GB2585857A - Improvements in and relating to dynamic HARQ in a non-terrestrial network - Google Patents

Abstract

A method of controlling HARQ operation in user equipment (UE) communicating with a base station (i.e. 5th Generation Node B, gNB) as part of a non-terrestrial network. The gNB signals S20, e.g. by RRC signalling, to the UE details of an expected Spectral Efficiency (SE), the gNB transmits data to the UE, the UE determines S21 a current SE on the basis of receiving the data transmitted by the gNB, the UE compares S22 the current SE to the expected SE and enables or disables HARQ operation in response to the comparison. If the current SE is less than the expected SE, then HARQ operation may be disabled S23. If the current SE is more than the expected SE, then the UE attempts S24 to decode the received data and, if successful S25, the UE enters S26 ACK-optional state and determines if HARQ feedback is provided. The data transmitted from the gNB to the UE may comprise modulation and coding scheme (MCS) data and HARQ information pointer, which includes a reference to a resource from where HARQ related information can be obtained.

Description

Improvements in and relating to dynamic HARQ in a Non-Terrestrial Network The present invention relates to the use of Hybrid Automatic Repeat Request (HARQ) in a Non-Terrestrial Network (NTN). It relates particularly to such use in a New Radio (NR) or Fifth Generation (5G) system but may have wider application.

HARQ uses a combination of high-rate forward error correcting (FEC) coding and Automatic Repeat Request (ARQ) error control. In standard ARQ, redundant bits are added to data to be transmitted using an error-detecting (ED) code such as a cyclic redundancy check (CRC). Receivers detecting a corrupted message will request a new message from the sender. In Hybrid ARQ, the original data is encoded with an FEC code, and the parity bits are either immediately sent along with the message or only transmitted upon request when a receiver detects an erroneous message. The ED code may be omitted when a code is used that can perform both FEC in addition to error detection, such as a Reed-Solomon code. The FEC code is chosen to correct an expected subset of all errors that may occur, while the ARQ method is used as a fall-back to correct errors that are uncorrectable using only the redundancy sent in the initial transmission. As a result, hybrid ARQ typically performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions.

As such, use of HARQ for all communications, even when it is not needed, can limit data throughput rates and so HARQ may only be enabled when signal conditions dictate.

The application of a high-altitude platform station (HAPS) and satellite nodes in NR can be important components of 5G. The deployment of non-terrestrial networks (NTNs) is very different from that of terrestrial networks, which can cause impacts on standard specifications due to different operating conditions and constraints.

A particular issue with NTN in the 5G context is the potential delaying of signals, caused both by the increased signal propagation time and the need to potentially retransmit certain data in an HARQ scenario. There is a general desire to make retransmission mechanisms at the physical layer more delay-tolerant. This may also include the capability to deactivate the HARQ mechanisms in certain situations.

It is an aim of embodiments of the present invention to address shortcomings in the prior art, whether mentioned herein or not.

Embodiments of the invention aim to flexibly enable and disable HARQ with a low overhead.

Embodiments of the invention introduce a procedure and signalling to support dynamic HARQ enabling/disabling for NR NTN. A radio resource control (RRC) signalling mechanism is designed to indicate the expected spectral efficiency for a certain user equipment (UE). Then, in the downlink control information (DCI) of a downlink transmission, the UE can use the modulation and coding scheme (MCS) level and the number of layers to infer whether HARQ is enabled or disabled.

Embodiments of the invention do not require frequent signalling for the indication and so signalling overhead is small. Moreover, a new HARQ state referred to here as ACK-optional is introduced for the UE to decide whether to provide the feedback based on its information on e.g. power level, propagation delay.

According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

According to a first aspect of the present invention, there is provided a method of controlling HARQ operation in a User Equipment, UE, operable to communicate with a base station, gNB, as part of a non-terrestrial network, wherein the method comprises the steps of: the gNB signalling to the UE details of an expected Spectral Efficiency, SE; the gNB transmitting data to the UE; the UE determining a current SE on the basis of receiving the data transmitted by the gNB; the UE comparing the current SE to the expected SE and enabling or disabling HARQ operation in response to the comparison.

In an embodiment, if the current SE is less than the expected SE, then HARQ operation is disabled.

In an embodiment, if the current SE is more than the expected SE then the UE attempts to decode the received data transmitted by the gNB.

In an embodiment, if the attempt to decode is successful, then the UE enters ACK-optional state, where the UE is operable to determine if HARQ feedback is provided.

In an embodiment, the UE is operable to determine if HARQ feedback is provided on the basis of one or more of: its own power level, propagation delay and traffic congestion.

In an embodiment, the gNB signalling to the UE details of an expected Spectral Efficiency, SE, comprises the use of RRC signalling.

In an embodiment, the expected SE is determined on the basis of present conditions or historical records.

In an embodiment, the data transmitted from the gNB to the UE comprises Modulation Coding Scheme, MCS, data and a HARQ information pointer, which includes a reference to a resource from where HARQ related information can be obtained.

According to a second aspect of the present invention, there is provided a base station arranged to signal to a UE details of an expected Spectral Efficiency, SE and to then transmit data to the UE.

According to a third aspect of the present invention, there is provided a UE operable to: receive details of an expected Spectral Efficiency, SE from a base station; receive data from the base station; determine a current SE on the basis of receiving the data transmitted by the base station; compare the current SE to the expected SE and enable or disable HARQ operation in response to the comparison.

Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which: Figure 1 shows a message exchange according to an embodiment of the present invention; Figure 2 shows a flowchart of a method according to an embodiment of the present invention; Figure 3 shows a method of calculating Spectral Efficiency according to an embodiment of the present invention; Figure 4 shows prior art DCI structure and an adapted version of the same according to an embodiment of the present invention; and Figure 5 shows a flowchart of a method according to an embodiment of the present invention.

In an embodiment of the invention, radio resource control (RRC) signalling related to the expected spectral efficiency (SE), which can be estimated using channel state information feedback or HARQ feedback or uplink channel sounding, is transmitted from the base station (gNB). Then, when the gNB transmits data in physical data shared channels (PDSCHs), the target user equipment (UE) calculates whether the current SE is above the expected SE and whether HARQ is enabled or disabled or if Acknowledgement (ACK) is optional according to the assigned modulation and coding scheme (MCS) and number of layers. Furthermore, a new DCI structure, separating MCS assignment and HARQ related information is provided.

Figure 1 illustrates a message exchange between UE 10 and gNB 20, according to an embodiment of the present invention. The UE is typically located on the surface of the earth and the gNB is hosted aboard a satellite or a HAPS.

The gNB 20 signals a threshold, via RRC signalling, to the UE 10 on the expected spectral efficiency (SE). Spectral efficiency is the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is utilized by the physical layer protocol. Expected SE may be determined on the basis of present conditions or historical records.

The gNB 20 indicates enabled/disabled/ACK optional HARQ status via Modulation Coding Scheme (MCS) levels and a number of layers in DCI.

As shown in Figure 1, the gNB 20 first signal a spectral efficiency (MCS and number of layers) threshold to a UE in message S10. Then, the gNB 20 may transmit, in message S11, data on PDSCHs with different MCS levels and numbers of layers. In order to receive the data, the UE 10 needs to decode downlink control information (DCI), which carries the assigned MCSs and number of layers. Therefore, the UE 10 can calculate the SE in this actual transmission (S11) and compare it to the RRC-configured SE threshold signalled in transmission S10.

If the current SE is less than the expected SE, then HARQ is disabled. The gNB 20 is able to control this via the RRC-configured SE threshold according to reports fed back from the UE (not shown). If the current SE is smaller than the expected SE, this means that the assigned MCS and number of layers are more conservative, such that the UE 10 has a higher chance of successfully decoding transmissions from the gNB 20.

On the contrary, when the current SE (from S11) is larger than the expected SE (from S10), then the UE 10 has a lower chance of successfully decoding transmissions. In this case, the UE 10 will attempt to decode PDSCH. If the decode is unsuccessful, the UE feeds back a NACK signal to the gNB (not shown). If the decode is successful, the UE decides whether to feed back an acknowledgement (ACK) (ACK optional). Further details of this mechanism are given later.

Figure 2 shows a flowchart of a method according to an embodiment of the present invention.

At step S20, the UE 10 receives a transmission from the gNB 20 via DCI signalling. This corresponds to 311 in Figure 1.

At step S21, the UE 10 calculates the current SE on the basis of the signal just received at S20.

At step S22, the current SE is compared to the expected SE and if the current SE is lower than the expected SE, then flow passes to step S23, where HARQ is disabled, since the UE 10 has a relatively higher chance of successfully decoding transmissions from the gNB 20.

At step S22, if the current SE is higher than the expected SE, then the UE 10 attempts to decode the PDSCH signal at step S24.

At step S25, it is determined if the decode operation at S24 was successful. If so, then HARQ is optional as per step S26.

If the decode operation is determined at step S25 to be unsuccessful, then at step S27 HARQ is enabled and NACK is fed back to the gNB to signal that the decode was unsuccessful.

There are two extreme options for operation of embodiments of the invention. Firstly, HARQ will effectively always be disabled if the gNB 20 sets an expected SE threshold value which is sufficiently high. Secondly, HARQ will effectively always be enabled if the gNB 20 sets an expected SE threshold value which is sufficiently low. In either case, the expected SE threshold value is set via RRC signalling from the gNB 20 to the UE 10.

In order to determine the current SE from the PDSCH signalling received by the UE 10, a process as illustrated in Figure 3 is followed.

Figure 3 illustrates the receipt of a DCI signal 30. From this, it is possible to extract and determine MCS 31 and, so, SE per layer, since there is a 1:1 mapping of MCS to SE.

At the same time, from the DCI signal 30, it is possible to extract the number of layers or data streams 33 and from this, the mapping of the layers 33.

The current SE 35 is determined as the product of the mapped layer 35 and the SE per layer 32.

As mentioned, the gNB 20 signals using RRC signalling to the UE 10, the value of the expected SE threshold. The value which is signalled may be by an integer, indicating a base single-layer SE (e.g. 10, equivalent to spectral efficiency 10/1024=0.0098 bps/Hz), and another integer, indicating a multiple of the base single-layer SE.

The signalling information may be represented as: ASN1SCAR7 DynafficHARQ::= SEQUENCE { base SpectralEfficency INTEGER eat SpectralEiriencv INTEGER ASN1S7CP

SysteminformationBlockType field descriptions

est-SpectralEfficiency Spectral efficiency as multiples of the singltlayerspectralefficiencypf the lowest,INCSitoptlzy:___ In an embodiment of the invention, a new DCI signalling structure is provided and this is illustrated in Figure 4. The left hand side illustrates the prior art structure and the right hand side illustrates a structure according to an embodiment of the present invention.

In the prior art structure, the MCS and HARQ related information are placed together so that the UE will acquire both items of information simultaneously.

In the modified structure on the right, the HARQ related information is replaced with a pointer called HARQ information pointer which points to the resource which stores the HARQ related information.

This has a benefit in that the UE 10 can then determine whether the current SE is larger than the expected SE, signalled by gNB 20. If so, then the UE 10 can read the HARQ information pointer and search the resource it points to. If not, then the UE 10 does not need to search for HARQ related information, as the gNB 20 will not have assigned HARQ. As a result, signalling overhead can be reduced.

Figure 5 illustrates the procedure followed in the event that the UE is required to acquire HARQ information as set out above.

Here, at step S40, the UE determines the current SE as set out in Figure 3. At step S41 the current SE is compared to the expected SE. If it is smaller, then flow is terminated at step S42, since HARQ is not required. This corresponds to step S23 in Figure 2.

If, however, the current SE is larger than the expected SE, then flow continues to S43 where the UE makes use of the HARQ information pointer to locate the HARQ related information to which it refers.

In the event that current SE is larger than the expected SE and the decode is successful, then the ACK-optional state may be entered and ACK may be provided to the gNB 20 by UE 10. This is as shown in steps S25 and S26 in Figure 2.

The UE 10 has the freedom to decide whether to provide feedback (ACK) considering e.g. its own power level, propagation delay or traffic congestion. Since the baseline position of the gNB is that no HARQ is provided, even if the UE chooses not to provide ACK feedback to the gNB, then performance will not be degraded.

By use of embodiments of the present invention, there is provided a flexible solution to dynamically enable/disable HARQ, with consequent lower signalling overhead.

Furthermore, in the ACK-optional state, the UE is able to determine itself whether to provide HARQ feedback, depending upon its circumstances, such as its own power level, propagation delay or traffic congestion.

At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as 'component, 'module' or 'unit used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of others.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

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Claims (10)

1. CLAIMS1. A method of controlling HARQ operation in a User Equipment, UE, operable to communicate with a base station, gNB, as part of a non-terrestrial network, wherein the method comprises the steps of: the gNB signalling to the UE details of an expected Spectral Efficiency, SE; the gNB transmitting data to the UE; the UE determining a current SE on the basis of receiving the data transmitted by the gNB; the UE comparing the current SE to the expected SE and enabling or disabling HARQ operation in response to the comparison.
2. 2. The method of claim 1 wherein if the current SE is less than the expected SE, then HARQ operation is disabled.
3. 3. The method of claim 1 or 2 wherein if the current SE is more than the expected SE then the UE attempts to decode the received data transmitted by the gNB.
4. 4. The method of claim 3 wherein if the attempt to decode is successful, then the UE enters ACK-optional state, where the UE is operable to determine if HARQ feedback is provided.
5. 5. The method of claim 4 wherein the UE is operable to determine if HARQ feedback is provided on the basis of one or more of: its own power level, propagation delay and traffic 25 congestion.
6. 6. The method any preceding claim wherein the gNB signalling to the UE details of an expected Spectral Efficiency, SE, comprises the use of RRC signalling.
7. 7. The method of claim 6 wherein the expected SE is determined on the basis of present conditions or historical records.
8. 8. The method of any preceding claim wherein the data transmitted from the gNB to the UE comprises Modulation Coding Scheme, MCS, data and a HARQ information pointer, which includes a reference to a resource from where HARQ related information can be obtained.
9. 9. A base station arranged to signal to a UE details of an expected Spectral Efficiency, SE and to then transmit data to the UE.
10. 10. A UE operable to: receive details of an expected Spectral Efficiency, SE from a base station; receive data from the base station; determine a current SE on the basis of receiving the data transmitted by the base station; compare the current SE to the expected SE and enable or disable HARQ operation in response to the comparison.