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Introduction

Antennas and Propagation ( William Stallings, “Wireless Communications and Networks” 2nd Ed, Prentice-Hall, 2005, Chapter 5). Introduction. An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy into space

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Introduction

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  1. Antennas and Propagation(William Stallings, “Wireless Communications and Networks” 2nd Ed, Prentice-Hall, 2005, Chapter 5) by Ya Bao http://eent3.sbu.ac.uk/staff/baoyb/acs

  2. Introduction • An antenna is an electrical conductor or system of conductors • Transmission - radiates electromagnetic energy into space • Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception

  3. Types of Antennas • Isotropic antenna (idealized) • Radiates power equally in all directions • Dipole antennas • Half-wave dipole antenna (or Hertz antenna) • Quarter-wave vertical antenna (or Marconi antenna) • Parabolic Reflective Antenna

  4. Radiation Patterns • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • Beam width (or half-power beam width) • Measure of directivity of antenna

  5. Radiation patterns

  6. Three-dimensional antenna radiation patterns. The top shows the directive pattern of a horn antenna, the bottom shows the omnidirectional pattern of a dipole antenna.

  7. or as separate graphs in the vertical plane (E or V plane) and horizontal plane (H plane). This is often known as a polar diagram

  8. outdoor enclosure featuring a wide band 2.5GHz panel antenna

  9. Other antennas Helical Antenna Patch (microstrip) antenna Multiband antenna: for GSM 900+GSM 1800+GSM 1900+Bluetooth; or GSM and 3G

  10. 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

  11. 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

  12. Propagation Models • Ground Wave (GW) Propagation: < 3MHz • Sky Wave (SW) Propagation: 3MHz to 30MHz • Effective Line-of-Sight (LOS) Propagation: > 30MHz

  13. Ground Wave Propagation • Follows contour of the earth. • Can propagate considerable distances. • Frequency bands: ELF, VF, VLF, LF, MF. • Spectrum range: 30Hz ~ 3MHz, e.g. AM radio.

  14. Sky Wave Propagation • Signal reflected from ionized layer of upper atmosphere back down to earth, which can travel a number of hops, back and forth between ionosphere and earth’s surface. • HF band with intermediate frequency range: 3MHz ~ 30MHz. • e.g: International broadcast.

  15. Line-of-Sight Propagation • Tx. and Rx. antennas are in the effective ‘line of sight’ range. • Includes both LOS and non-LOS (NLOS) case • For satellite communication, signal above 30 MHz not reflected by ionosphere. • For ground communication, antennas within effective LOS due to refraction. • Frequency bands: VHF, UHF, SHF, EHF, Infrared, optical light • Spectrum range : 30MHz ~ 900THz.

  16. LOS calculations dr • What is the relationship between h and d ? do optical horizon radio horizon earth h • For optical LOS: • where • h = antenna height (m) • d = distance between • antenna and horizon (km) • K = adjustment factor for • refraction, K = 4/3 • For effective or radio LOS:

  17. Line-of-Sight Equations Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3 • Maximum distance between two antennas for LOS propagation:

  18. LOS Wireless Transmission Impairments • Attenuation and attenuation distortion • Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise

  19. Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion

  20. Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength • d = propagation distance between antennas • c = speed of light ( 3  108 m/s) where d and  are in the same units (e.g., meters)

  21. Free Space Loss • Free space loss equation can be recast:

  22. Free Space Loss • Free space loss accounting for gain of other antennas can be recast as

  23. Categories of Noise • Thermal Noise • Intermodulation noise • Crosstalk • Impulse Noise

  24. Noise (1) • Thermal noise due to thermal agitation of electrons. • Present in all electronic devices and transmission media. • As a function of temperature. • Uniformly distributed across the frequency spectrum, hence often referred as white noise. • Cannot be eliminated – places an upper bound on the communication system performance. • Can cause erroneous to the transmitted digital data bits.

  25. Noise (2): Noise on digital data Error in bits

  26. Thermal Noise • The noise power density (amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor) is: • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803  10-23 J/K • T = temperature, in kelvins (absolute temperature) • 0oC = 273 Kelvin

  27. Thermal Noise • Noise is assumed to be independent of frequency • Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts (dBW),

  28. Noise Terminology • Intermodulation noise – occurs if signals with different frequencies share the same medium • Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies • Crosstalk – unwanted coupling between signal paths • Impulse noise – irregular pulses or noise spikes • Short duration and of relatively high amplitude • Caused by external electromagnetic disturbances, or faults and flaws in the communications system

  29. Signal to Noise Ratio – SNR (1) • Ratio of the power in a signal to the power contained in the noise present at a particular point in the transmission. • Normally measured at the receiver with the attempt to eliminate/suppressed the unwanted noise. • In decibel unit, where PS = Signal Power, PN = Noise Power • Higher SNR means better quality of signal.

  30. Signal to Noise Ratio – SNR (2) • SNR is vital in digital transmission because it can be used to sets the upper bound on the achievable data rate. • Shannon’s formula states the maximum channel capacity (error-free capacity) as: • Given the knowledge of the receiver’s SNR and the signal bandwidth, B. C is expressed in bits/sec. • In practice, however, lower data rate are achieved. • For a fixed level of noise, data rate can be increased by increasing the signal strength or bandwidth.

  31. Expression of Eb/N0 (1) • Another parameter that related to SNR for determine data rates and error rates is the ratio of signal energy per bit, Eb to noise power density per Hertz, N0; →Eb/N0. • The energy per bit in a signal is given by: • PS = signal power & Tb = time required to send one bit which can be related to the transmission bit rate, R, as Tb = 1/ R. • Thus, • In decibels: – 228.6 dBW

  32. Expression of Eb/N0 (2) BER versus Eb/N0 plot • As the bit rate R increases, the signal power PS relative to the noise must also be increased to maintain the required Eb/N0. • The bit error rate (BER) for the data sent is a function of Eb/N0 (see the BER versus Eb/N0 plot). • Eb/N0 is related to SNR as: Higher Eb/N0, lower BER where B = Bandwidth, R = Bit rate

  33. Wireless Propagation Mechanisms • Basic types of propagation mechanisms • Free space propagation • LOS wave travels large distance with obstacle-free • Reflection • Wave impinges on an object which is large compared to the wave-length  reflection Lamp post diffraction • Diffraction • Occurs when wave hits the sharp edge of the obstacles and bent around to propagate further in the ‘shadowed’ regions – Fresnel zones. • Scattering • Wave hits the objects smaller than  itself. e.g. street signs and lamp posts. scattering

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