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How to Orient outdoor WiMax MIMO Antennas

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  • How to Orient outdoor WiMax MIMO Antennas

    The first reaction is to orient both antennas to the same direction where the received signal strength is the strongest. This is WRONG. Well unless you have a clear line of sight to the wimax base station in which case open-loop MIMO is not needed/useful anyway.

    To take advantage of MIMOs spatial redundancy the antennas need to be oriented in different directions. MIMO works by using the space-domain to mitigate against multipath fading and combat errors in the channel. To get the best benefit the different channels between the separate transmit antennas and receive antennas need to be uncorrelated. The fact that MIMO loves obstructions and multipath at first glance seems counter intuitive but if you think about what the goal are it makes sense somewhat. The are different flavors of MIMO. Open loop MIMO used in 2.5GHz Clearwire WiMax includes Matrix-A or Space-time block coded mimo which simply replicates the transmit data and sends it in parallel coded streams or Matrix-B (SM-MIMO) which multiplexes the transmit data accross the several transmit antennas. In either of these cases the goal of MIMO is that if one stream experiences fading then you can still recover most of the information using the other stream. This is why orienting the antennas in the same direction doesnt make much sense since if that path experiences RF fades then then all the data streams are equally affected.
    KF7RCQ
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