Downlink Cell-Free Fixed Wireless Access: Architectures, Physical Realities, and Research Opportunities

Recently, a new paradigm of wireless access, termed as cell-free massive multiple-input multiple-output (MIMO), has drawn significant research interest. Its primary distinction from conventional massive MIMO aided cellular networks is the ability to eliminate the detrimental inter-cell interference (ICI), or to convert ICI, into extra power for the intended signal via a multi-cell cooperation approach originated from network MIMO. However, the information-theoretical limit of cell-free access is achieved at the expense of large network configuration overhead and high MIMO processing complexity. Because of the dynamic nature of wireless channels, the global channel state information (CSI) invoked for network MIMO quickly becomes outdated, leading to performance degradation. This article focuses on the cell-free implementation of fixed wireless access (FWA), a complementary solution to fiber-to-the-premise (FTTP) where the latter is prohibitively expensive. In particular, we discuss the centralization architectures and channel characteristics of cell-free FWA, as well as their joint implications on imperfect CSI performance. Moreover, measurement-based offline simulations show that the long coherence time (‘quasi-static’) assumption of real-world FWA channels is only valid against a completely motionless background, and, thus, it should not be used in FWA system design or performance analysis. Finally, we present new research opportunities for cell-free FWA in terms of physical infrastructure, data processing, and machine learning.