Signal Ending. Explanation?

It is not a coma, he has been hiding in a hospital. Moreover, someone else is helping him over the walkie talkie, since he did not die so the walkie talkie did not go to Hae Young, but he was saved by Hae Young in the past.

My interpretation is that it is not just past, present, and future anymore, lots of changes have taken place which resulted in the creation of parallel worlds/alternate universe/alternate timeline (whatever you want to call it), but at the same time, it is connected cause those who know about the walkie talkie still retain their conscious.

It is not possible for Jae Han from parallel work "C" to receive help from the future Hae Young of parallel work "C", cause Hae Young would not have the radio since Jae Han did not die. But I believe Hae Young conscious of Parallel World "B" has been transported to Parallel World "C" cause how the hell would he know about Parallel World "B" if that was not the case.

Proof of the parallel world theory is that Hae Young was moved to different teams in the beginning of the series, but in the end, the teams with Soo Hyeon, Gye Cheol, and Heon Gi is not there and he is still working in the old police station he worked in before finding the walkie talkie.

If you think about it, it is really confusing, don't know how the writer would be able to explain this in the 2nd season. But hope she does a good job.

pls ignore my comment ... this was written              almost a year after i watched it

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I don't know anything anymore 

I haven't been this confused since Ich Seh, Ich Seh

Actually, if you will analyze the ending, it's already done. It may seem like two transmissions happened but I think there are actually more transmissions. This is purely based on my opinion so I'm not saying that this is right but it could be.

First transmission- it wasn't shown but remember when Lee Jae Han first sent the transmission? Park Hae Young told him to go to that old hospital but Hae Young can't remember him and Jae Han died. That means that they failed on it so he told Hae Young to convince him on 1989 because he knows that the transmission will happen again.

Second transmission- this time, they changed what happened to the first one, that's why Park Hae Young knew that the past has changed when he was being brought to the hospital. He knows that there was first transmission. But they failed again. I think Jae Han died when he was discovered from that car and by that time, Kim Bum Joo might have been alive.

Third transmission- it wasn't shown too but this time they may be able to save Jae Han from dying and Kim Bum Joo died instead of him. They knew that they will never be able to change the past. They worked on it for 15 years, but I guess cold cases weren't left anymore. They could've changed some of those. This time, Jae Han could've died on the future, February 5. So this means Hae Young still has the walkie talkie until February 5, 2015.

Fourth transmission- this time, they were able to solve everything. Jae Han knew that they will be able to catch the real culprit in the future (2015) so he hid in that hospital. No more cold cases, because I guess they never gave up on it on the third transmission so the only thing left is the real case. Jae Han hid in that hospital for 15 years. He waited because he knew that would be the right time to end everything. Remember in the 2nd transmission, he was warned not to go in that old hospital but he went anyway with his team. He used that method to Hae Young and Detective Cha by warning them so that could mean that the two also brought back up with them. Since he did not die anymore, Hae Young doesn't have the walkie talkie but I guess they have a new way of communication. How could Jae Han knows Cha Soo Hyun's number? That could've happened on the third transmission!

The confusing part is the lighting of walkie talkie again. Remember when they prevented him from dying during the second transmission? His walkie talkie lighted up again but there was no transmission to Hae Young and then he was saved. If you will connect it, he could've died (like I said on the third transmission) on February 5 again (since it lighted up) but there was no transmission to Hae Young so they were able to prevent it during the fourth. So that means they were able to save him that day when those guys went to Jae Han. Idk if I made it clear hahaha but they ended it.

I had to watch the final 30 minutes twice, but I'm pretty sure I have it straight now.

1. In the third time loop, Lee Jae-han spoke to Cha Soo-hyeon rather than Park Hae-jung, and Cha warned Lee not to go to the hospital. But she also - and this is critical - cried about how his death had affected her. She said she'd waited for him for 15 years and he "came back dead." Since he was secretly in love with her, that made him determined to survive and keep his promise to come back. It also broke the loop which had already happened twice when he spoke with Park, who didn't remember him during that transmission. Now Lee had the option to do things different, so he had backup follow him the night he went to the hospital, and they were able to prevent his murder. That gave him a chance to reconnect him with Cha and let her know he had to go after Kim Bum-Joo's master, the congressman whose son r*ped the high school girl. It also changed the future of Ahn Chi-soo, who was arrested, and Kim Bun-Joo lost his status as the police superintendent and  went on the run. This solved a lot of future problems, including Park being framed for murder and shot.

2. Once Lee survived the attack in the forest, he had a chance to track Kim Bum-Joo to the warehouse where Kim had the bag containing a copy the disk incriminating the Chaebol whose faulty bridge collapsed. They were attacked by goons sent by the congressman, and Kim was killed, but Lee escaped with the bag containing the disk. He gave that disk, along with a note for Park, to Park's mother; and then he hid out at a river where he was found by the goons again. Before they attacked, he updated the note in the back of his case notebook, leaving a message that only Park would understand: the number of Park's home address. So Park went to see his mother, and she gave him the disk. He released the information and got the congressman in trouble, but the congressman still able to send his goons out again to find the disk copy.

3. Lee survived the attack at the river and recuperated at a hospital. After that, he knew he had to go into hiding, but like Park said, Lee wasn't the type to leave his family in the dark about his location. So he contacted his father - who now had the walkie-talkie because it would have been given to him once Lee was dead. That means Lee's father is the future contact Park speculated about, and Park himself saw the proof when he caught the father burning a bus ticket which would have taken him to the hospital to see his son. Lee's the father helped him hide out in the rehabilitation hospital for 15 years and warned him when the goons were going to attack him there, which must mean there was at least one more loop where Lee was killed at the hospital. Finally he worked out  a plan to save himself - and reunite with Cha and Park. A few days ahead of time, he texted Cha with a message only she would understand, letting her know that he was in the rehab hospital and in danger. The goons attacked the hospital, but Lee already knows they were coming and had a plan for escape. Plus, thanks to his messages to Cha and Park, they were on their way to help him at the crucial moment. I like to imagine they showed up at a rear exit to the hospital just as Lee stepped out the door.