Radio base station and random access method

Abstract

To provide an improved apparatus and method for executing a random access procedure between user equipment and a radio base station via an unlicensed cell.SOLUTION: In a mobile communication system, a base station device has a transmitter that transmits information on a random access preamble sequence to user equipment, and a receiver that receives the random access preamble sequence selected and transmitted by the user equipment on the basis of the information. The random access preamble sequence is transmitted with the frequency band width of an unlicensed band, the frequency band width being defined by the length of the random access preamble sequence and a sub carrier interval, and larger than a frequency band with which the random access preamble sequence is transmitted in a licensed band.SELECTED DRAWING: Figure 15B

Description

[Technical field]

[0001] The present disclosure relates to a method for performing a random access procedure between a user equipment and a radio base station in a mobile communication system. Further, the present disclosure provides a user equipment and a radio base station involved in the method(s) described herein. [Background technology]

[0002] Long Term Evolution (LTE) The third generation of mobile communication systems (3G), based on WCDMA radio access technology, is being deployed on a wide scale around the world. As a first step in enhancing or evolving this technology, High Speed ​​Downlink Packet Access (HSDPA) and Enhanced Uplink (also called High Speed ​​Uplink Packet Access (HSUPA)) have been introduced, providing a highly competitive radio access technology.

[0003] To meet the ever-increasing demands of users and to ensure competitiveness against new radio access technologies, 3GPP has introduced a new mobile communications system called Long Term Evolution (LTE), which is designed to accommodate carriers' demands for high-speed transmission of data and media, as well as high-capacity voice support, for the next decade.

[0004] The LTE (Long Term Evolution) work item (WI) specifications, called E-UTRA (Evolved UMTS Terrestrial Radio Access (UTRA)) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network (UTRAN), will be published eventually as Release 8 (LTE Release 8). The LTE system is a packet-based efficient radio access and radio access network that provides full IP-based functionality with low latency and low cost. LTE specifies multiple scalable transmission bandwidths (e.g., 1.4 MHz, 3.0 MHz, 5.0 MHz, 10.0 MHz, 15.0 MHz, and 20.0 MHz) to achieve flexible system deployment with a given spectrum. Orthogonal Frequency Division Multiplexing (OFDM)-based radio access is adopted for the downlink. Such radio access is inherently less susceptible to multipath interference (MPI) due to its low symbol rate, uses cyclic prefix (CP), and can accommodate various transmission bandwidth configurations. Single-Carrier Frequency Division Multiple Access (SC-FDMA) based radio access is adopted for the uplink because providing a large coverage area is prioritized over increasing peak data rates, given the limited transmission power of the user equipment (UE). In LTE Release 8 / 9, a number of key packet radio access technologies are adopted, such as multiple-input multiple-output (MIMO) channel transmission technology, to achieve a highly efficient control signaling structure.

[0005] LTE Architecture Figure 1 shows the overall architecture of LTE. E-UTRAN consists of eNodeBs, which terminate the E-UTRA user plane (PDCP / RLC / MAC / PHY) and control plane (RRC) protocols towards the user equipment (UE). The eNodeB (eNB) hosts the physical (PHY), medium access control (MAC), radio link control (RLC) and packet data control protocol (PDCP) layers, which include the user plane header compression and encryption functions. The eNB also provides the radio resource control (RRC) functions corresponding to the control plane. The eNB performs many functions, such as radio resource management, admission control, scheduling, enforcing uplink quality of service (QoS) through negotiation, broadcasting cell information, encrypting / decrypting user and control plane data, and compressing / decompressing downlink / uplink user plane packet headers. Multiple eNodeBs are connected to each other by the X2 interface.

[0006] The eNodeBs are also connected to the EPC (Evolved Packet Core) by the S1 interface, more specifically to the MME (Mobility Management Entity) by the S1-MME and to the Serving Gateway (SGW) by the S1-U. The S1 interface supports a many-to-many relationship between the MME / Serving Gateway and the eNodeBs. The SGW routes and forwards user data packets while acting as a mobility anchor for the user plane during handovers between eNodeBs and as an anchor for mobility between LTE and other 3GPP technologies (terminating the S4 interface and relaying traffic between 2G / 3G systems and the PDN GW). For idle user equipment, the SGW terminates the downlink data path and triggers paging when downlink data arrives for that user equipment. The SGW manages and stores the user equipment context (e.g. parameters of IP bearer services or network internal routing information). Furthermore, the SGW performs duplication of user traffic in case of lawful interception.

[0007] The MME is the main control node of the LTE access network. It is responsible for tracking of idle mode user equipment and paging procedures (including retransmissions). It is involved in the bearer activation / deactivation process and is also responsible for selecting the SGW for the user equipment at initial attach and during intra-LTE handover involving relocation of Core Network (CN) nodes. The MME is responsible for authenticating the user (by interacting with the HSS). Non-Access Stratum (NAS) signaling is terminated in the MME, which is also responsible for generating and assigning temporary identities to user equipment. The MME checks the authorization of the user equipment to enter the service provider's Public Land Mobile Network (PLMN) and enforces roaming restrictions for the user equipment. The MME is the termination point in the network for ciphering / integrity protection of the NAS signaling and is responsible for security key management. Lawful interception of signaling is also supported by the MME. In addition, the MME provides the control plane function for mobility between LTE and 2G / 3G access networks and terminates the S3 interface from the SGSN and the S6a interface towards the home HSS for roaming user equipment.

[0008] Component carrier structure in LTE The downlink component carrier of a 3GPP LTE system is further divided in the time-frequency domain in so-called subframes. In 3GPP LTE, each subframe is divided into two downlink slots as shown in Figure 2, where the first downlink slot comprises a control channel region (PDCCH region) within the first OFDM symbol. Each subframe consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), each OFDM symbol spanning the entire bandwidth of the component carrier. Each OFDM symbol therefore consists of several modulation symbols transmitted on the respective subcarrier. In LTE, the transmitted signal in each slot is divided into N DL RB ×N RB sc Book subcarrier and N DL symb It is described by a resource grid of N OFDM symbols. DL RB is the number of resource blocks in the bandwidth. DL RB depends on the downlink transmission bandwidth configured in the cell, and N min,DL RB ≦N DL RB ≦N max,DL RB In this case, N min,DL RB =6 and N max,DL RB N = 110 are the minimum and maximum downlink bandwidths, respectively, supported by the current version of the specification. RB sc is the number of subcarriers in one resource block. For the normal cyclic prefix subframe structure, N RB sc =12, N DL symb =7.

[0009] Considering a multi-carrier communication system, e.g. using OFDM, as used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be allocated by a scheduler is one "resource block". A physical resource block (PRB) is defined as consecutive OFDM symbols in the time domain (e.g. 7 OFDM symbols) and consecutive subcarriers in the frequency domain (e.g. 12 subcarriers of a component carrier), as illustrated in Fig. 2. Thus, in 3GPP LTE (Release 8), a physical resource block is composed of resource elements and corresponds to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see e.g. Section 6.2 of 3GPP LTE 2011-01-13 (current version 12.6.0), available on the 3GPP website and incorporated herein by reference).

[0010] A subframe consists of two slots, and therefore there are 14 OFDM symbols in a subframe when the so-called "normal" CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when the so-called "extended" CP is used. For the purpose of terminology, in the following, a time-frequency resource equivalent to the same consecutive subcarriers spread over an entire subframe is called a "resource block pair" or equivalently a "RB pair" or a "PRB pair". The term "component carrier" denotes a combination of several resource blocks in the frequency domain. In future releases of LTE, the term "component carrier" will no longer be used, and instead the terminology will be changed to "cell" to denote a combination of downlink and optionally uplink resources. The linking between the carrier frequencies of the downlink resources and the carrier frequencies of the uplink resources is indicated in the system information transmitted in the downlink resources.

[0011] Similar assumptions regarding the component carrier structure will apply to subsequent releases.

[0012] Carrier Aggregation in LTE-A for Support of Wider Bandwidth The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced has been decided, the actual available frequency bandwidth varies from region to region and country to country. However, following the decision on the outline of the available frequency spectrum, standardization of the air interface has begun in 3GPP (3rd Generation Partnership Project).

[0013] The bandwidth that the LTE-Advanced system can support is 100MHz, while the LTE system can only support 20MHz. Today, the lack of radio spectrum has become a bottleneck in the development of wireless networks, and as a result, it is difficult to find a wide enough spectrum band for the LTE-Advanced system. Therefore, it is urgent to find a way to obtain a wider radio spectrum band, and here, the possible answer is the carrier aggregation function.

[0014] In carrier aggregation, two or more component carriers are aggregated in order to support a wider transmission bandwidth of up to 100 MHz. In the LTE-Advanced system, several cells in the LTE system are aggregated into one wider channel, which is wide enough for 100 MHz even if these cells in LTE are in different frequency bands. All component carriers can be configured to be LTE Rel-8 / 9 compatible, at least when the bandwidth of the component carriers does not exceed the supported bandwidth of the LTE Rel-8 / 9 cells. Not all component carriers aggregated by a user equipment are necessarily LTE Rel-8 / 9 compatible. Existing mechanisms (e.g. barring) can be used to avoid Rel-8 / 9 user equipment camping on a component carrier.

[0015] Depending on its capabilities, a user equipment can simultaneously receive or transmit on one or multiple component carriers (corresponding to multiple serving cells). An LTE-A Rel-10 user equipment with receiving and / or transmitting capabilities for carrier aggregation can simultaneously receive and / or transmit on multiple serving cells, whereas an LTE Rel-8 / 9 user equipment can receive and transmit on only one serving cell if the component carrier structure follows the Rel-8 / 9 specifications.

[0016] Carrier aggregation is supported for both contiguous and non-contiguous component carriers, with each component carrier limited to a maximum of 110 resource blocks in the frequency domain (using 3GPP LTE (Release 8 / 9) numerology).

[0017] It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate different numbers of component carriers, possibly with different bandwidths in the uplink and downlink, originating from the same eNodeB (base station). The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the user equipment. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the user equipment. Currently, it is not possible to configure a mobile terminal with more uplink component carriers than downlink component carriers. In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier are the same in the uplink and downlink. Component carriers originating from the same eNodeB do not need to provide the same coverage.

[0018] The spacing of the center frequencies of contiguous aggregated component carriers is a multiple of 300 kHz to maintain compatibility with the 100 kHz frequency raster of 3GPP LTE (Release 8 / 9) while preserving the orthogonality of the 15 kHz spaced subcarriers. In some aggregation scenarios, the n x 300 kHz spacing can be facilitated by inserting a small number of unused subcarriers between consecutive component carriers.

[0019] The impact of aggregating multiple carriers only extends to the MAC layer, where one HARQ entity is required per aggregated component carrier, both in uplink and downlink. There is at most one transport block per component carrier (without SU-MIMO in uplink). A transport block and its (one or more) HARQ retransmissions (if they occur) must be mapped to the same component carrier.

[0020] When carrier aggregation is configured, the mobile terminal has only one RRC connection with the network. During RRC connection establishment / re-establishment, one cell provides security inputs (one ECGI, one PCI, and one ARFCN) and non-access stratum (NAS) mobility information (e.g. TAI), similar to LTE Release 8 / 9. After RRC connection establishment / re-establishment, the component carrier corresponding to that cell is called the downlink primary cell (PCell). In the connected state, there is always one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment. In the configured set of component carriers, the other cells are called secondary cells (SCells), and the carriers of the SCells are the downlink secondary component carriers (DL SCC) and uplink secondary component carriers (UL SCC). Currently, up to five serving cells (including the PCell) can be configured for one UE.

[0021] Configuration and reconfiguration, as well as addition and removal of component carriers can be performed by RRC. Activation and deactivation is done, for example, via MAC control elements. During intra-LTE handover, RRC can also add, remove or reconfigure SCells for use in the target cell. When adding a new SCell, dedicated RRC signaling is used to send the system information of the SCell (necessary for transmission / reception) (similar to handover in LTE Release 8 / 9). When a SCell is added to a UE, a serving cell index is configured for each SCell. The PCell always has serving cell index 0.

[0022] When a user equipment is configured to use carrier aggregation, at least one pair of uplink and downlink component carriers is always active. The downlink component carrier of this pair is sometimes called the "downlink anchor carrier". The same applies for the uplink. When carrier aggregation is configured, the user equipment can be scheduled on several component carriers at the same time, but at most one random access procedure can be ongoing at the same time. In cross-carrier scheduling, the PDCCH of one component carrier can schedule resources of another component carrier. For this purpose, a component carrier identification field (called "CIF") is introduced in the respective DCI (Downlink Control Information) format.

[0023] When cross-carrier scheduling is not performed, the uplink component carrier to which the grant applies can be identified by the link between the uplink component carrier and the downlink component carrier (established by RRC signaling). The link of the downlink component carrier to the uplink component carrier does not necessarily have to be one-to-one. In other words, two or more downlink component carriers can be linked to the same uplink component carrier. On the other hand, one downlink component carrier can only be linked to one uplink component carrier.

[0024] Random Access Procedure In LTE, the uplink transmission of a mobile terminal can be scheduled only if the mobile terminal's uplink transmission is time synchronized to maintain orthogonality with the uplink transmission from another UE. Therefore, the random access (RACH) procedure plays an important role as an opportunity for an unsynchronized mobile terminal (UE) to perform orthogonal transmission of uplink radio access. The random access procedure in LTE is essentially used to achieve uplink time synchronization in a user equipment that has not yet acquired uplink synchronization or a user equipment that has lost uplink synchronization. Once the user equipment achieves uplink synchronization, the eNodeB can schedule uplink transmission resources for that user equipment.

[0025] Additionally, the PRACH transmission and detection estimates the round-trip delay between the eNB and the UE. The design goal for the PRACH signal shape when operating LTE in licensed bands was to minimize overhead and the impact of interference on parallel uplink transmissions from other UEs, while at the same time providing sufficient accuracy of the round-trip delay estimation.

[0026] A further additional case is when the user equipment performs a random access procedure even if it is time synchronized, i.e. the user equipment uses the random access procedure to send a scheduling request (i.e. an uplink buffer status report) to the eNodeB when no separate uplink resources are allocated for sending the scheduling request (e.g. no Dedicated Scheduling Request (D-SR) channel is configured).

[0027] Therefore, the following scenarios are relevant for random access: 1. When a user equipment in RRC_CONNECTED state but not uplink synchronized wishes to transmit new uplink data or control information. 2. A user equipment in RRC_CONNECTED state but not uplink synchronized needs to receive downlink data and is therefore required to send corresponding HARQ feedback (i.e. ACK / NACK) in the uplink. This scenario is also referred to as Downlink data arrival. 3. When a user equipment in RRC_CONNECTED state hands over from the current serving cell to a new target cell, a random access procedure is performed with the aim of achieving uplink time synchronization in the target cell. 4. When timing advance is required for positioning purposes in RRC_CONNECTED state 5. When transitioning from RRC_IDLE to RRC_CONNECTED (e.g. when first accessing or when updating the tracking area) 6. Recovering from a radio link failure (i.e. re-establishing an RRC connection)

[0028] In LTE, two types of random access procedures are provided, allowing either contention-based (with risk of collision) or contention-free access. Note that contention-based random access is applicable to all six scenarios listed above, whereas contention-free random access procedure is only applicable to downlink data arrival and handover scenarios.

[0029] In the following, the contention-based random access procedure is explained in more detail with reference to Figure 3. A detailed description of the random access procedure is also given in 3GPP TS 2011-01-13 (current version 12.6.0), section 5.1, which is incorporated herein by reference.

[0030] Figure 3 shows the contention-based RACH procedure in LTE. The procedure consists of four "steps". First, the user equipment transmits a random access preamble to the eNodeB on the physical random access channel (PRACH) (301). The preamble is selected by the user equipment from a set of available random access preambles reserved by the eNodeB for contention-based access. cf is the number of signatures reserved by the eNodeB for the contention-free RACH. In LTE, there are a total of 64 preambles per cell that can be used for contention-free and contention-based random access. The set of contention-based preambles can be further divided into two groups, so that the selection of a preamble by the UE can convey one bit of information to indicate information about the amount of transmission resources required for the first scheduled transmission (referred to as msg3 in 3GPP TS 2013-010112) (see step 303 in Fig. 3). The system information broadcasted in the cell includes information on the signatures (preambles) belonging to each of the two subgroups and the meaning of each subgroup. The user equipment randomly selects one preamble from the subgroup that corresponds to the size of the transmission resources required for the transmission of msg3 (see step 303 below). When selecting the appropriate size to indicate, the UE can further take into account the current downlink path loss and the transmission power required for the message of step 303, in order to avoid granting resources for message sizes that require transmissions larger than the size allowed by the UE's maximum power.

[0031] After detecting the RACH preamble, the eNodeB sends a Random Access Response (RAR) message, i.e. the corresponding DCI in the PDCCH addressed to the (random access) RA-RNTI identifying the time-frequency slot in which the preamble was detected, on the Physical Downlink Shared Channel (PDSCH) (302). If multiple user equipments transmit the same RACH preamble on the same PRACH resource (also called collision), these user equipments receive the same random access response.

[0032] The RAR (Random Access Response) message conveys the identity of the detected RACH preamble, a Timing Alignment Command (TA command) for synchronizing subsequent uplink transmissions, an initial uplink resource allocation (grant) for transmitting the first scheduled transmission (see step 303), and the allocation of a T-CRNTI (Temporary Cell Radio Network Temporary Identifier) ​​which is used by the base station until the end of the RACH procedure to address the mobile terminal(s) for which a RACH preamble has been detected, since the eNodeB does not yet know the "true" identity of the mobile terminal at this point.

[0033] Furthermore, the RAR message may also contain a so-called back-off indicator, which may be set by the eNodeB in order to instruct the user equipment to back off (wait) for a certain period of time before retrying the random access. The user equipment monitors the PDCCH for a random access response within a given time window (set by the eNodeB). If no random access response is received within the set time window, the user equipment retransmits the preamble at the next PRACH opportunity, taking into account the back-off period, if specified.

[0034] In response to the RAR message received from the eNodeB, the user equipment transmits a scheduled first uplink transmission on the uplink resources allocated by the grant in the random access response (303). This scheduled uplink transmission carries the actual random access procedure message (e.g., RRC connection request, tracking area update, buffer status report, etc.). Furthermore, this uplink transmission contains either the C-RNTI of the user equipment in RRC_CONNECTED mode or a unique 48-bit user equipment ID if the user equipment is in RRC_IDLE mode. If a preamble collision occurs in step 301 (i.e., multiple user equipments sent the same preamble on the same PRACH resource), the colliding user equipments will receive the same T-CRNTI in the random access response and will also collide on the same uplink resource when transmitting their respective scheduled transmissions (303). This may result in interference, such that the transmissions from the colliding user equipments cannot be decoded at the eNodeB, and the user equipment will start the random access procedure again after reaching the maximum number of retransmissions of the scheduled transmission. If a scheduled transmission from one user equipment is successfully decoded by the eNodeB, the contention for the other user equipment remains unresolved.

[0035] To resolve this type of conflict, the eNodeB sends (304) a conflict resolution message addressed to the C-RNTI or the temporary C-RNTI, and in the case of the temporary C-RNTI, sends back the original 48-bit user equipment ID included in the scheduled transmission of step 303. After the collision, if the message sent in step 303 is successfully decoded, only the user equipment that detects its own identity (C-RNTI or unique user equipment ID) sends HARQ feedback (ACK). The other UEs know that a collision has occurred in step 301 and can immediately terminate the current RACH procedure and start a new one.

[0036] Figure 4 shows the contention-free random access procedure introduced as of 3GPP LTE Release 8 / 9. Compared to the contention-based random access procedure, the contention-free random access procedure is simplified. The eNodeB assigns a specific preamble to the user equipment for use in random access (401) such that there is no risk of collision (i.e., multiple user equipments do not transmit the same RACH preamble). The user equipment accordingly transmits the preamble signaled by the eNodeB in the uplink on the appropriate PRACH resource (402). Since contention-free random access avoids the case where multiple UEs transmit the same preamble, no contention resolution is required, and thus step 304 of the contention-based procedure shown in Figure 3 can be omitted. The contention-free random access procedure is essentially terminated after a successful reception of the random access response. In case of failure to receive the random access response, the next PRACH retransmission is autonomously initiated by the UE itself.

[0037] When carrier aggregation is configured, the first three steps of the contention based random access procedure are performed on the PCell and contention resolution (step 304) can be cross-scheduled by the PCell.

[0038] The initial setting of the transmit power of the preamble can be based on an open-loop estimation in which the path loss is fully compensated, where the open-loop estimation is designed such that the received power of the preamble is independent of the path loss.

[0039] Additionally, the eNB may configure an additional power offset depending, for example, on the desired received SINR, the measured uplink interference and noise levels in the time-frequency slot assigned to the RACH preamble, and (possibly) the format of the preamble. Furthermore, the eNB may configure power ramping of the preamble, such that the transmit power of each retransmitted preamble (e.g. in case of an unsuccessful PRACH transmission attempt) increases at regular intervals.

[0040] Random Access Preamble - Time, Frequency, Format The random access preamble transmission part of the random access procedure described above is mapped to the PRACH in the physical layer. The design of the preamble is crucial for the success of the random access procedure and is described in detail below. The RACH preamble is essentially a cyclic shift of a complex Zadoff-Chu (ZC) sequence, also known as the preamble signature. The LTE PRACH preamble consists of a complex sequence. However, unlike the W-CDMA preamble, the LTE PRACH preamble is also an OFDM symbol that must follow the DFT-S-OFDM structure of the LTE uplink and is constructed using a cyclic prefix (CP), thus enabling an efficient frequency domain receiver at the eNodeB. The physical layer random access preamble is a cyclic prefix of length T CP and the cyclic prefix of length T SEQand a sequence part of the cyclic prefix. The possible values ​​of these parameters are listed in the following table and depend on the frame structure and the random access configuration (e.g. preamble format that can be controlled by higher layers). Corresponding detailed information can be found in 3GPP TS 2.0 (current version 12.6.0), clause 5.7.1 "Time and frequency structure", which is incorporated herein by reference. For frequency division duplex operation, four random access preamble formats are defined, each format defined by the duration of the sequence and the duration of the cyclic prefix. The format configured in the cell is broadcast in the system information.

[0041] [Table 1]

[0042] T S is the assumed system sampling rate, which can be 1 / 30.72 μs, the basic time unit in LTE. The following table shows the T CP and T SEQ The values ​​shown are:

[0043] [Table 2]

[0044] The following table shows the subcarrier spacing and the corresponding symbol duration in the current LTE specification. For example, the preamble sequence duration for preamble formats 2 and 3 (1600 μs, see Table 2) is achieved by the repetition of the preamble symbol (800 μs) in the time domain.

[0045] [Table 3]

[0046] The duration of the series, T SEQ The lower bound of (683.33 μs) must be such that a UE located at the edge of the largest expected cell can unambiguously estimate the round trip time including the maximum expected delay spread in such a large cell (i.e., 16.67 μs). SEQ Further constraints on the size of the DFT (Discrete Fourier Transform) and IDFT (Inverse Discrete Fourier Transform) N DFT must be an integer.

[0047] To facilitate frequency multiplexing of PRACH and PUSCH resource allocations, a PRACH slot is provided with a bandwidth BW equal to an integer multiple of a resource block (i.e., an integer multiple of 180 kHz). PRACH The BW in LTE must be allocated. PRACH (6 PRBs, 1.08MHz) is constant for any system bandwidth, for simplicity's sake, PRACH is chosen to optimize both detection performance and timing estimation accuracy. The timing estimation accuracy determines the lower limit of the PRACH bandwidth. In practice, a minimum bandwidth of about 1 MHz is required to provide an acceptable timing accuracy in PUCCH / PUSCH transmissions of about ±0.5 μs one-shot accuracy.

[0048] Allocating 6 RBs (resource blocks) to the PRACH provides a good trade-off between PRACH overhead, detection performance, and timing estimation accuracy. Note that for the smallest system bandwidth (1.4 MHz, 6 RBs), the PRACH overlaps with the PUCCH. It is up to the eNodeB implementation to impose scheduling constraints during the PRACH slots to avoid collisions, or to allow PRACH and PUCCH collisions to occur and deal with the resulting interference.

[0049] In order to provide compatibility between the subcarriers of the preamble and the subcarriers of the PUSCH, the duration of the preamble should be fixed to an integer multiple of the duration of a PUSCH symbol, i.e., the subcarrier spacing of the PRACH should preferably be an integer fraction of the subcarrier spacing of the PUSCH.

[0050] As shown in Figure 6, PRACH is time and frequency multiplexed with PUSCH and PUCCH. Time-frequency resources of PRACH are semi-statically allocated in the PUSCH region and are repeated periodically. It is at the discretion of the eNodeB whether PUSCH transmissions can be scheduled in PRACH slots. 64 PRACH configurations are supported in LTE, each configuration consisting of a periodic PRACH resource pattern and an associated preamble format. A detailed list of PRACH configurations is given in Tables 5.7.1-2 and 5.7.1-3 of 3GPP TS 2.0, which is incorporated herein by reference. It is possible to schedule PUSCH transmissions in the same subframe together with the allocated PRACH resources. This decision is made by the eNB.

[0051] Random Access Preamble - Preamble Sequence Generation As mentioned above, 64 PRACH signatures are available in LTE compared to only 16 in WCDMA. This not only reduces the probability of collisions, but also allows one bit of information to be carried by the preamble in contention-based access and some signatures to be reserved for contention-free access. Therefore, an improved sequence design is required for the LTE PRACH preamble compared to WCDMA. In LTE, prime length Zadoff-Chu sequences are selected, which can improve the detection performance of the PRACH preamble. More detailed information can be found in 3GPP TS 2.0 (current version 12.6.0), section 5.7.2 "physical random access channel" (incorporated herein by reference).

[0052] The random access preamble is a Zadoff-Chu (ZC) sequence, which is generated from one or several root Zadoff-Chu sequences as follows: First, a root Zadoff-Chu sequence is chosen based on an indication of a logical sequence index (RACH_ROOT_SEQUENCE) broadcasted as part of the system information. The order of logical root sequences is cyclic, so logical index 837 is followed by index 0. The relationship between the logical root sequence index (indicated in the system information) and the physical root sequence index u is given by Table 5.7.2-4 of Non-Patent Document 1 for preamble formats 0 to 3, and by Table 5.7.2-5 for preamble format 4 (incorporated herein by reference).

[0053] The u-th root Zadoff-Chu sequence is defined by the following equation:

[0054]

number

[0055] where u is the physical root sequence index as described above, and the sequence length N ZC depends on the configured PRACH preamble format, i.e., for preamble formats 0 to 3, N ZC = 839, and in the case of preamble format 4, N ZC = 139 (see also Table 5.7.2-1 in Non-Patent Document 1).

[0056] From the u-th root Zadoff-Chu sequence, a sequence of length N is generated by cyclic shifting according to the following formula: CS A set of 64 random access preambles with a zero correlation zone of -1 is defined.

[0057]

number

[0058] The cyclic shift is given by:

[0059]

number

[0060] Parameter N CS is given by Tables 5.7.2-2 and 5.7.2-3 of 3GPP TS 2.0 and depends on the format of the preamble and the parameter zeroCorrelationZoneConfig provided by higher layers. Further information can be found in Section 5.7.2 of 3GPP TS 2.0.

[0061] If it is not possible to generate 64 preambles from one root Zadoff-Chu sequence, additional preamble sequences are derived from one or more root sequences with successive logical indices until all 64 preamble sequences are found.

[0062] In summary, the set of 64 preamble sequences available for use within a cell for the RACH procedure are generated by cyclic shifts of one or more root Zadoff-Chu sequences.

[0063] Random Access Preamble - Baseband Signal Generation The generation of the PRACH baseband signal is defined in section 5.7.3 of 3GPP TS 2010 / 0113611. The time-continuous random access signal s(t) is defined by the following equation:

[0064]

number

[0065] In the formula, 0≦t <T SEQ +T CP and β PRACH is the transmission power P PRACH is the amplitude scaling factor to fit

number

[0066] The position in the frequency domain is determined by the parameter n RA PRB It is controlled by the coefficient K=Δf / Δf RA takes into account the difference in subcarrier spacing between the random access preamble and the uplink data transmission. RA (subcarrier spacing of the random access preamble) and variable φ (a fixed offset that determines the frequency domain position of the random access preamble within a physical resource block) are both given by the following table (see Table 5.7.3-1 in Non-Patent Document 1).

[0067] [Table 4]

[0068] Note that the PUSCH has a subcarrier spacing of 15 kHz.

[0069] The time domain preamble sequence is of size N ZC The resulting frequency domain coefficients are then transformed into the frequency domain by the DFT of RA The PRACH transmission frequency spacing does not match the frequency spacing used for other uplink transmissions (such as PUSCH and PUCCH). The subcarrier mapping further incorporates the location of the PRACH in the frequency domain.

[0070] Figure 7 shows the mapping of the PRACH preamble to the assigned subcarriers and the mapping of the PUSCH subcarriers side by side. As is evident, the PRACH uses a guard band to avoid data interference at the preamble edges. The PRACH is transmitted in frequency domain resources corresponding to 6 consecutive PRBs (Physical Resource Blocks) (i.e., with a frequency bandwidth of 1.08 MHz). These PRBs can be located in the center of the nominal system bandwidth as shown in Figure 8, or in any other position within the nominal system bandwidth as shown in Figure 9.

[0071] Random Access Preamble - UE preamble sequence transmitter implementation In the following, a practical exemplary implementation of the PRACH function is briefly described. As shown in Fig. 10, the PRACH preamble can be generated at the system sampling rate by a large IDFT. The DFT block in Fig. 10 is dotted to indicate that it is optional, since the sequence can also be directly mapped to the frequency domain at the input of the IDFT. The cyclic shift can be performed in the time domain after the IDFT, or in the frequency domain before the IDFT through a phase shift.

[0072] Another option for generating the preamble consists of using a smaller IDFT (actually an IFFT) and shifting the preamble to the desired frequency position through time domain upsampling and filtering. A cyclic prefix can be inserted before the upsampling and time domain frequency shifting to minimize intermediate storage requirements.

[0073] LTE in Unlicensed Bands: Licensed Assisted Access (LAA) 3GPP launched a new study item on LTE operation in unlicensed spectrum in September 2014. The reason for extending LTE to unlicensed spectrum is the limited amount of licensed spectrum, combined with the ever-growing demand for wireless broadband data. Therefore, unlicensed spectrum is increasingly seen as a supplementary means for mobile operators to expand their service offerings. The advantage of LTE in unlicensed spectrum, compared to relying on other radio access technologies (RATs) such as Wi-Fi, is that operators and vendors can leverage existing and future investments in LTE / EPC hardware in radio and core networks by supplementing their LTE platform with access to unlicensed spectrum.

[0074] However, it must be taken into account that access to unlicensed spectrum can never be comparable in quality to licensed spectrum access, since it necessarily coexists with other radio access technologies (RATs), such as Wi-Fi, in unlicensed spectrum. Therefore, LTE operation in unlicensed spectrum will, at least initially, not be considered as a standalone operation in unlicensed spectrum, but rather as a complement to LTE in licensed spectrum. Based on this assumption, 3GPP has established the term Licensed Assisted Access (LAA) for the operation of LTE in unlicensed spectrum in conjunction with at least one licensed spectrum, although standalone operation of LTE in unlicensed spectrum (i.e. not assisted by licensed cells) in the future is not excluded.

[0075] A general LAA method currently intended in 3GPP is to make full use of the already established Carrier Aggregation (CA) framework of Release 12, which includes a so-called Primary Cell (PCell) carrier and one or more Secondary Cell (SCell) carriers, as described above. Carrier Aggregation (CA) generally supports both cell self-scheduling (scheduling information and user data are transmitted on the same component carrier) and cross-carrier scheduling between cells (scheduling information by PDCCH / EPDCCH and user data by PDSCH / PUSCH are transmitted on different component carriers).

[0076] Figure 11 shows a very basic scenario, where there is a licensed PCell, a licensed SCell 1 and various unlicensed SCells 2, 3, 4 (illustratively depicted as small cells). The transmitting / receiving network nodes for the unlicensed SCells 2, 3, 4 can be remote radio heads managed by the eNB or nodes attached to the network but not managed by the eNB. For simplicity, the connections from these nodes to the eNB or the network are not explicitly shown in the figure.

[0077] The basic approach currently envisaged in 3GPP is to operate the PCell in a licensed band while operating one or more SCells in an unlicensed band. The advantage of this approach is that the PCell can be used to reliably transmit control messages and user data (e.g., voice and video) that require high quality of service (QoS), but on the other hand, the SCell in the unlicensed band necessarily coexists with other radio access technologies (RATs), which may significantly degrade the QoS, although the degree varies depending on the scenario.

[0078] It was agreed that LAA (Licensed Assisted Access) will focus on the 5 GHz unlicensed bands. One of the most important issues is therefore the coexistence with Wi-Fi (IEEE 802.11) systems operating in these unlicensed bands. In order to support a fair coexistence between LTE and other technologies (such as Wi-Fi) and also to ensure fairness between different LTE operators in the same unlicensed bands, LTE channel access in unlicensed bands must follow a specific set of regulations, some of which vary depending on the geographical region and the specific frequency band. A comprehensive description of the regulatory requirements in all regions when operating in the 5 GHz unlicensed bands is given in 3GPP TS 2013-01-13 (incorporated herein by reference) and 3GPP TS 2013-01-13 (current version 13.0.0). Regulatory requirements that must be taken into account when designing an LAA procedure vary by region and band, but include Dynamic Frequency Selection (DFS), Transmit Power Control (TPC), Listen Before Talk (LBT), Discontinuous Transmission with limited maximum transmission time. 3GPP's intention is to target a single international framework for LAA, which basically means that all requirements for the different regions and the 5GHz band must be taken into account when designing a system.

[0079] For example, in Europe, specific limits for the Nominal Channel Bandwidth are set, as evident from section 4.3 of the European Standard 3GPP TS 2.0 (currently version 1.8.1), which is incorporated herein by reference. The Nominal Channel Bandwidth is the widest frequency band (including guard bands) that can be assigned to a channel. The Occupied Channel Bandwidth is the bandwidth that contains 99% of the power of the signal. A device is allowed to operate simultaneously on one or more adjacent or non-adjacent channels.

[0080] When devices transmit simultaneously on adjacent channels, these transmissions can be considered as one signal with an actual nominal channel bandwidth of "n" times the individual nominal channel bandwidth, where n is the number of adjacent channels. When devices transmit simultaneously on non-adjacent channels, consider each power envelope separately. The nominal channel bandwidth is always at least 5 MHz. The occupied channel bandwidth is in the range of 80% to 100% of the declared nominal channel bandwidth. In the United States, the minimum occupied channel bandwidth is 500 kHz according to IEEE 802.11b / g / n ...

[0081] The Listen Before Talk (LBT) procedure is defined as a mechanism for a device to apply Clear Channel Assessment (CCA) before using a channel. CCA uses at least the detection of energy to determine the presence or absence of another signal on a channel with the aim of determining whether the channel is occupied or free. European and Japanese regulations require the use of LBT in unlicensed bands. Apart from being a regulatory requirement, carrier sensing via LBT is one way to fairly share unlicensed spectrum. LBT is therefore considered an essential feature for a fair and friendly operation in unlicensed spectrum within the framework of one global solution.

[0082] In unlicensed frequency bands, channel availability cannot always be guaranteed. In addition, in certain regions, such as Europe and Japan, continuous transmission is prohibited and limitations are imposed on the maximum duration of transmission bursts in unlicensed frequency bands. Discontinuous transmission with a limited maximum duration of transmission is therefore a mandatory feature in LAA. Dynamic Frequency Selection (DFS) is required for certain regions and bands with the aim of detecting interference from radar systems and avoiding co-channel operation with these systems. The aim is to further achieve an approximately uniform loading of the frequency band. The operation of DFS and the corresponding requirements are related to the master / slave principle. To perform radar detection, the master detects radar interference, but can then rely on another device associated with the master.

[0083] In most regions, operation in the 5 GHz unlicensed band is limited to significantly lower transmission power levels compared to operation in licensed bands, resulting in a smaller coverage area. Even if licensed and unlicensed carriers transmit at the same power, the coverage area supported by an unlicensed carrier in the 5 GHz band is expected to be typically smaller than that of a licensed carrier in the 2 GHz band due to higher signal path loss and shadowing effects. A further requirement for certain regions and bands is the use of TPC (Transmit Power Control) with the aim of reducing the average level of interference caused to other devices operating in the same unlicensed band.

[0084] Detailed information can be found in the harmonized European standard 3GPP TS 2003-010621 (current version 1.8.0), which is incorporated herein by reference.

[0085] According to this European regulation on LBT, a device must perform a clear channel assessment (CCA) before occupying a radio channel by data transmission. Only after the channel is detected as clear, for example based on energy detection, a device is allowed to start transmitting on an unlicensed channel. In particular, a device must monitor the channel for a certain minimum time during CCA (e.g. 20 μs in Europe, see 3GPP TS 36.310, section 4.8.3). If the detected energy level exceeds the configured CCA threshold (e.g. -73 dBm / MHz in Europe, see 3GPP TS 36.310, section 4.8.3), the channel is considered to be occupied, and conversely, if the detected power level is lower than the configured CCA threshold, the channel is considered to be clear. If a channel is determined to be occupied, the device does not transmit on that channel for the next Fixed Frame Period. If the channel is classified as clear, the device is allowed to transmit immediately. The maximum duration of a transmission is limited with the aim of promoting fair resource sharing with other devices operating in the same band.

[0086] Energy detection in CCA is performed over the entire channel bandwidth (e.g. 20 MHz in the 5 GHz unlicensed band), i.e. the sum of the received power levels of all subcarriers of the LTE OFDM symbol in that channel is the energy level evaluated in the device performing CCA.

[0087] Furthermore, the total time that the equipment has transmission on a given carrier without re-evaluating its availability (i.e. LBT / CCA) is defined as the Channel Occupancy Time (see 3GPP TS 2011-010621, section 4.8.3.1). The Channel Occupancy Time may be in the range of 1 ms to 10 ms, and the maximum Channel Occupancy Time may be e.g. 4 ms as currently defined in Europe. Furthermore, there is also a minimum idle time during which the UE is not allowed to transmit after a transmission on an unlicensed cell, which is at least 5% of the Channel Occupancy Time. The UE may for example perform a new CCA shortly before the end of the idle period. This transmission behavior is shown diagrammatically in Figure 12, which is taken from 3GPP TS 2011-010621 (Figure 2: "Example of timing for Frame Based Equipment" in this document).

[0088] Figure 13 shows the timing between Wi-Fi transmission and LAA UE transmission in a particular frequency band (unlicensed cell). As can be seen from Figure 13, after the Wi-Fi burst, at least a CCA gap is required before the eNB "reserves" the unlicensed cell until the next subframe boundary, for example by transmitting a reservation signal. Then the actual LAA DL burst starts.

[0089] The RACH procedure is also supported in unlicensed bands. To date, it has been agreed that only contention-free PRACH transmissions are supported in unlicensed bands. As described above, unlike PRACH retransmissions in licensed bands, it is still under consideration whether PRACH retransmissions in unlicensed bands are explicitly scheduled by the eNB. However, while standardization work currently agrees that only contention-free random access is supported, this may change in the future and contention-based random access may be possible in unlicensed cells (indeed, the principles of the present invention are applicable to both contention-free and contention-based random access procedures).

[0090] Taking into account the various regulatory requirements, it is clear that the LTE specifications, and in particular the random access procedures for operation in unlicensed bands, require some modifications compared to the current Release 12 specifications, which are limited to operation in licensed bands. [Prior art documents] [Non-patent literature]

[0091] [Non-Patent Document 1] 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)” [Non-Patent Document 2] R1-144348, “Regulatory Requirements for Unlicensed Spectrum”, Alcatel-Lucent et al., RAN1#78bis, Sep. 2014 [Non-Patent Document 3] 3GPP TS 36.321 [Non-Patent Document 4] 3GPP Technical Report 36.889 [Non-Patent Document 5] ETSI EN 301 893 Summary of the Invention [Means for solving the problem]

[0092] Non-limiting exemplary embodiments provide an improved method for performing a random access procedure between a user equipment and a radio base station via an unlicensed cell. The independent claims provide non-limiting exemplary embodiments. Advantageous embodiments are the subject of the dependent claims.

[0093] According to some implementations of the aspects described herein, the random access procedure is improved, especially when it is performed via unlicensed cell(s). More specifically, mainly the preamble sequence part of the random access procedure (preamble generation, selection, and actual RF transmission) is improved. Therefore, further parts of the random access procedure are not the subject of the various described aspects, and remain (almost) the same as the random access procedure designed for example for licensed access.

[0094] In the following, the following scenario is assumed: in a mobile communication system, a user equipment and a radio base station are connected to each other via at least one unlicensed cell. The unlicensed cell can be operated as a single cell or can be supported by a further licensed cell that is additionally configured in the user equipment. The unlicensed cell is configured to have a certain frequency bandwidth, i.e. the unlicensed cell is operated by the radio base station and the user equipment in a channel having a certain frequency bandwidth (e.g. 10 MHz, 20 MHz, 40 MHz or a smaller bandwidth or a larger bandwidth, etc.) in an unlicensed frequency band.

[0095] In addition, transmissions in unlicensed cells are regulated to define at least a minimum frequency bandwidth threshold that indicates the minimum channel occupancy that transmissions via unlicensed cells must occupy. The minimum channel occupancy depends on the frequency bandwidth of the unlicensed cell and may therefore vary from channel to channel. The minimum channel occupancy may define a predefined percentage of the corresponding total frequency bandwidth of the unlicensed cell.

[0096] In the scenario defined in this way, essentially most of the transmissions performed by the UE (and eNodeB) via unlicensed cells have to comply with this minimum channel occupation requirement. This also applies to the random access procedure performed between the user equipment and the radio base station, e.g. to synchronize the uplink reference timing of the user equipment or to transmit a scheduling request to the radio base station. As part of the random access procedure, the user equipment selects an appropriate random access preamble sequence and then transmits this preamble sequence to the radio base station.

[0097] According to some aspects, the minimum frequency bandwidth threshold defined for transmissions via unlicensed cells is also taken into account when transmitting a random access preamble sequence to the radio base station as part of the random access procedure, in particular the random access preamble sequence is transmitted such that it complies with the minimum channel occupancy requirement by at least exceeding the minimum frequency bandwidth threshold defined for unlicensed cells.

[0098] For this purpose, a specific frequency bandwidth for transmitting the random access preamble sequence via the unlicensed cell can be determined, which frequency bandwidth is greater than a minimum frequency bandwidth threshold. Such a determination allows for flexible handling of different channel bandwidths of the unlicensed cells and therefore for handling and complying with different minimum channel occupancy requirements. This determination can be performed in the user equipment or the radio base station. In an exemplary implementation, the minimum frequency bandwidth threshold (e.g., determined by a simple calculation of a predefined percentage of the frequency bandwidth of the unlicensed cell) is known to both the user equipment and the radio base station, and thus the frequency bandwidth occupied by the transmission of the random access preamble sequence can be determined independently by the UE and the radio base station. Alternatively, this determination can be performed by one of the two entities (i.e., the UE or the radio base station) and the determination result can be communicated to the other entity. If the radio base station is the entity responsible for determining the actual frequency bandwidth of the preamble transmission signal, the radio base station maintains control over the frequency bandwidth actually used by the UE to transmit the random access preamble sequence. The transmission of such information by the radio base station to the UE can simply be done in the corresponding system information broadcast in the radio cell or, in case of contention-free random access, in a corresponding message transmitted at the beginning of the contention-free random access procedure (e.g. in the same message indicating the preamble to be used).

[0099] It should be noted in this context that contention-free and contention-based random access procedures are supported. In the contention-free random access procedure, the radio base station transmits corresponding indication information to the user equipment, indicating in particular which random access preamble sequence to select from the set of random access preamble sequences available to the UE (and the radio base station), which the user equipment follows. On the other hand, in the contention-based random access procedure, no such indication information is provided by the radio base station, and the user equipment autonomously selects the random access preamble sequence to transmit to the radio base station from the set of random access preamble sequences. The set of available random access preamble sequences for the contention-based random access procedure can be divided into two different subgroups, which are associated to different amounts of transmission resources requested via the transmission of the random access preamble sequence, just like in the currently defined standard random access procedure.

[0100] In summary, this enables the user equipment, when performing a random access procedure via an unlicensed cell, to comply with the minimum channel occupancy requirements defined for that unlicensed cell.

[0101] The random access procedure may continue as normal and thus may include the transmission of a random access response message from the radio base station to the user equipment. This random access response may include, for example, a corresponding uplink resource allocation, a timing alignment instruction, a temporary identifier of the user equipment, an identification of a random access preamble sequence previously transmitted by the user equipment. Furthermore, upon receiving such a random access response message from the radio base station, further messages may be transmitted from the user equipment to the radio base station using the allocated uplink resources. Furthermore, if a contention-based random access procedure is performed, contention resolution may be required and thus performed between the eNodeB and the UE.

[0102] In the following, two different aspects are described to achieve that the transmission of random access preamble sequences over unlicensed cells complies with the corresponding minimum channel occupancy (i.e. exceeds a minimum frequency bandwidth threshold).

[0103] According to the first aspect, the existing procedure for transmitting a random access preamble sequence is reused by repeating a normal preamble transmission at multiple different positions in the frequency domain, so as to eventually occupy at least the required frequency bandwidth of the unlicensed cell in order to comply with the regulatory requirements defined for such unlicensed cell. In particular, the random access preamble sequence is selected in a normal way and transmitted at the corresponding frequency positions. It should be noted that the normal / legacy random access preamble transmission occupies a predefined frequency bandwidth (6 PRBs, i.e. 1.08 MHz, as explained in the Background section). Furthermore, several repetitions of this transmission are performed at multiple different frequency positions, so that all preamble transmissions (with repetitions) occupy a frequency bandwidth that exceeds the minimum frequency bandwidth threshold of the unlicensed cell. The number of repetitions required to comply with this minimum channel occupancy depends on the actual frequency bandwidth threshold defined for the unlicensed cell, which in turn depends on the frequency bandwidth configured for the unlicensed cell. Furthermore, the number of repetitions also depends on the above-mentioned predetermined frequency bandwidth (i.e., 1.08 MHz) of the normal / legacy random access preamble transmission. In an exemplary implementation of the first aspect, the multiple different locations in the frequency domain at which the repetition of the preamble transmission is performed are such that the repeated transmissions are adjacent in the frequency domain.

[0104] The improved random access procedure provided for unlicensed cells according to the first aspect reuses the random access preamble sequence already defined for the normal / legacy random access procedure in licensed cells as explained above. The advantage of this approach is that no additional set(s) of random access preambles need to be defined in this respect. The same set of random access preamble sequences is available for performing the random access procedure via unlicensed cells and for performing the random access procedure via licensed cells. In particular, according to the first aspect, when performing the random access procedure via licensed cells, a further random access preamble sequence is selected from the already generated set and transmitted to the radio base station via the licensed cell, occupying the above-mentioned predetermined frequency bandwidth of the licensed cell (i.e. 6 PRBs, 1.08 MHz).

[0105] According to a further implementation of the first aspect, at least two random access preamble sequences are selected by the user equipment and transmitted together to the radio base station. In particular, at least a second random access preamble sequence is selected, which is different from the first selected random access preamble sequence. The transmission of the second random access preamble sequence is repeated in the same manner, but at a different frequency location than the transmission of the first random access preamble sequence. In particular, the first random access preamble sequence and the second random access preamble sequence are repeated and transmitted together to at least occupy a predetermined frequency bandwidth of the unlicensed cell in order to comply with a minimum channel occupancy.

[0106] According to a second aspect, the existing random access procedure, in particular the existing settings for transmitting the random access preamble sequence, is modified, i.e. the length of the random access preamble sequence and the subcarrier frequency spacing used to transmit the random access preamble sequence are selected such that, when combined, the corresponding transmission of the random access preamble sequence exceeds a minimum frequency bandwidth threshold. Note that in the following, a distinction is made between the length of the preamble sequence and the duration of the preamble in the time domain. The former determines the number of subcarriers used. The latter is given by one or more repeated preamble symbols and a cyclic prefix (the duration of the preamble symbol is given by the inverse of the subcarrier spacing of the preamble) and is not the subject of the various aspects of the invention.

[0107] It should be noted that the length of the random access preamble sequence (which basically corresponds to the number of frequency subcarriers used to transmit the random access preamble sequence) and the subcarrier frequency spacing (which basically determines how far apart the different frequency subcarriers are from each other) in combination (i.e., by simply multiplying the number of frequency subcarriers by the value of the subcarrier frequency spacing) define the overall frequency bandwidth of the preamble transmission. As a result, by adjusting these two parameters (i.e., the length of the preamble sequence and the subcarrier frequency spacing), the frequency shape / frequency bandwidth of the preamble signal can be controlled so that it complies with the frequency bandwidth requirement when it occupies on an unlicensed cell.

[0108] One or both of the two parameters can be controlled by the user equipment, or by the radio base station, or by a combination of both. In this respect, several different implementations of the second aspect are possible. For example, the subcarrier frequency spacing can be fixed, while the length of the preamble sequence can be flexibly determined according to the actual amount of frequency bandwidth that the preamble transmission must occupy (which depends on the system bandwidth of the unlicensed cell). Or, conversely, the length of the preamble sequence can be fixed, while the subcarrier frequency spacing can be flexibly adapted to different requirements of minimum channel occupancy. Furthermore, alternatively, both the length of the preamble sequence and the subcarrier frequency spacing can be flexibly controlled in the preamble transmission to occupy the necessary frequency bandwidth to comply with the minimum channel occupancy requirements of the unlicensed cell.

[0109] In an exemplary implementation of the second aspect, two different sets of random access preamble sequences may be generated by the user equipment, one for the licensed cell(s) and one for the unlicensed cell(s). It should be noted that, considering that the frequency bandwidth occupied by the transmission of the random access preamble is larger for the unlicensed cell than for the licensed cell, the length of the random access preamble sequence for the unlicensed cell is likely to be greater than the length of the random access preamble sequence for the licensed cell. As a result, the two different sets comprise random access preamble sequences of different lengths. In one exemplary implementation, assuming that the random access preamble sequence is generated from a suitable root sequence (e.g., Zadoff-Chu sequence), the corresponding root sequence for generating the random access preamble sequence used in connection with the unlicensed cell is longer than the root sequence used for generating the random access preamble sequence for the licensed cell. Thus, when performing a random access procedure via a licensed cell, a corresponding preamble is selected from the set of corresponding licensed cells, whereas when performing a random access procedure via an unlicensed cell, a corresponding preamble is selected from the set of corresponding unlicensed cells.

[0110] Thus, in one general first aspect, the techniques disclosed herein provide a method for performing a random access procedure between a user equipment and a radio base station in a mobile communication system, the method comprising: a user equipment configured with at least one unlicensed cell, the random access procedure being performed via the unlicensed cell having an unlicensed cell frequency bandwidth; a minimum frequency bandwidth threshold is defined for transmission via the unlicensed cell, the method comprising the following steps performed by the user equipment for the random access procedure: the user equipment selects a random access preamble sequence for the random access procedure and determines a frequency bandwidth for transmitting the random access preamble sequence via the unlicensed cell; the determined frequency bandwidth of the random access preamble sequence is at least the minimum frequency bandwidth threshold; the user equipment transmits the random access preamble sequence to the radio base station such that at least the determined frequency bandwidth of the unlicensed cell is occupied.

[0111] Accordingly, in one general first aspect, the techniques disclosed herein provide a user equipment for performing a random access procedure together with a radio base station in a mobile communication system. The user equipment is configured with at least one unlicensed cell, and the random access procedure is performed via the unlicensed cell having an unlicensed cell frequency bandwidth. A minimum frequency bandwidth threshold is defined for transmission via the unlicensed cell. A processor of the user equipment selects a random access preamble sequence for the random access procedure. The processor further determines a frequency bandwidth for transmitting the random access preamble sequence via the unlicensed cell. The determined frequency bandwidth is at least the minimum frequency bandwidth threshold. A transmitter of the user equipment transmits the random access preamble sequence to the radio base station such that at least the determined frequency bandwidth of the unlicensed cell is occupied.

[0112] Accordingly, in one general first aspect, the techniques disclosed herein provide a radio base station for performing a random access procedure together with a user equipment in a mobile communication system, the user equipment being configured with at least one unlicensed cell, the random access procedure being performed via the unlicensed cell having an unlicensed cell frequency bandwidth. A minimum frequency bandwidth threshold is defined for transmission via the unlicensed cell. A frequency bandwidth in which the user equipment transmits a random access preamble sequence via the unlicensed cell is determined, the determined frequency bandwidth being at least the minimum frequency bandwidth threshold. A receiver of the radio base station receives a random access preamble sequence for the random access procedure selected by the user equipment such that at least the determined frequency bandwidth of the unlicensed cell is occupied. The determined frequency bandwidth is at least the minimum frequency bandwidth threshold.

[0113] Further benefits and advantages of the disclosed embodiments will be apparent from the specification and drawings, and these benefits and / or advantages may be provided individually by various embodiments and features of the disclosure herein and by the drawings, and not all need be provided to obtain one or more of these benefits and / or advantages.

[0114] These general and specific aspects may be implemented using a system, a method, a computer program, or any combination thereof.

[0115] In the following, exemplary embodiments are explained in more detail with reference to the accompanying drawings. [Brief description of the drawings]

[0116] [Figure 1] 1 illustrates an example architecture of a 3GPP LTE system. [Diagram 2]1 illustrates an example downlink resource grid of a downlink slot of a subframe defined in 3GPP LTE (Release 8 / 9). [Diagram 3] 1 illustrates a contention-based RACH procedure defined in 3GPP LTE (as of Release 8 / 9), where contention may occur. [Figure 4] 1 illustrates the contention-free RACH procedure as defined in 3GPP LTE (as of Release 8 / 9). [Diagram 5] 1 shows the structure of the RACH preamble. [Figure 6] 1 illustrates multiplexing of PRACH transmission with PUSCH and PUCCH. [Figure 7] 1 shows the mapping of the PRACH preamble to the assigned subcarriers. [Figure 8] 1 illustrates different locations of the PRACH within the nominal frequency system bandwidth. [Figure 9] 1 illustrates different locations of the PRACH within the nominal frequency system bandwidth. [Figure 10] 1 illustrates an exemplary functional structure of a PRACH preamble transmitter. [Figure 11] 1 illustrates an example LAA scenario involving several licensed and unlicensed cells. [Figure 12] 4 shows the transmission behavior during LAA transmission. [Figure 13] Shows the timing between a Wi-Fi transmission burst and an LAA UE downlink burst in an unlicensed cell. [Figure 14A] 1 illustrates the frequency bandwidth of PRACH signal transmission according to a first embodiment using a repetition mechanism to comply with minimum channel occupancy requirements for a system bandwidth of 20 MHz for licensed cells. [Figure 14B]1 illustrates the frequency bandwidth of PRACH signal transmission according to a first embodiment using a repetition mechanism to comply with minimum channel occupancy requirements for a system bandwidth of 20 MHz for an unlicensed cell. [Figure 15A] 13A and 13B respectively show the frequency bandwidth of the PRACH signal transmission according to the first embodiment in the case of a system bandwidth of 10 MHz of the licensed cell. [Figure 15B] 1 illustrates the frequency bandwidth of PRACH signal transmission according to a first embodiment for a system bandwidth of 10 MHz for an unlicensed cell. [Figure 16] It is based on the implementation of FIG. 15B and in particular shows different subcarriers carrying PRACH signals of two adjacent PRACH transmissions / repetitions according to the first embodiment. [Figure 17A] 1 shows the power spectral density of PRACH transmissions over licensed cells. [Figure 17B] 1 illustrates the power spectral density of an improved PRACH transmission over an unlicensed cell according to a first embodiment. [Figure 18] 2 illustrates an exemplary implementation of a transmitter chain of a UE according to a first embodiment. [Figure 19] 1 shows a repetition pattern according to an improved first embodiment in which at least two preambles are selected to be transmitted through the repetition. [Figure 20] 1 shows a repetition pattern according to an improved first embodiment in which at least two preambles are selected to be transmitted through the repetition. [Figure 21] 1 shows a repetition pattern according to an improved first embodiment in which at least two preambles are selected to be transmitted through the repetition. [Figure 22A] 13 shows the frequency bandwidth of the PRACH signal transmission according to a second embodiment of adapting the parameters of the PRACH signal transmission to comply with the minimum channel occupancy requirements for a system bandwidth of 20 MHz of a licensed cell. [Figure 22B]13 illustrates the frequency bandwidth of PRACH signal transmission according to a second embodiment of adapting parameters of PRACH signal transmission to comply with minimum channel occupancy requirements for a system bandwidth of 20 MHz for an unlicensed cell. [Figure 23A] 13 illustrates the frequency bandwidth of the PRACH signal transmission according to the second embodiment for a system bandwidth of 10 MHz of the licensed cell. [Figure 23B] 13 shows the frequency bandwidth of PRACH signal transmission according to the second embodiment for a system bandwidth of 10 MHz for an unlicensed cell. [Figure 24A] 1 shows the power spectral density of PRACH transmissions over licensed cells. [Figure 24B] 5A and 5B respectively show the power spectral density of an improved PRACH transmission over an unlicensed cell according to the second embodiment. [Diagram 25] 22A and 22B, in particular according to a second embodiment, and shows various subcarriers carrying the PRACH signal. [Figure 26] 23A and 23B, in particular according to a second embodiment, and shows various subcarriers carrying the PRACH signal. [Figure 27] 1 illustrates an exemplary implementation of a transmitter chain of a UE according to a second embodiment. [Figure 28] 13 illustrates a frequency bandwidth of PRACH signal transmission for a system bandwidth of 40 MHz according to a third embodiment combining the first and second embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0117] A "mobile station" or "mobile node" or "user terminal" or "user equipment" is a physical entity in a communication network. A node may have several functional entities. A functional entity means a software or hardware module that performs a given set of functions and / or provides a given set of functions to another functional entity of the node or the network. A node may have one or more interfaces that attach the node to a communication device or medium through which the node can communicate. Similarly, a network entity may have logical interfaces that attach the functional entity to a communication device or medium through which the network entity can communicate with another functional entity or a correspondent node.

[0118] The term "radio resource" as used in the claims and in this application should be broadly understood to mean physical radio resources (eg, time-frequency resources).

[0119] The term "unlicensed cell" or "unlicensed carrier" as used in the claims and in this application should be understood broadly as a cell / carrier operating in an unlicensed frequency band having a particular frequency bandwidth. Correspondingly, the term "licensed cell" or "licensed carrier" as used in the claims and in this application should be understood broadly as a cell / carrier operating in a licensed frequency band having a particular frequency bandwidth. These terms should be understood, exemplarily, in the context of 3GPP as of Release 12 / 13 and the work item "Licensed-Assisted Access".

[0120] The term "minimum frequency bandwidth threshold" as used in the claims and in this application should be understood broadly as the minimum channel occupancy in the unlicensed cell(s). In other words, a transmission through an unlicensed cell occupies at least the amount set by this threshold in terms of frequency. The minimum channel occupancy is given, for example, by regulations defined for a particular geographical area, e.g., 80% of the system bandwidth in Europe. Thus, in Europe, a transmission in an unlicensed cell with 20 MHz must occupy at least 16 MHz.

[0121] The term "random access procedure" as used in the claims and in this application can be interpreted, in an exemplary embodiment, as the random access procedure of the 3GPP standard described in the Background section. The terms "random access preamble sequence", "preamble sequence", "preamble", "RACH preamble", and "preamble signature" can be used synonymously with each other to mean a complex sequence transmitted by a UE during a random access procedure (in an exemplary embodiment, a preamble message transmitted as described in connection with step 301 of FIG. 3 and step 401 of FIG. 4).

[0122] The term "repetition" as used in the claims and in this application should be interpreted broadly as "performing a particular operation several times." In this particular case, the transmission of the preamble is performed several times, but at different positions in the frequency domain.

[0123] The terms "occupy" and "occupy a frequency bandwidth" as used in the claims and in this application may be broadly interpreted to mean that a particular transmission of a signal / message / preamble is performed by using (all of) the frequencies in a particular frequency bandwidth.

[0124] As explained in the Background section, 3GPP is currently in the process of introducing Licensed Assisted Access (LAA). Although some agreement has already been reached on LAA, some important issues regarding LAA have yet to be agreed upon. Furthermore, in order to support RACH procedures in unlicensed bands, it is evident that it would be advantageous to make some modifications to the LTE specifications compared to the current specifications, which are limited to operation in licensed bands.

[0125] One simple solution for introducing the random access procedure in LAA is to apply the existing random access procedure in licensed cells to unlicensed cells, including the existing preamble format, signal shape and transmission procedure described in the background description section. In this case, CCA can be performed at the UE side just before the PRACH transmission opportunity, or alternatively at the eNodeB side before scheduling the RRACH transmission opportunity. A further alternative solution could omit the CCA related to the scheduling and transmission of the PRACH, however, omitting CCA may cause problems with other nodes operating on the same radio channel due to possible collision of transmissions. It should be noted that whether CCA is ultimately required or not depends on the regulations in the region where the system is operated (see background section and non-patent document 4).

[0126] However, this simple approach also has drawbacks. In particular, according to European regulations as explained in the background section, each transmission in unlicensed bands following CCA (Clear Channel Assessment) must occupy at least 80% of the nominal channel bandwidth. Similar regulations can be found in other countries, for example in the United States, where the minimum transmission bandwidth is 500 kHz (see Non-Patent Document 4). Assuming a nominal channel bandwidth of 20 MHz for LTE operation in unlicensed bands (see Non-Patent Document 4), the minimum channel occupancy of 80% as specified in Europe results in a minimum frequency bandwidth of 16 MHz. However, on the other hand, PRACH transmissions according to the existing definition in licensed bands occupy only 6 consecutive PRBs, regardless of the channel bandwidth, which corresponds to 1.08 MHz (i.e. only 5.4% of the nominal channel bandwidth of 20 MHz). Therefore, this simple solution (applying the existing definition of PRACH transmissions to unlicensed cells) does not meet the minimum channel occupancy requirements given by European regulations.

[0127] In addition, it should be noted that this minimum channel occupancy depends on the actual channel bandwidth of the unlicensed cell and may therefore vary from one unlicensed cell to another. In other words, the transmission of the random access preamble must be adapted to the channel bandwidth in order to be able to comply with the minimum channel occupancy requirement defined for the unlicensed cell. In contrast, the existing random access procedure, in particular the transmission of the random access preamble, has a fixed bandwidth, i.e. always uses 6 PRBs, regardless of the actual channel bandwidth of the (licensed) cell. Therefore, a further drawback of using the existing mechanisms to transmit the random access preamble via the unlicensed cell is the lack of flexibility to comply with the minimum channel occupancy requirement, which may in fact vary depending on the channel bandwidth of the unlicensed cell.

[0128] The inventors have devised the following exemplary embodiments with the objective of alleviating one or more of the problems discussed above.

[0129] Particular implementations of the various embodiments are implemented within the broad specifications given by the 3GPP standard, some of which are described in the Background section, with the addition of particularly important features relevant to the various embodiments as described below. It should be noted that the embodiments can be advantageously used in mobile communication systems such as, for example, 3GPP LTE-A (Release 10 / 11 / 12 / 13) as described in the Background section, but the embodiments are not limited to use in these particular exemplary communication networks.

[0130] The following description should not be understood as limiting the scope of the present disclosure, but as merely examples of embodiments for a deeper understanding of the present disclosure. Those skilled in the art should recognize that the general principles of the present disclosure described in the claims can be applied to various scenarios in ways not explicitly described herein. For the purpose of explanation, some assumptions are made, but these assumptions do not limit the scope of the following embodiments.

[0131] Furthermore, as mentioned above, the following embodiments can be implemented in a 3GPP LTE-A (Release 12 / 13) environment. These various embodiments mainly allow an improved random access procedure, in particular an improved transmission of the random access preamble. However, other features (i.e. features that are not changed by the various embodiments) can remain exactly the same as described in the Background section or can be changed without impact on the various embodiments, such as the features and procedures that lead to the execution of the improved random access procedure (such as the need for uplink synchronization and the need to transmit a scheduling request) and the remaining steps of the random access procedure (such as random access response, contention resolution, etc.).

[0132] In the following, three embodiments for solving the above problem(s) are described, which are illustrated by using the following example scenarios devised to easily explain the principles of the embodiments, however, these principles can also be applied to other scenarios, some of which are explicitly mentioned in the following description.

[0133] As explained in the Background section, 3GPP plans to enhance the current system by introducing Licensed Assisted Access (LAA), which includes the use of unlicensed cells operating in a channel(s) in an unlicensed frequency band. In the following, such a scenario is assumed, i.e., the UE is configured with at least one licensed cell and at least one unlicensed cell. Although the following description is based on such a scenario, the various embodiments focus on performing a random access procedure in the unlicensed cell, and therefore also apply to a scenario in which an unlicensed cell operates alone (i.e., without a corresponding licensed cell).

[0134] An unlicensed cell can be set up between an eNodeB and a UE in the usual way as described in the background section. An unlicensed cell is therefore operated in a particular channel in an unlicensed frequency band with a particular frequency bandwidth (also called nominal channel bandwidth in some European standards) (e.g. 10 MHz, 20 MHz, 40 MHz, or (in the future) smaller or larger bandwidth, etc.). As described in detail in the background section, operation in an unlicensed cell is regulated in various ways, for example in Europe, according to the European standard 3GPP TS 2010-2013-01 14:14 Page 15 of 20. In Europe (and elsewhere), a minimum channel occupancy is defined, among other things, for the channels of an unlicensed cell, for example in Europe the channel bandwidth occupied in an unlicensed cell is in the range of 80% to 100% of the declared nominal channel bandwidth of the unlicensed cell. Transmissions in an unlicensed cell must therefore (with very few exceptions) comply with this minimum channel occupancy requirement, such that the transmission occupies a corresponding frequency bandwidth portion of the total frequency bandwidth of the unlicensed cell. Considering that unlicensed cells can have different nominal channel bandwidths, the resulting minimum frequency bandwidth (which is a percentage of the nominal channel bandwidth) that needs to be occupied differs between channels with different nominal channel bandwidths.

[0135] In the following embodiments, it is assumed that both the eNodeB and the UE are aware of the specific minimum channel occupancy to be followed. The UE and the eNodeB are aware of the minimum frequency bandwidth threshold, which depends on the actual system bandwidth where the unlicensed cell is established. There are several possible different ways to achieve this. In one alternative, both the UE and the eNodeB determine the specific minimum frequency bandwidth threshold independently of each other, and both arrive at the same value by following the same decision rule. In another alternative, the eNodeB determines the specific minimum frequency bandwidth threshold and correspondingly informs the UE of this threshold, for example in a system information broadcast message, or in an RRC connection establishment message, or in the case of a contention-free random access procedure, in a random access preamble assignment message (see message 401 in FIG. 4) transmitted at the beginning of the random access procedure. According to yet another alternative, the UE determines the specific minimum frequency bandwidth threshold and correspondingly informs the eNodeB of this threshold. In either case, both the UE and the eNodeB have the same understanding of the minimum frequency bandwidth threshold that the transmission of the preamble should occupy at least.

[0136] This minimum frequency bandwidth threshold represents a lower limit of the frequency bandwidth that the transmission of the random access preamble must occupy. Furthermore, the actual used frequency bandwidth of the transmission of the random access preamble must also be known by both the UE and the eNodeB so that the eNodeB can successfully blind decode the random access preamble. The actual frequency bandwidth of the preamble transmission can be determined by the UE and / or the eNodeB, similar to when determining the minimum frequency bandwidth threshold, and this information can be exchanged between the two entities if necessary. Details will also be apparent from the detailed description of the various embodiments.

[0137] As mentioned above in the Background section, currently it is agreed that only contention-free RACH procedures are supported in unlicensed cells, the details of which are explained in the Background section. Therefore, the assumed scenario follows this initial agreement, but it should be understood that the principles of the present invention according to various embodiments are equally applicable to contention-based RACH procedures. In particular, as will become clear below, various embodiments of the present invention focus on the transmission of random access preambles, and therefore are equally applicable to contention-based RACH procedures in which the UE autonomously selects an appropriate random access preamble sequence (from an appropriate set of preambles) and to contention-free RACH procedures in which the UE receives corresponding indication information from the eNodeB regarding which random access preamble sequence (from the set of preambles) is used for the random access procedure. In the contention-based RACH procedure, similar to the method described in the Background section, it is possible to additionally transmit one bit of information giving information on the amount of transmission resources required to transmit the next message (msg3, 303 in Figure 3), thus allowing the UE to choose between two subgroups (the set of preambles available for the contention-based random access procedure is divided into these two subgroups).

[0138] In the following embodiments, it is further assumed that the random access procedure does not need to be modified, except for the transmission (and reception) of the random access preamble. As a result, the overall structure and sequence of the random access procedure exemplarily described in the Background section may remain the same, and only modifications are introduced into the random access procedure in relation to the transmission of the random access preamble, as described in the various embodiments below. For example, the standardized procedure for triggering the random access procedure and other messages of the random access procedure (such as the random access response message 302, 403, the scheduled transmission 303, the contention resolution message 304, the random access preamble allocation 401, etc.) may not need to be modified. Therefore, in order to avoid repetition of explanations, reference may be made to the corresponding paragraphs in the Background section above.

[0139] Therefore, assuming that a random access procedure is triggered in an unlicensed cell, the following embodiments provide several implementations of an improved random access procedure performed in an unlicensed cell.

[0140] First embodiment In the following, a first embodiment for solving the above problem(s) will be described in detail. Various implementation forms of the first embodiment will be described below by using the example scenario introduced above.

[0141] Briefly, according to the first embodiment, the existing definition of how to transmit a random access preamble to a radio base station is reused, but in addition to this, a repetition mechanism is introduced in the first embodiment as follows: The repetition mechanism in the UE allows the normal transmission of the random access preamble to be repeated as many times as necessary at different frequency positions in the frequency domain, such that the combined transmission of the random access preamble occupies at least the necessary frequency bandwidth to comply with the minimum channel occupancy requirement defined for unlicensed cells. This not only allows the existing definition and standardization of the preamble transmission to be reused as much as possible (and as much as necessary), but at the same time, the repetition scheme allows the entire PRACH transmission (i.e. including all preambles (repetitions)) to be flexibly adapted to different bandwidth requirements by simply adding further repetitions of the "standard" PRACH signal in terms of frequency until the minimum frequency bandwidth threshold is exceeded. More specifically, in an exemplary implementation of the first embodiment, the normal random access procedure in licensed cells, as detailed in the background section, is followed as much as possible. This includes, for example, the UE generating a set of random access preambles in a manner similar to that described in the background section. Thus, for example, it includes using the Zadoff-Chu root sequence explicitly indicated by the eNodeB and generating from this sequence 64 different random access preamble sequences by using cyclic shifts. The set of random access preambles thus generated is not only available for use in performing a random access procedure via licensed cells, but also for use in performing a random access procedure via unlicensed cells. Furthermore, the random access preambles may thus have the same structure as described in connection with FIG. 5 and may have the same sequence length for the various preamble formats (i.e. 839 for formats 0 to 3 or 139 for format 4).The same applies to the 1.25 kHz subcarrier spacing for preamble formats 0-3 and the 7.5 kHz subcarrier spacing for preamble format 4, which can be equally applied according to this implementation. Furthermore, the same PRACH duration (i.e., T) as before is used. CP and T SEQ (combining

[0142] Assuming a contention-free random access procedure, the UE receives from the eNodeB a corresponding indication as to which particular random access preamble of the generated set is to be used for the random access procedure. The UE thus selects the indicated random access preamble from the available set of preambles and prepares the transmission of that preamble to the eNodeB as follows:

[0143] A specific exemplary implementation of the first aspect will be described in detail. First, assume that a licensed cell and an unlicensed cell are configured with a nominal channel bandwidth of 20 MHz (the nominal channel bandwidth can also be referred to as the "system bandwidth"). The following description is made with reference to Figures 14A and 14B, which respectively show PRACH transmissions performed by corresponding UEs supporting LAA in licensed and unlicensed cells. As is evident from Figure 14A, the transmission of a random access preamble (PRACH) over a licensed cell is typically performed in 6 PRBs, i.e., has a frequency bandwidth of 1.08 MHz (6 x 180 kHz). The transmission of a corresponding random access preamble over an unlicensed cell must occupy at least a 16 MHz channel bandwidth of the unlicensed cell in order to comply with the 80% minimum channel occupancy parameter specified in Europe (see Figure 14B). With the aim of achieving this increased channel occupancy, the first embodiment proposes to introduce a repetition mechanism, which repeats the "normal" preamble transmission at multiple different frequency locations to configure the overall PRACH transmission above the minimum frequency bandwidth threshold of 16 MHz. As shown in Figure 14B, the normal PRACH transmission can be repeated as many times as necessary until the minimum channel occupancy of 16 MHz is exceeded. That is, in this particular case, 14 repetitions of the PRACH transmission are performed, thus occupying 16.2 MHz by transmitting the normal PRACH a total of 15 times, using 90 PRBs.

[0144] In the following exemplary scenario in Figures 15A and 15B, it is assumed that a licensed cell and an unlicensed cell with a nominal channel bandwidth of 10 MHz are configured. Correspondingly, as a result of a minimum channel occupancy of 80%, the combined random access preamble transmission must occupy a minimum frequency bandwidth of at least 8 MHz. As explained in relation to Figures 14A and 14B, Figure 15A discloses a PRACH transmission over the licensed cell (normally over 6 PRBs). In contrast, as shown in Figure 15B, in the case of an unlicensed cell, there are 7 repetitions and therefore 8 PRACH transmissions, occupying a total of 48 PRBs, 8.64 MHz (48 x 180 kHz).

[0145] In a particular exemplary implementation, different offsets φ can be used in different repetitions of the preamble transmission according to Fig. 14B and Fig. 15B, which can be directly derivable by the UE from an initial offset φ (7 or 2 depending on the format of the preamble) derived from the corresponding tables in the background art section. The different offsets can be selected such that the individual preamble transmissions are directly adjacent but do not overlap each other. Alternatively, although not shown in the figures, it may be possible to allow a slight overlap of two adjacent preamble transmissions such that only one guard band separates the two preamble transmissions (instead of two guard bands as is evident from Fig. 16). For this purpose, the frequency offset of the repetitions must be set depending on the overlap.

[0146] Figure 16 builds on the example scenario of Figure 15B and expands the diagram to show the different subcarriers and guard bands for two adjacent preamble transmissions out of the eight. As can be seen, a typical subcarrier frequency spacing of 1.25 kHz and 839 subcarriers that make up the PRACH signal are assumed (see also Figure 7 and the corresponding part in the Background section).

[0147] The required number of repetitions required to comply with the minimum channel occupancy requirements specified for unlicensed cells can be determined autonomously by the UE and the eNodeB by a simple calculation based on the frequency bandwidth occupied by the normal preamble transmission (i.e. 1.08 MHz) and the minimum frequency bandwidth threshold (e.g. 16 MHz for a 20 MHz system bandwidth, 8 MHz for a 10 MHz system bandwidth). Alternatively, the eNodeB can explicitly indicate to the UE the number of repetitions that the UE uses when transmitting the preamble. Alternatively, the number of repetitions for different system bandwidth configurations can be predefined in the standard. As is evident from Fig. 14B and Fig. 15B, for the assumed exemplary scenarios, 14 and 7 repetitions are required, respectively, in other words, 15 and 8 PRACH transmissions are required, respectively. The eNodeB can therefore successfully decode the PRACH transmission. Alternatively, the number of repetitions for each nominal channel bandwidth can be predefined in the standard and thus known to both the UE and the eNodeB.

[0148] In an exemplary implementation of the first aspect, it is assumed that each PRACH transmission is transmitted by the UE with the same transmission power as that used for normal PRACH transmission (in the licensed cell). In particular, Fig. 17A shows a PRACH transmission of a UE over a licensed cell with a particular transmission power and a particular power spectral density, which spans the normal 6 PRBs, 1.08 MHz. The transmission power is determined in a normal manner (e.g. by open-loop estimation with fully compensated path loss). The UE estimates the path loss by averaging measurements of Reference Signal Received Power (RSRP). Fig. 17A thus shows such a PRACH transmission over a licensed cell. Fig. 17B shows in a suitable manner a combined PRACH transmission over an unlicensed cell, which spans 48 PRBs, 8.64 MHz, as described above in relation to Fig. 15B for the first embodiment. As is evident from Figure 17B, this implementation of the first embodiment assumes that all different transmissions (i.e. all repetitions) of a normal PRACH transmission have the same power spectral density (i.e. are transmitted with the same transmit power), which can be implemented in the UE by applying the same transmit power value configured for the normal PRACH transmission to the repetitions at different frequency locations via the unlicensed cell.

[0149] Alternatively, the UE may transmit the various PRACH transmissions using different transmit power levels, rather than using the same transmit power value. For example, the various PRACH transmissions may all be transmitted at a lower transmit power (e.g., half the transmit power). One particular method for setting the transmit power is to set the transmit power of each of the various PRACH transmissions such that the total transmit power (i.e., the transmit power used to transmit all of the PRACHs (e.g., a total of 8 transmissions in the case of FIG. 17B)) is the same as the transmit power used to transmit one PRACH via the licensed cell. Thus, the power spectral density is reduced by the total number of PRACH transmissions (e.g., PSD / 8), but the total transmit power used by the UE for PRACH transmissions remains the same.

[0150] Furthermore, Fig. 18 shows an exemplary implementation of a transmitter of a UE according to the first embodiment, described in the background section in relation to Fig. 10. As is evident from Fig. 18, the repetition mechanism described above in the various implementations of the first embodiment can be implemented in the transmit chain between DFT and subcarrier mapping. By DFT and subcarrier mapping, the positioning of the PRACH signal in the frequency domain is achieved, and thus the generated preamble is processed in various DFTs to obtain the resulting frequency samples (N ZC ) to the corresponding frequency positions by subcarrier mapping, ZC The same generated preamble (left part) of can be repeated at multiple different frequency locations in the frequency domain as exemplarily shown in FIG. 14B and FIG. 15B.

[0151] Further implementations of this first embodiment provide improvements by allowing different preamble sequences to be used for different repetitions. These improved implementations are described in relation to Figures 19, 20, and 21. In brief, by allowing different preamble sequences to be used for different repetitions and appropriately determining the various repetition patterns between the eNodeB and the UE, additional information can be encoded in the overall PRACH transmission.

[0152] The additional information may for example include an indicator of the channel occupancy observed by the UE sending the PRACH. The observed channel occupancy may be defined by the ratio of successful and unsuccessful CCA (clear channel determination) at the UE prior to the PRACH transmission. A threshold may be defined for this ratio, e.g. 0.5. In this case, the transmitted PRACH conveys information whether this ratio is higher, equal or lower than the defined threshold. The eNB may utilize this information when scheduling downlink data transmissions to the UE and may expect a poor quality of service if this ratio is low.

[0153] In the above implementation of the first embodiment, in particular, it is assumed that only one preamble (among the available preambles) is used in the entire PRACH transmission (including repetitions) according to the standard procedure of normal PRACH transmission, i.e. the same preamble is repeatedly transmitted at different frequency positions. Thus, only one preamble (e.g. indicated by corresponding indication information from the eNodeB) is selected by the UE, and the preamble is used for each PRACH transmission. However, in a further implementation of the first embodiment, two or more different preambles transmitted by the UE in the same random access procedure via an unlicensed cell can be used, as described below.

[0154] First, it is assumed that two different preambles are selected by the UE to perform a random access procedure via an unlicensed cell. According to one implementation, both different preambles can be indicated by the eNodeB separately. Alternatively or additionally, a certain correspondence between the different preambles can be defined, so that when one specific random access preamble is indicated by the eNodeB (or when the UE autonomously selects one random access preamble in case of contention-based RACH), the UE correspondingly selects the further random access preamble(s) associated with the indicated (or autonomously selected) random access preamble. To improve the transmission performance, the specific correspondence can be optimized by appropriately defining the correspondence such that the PAPR (Peak-to-Average Power Ratio) or CM (Cubic Metric) of the entire transmission is minimized.

[0155] Thus, different PRACH transmissions are performed using different preambles. In the exemplary scenario of Fig. 19, which assumes a system bandwidth of 20 MHz, preamble A and preamble B are alternated in the frequency domain, thus forming a repetition pattern of ABABABAB... for transmitting 15 PRACH transmissions. Another exemplary repetition pattern is shown in Fig. 20, which also assumes two different preambles A, B, where preamble A is used in one (approximately) half of the total frequency bandwidth of the combined PRACH transmissions and preamble B is used in the other half (i.e., AAAAAAABBBBBBBB). In the exemplary implementation of Fig. 21, a total of three different preambles (preambles A, B, C) are assumed, with the exemplary repetition pattern AAAAABBBBBCCCCC.

[0156] In one example implementation, the repetition pattern to be used can be selected by the UE, e.g., from a limited number of preconfigured repetition patterns, which can be configured, e.g., by the eNodeB and signaled to the UE(s) in its cell, or can be predefined in a standard.

[0157] Each preconfigured repetition pattern can be associated with, for example, one particular piece of information, such that by the UE selecting a particular repetition pattern, the particular piece of information is already encoded. For example, when the eNodeB blindly decodes the repetitions of the PRACH, it successfully decodes the various PRACH transmissions according to the repetition pattern selected by the UE, thus deriving the encoded information.

[0158] The information regarding the required transmission resources may be the information that is coded by the repetition pattern. Considering two different repetition patterns that may be used by the UE, one repetition pattern may be associated with a larger amount of transmission resources, while the other repetition pattern may be interpreted as indicating that only a smaller amount of transmission resources is required by the UE.

[0159] Another important piece of information can be the statistics of the channel occupancy observed from the UE's point of view as mentioned above: the pattern ABABABAB... can indicate for example a channel occupancy higher than 0.5 and the pattern BABABABA... can indicate a channel occupancy less than or equal to 0.5.

[0160] Second embodiment In the following, a second embodiment for solving the above problem(s) is described in detail. The principle behind the second embodiment is completely different from the repetition mechanism described in relation to the first embodiment. Various implementation forms of the second embodiment are described in detail below by using the example scenario introduced above.

[0161] Briefly, in the second embodiment, instead of performing various repetitions of the normal PRACH signal as in the first embodiment, one or more configuration parameters for transmitting the PRACH signal are adapted to spread the signal over the required frequency bandwidth (i.e. to comply with the minimum channel occupancy requirements for unlicensed cells). The configuration parameters include the length of the RACH preamble sequence (i.e. N ZC ) and the subcarrier frequency spacing of the subcarriers used to transmit the RACH preamble (i.e., Δf RA ). These two parameters, in combination, essentially define the total frequency bandwidth of the PRACH signal transmission. The frequency bandwidth of a normal PRACH transmission is always 1.08 MHz, as explained in the Background section, regardless of the system bandwidth of the channel on which the PRACH is transmitted. For example, for preamble formats 0 to 3, the subcarrier frequency spacing is 1.25 kHz and they have 864 subcarriers (839 subcarriers + 2 × 12.5 subcarriers for guard bands) (see Figure 7), thus having a frequency bandwidth of 1.08 MHz. For preamble format 4, the subcarrier frequency spacing is 7.5 kHz and they have 144 subcarriers (139 subcarriers + 2 × 2.5 subcarriers for guard bands), also having a frequency bandwidth of 1.08 MHz. Note that the number of subcarriers used to transmit the PRACH signal is determined by the length N of the preamble sequence. ZC Because the preamble sequence is ZC are converted to frequency samples, which correspond to N ZCThis is because the preamble sequences are mapped to two subcarriers. This implementation method is generally applied in LTE because, as a property of ZC sequences, the DFT of a ZC sequence is also a weighted cyclically-shifted ZC sequence. Furthermore, it should be noted that when the length of the preamble sequence is a prime number, optimal cyclic cross-correlation is achieved between any pair.

[0162] Thus, by appropriately selecting different values ​​of these two parameters, the frequency bandwidth of the PRACH transmission can be controlled in order to comply with the minimum channel occupancy requirements specified for unlicensed cells. For this purpose, either one of the two parameters or both of the two parameters can be modified when compared to the normal / legacy PRACH signal transmitted in unlicensed cells. These two parameters (N ZC and Δf RA ), many different combinations are possible depending on the actual minimum frequency bandwidth threshold that the PRACH signal transmission must occupy at least.

[0163] In the following, as already done for the first embodiment, two different system bandwidths of 10 MHz and 20 MHz are assumed, respectively shown in Figures 22A, 22B, 23A and 23B, and furthermore, the same minimum channel occupancy requirement of 80% in Europe is assumed, and therefore the minimum frequency bandwidth thresholds of 8 MHz and 16 MHz, respectively, are followed when performing a random access procedure via an unlicensed cell (e.g. when transmitting a preamble from a UE to an eNodeB as part of the random access procedure).

[0164] For example, one can keep the subcarrier frequency spacing at 1.25 kHz (or 7.5 kHz for preamble format 4) as for normal / legacy PRACH transmissions for preamble formats 0-3, thus leaving only the length of the preamble sequence as a parameter to control depending on the sought minimum frequency bandwidth threshold. For 1.25 kHz and a frequency bandwidth threshold of 8 MHz, at least 6400 subcarriers are "necessary" to achieve a PRACH signal with a frequency bandwidth of 8 MHz. For an improved preamble design that maximizes the number of ZC sequences with optimal cross-correlation properties, a prime length preamble sequence should be chosen. In the case just described, a preamble length of 6421 should be chosen, resulting in a frequency bandwidth of 8.026 MHz.

[0165] For this, the preamble sequence length and therefore the number of subcarriers for transmitting the preamble signal can be kept the same as for normal / legacy PRACH transmission (i.e. 839 for preamble formats 0-3 and 139 for preamble format 4). In this particular case, it is possible to change the frequency bandwidth of the PRACH signal by adapting the subcarrier frequency spacing parameter. For example, for a preamble of length 839 (864 subcarriers in total including additional subcarriers for the two guard bands) and a frequency bandwidth threshold of 8 MHz, a subcarrier frequency spacing of at least 9.26 kHz is required.

[0166] Alternatively, both the preamble length and the RACH subcarrier frequency spacing can be changed to comply with the minimum channel occupancy requirements. In the above mentioned case with a 10 MHz system bandwidth for unlicensed cells, a subcarrier frequency spacing of 7.5 kHz can be assumed, in which case one needs to have at least 1067 subcarriers in total for the PRACH signal (including the subcarriers for the actual preamble and additional subcarriers for the guard band).

[0167] It should be noted that in general, in order to minimize the orthogonality loss in the frequency domain between the subcarriers of the preamble and the subcarriers of the surrounding uplink data transmission, the subcarrier frequency spacing adopted for the PRACH transmission should be an integer fraction (1 kHz, 2.5 kHz, 3 kHz, 5 kHz, 7.5 kHz, 15 kHz, etc.) of the subcarrier frequency spacing used for the PUSCH transmission (i.e., 15 kHz). Conversely, the subcarrier spacing of the PUSCH should be an integer multiple of the subcarrier spacing of the PRACH. Furthermore, in order to facilitate multiplexing of the PRACH and the PUSCH, the PRACH should be assigned a frequency bandwidth equal to an integer multiple of the frequency bandwidth of the resource block (i.e., an integer multiple of 180 kHz). Furthermore, for an improved preamble design that maximizes the number of ZC sequences with optimal cross-correlation properties, a prime length preamble sequence should be chosen. For optimal results, the design constraints just described are more easily achieved when both parameters (i.e., preamble length and subcarrier frequency spacing) are made variables, as explained below.

[0168] First, assume a system including unlicensed cells with a system bandwidth of 20 MHz and a corresponding minimum frequency bandwidth threshold of 16 MHz. Considering that the resulting frequency bandwidth of the PRACH transmission signal should be a multiple of the resource block bandwidth of 180 kHz, a total frequency bandwidth of 16.02 MHz spanning 89 PRBs can be assumed for the PRACH signal, which facilitates frequency multiplexing of PRACH and PUSCH as described above. In an exemplary implementation, a subcarrier frequency spacing of 15 kHz can be determined, resulting in a number of subcarriers of 1068. The nearest prime number less than 1068 is 1063, and therefore 5 subcarriers for two guard bands (i.e. 2.5 subcarriers per guard band) can be foreseen. This exemplary implementation of the PRACH signal according to the second embodiment is shown in FIG. 22B and FIG. 25. In such a configuration of the PRACH transmission signal, the subcarrier frequency spacing is an integer fraction of the subcarrier frequency spacing of the PUSCH (which minimizes the loss of orthogonality in the frequency domain) and the length of the preamble sequence is a prime number (which improves the cross-correlation properties).

[0169] Now, assume an exemplary system with a system bandwidth of 10 MHz for unlicensed cells and a corresponding minimum frequency bandwidth threshold of 8 MHz. Considering that the resulting bandwidth of the PRACH transmission signal should be a multiple of the resource block bandwidth of 180 kHz, a total frequency bandwidth of 8.1 MHz spanning 45 PRBs can be assumed for the PRACH signal. A subcarrier frequency spacing of 7.5 kHz can be assumed. This results in a total of 1080 subcarriers for the PRACH signal (including the subcarriers of the actual preamble and additional subcarriers for the guard bands). The nearest prime number less than 1080 is 1069, so 11 subcarriers for the two guard bands (i.e. 5.5 subcarriers per guard band) can be foreseen. This exemplary implementation of the PRACH signal according to the second embodiment is shown in FIG. 23B and FIG. 26.

[0170] As an alternative for the 20 MHz system, the preamble sequence length can be chosen to be a prime number 1069, which is the same length as for the 10 MHz system, and therefore the same preamble can be used for both bandwidths of the unlicensed cell, with the advantage that the UE does not have to provide preambles with different sequence lengths to support the two system bandwidths. Thus, assuming that the total frequency bandwidth should cover 16.2 MHz (i.e., covering 90 PRBs each with 180 kHz), a total of 1080 subcarriers, each with 15 kHz, are used to transmit the PRACH signal. This results in 5.5 subcarriers per guard band.

[0171] In both example implementations, the preamble lengths of 1069 and 1063, respectively, also impact the size of the DFT, but do not increase the IDFT (see FIG. 27) very significantly when compared to the preamble length of 839 already expected for the legacy PRACH. By keeping the length of the preamble sequence relatively small, the complexity of the DFT and IDFT operations does not increase significantly.

[0172] A similar method can be applied to set the parameters used for transmitting the PRACH signal in the case of an unlicensed cell having a different system bandwidth (eg, 40 MHz).

[0173] In summary, as mentioned above, there are several ways on how to set the preamble sequence length and the RACH subcarrier frequency spacing to achieve that the frequency bandwidth of the resulting PRACH transmission signal exceeds the minimum frequency bandwidth threshold imposed on the unlicensed cell. The corresponding parameter(s) can be selected either by the UE or by the eNodeB, and in the latter case the eNodeB must indicate the selection result to the UE.

[0174] In one particular implementation, different combinations of parameters (e.g., the combinations of parameters mentioned above) for various system bandwidths are pre-configured, so that for a system bandwidth of 20 MHz, a preamble length of 1069 and a subcarrier frequency spacing of 15 kHz can be selected. Similarly, for a system bandwidth of 10 MHz, a preamble length of 1069 and a subcarrier frequency spacing of 7 kHz can be selected.

[0175] As explained above, according to the second embodiment, the sequence length of the preamble can be changed as a function of the system bandwidth (i.e. the corresponding minimum frequency bandwidth threshold). Therefore, it is highly likely that a certain preamble (having a fixed length of 839 or 139) generated for performing a random access procedure via a licensed cell cannot be reused for performing a random access procedure via an unlicensed cell. Therefore, in a particular implementation of the second embodiment, at least a further set of random access preambles can be generated for this purpose, and thus a different set of preambles is available for performing a random access procedure via either a licensed cell or an unlicensed cell. According to the above-mentioned exemplary implementation of the second embodiment, a further set of preambles having a sequence length of 1069 can be generated. For example, a suitable root sequence having a sequence length of 1069 can be provided (e.g. provided by the eNodeB and indicated to the UE), from which the UE can generate a certain number of different preambles by performing a cyclic shift.

[0176] For example, 64 different preambles of length 1069 can be generated by performing cyclic shifts of the corresponding root sequences. On the other hand, fewer preambles (e.g. only 16) can also be generated for the set, taking into account that random access procedures are likely to be performed via unlicensed cells.

[0177] In an exemplary implementation of the second aspect, it is assumed that the PRACH transmission via the unlicensed cell is performed using the same transmission power as that set for the normal PRACH transmission via the licensed cell. A corresponding diagram of this implementation is presented in Fig. 24A and Fig. 24B. As can be seen from Fig. 24B, the power spectral density of the PRACH transmission via the unlicensed cell is significantly reduced when compared to the corresponding PRACH transmission via the licensed cell shown in Fig. 24A. Alternatively, the PRACH transmission via the unlicensed cell can be transmitted at a different transmission power value (higher or lower than the transmission power value used for the PRACH transmission via the licensed cell). For example, the transmission power can be increased to achieve essentially the same power spectral density over the extended frequency bandwidth as in the case of the transmission with the normal PRACH transmission via the licensed cell (see Fig. 24A). On the other hand, if the licensed cell is a macro cell having a large coverage area compared to an unlicensed cell having a small coverage area, the transmission power of the PRACH transmission via the unlicensed cell may also be reduced compared to the PRACH transmission via the licensed cell.

[0178] Moreover, Fig. 27 shows an exemplary implementation of a UE transmitter according to the second embodiment, which is similar to the implementation described in the Background section with reference to Fig. 10. The above-mentioned principles behind the second embodiment do not require substantial modification of the UE transmit chain. Instead, the DFT and IDFT sizes, as well as the sampling rate f, are modified to process the appropriate preamble transmitted via the unlicensed cell. s Different values ​​of are applied. The size of the DFT and IDFT directly corresponds to the sequence length of the preamble.

[0179] Third embodiment The following describes in detail a third embodiment for solving the above problem(s). This third embodiment is essentially a combination of the first and second embodiments, and thus can best combine the two principles. In short, one of the improved PRACH transmissions described according to the second embodiment can be repeated according to the repetition mechanism introduced by the first embodiment.

[0180] For example, the third embodiment is most advantageous for large system bandwidths, e.g., 40 MHz, in order to keep the RACH subcarrier frequency spacing at 15 kHz (for PUSCH) or less, without the need to significantly increase the preamble length, which may be disadvantageous in preamble generation and in the UE transmitter (especially DFT and IDFT) implementation. As an example, assuming a 40 MHz system bandwidth of the unlicensed cell, according to the third embodiment, the PRACH signal described in connection with FIG. 22B may be assumed, which may be repeated once (i.e. transmitted twice in total) in order to comply with the minimum channel occupancy of 80% of the 40 MHz system bandwidth of the unlicensed cell.

[0181] Another example is shown according to FIG. 28, in which a PRACH signal (see FIG. 23B and FIG. 26) having a frequency bandwidth of 8.1 MHz is assumed to be used and repeated three times, so that the combined PRACH transmission, for a total of four PRACHs, covers a sufficient frequency bandwidth of more than 32 MHz.

[0182] Hardware and Software Implementations of the Disclosure Another exemplary embodiment relates to implementing the various embodiments described above using hardware, software, or software cooperating with hardware. In this regard, a user terminal (mobile terminal) and an eNodeB (base station) are provided. The user terminal and the base station are configured to perform the methods described herein and include corresponding entities (receivers, transmitters, processors, etc.) appropriately involved in these methods.

[0183] It is further recognized that the various embodiments may be implemented or performed using a computing device (processor). The computing device or processor may be, for example, a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic device. The various embodiments may also be implemented or embodied by a combination of these devices. In particular, each functional block used in the description of each embodiment described above may be implemented by an LSI as an integrated circuit. These functional blocks may be individually formed as chips, or a single chip may be formed to include some or all of the functional blocks. These chips may include data input / output units coupled thereto. Depending on the degree of integration, the LSI may also be referred to as an IC, a system LSI, a super LSI, or an ultra LSI. However, the technology for implementing an integrated circuit is not limited to an LSI, and may be achieved by using a dedicated circuit or a general-purpose processor. Furthermore, an FPGA (field programmable gate array) that can be programmed after the LSI is manufactured, or a reconfigurable processor that can reconfigure the connections and settings of circuit cells arranged inside the LSI, may also be used.

[0184] Furthermore, the various embodiments may also be implemented by means of software modules, which are executed by a processor or directly in hardware. Also, a combination of software modules and hardware implementation is possible. The software modules may be stored on any kind of computer-readable storage medium, for example RAM, EPROM, EEPROM, flash memory, registers, hard disks, CD-ROM, DVD, etc. It should be noted further that individual features of the different embodiments may be the subject of another embodiment, either individually or in any combination.

[0185] It will be appreciated by those skilled in the art that various changes and / or modifications may be made to the present disclosure as set forth in the specific embodiments without departing from the concept or scope of the invention as broadly described, and the embodiments described herein are therefore to be considered in all respects as illustrative and not restrictive.

Claims

1. A transmitter for transmitting information regarding a random access preamble sequence to a user equipment; a receiver for receiving the random access preamble sequence selected and transmitted by the user equipment based on the information; having The random access preamble sequence is transmitted in a frequency bandwidth of an unlicensed band, the frequency bandwidth being defined by a length of the random access preamble sequence and a subcarrier spacing, and being larger than a frequency bandwidth in which the random access preamble sequence is transmitted in a licensed band. Base station equipment.

2. The random access preamble sequence is selected to have a length greater than the length of a random access preamble sequence used in the licensed band. The base station device according to claim 1 .

3. a random access preamble sequence is selected that has the same length as a random access preamble sequence used in the licensed band; The receiver receives the random access preamble sequence repeatedly transmitted at a plurality of different frequencies. The base station device according to claim 1 .

4. The frequency bandwidth is equal to or greater than the minimum bandwidth required for the unlicensed band. The base station device according to claim 1 .

5. The information about the random access preamble sequence is the length of the random access preamble sequence. The base station device according to claim 1 .

6. The information about the random access preamble sequence is the number of repetitions of the random access preamble sequence. The base station device according to claim 3 .

7. a transmitting step of transmitting information about a random access preamble sequence to a user equipment; receiving the random access preamble sequence selected and transmitted by the user equipment based on the information; having The random access preamble sequence is transmitted in a frequency bandwidth of an unlicensed band, the frequency bandwidth being defined by a length of the random access preamble sequence and a subcarrier spacing, and being larger than a frequency bandwidth in which the random access preamble sequence is transmitted in a licensed band. Random access method.

8. The random access preamble sequence is selected to have a length greater than the length of a random access preamble sequence used in the licensed band. The random access method according to claim 7.

9. a random access preamble sequence is selected that has the same length as a random access preamble sequence used in the licensed band; receiving the random access preamble sequence repeatedly transmitted on a plurality of different frequencies; The random access method according to claim 7.

10. The frequency bandwidth is equal to or greater than the minimum bandwidth required for the unlicensed band. The random access method according to claim 7.

11. The information about the random access preamble sequence is the length of the random access preamble sequence. The random access method according to claim 7.

12. transmitting information about a random access preamble sequence to a user equipment; receiving the random access preamble sequence selected and transmitted by the user equipment based on the information; Control the The random access preamble sequence is transmitted in a frequency bandwidth of an unlicensed band, the frequency bandwidth being defined by a length of the random access preamble sequence and a subcarrier spacing, and being larger than a frequency bandwidth in which the random access preamble sequence is transmitted in a licensed band. Integrated circuits.

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