KỸ THUẬT TRUYỀN DẪN VIBA SỐ - CHƯƠNG 1

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Mục đích môn học - Nghiên cứu các thành phần cơ bản trong quá trình phân tích và thiết kế hệ thống viba số :

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Nội dung Text: KỸ THUẬT TRUYỀN DẪN VIBA SỐ - CHƯƠNG 1

KỸ THUẬT TRUYỀN DẪN VIBA SỐ Nguyễn Huy Hùng Mobile: 0914908070 Email: hung_nguyenhuy@yahoo.com nghhung@ptithcm.edu.vn
Mục đích môn học Nghiên cứu các thành phần cơ bản trong  quá trình phân tích và thiết kế hệ thống viba số : Các kỹ thuật chủ yếu  Formatting and source coding   Modulation (Baseband and bandpass signaling)  Channel coding  Equalization  Synchronization Phân tích tuyến truyền dẫn vô tuyến  Hệ thống truyền dẫn viba số  2 20050121 Lecture 1
Tài liệu Course text book:  “Digital communications” by John G. Proakis  Additional recommended books:  “Communication systems engineering”, by John G. Proakis  and Masoud Salehi, Prentice Hall, 2002, 2nd edition, ISBN: 0130950076 “Digital communications: Fundamentals and Applications”  by Bernard Sklar,Prentice Hall, 2001,ISBN: 0130847887 “Communication Systems, Analysis and design”, by  H.P.E.Stern and S. A. Mahmoud, Prentice Hall, 2004, ISBN: 0131219294 3 20050121 Lecture 1
Today, we are going to talk about: What are the features of a Digital  communication system (DCS)? Why “digital” instead of “analog”?  What do we need to know before taking off  toward designing a DCS? Classification of signals  Random process  Autocorrelation  Power and energy spectral densities  Noise in communication systems  Signal transmission through linear systems  Bandwidth of signal  4 20050121 Lecture 1
Block diagram of a digital communication system Noise Received Transmitted Received info. signal signal Info. Transmitter Receiver Channel Source User S OURCE Transmitter Source Channel Formatter Modulator encoder encoder Receiver Source Channel Formatter Demodulator decoder decoder 5 20050121 Lecture 1
Digital communication system … Important features of a DCS:  Transmitter sends a waveform from a finite  set of possible waveforms during a limited time Channel distorts, attenuates the transmitted  signal and adds noise to it. Receiver decides which waveform was  transmitted from the noisy received signal Probability of erroneous decision is an  important measure for the system performance 6 20050121 Lecture 1
Digital versus analog Advantages of digital communications:  Regenerator receiver  Original Regenerated pulse pulse Propagation distance Different kinds of digital signal are treated  identically. Voice Data A bit is a bit! Media 7 20050121 Lecture 1
Phân loại tín hiệu Tín hiệu xác định và tín hiệu ngẫu nhiên  Deterministic signal: Có thể xác định giá trị  của tín hiệu tại bất kỳ thời điểm nào. Random signal: Không thể xác định được  giá trị của tín hiệu trước khi nó xãy ra Thermal noise in electronic circuits due to the  random movement of electrons  Reflection of radio waves from different layers of ionosphere 8 20050121 Lecture 1
Phân loại tín hiệu… Periodic and nonperiodic signals  A periodic signal A non-periodic signal Analog and discrete signals  A discrete signal Analog signals 9 20050121 Lecture 1
Phân loại tín hiệu .. Tín hiệu năng lượng và tín hiệu công suất  A signal is an energy signal if, and only if, it has  nonzero but finite energy for all time: A signal is a power signal if, and only if, it has  finite but nonzero power for all time: General rule: Periodic and random signals are power  signals. Signals that are both deterministic and non periodic are energy signals. 10 20050121 Lecture 1
Random process A random process is a collection of time functions, or  signals, corresponding to various outcomes of a random experiment. For each outcome, there exists a deterministic function, which is called a sample function or a realization. Random variables Real number Sample functions or realizations (deterministic function) time (t) 11 20050121 Lecture 1
Random process … Strictly stationary: If none of the statistics of the random process  are affected by a shift in the time origin. Wide sense stationary (WSS): If the mean and autocorrelation  function do not change with a shift in the origin time. Cyclostationary: If the mean and autocorrelation function are  periodic in time. Ergodic process: A random process is ergodic in mean and  autocorrelation, if and , respectively. 12 20050121 Lecture 1
Autocorrelation Autocorrelation of an energy signal  Autocorrelation of a power signal  For a periodic signal:  Autocorrelation of a random signal  For a WSS process:  13 20050121 Lecture 1
Spectral density Energy signals:  Energy spectral density (ESD):  Power signals:  Power spectral density (PSD):  Random process:  Power spectral density (PSD):  14 20050121 Lecture 1
Properties of an autocorrelation function For realvalued (and WSS in case of  random signals): Autocorrelation and spectral density form 1. a Fourier transform pair. Autocorrelation is symmetric around zero. 2. Its maximum value occurs at the origin. 3. Its value at the origin is equal to the 4. average power or energy. 15 20050121 Lecture 1
Noise in communication systems Thermal noise is described by a zeromean Gaussian  random process, n(t). Its PSD is flat, hence, it is called white noise.  [w/Hz] Power spectral density Autocorrelation function Probability density function 16 20050121 Lecture 1
Signal transmission through linear systems Input Output Linear system Deterministic signals:  Random signals:  Ideal distortionless transmission:  All the frequency components of the signal not only arrive with an identical time delay, but also are amplified or attenuated equally. 17 20050121 Lecture 1
Signal transmission … cont’d Ideal filters:  Non-causal! Low-pass Band-pass High-pass Realizable filters:  RC filters Butterworth filter 18 20050121 Lecture 1
Bandwidth of signal Baseband versus bandpass:  Baseband Bandpass signal signal Local oscillator Bandwidth dilemma:  Bandlimited signals are not realizable!  Realizable signals have infinite bandwidth!  19 20050121 Lecture 1
Bandwidth of signal – cont’d Different definition of bandwidth:  a) Half-power bandwidth d) Fractional power containment bandwidth b) Noise equivalent bandwidth e) Bounded power spectral density c) Null-to-null bandwidth f) Absolute bandwidth (a) (b) (c) (d) (e)50dB 20 20050121 Lecture 1