Do a full survey of all rooms using a toolk like Heatmapper and assure there are no other wireless devices on the same channel as your router, You may have to do this several times. If tou find conflicts, change your channel and set it to that channel. Do not allow it to automatically select the channel
You can also reorient the router to see if that changes the signal. Nearby objects can reflect or attenuate the signal and a small move can make a big difference.
You may also be able to get a high gain antenna which would boost the signal. Dlink makes one and there are others avaiulble and an online search may yeild many more.
Also make sure you have the latest firmware.
2There's a number of things that could affect this, and @DaveM has given some good suggestions. It is also possible that there is something in the construction of the building, either inside the walls or in the paint/wallpaper, that could be killing your signal. Your best solution will probably end up being to run a wire and set up another AP on the other side. – Iszi – 2013-03-05T16:44:22.890
Or set up a round-corner reflector. – Desmond Hume – 2013-03-05T17:45:42.400
possible duplicate of Which methods help to improve wireless signal?, http://superuser.com/questions/17897/how-to-extend-wi-fi-signal-across-rooms
– Ƭᴇcʜιᴇ007 – 2013-03-05T18:18:28.627