WO2014077742A1

WO2014077742A1 - Methods and apparatus for reference signal antenna mapping configuration

Info

Publication number: WO2014077742A1
Application number: PCT/SE2012/051244
Authority: WO
Inventors: Stefano Sorrentino; Robert Baldemair; Sara SANDBERG
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2012-11-13
Filing date: 2012-11-13
Publication date: 2014-05-22

Abstract

The present invention relates to a method for configuring reference signals for transmission on multiple transmit antenna ports of a wireless device. The method is performed in a radio network node of a wireless communication system. The method comprises determining (410) a configuration for transmission of reference signals by the wireless device. The configuration defines a mapping of a reference signal to each of the multiple transmit antenna ports. The reference signals are generated from a respective different base sequence. The method also comprises sending (420) the determined configuration to the wireless device, for configuring the wireless device to transmit the reference signals on the multiple transmit antenna ports in accordance with the determined configuration.

Description

METHODS AND APPARATUS FOR REFERENCE SIGNAL ANTENNA

MAPPING CONFIGURATION

TECHNICAL FIELD

The disclosure relates to configuration of reference signals for transmission on multiple transmit antenna ports of a wireless device, and more specifically to mapping of reference signals to the multiple transmit antenna ports.

BACKGROUND

3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3^rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.

Figure 1 illustrates a conventional radio access network in an LTE system. An eNodeB 101 a with a transmission point 102a serves a UE 103 located within the eNodeB's geographical area of service also called a cell 105a. The eNodeB 101 a is directly connected to the core network (not illustrated). The eNodeB 101 a is also connected via an X2 interface to a neighboring eNodeB 101 b with a transmission point 102b serving another cell 105b.

In LTE, Cell-specific Reference Signals (CRS) are transmitted in all downlink subframes. In addition to assisting downlink channel estimation, the CRS are also used for mobility measurements performed by the UEs. The CRS are generally intended for use by all the UEs in the coverage area. As of LTE Release-10, specific reference signals are provided for measuring the channel for the purpose of generating Channel State Information (CSI) feedback from the UE. The latter reference signals are referred to as CSI Reference Signals (CSI-RS). CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than reference signals used for demodulation. CSI-RS transmissions may take place every fifth, tenth, twentieth, fortieth, or eightieth subframe, as determined by a periodicity parameter and a subframe offset, each of which are configured by Radio Resource Control (RRC) signaling. A UE operating in connected mode can be requested by the base station to perform CSI reporting. This reporting can include, for example, reporting a suitable Rank Indicator (Rl) and one or more Precoding Matrix Indices (PMI), given the observed channel conditions, as well as a Channel Quality Indicator (CQI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback. The CSI feedback assists the RBS in scheduling, including deciding which subframe and resource blocks to use for the transmission, as well as deciding which transmission scheme and/or precoder to use. The CSI feedback also provides information that can be used to determine a proper user bit-rate for the transmission, i.e., for Link Adaptation (LA). In order to support mobility, a UE needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current serving cell, is then evaluated in order to determine whether a handover, for UEs in connected mode, or cell re-selection, for UEs in idle mode, should be carried out. For UEs in connected mode, the handover decision is taken by the network, based on measurement reports provided by the UEs. Examples of such reports are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) reports.

The concept of a "point" is heavily used in conjunction with techniques for Coordinated Multipoint (CoMP). In this context, a point 210a-c corresponds to a set of antennas covering essentially a same geographical area 21 1 a-c in a similar manner as illustrated in Figure 2. One transmitting and receiving node, such as an LTE RBS, may control one or several points. Thus, a point may correspond to one of the sectors at an RBS site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Different antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point is operated more or less independently from the other points, from a scheduling point of view.

When downlink CoMP is applied, the network needs to dynamically or semi- statically determine which transmission points are to serve each UE in the downlink. Additionally, the network needs to determine a set of points for which receiving feedback from the UE would be beneficial. Such a set of points for feedback reception is typically selected in a semi-static fashion, i.e., they are typically constant for several subframes. The corresponding feedback may be employed for scheduling, link adaptation and dynamic selection of the transmission points within the set of points for which feedback is available. The set of suitable transmission points for a UE typically changes dynamically, e.g. as the UE moves through the network. The network therefore needs to select, and continuously update, a set of candidate transmission points for the UE. The UE then sends more detailed feedback, e.g. precoding information, for the points in the candidate set, thereby enabling the network to select the best downlink transmission points. The techniques mentioned above will be collectively referred to as "point selection" in the following.

The points in the candidate set may be determined in a UE-centric manner, wherein the UE performs measurements on downlink signals, such as CSI-RS, and reports the results to the network. Alternatively, a network-centric approach may be used for point selection, wherein the network performs measurements such as pathloss measurements on uplink signals transmitted by the UE. For example uplink Sounding Reference Signals (SRS) may be used for this purpose. SRS are generated using a base sequence. Each SRS is characterized by a sequence-group and a sequence-index, which define the base sequence. Base sequences are typically cell-specific and are a function of the cell-ID. Different base sequences are semi-orthogonal. On top of the base sequence, a phase shift may be applied in frequency domain sometimes also called a Cyclic Shift (CS) as the phase shift in the frequency domain correspond to a cyclic shift in the time domain, and an Orthogonal Cover Code (OCC) may be applied in time domain over the slots. Even though CS is effective in multiplexing SRSs assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering UEs employ different base sequences.

One example of base sequences used to generate SRS is Zadoff-Chu sequences. Zadoff-Chu sequences have the advantage that they exhibit constant power in time and frequency, which is desirable as it provides SRS sequences with small power variations in time and frequency, resulting in high power amplifier efficiency and comparable channel estimation quality for all frequency components. A

Zadoff-Chu sequence is given by z_u («) = exp(- j ^π/_Ν un(n + 1)) and z_u (n) = exp(- i

j for odd and even sequence lengthN , respectively, j is the imaginary unit sqrt(-1 ), is the root sequence index, and 0≤« < N-1 yh_e sequence-group and/or the sequence-index define the root sequence index U . Another choice of base sequences are the Frank sequences, given by z_v (n) = exp(- j ²Y_Mvkl) _for n = kM + l, N = M² ,0≤n < N ,

M is an integer, j is the imaginary unit sqrt(-1 ), k and I are integer numbers, and v is the root sequence index. The sequence-group and/or the sequence-index define the root sequence index v. Uplink SRS

SRS are transmitted on the uplink to allow for the RBS to estimate the uplink channel state at different frequencies and time instances as compared to Physical Uplink Shared Channel (PUSCH) transmissions. The channel state estimates can then, for example, be used by the network scheduler to assign resource blocks of instantaneously good quality for PUSCH transmission, also referred to as uplink channel-dependent scheduling. Another alternative is to use the channel state estimates to select different transmission parameters such as the instantaneous data rate and different parameters related to uplink multi-antenna transmission. SRS transmission can also be used for uplink timing estimation, as well as for estimating downlink channel conditions assuming downlink/uplink channel reciprocity. Thus, an SRS is not necessarily transmitted together with any physical channel and if transmitted together with, for example, PUSCH, the SRS may cover a different, typically larger, frequency span. It may also be possible to employ SRS for mobility measurements, e.g. for cell and transmission/reception points association. SRS may also be used for uplink received signal strength measurements, which may e.g. be employed for adjusting the power transmitted by the corresponding UE. There are two types of SRS transmission defined for LTE uplink: periodic SRS transmission, which has been available from the first release of LTE (release 8); and aperiodic SRS transmission, introduced in LTE release 10.

Periodic SRS Transmission

Periodic SRS transmission, also known as Type 0 SRS, from a UE occurs at regular time intervals, from as often as once every 2 ms, i.e. every second subframe, to as infrequently as once every 160 ms, i.e. every 16th frame. When SRS is transmitted in a subframe, it occupies the last symbol of the subframe. As an alternative, in the case of Time Division Duplex (TDD) operation, SRS can also be transmitted within the Uplink Pilot Time Slot (UpPTS). In the frequency domain, SRS transmissions should cover the frequency band that is of interest for the scheduler. This can be achieved in two ways:

1 . By means of a wideband SRS transmission that allows for sounding of the entire frequency band of interest with a single SRS transmission.

2. By means of a more narrowband SRS transmission and hopping in the frequency domain in such a way that a sequence of SRS transmissions jointly covers the frequency band of interest.

The structure of SRS is similar to that of uplink demodulation reference signals. More specifically, an SRS is defined as a frequency-domain reference-signal sequence derived as a cyclic extension of prime-length Zadoff-Chu sequences. However, in the case of SRS, the reference-signal sequence is mapped to every second subcarrier, creating a comb-like spectrum. Taking into account that a resource block covers twelve subcarriers and that the bandwidth of the SRS transmission is always a multiple of four resource blocks, the lengths of the reference-signal sequences for SRS are thus always a multiple of 6^*4=24 subcarriers. The reference-signal sequence to use for SRS transmission within a cell is taken from the same sequence group as the demodulation reference signals used for channel estimation for Physical Uplink Control Channels (PUCCH). Similar to demodulation reference signals different phase rotations, typically referred to as cyclic shifts, can be used to generate different SRS that are orthogonal to each other. By assigning different phase rotations or cyclic shifts to different UEs, multiple SRS can thus be transmitted in parallel in the same subframe. However, it is then required that the SRSs all span the same frequency band. Another way to allow for SRS to be simultaneously transmitted from different terminals is to rely on the fact that each SRS only occupies every second subcarrier. Thus, SRS transmissions from two UEs can be frequency multiplexed by assigning them to different frequency shifts or combs. In contrast to the multiplexing of SRS transmission by means of different cyclic shifts described above, frequency multiplexing of SRS transmissions does not require the transmissions to cover identical frequency bands and is also less sensitive to differences in received signal strength.

To summarize, the following set of parameters defines the characteristics of an SRS transmission: · SRS transmission bandwidth - that is, the bandwidth covered by a single SRS transmission.

• Hopping bandwidth - that is, the frequency band over which the SRS transmission is frequency hopping.

• Frequency-domain position - that is, the starting point of the SRS transmission in the frequency domain.

• Transmission comb - that is, transmission on every n:th subcarrier. In the case of a transmission comb corresponding to transmission on every second subcarrier, the transmission comb may be defined as an odd or an even transmission comb. · Phase rotation or equivalently cyclic shift of the reference-signal sequence.

• SRS transmission time-domain period (from 2 to 160 ms) and subframe offset.

• SRS sequence A UE that is to transmit SRS is configured with these parameters by means of higher layer Radio Resource Control (RRC) signaling. In addition, all UEs within a cell should be informed in what subframes SRS may be transmitted within the cell. This is to assure that the SRS symbol should not be used for PUSCH transmission within these subframes by any UE. Aperiodic SRS Transmission

In contrast to periodic SRS, aperiodic SRS, also known as Type 1 SRS, are one- shot transmissions triggered by signaling on Physical Downlink Control Channel (PDCCH) as part of the scheduling grant or scheduling assignment. The frequency-domain structure of an aperiodic SRS transmission is identical to that of periodic SRS. Also, in the same way as for periodic SRS transmission, aperiodic SRS are transmitted within the last symbol of a subframe. Furthermore, the time instants when aperiodic SRS may be transmitted are configured per UE using higher-layer signaling. The frequency-domain parameters for aperiodic SRS such as the bandwidth, and the transmission comb, are configured by higher-layer RRC signaling. However, no SRS transmission will actually be carried out until the UE is explicitly triggered to do so by an explicit SRS trigger on PDCCH. When such a trigger is received, a single SRS is transmitted in the next available aperiodic SRS instant configured for the UE using the configured frequency-domain parameters. Additional SRS transmissions may then be carried out if additional triggers are received. Three different parameter sets can be configured for aperiodic SRS, for example differing in the frequency position of the SRS transmission and/or the transmission comb. Information on what parameters to use when the SRS is actually transmitted is included in the PDCCH information, which consists of two bits, three combinations of which indicate the specific SRS parameter set. The fourth combination simply indicates that no SRS should be transmitted.

Multi-antenna mapping In case the UE is provided with multiple transmit antennas to be used e.g. for Multiple Input Multiple Output (MIMO), the LTE specifications enable functionalities for transmitting SRS on only one antenna port or for transmitting SRS on all the transmit antenna ports. Sounding of a single transmit antenna or antenna port is useful in case the UE is not exploiting its MIMO capabilities, e.g., for uplink transmission mode 1 , while sounding of the multiple antenna ports is necessary to enable uplink spatial link adaptation, e.g., for transmission mode 2. The uplink transmission modes are specified in 3GPP TS 36.213 V1 1 .0.0 (2012- 09) section 8.0. If MIMO sounding is enabled, each transmit antenna port is assigned to a different cyclic shift. Additionally, different antenna ports for the same UE may be assigned to the same comb or to different combs, in order to enhance orthogonality among antenna ports as compared to orthogonality based on cyclic shift only.

However, these existing antenna mapping solutions developed for sounding and link adaptation are not optimal when transmitting reference signals for RSRP measurements. There is therefore a need for an improved multi-antenna mapping solution. SUMMARY

Sounding each antenna port individually on orthogonal resources, as done for SRS, is a waste of reference signal capacity when the reference signal is used for RSRP measurements. This is due to that RSRP is a function of the transmit power and average attenuation for a link, and not a function of the index of the transmit or receive antenna port over a link.

Additionally, RSRP measurements may be challenging in certain scenarios, especially at low signal to noise ratios. The current reference signal definition for SRS is based on the assumption that independent estimates are performed for each transmit antenna port. However, a better RSRP estimation may be achieved by estimating a common RSRP value for all transmit antenna ports in a single estimation step.

Furthermore, RSRP are average measurements based on long term channel properties and an average transmitted power. Therefore, estimating RSRP based on one of the antennas may give unnecessarily biased estimates if it is assumed that the channel realizations for the transmit antennas are independent or at least not fully correlated, and that there are inaccuracies in the output power of the amplifiers.

It is therefore an object to address some of the problems outlined above, and to provide improved mapping configurations of reference signals to multiple transmit antenna ports of a wireless device. This object and others are achieved by the methods, the wireless device, and the radio network node according to the independent claims, and by the embodiments according to the dependent claims.

In accordance with a first aspect of embodiments, a method for transmitting reference signals on multiple transmit antenna ports of a wireless device is provided. The method is performed in the wireless device of a wireless communication system. The method comprises receiving, from a radio network node, a configuration for transmission of reference signals. The configuration defines a mapping of a reference signal to each of the multiple transmit antenna ports. The reference signals are generated from a respective different base sequence. The method also comprises transmitting the reference signals on the multiple transmit antenna ports in accordance with the received configuration.

In accordance with a second aspect of embodiments, a method for configuring reference signals for transmission on multiple transmit antenna ports of a wireless device is provided. The method is performed in a radio network node of a wireless communication system. The method comprises determining a configuration for transmission of reference signals by the wireless device. The configuration defines a mapping of a reference signal to each of the multiple transmit antenna ports. The reference signals are generated from a respective different base sequence. The method also comprises sending the determined configuration to the wireless device, for configuring the wireless device to transmit the reference signals on the multiple transmit antenna ports in accordance with the determined configuration.

In accordance with a third aspect of embodiments, a wireless device of a wireless communication system is provided. The wireless device is adapted to transmit reference signals on multiple transmit antenna ports. The wireless device comprises a receiver adapted to receive, from a radio network node, a configuration for transmission of reference signals. The configuration defines a mapping of a reference signal to each of the multiple transmit antenna ports. The reference signals are generated from a respective different base sequence. The wireless device also comprises a transmitter adapted to transmit the reference signals on the multiple transmit antenna ports in accordance with the received configuration.

In accordance with a fourth aspect of embodiments, a radio network node of a wireless communication system is provided. The radio network node is adapted to configure reference signals for transmission on multiple transmit antenna ports of a wireless device. The radio network node comprises a processing circuit adapted to determine a configuration for transmission of reference signals by the wireless device. The configuration defines a mapping of a reference signal to each of the multiple transmit antenna ports. The reference signals are generated from a respective different base sequence. The radio network node further comprises a communication unit adapted to send the determined configuration to the wireless device, for configuring the wireless device to transmit the reference signals on the multiple transmit antenna ports in accordance with the determined configuration.

An advantage of embodiments is that improved RSRP estimations are enabled as compared to uplink measurements based on legacy SRS and SRS antenna port mapping, as the measurements of the reference signals transmitted on the multiple antenna ports may be combined to one higher quality RSRP measurement for a UE.

A further advantage of embodiments is that the estimation complexity is reduced, as an averaged RSRP measurement value over the multiple transmit antenna ports may be estimated in a single estimation step.

Other objects, advantages and features of embodiments will be explained in the following detailed description when considered in conjunction with the accompanying drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of a radio access network in LTE.

Figure 2 is a schematic illustration of a CoMP radio access network.

Figure 3 is a flowchart illustrating the method in the radio network node according to embodiments. Figure 4 is a flowchart illustrating the method in the wireless device according to embodiments.

Figure 5 is a block diagram schematically illustrating the wireless device and the radio network node according to embodiments.

DETAILED DESCRIPTION In the following, different aspects will be described in more detail with references to certain embodiments of the invention and to accompanying drawings. For purposes of explanation and not limitation, specific details are set forth, such as particular scenarios and techniques, in order to provide a thorough understanding of the different embodiments. However, other embodiments that depart from these specific details may also exist. Moreover, those skilled in the art will appreciate that the functions and means explained herein below may be implemented using software functioning in conjunction with a programmed microprocessor or general purpose computer, and/or using an application specific integrated circuit (ASIC). It will also be appreciated that while embodiments of the invention are primarily described in the form of methods, devices, and nodes, they may also be embodied in a computer program product as well as in a system comprising a computer processor and a memory coupled to the processor, wherein the memory is encoded with one or more programs that may perform the functions disclosed herein.

Embodiments are hereinafter described in a non-limiting general context in relation to an example scenario in E-UTRAN, where RSRP is measured by the network based on Reference Signals (RSs) transmitted by a wireless device on multiple antenna ports. However, it should be noted that the embodiments may be applied to any radio access network technology supporting measurements based on RSs. Furthermore, the RSRP measurement is only one example of a measurement that may benefit of RSs mapped to the multiple antenna ports according to embodiments of the invention. Other example measurements are RSRQ, i.e. , measurements of channel quality taking the interference into account. The wireless device may be any kind of wireless terminal with multiple antenna ports, such as a UE, a portable computer, or a smartphone. Hereinafter the wireless device will be exemplified by a MIMO UE.

The mapping of uplink RSs to the antenna ports of a MIMO UE may be performed in several ways. Some conventional examples comprise transmitting RSs only from a single antenna port or transmitting orthogonal RS sequences from all the different antenna ports. In the case of SRS transmissions in LTE, the SRSs from the different antenna ports are made orthogonal by assigning different cyclic shifts or a combination of cyclic shifts and Orthogonal Cover Codes (OCC) to a same base sequence to each port.

However, cyclic shifts are not provided for some RSs. One example of a RS which may not use cyclic shifts is a RS type similar to SRS, but with some important differences that make it possible to successfully receive and measure the RS at several different points, such as in a CoMP network deployment. The main difference of this RS, hereinafter referred to as the RSRP-RS, compared to SRS, is that the comb factor is significantly larger than two. Such a large comb factor enables frequency division multiplexing of many UEs. At the same time a large frequency span is provided, which enables frequency-domain averaging of the received power. Cyclic shift multiplexing of UEs is not provided. The advantage of using a large comb is that each UE transmits the RSRP-RS only on a few subcarriers, which means that the maximum transmit power per subcarrier is relatively high. This enables RSRP measurements of the RSRP-RS also at points that are farther away. The power control of the RSRP-RS may be adjusted to make sure that the signal may be received at all desired points.

As already mentioned above, RSRP measurements may be challenging at low signal to noise ratios. The current RS definition for SRS is based on the assumption that independent estimates are performed per each transmit antenna port. However, a better RSRP estimation and reduced estimation complexity may be achieved by estimating a common RSRP value for multiple transmit antenna ports in a single estimation step. RSRP is typically assumed to be constant for all transmission ports. However, RSRP estimations may be affected by instantaneous fading conditions. Therefore it may be beneficial to combine measurements based on different fading realizations. It is also observed that uplink RSRP coverage resolution may be limited by the transmission power of the UE. Depending on the UE implementation, it is possible that the UE is able to produce a maximum transmission power only when multiple transmission ports and thus PAs are transmitting simultaneously.

The sounding of each antenna port individually on orthogonal RS resources is thus a waste of RS capacity in the case of RSRP measurements, as the RSs do not need to and should preferably not be measured individually. Therefore, antenna mapping configurations for sounding RSs that are optimized for RSRP measurements are needed. Examples of situations when the methods according to embodiments of the invention are especially useful comprise the situation when RSs are used for RSRP measurements and the situation when orthogonality of the RSs through cyclic shifts is not possible to achieve, as e.g. for RSRP-RS described previously.

The problem of non-optimal RSRP measurements when using conventional SRS antenna mapping is addressed by a solution where a new configuration defining a mapping of a RS to each of multiple transmitting antenna ports of a MIMO UE is introduced. Each RS is generated from a base sequence which is different from any of the other RSs' base sequences. As an example, different Zadoff-Chu sequences may be used. As the RSs need not be orthogonal, they may be generated using different base sequences instead of using a same base sequence with different cyclic shifts as for SRS. RSs generated using different base sequences will provide quasi-orthogonal RSs.

A radio network node such as an eNodeB may thus determine the configuration to use for the antenna mapping of RSs, and send the configuration to the MIMO UE. The MIMO UE receives the configuration and applies the configuration when transmitting the RSs on its multiple antenna ports. The new mapping configurations allow for improved RSRP measurements based on the RSs.

In one example embodiment, a MIMO UE is configured to transmit from multiple transmit antenna ports on a same set of subcarriers, e.g. on a same comb. The signal associated to each antenna port is characterized by a different RS in terms of a different scrambling sequence. Such a scrambling sequence correspond to a base sequence used e.g. for generating SRSs. One example is a Zadoff-Chu sequence. Each RS is therefore characterized by a specific base sequence index. The receiver at the receiving point, e.g. at one of several coordinated multi-points, sees a composite channel given by the superposition of the channels and the different base sequences. An estimation of the RSRP based on a measurement of the RS is therefore less subject to bias introduced by coherent combination of the channels for the different antenna ports. This is due to that the combining weights for the antenna ports are independently scrambled for each used subcarrier. The transmission power may be split among the transmission antenna ports and the corresponding PAs, easing the transmission power requirements of each PA. Typically, RSRP is almost constant for all transmission ports but the measurements may be affected by instantaneous fading conditions. A further advantage of embodiments of the invention is that the received signal results of the combination of different antennas, which may each be associated with a different instantaneous fading realization. The receiver is thus able to estimate a better average of the signals. Consequently, the RSRP measurement for a MIMO UE will give a higher quality measurement when using the suggested antenna mapping configurations compared to using conventional SRS antenna mapping.

According to another embodiment, the MIMO UE may transmit the RSs from the different transmit antenna ports on different sets of subcarriers, e.g. on different combs. This is an advantageous alternative for UEs using RS with a short sequence length, as it allows improving the inter-antenna interference suppression.

According to still another embodiment, a combination of using a same set of subcarriers and different sets of subcarriers for the transmission on the different antenna ports is used. The interference suppression gain is low for a short sequence length, thus making it advantageous to assign different sets of subcarriers to different antenna ports such that the inter-antenna interference is reduced. In order to avoid that more than one sequence is needed for each MIMO UE, which would reduce the multiplexing capacity, UEs with a longer sequence length may be configured to use a same set of subcarriers for multiple antenna ports. In this way the multiplexing capacity is maximized while still achieving sufficient interference suppression.

In an example embodiment, UEs using a wide enough RS with regards to bandwidth which implies a long enough sequence, may be configured to transmit on a same comb and with different RS base sequences on its transmit antenna ports, whereas UEs with an insufficient RS sequence length transmit on different combs and with different RS base sequences. The criterion for deciding whether to configure the UE to use a same comb or different combs is thus the sequence length of the RS. For an RSRP-RS with a comb factor of 2, i.e. the RSRP-RS is transmitted on every second subcarrier, the sequence is probably long enough and the same comb may be used on multiple antenna ports. However, if the subcarrier spacing considering the comb factor is in the same order of magnitude or even larger than the coherence bandwidth of the associated channel, the inter- antenna interference becomes large and different combs should be used on the different antenna ports to improve the interference suppression. The network may thus make the best choice between using different combs or a same comb for a UE, based on the sequence length. Thereby all UEs may transmit close to orthogonal reference signals from each antenna port without adding severe overhead or being limited by the RS multiplexing capacity. One aspect common to all embodiments of the invention is that the specific mapping of sounding RS to antennas, or equivalently to antenna ports, is a function of the purpose of the RS. In particular, RSRP-RS are mapped to the antenna ports in a different way as compared to SRS. RSRP-RS and SRS may be transmitted at specific different time instances, possibly with a periodic pattern. Figure 3 is a flowchart illustrating a method for configuring RSs for transmission on multiple transmit antenna ports of a wireless device. The method is performed in a radio network node of a wireless communication system. The radio network node may e.g. be an eNodeB. The method comprises:

- 310: Determining a configuration for transmission of RSs by the wireless device. The configuration defines a mapping of a RS to each of the multiple transmit antenna ports. The RSs are generated from a respective different base sequence. The configuration may also define that the RSs are transmitted on a transmission comb.

- 320: Sending the determined configuration to the wireless device, for configuring the wireless device to transmit the RSs on the multiple transmit antenna ports in accordance with the determined configuration. The method in the wireless device is described below with reference to Figure 4.

In a first embodiment, the configuration may define that the RSs are all transmitted on a same set of subcarriers. If the RSs are transmitted on a transmission comb, they will thus be transmitted on the same transmission comb on multiple antenna ports of the wireless device. This would be a good alternative when the RS sequence length is long enough, e.g. for a transmission comb factor of two where the RSs are transmitted on every second subcarrier.

In a second embodiment, the configuration may define that the RSs are transmitted on a respective different set of subcarriers. If the RSs are transmitted on a transmission comb, they will thus be transmitted on different transmission combs on the multiple antenna ports. Transmitting on different transmission combs on the antenna ports would be a good alternative for UEs using RS with a short sequence length, as it allows improving the inter-antenna interference suppression.

In a third embodiment, a combination of the first and the second embodiment is used, and the configuration defines:

- that the RSs are all transmitted on a same set of subcarriers if a sequence length of the RSs exceeds a threshold, and - that the RSs are transmitted on a respective different set of subcarriers otherwise.

Different antenna mapping configurations will thus be used depending on the length of the RS sequence, to allow for a maximized multiplexing capacity while still achieving sufficient interference suppression. In an alternative embodiment, which may be implemented independently of the other embodiments described above, a same base sequence is used when generating the RSs for the multiple antenna ports of a UE, and the UE is configured to transmit the RSs on a respective different set of subcarriers on the different antenna ports, typically on different transmission combs. No cyclic shift of the base sequence is needed as full orthogonality is obtained between the different combs.

In one embodiment, which may be combined with any of the three embodiments described above, the method further comprises:

- 330: Retrieving a result from a measurement of the RSs transmitted by the wireless device on the multiple transmit antenna ports. The measurement may be a received power measurement, e.g. an RSRP measurement. The result is retrieved from at least one receiving point of the radio network node. If the method is performed in en eNodeB of a CoMP network with more than one receiving point, the RSRP may be retireved from a number of receiving points. As the RSRP measurements are done on RSs transmitted by the wireless device in accordance with the determined configuration of embodiments of the invention, the RSRP measurements may be of higher quality than if they were done on conventional SRS, as described above.

Figure 4 is a flowchart illustrating a method for transmitting RSs on multiple transmit antenna ports of a wireless device. The method is performed in the wireless device of a wireless communication system. The wireless device may e.g. be a MIMO UE. The method comprises:

- 410: Receiving, from a radio network node, a configuration for transmission of RSs. The configuration defines a mapping of a RS to each of the multiple transmit antenna ports. The RSs are generated from a respective different base sequence. The configuration may also define that the RSs are transmitted on a transmission comb.

- 420: Transmitting the RSs on the multiple transmit antenna ports in accordance with the received configuration.

In accordance with the first and third embodiment, the configuration may define that the RSs are all transmitted on a same set of subcarriers. If the RSs are transmitted on a transmission comb, they will thus be transmitted on the same transmission comb on multiple antenna ports. This would be a good alternative when the RS sequence length is long enough, e.g. for a transmission comb factor of two where the RSs are transmitted on every second subcarrier. In accordance with the second or third embodiment, the configuration may define that the RSs are transmitted on a respective different set of subcarriers. If the RSs are transmitted on a transmission comb, they will thus be transmitted on different transmission combs on the multiple antenna ports. Transmitting on different transmission combs on the antenna ports would be a good alternative for UEs using RS with a short sequence length, as it allows improving the inter-antenna interference suppression.

A wireless device 500 and a radio network node 550 of a wireless communication system are schematically illustrated in the block diagram in Figure 5. The radio network node 550 is adapted to configure RSs for transmission on multiple transmit antenna ports of the wireless device. The antenna ports are connected to their respective antennas, 508a and 508b. The radio network node comprises a processing circuit 551 adapted to determine a configuration for transmission of RSs by the wireless device. The configuration defines a mapping of a RS to each of the multiple transmit antenna ports. The RSs are generated from a respective different base sequence. The radio network node also comprises a communication unit 552 adapted to send the determined configuration to the wireless device, for configuring the wireless device to transmit the RSs on the multiple transmit antenna ports in accordance with the determined configuration. The communication unit may e.g. be the transmitter of an eNodeB. In one embodiment, the processing circuit 551 may be adapted to determine the configuration, defining also that the RSs are transmitted on a transmission comb.

In accordance with the first embodiment described above, the processing circuit 551 may be adapted to determine the configuration, defining that the RSs are all transmitted on a same set of subcarriers. In accordance with the second embodiment described above, the processing circuit 551 may be adapted to determine the configuration, defining that the RSs are transmitted on a respective different set of subcarriers.

In accordance with the third embodiment described above, the processing circuit 5 551 may be adapted to determine the configuration defining: that the RSs are all transmitted on a same set of subcarriers if a sequence length of the RSs exceeds a threshold, and that the RSs are transmitted on a respective different set of subcarriers otherwise.

In one embodiment which may be combined with any of the embodiments0 described above, the processing unit 551 may be further adapted to retrieve a result from a measurement of the RSs transmitted by the wireless device on the multiple transmit antenna ports. The measurement may be a received power measurement such as an RSRP measurement. The result may be retrieved from at least one receiving point 555a, 555b, of the radio network node. 5 The wireless device 500 is adapted to transmit RSs on multiple transmit antenna ports. The wireless device comprises a receiver 501 adapted to receive, from a radio network node, a configuration for transmission of RSs. The configuration defines a mapping of a RS to each of the multiple transmit antenna ports. The RSs are generated from a respective different base sequence. The wireless device0 also comprises a transmitter 502 adapted to transmit the RSs on the multiple transmit antenna ports in accordance with the received configuration. Furthermore, the wireless device comprises a processing circuit 503 for the processing needed in relation to the RS transmission.

According to one embodiment, the receiver 501 may be adapted to receive the5 configuration defining that the RSs are transmitted on a transmission comb.

According to the first and the third embodiment, the receiver 501 may be adapted to receive the configuration defining that the RSs are all transmitted on a same set of subcarriers.

According to the second and the third embodiment, the receiver 501 may be adapted to receive the configuration defining that the RSs are transmitted on a respective different set of subcarriers.

In an alternative way to describe the embodiments in Figure 5, the radio network node 550 comprises a Central Processing Unit (CPU) which may be a single unit or a plurality of units. Furthermore, the radio network node 550 comprises at least one computer program product (CPP) in the form of a non-volatile memory, e.g. an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive. The CPP comprises a computer program, which in turn comprises code means which when run on the radio network node 550 causes the CPU to perform steps of the procedure described earlier in conjunction with Figure 3. In other words, when said code means are run on the CPU, they correspond to the processing circuit 551 in the radio network node 550 of Figure 5

The above mentioned and described embodiments are only given as examples and should not be limiting. Other solutions, uses, objectives, and functions within the scope of the accompanying patent claims may be possible.

Claims

1 . A method for configuring reference signals for transmission on multiple transmit antenna ports of a wireless device, wherein the method is performed in a radio network node of a wireless communication system, the method comprising:

- determining (310) a configuration for transmission of reference signals by the wireless device, the configuration defining a mapping of a reference signal to each of the multiple transmit antenna ports, wherein the reference signals are generated from a respective different base sequence, and - sending (320) the determined configuration to the wireless device, for configuring the wireless device to transmit the reference signals on the multiple transmit antenna ports in accordance with the determined configuration.

2. The method according to claim 1 , wherein the configuration defines that the reference signals are transmitted on a transmission comb.

3. The method according to any of the preceding claims, wherein the configuration defines that the reference signals are all transmitted on a same set of subcarriers.

4. The method according to any of claims 1 -2, wherein the configuration defines that the reference signals are transmitted on a respective different set of subcarriers.

5. The method according to any of claims 1 -2, wherein the configuration defines: that the reference signals are all transmitted on a same set of subcarriers if a sequence length of the reference signals exceeds a threshold, and that the reference signals are transmitted on a respective different set of subcarriers otherwise.

6. The method according to any of the preceding claims, further comprising: - retrieving (330) a result from a measurement of the reference signals transmitted by the wireless device on the multiple transmit antenna ports, the result being retrieved from at least one receiving point of the radio network node.

5

7. The method according to claim 6, wherein the measurement is a received power measurement.

8. A method for transmitting reference signals on multiple transmit antenna ports 10 of a wireless device, wherein the method is performed in the wireless device of a wireless communication system, the method comprising:

- receiving (410), from a radio network node, a configuration for transmission of reference signals, the configuration defining a mapping of a reference signal to each of the multiple transmit antenna ports, wherein the reference

15 signals are generated from a respective different base sequence, and

- transmitting (420) the reference signals on the multiple transmit antenna ports in accordance with the received configuration.

9. The method according to claim 8, wherein the configuration defines that the 20 reference signals are transmitted on a transmission comb.

10. The method according to any of claims 8-9, wherein the configuration defines that the reference signals are all transmitted on a same set of subcarriers.

25 1 1 . The method according to any of claims 8-9, wherein the configuration defines that the reference signals are transmitted on a respective different set of subcarriers.

12. A radio network node (550) of a wireless communication system adapted to 30 configure reference signals for transmission on multiple transmit antenna ports of a wireless device, the radio network node comprising: - a processing circuit (551 ) adapted to determine a configuration for transmission of reference signals by the wireless device, the configuration defining a mapping of a reference signal to each of the multiple transmit antenna ports, wherein the reference signals are generated from a

5 respective different base sequence, and

- a communication unit (552) adapted to send the determined configuration to the wireless device, for configuring the wireless device to transmit the reference signals on the multiple transmit antenna ports in accordance with the determined configuration.

10

13. The radio network node according to claim 12, wherein the processing circuit (551 ) is adapted to determine the configuration defining that the reference signals are transmitted on a transmission comb.

15 14. The radio network node according to any of claims 12-13, wherein the processing circuit (551 ) is adapted to determine the configuration defining that the reference signals are all transmitted on a same set of subcarriers.

15. The radio network node according to any of claims 12-13, wherein the 20 processing circuit (551 ) is adapted to determine the configuration defining that the reference signals are transmitted on a respective different set of subcarriers.

16. The radio network node according to any of claims 12-13, wherein the 25 processing circuit (551 ) is adapted to determine the configuration defining: that the reference signals are all transmitted on a same set of subcarriers if a sequence length of the reference signals exceeds a threshold, and that the reference signals are transmitted on a respective different set of subcarriers otherwise.

30

17. The radio network node according to any of claims 12-16, wherein the processing unit (551 ) is further adapted to retrieve a result from a measurement of the reference signals transmitted by the wireless device on the multiple transmit antenna ports, the result being retrieved from at least one receiving point (555a, 555b) of the radio network node.

18. The radio network node according to claim 17, wherein the measurement is a received power measurement.

19. A wireless device (500) of a wireless communication system adapted to transmit reference signals on multiple transmit antenna ports, the wireless device comprising:

- a receiver (501 ) adapted to receive, from a radio network node, a configuration for transmission of reference signals, the configuration defining a mapping of a reference signal to each of the multiple transmit antenna ports, wherein the reference signals are generated from a respective different base sequence, and

- a transmitter (502) adapted to transmit the reference signals on the multiple transmit antenna ports in accordance with the received configuration.

20. The wireless device according to claim 19, wherein the receiver (501 ) is adapted to receive the configuration defining that the reference signals are transmitted on a transmission comb.

21 . The wireless device according to any of claims 19-20, wherein the receiver (501 ) is adapted to receive the configuration defining that the reference signals are all transmitted on a same set of subcarriers.

22. The wireless device according to any of claims 19-20, wherein the receiver (501 ) is adapted to receive the configuration defining that the reference signals are transmitted on a respective different set of subcarriers.