Browsing by Subject "synchronization"
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Item New advances in synchronization of digital communication receivers(Texas A&M University, 2005-02-17) Wang, YanSynchronization is a challenging but very important task in communications. In digital communication systems, a hierarchy of synchronization problems has to be considered: carrier synchronization, symbol timing synchronization and frame synchronization. For bandwidth efficiency and burst transmission reasons, the former two synchronization steps tend to favor non-data aided (NDA or blind) techniques, while in general, the last one is usually solved by inserting repetitively known bits or words into the data sequence, and is referred to as a data-aided (DA) approach. Over the last two decades, extensive research work has been carried out to design nondata-aided timing recovery and carrier synchronization algorithms. Despite their importance and spread use, most of the existing blind synchronization algorithms are derived in an ad-hoc manner without exploiting optimally the entire available statistical information. In most cases their performance is evaluated by computer simulations, rigorous and complete performance analysis has not been performed yet. It turns out that a theoretical oriented approach is indispensable for studying the limit or bound of algorithms and comparing different methods. The main goal of this dissertation is to develop several novel signal processing frameworks that enable to analyze and improve the performance of the existing timing recovery and carrier synchronization algorithms. As byproducts of this analysis, unified methods for designing new computationally and statistically efficient (i.e., minimum variance estimators) blind feedforward synchronizers are developed. Our work consists of three tightly coupled research directions. First, a general and unified framework is proposed to develop optimal nonlinear least-squares (NLS) carrier recovery scheme for burst transmissions. A family of blind constellation-dependent optimal "matched" NLS carrier estimators is proposed for synchronization of burst transmissions fully modulated by PSK and QAM-constellations in additive white Gaussian noise channels. Second, a cyclostationary statistics based framework is proposed for designing computationally and statistically efficient robust blind symbol timing recovery for time-selective flat-fading channels. Lastly, dealing with the problem of frame synchronization, a simple and efficient data-aided approach is proposed for jointly estimating the frame boundary, the frequency-selective channel and the carrier frequency offset.Item New leading/trailing edge modulation strategies for two-stage AC/DC PFC adapters to reduce DC-link capacitor ripple current(Texas A&M University, 2007-09-17) Sun, JingAC/DC adapters mostly employ two-stage topology: Power Factor Correction (PFC) pre-regulation stage followed by an isolated DC/DC converter stage. Low power AC/DC adapters require a small size to be competitive. Among their components, the bulk DC-link capacitor is one of the largest because it should keep the output voltage with low ripple. Also, the size of this capacitor is penalized due to the universal line voltage application. Synchronization through employing leading edge modulation for the first PFC stage and trailing edge modulation for the second DC/DC converter stage can significantly reduce the ripple current and ripple voltage of the DC-link capacitor. Thus, a smaller DC-link capacitance can be used, lowering the cost and size of the AC/DC adapter. Benefits of the synchronous switching scheme were already demonstrated experimentally. However, no mathematical analysis was presented. In this thesis, detailed mathematical analyses in per-unit quantity are given to facilitate the calculation of the DC-link capacitor ripple current reduction with Leading/Trailing Edge Modulation strategies. One of the limitations of leading/trailing edge modulation is that the switching frequencies of the two stages need to be equal to achieve the best reduction of the DC-link capacitor ripple current. The DC-link capacitor ripple current will become larger if the switching frequency of the DC/DC converter is larger than that of the PFC pre-regulator, which blocks us to employ higher frequency for isolated DC/DC converter to reduce its transformer size. This thesis proposed a new Leading/Trailing Edge Modulation strategy to further reduce the DC-link bulk capacitor ripple current when switching frequency of DC/DC converter stage is twice the switching frequency of PFC stage. This proposed pulse width modulation scheme was verified by simulation. Experimental results obtained through digital control based on FPGA are also presented in this thesis.Item Timing Synchronization at the Relay Node in Physical Layer Network Coding(2012-07-16) Basireddy, AshishIn recent times, there has been an increased focus on the problem of information exchange between two nodes using a relay node. The introduction of physical layer network coding has improved the throughput efficiency of such an exchange. In practice, the reliability of information exchange using this scheme is reduced due to synchronization issues at the relay node. In this thesis, we deal with timing synchronization of the signals received at the relay node. The timing offsets of the signals received at the relay node are computed based on the propagation delays in the transmitted signals. However, due to the random attenuation of signals in a fading channel, the near far problem is inherent in this situation. Hence, we aim to design near far resistant delay estimators for this system. We put forth four algorithms in this regard. In all the algorithms, propagation delay of each signal is estimated using a known preamble sent by the respective node at the beginning of the data packet. In the first algorithm, we carefully construct the preamble of each data packet and apply the MUSIC algorithm to overcome the near far problem. The eigenstructure of the correlation matrix is exploited to estimate propagation delay. Secondly, the idea of interference cancellation is implemented to remove the near far problem and delay is estimated using a correlator. Thirdly, a modified decorrelating technique is presented to negate the near far problem. Using this technique we aim to obtain an estimate of the weak user's delay that is more robust to errors in the strong user's delay estimate. In the last algorithm, pilot signals with desired autocorrelation and cross correlation functions are designed and a sliding correlator is used to estimate delay. Even though this approach is not near far resistant, performance results demonstrate that for the length's of preamble considered, this algorithm performs similar to the other algorithms.