WO2014126519A1

WO2014126519A1 - Select dm-rs pattern based on channel characteristics

Info

Publication number: WO2014126519A1
Application number: PCT/SE2013/051155
Authority: WO
Inventors: Meng Wang; Shaohua Li; Stefano Sorrentino; George JÖNGREN; David Hammarwall
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2013-02-12
Filing date: 2013-10-03
Publication date: 2014-08-21

Abstract

The present disclosure relates to Reference Signal (RS) patterns. Certain aspects of the disclosure relate to a transmitting node, such as an eNodeB, that selects 410 a demodulation reference signal (DM-RS) pattern from a plurality of DM-RS patterns based on current channel characteristics. The transmitting node also transmits 420 the thus selected RS pattern to a receiving node (e.g., a UE).

Description

SELECT DM-RS PATTERN BASED ON CHANNEL CHARACTERISTICS

TECHNICAL FIELD

Embodiments herein relate to a transmitting node, a receiving node and methods therein. In particular some embodiments relate to reference signals in a radio communications network. More particularly, some embodiments herein relate to methods and transmitting nodes (e.g. eNodeBs) for selecting a reference signal (RS) pattern from a plurality of RS patterns based on current channel characteristics, and for transmitting the thus selected RS pattern to a receiving node (e.g., a UE).

BACKGROUND

In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN generally covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a "NodeB" (Universal Mobile

Telecommunications System (UMTS)) or "eNodeB" (Long Term Evolution (LTE)). A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or, alternatively, an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is generally broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also

broadcasted in the cell. The base stations can communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.

In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.

The UMTS is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile

5 Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks o and further generations, and investigate enhanced data rate and radio capacity.

Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the LTE radio access, and the Evolved Packet Core (EPC), also5 known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E- UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an0 EPS system has an essentially "flat" architecture comprising radio base stations without reporting to RNCs.

In LTE systems, known reference symbols are inserted into Orthogonal Frequency Division Multiplexing (OFDM) time-frequency grid at regular time- frequency positions, i.e. resource elements (RE). Using the knowledge about the5 reference signals (RS), a receiver of a user equipment or a radio base station can perform channel estimation for demodulation and other purposes, e.g. acquiring channel state information.

In the LTE specification (see e.g. the technical specification 3GPP TS 36.21 1 (V.1 1 .1 .0), section 6.10), different types of reference signals are defined. In0 DL, the transmitted reference signals, from the radio base station, include Cell specific Reference Signal (CRS), Demodulation Reference Signals (DM-RS), Channel State Indicator Reference Signal (CSI-RS), Multicast/Broadcast Single Frequency Network (MBSFN) reference signals and positioning reference signals. Similarly, the reference signals transmitted in UL, i.e. from the user equipment, include UL DM-RS and Sounding Reference Signals (SRS). The corresponding defined procedures of RS sequence generation and mapping to REs form 'RS patterns'. To be specific, a RS pattern comprises:

- Overhead of the reference signal, i.e., the total number of REs/Resource Blocks (RB) used by the RS pattern in a regular period.

- Mapping to REs in time domain, frequency domain and code domain, or cyclic shift as a special case of code, which includes the orthogonal cover code (OCC) structure, e.g., an OCC can extend in time domain, in frequency, or in both.

- Transmission modes and ranks. The mapping procedures are defined

respectively for different transmission modes and ranks.

Thus, a RS pattern can be said to carry more information than merely a pattern (i.e. time-frequency grid). That is, a RS pattern can include the pattern (i.e. time-frequency grid) as well as the mapping method, the adopted transmission mode and ranks.

An example of a RS pattern, the pattern of DL CRS for one antenna port, is illustrated in Figure 1.

In LTE, the patterns of different types of reference signals are explicitly defined in the specification (see e.g. 3GPP TS 36.21 1 (V.1 1 .1 .0), section 6.10). The reference signals are generally designed to have a sufficiently high density and an optimized or good structure in both time and frequency domains to provide estimates for the entire time-frequency grid in the case of radio channels subject to high frequency and/or time selectivity. Generally, the design of RS patterns must take the most challenging channel characteristics into account and thus, high RS overhead and the associate structure design are typically required to guarantee proper demodulation under any channel condition.

The international patent application PCT/US201 1 /038822, which is titled "Reference Signal Patterns" and published under WO 201 1 /153286 A1 on

08.12.201 1 , describes a method for wireless communications. A set of available channel state information reference signals (CSI-RS) patterns is identified. The available CSI-RS patterns define resources for use in transmitting CSI-RS in a subframe from multiple antenna ports. Furthermore, a subset of the set of available CSI-RS patterns is identified bases on transmission configuration. One of the CSI-RS patterns is selected from the subset. Also, CSI-RS is transmitted in the subframe according to the selected CSI-RS pattern.

5

SUMMARY

It is in view of the above and other considerations that embodiments of the present invention have been made.

For example, the inventors have realized that the traditional reference o signals (RS) in LTE systems are generally designed to accommodate extreme channel conditions to guarantee proper quality of service. As radio

communications networks such as LTE evolve, additional capabilities and system performance enhancement are motivated by new indoor and outdoor scenarios. The specific channel characteristics of those new scenarios can be utilized to5 further improve system spectral efficiency. For this purpose, particularly, the

overhead and structure of reference signals can be optimized or otherwise improved according to the channel condition. Therefore, there is a need for adaptively configuring reference signals in LTE systems.

Among other things, embodiments herein are therefore aimed at performing0 adaptive switching between different RS patterns. In the current LTE specification (see e.g. 3GPP TS 36.21 1 (V.1 1 .1 .0), section 6.10), the RS pattern is solely defined for each type of RS. Instead, embodiments herein may provide a plurality of RS patterns for each type of RS, which are specifically designed for different channel characteristics, i.e., different deployments and UE behaviors. Among5 those candidate RS patterns, the RS pattern for transmission can be adaptively selected, for example, based on the criteria that the system performance is optimized or otherwise improved given the knowledge of current channel characteristics.

For example, a transmitting node, such as a user equipment in UL or a0 radio base station in DL, selects a reference signal pattern out of a number of reference signal patterns of a specific type of reference signal based on a criteria such as current channel statistics or similar. The transmitting node may receive, from a receiving node, a recommendation which reference signal pattern to use.

More particularly, according to one of its aspects, this disclosure presents a method performed by a transmitting node. The transmitting node may be

embodied as a radio base station (e.g. an eNodeB). A reference signal (RS) pattern is selected from a plurality of RS patterns based on current channel characteristics. Also, the selected RS pattern is transmitted to a receiving node. The receiving node may be a user equipment (UE). In a preferred embodiment, a demodulation reference signal (DM-RS) pattern is selected from a plurality of DM- RS patterns based on current channel characteristics. Also, the selected DM-RS pattern is transmitted to a receiving node.

In one embodiment, a set of available DM-RS patterns is identified. Also, a subset of the set of available DM-RS patterns is identified based on current channel characteristics. Furthermore, one of the available DM-RS patterns from the identified subset is selected.

The above-mentioned selection may comprise selecting the DM-RS pattern based on an estimation of a channel delay spread and/or a channel Doppler spread.

Typically, but not necessarily, a set of available DM-RS patterns is designed for different channel characteristics that are reflected in channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

In one embodiment, the method comprises receiving (from the receiving node) a recommendation of a DM-RS pattern to use for a downlink, DL, transmission (i.e. from the transmitting node to the receiving node). The recommendation may be received as part of a Channel State Information (CSI), Reporting. This CSI reporting may in addition involve a Channel Quality Indicator (CQI), a Pre-coding Matrix Indicator (PMI) and/or a Rank Indicator (Rl).

Said recommendation may be based on characteristics of channel delay spread. Additionally, or alternatively, said recommendation may be based on characteristics of channel Doppler spread. Furthermore, it is conceivable that said recommendation is based on characteristics of other parameters related to channel properties.

According to another aspect, this disclosure presents a transmitting node, e.g. a radio base station. The radio base station may be embodied as an eNodeB. 5 The transmitting node is configured to select a reference signal (RS)

pattern from a plurality of RS patterns based on current channel characteristics. Also, the selected RS pattern is transmitted to a receiving node. In a preferred embodiment, the transmitting node is configured to select a demodulation reference signal (DM-RS) pattern from a plurality of DM-RS patterns based on o current channel characteristics. Also, the selected DM-RS pattern is transmitted to a receiving node.

The transmitting node is also configured to transmit the selected DM-RS pattern to a receiving node. The receiving node may be embodied as a user equipment (UE). In a preferred embodiment, the transmitting node is configured to5 select a demodulation reference signal, DM-RS, pattern from a plurality of DM-RS patterns based on current channel characteristics and to transmit the selected DM-RS pattern to a receiving node.

In one embodiment, the transmitting node is further configured to identify a set of available DM-RS patterns; identify a subset of the set of available DM-RS0 patterns based on current channel characteristics; and select one of the available DM-RS patterns from the identified subset.

In one embodiment, the transmitting node is configured to select the DM- RS pattern based on an estimation of a channel delay spread and/or a channel Doppler spread. A set of available DM-RS patterns may be designed for different5 channel characteristics that are reflected in channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

In one embodiment, the transmitting node is configured to receive (from the receiving node) a recommendation of a DM-RS pattern to use for a downlink (DL) transmission. The transmitting node may be configured to receive said0 recommendation as part of a Channel State Information (CSI) Reporting, which in addition may involve a Channel Quality Indicator (CQI), a Pre-coding Matrix Indicator (PMI) and/or a Rank Indicator (Rl). Said recommendation may be based on characteristics of channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties. Through embodiments presented herein, the radio communications network may gain flexibility to select the RS patterns (e.g. DM-RS patterns) that achieve the optimal, or at least an improved, balance between RS overhead and receiver performance for different scenarios. In turn, this may lead to further enhancements in overall system performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described in more detail in relation to the enclosed drawings, in which:

Figure 1 is a schematic overview depicting a reference signal pattern of DL CRS for one antenna port within a pair of RBs;

Figure 2 is a schematic overview of a radio communications network;

Figure 3 is a schematic overview depicting a LTE system in which eNodeB is communicating with UEs;

Figure 4 is a flowchart of an example method performed by a transmitting node;

Figure 5 illustrates an example implementation with respect to the selection of a DM-RS pattern from a plurality of DM-RS patterns; and

Figure 6 is a block diagram depicting a transmitting node and a receiving node according to some embodiments herein.

DETAILED DESCRIPTION

Fig. 2 is a schematic overview depicting a radio communications network 1. The radio communications network 1 comprises one or more RANs and one or more CNs. The radio communications network 1 may use one or more of different technologies, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. In the radio communications network 1 , a user equipment 10, also known as a mobile station and/or a wireless terminal, is configured to communicate via a Radio Access Network (RAN) to one or more core networks (CN). It should be understood by persons skilled in the art that "user equipment" is a non-limiting term which means any wireless terminal, Machine Type Communication (MTC) device or node e.g. Personal Digital Assistant (PDA), laptop, mobile, sensor, relay node, mobile tablet or even a small base station (e.g. pico base station) communicating within respective cell.

The radio communications network 1 covers a geographical area which is divided into cell areas, e.g. a cell 11 being served by a radio base station 12. The radio base station 12 may also be referred to as a first radio base station. The radio base station 12 may be referred to as e.g. a NodeB, an evolved Node B (eNB, eNode B), a base transceiver station, Access Point Base Station, base station router, or any other network unit capable of communicating with a user equipment within the cell served by the radio base station depending e.g. on the radio access technology and terminology used. The radio base station 12 may serve one or more cells, such as the cell 1 1 .

A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The cell definition may also incorporate frequency bands and radio access technology used for transmissions, which means that two different cells may cover the same geographical area but use different frequency bands. Each cell is typically identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell 1 1 uniquely in the whole radio communications network 1 is also broadcasted in the cell 1 1 . The radio base station 12 is configured to communicate over the air or radio interface operating on radio frequencies with the user equipment 10 within range of the radio base station 12. The user equipment 10 is configured to transmit data over the radio interface to the radio base station 12 in Uplink (UL) transmissions and the radio base station 12 is configured to transmit data over an air or radio interface to the user equipment 10 in Downlink (DL) transmissions.

Furthermore, the radio communications network 1 comprises a core network node such as a Positioning node 13 for mobility management. Another, a different, or second, radio base station 14 may also be comprised in the radio communications network 1 . The second radio base station 14 may provide radio coverage over a second cell 15, another or a different cell, e.g. a cell neighboring the cell 1 1 .

In some versions of the radio communications network 1 , several base stations are typically connected, e.g. by landlines or microwave, to a controller node (not shown), such as a RNC or a Base Station Controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.

As radio communications networks, such as LTE, evolve, additional capabilities and system performance enhancement are motivated by e.g. new indoor and outdoor scenarios. Those scenarios may have specific channel characteristics that can be utilized to further improve system performance.

However, the traditional RS patterns are typically designed to accommodate different channel conditions and may hence lack the flexibility for some specific scenarios and the corresponding channel characteristics. This would potentially result in RS redundancy and performance loss. For example, a high-density RS pattern is generally not necessary for a scenario with relatively flat channel in both time and frequency domains. Meanwhile, the UEs could travel from one typical scenario to another. A RS pattern optimized for one scenario may thus not be sufficient for another.

In conclusion, traditional RS patterns in the LTE specification (e.g. 3GPP TS 36.21 1 (V.1 1 .1 .0)) lack flexibility for different scenarios with different channel characteristics.

Note that although terminology from 3GPP LTE is used throughout this disclosure to exemplify embodiments herein, this should not be seen as limiting the scope of the embodiments herein to only the aforementioned system. Other wireless systems, including e.g. WCDMA, WiMAX, and UMB, may also benefit from exploiting the ideas covered within this disclosure.

Also note that terminology such as base station and/or eNodeB should be considered non-limiting and does in particular not imply a certain hierarchical relation between the two; in general "eNodeB" could be considered as device 1 and "UE" could be considered as device 2 and these two devices communicate with each other over a radio channel. The UE may be a transmitting node in the UL and the radio base station may be the transmitting node in the DL.

Consequently, the radio base station may be a receiving node in the UL and the 5 UE may be the receiving node in the DL.

In the following, various exemplary embodiments will be illustrated in more detail. It should be noted that these embodiments are not to be construed as mutually exclusive. Hence, components from one embodiment may be tacitly assumed to be present in another embodiment and it will be conceivable to a o person skilled in the art how those components may be used in the other

exemplary embodiments.

Figure 3 illustrates a communication system exemplified as an LTE system in which a radio base station denoted as an eNodeB (in LTE) is communicating with one or several UEs. In the communication between the eNodeB and the5 UE(s), reference signals are transmitted in each subframe. In the sequel, we

describe embodiments of the invention from eNodeB and UE perspectives, respectively.

/. eNodeB side

0 In some embodiments, a plurality of RS patterns is available for at least one type of reference signal and for a specific transmission mode and, if applicable, rank. Typically, the available RS patterns are known at both eNodeB and UEs.

In some embodiments, a first RS pattern is used for the transmission to the first UE (as UE1 in Figure 3). In the transmission to the second UE (as UE2 in5 Figure 3), a second RS pattern is used. The RS patterns used for transmission can be updated regularly at a specific interval. In a further such embodiment said first and second UE may be the same UE.

In some embodiments, the first and second RS patterns used for transmission are selected from a set of available RS patterns to optimize the0 performance for different channel characteristics. For example, the information about the RS pattern that is (to be) used is part of a Downlink Control Information (DCI) message, in which case it typically applies to a single specific subframe, or is part of a Radio Resource Control (RRC) or Media Access Control (MAC) message, in which case it typically applies until further notice.

In some embodiments, the eNodeB receives a recommendation of a RS pattern to use for a DL transmission from said specific UE. In a further such 5 embodiment, said pattern recommendation is part of channel state information (CSI) reporting, which in addition involves a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), and/or a Rank Indicator (Rl). The eNodeB may follow said recommendation for at least one subsequent transmission and may transmit a Physical Downlink Shared Channel (PDSCH) message using said o recommended pattern. Also, a confirmation that eNodeB follows the

recommended RS may be signaled to the UE.

In some embodiments, it is the eNodeB that determines the RS pattern to be used based on several factors, which could for example include the UE recommendation, measurements performed at UE and reported via uplink, and/or5 a priori deployment information or measurements performed by the eNodeB in the uplink. These measurements may include an estimation of a channel delay spread, channel Doppler spread, or another parameter related to one of these measurements. The eNodeB may explicitly inform the UE (e.g. by signalling a message to the UE) what RS pattern is used for transmission.

0 In some embodiments, the available set of RS patterns is designed for different channel characteristics that are reflected in channel delay spread, channel Doppler spread and other parameters related to channel properties. In a further embodiment, the design of said RS patterns is embodied in the RS RE density in time and frequency domains, RS structure in the time-frequency grid,5 OCC applied in time and/or frequency domains, the OCC length, etc.

In some embodiments, the RS pattern is referred to DM-RS pattern or CSI- RS pattern.

In some embodiments, the method is applied in the downlink or the uplink. For uplink transmission from UE to eNodeB, the eNodeB may select a suitable RS0 pattern based on uplink measurements. Information about the selected RS pattern is then sent to the specific UE. In a further such embodiment, the UE follows the selected RS pattern by eNodeB. /./ eNodeB side - DM-RS patterns

An embodiment relating to DM-RS patterns will now be described in further detail. With respect to Fig. 4A, a flowchart of a method 400 performed by a

5 transmitting node, such as an eNodeB, is shown. A DM-RS pattern is selected 410 from a plurality of DM-RS patterns. The selection 410 is based on current channel characteristics, or current channel properties. Next, the selected DM-RS pattern is transmitted 420 to a receiving node, such as a UE. With respect to Fig. 4B, a flowchart of an example embodiment of the DM-RS selection 410 is further o detailed. In this example, a set of available DM-RS patterns is identified 41 1 . Also, a subset of the set of available DM-RS patterns is identified 412 based on current channel characteristics, or current channel properties. Furthermore, one of the available DM-RS patterns from the identified subset is selected 413.

The selection 410 may comprise selecting the DM-RS pattern based on an5 estimation of a channel delay spread. Alternatively, or additionally, the selection 410 may comprise selecting the DM-RS pattern based on a channel Doppler spread. Typically, but not necessarily, a set of available DM-RS patterns is designed for different channel characteristics that are reflected in channel delay spread and/or channel Doppler spread. It should be appreciated that other0 parameters related to channel properties could also be conceivable, e.g. SNR

(Signal-to-Noise Ratio), a spatial domain characteristic such as AoD (Angle-of- Departure) spread or AoA (Angle-of-Arrival) spread, and/or pathloss, shadow fading, etc.

With respect to Figures 5A-5D, an example implementation on how to5 select 410 a DM-RS pattern will be further explained. Fig. 5A illustrates an

example utilizing a look-up table implementation. As can be seen, four different DM-RS patterns can be defined. Or said differently, depending on various combinations of channel delay spread and channel Doppler spread, four different example DM-RS patterns can be defined. In this example implementation, Doppler0 Spread Value Range 1 corresponds to a low Doppler spread. The Doppler Spread Value Range 2 corresponds to a high Doppler spread, i.e. a Doppler spread that is comparatively higher than Doppler Spread Value Range 1. Furthermore, Delay Spread Value Range 1 corresponds to a low delay spread. The Delay Spread Value Range 2 corresponds to a comparatively higher delay spread, i.e. a delay spread that is comparatively higher than Delay Spread Value Range 1

Typically, but not necessarily, the spacing (in time) between the reference symbols (RSs) can be determined by considering the maximum channel Doppler spread (e.g. highest speed) to be supported. For example, if the highest supported speed is 500 km/h (i.e. kilometers per hour), the Doppler spread is fd=fc^*v/c where fc is the carrier frequency, v is the UE speed in meters per second, and c is the speed of light. Considering for example that fc=2 GHz and v =500 km/h, the Doppler spread is fd=950 Hz. According to Nyquist's sampling theorem and under the above assumptions, the minimum sampling frequency needed in order reconstruct the channel can thus be given by Tc=1 /(2fd)=0.5 ms . This implies that two reference symbols per Transmission Time Interval (TTI) are needed in the time domain in order to estimate the channel correctly or accurately. For low speed, for example, v< 250km/h, fd is about 500Hz, and Tc=1 ms. In this case, only one reference symbols per TTI may be needed. Turning back to fig. 5A, one example for the Doppler spread value range 1 is 0-500Hz and the Doppler spread value range 2 is 500Hz-1000Hz.

Persons skilled in the art should appreciate that for low channel Doppler spread values, the RS density in the time domain can be reduced. Similarly, for comparatively higher channel Doppler spread values, the RS density in the time domain can be increased. This is due to Nyquist's sampling theorem, see e.g. the reference book "LTE- The UMTS long term evolution, From theory to Practice", by Stefania Sesia, Issam Toufik, Matthew baker (201 1 ). Based on the Nyquist's sampling theorem, the RS density in the time domain is generally inversely proportional to the channel Doppler spread value.

Furthermore, persons skilled in the art should appreciate that for low channel delay spread values, the RS density in the frequency domain can be reduced. Similarly, for comparatively higher channel delay spread values, the RS density in the frequency domain can be increased. In the frequency domain, it should be appreciated that the frequency spacing is generally related to an expected coherence bandwidth of the channel. This is in turn related to the channel delay spread, i.e. the delay spread value. In particular the 90% and 50% coherence bandwidths can be given by B_c,90% = 1 /(50^*delay spread) and B_c,50%=1 /(5^*delay spread). B_c,x% is the bandwidth where the autocorrelation of the channel in the frequency domain is equal to x%, see e.g. "LTE- The UMTS long term evolution, From theory to Practice", by Stefania Sesia, Issam Toufik, Matthew baker (201 1 ). As a mere example, in the 3GPP Technical Specification TS 36.101 , the maximum channel delay spread is 991 ns, corresponding to

B_c,90% =20 kHz and B_c,50%=200 kHz. In LTE, for the current DM-RS, the frequency spacing between two reference symbols is 6^*15 kHz=90kHz (as shown in Fig5C and Fig5E, respectively). For a low channel delay spread (e.g. Delay Spread Value Range 1), for example, if the channel delay spread is half of 991 ns, the frequency spacing can be enlarged or otherwise increased, e.g. ,

1 1 ^*15kHz=165kHz (as shown in Fig. 5B and Fig5D, respectively).

For low channel delay spread values, the channel is generally relatively flat in frequency domain. Thus, only few samples must be used to regenerate channels. Similarly, when the channel is with low Doppler spreading, the channel variation in time domain may become smaller. Hence, fewer samples are generally needed to get sufficiently good channel estimation. Or said differently, the coherence bandwidth of the channel is generally inversely proportional to the channel delay spread. With large delay spread, the coherence bandwidth is thus smaller, and vice versa. With large coherence bandwidth, the channel is flat in frequency domain. In other words, the channel does not change much in terms of coherence bandwidth. Furthermore, the required spacing in time domain between reference symbols is generally proportional to the channel Doppler spread. In other words, with smaller Doppler spread less OFDM symbols are generally needed as reference symbols. So, with smaller delay spread, the coherent bandwidth is larger. Hence, the channel in one RB does not change much. Hence, it is not necessary to utilize many REs/RBs to estimate the channel sufficiently correctly, or sufficiently accurately. One example of different DM-RS patterns that can be utilized is schematically shown in Figs. 5A-5E. The person skilled in the art appreciates that other DM-RS patterns are equally possible. Thus, the exact DM- RS patterns should preferably be tested and evaluated for each specific case in dependence of operator needs, operator demands, end-user needs, end-user demands, system requirements, etcetera.

UE side

In some embodiments, a UE decodes a first PDSCH message in a first subframe using a first RS pattern, and a second PDSCH message, within the same transmission mode and, if applicable, rank, in a second subframe using a second PDSCH pattern. That is, the UE is capable of decoding, for a specific transmission mode and rank, a message using at least two different RS patterns.

In some embodiments the UE is capable of recommending, to the eNodeB, the RS pattern (e.g. a DM-RS pattern) to be used for at least one subsequent transmission. The UE may select said recommendation from a plurality RS patterns (e.g. a plurality of DM-RS patterns), such that the selected pattern results in the expected best performance, and may send a message to the eNodeB indicating said selection. Said recommendation may be based on characteristics of the wireless propagation channel, such as channel delay spread, channel Doppler spread and/or other parameters related to channel characteristics.

The RS pattern recommendation may advantageously (but not necessarily) be part of a CSI recommendation, wherein the PMI, Rl and/or CQI is based on the recommended RS pattern. In some embodiments, the UE is also configured to receive an indicator for which RS pattern is used in this transmission. The pattern indicator may part of a DCI message following some DCI format. When the DCI is decoded, the RS pattern used for current subframe is known. Hence, the information regarding RS pattern may need to be decoded on a subframe-to- subframe basis. In some embodiments, the pattern indicator may also be contained in RRC or MAC message. In this case, the RS pattern is not updated on a subframe-to-subframe basis. Instead, the UE keeps using the previously decoded RS pattern until further update in RRC or MAC message. In one such embodiment, the UE bases its derivations of CQI on the indicated RS pattern.

In some embodiments, the method is applied in the downlink or the uplink. Example advantages

1 . Through embodiments herein, the LTE system may gain flexibility to

adaptively select RS patterns for different channel characteristics. The overhead of reference signals can thus be optimized or otherwise improved according to different channel conditions.

2. Some embodiments herein may increase the spectral efficiency by

optimizing or otherwise improving RS resource allocation and hence, enhance the overall system performance.

3. Embodiments herein may be applied in downlink and/or uplink.

4. Embodiments herein are quite generic and may be applied to various

wireless communication systems such as GSM, WCDMA, WiMAX and UMB besides LTE.

In order to perform methods disclosed herein a transmitting node, as shown in Fig. 4, can be provided. The transmitting node may be radio base station or a user equipment. The transmitting node comprises a communication interface e.g. a receiver 401 and a transmitter 402 or a transceiver, configured to transmit a selected reference signal pattern. The transmitting node may select a reference signal pattern out of a plurality of reference signal patterns of a specific type of reference signals, which type may be a CRS, DM-RS, positioning reference signal, a MBSFN reference signal or similar. The transmitting node may adaptively switch the reference signal pattern based on e.g. received recommendation,

measurements performed at receiving node and reported via e.g. uplink, and/or a priori deployment information or measurements performed by the transmitting node.

The embodiments herein for selecting reference signal pattern may be implemented through one or more processors, such as a processing circuit 403 in the transmitting node depicted in Fig. 4, together with computer program code for performing the functions and/or method actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing embodiments herein when being loaded into the transmitting node. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server and downloaded to the transmitting node. The transmitting node may further comprise a memory 404. The memory may comprise one or more memory units and may be used to store for example data such as reference signal patterns, CSI, application to perform the methods herein when being executed on the transmitting node or similar.

In a preferred embodiment, the transmitting node is configured to select a DM-RS pattern from a plurality of DM-RS patterns based on current channel characteristics. The transmitting node may also be configured to transmit the selected DM-RS pattern to a receiving node. For example, the memory 404 and the computer program code may be configured to, with the processor 403, select a DM-RS pattern from a plurality of DM-RS patterns based on current channel characteristics. The transmitter (or, transceiver) 402 may be configured to transmit the selected DM-RS pattern to a receiving node. Furthermore, the transmitting node may be further configured to identify a set of available DM-RS patterns; identify a subset of the set of available DM-RS patterns based on current channel characteristics; and select one of the available DM-RS patterns from the identified subset. In one example implementation, the memory 404 and the computer program code may be configured to, with the processor 403, identify the set of available DM-RS patterns; identify the subset of the set of available DM-RS patterns based on current channel characteristics; and select one of the available DM-RS patterns from the identified subset. In one embodiment, the transmitting node may be configured to select the DM-RS pattern based on an estimation of a channel delay spread and/or a channel Doppler spread. That is, the memory 404 and the computer program code may be configured to, with the processor 403, select the DM-RS pattern based on an estimation of a channel delay spread and/or a channel Doppler spread. A set of available DM-RS patterns may be designed for different channel characteristics that are reflected in channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties. In one embodiment, the transmitting node is also configured to receive, from the receiving node, a recommendation of a DM-RS pattern to use for a downlink (DL) transmission. That is, the receiver 401 may be configured to receive said recommendation from the receiving node. For example, the transmitting node may be configured to receive said recommendation as part of a Channel State Information (CSI) Reporting, which in addition may involve a 5 Channel Quality Indicator (CQI), a Pre-coding Matrix Indicator (PMI) and/or a Rank Indicator (Rl). As mentioned earlier, said recommendation may be based on characteristics of channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

Embodiments herein also relate to a receiving node configured to receive a o reference signal but may also be configured to recommend a reference signal pattern out of a plurality of reference signal patterns of a specific type of reference signals, which type may be a CRS, DMRS, positioning reference signal, a MBSFN reference signal or similar. The recommendation may be performed in a

processing circuit 501 in the receiving node. The receiving node comprises a5 communication interface e.g. a receiver 502 and a transmitter 503 or a

transceiver, configured to transmit the recommended reference signal pattern. The receiving node may further comprise a memory 504. The memory 504 may comprise one or more memory units and may be used to store for example data such as reference signal patterns, CSI, application to perform the methods herein0 when being executed on the transmitting node or similar.

Those skilled in the art will also appreciate that the various "circuits" described may refer to a combination of analog and digital circuits, and/or one or more processors configured with software and/or firmware, e.g., stored in memory, that, when executed by the one or more processors, perform as described above.5 One or more of these processors, as well as the other digital hardware, may be included in a single application-specific integrated circuit (ASIC), or several processors and various digital hardware may be distributed among several separate components, whether individually packaged or assembled into a system- on-a-chip (SoC).

0 Several of the functional elements of the processing circuits discussed may be provided through the use of dedicated hardware, while others are provided with hardware for executing software, in association with the appropriate software or firmware. Thus, the term "processor" or "controller" as used herein does not exclusively refer to hardware capable of executing software and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random-access memory for storing software and/or program or application data, and non-volatile memory. Other hardware, conventional and/or custom, may also be included. Designers of communications receivers will appreciate the cost, performance, and maintenance tradeoffs inherent in these design choices.

In the drawings and specification, there have been disclosed exemplary embodiments. However, many variations and modifications can be made to these embodiments. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the embodiments herein being defined by the following claims. ABBREVIATIONS

RS reference signal

LTE long-term evolution

DL downlink

UL uplink

OFDM orthogonal frequency-division multiplexing

CRS cell-specific reference signals

DM-RS demodulation reference signals

CSI-RS channel-state information reference signals

MBSFN multicast-broadcast signal frequency network

SRS sounding reference signals

RE resource element

RB resource block

OCC orthogonal cover codes

CRS cell-specific reference signals

eNodeB E-UTRAN NodeB

UE user equipment

DCI downlink control information RRC radio resource control

MAC media access control

PDSCH physical downlink shared channel

CSI channel-state information

PMI precoding-matrix indicator

Rl rank indicator

CQI channel-quality indicator

GSM global system for mobile communications

WCDMA wideband code-division multiple access

WiMAX worldwide interoperability for microwave access

UMB ultra mobile broadband

Claims

1 . A method performed by a transmitting node, the method comprising:

selecting (410) a DM-RS pattern from a plurality of DM-RS patterns based on current channel characteristics; and

transmitting (420) the selected DM-RS pattern to a receiving node.

2. The method according to claim 1 , further comprising:

identifying (41 1 ) a set of available DM-RS patterns;

identifying (412) a subset of the set of available DM-RS patterns based on current channel characteristics; and

selecting (413) one of the available DM-RS patterns from the identified subset.

3. The method according to claim 1 or 2, wherein the selecting (410) comprises selecting the DM-RS pattern based on an estimation of a channel delay spread or a channel Doppler spread.

4. The method according to claim 1 or 2, wherein a set of available RS patterns is designed for different channel characteristics that are reflected in channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

5. The method according to any of the claims 1 -4, comprising:

receiving (405), from the receiving node, a recommendation of a RS pattern to use for a downlink, DL, transmission.

6. The method according to claim 5, wherein the recommendation is received (405) as part of a Channel State Information, CSI, Reporting, which in addition involves a Channel Quality Indicator, CQI, a Pre-coding Matrix Indicator, PMI and/or a Rank Indicator, Rl.

7. The method according to claim 5 or 6, wherein said recommendation is based on characteristics of channel delay spread, channel Doppler spread and/or other parameters related to channel properties.

8. The method according to any of the claims 1 -7, wherein the transmitting node is a radio base station and the receiving node is a user equipment.

9. A transmitting node configured to select a demodulation reference signal, DM-RS, pattern from a plurality of DM-RS patterns based on current channel characteristics and to transmit the selected DM-RS pattern to a receiving node.

10. The transmitting node according to claim 9, further being configured to identify a set of available DM-RS patterns; identify a subset of the set of available DM-RS patterns based on current channel characteristics; and select one of the available DM-RS patterns from the identified subset.

1 1 . The transmitting node according to claim 9 or 10, wherein the transmitting node is configured to select the DM-RS pattern based on an estimation of a channel delay spread or a channel Doppler spread.

12. The transmitting node according to claim 9 or 10, wherein a set of available RS patterns is designed for different channel characteristics that are reflected in channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

13. The transmitting node according to any of the claims 9-12, further being configured to receive, from the receiving node, a recommendation of a RS pattern to use for a downlink, DL, transmission.

14. The transmitting node according to claim 13, wherein the transmitting node is configured to receive said recommendation as part of a Channel State Information, CSI, Reporting, which in addition involves a Channel Quality Indicator, CQI, a Pre-coding Matrix Indicator, PMI and/or a Rank Indicator, Rl.

15. The transmitting node according to claim 13 or 14, wherein said recommendation is based on characteristics of channel delay spread, and/or channel Doppler spread and/or other parameters related to channel properties.

16. The transmitting node according to any of the claims 9-15, wherein the transmitting node is a radio base station.

17. The transmitting node according to any of the claims 9-16, wherein the receiving node is a user equipment.