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Chapter 4 Digital Transmission

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Chapter 4 Digital Transmission Chapter 4 Digital Transmission Digital-to-Digital Conversion Analog-to-Digital Conversion Transmission Modes Wireless System Lab, NCNU 1 Digital Transmission Before transmission, information is converted to Digital signal Analog signal Chapter 3 discusses advantages of digital transmission over analog transmission. Techniques used to transmit data digitally Digital-to-digital conversion. Analog-to-digital conversion. Transmission modes of data transmission. Wireless System Lab, NCNU 2 Digital Transmission Before transmission, information is converted to Digital signal Analog signal Chapter 3 discusses advantages of digital transmission over analog transmission. Techniques used to transmit data digitally Digital-to-digital conversion. Analog-to-digital conversion. Transmission modes of data transmission. Wireless System Lab, NCNU 3 Chapter 4 Digital Transmission Digital-to-Digital Conversion Wireless System Lab, NCNU 4 Digital-to-Digital Conversion How to represent digital data (0101011 bit stream) by using digital signals. Three techniques: Line coding Block coding Scrambling Line coding is always needed; block coding and scrambling may or may not be needed. Wireless System Lab, NCNU 5 Line Coding and Decoding Digital data: text, numbers, images, audio, or video, are converted as bit stream. Sender encodes digital data into digital signal. Receiver decodes the digital signal to digital data. Line coding maps binary information sequence into digital signal. (絞肉機) Ex: “1” mapped to +A square pulse; “0” to –A pulse Wireless System Lab, NCNU 6 Signal Element versus Data Element Data element The smallest entity that can represent a piece of information: this is bit. Signal element The shortest unit (timewise) of a digital signal. In other words Data element are what we need to send. Signal elements are what we can send. Wireless System Lab, NCNU 7 Signal Element v.s. Data Element A signal element (signal) can carry multiple data elements (bits). It is not an 1-to-1 mapping. Wireless System Lab, NCNU 8 Data Rate v.s. Signal Rate Data rate (bit rate) The number of data elements (bits) sent in 1s The unit is bits per second (bps) Called bit rate Signal rate (baud rate) The number of signal elements sent in 1s The unit is the baud Signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate Wireless System Lab, NCNU 9 Data Rate v.s. Signal Rate A signal carries r data bits, bit rate = r * signal rate. Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. The signal rate determines the effective bandwidth of digital signal. Which one is better? (c: 4bits in 2 signals, d: 4bits in 3 signals) Wireless System Lab, NCNU 10 Main Considerations of Line Code Transmitted power: Power consumption = $ Bit timing: Level transitions in signal (“+A””-A”) can help timing recovery Bandwidth efficiency: Excessive transitions wastes bw Low frequency content (DC component): Long periods of +A or of –A Waveform should not have low-frequency content, which cannot go through transformer and capacitor Built-in error detection. Immunity to Noise and Interference. Complexity/cost: Is the code implementable in chip? Wireless System Lab, NCNU 11 DC Components DC Components When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies (results of Fourier analysis). These frequencies around zero, call DC (direct- current) components. A channel may not pass low frequencies. Ex: telephone line cannot pass frequencies below 200 Hz. A system that uses electrical coupling. (Lower frequencies cannot go through a transformer or capacitor). Wireless System Lab, NCNU 12 Self-synchronization To correctly interpret the signals from the sender, the receiver's bit intervals must correspond exactly to the sender's bit intervals. If faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals. Self-synchronization Digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. Wireless System Lab, NCNU 13 Effect of Lack of Synchronization Bit intervals are different in the following example. Wireless System Lab, NCNU 14 Self-synchronization in Coding A widely-used coding scheme 0: negative, 1: positive. Without self-synchronization: If a string of “1”. 1 1 Which one? 1 1 1 1 1 With self-synchronization: Transitions (“+””-”) can provide timing information. If a string of “1”. 1 1 1 1 1 Wireless System Lab, NCNU 15 Example In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps? Solution At 1 kbps, the receiver receives 1001 bps instead of 1000 bps. (103*0.1%=1) At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps. (106*0.1%=1000) Wireless System Lab, NCNU 16 Typical Line Coding Schemes Wireless System Lab, NCNU 17 Line coding examples 1 0 1 0 1 1 1 0 0 Unipolar NRZ Polar NRZ NRZ-inverted (differential encoding) Bipolar encoding Manchester encoding Differential Manchester encoding Wireless System Lab, NCNU 18 Spectra of Line codes Assume 1s & 0s independent & equally-probable NRZ has high content at low frequency Bipolar tightly packed around ½ fT 1.2 Manchester wastes bandwidth NRZ 1 0.8 Bipolar 0.6 0.4 Manchester pow erdensity 0.2 0 0 1 2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 -0.2 0.2 fT Wireless System Lab, NCNU 19 Unipolar NRZ Scheme “1” maps to +V pulse, “0” maps to no pulse Unipolar (單極性): all the signal levels are on one side of the time axis, either above or below. NRZ: non-return-to-zero (during a bit interval) Wireless System Lab, NCNU 20 Properties of Unipolar NRZ High average Power: 0.5*V2 +0.5*02=V2/2 Long strings of “1” or “0” Poor timing Low-frequency content Simple Bit rate = signal rate. Wireless System Lab, NCNU 21 Polar NRZ Scheme 1 0 1 0 1 1 1 0 0 Unipolar NRZ Polar NRZ “1” maps to +V/2 pulse, “0” maps to –V/2 pulse Polar (極性): Voltages are on the both sides of the time axis. (e.g., ”1”: positive; “0”: negative) Better Average Power: 0.5*(V/2)2 +0.5*(-V/2)2=V2/4 Long strings of +V/2 or –V/2 Poor timing Low-frequency content Simple Wireless System Lab, NCNU 22 Polar NRZ-L and NRZ-I Schemes NRZ-L (NRZ-Level) : Voltage level determines the value of the bit. NRZ-I (NRZ-Invert) : Whether inversion or not determines the value of the bit. (No inversion: 0, inversion:1) NRZ-L and NRZ-I have a DC component problem. (PSD) Most of the energy is concentrated in frequencies between 0 and NI2. Wireless System Lab, NCNU 23 Problems in Polar NRZ-L and NRZ-I Synchronization problem: Long string of “0” in both schemes. Long string of “1” affects only NRZ-L. Sudden change of polarity 0: negative, 1: positive 1: negative, 0: positive Only affects NRZ-L. Wireless System Lab, NCNU 24 RZ scheme NRZ RZ Synchronization issue in NRZ: The receiver does not know when one bit ends and the next bit is starting. One possible solution: Return-to-zero (RZ) uses three values: positive, negative, and zero. Signal goes to 0 in the middle of each bit until the next bit. Bit rate = ½ signal rate. Wireless System Lab, NCNU 25 Polar RZ scheme No DC component (see its PSD). Disadvantages: two signal changes to encode a bit and therefore occupies greater bandwidth. Complexity: RZ uses three levels of voltage, more complex to create and detect. Not used today. Replaced by the better-performing Manchester schemes. Wireless System Lab, NCNU 26 Biphase: Manchester and Differential Manchester Manchester Differential Manchester Biphase: The voltage remains at one level during the first half, and transits to the other level in the second half. Transition at the middle of bit provides synchronization. Differential Manchester: the bit values are determined at the beginning of the bit. Next bit: “0”, transition; “1” no transition. Wireless System Lab, NCNU 27 Properties of Manchester and Differential Manchester No DC component. Disadvantage: Min. bandwidth of Manchester and differential Manchester is twice that of NRZ. Bit rate = ½ signal rate. Wireless System Lab, NCNU 28 Bipolar Code 1 0 1 0 1 1 1 0 0 Bipolar Encoding Three signal levels: {-A, 0, +A}. (Multilevel binary) “1” maps to +A or –A in alternation “0” maps to no pulse Every +pulse matched by –pulse so little content at low frequencies String of 1s will not produce a square wave Long string of 0s causes receiver to lose synch Wireless System Lab, NCNU 29 Bipolar Schemes: AMI and Pseudoternary Alternate mark inversion (AMI): “0”: zero voltage. “1”: alternating positive and negative voltages. Pseudoternary: “1”: zero. “0”: alternating positive and negative. No DC component. Spectrum centered at N/2 Wireless System Lab, NCNU 30 Average Bandwidth 1 0 1 0 1 1 1 0 0 Unipolar NRZ Polar NRZ NRZ-inverted Bipolar encoding Data rate = N. (r=1) Signal rate = N. (determining the bandwidth) By Nyquist bit rate formula: (a rough evaluation.) Rmax = 2*Bandwidth*log2L. N = 2*Bandwidth*log22 Bandwidth = N/2. Wireless System Lab, NCNU 31 Average Bandwidth 1 0 1 0 1 1 1 0 0 Manchester encoding Differential Manchester encoding Data rate = N. Signal rate = 2N. (r=½ ) By Nyquist bit rate formula: 2N = 2*Bandwidth*log22 BW = N.
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