Signal (2016)

Both the shows have Lee Je Hoon as the male lead being his usual clueless yet loveable self, meddling with different time zones while trying to solve the murder(s) of a close acquaintance.

Both dramas have a fantasy element that fate can be changed. But for worse or better? Can changing the past somehow lead to a different future?

Just like The Game, Signal deals about murder cases and supernatural occurrences that help investigation teams solve them. While Signal does this with a walkie-talkie that connects the present to the past, The Game deals about someone who can see the death of people. Both try to solve cases with their unique ability. Also both resolve around cases that shape each main character. the cases that shape them are quite similar to one another as well however due to spoilers i cannot go deeper into this.
Signal is a masterpiece of masterpieces, The Game seems to go into a similar direction with it as well.

Both establish communication with the past with a device. I think the first one was the movie Frequancy (2000).

The main serial case in both dramas have a lot of similarities, in which it may be based on the same murder case that sparked many different dramas and movies based on it. In both dramas there seem to be some hidden conspiracy which lead to people attempting to cover up the murder cases and led them remained unsolved.

although the guest is more about demons and possession, and signal is more about time slips(?), they both have similarities in terms of the main three working together to solve various crimes. they both also have similar mature / dark vibes with heavy themes.

Why Similar: Signal combines crime thriller elements with a complex narrative involving secret investigations and mysterious events. The show also features an intriguing storyline with twists and turns.
Plot: Detectives from different time periods communicate through a mysterious walkie-talkie to solve cold cases and prevent crimes.

Both dramas are about profiling.
Both are taking place in various time zones starting from the 80ties to the 2000s.
Both are thrillers with interesting cases. Both form teams that specialize in this.
And both have a male lead that is shaped due to his childhood and is doing a sort of narrative analysis's throughout the episodes.

Both Signal and Pride and Prejudice have a lead who went into law enforcement (cop/prosecutor)to find the truth/get justice for a family member that was murdered/falsely accused. Both dramas have leads that tackle cases as part of a close knit investigative unit. The cases seem to be unrelated but are in fact all related to a greater conspiracy connected to the death of the leads family member.
There are dirty cops/corrupt prosecutors in both. Great chemistry between the younger and older generations in the workplace.

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Both being series dealing with cold cases and both dramas have the same flavor in the cinematography and the OSTs.

Timeline Changes but communication through a Walkie Talkie
Thriller/Mystery
Serial Killer on the loose from the past
it will give you the vibes of what will happen next if the timeline changes

Supernatural element to solve muders. In signal, the use of the radio to solve cold cases. In cheo yong, the use of ghosts to solve murders, some that happened some time in the past with only the ghosts around as proof and other murders that have just happened. Both series highly recommended, although season 2 of cheo yong is much better!

Both are dealing with serial killers though Signal has a Sci-Fic element in it.
Both have action, suspense, thriller elements.
If you like Signal, try Inspector Koo and there is good chance you would like it.

Both are crime dramas produced by tVN, and both are inspired by the movie, "Memories of Murder".

the Japanese drama is an adaptation from the Korean one
I personally liked both so you should try the other if you liked one of them