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On the Downlink Capacity of Cell-Free Massive MIMO with Constrained Fronthaul Capacity

1
IMEC the Netherlands, 5656AE Eindhoven, The Netherlands
2
Department of Electrical Engineering, Technical University of Eindhoven, 5612AZ Eindhoven, The Netherlands
*
Author to whom correspondence should be addressed.
Entropy 2020, 22(4), 418; https://doi.org/10.3390/e22040418
Received: 14 February 2020 / Revised: 3 April 2020 / Accepted: 4 April 2020 / Published: 7 April 2020
(This article belongs to the Special Issue Wireless Networks: Information Theoretic Perspectives)
We investigate the downlink of a cell-free massive multiple-in multiple-out system in which all access points (APs) are connected in a linear-topolpgy fronthaul with constrained capacity and send a common message to a single receiver. By modeling the system as an extension of the multiple-access channel with partially cooperating encoders, we derive the channel capacity of the two-AP setting and then extend the results to arbitrary N-AP scenarios. By developing a cooperating mode concept, we investigate the optimal cooperation among the encoders (APs) when we limit the total fronthaul capacity, and the total transmit power is constrained as well. It is demonstrated that achieving capacity requires a water-pouring distribution of the total available fronthaul capacity over the fronthaul links. Our study reveals that a linear growth of total fronthaul capacity results in a logarithmic growth of the beamforming capacity. Moreover, even if the number of APs would be unlimited, only a finite number of them need to be activated. We found an expression for this number. View Full-Text
Keywords: channel capacity; distributed beamforming; cell-free MIMO; constrained fronthaul channel capacity; distributed beamforming; cell-free MIMO; constrained fronthaul
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Zhang, P.; Willems, F.M. On the Downlink Capacity of Cell-Free Massive MIMO with Constrained Fronthaul Capacity. Entropy 2020, 22, 418.

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