How can I raise my Armor Cl(3.5)?

I take the role of the Tank in the party. I am a lv 11 Human Fighter. My current AC is 21. My stats are STR 20 DEX 15 CON 18 INT15 WIS 15 CHA 17. I am Wearing Black Dragon Full Plate. I am using a Masterwork greatsword that acts like a flametounge (Pretty sure it's homebrew). I have the feats dodge, improved inititive, endurance, die hard, Exotic weapon, combat reflexes, Iron will, and Leadership.