WO2009024944A2

WO2009024944A2 - System and method employing a dedicated reference signal sequence for precoding weight identification

Info

Publication number: WO2009024944A2
Application number: PCT/IB2008/053356
Authority: WO
Inventors: Klaus Hugl; Karol Schober
Original assignee: Nokia Corporation; Nokia Inc.
Priority date: 2007-08-21
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: WO2009024944A3

Abstract

A communication system including a transmitter and a receiver that employs a dedicated reference signal sequence for precoding weight identification to decode user data. In one embodiment, the transmitter (145) includes a precoding weight selector (152) configured to select a precoding weight, and a dedicated reference signal sequence selector (156) configured to select a reference signal sequence based on the precoding weight and provide a dedicated reference signal sequence therefrom. The transmitter (145) also includes an encoder (150) configured to encode user symbols with the precoding weight to produce user data. In one embodiment, the receiver (175) includes a precoding weight detector (193) configured to identify the precoding weight applied to the dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences, and a data detector (194) configured to decode the user data using the precoding weight.

Description

SYSTEM AND METHOD EMPLOYING A DEDICATED REFERENCE SIGNAL SEQUENCE FOR PRECODING WEIGHT IDENTIFICATION

This application claims the benefit of U.S. Provisional Application No. 60/957,025 entitled "System and Method Employing a Dedicated Reference Signal Sequence for Precoding Weight Identification," filed on August 21, 2007, which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates, in general, to digital communication systems and, more particularly, to a system and method that employs a dedicated reference signal sequence for precoding weight identification to decode user data.

BACKGROUND

Broadcast and multicast communications are forms of point-to-multipoint communications wherein information is simultaneously transmitted from a single source to multiple destinations. The third generation partnership project ("3GPP") long term evolution ("LTE") work item describes an ongoing effort across the industry to improve the universal mobile telecommunications system ("UMTS") for mobile communications to cope with continuing new requirements and the growing base of users. The goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards. The 3GPP LTE work item should result in new recommendations for standards for the UMTS.

Generally, a cellular communication system includes a network infrastructure that includes a plurality of base stations that may be positioned at spaced-apart locations throughout a geographic area. Each of the base stations defines an area, referred to as a cell, from which the cellular communication system derives its name. The network infrastructure of which the base stations form portions may be coupled to a core network such as a packet data backbone or a public-switched telephone network. Communication devices such as computer servers, telephone stations, etc., are, in turn, coupled to the core network and are capable of communication by way of the network infrastructure and the core network. Portable transceivers, referred to as mobile stations, communicate with the base stations by way of radio links forming portions of an electromagnetic spectrum. The use of the cellular communication system is permitted, typically, pursuant to a service subscription and users (referred to as "subscribers") that communicate by way of the cellular communication system through utilization of the mobile stations.

Information communicated over a radio link may be susceptible to distortion such as dispersion as a result of non-ideal communication conditions. The distortion causes the information delivered to a receiving station (e.g., a base station or a mobile station) to differ from the corresponding information transmitted by a sending station (e.g., a base station or a mobile station). If the distortion is significant, the informational content will not be accurately recovered at the receiving station. For instance, fading caused by multi-path transmission distorts information communicated over a communication channel. If the communication channel exhibits significant levels of fading, the informational content may not be recoverable.

Various techniques such as spatial diversity may be employed to compensate for, or otherwise overcome, the distortion introduced upon the information transmitted over a communication channel to the receiving station. Spatial diversity is created through the use, at a sending station, of more than one transmit antenna from which information is transmitted, thereby creating spatial redundancy therefrom. The antennas are typically separated by distances sufficient to ensure that the information communicated by respective antennas fades in an uncorrelated manner. Additionally, the receiving stations sometimes use more than one receive antenna, preferably separated by appropriate distances. Communication systems that utilize both multiple transmitting antennas and multiple receiving antennas are often referred to as being multiple-input, multiple-output ("MIMO") systems. Communications in a MIMO system provide the possibility that higher overall capacity of the system, relative to conventional systems, can be achieved. As a result, an increased number of users may be serviced or more data throughput may be provided with improved reliability for each user. The advantages provided through the use of spatial diversity are further enhanced if the sending station is provided with information about the state or performance of the communication channel between the sending and receiving stations.

A sending station generally may not be able to measure channel characteristics of the communication channel directly, such as a channel (impulse) response or a channel correlation matrix representing a product of channel impulse response components for the multiple transmitting antennas. Thus, the receiving station typically measures the channel characteristics of the communication channel. In two-way communication systems, measurements made at the receiving station may be returned to the sending station to provide the channel characteristics to the sending station. Communication systems that provide this type of information to multiple- antenna sending stations are referred to as closed-loop transmit diversity systems. For an example of a transmit antenna diversity in mobile communication systems, see European Patent Application No. EP 1 074 098 Bl, International Publication No. WO 99/056407, entitled "Transmission Antenna Diversity," to Ari Hottinen, which is incorporated herein by reference. Communication channels extending from the network infrastructure of a cellular communication system to a mobile station are sometimes referred to as being downlink, or forward-link, channels. Conversely, the channels extending from the mobile station back to the network infrastructure are sometimes referred to as being uplink, or reverse-link, channels. The feedback information returned to the sending station (e.g., the network infrastructure such as a base station) from the receiving station (e.g., a mobile station) may be used to select values of antenna weightings (referred to as precoding weights) or alternatively, the mobile station feeds back the suggested precoding weight directly. The weightings are values including phase delays and amplitude factors by which information signals provided to individual antennas are weighted prior to their communication over a communication channel to the mobile station. A goal is to weight the information signals in amplitude and phase applied to the antennas in a manner that best facilitates communication of the information to the receiving station. Several closed-loop transmit diversity procedures may be utilized.

In order to signal the precoding weights used in downlink closed-loop beamforming and closed-loop MIMO applications from the sending station to the receiving station, such as in 3GPP LTE, a precoding weight vector (or matrix) may be signaled either by control signaling over a control channel (e.g., L1/L2 control channel) or dedicated reference signal sequences may be transmitted. A mobile station receiving the precoding weight vector (matrix) may identify the applied precoding weight vector (matrix), which is part of a precode codebook, by comparing the received dedicated reference signal sequence with the combination of known common precoding weights and reference signal sequences.

The use of control signaling over a control channel, such as a dedicated physical control channel ("DPCCH"), may yield good results for situations wherein there are relatively few precoding weight vectors that need to be signaled to the mobile station. However, in cases where a different precoding weight vector (matrix) may be allocated to the mobile station for each transmission band assigned to the mobile station, such as in the case of orthogonal frequency division multiplexing ("OFDM") systems like 3GPP LTE, the use of control signaling over a control channel may significantly increase the size of the control channel, which may negatively impact the performance of the communication system due to high control signaling overhead.

Accordingly, what is needed in the art is a system and method that effectively employs precoding weights with a transmission of user data and a dedicated reference signal sequence for use with sending and receiving stations of a communication system.

SUMMARY

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by embodiments of the present invention, which include a communication system including a transmitter and a receiver that employs a dedicated reference signal sequence for precoding weight identification to decode user data. In one embodiment, the transmitter includes a precoding weight selector configured to select a precoding weight, and a dedicated reference signal sequence selector configured to select a reference signal sequence based on the precoding weight and provide a dedicated reference signal sequence therefrom. The transmitter also includes an encoder configured to encode user symbols with the precoding weight to produce user data, and a multiplexer configured combine the user data with the dedicated reference signal sequence and a common reference signal sequence. The transmitter still further includes a modulator configured to apply a modulation carrier to modulate the user data, and the dedicated and common reference signal sequences, and a radio frequency circuit configured to convert the modulated user data, and the dedicated and common reference signal sequences into radio frequency signals for transmission.

In one embodiment, a receiver of the communication system includes a radio frequency circuit, coupled to a corresponding antenna, configured to convert the radio frequency signals detected by the antenna into a modulated receive signal including multiplexed user data, and dedicated and common reference signal sequences. The receiver also includes a demodulator configured to remove a modulation carrier signal applied to the modulated receive signal to the multiplexed user data, and dedicated and common reference signal sequences, and a demultiplexer configured to separate the user data from the dedicated and common reference signal sequences. The receiver still further includes a precoding weight detector configured to identify the precoding weight applied to the dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences, and a data detector configured to decode the user data using the precoding weight.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: FIGURE 1 illustrates a system level diagram of an embodiment of a communication system constructed according to the principles of the present invention;

FIGURE 2 illustrates a block diagram of an embodiment of a precoding weight detector and related subsystems constructed according to the principles of the present invention;

FIGUREs 3, 4, and 5 illustrate block diagrams of embodiments of portions of a transmitter and a receiver constructed according to the principles of the present invention;

FIGURE 6 illustrates a flow diagram of an embodiment of a sequence of events in a transmission of user data, and dedicated and common reference signal sequences that may be used to identify and verify a precoding weight used in a communication system according to the principles of the present invention; FIGURE 7 illustrates a flow diagram of an embodiment of a sequence of events in an identification of a precoding weight at a receiving station according to the principles of the present invention; and

FIGUREs 8 and 9 illustrate exemplary data plots demonstrating a difference in probability of identifying/verifying a selected precoding weight according to the principles of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently advantageous embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. The present invention will be described with respect to exemplary embodiments in a specific context, namely a 3GPP LTE communication system. The invention may also be applied, however, to other types of communication systems that utilize a weighting technique (beamshaping/beamforming, precoding) to identify different selected antennas and beams and need to communicate the precode weightings applied to a particular transmission.

Before addressing exemplary embodiments of the present invention, dedicated reference signal sequences may, for example, be used in wideband code-division multiple access ("WCDMA"), in order to verify the use of a precoding weight for closed loop transmit diversity as described in Annex A of 3GPP TS 25.214, entitled "3rd Generation Partnership Project: Technical Specification Group Radio Access Network: Physical Layer Procedures (FDD): (Release 6)," V 6.11.0, December 2006, which is incorporated herein by reference. In order to identify and verify the precoding weight used, it may be possible to compare a received signal from dedicated antenna specific reference signal sequences (i.e., dedicated reference signal sequences) with a received signal replicas computed at the receiver.

A transmitted dedicated reference signal sequence may typically be expressed as:

^ D - RS ^{= W} i ^{' S} D - RS ^■> where w, is the precoding weight and s_D__RS is a dedicated reference signal sequence. In a code division multiple access ("CDMA") system, the reference signal sequence may be presented in a time domain, however, in an OFDM system the reference signal sequence may be presented in a time domain and/or a frequency domain. Normally, a single reference signal sequence s_Z3__AS may be applied as in the case of WCDMA described in 3GPP TS 25.214.

A received dedicated reference signal sequence may typically be expressed as:

* D-RS ⁼ ** ^' X-D-RS ⁼ ** ^{' W}i ^{' S} D-RS ' where H is the propagation channel between a sending station and a receiving station. The received dedicated reference signal sequence may then be used at the receiving station that may also make use of known reference signal sequences s_Z3__AS and an estimate H of the propagation channel H to identify the precoding weight. Having introduced some foundational information, exemplary systems of the present invention will hereinafter be described.

Referring initially to FIGURE 1 , illustrated is a system level diagram of an embodiment of a communication system constructed according to the principles of the present invention. The communication system provides radio communications between communicating stations via communication channels. The communication system includes a sending station (e.g., a base station) 100 and a receiving station (e.g. a mobile station) 170. The communication channels may be defined by radio links, such as forward- link channels 105 and reverse-link channels 110. Information sent to the mobile station 170 may be communicated by the base station 100 over the forward- link channels 105 and information originating at the mobile station 170 may be communicated over reverse-link channels 110 to the base station 100. The communication system may be a digital communication system and network or a cellular communication system constructed pursuant to any of a number of different cellular communication standards. For instance, the base station and mobile station may be operable in an orthogonal frequency division multiplexing ("OFDM") communication system [e.g., for a wireless local area network

("WLAN") OFDM communication system the base station is often referred to as an access point and the mobile station is often referred to as a terminal]. Additionally, any communication system such as a worldwide interoperability for microwave access ("WiMAX") communication system may be employed in accordance with the principles of the present invention. The base station 100 forms part of a radio access network that may also include a radio network controller ("RNC") 115 coupled to a gateway 120 and a mobile switching center ("MSC") 125. The gateway 120 may be coupled to a packet data network ("PDN") 130 such as the Internet and the mobile switching center 125 may be coupled to a public switched telephone network ("PSTN") 135. A correspondent node ("CN") 137 may be coupled to the packet data network 130 and to the PSTN 135. The correspondent node 137 may represent a data source or a data destination from which, or to which, information is routed during operation of the communication system.

The base station 100 includes a receiver 140 and a transmitter 145. A forward- link signal to be communicated by the base station 100 to the mobile station 170 may be converted into a format for communication over the forward- link channels 105 by the transmitter 145. Closed- loop feedback information may be returned by the mobile station 170 to the base station 100 by way of the reverse-link channels 110. The mobile station 170 also includes a receiver 175 and a transmitter 180. The receiver 175 operates to receive, and operate upon, the forward-link signals transmitted by the base station 100 over the forward- link channels 105, and the transmitter 180 operates to transmit reverse-link signals over the reverse-link channels 110 to the base station

100.

The base station 100 and the mobile station 170 include multiple antennas, and the base station 100 and the mobile station 170 combination forms a multiple-input, multiple-output ("MIMO") system. For purposes of clarity, the base station 100 includes M base station antennas designated 147-1 to 147-M (hereinafter referenced as base station antennas 147). Also for purposes of clarity, the mobile station 170 includes N mobile station antennas designated 185-1 to 185 -N (hereinafter referenced as mobile station antennas 185).

The base station transmitter 145 includes an encoder (e.g., a multiplier) 150 that may be used to encode (e.g., multiply) user symbols with a selected precoding weight W₁ that may be selected from a plurality of precoding weights in a precoding weight selector 152. The multiplication of the user symbols with the selected precoding weight W₁ may produce user data suitable for transmission. The number of precoding weights in the precoding weight selector 152 may depend on the number of transmit antennas used by the base station transmitter 145. For example, in a four transmit antenna MIMO communication system, denoted 4TX SU-MIMO, there may be 16 possible precoding weight vectors, w_zwithze [l,16], in a codebook (Ωi) for single stream (rank = 1) transmissions and 16 possible precoding weight matrices W^ in a codebook (Q_k) for a dual stream (rank k = 2), triple stream (rank k = 3), and quadruple stream (rank k = 4) transmissions. The codebook for single stream transmissions (Ωi) may be expressed as:

Ωi = (W₁, W₂, . . . , Wi₆) , where individual precoding weight vectors may have a size of 4x1 for the 4TX antennas with each precoding weight vector expressible as:

W₂ = [Wj,i Wj,2 W_z,3 W_J,₄]^Γ or given more generally in the case of M base station antennas as: W₁ = [w_z,i W₁^ ■ ■ ■ W_Z,_M]^T- Similarly, corresponding codebooks for transmissions of rank two through four may be expressed as:

Ω* = (W^, W*,₂, ..., W^₆), where individual precoding weight matrices (M x k) may be expressible as: Wiii = [vfi,k,ι, w₂,k,z, ..., vfk,k,ι] for the ranks £ = 2 to 4, and i ranging from [1, 16].

Precoding weights vectors (for codebooks of rank one) and precoding weight matrices (for codebooks of rank two or more) may be referred collectively as precoding weights without loss of generality. User data (a product of the user symbols and the selected precoding weight) may be combined with a dedicated reference signal sequence W₁S_D-_RS,_! and common reference signal sequence S_C-_RS,_J (designated "CRSS") in a multiplexer 154. For an example of common reference signal sequences or cell specific reference signals (e.g., including a base station identification), see 3GPP TS 36.211, entitled "3rd Generation Partnership Project; Technical Specification

Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)," V 8.3.0, May 2005, which is incorporated herein by reference. In the case of multi-stream data transmission (rank k>l), the dedicated reference signal sequence may be generated by using the first precoding vector W₁,^ of the precoding matrix W^. Therefore, the dedicated reference signal sequence transmission might be applied with a smaller transmission rank than the actual data transmission it is time and/or frequency multiplexed with. In general, the whole precoding matrix W^ or a subset of the precoding matrix may also be used in the same manner with several reference signal sequences. The multiplexer 154 may time and/or frequency multiplex the user data, the dedicated reference signal sequence w_z SD-R_S,_J ^and the common reference signal sequence S_C-R_S,_J- The dedicated reference signal sequence, WISD-_RS,I, may be a selected reference signal sequence SD-R_SJ encoded (e.g., multiplied) with the selected precoding weight w_z. In an advantageous embodiment, the reference signal sequence S_D-_RS,_I corresponds to the selected precoding weights w_z in a way that: SD-RS.I ≠ S_D.RS,_J for all i ≠j. The dedicated reference signal sequence W₁ S_D-_RSJ may be precomputed in a dedicated reference signal sequence selector 156. Alternatively, the selected reference signal sequence s_D. R_Sj may be encoded (e.g. , multiplied) with the selected precoding weight W₁ in real-time and be provided to the multiplexer 154 by the dedicated reference signal sequence selector 156. Additionally, the dedicated reference signal sequence selector 156 may include a set of possible dedicated reference signal sequences, wherein the set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence. An output of the multiplexer 154, the user data, the dedicated reference signal sequence and the common reference signal sequence, may then be provided to a modulator 158 that modulates (e.g., apply a modulation carrier to) the user data, and the dedicated and common reference signal sequences to a desired carrier frequency. The modulated user data, and the dedicated and common reference signal sequences may then be converted into radio frequency ("RF") signals by RF circuitry 160 and then transmitted over the base station antennas 147. The mobile station receiver 175 includes RF circuitry 190 that may be used to convert RF signals received by the mobile station antennas 185 into a modulated receive signal including the multiplexed user data, and the dedicated and common reference signal sequences suitable for processing by subsequent circuitry. A demodulator 191 may be used to remove a modulation carrier applied to the modulated receive signal (and/or to transfer modulated receive signal to a suitable domain) to produce the multiplexed user data, and the dedicated and common reference signal sequences. A demultiplexer 192 may be used to separate (e.g., demultiplex) the user data and common reference signal sequence from the dedicated reference signal sequence. The received and demodulated dedicated reference signal sequence may be expressed as: ^_D-_RS ⁼ H ^• X_D-_RS = H ^• w_; ^• s_Z3__{AS ;} , where H is the propagation channel. A precoding weight detector 193 may then be used to identify or determine the precoding weight W₁ used by the transmitter for the user data transmission. The precoding weight detector 193 is configured to identify a precoding weight W₁ applied to a dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences. The precoding weight detector 193 may perform correlations of the received dedicated reference signal sequence with the set of signal replicas and the channel estimate H , identifying the precoding weight by selecting the one yielding an advantageous correlation value (e.g., largest correlation value). The set of signal replicas may include precoding weights combined with corresponding reference signal sequences, which may be calculated a priori.

Turning now to FIGURE 2, illustrated is a block diagram of an embodiment of a precoding weight detector and related subsystems constructed according to the principles of the present invention. The precoding weight detector includes a correlator 210 that may be used to perform correlations between the received and demodulated dedicated reference signal sequence Y_D-_RS and the set of signal replicas in a signal replica memory 220 combined with the channel estimates H in a channel estimates memory 230 from a channel estimator 235. The channel estimate H is computed by the channel estimator 235 in accordance with, for instance, a demultiplexed common reference signal sequence. Again, the set of signal replicas may include precoding weights combined with corresponding reference signal sequences, which may be computed a priori or in real time.

The result of each correlation may be stored in a memory 240 and the precoding weight resulting in an advantageous correlation result (e.g., the largest correlation value) may be identified as the precoding weight W₁ applied by the base station 100. The signal replica memory 220 and the channel estimate memory 230 may be a part of the memory 240 or vice versa. With reference back to FIGURE 1 , the applied precoding weight W₁ may then be provided to a data detector 194, which may use the channel estimate H and the applied precoding weight W₁ to decode (e.g., recover) the received user data Y_data = H • X_data = H • w_; • s_data . Of course, the subsystems illustrated in accordance with the precoding weight detector may be embodied in a single system or distributed within a receiver of a mobile station or terminal.

Turning now to FIGUREs 3, 4, and 5, illustrated are block diagrams of embodiments of portions of a transmitter (FIGUREs 3 and 4) and a receiver (FIGURE 5) constructed according to the principles of the present invention. The transmitters may utilize multiple independent multipliers 310, multiplexers 320, a user multiplexer 325, modulators 330, and RF circuits 340, while the receiver may utilize multiple independent RF circuits 510, demodulators 520, and demultiplexers 530. The transmitter and receiver may be employed in the communication system illustrated and described with respect to FIGURE 1. It should be understood that a transmitter and a receiver, in general, may be a portion of any sending station and receiving station, respectively, of a communication system.

Turning now to FIGURE 6, illustrated is a flow diagram of an embodiment of a sequence of events in a transmission of user data, and dedicated and common reference signal sequences that may be used to identify and verify a precoding weight used in a communication system according to the principles of the present invention. The sequence of events may be used to describe events taking place in a sending station such as base station of a communication system. The transmission of user data may begin with a selection of a precoding weight w_z at a step 610. The selection of a precoding weight may be made by the base station. In the case that a receiving station such as a mobile station feeds back a suggested precoding weight vector or matrix, the base station can follow the precoding weight recommendation of the mobile station or select another possible precoding weight. In the case that the mobile station feeds back the channel characteristics of the mobile radio channel, the precoding weight selection may be made by the base station and may be based on criteria such as achieving the maximum link throughput, maximize the received signal quality at the mobile station or, as another example, also taking into account the produced interference for other mobile stations. Typically, for a single mobile station, a single precoding weight in the case of single-stream (rank = 1) transmission or a single precoding matrix in the case of multi-stream MIMO transmission (rank k > 1) may be assigned.

After a precoding weight has been selected, then a corresponding reference signal sequence S_D-_RSJ may be selected at a step 620. The index i of the selected precoding weight W₁ may thereby directly identify the reference signal sequence S_D-_RSJ to be selected through an unambiguous mapping of the index i to the reference signal sequence S_D-_RSJ which may be known by the sending station and the receiving station. The mapping of the reference signal sequence to the corresponding precoding weight may be known to the base station and the receiving station and may either, for example, be defined in the specifications of the digital communication system (e.g., in the technical standards definition documentation) or agreed upon between both the sending and receiving stations by higher layer signaling. The mapping may generally mean that the reference signal sequences S_D-_RSJ should be known by both the sending and receiving stations. Typically, there are as many reference signal sequences as there are precoding weights. In such a situation, a single separate reference signal sequence may be assigned to each precoding weight (S_D-_RSJ ≠ S_D-_RS,_J)- Iⁿ the case that just a single reference signal sequence is available, the same reference signal sequence should be assigned to all the precoding weights (SD-R_SJ = $D-R_S,_J)- The reference signal sequence length and density in the transmission may be dependent on factors such as expected bit-error rate ("BER"), permitted overhead, and so forth. As mentioned below, the multiplexing of precoded user data and dedicated reference signal sequence may be done, so that the dedicated reference signal sequences are as equally distributed over the allocated user transmission bandwidth as possible to exploit frequency selectivity provided by, for instance, an OFDM system. However, if there are fewer reference signal sequences than there are precoding weights, then a single reference signal sequence may be assigned to several precoding weights. For example, a reference signal sequence may be assigned to a number of precoding weights that are relatively distant from each other (e.g., one possible measure of distance could be a chordal distance), meaning that S_D-_RS,_I ⁼ S_D-_RSJ, whereas for precoding weights with a small distance different reference signals may be assigned such that

$D-RS,ι ≠ SD-RSJ- With the precoding weight and the reference signal sequence selected, user data to be transmitted may be encoded with the selected precoding weight w_z at a step 630. The encoding of the user data and the selected precoding weight w_z may be expressed mathematically as ^_data ^{= w} _* ' ^s _dam • The reference signal sequence SD-RS,I should also be encoded with the selected precoding weight w_z, forming a dedicated reference signal sequence at a step 640. The encoding of the corresponding selected reference signal sequence S_D-_RSJ ^may be expressed mathematically as X_D__RS = w_; ^• s_D__RS , . Although the user data and the reference signals sequence SD-R_S,I ^may be encoded with the same selected precoding weight w_z, the resulting encoded user data, the dedicated reference signal sequence and a common reference signal sequence may be transmitted separately over a single communication channel, such as forward-link channels, to a mobile station at a step 650. Therefore, the user data itself may not be encoded with the selected reference signal sequence SD-R_S,I-

Turning now to FIGURE 7, illustrated is a flow diagram of an embodiment of a sequence of events in an identification of a precoding weight at a receiving station according to the principles of the present invention. The sequence of events may begin with the reception of encoded user data X_data = Yf₁ ^■ s_data and the dedicated reference signal sequence

X-_D-_RS ^{= w}i ' ^S _D-_RS i transmitted to the receiving station at a step 710. The receiving station may then estimate a channel response H from common antenna specific reference signal sequences

(i.e., common reference signal sequences) at a step 720. Using the channel response H and sets of known precoding weights and reference signal sequences, the receiving station may identify the selected precoding weight Yf₁ at a step 730. The receiving station may perform correlations of the received dedicated reference signal sequence Y_D__RS = H • X_D__RS = H • w, • s_D__{RS l} with a set of signal replicas in combination with the channel estimate H . Alternatively, the correlation may be performed with a product of each known precoding weight from the set of precoding weights and the corresponding reference signal sequences. The applied precoding weight Yf₁ may be identified for example as a result from an advantageous correlation value (e.g. , the largest correlation value).

After identifying the applied precoding weight Yf₁, the receiving station may demodulate and decode user data using the applied precoding weight Yf₁ and the channel response H at a step 740. The demodulating and the decoding of the user data Y_data = H • X_data = H w, • s_data may involve the receiving station's use of an equivalent channel H_egM = H • w_; .

Turning now to FIGURE 8, illustrated is an exemplary data plot demonstrating a difference in probability of identifying/verifying a selected precoding weight, assuming a reference symbol length often (10) and a four transmit antenna ("4TX") 3GPP LTE communication system according to the principles of the present invention. A first trace 810 displays signal to interference plus noise ("SINR") in decibels ("dB") versus error rate for a 4TX 3GPP LTE communication system using a single reference signal sequence and a second trace 820 displays SINR versus error rate for a 4TX 3GPP LTE communication system using 16 reference signal sequences. The use of more than one reference signal sequence significantly improves the identification/verification process. For example, at about one percent error probability, performance improvement is more than four decibels. A single reference signal sequence being independent of precoding weight is normally used in, for instance, WCDMA described in Chapter 7 of 3GPP TS 25.214. Using for each precoding weight a different reference signal sequence as illustratively demonstrated with respect to the second trace 820, the identification/verification probability can be dramatically improved compared to the state-of-the- art precoding weight identification based on a single reference signal sequence. Turning now to FIGURE 9, illustrated is an exemplary data plot demonstrating a difference in probability of identifying/verifying a selected precoding weight, a reference symbol length often (10) and a 4TX 3GPP LTE communication system with a different number of reference signal sequences according to the principles of the present invention. A first trace 910 displays SINR versus error rate for a 4TX 3GPP LTE communication system with a single reference signal sequence, a second trace 920 displays SINR versus error rate for a 4TX 3GPP LTE communication system with four reference signal sequences, and a third trace 930 displays SINR versus error rate for a 4TX 3GPP LTE communication system with 16 reference signal sequences. The traces show that by using more than one reference signal sequence performance can be significantly increased and even better performance may be achieved by having a separate reference signal sequence for each precoding weight. Should there be an inadequate number of available or defined reference signal sequences in the communication system to unambiguously assign a separate reference signal sequence for each precoding weight, it remains advantageous to use as many reference signal sequences as possible as illustrated demonstrated in the second and third traces 920, 930. The different reference signal sequences are preferably assigned to closest/neighboring precoding weights (e.g,. in the sense of chordal distance).

Thus, a communication system and method for exchanging information between a transmitter and a receiver thereof has been introduced. In one embodiment, the method includes selecting a precoding weight and a reference signal sequence based on the precoding weight. The method also includes encoding user data with the precoding weight and the reference signal sequence with the precoding weight to produce a dedicated reference signal sequence. The method also includes combining (e.g., multiplexing) the user data with the dedicated reference signal sequence and a common reference signal sequence, modulating and transmitting the user data, and the dedicated and common reference signal sequences. The method also includes estimating a channel response (e.g., channel estimates) with the common reference signal sequence and identifying the precoding weight from the transmitted dedicated reference signal sequence. The precoding weight may be identified by using a set of signal replicas and the channel response, wherein the set of signal replicas may include precoding weights combined with corresponding reference signal sequences. The method further includes decoding the user data using the identified precoding weight. In an exemplary embodiment, encoding the user data includes multiplying the user data with the precoding weight and encoding the reference signal sequence (which is dependent on the precoding weight) includes multiplying the reference signal sequence with the precoding weight to produce the dedicated reference signal sequence. In an advantageous embodiment, the precoding weight is identified by correlating the dedicated reference signal sequence with a set of signal replicas including precoding weights combined with corresponding reference signal sequences (e.g., computed a priori or in real time), and the channel estimates (e.g., computed/estimated in real time). The precoding weight is then identified in accordance with the largest correlation value. In another aspect, a transmitter of a communication system includes a precoding weight selector configured to select a precoding weight. The transmitter also includes a dedicated reference signal sequence selector configured to select a reference signal sequence based on the precoding weight, and provide a dedicated reference signal sequence therefrom (e.g. , encode the reference signal sequence with the precoding weight to produce the dedicated reference signal sequence in accordance with a multiplier). The transmitter also includes an encoder (e.g., a multiplier) configured to encode user symbols with a selected precoding weight to produce user data. A multiplexer of the transmitter is configured to combine (e.g., time and/or frequency multiplex) user data with the dedicated reference signal sequence and a common reference signal sequence. The dedicated reference signal sequence is a function of the selected precoding weight and the corresponding assigned reference signal sequence. A modulator of the transmitter is configured to apply a modulation carrier to modulate the user data, and the dedicated and common reference signal sequences. A plurality of radio frequency circuits, coupled to an antenna, are configured to convert the modulated user data, and the dedicated and common reference signal sequences into radio frequency signals suitable for transmission. In an exemplary embodiment, the dedicated reference signal sequence includes the selected precoding weight multiplied with the corresponding reference signal sequence. Additionally, the dedicated reference signal sequence selector may store a set of possible dedicated reference signal sequences, wherein the set of possible dedicated reference signal sequences includes each precoding weight in the set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

In another aspect, a receiver of a communication system includes a plurality of radio frequency circuits, coupled to a corresponding antenna, configured to convert radio frequency signals detected by the antenna into a modulated receive signal including user data, and dedicated and common reference signal sequences. A demodulator of the receiver is configured to remove a modulation carrier signal applied to the modulated receive signal to produce time and/or frequency multiplexed user data, and the dedicated and common reference signal sequence. A demultiplexer of the receiver is configured to separate (e.g., demultiplex) the user data from the dedicated and common reference signal sequences. A precoding weight detector of the receiver is configured to identify a precoding weight applied to the dedicated reference signal sequence from a set of signal replicas, including precoding weights combined with corresponding reference signal sequences, and a channel estimate. A data detector of the receiver is configured to decode the user data using the identified precoding weight. In an exemplary embodiment, the receiver includes a signal replica memory configured to store a set of signal replicas including precoding weights combined with corresponding reference signal sequences, which may be computed a priori or computed in real time. The real time estimated channel estimate might be stored in the signal replica memory or another memory such as a channel estimate memory. The channel estimate may be computed by a channel estimator in accordance with the common reference signal sequence. A correlator, of the precoding weight detector, is configured to correlate the dedicated reference signal sequence with the set of signal replicas combined with the channel estimate. The result of each correlation may be stored in a memory and the precoding weight resulting in the largest correlation value may be identified as the precoding weight.

As described above, the exemplary embodiment provides both a method and corresponding apparatus consisting of various modules providing functionality for performing the steps of the method. The modules may be implemented as hardware (including an integrated circuit such as an application specific integrated circuit), or may be implemented as software or firmware for execution by a computer processor. In particular, in the case of firmware or software, the exemplary embodiment can be provided as a computer program product including a computer readable storage structure embodying computer program code (i.e., software or firmware) thereon for execution by the computer processor.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the features and functions discussed above can be implemented in software, hardware, or firmware, or a combination thereof.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

WHAT IS CLAIMED IS:

1. An apparatus, comprising: a precoding weight selector configured to select a precoding weight; a dedicated reference signal sequence selector configured to select a reference signal sequence based on said precoding weight and provide a dedicated reference signal sequence therefrom; an encoder configured to encode user symbols with said precoding weight to produce user data; and a radio frequency circuit configured to convert said user data and said dedicated reference signal sequence into radio frequency signals for transmission.

2. The apparatus as recited in Claim 1 wherein said dedicated reference signal sequence includes said precoding weight multiplied with said reference signal sequence.

3. The apparatus as recited in Claim 1 wherein said dedicated reference signal sequence selector includes a set of possible dedicated reference signal sequences, wherein said set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

4. The apparatus as recited in Claim 1 further comprising a multiplexer configured to combine said user data with said dedicated reference signal sequence and a common reference signal sequence.

5. The apparatus as recited in Claim 1 further comprising a modulator configured to apply a modulation carrier to modulate said user data, said dedicated reference signal sequence and a common reference signal sequence.

6. The apparatus as recited in Claim 1 wherein said apparatus is a transmitter of a base station in a communication system.

7. An apparatus, comprising: means for selecting a precoding weight; means for selecting a reference signal sequence based on said precoding weight and providing a dedicated reference signal sequence therefrom; means for encoding user symbols with said precoding weight to produce user data; and means for converting said user data and said dedicated reference signal sequence into radio frequency signals for transmission.

8. The apparatus as recited in Claim 7 wherein said means for selecting said reference signal sequence includes a set of possible dedicated reference signal sequences, wherein said set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

9. A computer program product comprising program code stored in a computer readable medium configured to select a precoding weight, select a reference signal sequence based on said precoding weight and provide a dedicated reference signal sequence therefrom, encode user symbols with said precoding weight to produce user data, and convert said user data and said dedicated reference signal sequence into radio frequency signals for transmission.

10. The computer program product as recited in Claim 9 wherein said program code stored in said computer readable medium is configured to select said reference signal sequence in accordance with a set of possible dedicated reference signal sequences, wherein said set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

11. A method, comprising: selecting a precoding weight; selecting a reference signal sequence based on said precoding weight and providing a dedicated reference signal sequence therefrom; encoding user symbols with said precoding weight to produce user data; and converting said user data and said dedicated reference signal sequence into radio frequency signals for transmission.

12. The method as recited in Claim 11 wherein said dedicated reference signal sequence includes said precoding weight multiplied with said reference signal sequence.

13. The method as recited in Claim 11 wherein said reference signal sequence is selected in accordance with a set of possible dedicated reference signal sequences, wherein said set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

14. The method as recited in Claim 11 further comprising combining said user data with said dedicated reference signal sequence and a common reference signal sequence.

15. The method as recited in Claim 11 further comprising applying a modulation carrier to modulate said user data, said dedicated reference signal sequence and a common reference signal sequence.

16. An apparatus, comprising: a precoding weight detector configured to identify a precoding weight applied to a dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences; and a data detector configured to decode user data using said precoding weight.

17. The apparatus as recited in Claim 16 wherein said precoding weight detector includes a correlator configured to correlate said dedicated reference signal sequence with said set of signal replicas combined with a channel estimate.

18. The apparatus as recited in Claim 17 wherein a result of each correlation is stored in a memory and a precoding weight resulting in a largest correlation value provides said precoding weight.

19. The apparatus as recited in Claim 16, further comprising: a radio frequency circuit, coupled to a corresponding antenna, configured to convert radio frequency signals detected by said antenna into a modulated receive signal including multiplexed user data, said dedicated reference signal sequence and a common reference signal sequence; a demodulator configured to remove a modulation carrier signal applied to said modulated receive signal to said multiplexed user data, said dedicated reference signal sequence and said common reference signal sequence; and a demultiplexer configured to separate said user data from said dedicated reference signal sequence and said common reference signal sequence.

20. The apparatus as recited in Claim 16 further comprising a signal replica memory configured to store said set of signal replicas.

21. The apparatus as recited in Claim 16 wherein said apparatus is a receiver of a mobile station in a communication system.

22. An apparatus, comprising: means for identifying a precoding weight applied to a dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences; and means for decoding user data using said precoding weight.

23. The apparatus as recited in Claim 22 wherein said means for identifying said precoding weight includes correlating said dedicated reference signal sequence with said set of signal replicas combined with a channel estimate.

24. A computer program product comprising program code stored in a computer readable medium configured to identify a precoding weight applied to a dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences, and decode user data using said precoding weight.

25. The computer program product as recited in Claim 24 wherein said program code stored in said computer readable medium is configured to identify said precoding weight in accordance with correlating said dedicated reference signal sequence with said set of signal replicas combined with a channel estimate.

26. A method, comprising: identifying a precoding weight applied to a dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences; and decoding user data using said precoding weight.

27. The method as recited in Claim 26 wherein said identifying said precoding weight includes correlating said dedicated reference signal sequence with said set of signal replicas combined with a channel estimate.

28. The method as recited in Claim 27 wherein a result of each correlation is stored in a memory and a precoding weight resulting in a largest correlation value provides said precoding weight.

29. The method as recited in Claim 26, further comprising: converting radio frequency signals into a modulated receive signal including multiplexed user data, said dedicated reference signal sequence and a common reference signal sequence; removing a modulation carrier signal applied to said modulated receive signal to said multiplexed user data, said dedicated reference signal sequence and said common reference signal sequence; and separating said user data from said dedicated reference signal sequence and said common reference signal sequence.

30. The method as recited in Claim 26 further comprising storing said set of signal replicas.

31. A communication system, comprising: a transmitter, including: a precoding weight selector configured to select a precoding weight, a dedicated reference signal sequence selector configured to select a reference signal sequence based on said precoding weight and provide a dedicated reference signal sequence therefrom, an encoder configured to encode user symbols with said precoding weight to produce user data, a multiplexer configured combine said user data with said dedicated reference signal sequence and a common reference signal sequence, a modulator configured to apply a modulation carrier to modulate said user data, said dedicated reference signal sequence and said common reference signal sequence, and a radio frequency circuit configured to convert said modulated user data, said dedicated reference signal sequence and said common reference signal sequence into radio frequency signals for transmission; and a receiver, including: a radio frequency circuit, coupled to a corresponding antenna, configured to convert said radio frequency signals detected by said antenna into a modulated receive signal including multiplexed user data, a dedicated reference signal sequence and a common reference signal sequence, a demodulator configured to remove a modulation carrier signal applied to said modulated receive signal to said multiplexed user data, said dedicated reference signal sequence and said common reference signal sequence, a demultiplexer configured to separate said user data from said dedicated reference signal sequence and said common reference signal sequence, a precoding weight detector configured to identify said precoding weight applied to said dedicated reference signal sequence from a set of signal replicas including precoding weights combined with corresponding reference signal sequences, and a data detector configured to decode said user data using said precoding weight.

32. The communication system as recited in Claim 31 wherein said dedicated reference signal sequence selector includes a set of possible dedicated reference signal sequences, wherein said set of possible dedicated reference signal sequences includes each precoding weight in a set of precoding weights multiplied by a corresponding, mapped reference signal sequence.

33. The communication system as recited in Claim 31 wherein said precoding weight detector includes a correlator configured to correlate said dedicated reference signal sequence with said set of signal replicas combined with a channel estimate.