Ch 2

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Author:
Tjr31
ID:
32033
Filename:
Ch 2
Updated:
2010-08-31 11:20:47
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cwna
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cwna
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  1. Phase
    • The relationship between two or more signals
    • that share the same frequency
    • Peaks of 2 signals with
    • the same frequency are in exact alignment at the same time, are said to be in phase
    • Signals that have 0 degree
    • phase separation combine their amplitude resulting in much greater signal
    • strength, or twice the amplitude
    • If 2 RF signals are 180
    • degrees out of phase they cancel each others out and the received signal is
    • null
  2. Wave Propagation
    • The movement or motion of RF waves as they pass through the medium (the air)
    • Maybe impeded or completely stopped by materials in the path of the signal
    • RFsignals broaden as it travels farther away from the antenna
  3. Amplitude
    • The height, force or power of a wave
    • In WLAN, amplitude is referred to as either transmit amplitude or received amplitude
  4. radio frequency signal
    an electrical alternating current

    Generated by a transmitter

    Sent through a conductor/cable

    • RF electromagnetic signals travel using a
    • variety of movement behaviors



    Radiated from an antenna
  5. Absorption
    • When an RF signal is not allowed to pass
    • through an object, is not reflected by it and is not diffracted around it, the
    • signal will be absorbed to some extent by the object.

    • •Most materials will absorb a varying amount
    • of RF energy based upon material type.
  6. Reflection
    • When an RF signal strikes an object that does
    • not absorb the signal and does not cause the signal to be diffracted around the object, the signal is reflected by the object.

    •This is much like light striking a mirror and bouncing back toward the source.
  7. Two types of reflection
    • Sky reflection: occurs in frequencies below 1GHz (signal had a
    • very large wavelength)

    • –Microwave reflection: occurs between 1GHz and 300 GHz (smaller
    • wavelength) (WLAN)
    • In outdoor environment: microwave can reflect off large objects
    • and smooth surfaces such as buildings, roads, bodies of water and even earth surface
    • Indoor environment: microwave reflects off smooth surfaces such
    • as doors, walls and file cabinets
  8. Scattering
    Occurs when the same RF wave is reflected multiple times

    Occurs when electromagnetic signal’s wavelength is larger than pieces of whatever medium the signal is passing through

    • Causes multiple instances of the same signal
    • and degradation of signal quality
  9. Refraction
    A bending of an RF signal as it passes through a medium

    Happens when RF passes through a medium with different density, causing the direction of the wave to change

    Causes: Water vapor, changes in air temperature, and changes in air pressure
  10. Diffraction
    The bending of an RF signal as it wraps around an object

    Caused by partial blockage of the RF signal such as small hill or a building that sits between sender and receiver

    Receiver radio behind the obstruction is known as RF shadow

    Important when selecting antenna locations


    • This is also like Sun light bending around a
    • building. Yes, there will be RF shadows.
  11. Loss
    Weakening of the RF signal

    Also called Attenuation

    • Can be caused by several things
    • –Passing through various mediums

    –Cable

    –Impedance mismatch

    –Lightning arrestors

    –Connectors

    –Signal attenuators
  12. Free Space Path Loss
    • •Caused by a broadening of the wave as an RF
    • signal propagates (beam divergence)

    •No external impedance required

    • •The farther the signal travels, the weaker it
    • becomes
  13. Multipath
    • Multiple instances of the same signal
    • arriving out of phase at the intended receiver caused by reflection or some
    • other source of propagation interference

    • •Propagation behavior of reflection is the
    • main cause of high multipath environments

    • •Outdoor: multipah can be caused by a flat road, large body of
    • water, building or atmospheric conditions

    • •Indoor: reflected signals can be caused by
    • long hallways, walls, desks, floors, etc


    • •Due to differences in phase of the multiple
    • paths, the combined signal will often attenuate, amplify, or become corrupted (Ray fading)
  14. Four possible results in multipath:
    –Downfade:
    • Decrease signal strength.
    • Multiple RF signal paths arrive at the receiver at the same time and are out of
    • phase with the primary wave. Phase differences between 121 and 179 degrees
  15. multipath -
    Upfade:
    • Increased signal strength.
    • Multiple RF arrive at the same time in phase or partially out of phase (waves
    • between 0 and 120 degrees), never stronger than original signal
  16. multipath
    Nulling:
    signal cancellation. Multiple RF arrive at the receiver at the same time and are 180 degrees out of phase with the primary wave
  17. multipath
    Data corruption:
    • Difference in time between
    • primary RF and the reflected signals (delay spread) with multiple reflected
    • signals may cause problems demodulating RF signals information(bits may overlap with each other)
  18. multipath
    Solution:
    reduce transmit power or lower gain antenna
  19. Gain
    • •An increase in a signals strength or
    • amplitude

    •Two types: passive gain, active gain

    • •Active gain: Usually caused by the use of an
    • amplifier (requires external power source)

    •Antennae are passive amplifiers

    •Do not require external power source

    • •Antenna focus the signal more powerfully in
    • one direction than another
  20. RF
    • •RF electromagnetic signals travel using a
    • variety of movement behaviors

    •Propagation behaviors

    –Absorption

    –Reflection

    –Scattering

    –Refraction

    –Diffraction

    –Amplification

    –Attenuation
  21. RF Characteristics
    Wavelength

    Frequency

    Amplitude

    Phase
  22. Wavelength
    The distance between two successive peaks or valleys of a wave pattern
  23. Frequency
    • The number of times a specified event occurs
    • within a specified time period

    §Measured in Hz (1 Hz =1 cycle per second)
  24. Relationship between
    Wavelength and Frequency
    Inverse relationship

    •l=c/f, c=speed of light (300, 000,000 m/sec), f= frequency

    • •RF with smaller wavelength will attenuate faster
    • to an amplitude level below the sensitivity of a receiver radio

    •Lower frequency signals with the larger wavelength will be heard from a greater distance than the higher frequency with the smaller wavelength

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