Joint Scheduling and Hybrid-ARQ for MU-MIMO Downlink in the Presence of Inter-Cell Interference

User scheduling and multiuser multi-antenna (MU-MIMO) transmission are at the core of high-rate data-oriented downlink schemes foreseen in the next-generation cellular systems (e.g., LTE-Advanced). Scheduling exploits the knowledge of the users\´ instantaneous achievable coding rates. However, when it is applied independently in each cell, the level of inter-cell interference (ICI) experienced by each user is not known a priori at the cell scheduler as it depends on the local scheduling decisions at all other base stations. As a result, the instantaneous achievable rates are random variables, even under the optimistic assumption of perfect transmitter channel state information. In order to cope with this problem, we consider the joint design and operation of scheduling, MU-MIMO beamforming and Automatic Repeat reQuest (ARQ). We present a systematic method for joint scheduler/ARQ design and show that properly designed incremental-redundancy hybrid ARQ schemes can achieve (with finite average decoding delay) any desired fraction of the "optimistic" throughputs, i.e., of the throughputs that would be achieved if the transmitter knew the instantaneous user ICI and transmitted to each scheduled user at the maximum possible instantaneous rate.