This project demonstrates how the xr package can be used on Overleaf to resolve labels in other .tex files that have not been \include-d in the main file.
Make sure you enable the correct system call in latexmkrc, depending on the LaTeX engine you're using!
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
Block diagram of a QAM modulator used in digital communication technologies.
Diagrama en bloques de un modulador QAM usado en tecnologías digitales de comunicaciones.