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Why MIMO if antennas cannot transmit and receive simultaneously?

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  • 1.  Why MIMO if antennas cannot transmit and receive simultaneously?

    Posted Mar 03, 2017 05:42 PM

    Hy guys,

     

    My question is very simple. If antennas can only either transmit or receive at a time, why MIMO devices have separate antennas for transmitting and receiving? They are not used simultaneously.

     

    Regards,

    Julián



  • 2.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    EMPLOYEE
    Posted Mar 03, 2017 05:48 PM

    Multiple antennas can transmit and receive at the same time, colloquially we can say each antenna is a 'chain'. This has value on multiple fronts, I highly suggest you read http://www.arubanetworks.com/assets/wp/WP_80211acInDepth.pdf



  • 3.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    EMPLOYEE
    Posted Mar 03, 2017 05:50 PM

    It is in the wikipedia article here, first sentence:  https://en.wikipedia.org/wiki/MIMO

     

     



  • 4.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    Posted Mar 03, 2017 05:58 PM

    Hi guys,

     

    Maybe I didn't explain very well. I mean, if I am transmitting, I am not going to use my receiving antennas because no one else will be transmitting at the same time, otherwise collisions will occur. And if someone is transmitting and I am using my receiving antennas to listen to, I am not going to use my transmiting antennas because I am not going to transmit at that time, otherwise collisions will occur.

     

    Regards,

    Julián



  • 5.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    EMPLOYEE
    Posted Mar 06, 2017 09:57 AM

    Everything is covered in the links provided.



  • 6.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    Posted Mar 06, 2017 10:06 AM

    Thanks Jerrod,

     

    I will take a look to those links and be back if I miss anything.

     

    Warm regards,

    Julián



  • 7.  RE: Why MIMO if antennas cannot transmit and receive simultaneously?

    Posted Apr 01, 2020 01:45 PM

    Hi Julian

     

    Have you sorted out this question? I study for my Wireless exam and come across a youtube video from network chuck on 802.11ax.

    I did some searching and this is what i found:

     

    MU-MIMO is only on 802.11ac wave2, and only work downstream.

    meaning: the AP can transmit at the same time to multiple devices but transmitting to the AP,  the client devices still have to take their turns.

     

    so: upstream only 1 client can transmit downstream multiple clients can get transmission from the AP radios at the same time.

    802.11ax will have real MU-MIMO ergo transmitting and receiving to and from multiple devices at the same time.

     

    in the meantime I  asked a question that i didn't get:

    if an AP got multiple SSID-s each on different channels, how come multiple clients can't transmit at the same time to the AP?  they are not interfering with each other since they are on different channels. how do they do "Channel Assessment" (CCA) ? they actually SCAN every channel prior to transmission to see if the medium is idle?

    the answer was: the AP has a time allocation for each channel.

     

    so the way i see it : radio is a shared medium. either transmit or receive. 1 antenna can either transmit or receive. multiple antennas can form multiple spatial streams and transmit to multiple devices at a time but when the clients transmit uplink they still have to take their turns.

     

    Please, feel free to correct me if i am wrong.