Cell-Free Massive MIMO with Limited Backhaul

Abstract

We consider a cell-free Massive multiple-input multiple-output (MIMO) system and investigate the system performance for the case when the quantized version of the estimated channel and the quantized received signal are available at the central processing unit (CPU), and the case when only the quantized version of the combined signal with maximum ratio combining (MRC) detector is available at the CPU. Next, we study the max-min optimization problem, where the minimum user uplink rate is maximized with backhaul capacity constraints. To deal with the max-min non-convex problem, we propose to decompose the original problem into two sub-problems. Based on these sub-problems, we develop an iterative scheme which solves the original max-min user uplink rate. Moreover, we present a user assignment algorithm to further improve the performance of cell-free Massive MIMO with limited backhaul links. aaKeywords: Cell-free Massive MIMO, geometric programming, generalized eigenvalue problem, limited backhaul.

Max–Min Rate of Cell-Free Massive MIMO Uplink With Optimal Uniform Quantization

“…refers to coherence time. 4 In the following, we present a comparison between two cases of uplink transmission. To make a fair comparison between Case 1 and Case 2, we use the same total number of fronthaul bits for both cases, that is…”

“…In this section, we formulate the max-min rate problem for Case 2 of uplink transmission in cell-free Massive MIMO 4 Exploiting the constraint C m ≤ C fh , the largest number of quantization level is equal to α m,1 = 5 Future work is needed to investigate the performance analysis for different numbers of quantization bits as well as the numbers of APs. This will be presented in [15].…”

On the Energy Efficiency of Limited-Backhaul Cell-Free Massive MIMO

“…Theorem 1. The spectral efficiency of the kth user is given by (7) (defined at the top of this page), where τ c denotes the number of samples for each coherence interval, and…”

“…We study the case when only the quantized version of the weighted signal is available at the CPU and the CPU employs maximum ratio combining (MRC) detection. In [7]- [11], the authors show that exploiting optimal uniform quantization and wireless microwave links with capacity 100 Mbits/s, the performance of limited-backhaul cell-free Massive MIMO system closely approaches the performance of cell-free Massive MIMO with perfect backhaul links. We consider the energy efficiency maximization problem, where to tackle the non-convexity of the optimization problem, we decouple the original problem into two sub-problems, namely, receiver filter coefficient design, and power allocation.…”

Cell-Free Millimeter-Wave Massive MIMO Systems With Limited Fronthaul Capacity

“…The performance of cell-free massive MIMO using conventional sub-6 GHz frequency bands and assuming infinitecapacity fronthaul links has been extensively studied in, for instance, [10], [17]- [19]. Cell-free massive MIMO networks using capacity-constrained fronthaul links have also been considered in [20], [21] but assuming, again, the use of fully digital precoders in conventional sub-6 GHz frequency bands. Sub-6 GHz massive MIMO systems are often assumed to implement a fully-digital baseband signal processing requiring a dedicated RF chain for each antenna element.…”