I recently watched the series Altered Carbon. In that series, they showed a floating space station called "Head in the Clouds". There is a blue sky here, even clouds. This would imply a height of <15km. It seems to be kept up solely using thrusters.
Let's say we want to put a space station into orbit at 18km. The orbital speed at this height would be 7.9km/s. However, it would suffer orbital decay. Even the ISS does at a height of ~400km, and according to Wikipedia altitudes below 160km will suffer "very rapid orbital decay"Wikipedia.
So the question is; how much acceleration would be required to counter this orbital decay and keep a space station in orbit at 18km? How about the mesosphere? What is the function for acceleration required at a given height to keep it in orbit (i.e, orbital decay as a function of altitude)?