This letter propose a novel scheme for cognitive macro/femtocell downlink coordinated multipoint (CoMP) transmission to improve the capacity of femtocell without affecting the normal communications of macrocell users (MUs). In specific, the femtocell access point (FAP) assists the macrocell base station (MBS) by jointly transmiting to the MU in the vicinity of the femtocell. A portion of the spectrum originally assigned to the MU is reserved for CoMP. In return, the rest can be freely utilized by the femtocell. Moreover, the impact of non-ideal backhaul (NIB) is considered, which makes the joint spectrum splitting and power allocation non-convex. A universal dual bisection algorithm is proposed to achieve the globally optimal solution. Simulation results indicate that the capacity of femtocell is enhanced up to 200% relative to the traditional methods. Meanwhile, the data communications of MUs are well protected compared with the scheme that ignores the impact of NIB.