EP4335038A1 - Alignement de phase dans un système de communication mimo distribué - Google Patents

Alignement de phase dans un système de communication mimo distribué

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Publication number
EP4335038A1
EP4335038A1 EP21724249.4A EP21724249A EP4335038A1 EP 4335038 A1 EP4335038 A1 EP 4335038A1 EP 21724249 A EP21724249 A EP 21724249A EP 4335038 A1 EP4335038 A1 EP 4335038A1
Authority
EP
European Patent Office
Prior art keywords
reference signal
aps
phase
alignment
phase rotation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP21724249.4A
Other languages
German (de)
English (en)
Inventor
Pål FRENGER
Joao VIEIRA
Erik G. Larsson
Unnikrishnan KUNNATH GANESAN
Sarvendranath RIMALAPUDI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Telefonaktiebolaget LM Ericsson AB
Original Assignee
Telefonaktiebolaget LM Ericsson AB
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Telefonaktiebolaget LM Ericsson AB filed Critical Telefonaktiebolaget LM Ericsson AB
Publication of EP4335038A1 publication Critical patent/EP4335038A1/fr
Pending legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/022—Site diversity; Macro-diversity
    • H04B7/024—Co-operative use of antennas of several sites, e.g. in co-ordinated multipoint or co-operative multiple-input multiple-output [MIMO] systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B17/00—Monitoring; Testing
    • H04B17/10—Monitoring; Testing of transmitters
    • H04B17/11—Monitoring; Testing of transmitters for calibration
    • H04B17/12—Monitoring; Testing of transmitters for calibration of transmit antennas, e.g. of the amplitude or phase
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B17/00—Monitoring; Testing
    • H04B17/20—Monitoring; Testing of receivers
    • H04B17/24—Monitoring; Testing of receivers with feedback of measurements to the transmitter

Definitions

  • Embodiments presented herein relate to access points, systems, methods, computer programs, and a computer program product for phase-alignment between antenna panels at different access points in a distributed multiple -input multiple-output communication system.
  • Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple -output (MIMO) communication channel.
  • MIMO multiple-input multiple -output
  • Such systems and/or related techniques are commonly referred to as MIMO systems, or just MIMO for short.
  • D-MIMO Distributed MIMO
  • 6G 6 th generation
  • D-MIMO is a candidate for the physical layer of the 6 th generation (6G) telecommunication system.
  • D-MIMO is based on geographically distributing the antennas of the network and configure them to operate phase-coherently together.
  • D-MIMO Downlink Physical Broadcast
  • Beamforming may e.g. be analog, digital, or hybrid.
  • MIMO techniques can provide diversity, directivity, spatial multiplexing, etc.
  • D-MIMO techniques suitable for uplink communication can differ significantly from D-MIMO techniques suitable for downlink communication.
  • multiple access points where each AP has one or more antenna panel, are interconnected and configured such that two or more APs can cooperate in coherent decoding of data from a given user equipment (UE) served by the network, and such that two or more APs can cooperate in coherent transmission of data to a UE.
  • Each panel might comprise multiple antenna elements that are configured to operate phase-coherently together.
  • TDD time-division duplexing
  • UE user equipment
  • Each panel might comprise multiple antenna elements that are configured to operate phase-coherently together.
  • TDD time-division duplexing
  • This type of TDD operation is commonly referred to as reciprocity- based operation.
  • each antenna element in every panel has to be calibrated for uplink-downlink reciprocity. This in order to compensate for phase (and amplitude) mismatches between the receive and transmit branches of the hardware. This can be achieved, for example, by performing pairwise measurements between the antenna elements located in the same panel, and without interaction among different ones of the panels.
  • the APs need to agree on a common frequency reference to drive their mixer. This can be achieved by communication between the APs using an over-the-air protocols. For example, a master transmitter can broadcast a frequency correction burst. Cable-based (e.g. fiber or ethemet) technologies for this communication are also possible.
  • the APs need to agree on a common global phasor in order to be aligned in phase. Since a small time-shift of a narrowband signal is equivalent to a phase-shift, synchronizing to the global phasor can be viewed as performing a fine time synchronization. This is required for joint coherent beamforming in the downlink to work properly when multiple APs co-operate.
  • One solution is to connect all APs by means of a cable or fiber with precise calibration of all electronics to achieve this phase alignment. However, protocols that rely on measurements over-the-air are also possible. On example is to use pairwise bi directional measurements between the APs to obtain a common phase reference, or time-base.
  • the UEs it is also possible to involve the UEs in the synchronization and calibration tasks, but this is commonly considered undesirable for several reasons. For example, there might not even be a UE available, or the physical radio propagation channel between the APs and a UE may weak and/or fading. Relying on feedback from the UEs also places the burden of network synchronization on the UEs, which could result in a higher need for processing at the UE side, thus possibly reduced the battery life at the UE side. Involving the UEs introduces additional delay and overhead, resulting in reduced synchronization accuracy and capacity, since the measurements made by the UEs need to be communicated to the network before the measurements can be used for synchronization purposes of the APs.
  • synchronization can refer to time, phase, or frequency.
  • levels of synchronization such as either absolute synchronization, or relative synchronization
  • One aspect of synchronization concerns the aligning the phases of signals transmitted from different distributed APs.
  • One purpose of this phase alignment is that signals transmitted from different APs should add up coherently at the location of the receiving UE.
  • the coherent phase alignment does not have to be perfect to be beneficial.
  • SINR signal-to-interference-plus-noise ratio
  • the signals at a UE will add up to a larger resulting signal if the phase difference between the two components is less than +120 degrees (i.e. two vectors of the same length add up to a longer vector if the angle between them is less than 120 degrees).
  • Techniques exist for reciprocity calibration of a single antenna panel i.e., for antenna elements within an antenna panel.
  • phase alignment timing
  • frequency synchronization between antenna panels of different APs in a D-MIMO system. This cannot be performed locally at each AP but hence it requires interaction between different APs.
  • the antenna panels of the APs might be installed in such a way that there is no strong (line-of-sight) channel between antenna panels of different APs, and/or that the APs are not aware of the locations of antenna panels at other APs (or not even the location of other APs). Further, there might be strong interference, for example when the D-MIMO system is deployed for operation in an unlicensed frequency band. This can result in a loss of signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) and eventually to a loss of reliability.
  • SNR signal-to-noise ratio
  • SIR signal-to-interference ratio
  • An object of embodiments herein is to overcome the above noted issues and enable phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.
  • a method for phase-alignment between antenna panels at different APs in a distributed MIMO communication system Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity.
  • the method is performed by a first AP of the APs.
  • the method comprises determining a reference phase rotation a of a second reference signal received from a second AP of the APs.
  • the method comprises transmitting a third reference signal towards the second AP.
  • the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.
  • a first AP for phase-alignment between antenna panels at different APs in a distributed MIMO communication system.
  • Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity.
  • the first AP comprises processing circuitry (210).
  • the processing circuitry is configured to cause the first AP to determine a reference phase rotation a of a second reference signal received from a second AP of the APs.
  • the processing circuitry is configured to cause the first AP to transmit a third reference signal towards the second AP.
  • the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.
  • a computer program for phase -alignment between antenna panels at different APs comprising computer program code which, when run on processing circuitry of a first AP, causes the first AP to perform a method according to the first aspect.
  • a method for phase-alignment between antenna panels at different APs in a distributed MIMO communication system Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity.
  • the method is performed by a second AP of the APs.
  • the method comprises transmitting a second reference signal towards a first AP of the APs.
  • the method comprises determining a phase alignment factor d from a third reference signal received from the first AP.
  • the third reference signal has a phase rotation b compared to the second reference signal.
  • the phase alignment factor d is determined from the phase rotation b.
  • a second AP for phase-alignment between antenna panels at different APs in a distributed MIMO communication system.
  • Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity.
  • the second AP comprises processing circuitry (310).
  • the processing circuitry is configured to cause the second AP to transmit a second reference signal towards a first AP of the APs.
  • the processing circuitry is configured to cause the second AP to determine a phase alignment factor d from a third reference signal received from the first AP.
  • the third reference signal has a phase rotation b compared to the second reference signal.
  • the phase alignment factor d is determined from the phase rotation b.
  • a computer program for phase-alignment between antenna panels at different APs comprising computer program code which, when run on processing circuitry of a second AP, causes the second AP to perform a method according to the fourth aspect.
  • a seventh aspect there is presented a computer program product comprising a computer program according to at least one of the third aspect and the sixth aspect and a computer readable storage medium on which the computer program is stored.
  • the computer readable storage medium could be a non-transitory computer readable storage medium.
  • a distributed MIMO communication system comprising a first AP according to the second aspect and at least one a second AP according to the fifth.
  • these aspects enable phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.
  • these aspects enable a fast way for phase alignment (timing) and frequency synchronization since the phase alignment process can be considered as consisting of one uplink/downlink handshake per AP-pair (where the uplink/downlink handshake is defined by transmission and reception of the second reference signal and transmission and reception of the third reference signal).
  • Such uplink/downlink handshake can be performed simultaneously for a plurality of AP -pairs. Due to the quickness of the method, it guarantees that a bi-direction channel between two APs is phase reciprocal, which is a necessary condition for the alignment.
  • these aspects enable improved SNR and SIR in the detection of the phase alignment signals (i.e. the second and third reference signals), by virtue of beamforming gain achieved in the transmission of these signal, and the spatial processing gain when receiving these signal with multiple antennas.
  • these aspects enable the overhead for reference signals to be lowered in a typical deployment scenario (compared to full or pruned beam-sweep of all AP pairs). Even if time-repetition is required, the amount of repetition required cannot be larger than the amount of spatial repetition in a beam-sweeping procedure. At worst the required overhead is the same.
  • these aspects enable that pre-compensation of phase alignment can be done locally at each AP.
  • these aspects enable both phase errors and frequency errors to be corrected for.
  • phase alignment handshake (second and third reference signals) can be determined based on statistics of phase alignment compensation factor updates.
  • the periodicity of the beam-forming determination signal (first reference signal) can be adapted e.g. based on variations of observed SINR on the received phase alignment handshake signals.
  • Fig. 1 is a schematic diagram illustrating a communication system according to embodiments
  • FIGs. 2, 3, and 4 are flowcharts of methods according to embodiments
  • Figs. 5 and 6 are sequence diagrams of methods according to embodiments.
  • Fig. 7 is a schematic illustration of transmission of second reference signals according to embodiments.
  • Fig. 8 is a schematic illustration of reception of second reference signals and transmission of third reference signals according to embodiments
  • Fig. 9 is a schematic illustration of transmission of a first reference signal, a second reference signal, and a third reference signal according to embodiments
  • Fig. 10 is a schematic illustration of phase alignment error as a function of time according to embodiments.
  • Fig. 11 is a schematic diagram showing functional units of an AP according to an embodiment.
  • Fig. 12 shows one example of a computer program product comprising computer readable means according to an embodiment.
  • Fig. 1 is a schematic diagram illustrating a communication system 100 where embodiments presented herein can be applied.
  • the communication system 100 is a distributed MIMO communication system 100.
  • the communication system 100 comprises APs, five of which are identified at reference numerals 200a, 200b, 200c, 200d, 200e.
  • the herein disclosed embodiments are not limited to any particular number of APs 200a:200e as long as there are at least two APs 200a:200e.
  • Each AP 200a:200e could be a (radio) access network node, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, integrated access and backhaul (IAB) node, or the like.
  • NB node B
  • eNB evolved node B
  • IAB integrated access and backhaul
  • the APs 200a:200e operatively connected over interfaces 120 to a centralized node 300, which could represent a core network.
  • the APs 200a:200e are configured to provide network access to UEs, one of which is illustrated at reference numeral 400.
  • Each such UE 400 could be any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, smartphone, laptop computer, tablet computer, wireless modem, wireless sensor device, Internet of Things (IoT) device, network equipped vehicle, or the like.
  • IoT Internet of Things
  • the APs 200a:200e are configured for wireless communication with each other (as well as with the UE 400).
  • the APs 200a:200e use beamforming for this communication, as represented by beams 110a, 110b, 110c.
  • Beam 110a is an example of a wide beam, or omni-directional beam
  • beams 110b, 110c are examples of narrow beams, or so-called pencil beams.
  • Each AP 200a:”00b comprises at least one antenna panel.
  • each antenna panel is composed of a plurality of individual antenna elements.
  • the embodiments disclosed herein therefore relate to mechanisms for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100.
  • APs 200a:200e methods performed by the APs 200a:200e, computer program products comprising code, for example in the form of computer programs, that when run on processing circuitry of the APs 200a:200e, causes the APs 200a:200e to perform the methods.
  • AP 200a will hereinafter be referred to as a first AP, or reference AP (denoted AP R ), whereas AP 200b will be referred to as a second AP, or non-calibrated panel AP (denoted AP npa ). HOW to select the reference AP will be disclosed below.
  • the antenna panel at the first AP 200a is assumed to haveM R antenna elements.
  • the antenna panel at the second AP 200b is assumed to have M NPA antenna elements. It is assumed that the individual antenna panels are reciprocity -calibrated (but not phase aligned), such that, in mathematical representation, the physical radio propagation channel from the first AP 200a to the second AP 200b is equal to the (transpose of) the physical radio propagation channel from the second AP 200b to the first AP 200a. This can be achieved using known techniques for intra-panel reciprocity calibration.
  • a mechanism is disclosed for improved over-the-air phase-alignment (timing) and frequency synchronization based on pairwise transmission between APs 200a:200e.
  • FIG. 2 illustrating a method for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100 as performed by a first AP 200a of the APs 200a:200e according to an embodiment.
  • Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, but the antenna panels are not mutually phase aligned.
  • the first AP 200a determines a reference phase rotation a of a second reference signal received from a second AP 200b of the APs 200a:200e.
  • the first AP 200a transmits a third reference signal towards the second AP 200b.
  • the third reference signal is phase adjusted with a phase adjustment factor g determined from the reference phase rotation a.
  • this method enables phase alignment (timing) and frequency synchronization between antenna panels of different APs in a D-MIMO system.
  • this method enables a fast way for phase alignment (timing) and frequency synchronization since the phase alignment process can be considered as consisting of one uplink/downlink handshake per AP-pair (where the uplink/downlink handshake is defined by transmission and reception of the second reference signal and transmission and reception of the third reference signal).
  • Such uplink/downlink handshake can be performed simultaneously for a plurality of AP -pairs. Due to the quickness of the method, it guarantees that a bi-direction channel between two APs is phase reciprocal, which is a necessary condition for the alignment.
  • this method enables improved SNR and SIR in the detection of the phase alignment signals (i.e. the second and third reference signals), by virtue of beamforming gain achieved in the transmission of these signal, and the spatial processing gain when receiving these signal with multiple antennas.
  • this method enables the overhead for reference signals to be lowered in a typical deployment scenario (compared to full or pruned beam-sweep of all AP pairs). Even if time-repetition is required, the amount of repetition required cannot be larger than the amount of spatial repetition in a beam-sweeping procedure. At worst the required overhead is the same.
  • this method enables pre-compensation of phase alignment can be done locally at each AP.
  • this method enables both phase errors and frequency errors to be corrected for.
  • these aspects enable the overhead caused by the transmission of the reference signals to be adapted.
  • the periodicity of the phase alignment handshake (second and third reference signals) can be determined based on statistics of phase alignment compensation factor updates.
  • Embodiments relating to further details of phase-alignment between antenna panels at different APs 200a:200e as performed by the first AP 200a will now be disclosed.
  • the phase adjustment factor g is determined.
  • the reference phase rotation a is then dependent on this difference D.
  • the APs 200a:200e might use beamforming for communication between each other.
  • the second reference signal and the third reference signal are beamformed.
  • the first AP 200a Before receiving the second reference signal, the first AP 200a itself might transmit a first reference signal.
  • One purpose of this reference signal is to enable the second AP 200b:200e to estimate a direction towards the first AP 200a. In turn, this could allow the second AP 200b:200e to beamform the transmission of the second reference signal.
  • the first AP 200a is configured to perform (optional) step SI 02:
  • the first AP 200a transmits a first reference signal towards the second AP 200b.
  • the first reference signal is transmitted at least once prior to the first AP 200a receiving the second reference signal from the second AP 200b.
  • the periodicity of the beam-forming determination signal (first reference signal) can be adapted e.g. based on variations of observed SINR on the received phase alignment handshake signals.
  • the first reference signal is transmitted in a wider beam than the third reference signal. In some examples, the first reference signal is transmitted in a wide beam, or omni-directional beam 110a.
  • One efficient method to generate a wide beam from an antenna panel is that of dual-polarized array size invariant pre-coding.
  • the first reference signal is transmitted in a narrow beam, but this might require the first AP 200a to transmit the first signal in several narrow beams, for example by transmitting the first reference signal by performing a full beam-sweep.
  • the first reference signal and the third reference signal are by the first AP 200a beamformed using a phase-neutral beamformer.
  • One way to accomplish this is to use a fixed value (e.g. 1) for one component in all beamformers. This would avoid introducing phase ambiguities by the transmit and receive beamformers.
  • Each and every APs 210a:210e should thus use the same beamforming phase reference when calibrating and when transmitting data to the UEs.
  • a good approach is to rotate each transmit and receive beamformer used during phase alignment to a neutral phase e.g. by requiring that the first coefficient (y- in the beamforming vector (v) to be equal to 1. This does not change the direction of the transmit or receive beam.
  • the first reference signal is transmitted less frequently in time than the third reference signal.
  • at least two occurrences of the first reference signal might be transmitted.
  • at least two occurrences of the second reference signal are received and at least two occurrences of the third reference signal are transmitted between any two adjacent occurrences of the first reference signal. Further details relating thereto will be disclosed below with reference to Fig. 9.
  • each second AP 200b:200e there might be several second APs 200b:200e. All these second APs 200b:200e might be simultaneously phase-aligned with the first AP 200a.
  • one separate second reference signal is received from each of at least two second APs 200b:200e, and one separate value of the reference phase rotation is determined for each of the at least two second APs 200b:200e.
  • the second APs 200b:200e transmit second reference signals that are orthogonal with respect to each other. This enable the second reference signals of all the second APs 200b:200e to be simultaneously transmitted (and simultaneously received).
  • the second reference signal received from one of the at least two second APs 200b is orthogonal to the second reference signal received from any other of the at least two second APs 200c:200e.
  • each second AP 200b:200e might transmit its second reference signal in a very narrow beam. This could also enable the second reference signals of all the second APs 200b:200e to be simultaneously transmitted (and simultaneously received). Further details relating thereto will be disclosed below with reference to Fig. 7.
  • the first AP 200a might transmit one third reference signal to each of the second APs 200b:200e.
  • one separate third reference signal is transmitted towards each of the at least two second APs 200b:200e.
  • the third reference signal for a given one of the at least two second APs 200b:200e is phase adjusted with the phase adjustment factor determined from the reference phase rotation of the second reference signal received from this given one of the at least two second APs 200b:200e.
  • each third reference signal is phase adjusted with the phase adjustment factor determined for the intended receiver.
  • the first AP 200a transmits third reference signals that are orthogonal with respect to each other. That is, in some embodiments, the third reference signal transmitted towards one of the at least two second APs 200b is orthogonal to the third reference signal transmitted towards any other of the at least two second APs 200c:200e.
  • the third reference signals that are orthogonal with respect to each other enables all third reference signals to be simultaneously transmitted.
  • the third reference signals for all of the at least two second APs 200b:200e are simultaneously transmitted towards the at least two second APs 200b:200e.
  • the first AP 200a might transmit the third reference signals in very narrow beams. This could also enable the third reference signals to be simultaneously transmitted, without causing mutual interference at the second APs 200b:200e. Further details relating thereto will be disclosed below with reference to Fig. 8.
  • the first AP 200a is selected as reference AP.
  • One criterion depends on the geographical location where the APs 200a:”00e are deployed. Assume therefore that each of the APs 200a:200e has a deployment location.
  • the first AP 200a is selected from all the APs 200a:200e to transmit the third reference signal based on its deployment location in relation to the deployment locations of the other APs 200b:200e.
  • the AP being closest or the geographical center of all the APs (or a subset of the APs) is selected as reference AP.
  • the AP being on an edge between two subsets, or clusters, of APs is selected as reference AP.
  • the selection could be made by the centralized node 300.
  • the deployment location of the first AP 200a is either more centralized than the deployment location of the second AP 200b or is on an edge between a first sub-set of the APs 200a:200e and a second sub-set of the APs 200a:200e where the second AP 200b belongs to either the first sub-set or the second sub-set.
  • a decent quality AP-to-AP link (such as either line-of- sight (LOS) or without any significant reflection/refraction) is required for each pair or APs.
  • LOS line-of- sight
  • the network is divided into sub-sections and the herein disclosed methods are performed separately for each such subsection of the network.
  • one of the APs at the edge of this cluster, or sub-section of the network is selected as the next reference AP.
  • FIG. 3 illustrating a method for phase-alignment between antenna panels at different APs 200a:200e in a distributed MIMO communication system 100 as performed by a second AP 200b:200e of the APs 200a: 200e according to an embodiment.
  • Each of the antenna panels comprises multiple antenna elements calibrated for uplink-downlink reciprocity, but the antenna panels are not mutually phase aligned.
  • the second AP 200b:200e transmits a second reference signal towards a first AP 200a of the APs 200a:200e.
  • the second AP 200b:200e determines a phase alignment factor d from a third reference signal received from the first AP 200a.
  • the third reference signal has a phase rotation b compared to the second reference signal.
  • the phase alignment factor d is determined from the phase rotation b.
  • phase alignment factor d is determined.
  • the APs 200a:200e might use beamforming for communication between each other.
  • the second reference signal and the third reference signal are beamformed.
  • the same beamformer is used when transmitting the second reference signal as when receiving the third reference signal.
  • the first AP 200a before receiving the second reference signal, the first AP 200a itself might transmit a first reference signal.
  • One purpose of this reference signal is to enable the second AP 200b:200e to estimate a direction towards the first AP 200a.
  • the second AP 200b:200e is configured to perform (optional) step S202 and step S204:
  • the second AP 200b:200e receives a first reference signal from the first AP 200a.
  • the first reference signal is received by the second AP 200b:200e prior to the second AP 200b:200e transmitting the second reference signal towards the first AP 200a.
  • S204 The second AP 200b:200e estimates a dominant spatial direction from which the first reference signal emanates.
  • the second reference signal could then be beamformed to be transmitted in the dominant spatial direction in which the first reference signal was received. That is, in some embodiments, the second reference signal is beamformed in the thus estimated dominant spatial direction.
  • the first reference signal is transmitted less frequently in time than the third reference signal.
  • at least two occurrences of the first reference signal might be transmitted.
  • at least two occurrences of the second reference signal are transmited and at least two occurrences of the third reference signal are received between any two adjacent occurrences of the first reference signal.
  • the second reference signal is by the second AP 200b beamformed using a phase- neutral beamformer.
  • One way to accomplish this is to use a fixed value (e.g. 1) for one component in all beamformers.
  • the second APs 200b:200e transmit second reference signals that are orthogonal with respect to each other. That is, in some embodiments, the second reference signal transmited from the second AP 200b is orthogonal to any other second reference signal transmited from any other second AP 200c:200e.
  • the phase alignment factor d can by the second AP 200b:200e be applied for data transmission towards UE 400 served by the second AP 200b:200e.
  • the second AP 200b:200e is configured to perform (optional) step S210:
  • the second AP 200b:200e transmits a data signal towards UEs 400 served by the second AP 200b:200e.
  • the data signal when transmited is phase rotated by the phase alignment factor d.
  • the first AP 200a transmits a data signal towards the same UEs 400 (which are thus also served by the first AP 200b). This data signal when transmited does not need to be phase rotated.
  • the centralized node 300 provides control and coordination information to the APs 200a, 200b as well as to the UE 400 (via one or more of the APs 200a, 200b).
  • the first AP 200a transmits a first reference signal.
  • the first reference signal might be transmited in an omni-directional beam 110a at the first AP 200a.
  • the first reference signal is transmited in a wide beam, or omni directional beam 110a. Further, since the transmission of the first reference signal cannot benefit from any transmit beamforming gain, the first reference signal may be repeatedly transmited several times to accumulate enough energy at each of the receivers in the second APs 200b:200e. As further disclosed above, in other examples, the first reference signal is transmitted in a narrow beam, but this might require the first AP 200a to transmit the first signal in several narrow beams, for example by transmitting the first reference signal by performing a full beam-sweep. As further disclosed above, in some aspects, the first reference signal is transmitted less frequently in time than the third reference signal.
  • pilot waveforms (rows of F) are orthogonal.
  • the pilot waveforms are adapted based on knowledge of the propagation environment and specifically based on information about the location of the first AP 200a relative to the second APs 200b:200e.
  • S302 The second AP 200b receives the first reference signal.
  • the second AP 200b estimates the dominant spatial direction from which the first reference signal emanates.
  • the second AP 200b thereby estimates the dominant spatial direction towards the first AP 200a.
  • the second AP 200b might further determine a receive beamformer and a transmit beamformer based on the dominant spatial direction towards the first AP 200a.
  • the antenna panel at second AP 200b at time index n receives the time-domain signal y p n according to the following: where p represents transmission power, G is the channel matrix and w n is noise.
  • p represents transmission power
  • G is the channel matrix
  • w n is noise.
  • m and e denote frequency and phase (small time) offsets, respectively, which are a priori unknown to all APs 200a:200e.
  • the second AP 200b determines a beamforming vector, v (of dimension M p ), that matches the dominant spatial direction of G.
  • v is obtained through parametric estimation of G, treating m and e as explicit nuisance parameters.
  • second-order statistics of w n are jointly estimated with the dominant singular vector of G in order to suppress interference through spatial filtering.
  • S304 The second AP 200b beamforms, using a transmit precoder that is based on the transmit beamformer, a second reference signal that is transmitted in a beam 110b into the estimated dominant spatial direction towards the first AP 200a.
  • the first AP 200a receives the second reference signal.
  • the first AP 200a determines a reference phase rotation a of the second reference signal.
  • S306 The first AP 200a determines a receive beamformer and a transmit beamformer.
  • S307 The first AP 200a beamforms, using a transmit precoder that is based on the transmit beamformer, a third reference signal that is transmitted in a beam 110c towards the second AP 200b.
  • the second AP 200b receives the third reference signal using the receive beamformer.
  • the third reference signal has a phase rotation b compared to the second reference signal.
  • S310 The UE 400 transmits uplink reference signals that are received by the first AP 200a and the second AP 200b.
  • S311 The second AP 200b applies the determined phase alignment factor d.
  • the first AP 200a transmits data and/or control signals towards the UE 400.
  • the second AP 200b transmits data and/or control signals towards the UE 400 whilst applying the determined phase alignment factor d.
  • phase ambiguity the reference first AP 200a has is irrelevant.
  • the goal of the disclosed method is to enable calibration of the phase ambiguities of all APs (in the embodiment of Fig. 4 represented by the first AP 200a and the second AP 200b) to enable phase coherent downlink transmissions to UEs 400.
  • the APs 200a:200e transmit data signals to the same UE 400
  • the received data signals from the different APs 200a:200e should arrive phase aligned at the UE 400. This is accomplished by the disclosed embodiments.
  • Any phase ambiguity of the first AP 200a acting as reference AP is common to all APs 200a: 200e after phase alignment and can be equalized via one downlink reference signal (which is simultaneously transmitted by all APs 200a:200e).
  • Fig. 6 schematically illustrates a sequence diagram of transmission and reception of the second reference signal and transmission and reception of the third reference signal, together with the local clock phases at the first AP 200a and the second AP 200b.
  • the observed phase of the third reference signal at the second AP 200b is twice as large as the actual phase alignment error between the second AP 200b and the first AP 200a.
  • a phase alignment error is equivalent to a clock error and the transmission of a reference signal starts when the clock phase at the transmitting end is zero. This has the effect that the phase alignment error is applied twice to the observed phase of the reference signal at the second AP 200b when receiving the third reference signal.
  • the reference phase 0 is defined from the perspective the second AP 200b
  • the reference i6 phase 0 is defined from the perspective of the first AP 200a. This implies that the observed phase of the third reference signal at the second AP 200b equals — 2D in Fig. 6, which thus is equal to b in Fig. 5.
  • FIG. 7 schematically illustrates an example of transmission of second reference signals from four second APs 200b:200e towards the first AP 200a.
  • the second reference signals are denoted fi, f 2 , F 3 , and f 4 , and can, for example in the case of orthogonal frequency -division multiplexing (OFDM) transmission, be placed on orthogonal resource elements in a time -frequency resource grid.
  • OFDM orthogonal frequency -division multiplexing
  • Allocation of resource elements for transmission of the second reference signal in the time- frequency resource grid from each of the second APs 200b:200e is illustrated at 710b, 710c, 710d, and 710d.
  • Beams used by each of the second APs 200b:200e for transmission of the second reference signals are illustrated at 1 lOb-b, 1 lOb-c, 1 lOb-d, and 1 lOb-e.
  • the first AP 200a uses a fully digital receive beamformer, the first AP 200a can apply different receive beamforming coefficient to different resource elements in the time and frequency grid when receiving the second reference signals.
  • the time-frequency resource grid of resource elements of the second reference signals as received by the first AP 200a from all four second APs 200b:200e is illustrated at 720.
  • Beams used by the first AP 200a for reception of the second reference signals are illustrated at llOc-b, llOc-c, llOc-d, and llOc-e.
  • FIG. 8 schematically illustrates an example of reception of second reference signals by the first AP 200a from four second APs 200b:200e and transmission of third reference signals from the first AP 200a towards four second APs 200b:200e.
  • all four second APs 200b:200e are to be simultaneously phase aligned with the first AP 200a.
  • the time- frequency resource grid of resource elements of the second reference signals received by the first AP 200a from all four second APs 200b:200e is illustrated at 810.
  • At 820 is illustrated that the first AP 200a de-multiplexes the second reference signals from the separate second APs 200b:200e.
  • At 830b, 830c, 830d, 830e is illustrated the time-frequency resource grids for the thus separated second reference signals from the four second APs 200b:200e.
  • the first AP 200a determines one reference phase rotation a k for each of the four second APs 200b:200e.
  • At 860 is illustrated that the first AP 200a transmits the third reference signal towards the four second APs 200b:200e.
  • the first reference signal might be transmitted less frequently in time than the third reference signal.
  • Fig. 9 at 900 schematically illustrates transmission of a first reference signal, a second reference signal, and a third reference signal along a timeline.
  • first reference signal 910 is followed by a first occurrence of the second reference signal 920a and a first occurrence of the third reference signal 930a.
  • first downlink or uplink data 940a is transmitted, follows a second occurrence of the second reference signal 920b and a second occurrence of the third reference signal 930b.
  • second downlink or uplink data 940b is transmitted, follows a third occurrence of the second reference signal 920c and a third occurrence of the third reference signal 930c.
  • phase alignment occurs periodically.
  • phase alignment being represented by the transmission of the second reference signal and the third reference signal
  • phase alignment is opportunistically scheduled by the network node 200, or the centralized node 300, at instances when there is no or little user data to transmit.
  • the time delay between two adjacent occurrences of the second reference signal (and the third reference signal) can be adapted based on observed phase errors, drifts. According to one example, in case the standard deviation of phase alignment errors is smaller than a threshold, the rate of occurrence of the transmission of the second reference signal and the third reference signal is decreased; otherwise the rate of occurrence is increased.
  • the phase adjustment factor g will never, or at least very seldom, be exactly zero due to measurement noise and/or phase drifts.
  • phase alignment error 10 schematically depicts the phase alignment error as a function of time and which at (a) illustrates an example of a first phase alignment error 1010a evolving over time where the mean phase alignment error is zero, indicating no systematic frequency error, and which at (b) illustrates an example of a second phase alignment error 1010b evolving over time where the mean phase alignment error is above zero, indicating a systematic frequency error. If there is a constant drift, then the frequency of the local oscillator should be corrected accordingly.
  • the standard deviation of all compensation factor updates from all the second APs 200b:200e could be used as a metric when determining if the phase alignment period is too large or too small. This may e.g. be reported by the first AP 200a to the centralized node 300, either periodically or event triggered).
  • Fig. 11 schematically illustrates, in terms of a number of functional units, the components of an AP 200a:200e according to an embodiment.
  • Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210a (as in Fig. 12), e.g. in the form of a storage medium 230.
  • the processing circuitry 210 may further i8 be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).
  • ASIC application specific integrated circuit
  • FPGA field programmable gate array
  • the processing circuitry 210 is configured to cause the AP 200a:200e to perform a set of operations, or steps, as disclosed above.
  • the storage medium 230 may store the set of operations
  • the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the AP 200a:200e to perform the set of operations.
  • the set of operations may be provided as a set of executable instructions.
  • the processing circuitry 210 is thereby arranged to execute methods as herein disclosed.
  • the storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.
  • the AP 200a:200e may further comprise a communications interface 220 for communications with other functions, nodes, entities, and devices, in the distributed MIMO communication system 100.
  • the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components.
  • the processing circuitry 210 controls the general operation of the AP 200a:200e e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230.
  • Other components, as well as the related functionality, of the AP 200a:200e are omitted in order not to obscure the concepts presented herein.
  • the AP 200a:200e may be provided as a standalone device or as a part of at least one further device.
  • the AP 200a:200e may be provided in a node of the radio access network or in a node of the core network.
  • functionality of the AP 200a:200e may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts.
  • instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time.
  • a first portion of the instructions performed by the AP 200a:200e may be executed in a first device, and a second portion of the instructions performed by the AP 200a:200e may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the AP 200a:200e may be executed.
  • the methods according to the herein disclosed embodiments are suitable to be performed by an AP 200a:200e residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 11 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the computer programs 1220a, 1220b of Fig. 12.
  • Fig. 12 shows one example of a computer program product 1210a, 1210b comprising computer readable means 1230.
  • a computer program 1220a can be stored, which computer program 1220a can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein.
  • the computer program 1220a and/or computer program product 1210a may thus provide means for performing any steps of the first AP 200a as herein disclosed.
  • a computer program 1220b can be stored, which computer program 1220b can cause the processing circuitry 310 and thereto operatively coupled entities and devices, such as the communications interface 320 and the storage medium 330, to execute methods according to embodiments described herein.
  • the computer program 1220b and/or computer program product 1210b may thus provide means for performing any steps of the second AP 200b:200e as herein disclosed.
  • the computer program product 1210a, 1210b is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc.
  • the computer program product 1210a, 1210b could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory.
  • RAM random access memory
  • ROM read-only memory
  • EPROM erasable programmable read-only memory
  • EEPROM electrically erasable programmable read-only memory
  • the computer program 1220a, 1220b is here schematically shown as a track on the depicted optical disk, the computer program 1220a,

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Abstract

L'invention concerne des mécanismes d'alignement de phase entre des panneaux d'antenne au niveau de différents AP dans un système de communication MIMO distribué. Chacun des panneaux d'antenne comprend de multiples éléments d'antenne étalonnés pour une réciprocité de liaison montante-liaison descendante. Un procédé est effectué par un premier AP des AP. Le procédé comprend la détermination d'une rotation de phase de référence d'un deuxième signal de référence reçu d'un deuxième AP des AP. Le procédé comprend la transmission d'un troisième signal de référence vers le deuxième AP. Le troisième signal de référence est ajusté en phase avec un facteur d'ajustement de phase déterminé à partir de la rotation de phase de référence.
EP21724249.4A 2021-05-05 2021-05-05 Alignement de phase dans un système de communication mimo distribué Pending EP4335038A1 (fr)

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PCT/EP2021/061830 WO2022233406A1 (fr) 2021-05-05 2021-05-05 Alignement de phase dans un système de communication mimo distribué

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FI20065841A0 (fi) * 2006-12-21 2006-12-21 Nokia Corp Kommunikointimenetelmä ja -järjestelmä
US8995410B2 (en) * 2012-05-25 2015-03-31 University Of Southern California Airsync: enabling distributed multiuser MIMO with full multiplexing gain
EP4213564A1 (fr) * 2014-06-09 2023-07-19 CommScope Technologies LLC Réseaux d'accès radio utilisant plusieurs unites distantes
US10764840B2 (en) * 2017-05-05 2020-09-01 Qualcomm Incorporated Sounding reference signal (SRS) coordination, power control, and synchronization for distributed coordinated multipoint (CoMP)

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