Sparse Joint Transmission for Cloud Radio Access Networks With Limited Fronthaul Capacity

A cloud radio access network (C-RAN) is a promising cellular network, wherein densely deployed multi-antenna remote-radio-heads (RRHs) jointly serve many users using the same time-frequency resource. By extremely high signaling overheads for both channel state information (CSI) acquisition and data sharing at a baseband unit (BBU), finding a joint transmission strategy with a significantly reduced signaling overhead is indispensable to achieve the cooperation gain in practical C-RANs. In this paper, we present a novel sparse joint transmission (sparse-JT) method for C-RANs, where the number of transmit antennas per unit area is much larger than the active downlink user density. Considering the effects of noisy-and-incomplete CSI and the quantization errors in data sharing by a finite-rate fronthaul capacity, the key innovation of sparse-JT is to find a joint solution for cooperative RRH clusters, beamforming vectors, and power allocation to maximize a lower bound of the sum-spectral efficiency under the sparsity constraint of active RRHs. To find such a solution, we present a computationally efficient algorithm that guarantees to find a local-optimal solution for a relaxed sum-spectral efficiency maximization problem. By systemlevel simulations, we exhibit that sparse-JT provides significant gains in ergodic spectral efficiencies compared to existing joint transmissions. D. Han and N. Lee are with the Department of Electrical Engineering, POSTECH, Pohang, Gyeongbuk 37673, South Korea (e-mail: {dhhan, nylee}@postech.ac.kr). J. Park is with the School of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu, 41566, South Korea (e-mail: jeonghun.park@knu.ac.kr). S.-H. Park is with the Division of Electronics Engineering, College of Engineering, Jeonbuk National University, Jeonju, 54896, South Korea (e-mail: seokhwan@jbnu.ac.kr). This work was partly supported by Institute of Information & communications Technology Planning & Evaluation(IITP) (No.2021-0-00161, Post MIMO system research for massive connectivity and new wireless spectrum) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1013381). ar X iv :2 10 7. 13 81 9v 1 [ ee ss .S P] 2 9 Ju l 2 02 1