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Semidefinite relaxation (SDR) is a high-performance efficient approach to MIMO detection especially for the BPSK or QPSK constellations. Recently, a number of research endeavors have focused on extending SDR to the case of 16-QAM constellations. This paper reports two interesting and useful results on this problem. First, we show that two of the existing 16-QAM SDR receivers, namely the polynomial-inspired SDR (PI-SDR) and bound-constrained SDR (BC-SDR) methods, are equivalent. Second, we develop a specialized interior-point algorithm for the implementation of BCSDR. The proposed algorithm is computationally efficient exploiting the BC-SDR structures, and enables us to handle larger problem sizes in practice.
Ma et al. (2008) studied this question.