Achievable degrees of freedom for interference broadcast channels with asymmetric complex signaling

In multi-cell environments, interference alignment (IA) introduced by Cadambe and Jafar is an effective strategy for managing interference. However, this requires a large number of symbol extension in time/frequency domain to gurantee the optimal number of degrees of freedom (DOF). Recently, in the single-input single-output (SISO) case, a new idea of the IA scheme based on asymmetric complex signaling and symbol extension was proposed where at least the DOF of 1.2 is achievable for all complex channel values. In this paper, we prove that at least the DOF of 1.5 is achievable in multi-cell and multi-user interfering broadcast channels under the assumption of constant channel coefficients with no symbol extension. We also show that the achievable DOF varies with the condition of user scale and ω (√SNR) users are required to guarantee the DOF of 1.5. Furthermore, a simple IA scheme in conjunction with user selection which groups semi-aligned users is proposed to realize the achievable DOF.