Outage Constrained Transmit Approaches in Gaussian MIMO Relay Networks (OCTAGON)

使用多天线中继节点的协作通信是未来无线网络向越来越多的移动用户可靠地传递信息的重要技术。与众所周知的(多天线)点对点通道、多址通道和广播通道相比，源(S)在辅助节点(中继器)的帮助下向目的地(S)发送消息的协作通信领域仍然提供了一组丰富的挑战。即使假设有完美的CSI，最简单的协作通信单元、三节点中继信道的容量也是未知的。因此，有关中继信道的文献一般仅限于信息论容量的上下界。在这个研究项目中，我们打算研究(多天线)高斯中继信道，假设在发送节点只有不完美的CSI可用，而接收节点知道可靠译码所需的信道状态。在了解了基本的三节点中继信道后，我们将研究多址接入中继信道(MARC)，其中多个信源通过一个中继将信息传送到一个目的地，广播中继信道(BRC)，其中一个信源借助一个中继节点向多个目的地发送信息，以及多个中继支持从一个信源到一个目的地的通信。特别是，我们计划研究在给定中断概率上界的情况下，多天线中继系统中可达速率(S)的最大化问题。由于中断容量(在特定中断概率下的最大速率)是未知的，因此将研究基于(部分)译码转发(DF)方案的可实现下界和基于割集界限(CSB)的上限。由于中断概率的闭合形式表示缺失或过于复杂，因此所得到的机会约束问题的公式仍然很困难。因此，必须找到概率表达式的保守和乐观近似，以便解决关于传输协方差矩阵的优化，并由此限定三节点中继信道(具有和不具有半双工约束)和多中继情况的中断容量。对于更困难的多用户场景，即MARC和BC，目标是通过基于(部分)DF的内界和基于CSB的外界来表征中断容量区域。

Cooperative communications using multi-antenna relay nodes constitutes an important technology of future wireless networks to reliably convey information to a growing number of mobile users. In contrast to the well understood (multi-antenna) point-to-point channel, multiple-access channel, and broadcast channel, the field of cooperative communications where source(s) send messages to destination(s) with the help of assisting nodes (relays) still offers a rich set of challenges. Even if perfect CSI is assumed, the capacity of the simplest cooperative communication element, the three-node relay channel is still unknown. Therefore, the literature on relay channels is in general restricted to information-theoretic upper and lower capacity bounds. The even more complex assumption of fading channels further limits the state-of-the-art performance analysis to simplified scenarios and diversity-multiplexing tradeoff studies.In this research project, we intend to study (multi-antenna) Gaussian relay channels, assuming that only imperfect CSI is available at the transmitting nodes while the receiving nodes are aware of the channel states that are required for reliable decoding. After gaining knowledge for the basic three-node relay channel, we will investigate the multiple-access relay channel (MARC), where multiple sources convey information to one destination with the help of a single relay, the broadcast relay channel (BRC), where one source transmits information to multiple destinations with the help of a relay node, as well as networks where multiple relays support the communication from one source to one destination.Whereas the ergodic rate is the most important performance metric for fast fading channels, we focus on the case of slow fading, where the key event of interest is outage. In particular, we plan to study the maximization of the achievable rate(s) subject to a given upper bound on the outage probability in multi-antenna relay systems. Since the outage capacity (the maximum rate at a certain outage probability) is unknown, achievable lower bounds based on (partial) decode-and-forward (DF) schemes and an upper bound based on the cut-set-bound (CSB) will be investigated. The resulting chance constrained problem formulations are still difficult due to missing or too complex closed-form representations of the outage probabilities. Therefore, conservative and optimistic approximations of the probabilistic expressions have to be found in order to solve the optimizations with respect to the transmit covariance matrices and, therewith, bound the outage capacity of the three-node relay channel (with and without half-duplex constraints) and the multiple relay scenario. For the even more difficult multi-user scenarios, i.e., the MARC and the BC, the objective is to characterize the outage capacity region via inner bounds based on (partial) DF and an outer bound based on the CSB.