JP6027637B2 - Antenna port mapping method and apparatus for demodulated reference signal - Google Patents

Description

  The present invention generally relates to a demodulation reference signal (DM-RS) in an LTE or LTE advanced communication system. In particular, it relates to configuration of antenna ports for user-specific DM-RS.

  The 3rd Generation Partnership Project (3GPP) is responsible for standardization work for the UMTS (Global Mobile Communication Service) system and LTE (Long Term Evolution). LTE, which is considered as the next generation mobile communication system of the UMTS system, is a communication technology that realizes high-speed packet communication capable of achieving a high data rate in both downlink and uplink. The 3GPP specification in LTE is also called E-UTRAN (Evolved Global Land Access Network). LTE first edition, called Release 8 (Rel-8), reduces peak cost of 100Mbps, wireless network delay below 5ms, significant increase in spectral efficiency, and simplifies and reduces network control A network structure designed for such things can be provided. To support high data rates, LTE allows system bandwidths up to 20MHz. LTE can also operate in different frequency bands and can operate in both FDD (frequency division duplex) and TDD (time division duplex) modes. It is known that the modulation technique and transmission method used in LTE is OFDM (Orthogonal Frequency Division Multiplexing).

  Discussions have been made on support for bands up to 100 MHz in next-generation mobile communication systems such as IMT (International Mobile Communications) Advanced and / or LTE Advanced, which is an evolution of LTE. LTE Advanced is considered a future release of the LTE standard, and since this is an evolution of LTE, backward compatibility is important so that it can be deployed in the spectrum already occupied by LTE. In both LTE and LTE Advanced, the base station is known as Evolved Node Bs (eNBs or eNode B) and has multiple input and other output (MIMO) antenna configurations to provide high data rates to user terminals And spatial multiplexing is used. Another example of a system based on MIMO is the WiMax (Worldwide Interoperability for Microwave Access) system.

  In order to perform coherent demodulation for a plurality of different downlink physical channels, the user terminal needs an estimate of the downlink channel. More specifically, for OFDM transmission, the user terminal needs an estimate of the complex channel for each subcarrier. One way to enable channel estimation in the case of OFDM transmission is to insert a known reference symbol into the OFDM time frequency grid. In LTE, such reference symbols are collectively referred to as downlink reference signals.

  In the LTE system, two types of downlink reference signals are used. That is, a cell-specific downlink reference signal and a user-specific downlink reference signal. The cell-specific downlink reference signal is transmitted for each downlink subframe and occupies the entire downlink cell band. Cell-specific reference signals can be used for channel estimation and coherent demodulation except when spatial multiplexing is used. The reference signal unique to the user terminal is used for channel estimation and demodulation of the downlink common channel when spatial multiplexing is used. The user-specific reference signal is transmitted in a resource block allocated to a specific user terminal in order to transmit data on the downlink common channel. The reference signal unique to the user terminal is subjected to the same precoding as the data signal transmitted to the user terminal. The present invention is applicable to downlink reference signals specific to user terminals.

  FIG. 1 shows a typical partial OFDM time-frequency grid 50 in LTE. In general, the time-frequency grid 50 is divided into 1 millisecond subframes. FIG. 1 shows one subframe. Each subframe includes several OFDM symbols. In a standard cyclic prefix (CP) link, suitable for use in situations where multipath distortion is not expected to be extremely severe, one subframe contains 14 OFDM symbols. When the extended cyclic prefix is used, one subframe includes 12 OFDM symbols. In the frequency domain, physical resources are divided and adjusted to subcarriers having a 15 kHz interval. The number of subcarriers varies according to the allocated system band. The smallest element of the time-frequency grid 50 is a resource element. The resource element includes one OFDM symbol on one subcarrier.

  In order to schedule transmission on the downlink common channel (DL-SCH), downlink time frequency resources are allocated to units called resource blocks (RBs). Each resource block occupies 12 subcarriers (adjacent or distributed across the frequency spectrum) and half of one subframe. The term “resource block pair” refers to two resource blocks occupying all 1 millisecond subframes.

  Several resource elements within each subframe are reserved for transmitting downlink reference signals. FIG. 1 shows one exemplary resource allocation pattern for downlink reference signals to support downlink transmissions up to a maximum rank of 4. Twenty-four resource elements in one subframe are reserved for transmission of the downlink reference signal. More specifically, the demodulation reference signal is conveyed to OFDM symbols 5, 6, 12, and 13 (that is, the sixth, thirteenth, and fourteenth symbols). Resource elements for the demodulated reference signal are distributed in the frequency domain.

  Resource elements for demodulated reference signals are divided into two code division multiplexing (CDM) groups called CDM group 1 and CDM group 2. In LTE systems that support rank 1 to 4 transmissions, two CDM groups are used in combination with length 2 orthogonal cover codes (OCCs). The orthogonal cover code is applied to a group (cluster) of two reference signals. As used herein, the term “group” refers to a group of adjacent reference signals on the same subcarrier (in the time domain). In the embodiment shown in FIG. 1, the subcarriers including the demodulated reference signal each include two groups.

  FIG. 2 shows an exemplary reference element assignment for a spatial multiplexing system that supports transmissions up to rank 8. It should be noted that this resource allocation pattern is the same as the allocation pattern shown in FIG. A length 4 OCC is used instead of a length 2 OCC to support higher rank transmissions. A length 4 OCC is applied across two groups of resource elements.

  Up to 8 antenna ports are defined to support up to 8 spatial layers. Eight antenna ports can be mapped to two CDM groups, each using four OCCs. Therefore, an antenna port is uniquely identified by two parameters, a CDM group index and an OCC index. These two parameters are called index pairs. Currently, mapping of antenna ports and index pairs is not specified in the LTE standard. Some mappings are rank dependent and require different port mappings to be used for each transmission rank. Using different port mappings for different transmission ranks places a burden on the user terminal. That is, when the transmission rank changes, channel estimation must be changed

  The present invention provides an integrated, rank-independent mapping between antenna ports and group / code pairs. Each antenna port is uniquely associated with one code division multiplexing (CDM) group and one orthogonal cover code (OCC). The mapping between antenna ports and group / code pairs is chosen as follows. That is, for a given antenna port, the CDM group and OCC are the same for all transmission ranks.

  One exemplary embodiment of the present invention includes a method performed by a base station to transmit a demodulated reference signal to a user terminal. The method includes a step of determining a transmission rank for downlink transmission to the user terminal and a step of determining one or more reference signal antenna ports for the downlink transmission under the transmission rank. Each port is determined by a group / code pair including a code division multiplexing group and an orthogonal cover code, and the code division multiplexing group and code orthogonalization cover code are all transmitted at a given antenna port. Mapping a reference signal antenna port to a group / code pair for each transmission rank so that the ranks are the same, and transmitting the downlink reference signal on the reference signal antenna port .

  Yet another embodiment of the present invention includes a base station configured to perform the method as described above.

  One exemplary embodiment of the present invention includes a method performed by a user terminal that receives a demodulated reference signal transmitted by a base station. The user terminal method determines a transmission rank for downlink transmission to the user terminal, and determines one or more reference signal antenna ports for the downlink transmission under the transmission rank. Each port includes a group / code pair including a code division multiplexing group and an orthogonal cover code; and a code division multiplexing group and a code orthogonalization cover code for all antenna ports at a given antenna port. Mapping a reference signal antenna port to a group / code pair for each transmission rank so that the transmission rank is the same, and the downlink reference signal via the reference signal antenna port corresponding to the transmission rank Receiving.

  Yet another embodiment of the present invention includes a user terminal configured to perform the method as described above.

It is a figure which shows allocation of the resource element of the demodulation reference signal in an OFDM system in order to support transmission to rank 4. It is a figure which shows allocation of the resource element of the demodulation reference signal in an OFDM system in order to support transmission to rank 8. 1 is a diagram illustrating a typical MIMO communication system. FIG. FIG. 2 shows an exemplary transmit signal processor in an OFDM system. FIG. 6 shows the mapping of codewords to layers according to one embodiment for rank 1 to 4 transmissions. It is a figure which shows the typical transmission method of a demodulation reference signal. It is a figure which shows the reception method of a demodulation reference signal.

  FIG. 3 shows a multiple-input other-output (MIMO) wireless communication system including a base station 12 (referred to as an evolved node in LTE) and a user terminal 14. The present invention will be described in the environment of an LTE system. However, the present invention is applicable to other types of communication systems. Base station 12 includes a transmitter 100 that transmits a signal to a second station 14 via a communication channel 16. On the other hand, the user terminal 14 includes a receiver 200 that receives a signal transmitted from the base station 12. One skilled in the art will appreciate that each base station 12 and user terminal 14 includes both a transmitter 100 and a receiver 200 for bidirectional communication.

  In the base station 12, an information signal is input to the transmitter 100. The transmitter 100 includes a controller 110 that controls the overall operation of the transmitter 100 and the transmission signal processor 120. The transmit signal processor 120 performs error coding, mapping of input bits to complex modulation symbols, and generates a transmit signal for each transmit antenna. After conversion to high frequency, filtering and amplification, the transmitter 100 transmits a signal from the transmission antenna 130 to the user terminal 14 via the communication channel 16.

  The receiver 200 of the user terminal 14 performs demodulation and decoding on the signals received by the respective antennas. The receiver 200 includes a controller 210 that controls the operation of the receiver 200 and the received signal processor 220. The received signal processor 220 demodulates and decodes the signal transmitted from the first station 12. The output signal from the receiver 200 includes an estimate of the original information signal. When there is no error, the estimated value is the same as the original information signal input at the transmitter 12.

In the LTE system, when there are a plurality of antennas in both the base station 12 and the user terminal 14, spatial multiplexing can be used. FIG. 4 shows the main functional components of the transmission signal processor 120 in spatial multiplexing. The transmission signal processor 120 includes a layer mapping unit 122, a precoder 124, and a resource mapping unit 128. A series of information symbols (data symbols and reference symbols) are input to the layer mapping unit 122. This series of symbols is divided into one or two codewords. The layer mapping unit 122 maps the codeword to the _NL layer according to the transmission rank. Note that the number of layers need not be equal to the number of antennas 120. Different codewords are typically mapped to different layers. However, a single codeword is mapped to one or more layers. The number of layers corresponds to the selected transmission rank. After layer mapping, a set of N _L symbols (one symbol from each layer) is linearly combined by the precoder 124 and mapped to N _A antenna ports. The combination and mapping is described by a precoder matrix of size N _A × N _L. Resource mapping unit 128 maps the symbols transmitted on each antenna port 126 to resource elements allocated by the MAC scheduler.

  When the user terminal 14 is scheduled to receive downlink transmission on the downlink common channel (DL-SCH), the MAC scheduler at the transmitting station 12 assigns one or more resource block pairs to the user terminal 14. Assign to. As described above, some resource elements of each resource block are reserved for downlink reference signals. In order to support downlink transmission up to 8 layers, downlink reference signals specific to the user terminal are required for 8 layers. According to the present invention, eight individual reference signal antenna ports are defined to support transmissions up to eight layers. Each antenna port is uniquely associated with one code division multiplexing (CDM) group and one orthogonal cover code (OCC). For example, although the orthogonal code is also used for the OCC, it may include a length-2 or length-4 Walsh code. For convenience, the CDM group is defined by a group identifier having a value from 1 to 2, and the OCC is defined by a code identifier having a value from 1 to 4. The combination of CDM group and OCC is referred to herein as a group / code pair.

  In an exemplary embodiment, there are 2 CDM groups and 4 OCCs. Therefore, 8 combinations of CDM groups and OCCs are possible (2 groups x 4 OCCs), and support for 8 layers is possible. The mapping between antenna ports and group / code pairs is configured independently of rank. More specifically, the mapping between antenna ports and group / code pairs is chosen such that for a given antenna port, the CDM group and OCC are the same for all transmission ranks.

Table 1 below and FIG. 5 illustrate one possible mapping between antenna ports and group / code pairs according to one embodiment of the present invention.

OCC is a Walsh code given by a Walsh code matrix.

    As shown in Table 1, the antenna port mapping assigns CDM group 1 to ports 1, 2, 5, and 6 and assigns CDM group 2 to ports 3, 4, 7, and 8. OCC1 is assigned to ports 1 and 3, OCC2 is assigned to ports 2 and 4, OCC3 is assigned to ports 5 and 7, and OCC4 is assigned ports 6 and 8.

    This antenna port mapping described above is rank independent. Thus, a given antenna port can always use the same CDM group and OCC regardless of transmission rank. In addition, the antenna ports associated with a particular CDM group are nested. That is, for a set of antenna ports associated with a given CDM group, the antenna ports used for the lower transmission rank are a subset of the antenna ports used for the higher transmission rank. Thus, for antenna ports associated with CDM group 1, the ports used in transmission rank 1 are a subset of the ports used in transmission rank 2, and the ports used in transmission rank 2 are used in transmission rank 5. A port used in transmission rank 5 is a subset of ports used in transmission rank 6. The same nesting structure applies to the antenna ports associated with CDM group 2.

    Another useful feature for the antenna port mapping shown above is that for some antenna ports, a length 4 OCC is the same as a length 2 OCC. For example, in transmission rank 2, the same length Walsh code as length 2 appears in the length 4 Walsh codes in antenna port 1 and antenna port 2. In the case of a single user MIMO system, the user terminal 14 uses a length 2 OCC to perform channel estimation due to this feature. By using a length 2 OCC for channel estimation, the receiver 200 can interpolate and, as a result, generate a more accurate channel estimate. The improved channel estimation is effective for the user terminal 14 having high mobility. Thus, for transmission ranks 2, 4, and 5, the receiver may use a length-2 Walsh code to perform channel estimation at antenna port 1 and antenna port 2, as shown in FIG. it can. Similarly, for transmission ranks 3 and 4, the receiver can use a length-2 Walsh code to perform channel estimation at antenna port 3 and antenna port 4. If more than one layer is multiplexed into one CDM group, a length 4 OCC should be used for channel estimation.

    In the case of a multi-user MIMO system, when transparent MU-MIMO is used, the user terminal 14 does not know whether other user terminals 14 are scheduled in common at the same time. Due to this lack of information, the user terminal 14 uses a length-4 OCC for channel estimation even at low ranks. Therefore, there is a possibility that the performance is somewhat deteriorated particularly at a high speed. To take advantage of the length 2 OCC, we propose to use a 1-bit OCC length flag in the control signal. This can provide some further information to the user terminal about the OCC details, and as a result, improve the performance of MU-MIMO. Therefore, this 1-bit flag is also effective for dynamic switching between SU and MU.

    FIG. 6 illustrates an exemplary method 150 performed by the base station 12 that transmits a demodulated reference signal to the user terminal 14. When the user terminal 14 is scheduled to receive downlink transmission on the downlink common channel (DL-SCH), the base station determines a transmission rank for downlink transmission to the user terminal 14 ( Block 152), selecting one or more reference signal antenna ports for downlink transmission based on the transmission rank (Block 154). The transmit signal processor 120 at the base station 12 maps the antenna port to a specific CDM group and orthogonal cover code so that the CDM group and orthogonal cover code are the same for all transmission ranks at a given antenna port. Composed. Transmit signal processor 120 maps the demodulated reference signal to the reference signal antenna port according to the corresponding transmission rank (block 156) and transmits the demodulated reference signal via the selected antenna port. (Block 158).

  FIG. 7 illustrates an exemplary procedure 160 performed by a user terminal to receive a downlink reference signal from the base station 12. The user terminal 14 determines a transmission rank for downlink transmission to the user terminal 14 (block 162), and selects one or more reference signal antenna ports based on the transmission rank (block 164). The received signal processor 220 is configured to map the reference signal antenna port to the CDM group and orthogonal cover code so that the CDM group and orthogonal cover code are the same for all transmission ranks at a given antenna port ( Block 166). Receive signal processor 220 receives the reference signal via the selected antenna port (block 168) and processes the signal.

  Antenna port mapping is applicable to both single user MIMO and multi-user MIMO. Also, as with DwPTS and extended CP, it can also be applied to carriers of multiple components. The antenna port mapping method can be used to reduce the influence of randomization of peak power.

  The present invention may, of course, be carried out in other specific ways without departing from the scope or essential characteristics of the invention. Accordingly, the present embodiment is to be considered as illustrative in all respects and not restrictive, and all modifications within the meaning of the appended claims and the equivalents thereof are intended. It is intended to be included in it.

Claims (12)

A method performed by a base station that transmits a demodulation reference signal to a user terminal, comprising:
Determining a transmission rank for downlink transmission to the user terminal;
Determining one or more reference signal antenna ports for the downlink transmission based on the transmission rank, wherein each port is a group / code pair including a code division multiplexing group and an orthogonal cover code; A process determined by:
Mapping the reference signal antenna port to a group / code pair such that the code division multiplexing group and code orthogonalization cover code are the same for all transmission ranks at a given antenna port;
Via the reference signal antenna port corresponding to the transmission rank it possesses a step of transmitting a reference signal of the downlink,
The step of mapping the antenna ports to group / code pairs is further such that in a given code division multiplexing group, the antenna ports associated with a low transmission rank are a subset of the antenna ports associated with a high transmission rank. A method characterized by being configured as follows. The orthogonal cover cord includes a cover cord having a length of 4,
The step of mapping the antenna ports to group / code pairs can further be decomposed into length-2 orthogonal cover codes into two length-2 orthogonal cover codes for channel estimation at the selected antenna port. The method of claim 1, wherein the method is configured to be. Transmitting a control signal to a user terminal to indicate whether channel estimation is performed using either a length 2 orthogonal cover code or a length 4 orthogonal cover code at the selected antenna port; The method of claim 2 , further comprising: A method performed by a user terminal that receives a demodulation reference signal transmitted from a base station, comprising:
Determining a transmission rank for downlink transmission to the user terminal;
Determining one or more reference signal antenna ports for the downlink transmission based on the transmission rank, wherein each port is a group / code pair including a code division multiplexing group and an orthogonal cover code; A process determined by:
Mapping the reference signal antenna port to a group / code pair such that the code division multiplexing group and code orthogonalization cover code are the same for all transmission ranks at a given antenna port;
Possess a step of receiving a reference signal of the downlink by the reference signal antenna port corresponding to the transmission rank,
The step of mapping the antenna ports to group / code pairs is further such that in a given code division multiplexing group, the antenna ports associated with a low transmission rank are a subset of the antenna ports associated with a high transmission rank. A method characterized by being configured as follows. The orthogonal cover cord includes a cover cord having a length of 4,
The step of mapping the antenna ports to group / code pairs can further be decomposed into length-2 orthogonal cover codes into two length-2 orthogonal cover codes for channel estimation at the selected antenna port. 5. The method of claim 4 , wherein the method is configured to be. Step receives a control signal from the base station, in accordance with the control signal, which is executed in the selected antenna port, the channel estimation using either orthogonal cover code of the orthogonal cover code and the length 4 of length 2 6. The method of claim 5 , further comprising: A base station having a transmit signal processor and a transmit controller,
The transmission signal processor and the transmission controller are:
Determine the transmission rank for downlink transmission to the user terminal,
Based on the transmission rank, one or more reference signal antenna ports for the downlink transmission are determined, wherein each port is defined by a group / code pair including a code division multiplexing group and an orthogonal cover code. Decide
Map the reference signal antenna port to a group / code pair so that the code division multiplexing group and code orthogonalization cover code are the same for all transmission ranks at a given antenna port;
Configured to transmit the downlink reference signal through the reference signal antenna port corresponding to the transmission rank ;
Map antenna ports to group / code pairs such that in a given code division multiplexing group, the antenna ports associated with a lower transmission rank are a subset of the antenna ports associated with a higher transmission rank. A base station characterized in that it is configured . The orthogonal cover cord includes a cover cord having a length of 4,
Mapping the antenna ports to group / code pairs is such that, for the selected antenna port, the length 4 orthogonal cover code can be decomposed into two length 2 orthogonal cover codes for channel estimation. 8. The base station according to claim 7 , wherein the base station is configured as follows. In the selected antenna port, a control signal is further transmitted to the user equipment to indicate whether channel estimation is performed using either a length 2 orthogonal cover code or a length 4 orthogonal cover code. 9. The base station according to claim 8 , wherein the base station is configured to include. A user terminal including a reception signal processor and a reception controller,
The reception signal processor and the reception controller are :
Determining a transmission rank for downlink transmission to the user terminal;
Based on the transmission rank, one or more reference signal antenna ports for the downlink transmission are determined, wherein each port is defined by a group / code pair including a code division multiplexing group and an orthogonal cover code. Decide
Map the reference signal antenna port to a group / code pair so that the code division multiplexing group and code orthogonalization cover code are the same for all transmission ranks at a given antenna port;
Configured to receive the downlink reference signal by the reference signal antenna port corresponding to the transmission rank ; and
Map antenna ports to group / code pairs such that in a given code division multiplexing group, the antenna ports associated with a lower transmission rank are a subset of the antenna ports associated with a higher transmission rank. User terminal characterized by being configured . The orthogonal cover cord includes a cover cord having a length of 4,
Mapping the antenna ports into group / code pairs allows the length 4 orthogonal cover codes to be decomposed into two length 2 orthogonal cover codes for channel estimation at the selected antenna port. 11. The user terminal according to claim 10 , wherein Receiving a control signal from the base station and performing channel estimation using either a length 2 orthogonal cover code or a length 4 orthogonal cover code at a selected antenna port according to the control signal; 12. The user terminal according to claim 11 , wherein the user terminal is configured.