RF Considerations for wireless communications by pptfiles

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									    RF Considerations for
   wireless communications



Jose Antonio Echenique
        Agenda
   Introduction to wireless communications
       Wireless link implications
       Medium: the radio spectrum

   The three main parameters that define radio–frequency:
       Over-the-air data rate
       Receive Sensitivity
       Transmit power

   Other Considerations

   Barriers to Future Growth
        Introduction
   Wireless Communication System: Any electrical
    communication system that uses a naturally occurring
    communication channel, such as air, water, earth.

   Examples:
       Sonar
       Broadcast: Radio, TV, pagers, satellite TV, etc.
       Two Way: walkie talkie, cell phones, satellite phones,
       Wireless Local Area Networks, etc.
      Wireless link implications
   Communications channel is natural (air)
     poor   quality: fading, shadowing, weather, etc.


   Medium regulated by governments
     frequency   allocation, licensing, etc.


   Security issues
      Medium: the radio spectrum
   Wireless communications use the electromagnetic
    spectrum, which is regulated by government institutions
    such as the Federal Communications Commission
    (FCC).

   Regulations specify what bands of frequency can be
    used for different applications. For instance: FM radio
    has 88-108MHz and AM radio has 540-1600KHz .
          Medium: the radio spectrum
  SOUND            RADIO                     LIGHT               HARMFUL RADIATION




            VHF = VERY HIGH FREQUENCY                           3G CELLULAR          4G CELLULAR
            UHF = ULTRA HIGH FREQUENCY                             1.5-5.2 GHz          56-100 GHz
            SHF = SUPER HIGH FREQUENCY      1G, 2G CELLULAR
            EHF = EXTRA HIGH FREQUENCY             0.4-1.5GHz




Frequency and wave length:  = c/f :wave length , speed of light c  3x108m/s, frequency f
                                                                                 SOURCE: JSC.MIL
      RF CONSIDERATIONS
   The three main parameters that define radio–
    frequency:
     Over-the-air
                 data rate
     Receive Sensitivity
     Transmit power


          Range is a result of these three RF parameters and can be
           used to define them
     RF CONSIDERATIONS
   Over-the-air data rate
     Determined by data throughput requirements
     Speed calculation:
          In point-to-point systems:
               RF data rate = (Dpl+Dao+Dro)x(1+rt)/time
          In a multipoint application, unless a polling scheme
           or time-division multiple access (TDMA) scheme is
           used, the calculation is more complicated.
      RF CONSIDERATIONS
   Over-the-air data rate (Example)
     Assume  a remote unit needs to send 1000 bytes of
      payload data in a response to a 2-byte access point
      command every 75 milliseconds:
          Dao would be 32 bits and Dro 80 bits
          The total amount of data for both transmissions must occur in
           75 milliseconds
RF data rate =
  [((16b+32b+80b)+(8000b+32b+80b))x1.1]/0.075sec
  = 120.853 Kb/sec
      RF CONSIDERATIONS
   Receive Sensitivity
     Indicates  the level of signal strength that must be
      present to correctly receive data at a specified bit-
      error rate.
     Receive Sensitivity = Nt + Ns + 10log(BW) + SNRmin
             Nt is the thermal noise floor
             Ns is the system noise figure
             BW is the symbol rate
             SNRmin is the minimum signal-to-noise- ratio required for a
              given bit-error rate
      RF CONSIDERATIONS
   Transmit power
     Itis usually driven by regulatory and power-
      consumptions considerations
              For example, FCC allows up to 1 W of transmit power in
               the United States in the 2.4 Ghz band
      Other Considerations
   Antenna Selection

     Directionality
        Omni (360 degree coverage) directional

        Directional (limited range of coverage)



     Gain
        More gain means more coverage



     Polarization
        Other Considerations
   Modulation Techniques
                                           1   0   1

   Amplitude Shift Keying (ASK):
       very simple                                    t
       low bandwidth requirements
       very susceptible to interference   1   0   1

   Frequency Shift Keying (FSK):
       needs larger bandwidth                         t


                                           1   0   1
   Phase Shift Keying (PSK):
       more complex
                                                       t
       robust against interference
      Range Depends On...
   Frequency
   Transmit power
   Radio sensitivity
   Processing gain from access technique and redundancy
   Interference effects
    Barriers to Future Growth
 Irreducible size of antennas
 Rising level of RF emissions -
  interference problems and safety concerns
 Finite spectrum
 Lack of standards and interoperability of
  hardware
                   THANK YOU …


Reference:

http://www.ce-mag.com/archive/02/Spring/cutler2.html

“Unlicensed Wireless Data Communications, Part II:
Specifying RF Parameters” by Tim Cutler

								
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