Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Multi-carrier transmission techniques toward flexible and efficient wireless communication systems

制度:新 ; 文部省報告番号:甲2562号 ; 学位の種類:博士(国際情報通信学) ; 授与年月日:2008/3/15 ; 早大学位記番号:新470

Code-aided iterative techniques in OFDM systems

Inspired by the 'turbo principle', this thesis deals with two iterative technologies in orthogonal frequency division multiplexing (OFDM) systems: iterative interference cancelation in space-frequency block coded OFDM (SFBC-OFDM) and iterative channel estimation/ tracking in OFDM Access (OFDMA) with particular application to Worldwide Inter-operability for Microwave Access (WiMAX) systems. The linear matched filter (MF) decoding in SFBC-OFDM is simple yet obtains maximumlikelihood (ML) performance based on the assumption that the channel frequency response remains constant within a block. However, frequency response variations gives rise to inter-channel interference (lCI). In this thesis, a parallel interference cancelation (PIC) approach with soft iterations will be proposed to iteratively eliminate ICI in G4 SFBC-OFDM. Furthermore, the information from outer convolutional decoder is exploited and fed back to aid the inner PIC process to generate more accurate coded bits for the convolutional decoder. Therefore, inner and outer iterations work in a collaborative way to enhance the performance of interference cancelation. Code-aided iterative channel estimation/tracking has the ability of efficiently improving the quality of estimation/tracking without using additional pilots/training symbols. This technique is particularly applied to OFDMA physical layer ofWiMAX systems according to the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard. It will be demonstrated that the performance of the pilot-based channel estimation in uplink (UL) transmission and the channel tracking based on the preamble symbol in downlink (DL) transmission can be improved by iterating between the estimator and the detector the useful information from the outer convolutional codes. The above two issues will be discussed in Chapter 5 and Chapter 6, and before this, Chapter 2 to Chapter 4 will introduce some background techniques that are used throughout the thesis

PRE-FILTERING IN MC-CDMA DOWNLINK TRANSMISSIONS

Future wireless communication systems are expected to support high-speed and high-quality multimedia services. In theseapplications the received signal is typically affected byfrequency-selective fading, which must be properly counteracted toavoid a severe degradation of the system performance. MC-CDMA is a multiplexing technique that combines OFDM with direct sequence CDMA. It is robust to frequency-selective fading thanks to the underlying OFDM modulation and exploits frequency diversity by spreading the data of different users in the frequency domain. For these reasons it is considered as a promising candidate for the physical layer of future high-speed wireless communications. Recent publications show that MC-CDMA is particularly suitable for downlink transmissions, i.e., from the base station to the mobile terminals. In these applications orthogonal spreading codes are usually employed to provide protection against co-channel interference. In the presence of multipath propagation, however, signals undergo frequency-selective fading and the code orthogonality is lost. This gives rise to multiple-access interference, which strongly limits the system performance. In the past few years several advanced multi-user detection techniques have been proposed and discussed for interference mitigation. However, in spite of their effectiveness, all these methods are quite unattractive for downlink applications since they would entail high complexity and excessive power consumption at the remote units. As an alternative to multi-user detection, pre-filtering techniques can be employed in downlink transmissions to mitigate multiple-access interference and channel distortions. The idea behind pre-filtering is to vary the gain assigned to each subcarrier so that interference is reduced and the signal at the receiver appears undistorted. In this way, simple and low complex single-user detectors can be employed at the remote units, thereby moving most of the computational burden to the base station, where power consumption and computational resources are not critical issues. In general terms, the main contribution of this dissertation is threefold. First, we propose and discuss several linear and non-linear pre-filtering schemes for the downlink of MC-CDMA systems equipped with multiple transmit antennas and operating in a time-division-duplex mode. The resulting schemes are derived according to different optimization criteria and aim at combating the detrimental effects of MAI while maintaining the complexity of the remote units as low as possible. A second contribution comes from providing a unified framework for investigating pre-filtering in the downlink of both MC-CDMA and OFDMA systems. The use of a unified framework comprising both MC-CDMA and OFDMA allows a fair comparison between these multiple-access technologies under the same operating conditions. It turns out that OFDMA outperforms MC-CDMA when the system resources are optimally assigned to the active users according to the actual channel realization. As we shall see, in order to work properly, all the proposed schemes require explicit knowledge of the channel responses of the active users. In time-division-duplex systems this information can be achieved by exploiting the channel reciprocity between alternative uplink and downlink transmissions. If channel variations are sufficiently slow, the channel estimates of the active users can be derived at the base station during an uplink time-slot and reused for pre-filtering in the subsequent downlink time-slot. Thus, a third contribution comes from addressing the problem of channel acquisition in the uplink of an MC-CDMA system equipped with multiple receive antennas

Multi-Relay Communications in the Presence of Phase Noise and Carrier Frequency Offsets

Impairments like time varying phase noise (PHN) and carrier frequency offset (CFO) result in loss of synchronization and poor performance of multi-relay communication systems. Joint estimation of these impairments is necessary in order to correctly decode the received signal at the destination. In this paper, we address spectrally-efficient multi-relay transmission scenarios where all the relays simultaneously communicate with the destination. We propose an iterative pilot-aided algorithm based on the expectation conditional maximization (ECM) for joint estimation of multipath channels, Wiener PHNs, and CFOs in decode-and-forward (DF) based multi-relay orthogonal frequency division multiplexing (OFDM) systems. Next, a new expression of the hybrid Cramér-Rao lower bound (HCRB) for the multi-parameter estimation problem is derived. Finally, an iterative receiver based on an extended Kalman filter (EKF) for joint data detection and PHN tracking is employed. Numerical results show that the proposed estimator outperforms existing algorithms and its mean square error performance is close to the derived HCRB at differnt signal-to-noise ratios (SNRs) for different PHN variances. In addition, the combined estimation algorithm and iterative receiver can significantly improve average bit-error rate (BER) performance compared to existing algorithms. In addition, the BER performance of the proposed system is close to the ideal case of perfect channel impulse responses (CIRs), PHNs and CFOs estimation