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Correct Placement of 802.11n MIMO In-Building Antennas


  • Correct Placement of 802.11n MIMO In-Building Antennas

    I am wiring a concrete building with 3” to 5” concrete walls/floors with steel reinforcing rebar. We are drilling holes through some of these walls so that we could place the Antennas through but NOT the access points!

    We are going to use 802.11n MIMO routers and access points, which each have 3 external antennas. We want to remove the external antennas and attach 6 meter long extension cables to the antennas so that we can thread the antennas through the concrete walls into different rooms, I repeat DIFFERENT rooms. Each individual antenna would be no where near each other, at least 20 feet away from each other. This way, each access point, which has 3 external antennas, would have 1 antenna each into 3 different rooms all connected via external cable back to a single access point.

    Question is, considering these are MIMO access points, would this still work? I don't care if MIMO works or not, the goal is to get past the concrete walls without needing to buy 1 access point for each individual room. This will decide how we decide to wire the entire building.

    Unfortunately, No, this will not work reliably. Notice i say “reliably” and NOT that it will not work at all since I believe this would work sometimes albeit erratically. The main factor that determines how well this would work is the number of concurrent users in each room. If you only have one user at a time then this would be ok although you would not be getting the benefits of MIMO. If you have many concurrent users in each room then serious problems will arise especially if the radiation from other rooms’ antenna does not penetrate other rooms at all. Would not recommend this.

    The main purpose of MIMO or for that matter any antenna diversity technique (recall even before MIMO became widespread you could still find AP’s with 2 antennas which was for antenna diversity) is what’s called spatial multiplexing (taking advantage of the spatial or space domain for redundancy). The frequency domain and time domain are already extensively used in error correction coding but using the spatial domain is relatively new. So the idea is to mitigate against obstructions and multi-path propagation effects by creating different propagation paths from the transmitter to the receiver. This improves the effective signal-to-noise ratio (SNR) which allows for a higher modulation type such as 64-QAM to be used with a low Bit error rate (BER) and hence a higher datarate (or longer range for the same datarate). So simply put the MIMO antennas are intended to work together to carry data from a single transmitter vs being split up as you propose.

    The second problem you will encounter has to do with what’s know as a “hidden-node-problem”. If you assume that the radiation from each antenna does not propagate beyond the room that it’s installed in then when one rooms client device starts to transmit it doesn’t know if another is also trying to transmit. 802.11g uses an access control scheme called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) which cannot work in this case. Since the AP can only “talk” to one client at a time this will cause problems (lowered throughput due to collisions & repetitions). This really has nothing to do with MIMO and you would get the same problem if for example you only had one antenna port and used a splitter to connect two antennas in different rooms. For some AP's you can use RTS/CTS (Request to Send/ Clear to Send) parameters to somewhat overcome this but at lowered throughput.

    Finally since good quality 802.11g AP’s are never that expensive to begin with the recommended method would be to get one that utilizes Power-over-Ethernet (POE) and simply just run CAT5 cable to the different rooms and have a dedicated AP per room. For example: Engenius EAP-3660 600mW 802.11g Indoor AP. In fact this might cost less than a good quality low loss coaxial cable, high gain antenna & mounts. Other techniques such as broadband over powerline (BPL) or simply having higher power AP’s + wifi directional antennas + 2.4GHz amplifiers can work as well.

    Here are some medium range site setup drawings that can give you more ideas on this:


    This Q/A is really targeted for 802.11n WiFi MIMO but the case for 802.16e WiMax MIMO is a little different. The difference in this case is that for 4G WiMax MIMO is between the macrocellular WiMax BTS and the modem device and is hard to isolate different antennas from a single WiMax BTS.

    In the case of WiMax it is in fact advantageous to isolate the antennas from each other to make the RF channel from the BTS to either antenna more uncorrelated which improves MIMO performance. See for more on this.

    NOTE though that for MIMO to work in WiMax case you need to receive the multiple data streams from the same WiMax BTS so it doesn't benefit you to orient the multiple antennas to different BTS's (i.e the modem device can only be connected to one BTS at a time).
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