This application is a 35 U.S.C. § 371 national phase filing of International Application No. PCT/IB2017/055254, filed Aug. 31, 2017, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to transmit antenna selection for uplink Sounding Reference Signal (SRS) transmission.

BACKGROUND

The next generation mobile wireless communication system (Fifth Generation (5G)), or New Radio (NR), will support a diverse set of use cases and a diverse set of deployment scenarios. The later includes deployment at both low frequencies (100s of Megahertz (MHz)), similar to Long Term Evolution (LTE) today, and very high frequencies (mm waves in the tens of GHz).

Similar to LTE, NR will use Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (i.e. from a network node, or NR Base Station (gNB), to a User Equipment device (UE)). It is also referred to as Cyclic Prefix OFDM (CP-OFDM). In the uplink (i.e. from UE to gNB), both CP-OFDM and Discrete Fourier Transform (DFT) Spread OFDM (DFT-S-OFDM) will be supported. DFT-S-OFDM is also referred to as Single Carrier Frequency Division Multiple Access (SC-FDMA) in LTE.

The basic NR physical resource can thus be seen as a time-frequency grid similar to the one in LTE as illustrated inFIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. Although a subcarrier spacing of Δf=15 kHz is shown inFIG. 1, different subcarrier spacing values are supported in NR. The supported subcarrier spacing values (also referred to as different numerologies) in NR are given by Δf=(15×2α) kHz where a is a non-negative integer.

Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. For NR, a resource block is also 12 subcarriers in frequency, but for further study in time domain.

In the time domain, downlink and uplink transmissions in NR will be organized into equally-sized subframes similar to LTE as shown inFIG. 2, which illustrates the LTE time-domain structure with 15 Kilohertz (kHz) subcarrier spacing. In NR, the subframe length is 1 ms for all supported numerologies. A subframe is further divided into a number of slots of equal duration. There are two slots per subframe for 15 kHz subcarrier spacing as in LTE. For subcarrier spacing greater than 15 kHz, there are more than 2 slots per subframe. For convenience, subframe is used throughout the following description. However, it is understood that a subframe may be further divided into a number of slots and the discussions based on subframes are equally applicable to slots.

Downlink transmissions are dynamically scheduled, i.e., in each subframe the gNB transmits Downlink Control Information (DCI) about which UE data is to be transmitted to and which resource blocks in the current downlink subframe the data is transmitted on. This control signaling is typically transmitted in the first one or two OFDM symbols in each subframe in NR. The control information is carried on Physical Downlink Control Channel (PDCCH) and data is carried on Physical Downlink Shared Channel (PDSCH). A UE first detects and decodes PDCCH and if a PDCCH is decoded successfully, it then decodes the corresponding PDSCH based on the decoded control information in the PDCCH.

Uplink data transmissions are also dynamically scheduled using PDCCH. Similar to downlink, a UE first decodes uplink grants in PDCCH and then transmits data over the Physical Uplink Shared Channel (PUSCH) based on the decoded control information in the uplink grant such as modulation order, coding rate, uplink resource allocation, etc.

In addition to PUSCH, Physical Uplink Control Channel (PUCCH) is also supported in NR to carry Uplink Control Information (UCI) such as Hybrid Automatic Repeat Request (HARQ) related Acknowledgement (ACK), Negative Acknowledgement (NACK), or Channel State Information (CSI) feedback.

Although many details of NR PUCCH are still to be determined, it is envisioned that, similar to LTE, PUCCH resources will be pre-allocated in a cell and shared by all UEs.

In LTE, a number of Reference Signals (RS) are defined for downlink channel estimation. By measuring on a RS, a UE can estimate the effective channel the RS is traversing including the downlink radio propagation channel and both transmit and receive antenna gains. In more mathematical rigor, this implies that if a known RS signal xj(j=1,2, . . . , Ntx) is transmitted on the jth transmit antenna port of a base station at a time-frequency resource element, the received signal yi(i=1,2, . . . , Nrx) on the ith receive antenna port of a UE at the same time-frequency resource element can be expressed as
yi=hi,jxj+ni
where hi,jis the effective channel between the jth transmit antenna port and the ith receive antenna port at the time-frequency resource element, niis the receiver noise associated with the ith receive antenna port, Ntxis the number of transmit antenna ports at the base station, and Nrxis the number of receive antenna ports at the UE.

A UE can estimate the Nrx×Ntxeffective channel matrix H (H(i,j)=hi,j) for each time-frequency element over which RS is transmitted. The effective channel can thus be estimated over a Physical Resource Block (PRB), a subband (e.g. a few PRBs), or the whole system bandwidth (i.e. wideband).

For CSI feedback purpose, the channel rank, precoding matrix, and channel quality can be estimated based on the estimated channel. This is typically achieved by using a predesigned codebook for each rank, with each codeword in the codebook being a precoding matrix candidate. A UE searches through the codebook to find a rank, a codeword associated with the rank, and channel quality associated with the rank and precoding matrix to best match the effective channel. The rank, the precoding matrix and the channel quality estimation can be done per subband and/or wideband and reported in the form of a Rank Indicator (RI), a Precoding Matrix Indicator (PMI), and a Channel Quality Indicator (CQI).

The RSs for downlink channel estimation purpose includes:Cell Specific Reference Signal (CRS): CRSs are transmitted in every subframe and over the entire frequency band. Up to four CRS ports are supported in LTE, each associated with one antenna port. CRSs are transmitted on a grid of Resource Elements (REs) in each PRB. An example of the CRS RE locations in a PRB is shown inFIG. 3. The frequency locations of the CRS REs are cell dependent and may be shifted for cells with different physical cell Identifiers (IDs).Channel State Information Reference signal (CSI-RS): In LTE Release-10, a new reference signal was introduced for CSI estimation. The CSI-RS provides several advantages over CRS. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density per PRB as CRS. Therefore, the overhead of the CSI-RS is substantially less. Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements, i.e. which CSI-RS resource to measure on can be configured in a UE specific manner. In addition, up to 32 antenna ports can be supported by CSI-RS. CSI-RS is transmitted over the whole system bandwidth. In the time domain, CSI-RS can be transmitted periodically with preconfigured periodicity and subframe offset. CSI-RS can also be transmitted aperiodically, in which CSI-RS transmission is triggered dynamically by a DCI.Demodulation Reference Signal (DMRS): DMRS is also used for downlink channel estimation but mainly for data demodulations. Unlike CRS, DMRS transmission is UE specific, i.e. it is only transmitted when there is DL data transmission to a UE. There are eight DMRS ports (ports 7 to 15) defined in LTE and up to eight layers (rank=8) of data can be supported. The DMRS port used is dynamically indicated in the associated PDCCH. The DMRS are transmitted on certain fixed REs in a PRB. The RE pattern for port 7 and port 8 are shown inFIG. 3. Ports 7 and 8 occupy the same REs in a PRB and are multiplexed with orthogonal codes. DMRS is precoded with the same precoder as the data.
There is one reference signal transmitted per antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed.

Similar to LTE, CSI-RS and DMRS will also be supported in NR for downlink channel estimation. In addition to periodic and aperiodic CSI-RS, semi-persistent CSI-RS transmission will also be supported in NR, in which CSI-RS transmission is dynamically activated and deactivated.

The Sounding Reference Symbol (SRS) is used for uplink channel quality measurements for frequency-selective scheduling and link adaption. SRS is also used for uplink timing estimation. In TDD systems, SRS can also be used in downlink channel estimation since the same carrier frequency is used in both downlink and uplink.

In LTE Frequency Division Duplexing (FDD), SRS is only transmitted by a UE in the last OFDM symbol of a subframe configured for SRS transmission for the UE. In LTE Time Division Duplexing (TDD), SRS is also transmitted in OFDM symbols in Uplink Pilot Time Slot (UpPTS) of special subframes. The location of SRS in a PRB in a normal SRS subframe is shown inFIG. 4, where

DMRS are used for channel estimation in PUSCH demodulation.

The subframes in which SRS transmission can occur in a cell is referred to as cell specific SRS subframes. A UE can be configured to transmit SRS on a subset of the cell specific SRS subframes. An example is shown inFIG. 5, which is an example of cell specific and UE specific SRS subframes.

A UE can be configured with different SRS bandwidths. In general, two kinds of sounding bandwidths are supported, one is wideband and the other is narrowband. In case of wideband, channel measurement over the full system bandwidth can be performed in a single subframe. While in narrowband sounding, only part of the full system bandwidth can be measured in a subframe, thus multiple SRS subframes are needed for a full bandwidth channel measurement. Frequency hopping is supported for narrowband SRS so that different part of the frequency band can be measured in different subframes.

Furthermore, two types of sounding are supported, i.e. periodic (also referred to as type 0) and aperiodic (also referred to as type 1). In case of periodic SRS, a UE transmit SRS periodically at certain configured SRS subframes. In case of aperiodic SRS, a UE transmits SRS only when it is requested by Evolved or Enhanced Node B (eNB). The SRS subframes for periodic and aperiodic SRS are separately configured for a UE, both are within the cell specific SRS subframes.

The SRS bandwidth for a UE is configurable and is in the multiple of 4 PRBs. The minimum SRS bandwidth is 4 PRBs, which is also referred to as SRS subband. An example is shown inFIG. 6. In particular,FIG. 6is an example of wideband and narrowband SRS with 10 MHz system bandwidth.

In case of narrowband SRS with Frequency Hopping (FH), a SRS is transmitted on different part of the system bandwidth at different SRS subframes. For example, for a 10 MHz system and SRS bandwidth of 4 PRBs, a possible set of locations in the frequency domain for SRS transmission are shown inFIG. 7. In this example, the whole bandwidth can be measured after 12 SRS subframes.

A SRS signal is a phase-shifted Zadoff-Chu sequence. Different UEs can be multiplexed on the same time-frequency resources by assigning different phase shifts, known as Cyclic Shifts (CS). There are 8 cyclic shifts defined. In addition, a SRS signal is only transmitted on half of the subcarriers in the configured SRS bandwidth, either even-numbered or odd-numbered subcarriers, configurable through a parameter called comb. This is also referred to as Interleaved Frequency Division Multiple Access (IFDMA) with a repetition factor of 2. Therefore, up to 16 UEs can be multiplexed on the same SRS bandwidth.

In LTE Release (Rel.) 13, support for 4-comb was also introduced, which means that a SRS signal can be mapped to every 4thsubcarrier, thereby increasing the SRS multiplexing capacity provided that the channel is sufficiently flat so that every 4thsubcarrier is adequate.

UEs with different SRS bandwidths can be multiplexed on a SRS subframe with different comb values. UEs with the same SRS bandwidth can be multiplexed in a SRS subframe with different cyclic shifts.

The UE transmit antenna selection has been a part of the LTE specification since Rel-8. UE informs eNB that it supports transmit antenna selection by setting the ue-TxAntennaSelectionSupported field to TRUE and includes the field in the UE-EUTRA-Capability information element. When eNB wants to enable antenna selection, it sets the ue-TransmitAntennaSelection field to closedLoop and includes the field in the AntennaInfo information element. When eNB wants to disable antenna selection, it should set ue-TransmitAntennaSelection field to NULL in the AntennaInfo information element.

For UEs with two transmit antennas, if SRS transmit Antenna Selection (AS) is supported and enabled for the UE, UE transmits SRS alternating between the two antenna ports if frequency hopping is not enabled.

FIG. 8shows an example of SRS antenna selection with two transmit antenna ports (antenna ports with index 0 and 1). InFIG. 8, subband SRS is transmitted initially from antenna port 0. The next SRS is transmitted over different subband and from another antenna port. Antenna selection allows eNB to estimation DL channels for all antenna ports. However, it takes twice long for eNB to obtain channel estimation for both antenna ports assuming the same interval between two consecutive SRS transmissions. For example, it takes three SRS transmissions to cover the whole bandwidth (seeFIG. 8) without antenna selection. However, it takes six SRS transmissions to cover the whole bandwidth for both antennas with antenna selection. It takes five SRS transmissions to get the channel estimation over the whole bandwidth for a single antenna when antenna selection is enabled.

SRS will also be supported in NR for uplink channel sounding. Similar to LTE, configurable SRS bandwidth is supported. SRS can be configurable with regard to density in frequency domain (e.g., comb levels) and/or in time domain (including multi-symbol SRS transmissions). In addition to full band size, partial band size is also supported in NR, which is smaller than the largest transmission bandwidth supported by the UE. For the full band size, the size is equal to the largest transmission bandwidth supported by the UE. Aperiodic SRS transmission triggered by the network is supported in NR. The triggering is generally done through DCI dynamically. Periodic and semi-persistent NR-SRS transmissions are also supported in NR.

In NR, a SRS resource comprises of a set of REs within a time duration and frequency span and N antenna ports (N≥1). A UE can be configured with K≥1 NR-SRS resources. The maximum value of K is considered to be a UE capability. Out of K≥1 configured NR-SRS resources, for aperiodic transmission, the UE can be configured to transmit a subset of or all K NR-SRS resources. For periodic and semi-persistent transmission, out of K≥1 configured SRS resources, the UE can be configured to transmit K SRS resources.

Antenna selection in NR is expected to be similar to that in LTE.

SUMMARY

Systems and methods for selectively enabling antenna selection at a wireless device for uplink sounding reference signal transmission in a Time Division Duplexing (TDD) wireless communication system are disclosed. Embodiments of a method of operation of a network node are disclosed. In some embodiments, a method of operation of a network node to selectively enable antenna selection at a wireless device for uplink sounding reference signal transmission in a TDD wireless communication system comprises making a determination as to whether to enable or disable antenna selection at a wireless device for transmission of uplink sounding reference signals based on: an estimated speed of movement of the wireless device; both the estimated speed of movement of the wireless device and a Signal to Interference plus Noise Ratio (SINR) for one or more downlink channels to the wireless device; or the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and a rank used for downlink transmission to the wireless device. The method further comprises sending, to the wireless device, an indication that indicates whether the wireless device is to enable or disable antenna selection for transmission of uplink sounding reference signals in accordance with the determination.

In some embodiments, making the determination comprises making the determination as to whether to enable or disable antenna selection at the wireless device for transmission of uplink sounding reference signals based on the estimated speed of movement of the wireless device. In some other embodiments, making the determination comprises making the determination as to whether to enable or disable antenna selection at the wireless device for transmission of uplink sounding reference signals based on the estimated speed of movement of the wireless device and the SINR for the one or more downlink channels to the wireless device. In some other embodiments, making the determination comprises making the determination as to whether to enable or disable antenna selection at the wireless device for transmission of uplink sounding reference signals based on the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and the rank used for downlink transmission to the wireless device.

In some embodiments, making the determination comprises obtaining the estimated speed of movement of the wireless device, determining whether the estimated speed of movement of the wireless device is less than a first speed threshold, and making the determination to enable antenna selection if the estimated speed of movement of the wireless device is less than the first speed threshold.

In some embodiments, making the determination further comprises making the determination to disable antenna selection if the estimated speed of movement of the wireless device is not less than the first speed threshold. In some other embodiments, making the determination further comprises, if the estimated speed of movement of the wireless device is not less than the first speed threshold, determining whether the speed of movement of the wireless device is greater than a second speed threshold that is greater than the first speed threshold, and making the determination to disable antenna selection if the speed of movement of the wireless device is greater than the second speed threshold.

In some embodiments, making the determination further comprises, if the estimated speed of movement of the wireless device is not less than the first speed threshold, determining whether the SINR for the one or more downlink channels to the wireless device is greater than a first SINR threshold and making the determination to disable antenna selection if the speed of movement of the wireless device is not less than the first speed threshold and the SINR for the one or more downlink channels to the wireless device is greater than the first SINR threshold.

In some embodiments, making the determination further comprises, if the estimated speed of movement of the wireless device is not less than the first speed threshold, determining whether the speed of movement of the wireless device is greater than a second speed threshold, determining whether the SINR for the one or more downlink channels to the wireless device is greater than a first SINR threshold, and making the determination to disable antenna selection if the speed of movement of the wireless device is greater than the second speed threshold and the SINR for the one or more downlink channels to the wireless device is greater than the first SINR threshold.

In some embodiments, making the determination further comprises, if the SINR of the one or more downlink channels to the wireless device is not greater than the first SINR threshold, making the determination to enable antenna selection. In some other embodiments, making the determination further comprises determining whether the SINR of the one or more downlink channels to the wireless device is less than a second SINR threshold and making the determination to enable antenna selection if the SINR of the one or more downlink channels to the wireless device is less than the second SINR threshold.

In some embodiments, making the determination further comprises, if the SINR of the one or more downlink channels to the wireless device is not greater than the first SINR threshold, making the determination to enable antenna selection if a reported rank of the wireless device is rank 1 and making the determination to disable antenna selection if the reported rank of the wireless device is greater than rank 1. In some other embodiments, making the determination further comprises determining whether the SINR of the one or more downlink channels to the wireless device is less than a second SINR threshold that is less than the first SINR threshold and, if the SINR of the one or more downlink channels to the wireless device is less than the second SINR threshold, making the determination to enable antenna selection if a reported rank of the wireless device is rank 1 and making the determination to disable antenna selection if the reported rank of the wireless device is greater than rank 1.

In some embodiments, the network node is a radio access node.

Embodiments of a network node are also disclosed. In some embodiments, a network node for selectively enabling antenna selection at a wireless device for uplink sounding reference signal transmission in a TDD wireless communication system comprises at least one processor and memory comprising instructions executable by the at least one processor whereby the network node is operable to make a determination as to whether to enable or disable antenna selection at a wireless device for transmission of uplink sounding reference signals and send, to the wireless device, an indication that indicates whether the wireless device is to enable or disable antenna selection for transmission of uplink sounding reference signals in accordance with the determination. The determination is made based on: an estimated speed of movement of the wireless device; both the estimated speed of movement of the wireless device and a SINR for one or more downlink channels to the wireless device; or the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and a rank used for downlink transmission to the wireless device.

In some embodiments, the network node makes the determination as to whether to enable or disable antenna selection at the wireless device for transmission of uplink sounding reference signals based on the estimated speed of movement of the wireless device. In some other embodiments, the network node makes the determination as to whether to enable or disable antenna selection at the wireless device for transmission of uplink sounding reference signals based on the estimated speed of movement of the wireless device and the SINR for the one or more downlink channels to the wireless device. In some other embodiments, the network node makes the determination based on the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and the rank used for downlink transmission to the wireless device.

In some embodiments, a network node for selectively enabling antenna selection at a wireless device for uplink sounding reference signal transmission in a TDD wireless communication system is adapted to make a determination as to whether to enable or disable antenna selection at a wireless device for transmission of uplink sounding reference signals and send, to the wireless device, an indication that indicates whether the wireless device is to enable or disable antenna selection for transmission of uplink sounding reference signals in accordance with the determination. The network node is adapted to make the determination based on: an estimated speed of movement of the wireless device; both the estimated speed of movement of the wireless device and a SINR for one or more downlink channels to the wireless device; or the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and a rank used for downlink transmission to the wireless device.

In some embodiments, a network node for selectively enabling antenna selection at a wireless device for uplink sounding reference signal transmission in a TDD wireless communication system comprises a determining module and a sending module. The determining module is operable to make a determination as to whether to enable or disable antenna selection at a wireless device for transmission of uplink sounding reference signals based on: an estimated speed of movement of the wireless device; both the estimated speed of movement of the wireless device and a SINR for one or more downlink channels to the wireless device; or the estimated speed of movement of the wireless device, the SINR for the one or more downlink channels to the wireless device, and a rank used for downlink transmission to the wireless device. The sending module is operable to send, to the wireless device, an indication that indicates whether the wireless device is to enable or disable antenna selection for transmission of uplink sounding reference signals in accordance with the determination.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the embodiments in association with the accompanying drawing figures.

