Patent Description:
In some examples, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, otherwise known as user equipment (UEs). In LTE or LTE-A network, a set of one or more base stations may define an eNodeB (eNB). In other examples (e.g., in a next generation or <NUM> network), a wireless multiple access communication system may include a number of distributed units (DUs) (e.g., edge units (EUs), edge nodes (ENs), radio heads (RHs), smart radio heads (SRHs), transmission reception points (TRPs), etc.) in communication with a number of central units (CUs) (e.g., central nodes (CNs), access node controllers (ANCs), etc.), where a set of one or more distributed units, in communication with a central unit, may define an access node (e.g., a new radio base station (NR BS), a new radio node-B (NR NB), a network node, <NUM> NB, gNB, etc.). A base station or DU may communicate with a set of UEs on downlink channels (e.g., for transmissions from a base station or to a UE) and uplink channels (e.g., for transmissions from a UE to a base station or distributed unit).

The <NPL>, relates to TRS and discusses time domain pattern, QCL and the time/frequency offset tracking for idle mode. The TRS periodicity can thereby be determined by the speed of a UE.

The <NPL>, describes that periodic TRS is not present for IDLE mode UE.

There still exists a need for improved TRS transmission.

A solution is provided according to the subject matter of the independent claims.

Aspects of the present disclosure provide apparatus, methods, processing systems, and computer readable mediums for NR (new radio access technology or <NUM> technologies).

NR may support various wireless communication services, such as Enhanced mobile broadband (eMBB) targeting wide bandwidth (e.g. <NUM> beyond), millimeter wave (mmW) targeting high carrier frequency (e.g. <NUM> or beyond), massive MTC (mMTC) targeting non-backward compatible MTC techniques, and/or mission critical targeting ultra reliable low latency communications (URLLC). In LTE, the basic transmission time interval (TTI) or packet duration is <NUM> subframe. In NR, a subframe may still be <NUM>, but the basic TTI may be referred to as a slot. A subframe may contain a variable number of slots (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>,. slots) depending on the tone-spacing (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

Aspects of the present disclosure relate to aperiodic tracking reference signals.

For example, the wireless network may be a new radio (NR) or <NUM> network. UEs <NUM> and/or BSs <NUM> may be configured to perform the operations <NUM> and methods described herein for using an aperiodic TRS. The UEs <NUM> and/or BSs <NUM> may further be configured to perform complementary operations to the operations <NUM>.

As illustrated in <FIG>, the wireless network <NUM> may include a number of BSs <NUM> and other network entities. A BS may be a station that communicates with UEs. Each BS <NUM> may provide communication coverage for a particular geographic area. In 3GPP, the term "cell" can refer to a coverage area of a Node B and/or a Node B subsystem serving this coverage area, depending on the context in which the term is used. In NR systems, the term "cell" and gNB, Node B, <NUM> NB, AP, NR BS, NR BS, or TRP may be interchangeable. In some examples, the base stations may be interconnected to one another and/or to one or more other BSs or network nodes (not shown) in the wireless communication network <NUM> through various types of backhaul interfaces such as a direct physical connection, a virtual network, or the like using any suitable transport network.

The wireless communication network <NUM> may also include relay stations.

The wireless communication network <NUM> may support synchronous or asynchronous operation.

The BSs <NUM> may also communicate with one another, e.g., directly or indirectly via wireless or wireline backhaul.

A UE may also be referred to as a mobile station, a terminal, an access terminal, a subscriber unit, a station, a Customer Premises Equipment (CPE), a cellular phone, a smart phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a tablet, a camera, a gaming device, a netbook, a smartbook, an ultrabook, a medical device or medical equipment, a biometric sensor/device, a wearable device such as a smart watch, smart clothing, smart glasses, a smart wrist band, smart jewelry (e.g., a smart ring, a smart bracelet, etc.), an entertainment device (e.g., a music device, a video device, a satellite radio, etc.), a vehicular component or sensor, a smart meter/sensor, industrial manufacturing equipment, a global positioning system device, or any other suitable device that is configured to communicate via a wireless or wired medium. Some UEs may be considered evolved or machine-type communication (MTC) devices or evolved MTC (eMTC) devices. Some UEs may be considered Internet-of-Things (IoT) devices.

Consequently, the nominal FFT size may be equal to <NUM>, <NUM>, <NUM>, <NUM> or <NUM> for system bandwidth of <NUM>, <NUM>, <NUM>, <NUM> or <NUM> megahertz (MHz), respectively. For example, a subband may cover <NUM> (i.e., <NUM> resource blocks), and there may be <NUM>, <NUM>, <NUM>, <NUM> or <NUM> subbands for system bandwidth of <NUM>, <NUM>, <NUM>, <NUM> or <NUM>, respectively.

NR may utilize OFDM with a cyclic prefix (CP) on the uplink and downlink and include support for half-duplex operation using time division duplexing (TDD). A single component carrier (CC) bandwidth of <NUM> may be supported. NR resource blocks may span <NUM> subcarriers with a subcarrier bandwidth of <NUM> over a <NUM> duration. Each radio frame may consist of <NUM> half frames, each half frame consisting of <NUM> subframes, with a length of <NUM>. Consequently, each subframe may have a length of <NUM>. Each subframe may indicate a link direction (i.e., DL or UL) for data transmission and the link direction for each subframe may be dynamically switched. Each subframe may include DL/UL data as well as DL/UL control data. UL and DL subframes for NR may be as described in more detail below with respect to <FIG>. Alternatively, NR may support a different air interface, other than an OFDM-based. NR networks may include entities such central units (CUs) and/or distributed units (DUs).

As noted above, a RAN may include a CU and DUs. A NR BS (e.g., gNB, <NUM> Node B, Node B, transmission reception point (TRP), access point (AP)) may correspond to one or multiple BSs. NR cells can be configured as access cells (ACells) or data only cells (DCells). For example, the RAN (e.g., a CU or DU) can configure the cells. DCells may be cells used for carrier aggregation or dual connectivity, but not used for initial access, cell selection/reselection, or handover. In some cases DCells may not transmit synchronization signals (SS), but in some cases DCells may transmit SS. NR BSs may transmit downlink signals to UEs indicating the cell type. Based on the cell type indication, the UE may communicate with the NR BS. For example, the UE may determine NR BSs to consider for cell selection, access, handover, and/or measurement based on the indicated cell type.

For example, for RAN sharing, radio as a service (RaaS), and service specific ANC deployments, the TRP may be connected to more than one ANC.

The logical architecture <NUM> may be used to illustrate fronthaul definition. The logical architecture <NUM> may support fronthauling solutions across different deployment types. For example, the logical architecture <NUM> may be based on transmit network capabilities (e.g., bandwidth, latency, and/or jitter).

The logical architecture <NUM> may share features and/or components with LTE. The next generation AN (NG-AN) <NUM> may support dual connectivity with NR. The NG-AN <NUM> may share a common fronthaul for LTE and NR.

The logical architecture <NUM> may enable cooperation between and among TRPs <NUM>. There may be no inter-TRP interface.

Logical architecture <NUM> may have a dynamic configuration of split logical functions.

<FIG> illustrates an example physical architecture <NUM> of a distributed RAN, according to aspects of the present disclosure. The C-CU <NUM> may be centrally deployed.

<FIG> illustrates example components of the BS <NUM> and UE <NUM> illustrated in <FIG>, which may be used to implement aspects of the present disclosure. The BS may include a TRP and may be referred to as a Master eNB (MeNB) (e.g., Master BS, primary BS). Master BS and the Secondary BS may be geographically co-located.

One or more components of the BS <NUM> and UE <NUM> may be used to practice aspects of the present disclosure. For example, antennas <NUM>, Tx/Rx <NUM>, processors <NUM>, <NUM>, <NUM>, and/or controller/processor <NUM> of the UE <NUM> and/or antennas <NUM>, processors <NUM>, <NUM>, <NUM>, and/or controller/processor <NUM> of the BS <NUM> may be used to perform the operations <NUM> described herein and illustrated with reference to <FIG> and complementary operations.

For a restricted association scenario, the BS <NUM> may be the macro BS 110c in <FIG>, and the UE <NUM> may be the UE 120y. The BS <NUM> may also be a BS of some other type. The BS <NUM> may be equipped with antennas 434a through 434t, and the UE <NUM> may be equipped with antennas 452a through 452r.

The control information may be for the Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel (PCFICH), Physical Hybrid ARQ Indicator Channel (PHICH), Physical Downlink Control Channel (PDCCH), etc. The data may be for the Physical Downlink Shared Channel (PDSCH), etc. The processor <NUM> may process (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols, respectively. The processor <NUM> may also generate reference symbols, e.g., for the PSS, SSS, and cell-specific reference signal (CRS).

The processor <NUM> and/or other processors and modules at the BS <NUM> may perform or direct, e.g., the execution of the functional blocks illustrated in <FIG>, and/or other complementary processes for the techniques described herein. The processor <NUM> and/or other processors and modules at the UE <NUM> may perform or direct, e.g., the execution of the functional blocks illustrated in <FIG>, and/or other complementary processes for the techniques described herein. The memories <NUM> and <NUM> may store data and program codes for the BS <NUM> and the UE <NUM>, respectively.

The illustrated communications protocol stacks may be implemented by devices operating in a <NUM> system.

<FIG> is a diagram showing an example of a DL-centric subframe <NUM>. The DL-centric subframe <NUM> may include a control portion <NUM>. The control portion <NUM> may exist in the initial or beginning portion of the DL-centric subframe <NUM>. The DL-centric subframe600 may also include a DL data portion <NUM>. The DL data portion <NUM> may be referred to as the payload of the DL-centric subframe <NUM>.

The DL-centric subframe <NUM> may also include a common UL portion <NUM>. The common UL portion <NUM> may sometimes be referred to as an UL burst, a common UL burst, and/or various other suitable terms. The common UL portion <NUM> may include feedback information corresponding to various other portions of the DL-centric subframe. For example, the common UL portion <NUM> may include feedback information corresponding to the control portion <NUM>. Non-limiting examples of feedback information may include an ACK signal, a NACK signal, a HARQ indicator, and/or various other suitable types of information. The common UL portion <NUM> may include additional or alternative information, such as information pertaining to random access channel (RACH) procedures, scheduling requests (SRs), and various other suitable types of information. As illustrated in <FIG>, the end of the DL data portion <NUM> may be separated in time from the beginning of the common UL portion <NUM>. One of ordinary skill in the art will understand that the foregoing is merely one example of a DL-centric subframe and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

<FIG> is a diagram showing an example of an UL-centric subframe <NUM>. The UL-centric subframe <NUM> may include a control portion <NUM>. The control portion <NUM> may exist in the initial or beginning portion of the UL-centric subframe. The control portion <NUM> in <FIG> may be similar to the control portion described above with reference to <FIG>. The UL-centric subframe <NUM> may also include an UL data portion <NUM>. The UL data portion <NUM> may sometimes be referred to as the payload of the UL-centric subframe <NUM>. The UL portion may refer to the communication resources utilized to communicate UL data from the subordinate entity (e.g., UE) to the scheduling entity (e.g., UE or BS). In some configurations, the control portion <NUM> may be a physical UL control channel (PUCCH).

As illustrated in <FIG>, the end of the control portion <NUM> may be separated in time from the beginning of the UL data portion <NUM>. The UL-centric subframe <NUM> may also include a common UL portion <NUM>. The common UL portion <NUM> in <FIG> may be similar to the common UL portion <NUM> described above with reference to <FIG>. The common UL portion <NUM> may additionally or alternatively include information pertaining to channel quality indicator (CQI), sounding reference signals (SRSs), and various other suitable types of information. One of ordinary skill in the art will understand that the foregoing is merely one example of an UL-centric subframe and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

As discussed a UE <NUM> may receive signals transmitted by BS <NUM> on a DL. In order to determine parameters of a communication channel (e.g., on the DL) between UE <NUM> and BS <NUM>, BS <NUM> may transmit one or more reference signals (RSs) to UE <NUM>. The RS may be data that is known by both UE <NUM> and BS <NUM>. Therefore, the UE <NUM> may compare the decoded received RS transmitted from BS <NUM>, with the known RS to determine parameters of the communication channel in a process known as channel estimation. The channel estimation may be used to decode other data transmitted by the BS <NUM> to the UE <NUM> on the communication channel. For example, BS <NUM> may transmit a tracking reference signal (TRS) to UE <NUM>. The UE <NUM> may utilize the TRS to perform one or more of time/frequency tracking, estimation of Doppler spread, estimation of delay spread, estimation of power delay profile, etc. of the channel between BS <NUM> and UE <NUM>. In certain aspects, TRS is a device specific (e.g., UE-specific and transmitted on resources, such as resource blocks, allocated for a specific UE on the DL) RS and configured with higher-layer signaling (e.g., as part of a RRC signaling, media access control-control element (MAC-CE), downlink control information (DCI), etc.) in a device specific manner.

In certain aspects, the BS <NUM> is configured to transmit TRS periodically. For example, the BS <NUM> may transmit the TRS every Y number of slots (e.g., where Y is a positive integer). The BS <NUM> may further transmit the TRS over X number of slots (e.g., where X is a positive integer) for each transmission. Accordingly, the BS <NUM> may be configured to start transmission of TRS every Y number of slots and transmit TRS in X number of slots consecutively for each transmission, where X < Y.

Transmitting TRS periodically may not allow the UE <NUM> to account for sporadic changes in channel conditions (e.g., TX/RX conditions). For example, communication in NR may be bursty in nature, meaning that traffic (e.g., control and data transmissions) may be non-continuous and aperiodic in time and frequency. Further, adaptive link adaptation techniques may be used for the DL between BS <NUM> and UE <NUM> in order to maintain good link conditions between BS <NUM> and UE <NUM>. For example, one or more of TX power control, precoding/antenna/beam switching, dynamic cell selection, etc., may be used as adaptive link adaptation techniques for the DL. Accordingly, due to changes in channel conditions (e.g., due to adaptive link adaptation techniques or other changes in channel conditions) channel estimation of channel-related parameters such as average delay, delay spread, Doppler spread, Doppler shift, etc. of the channel between BS <NUM> and UE <NUM> may change. Though UE <NUM> may be able to perform channel estimation to determine updated channel-related parameters based on periodically transmitted TRS and/or beam management procedures (e.g., for mmW based systems), the channel estimation may be inaccurate for signals received between the periods where TRS is transmitted by BS <NUM> and where channel conditions have changed. Further, beam management procedures may not have enough resources allocated for accurate channel estimation. This may lead to performance degradation as UE <NUM> may use inaccurate channel estimates to decode received signals from BS <NUM>.

For example, <FIG> illustrates an example timeline of channel conditions of a DL between UE <NUM> and BS <NUM>. Periodically, BS <NUM> transmits TRS at time <NUM>, <NUM>, and <NUM> as shown to UE <NUM> for UE <NUM> to perform channel estimation utilizing TRS. As shown, the channel conditions between BS <NUM> and UE <NUM> may change from a first set of channel conditions to a second set of channel conditions at time <NUM>. Time <NUM> is between transmission of TRS at <NUM> and <NUM>. Accordingly, during time period <NUM>, UE <NUM> may have inaccurate channel estimates, and only be able to update channel estimates after time <NUM>.

In order to improve performance of decoding signals at UE <NUM>, techniques herein relate to BS <NUM> aperiodically transmitting TRS to UE <NUM>, and UE <NUM> using the aperiodically transmitted TRS for channel estimation. In certain aspects, an aperiodic TRS may be a single burst TRS transmission transmitted aperiodically. The aperiodic TRS may be transmitted over the same number of slots as periodic TRS for each transmission, or may be transmitted over a different number of slots. The aperiodic TRS may be configured using the same configuration as the periodic TRS (e.g., the same higher layer signaling) or using a different configuration.

In certain aspects, BS <NUM> determines to transmit aperiodic TRS when channel conditions change on the DL. For example, <FIG> illustrates an example timeline of channel conditions of a DL between UE <NUM> and BS <NUM>. Like shown in <FIG>, periodically, BS <NUM> transmits periodic TRS at time <NUM>, <NUM>, and <NUM> as shown to UE <NUM> for UE <NUM> to perform channel estimation utilizing TRS. As shown, the channel conditions between BS <NUM> and UE <NUM> may change from a first set of channel conditions to a second set of channel conditions at time <NUM>. Time <NUM> is between transmission of TRS at <NUM> and <NUM>. Accordingly, BS <NUM> may determine to and transmit an aperiodic TRS at time <NUM>, which is after the channel conditions change at time <NUM>, but before the next periodic TRS transmission at time <NUM>. UE <NUM> may utilize the aperiodic TRS to update channel estimates to accurately decode signals from BS <NUM>.

In certain aspects, aperiodic TRS may not be transmitted every time channel conditions change, as this may introduce too much overhead for aperiodic TRS transmission. Further, channel conditions changes may not always be significant enough that channel estimation needs to be updated in order to accurately decode signals at the UE <NUM> from BS <NUM>. Accordingly, in certain aspects, aperiodic TRS is transmitted only when one or more trigger conditions or thresholds are met.

In certain aspects, the transmission of aperiodic TRS may be triggered by the BS <NUM> or the UE <NUM>. In certain aspects, BS <NUM> can indicate the presence or transmission timing of an aperiodic TRS to UE <NUM> in order for UE <NUM> to be able to determine when aperiodic TRS is transmitted, so it can receive the aperiodic TRS and perform channel estimation. In certain aspects, BS <NUM> may indicate the transmission timing of an aperiodic TRS along with the signaling (e.g., as part of RRC signaling, MAC-CE, DCI, etc.) for a channel condition change (e.g., one or more of TX power control, precoding/antenna/beam switching, dynamic cell selection, or other adaptive link adaptation techniques). In certain aspects, the BS <NUM> indicates the transmission timing of an aperiodic TRS separately from the signaling for channel condition changes.

In the case where the channel condition change is due to a beam switching event (e.g., in mmW systems), the triggering of an aperiodic TRS may also trigger transmission of a phase tracking reference signal (PTRS) by BS <NUM> to UE <NUM>, used for phase tracking in the DL by UE <NUM>. In certain aspects, PTRS has higher density in the time domain, but lower density in the frequency domain than TRS. Accordingly, by combining PTRS and TRS in a mmW system, the overhead for transmitting PTRS and TRS (e.g., time multiplexed together) can be reduced by balancing the load between PTRS and TRS.

In certain aspects, the presence of PTRS in transmissions from BS <NUM> to UE <NUM> is implicitly determined by UE <NUM> based on modulation coding scheme (MCS), bandwidth, and subcarrier spacing (SCS) of transmissions from BS <NUM>. However, in certain aspects, the transmission of PTRS is explicitly triggered along with aperiodic TRS.

As discussed, in certain aspects, BS <NUM> may initiate a channel condition change on the DL (e.g., a beam switch from a first beam A to a second beam B for communication on the DL with UE <NUM>). For example, at time N, BS <NUM> may send a beam switch command on the DL control channel to UE <NUM> indicating a new beam (beam B) to use for communication on the DL. The UE <NUM> may decode the beam switch command and acknowledge receipt of the beam switch command to BS <NUM>. Accordingly, UE <NUM> may utilize beam B at time N+K<NUM>, where K<NUM> is the time it takes for UE <NUM> to switch from beam A to beam B.

BS <NUM> may also switch to utilizing beam B at time N+K<NUM> so that UE <NUM> and BS <NUM> communicate on beam B. In certain aspects, BS <NUM> determines whether to transmit an aperiodic TRS at time N+K<NUM>. For example, BS <NUM> determines if time N+K<NUM> is more than a threshold amount of time greater than the last beam management instance (e.g., channel estimation procedure) for beam B. If N+K<NUM> is more than a threshold amount of time greater than the last beam management instance, BS <NUM> determines to trigger aperiodic TRS. If N+K<NUM> is less than a threshold amount of time greater than the last beam management instance, BS <NUM> determines not to trigger aperiodic TRS. For example, BS <NUM> normally selects one of the beams from an active beam set at BS <NUM> to switch to. Such active beams in the active beam set are regularly (e.g., periodically) managed by a beam management process.

In certain aspects, regardless of the time separation between N+K<NUM> and the last beam management instance, BS <NUM> determines whether to transmit an aperiodic TRS at time N+K<NUM> based on whether the beam switch from beam A to beam B results in a large change in beam width and/or angle used for the DL between BS <NUM> and UE <NUM>. For example, if the beam width, pattern and/or direction (e.g., angle of departure and/or arrival) changes by a threshold amount(s), BS <NUM> determines to trigger aperiodic TRS. If the beam width and/or angle does not change by a threshold amount(s), BS <NUM> determines not to trigger aperiodic TRS. In certain aspects, BS <NUM> may indicate trigger of aperiodic TRS at time N+K<NUM> (or another suitable time) together with or separately from a beam switching command sent to UE <NUM>. BS <NUM> may indicate triggering of aperiodic TRS to UE <NUM> as discussed.

In certain aspects, though the beam switching is initiated by BS <NUM>, UE <NUM> may request triggering of aperiodic TRS (e.g., based on the same criteria as discussed for BS <NUM>). For example, the BS <NUM> may signal to UE <NUM> to initiate beam switching on the PDSCH. When UE <NUM> sends an ACK for the PDSCH transmission, it may also include a request for aperiodic TRS transmission (e.g., in PUCCH, MAC-CE, and/or scheduling request (SR)). BS <NUM> may then transmit the aperiodic TRS based on the request.

In certain aspects, UE <NUM> may initiate a channel condition change on the DL (e.g., a beam switch from a first beam A to a second beam B for communication on the DL with BS <NUM>). For example, at time N, UE <NUM> may send a beam switch request to BS <NUM> (e.g., in PUCCH, MAC-CE, and/or SR) indicating a new beam (beam B) to use for communication on the DL. In certain aspects, UE <NUM> may also determine whether to include (e.g., based on the same criteria as discussed for BS <NUM>) and then either include or not include a request for aperiodic TRS transmission along with (or separate from) the beam switch request. In certain aspects, if BS <NUM> accepts the beam switch request of UE <NUM>, UE <NUM> and BS <NUM> may utilize beam B at time N+K<NUM>, where K<NUM> is the time it takes for BS <NUM> to switch from beam A to beam B. Further, if the beam switching request includes (or the BS <NUM> separately receives from UE <NUM>) a request for aperiodic TRS transmission, the BS <NUM> transmits aperiodic TRS at time N+K<NUM> (or another suitable time).

Though certain aspects are described as between a BS <NUM> and UE <NUM>, certain aspects may also be used for communication between a BS <NUM> and another BS, or between UEs.

<FIG> illustrates example operations that may be performed by a wireless device (e.g., BS <NUM> or UE <NUM>) for using an aperiodic TRS in accordance with aspects of the present disclosure.

Operations <NUM> begin, at <NUM>, by determining a channel condition change of a downlink channel between a first wireless device and a second wireless device. Operations <NUM> continue at <NUM> by determining whether the channel condition change satisfies a trigger condition. Operations <NUM> continue at <NUM> by triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

The transceiver <NUM> is configured to transmit and receive signals for the communications device <NUM> via an antenna <NUM>, such as the various signal described herein.

The processing system <NUM> includes a processor <NUM> coupled to a computer-readable medium/memory <NUM> via a bus <NUM>. In certain aspects, the computer-readable medium/memory <NUM> is configured to store instructions that when executed by processor <NUM>, cause the processor <NUM> to perform the operations illustrated in <FIG>, or other operations for performing the various techniques discussed herein.

In certain aspects, the processing system <NUM> further includes a first determining component <NUM> for performing the operations illustrated at <NUM> in <FIG>. Additionally, the processing system <NUM> includes a second determining component <NUM> for performing the operations illustrated at <NUM> in <FIG>. The processing system <NUM> also includes a triggering component <NUM> for performing the operations illustrated at <NUM> in <FIG>. The first determining component <NUM>, second determining component <NUM>, and triggering component <NUM> may be coupled to the processor <NUM> via bus <NUM>. In certain aspects, the first determining component <NUM>, second determining component <NUM>, and triggering component <NUM> may be hardware circuits. In certain aspects, the first determining component <NUM>, second determining component <NUM>, and triggering component <NUM> may be software components that are executed and run on processor <NUM>.

Unless specifically stated otherwise, the term "some" refers to one or more.

For example, instructions for perform the operations described herein and illustrated in <FIG>.

Claim 1:
A method for wireless communication by a base station (<NUM>), the method comprising:
while transmitting periodically a periodic tracking reference signal to a user equipment (<NUM>):
determining, between a first periodic tracking reference signal and a second periodic tracking reference signal, a channel condition change of a downlink channel between the base station (<NUM>) and the user equipment (<NUM>);
determining whether the channel condition change satisfies a trigger condition; and
triggering transmission, after the channel condition change and before the second periodic tracking reference signal, of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.