Optimisation of wireless multimedia networks with MIMO antennas: a cross-layer approach

下一代无线通信的最终目标是在个人数字助理（PDA）等移动终端与计算机服务器之间提供无处不在的无缝连接，使用户可以随时随地享受高质量的多媒体服务。然而，基本的物理挑战，如信道衰落和干扰，给无线电资源带来了压力，使可靠的无线通信难以实现。为了应对快速增长的数据速率需求，特别值得注意的是所谓的MIMO天线（或在发送端和接收端都有多个天线的配置），它巧妙地利用天线之间空间特征的差异来再现传输带宽。该项目的目的是设计一种智能调度协议，通过适应信道变化，最大限度地利用从多天线基站到许多多天线移动终端的MIMO下行信道。具体地说，我们的目标是在满足用户错误率和延迟限制的同时最小化基站的总功率。该问题对于在无线信道中提供高质量的多媒体服务具有重要意义。提出的研究共同处理每个分组时间访问信道的用户的调度（即介质访问控制（MAC）层的主要功能）以及他们如何使用MIMO传输访问信道（即物理层的主要功能）。提出的联合MAC-PHY优化的优点可以解释如下。用户通常位于不同的位置，他们独立淡出。在特定的时间常数中，某些用户可能会陷入深度淡出，但有些用户可能不会，这就是所谓的多用户多样性。智能MAC调度通过适应用户间信道的时间变化，使信道利用率最大化。另一方面，从物理层的角度来看，MIMO传输可以优化，以充分利用用户之间空间中的信道变化。为了最大限度地利用渠道，正在体验良好渠道的用户应该获得渠道的访问权。然而，对于带有MIMO天线的信道，良好的感觉是不明确的，因为用户可能不会在所有天线上都拥有比其他用户更好的信道。此外，随着下行链路频谱复用的推进，用户间的干扰将进一步加剧好信道的定义。此外，就延迟而言，即使用户的信道不好，也应该在他/她的数据过期之前访问该信道。这些相互冲突的目标使优化变得复杂。为了实现我们的目标，我们需要克服上述挑战，最终能够设计出能够观察和适应信道变化的调度和传输。预计通过增强PHY和MAC之间的交互，系统吞吐量可以大大提高，远远超过目前使用的系统。

The ultimate goal of future-generation wireless communications is to provide ubiquitous seamless connections between mobile terminals such as personal digital assistant (PDA), and computer servers so that users can enjoy high-quality multimedia services at anytime anywhere without wires. Fundamental physical challenges such as channel fading and interference, however, have put strains on the radio resources that make reliable wireless communications difficult to achieve. In response to the rapidly increasing demand of the data-rate requirements, of particular attention is the so-called MIMO antenna (or a configuration that has multiple antennas at both the transmitter and the receiver), which cleverly exploits the difference between spatial signatures among antennas to reproduce transmission bandwidth.The aim of this project is to devise a smart scheduling protocol that maximises the utilisation of a MIMO downlink channel where communication occurs from a multi-antenna base station to many multi-antenna mobile terminals, through adaptation to the channel variations. Specifically, our aim is to minimise the total power of the base station while satisfying the users' error-rate and delay constraints. The problem is important for providing high-quality multimedia services in wireless channels. The proposed research jointly deals with scheduling of the users that access the channel for each packet time (i.e., the main function in the medium access control (MAC) layer) and how they access the channel using MIMO transmission (i.e., the main function in the physical (PHY) layer).The merit of the proposed joint MAC-PHY optimisation can be explained as follows. Users generally situated at different locations and they fade independently. At a particular constant in time, a certain user may be in deep fade but some may not be, and this contributes the so-called multiuser diversity. Intelligent MAC scheduling can maximise the channel utilisation by adapting to the channel variations in time among users. On the other hand, from the PHY point of view, MIMO transmission can be optimised to take full advantage of the channel variations in space among the users.To maximise the channel utilisation, users who are experiencing a good channel should gain access to the channel. However, for a channel with MIMO antenna, a sense of goodness is ill defined as a user may not have unanimously better channels for all antennas than another user's. Besides, as we are promoting spectral reuse in downlink, inter-user interference will further exacerbate the definition of a good channel. In addition, as far as delay is concerned, even a user who has a bad channel should gain access to the channel before his/her data expire. These mutually conflicting objectives complicate the optimisation.To accomplish our objective, we need to overcome the above challenges and in the end be able to design the scheduling and transmission jointly that can observe and adapt to the channel variations. It is anticipated that through enhanced interaction between PHY and MAC, the system throughput can be improved far exceeding systems in use today.