  In this paper, we investigate the spatial-wideband effects in cell-free
massive MIMO (CF-mMIMO) systems in mmWave bands. The utilization of mmWave
frequencies brings challenges such as signal attenuation and the need for
denser networks like ultra-dense networks (UDN) to maintain communication
performance. CF-mMIMO is introduced as a solution, where distributed access
points (APs) transmit signals to a central processing unit (CPU) for joint
processing. CF-mMIMO offers advantages in reducing non-line-of-sight (NLOS)
conditions and overcoming signal blockage. We investigate the synchronization
problem in CF-mMIMO due to time delays between APs. It proposes a minimum
cyclic prefix length to mitigate inter-symbol interference (ISI) in OFDM
systems. Furthermore, the spatial correlations of channel responses are
analyzed in the frequency-phase domain. The impact of these correlations on
system performance is examined. The findings contribute to improving the
performance of CF-mMIMO systems and enhancing the effective utilization of
mmWave communication.
