Performance of Asymmetric QPSK Modulation for Multi-Level ACK/NACK in LTE Uplink

Asymmetric modulation schemes are candidates for conveying quality feedback in the form of multi-level ACK/NACK in a system employing reliability-based HARQ. Asymmetric QPSK (A-QPSK) modulation, for example, enables to convey one ACK level and three NACK levels with unequal error protection of the ACK and NACK levels, each NACK level representing a certain quality level of a received packet. In this work, the performance of A-QPSK modulation for conveying multi-level ACK/NACK is assessed by deriving approximations of the error rates for AWGN channel and by means of link level simulations for the LTE uplink. It was found that use of A-QPSK modulation for conveying multi-level ACK/NACK provides valuable transmit power savings of about $0.6 - 0.8$ dB over the conventional symmetric QPSK modulation.