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CSE 4215/5431: Mobile Communications Winter 2011

This course provides an overview of mobile communications, including the similarities and differences with wired communication, the TCP/IP architecture, and the physical layer for mobile communications.

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CSE 4215/5431: Mobile Communications Winter 2011

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  1. CSE 4215/5431:Mobile Communications Winter 2011 Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4215 Some slides are adapted from the book website CSE 4215, Winter 2010

  2. Last class • Introduction to mobile communications • Similarities and differences with wired communication • Review of the TCP/IP architecture CSE 4215, Winter 2010

  3. Today • On the first lab • The physical layer for mobile communications CSE 4215, Winter 2010

  4. Lab 1: Evaluating mobility models • Why do we need mobility models? • What should the criteria be? CSE 4215, Winter 2010

  5. The Physical Layer • Let’s start with the very basic notions CSE 4215, Winter 2010

  6. Signals, channels and systems • What is a signal? • Baseband signal • Modulation • Bandwidth • Transmission/reception • What is a channel? • Bandwidth • Noise • Attenuation,Loss • What is a communication system? CSE 4215, Winter 2010

  7. Types of signals (a)continuous time/discrete time (b) continuous values/discrete values • analog signal = continuous time, continuous values • digital signal = discrete time, discrete values • Periodic signal - analog or digital signal that repeats over time • s(t +T ) = s(t ) -¥< t < +¥ • where T is the period of the signal • signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  • sine wave as special periodic signal for a carrier:s(t) = At sin(2  ft t + t) CSE 4215, Winter 2010

  8. Sine Wave Parameters

  9. Bandwidth • Of a signal • Of a channel Bandwidth vs bit rate CSE 4215, Winter 2010

  10. The underlying mathematics Fourier representation of periodic signals 1 1 0 0 t t ideal periodic signal real composition (based on harmonics) What about aperiodic signals ? CSE 4215, Winter 2010

  11. Frequency domain • Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency • Spectrum - range of frequencies that a signal contains • Absolute bandwidth - width of the spectrum of a signal • Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in CSE 4215, Winter 2010

  12. Transmitting rectangular signals • Observations • Any digital waveform will have infinite bandwidth • BUT the transmission system will limit the bandwidth that can be transmitted • AND, for any given medium, the greater the bandwidth transmitted, the greater the cost • HOWEVER, limiting the bandwidth creates distortions CSE 4215, Winter 2010

  13. Bit rates, channel capacity • Impairments, such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions CSE 4215, Winter 2010

  14. Nyquist Bandwidth • For binary signals (two voltage levels) • C = 2B • With multilevel signaling • C = 2B log2M • M = number of discrete signal or voltage levels CSE 4215, Winter 2010

  15. 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 CSE 4215, Winter 2010

  16. Shannon Capacity Formula • Equation: • 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 CSE 4215, Winter 2010

  17. Example of Nyquist and Shannon Formulations • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB • Using Shannon’s formula CSE 4215, Winter 2010

  18. Example of Nyquist and Shannon Formulations • How many signaling levels are required? CSE 4215, Winter 2010

  19. Modulation • Why? • How? CSE 4215, Winter 2010

  20. Frequencies for wireless communication VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length  = c/f wave length , speed of light c  3x108m/s, frequency f twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV CSE 4215, Winter 2010

  21. Frequencies for wireless communication VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication small antenna, beam forming large bandwidth available Wireless LANs use frequencies in UHF to SHF range some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies) weather dependent fading, signal loss caused by heavy rainfall etc. CSE 4215, Winter 2010

  22. Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) CSE 4215, Winter 2010

  23. Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed! Multiplexing channels ki k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f CSE 4215, Winter 2010

  24. Frequency multiplexing Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages no dynamic coordination necessary works also for analog signals Disadvantages waste of bandwidth if the traffic is distributed unevenly inflexible k1 k2 k3 k4 k5 k6 c f t CSE 4215, Winter 2010

  25. Time division multiplexing A channel gets the whole spectrum for a certain amount of time Advantages only one carrier in themedium at any time throughput high even for many users Disadvantages precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t CSE 4215, Winter 2010

  26. Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages better protection against tapping protection against frequency selective interference but: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t CSE 4215, Winter 2010

  27. Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages bandwidth efficient no coordination and synchronizationnecessary good protection against interferenceand tapping Disadvantages varying user data rates more complex signal regeneration Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t CSE 4215, Winter 2010

  28. Example • Lack of coordination requirement is an advantage. CSE 4215, Winter 2010

  29. Aside: Digital Communications • What is coding? • What is source coding? • What are line codes? • What is channel coding? CSE 4215, Winter 2010

  30. Transceivers • How are signals sent and received in wireless communications? CSE 4215, Winter 2010

  31. Antennas: isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna z y z ideal isotropic radiator y x x CSE 4215, Winter 2010

  32. Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipoleshape of antenna proportional to wavelength Example: Radiation pattern of a simple Hertzian dipole Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) /4 /2 y y z simple dipole x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) CSE 4215, Winter 2010

  33. Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z directed antenna x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) z z sectorized antenna x x top view, 3 sector top view, 6 sector CSE 4215, Winter 2010

  34. Antennas: diversity Grouping of 2 or more antennas multi-element antenna arrays Antenna diversity switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain cophasing needed to avoid cancellation /2 /2 /4 /2 /4 /2 + + ground plane CSE 4215, Winter 2010

  35. Antenna Gain • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) • Effective area • Related to physical size and shape of antenna CSE 4215, Winter 2010

  36. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area • f = carrier frequency • c = speed of light (» 3 ´ 108 m/s) •  = carrier wavelength CSE 4215, Winter 2010

  37. Back to modulation Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., /4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) CSE 4215, Winter 2010

  38. Modulation and demodulation analog baseband signal digital data digital modulation analog modulation radio transmitter 101101001 radio carrier analog baseband signal digital data analog demodulation synchronization decision radio receiver 101101001 radio carrier CSE 4215, Winter 2010

  39. Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth Phase Shift Keying (PSK): more complex robust against interference 1 0 1 t 1 0 1 t 1 0 1 t CSE 4215, Winter 2010

  40. Advanced Frequency Shift Keying bandwidth needed for FSK depends on the distance between the carrier frequencies special pre-computation avoids sudden phase shifts  MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK even higher bandwidth efficiency using a Gaussian low-pass filter  GMSK (Gaussian MSK), used in GSM CSE 4215, Winter 2010

  41. Example of MSK 0 1 1 0 1 0 1 bit data even 0 1 0 1 even bits odd 0 0 1 1 signal h n n hvalue - - + + odd bits low frequency h: high frequency n: low frequency +: original signal -: inverted signal highfrequency MSK signal t No phase shifts! CSE 4215, Winter 2010

  42. Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) Q I 1 0 Q 11 10 I 00 01 A t 01 11 10 00 CSE 4215, Winter 2010

  43. Quadrature Amplitude Modulation . Quadrature Amplitude Modulation (QAM) combines amplitude and phase modulation it is possible to code n bits using one symbol 2n discrete levels, n=2 identical to QPSK Bit error rate increases with n, but less errors compared to comparable PSK schemes Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 havethe same phase φ, but differentamplitude a. 0000 and 1000 havedifferent phase, but same amplitude Q 0010 0001 0011 0000 φ I a 1000 CSE 4215, Winter 2010

  44. Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers QPSK, 16 QAM, 64QAM Example: 64QAM good reception: resolve the entire 64QAM constellation poor reception, mobile reception: resolve only QPSK portion 6 bit per QAM symbol, 2 most significant determine QPSK HP service coded in QPSK (2 bit), LP uses remaining 4 bit Q 10 I 00 000010 010101 CSE 4215, Winter 2010

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