Digital communication systems


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Digital communication systems

  1. 1. DIGITAL COMMUNICATION SYSTEMS MSC -COURSE Prepared by : Nisreen Bashar AL-Madanat Under Supervision of : Dr. Ibrahim AL-Qatawneh
  2. 2. OUTLINES :                       Objectives Introduction Modes of communications Digital data communication Transmission timing Goals in communication systems Communication channels Channel capacity Noise in communication systems Digital transmission Multiplexing techniques Digital modulation and demodulation Selection of Encoding /Modulation schemes Matched filter Bandwidth efficiency MPSK Error rate Performance QAM Error performance Performance comparison of various Digital Modulation schemes Trade offs between BER ,POWER and BW Industry tends Summary References
  3. 3. OBJECTIVES : After completing this lecture the student will be able to :    Have an over all view about the communication system and its simplified block diagram. Discuss the main concepts of communication systems. Discuss and distinguish the types of communication systems and its channels and Modes.  Identify the digital data transmission methods and timing .  Consider the basic goals or tradeoffs in communication system design .  Identify the channel capacity and its related concepts.  Aware the effect of noise in communication systems.  Discuss the usefulness of digital transmission and its disadvantages.  Discuss Multiplexing techniques and its types and applications.  Identify modulation and its different types .  Compare between different digital modulation schemes.  Design and select a specific simple communication system based on the trade offs and the performance comparison .  Discuss industry trends.
  4. 4. INTRODUCTION    Main purpose of communication is to transfer information from a source to a recipient via a channel or medium. Communication systems are found whenever information is to be transmitted from one point to another. Basic block diagram of a communication system:
  5. 5. BRIEF DESCRIPTION Source: analog or digital  Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna  Channel: e.g. cable, optical fiber, free space  Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer  Recipient: e.g. person, (loud) speaker, computer 
  6. 6. DATA COMMUNICATION TERMS    Data - entities that convey meaning, or information Signals - electric or electromagnetic representations of data Transmission - communication of data by the propagation and processing of signals Types of information Voice,text data, video, music, email, numerical data , graphic data etc. Types of communication systems - Public Switched Telephone Network (voice,fax,modem) - Satellite systems - Radio,TV broadcasting - Cellular phones - Computer networks (LANs, WANs, WLANs)
  7. 7. INFORMATION REPRESENTATION    Communication system converts information into electrical electromagnetic/optical signals appropriate for the transmission medium. Analog systems convert analog message into signals that can propagate through the channel. Digital systems convert bits(digits, symbols) into signals Computers naturally generate information as characters/bits  Most information can be converted into bits  Analog signals converted to bits by sampling and quantizing (A/D conversion) 
  8. 8. MODES  OF COMMUNICATION Based on whether the systems communicate on only one direction or otherwise , the communication modes are as indicated in the following figure :
  9. 9. DIGITAL DATA COMMUNICATION In a digital communications system, there are two methods for data transfer: Parallel and Serial. Parallel connections have multiple wires running parallel to each other (hence the name), and can transmit data on all the wires simultaneously. Serial, on the other hand, uses a single wire to transfer the data bits one at a time. Parallel Data The parallel port on modern computer systems is an example of a parallel communications connection. The parallel port has 8 data wires, and a large series of ground wires and control wires. IDE hard disk connectors are another good example of parallel connections in a computer system. Serial Data The serial port on modern computers is a good example of serial communications. Serial ports have a single data wire, and the remainder of the wires are either ground or control signals. USB is a good example of other serial communications standards.
  11. 11. TRANSMISSION TIMING (SYNCHRONIZATION )  Synchronous and Asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.
  12. 12. SYNCHRONOUS TRANSMISSION      Data/Strobe Synchronous Transmission In synchronous transmission, the stream of data to be transferred is encoded as fluctuating voltages on one wire, and a periodic pulse of voltage is put on another wire (often called the "clock" or "strobe") that tells the receiver "here's where one bit/byte ends and the next one begins". Synchronous transmission is carried out under the control of the common master clock. No start and stop bits are used instead the bytes are transmitted as a block in a continuous stream of bits. The receiver operates at exactly the same clock frequency as that of transmitter. The grouping of these bits is the responsibility of the receiver.
  13. 13. SYNCHRONOUS TRANSMISSION Advantages:  Lower overhead and thus, greater throughput Disadvantages:  Slightly more complex .  Hardware is more expensive  High cost
  14. 14. ASYNCHRONOUS TRANSMISSION    In asynchronous transmission, there is only one wire/signal carrying the transmission. The transmitter sends a stream of data and periodically inserts a certain signal element into the stream which can be "seen" and distinguished by the receiver as a sync signal. That sync signal might be a single pulse (a "start bit" in asynchronous start/stop communication), or it may be a more complicated sync word.
  15. 15. ASYNCHRONOUS TRANSMISSION  • • •  • Advantages: Simple, doesn't require synchronization of both communication sides Cheap, timing is not as critical as for synchronous transmission, therefore hardware can be made cheaper. Set-up is very fast, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard. Disadvantages: Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes and thus carry no useful information.
  16. 16. GOALS IN COMMUNICATION SYSTEM DESIGN (TRADE –OFFS) To maximize transmission rate, R  To maximize system utilization, U  To minimize bit error rate, Pe  To minimize required systems bandwidth, W  To minimize system complexity, Cx  To minimize required power, Eb/No 
  17. 17. COMMUNICATION CHANNELS • Wire line Channels (Copper) • Microwave Radio • Satellite communication • Optical Fiber
  18. 18. CHANNEL CAPACITY • Channel Capacity (C) – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions  • Data rate (bps) – rate at which data can be communicated , impairments, such as noise, limit data rate that can be achieved  • Bandwidth (B) – the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)  • Noise (N) – impairments on the communications path  • Error rate - rate at which errors occur (BER) – Error = transmit 1 and receive 0; transmit 0 and receive 1 
  19. 19. SHANNON’S CHANNEL CAPACITY     Equation: C  B log 2 1  SNR  Channel capacity C (bits/sec) is the speed at which information can travel over a channel with an arbitrarily low error rate i.e. when a system is transmitting bits at or below C then for any BER e>0 there exists a code with block length n which will provide a BER < e Represents theoretical maximum that can be achieved In practice, only much lower rates achieved Formula assumes white noise (thermal noise)  Impulse noise is not accounted for  Attenuation distortion or delay distortion not accounted for  Mathematicians/Shannon.html
  20. 20. SIGNAL ENCODING CRITERIA • What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio (SNR)  Data rate  Bandwidth (B)  Inter-related quantities • Increase in SNR decreases bit error rate • Increase in data rate increases bit error rate • Increase in bandwidth allows an increase in data rate  Shannon Bound for AWGN non fading channel 
  21. 21. CONCEPTS RELATED TO CHANNEL CAPACITY   Shannon Bound for AWGN non fading channel Nyquist Bandwidth – For binary signals (two voltage levels) • C = 2B – With multilevel signaling (M-ary signalling) • C = 2B log 2 M • M = number of discrete signal or voltage levels • N= number of bits • M = 2^N
  22. 22. EXAMPLE OF NYQUIST AND SHANNON FORMULATIONS  • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB =24 dB B  4 M Hz  3 M Hz  1 M Hz SNR dB  24 dB  10 log10 SNR   SNR  251 Using Shannon’s formula C  106  log 2 1  251  106  8  8Mbps  How many signaling levels are required? C  2 B log 2 M   8  106  2  10 6  log 2 M 4  log 2 M M  16
  23. 23. NOISE IN COMMUNICATION SYSTEMS     Without noise/distortion/sync. problem, we will never make bit errors The term noise refers to unwanted electrical signals that are always present in electrical systems; e.g spark-plug ignition noise, switching transients, and other radiating electromagnetic signals. Can describe thermal noise as a zero-mean Gaussian random process. A Gaussian process n(t) is a random function whose amplitude at any arbitrary time t is statistically characterized by the Gaussian probability density function  1  n 2  1 p ( n)  exp       2  2    
  24. 24. WHITE NOISE    The primary spectral characteristic of thermal noise is that its power spectral density is the same for all frequencies of interest in most communication systems. N Gn ( f )  0 watts / hertz Power spectral density Gn(f ) 2 Autocorrelation function of white noise is Rn ( )  1{Gn ( f )}   The average power Pn of white noise is infinite  p ( n)       N0  ( ) 2 N0 df   2 The effect on the detection process of a channel with additive white Gaussian noise (AWGN) is that the noise affects each transmitted symbol independently. Such a channel is called a memoryless channel. The term “additive” means that the noise is simply superimposed or added to the signal
  25. 25. SYSTEM NOISE A constellation displaying significant noise.  Dots are spread out indicating high noise and most likely significant errors.  Dots are spread out causing errors to occur
  26. 26. DIGITAL TRANSMISSION ADVANTAGES Why Digital ?  – Increase System Capacity • compression, more efficient modulation  – Error control coding, equalizers,etc. possible to combat noise and interference => lower power needed  – Reduce cost and simplify designs  – Improve Security (encryption possible) • Digital Modulation  – Analog signal carrying digital data
  27. 27.   Digital techniques need to distinguish between discrete symbols allowing regeneration versus amplification Good processing techniques are available for digital signals, such as medium.      Data compression (or source coding) Error Correction (or channel coding)(A/D conversion) Equalization Security Easy to mix signals and data using digital techniques
  28. 28. DISADVANTAGES Requires reliable “synchronization”  Requires A/D conversions at high rate  Requires larger bandwidth  Non graceful degradation  Performance Criteria  Probability of error or Bit Error Rate 
  29. 29. MULTIPLEXING TECHNIQUES Multiplexing  Capacity of transmission medium usually exceeds capacity required for transmission of a single signal  Multiplexing - carrying multiple signals on a single medium  More efficient use of transmission medium
  30. 30. REASONS FOR WIDESPREAD USE OF MULTIPLEXING Cost per kbps of transmission facility declines with an increase in the data rate  Cost of transmission and receiving equipment declines with increased data rate  Most individual data communicating devices require relatively modest data rate support 
  31. 31. MULTIPLEXING TECHNIQUES  Frequency-division multiplexing (FDM)   Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal Time-division multiplexing (TDM)  Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal ** Statistical multiplexing  is a type of communication link sharing.
  32. 32. FDM VS TDM
  33. 33. STATISTICAL MULTIPLEXING       In statistical multiplexing , a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams The link sharing is adapted to the instantaneous traffic demands of the data streams that are transferred over each channel When performed correctly, statistical multiplexing can provide a link utilization improvement, called the statistical multiplexing gain. Statistical multiplexing is facilitated through packet mode or packet-oriented communication, which among others is utilized in packet switched computer networks In statistical multiplexing, each packet or frame contains a channel/data stream identification number, or (in the case of datagram communication) complete destination address information. The packets to be transmitted are stored in a common buffer and are then transmitted according to some specified scheduling rules.(the adv. Of this is that the transmitter is never Idle as long as there are packets to transmit.
  34. 34. COMPARISON WITH STATIC ,TDM AND FDM      Time domain statistical multiplexing (packet mode communication) is similar to (TDM), except that, rather than assigning a data stream to the same recurrent time slot in every TDM frame , each data stream is assigned time slots (of fixed length) or data frames (of variable lengths) that often appear to be scheduled in a randomized order, and experience varying delay (while the delay is fixed in TDM). Statistical multiplexing allows the bandwidth to be divided arbitrarily among a variable number of channels (while the number of channels and the channel data rate are fixed in TDM and FDM). Statistical multiplexing ensures that slots will not be wasted (whereas TDM can waste slots). The transmission capacity of the link will be shared by only those users who have packets. Statistical most costly . TDM and FDM divide a communication link into channels with fixed transmission rates , while in statistical it only uses the channel when it has packets to send.  TDM and FDM less efficient for irregular flow of packets .  The average delay for FDM and TDM is larger .
  35. 35. DIGITAL MODULATION AND DEMODULATION Modulation •All channels consist of some continuous parameter •Must map discrete states onto continuous property •Must have a decision circuit to map the state of the modulated channel into a discrete state •As number of levels or states M the behavior of the digital system does not approach that of an analog system, due to the decision circuit
  36. 36. MODULATION REVIEW Modulation - Converting digital or analog information to a waveform suitable for transmission over a given medium - Involves varying some parameter of a carrier wave (sinusoidal waveform) at a given frequency as a function of the message signal - General sinusoid  - If the information is digital changing parameters is called “keying” (e.g. ASK, PSK, FSK)
  37. 37. MODULATION – DEFINITION • Motivation -Smaller antennas (e.g., l /4 typical antenna size) • l = wavelength = c/f , where c = speed of light, f= frequency. • 3000Hz baseband signal => 15 mile antenna, 900 MHz => 8 cm - Frequency Division Multiplexing – provides separation of signals - medium characteristics - Interference rejection - Simplifying circuitry  • Modulation – shifts center frequency of baseband signal up to the radio carrier  • Basic schemes - Amplitude Modulation (AM) Amplitude Shift Keying (ASK) - Frequency Modulation (FM) Frequency Shift Keying (FSK) - Phase Modulation (PM) Phase Shift Keying (PSK) 
  38. 38. NUMBER OF LEVELS Digital communications relies on a finite number of discrete levels  Minimum number of levels is two (binary code)  Shannon Capacity helps determine optimum number of levels for a given bandwidth, SNR, and BER   Eb  r  log1  r   N0    r  R W
  39. 39. LIMITS ON COMMUNICATION CHANNELS  Eb   r  log 1  r  N0    Two types of communication channels  r<<1 – Power Limited    High dimensionality signaling schemes Binary r>>1 – Bandwidth Limited   Low dimensionality Multilevel Proakis and Salehi, pp. 738
  40. 40. DIGITAL MODULATION TYPES • Amplitude Shift Keying (ASK): - change amplitude with each symbol - frequency constant - low bandwidth requirements - very susceptible to interference  • Frequency Shift Keying (FSK): - change frequency with each symbol needs larger bandwidth  • Phase Shift Keying (PSK): - Change phase with each symbol - More complex - robust against interference 
  42. 42. AMPLITUDE-SHIFT KEYING o o One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of Carrier • where the carrier signal is Acos(2pfct) o o o o Very Susceptible to noise Used to transmit digital data over optical fiber Amplitude of carrier wave is modulated Equivalent BER vs SNR to baseband PAM Proakis and Salehi, pp. 306
  43. 43. ANGLE MODULATION (PSK AND FSK)  Frequency is time derivative of phase, PSK and FSK are somewhat equivalent. Proakis and Salehi, pp. 332
  44. 44. PSK: DIGITAL ANGLE MODULATION    Usually in digital communications PSK is chosen over FSK Easier to create multilevel codes Possibility of using differential phase shift keying (DPSK) Uses phase shifts relative to previous bit  Eliminates need for local oscillator at receiver  Use Gray Code to minimize effect of errors  Proakis and Salehi, pp. 631
  45. 45. BINARY FREQUENCY-SHIFT KEYING (BFSK) Two binary digits represented by two different frequencies near the carrier frequency  – where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts – B = 2([f2 – f1]/2 + fb) • Where fb = input bit rate  BER for BFSK
  46. 46. BI-PHASE SHIFT KEYING (BPSK)    BPSK is the simplest method of digital transmission. Data is transmitted by reversing the phase of the carrier. The amplitude of the carrier remains constant. Is a very robust transmission method but consumes significant bandwidth Data Amplitude  1 1 In Phase 0 180° Out of Phase 0 -1
  47. 47. QUADRATURE AMPLITUDE MODULATION • QAM is a combination of ASK and PSK – Two different signals sent simultaneously on the same carrier frequency – – Change phase and amplitude as function of input data – Simple case 8 QAM (two amplitudes – 4 phases)  Used in ADSL, Digital video broadcasting, Microwave,…  The amplitude of two carriers will be changed according to the data signal (Quadrature) 
  49. 49. THE I/Q DIAGRAM
  50. 50. BIT-ERROR RATE      QAM is a modulation type, where the amplitude of two carrier signals will be modulated according to the bits sequence to be transmitted. There are many kinds of QAM, depending on the number of bits in each sequence that will be transmitted as one symbol. Higher order QAM is more bandwidth efficient than the lower order. Higher order QAM is less power efficient than the lower orders because of the high bit-error rate. Rectangular QAM is easier to modulate and demodulate than nonrectangular QAM, but bit-error rate is higher in rectangular QAM.
  51. 51. D PHASE-SHIFT KEYING (PSK) Differential PSK (DPSK) – Phase shift with reference to previous bit • Binary 0 – signal burst of same phase as previous signal burst • Binary 1 – signal burst of opposite phase to previous signal burst 
  52. 52. PERFORMANCE IN AWGN CHANNELS  Similar to BPSK analysis have Pe for FSK, and DPSK
  53. 53. M-ARY SIGNALING/MODULATION What is M-ary signaling? – The transmitter considers ‘k’ bits at a times. It produces one of M signals where M = 2^k.  Example: QPSK (k = 2) 
  54. 54. M-ARY ERROR PERFORMANCE • MPSK, as M increases – the bandwidth remains constant, – the minimum distance between signals reduces => increase in symbol error rate  • MFSK, as M increases – the bandwidth increases – the performance improves but the minimum distance between signals remains the same 
  55. 55. SELECTION OF ENCODING/MODULATION SCHEMES • Performance in an AWGN channel – How does the bit error rate vary with the energy per bit available in the system when white noise present  • Performance in fading multipath channels – Same as above, but add multipath and fading  • Bandwidth requirement for a given data rate – Also termed spectrum efficiency or bandwidth efficiency – How many bits/sec can you squeeze in one Hz of bandwidth for a given error rate  • Cost – The modulation scheme needs to be cost efficient _ Circuitry should be simple to implement and inexpensive (e.g. detection, amplifiers) 
  56. 56. MATCHED FILTER In order to detect a signal at the receiver, a linear filter that is designed to provide the maximum output SNR in AWGN for a given symbol waveform is used.  This filter is called a “matched filter”  • If the transmitted signal is s(t), the impulse response of the matched filter can be shown to be
  57. 57.  This assumes that s(t) exists only for a duration of T seconds. Let us look at the output for k = 1. Compare with cross-correlation:  The output of the matched filter is the crosscorrelation of the received signal and the time shifted transmitted signal.
  60. 60. M-PSK ERROR RATE PERFORMANCE  Increasing M increases error rate and data rate
  63. 63. TRADEOFFS BETWEEN BER, POWER AND BANDWIDTH Trade BER performance for power – fixed data Rate  Trade data rate for power – fixed BER  Trade BER for data rate – fixed power 
  65. 65. SUMMARY
  66. 66. REFERENCES   John G. Proakis, Masoud Salehi, Communications Systems Engineering, Prentice Hall 1994 David Tipper/Associate Professor Department of Information Science and Telecommunications/University of Pittsburgh        Communication systems (4th Edition)/Simon haykin Constellations Demystified Presented by Sunrise Telecom Broadband Data communication and Network (4th Edition) by Behrouz.A.Forouzan with Sophia Chung Fegan . Fundamentals of DIGITAL COMMUNICATION by (Upamanyu madhow) Digital Communications, Fourth Edition, J.G. Proakis, McGraw Hill, 2000. Agilent Digital Modulation in Communications Systems An Introduction Application Note 1298 Digital Communications: Fundamentals and Applications, By “Bernard Sklar”, Prentice Hall, 2nd ed, 2001