HK1234548A - Antenna port mapping method and device for demodulation reference signals - Google Patents

Description

Antenna port mapping method and device for demodulation reference signals

Background

The present invention relates generally to demodulation reference signals (DM-RS) for LTE and LTE-advanced communication systems, and more particularly to configuration of antenna ports for user-specific DM-RS.

The 3 rd generation partnership project (3 GPP) is responsible for the standardization of UMTS (universal mobile telecommunications service) systems and LTE (long term evolution). LTE, which is a communication technology for realizing high-speed packet-based communication capable of achieving high data rates in both downlink and uplink, is considered as a next-generation mobile communication system of the UMTS system. The 3GPP work on LTE is also known as E-UTRAN (evolved universal terrestrial access network). The first version of LTE, referred to as release 8 (Rel-8), is capable of providing peak rates of 100Mbps, e.g., radio network delays of 5 ms or less, a significant increase in spectral efficiency, and a network architecture designed to simplify network operation, reduce cost, and the like. To support high data rates, LTE allows for system bandwidths up to 20 MHz. LTE is also capable of operating in different frequency bands and in both FDD (frequency division duplex) and TDD (time division duplex) modes. The modulation technique or transmission scheme used in LTE is called OFDM (orthogonal frequency division multiplexing).

For next generation mobile communication systems, which are evolutions of LTE, such as IMT-advanced (international mobile telecommunications) and/or LTE-advanced, support for bandwidths up to 100 MHz is under discussion. LTE-advanced can be seen as a future release of the LTE standard, and since it is an evolution of LTE, backward compatibility is important to enable LTE-advanced to be deployed in the spectrum that LTE already occupies. In both LTE and LTE-advanced radio base stations, called evolved NodeB (eNB or eNodeB), Multiple Input Multiple Output (MIMO) antenna configurations and spatial multiplexing can be used in order to provide high data rates to user terminals. Another example of a MIMO based system is the WiMAX (worldwide interoperability for microwave access) system.

In order to implement coherent demodulation of different downlink physical channels, the user terminal needs an estimate of the downlink channel. More precisely, in the case of OFDM transmission, the user terminal requires an estimation of the complex channel per subcarrier. One way to enable channel estimation in the case of OFDM transmission is to insert known reference symbols into the OFDM time-frequency grid. In LTE, these reference symbols are collectively referred to as downlink reference signals.

Two types of downlink reference signals are used in LTE systems: cell-specific downlink reference signals and user-specific downlink reference signals. The cell-specific downlink reference signals are transmitted in each downlink subframe and span the entire downlink cell bandwidth. The cell-specific reference signal can be used for channel estimation and coherent demodulation other than when spatial multiplexing is used. The user terminal specific reference signal is used for channel estimation and demodulation of the downlink shared channel when spatial multiplexing is used. The user-specific reference signals are transmitted within resource blocks assigned to specific user terminals for transmitting data on the downlink shared channel. The user terminal specific reference signals are subjected to the same precoding as the data signals transmitted to the user terminals. The present invention is applicable to user terminal specific downlink reference signals.

Fig. 1 shows a portion of an exemplary OFDM time-frequency grid 50 for LTE. In general, the time-frequency grid 50 is divided into a plurality of one millisecond subframes. One subframe is shown in fig. 1. Each subframe includes a plurality of OFDM symbols. For a conventional Cyclic Prefix (CP) link suitable for use in an environment where multipath dispersion is not expected to be very severe, one subframe includes fourteen OFDM symbols. If an extended cyclic prefix is used, one subframe includes twelve OFDM symbols. In the frequency domain, the physical resources are divided into adjacent subcarriers having a spacing of 15 kHz. The number of subcarriers varies according to the allocated system bandwidth. The smallest unit of the time-frequency grid 50 is a resource unit. The resource unit includes one OFDM symbol on one subcarrier.

To schedule transmissions on the downlink shared channel (DL-SCH), downlink time-frequency resources are allocated in units called Resource Blocks (RBs). Each resource block spans half and twelve subcarriers of a subframe (which may be contiguous or spread across the spectrum). The term "resource block pair" refers to two consecutive resource blocks that occupy a complete one millisecond subframe.

Certain resource elements within each subframe are reserved for transmission of downlink reference signals. Fig. 1 shows one exemplary resource allocation pattern for downlink reference signals supporting downlink transmission up to rank 4. Twenty-four resource elements within a subframe are reserved for transmission of downlink reference signals. More specifically, the demodulation reference signals are carried in OFDM symbols 5, 6, 12, and 13 (i.e., sixth symbol, seventh symbol, thirteenth symbol, and fourteenth symbol) of the OFDM subframe. The resource units of the demodulation reference signal are distributed in the frequency domain.

The resource elements of the demodulation reference signal are divided into two Code Division Multiplexing (CDM) groups, referred to herein as CDM group 1 and CDM group 2. In an LTE system supporting transmission ranks from 1-4, two CDM groups are used in conjunction with an Orthogonal Cover Code (OCC) of length 2. An orthogonal cover code is applied to the groups of two reference symbols. The term "group" as used herein refers to a group of adjacent reference symbols (in the time domain) in the same subcarrier. In the embodiment shown in fig. 1, the subcarriers containing demodulation reference symbols each comprise two groups.

Fig. 2 illustrates an exemplary allocation of resource units for a spatial multiplexing system supporting a transmission rank of up to eight. It may be noted that this resource allocation pattern is the same as the allocation pattern shown in fig. 1. To support a higher transmission rank, a length-4 OCC is used instead of a length-2 OCC. An OCC of length 4 is applied across the resource units of both groups.

Up to eight antenna ports may be defined to support up to 8 spatial layers. The 8 antenna ports can be mapped to two CDM groups, each using four OCCs. Thus, an antenna port can be uniquely identified by two parameters, a CDM group index and an OCC index (referred to herein as an index pair). Currently, mapping between antenna ports and index pairs has not been specified in the LTE standard. Some mappings may be rank dependent, which requires different port mappings to be used for each transmission rank. Using different port mappings for different transmission ranks creates a burden on the user terminals, which must perform channel estimation differently when the transmission rank changes.

Disclosure of Invention

The present invention provides a uniform, 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 such that for a given antenna port, CDM group and OCC will be the same for each transmission rank.

One exemplary embodiment of the present invention encompasses a method implemented by a base station for transmitting a demodulation reference signal to a user terminal. The method comprises determining a transmission rank of a 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 defined by a group/code pair comprising a code division multiplexing group and an orthogonal cover code; for each transmission rank, mapping reference signal antenna ports to group/code pairs such that for a given antenna port, the code division multiplexing group and code orthogonal cover codes are the same for each transmission rank; and transmitting the downlink reference symbols through the reference signal antenna port.

Yet another exemplary embodiment of the present invention encompasses a base station configured to implement the above-described method.

Another exemplary embodiment of the present invention encompasses a method implemented by a user terminal for receiving a demodulation reference signal transmitted by a base station. The user terminal method comprises determining a transmission rank for a 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 defined by a group/code pair comprising a code division multiplexing group and an orthogonal cover code; for each transmission rank, mapping reference signal antenna ports to group/code pairs such that for a given antenna port, the code division multiplexing group and orthogonal cover codes are the same for each transmission rank; and receiving the downlink reference symbols through the reference signal antenna ports corresponding to a transmission rank.

Yet another exemplary embodiment of the present invention includes a user terminal configured to implement the above-described method.

Drawings

Fig. 1 illustrates allocation of resource elements in an OFDM system for demodulation reference signals supporting a transmission rank of up to 4.

Fig. 2 illustrates allocation of resource elements in an OFDM system for demodulation reference signals supporting a transmission rank of up to 8.

Fig. 3 illustrates an exemplary MIMO communication system.

Fig. 4 illustrates an exemplary transmit signal processor for an OFDM system.

Fig. 5 shows codeword to layer mapping for transmission ranks from 1 to 4 according to an exemplary embodiment.

Fig. 6 illustrates an exemplary method for transmitting a demodulation reference signal.

Fig. 7 illustrates a method of receiving a demodulation reference signal.

Detailed Description

Fig. 3 shows a multiple input/multiple output (MIMO) wireless communication system 10, which includes a base station 12 (referred to as evolved NodeB in LTE) and a user terminal 14. The invention will be described in the context of an LTE system, but the invention is applicable to other types of communication systems. The base station 12 comprises a transmitter 100 for transmitting signals to the second station 14 over the communication channel 16, and the user terminal 14 comprises a receiver 200 for receiving signals transmitted by the base station 12. Those skilled in the art will recognize that base station 12 and user terminal 14 may each include both a transmitter 100 and a receiver 200 for bi-directional communication.

At the base station 12, an information signal is input to the transmitter 100. The transmitter 100 includes a controller 110 and a transmission signal processor 120 that control the overall operation of the transmitter 100. The transmission signal processor 120 performs error coding (error coding), maps input bits to complex modulation symbols, and generates a transmission signal for each transmission antenna 130. After up-frequency conversion, filtering and amplification, the transmitter 100 transmits the transmit signals from the respective transmit antennas 130 to the user terminals 14 over the communication channel 16.

The receiver 200 at the user terminal 14 demodulates and decodes the signals received at each antenna 230. The receiver 200 includes a controller 210 that controls the operation of the receiver 200 and a received signal processor 220. The received signal processor 220 demodulates and decodes the signal transmitted from the first station 12. The signal output from the receiver 200 comprises an estimate of the original information signal. In the absence of errors, the estimate will be the same as the original information signal input at the transmitter 12.

In an LTE system, spatial multiplexing can be used when there are multiple antennas at both the base station 12 and the user terminal 14. Fig. 4 shows the main functional components of the transmit signal processor 120 for spatial multiplexing. The transmission signal processor 120 includes a layer mapping unit122. A precoder 124 and a resource mapping unit 128. The sequence of information symbols (data symbols or reference symbols) is input to the layer mapping unit 122. The symbol sequence is divided into one or two codewords. Layer mapping unit 122 maps codewords to transmission ranksN _L In each layer. It should be noted that the number of layers is not necessarily equal to the number of antennas 130. Different codewords are typically mapped to different layers; however, a single codeword may be mapped to one or more layers. The number of layers corresponds to the selected transmission rank. After layer mapping, the precoder 124 will performN _L Sets of symbols (one symbol per layer) are linearly combined and mapped toN _A And an antenna port 126. The combination/mapping is of sizeN _A ×N _L The precoder matrix of (c). A resource mapping unit 128 maps symbols to be transmitted on each antenna port 126 to resource units assigned by the MAC scheduler.

When a user terminal 14 is scheduled to receive a downlink transmission on the downlink shared channel (DL-SCH), the MAC scheduler at the transmitting station 12 allocates one or more resource block pairs to the user terminal 14. As previously mentioned, some resource elements in each resource block are reserved for downlink reference signals. To support downlink transmissions containing up to eight layers, user terminal specific downlink reference signals for eight layers are required. In accordance with the present invention, eight unique reference signal antenna ports are defined to support transmissions having up to eight layers. Each antenna port is uniquely associated with one Code Division Multiplexing (CDM) group and one Orthogonal Cover Code (OCC). The OCC may, for example, include length 2 or length 4 walsh codes (walsh codes), although other orthogonal codes may also be used. For convenience, the CDM group may be identified by a group index having a value of 1 to 2, and the OCC may be identified by a code index having a value of 1 to 4. The combination of CDM groups and OCC is referred to herein as a group/code pair.

In the exemplary embodiment, there are two CDM groups and 4 OCCs. Therefore, there are eight possible combinations of CDM groups and OCCs (2 groups × 4 OCCs), enabling support of eight layers. The mapping between antenna ports and group/code pairs is designed to be rank independent. More specifically, the mapping between antenna ports and group/code pairs is chosen such that for a given antenna port, CDM groups and OCCs will be the same for each transmission rank.

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

OCC is the walsh code given by the walsh code matrix:

the antenna port mapping shown in table 1 allocates CDM group 1 to ports 1, 2, 5, and 6 and CDM group 2 to ports 3, 4, 7, and 8. OCC 1 is assigned to ports 1 and 3, OCC 2 is assigned to ports 2 and 4, OCC 3 is assigned to ports 5 and 7, and OCC 4 is assigned to ports 6 and 8.

This antenna port mapping described above is rank independent, such that a given antenna port will always use the same CDM group and OCC, regardless of the transmission rank. In addition, the antenna ports associated with a particular CDM group possess a nested property. That is, for the set of antenna ports associated with a given CDM group, the antenna ports for a low transmission rank will be a subset of the antenna ports for a higher transmission rank. Thus, for antenna ports associated with CDM group 1, the port for transmission rank 1 is a subset of the ports for transmission rank 2, the port for transmission rank 2 is in turn a subset of the ports for transmission rank 5, and the port for transmission rank 5 is in turn a subset of the ports for transmission rank 6. The same nesting property applies to the antenna ports associated with CDM group 2.

Another useful property of the antenna port mapping shown above is that the length-4 OCC on some antenna ports is exactly the same as the length-2 OCC. For example, for transmission rank 2, a length 4 walsh code at antenna ports 1 and 2 appears to be the same as a length 2 walsh code. In the case of a single-user MIMO system, this property enables the user terminal 14 to perform channel estimation using an OCC of length 2. Using a length 2 OCC for channel estimation allows receiver 200 to interpolate (interpolate) and thus provide a more accurate channel estimate. Improved channel estimation is beneficial for high mobility user terminals 14. Thus, for transmission ranks 2, 4, and 5, the receiver may use length-2 walsh codes on antenna ports 1 and 2 to perform channel estimation, as shown in fig. 5. Similarly, for transmission ranks 3 and 4, the receiver may use length-2 walsh codes on antenna ports 3 and 4 to perform channel estimation. When more than two layers are multiplexed into one CDM group, the OCC of length 4 should be used for channel estimation.

For multi-user MIMO, a user terminal 14 may not know whether other user terminals 14 are co-scheduled at the same time, such as when using transparent MU-MIMO. This lack of knowledge forces each user terminal 14 to use a length-4 OCC for channel estimation even for lower ranks, which can degrade performance somewhat more, especially for high speed scenarios. To take advantage of the length-2 OCC, we propose to introduce a 1-bit OCC length flag in the control signaling to provide the user terminal 14 with more information about the OCC details, which can improve performance in MU-MIMO accordingly. Thus, this 1-bit flag also enables dynamic SU/MU switching.

Fig. 6 illustrates an exemplary method 150 implemented by the base station 12 for transmitting demodulation reference signals to the user terminal 14. When the user terminal 14 is scheduled to receive a downlink transmission on a downlink shared channel (DL-SCH), the base station 12 determines a transmission rank for the downlink transmission to the user terminal 14 (block 152), and determines one or more reference signal antenna ports for the downlink transmission based on the transmission rank (block 154). Transmit signal processor 130 at base station 12 is configured to map the antenna ports to a particular CDM group and orthogonal cover codes such that for a given antenna port, the CDM group and orthogonal cover codes are the same for each transmission rank. The transmit signal processor 130 maps the demodulation reference signals to reference signal antenna ports corresponding to the transmission rank (block 156) and transmits the demodulation reference signals through the selected antenna ports (block 158).

Fig. 7 illustrates an exemplary process 160 implemented by the user terminal 14 to receive downlink reference signals from the base station 12. The user terminal 14 determines a transmission rank for the downlink transmission to the user terminal (block 162) and selects one or more reference signal antenna ports based on the transmission rank (block 164). Receive signal processor 230 is configured to map the reference signal antenna ports to CDM groups and OCCs such that, for a given antenna port, the CDM groups and OCCs are the same for each transmission rank (block 166). The receive signal processor 230 receives the reference signals through the selected antenna port (block 168) and processes the signals.

Antenna port mapping is applicable to both single-user MIMO and multi-user MIMO. It can also be applied to DwPTS and extended CP, and multi-component carrier. The antenna port mapping scheme can be used to reduce the peak power randomization effect.

The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims (10)

1. A method implemented by a base station for transmitting demodulation reference signals to a user terminal, the method comprising:

determining a transmission rank for a 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 defined by a group/code pair comprising a code division multiplexing group and an orthogonal cover code;

mapping reference signal antenna ports to group/code pairs for each transmission rank such that the code division multiplexing group and code orthogonal cover codes are the same for each transmission rank for a given antenna port; and

transmitting the downlink reference symbols through the reference signal antenna ports corresponding to the transmission rank.

2. The method of claim 1, wherein the mapping of antenna ports to group/code pairs is further configured such that: within a given code division multiplexing group, the antenna ports associated with a low transmission rank will be a subset of the antenna ports associated with a higher transmission rank.

3. The method of claim 1, wherein the orthogonal cover codes comprise length-4 cover codes, and wherein the mapping of antenna ports to group/code pairs is further configured such that: for the selected antenna port, the length-4 orthogonal cover code can be decomposed into two length-2 cover codes for channel estimation.

4. The method of claim 2, further comprising transmitting a control signal to the user terminal to indicate whether length 2 or length 4 orthogonal cover codes should be used for the selected antenna port to perform channel estimation.

5. A method implemented by a user terminal for receiving demodulation reference signals transmitted by a base station, the method comprising:

determining a transmission rank for a 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 defined by a group/code pair comprising a code division multiplexing group and an orthogonal cover code;

mapping reference signal antenna ports to group/code pairs for each transmission rank such that the code division multiplexing group and orthogonal cover codes are the same for each transmission rank for a given antenna port; and

receiving the downlink reference symbols through the reference signal antenna ports corresponding to the transmission rank.

6. The method of claim 5, wherein the mapping of antenna ports to group/code pairs is further configured such that: within a given code division multiplexing group, the antenna ports associated with a low transmission rank will be a subset of the antenna ports associated with a higher transmission rank.

7. The method of claim 5, wherein the orthogonal cover codes comprise length-4 cover codes, and wherein the mapping of antenna ports to group/code pairs is further configured such that: for the selected antenna port, the length-4 orthogonal cover code can be decomposed into two length-2 cover codes for channel estimation.

8. The method of claim 7, further receiving a control signal from the base station, and performing channel estimation using a length-2 or length-4 orthogonal cover code for a selected antenna port according to the control signal.

9. A base station comprising a transmit signal processor and a transmit controller, the base station configured to:

determining a transmission rank for a 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 defined by a group/code pair comprising a code division multiplexing group and an orthogonal cover code;

mapping reference signal antenna ports to group/code pairs for each transmission rank such that the code division multiplexing group and code orthogonal cover codes are the same for each transmission rank for a given antenna port; and

transmitting the downlink reference symbols through the reference signal antenna ports corresponding to the transmission rank.

10. The base station of claim 9, further configured to map antenna ports to group/code pairs such that within a given code division multiplexing group, antenna ports associated with a low transmission rank will be a subset of antenna ports associated with a higher transmission rank.