Method and apparatus for adjusting demodulation reference signal density based on explicit feedback in wireless communication network

Abstract

A method and system for adjusting a demodulation reference signal (DM-RS) density in a wireless communication network, the method comprising the steps of a receiver, especially a UE, submitting a request to change the DM-RS density to a transmitter, especially a base station, the transmitter monitoring feedback of the receiver related to reception quality, and the transmitter decides whether to indicate to change the DM-RS according to the feedback.

Description

Technical Field

[0001] Different types of reference signals (RS) are used in 3GPP 5G NR systems and subsequent systems. The present invention focuses on the transmission of demodulation reference signals (DM-RS) by the transmitter during the communication process with the receiver, which helps the receiver decode the data. DM-RS occupies one or more symbols within a time slot. It is used for channel estimation on the device side for coherent demodulation. Therefore, if the received DM-RS signal strength is insufficient, the receiver will not be able to decode the data packet with satisfactory error performance. In the downlink (DL), DM-RS is present in the resource blocks used for physical downlink shared channel (PDSCH) transmission, and also in the resource blocks used for physical downlink control channel (PDCCH). In the uplink (UL), DM-RS is present in the physical uplink shared channel (PUSCH) that allows the gNB to coherently demodulate the uplink data in the PUSCH, and also in the physical uplink control channel (PUCCH). Finally, in the sidelink (SL), DM-RS is present in both the physical sidelink shared channel (PSSCH) and the physical sidelink control channel (PSCCH).

[0002] Since the DM-RS itself does not carry any useful data, it is desirable to reduce or adjust its transmission according to the requirements of the receiver. In the current 3GPP specifications, the possibility of such adjustment is limited. In particular, if the receiver has channel prediction capabilities or can blindly decode the data (i.e., without first estimating the channel using DM-RS), the receiver may not need DM-RS to be transmitted by the transmitter. Therefore, the density of DM-RS can be reduced or the use of DM-RS can be avoided completely. On the other hand, if the receiver believes that higher accuracy is required, the density of DM-RS needs to be increased.

[0003] Previously known DM-RS density changes are based on HARQ ACK / NACK feedback or the rate of change of Doppler frequency and / or channel characteristics, where HARQ ACK / NACK feedback alone is insufficient to determine the channel prediction capability of the UE. For example, US2021028903 A1, US2020313818A1, and US2021226833 A1 disclose such feedback methods.

[0004] It is an object of the present invention to propose an efficient way of constituting DM-RS density variations during communication between a receiver and a transmitter.

[0005] The invention is described by the features of the independent claims 1, 14, 15 and 16. Advantageous embodiments can be found in the respective dependent claims. Summary of the invention

[0006] In summary, a core part of the concept of the present invention is to change the DM-RS density by request of the receiver (e.g., user equipment (UE)), especially if possible, even reduce the DM-RS density to zero. When submitting such a request, the transmitter (e.g., base station (e.g., gNB)) monitors the quality of feedback including HARQ response from the receiver, and decides whether to signal or indicate the requested DM-RS density change based on the feedback.

[0007] Therefore, the receiver's request can be based on its blind decoding capability or its own channel prediction capability. Therefore, blind decoding capability means that the UE is able to decode data without knowing the channel state information (CSI), which is usually achieved by the transmission and reception of DM-RS. Methods of blind decoding and channel prediction are known in the prior art. Channel prediction and blind decoding capabilities can be enhanced by using artificial intelligence (AI) or machine learning (ML) techniques.

[0008] In the first case (blind decoding), the receiver can request to completely turn off the DM-RS signal and send feedback to the transmitter (e.g., base station) only in the form of hybrid automatic repeat request acknowledgement / negative acknowledgement (HARQ ACK / NACK) while blind decoding. The transmitter makes a decision on the request based on the feedback, possibly in combination with other favorable conditions. Therefore, in the case of receiving an acknowledgement ("ACK") feedback, the transmitter indicates to turn off the DM-RS signal according to the request, or if the transmitter receives one or more negative acknowledgement ("NACK") feedback, it indicates to restore the DM-RS.

[0009] Regardless of the request, if the actual DM-RS density is zero and the transmitter receives "negative" feedback, ie one or more NACKs, the transmitter may make the decision to increase the DM-RS density every time.

[0010] In the second case (channel prediction), the receiver evaluates the prediction accuracy on a known RS with the current DM-RS density. Based on this evaluation, the receiver sends a request to the transmitter, such as requesting the UE to provide downlink communication feedback to the gNB via PUCCH or PUSCH, requesting the UE to provide sidelink communication feedback to another UE via PSSCH or PSCCH, or even requesting the gNB to provide uplink communication feedback to the UE via PDCCH or PDSCH, to change (or not change) the current DM-RS density. This indication can be based on a mapping between the error region evaluation and the DM-RS density. This mapping is then known in advance in the receiver via pre-configuration between the transmitter and the receiver or by a system information message sent by the network to the UE or is fixed in the specification. Therefore, in the event of a change in the DM-RS signal density, the receiver uses the new DM-RS density when performing data demodulation.

[0011] Therefore, in the scenario where the receiver has channel prediction or blind decoding capabilities, the proposed invention provides a system and method for the transmitter to adjust the DM-RS density or completely remove DM-RS transmission based on explicit feedback from the receiver. Therefore, reducing DM-RS transmission is always beneficial because it increases the achievable data rate.

[0012] Therefore, if the receiver believes that it can make accurate predictions of the channel or use blind decoding, it can reduce the density of DM-RS or avoid using DM-RS altogether. On the other hand, if the UE believes that it needs higher accuracy, it can use the same mechanism to request an increase in DM-RS density.

[0013] It is not important for the present invention whether the receiver uses blind decoding or channel prediction. What is important is that the receiver can request to turn off the DM-RS or request to change the density of the DM-RS. This can be based on any capabilities of the receiver, and the transmitter does not need to know these capabilities explicitly. Blind decoding and especially AI / ML-based channel prediction are examples of receiver capabilities.

[0014] Currently, DL and UL DM-RS are always configured by the gNB, where the UE has no say in the time and frequency domain placement of the DM-RS. Similarly, in SL, the DM-RS configuration is fixed in the specification. In the present invention, the receiver can use its blind decoding (using traditional methods or AI / ML-based methods) or channel prediction (using traditional methods or AI / ML-based methods) capabilities to influence the placement.

[0015] The method of the present invention is performed by a system, which includes: a receiver, especially a UE, which is equipped with a device for submitting a request for changing the DM-RS density to a transmitter, especially a base station; and a transmitter, which is equipped with a device for monitoring the feedback of the receiver related to the reception quality, and is equipped with a decision device for deciding whether to indicate a change in the DM-RS based on the feedback.

[0016] Another aspect of the present invention proposes a set of predefined DM-RS patterns with different time domain densities and frequency domain densities and signaling methods, wherein the transmitter and receiver are able to switch between these patterns. This aspect also includes a channel prediction error estimation method performed by the receiver to help switch between different DM-RS density patterns. Therefore, a system framework with ordered time domain and frequency domain DM-RS density can replace or supplement existing DM-RS formats.

[0017] The present invention is explained below for downlink (DL), but uplink (UL) and sidelink (SL) are also possible. In order to implement the present invention, the gNB and (multiple) UEs must be prepared to use flexible DM-RS time / frequency density. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Other advantages and characteristics of the invention will become more apparent on reading the following description, given by way of simple illustrative and non-limiting examples, and the accompanying drawings, in which:

[0019] Figure 1 The inventive steps performed by the UE and the gNB are shown,

[0020] Figure 2 It is a flowchart depicting the main steps from the UE perspective.

[0021] Figure 3 is a flowchart depicting the main steps from the perspective of gNB,

[0022] Figure 4 The DM-RS format is shown.

[0023] Figure 5 The DM-RS initial pattern is shown.

[0024] Figure 6 Shown is the DM-RS pattern representation option 1,

[0025] Figure 7 Shown is the DM-RS pattern representation option 2,

[0026] Figure 8 Yet another DM-RS pattern representation option 2 is shown,

[0027] Fig. 9 shows the mapping of error estimates to DM-RS density,

[0028] Fig.10 shows the mapping of error to DM-RS density option 1,

[0029] Fig.11 shows the mapping of error to DM-RS density Option 2,

[0030] Fig.12 Shows the UE Option 1 indication,

[0031] Fig.13 Shows the UE Option 2 indication,

[0032] Fig.14 Shows the UE Option 2.1 indication,

[0033] Fig.15Shows the UE Option 2.2 indication,

[0034] Fig.16 An example of indication of UE Option 2.2 is shown. DETAILED DESCRIPTION

[0035] The specific embodiments described below with reference to the accompanying drawings are intended as descriptions of various configurations and are not intended to represent the only configurations in which the concepts described herein may be practiced. The specific embodiments include specific details to provide a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In particular, although terminology from 3GPP 5G NR may be used in this disclosure to illustrate the embodiments herein, this should not be viewed as limiting the scope of the invention.

[0036] Figure 1 The message exchange of the present invention on an exemplary 5G New Radio (NR) wireless communication system including a user equipment (UE) and a base station (gNB) is shown. The wireless network may be an LTE network or some other wireless network, such as an LTE, 5G, or NR network. The wireless network may include one or more base stations. Depending on the wireless standard implemented, the base station may be referred to as a BS, NB, eNodeB (or eNB), gNodeB (or gNB), access point, etc. The base station provides radio communication coverage for a particular cell.

[0037] A user device may be referred to as a mobile station, a wireless terminal, etc. In some examples, a user device may be a cellular phone, a wireless modem, a wireless communication device, a handheld device, a laptop computer, etc. A user device may also be an IoT (Internet of Things) device, such as a wireless camera, a smart sensor or smart meter, a vehicle, a global positioning system device, or any other device configured to communicate over a wireless network.

[0038] exist Figure 1In FIG. 1 , it is shown that the UE 101 sends a request to change, in particular reduce, the DM-RS density to the gNB 103 in a first step 102. If the UE can perform blind decoding, it can request to switch off the DM-RS signal. Otherwise, when the UE has channel prediction capability, the UE can request to reduce the DM-RS to a certain specific pattern. At the same time, the UE continues to send HARQ ACK / NACK feedback to the gNB 104, and the gNB continues to monitor the HARQ feedback. Based on the HARQ feedback and possibly in combination with other feedback from the UE (e.g., channel state information feedback), the gNB checks whether the request to change the DM-RS density is feasible. Depending on the result of this check, the gNB sends a notification to the UE in step 105 accepting the requested change or rejecting the request. If the gNB determines that the request is not feasible, it can decide that an alternative DM-RS density is feasible. The gNB then sends a corresponding notification with an indication of the new DM-RS density to the UE.

[0039] If the UE uses channel prediction, its prediction accuracy is evaluated on the known RS (with the current DM-RS density). Based on this evaluation, the UE sends an indication to the gNB (via PUCCH / PUSCH) to change (or not change) the current DM-RS density. This indication can be based on a mapping between error values ​​and DM-RS density, which is either pre-configured or sent by the gNB to the UE in a system information message, or known in advance in the UE and gNB because it is fixed in the specification.

[0040] Figure 2 is a flow chart depicting the main steps of a state machine suitable for implementing the present invention in a user equipment (UE). The state machine starts at 201. In 202, the UE checks whether a change of the DM-RS format, in particular the DM-RS density, is required. If the answer is "no", at 203, the UE continues to perform data demodulation according to the current DM-RS format. If the answer is "yes", at step 204, the UE requests the gNB to change the DM-RS density and continues to send HARQ responses to the gNB according to the usual HARQ process until an indication is received from the gNB in ​​step 205 whether the DM-RS density should be changed. Then, in 206, the UE checks whether the change of DM-RS density was successful, which means whether the gNB indicated a change. If no change was indicated, in 203, the UE continues to perform data demodulation in the current format, which is the old format. If the gNB has indicated a change, in step 207, the UE updates the internal DM-RS format and performs data demodulation in 203 in the "current" format, which is the new format.

[0041] In step 208, the UE checks whether the DM-RS density of the current format is zero. If "yes", in step 209, the UE performs feedback according to the usual HARQ process. If the current density is not zero, the UE first evaluates the channel prediction error of the current DM-RS format in 210, and then performs the HARQ process in 209. The feedback in 209 affects the result of the check in step 202 whether the DM-RS format needs to be changed. The feedback of the UE can be performed in the physical uplink control channel (PUCCH) or through the appropriate physical uplink shared channel (PUSCH) resources provided by the gNB.

[0042] An advantageous method of updating the DM-RS by the UE is described as follows: Upon receiving an indication of a change in DM-RS density from the gNB, for example in downlink control information (DCI), the UE updates its own current DM-RS density as follows. In the case where the current DM-RS density is zero and the gNB uses binary indication, the UE switches to the default DM-RS density if the gNB accepts the change. Otherwise, the UE maintains the current DM-RS density at zero. On the other hand, if the current DM-RS density is zero and the gNB uses multi-bit indication, the UE switches to the corresponding DM-RS density mode indicated by those bits. If the UE's current DM-RS density is non-zero and the gNB is using binary indication, the UE switches to the DM-RS density it requested in the last uplink control information (UCI) transmission. On the other hand, if the current DM-RS density is non-zero and the gNB is using multi-bit indication, the UE switches to the corresponding DM-RS density indicated by those bits.

[0043] Figure 3 is a flow chart characterizing the steps of the counterpart, i.e. the base station (e.g., gNB), starting at point 301. In a first step 302, the gNB, as a transmitter, checks whether the current DM-RS density is zero. If not ("No"), in step 303, the gNB scans for submissions from the UE, wherein, when a submission is received, the gNB checks whether the submission contains a request to change the DM-RS density. As long as both are "No", the gNB remains in a loop starting with the "zero density" check in 302. If the gNB receives a request, it leaves the scan with "Yes" in 303 and continues to evaluate in 304 whether the requested change is feasible. If the requested change is not feasible, the gNB investigates in 305 whether an alternative change of the DM-RS is feasible. If also "No", the gNB indicates to the UE in step 306 that the DM-RS will not be changed, and the gNB continues to transmit in the next schedule with the "old" DM-RS density in step 307. The gNB then returns to the first step 302.

[0044] If the result of the check in step 305 is that an alternative change of DM-RS is feasible, the gNB continues to update the DM-RS density of the UE in step 308, then indicates the change to the UE in step 309 and transmits with the new DM-RS density in the next scheduling (step 310). The gNB then returns to the start of step 302.

[0045] If in step 304, the gNB finds that the requested DM-RS density is feasible ("Yes"), the gNB updates the density to the requested value in step 308 and continues as described above.

[0046] In step 302, if the gNB finds that the current density is zero ("yes"), the gNB checks in 311 whether it has received a NACK from the UE. If "yes", the gNB decides to update (i.e., increase) the DM-RS density of the UE in step 308 even without an explicit request from the UE, and continues the operation as described above. Otherwise, if the gNB has not received a NACK, it jumps to step 303 and scans whether a request has been received. If "no", the gNB restarts with the "zero check" in 302.

[0047] A favorable method of indication by the gNB is described as follows: Upon receiving a DM-RS density change request from a UE, the gNB advantageously evaluates the feasibility of the new pattern based on the patterns used for other users. After the evaluation, the gNB may indicate the UE using one of the following two options. The first option is a binary indication indicating whether the requested DM-RS density is accepted. This binary indication has lower overhead but less flexibility. An indication using multiple bits can indicate an index for updating the DM-RS density. This solution has higher overhead but better flexibility.

[0048] If the current DM-RS density of the UE is zero and the gNB has not received a DM-RS density change request from the UE, the gNB continues to monitor the HARQ ACK / NACK and can decide to change the DM-RS density with the following two indication options: Similarly, a binary indication can be used if the DM-RS density should be changed to the default density or initial density or should not be changed. This solution has low overhead but correspondingly poor flexibility. Using multiple bits to indicate the index of the new DM-RS density has higher overhead but better flexibility. In general, one or more bits can be used to signal the above gNB indication in the DCI.

[0049] A complete implementation requires specifying new DM-RS formats and indication methods, but these methods can also be implemented in a limited way within the current DM-RS framework of 3GPP. Therefore, the UE evaluates the error in the outlined prediction using, for example, a table that maps the error to the currently available DM-RS format (single symbol or dual symbol DM-RS and the number of DM-RS scenarios in the time slot). The UE can feedback an indication to increase or decrease the number of DM-RS scenarios and can also indicate the required number of DM-RS scenarios. The UE can also indicate a change from single symbol to dual symbol DM-RS and vice versa.

[0050] When implementing the method of the present invention, the gNB may indicate a new DM-RS format by changing the parameters "maxLength" and "dmrs-AdditionalPosition" in the "DMRSDownlinkConfig" information element (IE) (see 3GPP TS38.331, pages 424 and 425 for details).

[0051] Figure 4 It shows that the DM-RS format can be defined with different densities in the time domain and frequency domain. Therefore, the starting symbol and the maximum number of symbols of the DM-RS define the boundaries of the DM-RS scenario. The starting symbol can be indicated relative to the time slot boundary or the first data symbol (such as type A or type B in the current specification).

[0052] Figure 5 An example of an initial pattern of DM-RS that can be assigned by the gNB is shown. This initial pattern is fixed for a specific combination of starting symbol and maximum number of symbols. The initial pattern can be fixed in the specification, which is less complex, or semi-statically indicated to the UE, which involves signaling overhead. The UE can flexibly feedback the required DM-RS density pattern within the time-frequency block determined by the starting symbol and maximum number of symbols.

[0053] For each combination of starting symbol and maximum number of symbols, a set of ordered patterns from least dense to most dense can be defined. This can be done in two ways. The first is to specify a unique index for each pattern within the group and signal that index. The second is to specify a set of “neighboring” patterns for each pattern. In the second approach, the gNB can indicate an initial pattern, which should ideally have a medium density.

[0054] Figure 6An example of a first option for DM-RS pattern representation is shown. As can be seen in the figure, each individual pattern is assigned by an index in the codebook ("000" ... "111"). Therefore, the DM-RS pattern codebook can be fixed for all UEs, which means minimal overhead, but also no flexibility. In addition to this, it is also possible to fix the pattern for each (multiple) type of UE, which means no overhead, but some flexibility. Further, the pattern codebook can be indicated to each UE, for example via semi-static signaling. This solution requires considerable overhead, but has great flexibility.

[0055] Figure 7 An example of a second option for DM-RS pattern representation is shown, where for each defined pattern there can be eight other unique patterns corresponding to changes in DM-RS time domain density or frequency domain density. Again, these changes can be fixed in the specification - meaning no signaling overhead, or can be indicated semi-statically with signaling overhead. In the table shown, "↑" means increase, "↓" means decrease, and Meaning no change. Figure 8 An example of this second option is shown in .

[0056] Regarding channel prediction and error evaluation, the UE is allowed to use any method for channel prediction. Therefore, the UE must evaluate the channel prediction error based on the currently known DM-RS symbols. Based on this evaluation, the UE calculates two metrics, the time domain prediction error and the frequency domain prediction error (e.g., "normalized mean square error" "NMSE"), using the following formula:

[0057]

[0058] As can be found in the specification,

[0059] It means the UE predicted channel on the (k, l)th resource element (not using the DM-RS symbol of the current time slot)

[0060] It means that the UE on the (k, l)th resource element estimates the channel (using only the DM-RS of the current time slot)

[0061] is a set of time domain symbol indices assigned to DM-RS symbols according to the current DM-RS mode

[0062] is a set of frequency domain symbol indices assigned to DM-RS symbols according to the current DM-RS mode

[0063] and

[0064] is the total number of time domain symbols in the time slot assigned for DM-RS symbols in the current DM-RS mode is the total number of frequency-domain symbols in the slot assigned for DM-RS symbols in the current DM-RS mode.

[0065] Fig. 9 An example of a mapping from error assessment to DM-RS density is shown. The columns include quantization of the time domain (left column) and frequency domain (right column) error metrics into different ranges. The mapping from each combination of quantized error metrics to the next DM-RS pattern must be specified. To this end, two approaches are proposed: As option 1, the index of the next DM-RS pattern is specified for each combination. This approach does not depend on the current DM-RS pattern. Option 2 is to specify the next DM-RS pattern for each (current) DM-RS pattern and time-frequency error metric combination.

[0066] Fig.10 The list shown in is an example for option 1. As can be found from the list, each combination of quantization errors is mapped to a specific index.

[0067] In contrast, Fig.11 The mapping from error to DM-RS density under Option 2 is shown. For each combination of time domain error and frequency domain error, the table specifies a specific DM-RS pattern. This option has more flexibility, but the table must be stored in memory.

[0068] The following describes how the UE can find an indication to change the DM-RS density: If the current DM-RS density is zero, the UE does not send any indication / request other than the HARQ ACK / NACK. However, if the current DM-RS density is non-zero, the UE can indicate whether the density needs to be changed with a single bit in the indication part of the uplink control information (UCI) transmitted on the PUCCH or multiplexed with the PUSCH, where "0" can be used to indicate "no change" and "1" is used to indicate "change". If the density needs to be changed, the nature of the change must be indicated by a defined number of bits, i.e., whether a new mode needs to be used and which mode to use, or whether the current density needs to be increased or decreased and the degree of increase or decrease. In addition, the UE can also indicate the time (in milliseconds or in time slots / frames) when the new DM-RS density needs to take effect and the feasibility proof of the DM-RS density / mode (for example, the UE can directly indicate the channel prediction error). If the density does not need to be changed, no additional bits are required in the indication.

[0069] The remaining figures show examples of indication by UE.

[0070] Fig.12The first option of direct indexing is shown. The DM-RS pattern is directly signaled within the boundary between the start symbol and the maximum number of time-domain symbols through the four-bit index "0000", ... "XXXX". Each pattern corresponds to a specified DM-RS density.

[0071] Fig.13 shows the second option, which is one of the modes with a close index to the current mode. As can be seen from the table, for each mode there is a certain number of closest modes. These bits indicate which "closest" mode should be used as the next mode, where means “no change”, ↓ means “decrease”, and ↑ means “increase”.

[0072] Fig.14 and Fig.15 Similar instructions are shown in Fig.14 Allows to change the density in both time and frequency domains simultaneously, while Fig.15 You can only change the density in one domain at a time. Fig.15 In the example, the first single bit may indicate whether the density needs to be changed. The second single bit may indicate whether the time domain density or the frequency domain density needs to be changed (e.g., 0: time domain, 1: frequency domain). The third single bit (or multiple bits) may indicate increase or decrease (e.g., 0: decrease, 1: increase, or 00: decrease by two steps, 01: decrease by one step, 10: increase by one step, 11: increase by two steps, etc.). Fig.16 Shown Fig.15 A specific example of a situation.

Claims

1. A method for adjusting a demodulation reference signal (DM-RS) density in a wireless communication network, It is characterized by the following steps: - A receiver, in particular a UE, submits a request to a transmitter, in particular a base station, to change the DM-RS density, - the transmitter monitors the receiver's feedback related to reception quality, - The transmitter decides whether to indicate a change in DM-RS based on the feedback.

2. The method according to claim 1, wherein: - the receiver sends a request to the transmitter to turn off the DM-RS signal, - the transmitter monitors the HARQ feedback of the receiver and decides whether to change the DM-RS for the receiver based on the HARQ, - wherein, in case of HARQ ACK feedback, the transmitter indicates to turn off the DM-RS signal, or - If the transmitter receives one or more HARQ NACK feedbacks, the transmitter indicates to restore the DM-RS, -Wherein, when the DM-RS signal is turned off, the receiver can use blind decoding.

3. The method according to claim 1 or 2, wherein: The transmitter decides whether to indicate the change based on feedback based on the error estimate and, if possible, other feedback related to channel state information.

4. The method according to claim 3, characterized in that If the actual DM-RS density is zero, the transmitter decides whether to increase the DM-RS density based on the ACK / NACK feedback, possibly in combination with other types of feedback from the receiver.

5. The method according to claim 1, wherein: - the receiver performs error estimation using the current DM-RS density, - based on the evaluation, the receiver sends a request to the transmitter to change the current DM-RS density, - the transmitter indicates a change in DM-RS signal density, -in, In the case where the DM-RS signal density changes, the receiver uses the new DM-RS density when performing data demodulation.

6. The method according to claim 5, wherein: The receiver uses the prediction accuracy of the known RS or another suitable metric known to both the transmitter and the receiver for error estimation.

7. The method according to claim 6, in, After error evaluation, the UE selects an indication to be fed back to the transmitter based on a mapping between error and DM-RS pattern change.

8. The method according to claim 7, in, This mapping between error estimation and DM-RS density is known to the transmitter and receiver via pre-configuration between the transmitter and the receiver or a system information message sent by the transmitter to the receiver, or by a way fixed in the specification.

9. The method according to any one of the preceding claims, in, Define ordered patterns with varying density in the time domain and / or frequency domain.

10. The method according to claim 9, in, These modes may be fixed for all receivers or for each type of receiver or for each individual receiver.

11. The method according to claim 9 or 10, in, A single predefined mode is indicated by submitting a corresponding index or by indicating whether the mode should change in time and / or frequency to the next higher density mode or the next lower density mode.

12. The method according to any one of the preceding claims, in, If the current DM-RS density is zero, then even without an explicit request from the receiver, the transmitter can decide whether to change the density based on received ACK / NACK feedback, possibly in combination with other types of feedback from the receiver.

13. The method according to any one of the preceding claims, in, An appropriate error estimation method is specified and the receiver uses the error metric to select an indication and feed it back to the transmitter.

14. A system for executing the method according to any one of the preceding claims 1 to 13, the system comprising: A receiver, in particular a UE, provided with means for submitting a request to a transmitter, in particular a base station, to change the DM-RS density, and A transmitter is provided with means for monitoring feedback from the receiver related to reception quality and with decision means for deciding whether to instruct a change of the DM-RS according to the feedback.

15. A receiver, in particular a UE, for performing the method according to one of the preceding claims 1 to 13, comprising means for submitting a request to a transmitter to change the DM-RS density.

16. A transmitter, in particular a base station, for executing the method according to one of the preceding claims 1 to 13, comprising means for monitoring feedback from a receiver related to reception quality and being equipped with decision means for deciding whether to indicate a change of the DM-RS based on the feedback.

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