Here's my tip if you aren't a comfortable Pulse roamer.
You can do what @IWannaBeATiger suggested, which works great unless they have Fuze. Or the other thing you can do is just analyse the enemy team. See where they like to spawn and enter from and just don't pick Pulse on first defence round.
Once you start seeing patterns, you can start running Pulse on the first defence round and semi-accurately predict where they are going to enter from.
Another tip is that barricade yourself a Bunker. For example, if you're defending mining/train museum in Kafe, you can barricade the door to the bakery from red stairs and the door to kitchen in the hallway between redstairs and bar. You've just made a bunker. You can walk around with your sensor up all day in there and no one can rush you without breaking a barricade and giving themselves away. It also gives you a huge area to scan right below the obj, including the balcony of mining window, so you can run out Bakery windows and through a C4 up onto it and kill them in the douchebaggiest way in the game.
Once you start learning how to completely abuse the map as Pulse you will be fine. Just know that some maps like Plane and Favelas are poor Pulse maps for roaming as his gun is a little sub par compared to other roamers' and the maps are incredibly linear so cutting off roamers is easy.