Determining the optimum threshold values of MCS levels for retransmission packets in HARQ schemes

We analyze the optimum threshold value used in determining the modulation and coding scheme (MCS) level of retransmission packets in HARQ schemes, although the first packet is erroneous, it has a considerable energy that can enhance the system performance such as packet error rate (PER) and throughput. Using the existing energy of the first transmission packet, we can save the power for retransmission while satisfying the given target PER. The saved power can be used to adjust the MCS level for retransmitted packets or increase the data rate of other users. The proposed algorithm which adjust the transmit power at the retransmission can be applied to all types of HARQ schemes including Chase combining, partial incremental redundancy (IR), and full IR. Especially in the IR scheme, retransmitted packets convey inherent coding gain as well as energy because they contain additional redundant information fro the first failed attempt. Using this inherent coding gain, we can save more energy compared to the simplest HARQ form, the Chase combining scheme. The result shows that the saved energy can enhance the throughput of the overall system. In addition, the proposed scheme yields robustness about feedback information errors.