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I have read about MIMO (Multiple IN Multiple OUT) multiple antenna system help mitigate the multipath effects (like fading), otherwise occurring in a SISO i.e. single antenna system.

But I have been claims that MIMO antenna system is more productive if the multiple channels used by multiple antennas are highly uncorrelated with each other. If the channels are correlated then, the MIMO performance is as good as a SISO.

Any insights on why MIMO, to perform better, 'needs' to have an uncorrelated channel in a multipath environment.

Thanks.

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The whole point of MIMO is that you get multiple independent channels due to the multipath propagation. In this case, one channel could be the direct line of sight, and one could bounce off of an adjacent building. If the channels are sufficiently independent, different data can be sent down each channel. This means you can have greater throughput than if you had just 1 channel. And the more antennas you have at both ends, the more independent channels can be accessed, giving a higher potential throughput.

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  • \$\begingroup\$ So can we say, that MIMO performs better compared to SISO in multipath environment, but would perform similar (if not worse) in an 'ideal' or correlated channel environment? \$\endgroup\$ – eecs Oct 26 '16 at 4:12
  • \$\begingroup\$ Yeah, I think so. I think it depends on your definition of ideal. However, if there is only one path, then MIMO cannot perform any better than SISO. \$\endgroup\$ – alex.forencich Oct 26 '16 at 22:50
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There is a very simple intuitive explanation. With a fading signal, communication errors will occur in the signal fades. If the signal can take two or more independent paths, then each path will have fades that occur in different places - either in frequency or in time or both - from the other path. A MIMO receiver can exploit the stronger signal path at any time (or frequency) and thus will experience a lower overall error rate.

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  • \$\begingroup\$ That's diversity, not MIMO. \$\endgroup\$ – alex.forencich Nov 24 '16 at 9:24
  • \$\begingroup\$ The two concepts are inter-related, not exclusive. \$\endgroup\$ – Chris Hansen Dec 4 '16 at 20:05
  • \$\begingroup\$ MIMO is an extension of diversity. Diversity is actually more like SIMO (one transmitter, multiple receivers) where the receiver will pick the best signal to mitigate fading. However, MIMO can actually transmit different signals through independent multipath channels and get not only improved response in the case of fading, but also get a significantly higher data rate with the same RF bandwidth. \$\endgroup\$ – alex.forencich Dec 5 '16 at 0:28

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