数字接收机同步技术详解

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"Synchronization Techniques for Digital Receivers" 是一本专注于数字接收机同步技术的权威著作，由 Umberto Mengali 和 Aldo N. D'Andrea 共同撰写，出版于PLENUM PRESS。这本书涵盖了时间同步、载波频率同步和相位同步等核心主题，共计529页，是数字信号处理领域的经典参考资料。在数字通信系统中，同步技术扮演着至关重要的角色。时间同步确保了接收机能够准确地识别和解码发送端的信号，这通常通过时钟同步电路来实现，它们需要调整以与发送端的时钟保持一致。书中详细探讨了各种时间同步方法，包括基于滑窗技术、自同步机制以及利用特定同步码等策略。载波频率同步是另一个关键部分，因为数字信号通常调制在某个载波频率上。如果接收机不能正确跟踪并匹配这个频率，解调过程将受到严重影响。作者可能介绍了锁相环(PLL)、数字锁相环(DLL)等经典方法，以及用于提高频率同步精度的高级算法。相位同步则涉及到信号相位的精确对齐，这对于数据恢复至关重要。相位同步技术可能包括相位比较、相位锁定技术以及基于最大似然估计的相位估计算法。书中可能还讨论了如何处理相位噪声和快衰落环境下的同步问题。此外，书中可能还包括了对现代数字接收机设计的深入分析，如软件定义无线电(SDR)中的同步策略，以及在多径传播和干扰环境下的同步挑战。书中可能提供了丰富的实例、数学模型和仿真结果，以帮助读者理解这些概念，并具备解决实际问题的能力。该书不仅适合专业工程师、研究人员和研究生作为学习资料，也是数字通信和信号处理领域从业者的重要参考书。其包含的文献参考和索引进一步增强了其作为研究资源的价值。《Synchronization Techniques for Digital Receivers》全面地覆盖了数字接收机同步的各个方面，是理解和掌握这一复杂领域的宝贵资源。

Introduction

on detection performance. Various receiver configurations and modulation

formats are considered and, in each case, theoretical or simulation results are

illustrated. The purpose is to establish ball-park limits on allowable synchro

nization errors. The second part concentrates on estimation criteria and gives a

self-contained account of ML estimation methods. In particular, likelihood

functions for continuous-time and discrete-time observations are derived for the

additive white Gaussian noise (AWGN) channel and the concept of wanted and

unwanted parameters is discussed. These parameters play a fundamental role in

the computation of the MCRB. The third part gives closed-form expressions of

the MCRBs for timing, phase and frequency under various modulation condi

tions.

Chapter 3 investigates carrier frequency estimation with passband PAM

modulation. A distinction is made between two rather different situations, de

pending on whether the carrier frequency offset is expected to be small or com

parable with the symbol rate. Different estimation methods apply in the two

cases. In particular, data-aided or decision-directed schemes can be used with

small offsets whereas non-data-aided schemes are inevitable otherwise. As is

intuitively clear, data-aided and decision-directed methods are much more ac

curate than non-data-aided ones. In fact, circuits in the first category perform

close to the MCRB while the others are far from it.

Chapter 4 concentrates on frequency estimation with CPM modulation.

The same distinction between “small” and “large” frequency offsets is made as

in Chapter 3. As opposed to PAM modulation, however, few methods are

available for small frequency offsets and, what is worse, they are limited to bi

nary symbols and a modulation index equal to 1/2. On the contrary, a variety of

estimation schemes can be used with large frequency offsets. Their perfor

mance is far from the MCRB, however, especially with long frequency pulses.

In consequence, narrow-band tracking loops are needed to achieve small esti

mation variances. Of course, this translates into rather long acquisition times.

Chapter 5 is the longest and is concerned with phase estimation in PAM

modulations. Its first part focuses on phase recovery for transmissions over

AWGN channels. Costas loops are popular synchronization schemes for con

tinuous transmissions over these channels. They are easily designed to com

pensate for (small) frequency offsets and have excellent tracking performance

in the absence of phase noise. In any practical situation, however, some degree

of phase noise is inevitable due to oscillator imperfections. The resulting track

ing degradations can be limited by proper loop design. This subject is ade

quately addressed and criteria are provided to minimize the phase errors. The

central part of the chapter considers frequency-flat fading channels. Here, the

signal is affected by a multiplicative distortion (MD) which is modeled as a

slowly varying Gaussian random process. As samples of the MD (taken at the

symbol rate) are needed for coherent detection, the problem arises of estimating

Chapter 1

the MD random sequence from the received waveform. This is a generalization

of the carrier phase recovery problem on AWGN channels where the parameter

under estimation is a constant (the channel phase shift). MD estimation can be

accomplished in various ways. One is to exploit known symbols multiplexed

onto the data sequence. This leads to the so-called pilot symbol assisted

schemes, which work well for fading rates up to about 1% of the symbol rate.

Another solution is to perform ML joint channel and sequence estimation. This

approach is effective with Doppler rates up to about 5% of the symbol rate. The

last part of the chapter returns to the AWGN channel and explores open-loop

phase estimation methods for applications in packet transmissions.

Chapter 6 investigates phase recovery with CPM modulation. To some

degree it has the same structure as the previous one. Its size is reduced, how

ever, as many ideas developed earlier ^so apply to CPM formats. Decision-di

rected tracking loops are the most popular synchronization schemes for contin

uous transmission over the AWGN channel and are explored in the first part of

the chapter. Their performance is quite close to the MCRB at signal-to-noise

ratios of practical interest. With frequency-flat fading channels several approx

imate ML decoding schemes are available, with very good performance for

fading rates up to 5% of the symbol rate. The last part of the chapter focuses on

open-loop estimation methods for applications in packet transmissions.

Chapter 7 deals with clock synchronization in baseband transmissions.

The structure of the chapter reflects the fact that timing recovery consists of

two basic operations: (i) estimation of the positions of the signal pulses relative

to a local time reference; (ii) application of this information to the computation

of symbol-spaced signal samples {strobes) for use in the detection process. The

former is called timing measurement, the latter timing adjustment. Two ap

proaches to timing adjustment are investigated. In one case the strobes are ob

tained by sampling the received signal with a clock locked to the incoming data

stream (synchronous sampling). In the other, the sampling times are dictated by

a free-running oscillator and the strobes are computed by interpolating between

samples (non-synchronous sampling). Timing measurements are discussed in

the second part of the chapter. They can be performed by either open-loop or

closed-loop circuits. The former provide a direct estimate of the pulse positions

relative to a local time reference. The latter compute an error signal which is

proportional to the difference between the actual pulse positions and their esti

mates. The error signal is then exploited to update the estimates.

Chapter 8 investigates timing recovery with passband PAM modulation.

The chapter structure reflects the fact that carrier phase plays an important role

in timing estimation and, in consequence, it is useful to distinguish between

two scenarios. In the first one, carrier phase is estimated in conjunction with

timing. This leads to joint phase and timing synchronization schemes. In the

sccond scenario, the phase estimation problem is either postponed until after

Introduction

timing recovery or is not approached at all, as occurs with differential detection

receivers where no phase information is needed. In these circumstances it is

desirable to have timing circuits that are insensitive to phase variations.

Methods to achieve this goal are analyzed. Finally, the problem of timing re

covery with flat-fading is discussed. Here, the received signal is not only ro

tated (as happens with the AWGN channel) but is also attenuated. Thus, timing

recovery is a more complex function with fading channels. Two solutions are

proposed and compared. One is to use the same methods suitable for AWGN

channels. The other is to employ new schemes that take the fading channel fea

tures into account.

The last topic is timing recovery with CPM signals and is covered in

Chapter 9. The chapter is organized as the previous one except that only

AWGN channels are considered. This is so because timing recovery with fad

ing channels has not received much attention in the literature so far. Decision-

directed feedback synchronizers for joint phase and timing estimation are in

vestigated in the first part of the chapter. They have excellent tracking perfor

mance but may exhibit spurious locks, depending on the alphabet size and the

frequency response of the modulator. Methods to detect and correct spurious

locks are proposed. Phase information is not needed with differential detection

and the problem arises of estimating timing in a phase-independent fashion.

Timing algorithms that operate in this manner are investigated. They have good

performance with full response systems but fail with long frequency pulses.

The chapter concludes with two open-loop timing circuits for minimum shift

keying (MSK) and Gaussian minimum shift keying (GMSK).

References

[1] A.J.VUerbi，Principles of Coherent Communication, New York: McGraw-Hill，1966.

[2] The Theory of Synchronous Communications^ Englewood Cliffs: Prentice*

Hall, 1971.

[3] W.C,Lindsey, Synchronization Systems in Communication and Control, Englewood

Cliffs; Prentice-Hall, 1972.

[4] W^CXindsey and M.K.Simon, Telecommunication System Engineering, Englewood

Cliffs: Prentice-Hall, 1973,

[5] F.M.Gardner, Phaselock Techniques, New York: John Wiley&Sons, 1979.

[6] H.Meyr and G.Ascheid, Synchronization in Digital Communications, vol. 1, New York:

John Wiley&Sons, 1990.

[7] F.M.Gardner, Demodulator Reference Recovery Techniques Suited for Digital

Implementationy European Space Agency, Final Report，ESTEC Contract No.

6847/86/NL/DG, August, 1988.

[8] H.Meyr, S.Fechtel and M.Moeneclaey, Synchronization in Digital Communicationsy vol.

2, New York: John Wiley&Sons, to be published.

[9] J.B.Anderson，T.Aulin and C-E.Sundherg, Digital Phase Modulation, New York: Plenum

Press, 1986.

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