Step 203b: If the network device detects that J fourth frequency resources in the some or all fourth frequency resources are free, the network device uses at least one of the J fourth frequency resources as the target fourth frequency resource.

J is less than or equal to L, and J is a positive integer greater than or equal to 1.

Further optionally, in this embodiment of the present disclosure, the foregoing step 302a specifically includes the following content:

Step 302a1: The network device monitors the some or all fourth frequency resources in the L fourth frequency resources according to a seventh predetermined rule; and/or in this embodiment of the present disclosure, the foregoing step 302b specifically includes the following content:

Step 302b1: The network device selects at least one of the J fourth frequency resources as the target fourth frequency resource according to an eighth predetermined rule.

The seventh predetermined rule includes at least one of the following: fourth frequency resources satisfying a first condition are monitored, and a quantity of monitored fourth frequency resources is less than or equal to an eighth threshold; and the eighth predetermined rule includes at least one of the following: fourth frequency resources satisfying the first condition are selected, and a quantity of selected fourth frequency resources is less than or equal to a ninth threshold; where the first condition is that a frequency measurement value corresponding to the fourth frequency resource is greater than or equal to a second threshold.