Block Diagonalization Based Beamforming
Abstract: With increasing mobile penetration multi-user multi-antenna wireless communication systems are needed. This is to ensure higher per-user data rates along with higher system capacities by exploiting the excess degree of freedom due to additional antennas at the receiver with spatial multiplexing. The rising popularity of "Gigabit-LTE" and "Massive-MIMO" or "FD-MIMO" is an illustration of this demand for high data rates, especially in the forward link. In this thesis we study the MU-MIMO communication setup and attempt to solve the problem of system sumrate maximization in the downlink data transmission (also known as forward link) under a limited availability of transmit power at the base station. Contrast to uplink, in the downlink, every user in the system is required to perform interference cancellation due to signals intended to other co-users. As the mobile terminals have strict restrictions on power availability and physical dimensions, processing capabilities are extremely narrow (relative to the base station). Therefore, we study the solutions from literature in which most of the interference cancellation can also be performed by the base station (precoding). While doing so we maximize the sumrate and also consider the restrictions on the total transmit power available at the base station. In this thesis, we also study and evaluate different conventional linear precoding schemes and how they relate to the optimal structure of the solution which maximize the effective Signal to Noise Ratio (SINR) at every receiver output. We also study one of the suboptimal precoding solutions known as Block-diagonalization (BD) applicable in the case where a receiver has multiple receive antennas and compare their performance. Finally, we notice that in spite of the promising results in terms of system sumrate performance, they are not deployed in practice. The reason for this is that classic BD schemes are computationally heavy. In this thesis we attempt to reduce the complexity of the BD schemes by exploiting the principle of coherence and using perturbation theory. We make use of OFDM technology and efficient linear algebra methods to update the beamforming weights in a smart way rather than entirely computing them again such that the overall complexity of the BD technique is reduced by at least an order of magnitude. The results are simulated using the exponential correlation channel model and the LTE 3D spatial channel model which is standardized by 3GPP. The simulated environment consists of single cell MU-MIMO in a standardized urban macro environment with up to 100 transmit antennas at the BS and 2 receive antennas per user. We observe that with the increase in spatial correlations and in high SNR regions, BD outperforms other precoding schemes discussed in this thesis and the developed low complex BD precoding solution can be considered as an alternative in a more general framework with multiple antennas at the receiver.
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