No matter how high you make the tower, it would not work. If this were not so, then you would be able to see the sun at mid-lattitudes 24 hrs. per day--think about how far away the sun is, yet, when it's daytime at the other side of the earth, we still cannot see the sun. The only answer which is workable would be to set up relay satellites around the world so that signals are passed "horizontally" from satellite to satellite until the signal makes it around the world. Each relay satellite could aslo broadcast the signal down to the earth.