1 / 77

Introduction to Mobile Communications

Introduction to Mobile Communications. TCOM 552, Lecture #3 Hung Nguyen, Ph.D. 18 September, 2006. Outline. Channel Capacity Signal-to-Noise Ratio (SNR) Multiplexing Digital Modulation Analog Modulation Coding Simplex and Duplex Transission. About Channel Capacity.

maxime
Télécharger la présentation

Introduction to Mobile Communications

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Introduction to Mobile Communications TCOM 552, Lecture #3 Hung Nguyen, Ph.D. 18 September, 2006

  2. Outline • Channel Capacity • Signal-to-Noise Ratio (SNR) • Multiplexing • Digital Modulation • Analog Modulation • Coding • Simplex and Duplex Transission Hung Nguyen, TCOM 552, Fall 2006

  3. About Channel Capacity • Impairments, such as noise, limit data rate that can be achieved • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions Hung Nguyen, TCOM 552, Fall 2006

  4. Transmission Impairments • Signal received may differ from signal transmitted • Analog - degradation of signal quality • Digital - bit errors • Caused by • Attenuation and attenuation distortion • Delay distortion • Noise Hung Nguyen, TCOM 552, Fall 2006

  5. Attenuation • Signal strength falls off with distance • Depends on medium • Received signal strength: • must be enough to be detected • must be sufficiently higher than noise to be received without error • Attenuation is an increasing function of frequency Hung Nguyen, TCOM 552, Fall 2006

  6. Noise (1) • Additional EM energy and signals on the receiver • Thermal -- usually inserted by receiver circuits • Due to thermal agitation of electrons • Uniformly distributed • White noise • Intermodulation • Signals that are the sum and difference of original frequencies sharing a medium, and falling within the desired signal’s passband Hung Nguyen, TCOM 552, Fall 2006

  7. Noise (2) • Crosstalk • A signal from one line or channel is picked up by another • Impulse • Irregular pulses or spikes • e.g. External electromagnetic interference • Short duration • High amplitude • Multipath • See in later Sessions, causes distortions Hung Nguyen, TCOM 552, Fall 2006

  8. Signal-to-Noise Ratio • Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N) • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate Hung Nguyen, TCOM 552, Fall 2006

  9. Signals and Noise High SNR Lower SNR Hung Nguyen, TCOM 552, Fall 2006

  10. Concepts Related to Channel Capacity • Data rate - rate at which data can be communicated (bps) • Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) • Noise - average level of noise over the communications path • Error rate - rate at which errors occur • Error = transmit 1 and receive 0; transmit 0 and receive 1 Hung Nguyen, TCOM 552, Fall 2006

  11. Nyquist Bandwidth • For binary signals (two voltage levels) • C = 2B • With multilevel signaling • C = 2B log2 M • M = number of discrete signal or voltage levels Hung Nguyen, TCOM 552, Fall 2006

  12. Shannon Capacity Formula • Equation: • Represents theoretical maximum that can be achieved • In practice, somewhat lower rates achieved • Formula assumes white noise (thermal noise) • Worse when other forms of noise are included • Impulse noise • Attenuation distortion or delay distortion • Interference Hung Nguyen, TCOM 552, Fall 2006

  13. Example of Nyquist and Shannon Formulations • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB • Using Shannon’s formula Hung Nguyen, TCOM 552, Fall 2006

  14. Example of Nyquist and Shannon Formulations • How many signaling levels are required? Hung Nguyen, TCOM 552, Fall 2006

  15. 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 Hung Nguyen, TCOM 552, Fall 2006

  16. Multiplexing Hung Nguyen, TCOM 552, Fall 2006

  17. 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 Hung Nguyen, TCOM 552, Fall 2006

  18. 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 --- different users at different frequency bands or subbands • 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 --- different users at different time slots Hung Nguyen, TCOM 552, Fall 2006

  19. Frequency-division Multiplexing Hung Nguyen, TCOM 552, Fall 2006

  20. Time-division Multiplexing Hung Nguyen, TCOM 552, Fall 2006

  21. Multiplexing and Multiple Access • Both refer to the sharing of a communications resource, usually a channel • Multiplexing usually refers to sharing some resource by doing something at one site --- e.g., at the multiplexer • Often a static or pseudo-static allocation of fractions of the multiplexed channel, e.g., a T1 line. Often refers to sharing one resource. The division of the resource can be made on frequency, or time, or other physical feature • Multiple Access shares an asset in a distributed domain • i.e., multiple users at different places sharing an overall media, and using a scheme where it is divided into channels based on frequency, or time or another physical feature • Usually dynamic Hung Nguyen, TCOM 552, Fall 2006

  22. Factors Used to CompareModulation and Encoding Schemes • Signal spectrum • With fewer higher frequency components, less bandwidth required --- Spectrum Efficiency • For wired comms: with no DC component, AC coupling via transformer possible --- DC components cause problems • Transfer function of a channel is worse near band edges -- always better to constrain signal spectrum well inside the spectrum available • Synchronization and Clocking • Determining when 0 phase occurs -- carrier synch • Determining beginning and end of each bit position -- bit sync • Determining frame sync --- usually layer above physical Hung Nguyen, TCOM 552, Fall 2006

  23. Signal Modulation/Encoding Criteria: Demodulating/Decoding Accurately • What determines how successful a receiver will be in interpreting an incoming signal? • Signal-to-noise ratio = SNR • Signal power/noise power • Note: power = energy per unit time • Data rate (R) • Bandwidth (BW) • An increase in data rate increases bit error rate • An increase in SNR decreases bit error rate • An increase in bandwidth allows an increase in data rate Hung Nguyen, TCOM 552, Fall 2006

  24. Factors Used to CompareModulation/Encoding Schemes • Signal interference and noise immunity --- • Performance in the presence of interference and noise • For a given signal power level, the effect of noise and interference is then labeled the Power Efficiency • For digital modulation, Prob. Of Bit Error = function (SNR) where N includes the interference terms • More exactly, Prob. Bit Error = function (Energy per bit/Noise power density, with noise including interference and other noise like terms) --- see next chart • Cost and complexity • Usually the higher the signal and data rates require a higher complexity and greater the cost Hung Nguyen, TCOM 552, Fall 2006

  25. A Figure of Merit in Communications:Noise Immunity • For digital modulation one bottom line Figure of Merit (FOM) is Probability of Bit Error (Pe) -- Lowest for Most Accurate Decoding of Bit Stream • Prob. Bit Error= function of (Eb/N0) • Many functions for many different modulation and coding types have been computed - usually decreases with increasing Eb/N0 • Eb = energy per bit • N0 = noise spectral density; Noise Power N = (N0)* BW • Note: Includes Interference and Intermodulation and Crosstalk • (Eb/N0) is a critically important number for digital comms • Eb/N0 =(SNR)*(BW/R) ---- important formula -- derive it • SNR is signal to noise ratio, a ratio of power levels • BW is signal bandwidth, R is data rate in bits/sec • For analog modulation the FOM is SNR • Signal quality given by subjective statistical scores -- voice: 1-5 (high) • FM requires a lower SNR than AM for the same signal quality Hung Nguyen, TCOM 552, Fall 2006

  26. Basic Modulation/Encoding Techniques • Digital data to analog signal --- Digital Modulation • Amplitude-shift keying (ASK) • Amplitude difference of carrier frequency • Frequency-shift keying (FSK) • Frequency difference near carrier frequency • Phase-shift keying (PSK) • Phase of carrier signal shifted Hung Nguyen, TCOM 552, Fall 2006

  27. Basic Encoding Techniques Hung Nguyen, TCOM 552, Fall 2006

  28. Amplitude-Shift Keying • One binary digit represented by presence of carrier, at constant amplitude • Other binary digit represented by absence of carrier • where the carrier signal is A*cos(2πfct) Hung Nguyen, TCOM 552, Fall 2006

  29. Amplitude-Shift Keying • Susceptible to sudden gain changes • Inefficient modulation technique • On voice-grade lines, used up to 1200 bps • Used to transmit digital data over optical fiber Hung Nguyen, TCOM 552, Fall 2006

  30. 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 Hung Nguyen, TCOM 552, Fall 2006

  31. Binary Frequency-Shift Keying (BFSK) • Less susceptible to error than ASK • On voice-grade lines, used up to 1200bps • Used for high-frequency (3 to 30 MHz) radio transmission • Can be used at higher frequencies on LANs that use coaxial cable Hung Nguyen, TCOM 552, Fall 2006

  32. Multiple Frequency-Shift Keying (MFSK) • More than two frequencies are used • More bandwidth efficient but more susceptible to error • fi = fc + (2i – 1 – M)fd • fc = the carrier frequency • fd = the difference frequency • M = number of different signal elements = 2 L • L = number of bits per signal element Hung Nguyen, TCOM 552, Fall 2006

  33. Multiple Frequency-Shift Keying (MFSK) • To match data rate of input bit stream, each output signal element is held for: • Ts = LT seconds • where T is the bit period (data rate = 1/T) • So, one signal element encodes L bits Hung Nguyen, TCOM 552, Fall 2006

  34. Multiple Frequency-Shift Keying (MFSK) • Total bandwidth required • 2Mfd • Minimum frequency separation required 2fd = 1/Ts • Therefore, modulator requires a bandwidth of • Wd = 2L/LT = M/Ts Hung Nguyen, TCOM 552, Fall 2006

  35. Multiple Frequency-Shift Keying (MFSK) Hung Nguyen, TCOM 552, Fall 2006

  36. Phase Shift Keying (PSK) • The signal carrier is shifted in phase according to the input data stream • 2 level PSK, also called binary PSK or BPSK or 2-PSK, uses 2 phase possibilities over which the phase can vary, typically 0 and 180 degrees -- each phase represents 1 bit • can also have n-PSK -- 4-PSK often is 0, 90, 180 and 270 degrees --- each phase then represents 2 bits • Each phase called a ‘symbol’ • Each bit or groups of bits can be represented by a phase value (e.g., 0 degrees, or 180 degrees), or bits can be based on whether or not phase changes (differential keying, e.g., no phase change is a 0, a phase change is a 1) --- DPSK Hung Nguyen, TCOM 552, Fall 2006

  37. Phase-Shift Keying (PSK) • Two-level PSK (BPSK) • Uses two phases to represent binary digits Hung Nguyen, TCOM 552, Fall 2006

  38. 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 Hung Nguyen, TCOM 552, Fall 2006

  39. Phase-Shift Keying (PSK) • Four-level PSK (QPSK) • Each element represents more than one bit Hung Nguyen, TCOM 552, Fall 2006

  40. Quadrature PSK • More efficient use by each signal element (or symbol) representing more than one bit • e.g. shifts of /2 (90o) • In QPSK each element or symbol represents two bits • Can use 8 phase angles and have more than one amplitude -- then becomes QAM then (combining PSK and ASK) • QPSK used in different forms in a many cellular digital systems • Offset-QPSK: OQPSK: The I (0 and 180 degrees) and Q (90 and 270 degrees) quadrature bits are offset from each other by half a bit --- becomes a more efficient modulation, with phase changes not so abrupt so better spectrally, and more linear • p/4-QPSK is a similar approach to OQPSK, also used Hung Nguyen, TCOM 552, Fall 2006

  41. Multilevel Phase-Shift Keying (MPSK) • Multilevel PSK • Using multiple phase angles multiple signals elements can be achieved • D = modulation rate, baud • R = data rate, bps • M = number of different signal elements or symbols = 2L • L = number of bits per signal element or symbol • e.g., 4-PSK is QPSK, 8-PSK, etc Hung Nguyen, TCOM 552, Fall 2006

  42. Quadrature Amplitude Modulation • QAM is a combination of ASK and PSK • Two different signals sent simultaneously on the same carrier frequency Hung Nguyen, TCOM 552, Fall 2006

  43. Quadrature Amplitude Modulation Hung Nguyen, TCOM 552, Fall 2006

  44. Quadrature Amplitude Modulation (QAM) • The most common method for quad (4) bit transfer • Combination of 8 different angles in phase modulation and two amplitudes of signal • Provides 16 different signals (or ‘symbols’), each of which can represent 4 bits (there are 16 possible 4 bit combinations) Hung Nguyen, TCOM 552, Fall 2006

  45. Quadrature Amplitude Modulation Illustration -- Example of Constellation Diagram 90 135 45 amplitude 1 0 180 amplitude 2 225 315 270 • Notice that there are 16 circles or nodes, each represents a possible amplitude and phase, and each represents 4 bits • Obviously there are many such constellation diagrams possible --- the technical issue winds up being that as the nodes get closer to each other any noise can lead to the receiver confusing them, and making a bit error Hung Nguyen, TCOM 552, Fall 2006

  46. Performance of Digital Modulation Schemes • Bandwidth or Spectral Efficiency • ASK and PSK bandwidth directly related to bit rate • FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies • Determined by C/BW i.e. bps/Hz • Noise Immunity or Power Efficiency: In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK ---- i.e., x2 less power for same performance • Determined by BER as function of Eb/N0 Hung Nguyen, TCOM 552, Fall 2006

  47. Spectral Performance • Bandwidth of modulated signal (BT) • ASK, PSK BT = (1+r)R • FSK BT = 2DF+(1+r)R • R = bit rate • 0 < r < 1; related to how signal is filtered • DF = f2-fc = fc-f1 Hung Nguyen, TCOM 552, Fall 2006

  48. SPECTRAL Performance • Bandwidth of modulated signal (BT) • MPSK • MFSK • L = number of bits encoded per signal element • M = number of different signal elements Hung Nguyen, TCOM 552, Fall 2006

  49. BER vs.. Eb/N0 In Stallings Hung Nguyen, TCOM 552, Fall 2006

  50. BER vs.. Eb/N0 (cont’d) In Stallings Hung Nguyen, TCOM 552, Fall 2006

More Related