WO2015147387A1

WO2015147387A1 - Method for communicating using outdated channel state information in a 2-cell 2 user cellular network

Info

Publication number: WO2015147387A1
Application number: PCT/KR2014/007624
Authority: WO
Inventors: Changho Suh; Seiyun SHIN; Jaewoong CHO; Kilbom LEE; Jiwon Kang; Kitae Kim
Original assignee: Lg Electronics Inc.; Korea Advanced Institute Of Science And Technology
Priority date: 2014-03-23
Filing date: 2014-08-18
Publication date: 2015-10-01
Also published as: KR20160136274A

Abstract

Methods of communicating using outdated CSI for a UE and a BS are disclosed. The method for a UE comprises transmitting to a first BS 4 different linear combinations at each of 4 time slots included in a first time period, receiving from a second BS feedback signals for the linear combinations and transmitting to the first BS a reconstruction signal generated based on the feedback signals at a first time slot of a third time, wherein the feedback signals comprise channel coefficients information between the first UE and the second BS during the first time period.

Description

[DESCRIPTION]

[Invention Title]

METHOD FOR COMMUNICATING USING OUTDATED CHANNEL STATE INFORMATION IN A 2-CELL 2 USER CELLULAR NETWORK

[Technical Field]

[1] The present invention relates to a method of communicating using outdated channel state information in a 2-CELL, 2-USER cellular network.

[Background Art]

[2] Interference is the dominant limiting factor in the performance of wireless networks. The problem of interference arises in the multi-user environment where there are multiple transmitter-receiver pairs and transmitted signals are overheard by non-intended receivers.

Interference alignment (IA) has received attention as a potential solution to mitigate interference.

The IA technique was initially developed in the context of X-channel and K-user interference channel. It has been further investigated to show its great potential to a variety of practically- relevant network scenarios. Particularly for cellular networks, the IA technique demonstrates that near interference-free degree-of-freedom (DoF) can be achieved.

[3] While the I A schemes promise substantial theoretical gain, they come with challenge in practice. One such challenge is that the techniques require accurate channel state information at transmitters (CSIT). In conventional FDD systems, such CSIT is usually obtained by feedback from receivers, but the feedback is subject to delay. A conventional approach is to predict the current channel state and then apply the existing IA technique. For a fast fading scenario, however, the current CSI can be completely different from the fed-back CSI and such a prediction approach fails to achieve any DoF gain. Therefore, we developed new scheme.

[4] In the context of the multi-antenna broadcast channel, Maddah-Ali and Tse in "Completely Stale Transmitter Channel State Information is Still Very Useful, IEEE Transactions on Information Theory, vol. 58, no. 7, Jul. 2012" have recently shown that the completely outdated CSI can be still very useful. Specifically they developed an innovative transmission strategy that utilizes the past received signals to create signals of common interest to multiple receivers, hence significantly improving DoF by broadcasting them to receivers.

[5] However, the multi-antenna broadcast channel considered by Maddah-Ali and Tse represents only a single-cell downlink scenario. Hence it is not immediate whether or not there exists still DoF gain with outdated CSIT in general settings of cellular networks. [Disclosure]

[Technical Problem]

[6] Particularly for uplink scenarios in cellular networks, unlike the multi-antenna broadcast channel, transmit antennas are distributed at different locations. So it may not be possible for a transmitter to reconstruct previously received signals, since they may include other transmitters' signals that are not accessible to that transmitter. This leads us to investigate the role of outdated CSI for general contexts. As an initial effort, we consider a 2-cell uplink scenario, in which there are two users per cell.

[Technical Solution]

[7] We seek to explore the role of outdated channel state information (CSI) feedback for cellular networks. Specifically, for a fast fading scenario in which the current CSI is completely independent of the fed-back CSI, we develop a technique that achieves 6/5 degree-of-freedom (DoF) with outdated channel state information at transmitters (CSIT) which is strictly greater than DoF of 1 that can be achieved with no-CSIT. The result shows that delayed CSIT is useful for a wide variety of cellular network scenarios.

[Advantageous Effects]

[8] The embodiments of the present invention may have the following advantageous effects.

[9] First, capacity of a communication system can be improved by using outdated feedback information.

[10] Second, degree of freedom (DoF) can be achieved more than 1 even in a scenario where transmitters are distributed.

[11] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. [Description of Drawings]

[12] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:

[13] FIG. 1 illustrates a channel model according to the present invention;

[14] FIG. 2 illustrates a time period structure according to an exemplary embodiment of the present invention;

[15] FIG. 3 illustrates a flow chart for explanation of a method of communicating using outdated CSI of a UE according to an embodiment of the present invention;

[16] FIG. 4 illustrates a flow chart for explanation of a method of communicating using outdated CSI of a BS according to an embodiment of the present invention;

[17] FIG. 5 illustrates a block diagram of a structure of a UE and a base station according to an embodiment of the present invention.

[Best Mode]

[18] To achieve above-mentioned objects and other advantages in accordance with the purpose of the invention, as embodied and broadly described herein, a method of communicating using outdated CSI for a UE in a network environment which consists 2 BSs and 2 cells which include 2 UEs each, the method performed by a first UE of a first cell and comprises transmitting to a first BS 4 different linear combinations of 3 data symbols at each of 4 time slots included in a first time period, receiving from a second BS feedback signals for the linear combinations wherein the second BS overhears the linear combinations of the first UE as interference signals during the first time period, and transmitting to the first BS a reconstruction signal generated based on the feedback signals at a first time slot of a third time period including 2 time slots, wherein the feedback signals comprise channel coefficients information between the first UE and the second BS during the first time period.

[19] The received feedback signals may comprise a null space vector for the linear combinations from the first UE which are overheard by the second BS.

[20] The feedback signals may be multiplications of the null space vector and the overheard linear combinations, and the overheard linear combinations may be multiplications of the channel coefficients information and transmission precoder of the first UE.

[21] The first UE of the first cell and a second UE of the first cell may stop transmitting or receiving of any signals, during a second time period including 4 time slots.

[22] The first UE may stop transmitting or receiving of any signals, at a second time slot of the third time period.

[23] In other aspect of the present invention, a method of communicating using outdated CSI for a BS in a network environment which consists 2 BSs and 2 cells which include 2 UEs each, the method performed by a first BS and comprises receiving from each of 2 UEs of a first cell a first 4 different linear combinations of 3 data symbols at each of 4 time slots included in a first time period, receiving from each of 2 UEs of a second cell a second 4 different linear combinations of 3 data symbols for a second BS by overhearing the second 4 different linear combinations as interference signals at each of 4 time slots included in a second time period, transmitting to each of the 2 UEs of the second cell feedback signals for each of the second linear combinations, receiving from a first UE of the first cell and a first UE of the second cell a first reconstruction signals reconstructed by using the feedback signals at a first time slot of a third time period including 2 time slots, receiving from a second UE of the first cell and a second UE of the second cell a second reconstruction signals reconstructed by using the feedback signals at a second time slot of the third time period, and decoding the first linear combinations by using the first reconstruction signals and the second reconstruction signals, wherein the feedback signals comprise channel coefficients information between the first BS and each of the 2 UEs of the second cell.

[24] Each of the feedback signals may comprise null space vectors for the second linear combinations from the 2 UEs of the second cell.

[25] Each of the feedback signals may be a multiplication of the null space vector and the second linear combinations from each UEs, and the second linear combinations may be multiplications of the channel coefficients information and transmission precoder of each UEs.

[26] The decoding may comprise cancelling the feedback signal transmitted to the first UE of the second cell out from the first reconstruction signals and cancelling the feedback signal transmitted to the second UE of the second cell out from the second reconstruction signals.

[27] The decoding may further comprises decoding the cancelled out results and the first linear combinations together.

[28] It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[Mode for Invention]

[29] Most of the terms used herein are general terms that have been widely used in the technical art to which the present invention pertains. However, some of the terms used herein may be created reflecting intentions of technicians in this art, precedents, or new technologies. Also, some of the terms used herein may be arbitrarily chosen by the present applicant. In this case, these terms are defined in detail below. Accordingly, the specific terms used herein should be understood based on the unique meanings thereof and the whole context of the present invention.

[30] Embodiments described herein below are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment.

[31] In the description of the drawings, procedures or steps which render the scope of the present invention unnecessarily ambiguous will be omitted and procedures or steps which can be understood by those skilled in the art will be omitted.

[32] In the disclosure, 'include' or 'comprise' should be interpreted as that other components may further be included, not excluded, unless otherwise specified. The term '-unit', '-or(er)', 'module', etc. signifies at least one function or operation processing unit that can be implemented in hardware, software, or a combination thereof. In addition, it is to be understood that the singular forms 'a, 'an', and 'the' include plural referents unless the context clearly dictates otherwise.

[33] In the embodiments of the present invention, a description is made, centering on a data transmission and reception relationship between an eNB and a user equipment (UE). The eNB is. a terminal node of a network, which communicates directly with a UE. In some cases, a specific operation described as performed by the eNB may be performed by an upper node of the BS.

[34] Namely, it is apparent that, in a network comprised of a plurality of network nodes including an eNB, various operations performed for communication with a UE may be performed by the eNB, or network nodes other than the eNB. The term 'base station (BS)' may be replaced with the term 'fixed station', 'Node B', 'evolved Node B (eNode B or eNB)', an advanced base station (ABS), or an access point, etc.

[35] In addition, the term 'mobile station (MS)' may be replaced with the term 'user equipment (UE)', 'subscriber station (SS)', 'mobile subscriber station (MSS)', 'mobile terminal', 'advanced mobile station (AMS), 'terminal', etc.

[36] A transmitter refers to a fixed node and/or a mobile node for transmitting a data or voice service, and a receiver refers to a fixed node and/or a mobile node for receiving a data or voice service. Accordingly, in uplink, an MS becomes a transmitter and a base station becomes a receiver. Similarly, in downlink, an MS becomes a receiver and a base station becomes a transmitter.

[37] In addition, the expression that a device communicates with a 'cell' means that the device transmits and receives signals to and from an eNB of the corresponding cell. That is, an actual object to and from which the device transmits and receives signals may be a specific eNB. However, for convenience of description, the device transmits and receives signals to and from a cell formed by the specific cell. Similarly, the terms 'macro cell' and/or 'small cell' may refer to corresponding specific coverage and also refer to 'an eNB for supporting a macro cell' and/or 'a small cell eNB for supporting a small cell'.

[38] The embodiments of the present invention are supported by standard documents disclosed in at least one of the Institute of Electrical and Electronic Engineers (IEEE) 802.xx system, the 3rd generation partnership project (3 GPP) system, the 3 GPP long term evolution (LTE) system and the 3GPP2 system, all of which are wireless access systems. That is, the steps or the portions of the embodiments of the present invention which are not described in order to clarify the technical spirit are supported by the above-described documents.

[39] All the terms disclosed in the present specification may be described by the above- described standard documents. In particular, embodiments of the present invention can be supported by one or more of P802.16e-2004, P802.16e-2005, P802.16.1, P802.16p, and P802.16.1b standard documents that are standard documents of the IEEE 802.16 system.

[40] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. It is to be understood that the detailed description which will be disclosed along with the accompanying drawings is intended to describe the exemplary embodiments of the present invention, and is not intended to describe a unique embodiment through which the present invention can be carried out.

[41] The specific terms used in the following description are provided in order to facilitate the understanding of the present invention and may be changed in other forms without departing from the technical scope of the present invention.

[42] FIG. 1 illustrates a channel model according to the present invention.

[43] We consider 2-cell uplink scenario as illustrated in Fig. 1. There are two cells (cell a and β) and two user equipment(UE)s per each cell. Users (transmitters) and base stations (receivers) are equipped with single antenna. Users in cell a and β has a message for base station (BS) a and b, respectively. The received signals at BS a and b at time t are described as Equation 1. Equation 1

2 2

y^a(t) = h^a _k(t)v_ak(t)x_ak(t) + ^ gjj_k(t)v_pk(t)x_pk(t) + w^a(t) k=l k=l

2 2

y^b(t) = hg_k(t)v_pk(t)xp_k(t) + g£_k(t)v_ak(t)x_ak(t) + w^b(t)

k=l k=l

[45] where the superscripts and subscripts denote receivers and transmitters respectively, and k G {1,2} is user index. h^a _k G C and g^b _k G ⁽C indicate channel coefficients from a cell to intended BS and a cell to non-intended BS, respectively. v_ak G C^lx3 and x_ak G C^{3 x l} indicate the precoder vector and the vector containing data symbols. w^a G C and w^b G C indicates complex Gaussian noise. We assume that h _k G C and g _k G C are delayed channel state information at the transmitters (CSIT). In other words, all channel coefficients is available at each transmitter with one unit delay.

[46] We now describe the received signals in several phases. Each phase consists of multiple time slots. Time slot is a unit time interval which satisfies that one data symbol is transmitted during one time interval. And, one or more of time slots form a time period. Hereinafter, a time period may be referred to as 'phase'. The received signals in the nth phase can be described as Equation 2.

[47] Equation 2

2 2

y^a [n] = H^a _ak[n]V_ak[n]x_ak[n] + G_p ^a _k [n\V_pk [n\x_pk [n\ + w^a[n]

k=i Jc=i

2 2

y [n] = ^ H_p ^b _k [n]Vp_k[n]xp_k[n] + G^b _ak[n]V_ak [n]x_ak [n] + w^b [n]

fc=l fc=l

[48] If phase n consists of m time slots, then channel matrices, a precoder matrix and a transmitted data symbol are described as Equation 3

[49] Equation 3

H^a _k[n] = diag Ch^Ct_!), - , ^,,^))

XakW = Xak(tl) = · · · = Xak(tm) [50] We now develop a technique that achieves DoF of 6/5 with outdated CSIT which is strictly greater than DoF of 1 that can be achieved with no-CSIT. Our transmission strategy is based upon utilizing the past received equations to create equations of common interest to both base stations. Our model, however, provides a reconstruction of equations in a limited fashion since unlike the multi-antenna broadcast channel, transmit antennas in users are distributed at different locations. Hence, it may not be possible for a user to reconstruct previously received symbols, since they may include other users' symbols that are not accessible to that user. We next provide an achievable scheme that enables an efficient reconstruction. Our scheme consists of three phases.

[51] FIG. 2 illustrates a time period structure according to an exemplary embodiment of the present invention. In fig. 2, each box represents a time slot. Phase one (210), phase two (220) and phase three (230) may consist plurality of time slots, respectively. Time slot is a predetermined time unit that one data symbol may be transmitted within.

[52] Phase one (210) consists of four time slots. Users in cell a transmit signals while those in cell β transmit nothing in this phase. Each user simultaneously delivers linear combinations of three fresh symbols intended for BS a, as like Equation 4 below. The linear combination patterns of three symbols are predetermined between the users and BSs.

[53] Equation 4

[54] Disregarding the noise, the received signals in BS a and b are shown in Equation 5.

[55] Equation 5

[56] where V_al and V_a2 indicate 4x3 precoder matrices whose rank is 3. Both BS a and b receive four linear equations of six unknown symbols from cell a (3 symbols from first user and other 3 symbols from second user). Note that in order to resolve six unknown symbols; BS a needs to get two more linear equations, which are independent of the received equations. BS b saves overheard equations (received interference signals) for later usage, although these equations consist of data symbols intended for only BS a.

[57] In Phase two (220), similar to phase one (210), users in cell β transmit signals while those in cell a transmit nothing for four time slots. Each user simultaneously delivers linear combinations of three fresh symbols intended for BS b. Disregarding the noise, the received signal at BS a and b are shown below in Equation 6.

[58] Equation 6

2

y^a[2] = Gp^a _k[2]Vp_k[2]x_pk[2]

k=l

2

y^b [2] = ^ Hj_k[2]V_pk[2]x_pk[2]

k=l [59] Both BS a and b receive four linear equations of six unknown symbols from cell β. Note that two linearly independent equations are necessary for resolving six unknown symbols. Similarly, BS a saves overheard equations intended for BS b.

[60] In phase one (210) and two (220), we have spent extra time slots, i.e., using four time slots to deliver three fresh symbols. This enables BS b to generate vectors u_al and u_a2 which are shown in Equation 7.

[61] Equation 7

G^ [l]V_al [l]x_al [l] = 0

[62] Hence, by end of phase two (220), with the overheard equations BS b can compute Equation 8.

[63] Equation 8

Lai = uj₂y^b [l] =

[l]x_al [l] ^L«2 = _iy ^b[l] = uM₂ [l]V_a2 [l]x_a2[l].

[64] Similarly, BS a can compute Equation 9.

[65] Equation 9

Lpi = j₂y^a[2] = u5^" ₂Gjk[2]V_pi [2]x_pi [2]

L_P2 = uf_iy ^a[2] = ^^₂ [2] _β2 [2]χρ₂ [2]

[66] Recall that our challenge comes from the fact that each user has only access to its own data symbols. One way to overcome this challenge is to cancel out data symbols from one user. Observe that the four equations L_al, L_a2, L_pi and L_p2, consist of symbols from only one user; hence, they can be reconstructed by each user in a later phase since each user is aware of channel state information of previous phases. Moreover, these equations are independent of the equations received in phase one (210) and two (220). If one BS has these computed symbols overheard by the other BS, each BS then has enough equations to solve for its own symbols. Therefore the main mission of phase three (230) is to deliver these symbols to intended base stations efficiently.

[67] Phase three (230) consists of two time slots. In this phase, no new data symbols are transmitted. Instead, each user reconstructs the linear equation which is overheard from non- intended BS in the previous phases, i.e. L_al, L_a2, L_pi and L_p2.

[68] In first time slot of phase three, each user 1 in cell a and cell β reconstructs L_al and L_pi, respectively, and then transmits these equations simultaneously. Disregarding the noise, the received signals are as below in Equation 10.

[69] Equation 10

y^a(9) = h^a ₁(9)L_al + g^a ₁ (9)L_pi)

[70] Now the idea is to exploit the past received equations. BS a can obtain L_al from y^a(9) by exploiting the previously received equation L_pi. Similarly, BS b can obtain L_pi from y^b(9) by exploiting the previously received equation L_al. Now both BS a and b get one additional equation at the same time.

[71] In second time slot of phase three, similar to the previous time slot, each user 2 in cell a and cell β reconstructs L_a2 and Lp₂, respectively, and forward these equations simultaneously. The received signals are below in Equation 1 1.

[72] Equation 1 1

y^a(10) = h^a ₂ (10)L_a2 + g^lO)L_p2, y^b (10) = hJ₂ (10)L_p2 + g^₂ (10)L_a2.

[73] Note that BS a can obtain L_a2 from y^a(10) by exploiting the previously received equation Lp₂; BS b can obtain Lp₂ from y^b(10) by exploiting L_a2. Now both BS a and b get one additional equation at the same time. As a result, each base station finally obtains six linearly independent equations for decoding six unknown symbols. Therefore, all data symbols can be decoded at their corresponding base stations within ten time slots, achieving DoF of 6/5.

[74] FIG. 3 illustrates a flow chart for explanation of a method of communicating using outdated CSI of a UE according to an embodiment of the present invention. In FIG. 3, a method of communicating performed by the first UE in the first cell is explained. However, the method explained below may be applied similarly to the second UE of the first cell and UEs in the second cell also.

[75] In first time period which is phase 1 mentioned above, first UE of first cell transmits 4 different linear combinations of 3 data symbols to first BS (S310). The first UE transmits each of the 4 different linear combinations at each of time slots included in the first time period.

[76] The first UE receives feedback signals from second BS (S320). The second BS overhears the linear combinations being transmitted to the first BS as interference signals, and the feedback signals are for the overheard linear combinations.

[77] Received feedback signals comprise channel coefficients information between the first UE and the second BS. Further, the received feedback signals may further comprise a null space vector for the linear combinations. As described in FIG. 2, feedback signals may be multiplications of the null space vector and the overheard linear combinations, and the overheard linear combinations may be expressed in multiplications of the channel coefficients information and transmission precoder of the first UE.

[78] While, during the second time period (phase 2), the first UE stops transmitting and/or receiving of any signals. During the second time period, the UEs in the second cell transmit linear combinations to the second BS, respectively. [79] In the first slot of the third time period (phase 3), the UE transmits a reconstruction signal to the first BS (S330). The reconstruction signal is generated based on the received feedback signal in S320. By transmitting reconstruction signal, the first BS only requires 1 more equation to decode the received 4 linear combinations.

[80] In the second slot of the third time period, the first UE does not transmit any signals to the first BS, while the second UE of the first cell transmits its reconstruction signal to the first BS. The first BS obtains total 6 equations and thereby is able to decode all linear combinations.

[81] FIG. 4 illustrates a flow chart for explanation of a method of communicating using outdated CSI of a BS according to an embodiment of the present invention. In FIG. 4, a method of communicating performed by the first BS is explained. However, the method explained below may be applied similarly to the second BS also.

[82] In first time period (phase 1 ), first BS receives a first 4 different linear combinations from two UEs of first cell (S410). Subsequently, in second time period (phase 2), first BS receives a second 4 different linear combinations from two UEs of second cell (S420). The first BS receives the second linear combinations by overhearing linear combinations being transmitted from the two UEs of second cell to second BS as interference signals.

[83] The first BS generates feedback signals for the overheard linear combinations from UEs of the second cell, and transmits the feedback signals to each of the two UEs of the second cell (S430).

[84] The feedback signals comprise channel coefficients information between the first BS and each of the 2 UEs of the second cell. And, the feedback signals may further comprise null space vectors for the second linear combinations from the 2 UEs of the second cell. Similar to FIG. 3, each of the feedback signals may be multiplications of the null space vector and overheard second linear combinations, and the overheard linear combinations may be expressed in multiplications of the channel coefficients information and transmission precoder of each of the UEs.

[85] In a first slot of the third time period (phase 3), BS receives first reconstruction signals from first UE of the first cell and first UE of the second cell (S440). Each of the reconstruction signals may comprise feedback signals received from second BS and first BS, respectively. In a second slot of the third time period, BS receives second reconstruction signals from second UE of the first cell and second UE of the second cell (S450).

[86] By using the first and the second received reconstruction signals, the BS decodes the first 4 linear combinations (S460). The BS cancels the feedback signal transmitted to the first UE in S430 out from the first reconstruction signals, and cancels the feedback signal transmitted to the second UE in S430 out from the second reconstruction signals, respectively.

[87] Thereby, the first BS obtains 6 independent equations for the first 4 linear combinations and 2 reconstruction signals. As a result, the first BS decodes the cancelled out results and the first linear combinations altogether.

[88] FIG. 5 illustrates a block diagram of a structure of a UE 100 and a base station 200 according to an embodiment of the present invention.

[89] In FIG. 5, the UE 100 and the base station 200 may include radio frequency (RF) units 110 and 210, processors 120 and 220, and memories 130 and 230, respectively. Although FIG. 5 illustrates a 1 : 1 communication environment between the UE 100 and the base station 200, a communication environment between a plurality of UEs and the base station 200 can also be established. In addition, the base station 200 of FIG. 5 can be applied to both a macro cell eNB and a small cell eNB.

[90] The RF units 1 10 and 210 may include transmitters 1 12 and 212 and receivers 1 14 and 214, respectively. The transmitter and 1 12 and the receiver 1 14 of the UE 100 may be configured to transmit and receive signals to and from the base station 200 and other UEs and the processor 120 may be functionally connected to the transmitter 1 12 and the receiver 114 to control a process of transmitting and receiving signals to and from other devices by the transmitter 1 12 and the receiver 1 14. The processor 120 performs various processing processes on signals to be transmitted and then transmits the processed signals to the transmitter 112 and performs processing on the signals received by the receiver 1 14.

[91] As necessary, the processor 120 may store information contained in exchanged message in the memory 130. Based on this structure, the UE 100 can perform various methods according to the aforementioned embodiments of the present invention.

[92] The transmitter 212 and the receiver 214 of the base station 200 may be configured to transmit and receive signals to and from other eNBs and UEs and the processor 220 may be functionally connected to the transmitter 212 and the receiver 214 to control a process of transmitting and receiving signals to and from other devices by the transmitter 212 and the receiver 214. The processor 220 performs various processing processes on signals to be transmitted and then transmits the processed signals to the transmitter 212 and performs processing on the signals received by the receiver 214. As necessary, the processor 220 may store information contained in exchanged message in the memory 230. Based on this structure, the base station 200 can perform various methods according to the aforementioned embodiments of the present invention.

[93] The processors 120 and 220 of the UE 100 and the base station 200 requests (e.g., controls, manipulates, manages, etc.) operations of the UE 100 and the base station 200, respectively. The processors 120 and 220 may be connected to the memories 130 and 230 for storing program codes and data, respectively. The memories 130 and 230 may be connected to the processors 120 and 220 to stores operating system (OS), an application, and general files.

[94] The processors 120 and 220 according to the present invention can also be called a controller, a microcontroller, a microprocessor, a microcomputer, etc. The processors 120 and 220 may be embodied in the form of hardware, firmware, software, or a combination thereof. When an embodiment of the present invention is embodied using hardware, the processors 120 and 220 may include application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), or the like which is configured to embody the present invention.

[95] The embodiments of the present invention may be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable recording medium. In addition, a structure of data used in the above-described method may be recorded in a computer readable recording medium through various methods. Program storage devices used for description of a storage device containing an executable computer code for execution of the various methods according to the present invention is not understood as temporary objects such as carrier waves or signals. Examples of the computer readable recording medium include magnetic storage media (e.g., ROMs, floppy disks, hard disks, etc.) and optical recording media (e.g., CD-ROMs, or DVDs).

Claims

[CLAIMS]

[Claim 1 ]

A method of communicating using outdated channel state information (CSI) for a user equipment (UE) in a network environment which consists 2 base stations (BSs) and 2 cells which include 2 UEs each, the method performed by a first UE of a first cell and comprising:

transmitting to a first BS, 4 different linear combinations of 3 data symbols, at each of 4 time slots included in a first time period;

receiving from a second BS, feedback signals for the linear combinations, wherein the second BS overhears the linear combinations of the first UE as interference signals during the first time period; and

transmitting to the first BS, a reconstruction signal generated based on the feedback signals, at a first time slot of a third time period including 2 time slots,

wherein the feedback signals comprise channel coefficients information between the first UE and the second BS during the first time period.

[Claim 2]

The method of claim 1 , wherein the received feedback signals comprise a null space vector for the linear combinations from the first UE which are overheard by the second BS.

[Claim 3 ]

The method of claim 2, wherein the feedback signals are multiplications of the null space vector and the overheard linear combinations, and the overheard linear combinations are multiplications of the channel coefficients information and transmission precoder of the first UE.

[Claim 4]

The method of claim 1 , wherein the first UE of the first cell and a second UE of the first cell stop transmitting or receiving of any signals, during a second time period including 4 time slots.

[Claim 5]

The method of claim 1 , wherein the first UE stops transmitting or receiving of any signals, at a second time slot of the third time period.

[Claim 6]

A method of communicating using outdated channel state information (CSI) for a base station (BS) in a network environment which consists 2 BSs and 2 cells which include 2 UEs each, the method performed by a first BS and comprising: receiving from each of 2 UEs of a first cell, a first 4 different linear combinations of 3 data symbols, at each of 4 time slots included in a first time period;

receiving from each of 2 UEs of a second cell, a second 4 different linear combinations of 3 data symbols for a second BS by overhearing the second 4 different linear combinations as interference signals, at each of 4 time slots included in a second time period;

transmitting to each of the 2 UEs of the second cell, feedback signals for each of the second linear combinations;

receiving from a first UE of the first cell and a first UE of the second cell, a first reconstruction signals reconstructed by using the feedback signals, at a first time slot of a third time period including 2 time slots;

receiving from a second UE of the first cell and a second UE of the second cell, a second reconstruction signals reconstructed by using the feedback signals, at a second time slot of the third time period; and

decoding the first linear combinations by using the first reconstruction signals and the second reconstruction signals,

wherein the feedback signals comprise channel coefficients information between the first BS and each of the 2 UEs of the second cell.

[Claim 7]

The method of claim 6, wherein each of the feedback signals comprises null space vectors for the second linear combinations from the 2 UEs of the second cell.

[Claim 8]

The method of claim 7, wherein each of the feedback signals is a multiplication of the null space vector and the second linear combinations from each UEs, and the second linear combinations are multiplications of the channel coefficients information and transmission precoder of each UEs.

[Claim 9]

The method of claim 6, wherein the decoding comprises:

cancelling the feedback signal transmitted to the first UE of the second cell out from the first reconstruction signals; and

cancelling the feedback signal transmitted to the second UE of the second cell out from the second reconstruction signals.

[Claim 10] The method of claim 9, wherein the decoding further comprises:

decoding the cancelled out results and the first linear combinations together.