Summary: We propose a type IIB super-Poincaré algebra with \(SO(2,1)\) covariant central extension. Together with \(SO(2,1)\) and \(SO(9,1)\) generators, an \(SO(2,1)\) triplet (momenta), a Majorana-spinor doublet (supercharges) and a Rarita-Schwinger central charge generate a group, \(G\). We consider a coset \(G/H\) where \(H=(SO(2)\times \text{Lorentz})\), and the \(SL(2,\mathbb{R})\) 2-form doublet is obtained by the coset construction. It is shown that \(U(1)\) connections, whose strengths are associated with 2-forms, are recognized as coordinates of the enlarged space. We suggest that this is the fundamental algebra governing the superstring theories which explains the IIB \(SL(2,\mathbb{R})\) duality and geometrical origin of \(U(1)\) fields.