HARQ Feedback in Unlicensed Spectrum LTE: Design and Performance Evaluation

The uncertainty of channel availability for wireless systems in unlicensed bands necessitates new and robust designs for the feedback of uplink control information and channel state information. This work discusses the design of hybrid automatic repeat request (HARQ) feedback mechanisms and channels for unlicensed spectrum LTE. We describe in detail the motivation for new uplink physical channels that carry downlink HARQ feedback while adhering to unlicensed spectrum regulations. Finally, the performance of various control channel designs is evaluated by means of link-level simulations in order to provide insights on the trade-offs between required signal-to-noise ratio (SNR), HARQ-feedback payload size, transmission duration, and multi-user multiplexing capacity.