Someone helps me about some elements in Unity shader?

Hi. I'm a user using both Unity 5 and Unreal Engine 4 but not good at shader.
I want to port a Unity shader to UE4 like below :

======================================
inline float Linear01Depth( float z )
{
return 1.0 / (_ZBufferParams.x * z + _ZBufferParams.y);
}

float2 pixelSize = 1.0 / _ScreenParams.xy;
float depth = Linear01Depth( tex2D( _CameraDepthTexture, i.uv).r);
=======================================

Actually, I have no idea what these elements mean such as _ZBufferParams, _ScreenParams, _CameraDepthTexture and how to port them to UE4.

So if you know about how to port them to UE4 or what they mean, please let me know.
Thanks.