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 LTE uplink. It was found that use of AQPSK modulation for conveying multi-level ACK/NACK provides valuable transmit power savings over the conventional symmetric QPSK modulation, with gains of about 0.6 - 0.8dB for users at cell edge and about 1.6 - 1.9dB for users with moderate channel conditions. This translates into extra power requirements over BPSK-modulated binary ACK/NACK of about 2.3 - 2.5dB and 1.6 - 1.8dB, respectively.