The dollar remained firm on Wednesday but softened slightly against the yen ahead of a much-anticipated rate decision by the Federal Reserve later in the day. The U.S. dollar index, which measures the greenback against a basket of rivals, stayed mostly flat at 105.13 as traders awaited the Fed's rate decision. Markets expect the Fed will almost certainly keep rates on hold at 5.25% to 5.50%, putting the focus on the central bank's forward guidance.