Chip-Level LMMSE Based HARQ Chase Combining for HSUPA

Hybrid automatic repeat request (HARQ) is one of the main technologies used in HSUPA system to increase the uplink data throughput. Chase combining (CC) and incremental redundancy (IR) are the most known HARQ algorithms. The first provides a time diversity, while the second offers a coding gain. In this paper, we propose a method to increase the chase combining efficiency. In this method, all HARQ transmissions are considered as additional receive antennas, and combined at the chip-level using a linear minimum mean square error (LMMSE). It is shown that with our approach, the Chase combining diversity gain is considerably increased especially in the case of high order modulation.