Joint Power Allocation for Coordinated Two-Point Diversity Transmission Under a Coordinated Power Constraint in Rayleigh Fading Channels

We consider the joint power allocation problem in a coordinated two-point (Co2P) diversity transmission network with the constraint on the total coordinated transmission power, aimed at maximizing the ergodic cooperative capacity in Rayleigh fading channels. First, we obtain the relationship that yields the optimum reference-point power with respect to the mean branch signal-to-noise ratios. Then, we introduce two simple log-linear expressions (LLEs) for the reference-point power. One is the tangent line approximation to the optimum reference-point power at the balanced point where the optimum reference-point power and the optimum cooperative-point power are equal, and the other is the modified tangent line approximation based on a simple approximation to the exact slope. Using the LLEs, we also draw efficient criteria for turning off Co2P joint diversity transmission. The numerical results demonstrate that the LLEs are very efficient in terms of the ergodic cooperative capacity and the simplicity of mode switching as well as joint power allocation.