  Cell-free massive multiple-input-multiple-output (CF-mMIMO) is a
next-generation wireless access technology that offers superior coverage and
spectral efficiency compared to conventional MIMO. With many future
applications in unlicensed spectrum bands, networks will likely experience and
may even be limited by out-of-system (OoS) interference. The OoS interference
differs from the in-system interference from other serving users in that for
OoS interference, the associated pilot signals are unknown or non-existent,
which makes estimation of the OoS interferer channel difficult.
  In this paper, we propose a novel sequential algorithm for the suppression of
OoS interference for uplink CF-mMIMO with a stripe (daisy-chain) topology. The
proposed method has comparable performance to that of a fully centralized
interference rejection combining algorithm but has substantially less fronthaul
load requirements.
