Power minimization under constant throughput constraint in wireless networks with beamforming

In multi-access wireless communication systems, power control and adaptive modulation are two important means to increase spectral efficiency, combat with time varying fading environments, and reduce cochannel interference. The overall transmitted power is minimized by using adaptive MQAM modulation. Each link can select a different MQAM modulation range according to its current channel condition. Each link\´s time average throughput is constant. The overall network throughput is constant to ensure the network performance. The scheme can be interpreted as "water filling" each link\´s throughput in the time domain and allocating overall throughput to different links at each time. From the simulation results, our schemes reduce up to 80% overall transmitted power at BER=10^-3, 66% overall transmitted power at BER=10^-6, and increase average spectral efficiency by about 1.2 bit/s/Hz.