8 posts by 6 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: January 15, 2008:
  • Hi All

    Could someone please advise me on the advantages and disadvantages of have one or two antenna on an AP.

    Many thanks


  • ==Advantage==
    More data from different direction can come through.

    Wider coverage.

    Eliminate having two AP.

    Make sure channel selection are space apart so that they don't conflict.

    Making sure that coverage don't overlap (else, defeats the purpose of having two antennas).

    Making data flow to the right antenna and not the other antenna.

    These are some of the pros and cons, but do-able.

  • One antenna is used for TX and RX, the other is used for diversity RX.

    Using multipel receive antennas provides approximately 3 db of gain on the receive side of the link budget.

  • Be careful about what you read! 8-O

    For most APs, the purpose of a second antenna is to provide antenna diversity, to combat multipath.

    The AP will only transmit or receive from one antenna at a time, so throughput will NOT increase.

    Also, Cisco specifically states in their documentation not to use multiple antennas to provide increased coverage. The point is for the two antennas to cover the same basic service area and let the AP decide which antenna is picking up the best signal.

    Things change once you start talking about MIMO APs, so make sure you understand the difference.

  • I believe I said the exact same thing that you said. The only clarification is that the throughput will increase by using multiple receiove antennas at the AP(in the down link, not the up link).

    If you look at the link budget for the AP you will see the receive diversity gain, thus the improved throughput.

  • I?¡é?€??d have to disagree with the last poster if you are only considering 802.11 a/b/g?¡é?€?|Only one antenna is used for both TX/RX and the other is ignored. If the AP detects a multipathing condition, the AP will revert to the antenna with the best SNR to minimize or eliminate the mulitpath condition. What you?¡é?€??re describing is MIMO which is part of the 802.11n standard.

  • This is called "selective combining" and is assigned a 3 db gain (typically). If both antennas were processed this would be "maximum ratio combining" and would be assigned an effective gain of approximnately 5 db.

    There are several types of MIMO, spacial beamforming and spacial mutiplexing. MIMO uses DSP processing of each antenna to eliminate multipath and combines each of the processed received signals (using maximum ratio combining).

    Here is a good article explaining MIMO.

  • By (Deleted User)

    If you are not talking about 802.11n, then the 2nd antenna on an AP is to combat multipath. It is called ?¡é?€??antenna diversity?¡é?€??. Since in an indoor environment RF waves are reflected off objects a single antenna may not be in the optimal physical orientation to receive the best signal. Having two antenna separated by a half wavelength to several wavelengths increase the possibility that one of the antennas will be positioned in an advantageous orientation for the incoming RF wave.
    The AP can then make a decision on a frame by frame basis on which antenna to use to communicate with a station.

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