how to publish tf in gazebo simulation with depth camera for octomap_server

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I am using gazebo simulation for drone. I mounted a depth camera on the drone in gazebo. It can publish its odom and pointcloud2 correctly. I would like to use octomap_mapping.launch to get the octree for planning usage, and I change frame_id to /odom, cloud_in to pointcloud2. However, there is nothing when I check with Rviz.

I think the reason is because my gazebo simulation does not publish a tf, but I am not sure how to do that. Can anyone tell me what tf should I publish and how to do that?