Methods and devices for grant-based uplink transmission in an unlicensed band are provided. Uplink grant messages are transmitted to electronic devices (EDs) in order to indicate time-frequency resources that are allocated to the EDs for uplink transmission in an unlicensed spectrum band. For a given ED, in the event that a first listen-before-talk (LBT) operation for the time-frequency resource allocated to the ED fails, the ED performs a second LBT operation within the allocated time-frequency resource at a start time based on a start point configuration within the allocated time-frequency resource. If the second LBT operation succeeds, the ED transmits an uplink transmission within a remaining portion of the allocated time-frequency resource that includes an activation signal to indicate a start of the uplink transmission, and uplink payload data.