CS647: Advanced Topics in Wireless Networks Basics of Wireless Transmission

Transcription

1 CS647: Advanced Topics in Wireless Networks Basics of Wireless Transmission CS

2 Outline Frequencies Signals Antennas Signal propagation Multiplexing Spread spectrum Modulation CS

3 Types of Wave Sky wave Ionosphere ( km) Mesosphere (50-80 km) Transmitter Space wave Ground wave Earth Receiver CS Stratosphere (12-50 km) Troposphere (0-12 km)

4 Speed, Wavelength, Frequency Frequency and wave length: λ = c/f wave length λ, speed of light c 3x108m/s, frequency f System AC current FM radio Cellular Ka band satellite Ultraviolet light Frequency 60 Hz 100 MHz 800 MHz 20 GHz Hz Wavelength 5,000 km 3 m 37.5 cm 15 mm 10-7 m CS

5 Radio Frequency Bands Classification Band Initials Frequency Range Characteristics Extremely low Infra low ELF ILF < 300 Hz 300 Hz - 3 khz Ground wave Very low VLF 3 khz - 30 khz Low LF 30 khz khz Medium MF 300 khz - 3 MHz Ground/Sky wave High HF 3 MHz - 30 MHz Sky wave Very high VHF 30 MHz MHz Ultra high UHF 300 MHz - 3 GHz Space wave Super high SHF 3 GHz - 30 GHz Extremely high EHF 30 GHz GHz Tremendously high THF 300 GHz GHz CS

6 Frequencies for communication 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 µm 3 THz 1 µm 300 THz 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 VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VHF = Very High Frequency Frequency and wave length: λ = c/f wave length λ, speed of light c 3x10 8 m/s, frequency f CS

7 Frequencies for mobile 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. CS

8 Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Europe USA Japan Cellular Phones Cordless Phones Wireless LANs Others GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , RF-Control 27, 128, 418, 433, 868 AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , RF-Control 315, 915 PDC , , , PHS JCT IEEE RF-Control 426, 868 CS

9 Signals I physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f=1/t, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier: s(t) = A t sin(2 π f t t + ϕ t ) CS

10 Fourier representation of periodic signals g( t) = 1 2 c + n= 1 a n sin(2πnft) + n= 1 b n cos(2πnft) ideal periodic signal t 0 real composition (based on harmonics) t CS

11 Signals II Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase ϕ in polar coordinates) A [V] A [V] Q = M sin ϕ t[s] ϕ I= M cos ϕ ϕ f [Hz] Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!) CS

12 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 y z z x y x ideal isotropic radiator CS

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

14 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 x z x directed antenna side view (xy-plane) side view (yz-plane) top view (xz-plane) z z x x sectorized antenna top view, 3 sector top view, 6 sector CS

15 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 λ/4 λ/2 λ/4 λ/2 λ/2 λ/2 + + ground plane CS

16 Signal propagation ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission detection interference distance CS

17 Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum much more in real environments (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges shadowing reflection refraction scattering diffraction CS

18 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction LOS pulses multipath pulses signal at sender Time dispersion: signal is dispersed over time signal at receiver interference with neighbor symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts CS

19 Free-space Propagation h b h m Transmitter Distance d Receiver The received signal power at distance d: P r = AeGP t 2 4πd t where P t is transmitting power, A e is effective area, and G t is the transmitting antenna gain. Assume that radiated power is uniformly distributed over the surface of the sphere. CS

20 Antenna Gain The relationship between an effective aperture and received antenna gain G r can be given by: G r = 4π A e / λ 2 where λ is the wavelength, and A e is the effective area covered by the transmitter. By substituting A e, in terms of G r and λ, we obtain P r = G r G t P t / (4π d/λ) 2 Free Space path loss is defined as L f =P t /P = (1/G r r G t ) (4π d/λ) 2 L f indicates power loss in the free space. When G r =G t =1, L f = (4π d/λ) 2 = (4π f c d/c ) 2 where c = λ f c (c is speed of light) and f c is the carrier frequency. CS