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What’s up with MIMO? – Part Deux

In my first episode on MIMO (What’s up with MIMO), I introduced the concept of multiple antennas and the home audio analogy. In this example, I discussed one particular mode of MIMO that transmits the same stream across all antennas, and makes use of multipath to increase the chance of correctly decoding the received signal -- thus lowering the bit error rate. This method exploits what is known as spatial diversity, which is one of the simplest method to achieve MIMO gains. But MIMO is much more than that.There is also another mode of MIMO that is ideal in environments that experience few errors, which is called spatial multiplexing. In this mode, the transmitter would treat each antenna as a “separate channel” -- much like you would experience in a 5 channel home audio system. These systems send the audio stream that provides directionality to the whole experience. Of course, in MIMO systems, we don’t care much about directionality, but we certainly care about sending multiple streams to increase the overall effective bandwidth. So for instance, if an AP wanted to send a packet, it could split the packet up into three “chunks”, and transmit each “chunk” across each of its three antennas simultaneously. This would effectively reduce the transmit time -- and therefore increase the overall capacity of the network.

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6 Comments.


  1. Pat Calhoun

    William, it appears from the questions you are asking that you are trying to customize your AP 1250. I should note that Cisco does not endorse unauthorized modifications – further, there are certain FCC regulations that you should be aware of should this be your intent. It may be worth having a conversation with your Cisco wireless systems engineer.In any case, to address your specific questions:1. Does the cisco 1250 series WAP use both techniques? Do you have to configure it to use one method vs. the other?The AP 1250 uses the basic mandatory MIMO technique in the 11n standard, it has a combination of both spatial diversity and multiplexing. The method used depends upon various channel feedback factors, including signal strength, and is handled on a per-station basis. Given Cisco’s focus on improving the overall wireless experience, I would expect this to be an area of significant innovation.2. When it comes to spatial antenna diversity, is more better? Would greater spacing on 1250 antennas always be a good thing?Spatial and antenna diversity are different, but related. The main contributions to performance gains from spatial antenna diversity include independent multipaths, number of antennas and, as you mention, antenna spacing. The higher the antenna count, the better. Similarly, the wider the antenna separation, the more the paths become uncorrelated, which increases diversity. It is, however, important to note that performance improvements do not continue to increase as either one of these factors are increased. You end up at a point where you get diminishing return on performance improvements. 3. How would this be achieved, given that the only 2.4Ghz antennas officially for use w/ the 1250 are the AIR-ANT2430V-R triple-omni, and rubber-duck style antennas.I’m not quite sure where you got this information. The AP 1250 can be fitted with any TNC antenna. That said, I would point you to the Cisco webpage to look at the antennas we recommend, as well as their technical specifications: http://cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008022b11b.html. Again, I would remind you on the FCC guidelines before you opt to deploy any custom antennas.4. Is the dB loss from a TNC extender cable significant?Losses incurred by cables are inevitable and an unfortunate fact of life. But the amount of loss depends on the quality of cables and the freq. You can find the dB losses from our ultra-low loss cables in the link above. We have a 100 ft cable at Again, I would remind you on the FCC guidelines before you opt to deploy any custom antennas.4. Is the dB loss from a TNC extender cable significant?Losses incurred by cables are inevitable and an unfortunate fact of life. But the amount of loss depends on the quality of cables and the freq. You can find the dB losses from our ultra-low loss cables in the link above. We have a 100 ft cable at <5 dB loss, which should not sacrifice any performance.5. When you look at the 2.4Ghz TNC connectors on the 1250, which one is the receive-only connector?It would be the middle of the three.”

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  2. re: 802.11n spatial diversity vs spatial multiplexing:A bunch of questions:Does the cisco 1250 series WAP use both techniques? Do you have to configure it to use one method vs. the other?When it comes to spatial antenna diversity, is more better? Would greater spacing on 1250 antennas always be a good thing?How would this be achieved, given that the only 2.4Ghz antennas officially for use w/ the 1250 are the AIR-ANT2430V-R triple-omni, and rubber-duck style antennas.Is the dB loss from a TNC extender cable significant?When you look at the 2.4Ghz TNC connectors on the 1250, which one is the receive-only connector?

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  3. I read your article that is really a nice work.Can you tell me what the algorithm you are using in your technology

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  4. Pat Calhoun

    I would recommend the following article:An Overview of MIMO Communications — A Key to Gigabit WirelessPAULRAJ, A.J.; GORE, D.A.; NABAR, R.U.; BOLCSKEI, H.;Proceedings of the IEEE, Volume 92, Issue 2, Feb 2004 Page(s):198 – 218

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  5. When it comes to spatial antenna diversity, is more better? Would greater spacing on 1250 antennas always be a good thing?How would this be achieved, given that the only 2.4Ghz antennas officially for use w/ the 1250 are the AIR-ANT2430V-R triple-omni, and rubber-duck style antennas.

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  6. Has Cisco released any adapters in the market to receive data from cisco aironet 1250 AP?

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