In the TNG holodecks, as I understand it, the experience is a combination of force fields and directed photons (which come from the wall) to create the illusion of a 3d environment. So how do the Voyager holoemitters work? They've been used to create the holograms of other ships when trying to scare off hostile craft in an episode, and in the episode "Life Line", The doctor goes into a room where there is no direct line-of-sight with the mobile holoemitter that Barclay brings when the doctor arrives in the alpha-quadrant. If you can't see the emitter, how does it direct photons at your eyes, and therefore make the doctor visible. How is he still visible?
Great question. In several episodes, it is implied that there are either holo-emitters installed on the outside of the ship, or that the crew added them themselves when needed. This was done in several episodes, one where three Voyagers were created, and another (Basics I) where they also create ships (and accidentally put the doctor in space as well).
As for the doctor, if you can't see him, (because he's behind a wall), then it doesn't matter that the emitter can't reach you, it's the same as a regular person.
It's also mentioned that there are emitters throughout the lab to project anything that's needed, so a mobile emitter isn't required.