Performance of MIMO HARQ under Receiver Complexity Constraints

Hybrid Automatic Repeat reQuest (HARQ) schemes are becoming common in wireless systems. In addition to the channel characteristics, several system design considerations need to be taken into account when choosing the particular HARQ scheme to be used. For SISO systems, it has been shown that the coding gain of Incremental Redundancy (IR-) HARQ over Chase Combining (CC-) HARQ may be offset by adverse fading channel conditions. In this paper it is shown that, for MIMO systems, IR- HARQ may also be affected by practical complexity constraints on the implementation of the receiver that dictate the use of bit-level combining. In some cases the coding gain advantage of IR-HARQ may be offset by the inability to perform symbol combining in the receiver as opposed to the case when CC-HARQ is employed. A system compliant with the IEEE 802.16e standard that employs a Convolutional Turbo Code is used to demonstrate an example of such performance loss.