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Wireless QOTD April 8th

Discussion in 'Wireless' started by tripwire45, Apr 8, 2004.

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Multipath is a form of RF interference typical of outdoor wireless links. There are four solutions

  1. A. Antenna Diversity - not active which uses multiple antennas on a single input

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  2. B. Switching Diversity which uses multiple antennas on multiple receivers

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  3. C. Antenna Switching Diversity - active which uses multiple antennas on multiple inputs and a single

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  4. D. Phase Diversity which adjusts the phase of the antenna to the phase of the signal

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  1. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    I'm posting this early on the 8th (middle of the night for you...early evening for me) to give everyone a longer shot at it. We'll see how this works. Answer later.
     
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  2. Phil
    Honorary Member

    Phil Gigabyte Poster

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    Firstly I appreciate the earlier posting Trip, gives me a chance to have a go before going to work.

    I reckon A.

    Having done a bit of digging I found out multipath interference is caused by a signal bouncing off the environment around the transmitter and arriving at the receiver at different times, causing crc failures and the signal to have to be retransimitted which slows down the link. 802.11a and 802.11g networks use OFMD which uses lots of narrow subchannels and and so is far less susceptible to multipath than 802.11b which uses DSSS. DSSS transmits information continuously over a relatively wide channel, this leaves enough room for lower frequency elements of the signal to reflect off obstacles much differently than the higher frequency elements of the signal.

    The most common mitigation of multipath appears to be a single reciever with multiple antennae positioned sepparately so that if one is receiving a degraded signal the other should be be getting a better signal. The receiver only listens to one antennae at a time otherwise the differences in signal between them will cause it's own multipath affect.

    sources:
    wifiplanet, Multipath a Potential WLAN Problem
    Cisco, Multipath and Diversity

    Go on, tell me after all that I got the wrong answer anyway :D
     
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  3. mattwest

    mattwest Megabyte Poster

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    I was drawn to B (complete guess) but after reading Phil's post A makes sense....

    So one of those two! 8)

    Looking forward to finding out.

    (Lets hope we get more responders today!)
     
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  4. SimonV

    SimonV Petabyte Poster Administrator

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    I dont have all that much time to research today so think I'll have to go with A.
     
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  5. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    Hint: remember, all of the listed methods can be solutions for multipath. I'm looking for the most typically used solution. :wink:
     
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  6. mattwest

    mattwest Megabyte Poster

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    Still sticking with A but B is niggling at me.... :roll:
     
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  7. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    Time's up. Correct answer is C: Antenna Switching Diversity - active. Although all of the methods listed can be used as Multipath solutions, this one is used most often by WLAN vendors. A: Antenna Diversity -not active is used only rarely.

    Basically, multipath occurs because an RF signal propagates (gets wider) with distance travelled. If it travels far enough, the signal will not arrive at the receiving antenna before it strikes and reflects off of other objects such as metal roofs, water, etc...Often the waves reflected waves will arrive somewhat out of phase from the main signal resulting in a number of unfortunate issues (which will probably be the subject of a future QOTD).

    Antenna Switching Diversity- active uses multiple antennas on multiple inputs bringing the signal into a single receiver. The receiving radio is constantly sampling the signal quality of what each antenna receives and selects the best quality signal to accept. This helps to keep reception quality high and to minimize the effects of multipath.

    More tomorrow.
     
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