چكيده به لاتين
This thesis presents solutions for using three dimensional beamforming (3DBF) in downlink of multi-cell multi-user multiple input multiple output (MU-MIMO) systems with cooperation between base stations under two different scenarios from the channel state imformation (CSI) availability point of view.
In first scenario perfect CSI is available at transmitter. Each base station designs precoding vectors and performs 3DBF (also known as tilt angle adaptation). For achieving this goal an iterative algorithm is presented which introduces new coefficients in main objective function and by utilizing the relation between Minimum Mean Square Error of estimating symbols in user’s receiver and rate of the user, splits main objective to some simpler problems. Each derived problem can be solved in its own base station separately.
In second scenario base stations only have access to long term channel state information of user’s which includes pathloss and angle of each user equipment with antenna array of base stations. Then an algorithm is represented that maximizes an ergodic utility of user’s rate which is averaged over small scall fading realization of channels. In this case zero forcing beamfotming (ZFBF) vectors are used and egodic utility function is a lower bound on the sum rate of whole users in network. Optimization variables are allocated power to each user and tilt angle of antenna array of base stations. Some approximations are introduced to problem which make it possible to present an iterative algorithm to solve problem.
For instance and comparison of proposed method versus omnidirectional antenna array case with typical simulation setup, in first scenario computer simulations shows about 15% gain in sum energy efficiency for low transmit signal to noise ratio (SNR) region while this gain increases up to 40% for high transmit SNR region. For second scenario similar to first case gain achieved in energy efficiency increases from 10% to 30% by increasing available transmit power at each base station.
Keywords: Multi-cell network, multiple input multiple output system, 3D beamforming, array tilt angle, total instantaneous energy efficiency, total ergodic energy efficiency, convex optimization.
