Synchronisation in OFDMA and MIMO OFDMA Systems

随着移动通信系统对高速数据传输的需求和用户数量的不断增加，对高效的传输技术和改进的多址接入技术提出了更高的要求。MIMO和OFDMA被广泛认为是未来高速、高频谱效率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，增强移动系统的覆盖和容量，并通过消除处理多路径或多用户干扰的需要，大大降低MIMO接收机的复杂性至关重要。请注意，只有在实现完美同步的情况下才能保证这些优势，因为适当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，这对于可靠传输至关重要。众所周知，OFDMA具有对不完全同步的敏感性。对上行链路和MIMO环境中同步问题的精确解决方案尤其具有挑战性。然而，目前在这一领域的研究中，避免同步问题是非常普遍的，并且MIMO-OFDMA系统中的同步问题在文献中尚未得到详细的解决。这在实际应用中必须引起关注。该项目将为OFDMA和MIMO OFDMA系统的上行链路设计新颖的频率、定时和信道估计技术，具有足够的估计精度，降低了复杂性和系统开销。为了补偿的目的，需要一个下行控制通道将参数返回给每个用户。在时变场景中，这会导致过多的传输开销，并可能由于反馈延迟而导致过时的估计。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。这样就不需要将同步参数返回给每个用户，因此将减少传输开销，并允许系统更容易地跟踪信道变化。为了进一步提高系统性能，本项目将在上述系统中应用强大的信道编码，然后开发涡轮接收器，以充分利用编码特性。通过在这些过程之间迭代地交换软信息，涡轮接收器允许同步、信道估计和解码的连续改进，并有望显著提高整体性能。本课题还将在硬件平台上实现上述系统，并在实际环境中验证所提出的算法和接收机设计。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。本课题提出的新技术具有可接受的复杂性和系统开销，将为有前途的OFDMA和MIMO-OFDMA系统提供具有挑战性的同步问题的实用解决方案，并促进其在未来通信系统中的实际实施。因此，它被认为对未来通信的基础研究和实际发展至关重要。

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.