Beam selection and resource allocation in beamformed random access procedure

Abstract

Solutions for handling PRACH resource and beam selection are proposed in the following scenarios: First, a CFRA procedure is initiated, but all beams associated with the CFRA resource are below the RSRP threshold and CBRA fallback is not supported. The UE selects any downlink beam associated with the dedicated PRACH resource and then selects the dedicated PRACH resource associated with the selected downlink beam to continue performing the CFRA. Second, a CFRA procedure is initiated, but the active UL BWP is not paired with the active DL BWP. When CFRA is initiated, the UE uses the currently active UL and DL BWPs to perform the CFRA and switches to another BWP pair when all beams associated with the CFRA resource are below the RSRP threshold. Third, a CBRA procedure is initiated, but the active UL BWP is not configured with a PRACH resource. The UE selects a specific UL / DL BWP to perform the CBRA procedure.

Description

[0001] Cross-references to related applications

[0002] This application claims priority under 35 U.S.SC §119 to the following applications: U.S. Provisional Application No. 62 / 653,561, filed April 6, 2018, entitled “Modeling of Contention-free and Contention-based Randomaccess”, and U.S. Application No. 16 / 373881, filed April 3, 2019, the entire contents of which are hereby incorporated by reference. Technical Field

[0003] The disclosed embodiments generally relate to wireless network communications, and more particularly, to beam selection and resource allocation for a random-access channel (RACH) procedure in a 5G new radio (NR) wireless communication system with beamforming. Background Technology

[0004] The third-generation partnership project (3GPP) and Long-Term Evolution (LTE) mobile telecommunications systems offer high data rates, lower latency, and improved system performance. In 3GPP LTE networks, the evolved universal terrestrial radio access network (E-UTRAN) comprises multiple base stations, such as evolved Node Bs (eNBs) communicating with multiple mobile stations called user equipment (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been chosen as the LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by allocating different subbands of the system bandwidth (i.e., subcarrier groups, denoted as resource blocks (RBs)) to each user based on their existing channel conditions. In LTE networks, the Physical Downlink Control Channel (PDCCH) is used for downlink scheduling. The Physical Downlink Shared Channel (PDSCH) is used for downlink data. Similarly, the Physical Uplink Control Channel (PUCCH) carries uplink control information. The Physical Uplink Shared Channel (PUSCH) is used for uplink data.

[0005] In addition to control and data channels, the physical random-access channel (PRACH) is used for both contention-free and contention-based random access procedures. For a contention-free RACH (CFRA) procedure, the network provides dedicated PRACH resources to the UE to transmit a preamble (MSG1). Upon detecting the preamble on this dedicated PRACH resource, the network sends a random access response (RAR, MSG2). The network knows who transmitted the preamble, thus avoiding collisions. For a contention-based RACH (CBRA) procedure, the UE randomly selects a preamble to transmit from the common PRACH resource pool. Upon detecting a preamble transmission in a contention-based PRACH resource, the network sends RAR (MSG2). However, the network does not know which UE transmitted the preamble or the UE's UE ID sent to the network in MSG3. If multiple UEs select the same PRACH resource to transmit either MSG1 or MSG3, a RACH collision occurs. If MSG3 is decoded, the network sends MSG4 to notify the UE of a successful RACH procedure.

[0006] The growing bandwidth shortage faced by mobile operators has spurred exploration of the underutilized millimeter wave (mmWave) spectrum around 30 GHz and 300 GHz for next-generation 5G broadband cellular networks. The available spectrum in the mmWave band is two hundred times that of traditional cellular systems. mmWave wireless networks use narrow-beam directional communication and can support multi-gigabit data rates. The underutilized bandwidth of the mmWave spectrum has a wavelength range from 1 mm to 100 mm. The very small wavelength of the mmWave spectrum allows for the placement of numerous miniaturized antennas in small areas. This miniaturized antenna system can provide high beamforming gain by generating directional transmission through electrically controllable arrays.

[0007] 5G New Radio (NR) beamforming radio systems support RACH procedures on different downlink and uplink beams. A RACH procedure can include several preamble transmission trials, such as the radio resource control (RRC) configuration parameter `preambleTransMax`. If a dedicated RACH resource is allocated to the UE, the UE performs CFRA for the `preambleTransMax` trial; otherwise, the UE performs CBRA for the `preambleTransMax` trial. In NR, if CBRA fallback is supported, the RACH procedure includes both CFRA and CBRA trials, i.e., interleaved CFRA and CBRA transmissions. However, CBRA fallback is not supported in some cases.

[0008] Furthermore, to save power, NR introduces the concept of bandwidth parts (BWPs), which consist of continuous ranges of physical resource blocks (PRBs) in the frequency domain, and the bandwidth occupied by each BWP is a subset of the bandwidth of the associated carrier. In other words, the bandwidth of a BWP within a carrier is a subset of the carrier bandwidth, where the carrier bandwidth is divided into multiple continuous frequency bands with smaller bandwidths. A UE can be configured by the network to have several uplink BWPs and downlink BWPs, and the UE needs to monitor at most one uplink BWP and one downlink BWP simultaneously. The downlink BWP and uplink BWP currently being used / monitored by the UE are called active BWPs, for example, active downlink BWP and active uplink BWP, respectively. Therefore, since the UE only needs to monitor a smaller frequency range of the active BWP, rather than the entire carrier bandwidth, the power consumption for downlink monitoring can be reduced. Each uplink bandwidth part and downlink bandwidth part has its own identifier, i.e., a BWP ID. In FDD systems (i.e., paired spectrum systems), a UE can operate on active downlink BWPs and active uplink BWPs with different BWP IDs (e.g., using UL BWP#1 and DL BWP2); while in TDD systems (i.e., unpaired spectrum systems), a UE always operates on uplink and downlink BWPs with the same BWP ID. Each uplink BWP can optionally be configured with PRACH resources. If no PRACH resources are configured, the UE cannot transmit random access preambles or execute random access procedures on that uplink BWP.

[0009] When CFRA resources are not ideal and CBRA fallback is not supported, when CFRA resources are not ideal and the activated uplink BWP and downlink BWP are not paired (i.e. have the same BWP ID), and when CBRA is initiated but the activated UL BWP is not configured with PRACH resources, solutions need to be sought to handle PRACH resources and beam selection. Summary of the Invention

[0010] Solutions for handling PRACH resource and beam selection are proposed in the following scenarios: First, the CFRA procedure is initiated, but all beams associated with the CFRA resource are below the RSRP threshold and CBRA fallback is not supported. The UE continues CFRA by selecting any downlink beam associated with the dedicated PRACH resource and then selecting the dedicated PRACH resource associated with the selected downlink beam. Second, the CFRA procedure is initiated, but the active UL BWP and active DL BWP are not paired. When initiating CFRA, the UE uses the currently active UL and DL BWPs to perform CFRA and switches to another BWP pair when all beams associated with the CFRA resource are below the RSRP threshold. Third, the CBRA procedure is initiated, but the active ULBWP is not configured with PRACH resources. The UE selects a special UL / DL BWP to perform the CBRA procedure.

[0011] In one embodiment, the UE receives control information from a base station in a beamforming wireless communication network. The UE initiates a random access channel procedure, wherein a dedicated physical random access channel (PRACH) resource associated with a set of downlink beams is configured based on the control information. The UE determines that the signal quality of each of the downlink beams associated with the dedicated PRACH resource is below a predetermined threshold. The UE performs a contention-free random access (CFRA) procedure by first selecting a downlink beam from the set of downlink beams associated with the dedicated PRACH resource, and then selecting a PRACH resource associated with the selected downlink beam.

[0012] In another embodiment, the UE initiates a contention-free random access (CFRA) procedure in a beamforming wireless communication network. The UE is configured with multiple uplink and downlink bandwidth slices (BWPs) within its carrier bandwidth. The UE monitors the active uplink BWP and the active downlink BWP. The active uplink BWP has a configured dedicated physical random access channel (PRACH) resource, and the active downlink BWP is not paired with the active uplink BWP. The UE performs the CFRA procedure on both the active uplink BWP and the active downlink BWP. When it is determined that the dedicated PRACH resource becomes unavailable, the UE switches to the BWP pair to perform a contention-based RACH (CBRA) procedure.

[0013] In another embodiment, the UE operates in a beamforming wireless communication network with an active uplink bandwidth slice (BWP) and an active downlink BWP. The network configures the UE to have multiple BWPs on a carrier. The UE initiates a contention-based random access channel (CBRA) procedure. However, the active uplink BWP is not configured with physical random access channel (PRACH) resources. The UE then selects a specific uplink BWP and downlink BWP, wherein the uplink BWP is configured with PRACH resources and the downlink BWP is paired with the selected uplink BWP for the RACH procedure. The UE performs the CBRA procedure on the dedicated uplink and downlink BWPs.

[0014] Other embodiments and advantages are described in the following detailed description. This invention is not intended to be limited. The invention is defined by the claims. Attached Figure Description

[0015] Figure 1 According to a novel aspect, a beamforming wireless communication system supporting both contention-free RACH (CFRA) and contention-based RACH (CBRA) procedures is illustrated.

[0016] Figure 2 It is a simplified block diagram of a wireless transmitting and receiving device based on novel aspects.

[0017] Figure 3 The flowchart illustrates the sequence of CFRA and CBRA execution between the base station and user equipment in a beamforming wireless system.

[0018] Figure 4 An example of supporting CFRA and CBRA, as well as bandwidth slice (BWP) operation, is shown in a beamforming wireless system.

[0019] Figure 5A first embodiment of PRACH resources and beam selection for beamforming random access is shown according to a novel aspect.

[0020] Figure 6 A second embodiment of PRACH resources and beam selection for beamforming random access is shown according to a novel aspect.

[0021] Figure 7 A third embodiment of PRACH resources and beam selection for beamforming random access is shown according to a novel aspect.

[0022] Figure 8 This is a flowchart of a method for PRACH resource and beam selection for CFRA in a beamforming wireless communication system when CBRA fallback is not supported, according to a novel aspect.

[0023] Figure 9 This is a flowchart of a method for PRACH resource and beam selection for CFRA in a beamforming wireless communication system when the UL BWP and DL BWP are not paired, according to a novel aspect.

[0024] Figure 10 This is a flowchart of a method for performing CBRA in a beamforming wireless communication system when the activated UL BWP does not have a configured PRACH resource, according to a novel aspect. Detailed Implementation

[0025] Reference will now be made in detail to some embodiments of the invention, examples of which are shown in the accompanying drawings.

[0026] Figure 1 According to a novel aspect, a beamforming wireless communication system 100 supporting both contention-free RACH (CFRA) and contention-based RACH (CBRA) procedures is disclosed. The beamforming mmWave mobile communication network 100 includes a base station (BS) 101 and user equipment (UE) 102. The mmWave cellular network uses narrow-beam directional communication and is capable of supporting multi-gigabit data rates. Directional communication is achieved through digital and / or analog beamforming, wherein multiple antenna components are applied with multiple sets of beamforming weights to form multiple beams. Figure 1In the example, BS 101 is configured with multiple cells in the direction, and each cell is covered by a coarse set of TX / RX control beams. For example, cell 110 is covered by a set of eight downlink (DL) control beams CB1 to CB8. The set of DL beams CB1-CB8 covers the entire service area of ​​cell 110. Each DL beam transmits a set of known reference signals for initial time-frequency synchronization, identification of the control beam transmitting the reference signals, and measurement of the radio channel quality of the control beam transmitting the reference signals. In an NR system, each DL beam is used to transmit a corresponding system synchronization block (SSB) or a corresponding channel state information reference signal (CSI-RS).

[0027] When a downlink packet needs to be sent from the eNodeB to the UE, each UE receives a downlink assignment, such as a set of radio resources in the Physical Downlink Shared Channel (PDSCH). When a UE needs to send a packet to the eNodeB in the uplink, the UE receives an authorization from the eNodeB, which allocates a Physical Uplink Shared Channel (PUSCH) consisting of a set of uplink radio resources. The UE receives downlink or uplink scheduling information from a dedicated Physical Downlink Control Channel (PDCCH). Additionally, broadcast control information is also sent to all UEs in the cell via the PDCCH. The downlink or uplink scheduling information and broadcast control information carried by the PDCCH are collectively referred to as downlink control information (DCI). If the UE has data or RRC signaling, uplink control information (UCI) including HARQ ACK / NACK, CQI, MIMO feedback, and scheduling requests is carried by the Physical Uplink Control Channel (PUCCH) or PUSCH. In addition, the UE uses the Physical Random Access Channel (PRACH) to establish a connection with the base station. In the NR system, PRACH resources include predefined PRACH preambles and pre-allocated resource blocks, all of which are associated with corresponding DL beams.

[0028] In downlink (DL)-based beam management, the BS provides the UE with the opportunity to measure beamforming channels of different combinations of BS TX beams CB1-CB8 and UE RX beams 1-8. For example, BS 101 performs periodic beam scanning using reference signals (RS) carried on each BS TX beam. UE 102 measures the beamforming channel state using different UE RX beams 1-8 and reports the measurement results to the BS. For the RACH procedure, there is an association between PRACH resources and DL beams. UE measurements indicate that some DL beams have lower reference signal received power (RSRP), while others have higher RSRP. The UE can select a PRACH associated with a DL beam above a predetermined RSRP threshold to transmit a preamble. In other words, the UE uses the selected PRACH resource to implicitly tell the gNB which DL beam has a better RSRP for the UE. When the network detects the preamble, it can accordingly determine the DL beam to send the random access response; that is, the network selects the DL beam associated with the PRACH resource detected in the preamble. Therefore, after the UE successfully receives the RAR, the UE knows which UL beam and DL beam can be used to communicate with the gNB.

[0029] 5G New Radio (NR) beamforming radio systems support RACH procedures on different downlink and uplink beams. RACH procedures can include several preamble transmission trials, such as the radio resource control (RRC) configuration parameter `preambleTransMax`. If a dedicated PRACH resource is allocated to the UE, the UE performs contention-free random access (CFRA) for the `preambleTransMax` trial; otherwise, if a common PRACH resource is allocated to the UE, the UE performs contention-based random access (CBRA) for the `preambleTransMax` trial. In NR, if CBRA fallback is supported, the RACH procedure can include both CFRA and CBRA trials, i.e., interleaved CFRA and CBRA transmissions. However, CBRA fallback is not supported in some cases.

[0030] Furthermore, to save power, NR introduces the concept of Bandwidth Slices (BWPs), which consist of continuous ranges of Physical Resource Blocks (PRBs) in the frequency domain, occupying a subset of the bandwidth of the relevant carrier. That is, the bandwidth of a BWP within a carrier is a subset of the carrier bandwidth. The network can configure several uplink and downlink BWPs for a UE, and the UE needs to monitor at most one uplink and one downlink BWP simultaneously. The downlink and uplink BWPs currently being used / monitored by the UE are called active BWPs, for example, active downlink BWP and active uplink BWP. Therefore, since the UE only needs to monitor a smaller frequency range of the active BWP, rather than the entire carrier bandwidth, the power consumption of downlink monitoring can be reduced. Each uplink and downlink bandwidth slice has its own identifier, namely the BWP ID. In FDD systems (i.e., paired spectrum systems), the UE can operate on active downlink BWPs and active uplink BWPs with different BWP IDs (e.g., using UL BWP#1 and DL BWP 2); while in TDD systems (i.e., unpaired spectrum systems), the UE always operates on uplink and downlink BWPs with the same BWP ID. Each uplink BWP can optionally be configured with PRACH resources. If no PRACH resources are configured, the UE cannot send random access preambles and perform random access procedures on that uplink BWP.

[0031] According to a novel aspect, a solution for handling PRACH resources and beam selection is proposed in the following scenarios: First, a CFRA procedure is initiated, but all beams associated with the CFRA resource are below the RSRP threshold, and CBRA backoff is not supported or is not configured on the active UL BWP; Second, a CFRA procedure is initiated, but the active UL BWP is not paired with the active DL BWP, and all beams associated with the CFRA resource are below the RSRP threshold; Third, a CBRA procedure is initiated, but the active uplink BWP is not configured with PRACH resources for the initiated CBRA procedure.

[0032] Figure 2This is a simplified block diagram of wireless devices 201 and 211 according to novel aspects. For wireless device 201 (e.g., a transmitting device), antennas 207 and 208 transmit and receive radio signals. An RF transceiver module 206 is coupled to the antennas, receives RF signals from the antennas, converts the RF signals into baseband signals, and transmits the baseband signals to processor 203. RF transceiver 206 also converts the baseband signals received from the processor, converting them into RF signals, and transmits them to antennas 207 and 208. Processor 203 processes the received baseband signals and invokes different functional modules and circuits to execute features in wireless device 201. Memory 202 stores program instructions and data 210 to control the operation of device 201.

[0033] Similarly, for wireless device 211 (e.g., a receiving device), antennas 217 and 218 transmit and receive RF signals. An RF transceiver module 216, coupled to the antennas, receives the RF signals from the antennas, converts them into baseband signals, and transmits the baseband signals to processor 213. RF transceiver 216 also converts the baseband signals received from the processor, converts them into RF signals, and transmits them to antennas 217 and 218. Processor 213 processes the received baseband signals and invokes different functional modules and circuits to execute features in wireless device 211. Memory 212 stores program instructions and data 220 to control the operation of wireless device 211.

[0034] Wireless devices 201 and 211 also include several functional modules and circuits that can be implemented and configured to perform embodiments of the present invention. Figure 2 In the example, wireless device 201 is a transmitting device, which includes an encoder 205, a scheduler 204, a beamforming circuit 209, and a configuration circuit 221. Wireless device 211 is a receiving device, which includes a decoder 215, a PRACH circuit 214, a beamforming circuit 219, and a configuration circuit 231. Note that a wireless device can be both a transmitting device and a receiving device. Both a base station and a user equipment can be transmitting devices and / or receiving devices. Different functional modules and circuits can be implemented and configured by software, solid-state, hardware, and any combination thereof. When executed by processors 203 and 213 (e.g., by executing program codes 210 and 220), the functional modules and circuits allow transmitting device 201 and receiving device 211 to perform embodiments of the present invention accordingly.

[0035] In one example, base station 201 configures radio resources (PRACH) for the UE via configuration circuitry 221, schedules downlink and uplink transmissions for the UE via scheduler 204, encodes data packets to be transmitted via encoder 205, and transmits radio signals on various control beams on which beamforming weights are applied via beamforming circuitry 209. UE 211 obtains allocated radio resources for PRACH via configuration circuitry 231, receives and decodes downlink data packets via decoder 215, and transmits a random access preamble on PRACH resources on the UL beam selected via beamforming circuitry 219 via PRACH circuitry 214. In some cases, CFRA procedure is initiated, but CBRA backoff is not supported, or the UL BWP is not configured with CBRA PRACH resources, or the UL BWP is not paired with the DL BWP, or the CBRA procedure is initiated but the UL BWP does not have a configured PRACH. In a novel aspect, the PRACH processing circuit 214 selects PRACH resources and corresponding beam selection for the CFRA or CBRA program in the aforementioned scenario.

[0036] Figure 3The sequence of procedures for performing CFRA and CBRA in a beamforming radio system is illustrated between the base station and the user equipment. In step 311, gNB 301 transmits SS block (SSB) system information (SI) and CSI-RS reference signals to UE 302 via the corresponding DL beam. This DL beam is associated with PRACH resources, such as PRACH preambles and resource blocks. In step 321, UE 302 prepares for the RACH procedure: determining the CFRA or CBRA procedure and determining the PRACH resources and UL / DL beam selection. For the CFRA procedure, in step 331, UE 302 transmits a PRACH preamble (MSG1) to gNB 301 using one of the UE-specific PRACH resources, each of which is associated with a specific DL beam. After selecting the PRACH resource, the UE transmits the preamble on the selected PRACH and a specific UL beam (UL direction), where the specific UL beam is selected by the UE itself. If the UE fails during preamble transmission, for subsequent preamble retransmissions, the UE can select a new PRACH associated with the same or a different DL beam, and then transmit the preamble in the PRACH using a different UL beam (UL direction). In step 341, upon receiving the PRACH preamble, gNB 301 uses the DL beam to send the Random Access Response (RAR, MSG2) back to UE 302. Based on the PRACH resources on which the PRACH preamble transmission was detected, and the association between the PRACH resources and the DL beam, the DL TX beam for MSG2 can be obtained. For CFRA, the RACH procedure is completed because the network knows who sent the preamble; that is, only UEs configured with a dedicated PRACH will send the preamble on this PRACH.

[0037] For the CBRA procedure, UE 302 continues with step 351 and sends an uplink request (MSG3) with the UE identifier to gNB 301 using the selected UL beam. In step 361, gNB 301 sends a message (i.e., MSG4) as a response to MSG3 to UE 302 for contention resolution and completes the CBRA RACH procedure. The CFRA or CBRA RACH procedure may include several preamble transmission trials. For example, the RRC signaling configuration parameter preambleTransMax. In step 321, UE 302 decides whether to initiate CFRA or CBRA based on different network scenarios and conditions.

[0038] Figure 4An example of supporting CFRA and CBRA, as well as bandwidth slice (BWP) operation, is shown in a beamforming wireless system. In NR beamforming systems, there is a correlation between PRACH resources and DL beams (SSB or CSI-RS). Figure 4 As shown, CFRA PRACH 1 is associated with DL beam 1, CFRA PRACH 2 with DL beam 2, CBRA PRACH 3 with DL beam 3, and CBRA PRACH 4 with DL beam 4. For good communication, the UE will transmit a preamble on the PRACH associated with a high-quality (high RSRP) DL beam. In this way, when the network detects the preamble, it will transmit a RAR on the DL beam associated with that PRACH resource, and the UE will be able to successfully receive the RAR because the DL beam to which the network transmits the RAR to the UE has a high RSRP. In CFRA, if the UE finds that all dedicated PRACH resources used for CFRA are associated with beams below the RSRP threshold, and if the UE still transmits preambles on these dedicated CFRA resources, the network will transmit a RAR on a bad RSRP DL beam, and the RACH will therefore fail.

[0039] CBRA backoff means that when all DL beams are below the RSRP threshold, the UE is allowed to transmit preambles on PRACH resources from a contention-based PRACH resource pool, instead of always selecting PRACH resources from dedicated PRACH resources (410). Furthermore, if the UE performs CBRA backoff and uses contention-based PRACH resources for preamble retransmission, and if, during PRACH reselection for preamble retransmission, the UE finds that one or more DL beams associated with any dedicated PRACH resource have again become higher than the RSRP threshold during channel changes, the UE should switch back from CBRA to CFRA and perform CFRA again (i.e., perform CFRA using dedicated PRACH resources). Therefore, if CBRA backoff is supported, the RACH procedure can switch between CFRA and CBRA, and thus include CFRA and CBRA trials, i.e., interleaved CFRA and CBRA transmissions. Please note that when a CBRA fallback occurs, the RACH procedure is in progress and the preamble transmission counter continues. The only difference is that the UE switches to the already used contention-based public PRACH resource in the 4-step RACH procedure. In other words, both CFRA and CBRA are part of the same RACH procedure. However, CBRA is not supported in every scenario of the RACH procedure.

[0040] To save power, NR introduces the concept of Bandwidth Slicing (BWP). A bandwidth slice is a continuous frequency band whose bandwidth is less than the carrier bandwidth. The UE can be configured to monitor a smaller bandwidth to reduce power consumption. The UE can be configured to have up to 4 UL BWPs and 4 DL BWPs. The UE needs to monitor one UL BWP and one DL BWP at a time. For example, as... Figure 4 As shown, the UE is operated to monitor ULBWP 1 and DL BWP 2. During the CBRA RACH procedure, when the network receives the preamble on UL BWP 1, because the network does not know which UE sent the preamble (i.e., the preamble is merely a physical sequence and does not carry a UE ID), the network cannot determine on which DL BWP and how the RAR should be sent, i.e., it is DL BWP 2. In addition to this issue, the active UL BWP may not support CBRA fallback and may not have a configured PRACH. Therefore, it may be necessary to switch the active BWP (420).

[0041] Figure 5 A novel aspect illustrates a first embodiment of PRACH resources and beam selection for beamforming random access. In step 511, gNB 501 or UE 502 initiates a CFRA procedure for random access. In step 521, UE 502 discovers that CBRA fallback is not supported. This may occur under different circumstances. First, a CFRA procedure initiated by the network on an SPCELL or SCELL (i.e., a CFRA triggered by a PDCCH command) does not support CBRA fallback. Second, a CFRA procedure initiated by the UE for an MSG1-based SI (System Information) request does not support CBRA fallback. This is because a dedicated PRACH resource already exists on each SSB configured for each SI message supporting MSG1-based requests. Third, UE 502 is currently monitoring an active uplink BWP that does not have a configured contention-based PRACH resource. Additionally, UE 502 detects that all beams associated with the CFRA resource are below a predefined RSRP threshold. In step 531, although each dedicated PRACH resource is associated with a downlink beam below the RSRP threshold, UE 502 still selects a dedicated PRACH resource associated with any downlink beam. The downlink beam is selected based on its RSRP level; for example, the downlink beam selected for PRACH transmission is the beam with a relatively high RSRP among all those downlink beams associated with some dedicated PRACHs and below the RSRP threshold. In step 541, the UE continues to perform the CFRA procedure using the selected dedicated PRACH resource and downlink beam without attempting CBRA backoff.

[0042] Besides selecting one DL beam from those below the RSRP threshold as shown above, there are other alternatives. For example, when the UE detects that all DL beams associated with the dedicated PRACH resource are below the RSRP threshold, the UE stops the RACH procedure and may optionally indicate a random access problem to the upper layer or wait for processing from the network. The reason is that because all DL beams are below the RSRP threshold, even if the network can detect the preamble sent by the UE, the UE cannot successfully receive a random access response through one of these low RSRP DL beams. Therefore, to avoid wasting power, the UE should stop the RACH procedure when all DL beams associated with the dedicated PRACH resource are below the RSRP threshold. Another alternative is that when all DL beams associated with the dedicated PRACH resource are below the RSRP threshold, the UE stops the ongoing CFRA and triggers another contention-based RACH procedure. The reason is that since CBRA rollback is not allowed, the UE ignores the configured dedicated PRACH resources. That is, by re-initiating a new CBRA procedure, the UE can have a high RSRP beam that will be selected, which is independent of any dedicated PRACH configured by the network.

[0043] Figure 6 A second embodiment of PRACH resources and beam selection for beamforming random access is shown according to a novel aspect. In step 611, gNB 601 or UE 602 initiates a CFRA procedure for random access. UE 602 monitors the active UL BWP and DL BWP to reduce power. In step 621, UE 602 discovers that although the currently active uplink BWP has a configured contention-based PRACH resource, the active downlink BWP is not paired with an uplink BWP with the same BWP ID. Therefore, if CBRA fallback is necessary, UE 602 will not be able to perform the CBRA procedure on the currently active UL and DL BWPs. In step 631, UE 602 continues the CFRA procedure on the currently active UL and DL BWPs. In step 641, UE 602 detects that all beams associated with the CFRA resources are below a predefined RSRP threshold. In step 651, UE 602 switches to another configured BWP pair to perform the CBRA procedure for CBRA fallback. Note that in this embodiment, UE 602 does not switch to another BWP pair after step 621. Instead, UE 602 continues the CFRA procedure in step 631 and only switches to another BWP pair when all beams associated with the CFRA resource are below a predefined RSRP threshold (after step 641).

[0044] Figure 7A third embodiment of PRACH resources and beam selection for beamforming random access is shown according to a novel aspect. In step 711, gNB 701 or UE 702 initiates a CBRA procedure for random access. UE 702 monitors the active UL BWP and DL BWP to reduce power. In step 721, UE 702 finds that the currently active UL BWP does not have the configured PRACH resources. Therefore, UE 702 will not be able to perform the CBRA procedure on the currently active UL BWP. In step 731, UE 702 switches to a specific UL BWP and DL BWP that support CBRA. In one example, as configured by the network, this specific UL BWP and DL BWP constitute a specific BWP pair with the same BWP ID. In another example, as configured by the network, this specific UL BWP and DL BWP constitute one of the BWP pairs with the same BWP ID, and this specific BWP pair is the most power-efficient for the UE. In step 741, UE 702 performs CBRA on that particular UL BWP and DL BWP.

[0045] Figure 8 This is a flowchart illustrating a method for PRACH resource and beam selection for CFRA in a beamforming wireless communication system when CBRA fallback is not supported, according to a novel aspect. In step 801, the UE receives control information from a base station in the beamforming wireless communication network. In step 802, the UE initiates a random access channel procedure, wherein a dedicated physical random access channel (PRACH) resource associated with a set of downlink beams is configured based on the control information. In step 803, the UE determines that the signal quality of each of the downlink beams in the set associated with the dedicated PRACH resource is below a predetermined threshold. In step 804, the UE performs a contention-free random access (CFRA) procedure by first selecting a downlink beam from the set of downlink beams associated with the dedicated PRACH resource, and then selecting a PRACH resource associated with the selected downlink beam.

[0046] Figure 9This is a flowchart illustrating a method for PRACH resource and beam selection for CFRA in a beamforming wireless communication system when the UL BWP and DL BWP are not paired, according to a novel aspect. In step 901, the UE initiates a contention-free random access (CFRA) procedure in the beamforming wireless communication network. The UE is configured with multiple uplink and downlink bandwidth slices (BWPs) in the carrier bandwidth. In step 902, the UE monitors the active uplink BWP and the active downlink BWP. The active uplink BWP has a configured dedicated physical random access channel (PRACH) resource, and the active downlink BWP is not paired with the active uplink BWP. In step 903, the UE performs the CFRA procedure on the active uplink BWP and the active downlink BWP. In step 904, the UE switches to the BWP pair to perform a contention-based RACH (CBRA) procedure when it determines that the dedicated PRACH resource becomes unavailable.

[0047] Figure 10 This is a flowchart illustrating a method for performing a contention-based random access channel (CBRA) procedure in a beamforming wireless communication system when an active UL BWP does not have configured PRACH resources, according to a novel aspect. In step 1001, the UE operates in the beamforming wireless communication network on an active uplink bandwidth slice (BWP) and an active downlink BWP. The UE has multiple bandwidth slices (BWPs) configured in the carrier. In step 1002, the UE initiates a contention-based random access channel (CBRA) procedure. The active uplink BWP is not configured with physical random access channel (PRACH) resources. In step 1003, the UE selects a specific uplink BWP and downlink BWP, wherein the uplink bandwidth slice is configured with PRACH resources, and the downlink BWP is paired with the uplink bandwidth slice, i.e., has the same BWP ID. In step 1004, the UE performs the CBRA procedure on the specific uplink BWP and downlink BWP.

[0048] Although the invention has been described in conjunction with certain specific embodiments for guiding purposes, the invention is not limited thereto. Therefore, various modifications, adaptations, and combinations of the various features of the described embodiments can be made without departing from the scope of the invention as set forth in the claims.

Claims

1. A wireless communication method, comprising: In a wireless communication network, the user equipment (UE) receives control information sent by the base station. Initiating a random access procedure on the serving cell, wherein the dedicated physical random access channel (PRACH) resources and a set of downlink beams are configured by the network based on the control information; The UE performs a contention-free random access (CFRA) procedure on an active uplink UL bandwidth fragment BWP of a selected carrier in the serving cell by first selecting a downlink beam from the set of downlink beams configured by the network and the dedicated PRACH resource, and then selecting the PRACH resource associated with the selected downlink beam. The UE continues the CFRA procedure on the active UL BWP by selecting each PRACH resource from the dedicated PRACH resource for each random access preamble transmission, wherein the UL BWP does not have a contention-based PRACH resource configured for the UE.

2. The method according to claim 1, characterized in that, This CFRA procedure does not support competition-based RACH CBRA fallback.

3. The method according to claim 1, characterized in that, The CFRA procedure is triggered by a Physical Downlink Control Channel (PDCCH) command from the base station.

4. The method according to claim 1, characterized in that, The UE is configured with multiple bandwidth slices (BWPs) in the carrier bandwidth, and the UE operates on the active uplink BWP and the active downlink BWP.

5. A user equipment (UE), characterized in that, The UE includes a transceiver and a processor, which are configured to perform the following operations: Receive control information sent from base stations in the wireless communication network; Initiating a random access procedure on the serving cell, wherein the dedicated physical random access channel (PRACH) resources and a set of downlink beams are configured by the network based on the control information; The UE performs a contention-free random access (CFRA) procedure on an active uplink UL bandwidth fragment BWP of a selected carrier in the serving cell by first selecting a downlink beam from the set of downlink beams configured by the network and the dedicated PRACH resources, and then selecting the PRACH resources associated with the selected downlink beam. The UE continues the CFRA procedure on the active UL BWP by selecting each PRACH resource from the dedicated PRACH resources for each random access preamble transmission, wherein the UL BWP does not have contention-free PRACH resources configured for the UE.

6. The UE according to claim 5, characterized in that, This CFRA procedure does not support competition-based RACH CBRA fallback.

7. The UE according to claim 5, characterized in that, The CFRA procedure is triggered by a Physical Downlink Control Channel (PDCCH) command from the base station.

8. The UE according to claim 5, characterized in that, The UE is configured with multiple bandwidth slices (BWPs) in the carrier bandwidth, and the UE operates on the active uplink BWP and the active downlink BWP.

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