Iterative Receivers for LTE and WiMAX with Imperfect Synchronization

该提案考虑为4G（甚至5G）移动宽带无线接入技术的两个主要竞争者：LTE（长期演进）和WiMAX（微波接入的全球互操作性）设计迭代接收器。WiMAX空中接口使用OFDMA（正交频分多址）进行上行链路和下行链路传输。另一方面，LTE只在下行链路上使用OFDMA，在上行链路上使用SC-FDMA（单载波频分多址）。在不完全同步条件下，OFDMA和SC-FDMA都极易受到载波频偏（CFO）和时序误差等同步误差的影响，特别是在低信噪比（SNR）条件下。因此，在这两种系统中，同步参数和信道脉冲响应的估计至关重要。到目前为止，对该问题的研究大多集中在分离和顺序同步以及信道估计方面。更具体地说，信道估计是通过假设已经建立了完全同步来执行的，即使信道估计会因不完全同步而降低，反之亦然。在SD-FDMA和OFDMA系统中，数据以突发方式传输，每个突发都附加一个包含已知训练序列的前导，以方便初始同步和信道估计。然而，由于初始同步精度不足，信道估计及其时变，仍然需要在突发数据段中插入已知导频来更新和提高同步和信道估计精度，以降低频谱效率为代价保持系统的高性能。由于同步和信道估计是相互关联的，联合信道估计和同步可以提供更好的性能。因此，该项目的主要目标是开发可用于设计和优化接收器的工具和方法，这些接收器可以在LTE和WiMAX网络中以迭代（turbo）方式联合执行同步、信道估计和数据检测/解码。

This proposal considers the design of iterative receivers for the two leading contenders for 4G (and even for 5G) mobile broadband wireless access technology: LTE (Long term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access). The WiMAX air interface uses OFDMA (Orthogonal Frequency Division Multiple Access) for both uplink and downlink transmissions. On the other hand, LTE uses OFDMA only for its downlink, and uses SC-FDMA (Single Carrier Frequency Division Multiple Access) for its uplink. Under imperfect synchronization conditions, both OFDMA and SC-FDMA are highly susceptible to synchronization errors such as carrier frequency offset (CFO) and timing errors, especially for operation at low signal-to-noise ratio (SNR) regimes. Therefore, estimation of synchronization parameters and channel impulse responses (CIRs) are of crucial importance in these two systems. So far, most studies on the issue have been focused on separate and sequential synchronization and channel estimation. More specifically, channel estimation is performed by assuming that perfect synchronization has been established, even though channel estimation would be degraded by imperfect synchronization and vice versa. In SD-FDMA and OFDMA systems, data is transmitted in bursts, and each burst is appended with a preamble that contains known training sequences to facilitate the initial synchronization and channel estimation. However, insufficient accuracy of initial synchronization, and channel estimates as well as their time variation still require known pilot tones inserted in the data segment of the burst to update and enhance synchronization and channel estimation accuracy in order to maintain the high system performance at the cost of reduced spectral efficiency. Since synchronization and channel estimation are mutually related, joint channel estimation and synchronization can provide better performance. Hence, the main goal of this project is to develop tools and methodologies that can be used in the design and optimization of receivers that can jointly perform synchronization, channel estimation, and data detection/decoding in an iterative (turbo) fashion in LTE and WiMAX networks.