  Cell-free massive multiple-input-multiple-output (mMIMO) is an emerging
technology for beyond 5G with its promising features such as higher spectral
efficiency and superior spatial diversity as compared to conventional
multiple-input-multiple-output (MIMO) technology. The main working principle of
cell-free mMIMO is that many distributed access points (APs) cooperate
simultaneously to serve all the users within the network without creating cell
boundaries. This paper considers the uplink of a cell-free mMIMO system
utilizing the radio stripe network architecture with a sequential fronthaul
between the APs. A novel uplink sequential processing algorithm is developed
which is proved to be optimal in both the maximum spectral efficiency (SE) and
the minimum mean square error (MSE) sense. A detailed quantitative analysis of
the fronthaul requirement or signaling of the proposed algorithm and its
comparison with competing sub-optimal algorithms is provided. Key conclusions
and implications are summarized in the form of corollaries. Based on the
analytical and numerical simulation results, we conclude that the proposed
scheme can greatly reduce the fronthaul signaling, without compromising the
communication performance.
