How Resilient are Cell-Free Massive MIMO OFDM Systems to Propagation Delays?

This paper develops a mathematical framework to analyze the impact of inter-carrier and inter-symbol interference (ICI and ISI) in the uplink spectral efficiency (SE) of cell-free massive multi-input multi-output (CF-mMIMO) system using orthogonal frequency division multiplexing (OFDM). In a CF-mMIMO system, geographically separated access points (APs) jointly serve a large number of user-equipments (UEs). The distributed nature of the overall system results in different propagation delays in the signals received at the APs. This delay in receiving signals from different UEs can exceed the cyclic-prefix duration, leading to interference from adjacent subcarriers and consecutive OFDM symbols. Our analysis shows that ignoring this crucial aspect leads to a gross overestimation of the achievable SE. We also develop an interference-aware combining scheme to alleviate ISI and ICI in addition to multi-UE interference. We also account for the scenario in which each UE performs a timing-advance with respect to its nearest AP. Numerically, we illustrate that ICI and ISI can significantly limit the achievable SE, but we can considerably lessen the impacts of ICI and ISI by using nearest AP based timing advance and interference aware combining, and in many scenarios, obtain performance that is close to a time-aligned CF-mMIMO system.