WO2014153731A1 - Method and apparatus for interference alignment in a time division duplex system - Google Patents

Abstract

Embodiments of the present disclosure relate to a method and apparatus for interference alignment in a Time Division Duplex (TDD) system. The method may comprise: determining an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver; and determining, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction. In embodiments of the present disclosure, there is provided a coordinated interference alignment that may be implemented with high capacity and low latency, and it may reduce dimension of space spanned by interference signals, support more data streams in direct channels and achieve more diversity gains.

Description

METHOD AND APPARATUS FOR INTERFERENCE ALIGNMENT IN A TIME

DIVISION DUPLEX SYSTEM

FIELD OF THE INVENTION

[0001] Embodiments of the present disclosure generally relate to wireless communication techniques and more particularly relate to a method and apparatus for interference alignment in a time division duplex (TDD) system.

BACKGROUND OF THE INVENTION

[0002] With the constant increase of mobile data services and emergence of new-type applications, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications and LTE-Advanced (LTE-A) specifications. As the next generation cellular communication standard, an LTE or LTE -Advance system can operate in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.

[0003] To meet constantly increasing requirements on data rate and the coverage quality, in the 3 GPP LTE-A, there are proposed Heterogeneous Network (HetNet) technologies to improve the network performance. In a HetNet, there are deployed, for example, a Marcocell, a RRH and a small-type base station node operating at a low power, such as picocell, femtocell, relay, and etc. With the small-type base station node, a distance between an end user and a base station is shorten greatly and quality of receive signals can be enhanced, and furthermore, the transmission rate, the spectrum efficiency and the coverage for cell edge users can also be improved.

[0004] However, the use of a plurality of base stations might introduce some problems, especially interferences. For example, the Marcocell will interfere with the small-type base station such as the picocell, femtocell, or relay when it transmits signals, and vice visa; a User Equipment (UE) might also interfere with other UEs when it transmits signals to a base station.

[0005] In the TDD based LTE (TD-LTE) system, in order to adapt to the asymmetrical DL/UL data traffic, there has been advantageously proposed an asymmetrical DL/UL resource configuration scheme in which there are provided seven different semi-statically DL/UL configurations. Such an asymmetrical resource configuration scheme provides different DL/UL configuration patterns from which the base station can select a suitable configuration based on the UL data size and the DL data size. However, in such a system, it might result in cross-sub frame co-channel interference (CCI) due to the mismatched transmission directions in neighboring cells.

[0006] In 3 GPP T 36.828, the uplink-downlink interference between RRHs has been considered as the major impairment in TD-LTE systems and there had been proposed various interference cancelling approaches, including interference alignment. The interference alignment is an interference management scheme in which interference form different sources will be aligned at receiver side so as to cancel effect of the interference on a desired signal.

[0007] In US patent application publication No. 20110051837A1, there is disclosed an interference alignment scheme that aligns interference among three communication pairs. In the proposed scheme, beamformers are used to align interference at transmit side, and at the same time, receivers are used to cancel out the aligned interference.

[0008] In Chinese patent application publication No. 102651661 A, there is also disclosed an interference alignment method in a TD-LTE system. In this patent application, an interference alignment scheme focusing on a channel mismatch problem is disclosed, in which the Eigen beam forming (EBF) is combined with the interference alignment, smoothened and corrected channels are utilized for calculating the pre-coding vectors and receiving vectors for the interference alignment, and thus a better performance of the interference alignment is obtained.

[0009] However, the proposed interference alignment schemes have their own drawbacks. Particularly, they are designed for specific scenarios, have strict application constrains, and can not be extended to other scenarios.

[0010] Therefore, there is a need for an improved interference alignment scheme in the art. SUMMARY OF THE INVENTION

[0011] In view of the foregoing, the present disclosure provides a new solution for interference alignment in a TDD system so as to solve or at least partially mitigate at least a part of problems in the prior art.

[0012] According to a first aspect of the present disclosure, there is provided a method for interference alignment in a Time Division Duplex TDD system. The method may comprise determining an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver; and determining, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction.

[0013] In an embodiment of the present disclosure, the transmitter may be selected based on antenna configurations in the TDD system.

[0014] In another further embodiment of the present disclosure, the method may further comprise determining another interference alignment reference direction based on the transmission direction of the another transmitter; and determining, based on the another interference alignment reference direction, a transmission direction of a further transmitter interfering another receiver so as to align its interference at the another receiver with the another interference alignment reference direction.

[0015] In a further embodiment of the present disclosure, the transmitter that is selected from the plurality of transmitters may be a transmitter configured with a lower number of antennae.

[0016] In a yet embodiment of the present disclosure, the determining the interference alignment reference direction may further comprise determining the transmission direction of the transmitter that is selected from the plurality of transmitters according to a beamformer determination principle; and determining the interference alignment reference direction from the transmission direction of the transmitter and interference channel measurement information of the another transmitter.

[0017] In a still further embodiment of the present disclosure, the determining the transmission direction of the transmitter that is selected from the plurality of transmitters may be performed based on direct channel measurement information of the transmitter.

[0018] In a yet further embodiment of the present disclosure, the beamformer determination principle may comprise one of: signal to leakage plus noise ratio principle; maximum signal to interference plus noise ratio principle; maximum signal to noise ratio principle; and random precoding principle.

[0019] In a still yet further embodiment of the present disclosure, the determining a transmission direction of the another transmitter of the plurality of transmitter be performed based on the interference alignment reference direction and interference channel measurement information of the another transmitter.

[0020] According to a second aspect of the present disclosure, there is also provided an apparatus for interference alignment in a Time Division Duplex TDD system. The apparatus may comprise a reference direction determination unit configured to determine an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver; and a transmission direction determination unit configured to determine, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction.

[0021] According to a third aspect of the present disclosure, there is also provided a central control unit in a centralized radio access network, comprising an apparatus according to the second aspect of the present disclosure.

[0022] According to a fourth aspect of the present disclosure, there is further provided, a computer-readable storage media with computer program code embodied thereon, the computer program code configured to, when executed, cause an apparatus to perform actions in the method according to any one of embodiments of the first aspect.

[0023] According to a fifth aspect of the present disclosure, there is provided a computer program product comprising a computer-readable storage media according to the third aspect.

[0024] According to a sixth aspect of the present disclosure, there is provided an device for interference alignment in a Time Division Duplex TDD system, comprising a processor and at least one memory having computer codes stored therein, the computer codes are configured to, when executed, cause the processor to perform operations in the method according to any one of embodiments of the first aspect. [0025] In embodiments of the present disclosure, there is provided a coordinated interference alignment that may be implemented with high capacity and low latency. With these embodiments, dimension of space spanned by interference signals may reduced, more data streams can be supported in direct channels and more diversity gains can be achieved. Additionally, the interference alignment according to embodiments of the present disclosure has a better scalability and thus may be easily extended to different scenarios.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other features of the present disclosure will become more apparent through detailed explanation on the embodiments as illustrated in the embodiments with reference to the accompanying drawings, throughout which like reference numbers represent same or similar components and wherein:

[0027] Fig. 1 schematically illustrates a network in which embodiments of the present disclosure may be implemented;

[0028] Fig. 2 schematically illustrates a flowchart of a method for interference alignment in a TDD system according to an embodiment of the present disclosure;

[0029] Fig. 3A schematically illustrates an example of HetNet wherein two uplink pico cells are interfered by one downlink macro cell;

[0030] Fig. 3B schematically illustrates another example of HetNet wherein one uplink macro cell is interfered by two downlink pico cells;

[0031] Fig. 4 schematically illustrates a three-cell scene wherein two uplink transmission and one downlink transmission are operating according to an embodiment of the present disclosure;

[0032] Fig. 5A schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario 1-1 according to an embodiment of the present disclosure;

[0033] Fig. 5B schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario 1-2 according to an embodiment of the present disclosure;

[0034] Fig. 5C schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario 1-3 according to an embodiment of the present disclosure;

[0035] Fig. 5D schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario 1-3 according to an embodiment of the present disclosure;

[0036] Fig. 6 schematically illustrates a three-cell scene wherein one uplink transmission and two downlink transmission are operating according to an embodiment of the present disclosure;

[0037] Fig. 7A schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario II- 1 according to an embodiment of the present disclosure;

[0038] Fig. 7B schematically illustrates a diagram of a process of information exchange with in the BBU pool in scenario II-2 according to an embodiment of the present disclosure;

[0039] Fig. 8 schematically illustrates a block diagram of an apparatus for interference alignment in a TDD system according to an embodiment of the present disclosure;

[0040] Figs. 9A to 9D schematically illustrate CDF of achievable rate of three different schemes in ayleigh fading channel and in SCM fading channel in the three-cell scene as given in Fig. 4; and

[0041] Figs. 10A to 10D schematically illustrate CDF of achievable rate of three different schemes in Rayleigh fading channel and in SCM fading channel in the three-cell scene as given in Fig. 6.

DETAILED DESCRIPTION OF EMBODIMENTS

[0042] Hereinafter, a method and apparatus for interference alignment in a

TDD system will be described in details through embodiments with reference to the accompanying drawings. It should be appreciated that these embodiments are presented only to enable those skilled in the art to better understand and implement the present disclosure, not intended to limit the scope of the present disclosure in any manner.

[0043] In the accompanying drawings, various embodiments of the present disclosure are illustrated in block diagrams, flow charts and other diagrams. Each block in the flowcharts or block may represent a module, a program, or a part of code, which contains one or more executable instructions for performing specified logic functions. Besides, although these blocks are illustrated in particular sequences for performing the steps of the methods, as a matter of fact, they may not necessarily be performed strictly according to the illustrated sequence. For example, they might be performed in reverse sequence or simultaneously, which is dependent on natures of respective operations. It should also be noted that block diagrams and/or each block in the flowcharts and a combination of thereof may be implemented by a dedicated hardware-based system for performing specified functions/operations or by a combination of dedicated hardware and computer instructions.

[0044] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the/said [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, unit, step, etc., without excluding a plurality of such devices, components, means, units, steps, etc., unless explicitly stated otherwise. Besides, the indefinite article "a/an" as used herein does not exclude a plurality of such steps, units, modules, devices, and objects, and etc.

[0045] Additionally, in a context of the present disclosure, a user equipment (UE) may refer to a terminal, a Mobile Terminal (MT), a Subscriber Station (SS), a Portable Subscriber Station (PSS), Mobile Station (MS), or an Access Terminal (AT), and some or all of the functions of the UE, the terminal, the MT, the SS, the PSS, the MS, or the AT may be included. Furthermore, in the context of the present disclosure, the term "BS" may represent, e.g., a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), a radio header ( H), a remote radio head (RRH), a relay, or a low power node such as a femto, a pico, and so on.

[0046] For a better understanding of the present disclosure, the following description will be made to embodiments of the present disclosure by taking a cloud based TDD heterogeneous networks as an example. However, as can be appreciated by those skilled in the art, the present invention could be applied to any other suitable communication system.

[0047] First, reference will made to Fig. 1 to describe a cloud based TDD heterogeneous networks in which embodiments of the present disclosure may be implemented. As illustrated, in the centralized RAN (Radio Access Network) network, there are densely deployed a plurality of remote radio header (RRHs), a RRH is comparable to a cell and installed at each local site with only radio frequency (RF) front-end functionalities. All RRHs are connected with a central control unit (CCU) through an optical fiber network. All the processing units/capabilities (including a base-band) are pooled at the CCUs. Due to such a centralized RAN architecture, it is possible to provide a coordinated interference alignment that may be implemented with high capacity and low latency.

[0048] Hereinafter, reference is made to Fig. 2 to describe the method for interference alignment in a TTD system as provided in the present disclosure.

[0049] As illustrated in Fig. 2, fist at step S201, an interference alignment reference direction is determined from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver.

[0050] In the present disclosure, it is assumed that in the system, there are a plurality of transmitters interfering a target receiver. From the plurality of transmitters, it may select one transmitter and the selected transmitter may transmit signals in any direction while other transmitters may align their interference with that of the selected transmitter. Due to the fact the selected transmitter may transmit signals at will, it is preferable to select the transmitter based on antennae configurations in the TDD system. Particularly, a transmitter with a lower number of antennae may be selected. It is known that a lower number of antennae mean a lower degree of freedom (DOF) and thus a lower signal processing capability. Therefore, such a selection based on antennae configuration may enable an unresourceful transmitter to support more data streams in direct channel. However, it should also be appreciated that it is also feasible to select the transmitter randomly and it may benefit therefrom.

[0051] Once the transmitter that may transmit signals at will is determined, it may determine the transmission direction of the selected transmitter. The transmission direction of the selected transmitter may be determined according to a predetermined beamformer determination principle, such as using a signal to leakage plus noise ratio (SLNR) principle, maximum signal to interference plus noise ratio (SINR) principle, maximum signal to noise ratio (SNR) principle and so on. In such a case, the transmission direction of the selected transmitter may be determined based on its direct channel measurement information, particularly the channel matrix from the selected transmission to its intended receiver, which will be detailed hereinafter with reference specific scenarios. Or alternatively, the transmission direction of the selected transmitter may be determined according a random precoding principle, that is to say, it may be determined without any channel measurement information.

[0052] Then, from the transmission direction of the selected transmitter and interference channel measurement information of the transmitter (i.e., the cross channel matrix of the transmitter), it may determine a direction of the selected transmitter's interference received at interfered receiver. The direction of the selected transmitter's interference may be taken as an interference alignment reference direction at the interfered receiver, with which interference from other transmitters may be aligned.

[0053] Then, at step S202, a transmission direction of another transmitter of the plurality of transmitter may be determined based on the interference alignment reference direction so as to align its interference at the receiver with the interference alignment reference direction.

[0054] After the interference alignment reference direction is determined, one or more transmitter may align their respective interference with the interference alignment reference direction. That is to say, direction of interference received from these transmitters should be identical to the interference alignment reference direction. For a purpose of illustration and simplification, an example will be described by referring to one transmitter which will be called as "another transmitter" so as to distinguish from the transmitter selected from the plurality of transmitter.

[0055] The transmission direction of the another transmitter may be determined based on the interference alignment reference direction. In view of the fact that the interference alignment reference direction is a direction that the interference should have after it is transmitted through the interference channel of the another transmitter, the transmitter direction of the another transmitter may be derived from its interference channel measurement information and the interference alignment reference direction.

[0056] In such a way, the antennae may transmit signal in the determined transmitter direction so that the interference transmitted therefrom is aligned with that transmitted from the selected transmitter at the interfered receiver.

[0057] Additionally, there might be also a case wherein interference from a further transmitter needs to be aligned with the interference direction of the another transmitter. For example, the another transmitter and the further transmitter may interfere with another receiver which is different from the received mentioned hereinabove, and the another transmitter is further selected as a transmitter whose interference will be aligned with. In such a case, the another interference alignment reference direction may be determined based on the determined transmission direction of the another transmitter. Then, transmission direction of the further transmitter may be derived based on the another interference alignment reference direction so as to align its interference with the another interference alignment reference direction.

[0058] Next, reference will be made to Figs. 3 A to 7B to describe detailed operation process with reference to a plurality of specific scenes.

[0059] First, reference is first made to Figs. 3A and 3B to two HetNets to which the following description is referred, and in the two figures, desired channels are drawn with solid lines and interference channels are drawn with dash lines. Fig. 3A schematically illustrates an example of HetNet wherein two uplink pico cells are interfered by one downlink macro cell. In the HetNet as illustrated in Fig. 3A, two pico cells and a macro cell are operating in the uplink and downlink transmissions, respectively. Specifically, in one pico cell, UE_a transmits signals to RRH_a; in another pico cell, UE transmits signals to RRHp ; in the macro cell, RRH_y transmits signals to UE_y Therefore, in this case, UE_a will interfere with RRH^; UE will interfere with RRH_a>; and RRH_y will interfere with both RRH_a and RRHp. It is clear that, besides the inter-cell interference(ICI) between the uplink pico cells, the downlink transmissions from the macro cell can also corrupts the reception of the pico cells. Therefore, interference at RRH_a should be aligned and similarly interference at RRHp is required be aligned too.

[0060] Additionally, Fig. 3B schematically illustrates another example of HetNet wherein one uplink macro cell is interfered by two downlink pico cells. As illustrated, in two pico cells, RRH_a and RR¾ transmit signals to UE_a and UEp, respectively; in the macro cell, RRH_y receives signals from UE_y Therefore, in this case, both RR¾ and RRH_a, will interfere with RRH_y. Accordingly, interference at RRH₇ needs to be aligned.

[0061] It should be noted that, in the present disclose, it mainly focus on RRH-to-RRH interference since the UE-to-UE interference is statistically much weaker than the RRH-to-RRH interference and it may be neglected at UE. However, it should be appreciated that the present invention may also be applicable to cancel the UE-to-UE interference.

TWO UPLINKS AND ONE DOWN LINK CHANNEL MODEL

[0062] Fig. 4 schematically illustrates three-cell scene wherein two uplink transmission and one downlink transmission are operating according to an embodiment of the present disclosure. As illustrated, user equipment UE_a, UE_P and UE₇ are equipped withNJ antennae, Ν_β ^υΕ antennae &ηάΝ_γ ^υΕ antennae, respectively, while RRH_a,

RRH and RRH₇ are equipped with N_a ^RRH antennae, N^RRH antennae and N^RRH antennae, respectively. In addition, direct channels in cell i; i e [α; β; γ ] are denoted as Η_α, Η_β and Η_γ, while interference channels are denoted as Η_αβ, Η_αγ, Η_βα and Η_βγ.

[0063] Regarding to three-cell scene as illustrated in Fig. 4, the received signals at receivers can be expressed as:

y_a ^~ H_aV x_a + Η_αβνβΧβ +

+ n_a

½ ~ + Ι1_βαν_αχ_η + Η_βΊν_Ί χ,, + η_β

wherein x_t denotes the transmit symbol vector, x _t E. C^d,x' , z^' G [α; β; γ ]; d_t denotes the number of data streams for transmitter i; V, denotes the beamformer at transmitter i; and Hi denotes a zero-mean Gaussian noise vector at receiver i with a covariance matrix

[0064] Each receiver will processes the received signals by using receiver W" W" W" and thus the processed signals may be expressed by

(2a) (2b) l l ^' (2c) wherein W" is a linear zero-forcing (ZF) receiver if there is any interference from other cells, for /= a or β , W"≡ c^"^* , otherwise, W"≡ ^"^4' .

[0065] Additionally, it may be appreciated that, in the three-cell scene as illustrated in Fig. 4, the main object is to jointly design the transceiver in order to cancel out the aligned interference at RRH_a and RRH .

INTERFERENCE ALIGNMENT

Constrains on Number of Data streams

[0066] As is know, the Degrees of Freedom (DoFs) are always limited in both direct and interference channels, thus it is necessary to express the conditions on the number of data streams. For the direct channel in cell_{a >} the number of data streams d_a should meet d_a< min{ N_a ^uE , N^RRH }, while for the interference channel (or cross channel) in cell_a, the number of data streams d_a should further meet d_a < mm{ N"^E , N^RRH } . By combing the above two limitations, it can obtain constrains on the number of data streams in cell_a as follows.

[0067] Similarly, the data stream constrains of cell and celk. may be represented by

dpswz{ NT _t N™_t N™} (4)

d_y<mm{ N;^u , N^RRH , N^RRH , N^RRH } (5)

[0068] It may be appreciated that RRH_y interferes with both RRH_a and RRH and thus d_y, i.e., the data streams in cell_y have more limitations.

[0069] Additionally, for conventional interference network wherein the linear receiver to recover desired signals and no interference alignment is applied, it should further satisfy further data stream constrains. The data stream constrains at RRH_a and RRH may be respectively expressed as

[0070] However, due to the interference alignment of the present disclosure, a looser data stream constrain may be obtained as:

max { άβ,άγ} +d_a< N_a ^RRH (7a) max {da,d_Y} + άβ≤ N^RRH (7b)

Alignment Condition

[0071] Since RRH_a and RR% are equipped with N^RR " , N^RRH antennae respectively, dimensions of observation space at receivers are limited. Generally speaking, some of the dimensions are reserved for recovering intended data streams, while others are utilized to handle the ICI. As depicted in Fig. 4, it is required to align the interference from two independent interferers into a subspace of the observation space of RRH_a and RRH respectively.

[0072] It is assumed that d^m

d_y}, which indicate the number of aligned interference streams at RRH_a and RRHp respectively. Hence, the following conditions should be met to align interference at RRH_a and RR¾ respectively,

M rt"] - vjf- - Η, ν, .^ ι _(8b) where V[:;M] denotes the matrix containing the first M columns of V. Therefore, given the zero-forcing receiver, interference alignment based (IA-based) transceiver should be jointly designed to satisfy the following constrains:

W Y H_{i k}V_k

(9a) W H_{>: Y_;.

(9b)

Rank ^' W ^! }^■!.. ¥..[ (9c)

Rank W¾' ¾t'¾ ! (9d)

Rank \ Wj/ j (9e)

Channel State Information (CSI) Measurements

[0073] To implement align interference, it requires information on CSI. Therefore, RRH_a will measure its direct channel H_a and the interference channels Η_αβ and Η_αγ, RRHp will measure its direct channel Η_β and the interference channels Η_βα and Η_βγ, and RRH_y will measure its direct channel Η_γ. RRH_a and RRHp and RRH_y will report their respective measurement to BBUs serving the RRH_a, RRHp and RRH_y, respectively. BBUs are located in a BBU pool which is located in CCUs. Thus, it enables the jointly designing of the transceiver.

[0074] Next, different scenarios will be described in detail for the three-cell scene as illustrated in Fig. 4.

Scenario 1-1 : N a^UE >— N β_B ^RRH , ^' N β >— N^RRH [0075] In this scenario, UE_a and UEp are equipped with more antennae than

RRH and RRH_a respectively, which means UE_a and UEp have enough transmit antennae to align their interference with that of RRH,, at RRH_a and RRH respectively. Thus, RRH,, is able to transmit its signals in any direction.

[0076] In an exemplary embodiment of the present disclosure, it may use a principle of SLNR to implement the beamforming in an altruistic fashion at RRH_r However, it should be appreciated that beamforming may also be implemented based on any other appropriate criteria such as signal to maximum SINR, maximum SNR principle, random precoding principle and so on.

[0077] For the /th data stream of RRH,,, its beamformer may be denoted by V_y . Its LNR may xpressed as

[0078] According to the Rayleigh-Ritz quotient result, the beamformer for the /th data stream of RRJL under SLNR criteria ma be derived as

wherein eig(^') denotes the eigenvector of (^'). Based on equation (11), it can obtain beamformer for RRH,,

[0079] Next, the reference vector for interference alignment may be derived from the beamformer for RRHy, and a corresponding interference channel matrix. Specifically, Then the reference vectors for alignment that should be set respectively for RRH_a and RRHp may be expressed respective

(12b)

[0080] Hereinafter, for a purpose of illustration, it will take the determination of beamformer V_a for UE_a as an example; however, it should be appreciated that beamformer V for UE may be derived in a similar way.

[0081] To align interference at RR¾ it should meet the following equation:

[0082] If da <d_f , then d^™= d_a. Meanwhile, since it is assumed N™ >N^{R H} in this scenario 1-1, the solution to equation (3) may be infinite. Many ways may be used to solve the equation, for example it may use Moore-Penrose inverse (MP-Inv approach). Then, it can obtain: <ⁱ -^w (14) wherein (^') ^! denotes the Moore-Penrose inverse of matrix (^') . This solution may guarantee the least transmit power used to align the interference at RRH .

[0083] Or alternatively, it may use another way to determine the beamforming vector. For example, it may introduce more constraints to the alignment condition.

As an example of the constraints, one may consider add the requirement of interference leakage for each data stream because it is quite natural to minimize the interference leakage (Min-Interf approach). Thus, it may yield an optimization problem as given in following equation:

wherein l_p E. [1, d™" ]. It is clear that equation (15) may be viewed as a specific convex problem, and its numerical solution can be easily derived by using tools such as SeDuMi, CVX and so on.

[0084] On the other hand, if d_a>d_y , thent ^™"= d This means that for d_a - d_y vectors of V«, there is no interference to align with. Under this condition, the V« [ ;d_Y] may be determined as described with reference to equations (14) and (15) and the rest d_a - d vectors of V_a can be determined under the SLNR principle, maximum SINR principle maximum SNR principle, random precoding principle and so on. The approach for determining beamformer using SLNR principle has been described with reference to equation (10) and (11) and approaches for other principle is also known to the skilled in the art. Therefore, the determination of the rest d_a - d_y vectors of V_a is not elaborated herein.

[0085] In a similar way, the beamformer V for UEp may be derived accordingly.

Scenario 1-2: N γ^KKH >— N β_R ^KK + N a

[0086] In this scenario, the number of antennae equipped by RRH_y is more than the total number of antennae equipped by RRH_a and RRH _> which means that RRH_y has more resource to perform align interference. Thus, RRH_y will use transmit antennae of RRH_y to align its interference with that of UE_a and UEp at RRH and RRH_a respectively while UE_a and UE will transmit signals at will. For example, beamformers V_a and V for UE_a and UE may be determined at their own benefits, such as based on the SLNR principle, maximum SINR principle, maximum SNR principle, random precoding principle and so on. Based on the teaching herein and the basic knowledge in the art, the skilled in the art may readily determine beamformers V_a and V for UE_a and UE and thus detailed operations are not elaborated herein.

[0087] Once beamformers V_a and V for UE_a and UE are determined, the reference vectors at RRH_a and RRH may be determined based on their own interference channel information as follows

V_f? - H_< V _(16a)

V - Η_βαν_{α (16b)}

[0088] By concatenating the reference vector at RRH_a and RRH and based on the corresponding interference channel information, beamformer for RRFL V_y> may be determined cordance with

It is clear that the solutions are guaranteed as N^RRH > N^RRH + N

Scenario 1-3: N^m≥N^RR N^RRH≥N^R

[0089] In this scenario, the number of antennae equipped by UE_a is more than the number of antennae equipped by RR¾ and RRH_y is equipped with more antennae than RRH_a> Thus, in this scenario, UE_a may align its interference with that of RRH_y at RR¾ and RRH_y may align its reference with that of UE at RRH_a.

[0090] Therefore, in such as case, V for UE should be determined first according to arbitrary principle, which is good to its own benefits. After that, reference vectors at RRH_a may be determined as

i^d — (18)

[0091] Afterwards, based on the determined reference vector y , the beamformer V_Y can be calculated from the following equation.

V™' = H,,-, V., _{( 19)}

[0092] Then, based on the beamformer V_Y> the reference vector at RRH may be determined as

V ^' = R . V.

^{: ;} (20)

[0093] Next, from Y^r , it may further determine the beamformer V_a based on:

ΐ / ref rjr I ..-

^V :;] * *- £]_a V _(X (21)

[0094] In such a way, V_a, V and V₇ are determined. Scenario 1-4: N β >—N^RRH , ' N γ^RRH >—N β^RRH

[0095] This Scenario is symmetric to Scenario 1-3, and V_a, V and V₇ in this scenario may be determined in a similar way. That is, the beamformer determination process is as follows:

Step 1) determining V_a according to arbitrary principle,;

Step 2) determining Y^ref from V_{a ;} Step 3) determining V₇ from Y^re/ ;

Step 4) determining Y from Y_y; and

Step 5) determining V from Y^r .

[0096] Detailed operations may refer to Scenario 1-3 and not be elaborated herein.

Information Exchange

[0097] To implement the interference alignment, within the BBU pool, the channel matrices, reference vectors and beamforming information are required to be exchanged between the BBUs serving these RRHs.

[0098] First reference is made to Fig. 5A, which illustrates the information exchange in Scenario 1-1. As illustrated, first the BBU of RRH_y calculates its beamformer V_Y according to the SLNR or Maximum SNR principle, and then sends V_Y to the BBUs of RRH_a and RRH as well as UE_y, respectively. Afterwards, the BBUs of RRHa and RRH may derive the corresponding reference vectors Y^r and V^^e/ and then obtain V and V_a which are in turn sent to the BBUs serving RRH and RRH_a. Finally, RRH_a and RRH inform the beamformer V_a and V to UE_a and UEp, respectively.

[0099] Fig. 5B illustrates the information exchange in Scenario 1-2. As illustrated, the BBUs of RRH_a and RRH calculate their respective beamformers V_a and V according to the SLNR or Maximum SNR principle. The BBU of RRH_awill send the calculated V_ato the BBU of RRHp as well as UE_a; and likewise, the BBU of RRH will send the calculated Vp to the BBU of RRH_a as well as UE . Then the reference vectors Y^r and Y^r may be derived from V and V_a at the BBUs of RRH_a and RRH After that, Y^r and Y^r and corresponding interference channel measurement information Η_αγ and Η_βγ may be sent to the BBU of RRH_y which will derive its V_y from these information. Finally, RRH_y inform V_y to UE_y.

[00100] Fig. 5C illustrates the information exchange in Scenario 1-3. As illustrated, the BBU of RRH may determine its beamformer V according to the SLNR or Maximum SNR principle and sent the determined V to the BBU of RRH_a as well as UE . Then BBU of RRH_a may derive the reference vector Y^r from V and derive V_y fromV and Η_αγ. Subsequently, beamformer V_y is sent to the BBU of RRH_y and in turn to the BBU of RRH_P as well as UE₇. The BBU of RRH_P derives its V from V₇ and then derive beamformer V_a and further sends it to the BBU of RRH_a. Finally, RRH_a inform it to UE_a.

[00101] Fig. 5D illustrates the information exchange in Scenario 1-4, which gives a process symmetric to that given in Fig. 5C and thus is not elaborated herein.

ONE UPLINK AND TWO DOWN LINK CHANNEL MODEL

[00102] Fig. 6 schematically illustrates three-cell scene wherein one uplink transmission and two downlink transmission are operating according to an embodiment of the present disclosure. As illustrated, user equipment UE_a, UEp and UE₇ and RRH_a, RRHp and RRH_y have similar antennae to those in Fig. 4 and thus detailed description is omitted herein. In addition, similarly the direct channels in cell i; i e≡ [α; β; γ ] are denoted as Η_α, Η_β and Η_γ and the interference channels are denoted as Η_αβ, Η_αγ, Η_βα and

Η_βγ.

[00103] Regarding to three-cell scene as illustrated in Fig. 6, the received signals at receivers can be expressed as:

(22a) (22b)

[00104] With linear zero-forcing receiver, the estimated symbols can estimated as

(23a) (23b) k^ (23c)

[00105] Parameters contained in equations (22a) to (23 c) are similar to those in equations (la) to (2c) and thus there definitions are omitted herein.

INTERFERENCE ALIGNMENT Constrains on Number of Data streams

[00106] As is know, without interference alignment, the number of data streams should meet:

< <ηώι{ N™ , N , N , N^RRH } (24a)

[00107] If the interference align of the present disclosed is applied, it would have the following data streams constrains

max { ά_α,,ά_β) + d_y < N^RRH (25)

[00108] Furthermore, it may also obtain the following constrains:

d_a + dp≤N_a ^uE (26a) ά_α + ά_βγ< Ν_β ^υΕ (26b)

CSI Measurements

[00109] As described hereinbefore, the RRH_a may measure its direct channel H_a and the cross channel Η_αβ and Η_αγ, RRHp may measure its direct channel Η_β and the cross channel Η_βα and Η_βγ, and RRH_y may measure its direct channel Η_γ.

Scenario II-l N^RRH > N^R

[00110] In this scenario, RRH_a has enough antennae to align its interference with that of RRHp at RRH_r Thus, RRHp is able to transmit its signals in any direction. That is to say, beamformer V may be determined according to a certain rule such as a SLNR principle, a maximum SINR principle, a maximum SNR principle, random precoding principle and so on. Then reference interference vector at RRH,, may be derived as ' 7 ^:: (27)

[00111] With the derived reference vector, the beamformer of RRH_a can be obtained by

In such a way, it may determine beamformers V_a and V .

Scenario II-2 N^RRH≥ N^RRH [00112] This scenario is symmetric to Scenario II-3. Therefore, in this scenario, a beamformer V_a may be determined firstly according to a certain rule, then based on the beamformer V_a, the reference interference vector \T^ef may be derived and in turn V may be determined from the reference interference vector \T^ef .

Information Exchange

[00113] Fig. 7A illustrates the information exchange in Scenario II- 1. As illustrated, first, the BBU of RRHp calculates its beamformer V according to SLNR or any other appropriate principles, and then sends V to the BBU of RRH_y as well as UE , respectively. Afterwards, the BBU of RRH_y may derive the reference vector \^^ef and sends it to the BBU serving RRH_a. After that, RRH_a will determine the beamformer V_a from V_y ^ef , and send it to UE_a. Finally, RRH₇ may send its beamformer V_Y to UE_y.

[00114] Fig. 7B illustrates the information exchange in Scenario II-2, which gives a process symmetric to that given in Fig. 7A and thus is not elaborated herein.

[00115] Additionally, in the present disclosure, there is also provided an apparatus for interference alignment in a TDD system. Next, reference will be made to Fig. 8 to describe the apparatus as provided in the present disclosure.

[00116] As illustrated in Fig. 8, the apparatus 800 may comprises a reference direction determination unit 801 and a transmission direction determination unit 802. The reference direction determination unit 801 may be configured to determine an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver. The transmission direction determination unit may be configured to determine, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction.

[00117] In an embodiment of the present disclosure, the transmitter is selected based on antenna configurations in the TDD system.

[00118] In another embodiment of the present disclosure, the transmitter that is selected from the plurality of transmitters may be a transmitter configured with a lower number of antennae.

[00119] In a further embodiment of the present disclosure, the reference direction determination unit 801 may be further configured to: determine the transmission direction of the transmitter that is selected from the plurality of transmitters according to a beamformer determination principle; and determine the interference alignment reference direction from the transmission direction of the transmitter and interference channel measurement information of the transmitter.

[00120] In a yet embodiment of the present disclosure, the transmission direction of the transmitter that is selected from the plurality of transmitters may be determined based on its direct channel measurement information.

[00121] In a still embodiment of the present disclosure, the beamformer determination principle comprises one of: signal to leakage plus noise ratio principle; maximum signal to interference plus noise ratio principle; and maximum signal to noise ratio principle; and random precoding principle.

[00122] In a yet further embodiment of the present disclosure, the transmission direction determination unit may be further configured to determine the transmission direction of the another transmitter based on the interference alignment reference direction and interference channel measurement information of the another transmitter.

[00123] In a still further embodiment of the present disclosure, the reference direction determination unit 801 may be further configured to determine another interference alignment reference direction based on the transmission direction of the another transmitter; and wherein the transmission direction determination unit 802 may be further configured to determine, based on the another interference alignment reference direction, a transmission direction of a further transmitter interfering another receiver so as to align its interference at the another receiver with the another interference alignment reference direction.

[00124] It is noted that the apparatus 800 may be configured to implement functionalities as described with reference to Figs. 1 and 7. Therefore, for details about the operations of modules in these apparatus, one may refer to those descriptions made with respect to the respective steps of the methods with reference to Figs. 1 to 7.

[00125] It is further noted that the components of the apparatus 800 may be embodied in hardware, software, firmware, and/or any combination thereof. For example, the components of the apparatus 800 may be respectively implemented by a circuit, a processor or any other appropriate selection device. Those skilled in the art will appreciate that the aforesaid examples are only for illustration not limitation.

[00126] In some embodiment of the present disclosure, the apparatus 800 comprises at least one processor. The at least one processor suitable for use with embodiments of the present disclosure may include, by way of example, both general and special purpose processors already known or developed in the future. The apparatus 800 further comprises at least one memory. The at least one memory may include, for example, semiconductor memory devices, e.g., RAM, ROM, EPROM, EEPROM, and flash memory devices. The at least one memory may be used to store program of computer executable instructions. The program can be written in any high-level and/or low-level compilable or interpretable programming languages. In accordance with embodiments, the computer executable instructions may be configured, with the at least one processor, to cause the apparatus 800 to at least perform operations according to the method as discussed with reference to Figs. l to 7.

[00127] Besides, there is further provided a central control unit in a centralized radio access network, comprising an apparatus according to any one of embodiments of apparatus as described herein.

[00128] In addition, Figs. 9 and 10 further illustrate simulation results made on an embodiment of the present invention and the existing solution in the prior art. Parameters used in the simulations are listed in Table 1.

Table 1 Parameters used in the simulations

UE antenna gain OdBi

UE noise figure 9dB

UE power class 23dBm

UE Data streams 2

UE moving speed (for SCM only) Lower than 3m/s

Macro RU transmit power 64dBm

Macro RU noise figure 5dB

Macro RU Data streams 2

[00129] Figs. 9A to 9D illustrate performance of three transceiver schemes under both Rayleigh and SCM fading channels in the three-cell scene as illustrated in Fig. 4. Specifically, the three transceiver schemes include PM-lnv (Penrose-Moore based Pseudo Inverse) and Min-Interf (minimizing interference) according to interference alignment (IA-based) schemes of the present disclosure, and a SVD (singular value decomposition) which is purely selfish scheme and does not concern any interference generated by the SVD-based transmitter. Additionally, in the simulations, it uses a system configuration wherein the Macro RU is equipped with 8 antennae, each pico RU is equipped with 4 antennae and each UE is equipped with 2 antennae.

[00130] Similarly, Figs. 10A to 10D illustrate performance of the three transceiver schemes under both Rayleigh and SCM fading channels in the three-cell scene as illustrated in Fig. 6. However, different system configurations are used wherein the Macro RU is equipped with 4 antennae, each pico RU is equipped with 8 antennae and each UE is equipped with 4 antennae.

[00131] By reviewing the simulations as illustrated in Figs. 9A to 10D, it may observe that when there is interference between RRHs, the IA-based transceiver (both PM-lnv and Min-Interf) always outperforms the SVD-based scheme and the Min-Interf is slightly better than the PM-lnv. This can be attributed to aligning the interference at the corresponding receivers. However, in a case of no interference, the SVD-based scheme is optimal as widely acknowledged in both academia and industry. But due to the densely deployment of RRHs in the next generation of TDD-based wireless communication system, the interference between different RRHs will definitely be a huge issue. Thus, actually, the IA-based schemes of the present invention may achieve a better trade-off among different performance even in the case of no interference.

[00132] Therefore, in embodiments of the present disclosure, there is provided a coordinated interference alignment that may be implemented with high capacity and low latency. With these embodiments, dimension of space spanned by interference signals may reduced, more data streams can be supported in direct channels and more diversity gains can be achieved. Additionally, the interference alignment according to embodiments of the present disclosure has a better scalability and thus may be easily extended to different scenarios.

[00133] It should be appreciated that embodiments of the present disclosure has been described hereinbefore with reference to a three-cell scene. However, based on the teaching herein, the skilled in the art can extend it to other HetNet comprising more or less cells.

[00134] Additionally, it should be noted that it has taken different system antenna configuration scenarios to describe embodiments of the present disclosure; however, the skilled in the art can extend it to other system antenna configuration scenarios based on the teaching provided herein. Additionally, it will also be possible to select one transmitter from a plurality of transmitter interfering a receiver randomly, so that at least on other transmitter could align its interference with interference of the one transmitter at the receiver.

[00135] Beside, although constrains on data streams have been described in details in embodiments of the present disclosure, it may also implement the present invention by excluding one or more data streams when constrains on data streams are not meet.

[00136] Furthermore, it should be also noted that in the present disclosure, although embodiments of the present disclosure have been described with reference to CCUs, it is also possible to carry out them by other entity, such as, a BS, a base station controller (BSC), a gateway, a relay, a server, or any other applicable device.

[00137] Although embodiments of the present invention have been described with reference to the centralized RAN TDD system, the present invention may also be applicable in any other appropriate TDD system to benefit therefrom.

[00138] Besides, the present invention has been described with specific algorithm, but the present disclosure is not limited thereto, any other suitable algorithm may also be employed.

[00139] Additionally, based on the above description, the skilled in the art would appreciate that the present disclosure may be embodied in an apparatus, a method, or a computer program product. In general, the various exemplary embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the disclosure is not limited thereto. While various aspects of the exemplary embodiments of this disclosure may be illustrated and described as block diagrams, flowcharts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[00140] The various blocks shown in the companying drawings may be viewed as method steps, and/or as operations that result from operation of computer program code, and/or as a plurality of coupled logic circuit elements constructed to carry out the associated function(s). At least some aspects of the exemplary embodiments of the disclosures may be practiced in various components such as integrated circuit chips and modules, and that the exemplary embodiments of this disclosure may be realized in an apparatus that is embodied as an integrated circuit, FPGA or ASIC that is configurable to operate in accordance with the exemplary embodiments of the present disclosure.

[00141] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any disclosure or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular disclosures. 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.

[00142] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[00143] Various modifications, adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. Any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. Furthermore, other embodiments of the disclosures set forth herein will come to mind to one skilled in the art to which these embodiments of the disclosure pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings.

[00144] Therefore, it is to be understood that the embodiments of the disclosure are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are used herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

WHAT IS CLAIMED IS:

1. A method for interference alignment in a Time Division Duplex TDD system, comprising

determining an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver; and

determining, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction.

2. The method according to Claim 1, wherein the transmitter is selected based on antenna configurations in the TDD system.

3. The method according to Claim 1 or 2, further comprising,

determining another interference alignment reference direction based on the transmission direction of the another transmitter; and

determining, based on the another interference alignment reference direction, a transmission direction of a further transmitter interfering another receiver so as to align its interference at the another receiver with the another interference alignment reference direction.

4. The method according to any one of Claims 1 to 3, wherein the transmitter that is selected from the plurality of transmitters is a transmitter configured with a lower number of antennae.

5. The method according to any one of Claims 1 to 4, wherein the determining the interference alignment reference direction further comprises

determining the transmission direction of the transmitter that is selected from the plurality of transmitters according to a beamformer determination principle; and

determining the interference alignment reference direction from the transmission direction of the transmitter and interference channel measurement information of the transmitter.

6. The method according to Claim 5, wherein the determining the transmission direction of the transmitter that is selected from the plurality of transmitters is performed based on direct channel measurement information of the transmitter.

7. The method according to Claim 5, wherein the beamformer determination principle comprises one of:

signal to leakage plus noise ratio principle;

maximum signal to interference plus noise ratio principle;

maximum signal to noise ratio principle; and

random precoding principle.

8. The method according to Claim 1 to 7, wherein the determining a transmission direction of the another transmitter of the plurality of transmitter is preformed based on the interference alignment reference direction and interference channel measurement information of the another transmitter.

9. An apparatus for interference alignment in a Time Division Duplex TDD system, comprising

a reference direction determination unit configured to determine an interference alignment reference direction from a transmission direction of a transmitter that is selected from a plurality of transmitters interfering a receiver; and

a transmission direction determination unit configured to determine, based on the interference alignment reference direction, a transmission direction of another transmitter of the plurality of transmitter so as to align its interference at the receiver with the interference alignment reference direction.

10. The apparatus according to Claim 9, wherein the transmitter is selected based on antenna configurations in the TDD system.

11. The apparatus according to Claim 9 or 10, wherein the reference direction determination unit is further configured to determine another interference alignment reference direction based on the transmission direction of the another transmitter; and wherein the transmission direction determination unit is further configured to determine, based on the another interference alignment reference direction, a transmission direction of a further transmitter interfering another receiver so as to align its interference at the another receiver with the another interference alignment reference direction.

12. The apparatus according to any one of Claims 9 to 11, wherein the transmitter that is selected from the plurality of transmitters is a transmitter configured with a lower number of antennae.

13. The apparatus according to any one of Claims 9 to 12, wherein the reference direction determination unit is further configured to:

determine the transmission direction of the transmitter that is selected from the plurality of transmitters according to a beamformer determination principle; and

determine the interference alignment reference direction from the transmission direction of the transmitter and interference channel measurement information of the transmitter.

14. The apparatus according to Claim 13, wherein the transmission direction of the transmitter that is selected from the plurality of transmitters is determined based on direct channel measurement information of the transmitter.

15. The apparatus according to Claim 13, wherein the beamformer determination principle comprises one of:

signal to leakage plus noise ratio principle;

maximum signal to interference plus noise ratio principle; and

maximum signal to noise ratio principle; and

random precoding principle.

16. The apparatus according to any one of Claims 9 to 15, wherein the transmission direction determination unit is further configured to, determine the transmission direction of the another transmitter based on the interference alignment reference direction and interference channel measurement information of the another transmitter.

17. A central control unit in a centralized radio access network, comprising an apparatus according to any one of Claims 9 to 16.