Chapter 2 Baseband Transmission and Digital Multiplexing Learning

Chapter 2: Baseband Transmission and Digital Multiplexing

Learning Objectives LO 2. 1 – Know how to convert digital data to digital signal using various line codes for reliable baseband transmission. LO 2. 2 – Discuss intersymbol interference (ISI) problem in digital transmission. LO 2. 3 – Hypothesize Nyquist pulse shaping and outline equalizer schemes to minimize the effects of ISI. LO 2. 4 – Illustrate the concept of digital multiplexer and design TDM systems. LO 2. 5 – Integrate TDM-PCM encoded digital carrier systems and calculate their data rates.

2. 1 Line Coding and its Types 2. 1. 1 Desirable Properties of Line Codes 2. 1. 2 Types of Line-Coding Techniques 2. 1. 3 Power Spectra of Line Codes 2. 1. 4 Attributes of Line Coding Techniques – A Comparison 2. 1. 5 Application Areas of Line-Coding Techniques

Baseband Digital Transmission Waveform Coder Analog or Speech Signal Line Coding (Baseband Modulation) Regenerative Repeater Channel (Wireline) To Receiver Figure 2. 1. 1 Line Encoding – A process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the baseband channel

Process of Line-Coding Figure 2. 1. 2

2. 1. 1 Desirable Properties of Line Codes Transmission power efficiency Duty cycle Self-clocking capability or selfsynchronization Immunity to noise and interference The dc components Baseline wandering Bandwidth considerations Error detection capability Ease of detection and decoding

2. 1. 2 Types of Line-Coding Techniques Figure 2. 1. 7 Figure 2. 1. 8

Unipolar NRZ Line-Coding Waveform Figure 2. 1. 9

Unipolar RZ Line-Coding Waveform Figure 2. 1. 10

Polar NRZ-L Line-Coding Waveform Figure 2. 1. 11

Manchester Polar Line-Coding Waveform Figure 2. 1. 12

Differential Manchester Polar Line-Coding Figure 2. 1. 13

Bipolar NRZ-AMI Line-Coding Waveform Figure 2. 1. 15

Bipolar RZ Line-Coding Waveform Figure 2. 1. 16

BP-RZ-AMI Line-Coding Waveform Figure 2. 1. 17

HDB 3 NRZ-AMI Line-Coding Waveform Figure 2. 1. 18

B 8 ZS Line-Coding Waveform (Type – 1) Figure 2. 1. 19

B 8 ZS Line-Coding Waveform (Type – 2) Figure 2. 1. 20

2. 1. 3 Power Spectra of Line Codes Figure 2. 1. 24

2. 1. 4 Attributes of Line Encoding Techniques – A Comparison (Table 2. 1. 4)

…… Attributes of Line Encoding Techniques – A Comparison (Table 2. 1. 5)

2. 1. 5 Application Areas of Line-Encoding Techniques (Table 2. 1. 6)

2. 2 Intersymbol Interference (ISI) 2. 2. 1 Effects of ISI 2. 2. 2 Eye Diagram

2. 2. 1 Effects of ISI The presence of residual signals at the receiver output due to other symbols interfering with the required symbol is known as Intersymbol interference. Figure 2. 2. 1 Dispersed Pulse due to ISI

……. Effects of ISI Figure 2. 2. 2 Effects of ISI on Transmission of Pulses

Baseband Binary Data Transmission System Model Figure 2. 2. 3

2. 2. 2 Eye Diagram An eye pattern, also known as an eye diagram, is a practical technique for determining the effects of the degradations introduced by Intersymbol interference into the digital pulses as the signals travel through the channel to the receiver. Figure 2. 2. 4

A Typical Eye Diagram Figure 2. 2. 5

Monitoring Transmission Quality by an Eye Diagram Figure 2. 2. 6

2. 3 Nyquist Pulse Shaping and Equalization 2. 3. 1 Nyquist Criterion for Zero ISI 2. 3. 2 Raised-Cosine Pulse Shaping 2. 3. 3 Equalization to Reduce ISI 2. 3. 4 Self-Synchronization

Nyquist Pulse Shaping and Equalization One possible solution to reduce Inter Symbol Interference is to use a [(sin x)/x] pulse instead of a rectangular-shaped pulse waveform which can produce even a zero ISI. This is known as Nyquist pulse shaping. Equalization is a special technique that helps the demodulator at the baseband receiver to recover a rectangular pulse with the best possible signal-to-noise ratio, free of any intersymbol interference.

2. 3. 1 Nyquist Criterion for Zero ISI Figure 2. 3. 1 An Ideal Rectangular Pulse

…… Nyquist Criterion for Zero ISI Figure 2. 3. 2 Minimum Bandwidth Pulse that satisfies Nyquist’s First Criterion

2. 3. 2 Raised-Cosine Pulse Shaping Figure 2. 3. 3 Nyquist Pulse and Raised-cosine Spectrum

2. 3. 3 Equalization to Reduce ISI Equalizers are special filters inserted in the transmission path to equalize the distortion for all frequencies, creating a uniform transmission medium and reducing transmission impairments. Linear Adaptive Equalizers • Symbol spaced linear adaptive equalizers • Fractionally spaced linear adaptive equalizers Non-Linear Decision Feedback Adaptive Equalizers Adaptive MLSE Equalizer

……. Equalization to Reduce ISI

2. 3. 4 Self-Synchronization

2. 4 Digital Multiplexing and TDM 2. 4. 1 Synchronous TDM 2. 4. 2 Asynchronous TDM

Digital Multiplexing

2. 4. 1 Synchronous TDM

……. Synchronous TDM

2. 4. 2 Asynchronous TDM

2. 5 Digital Carrier Systems 2. 5. 1 North American Digital Carrier System 2. 5. 2 European Digital Carrier System

A Basic Digital Carrier System (DS 0)

2. 5. 1 North-American Digital Carrier System PCM Ch 1 PCM Ch 2 * * PCM Ch 24 Each @ 64 kbps Ist Level MUX #1 (24 DS 0 lines) Ist Level MUX #2 (24 DS 0 lines) Ist Level MUX #3 (24 DS 0 lines) @ 1. 544 Mbps (1) DS 1 (2) 2 nd Level MUX (4 DS 1 lines) @ 6. 312 Mbps DS 2 (1) (2) DS 2 * * DS 2 3 rd Level MUX (7 DS 2 lines) @ 274. 176 Mbps (7) Each @ 6. 312 Mbps (1) DS 3 (3) DS 3 Ist Level MUX #4 (24 DS 0 lines) @ 44. 736 Mbps DS 3 (2) * * (6) Each @ 44. 736 Mbps (4) Each @ 1. 544 Mbps Figure 2. 5. 2 4 th Level MUX (6 DS 3 lines) DS 4

T 1 Digital Carrier System Analog input signals CH 1 300 Hz – 3400 Hz CH 2 300 Hz – 3400 Hz Anti-aliasing filter PCM Encoder Sample & Hold circuit Sample pulse @ fs = 8 k. Hz CH 3 @64 kbps (8 -bit serial PCM code) 8 -bit A/D & parallel-toserial Buffer 1. 536 MHz 24 channel T D M M U X (Add framing pulse + Line coder) T 1 @ 1. 544 Mbps * * * CH 23 CH 24 300 Hz – 3400 Hz Anti-aliasing filter PCM Encoder Sample & Hold circuit Sample pulse @ fs = 8 k. Hz 8 -bit A/D & parallel-toserial 1. 536 MHz Figure 2. 5. 3 Buffer 1. 536 MHz

Table 2. 5. 1 Key Parameters for T Lines Digital Transmission Service Carrier Line# DS 1 T 1 No. of PCM voice channels Raw Data Net Data Rate (Mbps) Minimum bandwidth Typical medium used 24 DS 0 lines 1. 536 1. 544 772 k. Hz DS 1 C T 1 C 48 DS 0 lines (2 DS 1 lines) 3. 088 3. 096 1. 548 MHz Twisted-pair cable DS 2 T 2 96 DS 0 lines (4 DS 1 lines) 6. 176 6. 312 3. 156 MHz DS 3 T 3 672 DS 0 lines (28 DS 1 lines, 44. 184 or 7 DS 2 lines) 44. 736 22. 368 MHz DS 4 T 4 4032 DS 0 lines (168 DS 1 lines, or 42 DS 2 lines, or 6 DS 3 lines) 268. 416 274. 176 137. 088 MHz Coaxial cable, optical fiber cable DS-5 T 5 8064 DS 0 lines 516. 096 560. 16 280. 08 MHz optical fiber cable Twisted-pair cable, microwave Coaxial cable, microwave

2. 5. 2 European Digital Carrier System PCM Ch 1 PCM Ch 2 * * PCM Ch 32 Each @ 64 kbps Ist Level MUX #1 (32 PCM lines) Ist Level MUX #2 (32 PCM lines) Ist Level MUX #3 (32 PCM lines) Ist Level MUX #4 (32 PCM lines) @ 2. 048 Mbps (1) E 1 E 1 2 nd Level MUX (4 E 1 lines) E 1 (2) @ 8. 448 Mbps E 2 E 2 (1) (2) (3) (4) Each @ 8. 448 Mbps 3 rd Level MUX (4 E 2 lines) E 3 (3) E 3 @ 34. 368 Mbps E 3 (1) (2) (3) (4) Each @ 34. 368 Mbps (4) Each @ 2. 048 Mbps Figure 2. 5. 5 @ 139. 264 Mbps 4 th Level MUX (4 E 3 lines) E 4

Table 2. 5. 2 Key Parameters for E Lines Transmission Carrier No. of PCM voice No. of PCM control Transmission bit Line# channels Rate (Mbps) E 1 30 2 2. 048 E 2 (= 4 E 1 lines) 120 8 8. 448* E 3 (= 4 E 2 lines) 480 32 34. 368* E 4 (= 4 E 3 lines) 1920 128 139. 264* * Includes synchronization bits

About the Author T. L. Singal graduated from National Institute of Technology, Kurukshetra and post-graduated from Punjab Technical university in Electronics & Communication Engineering. He began his career with Avionics Design Bureau, HAL, Hyderabad in 1981 and worked on Radar Communication Systems. Then he led R&D group in a Telecom company and successfully developed Multi-Access VHF Wireless Communication Systems. He visited Germany during 1990 -92. He executed international assignment as Senior Network Consultant with Flextronics Network Services, Texas, USA during 2000 -02. He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2 G/3 G Cellular Networks in USA. Since 2003, he is in teaching profession in reputed engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored three text-books `Wireless Communications (2010)’, `Analog & Digital Communications (2012)’, and `Digital Communication (2015) with internationally renowned publisher Mc. Graw-Hill Education.

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