Methods, systems, and devices for wireless communications are described. The described techniques provide for a user equipment (UE) to adjust a random access message that is transmitted to a base station in an asynchronous uplink transmission. The UE may use a constrained mapping of time and frequency resources for the random access message or transmit all portions of the message in a continuous transmission. The UE may also adjust the timing of the portions of the message. The base station may use a portion of the message, such as a preamble, to perform channel estimation for another portion of the message, such as a payload, to improve decoding of the transmitted message. Additionally, the UE may configure a waveform of the message to improve the continuous transmission of the message, for example by reducing interference between portions of the message.