Physical layer characterization of a multi-user MISO system by efficient outage probabilities

In bottom-up cross-layer optimization, each layer offers a set of feasible operation points to the layer above, enabling the application to decide which parameter setup is optimum. In this paper, the capabilities of the physical layer in a multi-user MISO system are described in terms of the users' outage probabilities. Optimum descriptions are given by sets of efficient outage probability tuples. We first derive a closed-form expression for a user's outage probability in the presence of multi-user interference. Next, the set of efficient outage probability tuples under a simplified SDMA transmit strategy is derived. We show how a good approximate description of the efficient set is found by equidistant sampling. Finally, the performance of the proposed transmit strategy is compared with an orthogonal multiple access scheme, demonstrating that optimum mode selection at the physical layer can lead to significant performance gains at the application layer