Combined DL-UL Distributed Beamforming Design for Cell-Free Massive MIMO

TL;DR

The paper tackles the challenge of joint DL and UL beamforming in cell-free massive MIMO where DL and UL transmissions share a resource block. It develops fully distributed beamforming designs based on MMSE minimization and iterative bi-directional training (IBT), reusing DL beamformers for UL operation to reduce training overhead. To further improve efficiency, it introduces UE pairing that assigns a common DL multicast precoder to certain DL-only and UL-only UE pairs, reducing the number of precoders and associated IBT signaling. Through extensive simulations, the authors show that the combined DL-UL distributed beamforming approaches outperform separate DL and UL designs, particularly for short resource blocks, highlighting improved scalability and practical viability for CF-mMIMO with IBT.

Abstract

We consider a cell-free massive multiple-input multiple-output system with multi-antenna access points (APs) and user equipments (UEs), where the UEs can be served in both the downlink (DL) and uplink (UL) within a resource block. We tackle the combined optimization of the DL precoders and combiners at the APs and DL UEs, respectively, together with the UL combiners and precoders at the APs and UL UEs, respectively. To this end, we propose distributed beamforming designs enabled by iterative bi-directional training (IBT) and based on the minimum mean squared error criterion. To reduce the IBT overhead and thus enhance the effective DL and UL rates, we carry out the distributed beamforming design by assuming that all the UEs are served solely in the DL and then utilize the obtained beamformers for the DL and UL data transmissions after proper scaling. Numerical results show the superiority of the proposed combined DL-UL distributed beamforming design over separate DL and UL designs, especially with short resource blocks.

Figures (6)

• Figure 1: System model with $\mathcal{K} \! = \! \{ 1, 2, 3, 4, 5 \}$, $\mathcal{K}^{\textnormal{\tiny{DL}}} \! = \! \{ 1, 2, 3, 5 \}$, $\mathcal{K}^{\ul} \! = \! \{ 1, 3, 4 \}$, $\mathcal{K}^{\textnormal{\tiny{DL}}\textnormal{-}\ul} \! = \! \{ 1, 3 \}$, $\mathcal{K}^{\textnormal{\tiny{DL}}\textnormal{-only}} \! = \! \{ 2,5 \}$, and $\mathcal{K}^{\ul\textnormal{-only}} \! = \! \{ 4 \}$.
• Figure 2: Resource blocks containing UL and DL resources for both data and IBT signalling.
• Figure 3: Effective DL-UL sum rate vs. resource block number.
• Figure 4: Effective DL-UL sum rate vs. resource block size.
• Figure 5: Effective DL-UL sum rate vs. fraction of UEs in the DL and UL.