WO2009090618A2 - Procede et systeme de gestion de precodage dans un systeme de communication sans fil multi-utilisateur - Google Patents

Procede et systeme de gestion de precodage dans un systeme de communication sans fil multi-utilisateur Download PDF

Info

Publication number
WO2009090618A2
WO2009090618A2 PCT/IB2009/050164 IB2009050164W WO2009090618A2 WO 2009090618 A2 WO2009090618 A2 WO 2009090618A2 IB 2009050164 W IB2009050164 W IB 2009050164W WO 2009090618 A2 WO2009090618 A2 WO 2009090618A2
Authority
WO
WIPO (PCT)
Prior art keywords
channel estimation
response
precoding
predicted future
channel
Prior art date
Application number
PCT/IB2009/050164
Other languages
English (en)
Other versions
WO2009090618A3 (fr
Inventor
Gang Wu
Matthew Peter John Baker
Ni Ma
Xiaobo Zhang
Original Assignee
Nxp B.V.
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 Nxp B.V. filed Critical Nxp B.V.
Publication of WO2009090618A2 publication Critical patent/WO2009090618A2/fr
Publication of WO2009090618A3 publication Critical patent/WO2009090618A3/fr

Links

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
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0417—Feedback systems
    • 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
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
    • H04B7/0615—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
    • H04B7/0619—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal using feedback from receiving side

Definitions

  • the invention relates generally to wireless communications systems, and more particularly, to managing the precoding of wireless transmissions in a multi-user wireless communications network.
  • the 3 rd Generation Partnership Project (3 GPP) was established to produce globally applicable technical specifications and technical reports for a 3 rd generation mobile system based on evolved Global System for Mobile communications (GSM) core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).
  • GSM Global System for Mobile communications
  • UTRA Universal Terrestrial Radio Access
  • FDD Frequency Division Duplex
  • TDD Time Division Duplex
  • the scope was subsequently amended to include the maintenance and development of the GSM technical specifications and technical reports including evolved radio access technologies (e.g., General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE)).
  • GPRS General Packet Radio Service
  • EDGE Enhanced Data rates for GSM Evolution
  • LTE Long Term Evolution
  • UMTS Universal Mobile Telecommunication System
  • precoding is used to improve channel quality and throughput.
  • Conventional precoding typically relies on channel estimations that are made using recently received information (e.g., recently received pilot symbols). Although the channel estimations may accurately reflect the actual past channel responses, the channel estimations are backward-looking only. Because transmission conditions tend to vary over time, subsequent transmissions may exhibit different channel responses, which in turn may limit or negate the benefits of precoding.
  • precoding instead of precoding directly from channel estimations, previously generated channel estimations are used to predict future channel estimations, and the precoding is accomplished in response to the predicted future channel estimations instead of directly from the previously generated channel estimation. Because precoding is accomplished in response to a prediction about future channel estimations instead of directly from the previously generated channel estimations, which reflect past channel conditions, the precoding can be better matched to conditions that will be experienced in subsequent transmissions.
  • a method for operating a wireless communications system that supports multiple-input multiple-output (MIMO) communications using multiple transmit and/or receive antennas in a base station and/or multiple mobile stations involves generating a channel estimation, predicting a future channel estimation from the channel estimation, precoding data in response to the predicted future channel estimation, and transmitting the precoded data.
  • MIMO multiple-input multiple-output
  • a base station that supports MIMO communications between the base station and mobile stations includes a channel estimator configured to generate a channel estimation from received symbols, a channel estimation predictor configured to predict a future channel estimation from the channel estimation, a precoder configured to precode data in response to the predicted future channel estimation, and a transmitter configured to transmit the precoded data.
  • a mobile station that supports MIMO communications between a base station and the mobile station includes a channel estimator configured to generate a channel estimation from received symbols, a channel estimation predictor configured to predict a future channel estimation from the channel estimation, a precoding scheme selector configured to select a precoding scheme in response to the predicted future channel estimation, and a transmitter configured to transmit an indication of the precoding scheme to a base station.
  • Fig. 1 depicts a wireless communications system that supports MIMO communications between a base station and multiple mobile stations.
  • Fig. 2 depicts a wireless communications system that includes an evolved node B (eNB) and a user equipment (UE), where the UE is configured to predict a future channel estimation.
  • Fig. 3 depicts a non-adaptive filter for use in predicting a future channel estimation.
  • Fig. 4 depicts an embodiment of an eNB that is configured to implement precoding in response to predicted future channel estimates using singular value decomposition (SVD).
  • SVD singular value decomposition
  • Fig. 5 is a process flow diagram of a method for operating a wireless communications system that supports multi-user wireless communications between a base station and multiple mobile stations.
  • Fig. 6 illustrates the time offset between frames that carry pilot signals and frames that carry precoded data.
  • Fig. 7 illustrates the predetermined time offset used by the channel estimation predictor for codebook based precoding in a time division duplexing system.
  • Fig. 8 illustrates the predetermined time offset used by the channel estimation predictor for non-codebook based precoding in a time division multiplexing system.
  • Multi-user multiple-input multiple-output is an advanced spatial multiplexing technique for wireless transmission.
  • Fig. 1 depicts a wireless communications system 100 that includes a base station 102 (referred to herein as an evolved Node B (eNB)) and multiple mobile stations 104 (referred to herein as user equipments (UEs)).
  • the wireless communications system is operated in MIMO mode using frequency division duplexing (FDD) or time division duplexing (TDD).
  • FDD frequency division duplexing
  • TDD time division duplexing
  • the eNB is a wireless communications base station that supports MU-MIMO operation as specified in the 3GPP LTE specification.
  • the eNB includes four antennas 106 although the eNB can include more than four antennas.
  • Fig. 1 depicts a wireless communications system 100 that includes a base station 102 (referred to herein as an evolved Node B (eNB)) and multiple mobile stations 104 (referred to herein as user equipments (UEs)).
  • FDD frequency division duplex
  • the UEs are wireless communications mobile stations that support wireless operation as specified in the 3GPP LTE specification.
  • the UEs may have one or two antennas 108, although the UEs are not limited to two antennas (e.g., the UEs can include more than two antennas).
  • the eNB 102 includes a precoder 110 for precoding downlink transmissions.
  • precoding involves independently weighting the signals that are transmitted from the eNB to maximize channel throughput.
  • precoding is accomplished directly from channel estimations.
  • previously generated channel estimations are used to predict future channel estimations, and precoding is accomplished in response to the predicted future channel estimations instead of directly from the previously generated channel estimations. Because precoding is accomplished in response to predictions about future channel estimations instead of channel estimations that reflect the actual past conditions, the precoding can be better matched to conditions that will be experienced in subsequent transmissions.
  • Two categories of precoding include codebook based precoding and non- codebook based precoding.
  • An embodiment of the invention that is applicable to codebook based precoding is described below with reference to Figs. 2 and 3 and an embodiment of the invention that is applicable to non-codebook based precoding is described below with reference to Fig. 4.
  • Codebook based precoding is a technique that enables precoding information to be efficiently sent in a wireless communication, typically in the uplink direction from a UE to an eNB.
  • the technique involves establishing a codebook that includes a set of precoding matrices, with each precoding matrix in the set being uniquely identifiable by a codebook index.
  • a transmitter e.g., the eNB 102 transmits data such as pilot symbols and time offset information to a receiver (e.g., a UE 104).
  • the UE uses the pilot symbols and the time offset information to generate a channel estimation and the channel estimation is used by the receiver to select one of the precoding matrices from the set of precoding matrices.
  • the precoding matrix that maximizes the channel throughput is selected.
  • the corresponding codebook index is transmitted back to the eNB and the eNB uses the selected precoding matrix to precode subsequent downlink transmissions.
  • the UE may also generate channel quality information, for example, as channel quality indicators (CQIs), and provide the CQIs to the eNB along with the codebook indexes.
  • CQIs channel quality indicators
  • the codebook index(s) and CQIs are then used by the eNB to establish a precoding scheme.
  • An exemplary codebook based precoding scheme is referred to as the Per- User Unitary Rate Control (PU 2 RC) scheme.
  • PU 2 RC Per- User Unitary Rate Control
  • each UE 104 In operation, each UE 104 generates a channel estimation and calculates a CQI value for each vector in each matrix in the set E . Each UE also selects a preferred precoding matrix, which can be identified by a codebook index. The codebook index and the CQIs are then fed back to the eNB 102. The eNB gathers the feedback information, which indicates the index of a preferred precoding matrix and the CQIs for all the precoding vectors in the matrix. The eNB then groups the UEs that identify the same preferred precoding matrix, and selects a group with the highest group priority among the different groups. At the same time, inside the selected precoding group, the eNB allocates each precoding vector to the user with the highest priority. Finally, the eNB establishes a precoding scheme that corresponds to the selected group.
  • the precoding vectors in one precoding matrix can be assigned to multiple users or multiple streams of the same user.
  • the precoding matrix can be set to a unitary matrix, which can mitigate the interference between the different transmit antennas because of the orthogonality between the different precoding vectors in the precoding matrix.
  • Fig. 2 depicts a wireless communications system that includes an eNB 102 and a UE 104, where the UE is configured to predict future channel estimations.
  • the eNB includes a transmitter 120, a receiver 122, and a precoder 110 that is configured to precode data according to a codebook based precoding scheme that is selected in response to a predicted future channel estimation.
  • the UE includes a transmitter 124, a receiver 126, a channel estimator 128, a channel estimation predictor 130, and a channel feedback module 132.
  • the channel feedback module includes a precoding matrix selector 134 and a CQI calculator 136.
  • the transmitter and receiver of the UE support wireless communications.
  • the channel estimator generates a channel estimate from received signals
  • the channel estimation predictor generates a predicted future channel estimation from the channel estimation
  • the channel feedback module uses the predicted future channel estimation to select a precoding matrix, to identify the corresponding codebook index, and to calculate CQIs.
  • the UE transmits the codebook index and the CQIs to the eNB for use in precoding subsequent downlink transmissions. Operation of the system is now described in more detail with reference to
  • Fig. 2 Referring first to the operation of the channel estimator 128, assume that there are M transmit antennas (not shown) and TV receive antennas (not shown) in the wireless communications system and that a pilot signal transmitted at the mth transmit antenna is
  • the pilot signal, m is distinguished from the pilot signals at the other antennas by their different time or frequency allocations.
  • the received signal for the Mi UE can be expressed as: ym,n,k ⁇ s m f ⁇ m,n,k + r ⁇ m ⁇ k , j ,
  • a channel estimation for the Mi UE is expressed in a channel response matrix as:
  • the SNR of each precoding vector is calculated as the follows:
  • the CQI is obtained from the value of SNR.
  • the channel estimation, fi m , n ,k, from the channel estimator 128 is provided to the channel estimation predictor 130.
  • the channel estimation predictor predicts what the channel estimation will be in the future based on the received channel estimations.
  • the predicted future channel estimation at time, t is
  • Channel estimation predictions can be made using different techniques.
  • One technique for predicting a future channel estimation involves filtering, including, for example, adaptive filtering or non-adaptive filtering.
  • Adaptive filtering tracks variations in the channel response based on statistical characteristics derived from filter inputs.
  • Adaptive filtering techniques include, for example, least mean square (LMS) filtering, recursive least square (RLS) filtering, and Kalman filtering.
  • Non-adaptive filtering utilizes real-time information about the wireless propagation channel to update filter coefficients.
  • An example of a non-adaptive prediction filter is described with reference to Fig. 3.
  • Fig. 3 depicts a linear transversal prediction filter 140 that includes transversal units 142, weighting units 144, and summing units 146.
  • the prediction filter operates to generate the predicted future channel estimate at time, t, based on the previously generated channel estimations.
  • the filter operates to predict a future channel estimation, m > n > k , by using the channel estimations that were previously generated at a series of past times, Ki.n.kit ! f Ki ⁇ kif 2 ) ⁇ 5 Ki ⁇ kif ⁇ ) Using the fllter of Fig 3 ⁇ the calculation of the predicted future channel estimation is represented as:
  • Fig. 6 illustrates the time offset between frames in an FDD system.
  • the time offset is the time interval between frame 1 601, which carries pilot signals, and frame 3 603, which carries precoded data.
  • time offset information is sent to the UEs via a physical layer channel.
  • Fig. 7 illustrates the time offset between time slots in a TDD system. In Fig. 7, the time offset is predetermined. For example, the time offset is set by the time interval between downstream time slots, e.g., the time interval between time slots TSO and TS4.
  • the predicted future channel estimation is provided to the channel feedback module 132.
  • the precoding matrix selector 134 of the channel feedback module uses the predicted future channel estimation to select a precoding matrix and to identify the corresponding codebook index.
  • the CQI calculator 136 of the channel feedback module uses the predicted future channel estimation to calculate CQIs for the UE 104. Precoding matrix information (e.g., in the form of the corresponding codebook index) and the CQIs are then transmitted uplink to the eNB 102 for use in precoding.
  • Non-codebook based precoding involves generating precoding matrices directly from the channel estimations.
  • One non-codebook based technique for precoding utilizes singular value decomposition (SVD) to generate precoding matrices from channel estimations.
  • SVD singular value decomposition
  • precoding effectively converts the MIMO channel into a set of parallel flat fading sub-channels.
  • channel estimates are used to generate predicted future channel estimations and the predicted future channel estimations, instead of the previously generated channel estimations, are used in an SVD operation to generate precoding matrices.
  • FIG. 4 depicts an embodiment of an eNB 102 that is configured to implement precoding in response to predicted future channel estimations using singular value decomposition.
  • the eNB depicted in Fig. 4 includes transmitters 120, a receiver 122, a channel estimator 158, a channel estimation predictor 160, time offset information 161, an SVD module 162, and a precoder 110.
  • the channel estimator and channel estimation predictor are similar to those described with reference to Figs. 2 and 3 and the SVD module performs singular value decompositions as described above.
  • the precoder includes stream-specific coders 164 and summers 166, although other coder configurations are possible.
  • the channel estimator 158 generates channel estimations from the information received via an uplink channel and from time offset information. For example, the channel estimator uses known techniques to generate channel estimations. The channel estimations are provided to the channel estimation predictor 160, which uses the channel estimations to predict future channel estimations. As described above, the channel estimation predictor may utilize, for example, adaptive or non-adaptive filtering techniques to generate the predicted future channel estimations. The predicted future channel estimations generated by the channel estimation predictor are then provided to the SVD module 162 for use in singular value decomposition.
  • the SVD module generates precoding matrices, e.g., precoding matrices V 1 - V K , directly from the predicted future channel estimations and the precoding matrices are used by the precoder 110 to precode subsequent downlink transmissions.
  • precoding matrices e.g., precoding matrices V 1 - V K
  • V 1 - V K precoding matrices
  • the above-described technique takes advantage of channel reciprocity between the eNB and UEs in the TDD wireless communications system, wherein channel reciprocity involves essentially equivalent channel responses in the uplink and downlink directions.
  • Fig. 4 illustrates an example of the time offset in a TDD system. As indicated in Fig. 8, the time offset is predetermined to be equivalent to the interval of one time slot.
  • Fig. 5 is a process flow diagram of a method for operating a wireless communications system that supports multi-user wireless communications between a base station and multiple mobile stations.
  • a channel estimation is generated.
  • a future channel estimation is predicted from the channel estimation.
  • data is precoded in response to the predicted future channel estimation.
  • the precoded data is transmitted.

Landscapes

  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

L'invention concerne un procédé d'exploitation d'un système de communication sans fil soutenant des communications entrée multiple sortie multiple entre une station de base et des stations mobiles multiples. Ce procédé consiste à générer une estimation de voie, à prédire une estimation de voie future à partir de cette estimation de voie, à précoder des données en réponse à l'estimation de voie future prévue et à transmettre les données précodées.
PCT/IB2009/050164 2008-01-17 2009-01-17 Procede et systeme de gestion de precodage dans un systeme de communication sans fil multi-utilisateur WO2009090618A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US2171408P 2008-01-17 2008-01-17
US61/021,714 2008-01-17

Publications (2)

Publication Number Publication Date
WO2009090618A2 true WO2009090618A2 (fr) 2009-07-23
WO2009090618A3 WO2009090618A3 (fr) 2009-10-29

Family

ID=40791661

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/IB2009/050164 WO2009090618A2 (fr) 2008-01-17 2009-01-17 Procede et systeme de gestion de precodage dans un systeme de communication sans fil multi-utilisateur

Country Status (1)

Country Link
WO (1) WO2009090618A2 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2010080231A1 (fr) * 2009-01-06 2010-07-15 Qualcomm Incorporated Procédé et appareil d'estimation de canal à l'aide de multiples codes de description
WO2011121444A3 (fr) * 2010-03-30 2011-12-15 France Telecom Procédé et système d'allocation de ressources
WO2013132295A1 (fr) * 2012-03-08 2013-09-12 Telefonaktiebolaget L M Ericsson (Publ) Précodage utilisant un retour d'informations partiellement périmées
WO2023093973A1 (fr) * 2021-11-23 2023-06-01 Telefonaktiebolaget Lm Ericsson (Publ) Nœud de réseau et équipement utilisateur pour l'estimation d'un canal de propagation radio

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1816758A2 (fr) * 2006-02-03 2007-08-08 Samsung Electronics Co.,Ltd. Appareil et procédé pour déterminer le vecteur de formation de faisceaux dans un système de formation de faisceaux basé sur un guide de codification

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1816758A2 (fr) * 2006-02-03 2007-08-08 Samsung Electronics Co.,Ltd. Appareil et procédé pour déterminer le vecteur de formation de faisceaux dans un système de formation de faisceaux basé sur un guide de codification

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
QUAN ZHOU ET AL: "Joint Tomlinson-Harashima precoding and scheduling for multiuser MIMO with imperfect feedback" WIRELESS COMMUNICATIONS AND NETWORKING CONFERENCE, 2006. WCNC 2006. IE EE LAS VEGAS, NV, USA 3-6 APRIL 2006, PISCATAWAY, NJ, USA,IEEE, PISCATAWAY, NJ, USA, 1 January 2006 (2006-01-01), pages 1233-1238, XP031102389 ISBN: 978-1-4244-0269-4 *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2010080231A1 (fr) * 2009-01-06 2010-07-15 Qualcomm Incorporated Procédé et appareil d'estimation de canal à l'aide de multiples codes de description
US8787183B2 (en) 2009-01-06 2014-07-22 Qualcomm Incorporated Method and apparatus for channel estimation using multiple description codes
WO2011121444A3 (fr) * 2010-03-30 2011-12-15 France Telecom Procédé et système d'allocation de ressources
WO2013132295A1 (fr) * 2012-03-08 2013-09-12 Telefonaktiebolaget L M Ericsson (Publ) Précodage utilisant un retour d'informations partiellement périmées
US20130237212A1 (en) * 2012-03-08 2013-09-12 Ali S. Khayrallah Precoding with partially stale feedback
US9363002B2 (en) 2012-03-08 2016-06-07 Telefonaktiebolaget Lm Ericsson (Publ) Precoding with partially stale feedback
WO2023093973A1 (fr) * 2021-11-23 2023-06-01 Telefonaktiebolaget Lm Ericsson (Publ) Nœud de réseau et équipement utilisateur pour l'estimation d'un canal de propagation radio

Also Published As

Publication number Publication date
WO2009090618A3 (fr) 2009-10-29

Similar Documents

Publication Publication Date Title
US8472545B2 (en) Method and system for managing precoding in a multi-user wireless communications system
US8218422B2 (en) Coordinated linear beamforming in downlink multi-cell wireless networks
CN107483088B (zh) 大规模mimo鲁棒预编码传输方法
EP3681087B1 (fr) Procédé et appareil de rapports csi
EP2786508B1 (fr) Procédé et appareil de décodage dans des systèmes de communication avec des signaux corrélés et interférences.
CN102415002B (zh) 用于来自稀疏反馈数据的信道状态信息重构的方法
US20110299379A1 (en) Process for Beamforming Data to be Transmitted by a Base Station in a MU-MIMO System and Apparatus for Performing the Same
US8260209B2 (en) System and method for coordinated spatial multiplexing using second order statistical information
CN101686214B (zh) 一种进行信道质量指示估计的方法及装置
EP3163767B1 (fr) Procédé pour rapporter un index de matrice de précodage pour une communication en bande haute fréquence dans un système de communication sans fil et appareil associé
CN101288244A (zh) 在多用户多天线通信系统中发送/接收数据的设备及方法
EP2011251A1 (fr) Systeme ofdm mimo oriente par faisceau a complexite reduite
CN101378277A (zh) 多用户预编码及调度方法和实现该方法的基站
EP2498413B1 (fr) Procédé et appareil pour le traitement de la communication descendante et procédé et appareil auxiliaires correspondants
KR101698568B1 (ko) 프리코딩을 이용하는 다중―이용자 네트워크에서 통신하기 위한 방법 및 그의 디바이스
US10735853B2 (en) Stochastic transmission/reception method and apparatus for MU-MIMO scheme in MIMO radio communication system
US20110064160A1 (en) Method for Transmitting Data in a MIMO System and Communication System for Transmitting Data in a MIMO System
CN103378894B (zh) 在该无线电通信系统中执行的方法
CN105745893A (zh) 无线大量mimo系统中大尺度衰落系数估计
EP2425546A1 (fr) Procédé de communication dans un réseau
WO2009090618A2 (fr) Procede et systeme de gestion de precodage dans un systeme de communication sans fil multi-utilisateur
Smirani et al. On the distributed approaches with imperfect channel estimation for MIMO interference channel
Alonso ETSIT
CN105637777A (zh) 在mimo无线电通信系统中用于mu-mimo方案的随机发送/接收方法和装置
Sharma Channel prediction in MU-MIMO-OFDM downlink system

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 09701650

Country of ref document: EP

Kind code of ref document: A2

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 09701650

Country of ref document: EP

Kind code of ref document: A2