Bài giảng TRUYỀN THÔNG SỐ DIGITAL COMMUNICATION

Các bước quan trọng đầu tiên trong bất kỳ DCS:- Chuyển đổi các nguồn thông tin để tạo thành một tương thích với một hệ thống kỹ thuật số • Pulse điều chỉnh: (go u ch ế xung điện tử)- Chuyển đổi các tin nhắn kỹ thuật số để baseband dạng sóng- Baseband = tín hiệu có phổ rộng từ (hoặc gần) DC một số giá trị hữu hạn
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Bài giảng TRUYỀN THÔNG SỐ DIGITAL COMMUNICATION TRUYỀN THÔNG SỐDIGITAL COMMUNICATION Week 2 12• Format: The first important step in any DCS: – Transform the information source to a form compatible with a digital system• Pulse modulate: (điều chế xung) – Transform the digital messages to baseband waveforms – Baseband = signal whose spectrum extends from (or near) DC  some finite value (< a few MHz) 3 Formatting and transmission of baseband signal Digital info. Format Textualsource info. Pulse Transmit Analog modulate Sample Quantize Encode info. Pulse Channel Bit stream waveforms Format Analog info. Low-pass Decode Demodulate/ filter Receive Detect Textualsink info. Digital info. 4 Content1. Formatting Textual Data (Character Coding)2. Messages, Characters, and Symbols3. Formatting Analog Information4. Sources of Corruption5. Pulse Code Modulation6. Baseband Modulation 5 Content1. Formatting Textual Data (Character Coding)2. Messages, Characters, and Symbols3. Formatting Analog Information4. Sources of Corruption5. Pulse Code Modulation6. Baseband Modulation 6 Character Coding• The original form of most communicated data = textual or analog.• Data = text: Character Coding  digital format• E.g. ASCII (American Standard Code for Information Interchange), EBCDIC 78 Content1. Formatting Textual Data (Character Coding)2. Messages, Characters, and Symbols3. Formatting Analog Information4. Sources of Corruption5. Pulse Code Modulation6. Baseband Modulation 9• Characters are encoded into a sequence of bits = “a bit stream” or “baseband signal”• Groups of k bits = symbols• Symbol set size: M = 2k or M-ary system – k = 1: binary system – k = 2: quaternary system 10Example 11Example (cont.) 12 Content1. Formatting Textual Data (Character Coding)2. Messages, Characters, and Symbols3. Formatting Analog Information4. Sources of Corruption5. Pulse Code Modulation6. Baseband Modulation 13 Format analog signals• To transform an analog waveform into a form that is compatible with a digital communication, the following steps are taken: 1. Sampling 2. Quantization and encoding 3. Baseband transmission 14 Impulse Sampling Time domain Frequency domainxs (t ) = xδ (t ) × x(t ) X s ( f ) = Xδ ( f ) ∗ X ( f ) x(t ) | X( f )| | Xδ ( f ) | xδ (t ) xs (t ) | Xs( f ) | 1516Aliasing effect LP filter Nyquist rate aliasing 17 Sampling theorem Sampling Analog Pulse amplitude process signal modulated (PAM) signal• Sampling theorem: A bandlimited signal with no spectral components beyond , can be uniquely determined by values sampled at uniform intervals of – The sampling rate, is called Nyquist rate. 1819 Why Oversample?Transform analog signals to digital signals:Without Oversampling:• The signal  high performance analog LPF  sampled at Nyquist rate  ADCWith Oversampling:• The signal  low performance (less cost) analog LPF  sampled at (higher) Nyquist rate  ADC  high performance (low cost) digital filter (resample) 20