In the context of network information theory, broadcast channel (BC) represents a channel where there is only one transmitter and there are a number of receivers. Broadcast channel is a suitable model for the downlink in cellular networks, wireless local area networks (WLANs), and the downstream of digital subscriber lines (DSLs). For the last 35 years, the research on broadcast channel has achieved significant results, especially recent results on sum rate capacity and capacity region of the Gaussian multiple input multiple output broadcast channel (MIMO Gaussian BC). Those achievements have been greatly contributing to the advances in wired and wireless communications, especially in cellular networks.
In this thesis, we focus on linear beamforming for the Gaussian MIMO broadcast channels in which multiple antennas at both transmitter and receivers enable the transmission of several streams to each receiver. In particular, we generalize the uplink-downlink signal-to-interference-plus-noise (SINR) duality in broadcast channel to a MIMO scenario with general cross-talk matrix. The duality is utilized to find an optimal solution to the SINR-balancing problem in MIMO broadcast channel. An iterative algorithm is proposed to find optimal beamforming vectors as well as optimal power allocation for the streams while maximizing the minimum of individual SINR's. The results provide a general tool for characterizing the rate regions achieved by different coding strategies.
In the second part of this thesis, we consider superposition coding and successive decoding for MIMO BC with the use of a "common information". The scheme is named CMHP by some authors. We propose a lower bound to the rate region of CMHP in a 2-user MIMO BC in which each user has 2 antennas and the transmitter has 4 antennas.
Numerical results are provided for illustrating the analysis, and open problems are also discussed.
네트워크 정보이론에서 브로드캐스트 채널은 하나의 송신기가 다수의 수신기로 신호를 전송하는 상황을 나타낸다. 본 논문은 다중안테나 가우시안 브로드캐스트 채널에서 공통정보가 있는 경우의 선형 빔포밍과 수퍼포지션 코딩을 고려하였다. 일반적인 혼선행렬로 표현된 다중안테나 브로드캐스트 채널에서의 상/하향링크의 쌍대성을 증명하였고, 이를 SINR-균형화 문제에 적용하였다. 이 문제에서 얻어진 해는 다양한 코딩방식들의 달성 가능한 영역을 규명하는데 이용되었다. 그 결과로서, CMHP를 이용한 전송방식을 제안하였다. 최적화 알고리즘을 제안하였으며, 해석과 비교를 위해 시뮬레이션 결과들을 제시하였다.
