Achievable DoF Bounds for Cache-Aided Asymmetric MIMO Communications

Integrating coded caching (CC) into multiple-input multiple-output (MIMO) communications can significantly enhance the achievable degrees of freedom (DoF) in wireless networks. This paper investigates a practical cache-aided asymmetric MIMO configuration with cache ratio $γ$, where a server equipped with $L$ transmit antennas communicates with $K$ users, each having $G_k$ receive antennas. We propose three content-aware MIMO-CC strategies: the \emph{min-G} scheme, which treats the system as symmetric by assuming all users have the same number of antennas, equal to the smallest among them; the \emph{Grouping} scheme, which maximizes spatial multiplexing gain separately within each user subset at the cost of some global caching gain; and the \emph{Phantom} scheme, which dynamically redistributes spatial resources using virtual or ``phantom’’ antennas at the users, bridging the performance gains of the min-$G$ and Grouping schemes. These strategies jointly optimize the number of users, $Ω$, and the parallel streams decoded by each user, $β_k$, ensuring linear decodability for all target users. Analytical and numerical results confirm that the proposed schemes achieve significant DoF improvements across various system configurations.

💡 Research Summary

This paper investigates the degrees‑of‑freedom (DoF) performance of cache‑aided multiple‑input multiple‑output (MIMO) downlink systems where the base station is equipped with L transmit antennas and the K users have heterogeneous numbers of receive antennas Gk. The cache ratio is defined as γ = M/N, where each user stores M files out of a library of N files. After a placement phase that distributes sub‑files across user caches, the delivery phase transmits S vectors x(s) ∈ ℂL over S time slots. In each slot a subset of Ω(s) target users is served, each receiving βk(s) parallel streams of size f(s). Assuming perfect zero‑forcing at high SNR, each stream achieves a rate proportional to log SNR, and the overall DoF reduces to the total number of delivered streams, i.e., DoF = ∑ Ω(s)·β(s).

Existing works on MIMO‑CC assume all users have the same number of receive antennas G, leading to the well‑known DoF bound DoF = Ω·β with Ω and β chosen to satisfy a linear decodability constraint. However, real‑world 5G/6G deployments feature devices ranging from high‑end smartphones to low‑power IoT nodes, resulting in asymmetric antenna configurations that have not been analytically addressed.

To fill this gap the authors propose three delivery schemes:

1. min‑G scheme – The system is forced into a symmetric model by taking Ĝ = mink Gk as the effective receive antenna count for every user. Files are split into K·γ sub‑files, each user caches the sub‑files that contain its index, and the symmetric MIMO‑CC algorithm of