KỸ THUẬT TRUYỀN DẪN VIBA SỐ Nguyễn Huy Hùng Mobile: 0914908070 Email: hung_nguyenhuy@yahoo.com nghhung@ptithcm.edu.vn 2005-01-21 Lecture 1 2 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 2005-01-21 Lecture 1 3 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, 2 nd edition, ISBN: 0-13-095007-6  “Digital communications: Fundamentals and Applications” by Bernard Sklar,Prentice Hall, 2001,ISBN: 0-13-084788-7  “Communication Systems, Analysis and design”, by H.P.E.Stern and S. A. Mahmoud, Prentice Hall, 2004, ISBN: 0-13-121929-4 2005-01-21 Lecture 1 4 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 2005-01-21 Lecture 1 5 Block diagram of a digital communication system SOURCE Info. Transmitter Transmitted signal Received signal Receiver Received info. Noise Channel Source User Formatter Source encoder Channel encoder Modulator Formatter Source decoder Channel decoder Demodulator Transmitter Receiver 2005-01-21 Lecture 1 6 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 2005-01-21 Lecture 1 7 Digital versus analog  Advantages of digital communications:  Regenerator receiver  Different kinds of digital signal are treated identically. Data Voice Media Propagation distance Original pulse Regenerated pulse A bit is a bit! 2005-01-21 Lecture 1 8 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 2005-01-21 Lecture 1 9 Phân loại tín hiệu…  Periodic and non-periodic signals  Analog and discrete signals A discrete signal Analog signals A non-periodic signal A periodic signal 2005-01-21 Lecture 1 10 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. [...]... also are amplified or attenuated equally 200 5-0 1- 2 1 Lecture 1 17 Signal transmission … - cont’d  Ideal filters: Non-causal! Low-pass Band-pass  High-pass Realizable filters: RC filters 200 5-0 1- 2 1 Butterworth filter Lecture 1 18 Bandwidth of signal  Baseband versus bandpass: Baseband signal Bandpass signal Local oscillator  Bandwidth dilemma:   200 5-0 1- 2 1 Bandlimited signals are not realizable! Realizable... 200 5-0 1- 2 1 Lecture 1 12 Autocorrelation  Autocorrelation of an energy signal  Autocorrelation of a power signal   For a periodic signal: Autocorrelation of a random signal  For a WSS process: 200 5-0 1- 2 1 Lecture 1 13 Spectral density  Energy signals:   Power signals:   Energy spectral density (ESD): Power spectral density (PSD): Random process:  Power spectral density (PSD): 200 5-0 1- 2 1 Lecture... density (PSD): 200 5-0 1- 2 1 Lecture 1 14 Properties of an autocorrelation function  For real-valued (and WSS in case of random signals): 1 2 3 4 200 5-0 1- 2 1 Autocorrelation and spectral density form a Fourier transform pair Autocorrelation is symmetric around zero Its maximum value occurs at the origin Its value at the origin is equal to the average power or energy Lecture 1 15 Noise in communication systems... realizable! Realizable signals have infinite bandwidth! Lecture 1 19 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) 200 5-0 1- 2 1 Lecture 1 (e)50dB 20 ... outcome, there exists a deterministic function, which is called a sample function or a realization Real number Random variables Sample functions or realizations (deterministic function) time (t) 200 5-0 1- 2 1 Lecture 1 11 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... average power or energy Lecture 1 15 Noise in communication systems   Thermal noise is described by a zero-mean 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 200 5-0 1- 2 1 Lecture 1 16 Signal transmission through linear systems Input Output Linear system    Deterministic signals: Random . system 200 5-0 1- 2 1 Lecture 1 18 Signal transmission … - cont’d  Ideal filters:  Realizable filters: RC filters Butterworth filter High-pass Low-pass Band-pass Non-causal! 200 5-0 1- 2 1 Lecture 1 19 Bandwidth. KỸ THUẬT TRUYỀN DẪN VIBA SỐ Nguyễn Huy Hùng Mobile: 0 914 908070 Email: hung_nguyenhuy@yahoo.com nghhung@ptithcm.edu.vn 200 5-0 1- 2 1 Lecture 1 2 Mục đích môn học  Nghiên. ionosphere 200 5-0 1- 2 1 Lecture 1 9 Phân loại tín hiệu…  Periodic and non-periodic signals  Analog and discrete signals A discrete signal Analog signals A non-periodic signal A periodic signal 200 5-0 1- 2 1 Lecture