Interference mitigation for downlink in a wireless communication system

Techniques for mitigating interference in a wireless communication system are described. In an aspect, pertinent transmission parameters for a served UE may be sent to at least one interfered UE to support interference mitigation. In one design, information for at least one transmission parameter for a data transmission sent by a first cell to a first UE may be transmitted to at least one UE served by a second cell to enable the at least one UE to perform interference mitigation for the data transmission sent by the first cell to the first UE. The information may be transmitted by either the first cell or the second cell. In another aspect, a cell may send transmission parameters for a UE via a pilot. In yet another aspect, scrambling may be performed by a cell at symbol level to enable an interfered UE to distinguish between modulation symbols of desired and interfering transmissions.

BACKGROUND

The present disclosure relates generally to communication, and more specifically to techniques for mitigating interference in a wireless communication system.

Wireless communication systems are widely deployed to provide various communication content such as voice, video, packet data, messaging, broadcast, etc. These wireless systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA) systems.

A wireless communication system may include a number of base stations that can support communication for a number of user equipments (UEs). A UE may communicate with a base station via the downlink and uplink. The downlink (or forward link) refers to the communication link from the base station to the UE, and the uplink (or reverse link) refers to the communication link from the UE to the base station.

A UE may communicate with a serving cell and may be within range of one or more neighbor cells. The term “cell” can refer to a coverage area of a base station and/or a base station subsystem serving the coverage area. The UE may receive a data transmission sent by the serving cell to the UE on the downlink. The UE may also receive other data transmissions sent by the neighbor cells to other UEs. These other data transmissions may appear as interference to the UE and may impact the ability of the UE to recover the data transmission from the serving cell. It may be desirable to mitigate the interference on the downlink in order to improve performance.

SUMMARY

Techniques for mitigating interference in a wireless communication system are described herein. In an aspect, pertinent transmission parameters for a served UE may be sent to at least one interfered UE to enable each interfered UE to perform interference mitigation. The transmission parameters may include a modulation order or modulation scheme, a traffic-to-pilot ratio (T2P), precoding information, a transmission rank, downlink resources, and/or other parameters for a data transmission to the served UE.

In one design, information for at least one transmission parameter for a data transmission sent by a first cell to a first UE may be obtained. The information for the at least one transmission parameter may be transmitted to at least one UE served by a second cell to enable the at least one UE to perform interference mitigation for the data transmission sent by the first cell to the first UE. The information may be obtained and transmitted by either the first cell or the second cell.

In one design, the first UE may obtain a received signal comprising a first data transmission sent by the first cell to the first UE and a second data transmission sent by the second cell to a second UE. The first UE may also obtain information for the at least one transmission parameter for the second data transmission, e.g., from the first cell or the second cell. The first UE may perform interference mitigation for the second data transmission based on the information for the at least one transmission parameter to recover the first data transmission sent to the first UE.

In another aspect, a cell may send transmission parameters for a UE via a pilot sent to the UE. In one design, the cell may generate a data transmission based on at least one transmission parameter. The cell may also generate a pilot (e.g., a dedicated pilot or a UE-specific reference signal) comprising information for the at least one transmission parameter. In one design, the cell may generate a pseudo-random number (PN) sequence based on the information for the at least one transmission parameter and may then generate modulation symbols for the pilot based on the PN sequence. The cell may transmit the pilot and the data transmission to a recipient UE. Other UEs may use the information for the at least one transmission parameter in the pilot to perform interference mitigation for the data transmission.

In yet another aspect, scrambling may be performed by a cell at symbol level to enable an interfered UE to distinguish between modulation symbols of a desired transmission and modulation symbols of an interfering transmission. In one design, a cell may generate modulation symbols for a data transmission for a recipient UE and may scramble the modulation symbols based on a scrambling sequence to obtain scrambled symbols. The cell may generate the scrambling sequence based on a cell identity (ID) and/or a UE ID. The cell may transmit the scrambled symbols for the data transmission. The recipient and interfered UEs may each perform descrambling with its scrambling sequence and may be able to distinguish between the modulation symbols of its data transmission and the modulation symbols of interfering transmissions, which may be useful for interference mitigation.

DETAILED DESCRIPTION

FIG. 1shows a wireless communication system100, which may be an LTE system, a CDMA system, etc. System100may include a number of base stations and other network entities. For simplicity, only two base stations110and112are shown inFIG. 1. A base station may be an entity that communicates with UEs and may also be referred to as a Node B, an evolved Node B (eNB), an access point, etc. Each base station may provide communication coverage for a particular geographic area. To improve system capacity, the overall coverage area of a base station may be partitioned into multiple (e.g., three) smaller areas. In 3GPP, the term “cell” can refer to the smallest coverage area of a base station and/or a base station subsystem serving this coverage area. The base stations may communicate with other network entities (e.g., other base stations and/or network controllers) via a backhaul.

A number of UEs may be dispersed throughout the system, and each UE may be stationary or mobile. For simplicity, only two UEs120and122are shown inFIG. 1. A UE may also be referred to as a mobile station, a terminal, an access terminal, a subscriber unit, a station, etc. A UE may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a smart phone, a netbook, a smartbook, etc. A UE may communicate with a serving cell and may be within range of one or more neighbor cells. The UE may receive a desired transmission from the serving cell and may also receive interfering transmissions from the neighbor cells on the downlink.

In the description herein, the terms “cell” and “base station” may be used interchangeably. A serving cell is a cell or base station designated to serve a UE on the downlink. A neighbor cell is a cell or base station not serving a UE. The terms “transmission” and “signal” may be used interchangeably.

InFIG. 1, UE120may communicate with its serving cell110, and UE122may communicate with its serving cell112. UE120may receive a desired transmission from serving cell110as well as interfering transmissions from neighbor cell112. The interfering transmissions may be intended for UE122and/or other UEs served by cell112. UE120may thus be an interfered UE for the transmissions from cell112to UE122and other UEs. In general, UE120may receive any number of interfering transmissions from any number of neighbor cells. For simplicity, much of the description below assumes the example shown inFIG. 1, with one interfering transmission from one neighbor cell112to one UE122.

Interference mitigation may be performed by a UE to mitigate interference on the downlink from neighbor cells. Interference mitigation refers to a process to address (e.g., suppress) interference in a received signal in order to improve the likelihood of recovering a desired transmission in the received signal. Interference mitigation may be accomplished via interference cancellation. Interference mitigation deals with interference that is present in a received signal whereas interference avoidance attempts to completely avoid interference, e.g., by sending transmissions on different frequency regions and/or in different time intervals. Interference mitigation may be used to improve system capacity, extend coverage, and/or improve data transmission performance of UEs that are exposed to strong interfering cells.

Interference mitigation may be categorized into two main classes:Packet-level interference mitigation—mitigate interference by exploiting the code structure of an interfering transmission, andSymbol-level interference mitigation—mitigate interference based on knowledge or assumption of a modulation order of an interfering transmission.

An example of packet-level interference mitigation is post-decoding interference cancellation, which may be performed as follows. UE120may obtain a received signal comprising a desired transmission from serving cell110and an interfering transmission from neighbor cell112, which may be intended for UE122. UE110may process the received signal and decode the interfering transmission in the received signal. If the interfering transmission is decoded successfully, then UE120may estimate the interference due to the interfering transmission based on the decoded data and may then subtract the estimated interference from the received signal to obtain an interference-canceled signal having a higher signal-to-noise-and-interference ratio (SINR). UE120may then process the interference-canceled signal and decode the desired transmission. Data transmission performance may improve (e.g., a higher data rate may be supported) by the higher SINR obtained with interference cancellation.

Post-decoding interference cancellation may be supported if UE120can obtain pertinent transmission parameters to receive and decode the interfering transmission. These parameters may include a modulation and coding scheme (MCS) used for the interfering transmission, downlink resources (e.g., resource blocks) on which the interfering transmission is sent, etc. Neighbor cell112may send control information (e.g., a downlink grant) comprising these parameters to UE122. Neighbor cell112may send the downlink grant with power control and/or rate control so that the downlink grant can be reliably received by intended UE122. Hence, UE120may not be able to decode the downlink grant sent to UE122with power and/or rate control.

Furthermore, even if UE120can obtain the pertinent transmission parameters for the interfering transmission, UE120may be unable to successfully decode the interfering transmission. The MCS for the interfering transmission may be selected based on the channel quality between neighbor cell112and UE122. The channel quality between neighbor cell112to its UE122will likely be better than the channel quality between neighbor cell112and UE120. This may be due to the fact that each UE typically associates with the strongest cell, which may be serving cell110for UE120. Consequently, UE120may receive the interfering transmission from neighbor cell112with a lower SINR than the required SINR for the MCS used for the interfering transmission.

Symbol-level interference mitigation may be performed when packet-level interference mitigation is not practical or when lower complexity is desired. Symbol-level interference mitigation does not require UE120to correctly decode an interfering transmission. Instead, UE120may demodulate the interfering transmission and may estimate and cancel the interference due to the modulation symbols. Symbol-level interference mitigation may be categorized into two main classes:Soft-symbol interference cancellation—modulation symbols of an interfering transmission are estimated and subtracted from a received signal to improve SINR, andJoint demodulation—modulation symbols of an interfering transmission and a desired transmission are jointly demodulated, which may lead to calculation of “soft bits” or log-likelihood ratios (LLRs) of the desired transmission to be applied to a decoder for the desired transmission.

Soft-symbol interference cancellation or joint demodulation may also be performed iteratively with decoding. In this case, demodulation with soft-symbol interference cancellation or joint demodulation may be performed on received symbol to obtain demodulated symbols, then decoding may be performed on the demodulated symbols to obtain decoder output, and the processing may be repeated one or more times with the output of the demodulation step being provided to the decoding step, and vice versa. The demodulation step may thus alternate with the decoding step to obtain better estimates of modulation symbols of the desired transmission

In an aspect, pertinent transmission parameters for a data transmission for a served UE may be sent to at least one interfered UE to enable each interfered UE to perform interference mitigation. Symbol-level interference mitigation may be supported by providing the interfered UE(s) with pertinent transmission parameters to demodulate an interfering transmission and to estimate interference due to these modulation symbols. These parameters may include one or more of the following:Modulation order/modulation scheme used for the interfering transmission,T2P of the interfering transmission,Precoding information for the interfering transmission,Transmission rank of the interfering transmission,Downlink resources used for the interfering transmission, and/orOther transmission parameters.

UE120may demodulate the interfering transmission based on the modulation order used for the interfering transmission. This modulation order may be selected from a set of modulation orders supported by the system. The supported modulation orders may include binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-level quadrature amplitude modulation (16-QAM), 64-QAM, etc. In one design, UE120may obtain the modulation order (rather than the MCS) of the interfering transmission as described below and may perform demodulation based on the modulation order. In another design, UE120may assume a particular modulation order (e.g., QPSK) for the interfering transmission and may perform demodulation based on the assumed modulation order. UE120may achieve maximum gain when the actual modulation order matches the assumed modulation order and may obtain substantial gain even when the actual modulation order (e.g., 16-QAM or 64-QAM) does not match the assumed modulation order (e.g., QPSK). In yet another design, UE120may perform demodulation based on multiple hypotheses for modulation order and may select a hypothesis associated with the highest likelihood of correct demodulation. The best hypothesis may be identified based on one or more metrics computed for each hypothesis, such as absolute values of LLRs in the case of joint demodulation, post-cancellation SINR in the case of demodulation with interference cancellation, etc.

UE120may estimate the wireless channel from neighbor cell112to UE120and may use the channel estimate to perform interference mitigation. UE120may perform channel estimation in different manners depending on how a reference signal (or pilot) and data are transmitted by neighbor cell112. UE120may use the T2P and the precoding information for the interfering transmission to derive the channel estimate, as described below. The precoding information may convey weights used for precoding by neighbor cell112for UE122. The precoding information may comprise a precoding matrix indicator (PMI), a transmission mode indicator, etc. The PMI may indicate a specific vector of weights used for precoding. The RI may indicate the number of data streams or packets to transmit. The transmission mode indicator may indicate whether closed-loop precoding or open-loop precoding, such as large delay cyclic delay diversity (LD-CDD) in LTE, is used. Closed-loop precoding or open-loop precoding may be performed based on specified precoding weights.

UE120may demodulate the interfering transmission based further on the transmission rank of the interfering transmission. The transmission rank may indicate the number of data streams transmitted by neighbor cell112to UE122. UE120may also demodulate the interfering transmission based on the downlink resources used for the interfering transmission.

The received signal at UE120may be expressed as:
X(k)=H1·S1(k)+H2·S2(k)+N,Eq (1)
where S1(k) is a desired transmission from serving cell110to UE120,

S2(k) is an interfering transmission from neighbor cell112,

H1is a channel gain from serving cell110to UE120,

H2is a channel gain from neighbor cell112to UE120,

X(k) is the received signal at UE120, and

N is additive noise at UE120.

In equation (1), k may be an index for downlink resources used to send the desired and interfering transmissions. For simplicity, equation (1) assumes a flat fading channel with a constant channel gain for all downlink resources. The channel gain may also be a function of resource index k. Also for simplicity, equation (1) assumes only one interfering transmission from one neighbor cell.

UE120may perform minimum mean square error (MMSE) demodulation on the received signal by treating the interfering transmission as interference, as follows:

S^1⁡(k)=X⁡(k)·H^1*H^12+σn⁢⁢12,Eq⁢⁢(2)
where Ĥ1is an estimate of the channel gain from serving cell110to UE120,

Ŝ1(k) is an estimate of the desired transmission,

σn12is a variance of total noise and interference for the desired transmission,which is H2·S2(k)+N, and

“*” denotes a complex conjugate.

For simplicity, equation (2) assumes one receive antenna at UE120. Multiple-input multiple-output (MIMO) detection based on MMSE may also be performed if UE120is equipped with multiple receive antennas. Since the interfering transmission is treated as interference in equation (2), UE120may obtain a lower SINR for the desired transmission.

In one design, UE120may perform soft-symbol interference cancellation to improve performance. For soft-symbol interference cancellation, UE120may first perform demodulation to obtain an estimate of the interfering transmission based on the following criterion:
Ŝ2(k)=E{S2(k)|X(k),Ĥ1,Ĥ2,σn2,Q2}  Eq (3)
where Ĥ2is an estimate of the channel gain from neighbor cell112to UE120,

Ŝ2(k) is an estimate of the interfering transmission, and

Q2denotes a modulation order of the interfering transmission assumed by UE120,

σn2is a variance of additive noise N in equation (1), and

E{U|V} denotes an expected value of U when V is observed.

As shown in equation (3), UE120may obtain an estimate Ŝ2(k) of the interfering transmission based on the received signal X(k), the channel estimates Ĥ1and Ĥ2, the noise variance σn2, and the modulation order Q2of the interfering transmission.

UE120may then estimate and subtract the interference due to the interfering transmission from the received signal, as follows:
Y(k)=X(k)−Ĥ2·Ŝ2(k)=H1·S1(k)+Nr,  Eq (4)
where Ĥ2·Ŝ2(k) is estimated interference due to the interfering transmission,

Y(k) is an interference-canceled signal, and

Nris residual noise and interference for the desired transmission.

UE120may then perform demodulation on the interference-canceled signal, as follows:

S^1⁡(k)=Y⁡(k)·H^1*H^12+σn⁢⁢r2,Eq⁢⁢(5)
where σnr2is a variance of Nr.

Since the interfering transmission is removed in the interference-canceled signal, UE120may obtain a higher SINR for the desired transmission due to soft-symbol interference cancellation. UE120may further process (e.g., decode) the demodulated symbols for the desired transmission to recover data sent by serving cell110to UE120.

In another design, UE120may perform joint demodulation to improve performance UE120may estimate the wireless channels for serving cell110and neighbor cell112. UE120may then perform joint demodulation based on maximum a posteriori probability (MAP) criterion, which may be equivalent to minimizing a distance metric in some cases. The MAP criterion may be expressed as follows:

UE120may perform joint demodulation based on maximum likelihood (ML) estimation or some other technique known in the art. UE120may perform joint demodulation with (i) channel estimates Ĥ1and Ĥ2for the serving and neighbor cells, respectively, (ii) a known modulation order of the desired transmission, and (iii) a known or assumed modulation order of the interfering transmission. UE120may obtain demodulated symbols Ŝ1(k) and Ŝ2(k) for both the desired and interfering transmissions from the joint demodulation. UE120may discard the demodulated symbols for the interfering transmission and may process (e.g., decode) the demodulated symbols for the desired transmission to recover the data sent by serving cell110to UE120.

Regardless of the interference mitigation technique selected for use, UE120may need to estimate the wireless channel from neighbor cell112to UE120in order to perform interference mitigation. UE120may perform channel estimation in different manners depending on how the reference signal (or pilot) is transmitted.

In one design, a cell may transmit a common pilot to all UEs. The common pilot may also be referred to as a cell-specific reference signal (CRS), etc. The cell may transmit the common pilot from each antenna at the cell without any precoding. The cell may transmit data to a UE with or without precoding and at a suitable transmit power level. The transmit power level for data may be specified relative to the transmit power level for pilot and may be given by a T2P value.

UE120may estimate the channel gain for each antenna of each cell of interest based on the common pilot transmitted from that cell on that antenna. UE120may then obtain a channel estimate for each cell m, as follows:
Hm=Gm1·Wm1√{square root over (Pm)}+ . . . +GmN·WmN·√{square root over (Pm)},  Eq (8)
where Gm1through GmNare channel gains for N antennas at cell m, where N≧1,

Wm1through WnNare precoding weights for the N antennas at cell m,

Pmis a power gain determined by the T2P of the interfering transmission, and

Hmis the channel gain from cell m to UE120.

As shown in equation (8), the actual channel observed by a data transmission from cell m may depend on precoding weights for the N antennas at cell m and the T2P used for the data transmission. Cell112may send the precoding weights and T2P directly or indirectly to recipient UE122. For example, cell112may send PMI identifying the precoding weights in a downlink grant and may send the T2P via upper layer signaling to UE122. Cell112may also send the precoding weights indirectly via a selected transmission mode, such as LD-CDD in LTE. UE122may be able to determine the precoding weight based on the direct or indirect signaling from cell112. UE122may then reconstruct the actual channel based on estimated channel gains for the N antennas of cell112and the transmission parameters (e.g., the PMI and/or T2P) signaled to UE122. These parameters may be made available to interfered UE120served by a different cell110, so that UE120can perform interference mitigation.

In another design, a cell may transmit a dedicated pilot to a specific UE being served. The dedicated pilot may also be referred to as a dedicated reference signal (DRS), a UE-specific reference signal (UE-RS), etc. The cell may transmit the dedicated pilot on some of the downlink resources used for data transmission and may process (e.g., precode) the dedicated pilot in similar manner as for the data transmission. The dedicated pilot would then observe the same overall channel as the data transmission. The recipient UE may obtain a channel estimate for the cell based on the dedicated pilot, without having to know the processing (e.g., precoding and power scaling) performed by the cell for the dedicated pilot and the data transmission. Similarly, an interfered UE may also obtain a channel estimate for the cell based on the dedicated pilot transmitted by the cell.

As noted above, pertinent transmission parameters for a served UE may be sent to an interfered UE to enable to the interfered UE to perform interference mitigation. These parameters may include any of the parameters described above, e.g., the modulation order, MCS, T2P, precoding information, transmission rank, downlink resources, etc. These parameters may be sent in various manners.

In one design, a given cell m may send transmission parameters for other UEs served by neighbor cells to UEs served by cell m. Cell m may send transmission parameters for its UEs to the neighbor cells (e.g., via the backhaul, as shown inFIG. 1). Cell m may also receive transmission parameters for other UEs served by the neighbor cells, e.g., via the backhaul. Cell m and the neighbor cells may exchange transmission parameters for certain UEs (e.g., UEs located near the edge of coverage) instead of all UEs. The cell-edge UEs may be identified based on the locations of the UEs served by these cells and/or pilot measurements made by the UEs. Cell m may send the transmission parameters for the other UEs to its UEs, e.g., via broadcast signaling to all UEs, or unicast signaling to specific UEs, or multicast signaling to groups of UEs. A UE served by cell m may receive transmission parameters for one or more other UEs served by the neighbor cells and may perform interference mitigation based on the transmission parameters. This design may allow each cell to reliably send transmission parameters for other UEs in neighbor cells to the UEs served by that cell using existing signaling scheme.

In another design, a given cell m may send transmission parameters for its UEs to other UEs by neighbor cells. Cell m may send these transmission parameters via broadcast, unicast, or multicast signaling at a sufficiently high transmit power level to enable other UEs in the neighbor cells to reliably receive the transmission parameters. A UE served by cell m may receive signaling comprising transmission parameters for one or more other UEs served by one or more neighbor cells. The UE may perform interference mitigation based on the transmission parameters. This design may allow each cell to send transmission parameters for its UEs to other UEs in neighbor cells without having to exchange the parameters via the backhaul.

In general, transmission parameters for a given UE z may be sent by a serving cell, a neighbor cell, and/or some other entity to one or more interfered UEs. In one design, the sent transmission parameters may be actual parameters that are actually used for UE z. This design may allow the interfered UEs to demodulate the data transmission to UE z. In another design, the sent transmission parameters may be default parameters that are likely to be used for UE z. For example, the system may support QPSK, 16-QAM and 64-QAM, and the sent transmission parameters may convey a default modulation order of QPSK. In this case, the modulation order used for UE z may likely be QPSK but may also be 16-QAM or 64-QAM. In this design, the interfered UEs may perform demodulation based on the default modulation order. The interfered UEs may obtain the maximum gain when the actual modulation order is QPSK and may obtain substantial gain even when the actual modulation order is 16-QAM or 64-QAM. A cell may send different default values of a given transmission parameter for different sets of resources (e.g., different sets of frequency subbands, different subframes, or different sets of time frequency blocks) that may be used for data transmission. The cell may use the default parameter values for each set of resources, to the extent possible, in order to improve interference mitigation performance by the interfered UEs.

In another aspect, a cell may send transmission parameters for a UE via a dedicated pilot sent to the UE. The cell may generate the dedicated pilot based on a PN sequence, or a scrambling sequence, or a constant amplitude zero auto correlation (CAZAC) sequence, or some other sequence. For clarity, the following description assumes the use of a PN sequence. The cell may encode or scramble the dedicated pilot based on the transmission parameters, e.g., by generating the PN sequence based on these parameters.

FIG. 2shows a block diagram of a design of a transmit processor200that can generate a dedicated pilot carrying transmission parameters for data transmission for a UE. Transmit processor200may be part of a base station/cell. Within transmit processor200, an encoder210may receive data for the UE being served, encode the data based on a selected coding scheme or code rate, and provide coded data. A symbol mapper212may map the coded data to modulation symbols based on a selected modulation order. The modulation symbols for data may be referred to as data symbols.

A PN generator220may receive a set of parameters for the UE being served and may generate a PN sequence based on this parameter set. The parameter set may include a cell ID of the cell transmitting the data and dedicated pilot, one or more transmission parameters for the data transmission, and/or other parameters. The transmission parameters for the data transmission may include the selected modulation order for the data transmission and/or any of the parameters described above. In one design, PN generator220may generate a seed value based on the parameter set and may initialize a linear feedback shift register (LFSR) based on the seed value. The LFSR may then generate the PN sequence based on a particular polynomial generator. A symbol mapper222may map the bits in the PN sequence to modulation symbols based on a modulation order used for the dedicated pilot, which may be different from the modulation order used for data transmission. The modulation symbols for the pilot may be referred to as pilot symbols.

A multiplexer (Mux)230may receive the data symbols from symbol mapper212and the pilot symbols from symbol mapper222. Multiplexer230may provide the data symbols to resources used for data transmission and may provide the pilot symbols to resources used for the dedicated pilot. The resources for data transmission and the resources for the dedicated pilot may be part of the resources allocated to the recipient UE for data transmission.

As shown inFIG. 2, the dedicated pilot may carry a signature of a downlink grant for the recipient UE. The recipient UE can obtain the transmission parameters from a downlink grant sent by the cell and can readily generate the PN sequence for the dedicated pilot. An interfered UE may demodulate the dedicated pilot by evaluating different hypotheses for the set of parameters sent in the dedicated pilot. For example, only the modulation order may be sent in the dedicated pilot, and only three modulation orders of QPSK, 16-QAM and 64-QAM may be supported by the system. The interfered UE may then demodulate the dedicated pilot for three hypotheses of QPSK, 16-QAM and 64-QAM and may obtain a demodulation metric for each hypothesis. The interfered UE may obtain a channel estimate as well as the modulation order of the data transmission based on the hypothesis with the best demodulation metric.

For symbol-level interference mitigation, it may be desirable for an interfered UE to be able to distinguish between modulation symbols of a desired transmission and modulation symbols of an interfering transmission. The wireless channel for a serving cell may often be different from the wireless channel for a neighbor cell. Hence, the interfered UE can typically distinguish between the modulation symbols of the desired and interfering transmissions after performing channel estimation. However, the distinction between the modulation symbols of the desired and interfering transmissions may be limited in certain cases, e.g., when the wireless channels for the serving cell and the neighbor cell are relatively close. Furthermore, uncertainty in channel knowledge (e.g., due to an unknown transmission parameter such as T2P) may lead to limited ability to distinguish between the modulation symbols of the desired and interfering transmissions, especially in the presence of noise.

In yet another aspect, scrambling may be performed at symbol level to enable an interfered UE to distinguish between modulation symbols of desired and interfering transmissions. A given cell may perform symbol-level scrambling by multiplying modulation symbols (or data symbols) of a data transmission to a served UE with a scrambling sequence of modulation symbols (or scrambling symbols). The scrambling sequence may be specific for the cell (e.g., generated based on a cell ID) and/or may be specific for the served UE (e.g., generated based on a UE ID). For other cell interference mitigation, it may be better to use cell-specific scrambling, so that the scrambling sequence may be known to interfered UEs in neighbor cells. In any case, the scrambling symbols can map the data symbols for the served UE to scrambled symbols that can be distinguished from the data symbols for an interfered UE. In one design, the scrambling symbols may be generated based on a modulation order/scheme that is different from the modulation orders available for data transmission. In another design, the scrambling symbols may be generated based on a modulation order that can map the data symbols for the served UE so that the scrambled symbols of the interfering transmission do not appear as valid scrambled symbols from the serving cell.

In one design, the scrambling symbols may be generated based on 8-PSK. This design may result in the scrambled symbols being rotated in quadrature as well as along diagonal axes. The scrambled symbols transmitted by the serving cell (e.g., with the data symbols being generated based on QPSK, 16-QAM, or 64-QAM) may then be readily distinguished from the scrambled symbols transmitted by the interfering cell. In contrast, if the scrambling symbols are generated based on QPSK and the data symbols are generated based on QPSK, 16-QAM, or 64-QAM, then the scrambled symbols from the serving and interfering transmissions may resemble each other. In general, the scrambled symbols should be defined by a signal constellation that does not resemble any signal constellation for data symbols.

Symbol-level scrambling for data transmission may improve robustness of interference mitigation. Symbol-level scrambling is different from scrambling on information bits provided to an encoder or code bits generated by the encoder. Symbol-level scrambling may allow the data symbols of the desired and interfering transmissions to be distinguished even if these transmissions undergo the same wireless channel.

FIG. 3shows a block diagram of a design of a transmit processor300that can perform symbol-level scrambling. Within transmit processor300, an encoder310may receive data for a UE being served, encode the data based on a selected coding scheme or code rate, and provide coded data. A symbol mapper312may map the coded data to modulation symbols (or data symbols) based on a selected modulation order. A PN generator320may receive one or more scrambling parameters and may generate a PN sequence based on the scrambling parameter(s). The scrambling parameter(s) may include a cell ID for cell-specific scrambling, or a UE ID of the served UE for UE-specific scrambling, or some other parameter, or a combination thereof. A symbol mapper322may map the bits in the PN sequence to modulation symbols (or scrambling symbols) based on a modulation order (e.g., 8-PSK) used for the scrambling sequence. A multiplier330may receive the data symbols from symbol mapper312and the scrambling symbols from symbol mapper322. Multiplier330may multiply each data symbol with a corresponding scrambling symbol to generate a corresponding scrambled symbol.

The received signal at UE120, when symbol-level scrambling is performed by each cell, may be expressed as:
X(k)=H1·S1(k)·Q1(k)+H2·S2(k)·Q2(k)+N,Eq (9)
where Q1(k) is a scrambling sequence for the desired transmission, and

Q2(k) is a scrambling sequence for the interfering transmission.

UE120may descramble the received signal based on the scrambling sequence for the desired transmission to obtain a descrambled signal, which may be expressed as:
Z(k)=X(k)·Q1*(k)=H1·S1(k)+H2·S2(k)·Q2(k)·Q1*(k)+N·Q1*(k),  Eq (10)
where Z(k) is a descrambled signal for serving cell110.

As shown in equation (10), the descrambled signal includes a desired transmission corresponding to H1·S1(k) as well as a scrambled interfering transmission corresponding to H2·S2(k)·Q2(k)·Q1*(k). The desired transmission may comprise data symbols for a selected modulation order. The scrambled interfering transmission may comprise scrambled symbols, which may be rotated by a scrambling sequence Q2(k)·Q1*(k) and may thus be distinguishable from the data symbols.

FIG. 4shows a block diagram of a design of a receive processor400that can perform symbol-level descrambling. Receive processor400may be part of a UE. Within receive processor400, a received signal comprising received symbols may be provided to a multiplier412. A unit414may receive a scrambling sequence for a desired transmission and may provide a conjugated scrambling sequence as a descrambling sequence. Multiplier412may multiply each received symbol with a corresponding symbol in the descrambling sequence and provide a corresponding descrambled symbol. A demodulator420may perform demodulation with symbol-level interference mitigation based on channel estimates for the serving cell and one or more interfering cells as well as one or more transmission parameters such as modulation order, T2P, etc. Demodulator420may perform demodulation with soft-symbol interference cancellation as shown in equations (3) to (5) or may perform joint demodulation based on the criterion shown in equation (6). In either case, demodulator420may provide demodulated symbols for the serving cell. A decoder430may receive and decode the demodulated symbols and provide decoded data for UE120.

UE120may perform demodulation and decoding for a desired transmission from serving cell110in various manners. In a first design, UE120may first perform demodulation and decoding without interference mitigation to recover data sent by serving cell110. If decoding is unsuccessful, then UE120may next perform demodulation and decoding with interference mitigation to recover the data sent by serving cell110. In a second design, UE120may perform demodulation and decoding with interference mitigation (and may not attempt to perform demodulation and decoding without interference mitigation) to recover the data sent by serving cell110. In a third design, UE120may perform demodulation and decoding using either the first design or the second design based on one or more factors such as the channel quality for serving cell110. For example, UE120may perform demodulation and decoding using the first design if the channel quality is sufficiently good and using the second design otherwise.

In another design, UE120may perform demodulation for the desired transmission by treating an interfering transmission as interference, e.g., as shown in equation (2), and may obtain a metric for this demodulation. UE120may also perform demodulation for the desired transmission with interference mitigation based on assumed transmission parameters for the interfering transmission and may obtain a metric for this demodulation. UE120may then select the demodulation output with the better metric.

UE120may perform demodulation and decoding with interference mitigation in various manners. In one design, UE120may estimate and cancel the interference from all interfering transmissions to obtain an interference-canceled signal and may then decode the interference-canceled signal to recover data sent by serving cell110to UE120. In another design, UE120may estimate and cancel the interference from one interfering transmission at a time (e.g., starting with the strongest interfering transmission) and may decode the interference-canceled signal, after canceling the interference from each interfering transmission, to recover the data sent by serving cell110to UE120. In yet another design, UE120may estimate and cancel the interference from one set of interfering transmissions at a time and may decode the interference-canceled signal after canceling the interference from each set of interfering transmissions. UE120may also perform demodulation and decoding with interference mitigation in other manners.

UE120may perform interference mitigation in various manners. In one design, UE120may perform interference mitigation for each interfering transmission based on known or assumed transmission parameters for that interfering transmission. In another design, UE120may perform blind demodulation for an interfering transmission based on different hypotheses for transmission parameters used for the interfering transmission. This design may be used if the transmission parameters (i) are not sent by any cell or (ii) are sent but not received by the interfered UE for any reason. The interfered UE may perform demodulation for the interfering transmission based on each hypothesis and may obtain a metric for the hypothesis. For example, the interfered UE may perform demodulation based on a hypothesis of QPSK, and another hypothesis of 16-QAM, and yet another hypothesis of 64-QAM. The interfered UE may then select the hypothesis with the best metric and may decode the demodulated symbols associated with the selected hypothesis.

For clarity, various techniques for interference mitigation have been described specifically for data transmission on the downlink. Some or all of these techniques may also be used for interference mitigation for data transmission on the uplink. A cell may perform interference mitigation as described above to mitigate interference due to interfering transmissions from UEs served by neighbor cells.

The techniques described herein may be used for interference mitigation for interfering transmissions from neighbor cells, which may be referred to as inter-cell interference mitigation. The techniques may also be used for intra-cell interference mitigation for multi-user MIMO (MU-MIMO). For MU-MIMO, multiple UEs may be scheduled for data transmission on the same time-frequency resources with spatial separation (e.g., via beamforming) at a cell. MU-MIMO is typically designed to be transparent to each UE being scheduled. Hence, a given UE may not be aware of other UE(s) being scheduled concurrently on the same time-frequency resources. The UE may have limited information about the other data transmission(s) sent by the cell to the other UE(s). For example, the UE may not know the presence of the other UE(s), the transmission rank, MCS, resources, etc. Interference mitigation for MU-MIMO (or intra-cell interference mitigation) may be performed in similar manner as inter-cell interference mitigation. The main differences between intra-cell interference mitigation and inter-cell interference mitigation may be as follows:Inter-cell interference mitigation may require communication between cells to convey transmission parameters whereas intra-cell interference mitigation may be localized to a single cell and may not require inter-cell communication; andAny cell-specific aspect in the inter-cell context may be localized to a single cell in the intra-cell context. For example, cell ID in cell-specific scrambling (e.g., for UE-RS scrambling and/or data scrambling to differentiate between the serving and interfering cells) may be replaced with multiple cell IDs for the same cell and assigned (e.g., semi-statically or dynamically) to different UEs being scheduled concurrently for MU-MIMO.

A UE may perform interference mitigation for MU-MIMO in similar manner as an interfered UE performing interference mitigation for an interfering transmission from a neighbor cell. The UE may receive information for at least one transmission parameter for one or more other UEs scheduled concurrently with the UE for a MU-MIMO transmission. The UE may perform packet-level or symbol-level interference mitigation for each co-scheduled UE, as described above. The UE may also perform soft-symbol interference cancellation or joint demodulation for each co-scheduled UE, as also described above.

FIG. 5shows a design of a process500for performing interference mitigation. Process500may be performed by a first UE for data transmission on the downlink (as described below), or by a base station for data transmission on the uplink, or by some other entity. The first UE may obtain a received signal comprising a first data transmission sent by a first cell to the first UE and a second data transmission sent by a second cell to a second UE (block512). The first UE may also obtain information for at least one transmission parameter for the second data transmission (block514). The first UE may perform interference mitigation for the second data transmission, based on the information for the at least one transmission parameter, to recover the first data transmission sent to the first UE (block516).

In one design of block516, the first UE may perform interference mitigation for the second data transmission at the packet level by (i) decoding the second data transmission to recover at least one packet sent by the second cell to the second UE, (ii) estimating interference due to the second data transmission based on the at least one packet, and (iii) canceling the estimated interference. In another design, the first UE may perform interference mitigation for the second data transmission at the symbol level, without decoding the second data transmission to recover any packet sent by the second cell to the second UE.

In one design of symbol-level interference mitigation, the first UE may perform soft-symbol interference cancellation. The first UE may first demodulate the received signal based on the information for the at least one transmission parameter to obtain demodulated symbols for the second data transmission. The first UE may then estimate interference due to the second data transmission based on the demodulated symbols for the second data transmission. The first UE may subtract the estimated interference from the received signal to obtain an interference-canceled signal. The first UE may then demodulate the interference-canceled signal to obtain demodulated symbols for the first data transmission.

In another design of symbol-level interference mitigation, the first UE may perform joint demodulation on the received signal based on the information for the at least one transmission parameter to obtain demodulated symbols for both the first and second data transmissions. The first UE may discard the demodulated symbols for the second data transmission and may process (e.g., decode) the demodulated symbols for the first data transmission.

In yet another design of symbol-level interference mitigation, the first UE may perform iterative demodulation and decoding. The first UE may perform demodulation with interference mitigation and decoding for a plurality of iterations. The output of demodulation in each iteration may be used for decoding in the same iteration. The output of decoding in each iteration, except for the final iteration, may be used for demodulation in the next iteration.

In one design, the first UE may perform demodulation with interference mitigation for a plurality of hypotheses. Each hypothesis may correspond to a different set of one or more values for one or more transmission parameters. The first UE may select the demodulation output for the hypothesis with the best metric. In another design, the first UE may perform demodulation with interference mitigation for a single hypothesis, e.g., based on known or assumed value for the at least one transmission parameter.

In one design, the first UE may obtain a channel estimate for a wireless channel from the second cell to the first UE based on information for one or more transmission parameters, e.g., T2P, precoding information, transmission rank, etc. This design may be used if the second cell transmits a common pilot (or a cell-specific reference signal) for all UEs served by the second cell. This design may also be used if the second cell transmits a dedicated pilot (or a UE-specific reference signal) to the second UE. In any case, the first UE may perform demodulation with interference mitigation based on the channel estimate to obtain demodulated symbols for the first data transmission.

In one design, the first UE may obtain the information for the at least one transmission parameter from signaling transmitted by the first cell or the second cell. In another design, the first UE may obtain the information for the at least one transmission parameter from a pilot (e.g., a dedicated pilot) transmitted by the second cell to the second UE. The first UE may also obtain the information for the at least one transmission parameter in other manners. The information for the at least one transmission parameter may comprise a modulation order, or an MCS, or a T2P, or precoding information (e.g., PMI, transmission mode indicator, etc.), or a transmission rank, or assigned resources for the second data transmission, or some other information, or a combination thereof. In one design, the first UE may obtain actual value of the at least one transmission parameter used by the second cell for the second data transmission. In another design, the first UE may obtain default value of the at least one transmission parameter likely to be used by the second cell for the second data transmission.

FIG. 6shows a design of an apparatus600for performing interference mitigation. Apparatus600includes a module612to obtain at a first UE a received signal comprising a first data transmission sent by a first cell to the first UE and a second data transmission sent by a second cell to a second UE, a module614to obtain information for at least one transmission parameter for the second data transmission, and a module616to perform interference mitigation for the second data transmission, based on the information for the at least one transmission parameter, to recover the first data transmission sent to the first UE.

FIG. 7shows a design of a process700for sending transmission parameters to support interference mitigation. Process700may be performed by a cell (as described below) or by some other entity. Information for at least one transmission parameter for a data transmission sent by a first cell to a UE may be obtained (block712). The information for the at least one transmission parameter may be transmitted to at least one UE served by a second cell to enable the at least one UE to perform interference mitigation for the data transmission sent by the first cell to the UE (block714).

In one design, blocks712and714may be performed by the first cell. In another design, blocks712and714may be performed by the second cell, which may receive the information for the at least one transmission parameter from the first cell via the backhaul. In any case, the information for the at least one transmission parameter may comprise a modulation order, or an MCS, or a T2P, or precoding information, or a transmission rank, or assigned resources for the data transmission, or some other information, or a combination thereof.

In one design, information for transmission parameters for UEs or cells that are potential interferers may be sent to other UEs for interference mitigation. The UE or the first cell may be identified as potential interferer to the at least one UE based on pilot measurement from the UE, or the location of the UE, or pilot measurements from the at least one UE, or the location of the at least one UE, or some other information, or a combination thereof.

FIG. 8shows a design of an apparatus800for sending transmission parameters to support interference mitigation. Apparatus800includes a module812to obtain information for at least one transmission parameter for a data transmission sent by a first cell to a UE, and a module814to transmit the information for the at least one transmission parameter to at least one UE served by a second cell to enable the at least one UE to perform interference mitigation for the data transmission sent by the first cell to the UE.

FIG. 9shows a design of a process900for sending transmission parameters via a pilot. Process900may be performed by a cell (as described below) or by some other entity. The cell may generate a data transmission based on at least one transmission parameter (block912). The cell may also generate a pilot (e.g., a dedicated pilot) comprising information for the at least one transmission parameter, which may include any of the information listed above (block914). In one design, the cell may generate a PN sequence based on the information for the at least one transmission parameter and may then generate modulation symbols for the pilot based on the PN sequence. The cell may transmit the pilot and the data transmission to a recipient UE (block916). Other UEs may use the information for the at least one transmission parameter in the pilot to perform interference mitigation for the data transmission.

FIG. 10shows a design of an apparatus1000for sending transmission parameters via a pilot. Apparatus1000includes a module1012to generate a data transmission based on at least one transmission parameter, a module1014to generate a pilot comprising information for the at least one transmission parameter, and a module1016to transmit the pilot and the data transmission.

FIG. 11shows a design of a process1100for performing symbol level scrambling. Process1100may be performed by a cell (as described below) or by some other entity. The cell may generate modulation symbols for a data transmission (block1112). The cell may scramble the modulation symbols based on a scrambling sequence to obtain scrambled symbols (block1114). The cell may transmit the scrambled symbols for the data transmission (block1116).

In one design, the cell may generate the scrambling sequence based on a cell ID, or a UE ID, or both. In one design, the cell may generate the scrambling sequence based on (i) a modulation order that is not used for the data transmission, or (ii) a modulation order that does not map the modulation symbols for the data transmission to other valid modulation symbols, or (iii) 8-PSK or some other suitable modulation order.

FIG. 12shows a design of an apparatus1200for performing symbol level scrambling. Apparatus1200includes a module1212to generate modulation symbols for a data transmission, a module1214to scramble the modulation symbols based on a scrambling sequence to obtain scrambled symbols, and a module1216to transmit the scrambled symbols for the data transmission.

FIG. 13shows a design of a process1300for performing symbol level descrambling. Process1300may be performed by a UE (as described below) or by some other entity. The UE may descramble received symbols based on a scrambling sequence to obtain descrambled symbols (block1312). In one design, the UE may generate the scrambling sequence based on a cell ID, or a UE ID, or both. The UE may demodulate the descrambled symbols to obtain demodulated symbols (block1314).

In one design, the UE may obtain the received symbols comprising a first data transmission sent by a first cell to the UE and a second data transmission sent by a second cell to a second UE. The UE may also obtain information for at least one transmission parameter for the second data transmission. The UE may demodulate the descrambled symbols with interference mitigation based on the information for at least one transmission parameter to recover the first data transmission sent to the UE.

FIG. 14shows a design of an apparatus1400for performing symbol level descrambling. Apparatus1400includes a module1412to descramble received symbols based on a scrambling sequence to obtain descrambled symbols, and a module1414to demodulate the descrambled symbols (e.g., with interference mitigation) to obtain demodulated symbols.

FIG. 15shows a block diagram of a design of base station110and UE120inFIG. 1. Base station110may be equipped with T antennas1534athrough1534t, and UE120may be equipped with R antennas1552athrough1552r, where in general T≧1 and R≧1.

At base station110, a transmit processor1520may receive data from a data source1512for one or more UEs, process (e.g., encode and modulate) the data for each UE based on one or more MCSs selected for that UE, and provide data symbols for all UEs. Transmit processor1520may also receive control information (e.g., grants, transmission parameters for UEs communicating with neighbor cells, etc.) from a controller/processor1540. Processor1520may process the control information and provide control symbols. Processor1520may also generate pilot/reference symbols for one or more pilots/reference signals, e.g., dedicated pilot, common pilot, etc. A transmit (TX) MIMO processor1530may perform spatial processing (e.g., precoding) on the data symbols, the control symbols, and/or the pilot symbols, if applicable, and may provide T output symbol streams to T transmitters (TMTRs)1532athrough1532t. Each transmitter1532may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each transmitter1532may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. T downlink signals from transmitters1532athrough1532tmay be transmitted via T antennas1534athrough1534t, respectively.

At UE120, antennas1552athrough1552rmay receive the downlink signals from base station110and may provide received signals to receivers (RCVRs)1554athrough1554r, respectively. Each receiver1554may condition (e.g., filter, amplify, downconvert, and digitize) its received signal to obtain input samples. Each receiver1554may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector1556may obtain received symbols from all R receivers1554athrough1554r, perform MIMO detection on the received symbols if applicable, and provide detected symbols. A receive processor1558may process (e.g., demodulate with interference mitigation and decode) the detected symbols, provide decoded data for UE120to a data sink1560, and provide decoded control information to a controller/processor1580.

On the uplink, at UE120, a transmit processor1564may receive and process data from a data source1562and control information from controller/processor1580. Processor1564may also generate pilot/reference symbols for pilot/reference signal. The symbols from transmit processor1564may be precoded by a TX MIMO processor1566if applicable, further processed by transmitters1554athrough1554r(e.g., for SC-FDM, OFDM, etc.), and transmitted to base station110. At base station110, the uplink signals from UE120may be received by antennas1534, processed by receivers1532, detected by a MIMO detector1536if applicable, and further processed by a receive processor1538to obtain decoded data and control information sent by UE120. Processor1538may provide the decoded data to a data sink1539and the decoded control information to controller/processor1540.

Controllers/processors1540and1580may direct the operation at base station110and UE120, respectively. Processor1540and/or other processors and modules at base station110may perform or direct process700inFIG. 7, process900inFIG. 9, process1100inFIG. 11, and/or other processes for the techniques described herein. Processor1580and/or other processors and modules at UE120may perform or direct process500inFIG. 5, process1300inFIG. 13, and/or other processes for the techniques described herein. Memories1542and1582may store data and program codes for base station110and UE120, respectively. A scheduler1544may schedule UEs for data transmission on the downlink and/or uplink.