Link adaptation method and device

Abstract

In the present invention, an OFDM-CSI-RS is transmitted from a radio base station to a radio terminal, an OTFS-CSI-RS is transmitted from the radio base station to the radio terminal, the radio terminal receives the OFDM-CSI-RS and assesses CQI, the radio terminal receives the OTFS-CSI-RS and estimates a Doppler shift, the CQI and Doppler shift information are fed back from the radio terminal to the radio base station, the radio base station receives the CQI and the Doppler shift information, determines an MCS, and notifies the radio terminal of the MCS determined by the radio base station.

Description

Link adaptation method and device

[0001] The present invention relates to a link adaptation method and apparatus in a mobile communication system, and more particularly to a link adaptation method and apparatus for setting an MCS (Modulation and Coding Scheme) of a wireless access link in accordance with the signal quality and moving speed of a wireless terminal in a mobile communication system.

[0002] Orthogonal frequency division multiplexing (OFDM), which is used in fourth and fifth generation mobile communication services, has high spectral efficiency and excellent resistance to multipath fading. These services also use link adaptation, which switches MCS depending on the communication quality of the communication terminal, and use the signal-to-interference-plus-noise power ratio (SINR) or its corresponding channel quality indicator (CQI) as an indicator of communication quality.

[0003] Orthogonal time-frequency space (OTFS) transmission has attracted attention as a transmission method that can accommodate time variations caused by different Doppler shifts.

[0004] Patent Document 1 discloses a technique for simultaneous operation of an OTFS modulation communication system and an LTE system using OFDM.

[0005] Non-Patent Document 1 presents a fair comparison between OTFS and OFDM in a sparsity environment.

[0006] International Publication No. 2019 / 241436

[0007] IEEE, OTFS vs. OFDM in the Presence of Sparsity: A Fair Comparison

[0008] Figure 2 compares the throughput of each modulation method, OFDM and OTFS, for each Doppler frequency. In OFDM, the Doppler shift can cause data communication quality to become unstable in high-speed moving environments, which can reduce communication reliability.

[0009] Although there is potential for improvement if the MCS of the wireless access link can be adaptively switched according to the effects of Doppler shift, the OFDM modulation method does not include Doppler shift information in the CQI, so it is necessary to periodically set the MCS according to the Doppler shift.

[0010] The present invention provides a link adaptation method and apparatus that can adaptively switch MCS in response to the effect of Doppler shift.

[0011] The present invention provides a link adaptation method for a radio access link in a mobile communication system, in which a radio base station transmits an OFDM-modulated reference signal (e.g., CSI-RS (OFDM-CSI-RS)) to a radio terminal, the radio base station transmits an OTFS-modulated reference signal (e.g., CSI-RS (OTFS-CSI-RS)) to the radio terminal, the radio terminal receives the OFDM-CSI-RS and determines channel quality information (e.g., CQI), the radio terminal receives the OTFS-CSI-RS and estimates a Doppler shift, the radio terminal feeds back the CQI and Doppler shift information to the radio base station, the radio base station receives the CQI and Doppler shift information and determines an MCS for the radio access link, and the radio base station notifies the radio terminal of the determined MCS via a downlink control channel (e.g., PDCCH).

[0012] The present invention can be realized not only as a link adaptation method for a wireless access link including such a characteristic procedure, but also as a link adaptation device using such a characteristic procedure as means.

[0013] According to the present invention, the MCS of a wireless access link in a mobile communication system can be adaptively set according to the moving speed of a wireless terminal, thereby improving the stability of OFDM transmission and increasing the reliability of communication.

[0014] Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which the same or similar elements are designated by the same reference numerals.

[0015] The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. Figure 1 is a sequence flow diagram illustrating an example of a link adaptation method according to one embodiment of the present invention. Figure 2 is a graph comparing the throughput of OFDM and OTFS modulation schemes at different Doppler frequencies. Figure 3 is a diagram illustrating an example of a lookup table for determining an MCS based on a CQI and a Doppler index. Figure 4 is a diagram illustrating an example of a lookup table for determining a Doppler index based on a Doppler frequency. Figure 5 is a diagram illustrating a specific example (part 1) of determining an MCS based on a CQI and a Doppler shift. Figure 6 is a diagram illustrating a specific example (part 2) of determining an MCS based on a CQI and a Doppler shift. Figure 7 is a diagram illustrating a specific example (part 3) of determining an MCS based on a CQI and a Doppler shift. Figure 8 is a diagram illustrating a specific example (part 4) of determining an MCS based on a CQI and a Doppler shift. Figure 9 is a sequence flow diagram illustrating an example of a link adaptation method according to another embodiment of the present invention.

[0016] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0017] FIG. 1 is a sequence flow illustrating an example of a link adaptation method in a mobile communication system according to one embodiment of the present invention, showing a communication sequence in a radio access link between a 5G base station (gNB) and a 5G terminal (UE).

[0018] Note that the 4G / 5G-specific CSI-RS (Channel State Information-Reference Signal) is adopted as the reference signal for downlink propagation path estimation and quality measurement, and similarly, the CQI (Channel Quality Indicator) is adopted as channel quality information, and the PDCCH (Physical Downlink Control Channel) is adopted as the downlink control channel; however, the present invention is not limited to these, and various reference signals, quality information, and control channels can be adopted depending on the assumed wireless communication system.

[0019] In this embodiment, the gNB periodically transmits an RRC configuration (OFDM CSI-RS config, OTFS CSI-RS Config) to the UE, and then notifies the UE of the transmission conditions for the OFDM-modulated CSI-RS (OFDM CSI-RS) transmitted in step 1 and the OTFS-modulated CSI-RS (OTFS-CSI-RS) transmitted in step 2.

[0020] In step 1, the gNB transmits OFDM-CSI-RS to the UE under the conditions (e.g., OFDM CSI-RS config) notified to the UE. Based on the OFDM-CSI-RS received from the gNB, the UE calculates a CQI indicating the downlink propagation quality of the radio access link.

[0021] In step 2, the gNB transmits the OTFS-CSI-RS to the UE under the conditions (e.g., OTFS CSI-RS Config) notified to the UE. The UE estimates the Doppler shift based on the OTFS-CSI-RS received from the gNB.

[0022] In step 3, downlink CQI is fed back as OFDM-CSI from the UE to the gNB. In step 4, information on the estimated Doppler shift (Doppler shift information) is fed back as OTFS-CSI from the UE to the gNB.

[0023] The gNB determines the MCS to be used for the downlink of the radio access link based on the CQI and Doppler shift information fed back from the UE. Here, MCS is information that indexes the combination of modulation schemes, coding rates, etc.

[0024] 3 and 4 are diagrams illustrating an example of a method in which a gNB determines an MCS for a radio access link based on CQI and Doppler shift information fed back from a UE.

[0025] In this embodiment, a lookup table (LUT1) that defines the correspondence between CQI, Doppler index, and MCS is set in advance in the gNB, as shown in an example of Fig. 3. Furthermore, a LUT2 that defines the relationship between Doppler frequency and Doppler index is set in advance in the gNB or UE, as shown in an example of Fig. 4.

[0026] If the LUT2 is pre-configured in the gNB, the UE will feed back the Doppler frequency as the Doppler shift information, and the gNB will apply the fed back Doppler frequency to its own LUT2 to determine the corresponding Doppler index.

[0027] On the other hand, if LUT2 is pre-configured in the UE, the UE applies the Doppler frequency to its own LUT2 to determine the corresponding Doppler index, which it then feeds back to the gNB as Doppler shift information.

[0028] The gNB can determine the MCS for the radio access link by applying the CQI and Doppler index to LUT1 in Figure 3.

[0029] Figures 5 to 8 are diagrams that show in more detail how the gNB determines the MCS, and show the correspondence between the combination of "modulation method," "code rate × 1024," and "efficiency" in the MCS for each CQI and the Doppler shift.

[0030] In the example of the correspondence relationship (part 1) shown in FIG. 5, when the Doppler shift is 0.025 or more, even if the CQI fed back from the UE is "3", the MCS of the wireless access link is determined to be the MCS corresponding to 16QAM so that the modulation scheme is converted from 64QAM to 16QAM.

[0031] Similarly, when the Doppler shift is 0.1 or greater, even if the CQI index fed back from the UE is "2," the MCS for the radio access link is determined to be the MCS corresponding to QPSK so that the modulation scheme is converted from 16QAM to QPSK.

[0032] In the example of the correspondence relationship (part 2) shown in FIG. 6 , when the Doppler shift is 0.1 or more, even if the CQI index fed back from the UE is “3”, the MCS of the radio access link is determined to be the MCS corresponding to 16QAM so that the modulation scheme is converted from 64QAM to 16QAM.

[0033] Similarly, when the Doppler shift is 0.175 or greater, even if the CQI index fed back from the UE is "2," the MCS for the wireless access link is determined to be the MCS corresponding to QPSK so that the modulation scheme is converted from 16QAM to QPSK.

[0034] In the example of the correspondence relationship (part 3) shown in FIG. 7 , when the Doppler shift is 0.1 or more, even if the CQI index fed back from the UE is “2”, the MCS of the wireless access link is determined to be the MCS corresponding to QPSK so that the modulation scheme is converted from 16QAM to QPSK.

[0035] In the example of the correspondence relationship (part 4) shown in FIG. 8 , when the Doppler shift is 0.025 or more, even if the CQI index fed back from the UE is “4”, the MCS of the wireless access link is determined to be the MCS corresponding to 64QAM so that the modulation scheme is converted from 256QAM to 64QAM.

[0036] Similarly, when the Doppler shift is 0.125 or greater, even if the CQI index fed back from the UE is "3," the MCS of the radio access link is determined to be an MCS compatible with 16QAM so that the modulation scheme is converted from 64QAM to 16QAM.

[0037] Similarly, when the Doppler shift is 0.175 or greater, even if the CQI index fed back from the UE is "2," the MCS for the wireless access link is determined to be the MCS corresponding to QPSK so that the modulation scheme is converted from 16QAM to QPSK.

[0038] Returning to Figure 1, once the MCS is determined based on the estimation results of the CQI and Doppler shift as described above, in step 5, the gNB notifies the UE of the determined MCS for the radio access link along with allocation resource information via the downlink control channel (PDCCH), and transmits a shared channel (PDSCH: Physical Downlink Shared Channel) using that MCS in step 6. The UE receives and decodes the PDCCH, and receives the PDSCH based on the control information.

[0039] In the above embodiment, an example in which the UE measures the Doppler shift has been described. Alternatively, for example, the UE may transmit an OTFS-modulated reference signal, and the gNB may measure the Doppler shift. An example of the processing flow in this case is shown in FIG. 9.

[0040] In the process of Figure 9, the gNB transmits an RRC configuration (OFDM CSI-RS config, OTFS SRS config, OFDM SRS-config) to the UE, and then notifies the UE of the transmission conditions for the OFDM-modulated CSI-RS (OFDM CSI-RS) to be transmitted in step 1 and the OTFS-modulated SRS (OTFS-SRS) to be transmitted in step 2.

[0041] Step 1 is the same as step 1 in Figure 1. Meanwhile, in step 2, the UE transmits an OTFS-SRS to the gNB under the conditions (e.g., OTFS SRS Config) notified to the UE. The gNB estimates the Doppler shift based on the OTFS-SRS received from the UE. Furthermore, the UE periodically transmits an OFDM SRS to the gNB under the conditions (e.g., OFDM SRS-config) notified to the UE.

[0042] In procedure 3, downlink CQI is fed back as OFDM-CSI from the UE to the gNB. On the other hand, in this processing example, Doppler shift estimation is performed in the gNB, so Doppler shift information is not fed back.

[0043] The gNB determines the MCS to be adopted for the downlink of the radio access link based on the CQI fed back from the UE and the Doppler shift estimated by the gNB. Subsequent steps 4 and 5 are the same as steps 5 and 6 in Figure 1. Furthermore, the method of determining the MCS can be applied as is to the methods described using Figures 3 to 8.

[0044] In the above embodiment, the present invention has been described using an example of applying it to the downlink channel of the wireless access link between the UE and the gNB, but the present invention is not limited to this and can also be applied to the downlink channel of the wireless access link between the UE and the wireless access point (AP).

[0045] According to the above embodiment, the MCS of a wireless access link in a mobile communication system can be adaptively set according to the moving speed of a wireless terminal, thereby improving the stability of OFDM transmission and increasing the reliability of communications. This makes it possible to contribute to Goal 9 "Build resilient infrastructure and promote inclusive and sustainable industrialization" and Goal 11 "Make cities inclusive, safe, resilient and sustainable" of the Sustainable Development Goals (SDGs) led by the United Nations.

[0046] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention.

[0047] This application claims priority based on Japanese Patent Application No. 2023-170117 filed on September 29, 2023 and Japanese Patent Application No. 2023-170123 filed on September 29, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A link adaptation method for a wireless access link in a mobile communication system, comprising: transmitting an OFDM modulated reference signal from a wireless base station to a wireless terminal; transmitting an OTFS modulated reference signal from the wireless base station to the wireless terminal; the wireless terminal receiving the OFDM modulated reference signal and determining channel quality information of the wireless access link; the wireless terminal receiving the OTFS modulated reference signal and estimating a Doppler shift; feeding back the channel quality information and Doppler shift information from the wireless terminal to the wireless base station; the wireless base station receiving the channel quality information and Doppler shift information and determining an MCS for the wireless access link; and the wireless base station notifying the wireless terminal of the determined MCS.

2. The link adaptation method according to claim 1, characterized in that the wireless base station is provided with a lookup table that sets the correspondence between Doppler frequencies and Doppler indexes, the wireless terminal transmits Doppler frequencies as the Doppler shift information, and the wireless base station applies the received Doppler frequencies to the lookup table to determine the Doppler index.

3. The link adaptation method according to claim 1, characterized in that the wireless terminal is provided with a lookup table that sets the correspondence between Doppler frequency and Doppler index, and the wireless terminal determines the Doppler index by applying the Doppler frequency to the lookup table, and transmits the Doppler index as the Doppler shift information.

4. The link adaptation method according to claim 2 or 3, characterized in that the radio base station is provided with a lookup table that sets the correspondence between channel quality information, Doppler index and MCS, and the MCS is determined by applying the Doppler index to the lookup table.

5. The link adaptation method according to claim 1, characterized in that the radio base station comprises a lookup table in which a correspondence relationship between channel quality information, MCS and Doppler shift reference value is set, and when a Doppler shift of the received Doppler shift information exceeds a Doppler shift reference value corresponding to the received channel quality information, the MCS corresponding to the received Doppler shift information is determined as an MCS for the radio access link.

6. A link adaptation method according to any one of claims 1 to 5, characterized in that the OFDM modulated reference signal is OFDM-CSI-RS, the OTFS modulated reference signal is OTFS-SRS, and the channel quality information is CQI.

7. A link adaptation device for a wireless access link in a wireless base station of a mobile communication system, comprising: means for transmitting an OFDM modulated reference signal to a wireless terminal; means for transmitting an OTFS modulated reference signal to the wireless terminal; means for receiving feedback of channel quality information determined by the wireless terminal upon receiving the OFDM modulated reference signal and of a Doppler shift estimated by the wireless terminal upon receiving the OTFS modulated reference signal; means for determining an MCS for the wireless access link based on the channel quality information and the Doppler shift; and means for notifying the wireless terminal of the determined MCS.

8. The link adaptation device according to claim 7, characterized in that the OFDM modulated reference signal is OFDM-CSI-RS, the OTFS modulated reference signal is OTFS-SRS, and the channel quality information is CQI.

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