This application discloses a communication method, apparatus, and system. A network device sends a first downlink common signal (for example, an SSB) to a terminal device. The terminal device receives system information from the network device based on the SSB. The system information carries information indicating a first frequency domain resource and a second frequency domain resource. The first frequency domain resource and the second frequency domain resource may be two carriers or two BWPs, and the two frequency domain resources correspond to different uplink-downlink configurations. The terminal device selects a first random access resource from a random access resource set based on an association relationship between an SSB and a random access resource to send a random access preamble. The random access resource set includes a random access resource on the first frequency domain resource and a random access resource on the second frequency domain resource.