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WiMax Antenna Primer - Using One External Antenna vs Two Antennas


  • WiMax Antenna Primer - Using One External Antenna vs Two Antennas

    Here is an excellent primer from Motorolas Networks & Enterprise Group that addresses a lot of the issues we have been discussing here on MIMO multiantenna transmission techniques and when you can use one antenna instead of two antennas.

    WiMAX Antennas Primer - A guide to MIMO and beamforming

    The key takeaway is that when a CPE (customer premise equipment) has multiple antennas and is using MIMO Matrix A with Space-Time Block Coding, if an external directional high-gain wimax antenna is mounted to provide a line-of-sight (LOS) or near-line-of-sight (nlos) to a base-station, then one antenna could work just as well as two. This is because for MIMO Matrix A the datastream is replicated and separately encoded such that if one parallel path has a good enough SNR then the MIMO benefit of a second path is not as high. Same goes when using antenna diversity techniques such as MRC (Maximal ratio combining) that combines the signal from the two antennas based on their SNR's.

    However if you are using MIMO Matrix B then you do need both antennas always since the datastream is divided up first then multiplexed onto the different transmit antennas (hence the name SM-MIMO - spatial multiplexing since you are multiplexing the space-domain).

    Finally when the MIMO CPE radio uses closed-loop multiantenna techniques such as beamforming including EBF (Eigen Beamforming) and MRT (Maximum Ratio Transmission) multiple receive antennas are important to boost performance mainly for external directional antennas that can be physically separated. The small integrated patch antennas don't provide enough flexibility to modulate the amplitude and phase array of the radiated output signal. However by sending measured channel information back to the transmitter they can affect the transmitter radiation characteristics optimally to avoid interference. A single receive external antenna is just as good as using the integrated antennas.

    The Clearwire Motorola CPE i150 for example supports Maximum-
    Ratio Combining (MRC), Beamforming, Open-loop MIMO Matrix A (STBC) and Matrix B (SM-MIMO).

    Simple Summary on When two External Antennas are needed and how to determine if they are needed:
    when in doubt and on a budget simply attach one external antenna and observe the performance boost and if your RSSI/CINR are acceptable and stable then you need not do more. If you don't have budget constraints and don't want to worry about whether you are getting the best performance simply start with two antennas.
    • Do you have a clear line-of-sight (LOS) to a WiMax BTS? If you do then you might getaway with using one antenna provided your outdoor RSS is very strong. If you held your WiMax modem at the outdoor location where you would mount your antenna, typically on the roof, and your RSSI/CINR is very good AND you will not need to run a very long cable to get into the building (a long cable would lead to too much attenuation of the strong outdoor signal).
    • If you cannot achieve a clear LOS for example have a lot of adjacent buildings shielding the LOS to the BTS then two antennas will improve performance. Obstructions cause multipath propagation which helps with spatial (space) redundancy since it guarantees that the signals received by the multiple antennas are very uncorrelated - which is a very good thing for MIMO**. In this case consider an omnidirectional antenna or even better a directional antenna on the primary antenna port and omnidirectional on the secondary. Here is an illustration of the latter:
    • If you are primarily interested in the reverse-link channel (uplink) for example have an application where you are ONLY interested in the upload e.g Video Surveillance backhaul, you don't need to worry about a secondary antenna. This is because MIMO for most modems currently in production is ONLY implemented in the forward channel (downlink). This is also the reason that it's VERY important that if you are only using one external WiMax antenna you put it on the Primary antenna port otherwise your uplink performance could be severely degraded.
    • A frequently encountered question:
      I have one outdoor antenna which improved my performance but still not to what I would like. How do I know if I need a second antenna?
      A simple experiment is to take the modem outdoors at the approximate location where you have mounted the outdoor antenna. If the modem without the antenna hookup performs better than when it's indoors with the antenna hookup then you likely need a second antenna. The idea is that in the when the modem is outdoors without an external antenna hookup both the internal antennas are receiving a strong signal so MIMO works fine. When the modem is indoors with only one antenna hookup (and this must be in the primary antenna port) then only the primary antenna is receiving a strong signal but the secondary is receiving no signal or a very low SNR signal so MIMO doesn't really work.

    **for those techy's out there - it can be shown that MIMO diversity performance gain is related to the rank of the channel matrix.
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