transmit diversity receive diversity

What's mean of transmit diversity and receive diversity in MIMO system?
Please give some defination, explanation or reference. Thanks.

Try this link:
http://www.merl.com/reports/docs/TR2003-37.pdf

transmit diversity and receive diversity in MIMO mean the RF system will chose the best TX antennas or RX antennas depends on transmitted or received signal quality.

hi
the link: h**p://www.merl.com/reports/docs is a very good link