WO2016195393A1 - Method and apparatus for reducing complexity in acquiring uplink synchronisation taking account of beamforming effect in wirless communication system - Google Patents

Abstract

A method and an apparatus for calculating correlation in a wireless communication system are provided. A base station classifies a plurality of beams into a plurality of beam groups according to the quality of reception signals respectively corresponding to the plurality of beams, and applies different correlation calculation intervals to the respective classified plurality of beam groups. The plurality of beams can be classified into the plurality of beam groups based on correlation outputs according to the quality of the reception signals, and a first correlation calculation interval that is applied to a first beam group having a correlation output that is bigger than a correlation output threshold can be smaller than a second correlation calculation interval that is applied to a second beam group having a correlation output that is smaller than the correlation output threshold.

Description

Method and apparatus for reducing complexity of uplink synchronization acquisition considering beamforming effect in wireless communication system

The present invention relates to wireless communication, and more particularly, to a method and apparatus for reducing the complexity of uplink synchronization acquisition considering beamforming effects in a wireless communication system.

3rd generation partnership project (3GPP) long-term evolution (LTE) is a technology that enables high-speed packet communication. Many schemes have been proposed for the purposes of LTE, including reduced costs for users and suppliers, improved quality of service, and increased coverage and system capacity. 3GPP LTE is a higher layer requirement that requires reduced cost per bit, increased service availability, flexible frequency usage, simple structure, open interface and proper power consumption of the terminal.

In order to increase the efficiency of limited resources, the transceivers are equipped with multiple antennas to obtain additional spatial area for resource utilization to obtain diversity gain or to transmit data in parallel through each antenna. As for height, what is called a multi-antenna technique is actively developed. Beamforming and / or precoding may be used as a method for increasing signal-to-noise ratio (SNR) in a multiple antenna technology. Beamforming and / or precoding may be used to maximize the SNR through the feedback information in a closed loop system where feedback information is available at the transmit end. Beamforming can be broadly classified into analog beamforming and digital beamforming.

Massive multiple-input multiple-output (MIMO) is a multi-antenna technology that seeks to achieve high energy efficiency with high transmission rates by mounting more than a few dozen antennas in a base station. If the existing analog beamforming and / or digital beamforming is applied to large MIMO, the complexity of signal processing and / or hardware implementation is very large, or the performance increase using multiple antennas is insignificant, Flexibility can be inferior. Accordingly, the use of hybrid beamforming combining existing analog beamforming and digital beamforming in large MIMO is being discussed.

Due to the recent rapid spread of mobile smart devices and the emergence of big data, mobile traffic is expected to double every year and increase more than 1000 times in 10 years. Due to the explosion of mobile traffic, the burden on the mobile network operators is increasing, and the existing 4G mobile communication with limited additional frequency cannot accommodate the exploding mobile traffic. Therefore, the development of millimeter wave (mmWave) based fifth generation mobile communication technology that can secure a broadband is being discussed. The millimeter wave is a frequency in the 30-300 GHz band, commonly called an extreme high frequency (EHF) band, and refers to a band having a wavelength length of 1 cm-1 mm. This wave propagates between the radio frequency band currently used and infrared rays (a wavelength of about 0.1 mm) and is very close to light, and is currently used in high resolution wave radar and microwave spectroscopy. Millimeter waves are smaller in diffraction and straightness than conventional radio waves, and are more diffractive and less linear than laser light. If millimeter waves are used for communication, it is thought that ultra-multiple communication far exceeding the amount of microwaves is possible. This is because energy absorption by oxygen and water molecules in the atmosphere is relatively large compared to conventional cellular frequencies, resulting in high path loss.

When hybrid beamforming and the like are introduced, channel characteristics may be different for each beam, and thus channel delays may be different for each beam. Therefore, in consideration of such characteristics, a method of estimating a channel delay and determining a timing advance value for each beam may be required. Also in this case, since the complexity of the timing advance calculation may increase, a method that can reduce the complexity of the calculation may also be required.

An object of the present invention is to provide a method and apparatus for reducing the complexity of obtaining uplink sync considering the beamforming effect in a wireless communication system. The present invention provides a method and apparatus for reducing the complexity of calculation of timing misalignment per beam caused by a change in channel characteristics per beam during uplink beam scanning. In addition, the present invention provides a specific method and apparatus for reducing the complexity of the correlation calculation when estimating the channel delay per beam and calculating and updating a timing advance value based on the beam delay.

In one aspect, a method for calculating correlation by a base station in a wireless communication system is provided. The method includes classifying a plurality of beams into a plurality of beam groups according to the quality of a received signal corresponding to each of the plurality of beams, and applying different correlation calculation intervals to each of the classified plurality of beam groups. do.

In another aspect, a base station is provided in a wireless communication system. The base station includes a memory, a transceiver, and a processor connected to the memory and the transceiver. The processor is configured to classify the plurality of beams into a plurality of beam groups according to the quality of a received signal corresponding to each of the plurality of beams, and apply different correlation calculation intervals to each of the classified plurality of beam groups.

When the channel delay is estimated for each beam and the timing advance value is calculated and updated based on the beam delay, the complexity of the calculation can be reduced.

1 illustrates a cellular system.

2 shows a structure of a radio frame of 3GPP LTE.

3 is a block diagram of a transmitter including an analog beamformer and an RF chain.

4 is a block diagram of a transmitter including a digital beamformer and an RF chain.

5 is a block diagram of a transmitter including a hybrid beamformer.

6 shows an example of timing alignment of UL transmissions to which TA is not applied.

7 shows an example of timing alignment of UL transmissions to which TA is applied.

8 shows an example of application of a TA command for TA update.

9 is a conceptual diagram illustrating that channel characteristics change for each beam.

10 is a conceptual diagram illustrating that channel characteristics are different for each beam.

11 conceptually illustrates the complexity of calculation for uplink synchronization acquisition according to the increase of the effective multipath.

12 illustrates an example of applying a correlation calculation interval to a strong beam group according to an embodiment of the present invention.

13 illustrates an example of applying a correlation calculation interval to a weak beam group according to an embodiment of the present invention.

14 illustrates a method for calculating correlation by a base station according to an embodiment of the present invention.

15 is a block diagram of a wireless communication system in which an embodiment of the present invention is implemented.

The following techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), and the like. It can be used in various wireless communication systems. CDMA may be implemented by a radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. TDMA may be implemented with wireless technologies such as global system for mobile communications (GSM) / general packet radio service (GPRS) / enhanced data rates for GSM evolution (EDGE). OFDMA may be implemented by wireless technologies such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), and the like. IEEE 802.16m is an evolution of IEEE 802.16e and provides backward compatibility with systems based on IEEE 802.16e. UTRA is part of a universal mobile telecommunications system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is part of evolved UMTS (E-UMTS) using evolved-UMTS terrestrial radio access (E-UTRA), which employs OFDMA in downlink and SC in uplink -FDMA is adopted. LTE-A (advanced) is the evolution of 3GPP LTE.

For clarity, the following description focuses on LTE-A, but the technical features of the present invention are not limited thereto.

1 illustrates a cellular system. Referring to FIG. 1, cellular system 10 includes at least one base station (BS) 11. BS 11 provides communication services for specific geographic regions (generally called cells) 15a, 15b, 15c. The cell can in turn be divided into a number of regions (called sectors). A user equipment (UE 12) may be fixed or mobile, and may have a mobile station (MS), a mobile terminal (MS), a user terminal (UT), a subscriber station (SS), a wireless device, a PDA, and the like. (personal digital assistant), wireless modem (wireless modem), a handheld device (handheld device) may be called other terms. BS 11 generally refers to a fixed point of communication with UE 12 and may be referred to in other terms, such as an evolved-NodeB (eNB), a base transceiver system (BTS), an access point, and the like.

The UE typically belongs to one cell, and the cell to which the UE belongs is called a serving cell. A BS that provides a communication service for a serving cell is called a serving BS. The cellular system includes another cell adjacent to the serving cell. Another cell adjacent to the serving cell is called a neighbor cell. A BS that provides communication service for a neighbor cell is called a neighbor BS. The serving cell and the neighbor cell are determined relatively based on the UE.

This technique can be used for downlink (DL) or uplink (UL). In general, DL means communication from BS 11 to UE 12, and UL means communication from UE 12 to BS 11. In the DL, the transmitter may be part of the BS 11 and the receiver may be part of the UE 12. In the UL, the transmitter is part of the UE 12 and the receiver may be part of the BS 11.

2 shows a structure of a radio frame of 3GPP LTE. Referring to FIG. 2, a radio frame consists of 10 subframes, and one subframe consists of two slots. Slots in a radio frame are numbered with slots # 0 through # 19. Transmission time interval (TTI) is a basic scheduling unit for data transmission. In 3GPP LTE, one TTI may be equal to the time taken for one subframe to be transmitted. One radio frame may have a length of 10 ms, one subframe may have a length of 1 ms, and one slot may have a length of 0.5 ms.

One slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain and a plurality of subcarriers in the frequency domain. The OFDM symbol is used to represent one symbol period since 3GPP LTE uses OFDMA in downlink, and may be called another name according to a multiple access scheme. For example, when SC-FDMA is used as an uplink multiple access scheme, it may be referred to as an SC-FDMA symbol. A resource block (RB) includes a plurality of consecutive subcarriers in one slot in resource allocation units. The structure of the radio frame is merely an example. Accordingly, the number of subframes included in the radio frame, the number of slots included in the subframe, or the number of OFDM symbols included in the slot may be variously changed. 3GPP LTE defines that one slot includes seven OFDM symbols in a normal cyclic prefix (CP), and one slot includes six OFDM symbols in an extended CP.

The necessity of hybrid beamforming is described. Beamforming techniques using multiple antennas are analog beamforming techniques (hereinafter referred to as analog beamforming) and digital beamforming techniques (depending on where the beamforming weight vector (or precoding vector) is applied). Hereinafter, it may be classified into digital beamforming.

3 is a block diagram of a transmitter including an analog beamformer and an RF chain. Analog beamforming is a representative beamforming technique applied to early multi-antenna structures. Analog beamforming branches the analog signal, which has completed digital signal processing, into a plurality of paths, and forms a beam by setting a phase shift (PS) and a power amplifier (PA) in each path. Referring to FIG. 3, the PS and the PA connected to each antenna process an analog signal derived from a single digital signal in analog beamforming. That is, the PS and the PA process complex weights in the analog stage. Here, the RF (radio frequency) chain refers to a processing block in which a baseband signal is converted into an analog signal. In analog beamforming, the beam accuracy is determined by the characteristics of the PS and PA devices, and the control characteristics of the devices are advantageous for narrowband transmission. On the other hand, due to a hardware structure that is difficult to implement multi-stream transmission, the multiplexing gain for increasing the transmission rate is relatively small, and it is difficult to form beams for each user based on orthogonal resource allocation.

4 is a block diagram of a transmitter including a digital beamformer and an RF chain. Unlike analog beamforming, digital beamforming uses baseband processing to form beams at the digital stage to maximize diversity and multiplexing gain in a MIMO environment. Referring to FIG. 4, a beam may be formed by performing precoding in baseband processing. The RF chain may comprise a PA. Accordingly, the complex weight derived for beamforming may be directly applied to the transmission data. Digital beamforming may form a beam differently for each user, and thus may simultaneously support multi-user beamforming. In addition, digital beamforming is capable of forming an independent beam for each user to which orthogonal resources are allocated, and thus has high scheduling flexibility. In addition, digital beamforming may form an independent beam for each subcarrier when a technique such as MIMO-OFDM is applied in a broadband transmission environment. Therefore, digital beamforming can maximize the maximum data rate of a single user based on increased system capacity and enhanced beam gain. Therefore, MIMO technology based on digital beamforming has been introduced in 3G / 4G systems.

On the other hand, a huge massive multiple-input multiple-output (MIMO) that considerably increases the number of transmit and receive antennas may be considered. A general cellular system assumes 8 maximum transmit / receive antennas applied to a MIMO environment. However, in a large MIMO environment, the maximum transmit / receive antennas may increase to tens or hundreds. If the existing digital beamforming is applied in a large MIMO environment, the digital signal processing for hundreds of transmit antennas must be performed through baseband processing, and thus the complexity of signal processing becomes very large, and as many RF antenna chains are required as the number of transmit antennas. The complexity of the hardware implementation is very large. In addition, independent channel estimation is required for all transmit antennas, and in the case of a frequency division duplex (FDD) system, feedback information for a huge MIMO channel including all antennas is required, so that pilot and feedback overhead becomes very large. On the other hand, if the conventional analog beamforming is applied in a large MIMO environment, the hardware complexity of the transmitter is relatively low, while the performance increase using multiple antennas is insignificant, and the flexibility of resource allocation is inferior. In particular, it is very difficult to control the beam for each frequency during broadband transmission.

Therefore, in a large MIMO environment, hybrid beamforming in which analog beamforming and digital beamforming are combined is required, rather than using only one of analog beamforming and digital beamforming as a beamforming technology. That is, a hybrid type transmitter stage structure may be required to reduce the complexity of hardware implementation of the transmitter stage according to the characteristics of analog beamforming, and to maximize the beamforming gain using a large number of transmit antennas according to the characteristics of digital beamforming. have.

Hybrid beamforming will be described. As described above, hybrid beamforming aims to configure a transmitter that can take advantage of analog beamforming and digital beamforming in a large MIMO environment.

5 is a block diagram of a transmitter including a hybrid beamformer. Referring to FIG. 5, basically, hybrid beamforming may form coarse beams through analog beamforming, and beams for multi-stream or multi-user transmission may be formed through digital beamforming. In other words, the hybrid beamforming has a structure in which analog beamforming and digital beamforming are simultaneously performed to reduce the implementation complexity or hardware complexity of the transmitter.

The technical issues of hybrid beamforming are as follows.

(1) Difficulties in optimizing analog / digital beamforming design: Digital beamforming can form independent beams for each user with the same time-frequency resources, but analog beamforming forms a common beam with the same time-frequency resources. There is a limit to this. Therefore, issues such as a limitation of the maximum supportable rank according to the number of RF chains, difficulty of controlling subband beams with an RF beamformer, and / or optimization of beam forming resolution (resolution / granularity) may occur.

(2) Need to specify a common signal transmission method: Analog beamforming forming a beam only in a specific direction on the same time-frequency resource cannot simultaneously form multiple beams in all terminal directions. Therefore, a problem may occur in that a DL / UL control channel, a reference signal, a broadcast channel, a synchronization signal, and the like may not be simultaneously transmitted to all terminals that may be distributed in all regions of a cell. In addition, a problem may occur when the UE transmits a physical random access channel (PRACH), a physical uplink control channel (PUCCH), and / or a sounding RS (SRS) on the UL.

(3) Additional pilot and feedback design required for analog / digital beam determination: When performing the analog / digital beam estimation, the digital beam can be estimated using the existing orthogonal pilot allocation method, but the analog beam As many times as the number of beam candidates are required. That is, it means that the time delay required for the estimation of the analog beam is large, resulting in system loss. In addition, when estimating a digital beam and an analog beam simultaneously, the complexity may increase significantly.

(4) Analog beam based spatial division multiple access (SDMA) and FDMA support difficulties: While digital beamforming can freely form beams for multiple users / streams, analog beamforming forms the same beam for the entire transmission band. Therefore, it is difficult to perform independent beam formation for each user or stream. In particular, because it is difficult to support FDMA (e.g. OFDMA) through orthogonal frequency resource allocation, it may be difficult to optimize frequency resource efficiency.

Among the technical issues of hybrid beamforming described above, the present invention described below can provide a method for optimizing an analog / digital beam design for hybrid beamforming.

Timing advance (TA) will be described. In order to maintain orthogonality of UL received from different terminals in 3GPP LTE, the timing of UL received from different terminals is aligned at the receiver of the base station. That is, the UL transmission and the DL transmission may be aligned on the time axis with respect to the base station. Timing alignment of the UL transmission is one of the most basic ways to avoid interference between terminals in the cell. TA may be applied to the UL transmission of the terminal to implement timing alignment of the UL transmission. The terminal may set a TA value in response to the received DL timing, and thus may correspond to different propagation delays between different terminals.

6 shows an example of timing alignment of UL transmissions to which TA is not applied. Referring to FIG. 6, UE1 is located relatively close to the base station, so that the propagation delay TP1 is relatively short, and UE2 is far from the base station, so that the propagation delay TP2 is relatively long (that is, TP1 <TP2). The criterion of propagation delay is the DL symbol timing received by the UE. Assuming the DL symbol timing at the base station is T, the DL symbol timing received at UE1 is T + TP1 delayed by the propagation delay TP1 of UE1. Assuming that UE1 performs UL transmission without delay, the UL symbol timing at UE1 is T + TP1, and the UL symbol timing received at the base station is T + 2 * TP1 delayed again by the propagation delay TP1. Similarly, the DL symbol timing received at UE2 is T + TP2 delayed by the propagation delay TP2 of UE2. Assuming that UE2 performs UL transmission without delay, the UL symbol timing at UE2 is T + TP2, and the UL symbol timing received at the base station is T + 2 * TP2 delayed again by the propagation delay TP2. That is, the timing of the UL transmission transmitted by the UE1 and the UL transmission transmitted by the UE2 is shifted by 2 * (TP2-TP1) from the position of the base station.

7 shows an example of timing alignment of UL transmissions to which TA is applied. By applying the TA, the timing alignment at the base station can be made appropriately. The propagation delay measured for the application of TA is converted into a round trip delay (RTD), and its value is (propagation delay * 2). Referring to FIG. 7, assuming that DL symbol timing at a base station is T, DL symbol timing received at UE1 is T + TP1 delayed by propagation delay TP1 of UE1. UE1 applies a TA value having a size of TP1 * 2, so that the UL transmission timing in UE1 becomes T-TP1. Accordingly, the UL symbol timing received at the base station becomes T, and DL transmission and UL reception are aligned at the base station. Similarly, the DL symbol timing received at UE2 is T + TP2 delayed by the propagation delay TP2 of UE2. UE2 applies a TA value having a size of TP2 * 2, so that the UL transmission timing in UE2 becomes T-TP2. Accordingly, the UL symbol timing received at the base station becomes T, and DL transmission and UL reception are aligned at the base station. It can be seen that the farther the propagation delay from the base station is, the longer the UL transmission needs to be performed first for timing alignment in the base station.

For the initial TA procedure, the UE performs initial receiver synchronization for DL transmission transmitted from the base station and performs TA using a random access procedure. The base station measures the UL timing through the random access preamble transmitted by the terminal, and transmits an initial TA command corresponding to the UL timing through a random access respond (RAR). The size of the TA command may be 11 bits.

In addition, depending on the situation, the TA may be updated. The base station may perform a TA update command using all available UL reference signals (RSs). That is, the base station may perform a TA update command using SRS, channel quality indicator (CQI), acknowledgment / non-acknowledgement (ACK / NACK), and the like. In general, SRS may be advantageous because the accuracy of timing estimation increases as the UL RS transmitted through a wide bandwidth increases, but there may be a limitation in using the SRS due to power limitation for a terminal located at a cell boundary. However, since the TA update depends on the implementation of the base station, the standard does not describe any constraints.

8 shows an example of application of a TA command for TA update. Referring to FIG. 8, a TA command for updating a TA is applied when a UE receives a TA command and transmits a first UL subframe after 5 ms-round trip time (RTT). This is because a UL subframe may not exist at a time according to a UL / DL configuration in a time division duplex (TDD) frame or a half-duplex frequency division duplex (FDD) frame. RTT may be propagation delay * 2.

Until now, it has been assumed that 3GPP LTE uses an omni antenna having a single channel characteristic, and accordingly, UL timing alignment is performed only for a single channel characteristic. However, when the UE performs narrowband beamforming using many antennas according to the introduction of hybrid beamforming, the scattering environment that each beam undergoes may change, and the ratio of timing misalignment may vary for each beam. have. This characteristic grows larger in the millimeter wave (mmWave) band. This is because the distribution of the scatter due to beamforming is changed in the millimeter wave band and the path attenuation with distance increases more. In the millimeter wave band, since the hybrid beamforming structure is oriented in consideration of the ease of implementation and the complexity of the baseband, the above-described beam scanning procedure may be necessary. In addition, this characteristic may become larger as the symbol period of the data and the UL RACH is the same.

9 is a conceptual diagram illustrating that channel characteristics change for each beam. As described above, in the hybrid beamforming, beam scanning must be performed in the time domain. However, as the direction formed by the beamforming is directed to a specific scatter, the multipath characteristic may change and thus the channel characteristic may change. Referring to FIG. 9, the terminal performs UL transmission through beam # 1, beam # 2, and beam # 3, and multipath characteristics are changed to beam rice according to the direction of beamforming. Accordingly, the base station experiences different channel characteristics for each beam when performing beam scanning.

10 is a conceptual diagram illustrating that channel characteristics are different for each beam. Referring to FIG. 10, channel characteristics of beams # 1 and # 2 are different from each other. In particular, the TA time point τ ₀ corresponding to the propagation delay is different for each beam.

In order to solve the above problems, that is, channel characteristics vary from beam to beam due to the introduction of hybrid beamforming in the millimeter wave band, the timing shift is different from beam to beam. Can be. That is, in consideration of the characteristics of the analog beam scanning procedure for performing hybrid beamforming, a TA value for each beam may be estimated. In addition to the existing TA procedure based on the omni antenna, a TA procedure optimized for each beam based on beamforming may be proposed for data transmission.

In more detail, the base station may estimate the TA value for each of K different preambles or UL RSs on a time axis to which different K beamforming is applied by one UE during UL beam scanning. That is, the base station estimates the TA value changed by the beam characteristics during the UL beam scanning for each beam. When different K preambles are transmitted with different beamforming applied to each other, channel characteristics may vary for each beam, and thus a delay profile may vary for each beam. The reason for estimating the TA value differently for each beam is that the beam scanning process must be accompanied by the characteristics of the analog stage. The base station can estimate the UL channel delay and the TA value for each beam through the basic promise for beam scanning with the terminal. Assuming that the detection of the preamble or the UL RS is performed in the time domain, the channel delay m _K ^* maximizing the output in the beam K can be obtained by Equation 1.

<Equation 1>

In Equation 1, i is a time index, m is a timing offset, N is the total length (or OFDM symbol length) of a time signal, L is a multipath channel delay length by a PDF (probability distribution function), and Y [i] is The signal received at time i, S [i], represents the signal transmitted at time i. Through Equation 1, individual channel delays and TA values for different K beams can be estimated. Accordingly, a TA command according to a beam suitable for a UE can be performed.

In addition, the base station may perform a TA command to the terminal based on the channel delay or the TA value estimated for each beam. In more detail, the base station may perform a TA command in consideration of a channel delay or a TA value of at least one beam among different K beams detected by beam scanning. The base station may determine the final TA value of the terminal based on various criteria. The base station may determine the final TA value of the terminal based on one beam. For example, the base station may determine the final TA value of the terminal based on the TA value of the beam having the best reception quality among the detected beams. Alternatively, the base station may determine the final TA value of the terminal based on the plurality of beams. For example, the base station may determine the final TA value of the terminal based on the TA values of several beams whose signal strength or signal quality of the received beam is greater than or equal to a predetermined reference among the detected beams.

As described above, the detection of the preamble or the UL RS is basically performed in the time domain, and according to Equation 1 described above, m _k ^* for maximizing the output of the beam K in the search window is searched for the beam K. Uplink synchronization can be obtained. Meanwhile, referring to Equation 1, in order to obtain accurate uplink synchronization for each beam, the multipath channel delay length L is used instead of the single path.

It can be seen that this L iteration is calculated repeatedly.

11 conceptually illustrates the complexity of calculation for uplink synchronization acquisition according to the increase of the effective multipath. Referring to FIG. 11, it can be seen that within the search window corresponding to the length N of the ZC sequence, the calculation for uplink synchronization acquisition is repeated L times for the multipath channel length L. FIG. That is, as the uplink synchronization is obtained in consideration of the multipath channel, the complexity of the calculation increases. In particular, after acquiring the uplink synchronization in consideration of the multipath channel, the complexity of the calculation is large even when the uplink synchronization is reacquired or the TA value is recalculated according to the movement of the terminal or the change of the channel condition.

In order to solve the above-described problem, the present invention obtains the uplink synchronization in consideration of the multipath channel, multi-path in the case of re-acquiring uplink synchronization or re-calculation of the TA value in accordance with the movement of the terminal or the change of channel conditions We propose a method to adaptively reduce the complexity of recalculation according to the channel delay length L. That is, the present invention proposes a method of lowering the amount of computation occurring in the correlation calculation according to the multipath channel delay length L when re-acquiring uplink synchronization or recalculating the TA value.

In more detail, according to an embodiment of the present invention, when the base station re-acquires uplink synchronization or recalculates or updates a TA value, the base station may perform reception of a beam-specific received signal obtained by uplink beam scanning previously performed. A plurality of beams may be classified into several beam groups according to quality, and a correlation calculation interval for a multipath channel delay length L may be differently applied according to each beam group. The base station classifies the plurality of beams into G _N beam groups according to the quality of the received signal obtained by the uplink beam scanning previously performed for each terminal. Correlation calculation interval I _gap is defined for each beam group. Table 1 shows an example of grouping for a plurality of beams per terminal and a correlation calculation interval for each beam group.

Beam group	Threshold (based on correlation plot output)	Correlation Calculation Interval
Beam group 1	P ≥ P th, 1	I gap, 1
Beam group 2	P th, 2 ≤ P <P th, 1	I gap, 2
Beam group 3	P th, 3 ≤ P <P th, 2	I gap, 3
...	...	...
Beam group G N	P <P th, GN -1	I gap, GN

In Table 1, G _N represents the total number of beam groups, P _th represents a threshold value of the correlation output per beam, and Igap represents a correlation calculation interval for each beam group. That is, a plurality of beams are classified into a plurality of beam groups based on the correlation output, and a correlation calculation interval is defined differently for each beam group.

When re-acquiring uplink synchronization or recalculating or updating a TA value, the amount of computation can be selectively lowered by adaptively applying a correlation calculation interval for each beam group through the aforementioned beam grouping. For example, assume that the base station classifies the entire beam into two beam groups, a strong beam group and a weak beam group, for each terminal. According to the proposal of the present invention, when re-acquiring uplink synchronization or recalculating or updating the TA value, the base station calculates the correlation according to the correlation calculation interval 1 for all K beams for the multipath channel delay length L. Instead, the correlation calculation interval may be applied adaptively for each beam group. For example, the base station calculates the correlation with the correlation calculation interval 1 for beams belonging to the strong beam group (i.e., calculating the finer correlation), and the correlation calculation interval 5 or 10 for beams belonging to the weak beam group. Correlation can be calculated (ie, coarser correlation). In this case, it may be assumed that even if the terminal moves, the terminal may move to a beam having a higher degree of correlation with the previous beam.

Hereinafter, in a specific embodiment, based on the signal quality according to the correlation output of the total K beams obtained in the uplink beam scanning performed previously, the base station selects the entire K beams from the strong beam group / middle beam group / weak. Assume that the case is classified into three beam groups of the beam group. When the base station reacquires uplink synchronization or recalculates or updates a TA value, the base station may apply different correlation calculation intervals according to beam groups for respective terminals. Table 2 shows an example of classifying all K beams into three beam groups of a strong beam group, a middle beam group, and a weak beam group for each terminal.

Terminal	Beam group
	Strong beam group	Medium beam group	Weak beam group
One	K strong = {1,2,3}	K inter = {4,5,6}	K weak = {others}
2	K strong = {2,3,4}	K inter = {1,5,6}	K weak = {others}
3	K strong = {4,5,6}	K inter = {1,2,3}	K weak = {others}
4	K strong = {7,8,9}	K inter = {1,4,6}	K weak = {others}
...	...	...	...

As such, when the total K beams are classified into three beam groups of the strong beam group, the middle beam group, and the weak beam group for each terminal, the base station determines the correlation calculation interval for the multipath channel delay length L for the strong beam group. The TA value can be recalculated or updated to a minimum (ie, I _min ). That is, when re-acquiring uplink synchronization or recalculating or updating a TA value, the base station can perform a finer correlation calculation only for the beam group showing a relatively high correlation output in the uplink beam scanning previously performed. have. That is, the correlation for the multipath channel delay length L is calculated according to the minimum correlation calculation interval I _min . For example, Equation 1 described above may be modified as in Equation 2 below with respect to the strong beam group.

<Equation 2>

That is, according to Equation 2, the correlation is calculated according to the minimum correlation calculation interval I _min only for beams belonging to the strong beam group K _strong . In general, a minimum correlation calculation interval I _min = 1 applied to a strong beam group may be represented, and Equation 2 may be expressed by Equation 3 below.

<Equation 3>

12 illustrates an example of applying a correlation calculation interval to a strong beam group according to an embodiment of the present invention. Referring to FIG. 12, for the beam belonging to the strong beam group as described above, the correlation for the multipath channel delay length L is calculated according to the minimum correlation calculation interval I _min = 1.

On the other hand, when the total K beams are classified into three beam groups of the strong beam group, the middle beam group, and the weak beam group for each terminal, the base station determines the multipath channel delay length L for the middle beam group or the weak beam group. TA values can be recalculated or updated by applying correlation calculation intervals I _inter (> I _min ) and I _max (I _min ), respectively. That is, when re-acquiring uplink synchronization or recalculating or updating a TA value, the base station is based on a larger correlation calculation interval for a beam group having a relatively low correlation output in uplink beam scanning previously performed. The approximate correlation can be calculated. For example, Equation 1 described above may be modified as Equation 4 below with respect to a weak beam group.

<Equation 4>

That is, according to Equation 4, the correlation is calculated according to the correlation calculation interval I _max for the beam belonging to the weak beam group K _weak . In general, the correlation calculation interval I _max applied for a weak beam group may be a few samples or more, for example, I _max = 5. In this case, Equation 4 may be expressed by Equation 5 below.

<Equation 5>

13 illustrates an example of applying a correlation calculation interval to a weak beam group according to an embodiment of the present invention. Referring to FIG. 13, for the beam belonging to the weak beam group as described above, the correlation for the multipath channel delay length L is calculated according to the correlation calculation interval I _max = 5.

The above-described method may be applied to the intermediate beam group as it is.

On the other hand, in the above description, applying different correlation calculation intervals for each beam group may include the granularity and / or quantization level of the channel delay and / or TA value calculated for each beam using different correlation calculation intervals. ) Or different bit lengths. More specifically, since the strong beam group is finely correlated according to the minimum correlation calculation interval, the beam delay per channel and / or the TA value may be high, and used to transmit the TA value. The number of bits may be many. On the contrary, since the weak beam group is roughly calculated according to the maximum correlation calculation interval, the beam delay per channel and / or the precision of the TA value may be low, and the amount of bits used to transmit the TA value may be reduced. It may be less. For example, a large number of bits are allocated to transmit the channel delay and / or TA value of a beam belonging to a strong beam group, and to transmit the channel delay and / or TA value of a beam belonging to a weak beam group. A small number of bits can be allocated. Alternatively, a TA value may not be transmitted for beams belonging to a weak beam group.

14 illustrates a method for calculating correlation by a base station according to an embodiment of the present invention. The above description of the present invention can be applied to the embodiment of FIG. 14.

In step S100, the base station classifies the plurality of beams into a plurality of beam groups according to the quality of the received signal corresponding to each of the plurality of beams. The plurality of beams may be classified into the plurality of beam groups based on a correlation output according to the quality of the received signal. Correlation output according to the quality of the received signal may be obtained through uplink beam scanning. In addition, the plurality of beam groups may be classified for each terminal.

In step S110, the base station applies different correlation calculation intervals to each of the classified plurality of beam groups. A first correlation calculation interval applied to a first beam group in which the magnitude of the correlation output is greater than a correlation output threshold is applied to a second beam group in which the magnitude of the correlation output is smaller than the correlation output threshold. The second correlation may also be less than the calculation interval.

More specifically, the plurality of beam groups include a strong beam group including a beam having a magnitude of the correlation output greater than a first correlation output threshold, and the magnitude of the correlation output is greater than the first correlation output threshold. An intermediate beam group may include a beam group that is small and larger than a second correlation output threshold, and a weak beam group that includes a beam whose magnitude is less than a second correlation output threshold. The smallest correlation calculation interval may be applied to the strong beam group. In this case, the correlation may be calculated according to Equation 2, and the smallest correlation calculation interval may be 1. The largest correlation calculation interval may be applied to the weak beam group. In this case, the correlation may be calculated according to Equation 4 described above.

The base station may calculate a correlation for each of the plurality of beam groups by applying different correlation calculation intervals to each of the plurality of beam groups, and calculate a TA for each beam using the calculated correlation. The base station may also transmit the timing advance calculated for each beam to each terminal using bits of different lengths for each of the plurality of beam groups. In this case, the length of the bits used to transmit the timing advance of the beam belonging to the strong beam group among the plurality of beam groups may be longer than the length of the bits used to transmit the timing advance of the beam belonging to the weak beam group. .

15 is a block diagram of a wireless communication system in which an embodiment of the present invention is implemented.

The base station 800 may include a processor 810, a memory 820, and a transceiver 830. Processor 810 may be configured to implement the functions, processes, and / or methods described herein. Layers of the air interface protocol may be implemented by the processor 810. The memory 820 is connected to the processor 810 and stores various information for driving the processor 810. The transceiver 830 is connected to the processor 810 to transmit and / or receive a radio signal.

The terminal 900 may include a processor 910, a memory 920, and a transceiver 930. Processor 910 may be configured to implement the functions, processes, and / or methods described herein. Layers of the air interface protocol may be implemented by the processor 910. The memory 920 is connected to the processor 910 and stores various information for driving the processor 910. The transceiver 930 is connected to the processor 910 to transmit and / or receive a radio signal.

Processors 810 and 910 may include application-specific integrated circuits (ASICs), other chipsets, logic circuits, and / or data processing devices. The memory 820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium, and / or other storage device. The transceivers 830 and 930 may include a baseband circuit for processing radio frequency signals. When the embodiment is implemented in software, the above-described technique may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in the memory 820, 920 and executed by the processor 810, 910. The memories 820 and 920 may be inside or outside the processors 810 and 910, and may be connected to the processors 810 and 910 by various well-known means.

In the exemplary system described above, methods that may be implemented in accordance with the above-described features of the present invention have been described based on a flowchart. For convenience, the methods have been described as a series of steps or blocks, but the claimed features of the present invention are not limited to the order of steps or blocks, and certain steps may occur in the same order as other steps and in a different order than described above. In addition, those skilled in the art will appreciate that the steps shown in the flowcharts are not exclusive and that other steps may be included or one or more steps in the flowcharts may be deleted without affecting the scope of the present invention.

Claims (15)

1. In the method for calculating the correlation by the base station in a wireless communication system,

   Classifying the plurality of beams into a plurality of beam groups according to the quality of a received signal corresponding to each of the plurality of beams; And

   And applying different correlation calculation intervals to each of the classified plurality of beam groups.
2. The method of claim 1,

   And the plurality of beams are classified into the plurality of beam groups based on a correlation output according to the quality of the received signal.
3. The method of claim 2,

   A first correlation calculation interval applied to a first beam group in which the magnitude of the correlation output is greater than a correlation output threshold is applied to a second beam group in which the magnitude of the correlation output is smaller than the correlation output threshold. And less than a second correlation calculation interval.
4. The method of claim 2,

   Wherein the plurality of beam groups comprise a strong beam group comprising beams having a magnitude of the correlation output greater than a first correlation output threshold, the magnitude of the correlation output being less than the first correlation output threshold and a second correlation A middle beam group comprising a beam larger than the power output threshold and a weak beam group comprising a beam whose magnitude of the correlation output is less than a second correlation power threshold.
5. The method of claim 4, wherein

   And the smallest correlation calculation interval is applied to said strong beam group.
6. The method of claim 5,

   And applying the smallest correlation calculation interval to the strong beam group to calculate a correlation according to the following equation.

   

   Where i is the time index, m is the timing offset, N is the total length of the time signal (or orthogonal frequency division multiplexing (OFDM) symbol length), L is the multipath channel delay length by the PDF (probability distribution function), and Y [ i] denotes a signal received at time i, S [i] denotes a signal transmitted at time i, K _strong denotes the strong beam group, and I _min denotes the smallest correlation calculation interval.
7. The method of claim 5,

   And wherein the smallest correlation calculation interval is one.
8. The method of claim 4, wherein

   And the largest correlation calculation interval is applied to the weak beam group.
9. The method of claim 8,

   And applying the largest correlation calculation interval to the weak beam group, wherein the correlation is calculated according to the following equation.

   

   Where i is the time index, m is the timing offset, N is the total length of the time signal (or OFDM symbol length), L is the multipath channel delay length by PDF, and Y [i] is the signal received at time i, S [i] denotes a signal transmitted at time i, K _weak denotes the weak beam group, and I _max denotes the largest correlation calculation interval.
10. The method of claim 2,

    The correlation output according to the quality of the received signal is obtained through uplink beam scanning.
11. The method of claim 1,

    Calculating a correlation for each of the plurality of beam groups by applying different correlation calculation intervals to each of the plurality of beam groups; And

    And calculating a timing advance (TA) for each beam by using the calculated correlation.
12. The method of claim 11,

    And transmitting timing advances calculated for each beam to each terminal using bits of different lengths for each of the plurality of beam groups.
13. The method of claim 12,

    Characterized in that the length of the bits used to transmit the timing advance of the beam belonging to the strong beam group among the plurality of beam groups is longer than the length of the bits used to transmit the timing advance of the beam belonging to the weak beam group. How to.
14. The method of claim 1,

    The plurality of beam groups are classified by terminal.
15. A base station in a wireless communication system,

    Memory;

    A transceiver; And

    Including a processor connected to the memory and the transceiver,

    The processor,

    Classify the plurality of beams into a plurality of beam groups according to the quality of a received signal corresponding to each of the plurality of beams,

    And apply different correlation calculation intervals to each of the classified plurality of beam groups.

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