In forward rendering what's the go to technique to smooth out rigid shadow due to low resolution shadowmap texture? I am building my own graphic engine.

My depth texture is significantly smaller than that of the screen.

The low resolution depth map results in rigid shadow like this...



I am sure this is a common issue.

I would like to learn "go to" technique to resolve this issue.
Percentage Closer Filtering with some amount of blur (ie. a minimum filter size in texture space) is the most basic method I know of. For example, see GPU Gems. My experience is that it is also quite robust.