Performance Bounds and Improvements to HARQ in LTE under Unknown and Highly Variable Interference Due to MIMO

We consider the performance of Hybrid ARQ (HARQ) in LTE in the presence of inter-cell interference (ICI) due to MIMO transmissions. Such ICI is a function of the MIMO transmission parameters used in neighboring cells, including the beamforming vectors used and the powers applied to these vectors. Since these are often adapted for different transmissions and generally not known to neighboring cells, the prevailing ICI and channel quality experienced by users in such scenarios varies and may often be unknown to the transmitter. Systems that can operate efficiently in the presence of variable/unknown ICI levels are therefore of particular interest for such scenarios. We show that MIMO-ICI can be efficiently handled by use of HARQ. However, the 4-Redundancy Version HARQ design of LTE can have a significant loss in throughput with respect to what can be achieved. Handling MIMO-ICI requires a more aggressive use of Incremental Redundancy (IR) that does not target successful decoding in the first few (e.g.~1 to 4) transmissions (TXs). Rather, the system is less reliant on fine-grained control of modulation/code-rate settings, and net TX rates are adapted implicitly with prevailing ICI conditions via the number of TXs (often exceeding 4) required for successful decoding. This points to a fundamentally different, simplified, and higher-throughput system design that is well suited to delay tolerant transmission.