arXiv:2607.24227v2 Announce Type: replace Abstract: Future wireless multiple-input multiple-output (MIMO) systems will integrate sub-6 GHz and millimeter wave (mmWave) bands to support high data rates for latency-critical applications. The coexistence of these bands enables the use of reliable out-of-band sub-6 GHz information to assist mmWave link establishment, particularly for channel estimation. In this paper, we propose a novel beam-domain channel estimation method for mmWave MIMO that leverages sub-6 GHz beam-domain information to improve estimation accuracy. We compare the proposed method with conventional in-band methods. Simulation results show that our proposed method outperform existing baselines in terms of spectral efficiency, in both line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios.