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EP-3864925-B1 - EFFICIENT MESSAGING IN A PROCEDURE FOR ACCESSING A COMMUNICATION CHANNEL

EP3864925B1EP 3864925 B1EP3864925 B1EP 3864925B1EP-3864925-B1

Inventors

  • WU, CHIH-HSIANG

Dates

Publication Date
20260506
Application Date
20200131

Claims (11)

  1. A method in a base station for granting access to a communication channel, the method comprising: receiving at the base station (104) from a user device (102), a request to access the communication channel, the request including a random access preamble and a payload; attempting to decode the payload; in a first instance when the data payload was not successfully decoded; generating a first response message including a first MAC protocol data unit, MAC PDU, having a first MAC subheader in a first MAC subheader format including extension, E, type, T, and random access preamble identifier, RAPID, fields, the first MAC subheader format indicating a four-step random access channel, RACH, procedure; and transmitting to the user device (102), the first response message; in a second instance when the data payload was successfully decoded; generating a second response message including a second MAC PDU having a second MAC subheader in a second MAC subheader format different from the first MAC subheader format and including a reserved, R, field, the second MAC subheader format indicating a two-step RACH procedure; and transmitting to the user device (102), the second response message.
  2. The method of claim 1, wherein: in the first instance, the method further comprises: receiving from the user device (102), an additional message including the payload; and transmitting to the user device (102), an additional response message; and in the second instance, transmitting the second response message includes transmitting to the user device (102), the second response message including an indication of an uplink grant.
  3. The method of claim 1 or 2, wherein generating the first response in the first instance includes: generating the first response message including a random access response, RAR, as a Msg2 in a four-step random access channel, RACH, procedure.
  4. The method of claim 1 or 2, wherein generating the second response in the second instance includes: generating the second response message including an RAR and a contention resolution as a MsgB in a two-step RACH procedure.
  5. The method of any of the preceding claims, further comprising: generating a radio network temporary identifier, RNTI,; generating a downlink control information, DCI, element; generating a cyclic redundancy check, CRC, of the DCI element; and transmitting to the user device (102) on a physical downlink control channel, PDCCH, the DCI and the CRC scrambled with the RNTI; and transmitting to the user device (102) on a physical data shared channel, PDSCH, the first or second MAC PDU.
  6. A base station comprising processing hardware configured to implement a method of any of claims 1-5.
  7. A method in a user device (102) for requesting access to a communication channel, the method comprising: transmitting at the user device (102) to a base station (104), a request to access the communication channel, the request including a random access preamble and a payload; receiving from the base station (104), a response message to the request, the response message indicating a format of the response message in accordance with whether the payload was decoded successfully by the base station; determining the format of the response message based on a MAC subheader format of a MAC subheader of a MAC protocol data unit, PDU, included in the response message; wherein in the event the payload was decoded successfully by the base station (104), the MAC subheader format comprises a first MAC subheader format including extension, E, type, T, and random access preamble identifier, RAPID, fields and indicating a four-step random access channel, RACH, procedure, and in the event the payload was not decoded successfully by the base station, the MAC subheader format comprises a second MAC subheader format including a reserved, R, field and indicating a two-step RACH procedure, wherein the first MAC subheader format differs from the second MAC subheader format; and decoding a portion of the response message according to the determined format.
  8. The method of claim 7, wherein determining the format of the response message includes: in a first instance, determining the response message is in a first format based on the subheader including a first subheader format in the response message indicating that the payload was not successfully decoded; decoding the portion of the response message according to the first format; transmitting to the base station, an additional message including the payload; and receiving from the base station, an additional response message; and in a second instance, determining the response message is in a second format based on the subheader including a second subheader format in the response message indicating that the payload was successfully decoded; and decoding the portion of the response message according to the second format, wherein receiving the response message includes receiving from the base station (104), the response message including an indication of an uplink grant.
  9. The method of claim 8, wherein in the first instance, decoding the portion of the response message includes decoding a random access response, RAR, in the response message according to a Msg2 format in a four-step random access channel, RACH, procedure; and wherein transmitting an additional message includes transmitting the payload as a Msg3 in the four-step RACH procedure.
  10. The method of claim 8, wherein in the second instance, decoding the response message includes decoding an RAR and a contention resolution in the response message according to a MsgB format in a two-step RACH procedure.
  11. A user device (102) comprising processing hardware configured to implement a method of any of claims 7-10.

Description

FIELD OF THE DISCLOSURE This disclosure relates to wireless communications and, more particularly, to synchronizing wireless communications in a channel access procedure. BACKGROUND The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. To synchronize communication over a radio interface in an unlicensed portion of the radio spectrum, a user device (referred to as user equipment, or "UE") and a base station can use a random access channel (RACH) procedure, for example. Some standards define two types of RACH procedures: a two-step procedure and a four-step procedure. In the four-step procedure as shown in Fig. 2, (1) a user device sends 202 a random access preamble to the base station (also referred to herein as "Msg1"); (2) the base station sends 204 a random access response (RAR) to the user device (also referred to herein as "Msg2"); (3) the user device sends 206 a scheduled transmission to the base station (also referred to herein as "Msg3"); and (4) the base station sends 208 a contention resolution to the user device (also referred to herein as "Msg4"). The two-step procedure as shown in Fig. 3, condenses steps (1) and (3) into a first step and steps (2) and (4) into a second step, such that (1) a user device sends 302 a random access preamble and a scheduled transmission to the base station (also referred to herein as "MsgA"); and (2) the base station sends 304 an RAR and contention resolution to the user device (also referred to herein as "MsgB"). In some scenarios, a user device sends a message to the base station including a random access preamble and a payload in accordance with the two-step procedure. If the base station fails to decode the payload, the base station may transmit only an RAR to the user device instead of a message including both the RAR and contention resolution. However, the user device does not know whether the base station transmitted the RAR or MsgB (i.e., a combination of the RAR and contention resolution) and may fail to decode the transmission from the base station according to the appropriate procedure. In a paper entitled "2-step RACH msgA and msgB contents" (MediaTek Inc., 3GPP TSG-RAN WG2 Meeting #104, Spokane US, 12th - 16th November 2018), the authors discuss contents of MsgA and MsgB for 2-step RACH. The patent document WO 2018/204863 A1, 08.11.2018, discloses a two-step random access procedure. SUMMARY The invention is defined in the independent claims. particular embodiments of the invention are set out in the dependent claims. According to a first aspect of the present invention, there is provided a method as set out in claim 1. According to a second aspect of the present invention, there is provided a base station as set out in claim 6. According to a third aspect of the present invention, there is provided a method as set out in claim 7. According to a fourth aspect of the present invention, there is provided a user device as set out in claim 11. Generally speaking, the techniques of this disclosure allow a user device to determine, during a channel access procedure, which communication scheme the base station is following, and whether the base station needs additional information from the user device. In some implementations, the channel access communications are RACH transmissions from the base station. The user device determines how to decode the RACH transmissions based on whether the base station generates the RACH transmissions according to a two-step or four-step RACH procedure format (i.e., whether the base station is using a format that includes an RAR and a resolution or a format that does not include the resolution). More specifically, in response to receiving an RACH communication from a user device, a base station attempts to decode the payload of the RACH communication. If the base station fails to decode the payload, the base station may determine to proceed with the four-step RACH procedure. To make the user device aware of the switch to the four-step RACH procedure, in a response to the RACH communication sent to the user device, the base station includes an indication to fall back to the four-step RACH procedure. The indication may be the presence of a first radio network temporary identifier (RNTI) in the response, where the user device associates the first RNTI with a switch to the four-step RACH procedure. In other implementations, the indication may be included in a media access control (MAC) layer packet data unit (PDU) or in a downlink control information (DCI) element. In yet other implementations, the response to the RACH communication may include an indication of the size of a portion of