Cell-Free Massive MIMO Versus Small Cells

Abstract

Abstract-A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs) which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughputs lead to max-min power control algorithms. Max-min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard.Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly 5-fold improvement in 95%-likely per-user throughput over the small-cell scheme, and 10-fold improvement when shadow fading is correlated.

Resource Allocation for Energy Efficiency in 5G Wireless Networks

“…Cell-free massive MIMO has been investigated as an emerging 5G technology in the wireless system because of its ability to provide good service to all users [33,34]. As such system, a numerous low-powered and distributed APs serve a small number of single-antenna users are randomly located in a wide network area [24,34].…”

“…As such system, a numerous low-powered and distributed APs serve a small number of single-antenna users are randomly located in a wide network area [24,34]. In cell-free systems, the inter-cell interference is no longer the main problem that affects the spectral efficiency of wireless networks.…”

“…In cell-free systems, the inter-cell interference is no longer the main problem that affects the spectral efficiency of wireless networks. Rather, the inter-user interference is a more important role in cell-free [34]. To this end, ZF precoding design is an efficient approach to deal with the term of multiuser interference [37], although the implementation of ZF processing requires a higher complexity than conjugate beamforming [34].…”

“…When a massive APs number in large-scale networks, EE performance in terms of bits/Joule becomes a major figure-of-merit [36,37] which does not consider in the literature of cell-free networks [24,34]. It is important noted that the strategy of beamforming design is significantly necessary to improve the EE performance because of a huge paramount energy consumption to employ signal processing techniques in cell-free massive MIMO.…”

User-Centric Networking for Dense C-RANs: High-SNR Capacity Analysis and Antenna Selection

“…a centralized baseband unit (BBU) [6,7]. The performance of distributed antenna arrays and best base station (BS) selection schemes in C-RANs was assessed in [8], where both the users and RRHs are equipped with a single antenna.…”