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minimum distance between two antennas in MIMO STC?

I am doing a project on MIMO - STC. I have read that the minimum separation required between two antennas so that the signals undergo independent fading is half the wavelength of the signal.

 

Can anyone explain the reason for it?, relevant links or papers will also be greatly appreciated.

 

Thanks 

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Hi Ujjval Shah,

 

Thank you for posting your questions. In order to understand the minimum separation required between two antennas, you should be familiar with the term "Diversity". In this case, diversity means that the same information reaches the receiver on statistically independent channels. Similar to what you mentioned, diversity is most efficient when the different transmission channels carry independently fading copies of the same signal. Any correlation between the fading of the channels decreases the effectiveness of diversity. The  " Correlation coefficient " characterizes the correlation between signals on different transmission channels. Signals are often said to be effectively decorrelated if the correlation coefficient is below a certain threshold. This threshold defines the minimum separation between the two antennas. 

In order to understand this better, I recommend checking  " Spatial Diversity" topic in any wireless communications textbook. Some tectbooks have even tables for the numerical evaluations of the correlation coefficient as a function of antenna spacings.

 

I hope this answer helps

 

Best Regards,

Faris A

Bueller
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Hi Faris A,

 

Thanks for your reply!

 

I have been working on a simulation model of MIMO in LabVIEW for my undergraduation major project for past three months so I assure you that you can safely assume that I totally understand the concept of spatial diversity and related phenomena of channel fading and coding techniques. In the process I have been referring to a number of published papers and books on MIMO and Space time coding techniques; all of them mention that for the channels to be uncorrelated they should be atleast half the wavelength apart but none of them have given an explanation for that. I am looking for that specific explanation

 

As suggested I have already referred the books like Wireless Communication by Theodore Rappaport and Space Time Coding by Hamid Jafarkhani but without any success.

 

I would really appreciate if you can suggest a specific book/paper, where i can find the related information.

 

Thanks for your help.

 

Best Regards,

 

Ujjval 

Message Edited by Ujjval Shah on 04-14-2009 12:10 AM
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Hi Ujival,

 

I think that " Wireless Communications" by Andreas F.Molisch is a good resource to know that. In one of the chapters they have numerical evaluations of the correlation coefficient versus distance between antennas. ( I think chapter 13)

I also want to add that in that book there's a chapter about MIMO , but in order to understand the antenna spacing, you need to read some of the previous chapters.

 

Please let me know if I can provide further assistance.

 

Best Regards,

Faris A

Bueller
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Hi Faris A,

 

thanks for your kind help.

i looked for this book, but unfortunately it has already been issued to someone and i cant find any other copy; neither soft nor hard; guess will have to wait.

 

Anyways, Thanks once again. this should solve my problem. 🙂

 

regards,

 ujjval 

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hey ujjwal im in my final year and im doing a project on MIMO

at dis time im still not over with my literature survey but i dnt want to repeat the work already been done in dis field so can u please give some details of work done by you so that i would be able to do something good in it

 

please do contact me my email id is: siddharth.vadhera@gmail.com

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Hi. This is a first for me, joining a forum.
Honours degree in telecoms and lots of experience, in classrooms at least. After a break of a few years, just trying to get back up to date. Can't quite get my head around mimo yet so I don't know how valid this answer is. Gsm receive antennas at base station, almost always a pair for spatial diversity. The theory there is that the further apart the two receive antennas are, the better. However at ten wavelengths, the signals are pretty much uncorrelated and there is little to be gained beyond that. So gsm 900 about 3m apart, wavelength approx 30 cm. gsm 1800 about 1.5 m.
I appreciate there is more to mimo that receive spatial diversity, but the reasoning that two signals will be largely uncorrelated with a 10wavelength separation is sound.
I think the half wavelength idea is wrong and a common misconception. The significance of half wavelength is that half wavelength is the difference in the path length between constructive addition of received signals and cancellation . . Ie fading.
There are lots of articles that use the complex maths, and lots of articles that brush the surface without any good explanation of the principles. There must be one somewhere. I will write on when I've sorted it.
Bit rusty so hope this is of some value.
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But on the other hand . . . Half wavelength separation of transmit antennas gives max benefit for making the beam directional. Common practice for multi element antenna arrays for the individual elements to have half wavelength separation. To be continued when my brain wakes up!
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