Base Station Cooperation
This project considers cooperative transmission by base stations in multiuser, multi-cell multiple input multiple output (MIMO) systems. A key result is that in such systems, the multiuser interference is asynchronous by nature. We establish an accurate mathematical model for the asynchronity and show that it leads to a significant performance degradation of existing designs, which were derived based on the idealized assumption of synchronous interference. We therefore propose three new linear pre-coding algorithms that are shown to be better in mitigating the impact of the asynchronous interference and improve overall system spectral efficiency.
Background & Objective: In the downlink of conventional cellular systems, base stations send out signals to the mobile stations in their cells without regard to the mobile stations in neighboring cells. The resulting interference can be mitigated by the use of different frequency bands or different codes in the cells, but such an approach decreases the spectral efficiency of the overall system. A more efficient method is based on a cooperation between neighboring base stations that reduces interference without the use of different transmission frequencies. Each base station knows, and transmits - with linear precoding - the signals to be transmitted to the mobile stations not only in its own cell, but also its neighbors. The precoding is designed to optimize the signal-to-noise and interference ratio at all the mobile stations. The major aspect of our work is to take into account the fact that the interference is inherently asynchronous to the desired signals.
Technical Discussion: In recent years, several advanced techniques have been proposed to better mitigate the effect of inter-cell CCI in cellular MIMO systems. all the schemes proposed for BS cooperation invariably assume that both the desired and the interfering signals from different BSs arrive at each of the MSs synchronously (at the same time). The BSs can align their transmissions so that the signals intended for any MS arrive at that MS synchronously. However, even under the assumption of perfect BS cooperation, the BSs cannot simultaneously control when these signals are received as interference by other MSs. This severely degrades the performance of the previously proposed designs, which do not account for it, especially, in high data rate regimes. In our work, we develop a mathematical formulation of the asynchronous interference problem, and use it to derive three new algorithms for linear precoding design that provide different trade-offs between complexity and performance in different SNR regimes.
Outside Collaborations: This work was done in collaboration with Dr. Hongyuan Zhang and Prof. Huaiyu Dai from North Carolina State University. Dr. H. Zhang is now with Marvell Semiconductors.
Future Direction: We will investigate the application of our framework and algorithms in wideband OFDM systems.
Contacts:
Andreas F. Molisch
Jinyun Zhang
Technology Area: Digital Communications
Modification Date: September 12, 2007
