Patent Publication Number: US-2023163932-A1

Title: Coverage enhancement

Description:
TECHNICAL FIELD 
     Various examples generally relate to enhancing coverage of a wireless communication system. Various examples specifically relate to setting a time-frequency resource mapping of resource elements allocated to reference signals used for channel estimation. 
     BACKGROUND 
     Wireless communication is widespread. A communications network can connect to multiple wireless communication devices (UEs), thereby forming a communication system. Coverage is an important aspect to consider when designing a communication system. This helps to guarantee quality of service even for UEs faraway from access points of the communications network. 
     Measures are taken to facilitate coverage enhancement (CE). 
     For example, in the Third Generation Partnership Project (3GPP), there is a study item, named FS_NR_CovEnh, for 3GPP Release 17. The objective of this study item is to “study potential coverage enhancement solutions for specific scenarios for both FR1 and FR2.” 
     SUMMARY 
     Accordingly, there is a need for advanced techniques to increase coverage for communication systems. 
     This need is met by the features of the independent claims. The features of the dependent claims define embodiments. 
     A method of operating a wireless communication device is provided. The method includes receiving at least one downlink control message from a communications network, the at least one downlink control message being indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network. By means of the estimation of the radio channel, it is possible to coherently decode data signals encoding data of a uplink data transmission. The uplink reference signals and the data signals use the same precoding. The method also includes transmitting the uplink reference signals on the radio channel using a time-frequency resource mapping. The time-frequency resource mapping is set in accordance with the frequency density. The time-frequency resource mapping allocates time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     For example, prior to transmitting the uplink reference signals using the time-frequency resource mapping, the uplink reference signals may be transmitted using a further time-frequency resource mapping. 
     It would be possible that the frequency density associated with the time-frequency resource mapping is reduced if compared to a further frequency density associated with the further time-frequency resource mapping. Thereby, the adjustment of the frequency density can be achieved. 
     It would be possible that an average frequency density is adjusted. 
     It would be possible that—when reducing the frequency density—the minimum frequency spacing between nearest-neighbor resource elements allocated to the reference signals is increased. 
     It would be possible that—when adjusting the frequency density—a type of the reference signals—e.g., characterized by the count of antenna ports used for said transmitting—remains fixed. 
     It would be possible that—when adjusting the frequency density—a pattern of the time-frequency resource mapping remains fixed. 
     It would be possible that multiple adjustment of the frequency density are executed. It would be possible that multiple adjustment of the frequency density are executed per radio frame, e.g., the frequency density can be adjusted multiple times within 10 ms. It would be possible that the frequency density is adjusted at least twice within 50 ms or within 100 ms. 
     A computer program or a computer-program product or a computer-readable storage medium includes program code. The program code can be loaded and executed by a control circuitry. Upon loading and executing, the at least one processor performs a method of operating a wireless communication device. The method includes receiving at least one downlink control message from a communications network, the at least one downlink control message being indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network. By means of the estimation of the radio channel, it is possible to coherently decode data signals encoding data of a uplink data transmission. The uplink reference signals and the data signals use the same precoding. The method also includes transmitting the uplink reference signals on the radio channel using a time-frequency resource mapping. The time-frequency resource mapping is set in accordance with the frequency density. The time-frequency resource mapping allocates time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     A wireless communication device includes a control circuitry configured to receive at least one downlink control message from a communications network. The at least one downlink control message is indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network to coherently decode data signals encoding data of a uplink data transmission. The uplink reference signals and the data signals use the same precoding. The control circuitry is further configured to transmit the uplink reference signals on the radio channel using a time-frequency resource mapping set in accordance with the frequency density. The time-frequency resource mapping allocates time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     A method of operating an access node of a communications network is provided. The method includes transmitting at least one downlink control message to a wireless communication device, the at least one downlink control message being indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network to coherently decode data signals encoding data of uplink data transmission. The uplink reference signals and the data signals use the same precoding. The method further includes receiving the uplink reference signals on the radio channel using a time-frequency resource mapping set in accordance with the frequency density, the time-frequency resource mapping allocating time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     A computer program or a computer-program product or a computer-readable storage medium includes program code. The program code can be loaded and executed by a control circuitry. Upon loading and executing, the at least one processor performs a method of operating an access node of a communications network. The method includes transmitting at least one downlink control message to a wireless communication device, the at least one downlink control message being indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network to coherently decode data signals encoding data of uplink data transmission. The uplink reference signals and the data signals use the same precoding. The method further includes receiving the uplink reference signals on the radio channel using a time-frequency resource mapping set in accordance with the frequency density, the time-frequency resource mapping allocating time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     An access node of a communications network is provided. The access node includes a control circuitry. The control circuitry is configured to transmit at least one downlink control message to a wireless communication device, the at least one downlink control message being indicative of a frequency density of uplink reference signals. The uplink reference signals are for an estimation of a radio channel between the wireless communication device and the communications network to coherently decode data signals encoding data of uplink data transmission. The uplink reference signals and the data signals use the same precoding. The control circuitry is further configured to receive the uplink reference signals on the radio channel using a time-frequency resource mapping set in accordance with the frequency density, the time-frequency resource mapping allocating time-frequency resource elements of a plurality of time-frequency resource elements of a time-frequency resource grid of the radio channel to the uplink reference signals. 
     A method of operating a wireless communication device in a coverage enhancement mode of a data transmission comprising data is provided. The method comprises transmitting a redundancy version of the data using a code rate that is below 1/100, wherein a length of the redundancy version is longer than one or more slots, e.g., at least longer than 10 slots. 
     A method of operating a wireless communication device in a coverage enhancement mode of a data transmission comprising data is provided. The method comprises transmitting multiple repetitions of a given redundancy version contemporaneously in time and offset in frequency domain. 
     It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically illustrates a cellular network and a UE forming a communication system according to various examples. 
         FIG.  2    schematically illustrates a time-frequency resource grid of a radio link between the UE and the cellular network according to various examples. 
         FIG.  3    schematically illustrates a base station of the cellular network according to various examples. 
         FIG.  4    schematically illustrates the UE according to various examples. 
         FIG.  5    is a flowchart of a method according to various examples. 
         FIG.  6    schematically illustrates dynamically adjusting a frequency density of reference signals according to various examples. 
         FIG.  7    schematically illustrates different frequency densities of reference signals implemented by different time-frequency resource mappings of resources allocated to the reference signals according to various examples. 
         FIG.  8    schematically illustrates allocating resources to reference signals that are orthogonally polarized according to various examples. 
         FIG.  9    is a signaling diagram according to various examples. 
         FIG.  10    is a flowchart of a method according to various examples. 
         FIG.  11    schematically illustrates an adjustment of the frequency density of reference signals according to various examples. 
         FIG.  12    schematically illustrates an adjustment of the frequency density of reference signals according to various examples. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Some examples of the present disclosure generally provide for a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and/or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of microcontrollers, a graphics processor unit (GPU), integrated circuits, memory devices (e.g., FLASH, random access memory (RAM), read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), or other suitable variants thereof), and software which co-act with one another to perform operation(s) disclosed herein. In addition, any one or more of the electrical devices may be configured to execute a program code that is embodied in a non-transitory computer readable medium programmed to perform any number of the functions as disclosed. 
     In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only. 
     The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof. 
     Hereinafter, techniques of wireless communication are described. Respective communication systems can include one or more access nodes of a communications network and one or more UEs connectable to the communications network via the one or more access nodes. A specific implementation of a communications network is a cellular network having multiple cells each being served by one or more access points being implemented as base stations (BSs). 
     Communication of signals from a UE to the cellular network is referred to as uplink (UL) communication and communication of signals from the cellular network to a UE is referred to as downlink (DL) communication. The techniques described herein can be applicable to UL communication, as well as DL communication. 
     A message communicated in the uplink can be referred to as uplink message and a message communicated in the downlink can be referred to as downlink message. 
     For illustrative purposes, various examples are described in the context of UL communication in a 3GPP New Radio (NR) communication system operating in frequency range 2 (FR2). However, similar techniques can be applied to other kinds and types of communication systems and/or to DL communication or even peer-to-peer communication, sometimes labeled sidelink in the context of a cellular network. 
     Various techniques are described herein which enhance the coverage of a data transmission in a communication system. This means that for a given data rate, the distance at which transmissions can be received reliably is increased. Conversely, it can also mean that for a given distance, the maximum data rate at which transmissions can be reliably received is increased. 
     Some of the techniques to enhance coverage are summarized in TAB. 1 below: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Options for enhancing coverage. The techniques A, B, 
               
               
                 and C all involve a transmission of RSs. The techniques 
               
               
                 D and E relate to CE protection of a data of a data 
               
               
                 transmission by appropriately setting CE parameters. 
               
            
           
           
               
               
               
            
               
                 Tech- 
                 Brief 
                   
               
               
                 nique 
                 description 
                 Example details 
               
               
                   
               
               
                 A 
                 Configuring a 
                 The transmission of RSs can be configured 
               
               
                   
                 transmission of 
                 by setting one or more properties of a time- 
               
               
                   
                 reference 
                 frequency resource mapping of the 
               
               
                   
                 signals (RSs) 
                 resources allocated to the RSs. For example, 
               
               
                   
                 by setting a 
                 adjustment of a frequency density and/or a 
               
               
                   
                 time-frequency 
                 time density of the resources would be 
               
               
                   
                 resource 
                 possible. For example, the frequency density 
               
               
                   
                 mapping 
                 may be adjusted by adjusting a count of RSs 
               
               
                   
                 of resources 
                 per frequency unit and per time unit. For 
               
               
                   
                 allocated to 
                 example, the frequency density may be 
               
               
                   
                 the RSs 
                 adjusted by adjusting a frequency spacing 
               
               
                   
                   
                 between adjacent RSs. The frequency 
               
               
                   
                   
                 density may be adjusted while the timing of 
               
               
                   
                   
                 resource elements allocated to the RSs is 
               
               
                   
                   
                 kept constant; likewise, the time density may 
               
               
                   
                   
                 be adjusted while the frequencies of resource 
               
               
                   
                   
                 elements allocated to the RSs is kept 
               
               
                   
                   
                 constant. 
               
               
                   
                   
                 By configuring the transmission of the RSs, it 
               
               
                   
                   
                 is possible to free-up additional resources for 
               
               
                   
                   
                 protective measures to obtain the CE for the 
               
               
                   
                   
                 data transmission. Example protective 
               
               
                   
                   
                 measures include transmitting error 
               
               
                   
                   
                 protection bits of a forward error correct, 
               
               
                   
                   
                 transmitting additional repetitions of data, 
               
               
                   
                   
                 etc.. Likewise, transmit power from the freed- 
               
               
                   
                   
                 up RSs can be reallocated to the data 
               
               
                   
                   
                 transmission, to obtain further CE. 
               
               
                 B 
                 Configuring a 
                 This can be seen as an extreme case of 
               
               
                   
                 transmission 
                 technique A, in which the frequency density 
               
               
                   
                 of RSs by 
                 and time density is reduced to 0. This can be 
               
               
                   
                 temporarily 
                 referred to RS-less operation. For instance, 
               
               
                   
                 suspending 
                 the RSs may be suspended upon activating 
               
               
                   
                 transmitting 
                 a CE transmission mode for the data 
               
               
                   
                 of the RSs 
                 transmission. 
               
               
                   
                   
                 In this case, the receiver needs to decode the 
               
               
                   
                   
                 received data signals blindly, i.e., in the 
               
               
                   
                   
                 absence of channel estimates. 
               
               
                   
                   
                 The transmission of RSs can be suspended 
               
               
                   
                   
                 until further notice or for a predetermined 
               
               
                   
                   
                 amount of time, e.g., measured in multiples 
               
               
                   
                   
                 of subframes or frames. The transmission of 
               
               
                   
                   
                 RSs can be suspended until detection of a 
               
               
                   
                   
                 predetermined trigger event. 
               
               
                 C 
                 Configuring a 
                 By using polarized RSs, the polarization of 
               
               
                   
                 transmission 
                 the radio channel can be estimated and, 
               
               
                   
                 of RS by 
                 thus, the communication quality can be 
               
               
                   
                 defining a 
                 increased. This leads to CE. 
               
               
                   
                 polarization 
                 The transmitter can increase the amount of 
               
               
                   
                 of RSs 
                 power reaching the receiver by aligning its 
               
               
                   
                   
                 transmissions with the direction of 
               
               
                   
                   
                 polarization of the radio channel at the 
               
               
                   
                   
                 transmit end of the communication link. 
               
               
                   
                   
                 Similarly, the receiver should also align 
               
               
                   
                   
                 reception to the direction of polarization of 
               
               
                   
                   
                 the channel at the receive link-end. For this 
               
               
                   
                   
                 to be possible, UL RSs can be transmitted 
               
               
                   
                   
                 using two linearly independent polarizations 
               
               
                   
                   
                 in distinct resources, i.e., a 0°-polarized RS 
               
               
                   
                   
                 and a 90°-polarized RS (0° and 90° are 
               
               
                   
                   
                 relatively defined). Compared to RS 
               
               
                   
                   
                 transmissions using a single polarization, the 
               
               
                   
                   
                 proposed technique requires double as many 
               
               
                   
                   
                 resources. 
               
               
                   
                   
                 This may be implemented in one of the 
               
               
                   
                   
                 following ways: (i) By using a feedback 
               
               
                   
                   
                 signaling indicating an observed polarization 
               
               
                   
                   
                 of the transmitted RS. In an example, the UE 
               
               
                   
                   
                 transmits two RSs of different polarization to 
               
               
                   
                   
                 the BS, the BS estimates a polarization of the 
               
               
                   
                   
                 received RS, the BS then signals the 
               
               
                   
                   
                 estimated optimal polarization direction to 
               
               
                   
                   
                 UE, and the UE aligns subsequent data 
               
               
                   
                   
                 transmissions according to the polarization 
               
               
                   
                   
                 feedback by the BS. The UE can continue to 
               
               
                   
                   
                 transmit the polarized RSs to the BS. (ii) By 
               
               
                   
                   
                 sending RSs in the reverse direction, i.e., DL. 
               
               
                   
                   
                 These DL RS can be used by the UE to 
               
               
                   
                   
                 estimate a suitable polarization for UL data 
               
               
                   
                   
                 transmissions and associated UL RS 
               
               
                   
                   
                 transmissions. In this case, no feedback 
               
               
                   
                   
                 channel is required. 
               
               
                 D 
                 Ultra-low 
                 The code rate of a forward error correction 
               
               
                   
                 code rate 
                 code is the proportion of the data-stream that 
               
               
                   
                   
                 is useful (non-redundant). The code rate thus 
               
               
                   
                   
                 is associated with the ratio of the data rate 
               
               
                   
                   
                 that is allocated for a time instance and the 
               
               
                   
                   
                 maximum data rate that ideally can be 
               
               
                   
                   
                 allocated in the same time instance. A lower 
               
               
                   
                   
                 code rate means that more redundancy bits 
               
               
                   
                   
                 are inserted during the channel coding 
               
               
                   
                   
                 process and a higher code rate means that 
               
               
                   
                   
                 less redundancy bits are inserted. 
               
               
                   
                   
                 A code rate of less than, e.g., 1/100 can be 
               
               
                   
                   
                 applied. Lower code rates tend to lead to CE. 
               
               
                   
                   
                 To achieve such low code rates, 
               
               
                   
                   
                 transmissions may span several slots in time. 
               
               
                   
                   
                 In extremely poor coverage conditions, UEs 
               
               
                   
                   
                 may request, and the BS may schedule a 
               
               
                   
                   
                 single repetition of a given redundancy 
               
               
                   
                   
                 version of the data of the data transmission 
               
               
                   
                   
                 to extend over several slots with the purpose 
               
               
                   
                   
                 of overcoming temporary or permanent 
               
               
                   
                   
                 unusually large pathlosses. I.e., as a general 
               
               
                   
                   
                 rule, the length of the given redundancy 
               
               
                   
                   
                 version (e.g., the lowest redundancy version) 
               
               
                   
                   
                 can be longer than one or two slots. 
               
               
                   
                   
                 Repetitions of the given redundancy version 
               
               
                   
                   
                 may not be required. The net result is an 
               
               
                   
                   
                 encoding with ultra-low code rates. Ultra-low 
               
               
                   
                   
                 code rates can also be used in conjunction 
               
               
                   
                   
                 with blind decoding (see technique B above). 
               
               
                 E 
                 Repetitions of 
                 The same redundancy version of data can be 
               
               
                   
                 redundancy 
                 transmitted multiple times. Associated RSs 
               
               
                   
                 versions of 
                 can be transmitted in all, some or none of the 
               
               
                   
                 data of the data 
                 repetitions. 
               
               
                   
                 transmission 
                 In poor coverage conditions communication 
               
               
                   
                   
                 is typically limited by the amount of available 
               
               
                   
                   
                 power while, at the same time, there might be 
               
               
                   
                   
                 an excess of REs available for 
               
               
                   
                   
                 communication. Under such conditions, 
               
               
                   
                   
                 communication reliability can be improved by 
               
               
                   
                   
                 including additional redundancy in all the 
               
               
                   
                   
                 available resources, by repetitions of the 
               
               
                   
                   
                 same redundancy version of the data. 
               
               
                   
                   
                 Different redundancy versions can be offset 
               
               
                   
                   
                 in frequency domain, e.g., of the same slot or 
               
               
                   
                   
                 subframe (i.e., concurrent transmission of a 
               
               
                   
                   
                 packet in the frequency domain). This could 
               
               
                   
                   
                 be beneficial, e.g., for communication with 
               
               
                   
                   
                 users at the cell edge when a large 
               
               
                   
                   
                 bandwidth is available which cannot be used 
               
               
                   
                   
                 completely because no coding schemes are 
               
               
                   
                   
                 available with sufficiently low code rates. 
               
               
                   
               
            
           
         
       
     
     The techniques to increase coverage disclosed herein, in particular according to TAB. 1, can be applied jointly or independently. The techniques disclosed herein apply to data transmissions of payload data and/or of control data (e.g., Layer 3 RRC control data), to transmissions in the UL directions and/or in the DL direction. Likewise, they can be applied to various frequency ranges of interest. 
     It has been found that UL communication in FR2 has the potential to benefit significantly from the techniques described herein, in terms of increased coverage. There are two reasons for this. First, it follows from the well-known Friis transmission equation in free space that the pathloss increases with the square of the carrier frequency. Thus, for a given transmit power constraint, the area covered by a wireless communication system shrinks as the frequency of operations increases. Second, UEs are typically battery-powered and therefore coverage is more limited for UL communication compared to DL communication (the BS is connected to the energy grid). The reader will appreciate, however, that the method herein disclosed can also be applied to DL communication, as well as to sidelink or device-to-device communications. Moreover, the method disclosed is not limited to FR2 but can also be applied to bands in other frequency ranges, such as frequency range 1 (FR1). 
     Various techniques described herein rely on channel estimation of the radio channel between the cellular network and the UE. The channel estimation can facilitate coherent decoding of data signals at a receiver, e.g., for an Orthogonal Frequency Division Multiple (OFDM) modulation. Such RSs are, thus, also referred to as demodulation RSs (DM-RS). 
     RSs (sometimes also referred to as pilot signals) are transmitted on the radio channel, e.g., in the UL. 
     As a general rule, RSs have a well-defined transmit shape and, thus, it is possible to estimate the impact of the radio channel on the observed receive shape by using the transmit shape as a baseline. One or more receive properties—e.g., amplitude and phase—may be determined for this purpose at the receiver. 
     The RSs may, in some examples, be precoded. I.e., a spatial filter filter may be applied to the respective waveforms, using amplitude and phase relationships between different antenna elements. RSs are to be distinguished from signals encoding data (data signals)—e.g., control data and/or payload data; because the data is a-priori unknown to the receiver, also the transmit shape is not well-defined or known to the receiver. The RSs can be specific to a UE, i.e., uniquely associated with a UE. This can be due to beamforming and can be different to non-precoded RSs, e.g., channel reference signal (CRS) used in 3GPP 4G. 
     As a general rule, the RSs can be associated with a payload transmission including payload data: The RSs can facilitate estimating the channel to then perform coherent decoding of the data signals encoding the data of the data transmission based on the channel estimate, to recover the data. For this purpose, a time-frequency resource mapping of the RSs to a resource grid of the radio channel including a grid of time-frequency resource elements (REs) can allocate REs to the RSs across the frequency range covered by the data transmission. In particular, it is possible that the REs allocated to the RSs are interspersed with the REs allocated to the data transmission: I.e., it is possible that one set of REs is scheduled by a scheduler (i.e., a scheduling control message may be indicative of the set of REs) and that these REs are then allocated by the UE to the RSs or data signals, by selecting the REs allocated to the RSs from the set and selecting the REs allocated to the data signals from the set. Thus, different REs can be allocated to the RSs than to the data signals. 
     It would be possible that the time-frequency resources allocated to the RSs are restricted to such frequencies that are actually used for the data transmission. The RSs are pre-coded in the same manner as the data transmission, i.e., using the same spatial filters. 
     The RSs can be tailored to have a small power variation in the frequency domain to allow for accurate estimation of the radio channel across the entire frequency range spend by the data transmission. For this, a pseudo-random sequence—e.g., a Gold sequence—can be used, wherein the sequence may be generated across all time-frequency resources in the frequency range. 
     An example of RSs subject to the techniques described herein are 3GPP NR DM-RS, see, e.g., Dahlman, Erik, Stefan Parkvall, and Johan Skold. 5G NR: The next generation wireless access technology. Academic Press, 2018, chapter 9.11.1 DEMODULATION RSS FOR OFDM-BASED DOWNLINK AND UPLINK. 
     The DM-RS can be associated with Physical Uplink Shared Channel (PUSCH) data transmissions. DM-RSs can also be associated with data transmission on the Physical Uplink Control Channel (PUCCH), or—in the DL—with dated transmissions on the Physical Downlink Shared Channel (PDSCH) or the Physical Downlink Control Channel (PDCCH). 
     According to various techniques described herein, the time-frequency mapping of the RSs to the time-frequency resource grid of the radio channel can be dynamically changed or adjusted (see TAB. 1, techniques A and B). This may be network-controlled, e.g., using Layer 3 or Layer 2 or Layer 1 control signaling. It could also be initiated by the UE. 
     As a general rule, various options are available for dynamically changing the time-frequency resource mapping. For example, a time spacing between subsequently transmitted RSs may be adjusted. Alternatively or additionally, a frequency spacing between adjacent RSs—e.g., located in the same transmission slot—can be adjusted. The frequency density of the RSs may be adjusted. A hopping pattern defining a variation in the time position and/or frequency position—e.g., defined with respect to RBs—from RS occasion to RS occasion could be adjusted. By such techniques, it is possible to tailor the time-frequency resource mapping. In particular, the time domain and/or frequency domain density of RSs can be tailored in view of the coverage situation. For instance, it has been found that for a poor-coverage situation (e.g., cell edge), a different time domain and/or frequency domain density of the RSs can be helpful if compared to a good-coverage situation. 
     In particular, the frequency density of RSs can be adjusted by means of the parameter pilotSeparation, denoted Δ P , which determines the distance between two REs adjacent in the frequency domain and which are allocated to RSs (i.e., it defines a frequency spacing). For example, a setting Δ P =2, representative of Rel. 16 NR 5G for PUSCH transmissions, corresponds to ½ of the resource elements (REs) in a symbol being allocated to PUSCH DM-RSs. On the other hand, by setting Δ P =24, only every 24th RE in a symbol is allocated to PUSCH DM-RSs. Hence, not all resource blocks (RBs; each RB includes multiple REs) allocated to a UE for PUSCH transmissions would carry PUSCH DM-RSs: the frequency-density of the PUSCH DM-RS would be smaller than one per RB. Different RBs can carry a different count of PUSCH DM-RS. Thereby, low frequency densities of the DM-RSs can be achieved, which can be helpful to extend coverage where the freed-up resource elements are used to protect the data transmission, by taking CE measures. 
     As will be appreciated from the above, by virtue of the proposed adjustment mechanism for the frequency density of the RSs, the density of RSs can be seen belonging to a continuum. RSs can be equally spread out over the covered bandwidth. At one end of the spectrum of available densities, data transmissions of payload or control data without associated RSs, so-called RS-less transmissions, may be used. 
     This corresponds to Δ P =∞ (see TAB. 1, technique B). At the other end of the spectrum, Δ P =1, and all the REs in the OFDM symbol are allocated for RSs. 
     As a general rule, it would be possible that one or more other configuration parameters of the RSs transmission are fixed, when adjusting the frequency density. For example, the frequency density can be adjusted while a count of antenna ports used for transmitting the RSs remains fixed. Alternatively or additionally, the frequency density can be adjusted while a pattern of the time-frequency resource mapping of the REs allocated to the RSs remains fixed; i.e., all frequency spacings encountered within the frequency range are equally adjusted, without adding further frequency spacings. By selectively adjusting the frequency density—while keeping one or more other configuration parameters of the RS transmission fixed—it can be possible to avoid a significant reconfiguration of the operation at the UE and/or the BS, which facilitates fast adjustments. Thereby, changes to the channel can be tracked, in particular when—along with adjustments to frequency density—the UE switches from a first setting of a CE parameter to a second setting of the CE parameter. 
     For illustration, it would be possible that—along with commencing said transmitting of the RSs in accordance with the time-frequency resource mapping that is set in accordance with the adjusted frequency density —, the UE switches from a first setting of a CE parameter used for the uplink data transmission to a second setting of the CE parameter used for the uplink data transmission. This means that the setting of the CE parameter for the data transmission can be correlated with the adjustment of the frequency density (or, generally, any re-configuration of the RS transmission). In short, the re-configuration of the RS transmission and occurs for the purpose of CE, i.e., the CE parameter is set in a correlated fashion. For example, the switch between setting of the CE parameter may occur at the same time at which the adjusted frequency density is implemented; or it would be possible to consider a predefined time offset. 
     A few CE parameters that can be subject to the techniques described herein are summarized in TAB. 2 below. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 CE parameters that can be set, e.g., in a correlated 
               
               
                 manner with adjusting a frequency density of RSs. 
               
            
           
           
               
               
            
               
                 CE parameter 
                 Details 
               
               
                   
               
               
                 Transmit power of data 
                 A higher transmit power tends to increase the 
               
               
                 signals 
                 coverage. At the same time, the maximum 
               
               
                   
                 usable transmit power may be limited by a 
               
               
                   
                 transmit power budget per time and frequency, 
               
               
                   
                 e.g., due to hardware restrictions of the 
               
               
                   
                 transmitter circuitry. Thus, an aggregate 
               
               
                   
                 transmit power may be limited to some 
               
               
                   
                 threshold. Thus, where the frequency density 
               
               
                   
                 of the RSs is reduced, the transmit power 
               
               
                   
                 budget may be redistributed to the data 
               
               
                   
                 signals, from what was previously used for 
               
               
                   
                 transmitting the RSs. 
               
               
                   
                 In detail: in poor coverage conditions, wireless 
               
               
                   
                 communications are typically limited by the 
               
               
                   
                 available power for transmissions (see TAB. 2, 
               
               
                   
                 “Transmit power of data signals”). This means 
               
               
                   
                 that data rates can only be effectively increased 
               
               
                   
                 if more power is made available to the 
               
               
                   
                 transmitter and/or receiver. Here, hardware 
               
               
                   
                 restrictions of the wireless interface of the UE 
               
               
                   
                 can impose a transmit power budget. Because 
               
               
                   
                 of this, UEs at the cell edge will typically use 
               
               
                   
                 waveforms with lower peak-to-average ratio 
               
               
                   
                 (PAPR), such DFT-spread OFDM, for UL data 
               
               
                   
                 transmissions, such as PUSCH transmissions. 
               
               
                   
                 Likewise, UL data transmission of control data, 
               
               
                   
                 such as PUCCH transmissions, will typically 
               
               
                   
                 resort to sequences with low PAPR, such as 
               
               
                   
                 Zadoff-Chu sequences. To provide additional 
               
               
                   
                 transmit power to the data transmission, it is 
               
               
                   
                 possible to reduce the frequency density of UL 
               
               
                   
                 RSs. Then, the available transmit power budget 
               
               
                   
                 can be redistributed to provide higher transmit 
               
               
                   
                 power for data signals including the data of the 
               
               
                   
                 data transmission. 
               
               
                 Redundancy repetition 
                 A redundancy version of data—e.g., defined 
               
               
                 count of the data 
                 by a certain error correction checksum and 
               
               
                   
                 code rate—may be repeatedly transmitted so 
               
               
                   
                 that at the receiver, after combining the 
               
               
                   
                 respective data signals, a coherent decoding of 
               
               
                   
                 the combinations of combined data signals is 
               
               
                   
                 possible. This is different to Automatic Repeat 
               
               
                   
                 Request retransmissions which are on-demand 
               
               
                   
                 and triggered after a decoding attempt. Thus, 
               
               
                   
                 redundancy repetitions may be labeled blind 
               
               
                   
                 repetitions. Thus, upon reducing the frequency 
               
               
                   
                 density of the RSs, more resource element 
               
               
                   
                 become available for carrying redundancy 
               
               
                   
                 repetitions of the data. Thus the count of 
               
               
                   
                 repetitions can be increased. 
               
               
                   
                 As a general rule, redundancy repetitions may 
               
               
                   
                 be arranged offset in time domain and/or 
               
               
                   
                 frequency domain. 
               
               
                   
                 Also see Tab 1, technique E. 
               
               
                 Code rate of the data 
                 Upon reducing the frequency density of the 
               
               
                 of the uplink data 
                 RSs, more resource elements become 
               
               
                 transmission 
                 available for carrying redundancy bits. 
               
               
                   
                 Coverage can be enhanced. 
               
               
                   
               
            
           
         
       
     
     According to the techniques described herein, the parameter pilotSeparation—or another parameter of the configuration of the RSs transmission—is dynamic and can be set by control signaling. For example, the said parameter can be part of a L3 Radio Resource Control (RRC) information element for PUSCH-DM-RS configuration. Alternatively, or additionally, it can also set via L1 (PHY) and/or layer L2 (MAC) control signaling. For example, a DL control message may be indicative of the parameter in explicit or implicit form. For instance, a relative change of the parameter may be signaled, with respect to a current value of the parameter. Thereby, incremental changes can be implemented. It would also be possible that the DL control message includes an indicator indicative of an entry of a codebook that is predefined and includes multiple candidate values for the parameter. In particular, the codebook may include not less than 3 entries, preferably not less than 10 entries, more preferably not less than 100 entries. The codebook can be pre-configured in the UE and the BS, e.g., by negotiations. The codebook is synchronized between both sides, i.e., known to both sides. The codebook could be fixed by standardization. 
     As a general rule, various triggers are conceivable for adjusting a parameter of the configuration of the RS transmission such as the frequency density. Some of these trigger criteria are summarized in TAB. 3 below. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Trigger criteria for adjusting a configuration of the RS transmission, 
               
               
                 e.g., for adjusting the frequency density. A combination 
               
               
                 of one or more such trigger criteria would be possible. A 
               
               
                 hierarchy between trigger criteria could be implemented, 
               
               
                 e.g., using a primary and secondary decision criteria. 
               
            
           
           
               
               
            
               
                 Trigger 
                 Explanation 
               
               
                   
               
               
                 Coherence 
                 In general, is suffices to allocate one RE per coherence 
               
               
                 bandwidth 
                 bandwidth interval to RSs. Note that the coherence 
               
               
                 of the radio 
                 bandwidth depends on the characteristics of the multipath 
               
               
                 channel 
                 propagation. In particular, it depends on the delay spread 
               
               
                   
                 of the radio channel. Various techniques are based on the 
               
               
                   
                 finding that, when precoding is used—as is typical for 
               
               
                   
                 higher frequencies exhibiting higher pathloss—, there can 
               
               
                   
                 be a tendency of reduced multipath propagation. 
               
               
                   
                 Accordingly, the coherence bandwidth can be large. 
               
               
                   
                 For instance, the coherence bandwidth of the radio 
               
               
                   
                 channel drops below a certain value that is predefined, the 
               
               
                   
                 parameter of the configuration of the RS transmission can 
               
               
                   
                 be adjusted. 
               
               
                   
                 It would also be possible that the parameter of the 
               
               
                   
                 configuration of the RS transmission—e.g., the frequency 
               
               
                   
                 density—is continuously readjusted based on tracking 
               
               
                   
                 changes to the coherence bandwidth. 
               
               
                 Quality of 
                 The quality of the received signal may be measured, e.g., 
               
               
                 the received 
                 in terms of the signal-to-noise ratio (SNR) and/or the 
               
               
                 signal 
                 signal-to-noise-plus-interference ration (SINR). More 
               
               
                   
                 specifically, in order to improve the quality of the channel 
               
               
                   
                 estimates, one might need to allocate additional REs for 
               
               
                   
                 RSs in poor channel propagation conditions, such as when 
               
               
                   
                 the UE is located far away from the BS, or when the line- 
               
               
                   
                 of-sight between the UE and the BS is temporarily blocked, 
               
               
                   
                 e.g., by building or by the user&#39;s body. 
               
               
                 CE 
                 One or more of the CE parameters as described in TAB. 2, 
               
               
                 parameter 
                 such as data rate or code rate, can be used to determine 
               
               
                   
                 whether an adjustment of the configuration of the RS 
               
               
                   
                 transmission is required. 
               
               
                   
                 For example, transmissions at low data rates and/or using 
               
               
                   
                 low code rates may be more tolerant to the quality of the 
               
               
                   
                 channel estimates available to the receiver, thereby 
               
               
                   
                 allowing for a low density of RSs. This is because, in this 
               
               
                   
                 case, more robust modulation and/or coding schemes are 
               
               
                   
                 typically used. Conversely, transmissions at high data 
               
               
                   
                 rates and/or high code rates may call for denser RSs. 
               
               
                   
               
            
           
         
       
     
     For illustration, it would be possible that the UE determines a current value associate with one or more of such trigger criteria as summarized in TAB. 3 can then transmits an UL control message to the cellular network including an indicator indicative of a requested frequency density of the RSs that is based on this current value. The UE could also transmit the current value. Alternatively or additionally, it would also be possible that a BS determines the current value and transmits the DL control message that is indicative of the frequency density of the RSs to the UE. 
       FIG.  1    schematically illustrates a cellular NW  100 . The example of  FIG.  1    illustrates the cellular NW  100  according to the 3GPP 5G architecture. Details of the 3GPP 5G architecture are described in 3GPP TS 23.501, version 15.3.0 (2017-09). While  FIG.  1    and further parts of the following description illustrate techniques in the 3GPP 5G framework of a cellular NW, similar techniques may be readily applied to other communication protocols. Examples include 3GPP LTE 4G—e.g., in the MTC or NB-IOT framework—and even non-cellular wireless systems, e.g., an IEEE Wi-Fi technology. 
     In the scenario of  FIG.  1   , a UE  101  is connectable to the cellular NW  100 . For example, the UE  101  may be one of the following: a cellular phone; a smart phone; an IOT device; a MTC device; a sensor; an actuator; etc. The UE  101  has a respective identity  451 , e.g., a subscriber identity. 
     The UE  101  is connectable to a core NW (CN)  115  of the cellular NW  100  via a RAN  111 , typically formed by one or more BSs  112  (only a single BS  112  is illustrated in  FIG.  1    for sake of simplicity). A radio channel  114  is present between the RAN  111 —specifically between one or more of the BSs  112  of the RAN  111 —and the UE  101 . The radio channel  114  can be estimated by means of RSs. The BS  112  can transmit DL RSs and the UE  101  can transmit UL RSs. 
     The radio channel  114  implements a time-frequency resource grid  300 , as illustrated in  FIG.  2   . Typically, Orthogonal Frequency Division Multiplexing (OFDM) is used: here, a carrier includes multiple subcarriers  307 . The subcarriers  307  (in frequency domain) and the symbols (in time domain) then define time-frequency REs  309  of the time-frequency resource grid  300 . Multiple REs  309  can be grouped in RBs  308 . Scheduling of resources for transmission can be implemented on the granularity of RBs  308 . I.e., scheduling control messages—e.g., DL control information (DCI) communicated on the PDCCH—can include indicators indicative of one or more RBs. The time-sequence is further structured by means of slots  303 , subframes  302  (each subframe includes two slots  303 ), and frames  301  (each frame includes ten subframes  302 ). 
     For example, for 3GPP NR: 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Time units of a time-frequency resource grid 300 in 3GPP NR. It is 
               
               
                 noted that the concept of a slot is slightly different in 3GPP 4G 
               
               
                 Long Term Evolution (LTE) and 5G NR. In 5G, the slot is the basic 
               
               
                 unit and has the same meaning as a 4G subframe. Furthermore, its 
               
               
                 duration scales with the numerology. By contrast a 5G subframe and 
               
               
                 frame have fixed durations (1 m and 10 ms), as illustrated above. 
               
            
           
           
               
               
               
            
               
                   
                 Time Unit 
                 Duration 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Frame 301 
                 10 
                 ms 
               
               
                   
                 Subframe 302 
                 1 
                 ms 
               
               
                   
                 Slot 303 
                 0.5 
                 ms 
               
               
                   
                   
               
            
           
         
       
     
     Different REs  309  or RBs  308  can be allocated to different logical channels of the radio channel  114 . Examples include: PUCCH, PUSCH, PDSCH, and/or PUSCH. 
     Also illustrated in  FIG.  2    is a frequency range  306  used for a data transmission. The frequency range  306  is a certain fraction of the overall bandwidth of the radio channel  114 , but, as a general rule, it would be possible that the frequency range  306  equals the overall bandwidth. 
     Referring again to  FIG.  1   : The CN  115  includes a user plane (UP)  191  and a control plane (CP)  192 . Application data is typically routed via the UP  191 . For this, there is provided a UP function (UPF)  121 . The UPF  121  may implement router functionality. Payload data may pass through one or more UPFs  121 . In the scenario of  FIG.  1   , the UPF  121  acts as a gateway towards a data NW  180 , e.g., the Internet or a Local Area NW. Payload data can be communicated between the UE  101  and one or more servers on the data NW  180 . 
     The cellular NW  100  also includes a mobility-control node, here implemented by an Access and Mobility Management Function (AMF)  131  and a Session Management Function (SMF)  132 . These entities consume and generate control data. 
     The cellular NW  100  further includes a Policy Control Function (PCF)  133 ; an Application Function (AF)  134 ; a NW Slice Selection Function (NSSF)  134 ; an Authentication Server Function (AUSF)  136 ; and a Unified Data Management (UDM)  137 .  FIG.  1    also illustrates the protocol reference points N1-N22 between these nodes. 
     The AMF  131  provides one or more of the following functionalities: connection management sometimes also referred to as registration management; NAS termination for communication between the CN  115  and the UE  101 ; connection management; reachability management; mobility management; connection authentication; and connection authorization. 
     A data connection  189  for data transmission is established by the SMF  132  if the respective UE  101  operates in the connected mode. The data connection  189  is characterized by UE subscription information hosted by the UDM  137 . To keep track of the current mode of the UE  101 , the AMF  131  sets the UE  101  to CM-CONNECTED or CM-IDLE. During CM-CONNECTED, a non-access stratum (NAS) connection is maintained between the UE  101  and the AMF  131 . 
     The SMF  132  provides one or more of the following functionalities: session management including session establishment, modify and release, including bearers set up of UP bearers between the RAN  111  and the UPF  121 ; selection and control of UPFs; configuring of traffic steering; roaming functionality; termination of at least parts of NAS messages; etc. As such, the AMF  131  and the SMF  132  both implement CP mobility management needed to support a moving UE. 
     The data connection  189  is established between the UE  101  and the RAN  111  and on to the UP  191  of the CN  115  and towards the DN  180 . For example, a connection with the Internet or another packet data NW can be established. Payload data of a data transmission can be transmitted along the data connection  189 . To establish the data connection  189 , i.e., to connect to the cellular NW  100 , it is possible that the respective UE  101  performs a random access (RACH) procedure, e.g., in response to reception of a paging signal or in response to UE-originating UL data being buffered for transmission. This establishes at least a RAN-part of the data connection  189 . A server of the DN  180  may host a service for which payload data is communicated via the data connection  189 . The data connection  189  may include one or more bearers such as a dedicated bearer or a default bearer. The data connection  189  may be defined on the RRC layer, e.g., generally Layer 3 of the OSI model. 
       FIG.  3    schematically illustrates the BS  112 . The BS  112  includes a control circuitry  1122 , e.g., implemented by one or more processors. The control circuitry  1122  can load program code from a memory  1123 . The BS  112  can communicate on the radio channel  114  using an interface  1125 . The control circuitry  1122 , upon executing the loaded program code, can perform techniques as described herein, e.g.: configuring a data transmission to or from the UE  101 , e.g., by determining one or more CE parameters; configuring a RS transmission to or from the UE, e.g., by configuring a time-frequency resource mapping of RE allocated to the RSs; transmitting one or more DL control messages to the UE  101 , e.g., thereby configuring a RS transmission (TAB  1 , techniques A-C); determining a frequency density of RSs; controlling the interface  1125  to decode an UL transmission, e.g., perform blind decoding or a coherent decoding based on a channel estimate determined based on a receive property—e.g., a receive amplitude and/or a receive phase—of a received UL RS; etc. 
       FIG.  4    schematically illustrates the UE  101 . The UE  101  includes a control circuitry  1012 , e.g., implemented by one or more processors. The control circuitry  1012  can load program code from a memory  1013 . The UE  101  can communicate on the radio channel  114  using an interface  1015 . The control circuitry  1012 , upon executing the loaded program code, can perform techniques as described herein, e.g.: transmitting UL RSs for estimating the radio channel  114 ; monitoring for DL RSs; transmitting and/or receiving (communicating) RSs in accordance with a time-frequency resource mapping that is set in accordance with a frequency density that may be network-controlled; setting the time-frequency resource mapping in accordance with a DL control message received from the cellular network  100 , e.g., in accordance with a respectively indicated frequency spacing between adjacent RSs; suspending transmitting the UL RSs, e.g., in response to detecting a respective trigger criterion associated with the UL data transmission; etc. 
       FIG.  5    is a flowchart of a method according to various examples. For instance, the method of  FIG.  5    may be executed by a UE, e.g., by the control circuitry of the UE upon loading program code from a local memory of the UE. The techniques of  FIG.  5    will be explained hereinafter in connection with a scenario in which the method of  FIG.  5    is executed by the UE  101 . 
     Optional blocks are labelled with dashed lines in  FIG.  5   . 
     At optional box  3001 , the UE transmits an UL control message to the cellular network. The UL control message is associated with a DM-RS transmission. 
     The DM-RSs are for estimating the radio channel between the UE and the cellular network. Based on such channel estimation, a BS of the cellular network can coherently decode signals received from the UE. The DM-RSs can be associated with a data transmission—e.g., payload data and/or control data, e.g., on the PUSCH or PUCCH—i.e., be interspersed with RE allocated to the data transmission and/or using the same precoding as used for transmitting of data signals including data of the data transmission. 
     The UL control message implements a request for a DM-RS configuration, e.g., for an adjustment or change thereof. For example, an adjustment that would allow increased repetition, etc. For instance, the UL control message could request an adjustment of one or more UL DM-RS properties. 
     For instance, the one or more UL DM-RS properties may be selected from the group comprising: frequency density of the DM-RSs; time density of the DM-RSs; frequency hopping pattern; parameter of a time-frequency resource mapping of REs allocated to the DM-RSs; frequency-domain spacing between DM-RSs allocated to REs that are adjacent in frequency domain; time-domain spacing between DM-RSs allocated to the REs that are adjacent time domain; count of DM-RSs per RB; polarization used for UL DM-RS; suspending the UL DM-RS transmission; etc. 
     It would be possible that the UE determines the requested DM-RS configuration. One or more decision criteria can be taken into account, e.g., summarised above in TAB. 3. 
     In other examples, it is possible that the configuration is determined at the cellular network. A re-configuration can be triggered at the cellular network. At least in such scenarios it is not required to execute box  3001 . 
     Next, at box  3002 , a DL control message is received from the communications network. The DL control message is indicative of a UL DM-RS configuration. For instance, the DL control message could be indicative of an adjustment of one or more UL DM-RS properties (e.g., as explained in connection with box  3001  above). One or more respective settings of a configuration of the DM-RS may be indicated, e.g., by one or more respective indicators. 
     Then, at box  3003 , the DM-RSs are transmitted in accordance with the configuration as indicated by the DL control message received at box  3002 . For instance, considering a scenario in which the DL control message received at box  3002  is indicative of a frequency density of the UL DM-RSs, it is then possible that the UL RSs are transmitted, at box  3003 , on the radio channel using a time-frequency resource mapping that is set in accordance with the frequency density. 
     As illustrated in  FIG.  5   , executing box  3003  can be correlated with executing box  3030 . Thus, along with commencing said transmitting of the UL RSs at box  3003 , at box  3030  there is a switch between different settings of a CE parameter of the UL data transmission that is associated with the DM-RS. Example CE parameters have been summarized above in TAB. 2. 
     Such correlated execution of box  3003  along with box  3030  can mean that, e.g., box  3003  is executed upon detecting the switch of the setting of the CE parameter at box  3030 , or vice versa that the CE parameter at box  3030  is switched upon commencing to transmit the UL RSs at box  3003  using the configuration of box  3002 . Such correlated execution could also mean that the UE receives respective commands from the cellular network, to contemporaneously or at least time aligned execute box  3003  at box  3030 . The correlated execution could also mean that the setting of the CE parameter at box  3030  is determined based on the configuration of the DM-RS received at box  3002  used for transmitting at box  3030 . For instance, a predefined correlation may be used by the UE to determine the new setting of the CE parameter, based on the configuration of the DM-RS, e.g., the frequency density. The correlation “translates” the configuration of the DM-RS into the setting of the CE parameter. An example implementation of the correlation as mapping table is illustrated below in TAB  5 : 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Example implementation of the correlation between CE parameter (here: 
               
               
                 repetition count of redundancy version retransmissions) and configuration 
               
               
                 of DM-RS transmission (here: frequency density). 
               
            
           
           
               
               
               
               
            
               
                   
                 Entry 
                 Setting repetition count 
                 Frequency density 
               
               
                   
                   
               
               
                   
                 A 
                 Low 
                 High 
               
               
                   
                 B 
                 Medium 
                 Medium 
               
               
                   
                 C 
                 High 
                 Low 
               
               
                   
                   
               
            
           
         
       
     
     By using such correlated execution of box  3003  along with box  3030 , the CE for the data transmission can be facilitated by the (re-)configuration of the DM-RS transmission. 
     At box  3004 , it is checked whether a DM-RS-free operation should commence. If this is the case, then, at box  3005 , transmitting the DM-RSs is suspended. 
     For example, it would be possible to check, at box  3004 , whether a predefined CE transmission mode for the UL data transmission associated with the DM-RSs is activated. In the affirmative, box  3005  may be executed. This is labeled DM-RS-free operation. 
     For illustration, the predefined CE transmission mode may be characterized by a code rate of the data of the data transmission associated with the DM-RSs that is below a predefined threshold. Typically, the predefined threshold could be 1/100 or even smaller. Alternatively or additionally, the CE transmission mode could be characterized by repetitions of a given redundancy version of the data. More specifically, it would be possible that the CE transmission mode is characterized by a redundancy repetition count being above a predefined threshold. Typically, the predefined threshold could be 1000 or even 5000 or larger. 
     The CE transmission mode may be characterized by other criteria defined in connection with CE parameters as summarized in TAB. 2. 
     Once it is judged, at box  3006 , that the DM-RS-free operation is to be stopped, then transmission of DM-RSs can be restarted, at a further iteration of box  3003 . 
     For instance, at box  3006 , it may be checked whether a predetermined amount of time since executing  3004  has expired, e.g., based on a timer initialized when executing  3004 . The amount of time may be measured in subframes or frames, e.g., after at least ten subframes etc. 
     At box  3006  one or more further criteria may be checked, e.g., reception of a DL control message from the cellular NW, or whether the predefined CE transmission mode—discussed in connection with box  3004 —has ended. 
     Accordingly, as will be appreciated from the above, the method of  FIG.  5    facilitates a dynamic adjustment of the properties of UL DM-RSs, in particular, of the frequency density of the UL DM-RSs. Details with respect to such dynamic adjustment are illustrated in connection with  FIG.  6   . 
       FIG.  6    schematically illustrates a temporal evolution of the frequency density  401 - 404  of the DM-RSs. As illustrated, from time to time, different frequency density is  401 - 404  are implemented. For instance, the adjustment from the frequency density  401  to the lower frequency density  402  is triggered by a DL control message  411  received at the UE  101 . Likewise, the adjustment from the frequency density  402  to the frequency density  403  is triggered by a further DL control message  412 . For instance, the further DL control message  412  could be indicative of an incremental or relative change, i.e., a difference between the frequency density  402  and the frequency density  403 . Differently, the DL control message  411  could be indicative of an entry of a predefined codebook specifying the frequency density  402 . These are just examples. 
     As a general rule, it is not required that all adjustment of the frequency density  401 - 402  are immediately in response to reception of a respective DL control message  411 - 412  (albeit this is a possibility). For instance, it would be possible that a respective timer is defined and initialized upon reception of the DL control message  412  and, upon expiry of that time, the adjustment from the frequency density  403  to the frequency density  404  is triggered. Other trigger criterion for executing the adjustment of the frequency density are conceivable, such as changes of the channel quality of the radio channel  114 ; i.e., once the channel quality fulfils a predefined criterion, the previously configured adjustment can be executed. 
     As a general rule, the downlink control message may parameterize one or more trigger criteria to execute the (re-)configuration of the UL DM-RS transmission and then the UE can monitor whether the one or more trigger criteria are fulfilled (e.g., SINR, SNR, path loss, fading, coherence bandwidth, or generally any trigger criterion specified in TAB. 3). 
     Above, aspects with respect to dynamically adjusting the frequency density of the RSs have been explained. Next, details with respect to implementing different frequency densities of the RSs are discussed in connection with  FIG.  7   . 
       FIG.  7    schematically illustrates aspects with respect to the allocation of REs  311  (selected from all available REs  309  of the time-frequency resource grid  300 ) to RSs (DM-RS REs  311 ). I.e.,  FIG.  7    illustrates a time-frequency resource mapping  390  of the DM-REs  311  to the resource grid  300 , for multiple RBs  308 . The time-frequency resource mapping  390 —in the illustrated example—is defined for a subframe  304  and can be repeated from subframe. A front-loaded scenario is shown in which the DM-RS REs  311  are all in the first slot  303  of the subframe  304 . Another option would be back-loaded DM-RSs, or DM-RSs located in symbols interior with respect to an associated data allocation. 
     “Front-loaded” can means that RSs are transmitted before the data transmissions they assist. That in, in the first (and possibly) second symbols of the slot or UL data allocation, depending on the pilot type. Additional pilots might also be present in later symbols. 
     Other REs not allocated to the DM-RSs can be allocated to the data transmission. For instance, all remaining REs in the subframes  304  could be allocated to the data transmission, or at least 80% or at least 50%. There may be some RE allocated to other RSs, etc. It would also be possible that RBs  308  including at least one DM-RS REs  311  do not include any other signals, e.g., for DFT-spread OFDM uplink. 
     As a general rule, the allocation can be with respect to a reference subcarrier acting as baseline. The reference subcarrier could be network-configured. This would provide for an absolute allocation of the DM-RS REs  311 . RBAs illustrated in  FIG.  7   , the DM-RS REs  311  are essentially spread across the entire frequency range  306  of the data transmission. 
     In  FIG.  7   , different frequency spacings  321 - 323  (e.g., see parameter Δ P  discussed above) are illustrated for the different frequency densities  401 - 403 . As illustrated in  FIG.  7   , for a lower frequency density  401 - 403 , the frequency spacing  321 - 323  increases. 
     As a general rule—and as illustrated in  FIG.  7   —the frequency spacings can be approximately constant across the entire frequency range  306 . E.g., a variation may be less than 80% or less than 20%. 
     In  FIG.  7   , the frequency density  401  is two per RB  308  and the frequency density  402  is 0.5 per RB  308  and the frequency density  403  is two-thirds per RB  308  (not counting RBs  308  of the second slot of the subframe  304  that are generally not used for the DM-RS transmission). These are just examples. 
     For the time-frequency resource mappings  390  implementing the frequency density  402  the frequency density  403 , some RBs  308  include no REs  309  allocated to the DM-RSs while other RBs  308  include one or two REs  311  (again, only considering those slots that carry any DM-RSs)—i.e., different RBs  308  of the first slot include different counts of DM-RSs. 
       FIG.  8    schematically illustrates aspects with respect to the allocation of REs  312 ,  313  (selected from all available REs  309  of the time-frequency resource grid  300 ) to RSs (DM-RS REs  312 ,  313 ). 
     In the illustrated example, the DM-RS REs  312  are allocated to the DM-RSs having a 0° polarization and the DM-RS REs  313  are allocated to the DM-RSs having a 90° polarization (relatively defined with respect to each other, i.e., orthogonal). As will be appreciated, the frequency domain density for both subgroups of demodulation reference signals, i.e., 0° and 90° polarization is the same, i.e., one-third per RB  308 . This is just an example. As a general rule, the frequency domain density may differ for the two subgroups, e.g., to track a dominant polarization of the radio channel  114 . As a general rule, the frequency densities for the two subgroups can be separately configured (cf.  FIG.  5   , box  3002 ). In general, the polarizations do not need to be at exactly 90° and 0° in an absolute direction. It can be appreciated that any two independent polarization, such as any two orthogonal polarizations, will serve equally well. 
       FIG.  9    is a signaling diagram of communication between the UE  101  and the BS  112 . 
     At optional box  5001 , the UE  101  determines a requested frequency density for DM-RSs  31  repeatedly transmitted by the UE  101  to the BS  112  (for sake of legibility, the arrows indicating the DM-RSs  31  are not connected all the way through to the BS  112  and  FIG.  9   ; but the BS  112  attempts to receives these DM-RSs  31 ). 
     Such determination could be based on at least one of a coherence bandwidth of the radio channel  114 , a receive quality of signals received on the radio channel  114 , or a CE parameter used for the UL data transmission (see TAB. 2). 
     Then, optionally, at  5002 , an UL control message  4001  as transmitted to the BS  112 . The UL control message  4001  is indicative of the requested frequency density of the DM-RSs  31 . 
     Thus, box  5001  and  5002  correspond to box  3001  of  FIG.  5   . 
     The BS  112 , at box  5003 , determines the frequency density to be used for the DM-RSs  31 . This could be based on at least one of the coherence bandwidth of the radio channel  114 , a receive quality of signals transmitted on the radio channel  114 , a CE parameter used for the UL data transmission (cf. TAB. 2) or—where applicable—the UL control message  4001 . 
     It would be possible that the BS  112 , at box  5003 , also determines a setting of a CE parameter (cf. TAB. 2) to switched to for the payload data transmission  4012  when using the newly-determined frequency density. It would be possible that the frequency density is determined based on the determined setting, or vice versa. 
     The BS  112 , at  5004 , transmits a DL control message  4002  that is indicative of the frequency density of the DM-RSs  31  determined at box  5003 . Thus, the DL control message  4002  could implement one of the DL control messages  411 ,  412  discussed above. 
     For illustration, the DL control message  4002  could be a L2 or L1 control message. This would facilitate a fast adjustment of the frequency density, without requiring a complete reconfiguration—e.g., on L3—of the DM-RS transmission. 
     As a general rule, a DL control message  4002  could be transmitted multiple times within a frame, e.g., once per subframe. Thereby, it would be possible to dynamically adjust the frequency spacing on a short timescale, flexibly tracking the condition of the radio channel  114 . There could be a fixed resource allocation available, e.g., on PDCCH, to adjust the frequency spacing (or another configuration parameter of the DM-RS transmission). 
       5004  thus corresponds to box  3002  of  FIG.  5   . 
     The DL control message  4002 —or another DL control message (not shown in  FIG.  9   )—could also be indicative of any determining setting of the CE parameter. 
     As illustrated in  FIG.  9   , upon receiving the DL control message  4001 , the frequency density  402  is implemented, following the frequency density  401  (cf. box  3003  of  FIG.  5   ). 
     At  5005 , the BS  112  transmits an UL scheduling grant  4011  to the UE  101 . The UL scheduling grant is indicative of a set of REs  309  of the time-frequency resource grid  300 , e.g., by indicating one or more RBs  308 . The UE  101  could then select time frequency REs  311 - 313  allocated to the DM-RSs  31  and further time-frequency REs allocated to payload signal encoding payload data of the payload data transmission  4012 , executed at  5006 ,  5007 , and  5008 , from the indicated set. This selection can be in accordance with the time-frequency resource mapping  390  that is set in accordance with the frequency density as indicated by the DL control message  4002 . 
     The payload data transmission  4012  can be in accordance with the setting of the CE parameter, newly-determined in some examples at  5003 , as described above (cf. box  3030  of  FIG.  5   ). 
     At  5009 , the BS  112  determines that DM-RS-free operation shall commence and a respective control message  4003  is transmitted by the BS  112  at  5010 , to inform the UE  101  (cf.  FIG.  5   , box  3004 ). Accordingly, a zero frequency density  409  is activated and DM-RSs  31  are not transmitted (i.e., Δ P =∞). The data signals encoding the payload data  4012  transmitted at  5011  and  5012  are blind decoded by the BS  112 . 
     The DM-RS-free operation is determined to stop at box  5013  (cf.  FIG.  5   , box  3006 ) and upon transmitting a respective DL control message  4004  to activate the DM-RSs, DM-RSs  31  are transmitted again. 
       FIG.  10    is a flowchart of a method according to various examples. For instance, the method of  FIG.  10    could be executed by a BS. For instance, the method of  FIG.  10    could be implemented by the control circuitry  1122  of the BS  112  of the cellular network  100 . 
     Optional boxes are illustrated using dashed lines in  FIG.  10   . 
     At optional box  3101 , and UL control message indicative of a requested adjustment of a configuration used for transmitting DM-RSs is received from a UE. Box  3101  corresponds to box  3001 . 
     At box  3102 , a configuration for DM-RSs is determined and, subsequently, provided to the UE, using one or more control messages transmitted in the DL. Box  3102  corresponds to box  3002 . 
     Then, at box  3103 , DM-RSs are received in accordance with the configuration is provided at box  3102 . Box  3103  can be correlated to box  3130  where a setting of a CE parameter is switched. Respective details have been explained above in connection with box  3003  and  3030 . 
     At box  3104  it is checked whether transmission of DM-RSs is to be suspended, as explained above in connection with box  3004 . In the affirmative, at box  3105 , the BS blindly decodes data signals including data of a data transmission, because there are no DM-RSs transmitted by the UE. 
     The decision at box  3104  could be based on activation of a CE transmission mode used for the data transmission, as explained above in connection with box  3004 . 
     Then, at box  3106 , it can be judged to switch on the transmission of DM-RSs again. 
     Next, details with respect to adjusting a frequency density of the RSs will be explained in connection with  FIG.  11    and  FIG.  12   . 
       FIG.  11    schematically illustrates aspects with respect to changing the frequency density of the RSs by changing the frequency spacing between adjacent RSs  31 .  FIG.  11    is a histogram illustrating the various frequency spacings of the RSs  31  across the frequency range  306 . As illustrated in  FIG.  11    by the full bar, there is only a single—comparably small—frequency spacing  371  (here, the size of the frequency spacing  371  is the size of a RB  308 ) encountered, meaning that all RSs  31  are evenly distributed across the frequency range  306 . There are no clusters of RSs  31  in frequency domain having a smaller frequency spacing. It can be said that the spectrum frequency spacings only has a single contribution. This histogram of frequency spacings defines a pattern of the time-frequency resource mapping  390 , as illustrated in the bottom part of  FIG.  11   . 
     The adjustment of the frequency density is obtained by evenly scaling the frequency spacing  371  across the frequency range  306 , to the frequency spacing  372 . The pattern of the time-frequency resource mapping  390  is preserved, since no new contributions are added to the frequency spectrum, all that is observed is a shift of the only, single contribution. 
     Thus, as a general rule, in adjustment of the frequency density can be achieved by changing the one or more frequency spacings between adjacent RSs  31 , without changing the count of frequency spacings across the frequency range  306 . The pattern of the time-frequency resource mapping  390  can remain unaffected by the adjustment of the frequency density. This makes an adjustment simple and thus allows to reduce the complexity of the adjustment; in turn, the adjustment can be carried out quickly, e.g., multiple times withing a frame. This enables to track the radio channel  114  accurately. 
     A different scenario is illustrated in  FIG.  12   . Here, the count of frequency spacings is changed by introducing a new contribution to the spectrum of frequency spacings, by forming clusters of closely-packed RSs having the spacing  374 . The pattern of the time-frequency resource mapping  390  is changed by the adjustment. Also, the average frequency density is reduced (from 1 per RB  308  to 0.5 per RB  308 ). 
     For instance, 3GPP NR type 1 and type 2 DM-RS are characterized by a change in the pattern of the time-frequency resource mapping  390 . See Dahlman, Erik, Stefan Parkvall, and Johan Skold. 5G NR: The next generation wireless access technology. Academic Press, 2018;  FIG.  9 . 18    vs.  FIG.  9 . 19   . For type 1, a new cluster of RSs is introduced. A switching between DM-RS type 1 and type 2 is comparably complex and slow (typically requiring RRC re-configuration) and thus limited in its suitability to track changes on the radio channel and adjusting the frequency density or another parameter of the DM-RS transmission along with changes to the setting of a CE parameter, as explained above. 
     As a general rule, a change of one or more configuration parameters of the RSs transmission could be implemented by specifying a new type, e.g., type 3 DM-RS. It would also be possible to specify changes with respect type 1 or type 2 DM-RS of 3GPP NR. 
     Summarizing, at least the following examples have been described above: 
     Example 1: A method to control a density of RSs used for demodulation of associated payload data transmissions, in the UL and/or in the DL, including: A signaling, from the UE to BS, (optionally) requesting a desired density of pilot signals; and a signaling, from the BS to the UE, configuring the density (in frequency domain) of RSs. 
     Example 2: The method according to Example 1, wherein the said density depends on one more of the following factors: (in spec this is implementation, i.e. transparent)
         a. A coherence bandwidth of the radio channel (could be zero, i.e., o need for DMRS) as estimated by the UE and/or the BS, in the DL and/or in the UL;   b. A signal-to-noise ratio (SNR) or a signal-to-interference-plus-noise ratio (SINR) as estimated by the BS or the UE, in the DL and/or in the UL;   c. A desired or actual coding rate of the associated payload data transmissions, in the DL and/or in the UL.       

     Example 3: The method according to any one of the preceding Examples, wherein the RSs are divided into two groups, each using a different transmit polarization, so that the associated data transmission can track the polarization of the radio channel. 
     Example 3 may be implemented in one of the following ways:
         a. By using a feedback signaling indicating an observed polarization of the transmitted RSs. In an example (Note that, typically, the following steps i. and v. are executed concurrently):
           i. the UE transmits PUSCH DM-RS to the BS,   ii. the BS estimates a polarization of the received PUSCH DM-RS (per relevant bandwidth, which depend on the DMRS density in frequency domain),   iii. the BS signals the estimated optimal polarization direction to UE,   iv. the UE aligns subsequent PUSCH data transmissions according to the polarization feedback by the BS,   v. the UE transmits more PUSCH DM-RS to the BS, and so on.   
           b. By sending RSs in the reverse direction if compared to scenario a above. For example, suitable designed PDSCH DM-RS (or CSI-RS) can be used by the UE to estimate a suitable polarization for PUSCH transmissions and associated PUSH DM-RS transmissions. In this case, no feedback channel is required.       

     Example 4: Ultra-low coding rates for reliable data transmissions of payload data and/or control data, e.g., Layer 3 RRC control data. For example, coding rates lower than 1/100 might be signaled to UEs located at the cell edge. Transmissions with ultra-low coding rates may span multiple slots. 
     Example 5: Blind decoding of payload data transmissions, i.e., without associated RSs. For example, blind decoding might be signaled for data transmissions of payload data and/or control data with ultra-low coding rates. (DMRS less) 
     Example 6: Repetitions, in the frequency domain, of payload data transmissions, using the same or different redundancy versions. Associated RSs can be transmitted in all, some or none of the repetitions. 
     Further, at least the following EXAMPLES have been described above. 
     Although the invention has been shown and described with respect to certain preferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the appended claims. 
     For illustration, above, various scenarios have been discussed in the context of UL RSs transmitted from the UE to the communications network. Similar techniques may be readily applied for DL RSs. For example, a frequency density of DL RSs may be signalled to the UE and the UE may then monitor for the DL RSs using a time-frequency resource mapping set in accordance with the frequency mapping. 
     For further illustration, various scenarios have been discussed in the context of a scenario in which the logic for determining the frequency density (or another configuration of a transmission of RSs) resides at the network. In other scenarios, it would be possible that such logic resides at the UE. 
     For further illustration, various examples have been described in connection with DM-RSs, but similar techniques may be implemented with other kinds and types of RSs. 
     For still further illustration, while various examples happen described in which a frequency density of reference signals is adjusted, in other scenarios, other configuration parameters of the configuration of a reference signal transmission can be adjusted, e.g., a frequency spacing (keeping the average frequency density constant), used polarizations, and so forth.