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I have a set of four commercial cellular tranceivers that support 3G and 4G(LTE).

These phones have two antenna connectors (for MIMO/diversity) in an embeddable industrial form factor (eg. just bare electronics, not a handset).

I have been asked to see if there is a way to reduce the necessary number of antennas from the obvious 8, down to 1 or 2 while still keeping all phones fully utilised simultaneously all the time (e.g. all 4 phones are running 90% duty cycle or more, all the time).

My question: Is this possible, and if so how?

My thoughts on doing this (I am not a radio engineer so please forgive my ignorance) are as follows:

Use one 4-into-1 bidirectional combiner for the 4 "Main" antennas and a second 4-into-1 bidirectional combiner for the "Aux" antennas. This would leave me with the requisite 2 MIMO antennas on the roof. But would I need additional amplifiers? Do such combiners exist? I don't really know how to search for such things due to my lack of knowledge.

Is it possible to achieve all this with a single antenna?

Notes:

  • The main reason for thinking about this problem is that we don't want to route out 8 antennas when we can get away with 1 or 2.
  • This whole idea sounds REALLY expensive to me, but lets ignore that minor issue for now, I'm mostly after a feasibility check.
  • Personally I'd rather just run out 8 antennas (nice and cheap and definitely works) but this system needs to be scalable up to perhaps 12 phones (24 antennas would look like Cthulu's grandma!).
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  • \$\begingroup\$ instant upvote :) \$\endgroup\$ – Marcus Müller May 25 '16 at 14:43
  • \$\begingroup\$ 1) you could possibly share antenna's that share the same frequency band 2) you can use a shared antenna only for one thing at a time (send or receive and 3G or LTE) 3) MIMO/diversity can only work when using at least 2 antennas 4) these 2 antennas are then both used by the same transceiver so 3G or LTE, not both at the same time. \$\endgroup\$ – Bimpelrekkie May 25 '16 at 14:46
  • \$\begingroup\$ This question was moved from the Ham Radio site, which I had hoped would have some specialist knowledge, not realising that the Ham Radio site has like 7 users. Thanks @MarcusMüller :) \$\endgroup\$ – Wossname May 25 '16 at 14:48
  • \$\begingroup\$ It's just as well - questions about non-ham applications of radio are off-topic there anyway. \$\endgroup\$ – Dave Tweed May 25 '16 at 14:51
  • \$\begingroup\$ 5) for diversity to work, you will need some distance ( I think at least 1/4 lambda, lambda is the wavelength) between the antennas 6) yes combiners exist but you cannot combine two "diversity" signals from the same transceiver. 7) do get documentation from the manufacturer how to use these transceivers at their RF side !!! Using them the wrong way can destroy their RF input/output, that will be an expensive repair ! \$\endgroup\$ – Bimpelrekkie May 25 '16 at 14:52
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So:

I have been asked to see if there is a way to reduce the necessary number of antennas from the obvious 8, down to 1 or 2 while still keeping all phones fully utilised simultaneously all the time (e.g. all 4 phones are running 90% duty cycle or more, all the time).

The question whether you can remove the numbers of antennas is pretty much up to the device manufacturer – however, MIMO basically works because your antennas receive something different. Talking very abstractingly, a MIMO system uses a Matrix representation of your N transmit antennas – to – M receive antennas system, and finds suitable decompositions of that matrix so that the amount of data transportable can be maximized.

If all but one receive antenna are deaf, the whole MIMO collapses to MISO, but all algorithms that actually get deployed somewhere should still work – it's not unlikely that one antenna has an SNR that is so much better than all the others, that diversity gain is negligible.

The same goes for TX.

The only thing I can see for TX is that with an unconnected antenna port, you might damage the TX amplifier with the reflected power. But that's something that the manufacturer of your devices will be able to tell you. In any case, you should alternatively be able just to terminate these connectors with a matching terminator – again, the MIMO would collapse, but SISO is just a "special case" of MIMO, if you will so.

Combining multiple antennas into one antenna doesn't make sense – as said in the comments to your question, for MIMO to do anything, it's crucial that the receive signals are diverse, which means if you just used the same antenna signal for all 4 channels, you'd win nothing; the only thing you'd do is split your receive power by 4, and that will reduce SNR. So don't do that.

The main reason for thinking about this problem is that we don't want to route out 8 antennas when we can get away with 1 or 2.

The main reason I'm not driving a Benz is because I can get away with a bike. However, if I had a Benz, I might be faster, have less outage probability and be more efficient at work.

This whole idea sounds REALLY expensive to me, but lets ignore that minor issue for now, I'm mostly after a feasibility check.

Why would even your approach sound expensive? The power splitters/combiners you mention would be easy to realize in microstrip on a PCB (in fact, just as a couple of antennas.

Personally I'd rather just run out 8 antennas (nice and cheap and definitely works) but this system needs to be scalable up to perhaps 12 phones (24 antennas would look like Cthulu's grandma!).

A nice old lady. Has the best cookies in town, but once you're there for tea, you gotta listen to stories about her grandson until your ears grow back and you develop scales. Anyway:

Really, I'd go with your approach, here. Make use of let's say two patch antennas (cheap as hell, not very large for 900 MHz or 1800 MHz or so), spaced at least half a wavelength apart, and two inverted-F or something more "linearly" shaped so that you get waves from different directions.

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  • \$\begingroup\$ I think this answer is what I secretly wanted to hear (don't tell my boss!). I think I assumed a great deal of expense because of some equipment we had to buy several years ago for a different set of radio gear (unrelated to cellular) which cost upwards of 5 digits. \$\endgroup\$ – Wossname May 26 '16 at 10:13
  • \$\begingroup\$ I'll accept this as the answer (the microstrip part) because I now think it is very clear that the skills to accomplish this in a timely fashion are not available to us at the moment. I'm not an RF engineer and would be forced to seek design consultation externally since nobody else in my company is an RF guy either. It's a case of "easier said than done". Thank you for your time and advice Marcus. :) \$\endgroup\$ – Wossname May 26 '16 at 10:19
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    \$\begingroup\$ @Wossname by the way, 24 antennas doesn't sound sooo scary: Imagine a little 25cm x 25cm x 25cm cube of PCBs, with up to 9 patch antennas on each (but the downwards-facing) side. PCB antennas are cheap (<10USD a piece); I've had good experiences with Kent's antennas, but I don't think he currently has exactly the product you're looking for. But really, 24 of something similar to [this](www.aliexpress.com/item/GSM-GPRS-3G-Internal-circuit-board-antenna/32469736064.html) cheap low-quality stuff would be sufficient. (not affiliated with either) \$\endgroup\$ – Marcus Müller May 26 '16 at 12:52
  • \$\begingroup\$ if we were to build our own it would most likely end up being a wooden pole with a bunch of magmount style antennas screwed to it all inside a section of plastic weatherproof tubing. This at least might pass for a "single" antenna as far as the customer is concerned as it would be a single object to attach to the mast. It would look quite comical no doubt. \$\endgroup\$ – Wossname May 26 '16 at 13:37

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