상세정보

The ultimate goal of a communication system is the reliable transmission of information at the highest possible data rate. However, several obstacles, such as inter-symbol interference (ISI), carrier frequency offset (CFO), carrier phase offset (CPO), non-linearity, and thermal noise, can interfere with achieving this goal. In particular, ISI is a major obstacle. Usually, dispersion of the channel in band-limited time-dispersive channels causes ISI, which refers to an effect of neighboring signals on the current signal and distorts the transmitted signal. Unless it is handled properly, it can result in severe bit error rate performance degradation. Thus, various channel equalization schemes, to compensate for channel dispersion in receivers, have been developed to combat ISI. Other obstacles are the CFO and CPO. CFO is caused by the mismatch of carrier frequencies in the transmitter and receiver. CPO is the result of three major components: phase instability in oscillators, the phase due to transmission delay, and thermal noise (such as additive white Gaussian noise). CFO and CPO often degrade the system performance of digital receivers. Thus, several carrier recovery algorithms have been studied. In optical communication systems, non-linear effects also give rise to many difficulties. This thesis deals with interference mitigation schemes, such as channel equalization and carrier recovery, from the viewpoint of digital signal processing.

In the first part of this thesis, we propose a reduced complexity maximum likelihood sequence detection (MSLD) equalizer for wireless communications based on bidirectional decision feedback equalizers (DFEs). We apply reduced length two-level decisions produced by a bidirectional DFEs. Therefore, the computationally expensive MLSD algorithm is applied sparingly for the two-level signals with the effective channel length shorter than the original channel, regardless of the original constellation size of the transmitted symbols.
In the second part of this thesis, we propose a blind adaptive carrier phase offset recovery algorithm based on output energy maximization (OEM) approach for the VSB signals. Unlike the most conventional modulation schemes, the VSB signals in practice have an asymmetric energy balance between the in-phase and quadrature components. We investigate this energy imbalance of the VSB signals and propose a blind adaptive carrier phase offset recovery scheme by maximizing the energy of the in-phase component of the VSB signals. We verify the performance of the proposed OEM algorithm with a mathematical analysis and simulation.
Finally, we analyze the linear equalizers used in optically amplified on-off-keyed (OOK) systems to combat chromatic dispersion (CD) and polarization mode dispersion (PMD), and we derive the mathematical minimum mean squared error (MMSE) performance of these equalizers in non-linear channels. No theoretical studies on the MMSE solutions for these equalizers have been performed. We model the optical OOK systems as square-law non-linear channels and compute the MMSE equalizer coefficients directly from the estimated optical channel, signal power, and optical noise variance. The accuracy of the calculated MMSE equalizer coefficients and MMSE performance of these equalizers have been verified by simulations using adaptive algorithms.

Contents
Abstract
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  iv
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Introduction to Communication Systems . . . . . . . . . . . . . . . .  1
1.1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1
1.1.2 Issues in Communication Systems . . . . . . . . . . . . . . . . . .  2
1.2 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Interferences in Communication Systems . . . . . . . . . . . . . . . . 9
2.2.1 Multipath Propagation . . . . . . . . . . . . . . . . . . . . . . .  9
2.2.2 Inter-symbol Interference (ISI) . . . . . . . . . . . . . . . . . .  14
2.2.3 Carrier Frequency/Phase Offset . . . . . . . . . . . . . . . . . . . 18
2.3 Channel Equalization . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.1 Maximum Likelihood (ML) Detection . . . . . . . . . . . . . . . . .  22
2.3.2 Linear Equalization . . . . . . . . . . . . . . . . . . . . . . . .  23
2.3.3 Decision Feedback Equalization (DFE) . . . . . . . . . . . . . . . . 25
3 Channel Equalization Based on Bidirectional DFEs . . . . . . . . . . . . 27
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 System Model Based on BDFE . . . . . . . . . . . . . . . . . . . . . . 30
3.3 Symbol Arbitration . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3.1 BAD Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
3.3.2 Proposed Reduced Complexity MLSD Based on BDFE . . . . . . . . . . . 33
3.4 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . .  35
3.4.1 Computational Complexity Comparison . . . . . . . . . . . . . . . .  35
3.4.2 Performance Comparison . . . . . . . . . . . . . . . . . . . . . . . 38
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
4 Carrier Phase Offset Recovery of VSB Signals . . . . . . . . . . . . . . 45
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 ATSC 8-VSB Signal and System Model . . . . . . . . . . . . . . . . . . 48
4.3 Carrier Phase Offset Recovery . . . . . . . . . . . . . . . . . . . .  52
4.3.1 Existing Carrier Phase Offset Recovery Schemes . . . . . . . . . . . 52
4.3.2 Proposed Output Energy Maximization (OEM) Approach . . . . . . . . . 53
4.4 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . .  58
4.4.1 Tracking Ability Analysis . . . . . . . . . . . . . . . . . . . . .  58
4.4.2 Performance Comparison . . . . . . . . . . . . . . . . . . . . . . . 62
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  72
5 Linear Equalization in Square-law Non-linear Channels . . . . . . . . .  73
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.2 OOK Optical Communication System Model . . . . . . . . . . . . . . . . 75
5.3 Square-law Non-linear Channels . . . . . . . . . . . . . . . . . . . . 77
5.4 MMSE Linear Equalizer in Square-law Non-linear Channels . . . . . . .  79
5.5 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . .  84
5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  89
6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  90
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100