Synchronisation in OFDMA and MIMO OFDMA Systems

随着对高数据传输速率的快速增长的需求和移动的通信系统中用户数量的增加，需要有效的传输技术和改进的多址技术。MIMO和OFDMA被广泛认为是在即将到来的高速、高频谱效率的无线网络中满足这些要求的两种关键技术。MIMO和OFDMA技术的联合设计对于促进多用户/多速率传输以增强移动的系统的覆盖范围和容量以及通过消除处理多径或多用户干扰的需要而大大降低MIMO接收机的复杂度是至关重要的。注意，只有在实现完美同步的情况下才能保证这些优点，因为正确的频率和定时同步对于保持活动用户之间的正交性是必要的，这对于可靠的传输是必不可少的。众所周知，OFDMA对不完美同步敏感。上行链路和MIMO环境中同步问题的精确解决方案特别具有挑战性。然而，在该领域的当前研究中避免同步问题是非常常见的，并且在文献中还没有详细地解决MIMO-OFDMA系统中的同步问题。本计画将针对OFDMA与MIMO OFDMA系统的上行链路，设计新颖的频率、时基与信道估计技术，以达到足够的估计精度，并降低复杂度与系统开销。为了补偿的目的，需要下行链路控制信道来向每个用户返回参数。在时变场景中，这会导致过多的传输开销，并且由于反馈延迟而可能导致过时的估计。为了解决这个问题，本项目将为上述系统开发计算效率高的迭代接收器，以执行联合同步，信道估计和数据检测。这样就不需要向每个用户返回同步参数，因此将减少传输开销，并允许系统更容易地跟踪信道变化。为进一步提升系统效能，本计画将应用强大的通道编码于上述系统，并开发涡轮接收机以充分利用编码特性。Turbo接收器允许通过在这些过程之间迭代地交换软信息来连续改进同步、信道估计和解码，并且预期显著地提高整体性能。本计画亦将在硬体平台上实作上述系统，并在实际环境中验证所提出之演算法与接收机设计。从硬件设计中获得的实验结果对无线通信行业具有重要意义。本研究所提出的新技术，在复杂度和系统开销都可以接受的情况下，将为未来的OFDMA和MIMO-OFDMA系统中具有挑战性的同步问题提供实用的解决方案，并促进其在未来通信系统中的实际实现。因此，它被认为是至关重要的基础研究和未来通信的实际发展。

With the rapidly increasing demand for high data transmission rates and the increasing number of subscribers in mobile communication systems, efficient transmission techniques and improved multiple access technology are required. MIMO and OFDMA are widely recognised as two of the key techniques to meet these requirements in forthcoming high-speed, high-spectrum efficiency wireless networks. Joint design of MIMO and OFDMA techniques is crucial to facilitate multiuser/multirate transmissions to enhance the coverage and capacity of mobile systems, and to greatly reduce the complexity of the MIMO receiver by eliminating the need for dealing with multipath or multiuser interference. Note that these advantages are guaranteed only if perfect synchronization is achieved, because proper frequency and timing synchronization is necessary to maintain orthogonality among the active users, which is essential for reliable transmission. It is well known that OFDMA suffers from sensitivity to imperfect synchronization. The exact solutions to the synchronisation problems in the uplink and MIMO environments are particularly challenging. However, it is very common to avoid synchronisation problems in the current research in this area and the synchronisation problems in MIMO-OFDMA systems have not yet been addressed in any detail in the literature. This must be an issue of concern in practical applications.This project will devise novel frequency, timing and channel estimation techniques, with sufficient estimation accuracy and reduced complexity and system overhead, for the uplink of the OFDMA and MIMO OFDMA systems. For the purpose of compensation, a downlink control channel is required to return the parameters to each user. In a time-varying scenario, this results in excessive transmission overhead and possibly outdated estimates due to feedback delays. To solve this problem, this project will develop computationally efficient iterative receivers for the above systems to perform joint synchronisation, channel estimation and data detection. There is then no need to return the synchronization parameters to each user, and hence will reduce the transmission overhead and allow systems to keep track of channel variations more easily. To further improve system performance, this project will apply powerful channel coding to the above systems and then develop turbo receivers to fully exploit the code properties. The turbo receiver allows successive refinement of synchronisation, channel estimation and decoding through iteratively exchanging soft information between these procedures and is expected to significantly improve the overall performance. This project will also implement the above systems on the hardware platform, and verify the proposed algorithms and receiver designs for a practical environment. The experimental results obtained from the hardware design must have significant importance for the wireless communications industry. The novel techniques derived in this project, with acceptable complexity and system overhead will provide practical solutions to the challenging synchronization problems for the promising OFDMA and MIMO-OFDMA systems, and facilitate their practical implementation in future communications systems. Therefore it is believed to be crucially important for fundamental research and practical development of future communications.