信号符号定时同步算法.pdf

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Symbol Timing Synchronization r(t) MF MF x y decision − )n0 sin2 ω ( )n0 2 ω cos ( t = kTs + τ symbol timing synchronization Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 87

Symbol Timing Synchronization in Analog Systems r(t) MF MF − )t0 sin2 ω ( )t0 2 ω cos ( samples taken on clock edge x y decision loop aligns clock edge with symbol boundary times TED F(z) VCC Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 88

Symbol Timing Synchronization in Analog Systems Matched Filter Output VCC output clock edges occur too late so loop increases frequency of VCC clock edges are aligned with optimum sampling instants Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 89

The Eye Diagram and the Effects of Symbol Timing Offset for QPSK I Q 2 1.5 1 0.5 0 -0.5 -1 -1.5 -Ts/2 0 Ts/2 -2 -2 -1.5 -0.5 -1 Constellation Corresponding to 1 I and Q channels sampled with no timing error 0 0.5 1.5 2 Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 90

The Eye Diagram and the Effects of Symbol Timing Offset for QPSK I Q 2 1.5 1 0.5 0 -0.5 -1 -1.5 -Ts/2 0 Ts/2 -2 -2 -1.5 -0.5 -1 Constellation Corresponding to 1 I and Q channels sampled with timing error = 0.125Ts 0 0.5 1.5 2 Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 91

The Eye Diagram and the Effects of Symbol Timing Offset for QPSK I Q 2 1.5 1 0.5 0 -0.5 -1 -1.5 -Ts/2 0 Ts/2 -2 -2 -1.5 -0.5 -1 Constellation Corresponding to 1 I and Q channels sampled with timing error = 0.25Ts 0 0.5 1.5 2 Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 92

The Eye Diagram and the Effects of Symbol Timing Offset for QPSK 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2 I Q -Ts/2 0 Ts/2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Constellation Corresponding to I and Q channels sampled with timing error = 0.5Ts Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 93

Timing Error Detectors optimum sampling time corresponds to point where slope of eye = 0 matched filter output sampled too early: the slope is positive the timing should be delayed (i.e. sample clock period increased) so that the next sample taken a little later. -Ts/2 0 Ts/2 -Ts/2 0 Ts/2 matched filter output sampled too late: the slope is negative the timing should be advanced (i.e. sample clock period decreased) so that the next sample taken a little sooner. -Ts/2 0 Ts/2 Fall Semester 2001 ECEn 485: Introduction to Digital Communication Theory 94

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