SUBF vs. MUBF in a Gaussian MIMO Broadcast Channel with Partial Channel State Information

In a Gaussian MIMO broadcast channel, serving more than one user at a time is generally more beneficial in terms of average sum-rate. Under partial channel state information (CSI) feedback constraints, however, serving only one user at a time can be a better multiple access method when the number of active users in the system is smaller than a certain threshold. This paper uses single user beam-forming (SUBF) to refer to the transmitting scheme that serves only one user at a time and multi-user beam-forming (MUBF) to refer to the scheme without such a constraint, i.e., a scheme that is allowed to serve more than one user at a time. It turns out that the aforementioned user threshold varies with other system parameters, especially, it increases as the number of transmit antennas increases. Monte Carlo simulations and a statistical analysis are provided to verify the results