Transmission mode selection in a heterogeneous network using opportunistic beamforming

In this paper, we consider a heterogeneous network where the statistics of fading channels between the Base Station (BS) and various users are different. The network can operate either in a single-user mode (i.e. the BS transmits to one user at a time) or in a multi-user mode via opportunistic beamforming. Given the profile of users' fading channel statistics, we are able to derive the exact downlink capacities associated with different modes assuming a 2-Tx BS, as well as determine the optimal mode that maximizes the throughput. The switching point between transmission modes depends on the signal-to-noise ratio (SNR), the distribution of users and the path loss of the fading channel. Simulations are provided to verify the derived expressions and to illustrate the movement of the switching point. In addition, a method is proposed to address the fairness issue in user scheduling, which allows for a reasonable trade-off between performance and fairness.