Total/Scattered field formulat in BOR FDTD

hi! i've been doing FDTD for symmetric bodies(Body of revolution/BOR FDTD) but i have a problem introducing a PLANE WAVE using the TOTALfield-SCATTEREDfield formulation. can anyone help me with this?

Hi
In the book of sollivan says has two reason:
In order to simulate a plane wave in a 2D FDTD program, the problem space will be divided up into two regions, the total field and the scattered field .There are two primary reasons for doing this: (1) The propagating plane wave should not interact with the absorbing boundary conditions; (2) the load on the absorbing boundary conditions should be minimized. These boundary conditions are not perfect, i.e., a certain portion of the impinging wave is reflected back into the problem space.