# Patent

## Title
Method and apparatus for interference alignment in a wireless communication system

## Abstract

The present invention relates to a method and apparatus for interference alignment in a wireless communication system. A method in which a terminal aligns interferences in a wireless communication system, which includes a first base station governing a first cell and a second base station governing a second cell, comprises: a step in which the terminal measures a serving channel from the first base station which is a serving base station and an interference channel from the second base station; a step of transmitting null space information orthogonal to the interference channel to the first base station; a step of measuring channels from the first base station and the second base station to which an inference alignment plane generated based on the null space information is applied, and generating a receiving filter; and a step of transmitting, to the first base station, the interference alignment plane, the receiving filter, and effective channel measurement value based on the channel from the first base station.

## Background
TECHNICAL FIELD
The present disclosure relates to a wireless communication system and, in particular, to an interference alignment method and apparatus in uplink and downlink.
BACKGROUND ART
There have been many inventions in the field of the cellular communication systems and fourth generation communication technologies for in the last few years. The network MIMO systems referred to as multi-cell MIMO systems are focused on the IMT-Advanced and beyond systems. In such multi-cell MIMO networks, the inter-cell interference is one of the significant problems. In the well-designed multi-cell MIMO systems, the inter-cell coordination can be accomplished through cooperative signal configuration. There are many methods well-known for making in possible to reach certain interference channel capacity through efficient cooperative multi-cell processing design. One of the well-known methods is interference alignment.
The interference alignment is a cooperative processing of the transmitters/receivers. This is to generate overlapped signal spaces occupied by unwanted interferences at the respective receivers while maintain the wanted signal spaces. On the time or frequency varying channel, the interference alignment is capable of achieving symbol extension in time and frequency domain. In the present disclosure, the main focus is on the space rather than time and frequency. The spatial freedom degree is conceived along with constant channel coefficient.
The channel alignment for three users (K=3) interference channel having multiple antennas (M>1) is known to have the external and internal boundaries of freedom degree that are equal to each other. When the external and internal boundaries of the freedom degree match each other, it is regarded that the interference alignment has achieved the optimal freedom degree. However, it has not been known whether such a result can be extended to the case of normal number of users (K>3) and plural receive and transmit antennas. In the different networks such as X networks and cellular networks, the constant channel coefficient of the interference alignment for achieving the optimal freedom degree is not known yet.
DISCLOSURE OF INVENTION
Technical Problem
The present disclosure considers the interference alignment problem on the 2-cell cellular networks having constant channel coefficient. The present disclosure is directed to the realistic scenario having different numbers of transmit/receive antennas instead of the assumption of equal numbers of transmit/receive antennas as (in the invention of interference alignment for symbol extension). The two cells have the same structure (i.e. homogeneous network) and, in this structure, each cell includes K users having N antennas and one base station having M antennas. The present disclosure is directed to the scenario of M>N. The interference alignment of multiple access channels (MAC) is considered first and then, the MAC interference alignment is extended to Broadcast Channel (BC) interference alignment using the duality between the MAC and BC. The proposed interference alignment method is referred to as null space interference alignment. The cases in which the transmit antennas are correlated and networks are heterogeneous are considered briefly.
Solution to Problem
In accordance with an aspect of the present invention, an interference alignment method of a base station in a wireless communication system including a first base station managing a first and a second base station managing a second cell includes measuring, at the first base station, a channel of a serving base station and interference channel of neighbor base station from terminals located in the first and second cells, generating an interference alignment plane based on the measurement result, calculating a transmission precoder for the terminal located in the second cell, the transmission precoder preventing the terminal located in the second cell from being influenced by interference a channel from the first cell to which the interference alignment plane is applied, and transmitting the transmission precoder to the terminal located in the second cell.
In accordance with another aspect of the present invention, an interference alignment method of a terminal in a wireless communication system including a first base station managing a first and a second base station managing a second cell includes measuring, at the terminal, a serving channel from the first base station as a serving base station and an interference channel from the second base station, transmitting null space information orthogonal with the interference channel to the first base station, measuring the channels from the first and second base stations to which an interference alignment plane generated based on the null space information is applied and generating a reception filter, and transmitting the interference alignment plane, the reception filter, and valid channel measurement value based on the channel from the first base station to the first base station.
In accordance with an aspect of the present disclosure, a base station for aligning interference in a wireless communication system including a first base station managing a first and a second base station managing a second cell includes a transceiver which transmits and receives signals to and from neighbor base station or a terminal and a control unit which controls measuring, at the terminal, a serving channel from the first base station as a serving base station and an interference channel from the second base station, transmitting null space information orthogonal with the interference channel to the first base station, measuring the channels from the first and second base stations to which an interference alignment plane generated based on the null space information is applied and generating a reception filter, and transmitting the interference alignment plane, the reception filter, and valid channel measurement value based on the channel from the first base station to the first base station.
In accordance with another aspect of the present invention, a terminal for aligning interference in a wireless communication system including a first base station managing a first and a second base station managing a second cell includes a transceiver which transmits and receives signals to and from a serving base station or a neighbor base station and a control unit which controls measuring, at the first base station, a channel of a serving base station and interference channel of neighbor base station from terminals located in the first and second cells, generating an interference alignment plane based on the measurement result, calculating a transmission precoder for the terminal located in the second cell, the transmission precoder preventing the terminal located in the second cell from being influenced by interference a channel from the first cell to which the interference alignment plane is applied, and transmitting the transmission precoder to the terminal located in the second cell.
Advantageous Effects of Invention
According to the present disclosure, the base station and the terminal are capable of cancelling the interference from neighbor base stations and terminals efficiently in uplink and downlink.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram illustrating architecture of MAC (uplink) wireless network according to an embodiment of the present disclosure.
FIG. 2 is a flowchart illustrating the interference alignment procedure of the base station in uplink (MAC).
FIG. 3 is a diagram illustrating architecture of B (downlink) wireless network according to an embodiment of the present disclosure.
FIG. 4 is a flowchart illustrating the interference alignment procedure of the base station in downlink (MAC).
FIG. 5 is a diagram illustrating architecture of wireless communication network including macro and femto cells according to an embodiment of the present disclosure.
FIG. 6 is a block diagram illustrating a configuration of the base station according to an embodiment of the present disclosure.
FIG. 7 is a block diagram illustrating a configuration of the terminal according to an embodiment of the present disclosure.
FIGS. 8 to 11 are graphs illustrating simulation results showing validity of the null spatial interference alignment method proposed in the present invention.



MODE FOR THE INVENTION
1 Null Space Interference Alignment on 2-Cell MIMO MAC
The detailed description on the proposed method is mainly directed to the MAC scenario. The extension to the BC scenario may be understood from the duality of MAC and BC.
1.1 MAC Channel Model
FIG. 1 is a diagram illustrating architecture of MAC (uplink) wireless network according to an embodiment of the present disclosure.
In the present disclosure, a structure of interference alignment mechanism for 2-cell K-user uplink MIMO is proposed. The network includes cell a and cell b. In each cell, the base station α(αε{a,b}) may receive K independent messages from K independent users within cell α.
In the network, 2K users transmit signals to the base stations simultaneously. While the base station α listen to K users within cell α, other K users within cell α incur interference to the base station α (see FIG. 1). Here, α={a,b}/α. Then, the channel can be expressed as follows.




  
    
      
        
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In equation (1), YαεCM×1 and ZαεCM×1 denote the received signal and noise vectors respectively. The noise vector Zα is generated such that each entry is characterized by zero mean and unit variance complex Gaussian distribution, i.e. independently and identically distributed (i.i.d.). The vectors xαmεCN×1 and xαmεCN×1 are the transmission vectors related to each base station α and user m within the base station α. The matrix HαmεCM×N denotes the channel from the user m within the cell α to the base station α, GαmεCM×N denotes an interference channel from user m within the cell α to the base station α. The entry of each of Hαm and Gαm is CN(0,1) and i.i.d. and all channel matrixes are independent. The transmission vector xαm is as follows.

xαm=Wαmsαm  (2)

In equation (2), sαmεCβ×1 is the symbol carrying message Wαm, WαmεCN×β is the beamforming vector for transmitting the symbol sαm for β>0. At each transmitter, the average power ρ is restricted to E[∥xαm∥2]≦ρ as an example.
1.2 Null Spatial Interference Alignment Framework
In the present disclosure, a transmission and reception method for aligning K independent interferences from the interference cells completely on the low dimensional space (<max(M, N)) is proposed in order for the zero-forcing-based signaling to null out all the interference completely. In the legacy interference alignment method for cellular networks, it is known that the per-cell freedom degree is K and each user secures one interference free signal dimension under the assumption of M=N=K+1.
The present disclosure considers the case of M>N for handling realistic scenario on the cellular networks (where the base station has the antennas greater in number than the user), and this method takes notice of transmitting plural streams of β>0 (not limited to one data stream transmission). According to this configuration, the conditions for interference alignment are characterized. The conditions for the number of antennas that are required to solve  signal dimension are conceived. In order to build an achievable method, the null space interference alignment concept is used. The null space interference alignment requires base stations using the null space planes selected to reflect the reception signal. In order to mitigate the out-of-cell freedom degree loss cooperatively, such a plane (or surface, hereinafter the terms ‘surface’ and ‘plane’ are used interchangeably) is designed such that the out-of-cell interferences are overlapped. The dimensions of the interference-free signals at the respective base station depend on the overlapped null space dimension so called geometric multiplicity with γ>0 in this disclosure.
The 2-cell MAC interference alignment method consists of two steps, i.e. out-of-cell interference alignment and inter-user interference removal. The out-of-cell interference alignment is achieved by selecting a precoder WαmεCN×β for projecting the received signal vectors to the interference alignment plane Pα and forcing null to the projected valid channel. If the out-of-cell interference alignment is performed, inter-user interference cancellation follows through channel inversion (or zero forcing). Detailed description are made of detailed conditions of out-of-cell interference alignment for M=Nγ+β and ≧N≧β.
If M>N and if number of users of each cell is K, the out-of-cell interference alignment plan for cell α is defined as PαεC×M, and a set of the user indices is defined as ={1,2, . . . K}. Pα is designed for rank(PαGαm)<N−β in ∀mε. Since the homogeneous networks are assumed, the condition is as follows.

dim(null(PαGαm))≧β in ∀mε  (3)

Inequality (3) means that Pα is designed to distributed the available dimensions to nullify the out-of-cell interferences induced in {Gαm}mε. If the condition of inequality (3) is fulfilled, each user of cell α configures the zero-forcing transmission vector WαmεCN×β to become span(Wαm)⊂null(PαGαm), i.e. equation (4).

PαGαmwαm=0 in ∀mε  (4)

In this way, the base station α guarantees cancellation of out-of-cell interference. As described above, the interest is focused on the realist case of multi-user cellular networks, i.e. M=Nγ+β and ≧N≧β. In this case, certain PαεC×M has to fulfill the condition rank(Pα)= in order for the base stations to decode  streams.
In order to see the practicability of inequality (3), it is necessary to check the presence of the interference alignment plane Pα fulfilling inequality (3) and conceive detailed concept for designing Pα. Then, null space interference alignment criteria for pairs different in numbers of transmit and receive antennas are characterized. For convenience sake, γ is defined.
Definition: In AiεCM×N and Null(Ai*)=NAiεCM×(M−N), the null spaces corresponding to a set of matrix {A1, . . . , AK} are referred to as {NA1, . . . , NAK}. If the all γ-tuples of the null spaces {NAms, . . . , NAma} are non-empty intersection, i.e. if {∩i=1a NAmi}≠∅ in ∀miε, the set {A1, . . . , AK} is referred to as null spaces having geometric multiplicity γ.
The null space of GαmεCM×N is defined as Null(G*αm)=NαmεCM×(M−N) in ∀m ε. Then the maximum geometric multiplicity γ of the set {Gα, 1, . . . , Gα, K} can be characterized as follows.
Thesis 1: when {Gα1, . . . , GαK} under the conditions GαmεCM×N and M>N, the maximum geometric multiplicity γ is given by




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for





  
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This means that the γ-tuple combinations (non-repetitive) of the null spaces {Nαms , . . . , Nαmi } of {Gα,1, . . . , Gα,K} has non-empty intersection. The value of




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denotes the maximum geometric multiplicity. If M=Nγ+β and ≧N≧β, M and N fulfill (γ−1)N<M−N≦γN always.

The null space interference alignment condition of inequity (3), i.e. dim(null(PαGαm))≧β in ∀m ε has the meaning as follows.

dim(ran(Nαm)∩ran(Pα*))≧β  (5)

In order to fulfill the condition of inequity (5), at least β rows of Pα has to be Nαm in ∀mε. It is assumed that the set {Gα,1, . . . , Gα, K} has null spaces of geometric multiplicity of γ≧1. Once {Gα,1, . . . , Gα, K} is fulfilled under γ≧1, Pα capable of inequity (5) (or inequity (3)) exists always.
The kth γ-tuple index set is defined as {πi}kγ+k−1 having πi=((i−1)mod K)+1 in kε. Then, the orthonormal basis of the intersection null space related to {πi}kγ+k−1 is expressed as Nπ(k)εCNγ+β)×β.




  
    
      
        
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A simple method for configuring Pαfulfilling inequity (5) is to assign (k−1)β+1 as β columns of Nα(k) given in equation (7) to the ()th row of Pα.

Pα=[Nα(1)Nα(2) . . . Nα(K)]*   (7)

Here, it is not unique to map the columns of Nα(k) to the rows of Pα. Any mapping from β column of Pα to β row of Pα is possible (the only problem is the expression). In this way, inequity (5) is fulfilled always.
The above-described configuration of Pα is based on the assumption in that {Gα,1, . . . , Gα,K}, has γ≧1 and M=Nγ+β. Since Pα has γβ. rows in the null space of {Gα,1, . . . , Gα, K}, the out-of-cell interference channel PαGα,m projected in mε has the non-zero rows of (K−γ)β. Accordingly, if equations (8) and (9) are as follows in 1≦γ≦K, inequity (5) is fulfilled.

N=(K−γ)β+β  (8)

M=Nγ+βif 1≦γ≦K−1 and M=Nγ+Kβif γ=K.   (9)

Once Pα is designed, the inter-user interference is removed with matrix inversion. The matrix is defined as follows.

Hα=[PαHα1wa1PαHα2wα2 . . . PαHαKwαK]

Also, the zero-forcing vector vai* for user i is a set of normalized rows from (i−1 )β+1 to iβ of the matrix Hα−1 and selected by the base station α. The symbols from the user i of the cell α are decoded by multiplying vai* to PαYα.
1.3 Steps: MAC Interference Alignment (Uplink MAC Interference Alignment Method)
Descriptions are made of the detailed steps of the proposed interference alignment method. The network determines the geometric multiplicity γ first, and N and M are determined by equations (8) and (9). The index set is expressed as ={1, . . . , K}.
Step 1: The base station α measures Hαm and G αm for ∀mε. The base station α maps the column β of Nα(m) and ith row of Pα so as to form an interference alignment plane as shown in equation (7).
The interference plane Pα may be called the first step combiner of the receivers which may project the inter-cell interference signals in a specific subspace. The subspace is calculated prudently to null the out-of-cell interference while the intended signal within the serving cell is recovered completely.
Step 2: The base station α calculates the transmit precoder for the mth user of the cell a and feeds back the precoder weights to the users of the cell α to achieve wαm εNull(PαGαm) for ∀mε (i.e. to be mapped to the null space of the equivalent (effective) channel with the application of the interference alignment plane to the interference channel==to avoid the interference from the equivalent channel with the application of the interference alignment plane to the interference channel).
This means to design the transmit precoder for the mobile terminals to achieve PαGαmWαm=0 (i.e. to null out interference channels from all users of cell α at BS α==(mathematically) to null out the interference with the application of the interference plane at BS α when receiving the interference channel to which the transmit precoder of the respective interference users is applied. The cooperative design of the interference plane Pα and Wαm removes the inter-cell interference.
Step 3: The base station α generates the valid channel matrix Hα=[PαHα1 wa1PαHα2wα2 . . . PαHαKwαK] and calculates a receive zero-forcing vector vαm for decoding symbols from mth user of the cell α as the mth row of Hα−1.
The zero-forcing vector vαm may be called the second step combiner performed after the interference plane Pα. The aim is to null out intra-cell interference occurring by the co-scheduled users.
The proposed interference alignment method achieves the freedom degree  per cell.
α indicates a or b. Accordingly, above-described operation is applicable to both the base stations a and b.
In detail, in case of the base station a,
Step 1: The base station a measure Ham and Gam for ∀mε. The base station a maps the column β of Na(m) to the ith row of Pa to form the interference alignment plane as shown in equation (7)
Step 2: The base station a calculates the transmit precoder for mth user of cell b to achieve wbmεNull(PaGam) for ∀mε and feeds back the precoder weights to the users of cell b (to be mapped to the null space of the equivalent (effective) channel acquired by applying interference alignment plane to the interference channel ==to avoid the interference from the channel equivalent to the interference channel to which the interference alignment plane is applied.
Step 3: The base station a forms an effective channel matrix Ha=[PaHa1wa1PaHa2 . . . PaHaKwaK] (BS a establish an equivalent channel for receiving the transmit signal to which the precoder weights received by the respective users of the cell a are applied) and calculates the receive zero-forcing vector vam for decoding the symbol from the mth user of the cell α as the mth row of Ha−1.
The detail of the above procedure is described with reference to FIG. 2.
FIG. 2 is a flowchart illustrating the interference alignment procedure of the base station in uplink (MAC).
In more detail, the BS a measures its own channel and interference channels to all users located within the coverage of BS and BS b at step S210 (Step 1 above). Then, the BS a forms an interference alignment plane at step S220.
The BS a calculates the transmit precoder for the terminals belonging to the BS 2 at step S230 (Step 2 above). The transmit precoder is calculated in the way of preventing the terminals belonging to the BS b from being interfered by the BS a.
The BS a transmit the calculated precoder to the corresponding terminals at step S240.
The BS a forms a channel in the way that the terminals located within its cell are capable of removing interference from the neighbor cell, i.e. BS b at step S250 (step 3 above). The BS a calculates the receive zero-forcing vector value for decoding the symbol transmitted by a certain user.
Meanwhile, in case of base station b,
Step 1: The base station b measures Hbm and Gbm for ∀mε. The base station b maps the column of β of Nb(m) to the ith row of Pb as shown in equation (7) to form an interference alignment plane.
Step 2: The base station b calculates a transmit precoder for the mth user of cell a to achieve wamεNull(PbGbm) for ∀mε and feeds back the precoder weights to the users of the cell a.
Step 3: The base station b forms the valid channel matrix Hb=[PbHb1wb1PbHb2wb2 . . . PbHbKwbK] and calculates the receive zero-forcing vector vbm for decoding the symbol from the mth user of the cell b.
1.4 steps: BC Interference Alignment (Downlink Interference Alignment Method)
FIG. 3 is a diagram illustrating architecture of B (downlink) wireless network according to an embodiment of the present disclosure.
Using the duality of MAC and BC, the interference alignment method for BC to achieve the same freedom degree as MAC is described. Suppose that γ is given and M and N are determined as in the case of MAC. According to the duality, the downlink channels have the reciprocal relation with the uplink channels. That is, when HαmεCN×M and GαmεCN×M, Hαm=HαmT and Gαm=GαmT come into existence.
Step 1: The user αm measures Hαm and Gαm. The user αm feeds back Null(Gαm)=Nαm to the base station α.
The feedback information may be quantized or non-quantized.
Step 2: The base station α transmits Nam for ∀mε to the base station α form the out-of-cell interference alignment plane Pα by mapping the column β of Nα(m) (γ-tuple intersection null space composed by {Nαm}mε) to Pα.
Step 3: the user αm measures HαmPα and GαmPα(measures channels to which interference alignment plane is applied), renders Wαm*GαmPα=0 (null out interference when the user am receives the transmit signal to which the interference alignment plane applied at the interference cell base station), and forms received combination weight (reception filter) Wαm to achieve Wαm*GαmPα=0.
Hαm at step 1 and Hαm at step 3 may be identical with or different from each other.
HαmPα and GαmPα may be measured based on the user-specific precoded reference signals or common non-precoded reference signals.
In the case of using the user-specific precoded reference signals, the user αm measures HαmPα and GαmPα directly. At this time, the user-specific precoded reference signals transmitted by the base station α are precoded by Pα, and the user-specific precoded reference signals transmitted by the base station α  are precoded by Pα.
In the case of using the non-precoded reference signals, the user αm measures Hαm based on the common non-precoded reference signals transmitted by the base station α, and the base station α measures Gαm based on the common non-precoded reference signals transmitted by the base station α. The base station α informs the user αm of the Pαin DL. Alternatively, the base station α may share Pα with the base station α. Then the base station α informs the user αm of Pα in DL. The DL indication of Pα and Pα may be implemented in quantized or non-quantized method.
Step 4: The user αm feeds back wαm*HαmPα (equivalent channel received through the reception filter as the transmit signal to which the base station α has applied the interference alignment plane) to the base station α.
The feedback information may be quantized or non-quantized.
Step 5: The base station α forms Hα=[Wα1*Hα1Pα; . . . ; Wα{tilde over (K)}*Hα{tilde over (K)}Pα] and calculates the zero-forcing transmission weight Vαm for the user as the mth column of Hα−1 (by combining the equivalent channels fed back from the respective users, the base station a designs the transmit filter. The zero-forcing transmit filter may be an example of the transmit filter.
The above procedure is described in detail with reference to FIG. 4.
FIG. 4 is a flowchart illustrating the interference alignment procedure of the base station in downlink (MAC).
The first terminal measures the channel from the first base station serving the first terminal and interference channel from other cells (i.e. interference cells) at step S405 (step 1 above). Next, the first terminal transmits the channel information from the interference cell, i.e. the null space information on the channel of the interference cell, to the first base station, at step S410. The null space information is orthogonal with the measured interference channel.
Then the first base station transmits the null space information to the second base station as the interference base station at step S415. The second base station generates an interference alignment plane at step S420.
The terminal measures the channel from the first base station and the interference channel again using the signals precoded with the interference alignment plane and transmitted by the first and second base stations at step S425.
The terminal generates a reception filter based on the measurement result at step S430. The reception filter is characterized by removing the interference when the interference signal to which interference alignment plane is applied in the interference cell is received through the interference channel.
The terminal transmits the interference alignment plane, channel, and valid channel measurement value in consideration of the reception filter of the terminal from the first base station at step S435 (step 4 above).
The first base station receives the valid channel feedback from the terminal to design the transmission filter at step S440 (step 5 above) and transmits data to the terminal at step S445.
α indicates a or b. Accordingly, the above described operation is applicable to both the base stations a and b.
In detail, in the case of the base station a,
Step 1: The user am measures Hαm and Gαm. The user am feeds back Null(Gαm)=Nαm to the base station a.
The feedback information may be quantized or non-quantized.
Step 2: The base station a transmits for Nαm to the ∀mε to the base station be to form the out-of-cell interference alignment plane Pb ∀mε by mapping the column β of Nα(m) (γ-tuple intersection null space formed by {Nαm}mε) to Pb.
Step 3: The user am measures HamPa and GamPb and forms a reception combination weight (reception filter) to achieve Wam*GamPb=0.
Ham of step 1 and Ham of step 3 may be identical with or different from each other.
The measurement of HamPa and GamPb may be performed based on the user-specific precoded reference signals or common non-precoded reference signals.
In the case of using the user-specific precoded reference signals, the user am measures HamPa and GamPb directly. At this time, the user-specific precoded reference signals transmitted by the base station a is precoded with Pa, and the user-specific precoded reference signals transmitted by the base station be are precoded with Pb.
In the case of using the common non-precoded reference signals, the user am measures Ham based on the common non-precoded reference signals transmitted by the base station a and Gam based on the common non-precoded reference signals transmitted by the base station b. The base station a informs the user am of Pa in DL. The base station b informs the user am of Pb in DL. Alternatively, the base station b may share Pb with the base station a. Then the base station a may inform the user am of Pb in DL. The indication of Pa and Pb may be performed through quantized or non-quantized method.
Step 4: The user am feeds back Wam*HamPa to the base station a.
The feedback information may be quantized or non-quantized.
Step 5: The base station a forms Ha=[Wa1*Ha1Pa; . . . ; Wa{tilde over (K)}*Ha{tilde over (K)}Pa] and calculates zero-forcing transmission weight Vαm for the user am with mth column of Ha−1.
In detail, in the case of base station b,
Step 1: The user bm measures Hbm and Gbm. The user bm feeds back Null(Gbm)=Nbm to the base station b.
The feedback information may be quantized or non-quantized.
Step 2: The base station b transmits Nbm for ∀mε to the base station a to form the out-of-cell interference alignment plane Pa by mapping the column β of Nb(m) (γ-tuple intersection null space composed by {Nbm}mε) to Pa.
Step 3: The user bm measures HbmPb and GbmPa and forms reception combination weight (reception filter) Wbm to achieve Wbm*GbmPa=0.
Hbm of step 3 and Hbm of step 1 may be identical with or different from each other.
HbmPb and GbmPa may be measured based on the user-specific precoded reference signals or common non-precoded reference signals.
In the case of using the user-specific precoded reference signals, the user am measures HbmPb and GbmPa directly. At this time, the user-specific precoded reference signals transmitted by the base station b is precoded with Pb, and the user-specific precoded reference signals transmitted by the base station a is precoded with Pa.
In the case of using the common non-precoded reference signals, the user bm measures Hbm based on the common non-precoded reference signals transmitted by the base station b and Gbm. based on the common non-precoded reference signals transmitted by the base station a. The base station a informs the user bm of Pa in DL. Alternatively, the base station a may share Pa with the base station b. Then the base station b informs the user bm of Pa in DL. The DL indication of Pb and Pa may be performed through quantized or non-quantized method.
Step 4: The user bm feeds back Wbm*HbmPb to the base station b.
The feedback information may be quantized or non-quantized.
Step 5: The base station b formsHb=[Wb1*Hb1Pb; . . . ; Wb{tilde over (K)}*Hb{tilde over (K)}Pb] and calculates zero-forcing transmission weight Vbm for the user bm with the mth column of {right arrow over (H)}b−1.
Due to the duality of MAC and BC, the interference alignment proposed for BC achieves the freedom degree of  per cell which fulfills M≧N for 2-cell homogeneous cellular networks. For BC, the proposed interference alignment method requires two CSI feedback (steps 1 and 4). Accordingly, the proposed method is appropriate for slow fading channels for which two forward-reverse link accesses is possible in the channel coherence time.
Hereinafter, a modified example characterized by large feedback overhead and frequent message exchange between the base stations in spite of the gain requiring a single feedback.
Step 1: The user αm measures Ham and Gam. The user αm feeds back both Ham and Gam to the base station α.
Step 2: The base station αm transmits the matrices for Gαm for ∀mε to the base station α. The base station α maps the column βof Nα(m) (γ-tuple intersection null space composed by {Nαm}mε) to Pα to form the out-of-cell interference alignment plane Pα. In order to accomplish Wαm*GαmPα=0, the base station a forms the reception combination weight (reception filter) Wαm for all the users αm based on Pα and Gαm.
Step 3: The base station α transmits Wαm for ∀mε to the base station α. The base station α calculates Wαm*HαmPα based on Hαm prior feedback performed at step 1.
Step 4: The base station α generates Hα=[Wα1*Hα1Pα; . . . ; Wα{tilde over (K)}*Hα{tilde over (K)}Pα] and calculates zero-forcing transmission weight Vαm for the user αm as the mth column of Hα−1. The base station α notifies the user αm of the reception combination weight Wαm in DL (in the quantized or non-quantized way).
α indicates a or b. Accordingly, the above described operation is applicable to both the base stations a and b.
In detail, in the case of the base station a,
Step 1: The user am measures Hαm and Gαm. The user am feeds back both the Hαm and Gαm to the base station a.
Step 2: The base station a transmits the matrices Gam for ∀mε to the base station b. The base station b forms the out-of-cell interference alignment plane Ft by mapping the column β of Na(m) γ-tuple intersection null space composed by to {Nam}mε) to Pb. In order to accomplish Wam*GamPb=0, the base station generates the reception combination weight (reception filter) Wam for all the users am based on the known Pb and Gam 
Step 3: The base station b transmits Wam for ∀mε to the base station a. The base station a calculates Wam*HamPa based on the Ham prior feedback performed at step 1.
Step 4: The base station a forms Ha=[Wa1*Ha1Pa; . . . ; Wa{tilde over (K)}Ha{tilde over (K)}Pa] and calculates zero-forcing transmission weight Vam the user am as the mth column of Ha−1. The base station a notifies the user am of the reception combination weight vam in DL (in the quantized or non-quantized way).
In detail, in the case of the base station b,
Step 1: The user bm measures Hαm and Gαm. The user bm feeds back both Hαm and Gαm. to the base station b.
Step 2: The base station b transmits the matrices Gbm for ∀mε to the base station a. The base station a forms the out-of-cell interference alignment plane Pa by mapping the column β of Nb(m) (γ-tuple intersection null space composed by {Nbm}mε) to Pa. In order to accomplish Wbm*GbmPa=0, the base station a generates the reception combination weight (reception filter) for all the users bm based on the known Pa and Gbm.
Step 3: The base station a transmits Wbm for ∀mε to the base station b. The base station calculates Wbm*HbmPb based on the Hbm prior feedback performed at step 1.
Step 4: The base station generates Hb=[Wb1*Hb1Pb; . . . ; Wb{tilde over (K)}*Hb{tilde over (K)}Pb] and calculates zero-forcing transmission weight Vbm for the user bm as the mth column of {tilde over (H)}b−1. The base station b notifies the user bm in DL of the reception combination weight wbm (in quantized or non-quantized way).
2. Other Particulars
2.1 Degenerative Channel (Channel Correlation in View of Transmission)
In view of transmission, the antenna correlation exists often to degenerate channel state. There is a doubt on whether the channel degeneration degrades the interference alignment performance. An interesting scenario about BC (MAC is also handled in similar way) is considered. The channel correlation matrices in view of transmission from the serving and interference base stations are defined as Rαm=E[Hαm*Hαm] and Sαm=E[Gαm*Gαm] for rank(Rαm)=rank(Sαm)=r. Then the channels are modeled by Hαm=Hαm,wRαm1/2Gαm=Gαm,wSαm1/2. Here, Hαm,w and Gαm,w are N×M zero averaging and unit variation complex Gaussian matrices i.i.d. If r≧N, it is identical with the null spatial interference alignment scenario described in the previous section, the interest is focused on the case fulfilling r<N herein. Since the channel correlation is a long term statistical data, it is assumed that the base station α knows Rαm for all users. In this case, since r<N, the user eliminates the out-of-cell interference always. Accordingly, β=N−r defined. In this scenario, the conventional zero-forcing transmission for =M is possible.
2.1.1 Steps.
It is noted that there is no need to configure the interference alignment in section 3.1.1 since the user always can null out the signals from other interference cells for r<N.
As described later, in 3.1.1, the operation of calculating the interference alignment plane is omitted as compared to the procedure described in section 2.3 and 2.4.
This is a special case, i.e. the case capable of being applied for r<N, and thus the description is directed to the case capable of achieving the same effect by omitting specific operation or process other than using the interference alignment method through the two-stage feedback scheme of the previous sections 2.3 and 2.4.
Step 1: The user αm measures Hαm and Gαm and calculates N×β zero-forcing reception vector Wαm for Wαm*Gαm=0 (since r<N, i.e. Null(G*αm)=Wαm, it is always possible).
Step 2: The user αm feeds back Wαm*Hαm to the base station α.
This feedback information may be quantized or non-quantized. The quantized feedback may be performed by designing a codebook projected on Rαm .
Step 3: The base station α generates Hα=[Wα1*Hα1; . . . ; WαM*HαK] and selects a transmit beamforming weight for the mth user from Hα−1.
α indicates a or b. Accordingly, the above described operation is applicable to both the base stations a and b.
In detail, in the case of the base station a,
Step 1: The user am measure Ham and Gam and calculates N×β zero-forcing reception vector for Wam for Wam*Gam=0 (since r<N, i.e. Null(G*am)=Wam, it is possible always).
Step 2: The user am feeds back Wam*Ham to the base station a.
This feedback information may be quantized or non-quantized. The quantized feedback is performed by designing the codebook projected on Ram.
Step 3: The base station a generates the matrix Ha=[Wa1*Ha1; . . . ; WaM*HaK] and selects transmission beamforming weight for the mth user from Ha−1.
In detail, in the case of base station b,
Step 1: The user bm measures Hbm and Gbm and calculates N×β zero-forcing reception vector Wbm for Wbm*Gbm=0 (since r<N, i.e. Null(G*bm)=Wbm, it is possible always).
Step 2: The user bm feeds back Wbm*Hbm to the base station b.
This feedback information may be quantized or non-quantized.
The quantized feedback is performed by designing the codebook projected on Rbm.
Step 3: The base station b generates the matrix Hb=[Wb1*Hb1; . . . ; WbM*HbK] and selects the transmission beamforming weight for the mth user from Hb−1.
When r≦N, the proposed interference alignment method achieve the freedom degree of  per cell. In case of BC, one time CSI feedback is enough and there is no need to design the out-of-cell interference alignment plane dependent on Gαm. In the null spatial interference alignment method without channel correlation, the interference alignment plane aims to degenerate the valid channel ranks less than N for all the users. Actually, since the channel correlation matrices Rαm1/2 and Sαm1/2 responsible for a function similar to the null spatial interference alignment plane and the correlation matrices are independent among each other, the interference alignment for =M is done.
Here, an important aspect of the proposed interference alignment is addressed. Typically, a strong channel correlation from the base station's view point is a curse for multi-user MIMO. However, in view of the eligibility of the null spatial interference alignment for K users, the potential antenna correlation is a blessing.
This method is more appropriate for the highly-transmit correlated channel of the base station. This method may be applied to other types of transmission scenarios. Although the above method is applicable, its performance may be inferior to that of the method described in section 2.4.
2.2 Hybrid Method
In section 2.4, a description is made of the BC interference alignment requiring two-stage feedback. Section 3.1 shows the method simplified dramatically in the high transmit correlation at the base station. In the case of the hybrid method of this section, the base station switches between the methods of sections 2.4 and 3.1 according to the long term correlation matrix.
In the hybrid method, all users of the network measures their long term and/or broadband transmit matrices respectively based on channel measurements. Also, all the users report their long term and/or correlation matrices to the respective base stations in the quantized or non-quantized scheme. On the basis of cooperation through correlation matrices exchange, the base station determines the use of the method of section 2.4 or 3.1.
2.3 Interference Alignment in Heterogeneous Networks
FIG. 5 is a diagram illustrating architecture of wireless communication network including macro and femto cells according to an embodiment of the present disclosure.
As a modification of the null spatial interference alignment method, the heterogeneous networks composed of the macro and femto (or pico) cells are considered. The femto cells are appropriate for use as low power and short range data access points and capable of providing improved coverage and throughput. The cross-tire interference between the macro and femto cells is the challenge for the deployment of the heterogeneous networks. In the femto cell, the signal user transmission is known as a scheme capable of providing superior coverage and spatial reuse related to the multi-user transmission. Accordingly, it is presumed that the signal user transmission gives array gain in the femto cell and the K multi-user transmissions gives multiplexing gain.
Each of the users of the network has N receive antennas (2≦N≦K). If the number of transmit antennas of the femto cell base station (base station a) is L(≧N), the interference alignments of the femto and macro cells are expressed as paεCL×1 and PbεCM×K. The macro cell base station (base station b) has M(=K+1 transmit antennas. Here, β=1 is assumed.
The interference alignment plane for the femto cell paεCL×1 may be designed randomly. This is because the user bm may configure wbm anytime under 2≦N≦K to null out the interference from the femto cell base station 1 (i.e. wbk* Gbkpa=0). Accordingly, once pa is known to all of the users, the out-of-cell interference alignment for the macro cell is achieved without backhaul from the base station b to the base station a. In order to design the interference alignment plane PbεCM×K for the macro cell, the first to (K+1−N)th columns has to span across Null (Ga1) In order to accomplish Span(Pb′)⊂Null(Ga1) the matrix having K+1−N columns is expressed as Pb′εCM×(K+1−N). Then the interference alignment plane Pb for the macro cell may be designed as Pb=[Pb′Pb′′] for Pb′′εCM×(N−1) .
Step 1: The user a1 measures Ha1 and Ga1. The user a1 calculates wa1 to achieve wa1*Ga1Pb=0 and feeds back Pb′ to the base station a.
The user bk measures Hbk and Gbk. The user bk calculates wbk to achieve wbk*GbkPa=0 and feeds back wbk*Hbk to the base station b.
Step 2: The base station a sends Pb′ to the base station b.
Step 3: The base station b configures Pb=[Pb′, Pb′′] and calculates zero-forcing transmission weight vbk for the user bk to achieve vbkεNull({(wbi*HbiPb)*}iε{1mK}/k).
The proposed method may achieve the freedom degree of 1 for the femto cell users through one time CSI feedback and freedom degree per K cells for the macro cell users (state 2).
3 Base Station and Terminal Configurations
FIG. 6 is a block diagram illustrating a configuration of the base station according to an embodiment of the present disclosure. As shown in FIG. 6, the mobile terminal of the present disclosure includes a transceiver 610 and a controller 620.
The transceiver transmits and receives data or signals to and from another terminal or adjacent base station.
The control unit 620 controls signal flows among the components of the base station for the operations of the base station. As shown in FIG. 6, the control unit 620 includes a measurement module, an interference alignment plane generator 622, and a transmission precoder calculation module 623.
The measurement module 621 measures the channel to the base station and the interference channels to the neighbor base station from the terminals located in the coverage of the serving and neighbor base stations.
The interference alignment plane generation module 622 generates the interference alignment plane based on the measurement result. In this case, the interference measurement plane is generated by mapping a specific column β of the null space matrix which is orthogonal with the interference channel to a specific row i of the interference alignment plane.
The transmission precoder calculation module 623 calculates the transmission precoder for the terminal located in the coverage of the neighbor base station in such a way of avoiding the interference from the channel acquired by applying the interference alignment plane the measured interference channel. The transmission precoder calculation module 623 transmits the calculated transmission precoder to the terminal located in the coverage of the neighbor cell.
Although the description is directed to the case where the control unit 620 includes independent blocks responsible for different functions, the present disclosure is not limited thereto. For example, the channel measurement operation may be performed by the control unit 620 in itself.
FIG. 7 is a block diagram illustrating a configuration of the terminal according to an embodiment of the present disclosure. As shown in FIG. 7, the terminal includes a transceiver 710 and a control unit 720.
The transceiver 710 transmits and receives data or signals to and from the serving or neighbor base station.
The control unit 720 controls signal flows among the function blocks for overall operations of the terminal. As shown in FIG. 7, the control unit 702 may include a measurement module 721 and a reception filter generation module.
The measurement module 721 measures the channel from the serving base station and the interference channels from neighbor base stations. The measurement module 721 also transmits the null space information orthogonal with the interference channel to the serving base station.
The null space information is transmitted from the serving base station to the neighbor base station.
The neighbor base station generates the interference alignment plane based on the null space information and shares the interference alignment plane with the serving base station.
Then the reception filter generation module 722 measures the channel from the serving and neighbor base stations to which the interference alignment plane has been applied. The reception filter is designed to remove the interference when the terminal receives the signal to which the interference alignment plane is applied by the neighbor base station.
The reception filter generation module 722 calculates a valid channel measurement value based on the interference alignment plane, reception filter, and channel received from the serving base station.
Then the serving base station design the transmission filter by combining equivalent valid channel measurement values received the respective terminals.
4 Simulation
Though this simulation, the validity of the null spatial interference alignment strategy proposed in the present disclosure is verified. The proposed interference alignment method requiring on time CSI feedback is simulated in the MAC, degenerated BC, and femto cell networks. In this simulation, it is assumed that the beamforming scenario attempts a single stream transmission per user without out-of-cell interference and inter-user interference.
FIG. 8 shows the sum throughput for MAC with M=5, N=2, and K=3 (i.e. γ=2). The achievable per-cell freedom degree is K=3. According to the algorithm described in section 2.3, one time feedback of the quantized transmit beamformer for out-of-cell users of base station a to base station Ft is required. As the number of feedback bits, the sum rate approximates the complete CSI sum throughput.
FIG. 9 shows the sum throughput on degenerated BC with M=4, N=3, and K=4. It is assumed that the correlation at the transmitter degenerate the channel rank which is less than N=3 (i.e. r=2<N). Since the correlation matrix acts as the interference alignment plane for the respective users on the degeneration BC, it is not necessary for the base station to design the interference measurement plane. As shown in the drawing, as the CSI resolution increases, the achievable sum throughput increases.
In the final simulation, the interference alignment in the femto cell BC networks with M=4, L=2, N=2, and K=2. In order to observe the interference limited behavior of the femto cell, it is assumed that the femto cell is located at a distance near the macro cell. That is, it is assumed the total power of the interference from the macro cell base station to the femto cell base station is greater than twice.
FIG. 10 shows the thumb throughput achievable for macro and femto cell users with the application of the interference alignment method according to an embodiment of the present invention. The achievable throughput for the femto cell user is limited due to the high interference limitation. In order to address the high cross-tier interference, a user scheduling may be considered for use in the femto cell to give additional throughput gain. In order to achieve potential gain in the femto cell, a user scheduling for the femto cell may be considered. The scheduler selects the user for which channel matches pa validly. That is, the user maximizing ∥Ha1pa∥ is selected.
FIG. 11 shows throughput without user scheduling and the user scheduling throughput of user scheduling with KK=4 comparatively.

## Claims

1. An interference alignment method of a base station in a wireless communication system including a first base station managing a first cell and a second base station managing a second cell, the method comprising:
measuring, at the first base station, a channel of a serving base station and interference channel of neighbor base station from terminals located in the first and second cells;
generating an interference alignment plane based on a result of the measurement;
calculating a transmission precoder, based on the interference alignment plane, for the terminal located in the second cell, the transmission precoder preventing the terminal located in the second cell from being influenced by interference a channel from the first cell to which the interference alignment plane is applied; and
transmitting the transmission precoder to the terminal located in the second cell,
wherein the interference alignment plane is generated by mapping a predetermined column of a null space matrix orthogonal with the interference channel to a predetermined row of the interference alignment plane.

2. The method of claim 1, further comprising forming a channel to the terminals located in the first cell, the channel being designed for preventing the terminals located in the first cell from being influenced by the interference from the second cell.

3. An interference alignment method of a terminal in a wireless communication system including a first base station managing a first cell and a second base station managing a second cell, the method comprising:
measuring, at the terminal, a serving channel from the first base station as a serving base station and an interference channel from the second base station;
transmitting null space information orthogonal with the interference channel to the first base station;
measuring the channels from the first and second base stations to which an interference alignment plane generated based on the null space information is applied and generating a reception filter; and
transmitting the interference alignment plane, the reception filter, and valid channel measurement value based on the channel from the first base station to the first base station,
wherein the interference alignment plane is generated by mapping a predetermined column of a null space matrix orthogonal with the interference channel to a predetermined row of the interference alignment plane.

4. The method of claim 3, wherein the reception filter is configured to null out the interference when a signal to which the interference alignment plane is applied is received from the second base station as an interference base station.

5. The method of claim 3, further comprising receiving data transmitted by the first base station through a transmission filter, the transmission filter being generated by the first base station based on the valid channel measurement value.

6. The method of claim 5, further comprising forming a channel for the terminal located in the first cell to prevent the terminals located in the first cell from being influenced by the interference from the second cell.

7. A terminal for aligning interference in a wireless communication system including a first base station managing a first cell and a second base station managing a second cell, the terminal comprising:
a transceiver configured to transmit and receive signals to and from a serving base station or a neighbor base station; and
a controller configured to control to:
measure, at the terminal, a serving channel from the first base station as a serving base station and an interference channel from the second base station,
transmit null space information orthogonal with the interference channel to the first base station,
measure the channels from the first and second base stations to which an interference alignment plane generated based on the null space information is applied and generate a reception filter, and
transmit the interference alignment plane, the reception filter, and valid channel measurement value based on the channel from the first base station, to the first base station,

wherein the interference alignment plane is generated by mapping a predetermined column of a null space matrix orthogonal with the interference channel to a predetermined row of the interference alignment plane.

8. The terminal of claim 7, wherein the reception filter is configured to null out the interference when a signal to which the interference alignment plane is applied is received from the second base station as an interference base station.

9. The terminal of claim 7, wherein the controller is configured to control to receive data transmitted by the first base station through a transmission filter which is generated by the first base station based on the valid channel measurement value.