one MEM is shown in the BS 901 or UE 902, there may be several physically distinct memory units in the BS 901 or UE 902. The DPs 903 and 907 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non limiting examples. Either or both of the UE

902 and the BS 901 may have multiple processors, such as for example an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

[00110] Exemplary embodiments of the present invention have been described above with reference to block diagrams and flowchart illustrations of methods, apparatuses (i.e., systems). It will be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, respectively, can be implemented by various means including computer program instructions. These computer program instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart block or blocks. [00111] The foregoing computer program instructions can be, for example, sub-routines and/or functions. A computer program product in one embodiment of the invention comprises at least one computer readable storage medium, on which the foregoing computer program instructions are stored. The computer readable storage medium can be, for example, an optical compact disk or an electronic memory device like a RAM (random access memory) or a ROM (read only memory).

[00112] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any implementation or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular implementations. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub- combination.

[00113] It should also be noted that the above described embodiments are given for describing rather than limiting the invention, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims. The protection scope of the invention is defined by the accompanying claims. In addition, any of the reference numerals in the claims should not be interpreted as a limitation to the claims. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.

Claims

WHAT IS CLAIMED IS:

1. A method for calibrating antennas between a first base station (BS) and a second BS, comprising:

obtaining uplink channel information of the first BS based on a training sequence sent from a user equipment;

obtaining uplink equivalent channel information of the second BS via a weighted training sequence sent from the user equipment, wherein the weighted training sequence is obtained by weighting the training sequence with a ratio of downlink channel information of the first BS to that of the second BS; and

calculating an ambiguity factor between the first BS and the second BS based at least on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS, wherein the ambiguity factor is used for calibrating the antenna in at least one of the first BS and the second BS.

2. The method of claim 1, wherein at least one of the first BS and the second BS comprises multiple antennas, and the multiple antennas have been calibrated by a self calibration process.

3. The method of claim 1 or 2, wherein calculating an ambiguity factor comprises: jointly estimating the ambiguity factor based on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS obtained through multiple user equipments and/or during multiple frames.

4. The method of claim 1 or 2, where the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the first BS and the second BS higher than a predetermined threshold.

5. The method of claim 1 or 2, wherein weighting the training sequence comprises: weighting the training sequence with an amplitude and/or phase of the ratio of downlink channel information of the first BS to that of the second BS.

6. A method for a user equipment to assist antennas calibration between two base stations, comprising: transmitting a training sequence to the two base stations;

estimating downlink channel information of the two base stations;

weighting the training sequence with a ratio of the downlink channel information of the two base stations; and

transmitting the weighed training sequence to at least one of the two base stations which is serving the user equipment.

7. The method of claim 6, the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the two base stations higher than a predetermined threshold.

8. The method of claim 6 or 7, wherein the user equipment repeats the estimating, weighting, and transmitting during multiple frames.

9. The method of claim 6 or 7, wherein the estimating is based on training sequences sent from the two base stations.

10. An apparatus for calibrating antennas between a first base station (BS) and a second BS, comprising:

an obtainment unit, configured to obtain uplink channel information of the first BS based on a training sequence sent from a user equipment, and to obtain uplink equivalent channel information of the second BS via a weighted training sequence sent from the user equipment, wherein the weighted training sequence is obtained by weighting the training sequence with a ratio of downlink channel information of the first BS to that of the second BS; and

a calculation unit, configured to calculate an ambiguity factor between the first BS and the second BS based at least on the uplink channel infomiation of the first BS and the uplink equivalent channel infomiation of the second BS, wherein the ambiguity factor is used for calibrating the antenna in at least one of the first BS and the second BS.

11. The apparatus of claim 10, wherein at least one of the first BS and the second BS comprises multiple antennas, and the multiple antennas have been calibrated by a self calibration process.

12. The apparatus of claim 10 or 1 1 , wherein the calculation unit is further configured to jointly estimate the ambiguity factor based on the uplink channel information of the first BS and the uplink equivalent channel information of the second BS obtained through multiple user equipments and/or during multiple frames.

13. The apparatus of claim 10 or 11 , where the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the first BS and the second BS higher than a predetermined threshold.

14. The apparatus of claim 10 or 11 , wherein the weighted training sequence is weighted with an amplitude and/or phase of the ratio of downlink channel inforaiation of the first BS to that of the second BS.

15. An apparatus for a user equipment to assist antennas calibration between two base stations, comprising:

a transmitting unit, configured to transmit training sequence to the two base stations; an estimation unit, configured to estimate downlink channel information of the two base station; and

a weighting unit, configured to weight the training sequence with a ratio of the downlink channel information of the two base stations;

wherein the transmitting unit is further configured to transmit the weighed training sequence to at least one of the two base stations which is serving the user equipment.

16. The apparatus of claim 15, the user equipment is an edge user equipment having signal-to-noise-ratios (SNRs) of links with the two base stations higher than a predetermined threshold.

17. The apparatus of claim 15 or 16, wherein the user equipment repeats the estimating, weighting, and transmitting during multiple frames.

18. The apparatus of claim 15 or 16, wherein the estimation unit is configured to estimate the downlink channel information based on training sequences sent from the two base stations.