Activity, Channel, and Carrier Frequency Offset Estimation for Cell-Free Massive MIMO

This letter proposes a joint user equipment (UE) activity, channel, and carrier frequency offset (CFO) estimation method for cell-free massive MIMO (CF-mMIMO) systems. We formulate a system model in which CFOs arise not only among UEs but also among access points (APs), and introduce a decomposition of the double-sided CFOs based on their heterogeneous observation structure. Leveraging the resulting separated signal model, the proposed receiver alternately estimates the CFOs for both UEs and APs across the frequency–time domains. Furthermore, we demonstrate that calibrating the estimated CFOs using demodulated data symbols enables the proposed method to achieve performance approaching that of an ideal scenario with perfectly known CFOs.