Research On Synchronization Techniques In OFDMA Systems

Orthogonal frequency division multiple access(OFDMA) is a multi-user version of the popular orthogonal frequency division multiplexing(OFDM) ditigital modulation scheme. OFDMA has become the promising technology for the next generation mobile communication networks. OFDMA inherits from OFDM the weakness of being sensitive to synchronization errors, especially to carrier frequency offset(CFO). In OFDMA systems, CFO between the transmitter and the receiver results in the loss of orthogonality among subcarriers and inter-carrier interference(ICI), which further introduces multi-access interference(MAI) and degrades the system performance. Frequency synchronization in the uplink of OFDMA includes two aspects: CFO estimation amd correction.3GPP has recently launched the study item "3G long term evolution (LTE)". 3G LTE has decided to adopt OFDMA as the downlink modulation technology. LTE provides opportunity and challenge and cell search for LTE is one of hot topics.Hence, the dissertation deals with cell search for 3G LTE systems and CFO estimation and correction for the uplink of OFDMA systems. The main contribution of the disseration is as follows.1. Cell search for LTE systems is studied. For co-existence with the current UTRA LCR-TDD system, the long term evolution of LCR-TDD system, i.e., EUTRA LCR-TDD should adopt the same frame structure as the current LCR-TDD does. We propose a cell search scheme for EUTRA LCR-TDD system and have submitted our proposals to LTE meetings for discussion. LTE cell search is required to support scalable bandwidths from 1.25MHz to 20MHz, so synchronization channel(SCH) always occupies the central 1.25MHz of the total system bandwidth. With the small SCH bandwidth, synchronization performance can be guaranteed by transmitting several SCHs at a radio same frame while not increasing cell search time and however, decreasing the system efficiency. To the problem, we propose a oversampling-based cell search method. Theory analysis and simulations are presented to prove the proposed method. Furtheremore, the GCL(Generalized Chirp Like)-based cell identification methods are studied. The features of the proposed cell search methods are: Cell search is hierarchical in that SCH is common to all cells and cell identification is based on cell specific reference symbols; Cell specific sequences adopt PN sequences; To support scalable bandwidths, both SCH and cell specific reference symbols only occupy the central 1.25MHz of the total system bandwidth; To mitigate inter-cell interference, neighboring three cell specific reference symbols are frequency division multiplexed(FDM); To support a large number of target cells, a cell specific sequence is either QPSK modulated or divided into two halfves and then BPSK modulated onto the first and the second reference symbols respectively; To combat channel fading, cell identification is based on the frequency domain differential cross-correlations and maximal ratio combining(MRC).2. The effect of synchronization errors in the uplink of OFDMA systems is studied. The OFDMA uplink performance in the presence of timing, carrier frequency or sampling frequency offsets are analyzed theoretically. The effect of carrier frequency offsets in the uplink of OFDMA is related with subcarrier assignment scheme which is discussed through simulations.3. CFOs correction issue in OFDMA uplink is studied. In the uplink of OFDMA, assuming that CFOs have been estimated, there are two methods to correct CFOs: feedback and compensation. The former is to return CFO estimates to subscribers for adjusting their transmitter carrier frequencies. Feedback is simple and effective with corresponding overhead. The latter is to compensate for CFOs directly at the BS to restore desired signals and it usually needs advanced signal processing techniques. This paper focus on the following CFO compensation receivers: single user detector, Post-DFT processing receiver, interference cancellation receiver and the interleaved multiuser signal structure-based minimum mean square error(MMSE) detector. In this paper, the signal-to-interference ratio(SIR) expression for the interleaved multiuser signal structure-based MMSE detector is first derived. As concerning the performance, the interleaved multiuser signal structure-based MMSE detector is optimal in that it can eliminate ICI completely and decrease MAI at the same time. However, its computation is complex involving matrix inverse.4. CFOs estimation issue for the uplink of OFDMA is investigated. In the uplink of OFDMA, each user has its own CFO and CFO estimation in this case becomes a multi-parameter estimation problem. Based on the multi-user signal structure in the interleaved OFDMA uplink, CFOs can be estimated utilizing the subspace decomposition method. The subspace-based estimator belongs to blind estimation with high system efficiency and, however, with high computation. This paper proposes a pilot-aided CFO estimator for the interleaved OFDMA uplink. Pilot symbol is composed of one OFDMA symbol. At the basesation, to estimate a particular user's CFO, first correlate pilot signal with the user's pilot sequence, then detect the prominent frequency component of the correlations utilizing the periodgram in power spectrum density(PSD) estimation. In order that the proposed estimator is effective, ZCZ sequences are used to mitigate multipath fading and multiuser interference. Furthermore, this paper also proposes an interference cancellation-based CFO estimator requiring that each user transmits two consecutive same OFDM symbols. Since each user transmits training symbols on its own subcarriers, the estimator does't adhere to a particular subcarrier assignment. At the receiver, an initial CFO estimates are first obtained based on original training signal. Then, using the initial CFO estimates, interference cancellation is performed. Finally, based on the interference cancelled training signal, CFOs can be estimated with high precision.