Iterative Receivers for LTE and WiMAX with Imperfect Synchronization

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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.