Burning Ice (2017)

In both stories, there's a character who helped two people to conceal their crimes. One is a smart detective who seeks justice/revenge for his family, while the other is a Math genius teacher who felt protective of the woman he likes.

+ Both dramas are extremely well connected crime dramas where the cases are linked to the characters on a personal level.
+ both thematize the psychological aspect => While in Signal the psychological aspect is more debated and analyses, in Burning Ice you can really see psychological methods being used.
+ Both dramas show very well character development.
+ both are connected to past events

If you enjoy the cases in Signal you are bound to enjoy Burning Ice.

Both dramas have a couple that is meeting due to unusual circumstance again but where one of them makes the decision to help the other. By doing so all actors get involved in a case that challenges not only the relationship they have, but also the characters development too.
There are a lot more similarities between them however naming them would spoiler too much.
IF you enjoy one there is a high chance you will enjoy the other drama too.

Both are crime dramas that debate the morals behind of ones personal justice. Both deal about characters that change due to things that have happened to them. Both are absolutely mind blowing in terms of character complexity, story development and cases. Absolutely unforgettable experience.

Can't explain too much without spoiling stuff, but both dramas are about normal people and how interacting with murderers changes them. They give off similar, suffocating and dark vibe.

Both dramas have cases where the killer is playing mind games on his victims. In both dramas characters end up questioning their own morality and have to make decisions that turn them from good to bad.