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When MIMO is proposed it is shown to achieve higher rate than SISO. How to measure the gain? Well, from Shannon theorem we know that for an SISO link, the capacity is given by log(1+SNR) and for the m by n MIMO system the capapcity is shown to be min(m,n)lolg(1+SNR). Then, it is now a convention to refer to min(m,n) as multiplexing gain. It can be interpreted as multiple streams are multiplexed in spatial domain.
While the above line of thought focuses on transmision rate, i.e., capacity, another line of work studies the diversity order provided by multiple antennas. The latter is initiated by Alamouti who shows that a 2 tx antenna system can achieve the same diversity as a 2 rx antenna system. The idea here is to use multiple antennas to compensate channel fading to make the link more reliable, as opposed to shooting for higher data rate. The above two lines of work was bridged by Lizhong Zheng and David Tse, who tell us that with an MIMO system we can achieve higher data rate, as well as achieve higher reliablity. However, that is a tradeoff between the two.
Note that the tradeoff study is focus on high SNR and slow fading channel.