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EP-4505641-B1 - CONTROL OF REFERENCE SIGNALING FOR CALIBRATION OF A D-MIMO SYSTEM

EP4505641B1EP 4505641 B1EP4505641 B1EP 4505641B1EP-4505641-B1

Inventors

  • VIEIRA, Joao
  • SARAJLIC, MURIS
  • MALEKI, Sina

Dates

Publication Date
20260506
Application Date
20230302

Claims (17)

  1. A method of a control node for a distributed multiple-input multiple-output, D-MIMO, system comprising a plurality of access points, the method comprising, during a calibration process with over-the-air reference signaling between access points, wherein the calibration process comprises a plurality of reference signaling occasions, for (360) at least one particular reference signaling occasion: dynamically selecting (320), among the plurality of access points and based on information obtained by measurements performed during a previous reference signaling occasion, a first access point, wherein the information obtained by measurements performed during the previous reference signaling occasion comprises one or more of: a respective channel coherence time for the first access point relative to each of one or more other access points among the plurality of access points, and a respective reference signaling reliability for the first access point relative to each of one or more other access points among the plurality of access points, wherein the respective reference signaling reliability for the first access point is based on signal strength measurements performed by the first access point; and controlling (340) the first access point to act as a reference signal transmitter during the particular reference signaling occasion.
  2. The method of claim 1, wherein the dynamically selected first access point and the reference signal transmitter of the previous reference signaling occasion form respective ends of a bi-directional link.
  3. The method of any of claims 1 through 2, wherein the measurements performed during the previous reference signaling occasion comprises Doppler measurements and/or signal strength measurements.
  4. The method of any of claims 1 through 3 further comprising, for the at least one particular reference signaling occasion: dynamically selecting (330), among the plurality of access points excluding the first access point, a second access point; controlling (340) the second access point to perform measurements during the particular reference signaling occasion; and informing the second access point that the first access point acts as the reference signal transmitter during the particular reference signaling occasion.
  5. The method of claim 4 further comprising excluding a third access point from the dynamic selection of the second access point, responsive to one or more of: that the information obtained by measurements performed during the previous reference signaling occasion indicates a signal transfer quality between the first access point and the third access point that falls on a side of a quality threshold that indicates low quality; and that channel transfer data is otherwise acquired during the calibration process for calibration of the first and third access points.
  6. The method of any of claims 1 through 5 further comprising receiving (350), prior to a subsequent reference signaling occasion, information (503) obtained by measurements performed during the particular reference signaling occasion.
  7. The method of any of claims 1 through 6, wherein the dynamic selection of the first access point is further based on a system time parameter associated with a coherence assumption, and/or is further based on a movement estimation obtained by a sensor co-located with the first access point.
  8. The method of any of claims 1 through 7, wherein - when the information obtained by measurements performed during the previous reference signaling occasion comprises the respective channel coherence time for the first access point - the respective channel coherence time for the first access point is based on Doppler measurements performed by the first access point.
  9. The method of any of claims 1 through 8, wherein - when the information obtained by measurements performed during the previous reference signaling occasion comprises the respective channel coherence time for the first access point - the dynamically selected first access point fulfills a coherence condition relative the reference signal transmitter of the previous reference signaling occasion, wherein the coherence condition preferably comprises that a time interval between the previous reference signaling occasion and the particular reference signaling occasion is shorter than the channel coherence time for the first access point relative the reference signal transmitter of the previous reference signaling occasion.
  10. The method of claim 9, wherein - among access points fulfilling the coherence condition at the particular reference signaling occasion - the dynamically selected first access point has a shortest time, at the particular reference signaling occasion, until the coherence condition is no longer fulfilled.
  11. The method of any of claims 1 through 10, wherein - when the information obtained by measurements performed during the previous reference signaling occasion comprises the respective reference signaling reliability for the first access point - the dynamically selected first access point fulfills a reliability condition relative the reference signal transmitter of the previous reference signaling occasion, wherein the reliability condition preferably comprises that a received signal strength metric for the reference signal transmitter of the previous reference signaling occasion, as obtained at the first access point, exceeds a signal strength metric threshold.
  12. The method of any of claims 1 through 11 further comprising: determining (310) an overall channel scenario status of the plurality of access points as stationary or variable; and performing the calibration process by dynamically selecting the first access point only when the overall channel scenario status is determined to be variable.
  13. The method of any of claims 1 through 12 further comprising, after the plurality of reference signaling occasions, determining (370) calibration coefficients for the plurality of access points based on channel transfer data acquired by measurements performed during the plurality of reference signaling occasions.
  14. A computer program product comprising a non-transitory computer readable medium (1000), having thereon a computer program comprising program instructions, the computer program being loadable into a data processing unit of a control node for a distributed multiple-input multiple-output, D-MIMO, system and configured to cause execution of the method according to any of claims 1 through 13 when the computer program is run by the data processing unit.
  15. An apparatus for a control node for a distributed multiple-input multiple-output, D-MIMO, system comprising a plurality of access points, the apparatus comprising controlling circuitry (720) configured to cause, during a calibration process with over-the-air reference signaling between access points, wherein the calibration process comprises a plurality of reference signaling occasions, for at least one particular reference signaling occasion: dynamic selection, among the plurality of access points and based on information obtained by measurements performed during a previous reference signaling occasion, of a first access point, wherein the information obtained by measurements performed during the previous reference signaling occasion comprises one or more of: a respective channel coherence time for the first access point relative to each of one or more other access points among the plurality of access points, and a respective reference signaling reliability for the first access point relative to each of one or more other access points among the plurality of access points, wherein the respective reference signaling reliability for the first access point is based on signal strength measurements performed by the first access point; and control of the first access point to act as a reference signal transmitter during the particular reference signaling occasion.
  16. A control node for a distributed multiple-input multiple-output, D-MIMO, system, the control node comprising the apparatus of claim 15.
  17. A distributed multiple-input multiple-output, D-MIMO, system comprising the control node of claim 16.

Description

TECHNICAL FIELD The present disclosure relates generally to the field of wireless communication. More particularly, it relates to calibration of distributed multiple-input multiple-output (D-MIMO) systems. BACKGROUND A distributed multiple-input multiple-output (D-MIMO) system typically comprises a control node and a plurality of access points. Typically, each access point (AP) has one or more (comparatively few, e.g., two) antenna elements, and the plurality of access points are spatially spread out to collectively provide a multi-antenna transceiver system. There exist various approaches for calibration of multi-antenna transceiver systems. A first group of such approaches involves using an internal calibration network in a multi-antenna transceiver device, and a second group of such approaches involves over-the-air (OTA) signaling. US 2020/0244376 A1 describes over-the-air reciprocity calibration for D-MIMO systems, and WO 2021/213658 A1 describes over-the-air beamforming calibration for a multi-antenna transceiver. Approaches involving an internal calibration network typically entails high complexity in terms of hardware and/or software. For example, using an internal calibration network typically requires dedicated hardware (e.g., cables) for calibration, which is not needed in OTA-based calibration. Furthermore, internal calibration network approaches are typically not well suited for D-MIMO (due to the distributed nature of the access point deployment). Approaches involving over-the-air signaling typically entails signaling overhead and/or suffers from being dependent on the radio channel. Thus, there is a need for alternative approaches for calibration of a multi-antenna transceiver system. Preferably, such approaches are suitable for calibration of a D-MIMO system. Also preferably, such approaches require less signaling overhead than over-the-air signaling approaches of the prior art. Furthermore, it is preferable that the negative impact of being dependent on the radio channel is mitigated. It is also beneficial if the calibration approach is equally applicable when the number of access points increases. SUMMARY It should be emphasized that the term "comprises/comprising" (replaceable by "includes/including") when used in this specification is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Generally, when an arrangement is referred to herein, it is to be understood as a physical product; e.g., an apparatus. The physical product may comprise one or more parts, such as controlling circuitry in the form of one or more controllers, one or more processors, or the like. It is an object of some embodiments to solve or mitigate, alleviate, or eliminate at least some of the above or other disadvantages. A first aspect is a method of a control node for a distributed multiple-input multiple-output (D-MIMO) system comprising a plurality of access points. The method comprises - during a calibration process with over-the-air reference signaling between access points, wherein the calibration process comprises a plurality of reference signaling occasions, and for at least one particular reference signaling occasion - dynamically selecting (among the plurality of access points and based on information obtained by measurements performed during a previous reference signaling occasion) a first access point, and controlling the first access point to act as a reference signal transmitter during the particular reference signaling occasion. In some embodiments, the dynamically selected first access point and the reference signal transmitter of the previous reference signaling occasion form respective ends of a bi-directional link. In some embodiments, the measurements performed during the previous reference signaling occasion comprises Doppler measurements and/or signal strength measurements. In some embodiments, the method further comprises - for the at least one particular reference signaling occasion - dynamically selecting (among the plurality of access points excluding the first access point) a second access point, and controlling the second access point to perform measurements during the particular reference signaling occasion. In some embodiments, the method further comprises informing the second access point that the first access point acts as the reference signal transmitter during the particular reference signaling occasion. In some embodiments, the method further comprises excluding a third access point from the dynamic selection of the second access point, responsive to that the information obtained by measurements performed during the previous reference signaling occasion indicates a signal transfer