Near-far effects in adaptive SDMA systems

In an adaptive SDMA system users can share the same channel which is made possible by using adaptive array antennas at the base station site and forming multiple independent beams to serve the users. Though the enormous capacity enhancements of this approach are obvious, there are two constraints which limit its performance: (1) any two users sharing the same channel should be located in different directions and (2) the difference between the received power levels should not be too large. If one of these constraints is violated, the users cannot be separated, i.e. one of the connections is blocked or lost. While the capacity increase of SDMA taking into account the first constraint have been analysed by Tangemann (see Proc. PIMRC ´94, The Hague, The Netherlands, p.745, 1994), this paper is focused on the implications of the second constraint, which represents a near-far problem similar to those encountered in CDMA systems. To solve the near-far problem, grouping of the mobile users into power classes is suggested. The resulting channel allocation scheme is analysed, and it will be shown that the gain reduction caused by the second constraint will be only marginal in most realistic environments, i.e. still a large capacity gain can be achieved