Multiuser Antenna Partitioning for Cellular MIMO-CDMA Systems
Submitted to the IEEE Transactions on Vehicular Technology, June 2005.
Improving the capacity of CDMA systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multi-antenna technologies (called MIMO) are an obvious candidate for increasing downlink capacity in particular, but nearly all research on MIMO-CDMA has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. In this paper, we consider the downlink of a heavily-loaded multicell CDMA system with multiple transmit and receive antennas. Straightforward application of known MIMO techniques to such a system does not actually increase the number of supportable users, due primarily to increased other-cell interference. To overcome this, a novel MIMO-CDMA system design based on user partitioning is developed, in which each user is assigned to a single transmit antenna either without regard to channel knowledge (static), or based on antenna selection feedback bits (dynamic). These proposed multiuser antenna partitioning techniques have a minimal increase in complexity and would require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems is derived and it is shown that in particular, the dynamic partitioning scheme has a very large gain over both conventional CDMA and the static MIMO CDMA scheme. This gain can be accredited to multiuser selection diversity, which in this scheme due to the large number of users comes without the typical expense of a loss in spatial multiplexing gain.
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