WO2016122055A1

WO2016122055A1 - Method and apparatus for performing random access procedure in wireless communication system

Info

Publication number: WO2016122055A1
Application number: PCT/KR2015/006313
Authority: WO
Inventors: Kitae Kim; Jiwon Kang; Kilbom LEE; Kungmin PARK; Heejin Kim
Original assignee: Lg Electronics Inc.
Priority date: 2015-01-28
Filing date: 2015-06-22
Publication date: 2016-08-04

Abstract

A method and apparatus for performing a random access procedure in a wireless communication system is provided. A user equipment (UE) transmits a random access channel (RACH) preamble, which is generated by using a combination of multiple sequences, to an evolved NodeB (eNB), and receives a random access response from the eNB. The UE transmits a radio resource control (RRC) connection request message to the eNB, if a priority value related to the random access response is detected.

Description

METHOD AND APPARATUS FOR PERFORMING RANDOM ACCESS PROCEDURE IN WIRELESS COMMUNICATION SYSTEM

The present invention relates to wireless communications, and more particularly, to a method and apparatus for performing a random access procedure in a wireless communication system.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

In 3GPP LTE, a random access procedure may be performed. A major use of the random access procedure is uplink (UL) initial access and the transmission of a short message. In wideband code division multiple access (WCDMA), initial network access and the transmission of a short message are performed through the random access procedure. In contrast, in 3GPP LTE, a short message is not transmitted through the random access procedure. Furthermore, in 3GPP LTE, a random access channel (RACH) for the random access procedure may be transmitted separately from an UL data transmission channel. More specifically, in 3GPP LTE, the random access procedure may be performed in the following cases.

(1) A UE in radio resource control (RRC) connected state (RRC_CONNECTED), but not uplink-synchronized, needing to send new UL data or control information (e.g. an event-triggered measurement report)

(2) A UE in RRC_CONNECTED, but not uplink-synchronized, needing to receive new downlink (DL) data, and therefore to transmit corresponding acknowledgement/non-acknowledgement (ACK/NACK) in the UL

(3) A UE in RRC_CONNECTED, handing over from its current serving cell to a target cell

(4) For positioning purposes in RRC_CONNECTED, when timing advance is needed for UE positioning

(5) A transition from RRC idle state (RRC_IDLE) to RRC_CONNECTED, for example for initial access or tracking area updates

(6) Recovering from radio link failure

In general, a UE that has not obtained UL synchronization or has not maintained UL synchronization obtains UL timing synchronization using a random access procedure. If the UL synchronization of a UE has been obtained, a base station (BS) may schedule transmission resources with orthogonality. A UE may perform UL transmission only when UL synchronization has been formed between the UE and a BS and receive scheduling for data transmission from the BS. That is, through the random access procedure, a UE that has not been synchronized may perform wireless access using a transmission scheme that is orthogonal or that is not overlapped to a maximum extent.

In general, an RACH preamble transmitted in a random access procedure in order to obtain UL synchronization is managed based on non-coherent detection. Accordingly, a BS may recognize RACH preambles, simultaneously transmitted by a plurality of UEs at a specific point of time, as a collision. A BS needs to detect an RACH preamble transmitted by a specific UE without a collision depending on the service type of a UE or emergency access, and thus the random access procedure of a specific UE needs to be successfully performed. That is, a random access procedure for preventing a collision between RACH preambles needs to be improved.

The present invention provides a method and apparatus for performing a random access procedure. The present invention provides a random access procedure enhancement using multi quality of service (QoS) preamble. The present invention provides a method and apparatus for providing priority access with consideration of a service type of a user equipment (UE) and/or emergency access by enhancing structure of a random access preamble of a conventional random access procedure.

In an aspect, a method for performing, by a user equipment (UE), a random access procedure in a wireless communication system is provided. The method includes transmitting a random access channel (RACH) preamble, which is generated by using a combination of multiple sequences, to an evolved NodeB (eNB), receiving a random access response from the eNB, and transmitting a radio resource control (RRC) connection request message to the eNB, if a priority value related to the random access response is detected.

In another aspect, a user equipment (UE) includes a memory, a transceiver, and a processor coupled to the memory and the transceiver, and configured to control the transceiver to transmit a random access channel (RACH) preamble, which is generated by using a combination of multiple sequences, to an evolved NodeB (eNB), control the transceiver to receive a random access response from the eNB, and control the transceiver to transmit a radio resource control (RRC) connection request message to the eNB, if a priority value related to the random access response is detected.

A random access response can be preferentially assigned to a specific UE without designing an existing random access procedure again.

FIG. 1 shows a wireless communication system.

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

FIG. 3 shows a resource grid for one downlink slot.

FIG. 4 shows structure of a downlink subframe.

FIG. 5 shows structure of an uplink subframe.

FIG. 6 shows a contention-based random access procedure.

FIG. 7 shows an example of a structure of an RACH preamble.

FIG. 8 shows an example of a CAZAC sequence set.

FIG. 9 shows another example of a CAZAC sequence set.

FIG. 10 shows an example of output of a receiver that has receives a sequence within a CAZAC sequence set.

FIG. 11 shows an example of an asymmetric CAZAC sequence set.

FIG. 12 shows another example of an asymmetric CAZAC sequence set.

FIG. 13 shows an example in which RACH preamble collide against each other in a contention-based random access procedure.

FIG. 14 shows time-frequency resources on which a UE transmits an RACH preamble.

FIG. 15 shows an example of a collision between RACH preambles.

FIG. 16 shows an example of a method of performing a random access procedure according to an embodiment of the present invention.

FIG. 17 shows an example of a random access response according to an embodiment of the present invention.

FIG. 18 shows an example when an eNB according to an embodiment of the present invention detects an RACH preamble.

FIG. 19 shows an example of a method for performing a random access procedure according to an embodiment of the present invention.

FIG. 20 shows another example of a method for performing a random access procedure according to an embodiment of the present invention.

FIG. 21 shows another example of a method for performing a random access procedure according to an embodiment of the present invention.

FIG. 22 shows a wireless communication system to implement an embodiment of the present invention.

Techniques, apparatus and systems described herein may be used in various wireless access technologies such as 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), etc. The CDMA may be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. The TDMA may be implemented with a radio technology such as global system for mobile communications (GSM)/general packet radio service (GPRS)/ enhanced data rates for GSM evolution (EDGE). The OFDMA may be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved-UTRA (E-UTRA) etc. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved-UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE employs the OFDMA in downlink (DL) and employs the SC-FDMA in uplink (UL). LTE-advance (LTE-A) is an evolution of the 3GPP LTE. For clarity, this application focuses on the 3GPP LTE/LTE-A. However, technical features of the present invention are not limited thereto.

FIG. 1 shows a wireless communication system. The wireless communication system 10 includes at least one evolved NodeB (eNB) 11. Respective eNBs 11 provide a communication service to particular

geographical areas

15a, 15b, and 15c (which are generally called cells). Each cell may be divided into a plurality of areas (which are called sectors). A user equipment (UE) 12 may be fixed or mobile and may be referred to by other names such as mobile station (MS), mobile terminal (MT), user terminal (UT), subscriber station (SS), wireless device, personal digital assistant (PDA), wireless modem, handheld device. The eNB 11 generally refers to a fixed station that communicates with the UE 12 and may be called by other names such as base station (BS), base transceiver system (BTS), access point (AP), etc.

In general, a UE belongs to one cell, and the cell to which a UE belongs is called a serving cell. An eNB providing a communication service to the serving cell is called a serving eNB. The wireless communication system is a cellular system, so a different cell adjacent to the serving cell exists. The different cell adjacent to the serving cell is called a neighbor cell. An eNB providing a communication service to the neighbor cell is called a neighbor eNB. The serving cell and the neighbor cell are relatively determined based on a UE.

This technique can be used for DL or UL. In general, DL refers to communication from the eNB 11 to the UE 12, and UL refers to communication from the UE 12 to the eNB 11. In DL, a transmitter may be part of the eNB 11 and a receiver may be part of the UE 12. In UL, a transmitter may be part of the UE 12 and a receiver may be part of the eNB 11.

The wireless communication system may be any one of a multiple-input multiple-output (MIMO) system, a multiple-input single-output (MISO) system, a single-input single-output (SISO) system, and a single-input multiple-output (SIMO) system. The MIMO system uses a plurality of transmission antennas and a plurality of reception antennas. The MISO system uses a plurality of transmission antennas and a single reception antenna. The SISO system uses a single transmission antenna and a single reception antenna. The SIMO system uses a single transmission antenna and a plurality of reception antennas. Hereinafter, a transmission antenna refers to a physical or logical antenna used for transmitting a signal or a stream, and a reception antenna refers to a physical or logical antenna used for receiving a signal or a stream.

FIG. 2 shows structure of a radio frame of 3GPP LTE. Referring to FIG. 2, a radio frame includes 10 subframes. A subframe includes two slots in time domain. A time for transmitting one subframe is defined as a transmission time interval (TTI). For example, 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 time domain. Since the 3GPP LTE uses the OFDMA in the DL, the OFDM symbol is for representing one symbol period. The OFDM symbols may be called by other names depending on a multiple-access scheme. For example, when SC-FDMA is in use as a UL multi-access scheme, the OFDM symbols may be called SC-FDMA symbols. A resource block (RB) is a resource allocation unit, and includes a plurality of contiguous subcarriers in one slot. The structure of the radio frame is shown for exemplary purposes only. Thus, the number of subframes included in the radio frame or the number of slots included in the subframe or the number of OFDM symbols included in the slot may be modified in various manners.

The wireless communication system may be divided into a frequency division duplex (FDD) scheme and a time division duplex (TDD) scheme. According to the FDD scheme, UL transmission and DL transmission are made at different frequency bands. According to the TDD scheme, UL transmission and DL transmission are made during different periods of time at the same frequency band. A channel response of the TDD scheme is substantially reciprocal. This means that a DL channel response and a UL channel response are almost the same in a given frequency band. Thus, the TDD-based wireless communication system is advantageous in that the DL channel response can be obtained from the UL channel response. In the TDD scheme, the entire frequency band is time-divided for UL and DL transmissions, so a DL transmission by the eNB and a UL transmission by the UE cannot be simultaneously performed. In a TDD system in which a UL transmission and a DL transmission are discriminated in units of subframes, the UL transmission and the DL transmission are performed in different subframes.

FIG. 3 shows a resource grid for one downlink slot. Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols in time domain. It is described herein that one DL slot includes 7 OFDM symbols, and one RB includes 12 subcarriers in frequency domain as an example. However, the present invention is not limited thereto. Each element on the resource grid is referred to as a resource element (RE). One RB includes 12×7 resource elements. The number N^DL of RBs included in the DL slot depends on a DL transmit bandwidth. The structure of a UL slot may be same as that of the DL slot. The number of OFDM symbols and the number of subcarriers may vary depending on the length of a CP, frequency spacing, etc. For example, in case of a normal cyclic prefix (CP), the number of OFDM symbols is 7, and in case of an extended CP, the number of OFDM symbols is 6. One of 128, 256, 512, 1024, 1536, and 2048 may be selectively used as the number of subcarriers in one OFDM symbol.

FIG. 4 shows structure of a downlink subframe. Referring to FIG. 4, a maximum of three OFDM symbols located in a front portion of a first slot within a subframe correspond to a control region to be assigned with a control channel. The remaining OFDM symbols correspond to a data region to be assigned with a physical downlink shared chancel (PDSCH). Examples of DL control channels used in the 3GPP LTE includes a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), a physical hybrid automatic repeat request (HARQ) indicator channel (PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of a subframe and carries information regarding the number of OFDM symbols used for transmission of control channels within the subframe. The PHICH is a response of UL transmission and carries a HARQ acknowledgment (ACK)/non-acknowledgment (NACK) signal. Control information transmitted through the PDCCH is referred to as downlink control information (DCI). The DCI includes UL or DL scheduling information or includes a UL transmit (Tx) power control command for arbitrary UE groups.

The PDCCH may carry a transport format and a resource allocation of a downlink shared channel (DL-SCH), resource allocation information of an uplink shared channel (UL-SCH), paging information on a paging channel (PCH), system information on the DL-SCH, a resource allocation of an upper-layer control message such as a random access response transmitted on the PDSCH, a set of Tx power control commands on individual UEs within an arbitrary UE group, a Tx power control command, activation of a voice over IP (VoIP), etc. A plurality of PDCCHs can be transmitted within a control region. The UE can monitor the plurality of PDCCHs. The PDCCH is transmitted on an aggregation of one or several consecutive control channel elements (CCEs). The CCE is a logical allocation unit used to provide the PDCCH with a coding rate based on a state of a radio channel. The CCE corresponds to a plurality of resource element groups.

A format of the PDCCH and the number of bits of the available PDCCH are determined according to a correlation between the number of CCEs and the coding rate provided by the CCEs. The eNB determines a PDCCH format according to a DCI to be transmitted to the UE, and attaches a cyclic redundancy check (CRC) to control information. The CRC is scrambled with a unique identifier (referred to as a radio network temporary identifier (RNTI)) according to an owner or usage of the PDCCH. If the PDCCH is for a specific UE, a unique identifier (e.g., cell-RNTI (C-RNTI)) of the UE may be scrambled to the CRC. Alternatively, if the PDCCH is for a paging message, a paging indicator identifier (e.g., paging-RNTI (P-RNTI)) may be scrambled to the CRC. If the PDCCH is for system information, a system information identifier and a system information RNTI (SI-RNTI) may be scrambled to the CRC. To indicate a random access response that is a response for transmission of a random access preamble of the UE, a random access-RNTI (RA-RNTI) may be scrambled to the CRC.

FIG. 5 shows structure of an uplink subframe. Referring to FIG. 5, a UL subframe can be divided in a frequency domain into a control region and a data region. The control region is allocated with a physical uplink control channel (PUCCH) for carrying UL control information. The data region is allocated with a physical uplink shared channel (PUSCH) for carrying user data. When indicated by a higher layer, the UE may support a simultaneous transmission of the PUSCH and the PUCCH. The PUCCH for one UE is allocated to an RB pair in a subframe. RBs belonging to the RB pair occupy different subcarriers in respective two slots. This is called that the RB pair allocated to the PUCCH is frequency-hopped in a slot boundary. This is said that the pair of RBs allocated to the PUCCH is frequency-hopped at the slot boundary. The UE can obtain a frequency diversity gain by transmitting UL control information through different subcarriers according to time.

UL control information transmitted on the PUCCH may include a HARQ ACK/NACK, a channel quality indicator (CQI) indicating the state of a DL channel, a scheduling request (SR), and the like. The PUSCH is mapped to a UL-SCH, a transport channel. UL data transmitted on the PUSCH may be a transport block, a data block for the UL-SCH transmitted during the TTI. The transport block may be user information. Or, the UL data may be multiplexed data. The multiplexed data may be data obtained by multiplexing the transport block for the UL-SCH and control information. For example, control information multiplexed to data may include a CQI, a precoding matrix indicator (PMI), an HARQ, a rank indicator (RI), or the like. Or the UL data may include only control information.

A random access procedure may be divided into a contention-based random access procedure and a non-contention-based random access procedure. First, in the contention-based random access procedure, different UEs are permitted to simultaneously access an eNB using the same RACH preamble. Accordingly, a contention may occur. In order to process such a contention, an additional contention resolution step is required.

FIG. 6 shows a contention-based random access procedure.

At step S60, a UE transmits an RACH preamble to an eNB. The RACH preamble may be called a "message 1". The RACH preamble may include an RA-RNTI. The RA-RNTI may be determined as (1+t_id+10*f_id). t_id is the index of the first subframe of the specified physical random access channel (PRACH) (0≤t_id<10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≤f_id<6). The eNB may obtain the RA-RNTI by decoding the RACH preamble.

At step S61, the eNB transmits a random access response to the UE. The random access response may be called a "message 2". The random access response may include the RA-RNTI obtained by decoding the RACH preamble by the eNB, a TA, a temporary C-RNTI, and a resource block assignment (i.e., an UL grant for an L2/L3 message). The UE may obtain the resource block assignment and a modulation and coding scheme (MCS) configuration by decoding the random access response. The eNB may be configured to receive an RRC connection request message through the DCI format 0.

At step S62, the UE transmits an L2/L3 message, that is, an RRC connection request message, to the eNB. The RRC connection request message may be called a "message 3". The UE may transmit the RRC connection request message using the temporary C-RNTI obtained from the random access response.

At step S63, if the RRC connection request message transmitted by the UE has been successfully decoded, the eNB transmits HARQ ACK to the corresponding UE. Accordingly, the UE may be aware that the random access procedure has been successful. Such a process is called a contention resolution process. More specifically, the eNB transmits an RRC connection setup message to the UE using the temporary C-RNTI in response to the RRC connection request message. The RRC connection setup message may be called a "message 4". The RRC connection setup message may include a C-RNTI. From this point of time, the UE and the eNB may exchange messages using the C-RNTI.

If the UE has not received the HARQ ACK, the UE may return to step S60 and transmit the RACH preamble to the eNB.

In a non-contention-based random access procedure, a contention is not permitted for a reason of timing restriction. An eNB may indicate that each UE has to transmit which RACH preamble when. To this end, a UE has to be in a connected state (RRC_CONNECTED) prior to the random access procedure. For example, a non-contention-based random access procedure may be performed during handover. In the non-contention-based random access procedure, first, an eNB transmits an RACH preamble assignment to a UE. The UE transmits an RACH preamble, including an RA-RNTI and an indication of the size of an L2/L3 message, to the eNB based on the received RACH preamble assignment. The eNB that has received the RACH preamble transmits a random access response, including a TA, a C-RNTI, and an UL grant for an L2/L3 message, to the UE. Accordingly, the non-contention-based random access procedure may be completed.

FIG. 7 shows an example of a structure of an RACH preamble. Referring to FIG. 7, the RACH preamble consists of a CP, a sequence, and a guard time (GT). The CP absorbs maximum channel delay spread and a round trip time (RTT), and the GT absorbs the RTT. The CP is generated by inserting the latter part of an existing OFDM symbol into the CP interval of the RACH preamble. Accordingly, a periodic correlation in an eNB that receives the RACH preamble may be possible. A UE may transmit an RACH preamble, assuming that it has been synchronized with the eNB (i.e., a timing advance (TA) = 0ms). Accordingly, an RACH preamble transmitted by a UE close to the eNB is received by the eNB without a delay, and an RACH preamble transmitted by a UE distant from the eNB is received by the eNB with a propagation delay. In this case, the eNB may perform a random access procedure depending on the location where the RACH preamble transmitted by each UE was detected through a periodic correlation because it is aware of a sequence transmitted by each UE.

Several sequences may be used for an RACH preamble. Representatively, an auto-correlation-based Zadoff-Chu (ZC) sequence and a cross-correlation-based pseudo-random sequence may be used for an RACH preamble. In general, the auto-correlation-based sequence may be used in a situation in which intra-cell interference is small, and the cross-correlation sequence may be used in a situation in which intra-cell interference is great. In 3GPP LTE, a ZC sequence of 839 in length may be used for an RACH preamble. The ZC sequence used for the RACH preamble may satisfy the following conditions.

- Intra-cell interference between different RACH preambles using the same frequency-time RACH resources is relatively small.

- Intra-cell interference may be optimized depending on the size of a cell. That is, in order to improve detection performance of an eNB, more orthogonal preambles may be generated for a smaller cell.

- Detection performance is improved as the number of orthogonal preambles is increased. (3GPP LTE is 64 signatures, and WCDMA is 16 signatures)

- Detection complexity of an eNB is relatively small.

- A high-speed UE can be supported.

When intra-cell interference between signatures is great, a PN sequence may be used for an RACH preamble.

A constant amplitude zero auto-correlation (CAZAC) sequence is described below.

FIG. 8 shows an example of a CAZAC sequence set. Referring to FIG. 8, each sequence within the CAZAC sequence set has a different cyclic shifting (0, 1, 2, or 3). Furthermore, each sequence within the CAZAC sequence set is mapped to specific bits. In this case, each sequence becomes a single signature having specific information. Referring to FIG. 8, [0 0] is mapped to a sequence having a cyclic shifting of 0, [0 1] is mapped to a sequence having a cyclic shifting of 1, [1 0] is mapped to a sequence having a cyclic shifting of 2, and [1 1] is mapped to a sequence having a cyclic shifting of 3, respectively. That is, in FIG. 8, a transmitter may transmit information of 2 bits.

When the transmitter transmits a single sequence within a CAZAC sequence set, a receiver may identify the sequence and determine information mapped to a corresponding sequence. For example, when the receiver identifies the sequence having the cyclic shifting of 0, it may be aware that the transmitter has transmitted information of [0 0]. Assuming an additive white Gaussian noise　(AWGN) channel and an environment not including a noise, the receiver may identify a transmitted sequence by finding an output value having the greatest size.

Performance of the CAZAC sequence set defined in FIG. 8 may be greatly deteriorated in a multi-path environment. In order to solve such a problem, in commercialized systems, such as 3GPP LTE, a CAZAC sequence set may be defined by taking into consideration a zero-correlation zone (ZCZ), that is, a valid delay period.

FIG. 9 shows another example of a CAZAC sequence set. Referring to FIG. 9, each sequence within the CAZAC sequence set has a different cyclic shifting (0, 4, 8, or 12). That is, compared to the CAZAC sequence set of FIG. 8, each sequence within the CAZAC sequence set of FIG. 9 is generated by setting an interval between cyclic shiftings to 4. In this case, the interval may be determined by a channel valid delay period L (i.e., the location of the last tap of a channel in a time axis).

FIG. 10 shows an example of output of a receiver that has receives a sequence within a CAZAC sequence set. FIG. 10 corresponds to the output of the receiver that has received a sequence having a cyclic shifting of 0 in FIG. 9, assuming an environment not including a receiver noise. Referring to FIG. 10, the size W of a ZCZ may be determined to a maximum of cyclic shifting (=L) of a sequence. A receiver first selects the greatest output value y_i in each of ZCZs one by one, performs a comparison on the selected output values y_i, and selects a ZCZ having the greatest output value. That is, the receiver may select the first ZCZ. Four output values {y₀, y₁, y₂, y₃} having different sizes are present in the first ZCZ unlike in other ZCZs, and this has been generated due to the influence of delay spread. However, the receiver may identify a sequence transmitted by a transmitter by setting the size W of a ZCZ greater than the channel valid delay period L.

Meanwhile, more signatures may be defined as the interval of cyclic shiftings between sequences is reduced. For example, in FIG. 9, if the interval between cyclic shiftings is set to 1, a total of 16 signatures may be defined.

A multi quality of service (QoS) sequence is described below. The multi-QoS sequence may be generated based on an asymmetric CAZAC sequence set. The asymmetric CAZAC sequence set means a CAZAC sequence set having different intervals of cyclic shiftings between sequences in a time domain. In contrast, the CAZAC sequence set described in FIG. 8 and FIG. 9 may be called a symmetric CAZAC sequence set because the interval of cyclic shiftings between sequences is the same, that is, 1 and 4. In an asymmetric CAZAC sequence set, bits may be mapped to each sequence so that sequences having a small difference between cyclic shiftings share a specific bit. Furthermore, information having higher importance may be mapped to a shared bit.

FIG. 11 shows an example of an asymmetric CAZAC sequence set. Referring to FIG. 11, each sequence within a CAZAC sequence set has a different cyclic shifting (0, 3, 8, or 11), and the interval of cyclic shiftings between sequences is not the same (3 and 5). In this case, sequences having a small difference between cyclic shiftings may be grouped into a single group. That is, in FIG. 11, sequences having cyclic shiftings of 0 and 3 are grouped into a group A, and sequences having cyclic shiftings of 8 and 11 are grouped into a group B. Furthermore, in each group, bits may be mapped to each sequence so that the first bits are the same and the second bits are different. In FIG. 11, [0] is mapped to the sequences of the group A as the first bit, and [1] is mapped to the sequences of the group B as the first bit. As a result, in FIG. 11, [0 0] is mapped to a sequence having a cyclic shifting of 0, [0 1] is mapped to a sequence having a cyclic shifting of 3, [1 1] is mapped to a sequence having a cyclic shifting of 8, and [1 0] is mapped to a sequence having a cyclic shifting of 11, respectively.

If an asymmetric CAZAC sequence set is configured as described above, the interval of cyclic shifts between sequences within each group is 3, whereas the interval of cyclic shifts between groups is 5. In this case, a probability that a sequence having a cyclic shifting of 3 may be mistaken for a sequence having a cyclic shifting of 0 is higher than a probability that it may be mistaken for a sequence having a cyclic shifting of 8. That is, a probability that an error may occur in the first bit shared within a group is smaller than a probability that an error may occur in the second bit not shared within a group. Accordingly, information having higher importance may be mapped to the first bit, and information having relatively lower importance may be mapped to the second bit.

As a result, the asymmetric CAZAC sequence set is generated by making different the interval of cyclic shifts between sequences. Accordingly, a mistaken probability within a group may be different from a mistaken probability between groups. Information having different importance, that is, different QoS, is mapped based on a mistaken probability within a group and between groups, and thus a multi-QoS sequence may be finally transmitted.

The following information is an example of information having high importance.

- A packet ID: a UE reads a packet ID and determines whether a currently received packet is its own packet. If the received packet is not its own packet, the UE no longer decode the packet and may reduce power by discarding the received packet. That is, if a packet ID is erroneously determined, a system yield may be greatly reduced because the packet itself is lost. Accordingly, a packet ID may be taken as information having high importance.

- A basic service set ID (BSSID) of a Wi-Fi system: In the next-generation Wi-Fi system, a dense environment including many BSSs may be supported. In this case, when a UE reads a BSSID and recognizes that the transmission of a packet is generated within its own BSS, the UE may delay the transmission of its own packet although a channel is determined to be idle. The reason for this is that if its own packet is transmitted, decoding may be difficult due to a collision problem because an AP, that is, a recipient, has to receive a plurality of packets. Accordingly, a BSSID may be taken into consideration as information having high importance.

- Bandwidth information: For example, if information of 2 bits is mapped to each sequence within a CAZAC sequence set as in the aforementioned embodiment, bandwidth information of 20 MHz, 40 MHz, 80 MHz, or 160 MHz may be mapped.

For example, information having low importance may include the number of Tx antennas or the location of an enhanced PDCCH (ePDCCH). In general, this information may be detected using a blind decoding method. However, blind decoding needs to be performed several times, and this may increase the latency and complexity of a system. If this information is transmitted through a signaling method, the disadvantages may be reduced or obviated. Furthermore, if information transmitted through a signaling method is not matched, it may be detected again using an existing blind decoding method. That is, since an error can be restored, an influence attributable to a failure in the transfer of the information is relatively small. As a result, information that may be restored may be classified as information having low importance.

FIG. 12 shows another example of an asymmetric CAZAC sequence set. Referring to FIG. 12, four sequences A, B, C, and D each having a length of N=6W form an asymmetric CAZAC sequence set. The sequences A/B form a single group, and the sequences C/D form another group. The interval of cyclic shifts within each group is W, and the interval of cyclic shifts between the group is 2W. In this case, it may be seen that a probability that a sequence may be mistaken for one of two neighbor sequences is different. For example, a probability that a sequence B may be mistaken for a sequence A is greater than a probability that the sequence B may be mistaken for a sequence C. Accordingly, bits may be mapped to each sequence so that sequences within a group having a small interval between cyclic shifts share a specific bit. In FIG. 12, the first bit mapped to a sequence becomes a shared bit within each group, and the second bit becomes a bit not shared within each group. [0] is mapped to the sequences A/B as the first bit, and [1] is mapped to the sequences C/D as the first bit. Furthermore, information having high importance may be mapped to a bit shared within a group, and information having low importance may be mapped to a bit not shared within a group. For example, a cell ID having high importance may be mapped to a bit shared within a group, and control channel information having low importance may be mapped to a bit not shared within a group.

A method of generating a multi-QoS sequence from the aforementioned asymmetric CAZAC sequence set is as follows. The sequence of each group may be represented by Equation 1 below. In Equation 1, s_i(j) indicates the i-th sequence (i=0, 1, 2, ...) of a j-th group (j=1, 2, 3, ...).

A multi-QoS sequence may be generated by Equation 2 below based on the sequence of each group expressed in Equation 1.

Referring to Equation 2, the multi-QoS sequence is calculated by assigning weight α_j to the same i-th sequence of each sequence group. For example, assuming that the first sequence of a first group is s₀ ⁽¹⁾=[s₀,s₁,s₂,s₃,...,s₁₉]^T, the first sequence of a second group is s₀ ⁽²⁾=[s₁₈,s₁₉,s₀,s₁,...,s₁₇]^T, and weight α₁=α₂=1 of each sequence, a multi-QoS sequence s finally transmitted by a UE may be generated by Equation 3 below.

A collision between RACH preambles in a random access procedure is described below.

FIG. 13 shows an example in which RACH preamble collide against each other in a contention-based random access procedure. All UEs use the contention-based random access procedure described above in FIG. 6 in order to obtain UL synchronization and may transmit its own RACH preamble using specific time-frequency resources. In this case, if two or more RACH preambles are transmitted by a plurality of UEs using identical time-frequency resources, a collision occurs when an eNB detects RACH preambles. That is, in FIG. 13, if the RACH preamble of a UE #1 and the RACH preamble of a UE #2 are transmitted through the same time-frequency resources, a collision occurs between the RACH preambles. Accordingly, a corresponding UE is unable to receive a random access response to the RACH preamble.

An eNB that has received an RACH preamble may transmit a random access response after 3 ms. However, the size of ra- ResponseWindowSize of an RACH -ConfigCommon information element (IE) is set to 2~10 ms. Accordingly, a difference of a maximum of 12 ms may be generated between the reception of the RACH preamble and the transmission of the random access response. As a result, a UE may recognize whether RACH preambles collide against each other even after a maximum of 12 ms, waits for the reception of the random access response during a maximum of 12 ms, and retransmits the RACH preamble. If the UE #1 and the UE #2 that retransmit the RACH preambles due to a collision between the RACH preambles use the same RACH preamble, a collision may occur again.

FIG. 14 shows time-frequency resources on which a UE transmits an RACH preamble. In FIG. 14, it is assumed that PRACH resources according to a PRACH configuration index 6 is set in a frame structure type 1, that is, in an FDD frame. An area checked in FIG. 14 is a PRACH resource region, that is, an area in which the UE maps a sequence for the RACH preamble and transmits the sequence. If a UE #1 and a UE #2 select the same preamble sequence, select the same PRACH resource region, and transmit RACH preambles, a collision occurs between the RACH preambles.

FIG. 15 shows an example of a collision between RACH preambles. Referring to FIG. 15, two correlation peaks are detected in a single ZCZ. Accordingly, an eNB recognizes a collision assuming that two or more UEs have selected the same RACH preamble.

In a situation in which RACH preambles transmitted by two or more UEs collide against each other as described above, a random access procedure needs to be improved so that an RACH preamble transmitted by a specific UE that wants to obtain UL synchronization is preferentially detected. For example, if a specific UE retransmits an RACH preamble, although a collision occurs in the corresponding RACH preamble, an eNB may detect the retransmitted RACH preamble and transmit a random access response to the corresponding UE. Alternatively, if an RACH preamble transmitted by a specific UE corresponds to a specific service, such as emergency access, although a collision occurs in the RACH preamble, an eNB may preferentially detect the RACH preamble and transmit a random access response to the UE that has transmitted the corresponding RACH preamble.

Hereinafter, a method of performing a random access procedure according to an embodiment of the present invention is described. In accordance with an embodiment of the present invention, there may be proposed a method of selecting, by an eNB, an RACH preamble transmitted by a specific UE and transmitting a random access response to only the specific UE that has transmitted the RACH preamble when RACH preambles transmitted by a plurality of UEs collide against each other. More specifically, in accordance with an embodiment of the present invention, an asymmetric CAZAC sequence set and a multi-QoS sequence based on the asymmetric CAZAC sequence set tare defined, and an RACH preamble may be newly designed and generated based on the multi-QoS sequence. Accordingly, the RACH preamble of an existing random access procedure can be improved. An eNB may detect an RACH preamble based on a multi-QoS sequence in addition to the detection of an existing RACH preamble. In particular, the eNB may preferentially detect the RACH preamble based on the multi-QoS sequence.

At step S100, a UE #1 transmits an RACH preamble to an eNB using a multi-QoS sequence. That is, the UE #1 transmits the RACH preamble generated based on a combination of sequences. The RACH preamble using the multi-QoS sequence may correspond to a specific service type, such as emergency access, or may correspond to a retransmitted RACH preamble. That is, a UE that tries to preferentially obtain UL synchronization may generate a new RACH preamble obtained by combining a sequence of another group with a sequence for an existing RACH preamble and transmit the new RACH preamble. At step S101, a UE #2 transmits an existing RACH preamble not using a multi-QoS sequence to the eNB. It is assumed that the RACH preambles transmitted by the UE #1 and the UE #2 are transmitted through the same time-frequency resources.

The eNB detects the transmitted RACH preambles and may be aware that the new RACH preamble transmitted by the UE #1 collides against the existing RACH preamble transmitted by the UE #2. Furthermore, the eNB may detect the RACH preamble based on the multi-QoS sequence and may be aware of the existence of the UE #1 that tries to preferentially obtain UL synchronization. In this case, the eNB is unable to be aware that which UE has transmitted the RACH preamble based on the multi-QoS sequence. That is, the RACH preamble based on the multi-QoS sequence may be used as only information that notifies the eNB that there is a UE trying to preferentially receive a random access response although there is a collision between RACH preambles.

At step S110, the eNB transmits a random access response. If RACH preambles transmitted by a plurality of UEs collide against each other and an RACH preamble which is based on a multi-QoS sequence is detected, the eNB may configure the random access response so that it includes a legacy random access preamble identifier (RAPID) and an additional priority value field. That is, the eNB may transmit a new random access response to which a priority value field has been added in addition to an existing random access response for the UE #1 that has transmitted the RACH preamble based on the multi-QoS sequence. In this case, the priority value field indicates that the corresponding random access response is a random access response for the UE #1 that has transmitted the RACH preamble based on the multi-QoS sequence.

FIG. 17 shows an example of a random access response according to an embodiment of the present invention. It is assumed that the legacy RAPID of an RACH preamble transmitted by a UE is 15. Referring to FIG. 17, the random access response according to an embodiment of the present invention includes a legacy RAPID=15 and further includes a priority value field for a UE that tries to preferentially obtain UL synchronization. That is, the priority value field indicates that a corresponding random access response is a random access response for the UE that tries to preferentially obtain UL synchronization.

Referring back to FIG. 16, all of UEs that have transmitted the RACH preamble of the same legacy RAPID may receive random access responses. That is, if the UE #1 and the UE #2 have transmitted the RACH preambles having the same legacy RAPID, both the UE #1 and the UE #2 may receive random access responses having the corresponding legacy RAPID. In this case, only the UE #1 that has transmitted the RACH preamble based on the multi-QoS sequence may check the legacy RAPID and the newly added priority value field included in the random access response.

At step S120, the UE #1 that has transmitted the RACH preamble based on the multi-QoS sequence checks the legacy RAPID and priority value field in the random access response and transmits an RRC connection request message to the eNB. The UE #2 that has transmitted the existing RACH preamble retransmits the RACH preamble without transmitting an RRC connection request message.

At step S130, the UE #1 receives an RRC connection setup message indicating that a contention resolution has been successfully performed from the eNB.

In the aforementioned description, the priority value according to an embodiment of the present invention has been illustrated as being inserted into a random access response as an additional field, but the present invention is not limited thereto. The priority value may be delivered in various ways. For example, the priority value may be delivered through the modulation of a physical channel/signal through which a random access response is transmitted. For example, if a random access response to an RACH preamble based on a multi-QoS sequence is to be transmitted, when a PDCCH including information about the assignment of PDSCH resources in which the random access response is transmitted is configured, CRC scrambling may be performed based on a value obtained by combining an RA-RNTI and a priority value instead of performing CRC scrambling based on the RA-RNTI only. Alternatively, if a random access response to an RACH preamble based on a multi-QoS sequence is to be transmitted, a PDSCH in which the random access response is transmitted or a PDCCH including information about the assignment of corresponding resources is configured, a physical signal itself may be modulated into a predetermined pattern. For example, a signal may be modulated using a method of inverting the phase of a physical signal or multiplying a predetermined sequence (or code) so that a UE recognizes a priority value.

A method of detecting, by an eNB, an RACH preamble according to an embodiment of the present invention is described in detail below. The eNB may detect the final reception output by Equation 4 below. In Equation 4, N_ZC is the length of a signal or sequence, r is a reception vector in a time axis, s⁽ⁱ⁾ is i-th cyclically shifted sequence vector, and y_i is the final reception output.

For an RACH preamble based on a multi-QoS sequence, unlike in an existing RACH preamble, several peaks are detected pair-wise if a channel delay is not generated. For example, it is assumed that a sequence length N_ZC=839, cyclic shift lengths N_CS=119, N_off,2=20, and N_off,3=30, and the UE #1 generates the final RACH preamble by summing the fourth sequences of first, second, and third sequence groups. The final RACH preamble transmitted by the UE #1 may be expressed by Equation 5 below.

In this case, it is assumed that a ratio of the transmission power of sequences is set to 3:1:2. Accordingly, assuming that total transmission power is P, weight α₁, α₂, and α₃ for the respective sequences may be set by Equation 6 below.

It is assumed that the UE #2 transmits a conventional RACH preamble based on only the fourth sequence of a first sequence group. Furthermore, it is assumed that a delay is generated in the channel of the UE #2 and has a size of τ_D=10. The RACH preamble transmitted by the UE #2 may be expressed by Equation 7 below.

FIG. 18 shows an example when an eNB according to an embodiment of the present invention detects an RACH preamble. RACH preambles transmitted by the UE #1 and the UE #2 may be detected in the same ZCZ because they include the same legacy RAPID. Referring to FIG. 18, an RACH preamble S_A transmitted by the UE #1 is detected as having three peaks within the area of the window size N_CS=119 of a fourth ZCZ, and an RACH preamble S_B transmitted by the UE #2 is detected as a single peak with a channel delay of τ_D=10. The eNB may be aware that there is a UE that tries to preferentially obtain UL synchronization because the eNB has detected the RACH preamble having three peaks. Accordingly, the eNB may transmit a random access response, including a legacy RAPID and a priority value field, for a corresponding UE. The eNB is unable to be aware that which UE has transmitted an RACH preamble based on a multi-QoS sequence, but the UE #1 that has received the random access response may be aware that the received random access response is its own random access response by detecting the priority value field.

In step S200, the UE transmits a preamble, which is generated by using a combination of multiple sequences, to the eNB. The method may further comprise generating the combination of multiple sequences. Each of the multiple sequences may belong to different sequence groups, respectively. Further, the different sequence groups may have different cyclic shifting from each other, respectively. The combination of multiple sequences may correspond to sum of the multiple sequences. The combination of multiple sequences may corresponds to sum of the multiple sequences with a weight value for each of the multiple sequences. The preamble may be a RACH preamble for an emergency access, or a retransmitted RACH preamble.

In step S210, the UE receives a random access response from the eNB. The random access response may include a priority value field. A value of the priority value field may be 1. The priority value may be included in the priority value field. The random access response may further include a RAPID.

In step S220, the UE transmits a RRC connection request message to the eNB if a priority value related to the random access response is detected. The priority value related to the random access response may be the priority value included in the priority value field of the random access response. For example, upon detecting the priority value field, the UE may transmit an RRC connection request message to the eNB. The UE may further receive a RRC connection setup message from the eNB.

In step S300, the eNB receives a first preamble which is generated by using a combination of multiple sequences and a second preamble which is generated by using one sequence. The eNB may not know which UE transmits which preamble.

In step S310, the eNB detects that the first preamble and the second preamble are collided with each other.

In step S320, the eNB transmits a random access response indicating priority access of a UE which has transmitted the first preamble. The priority access may be indicated by a priority value. The priority access may be indicated by a priority value field included in the random access response.

In step S400, the first UE transmits a first preamble which is generated by using a combination of multiple sequences. In step S401, the second UE transmits a second preamble which is generated by using one sequence.

In step S410, the eNB detects that the first preamble and the second preamble are collided with each other. Further, the eNB detects the first preamble which is generated by using the combination of multiple sequences. The eNB may not know which UE transmits which preamble.

In step S420, the eNB transmits a random access response indicating priority access of a UE which has transmitted the first preamble, i.e. the first UE. The priority access may be indicated by a priority value. The priority access may be indicated by a priority value field included in the random access response. Both the first UE and the second UE receive the random access response, and only the first UE, which has transmitted the first preamble, may keep performing the random access procedure. The second UE, which has transmitted the second preamble, may retransmit a preamble.

In step S430, the first UE transmits a RRC connection request to the eNB. In step S440, the eNB transmits a RRC connection setup to the first UE.

An eNB 800 may include a processor 810, a memory 820 and a transceiver 830. The processor 810 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 810. The memory 820 is operatively coupled with the processor 810 and stores a variety of information to operate the processor 810. The transceiver 830 is operatively coupled with the processor 810, and transmits and/or receives a radio signal.

A UE 900 may include a processor 910, a memory 920 and a transceiver 930. The processor 910 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 910. The memory 920 is operatively coupled with the processor 910 and stores a variety of information to operate the processor 910. The transceiver 930 is operatively coupled with the processor 910, and transmits and/or receives a radio signal.

The

processors

810, 910 may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The

memories

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, 930 may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in

memories

820, 920 and executed by

processors

810, 910. The

memories

820, 920 can be implemented within the

processors

810, 910 or external to the

processors

810, 910 in which case those can be communicatively coupled to the

processors

810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.

Claims

A method for performing, by a user equipment (UE), a random access procedure in a wireless communication system, the method comprising:

transmitting a random access channel (RACH) preamble, which is generated by using a combination of multiple sequences, to an evolved NodeB (eNB);

receiving a random access response from the eNB; and

transmitting a radio resource control (RRC) connection request message to the eNB, if a priority value related to the random access response is detected.
The method of claim 1, wherein each of the multiple sequences belongs to different sequence groups, respectively.
The method of claim 2, wherein the different sequence groups have different cyclic shifting from each other, respectively.
The method of any one of claims 1 to 3, wherein the combination of multiple sequences corresponds to a sum of the multiple sequences.
The method of any one of claims 1 to 4, wherein the combination of multiple sequences corresponds to a sum of the multiple sequences with a weight value for each of the multiple sequences.
The method of any one of claims 1 to 5, wherein the priority value is included in the random access response.
The method of any one of claims 1 to 6, wherein the random access response includes a priority value field and the priority value is included in the priority value field.
The method of any one of claims 1 to 7, wherein the priority value has a value of 1.
The method of any one of claims 1 to 8, wherein the RACH preamble is a RACH preamble for an emergency access.
The method of any one of claims 1 to 9, wherein the RACH preamble is a retransmitted RACH preamble.
The method of any one of claims 1 to 10, wherein the random access response includes a random access preamble identifier (RAPID).
The method of any one of claims 1 to 11, further comprising receiving a RRC connection setup message from the eNB.
The method of any one of claims 1 to 12, wherein the priority value indicates that the received random access response is a random access response for the UE that has transmitted the RACH preamble, which is generated by using the combination of multiple sequences.
The method of any one of claims 1 to 13, further comprising generating the combination of multiple sequences.
15. The method of any one of claims 1 to 14, wherein the step of transmitting the RRC connection request message comprises transmitting the RRC connection request message to the eNB, upon detecting a priority value field included in the random access response.
A user equipment (UE) comprising:

a memory;

a transceiver; and

a processor coupled to the memory and the transceiver, and configured to:

control the transceiver to transmit a random access channel (RACH) preamble, which is generated by using a combination of multiple sequences, to an evolved NodeB (eNB);

control the transceiver to receive a random access response from the eNB; and

control the transceiver to transmit a radio resource control (RRC) connection request message to the eNB, if a priority value related to the random access response is detected.
The UE of claim 16, wherein each of the multiple sequences belongs to different sequence groups, respectively.
The UE of claim 17, wherein the different sequence groups have different cyclic shifting from each other, respectively.
The UE of any one of claims 16 to 18, wherein the combination of multiple sequences corresponds to a sum of the multiple sequences.
The UE of any one of claims 16 to 19, wherein the combination of multiple sequences corresponds to a sum of the multiple sequences with a weight value for each of the multiple sequences.
The UE of any one of claims 16 to 20, wherein the priority value indicates that the received random access response is a random access response for the UE that has transmitted the RACH preamble, which is generated by using the combination of multiple sequences.
The UE of any one of claims 16 to 21, wherein the processor is further configured to generate the combination of multiple sequences.
The UE of any one of claims 16 to 22, wherein the processor is further configured to control the transceiver to transmit the RRC connection request message to the eNB, upon detecting a priority value field included in the random access response.