NETWORK SIGNALING FOR NETWORK-ASSISTED INTERFERENCE CANCELLATION AND MITIGATION - Patent application

Abstract

To provide a method, a device, and a base station for a cellular communication network to determine whether communication between one base station-UE pair interferes with another UE in the same cell or a different cell.SOLUTION: A cellular network 100 identifies interference parameters associated with interference signals 110 that may be received by a UE 105. The interference signals 110 may be generated by a base station 101 itself, such as communications with other UEs 102, or by a neighboring base station 104. The base station 101 transmits the interference parameters to the UE 105. The UE 105 receives the one or more parameters comprising information about signals expected to cause intra-cell or inter-cell interference, and then processes received signals using the one or more parameters to suppress the intra-cell or inter-cell interference signal 110.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] TECHNICAL FIELD This application relates generally to wireless communications, and more particularly to wireless telephone communications. [Background technology]

[0002] In one example, a cellular wireless network includes multiple base stations, where each base station transmits (downlink) to and receives (uplink) from mobile users in its coverage area. In the downlink, each user receives data from its serving base station (or serving cell). Signals from neighboring base stations may impose inter-cell interference. Because all base stations in a particular cellular wireless network operate in the same spectrum, interference is considered to be the main obstacle to cellular communications. This becomes even more problematic as base station density continues to grow exponentially due to the deployment of low-power, small form factor base stations (small cells). Thus, mitigating co-channel interference becomes an increasingly important factor for continued data rate and spectral efficiency improvements in cellular networks. Summary of the Invention

[0003] In the described example, the cellular communication network knows which base stations communicate with which UEs, and may determine that communications between one base station and UE pair may interfere with another UE in the same or a different cell. The network may inform the base station of communications in neighboring cells that may cause interference. Alternatively, the base station may determine that communications to UEs in its cell may cause interference.

[0004] The base station identifies interference parameters associated with interfering signals that may be received by a UE. These interfering signals may be generated by the base station itself, such as in communication with other UEs, or by neighboring base stations. The base station transmits the interference parameters to the UE. The one or more parameters identify a number of interfering signals that may affect the user equipment. The interference parameters may be configured semi-statically, such as by RRC signaling, or may be configured dynamically by the network or the base station.

[0005] The base station may include a processor circuit that generates a bitmap indicating whether each of a plurality of interference sources is present and then transmits the bitmap to the user equipment.

[0006] In other embodiments, the base station may signal a first set of parameters to the UE on a wideband basis and a second set of parameters to the UE on a narrowband basis, e.g., the first set of parameters may be applicable to all PRBs in the assigned frequency resources and the second set of parameters may be applicable to each PRB pair or each PRG in the UE's assigned frequency resources.

[0007] For example, the interference parameters include the demodulation reference signal (DMRS) antenna port, the DMRS antenna port scrambling sequence initialization identification number (n SCID ), the power level of the interfering signal, the transmission rank of the interfering signal, the modulation order of the interfering signal, the code rate of the interfering signal, the Radio Network Temporary Identifier (RNTI) of the interfering signal, the cell ID of a nearby interfering cell, and the system frame number (SFN) of the interfering signal.

[0008] The UE receives one or more parameters that include information about signals that are expected to cause intra-cell or inter-cell interference, and the UE processes the received signals using the one or more parameters to mitigate the intra-cell or inter-cell interference. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view of inter-cell interference in a cellular network.

[0010] [Diagram 2] FIG. 1 is a perspective view of intra-cell interference in a cellular network.

[0011] [Diagram 3] FIG. 2 is a block diagram of an overview of physical channel processing for the LTE downlink in a base station.

[0012] [Figure 4] 4 is a flow chart of a process for notifying user equipment of possible intra-cell and inter-cell interference signals according to one embodiment.

[0013] [Diagram 5] 3 is a detailed internal block diagram of a mobile user equipment and a base station operating in a network system such as the network of FIG. 1 or FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Figure 1 shows inter-cell interference in a cellular network. When inter-cell interference occurs, the interference comes from a neighboring base station transmitting to other users in neighboring cells. Because a typical cellular network does not have coordination between different base stations, inter-cell interference appears to mobile users as random radio signals.

[0015] The system 100 is a cellular network, such as a 3GPP Long Term Evolution (LTE) system. A base station 101, such as an LTE eNodeB, serves a user equipment (UE) 102 and other devices (not shown in FIG. 1 ) in a cell 103. A base station 104 serves a UE 105 and other devices (not shown in FIG. 1 ) in a cell 106. The UE 102 receives downlink communications 107 from the base station 101, and the UE 105 receives downlink communications 108 from the base station 104. Unfortunately, these downlink transmissions also reach neighboring cells. Thus, the UE 102 receives both the intended downlink communications 107 from the base station 101 and the unwanted communications 108 from the neighboring base station 104. Similarly, the UE 105 receives both the intended downlink communications 108 from the base station 104 and the unwanted communications 110 from the neighboring base station 101. These unwanted communication signals cause inter-cell interference at the UEs 102, 105.

[0016] Figure 2 shows intra-cell interference in a cellular network. Inter-cell interference occurs when one serving base station transmits to one or more co-scheduled users in the same cell. For example, multi-user multiple-input and multiple-output (MU-MIMO) in LTE follows a transparent design principle throughout Releases 8-11, in which the UE has no information about the existence or attributes of any co-scheduled users in the same cell. Thus, the UE does not know whether it is scheduled in single-user MIMO (SU-MIMO) mode or paired with another user in MU-MIMO mode.

[0017] Base station 201 serves both UE 202 and UE 203 in cell 204. Downlink transmission 205 is intended for UE 202, and downlink transmission 206 is intended for UE 203. However, UE 202 may also receive transmission 207 intended for UE 203 or some other device in cell 204. Transmission 207 causes intra-cell interference to UE 202. Similarly, transmission 208 intended for UE 202 or some other device in cell 204 causes intra-cell interference to UE 203.

[0018] In recent years, advanced MIMO receivers have been increasingly implemented by mobile handset manufacturers and chipset vendors to achieve better MIMO decoding performance. In addition to decoding its own signals, the advanced MIMO receiver can blindly suppress / decode intra-cell / inter-cell interference through brute-force search, which can significantly improve downlink signal-to-noise ratio (SNR) and data throughput. Although this is feasible and does not require standardization support, it is still very challenging in terms of UE complexity, power consumption, and chipset size, despite recent improvements in UE receiver design. Alternatively, if the network can signal interference attributes to the UE, the UE can utilize such information with reasonably low UE complexity to achieve better interference cancellation and suppression. Such additional downlink signaling of interference attributes can become a new downlink control signaling in cellular networks.

[0019] These issues are described herein, along with suggestions for possible information regarding downlink interference that should be signaled by the network, methods for signaling such information by the network, and mechanisms in the mobile receiver for receiving and using such signaled interference information.

[0020] Figure 3 shows an overview of the LTE downlink physical channel processing at the base station. A codeword 301 undergoes scrambling 302 and modulation mapping 303. A layer mapper 304 produces layers 305 which are precoded 306 and then mapped 307 to resource elements. An OFDM signal is then generated 308 and transmitted via the assigned antenna ports 309.

[0021] In conventional wireless systems, a UE decodes only its own signals, such as signals intended for the UE on a physical downlink shared channel (PDSCH). MIMO receivers can be classified as linear or nonlinear. In a linear MIMO receiver, an equalization matrix (R×Nr) is applied to the received signal vector (Nr×1) to remove inter-layer interference, where R is the number of data layers and Nr is the number of receive antennas. Then, layer-by-layer demodulation and decoding are performed (such as descrambling, deinterleaving, demodulation, and decoding). Common linear MIMO receivers include a zero-forcing receiver, a linear MMSE receiver, and a linear MMSE receiver with interference cancellation combining.

[0022] In a nonlinear MIMO receiver, the decoding and demodulation of multiple layers are not independent but are interrelated.

[0023] In a maximum likelihood (ML) receiver, the decoder exhaustively searches all possible quadrature amplitude modulation (QAM) constellation combinations in the R layer for the best Nr×1 QAM symbol vector. The ML decoder is optimal in terms of symbol error rate, but has a complexity that grows exponentially in R.

[0024] Successive interference cancellation receivers are also possible, where the receiver proceeds to decode the first layer, reconstruct the first layer, subtract the resulting interference from the residual signal, and then decode the second layer. Both soft and hard interference cancellation are possible. With soft interference cancellation, in one scenario, the inter-layer interference is constructed by estimating the soft QAM constellation of the first layer without channel decoding. Alternatively, the transport block can be decoded, and the soft information and parity bits can be used to reconstruct the interference. With hard interference cancellation, the first layer is channel decoded to generate the original transport block (TB), where the information bits of the first layer are re-encoded, re-modulated, and re-scrambled to generate the inter-layer interference subtracted from the residual signal.

[0025] In an advanced interference cancellation / suppression receiver, similar algorithms may be applied as in the SU-MIMO case, except that the target UE decodes its own signal (source signal) and signals intended for other users (interfering signals). Therefore, to facilitate UE receiver operation, information about the presence and / or attributes of co-scheduled users is required and may be signaled by the network. The following sections describe information that may be signaled by the network for interference cancellation and / or suppression.

[0026] In one embodiment, the network may send one or more parameters related to the interfering signals, such as a subset of the parameters described hereinafter, such that the UE may utilize the information represented by these parameters for intra-cell and / or inter-cell interference cancellation and / or mitigation.

[0027] The network may identify a demodulation reference signal (DMRS) antenna port on which the UE may estimate that an interfering signal is transmitted. For example, when a target UE receives a PDSCH on antenna port 7, the network may signal to the UE that an interfering signal is being transmitted to another UE on antenna port 8. The UE may process the received signal accordingly to minimize interference.

[0028] At each DMRS antenna port of the interference signal, the scrambling sequence initialization identification number (n SCID ) is signaled to the target UE. SCID is used by the target UE to generate the scrambling sequence for the DMRS port of the interfering signal.

[0029] The network may signal the power level of the interfering signal to the UE. The signaled power level may be the power of the DMRS antenna port of the interfering signal and / or the power of the PDSCH of the interfering signal. The interfering signal power level may be signaled either in the form of absolute power (such as dBm) or as a relative power ratio to a reference power. For example, if the PDSCH of the target UE is used as the default reference power, the network may signal to the target UE the ratio of the PDSCH power of the interfering signal to the PDSCH power of the target UE. The target UE may use such information to derive appropriate receiver weights for channel estimation, PDSCH decoding, and interference cancellation and suppression.

[0030] The network may signal the transmission rank (e.g., the number of PDSCH layers) of the interfering signal. The number of PDSCH layers is equivalent to the number of DMRS antenna ports in a DMRS-based transmission. One reason for the network to provide this signaling is due to the fact that the DMRS pattern, overhead, PDSCH mapping pattern, and PDSCH power are functions of the transmission rank.

[0031] For rank-1 / 2 PDSCH transmission, the DMRS overhead is 12 resource elements (RE) / physical resource block (PRB). For rank-3 / 4 PDSCH transmission, the DMRS overhead is 24 RE / PRB. Because the PDSCH is rate-matched per DMRS, the PDSCH mapping pattern can be different for rank-1 / 2 and even rank-3 / 4. For rank-1 / 2, the power of the PDSCH at each antenna port is equal to the power of the corresponding DMRS antenna port. For rank-3 / 4, the power of the PDSCH at each antenna port is 3 dB lower than (e.g., 50% of) the power of the corresponding DMRS antenna port.

[0032] Therefore, by signaling the rank of the interfering PDSCH signal, the network can allow the target UE to accurately interpret the DMRS and PDSCH transmission characteristics of the interfering signal. Alternatively, instead of signaling the channel rank, the network can use one bit to signal the DMRS overhead (12RE / PRB or 24RE / PRB) or the PDSCH mapping pattern.

[0033] The network may signal the modulation order of the interfering signal (e.g., quadrature phase shift keying (QPSK), 16QAM, and 64QAM). This information may be used by UEs using soft interference cancellation based on which UEs can decode the symbol-level QAM of the interfering signal without performing channel decoding.

[0034] The code rate of the interfering signal may be used by the UE using hard interference cancellation based on which UE can decode the transport block of the interfering signal via channel decoding. LTE systems currently do not support explicit signaling of the code rate. Instead, the network may signal an index for the transport block size or modulation / coding scheme (MCS). When combined with the frequency assignment size, the UE may interpret the code rate. Similarly, in network-assisted interference cancellation and suppression (NAICS), the network may signal an index for the transport block size or MCS of the interfering signal to the UE. For example, to obtain the transport block size (TBS) of the interfering signal, the UE may estimate that the interfering signal and the source PDSCH signal have the same frequency allocation (e.g., the interfering and target PDSCH are assigned to the same set of PRBs).

[0035] The network receives n RNTIs corresponding to the radio network temporary identifiers (RNTIs) of the relevant interference signals. RNTI n RNTI is the UE-ID of the interfering UE targeted by the interfering signal. RNTI is needed to recover the scrambling sequence for the interfering PDSCH signal and is also needed if the target UE wants to decode the interfering signal transport block before performing hard interference cancellation. RNTI may be signaled, for example, in the form of a virtual UE-ID.

[0036] The network may signal the cell ID of the interfering cell if the interference comes from the transmission of a neighboring base station (inter-cell interference). This information is needed to recover the scrambling sequence of the interfering PDSCH signal. The cell ID of the interfering cell may be signaled explicitly or in the form of a virtual cell ID, which is used by the UE to generate the scrambling sequence of the interfering DMRS signal.

[0037] If the system frame numbers (SFNs) of the source and interfering signals are not aligned, the network may signal the SFN value of the interfering signal to the target UE, where the SFN value of the interfering cell is needed to generate the scrambling sequence of the interfering signal.

[0038] The target UE may receive multiple interference signals. For example, the UE may receive a strong inter-cell interference signal from one neighboring cell and an intra-cell interference signal originating from the same serving cell transmitting to a user scheduled simultaneously with MU-MIMO. In another example, a single-cell MU-MIMO may simultaneously schedule four users in the same subframe in the same cell, in which case three intra-cell interference signals are present to the UE. Thus, the network may signal the number of interference signals the UE can expect.

[0039] Alternatively, the network may signal a bit sequence, where each bit indicates the presence of a possible interference source (e.g., 0 indicates no interference, 1 indicates interference is present, etc.). The length of the bitmap (whether fixed or Radio Resource Control (RRC) signaled) may depend on the number of interference signals that the UE may expect in a typical cellular deployment, while accounting for network signaling overhead. If the length of the bitmap is RRC signaled, the network may indicate to the UE how many interference signals the UE should expect. For example, in a typical homogeneous deployment scenario with only macro base stations, there may be only a few dominant interference signals, so the bitmap may have a shorter length. In a dense urban deployment scenario with many low-power small cell base stations, the UE may receive a large number of interference signals, so the bitmap may have a longer length. The interference parameters defined above or a subset of them may be signaled independently for each possible interference signal.

[0040] In one embodiment, the aforementioned interference parameters may be signaled on a wideband basis, where the UE may assume that the signaled parameters apply to all PRBs in the UE's assigned frequency resources. Alternatively, these parameters may be signaled on a narrowband basis, such as for each PRB pair or each Precoding Resource Block Group (PRG) in the target UE's assigned frequency resources. Some parameters may be signaled on a wideband basis, while other parameters are signaled on a narrowband basis. For example, the cell ID and SFN of the interfering signal may be signaled on a wideband basis, while the n of the interference may be signaled on a narrowband basis. RNTI , DMRS port, n SCID is signaled on a narrowband basis.

[0041] The interference parameters may be semi-statically configured by RRC signaling or may be provided to the UE via dynamic signaling (such as in a downlink (DL) grant). For example, the cell ID and SFN of an interfering signal may be configured by RRC signaling and may remain constant for a relatively long time, since the neighboring cells that generate inter-cell interference may not change frequently. In contrast, the n RNTI , DMRS port, DMRS scrambling may change per subframe and may be dynamically signaled.

[0042] PDSCH rate matching is UE specific and depends on an RRC configured UE specific signal (such as CSI-RS or zero power CSI-RS). In one embodiment, the UE may assume that the interfering signal is rate matched in the same manner as the source signaled. In another embodiment, the UE may be signaled with the actual PDSCH mapping / rate matching pattern of the interfering signal. However, this may incur significant signaling overhead and is not preferred.

[0043] FIG. 4 illustrates a process for notifying a UE of possible intra-cell and inter-cell interference signals according to one embodiment. The cellular communication network knows which base stations are communicating with which UEs and may determine that communications between one base station and UE pair may interfere with another UE in the same or a different cell. The network may notify the base station of communications in neighboring cells that may cause interference. Alternatively, the base station may determine that communications to a UE within the base station's cell may cause interference.

[0044] In step 401, the base station identifies interference parameters associated with interfering signals that may be received by the UE. The interfering signals may be generated by the base station itself, such as in communication with other UEs, or by neighboring base stations. In step 402, the base station transmits the interference parameters to the UE. The one or more parameters identify a number of interfering signals that may affect the user equipment. The interference parameters may be configured semi-statically, such as by RRC signaling, or may be configured dynamically by the network or the base station.

[0045] The base station may include a processor circuit that generates a bitmap indicating whether each of a plurality of interference sources is present and transmits the bitmap to the user equipment.

[0046] In other embodiments, the base station may signal a first set of parameters to the UE on a wideband basis and a second set of parameters to the UE on a narrowband basis, e.g., the first set of parameters may be applicable to all PRBs in the assigned frequency resources and the second set of parameters may be applicable to each PRB pair or each PRG in the UE's assigned frequency resources.

[0047] For example, the interference parameters include the demodulation reference signal (DMRS) antenna port, the DMRS antenna port scrambling sequence initialization identification number (n SCID), the power level of the interfering signal, the transmission rank of the interfering signal, the modulation order of the interfering signal, the code rate of the interfering signal, the Radio Network Temporary Identifier (RNTI) of the interfering signal, the cell ID of the nearby interfering cell, and the System Frame Number (SFN) of the interfering signal.

[0048] In step 403, the UE receives one or more parameters containing information about signals expected to cause intra-cell or inter-cell interference, and in step 404, the UE processes the received signals using the one or more parameters to mitigate the intra-cell or inter-cell interference.

[0049] Figure 5 illustrates internal details of a mobile UE 501 and a base station, such as an eNB 502, operating in a network system, such as the network of Figure 1 or Figure 2. The mobile UE 501 may represent any of a variety of devices, such as a server, a desktop computer, a laptop computer, a mobile phone, a personal digital assistant (PDA), a smartphone, or other electronic device. In some embodiments, the electronic mobile UE 501 communicates with the eNB 502 based on LTE or Evolved Universal Terrestrial Radio Access (E-UTRA) protocols. Alternatively, another communication protocol, now known or later developed, may be used.

[0050] The mobile UE 501 includes a processor 503 coupled to a memory 504 and a transceiver 505. The memory 504 stores applications 506 (e.g., software) for execution by the processor 503. Such applications may include any known or future application useful to an individual or organization. These applications may be categorized as an operating system (OS), device drivers, databases, multimedia tools, presentation tools, Internet browsers, emailers, Voice-Over-Internet Protocol (VOIP) tools, file browsers, firewalls, instant messaging, financial tools, games, word processors, or other categories. Regardless of the exact nature of the application, at least some of the applications may instruct the mobile UE 501 to periodically or continuously transmit UL signals to the eNB (base station) 502 via the transceiver 505.

[0051] The transceiver 505 includes uplink logic, which may be implemented by execution of instructions that control the operation of the transceiver. Some of these instructions may be stored in memory 504 and executed when required by the processor 503. Components of the uplink logic may participate in the physical (PHY) layer and / or medium access control (MAC) layer of the transceiver 505. The transceiver 505 includes one or more receivers 507 and one or more transmitters 508.

[0052] The processor 503 may send or receive data to various input / output devices 509. A subscriber identity module (SIM) card stores and retrieves information used to make calls over the cellular system. A Bluetooth baseband unit may be provided for wireless connection to a microphone and a headset for receiving and transmitting voice data. The processor 503 may send information to a display unit for interaction with a user of the mobile UE 501 during the call process. The display may also display images received from the network, from a local camera, or from other sources such as a Universal Serial Bus (USB) connector. The processor 503 may also send video streams received from various sources, such as the cellular network via the RF transceiver 505 or the camera, to the display.

[0053] During transmission and reception of voice or other application data, the transmitter 508 may be or become asynchronous with its serving eNB. In this case, the transmitter sends a random access signal. As part of this procedure, the transmitter determines a preferred size for the next data transmission, called a message, by using a power threshold value provided by the serving eNB. In this embodiment, the message preferred size determination may be implemented by the processor 503 executing instructions stored in the memory 504. In other examples, the message size determination may be implemented by a separate processor and / or memory unit, by a hardwired state machine, or by other types of control logic.

[0054] In one embodiment, the UE 501 receives interference parameters from the base station 502. The processor 503 uses the interference parameters to identify and suppress interfering signals received at the receiver 507.

[0055] The eNB 502 includes a processor 510 coupled to a memory 511, symbol processing circuitry 512, and a transceiver 513 via a backplane bus 514. The memory stores applications 515 for execution by the processor 510. Such applications may include any known or future applications useful for managing wireless communications. At least some of the applications 515 may instruct the eNB 502 to manage transmissions to and from the mobile UE 501.

[0056] The transceiver 513 includes an uplink resource manager that enables the eNB 502 to selectively assign uplink Physical Uplink Shared Channel (PUSCH) resources to the mobile UE 501. Components of the uplink resource manager may be involved in a physical (PHY) layer and / or a medium access control (MAC) layer of the transceiver 513. The transceiver 513 includes at least one receiver 517 for receiving transmissions from various UEs within range of the eNB 502 and at least one transmitter 516 for transmitting data and control information to various UEs within range of the eNB 502.

[0057] The uplink resource manager executes instructions that control the operation of the transceiver 513. Some of these instructions may be located in the memory 511 and executed when needed on the processor 510. The resource manager controls the transmission resources allocated to each UE 501 served by the eNB 502 and broadcasts control information via the PDCCH. The UE 501 may receive TTD UL / DL configuration instructions from the eNB 502.

[0058] The symbol processing circuitry 512 performs demodulation using known techniques. The random access signal is demodulated in the symbol processing circuitry 512. During transmission and reception of voice data or other application data, the receiver 517 may receive a random access signal from the UE 501. The random access signal is encoded to request a preferred message size for the UE 501. The UE 501 determines the preferred message size by using a message threshold provided by the eNB 502.

[0059] Modifications can be made to the exemplary embodiments described and many other embodiments are possible within the scope of the claims of the invention.

Claims

1. 1. A method comprising: Receiving, at a user equipment (UE), a plurality of parameters including information regarding signals expected to cause intra-cell or inter-cell interference; processing a received signal at the UE using the parameters to mitigate the intra-cell or inter-cell interference; Including, The method, wherein the plurality of parameters is configured by Radio Resource Control (RRC) signaling, the plurality of parameters including a first parameter indicative of a power level of an interfering signal and a second parameter indicative of a cell identity of a nearby interfering cell.

2. 2. The method of claim 1 , The method further includes receiving, at the UE, a bitmap indicating whether each of a plurality of interference sources associated with the intra-cell or inter-cell interference is present.

3. 2. The method of claim 1 , The method, wherein the plurality of parameters further includes a third parameter indicative of a transmission rank of a signal from the neighboring interfering cell.

4. A user equipment in communication with a base station, comprising: receiving a plurality of parameters transmitted by the base station, the plurality of parameters including information regarding signals expected to cause intra-cell or inter-cell interference; processing a received signal using the plurality of parameters to combat the intra-cell or inter-cell interference; It is configured as follows: The user equipment, wherein the plurality of parameters is configured by Radio Resource Control (RRC) signaling, the plurality of parameters including a first parameter indicative of a power level of an interfering signal and a second parameter indicative of a cell identity of a nearby interfering cell.

5. 5. A user equipment as claimed in claim 4, comprising: The user equipment, wherein the plurality of parameters further includes a third parameter indicative of a transmission rank of a signal from the neighboring interfering cell.

6. 1. A base station for communicating with user equipment, comprising: identifying a plurality of parameters associated with an interfering signal generated by the base station or by a neighboring base station and received by the user equipment; transmitting said plurality of parameters to said user equipment; It is configured as follows: The base station, wherein the plurality of parameters is configured by Radio Resource Control (RRC) signaling, the plurality of parameters including a first parameter indicative of a power level of an interfering signal and a second parameter indicative of a cell identity of a nearby interfering cell.

7. 7. A base station according to claim 6, The base station, wherein the plurality of parameters further includes a third parameter indicative of a transmission rank of a signal from the neighboring interfering cell.

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