WO2020042107A1

WO2020042107A1 - Determination of downlink channel state information in massive mimo systems

Info

Publication number: WO2020042107A1
Application number: PCT/CN2018/103326
Authority: WO
Inventors: Huan Sun
Original assignee: Nokia Shanghai Bell Co., Ltd.; Nokia Solutions And Networks Oy; Nokia Technologies Oy
Priority date: 2018-08-30
Filing date: 2018-08-30
Publication date: 2020-03-05
Also published as: CN112673580A; CN112673580B

Abstract

Embodiments of the present disclosure provide method, device and computer readable medium for determination of downlink CSI in massive MIMO systems. In example embodiments, methods implemented at a terminal device and at a network device are provided respectively. The method implemented at the terminal device comprises: determining first and second CSI based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively; determining, based on the first CSI, an updated codebook specific to the terminal device; quantizing the second CSI based on the updated codebook; and transmitting, via the first antenna, an uplink SRS and the quantized CSI to a network device. The method implemented at the network device comprises: receiving, from a terminal device, an uplink SRS and a quantized CSI; determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device; determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.

Description

DETERMINATION OF DOWNLINK CHANNEL STATE INFORMATION IN MASSIVE MIMO SYSTEMS

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to the field of communications, and in particular, to a method, device and computer readable storage medium for determination of downlink channel state information (CSI) in massive multiple input multiple output (MIMO) systems.

BACKGROUND

In order to serve more users and provide high capacity for each user, MIMO antenna arrays or "massive MIMO" technologies have been introduced to the fourth (4G) and fifth generation (5G) mobile communication systems. Massive MIMO solutions require high accurate downlink CSI for each user at a network device in order to perform a multi-user scheduling and a transmit pre-coder design for downlink data transmission.

As known, a time division duplexing (TDD) technology is more suitable for massive MIMO solution implementation as the network device in this case can easily get the accurate downlink CSI for each user with small system overhead. In the TDD massive MIMO system, both the network device and a terminal device of the user are equipped with multiple antennas. The network device gets downlink CSI by measuring the user’s uplink sounding reference signals (SRSs) and exploiting the properties of channel reciprocity between a downlink channel and an uplink channel.

However, as only a subset of all antennas at the terminal device side can be used for transmission of uplink signals in a certain time slot, the terminal device should switch its antenna group to transmit SRSs in multiple continuous predefined timeslots in order to help the network device to get full downlink CSI. For different SRS configuration periods, the network device may spend a long time period to get the user’s full CSI, and thus the CSI may be outdated comparing to the real channel for downlink data transmission. Thereby, the multi-user scheduling and transmit pre-coder design may be impacted by the outdated CSI, and then multi-user interference may not be well pre-compressed and the system performance may be degraded.

SUMMARY

In general, example embodiments of the present disclosure provide a method, device and computer readable storage medium for determination of downlink CSI.

In a first aspect, there is provided a method implemented at a terminal device. The method comprises: determining first and second CSI based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively; determining, based on the first CSI, an updated codebook specific to the terminal device; quantizing the second CSI based on the updated codebook; and transmitting, via the first antenna, an uplink SRS and the quantized CSI to a network device.

In a second aspect, there is provided a method implemented at a network device. The method comprises: receiving, from a terminal device, an uplink SRS and a quantized CSI; determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device; determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.

In a third aspect, there is provided a terminal device. The terminal device comprises a processor and a memory coupled to the processor. The memory stores instructions that when executed by the processor, cause the terminal device to perform actions. The actions comprise: determining first and second CSI based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively; determining, based on the first CSI, an updated codebook specific to the terminal device; quantizing the second CSI based on the updated codebook; and transmitting, via the first antenna, an uplink SRS and the quantized CSI to a network device.

In a fourth aspect, there is provided a network device. The network device comprises a processor and a memory coupled to the processor. The memory stores instructions that when executed by the processor, cause the terminal device to perform actions. The actions comprise: receiving, from a terminal device, an uplink SRS and a quantized CSI; determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device; determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.

In a fifth aspect, there is provided a computer readable storage medium having instructions stored thereon. The instructions, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the present disclosure.

In a sixth aspect, there is provided a computer readable storage medium having instructions stored thereon. The instructions, when executed on at least one processor, cause the at least one processor to carry out the method according to the second aspect of the present disclosure.

It is to be understood that the summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the more detailed description of some embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein:

Fig. 1 shows an example communication network in which embodiments of the present disclosure can be implemented;

Fig. 2 shows an example scenario of antenna switching based uplink SRS transmission in which embodiments of the present disclosure can be implemented;

Fig. 3 shows an example interaction between a network device and a terminal device in accordance with some embodiments of the present disclosure;

Fig. 4 shows a flowchart of an example method implemented at a terminal device for determination of downlink CSI in accordance with some embodiments of the present disclosure;

Fig. 5 shows a flowchart of an example method for determination of an updated codebook in accordance with some embodiments of the present disclosure;

Fig. 6 shows a flowchart of an example method implemented at a network device for determination of downlink CSI in accordance with some embodiments of the present disclosure;

Fig. 7 shows link level simulations comparison between the present solution and traditional solutions; and

Fig. 8 is a simplified block diagram of a device that is suitable for implementing embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitations as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

As used herein, the term “communication network” refers to a network that follows any suitable communication standards or protocols such as long term evolution (LTE) , LTE-Advanced (LTE-A) and 5G NR, and employs any suitable communication technologies, including, for example, Multiple-Input Multiple-Output (MIMO) , OFDM, time division multiplexing (TDM) , frequency division multiplexing (FDM) , code division multiplexing (CDM) , Bluetooth, ZigBee, machine type communication (MTC) , eMBB, mMTC and uRLLC technologies. For the purpose of discussion, in some embodiments, the LTE network, the LTE-Anetwork, the 5G NR network or any combination thereof is taken as an example of the communication network.

As used herein, the term “network device” refers to any suitable device at a network side of a communication network. The network device may include any suitable device in an access network of the communication network, for example, including a base station (BS) , a transmission point (TRP) , a relay, an access point (AP) , a node B (NodeB or NB) , an evolved NodeB (eNodeB or eNB) , a gigabit NodeB (gNB) , a Remote Radio Module (RRU) , a radio header (RH) , a remote radio head (RRH) , a low power node such as a femto, a pico, and the like. For the purpose of discussion, in some embodiments, the eNB is taken as an example of the network device.

The network device may also include any suitable device in a core network, for example, including multi-standard radio (MSR) radio equipment such as MSR BSs, network controllers such as radio network controllers (RNCs) or base station controllers (BSCs) , Multi-cell/multicast Coordination Entities (MCEs) , Mobile Switching Centers (MSCs) and MMEs, Operation and Management (O&M) nodes, Operation Support System (OSS) nodes, Self-Organization Network (SON) nodes, positioning nodes, such as Enhanced Serving Mobile Location Centers (E-SMLCs) , and/or Mobile Data Terminals (MDTs) .

As used herein, the term “terminal device” refers to a device capable of, configured for, arranged for, and/or operable for communications with a network device or a further terminal device in a communication network. The communications may involve transmitting and/or receiving wireless signals using electromagnetic signals, radio waves, infrared signals, and/or other types of signals suitable for conveying information over air. In some embodiments, the terminal device may be configured to transmit and/or receive information without direct human interaction. For example, the terminal device may transmit information to the network device on predetermined schedules, when triggered by an internal or external event, or in response to requests from the network side.

Examples of the terminal device include, but are not limited to, user equipment (UE) such as smart phones, wireless-enabled tablet computers, laptop-embedded equipment (LEE) , laptop-mounted equipment (LME) , and/or wireless customer-premises equipment (CPE) . For the purpose of discussion, in the following, some embodiments will be described with reference to UEs as examples of the terminal devices, and the terms “terminal device” and “user equipment” (UE) may be used interchangeably in the context of the present disclosure.

As used herein, the term “cell” refers to an area covered by radio signals transmitted by a network device. The terminal device within the cell may be served by the network device and access the communication network via the network device.

As used herein, the term “circuitry” may refer to one or more or all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) combinations of hardware circuits and software, such as (as applicable) : (i) a combination of analog and/or digital hardware circuit (s) with software/firmware and (ii) any portions of hardware processor (s) with software (including digital signal processor (s)) , software, and memory (ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

(c) hardware circuit (s) and or processor (s) , such as a microprocessor (s) or a portion of a microprocessor (s) , that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

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. The term “includes” and its variants are to be read as open terms that mean “includes, but is not limited to. ” The term “based on” is to be read as “at least in part based on. ” The term “one embodiment” and “an embodiment” are to be read as “at least one embodiment. ” The term “another embodiment” is to be read as “at least one other embodiment. ” The terms “first, ” “second, ” and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below.

In some examples, values, procedures, or apparatus are referred to as “best, ” “lowest, ” “highest, ” “minimum, ” “maximum, ” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, higher, or otherwise preferable to other selections.

As described above, as only a subset of all antennas at a terminal device can be used for transmission of uplink signals in a certain time slot, the terminal device should switch its antenna group to transmit SRSs in multiple continuous predefined timeslots in order to help a network device to get full downlink CSI. However, for different SRS configuration periods, the network device may spend a long time period to get the user’s full CSI, and thus the CSI may be outdated comparing to the real channel for downlink data transmission. In view of this, a traditional scheme has been proposed which is based on jointed uplink SRS transmission and uplink CSI feedback.

In this scheme, at one given time slot, the terminal device transmits the uplink SRS via a subset of its antennas, and quantizes CSI of sub channel (s) between the network device and those unused antennas (not be used for SRS transmission) via a common codebook and feeds back the quantized CSI to the network device. Then the network device derives full downlink CSI for the terminal device by combining CSI of sub channel (s) measured by uplink SRS and CSI of other sub channel (s) restored by feedback via the same codebook.

However, this scheme may face more challenges for massive MIMO. Given the codebook size in order to control the uplink overhead, the quantization error will increase as the number of transmit antennas increases in massive MIMO systems. The reason is that using a codebook of a given size to quantize a larger spatial space will degrade the quantization resolution. If the codebook size is not increased with the increasing number of the transmit antennas, the traditional solution should be further investigated and optimized.

Embodiments of the present disclosure provide an improved solution for determination of downlink CSI uses an adaptive codebook technology so as to solve the problem above and one or more of other potential problems. Comparing with the traditional schemes, the solution for determination of downlink CSI in accordance with embodiments of the present disclosure achieves a higher quantization resolution, a lower uplink overhead and a more accurate downlink CSI determination. Principle and implementations of the present disclosure will be described in detail below with reference to Figs. 1-8.

Fig. 1 illustrates an example communication network 100 in which embodiments of the present disclosure can be implemented. As shown in Fig. 1, the network 100 includes a network device 110 and a terminal device 120 served by the network device 110. It is to be understood that the number of network devices and terminal devices as shown in Fig. 1 is only for the purpose of illustration without suggesting any limitations. The network 100 may include any suitable number of devices adapted for implementing embodiments of the present disclosure. It is to be understood that the network device 110 may schedule multiple users (such as the terminal device 120) for downlink transmission simultaneously.

As shown in Fig. 1, the network device 110 and the terminal device 120 may communicate with each other. The terminal device 120 may have multiple antennas for communication with the network device 110. For example, the terminal device 120 may include four

antennas

121, 122, 123, and 124. It is to be understood that the number of antennas as shown in Fig. 1 is only for the purpose of illustration without suggesting any limitations. The terminal device 120 may provide any suitable number of antennas adapted for implementing embodiments of the present disclosure. In addition, it is to be understood that the network device 110 may also have multiple antennas for communication with the terminal device 120 and that all of the antennas at the network device 110 can be used for downlink signals transmission and uplink signals receiving. It is not shown here to avoid obscuring the present invention.

The communications in the network 100 may conform to any suitable standards including, but not limited to, Long Term Evolution (LTE) , LTE-Evolution, LTE-Advanced (LTE-A) , Wideband Code Division Multiple Access (WCDMA) , Code Division Multiple Access (CDMA) and Global System for Mobile Communications (GSM) and the like. Furthermore, the communications may be performed according to any generation communication protocols either currently known or to be developed in the future. Examples of the communication protocols include, but not limited to, the first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the fifth generation (5G) communication protocols.

In some embodiments, all of the antennas at the terminal device 120 can be used for downlink signals reception in order to achieve a receiving combining gain, while only a subset of the antennas can be used for uplink signals transmission in one certain time instance. In order to help the network device 110 to get full downlink CSI, the terminal device 120 will switch its antenna group to transmit uplink reference signals such as SRSs in multiple continuous predefined timeslots. Fig. 2 shows an example scenario 200 of antenna switching based uplink SRS transmission in which embodiments of the present disclosure can be implemented. The scenario 200 can be implemented at the terminal device 120 shown in Fig. 1. For the purpose of discussion, the scenario 200 will be described with reference to Fig. 1.

As shown in Fig. 2, T _SF denotes the duration of one sub-frame, T _SRS stands for a period of SRS transmission and T _FR presents the duration of one frame. For example, t ₁, t ₆, t ₁₁, t ₁₆ and t ₂₁ are uplink time slots for the terminal device 120. Assume that in each of the uplink time slots, only one antenna is used for uplink SRS transmission in embodiments of the present disclosure. It is to be understood that in an alternative embodiment, more antennas can be used for uplink SRS transmission in each of the uplink time slots.

For example, in the time slot t ₁, an uplink SRS is transmitted only via the antenna 121. By antenna switching, in the time slot t ₆, an uplink SRS will be transmitted only via the antenna 122. In the time slot t ₁₁, an uplink SRS will be transmitted only via the antenna 123. In the time slot t ₁₆, an uplink SRS will be transmitted only via the antenna 124. In the time slot t ₂₁, an uplink SRS will in turn be transmitted only via the antenna 121. And so on, for each of subsequent uplink time slots. In some embodiments, if T _SRS=5, then time slot t _5 (n-1) +i will be used for SRS transmission of the ith antenna, wherein n denotes an index of current frame and i denotes an index of an antenna of a terminal device.

In the following text, a subset of antennas which are transmitting uplink SRS in an uplink time slot may be also collectively referred to as a first antenna, while other antennas which are not transmitting uplink SRS in the same uplink time slot may be also collectively referred to as a second antenna.

According to the concept of the present disclosure, for sub channel (s) between the network device 110 and the first antenna of the terminal device 120, the network device 110 measures the uplink SRS and then derives the corresponding downlink CSI for the sub channel by exploiting the channel reciprocity between downlink and uplink channels. Meanwhile, for sub channel (s) between the network device 110 and the second antenna of the terminal device 120, the network device 110 will restore the corresponding downlink CSI for the sub channel by feedback from the terminal device 120 using a codebook which is adaptively updated based on the antenna switching as described in connection with Fig. 2.

Fig. 3 shows an example interaction 300 between a network device and a terminal device in accordance with some embodiments of the present disclosure. The interaction 300 can be implemented at the network device 110 and the terminal device 120 shown in Fig. 1. For the purpose of discussion, the interaction 300 will be described with reference to Figs. 1 and 2. In this example, the interaction 300 will be described for the time slot t ₁ as shown in Fig. 2. It is to be understood that the interaction 300 may include additional acts not shown and/or may omit some shown acts, and the scope of the present disclosure is not limited in this regard.

As shown in Fig. 3, the network device 110 transmits 301 downlink reference signals to terminal devices in its serving cell such as the terminal device 120. For example, referring back to Fig. 2, in a downlink time slot preceding the time slot t ₁, the network device 110 may transmit 301 the downlink reference signals. In some embodiments, the network device 110 may periodically transmit channel state information reference signals (CSI-RSs) . The transmission manner and the form of the downlink reference signals are not limited to this and may be in any suitable way.

Upon receiving the downlink reference signals from the network device 110, the terminal device 120 determines 302 a corresponding CSI for each of sub channels between the network device 110 and each of the

antennas

121, 122, 123 and 124, for example, by measuring the downlink reference signals in the corresponding sub channels. It is to be understood that the measurement may be carried out in any suitable way.

Upon determining the CSI for each of the sub channels, the terminal device 120 determine 303 an updated codebook specific to the terminal device 120 based on the CSI (which is also referred to as first CSI) for the sub channel between the network device 110 and the antennas 121. In some embodiments, the updated codebook is determined upon the first CSI and history CSI (which is also referred to as history second CSI) for each of the sub channels between the network device 110 and each of the

antennas

122, 123 and 124. In some embodiments, the terminal device 120 may replace history CSI (which is also referred to as history first CSI) for the sub channel between the network device 110 and the antenna 121 with the first CSI and derive the updated codebook based on the first CSI and the history second CSI. As used herein, history CSI refers to CSI for a sub channel between a network device and any of antennas of a terminal device, obtained by a terminal device from downlink reference signals in one or more previous downlink time slots. As to the implementation for the determination 303, it will be described later in detail with reference to Fig. 5.

Based on the updated codebook, the terminal device 120 quantizes 304 the CSI (which is also referred to as second CSI) for each of the sub channels between the network device 110 and each of the

antennas

122, 123 and 124. For example, the quantization 304 may be carried out by matching the CSI with a code-word in the updated codebook. It is to be understood that the quantization may be carried out in any suitable technologies. In the time slot t ₁, the terminal device 120 transmits 305, via the antenna 121, an uplink SRS and the quantized CSI to the network device 110.

Upon receiving the uplink SRS and the quantized CSI, the network device 110 determines 306, based on the uplink SRS, a portion of downlink CSI corresponding to the sub channel between the network device 110 and the antenna 121 (which is also referred to as first downlink CSI) , for example, by measuring the uplink SRS in this sub channel. It is to be understood that the measurement may be carried out in any suitable way.

Based on the determined 306 portion of downlink CSI, the network device 110 determines 307 an updated codebook specific to the terminal device 120. In some embodiments, the updated codebook is determined upon the first downlink CSI and history downlink CSI (which is also referred to as history second downlink CSI) for each of the sub channels between the network device 110 and each of the

antennas

122, 123 and 124. In some embodiments, the network device 110 may replace history downlink CSI (which is also referred to as history first downlink CSI) for the sub channel between the network device 110 and the antenna 121 with the first downlink CSI and derive the updated codebook based on the first downlink CSI and the history second downlink CSI. As used herein, history downlink CSI refers to downlink CSI for a sub channel between a network device and any of antennas of a terminal device obtained by a network device from uplink SRS in one or more previous uplink time slots. The process of the determination 307 is similar with that of the determination 303, except implemented by the network device based on the first downlink CSI instead of implemented by the terminal device based on the first CSI. Its details see the contents described later in detail with reference to Fig. 5.

Based on the updated codebook determined at 307 and the quantized CSI received at 305, the network device 110 determines 308 the remaining portions of downlink CSI (which is also referred to as second downlink CSI) corresponding to the sub channels between the network device 110 and each of the

antenna

122, 123 and 124. For example, the determination 308 may be carried out by matching the quantized CSI with a code-word in the updated codebook. It is to be understood that the quantization may be carried out in any suitable technologies. Thereby, the network device 110 obtains full downlink CSI associated with the terminal device 120.

As used herein, the first downlink CSI refers to the CSI for the sub channel (s) between the network device and the first antenna, and the second downlink CSI refers to the CSI for the sub channel (s) between the network device and the second antenna.

Corresponding to the above concept described with reference to Figs. 1-3, the present disclosure provides methods implemented at a terminal device and a network device respectively. Fig. 4 shows a flowchart of an example method 400 implemented at a terminal device for determination of downlink CSI in accordance with some embodiments of the present disclosure. The method 400 can be implemented at the terminal device 120 shown in Fig. 1. For the purpose of discussion, the method 400 will be described with reference to Fig. 1. It is to be understood that the method 400 may include additional acts not shown and/or may omit some shown acts, and the scope of the present disclosure is not limited in this regard.

At block 410, the terminal device 120 determines first and second CSI based on downlink reference signals received from the network device 110 via first and second antennas of the terminal device respectively. In present example, the first antenna includes the antenna 121 which is used for uplink SRS transmission in time slot t ₁, and the second antenna includes the remaining

antennas

122, 123 and 124 which are not used for uplink SRS transmission in time slot t ₁. The process in block 410 is similar with

operations

301 and 302 described in Fig. 3, and thus omitted here.

At block 420, the terminal device 120 determines, based on the first CSI, an updated codebook specific to the terminal device. In some embodiments, the terminal device 120 may determine the updated codebook in response to receiving an indication for the use of the updated codebook from the network device 110. For example, according to a change in a network condition, antenna configuration or other factors, the network device 110 may select to use an adaptive codebook and inform it to the terminal device 120. In some embodiments, an additional 1 bit signal can be designed to indicate the use of an updated codebook. It is to be understood that other indications also can be employed to indicate the use of an updated codebook. In this way, an operation based on an adaptive codebook can be aligned at a network device and at a terminal device, including CSI quantization at a terminal device side and recovering at a network device side.

The process in block 420 is similar with operations 303 described in Fig. 3 and now is described in detail below with reference to Fig. 5. Fig. 5 shows a flowchart of an example method 500 for determination of an updated codebook in accordance with some embodiments of the present disclosure. The method 500 can be implemented at the network device 110 or terminal device 120 shown in Fig. 1. For the purpose of discussion, the method 500 will be described with reference to Fig. 1. It is to be understood that the method 500 may include additional acts not shown and/or may omit some shown acts, and the scope of the present disclosure is not limited in this regard.

At block 510, the terminal device 120 may update, based on the first CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas. In some embodiments, the first and second CSI may be maintained in a matrix form (which is also referred to as a downlink channel matrix) . For example, assuming that a terminal device has N antennas (N is an integer larger than 1) , the downlink channel matrix H ^iter (t) may be written as:

H ^iter (t) = [h ₁ (t) , h ₂ (t) , ..., h _i (t) , ..., h _N (t) ] (1)

wherein h _i (t) denotes a measured or quantized CSI for a sub channel between a network device and the ith antenna of the terminal device in time slot t.

In some embodiments, upon determining the first and second CSI at block 410, the terminal device 120 may update the element h ₁ (t) of the matrix H ^iter (t) with the first CSI, while remaining other elements of the matrix H ^iter (t) unchanged. In some embodiments, the downlink channel matrix may be initialized as a null matrix. By antenna switching in uplink time slots, the elements of matrix H ^iter (t) are iteratively updated, and thus the resulting CSI is more and more accurate.

At block 520, the terminal device 120 may determine a covariance matrix from the updated downlink channel matrix. According to the concept of the present disclosure, second order statistic properties of iterative channel matrix H ^iter (t) is investigated. In some embodiments, the covariance matrix R ^iter can be written as:

wherein the parameter T is a slide window for downlink channel covariance matrix calculation.

As the wireless channels usually satisfy a wide sense stationary uncorrelated scattering (WSSUS) model and properties, R ^iter approximates to an ideal downlink channel covariance matrix R ^theoretical in a larger average period T. Ideal covariance matrix R ^theoretical is calculated based on ideal downlink channel information. Usually, a terminal device can get R ^theoretical by downlink CSI-RS measurement, but a network device cannot get the R ^theoretical due to uplink SRSs transmitted in multiple time slots, and thus the network device can use the R ^iter for user specific adaptive codebook design.

By switching antenna and exploiting the channel reciprocity between TDD downlink and uplink channels, the terminal device 120 can get the covariance matrix R ^iter. In each of the uplink time slots, a corresponding covariance matrix R ^iter can be determined.

At block 530, the terminal device 120 may obtain the updated codebook based on the covariance matrix and a common codebook. In some embodiments, for example, assuming that there is a common codebook C with size S and the code-word C _i, 1 ≤ i ≤ S, the updated codebook can be obtained by multiplying each code-word C _i of the codebook C with covariance matrix R ^iter and then normalizing the code-words. Then an adaptive codebook specific to a terminal device is generated.

For a given time slot, if an antenna or a subset of antennas is selected for uplink SRS transmission, the sub channel between TRP and the antenna (s) which do not transmit SRS in the given time slot will be quantized and feed back to the network device by the terminal device. This quantized downlink channel is recovered by the network device via the corresponding adaptive codebook, and thus downlink CSI updating is accelerated.

Traditionally, a codebook design in NR or LTE is used for all users, it covers a large spatial space with limited code-words in order to reduce uplink overhead. Thus, the traditional codebook has a low spatial resolution for massive MIMO systems. In contrast, an adaptive codebook design in accordance with the present disclosure can trace sub channel properties associated with individual antennas of a terminal device and improve a sub-spatial space quantization resolution.

In addition, covariance matrix are aware at both a network device and a terminal device, it is not needed to be feedback in the uplink channel and thus results in lower uplink overhead. Furthermore, as the adaptive codebook design is based on a common codebook, the codebook size is not changed, so the uplink overhead is not changed when comparing with the traditional solution.

In addition to the embodiment as shown in Fig. 5, in alternative embodiments, the first and second CSI may be maintained in any other suitable forms of matrix existing or future developed. The determination manner for the updated codebook is not intended to be limited to the listed example.

Referring back to Fig. 4, at block 430, the terminal device 120 quantizes the second CSI based on the updated codebook. The process in block 430 is similar with operation 304 described in Fig. 3, and thus omitted here. At block 440, the terminal device 120 transmits, via the first antenna, an uplink SRS and the quantized CSI to the network device 110. The process in block 440 is similar with the operation 305 described in Fig. 3, and thus omitted here.

Based on joint adaptive codebook and antenna switching SRS transmission, accurate downlink CSI associated with the certain terminal device can be obtained, and therefore a multi-user paring and transmit pre-coder design can be improved significantly.

Fig. 6 shows a flowchart of an example method 600 implemented at a network device for determination of downlink CSI in accordance with some embodiments of the present disclosure. The method 600 can be implemented at the network device 110 shown in Fig. 1. For the purpose of discussion, the method 600 will be described with reference to Fig. 1. It is to be understood that the method 600 may include additional acts not shown and/or may omit some shown acts, and the scope of the present disclosure is not limited in this regard.

At block 610, the network device 110 receives, from the terminal device 120, an uplink SRS and a quantized CSI. In the present example, the uplink SRS and the quantized CSI may be transmitted via the antenna 121. The process in block 610 is similar with the operation 305 described in Fig. 3, and thus omitted here.

At block 620, the network device 110 determines, based on the SRS, first downlink CSI associated with a first antenna of the terminal device 120. The process in block 620 is similar with the operation 306 described in Fig. 3. In some embodiments, the network device 110 may obtain the first downlink CSI by measuring the SRS, and determine the association of the first downlink CSI with the first antenna. In this case, the network device 110 may determine which antenna is the first antenna in current time slot, that is, via which antenna the SRS is transmitted.

In some embodiments, the network device 110 may determine which antenna is the first antenna in current time slot as follows. Let L = N × T _SRS , N is the number of antennas of a terminal device, and T _SRS stands for a period of SRS transmission. At time slot t, the index μ of last antenna transmitting the uplink SRS can be determined as

wherein

is a function to find the minimum largest integer than an input parameter, mod (□) is a modular function.

In some embodiments, the sub channel between a network device and the ith antenna of the terminal device with index no large than μ is updated in time slot t _i , and it can be denoted as

t _i =t- (μ-1) T _SRS -mod (t, T _SRS) , i ≤ μ (4)

Meanwhile, the sub channel between the network device and the jth antenna of the terminal device with index larger than μ is updated in time slot t _j, and it can be denoted as

t _j =t-mod (t, T _SRS) + (i-μ) T _SRS -L (5)

In this way, it facilitates the subsequent determination of the updated codebook. In other words, it determines the CSI associated with which antenna will be updated in current time slot. It is to be understood that any other suitable manners can also be employed to determine which antenna will be updated. For example, the network device may record the antenna switching associated with each of the terminal devices, and thus know the source of the received SRS.

At block 630, the network device 110 determines, based on the first downlink CSI, an updated codebook specific to the terminal device. In some embodiments, the network device 110 may determine the updated codebook in response to receiving an indication for the use of the updated codebook from the terminal device 120. For example, according to a change in a network condition, antenna configuration or other factors, the terminal device 120 may select to use an adaptive codebook and inform it to the network device 110. In some embodiments, an additional 1 bit signal can be designed to indicate the use of an updated codebook. It is to be understood that other indications also can be employed to indicate the use of an updated codebook. In this way, an operation based on an adaptive codebook can be aligned at a network device and at a terminal device, including CSI quantization at a terminal device side and recovering at a network device side.

The process in block 630 is similar with the operation 307 described in Fig. 3. The process in the operation 307 is similar with that in the operation 303, except implemented by the network device based on the first downlink CSI instead of implemented by the terminal device based on the first CSI. Its details see that described in connection with Fig. 5, and thus omitted here for concise.

At block 640, the network device 110 determines, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device. The process in block 640 is similar with that in the operation 308, and thus omitted here for concise. In this way, the network device 110 can get full downlink CSI associated with the terminal device 120 rapidly and accurately with low uplink overhead.

Comparing with the traditional solutions, the present solution uses the adaptive codebook technologies to improve the given codebook quantization resolution. It helps the network device to get more accurate full user downlink CSI in time, which will be used to improve the multi-user scheduling as well as multi-user interference pre-cancellation. More important point is that the present solution can significantly improve the massive MIMO system performance without inducing any additional system overhead in an uplink channel.

Fig. 7 shows link level simulations comparison between the present solution and traditional solutions, wherein the relationship between a spectral efficiency (SE) and a signal noise ratio (SNR) is shown. The key parameters for the simulation are listed in Table 1 below.

Table 1

There are six schemes are evaluated and compared in Fig. 7 in order to comprehensively show the performance of each of the schemes.

Solution 1, as shown by 710 in Fig. 7, relates to “perfect full CSI-upper bound” . In this case, assume that at each sub-frame, the network device can get perfect downlink information, and implement the optimum multi-user scheduling and transmit pre-coder design. Its performance is the upper bound of massive MIMO systems.

Solution 2, as shown by 720 in Fig. 7, relates to “Ideal channel based adaptive codebook with SRS period = 4ms” . In this scheme, an adaptive codebook is implemented based on ideal downlink channel covariance matrix for both a network device and a terminal device. The purpose of this scheme is to verify the impact ofR ^iter on the multi-user performance. The SRS transmission period is 4ms.

Solution 3, as shown by 730 in Fig. 7, relates to “Iterative channel based adaptive codebook with SRS period =4ms” . In this scheme, an adaptive codebook is implemented base on iterative downlink channel covariance as shown in the equation (2) for both a network device and a terminal device. For comparing with the ideal downlink channel covariance based adaptive codebook, the SRS period is configured to 4ms too.

Solution 4, as shown by 740 in Fig. 7, relates to “Full CSI with SRS period =4ms” . In this scheme, H ^iter for downlink CSI associated with a certain terminal device is used at a network device for multi-user scheduling and transmit pre-coder design.

Solution 5, as shown by 750 in Fig. 7, relates to “SRS+R13 codebook” . It’s the traditional solution using part SRS transmission and common codebook base channel quantization and feedback.

Solution 6, as shown by 760 in Fig. 7, relates to “FDD R13 codebook” . In this scheme, downlink CSI associated with a certain terminal device is only quantized by the existing codebooks. It is used to check the FDD massive MIMO performance.

In the solution 5 and solution 6, assuming that there is time delay between the uplink SRS transmission, codebook feedback and the downlink transmission, it’s the upper bound performance of the traditional solution and FDD massive MIMO systems.

From the simulation results as shown in Fig. 7, some conclusions can be made as below:

The present solution (solution 3) outperforms the traditional solutions (solution 4/5) in all SNR region without introducing additional uplink overhead, it can be used for cell center use with high SNR as well for cell edge use with low SNR.

The present solution (solution 3) can achieve the same performance as the ideal channel based adaptive codebook scheme (solution 2) . It means that ifmore accurate CSI is required, it is necessary to enlarge the common codebook size with SNR increasing. A tradeoff between the performance and overhead should be further optimized.

Traditional solutions can only be used in low SNR region or only be used for cell edge users, this limits its application scopes.

For limited size codebook, non-ideal TDD massive MIMO system outperforms FDD massive MIMO systems. The performance gap is increasing significantly with the SNR increasing.

In some embodiments, an apparatus capable of performing the method 400 (for example, the terminal device 120) may comprise means for performing the respective steps of the method 400. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some embodiments, the apparatus comprises: means for determining first and second CSI based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively; means for determining, based on the first CSI, an updated codebook specific to the terminal device; means for quantizing the second CSI based on the updated codebook; and means for transmitting, via the first antenna, an uplink SRS and the quantized CSI to a network device.

In some embodiments, the means for determining the updated codebook comprises: means for updating, based on the first CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas; means for determining a covariance matrix from the updated downlink channel matrix; and means for obtaining the updated codebook based on the covariance matrix and a common codebook.

In some embodiments, the downlink channel matrix is initialized as a null matrix.

In some embodiments, the means for determining the updated codebook comprises: means for receiving, from the network device, an indication for the use of the updated codebook; and means for determining the updated codebook in response to the reception of the indication.

In some embodiments, an apparatus capable of performing the method 600 (for example, the network device 110) may comprise means for performing the respective steps of the method 600. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some embodiments, the apparatus comprises: means for receiving, from a terminal device, an uplink SRS and a quantized CSI; means for determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device; means for determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and means for determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.

In some embodiments, the means for determining the updated codebook comprises: means for updating, based on the first downlink CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas; means for determining a covariance matrix from the updated downlink channel matrix; and means for obtaining the updated codebook based on the covariance matrix and a common codebook.

In some embodiments, the means for determining the first downlink CSI comprises: means for obtaining the first downlink CSI by measuring the SRS; and means for determining the association of the first downlink CSI with the first antenna.

In some embodiments, the means for determining the updated codebook comprises: means for receiving, from the terminal device, an indication for the use of the updated codebook; and means for determining the updated codebook in response to the reception of the indication.

Fig. 8 is a simplified block diagram of a device 800 that is suitable for implementing embodiments of the present disclosure. The device 800 can be considered as a further example implementation of the network device 110 or the terminal device 120 as shown in Fig. 1. Accordingly, the device 800 can be implemented at or as at least a part of the network device 110 or the terminal device 120.

As shown, the device 800 includes a processor 810, a memory 820 coupled to the processor 810, a suitable transmitter (TX) and receiver (RX) 840 coupled to a processing means 850 configured by the processor 810 and the memory 820, and a communication interface coupled to the TX/RX 840. The memory 810 stores at least a part of a program 830. The TX/RX 840 is for bidirectional communications. The TX/RX 840 has at least one antenna to facilitate communication, though in practice an Access Node mentioned in this application may have several ones. The communication interface may represent any interface that is necessary for communication with other network elements, such as X2 interface for bidirectional communications between eNBs, S 1 interface for communication between a Mobility Management Entity (MME) /Serving Gateway (S-GW) and the eNB, Un interface for communication between the eNB and a relay node (RN) , or Uu interface for communication between the eNB and a terminal device.

The program 830 is assumed to include program instructions that, when executed by the associated processor 810, enable the device 800 to operate in accordance with the embodiments of the present disclosure, as discussed herein with reference to Fig. 4 or 6. The embodiments herein may be implemented by computer software executable by the processor 810 of the device 800, or by hardware, or by a combination of software and hardware. The processor 810 may be configured to implement various embodiments of the present disclosure. Furthermore, a combination of the processor 810 and memory 820 may form the processing means 850 adapted to implement various embodiments of the present disclosure.

The memory 820 may be of any type suitable to the local technical network and may be implemented using any suitable data storage technology, such as a non-transitory computer readable storage medium, semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one memory 820 is shown in the device 800, there may be several physically distinct memory modules in the device 800. The processor 810 may be of any type suitable to the local technical network, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 800 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representation, it will be appreciated that the blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the process or method as described above with reference to Fig. 4 or 6. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions/operations specified in the flowcharts and/or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

In the context of the present disclosure, the computer program codes or related data may be carried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer readable media.

The above program code may be embodied on a machine readable medium, which may be any tangible medium that may contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. The machine readable medium may be a machine readable signal medium or a machine readable storage medium. A machine readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the machine readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM or Flash memory) , an optical fiber, a portable compact disc read-only memory (CD-ROM) , an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

Although the present disclosure has been described in language specific to structural features and/or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

A terminal device comprising:

a processor; and

a memory coupled to the processor and storing instructions thereon, the instructions, when executed by the processor, causing the terminal device to perform actions comprising:

determining first and second channel state information (CSI) based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively;

determining, based on the first CSI, an updated codebook specific to the terminal device;

quantizing the second CSI based on the updated codebook; and

transmitting, via the first antenna, an uplink sounding reference signal (SRS) and the quantized CSI to a network device.
The terminal device of Claim 1, wherein determining the updated codebook comprises:

updating, based on the first CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas;

determining a covariance matrix from the updated downlink channel matrix; and

obtaining the updated codebook based on the covariance matrix and a common codebook.
The terminal device of Claim 2, wherein the downlink channel matrix is initialized as a null matrix.
The terminal device of Claim 1, wherein determining the updated codebook comprises:

receiving, from the network device, an indication for the use of the updated codebook; and

determining the updated codebook in response to the reception of the indication.
A network device comprising:

a processor; and

a memory coupled to the processor and storing instructions thereon, the instructions, when executed by the processor, causing the network device to perform actions comprising:

receiving, from a terminal device, an uplink sounding reference signal (SRS) and a quantized channel state information (CSI) ;

determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device;

determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and

determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.
The network device of Claim 5, wherein determining the updated codebook comprises:

updating, based on the first downlink CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas;

determining a covariance matrix from the updated downlink channel matrix; and

obtaining the updated codebook based on the covariance matrix and a common codebook.
The network device of Claim 6, wherein the downlink channel matrix is initialized as a null matrix.
The network device of Claim 5, wherein determining the first downlink CSI comprises:

obtaining the first downlink CSI by measuring the SRS; and

determining the association of the first downlink CSI with the first antenna.
The network device of Claim 5, wherein determining the updated codebook comprises:

receiving, from the terminal device, an indication for the use of the updated codebook; and

determining the updated codebook in response to the reception of the indication.
A method implemented at a terminal device, comprising:

determining first and second channel state information (CSI) based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively;

determining, based on the first CSI, an updated codebook specific to the terminal device;

quantizing the second CSI based on the updated codebook; and

transmitting, via the first antenna, an uplink sounding reference signal (SRS) and the quantized CSI to a network device.
The method of Claim 10, wherein determining the updated codebook comprises:

updating, based on the first CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas;

determining a covariance matrix from the updated downlink channel matrix; and

obtaining the updated codebook based on the covariance matrix and a common codebook.
The method of Claim 11, wherein the downlink channel matrix is initialized as a null matrix.
The method of Claim 10, wherein determining the updated codebook comprises:

receiving, from the network device, an indication for the use of the updated codebook; and

determining the updated codebook in response to the reception of the indication.
A method implemented at a network device, comprising:

receiving, from a terminal device, an uplink sounding reference signal (SRS) and a quantized channel state information (CSI) ;

determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device;

determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and

determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.
The method of Claim 14, wherein determining the updated codebook comprises:

updating, based on the first downlink CSI, a corresponding portion of a downlink channel matrix associated with the first and second antennas;

determining a covariance matrix from the updated downlink channel matrix; and

obtaining the updated codebook based on the covariance matrix and a common codebook.
The method of Claim 15, wherein the downlink channel matrix is initialized as a null matrix.
The method of Claim 14, wherein determining the first downlink CSI comprises:

obtaining the first downlink CSI by measuring the SRS; and

determining the association of the first downlink CSI with the first antenna.
The method of Claim 14, wherein determining the updated codebook comprises:

receiving, from the terminal device, an indication for the use of the updated codebook; and

determining the updated codebook in response to the reception of the indication.
An apparatus implemented at a terminal device, comprising:

means for determining first and second channel state information (CSI) based on downlink reference signals received from a network device via first and second antennas of the terminal device respectively;

means for determining, based on the first CSI, an updated codebook specific to the terminal device;

means for quantizing the second CSI based on the updated codebook; and

means for transmitting, via the first antenna, an uplink sounding reference signal (SRS) and the quantized CSI to a network device.
An apparatus implemented at a network device, comprising:

means for receiving, from a terminal device, an uplink sounding reference signal (SRS) and a quantized channel state information (CSI) ;

means for determining, based on the SRS, first downlink CSI associated with a first antenna of the terminal device;

means for determining, based on the first downlink CSI, an updated codebook specific to the terminal device; and

means for determining, based on the updated codebook and the quantized CSI, second downlink CSI associated with a second antenna of a terminal device.
A computer readable storage medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to carry out the method according to any of claims 10 to 13.
A computer readable storage medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to carry out the method according to any of claims 14 to 18.