No minimum has been disclosed, but as a general rule of thumb the better (faster, more stable) your broadband the better.
There is a thread where someone is playing on a 14400 modem due to hitting a bandwidth cap, who seems to be doing 'okay'.
I am on BT rural broadband, and have mixed results, including dropped market connections, failing to login at the beginning, inability to see friends in the same system, a long exit from Supercruise, much rubber-banding and frequent random screen freezes.