Patent Publication Number: US-2023143268-A1

Title: Methods and Devices for Channel Estimation

Description:
TECHNICAL FIELD 
     The present disclosure relates to wireless communication, and more particularly, to methods and devices for channel estimation. 
     BACKGROUND 
     Multiple Input Multiple Output (MIMO), particularly Multi-User MIMO (MU-MIMO) is a technique that can increase system throughput very effectively. Theoretically, when a number, M, of antennas are equipped at a network device (e.g., an evolved NodeB, or eNB), an M-layer MU-MIMO system can be supported or an increase of throughput by a factor of M can be achieved. 
     In order for a MIMO system to work properly, it is important that the network device knows channels to/from all terminal devices (e.g., User Equipments, or UEs) to be grouped. For example, an eNB can allocate a Sounding Reference Signal (SRS) to a UE. Upon receiving the SRS from the UE, the eNB can estimate an uplink channel from the UE to the eNB based on measurement of the SRS. In a Time Division Duplex (TDD) system, in addition to the uplink channel, the eNB can obtain a downlink channel from the eNB to the UE due to channel reciprocity in TDD system. 
     SUMMARY 
     It is an object of the present disclosure to provide methods and devices for channel estimation. 
     According to a first aspect of the present disclosure, a method in a network device for channel estimation is provided. The method includes: transmitting to a terminal device an instruction to precode each of a number, L, of DeModulation Reference Signals (DMRSs) using a number, N, of linearly independent precoders, respectively; receiving from the terminal device L*N precoded DMRSs; estimating an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs; and determining the uplink channel from the equivalent channel based on the N precoders. 
     In an embodiment, the L*N precoded DMRSs can be received in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being received in one of the N TTIs. 
     In an embodiment, each of the L DMRSs can be used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the equivalent channel can be a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the equivalent channel can be an average of N equivalent channels each being a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the operation of determining can include: combining the N precoders into a precoding matrix; and deriving the uplink channel as a function of the equivalent channel and an inverse of the precoding matrix. 
     In an embodiment, the instruction can include an indication of the N precoders and/or can be transmitted to the terminal device via Downlink Control Information (DCI). 
     In an embodiment, the method can further include: determining a downlink channel from the network device to the terminal device based on the uplink channel. 
     According to a second aspect of the present disclosure, a network device is provided. The network device includes: a transmitting unit configured to transmit to a terminal device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. The network device further includes: a receiving unit configured to receive from the terminal device L*N precoded DMRSs. The network device further includes an estimating unit configured to estimate an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs. The network device further includes a determining unit configured to determine the uplink channel from the equivalent channel based on the N precoders. 
     According to a third aspect of the present disclosure, a network device is provided. The network device includes one or more processors and one or more memories. The one or more memories contain instructions executable by the processors, whereby the network device is operative to perform the method according to the above first aspect. 
     According to a fourth aspect of the present disclosure, a computer readable storage medium is provided. The computer readable storage medium has computer program instructions stored thereon. The computer program instructions, when executed by a processor in a network device, cause the network device to perform the method according to the above first aspect. 
     According to a fifth aspect of the present disclosure, a method in a terminal device for facilitating channel estimation is provided. The method includes: receiving from a network device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively; and transmitting to the network device L*N precoded DMRSs. 
     In an embodiment, the L*N precoded DMRSs can be transmitted in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being transmitted in one of the N TTIs. 
     In an embodiment, each of the L DMRSs can be used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L is equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the instruction can include an indication of the N precoders and/or can be received from the network device via Downlink Control Information (DCI). 
     According to a sixth aspect of the present disclosure, a terminal device is provided. The terminal device includes a receiving unit configured to receive from a network device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. The terminal device further includes a transmitting unit configured to transmit to the network device L*N precoded DMRSs. 
     According to a seventh aspect of the present disclosure, a terminal device is provided. The terminal device includes one or more processors and one or more memories. The one or more memories contain instructions executable by the processors, whereby the terminal device is operative to perform the method according to the fifth aspect. 
     According to an eighth aspect of the present disclosure, a computer readable storage medium is provided. The computer readable storage medium has computer program instructions stored thereon. The computer program instructions, when executed by a processor in a terminal device, cause the terminal device to perform the method according to the fifth aspect. 
     According to a ninth aspect of the present disclosure, a communication system is provided. The communication system includes a host computer including: processing circuitry configured to provide user data; and a communication interface configured to forward the user data to a cellular network for transmission to a UE. The cellular network includes a base station having a radio interface and processing circuitry. The base station&#39;s processing circuitry is configured to perform the method according to the first aspect. 
     In an embodiment, the communication system can further include the base station. 
     In an embodiment, the communication system can further include the UE. The UE is configured to communicate with the base station. 
     In an embodiment, the processing circuitry of the host computer can be configured to execute a host application, thereby providing the user data. The UE can include processing circuitry configured to execute a client application associated with the host application. 
     According to a tenth aspect of the present disclosure, a method is provided. The method is implemented in a communication system including a host computer, a base station and a UE. The method includes: at the host computer, providing user data; and at the host computer, initiating a transmission carrying the user data to the UE via a cellular network comprising the base station. The base station can perform the method according to the first aspect. 
     In an embodiment, the method further can include: at the base station, transmitting the user data. 
     In an embodiment, the user data can be provided at the host computer by executing a host application. The method can further include: at the UE, executing a client application associated with the host application. 
     According to an eleventh aspect of the present disclosure, a communication system is provided. The communication system includes a host computer including: processing circuitry configured to provide user data; and a communication interface configured to forward user data to a cellular network for transmission to a UE. The UE includes a radio interface and processing circuitry. 
     The UE&#39;s processing circuitry is configured to perform the method according to the fifth aspect. 
     In an embodiment, the communication system can further include the UE. 
     In an embodiment, the cellular network can further include a base station configured to communicate with the UE. 
     In an embodiment, the processing circuitry of the host computer can be configured to execute a host application, thereby providing the user data. The UE&#39;s processing circuitry can be configured to execute a client application associated with the host application. 
     According to a twelfth aspect of the present disclosure, a method is provided. The method is implemented in a communication system including a host computer, a base station and a UE. The method includes: at the host computer, providing user data; and at the host computer, initiating a transmission carrying the user data to the UE via a cellular network comprising the base station. The UE can perform the method according to the fifth aspect. 
     In an embodiment, the method can further include: at the UE, receiving the user data from the base station. 
     According to a thirteenth aspect of the present disclosure, a communication system is provided. The communication system includes a host computer including: a communication interface configured to receive user data originating from a transmission from a UE to a base station. The UE includes a radio interface and processing circuitry. The UE&#39;s processing circuitry is configured to: perform the method according to the fifth aspect. 
     In an embodiment, the communication system can further include the UE. 
     In an embodiment, the communication system can further include the base station. The base station can include a radio interface configured to communicate with the UE and a communication interface configured to forward to the host computer the user data carried by a transmission from the UE to the base station. 
     In an embodiment, the processing circuitry of the host computer can be configured to execute a host application. The UE&#39;s processing circuitry can be configured to execute a client application associated with the host application, thereby providing the user data. 
     In an embodiment, the processing circuitry of the host computer can be configured to execute a host application, thereby providing request data. The UE&#39;s processing circuitry can be configured to execute a client application associated with the host application, thereby providing the user data in response to the request data. 
     According to a fourteenth aspect of the present disclosure, a method is provided. The method is implemented in a communication system including a host computer, a base station and a UE. The method includes: at the host computer, receiving user data transmitted to the base station from the UE. The UE can perform the method according to the fifth aspect. 
     In an embodiment, the method can further include: at the UE, providing the user data to the base station. 
     In an embodiment, the method can further include: at the UE, executing a client application, thereby providing the user data to be transmitted; and at the host computer, executing a host application associated with the client application. 
     In an embodiment, the method can further include: at the UE, executing a client application; and at the UE, receiving input data to the client application, the input data being provided at the host computer by executing a host application associated with the client application. The user data to be transmitted is provided by the client application in response to the input data. 
     According to a fifteenth aspect of the present disclosure, a communication system is provided. The communication system includes a host computer including a communication interface configured to receive user data originating from a transmission from a UE to a base station. The base station includes a radio interface and processing circuitry. The base station&#39;s processing circuitry is configured to perform the method according to the first aspect. 
     In an embodiment, the communication system can further include the base station. 
     In an embodiment, the communication system can further include the UE. The UE can be configured to communicate with the base station. 
     In an embodiment, the processing circuitry of the host computer can be configured to execute a host application; the UE can be configured to execute a client application associated with the host application, thereby providing the user data to be received by the host computer. 
     According to a sixteenth aspect of the present disclosure, a method is provided. The method is implemented in a communication system including a host computer, a base station and a UE. The method includes: at the host computer, receiving, from the base station, user data originating from a transmission which the base station has received from the UE. The base station can perform the method according to the first aspect. 
     In an embodiment, the method can further include: at the base station, receiving the user data from the UE. 
     In an embodiment, the method can further include: at the base station, initiating a transmission of the received user data to the host computer. 
     With the embodiments of the present disclosure, a network device transmits to a terminal device an instruction to precode each of L DMRSs using N linearly independent precoders, respectively. Upon receiving from the terminal device L*N precoded DMRSs, the network device estimates an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs and determines the uplink channel from the equivalent channel based on the N precoders. In this way, the network device can obtain an estimate of the actual uplink channel, and possibly the actual downlink channel, based on the precoded DMRSs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages will be more apparent from the following description of embodiments with reference to the figures, in which: 
         FIG.  1    is a flowchart illustrating a method in a network device for channel estimation according to an embodiment of the present disclosure; 
         FIG.  2    is a flowchart illustrating a terminal device for facilitating channel estimation according to an embodiment of the present disclosure; 
         FIG.  3    is a block diagram of a network device according to an embodiment of the present disclosure; 
         FIG.  4    is a block diagram of a network device according to another embodiment of the present disclosure; 
         FIG.  5    is a block diagram of a terminal device according to an embodiment of the present disclosure; 
         FIG.  6    is a block diagram of a terminal device according to another embodiment of the present disclosure; 
         FIG.  7    schematically illustrates a telecommunication network connected via an intermediate network to a host computer; 
         FIG.  8    is a generalized block diagram of a host computer communicating via a base station with a user equipment over a partially wireless connection; and 
         FIGS.  9  to  12    are flowcharts illustrating methods implemented in a communication system including a host computer, a base station and a user equipment. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “wireless communication network” refers to a network following any suitable communication standards, such as LTE-Advanced (LTE-A), LTE, Wideband Code Division Multiple Access (WCDMA), High-Speed Packet Access (HSPA), and so on. Furthermore, the communications between a terminal device and a network device in the wireless communication network may be performed according to any suitable generation communication protocols, including, but not limited to, Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), and/or other suitable 1G (the first generation), 2G (the second generation), 2.5G, 2.75G, 3G (the third generation), 4G (the fourth generation), 4.5G, 5G (the fifth generation) communication protocols, wireless local area network (WLAN) standards, such as the IEEE 802.11 standards; and/or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, and/or ZigBee standards, and/or any other protocols either currently known or to be developed in the future. 
     The term “network device” refers to a device in a wireless communication network via which a terminal device accesses the network and receives services therefrom. The network device refers to a base station (BS), an access point (AP), or any other suitable device in the wireless communication network. The BS may be, for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), or gNB, a Remote Radio Unit (RRU), a radio header (RH), a remote radio head (RRH), a relay, a low power node such as a femto, a pico, and so forth. Yet further examples of the network device may include multi-standard radio (MSR) radio equipment such as MSR BSs, network controllers such as radio network controllers (RNCs) or base station controllers (BSCs), base transceiver stations (BTSs), transmission points, transmission nodes. More generally, however, the network device may represent any suitable device (or group of devices) capable, configured, arranged, and/or operable to enable and/or provide a terminal device access to the wireless communication network or to provide some service to a terminal device that has accessed the wireless communication network. 
     The term “terminal device” refers to any end device that can access a wireless communication network and receive services therefrom. By way of example and not limitation, the terminal device refers to a mobile terminal, user equipment (UE), or other suitable devices. The UE may be, for example, a Subscriber Station (SS), a Portable Subscriber Station, a Mobile Station (MS), or an Access Terminal (AT). The terminal device may include, but not limited to, portable computers, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, a tablet, a wearable device, a personal digital assistant (PDA), portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, wearable terminal devices, vehicle-mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), USB dongles, smart devices, wireless customer-premises equipment (CPE) and the like. In the following description, the terms “terminal device”, “terminal”, “user equipment” and “UE” may be used interchangeably. As one example, a terminal device may represent a UE configured for communication in accordance with one or more communication standards promulgated by the 3rd Generation Partnership Project (3GPP), such as 3GPP&#39;s GSM, UMTS, LTE, and/or 5G standards. As used herein, a “user equipment” or “UE” may not necessarily have a “user” in the sense of a human user who owns and/or operates the relevant device. In some embodiments, a terminal device may be configured to transmit and/or receive information without direct human interaction. For instance, a terminal device may be designed to transmit information to a network on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the wireless communication network. Instead, a UE may represent a device that is intended for sale to, or operation by, a human user but that may not initially be associated with a specific human user. 
     The terminal device may support device-to-device (D2D) communication, for example by implementing a 3GPP standard for sidelink communication, and may in this case be referred to as a D2D communication device. 
     As yet another example, in an Internet of Things (IOT) scenario, a terminal device may represent a machine or other device that performs monitoring and/or measurements, and transmits the results of such monitoring and/or measurements to another terminal device and/or network equipment. The terminal device may in this case be a machine-to-machine (M2M) device, which may in a 3GPP context be referred to as a machine-type communication (MTC) device. As one particular example, the terminal device may be a UE implementing the 3GPP narrow band internet of things (NB-IoT) standard. Particular examples of such machines or devices are sensors, metering devices such as power meters, industrial machinery, or home or personal appliances, for example refrigerators, televisions, personal wearables such as watches etc. In other scenarios, a terminal device may represent a vehicle or other equipment that is capable of monitoring and/or reporting on its operational status or other functions associated with its operation. 
     As used herein, a downlink, DL transmission refers to a transmission from the network device to a terminal device, and an uplink, UL transmission refers to a transmission in an opposite direction. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be liming of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and/or “including”, when used herein, specify the presence of stated features, elements, and/or components etc., but do not preclude the presence or addition of one or more other features, elements, components and/or combinations thereof. 
     In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs. 
     As discussed above, the channel estimation can be based on the SRS. However, the capacity of SRS is limited. In a typical Long Term Evolution (LTE) TDD configuration (TDD Configuration 2, Special Subframe Configuration 7, Cyclic Shift 4, Comb 2), up to 32 UEs can each be allocated with an SRS every 10 ms. However, there may be quite many UEs in a cell, and this 32 SRS capacity may need be shared among hundreds of UEs. In order to overcome the capacity issue of SRS, it is possible to estimate a channel by measuring an uplink DeModulation Reference Signal (DMRS). Thus, more UEs can be grouped using the MU-MIMO technique and the system throughput can be improved. If one single UE is equipped with more than one antenna, it is also possible to enable spatial multiplexing per UE to increase uplink throughput for the UE. 
     Furthermore, codebook based solution has been proposed for uplink transmission such that an eNB can control how layers are multiplexed for transmission. The DMRS transmitted from a UE to an eNB is also precoded according to a codebook configured by the eNB. When the DMRS is precoded, the channel estimated based on the measurement of the DMRS would not be the actual channel and thus cannot be used directly e.g., to determine beamforming weights for MIMO transmissions. 
       FIG.  1    is a flowchart illustrating a method  100  for channel estimation according to an embodiment of the present disclosure. The method  100  can be performed at a network device. 
     At block  110 , the network device transmits to a terminal device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. In an example, the instruction can include an indication of the N precoders. The instruction can be transmitted to the terminal device via Downlink Control Information (DCI), e.g., over Physical Downlink Control Channel (PDCCH). 
     Here, each of the L DMRSs can be used for one-layer transmission. L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. The number of antennas at the terminal device can be larger than or equal to 2. In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     Preferably, the N linearly independent precoders can be orthogonal to each other. 
     At block  120 , the network device receives from the terminal device L*N DMRSs that are precoded using the N precoders, respectively. The L*N precoded DMRSs can be received in a relatively short time period with respect to channel variation. Preferably, the L*N precoded DMRSs can be received in N consecutive TTIs or slots, with L precoded DMRSs that are precoded using one of the N precoders being received in one of the N TTIs. Each TTI or slot can have a duration of e.g., 1 ms. Alternatively, the L*N precoded DMRSs can be received in N adjacent Physical Resource Blocks (PRBs) in one TTI, with L precoded DMRSs that are precoded using one of the N precoders being received in one of the N PRBs. 
     At block  130 , the network device estimates an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs. 
     In an example, the equivalent channel can be a combination of the uplink channel and one of the N precoders. The equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     Alternatively, the equivalent channel can be an average of N equivalent channels each being a combination of the uplink channel and one of the N precoders. The equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     At block  140 , the network device determines the uplink channel from the equivalent channel based on the N precoders. 
     In particular, in the block  140 , the N precoders can be combined into a precoding matrix and the uplink channel can be derived as a function of the equivalent channel and an inverse of the precoding matrix. It is assumed here that the uplink channel remains substantially constant during the short time period (e.g., N consecutive TTIs). 
     In a TDD system, for example, a downlink channel from the network device to the terminal device can be determined based on the uplink channel, by utilizing the channel reciprocity. 
     The principle of the above method can be described mathematically as follows. 
     It is assumed here that the network device is equipped with M antennas and the terminal device is equipped with N antennas, where M≥2, N≥2. It is also assumed that L=1 and the number of precoders, and thus the number of precoded DMRSs, is N. 
     Let s denote the DMRS and an N-dimensional vector P i , i=1, 2, . . . , N, denote the i-th precoder. The precoded DMRS can be represented as: 
         d   i   =P   i   s,   (1)
 
     where d i  is an N-dimensional vector and denotes the i-th precoded DMRS that is precoded P i . 
     A signal received at the network device can be represented as: 
         r   i   =H   i   d   i   +n   i   =H   i   P   i   s+n   i ,  (2)
 
     where r i  is an M-dimensional vector and denotes the signal received at the network device, H i  is an M*N matrix and denotes the uplink channel from the terminal device to the network device, and n i  is an N-dimensional vector and denotes a noise at the network device. 
     Let H′ i =H i P i  denotes an equivalent channel, the above equation (3) can be rewritten as: 
         r   i   =H′   i   s+n   i .  (3)
 
     The equivalent channel H′ i  can be estimated at the network device by using any appropriate channel estimation technique. 
     The N precoders can be combined into a precoding matrix: 
         P =[ P   1   P   2    . . . P   N ].  (3)
 
     As discussed above, as the uplink channel typically remains substantially constant during a short time period (e.g., N consecutive TTIs), i.e., assuming H 1 =H 2 = . . . H N , the uplink channel H i  can be estimated as: 
         H   i   =H′   i   P   −1 .  (4)
 
     Alternatively, an average of the N equivalent channels can be calculated as: 
         H′   avg =( H′   1   +H′   2   + . . . H′   N )/ N.   (5)
 
     The uplink channel can be estimated as: 
         H   UL   =H′   avg   P   −1 .  (6)
 
     Accordingly, when the channel reciprocity is applicable, the downlink channel can be calculated as: 
         H   DL   =H   i   T ,  (7)
 
       or 
         H   DL   =H   UL   T ,  (8)
 
     where ( ) T  denotes transposition. 
       FIG.  2    is a flowchart illustrating a method  200  for facilitating channel estimation according to an embodiment of the present disclosure. The method  200  can be performed at a terminal device. 
     At block  210 , the terminal device receives from a network device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. In an example, the instruction can include an indication of the N precoders. The instruction can be received via DCI, e.g., over PDCCH. 
     Here, each of the L DMRSs is used for one-layer transmission. L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. The number of antennas at the terminal device can be larger than or equal to 2. In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     Preferably, the N linearly independent precoders can be orthogonal to each other. 
     At block  220 , the terminal devices transmits to the network device L*N precoded DMRSs. Here, the L*N precoded DMRSs can be transmitted in a relatively short time period with respect to channel variation. Preferably, the L*N precoded DMRSs can be transmitted in N consecutive TTIs or slots, with L precoded DMRSs that are precoded using one of the N precoders being transmitted in one of the N TTIs. Alternatively, the L*N precoded DMRSs can be transmitted in N adjacent PRBs in one TTI, with L precoded DMRSs that are precoded using one of the N precoders being transmitted in one of the N PRBs. 
     Correspondingly to the method  100  as described above, a network device is provided.  FIG.  3    is a block diagram of a network device  300  according to an embodiment of the present disclosure. 
     As shown in  FIG.  3   , the network device  300  includes a transmitting unit  310  configured to transmit to a terminal device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. The network device  300  further includes a receiving unit  320  configured to receive from the terminal device L*N precoded DMRSs. The network device  300  further includes an estimating unit  330  configured to estimate an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs. The network device  300  further includes a determining unit  340  configured to determine the uplink channel from the equivalent channel based on the N precoders. 
     In an embodiment, the L*N precoded DMRSs can be received in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being received in one of the N TTIs. 
     In an embodiment, each of the L DMRSs can be used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the equivalent channel can be a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the equivalent channel can be an average of N equivalent channels each being a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the determining unit  340  can be configured to combine the N precoders into a precoding matrix; and derive the uplink channel as a function of the equivalent channel and an inverse of the precoding matrix. 
     In an embodiment, the instruction can include an indication of the N precoders and can be transmitted to the terminal device via Downlink Control Information (DCI). 
     In an embodiment, the determining unit  340  can further be configured to determine a downlink channel from the network device to the terminal device based on the uplink channel. 
     The transmitting unit  310 , the receiving unit  320 , the estimating unit  330  and the determining unit  340  can be implemented as a pure hardware solution or as a combination of software and hardware, e.g., by one or more of: a processor or a micro-processor and adequate software and memory for storing of the software, a Programmable Logic Device (PLD) or other electronic component(s) or processing circuitry configured to perform the actions described above, and illustrated, e.g., in  FIG.  1   . 
       FIG.  4    is a block diagram of a network device  400  according to another embodiment of the present disclosure. 
     The network device  400  includes one or more processors  410  and one or more memories  420 . The memories  420  contain instructions executable by the processors  410  whereby the network device  400  is operative to perform the actions, e.g., of the procedure described earlier in conjunction with  FIG.  1   . Particularly, the memories  420  contain instructions executable by the processors  410  whereby the network device  400  is operative to: transmit to a terminal device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively; receive from the terminal device L*N precoded DMRSs; estimate an equivalent channel associated with an uplink channel from the terminal device to the network device based on one or more of the L*N precoded DMRSs; and determine the uplink channel from the equivalent channel based on the N precoders. 
     In an embodiment, the L*N precoded DMRSs can be received in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being received in one of the N TTIs. 
     In an embodiment, each of the L DMRSs is used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the equivalent channel can be a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the equivalent channel can be an average of N equivalent channels each being a combination of the uplink channel and one of the N precoders, and the equivalent channel can be estimated based on L of the L*N precoded DMRSs that are precoded using the one precoder. 
     In an embodiment, the determining unit  340  can be configured to combine the N precoders into a precoding matrix; and derive the uplink channel as a function of the equivalent channel and an inverse of the precoding matrix. 
     In an embodiment, the instruction can include an indication of the N precoders and can be transmitted to the terminal device via Downlink Control Information (DCI). 
     In an embodiment, the memories  420  can further contain instructions executable by the processors  410  whereby the network device  400  is operative to determine a downlink channel from the network device to the terminal device based on the uplink channel. 
     Correspondingly to the method  200  as described above, a terminal device is provided.  FIG.  5    is a block diagram of a terminal device  500  according to an embodiment of the present disclosure. 
     As shown in  FIG.  5   , the terminal device  500  includes a receiving unit  510  configured to receive from a network device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively. The terminal device  500  includes a transmitting unit  520  configured to transmit to the network device L*N precoded DMRSs. 
     In an embodiment, the L*N precoded DMRSs can be transmitted in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being transmitted in one of the N TTIs. 
     In an embodiment, each of the L DMRSs can be used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the instruction can include an indication of the N precoders and can be received from the network device via Downlink Control Information (DCI). 
     The receiving unit  510  and the transmitting unit  520  can be implemented as a pure hardware solution or as a combination of software and hardware, e.g., by one or more of: a processor or a micro-processor and adequate software and memory for storing of the software, a Programmable Logic Device (PLD) or other electronic component(s) or processing circuitry configured to perform the actions described above, and illustrated, e.g., in  FIG.  2   . 
       FIG.  6    is a block diagram of a terminal device  600  according to another embodiment of the present disclosure. 
     The terminal device  600  includes one or more processors  610  and one or more memories  620 . The memories  620  contain instructions executable by the processors  610  whereby the terminal device  600  is operative to perform the actions, e.g., of the procedure described earlier in conjunction with  FIG.  2   . Particularly, the memories  620  contain instructions executable by the processors  610  whereby the terminal device  600  is operative to: receive from a network device an instruction to precode each of a number, L, of DMRSs using a number, N, of linearly independent precoders, respectively; and transmit to the network device L*N precoded DMRSs. 
     In an embodiment, the L*N precoded DMRSs can be transmitted in N consecutive Transmission Time Intervals (TTIs), with L precoded DMRSs that are precoded using one of the N precoders being transmitted in one of the N TTIs. 
     In an embodiment, each of the L DMRSs can be used for one-layer transmission, where L can be smaller than a number of antennas at the terminal device and L*N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the number of antennas at the terminal device can be larger than or equal to 2. 
     In an embodiment, L can be equal to 1 and the number N can be larger than or equal to the number of antennas at the terminal device. 
     In an embodiment, the N precoders can be orthogonal to each other. 
     In an embodiment, the instruction can include an indication of the N precoders and can be received from the network device via Downlink Control Information (DCI). 
     The present disclosure also provides at least one computer program product in the form of a non-volatile or volatile memory, e.g., a non-transitory computer readable storage medium, an Electrically Erasable Programmable Read-Only Memory (EEPROM), a flash memory and a hard drive. The computer program product includes a computer program. The computer program includes: code/computer readable instructions, which when executed by the processors  410 , cause the network device  400  to perform the actions, e.g., of the procedure described earlier in conjunction with  FIG.  1   ; or code/computer readable instructions, which when executed by the processors  610 , cause the terminal device  600  to perform the actions, e.g., of the procedure described earlier in conjunction with  FIG.  2   . 
     The computer program product may be configured as a computer program code structured in computer program modules. The computer program modules could essentially perform the actions of the flow illustrated in  FIG.  1  or  2   . 
     The processor may be a single CPU (Central processing unit), but could also comprise two or more processing units. For example, the processor may include general purpose microprocessors; instruction set processors and/or related chips sets and/or special purpose microprocessors such as Application Specific Integrated Circuit (ASICs). The processor may also comprise board memory for caching purposes. The computer program may be carried by a computer program product connected to the processor. The computer program product may comprise a non-transitory computer readable storage medium on which the computer program is stored. For example, the computer program product may be a flash memory, a Random-access memory (RAM), a Read-Only Memory (ROM), or an EEPROM, and the computer program modules described above could in alternative embodiments be distributed on different computer program products in the form of memories. 
     With reference to  FIG.  7   , in accordance with an embodiment, a communication system includes a telecommunication network  710 , such as a 3GPP-type cellular network, which comprises an access network  711 , such as a radio access network, and a core network  714 . The access network  711  comprises a plurality of base stations  712   a ,  712   b ,  712   c , such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area  713   a ,  713   b ,  713   c . Each base station  712   a ,  712   b ,  712   c  is connectable to the core network  714  over a wired or wireless connection  715 . A first user equipment (UE)  791  located in coverage area  713   c  is configured to wirelessly connect to, or be paged by, the corresponding base station  712   c . A second UE  792  in coverage area  713   a  is wirelessly connectable to the corresponding base station  712   a . While a plurality of UEs  791 ,  792  are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding base station  712 . 
     The telecommunication network  710  is itself connected to a host computer  730 , which may be embodied in the hardware and/or software of a standalone server, a cloud-implemented server, a distributed server or as processing resources in a server farm. The host computer  730  may be under the ownership or control of a service provider, or may be operated by the service provider or on behalf of the service provider. The connections  721 ,  722  between the telecommunication network  710  and the host computer  730  may extend directly from the core network  714  to the host computer  730  or may go via an optional intermediate network  720 . The intermediate network  720  may be one of, or a combination of more than one of, a public, private or hosted network; the intermediate network  720 , if any, may be a backbone network or the Internet; in particular, the intermediate network  720  may comprise two or more sub-networks (not shown). 
     The communication system of  FIG.  7    as a whole enables connectivity between one of the connected UEs  791 ,  792  and the host computer  730 . The connectivity may be described as an over-the-top (OTT) connection  750 . The host computer  730  and the connected UEs  791 ,  792  are configured to communicate data and/or signaling via the OTT connection  750 , using the access network  711 , the core network  714 , any intermediate network  720  and possible further infrastructure (not shown) as intermediaries. The OTT connection  750  may be transparent in the sense that the participating communication devices through which the OTT connection  750  passes are unaware of routing of uplink and downlink communications. For example, a base station  712  may not or need not be informed about the past routing of an incoming downlink communication with data originating from a host computer  730  to be forwarded (e.g., handed over) to a connected UE  791 . Similarly, the base station  712  need not be aware of the future routing of an outgoing uplink communication originating from the UE  791  towards the host computer  730 . 
     Example implementations, in accordance with an embodiment, of the UE, base station and host computer discussed in the preceding paragraphs will now be described with reference to  FIG.  8   . In a communication system  800 , a host computer  810  comprises hardware  815  including a communication interface  816  configured to set up and maintain a wired or wireless connection with an interface of a different communication device of the communication system  800 . The host computer  810  further comprises processing circuitry  818 , which may have storage and/or processing capabilities. In particular, the processing circuitry  818  may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The host computer  810  further comprises software  811 , which is stored in or accessible by the host computer  810  and executable by the processing circuitry  818 . The software  811  includes a host application  812 . The host application  812  may be operable to provide a service to a remote user, such as a UE  830  connecting via an OTT connection  850  terminating at the UE  830  and the host computer  810 . In providing the service to the remote user, the host application  812  may provide user data which is transmitted using the OTT connection  850 . 
     The communication system  800  further includes a base station  820  provided in a telecommunication system and comprising hardware  825  enabling it to communicate with the host computer  810  and with the UE  830 . The hardware  825  may include a communication interface  826  for setting up and maintaining a wired or wireless connection with an interface of a different communication device of the communication system  800 , as well as a radio interface  827  for setting up and maintaining at least a wireless connection  870  with a UE  830  located in a coverage area (not shown in  FIG.  8   ) served by the base station  820 . The communication interface  826  may be configured to facilitate a connection  860  to the host computer  810 . The connection  860  may be direct or it may pass through a core network (not shown in  FIG.  8   ) of the telecommunication system and/or through one or more intermediate networks outside the telecommunication system. In the embodiment shown, the hardware  825  of the base station  820  further includes processing circuitry  828 , which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The base station  820  further has software  821  stored internally or accessible via an external connection. 
     The communication system  800  further includes the UE  830  already referred to. Its hardware  835  may include a radio interface  837  configured to set up and maintain a wireless connection  870  with a base station serving a coverage area in which the UE  830  is currently located. The hardware  835  of the UE  830  further includes processing circuitry  838 , which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The UE  830  further comprises software  831 , which is stored in or accessible by the UE  830  and executable by the processing circuitry  838 . The software  831  includes a client application  832 . The client application  832  may be operable to provide a service to a human or non-human user via the UE  830 , with the support of the host computer  810 . In the host computer  810 , an executing host application  812  may communicate with the executing client application  832  via the OTT connection  850  terminating at the UE  830  and the host computer  810 . In providing the service to the user, the client application  832  may receive request data from the host application  812  and provide user data in response to the request data. The OTT connection  850  may transfer both the request data and the user data. The client application  832  may interact with the user to generate the user data that it provides. 
     It is noted that the host computer  810 , base station  820  and UE  830  illustrated in  FIG.  8    may be identical to the host computer  730 , one of the base stations  712   a ,  712   b ,  712   c  and one of the UEs  791 ,  792  of  FIG.  7   , respectively. This is to say, the inner workings of these entities may be as shown in  FIG.  8    and independently, the surrounding network topology may be that of  FIG.  7   . 
     In  FIG.  8   , the OTT connection  850  has been drawn abstractly to illustrate the communication between the host computer  810  and the use equipment  830  via the base station  820 , without explicit reference to any intermediary devices and the precise routing of messages via these devices. Network infrastructure may determine the routing, which it may be configured to hide from the UE  830  or from the service provider operating the host computer  810 , or both. While the OTT connection  850  is active, the network infrastructure may further take decisions by which it dynamically changes the routing (e.g., on the basis of load balancing consideration or reconfiguration of the network). 
     The wireless connection  870  between the UE  830  and the base station  820  is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to the UE  830  using the OTT connection  850 , in which the wireless connection  870  forms the last segment. More precisely, the teachings of these embodiments may improve the data rate and thereby provide benefits such as reduced user waiting time. 
     A measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection  850  between the host computer  810  and UE  830 , in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection  850  may be implemented in the software  811  of the host computer  810  or in the software  831  of the UE  830 , or both. In embodiments, sensors (not shown) may be deployed in or in association with communication devices through which the OTT connection  850  passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software  811 ,  831  may compute or estimate the monitored quantities. The reconfiguring of the OTT connection  850  may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not affect the base station  820 , and it may be unknown or imperceptible to the base station  820 . Such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling facilitating the host computer&#39;s  810  measurements of throughput, propagation times, latency and the like. The measurements may be implemented in that the software  811 ,  831  causes messages to be transmitted, in particular empty or ‘dummy’ messages, using the OTT connection  850  while it monitors propagation times, errors etc. 
       FIG.  9    is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIGS.  7  and  8   . For simplicity of the present disclosure, only drawing references to  FIG.  9    will be included in this section. In a first step  910  of the method, the host computer provides user data. In an optional substep  911  of the first step  910 , the host computer provides the user data by executing a host application. In a second step  920 , the host computer initiates a transmission carrying the user data to the UE. In an optional third step  930 , the base station transmits to the UE the user data which was carried in the transmission that the host computer initiated, in accordance with the teachings of the embodiments described throughout this disclosure. In an optional fourth step  940 , the UE executes a client application associated with the host application executed by the host computer. 
       FIG.  10    is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIGS.  7  and  8   . For simplicity of the present disclosure, only drawing references to  FIG.  10    will be included in this section. In a first step  1010  of the method, the host computer provides user data. In an optional substep (not shown) the host computer provides the user data by executing a host application. In a second step  1020 , the host computer initiates a transmission carrying the user data to the UE. The transmission may pass via the base station, in accordance with the teachings of the embodiments described throughout this disclosure. In an optional third step  1030 , the UE receives the user data carried in the transmission. 
       FIG.  11    is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIGS.  7  and  8   . For simplicity of the present disclosure, only drawing references to  FIG.  11    will be included in this section. In an optional first step  1110  of the method, the UE receives input data provided by the host computer. Additionally or alternatively, in an optional second step  1120 , the UE provides user data. In an optional substep  1121  of the second step  1120 , the UE provides the user data by executing a client application. In a further optional substep  1111  of the first step  1110 , the UE executes a client application which provides the user data in reaction to the received input data provided by the host computer. In providing the user data, the executed client application may further consider user input received from the user. Regardless of the specific manner in which the user data was provided, the UE initiates, in an optional third substep  1130 , transmission of the user data to the host computer. In a fourth step  1140  of the method, the host computer receives the user data transmitted from the UE, in accordance with the teachings of the embodiments described throughout this disclosure. 
       FIG.  12    is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to  FIGS.  7  and  8   . For simplicity of the present disclosure, only drawing references to  FIG.  12    will be included in this section. In an optional first step  1210  of the method, in accordance with the teachings of the embodiments described throughout this disclosure, the base station receives user data from the UE. In an optional second step  1220 , the base station initiates transmission of the received user data to the host computer. In a third step  1230 , the host computer receives the user data carried in the transmission initiated by the base station. 
     The disclosure has been described above with reference to embodiments thereof. It should be understood that various modifications, alternations and additions can be made by those skilled in the art without departing from the spirits and scope of the disclosure. Therefore, the scope of the disclosure is not limited to the above particular embodiments but only defined by the claims as attached.