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How to Correctly Orient Outdoor WiMax MIMO Antennas

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  • How to Correctly 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 not necessarily the best approach. 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 or sufficiently separated if oriented in the same direction. 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.

    NOTE that the above does not mean that you should necessarily point the antennas to different WiMax BTS (Wimax base-stations). A WiMax CPE/modem can ONLY be connected to one BTS at a time so the idea that pointing to different towers will improve performance doesn't make much sense**. In fact this idea is the same for any MIMO implementation e.g 802.11n WiFi. See Correct Placement of 802.11n MIMO In-Building Antennas for more discussion on this.

    The fact that MIMO loves obstructions and multipath at first glance seems counter intuitive but if you think about what the goal is it makes sense somewhat. The are different flavors of MIMO. Open loop MIMO used in 2.5GHz Clearwire WiMax modems 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 across 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 that are in close proximity to each other in the same direction doesn't make much sense since if that path experiences RF fades then all the data streams are equally affected.

    Because of the reasons above omnidirectional wimax antennas are actually a better choice than their directional counterparts (such as the WiMax Yagi or Wimax Panel) in locations that experience plenty of multipath reflections (i.e areas where you cannot guarantee a clear line of sight to the WiMax BTS) even in spite of the fact that Omnis typically have lower possible antenna gain than directional antennas. The reason again is that being able to receive signals from multiple directions allows for a better spatial redundancy and ensures the decorrelation of the signals received by the different antennas attached to the mobile device. Here is a generic example setup that illustrates this idea: http://www.rfwel.com/support/tech-dr...ENERIC_USB.pdf



    ** Note that even if you are targeting network redundancy as the primary goal this is still not the best approach based on how MIMO works. i.e if you plan have a strong signal from a different tower for a good handoff when the primary tower goes down. The reason is because MIMO needs the multiple datastreams from a single tower each time. A better approach to achieve this redundancy would be either to add another pair of antennas with a splitter/combiner (problem is insertion loss of as much as 3dB for 50% power split) or even better to use a multi-WAN router and another modem with antennas oriented to a different tower. The latter solution has the added advantage of improving speed/latency.
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