1. © Copyright 2014 Acehub Vista Sdn. Bhd. 1 ME1110 Analog and Digital Communications http://dreamcatcher.asia/cw This courseware product contains scholarly and technical information and is protected by copyright laws and international treaties. No part of this publication may be reproduced by any means, be it transmitted, transcribed, photocopied, stored in a retrieval system, or translated into any language in any form, without the prior written permission of Acehub Vista Sdn Bhd. The use of the courseware product and all other products developed and/or distributed by DreamCatcher TM are subject to the applicable License Agreement. For further information, see Courseware Product License Agreement.

2. © Copyright 2014 Acehub Vista Sdn. Bhd. 2 3. Baseband Pulse Transmission

3. © Copyright 2014 Acehub Vista Sdn. Bhd. 3 Definition of Baseband In communications, baseband is the term that describes signals and systems whose range of frequencies is measured from close to 0 Hz to a cut-off frequency, a maximum bandwidth or highest signal frequency. It is sometimes used as a noun for a band of frequencies starting close to zero. Baseband can often be considered a synonym to low-pass or non-modulated. fcfc BW

4. © Copyright 2014 Acehub Vista Sdn. Bhd. 4 Possible Types of Signal Transmission Analog source Analog destination Analog source Modulator (Tx) Analog destination Demodulator (Rv) channel Digital source Coder Digital destination Decoder Digital channel Analog signal and baseband transmission Analog transmission using modulation and demodulation Digital signal transmission on digital channel Baseband channel * microphone * Radio, TV * Data file in computer

5. © Copyright 2014 Acehub Vista Sdn. Bhd. 5 Possible Types of Signal Transmission (Cont) Analog source A/D & coding Analog destination Decoding & D/A Digital channel Digital signal transmission by modem Analog signal digitized transmission Digital source Modem Digital destination Modem Analog channel Analog signal digitized transmitted by modem Analog source A/D & coding Analog destination Decoding & D/A Analog channel Modem * Data through internet * Digitized voice, CD * Modern comm system

6. © Copyright 2014 Acehub Vista Sdn. Bhd. 6 In baseband transmission, a bit stream (e.g. a PCM signal) is encoded into pulses without modulating a high- frequency carrier. The digital pulses are then transmitted over a baseband or low-pass channel. The main concern is shaping the transmitted pulses to bring the Inter-Symbol Interference (ISI) under control. In baseband transmission, the required bandwidth is proportional to the bit rate; to send bits faster, more bandwidth is required. Baseband Pulse Transmission

7. © Copyright 2014 Acehub Vista Sdn. Bhd. 7 Baseband Pulse Transmission (cont.) Transmission Medium (Wired) Digital Signal :101011.. Line Code Pulse Shaping 101011.. 5V 0V Pulse Recovery Circuit Wide bandwidth Time 5V

8. © Copyright 2014 Acehub Vista Sdn. Bhd. 8 Bandwidth of Baseband Channel Frequency Amplitude 0 f Frequency Amplitude 0 f Baseband or Low-Pass Channel with Wide-bandwidth Baseband or Low-Pass Channel with Narrow-bandwidth

9. © Copyright 2014 Acehub Vista Sdn. Bhd. 9 Bandwidth of Baseband Channel (cont.) Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if the low-pass channel has an infinite or very wide bandwidth. Input Signal Bandwidth (Ideal Pulse) f1f1 f2f2 Wide-bandwidth Channel f1f1 f2f2 0 ∞ t f f f t Output Signal Bandwidth Input Signal (Ideal Pulse) Output Signal

10. © Copyright 2014 Acehub Vista Sdn. Bhd. 10 Digital vs. Analog Analog: Continuous signal Decision on symbol from a pre-defined alphabet Digital : Discrete signal Every amplitude needed for reception

11. © Copyright 2014 Acehub Vista Sdn. Bhd. 11 Binary 1’s and 0’s, such as PCM bit stream, may be represented in various rectangular pulses called line codes. There are two major categories of line codes: RZ (return-to-zero) and NRZ (nonreturn-to-zero). In RZ coding, a waveform returns to zero-volt level for a portion (usually at the middle) of the bit interval. Line Coding

12. © Copyright 2014 Acehub Vista Sdn. Bhd. 12 Unipolar Signalling: a binary 1 is represented by a positive level (+A volts) and a binary 0 by zero level. Polar Signalling: binary 1’s and 0’s are represented by equal positive and negative levels (+A and -A volts), respectively. Bipolar Signalling: binary 1’s are represented by alternately positive and negative levels. The binary 0’s are represented by zero level. Also called AMI (Alternate Mark Inversion). Manchester Signalling: each binary 1 is represented by a positive half-bit period pulse followed by a negative half-bit period pulse while a binary 0 is represented by a negative half-bit period pulse followed by a positive half-bit period pulse. Also called Split Phase Encoding. Binary Line Codes

13. © Copyright 2014 Acehub Vista Sdn. Bhd. 13 Binary Line Codes (cont.) 1 1 1 1 0 00 Time A -A A A A A 0 0 0 0 0 Volts TbTb (a) Binary data (c) Polar NRZ (d) Unipolar RZ (f) Manchester NRZ (e) Bipolar RZ (b) Unipolar NRZ Figure 3.1 – Binary Signaling Format

14. © Copyright 2014 Acehub Vista Sdn. Bhd. 14 Self-synchronisation: When there is a long string of 1’s or 0’s (DC component), a receiver is unable to identify a bit interval. The receiver will lose bit synchronisation with transmitter. A line code (e.g. Manchester NRZ) builds in a timing information by having a transition within a bit period. Receiver will extract this timing information to identify a bit interval for maintaining bit synchronisation with transmitter. Low probability of bit error: A line code is able to reduce the effects of noise and ISI during transmission over a baseband channel. This leads to reduced bit errors and hence low bit error probability. Transparency: The data protocol and line code are designed so that every possible sequence of data can be transmitted and received. Line Code Properties

15. © Copyright 2014 Acehub Vista Sdn. Bhd. 15 Advantages: Simplicity in implementation. Does not require a lot of bandwidth for transmission. Disadvantages: Presence of DC level (indicated by spectral line at 0 Hz). Contains low frequency components. Does not have any error correction capability. Does not posses any clocking component for ease of synchronisation. Is not Transparent. Long string of zeros causes loss of synchronisation. Unipolar NRZ

16. © Copyright 2014 Acehub Vista Sdn. Bhd. 16 Advantages: Simplicity in implementation. Presence of a spectral line at symbol rate which can be used as symbol timing clock signal. Disadvantages: Presence of DC level (indicated by spectral line at 0 Hz). Continuous part is non-zero at 0 Hz. Does not have any error correction capability. Occupies twice as much bandwidth as Unipolar NRZ. Is not Transparent Unipolar RZ

17. © Copyright 2014 Acehub Vista Sdn. Bhd. 17 Advantages: Simplicity in implementation. No DC component. Disadvantages: Continuous part is non-zero at 0 Hz. Does not have any error correction capability. Does not posses any clocking component for ease of synchronisation. Is not transparent. Polar NRZ

18. © Copyright 2014 Acehub Vista Sdn. Bhd. 18 Advantages: No DC component. Occupies less bandwidth than unipolar and polar RZ schemes. Suitable for transmission over AC coupled lines. Possesses single error detection capability. Clock can be extracted by rectifying (a copy of) the received signal. Disadvantages: Is not Transparent. Bipolar RZ

19. © Copyright 2014 Acehub Vista Sdn. Bhd. 19 Advantages: No DC component. Suitable for transmission over AC coupled lines. Easy to synchronise with. Is Transparent. Disadvantages: Because of the greater number of transitions it occupies a significantly large bandwidth. Does not have error detection capability. Manchester

20. © Copyright 2014 Acehub Vista Sdn. Bhd. 20 An eye pattern derives its name as it resembles a human eye. It is the synchronised superposition of all possible realisations of the signal of interest viewed within a signalling interval. To construct an eye pattern, we plot the received signal against time on a fixed-interval axis, at the end of the fixed time interval, wrap around to the beginning of the time axis. Thus the pattern consists of many overlapping curves. An eye pattern provides the following information: Timing error allowed on the sampler is given by the horizontal width of the eye. The preferred time for sampling is at the point where the vertical opening of the eye is the largest. Sensitivity to timing error is given by the slope of the open eye (evaluated at, or near, the zero-crossing point). Noise margin of the system is given by the eye opening height. Eye Patterns

21. © Copyright 2014 Acehub Vista Sdn. Bhd. 21 Eye Patterns (cont.) Figure 3.2 – Eye Patterns Interpretation Source : Ref [5]

22. © Copyright 2014 Acehub Vista Sdn. Bhd. 22 When a received line code waveform contains channel impairments (such as distortion, ISI, and noise), traces from the upper portion of the eye pattern cross traces from the lower portion. This causes the eye opening becomes small. When amount of channel impairments increases, the opening becomes even smaller. This greatly reduces the noise margin and results in more bit errors. Hence, eye patterns are used to assess the quality of a received line code waveform. In addition, eye patterns are a tool for evaluating the performance of a baseband pulse transmission system. Eye Patterns (cont.)

23. © Copyright 2014 Acehub Vista Sdn. Bhd. 23 Eye Patterns (cont.) Waveform 0 00 11 1 TbTb Time (a) Distortion (b) Distortion and ISI (c) Distortion, ISI and Noise Eye Pattern t Noise Margin Maximum Distortion Best Sampling Time Figure 3.3 – Distorted polar NRZ waveform and corresponding eye pattern Source : Ref [1]

24. © Copyright 2014 Acehub Vista Sdn. Bhd. 24 Bandwidth of Digital Signal t t V 2 (t) V 1 (t) T = 10nsec  = 3.0nsec V 1 (f) V 2 (f) |V(f)| f Observation: pulse with smaller rise/fall time extends further out on the frequency axis (Sinc response). Observation: pulse with smaller rise/fall time extends further out on the frequency axis (Sinc response). V 0 (t) T = 10nsec  = 0.5nsec t T = 10nsec  = 0 nsec Ideal V 0 (f) (Infinite BW)

25. © Copyright 2014 Acehub Vista Sdn. Bhd. 25 A baseband pulse transmission system consists of three lowpass filtering at transmitter, channel, and receiver, respectively. The combined effect is called system lowpass filtering. When line code flat-top pulses are transmitted over the system, the system filtering will cause the pulses to have rounded tops at system output. System Filtering Figure 3.4 – General Baseband Pulse Transmission System System filtering P(f ) Transmitting filter H T (f) Channel (filter) Characteristics H C (f) Receiver filter H R (f) Input Flat- top pulses X in (t)X C (t)X out (t) Recovered pulse is rounded (is to send to sampling and decoding circuits) Source : Ref [1]

26. © Copyright 2014 Acehub Vista Sdn. Bhd. 26 Effect of Bandlimiting the Pulse Spectrum t t Low Pass Filter  Causes “ringing” or ripples in time domain  Then, pulse spread in time domain causes inter-symbol interference

27. © Copyright 2014 Acehub Vista Sdn. Bhd. 27 If these flat-top pulses are filtered improperly by the system, they will spread in time to form so-called the tails or ripples. These tails spread into adjacent bit intervals to interfere with the interpretation of the adjacent symbols. This interference is known as ISI (inter-symbol interference). If an ISI causes a wrong interpretation of a symbol (e.g. a pulse symbol is interpreted as binary 0 instead of binary 1, and a zero- level symbol is interpreted as binary 1 instead of binary 0), then a bit error arises. ISI is a major source of bit errors in the receivers. Inter-symbol Interference

28. © Copyright 2014 Acehub Vista Sdn. Bhd. 28 Inter-symbol Interference (cont.) Input waveform, X in (t) Individual pulse response Received waveform, X out (t) (sum of pulse responses) 1 1 1 0 0 0 1 0 Sampling points (transmitter clock) Sampling points (receiver clock) Sampling points (receiver clock) TsTs TsTs O O O O O O t t t t t t TsTs TsTs TsTs TsTs Inter-symbol interference Figure 3.5 – Examples of ISI on received pulse in a binary communication systemSource : Ref [1]

29. © Copyright 2014 Acehub Vista Sdn. Bhd. 29 Baseband system model Equivalent model Tx Filter ChannelRx Filter Detector Equivalent System Detector filtered noise Inter-symbol Interference Detection filtered noise Figure 3.6 – ISI detection process Source : Ref [1]

30. © Copyright 2014 Acehub Vista Sdn. Bhd. 30  Two approaches are used to control ISI: pulse shaping and equalisation.  Pulse shaping is a process of using system filtering to change the shape of transmitted pulses. Two pulse shaping or Nyquist filtering techniques: ideal Nyquist and raised cosine.  Harry Nyquist stated that it is possible to transmit independent symbols through a baseband system at a symbol rate R S  2W without ISI, where W is called minimum Nyquist bandwidth.  This occurs when the system spectrum P(f) is made rectangular with bandwidth W. The system time response p(t) is a sinc pulse.  The received sinc pulses have been shaped in such a way that their tails will cross the zero level at every sampling instants to eliminate the ISI. This is ideal Nyquist pulse shaping. Pulse Shaping: Ideal Nyquist Filtering

31. © Copyright 2014 Acehub Vista Sdn. Bhd. 31 (b) Shaped sinc pulse at system output (also called system’s time impulse response) p(t) = sinc(2Wt) (a) System’s ideal spectrum (zero ISI) P(f ) = X(f)  H e (f) =1/(2W)  rect[ f / (2W)] Pulse Shaping: Ideal Nyquist Filtering (cont.) = min. Nyquist bandwidth -0.2 -0.4 t/T b P(t) Sampling Instant TbTb Signaling Interval TbTb 0.2 0.4 0.6 0.8 1 -3-2 1 2 03 W = 1/2T b 0 f |P(f)| -W = -1/2T b Figure 3.7(a) – Criteria for zero ISI

32. © Copyright 2014 Acehub Vista Sdn. Bhd. 32  Nyquist filter - achieves zero crossings at integer multiples of symbol period  Zero crossings at symbol interval - no ISI at sample point Pulse Shaping: Ideal Nyquist Filtering (cont.) Figure 3.7(b) – Criteria for zero ISI

33. © Copyright 2014 Acehub Vista Sdn. Bhd. 33 Received sinc pulses with their tails crossing zero level at all sampling instants. Pulse Shaping: Ideal Nyquist Filtering (cont.) Figure 3.8 – Zero ISI transmission Source : Ref [5]

34. © Copyright 2014 Acehub Vista Sdn. Bhd. 34 Two problems in ideal Nyquist filtering: (1) The rectangular spectrum is not physically realisable. (2) A small sampling instant error (i.e. a sampling instant is slightly shifted) will result in ISI. In order to solve these two problems, raised cosine filtering is recommended. A sinusoidal (raised cosine) roll-off is introduced to the system spectrum. The degree of roll-off is determined by a roll-off factor r, which varies from 0 to 1. When r = 0, we have the rectangular spectrum with bandwidth W. At r = 1, the system spectrum has the most gradual roll-off with bandwidth 2W. System spectra with sinusoidal roll-off are physically realisable. As r increases from 0 to 1, the transmitted sinc pulses are shaped to have their tail amplitudes reduced. At r = 1, the tails have additional zero-crossings. Smaller tail amplitudes and additional zero-crossings will help reduce ISI, which is caused by sampling instant errors. Pulse Shaping: Raised Cosine Filtering

35. © Copyright 2014 Acehub Vista Sdn. Bhd. 35 Pulse Shaping: Raised Cosine Filtering (cont.) The raised cosine Nyquist filter has the transfer function : f  = B – f 0 f 1 = f 0 - f  r = f  / f 0 B is the absolute bandwidth f 0 is the 6dB bandwidth of the filter r is the roll-off factor |P cosine (f)| 1 0.5 Bf0f0 f1f1 ff ff f  B B  f0 f0  f1 f1 Figure 3.9(a) – Raised cosine filter characteristics Source : Ref [1]

36. © Copyright 2014 Acehub Vista Sdn. Bhd. 36  Smaller r, less bandwidth but higher sidelobe levels  r=0.2 to 0.5 typically used currently  Bandwidth is given as Pulse Shaping: Raised Cosine Filtering (cont.) r = 0 r = 0.5 r = 1.0 r = 0 r = 0.5 r = 1.0 Figure 3.9(b) – Raised cosine filter characteristics where R s is symbol rate

37. © Copyright 2014 Acehub Vista Sdn. Bhd. 37 Pulse Shaping: Raised Cosine Filtering (cont.) Figure 3.11 – Time response for different roll-off factors p cosine (t) t Source : Ref [1]

38. © Copyright 2014 Acehub Vista Sdn. Bhd. 38 Raised Cosine Filters and Root Raised Cosine Filters Root Raised Cosine Root Raised Cosine DEMOD Transmitter  Spectral containment Receiver  Filter noise  Max SNR @ decision pt Data Root Raised Cosine filters at TX & RX form overall Raised Cosine response

39. © Copyright 2014 Acehub Vista Sdn. Bhd. 39 FIR Implementation

40. © Copyright 2014 Acehub Vista Sdn. Bhd. 40 What is the minimum required bandwidth of a baseband channel if we need to send 1 Mbps by using baseband transmission? Review Question Therefore, minimum bandwidth to send 1Mbps, is given as: B = Rs/2 = 1 Mbps/2 = 500 kHz. Ans From, the minimum bandwidth is when r=0 ?

41. © Copyright 2014 Acehub Vista Sdn. Bhd. 41 Probability of Error For a simple threshold detection system: Erroneous decision made when noise causes signal to cross threshold Probability of error related to signal-to-noise ratio

42. © Copyright 2014 Acehub Vista Sdn. Bhd. 42 Probability of Error (cont.) Assumes AGWN Perfect sampling Bipolar signal (+1,-1) Typical operating points: Wireless voice10 -3 Wireless data10 -6 Lightwave<10 -9 to 10 -12

43. © Copyright 2014 Acehub Vista Sdn. Bhd. 43 Matched Filter Filter at RX to reduce noise Choose filter response to be mirror image of incoming signal Filter is ‘matched’ e.g. RRCF pair Signal output from filter: E Noise output from filter: N O Output SNR depends on signal energy (E) and noise power spectral density (N O )

44. © Copyright 2014 Acehub Vista Sdn. Bhd. 44 Probability of Error vs. Eb/No  Performance of bipolar NRZ in AWGN  E b /N o - measure of power efficiency  Typical implementation: RRCF pair

45. © Copyright 2014 Acehub Vista Sdn. Bhd. 45 Increasing Number of Symbol States  Represent groups of bits as symbols  Increase bandwidth efficiency by reducing symbol rate  e.g. Two bits encoded into one symbol  Symbol period: T S =2T b  R S =0.5R b  Trade-off against noise immunity k – Bit digital- to-analog converter L – level multilevel signal D symbols/sec = R/l and R bits/sec w 1 (t) or w 2 (t) Binary signal D symbols/sec = R bits/sec L = 2 k Binary Input (l = 2 bits) Output Level (V) 11 10 00 01 + 1.5 + 0.5 - 0.5 - 1.5

46. © Copyright 2014 Acehub Vista Sdn. Bhd. 46 Example Modem uses 32 phases to transmit 5 bits of information Phase change every 0.5 ms (symbol period T S ) Symbol rate:R S =1/T S =2000 symbols/sec Channel capacity:5xR S =10,000 bits/sec Do not confuse symbol (baud) rate with information (bit) rate!

47. © Copyright 2014 Acehub Vista Sdn. Bhd. 47 Bit Rate Vs Baud/Symbol Rate Bit rate is the number of bits per second. Baud or Symbol rate is the number of signal elements per second. In the analog transmission of digital data, the baud or symbol rate is less than or equal to the bit rate. The term “baud” originates from the French engineer Emile Baudot, who invented the 5-bit teletype code. Baud rate refers to the number of signal or symbol changes that occur per second. A symbol is one of several voltage, frequency, or phase changes.

48. © Copyright 2014 Acehub Vista Sdn. Bhd. 48 Limitation on Symbol States Bandwidth efficiency achieved by increasing symbol states Limit set by POWER EFFICIENCY n bits encoded into M=2 n symbol states Example: Information rate:2 Mbits/s Channel bandwidth:75 kHz Symbol rate:150 ksymbol/s Bits/sym required:(2x10 6 /150x10 3 )=14 bits/symbol No. of symbol states:M=2 14 =16,384 states Assume symbols correctly identified if separated by 5 mV Pk-pk voltage of waveform:82 V

49. © Copyright 2014 Acehub Vista Sdn. Bhd. 49 Power-Bandwidth Efficiency Trade-off Increase symbol states to increase bandwidth efficiency Harder to distinguish symbols in presence of noise Hence reduced power efficiency Power/BW efficiency trade-off

50. © Copyright 2014 Acehub Vista Sdn. Bhd. 50 A computer has an output bit rate of 56 kbps, and three successive binary digits are coded into one of eight possible amplitude levels. The resulting eight-level pulses are then shaped using raised cosine filtering during transmission over a baseband channel. Find the transmission bandwidths for the following roll-off factors: r = 0.25, 0.5, 0.75 and 1. Review Question ?

51. © Copyright 2014 Acehub Vista Sdn. Bhd. 51 Review Question Ans Bit rate R S =R b = 56 kbps, number of bits per symbol k = 3, 8-level signalling Roll-off FactorTransmission Bandwidth r = 0.25B T = 11.67 kHz r = 0.5B T = 14 kHz r = 0.75B T = 16.4 kHz r = 1B T = 18.7 kHz

52. © Copyright 2014 Acehub Vista Sdn. Bhd. 52 Review Question An analogue source is sampled, quantised, and encoded into a binary PCM signal. Each quantised sample is encoded into a code-word, which is then line coded into three information pulses plus a synchronising pulse. The line code pulses can take on four possible levels, and are transmitted over a channel of bandwidth 6 kHz using RRC filtering r=0.5. 1.Determine the symbol rate of the line code pulses. 2.Find the information bit rate of the PCM signal. ?

53. © Copyright 2014 Acehub Vista Sdn. Bhd. 53 Review Question Ans No. of signalling levels M = 4 = 2 k, No. of bits per symbol k = 2 Channel bandwidth = transmission bandwidth B T = 6 kHz (1)

54. © Copyright 2014 Acehub Vista Sdn. Bhd. 54 Review Question (2) Only ¾ symbols carry information out of 8,000 symbols in one second Info symbol rate = ¾ x 8,000 = 6,000 info symbols/s Info bit rate = info symbol rate x k = 6,000 x 2 = 12,000 bps Ans

55. © Copyright 2014 Acehub Vista Sdn. Bhd. 55 References [1] Leon W. Couch II, “Digital and Analog Communication Systems”, Pearson Education 7th Edition., N.J., 2007. [2] Bernard Sklar, “Digital Communications : Fundamentals and Applications”, Prentice Hall, 2nd Edition, 2001. [3] B. P. Lathi, “Modern Digital and Analog Communication Systems”, Oxford University Press 3rd Edition., N.Y., 1998. [4] Digital Modulation in Communications Systems — An Introduction, Keysight Application Note 1298, Literature Number 5965-7160E. [5] Simon Haykin, “Communication Systems”, John Wiley & Sons, 4 th Edition., N.Y., 2001