Diversity-Multiplexing Tradeoff of MIMO Systems
MIMO (Multiple-Input Multiple-Output) systems have shown their ability in providing great performance improvements over the SISO (Single-Input Single-Output) system thanks to their higher spectral efficiency. It has been well understood that multiple antennas can not only be used to achieve diversity gain, but also as an effective way to increase the degrees of freedom of the channel. With multiple antennas at both the transmitter and receiver, independent information streams can be transmitted through the parallel spatial channels so that the data rate is increased. Therefore, a MIMO system can provide both diversity gain and multiplexing gain.
Recently, Zheng and Tse proposed an important performance evaluator for MIMO systems: diversity-multiplexing tradeoff function . Such function allows us to compare the performance between diversity-based schemes and multiplexing-based schemes. It is found that a higher spatial multiplexing gain comes at the price of sacrificing diversity and vice versa. An optimal diversity-multiplexing tradeoff curve is characterized in  and it was shown that no existing scheme, except for the Alamouti's scheme with 2 transmit antennas and 1 receive antenna, can achieve this optimal tradeoff.
In our work, we further derive the optimal diversity-multiplexing tradeoff function when group detection is adotped . Various rate allocation schemes are proposed and the tradeoff functions for GZF and GSIC are evaluated.
Besides, we also present a a novel group space-time architecture, Quasi-Orthogonal Group Space-time (QoGST), in which the transmit stream is partitioned into different groups and all the groups are encoded together via an inter-group STBC . The analysis on the diversity-multiplexing tradeoff shows that this new architecture can efficiently bridge the large gap between the tradeoff achieved by the existing diversity-based scheme or multiplexing-based scheme and the optimal one.
 Lizhong Zheng and D. N. C. Tse, "Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels," IEEE Trans. Inf. Theory, vol. 49, no. 5, pp. 1073-1096, May 2003.
 Sana Sfar, Lin Dai and Khaled B. Letaief, "Optimal Diversity-Multiplexing Tradeoff for Group Detection," IEEE Trans. Commun., vol. 53, no. 7, pp. 1178-1190, July 2005.
 Lin Dai, Sana Sfar and Khaled B. Letaief, "A Quasi-Orthogonal Group Space-time Architecture to Achieve a Better Diversity-Multiplexing Tradeoff," IEEE Trans. Wireless Commun., vol. 6, no. 4, pp. 1295-1307, Apr. 2007.