[Paper Review] Cell-Free Massive MIMO with Channel Aging and Pilot Contamination

Jiakang Zheng, Jiayi Zhang+2 more|arXiv (Cornell University)|Aug 25, 2020

Advanced MIMO Systems Optimization13 references4 citations

TL;DR

This paper investigates cell-free massive MIMO systems under channel aging and pilot contamination, proposing a channel estimator and deriving closed-form uplink spectral efficiency (SE) expressions for large-scale fading decoding (LSFD) and matched-filter (MF) receivers. Results show that cell-free massive MIMO outperforms small-cell systems in both static and high-mobility scenarios, with superior robustness to channel aging and improved SE gains, especially under fractional power control (FPC).

ABSTRACT

In this paper, we investigate the impact of channel aging on the performance of cell-free (CF) massive multiple-input multiple-output (MIMO) systems with pilot contamination. To take into account the channel aging effect due to user mobility, we first compute a channel estimate. We use it to derive novel closed-form expressions for the uplink spectral efficiency (SE) of CF massive MIMO systems with large-scale fading decoding and matched-filter receiver cooperation. The performance of a small-cell system is derived for comparison. It is found that CF massive MIMO systems achieve higher 95\%-likely uplink SE in both low- and high-mobility conditions, and CF massive MIMO is more robust to channel aging. Fractional power control (FPC) is considered to compensate to limit the inter-user interference. The results show that, compared with full power transmission, the benefits of FPC are gradually weakened as the channel aging grows stronger.

Motivation & Objective

• To analyze the impact of channel aging on cell-free massive MIMO systems with pilot contamination.
• To compare the uplink spectral efficiency (SE) of cell-free massive MIMO with that of conventional small-cell systems under realistic mobility conditions.
• To evaluate the effectiveness of fractional power control (FPC) in mitigating inter-user interference under channel aging.
• To derive novel closed-form expressions for uplink SE under large-scale fading decoding (LSFD) and matched-filter (MF) reception with channel aging.
• To quantify the robustness of cell-free massive MIMO to temporal channel variations due to user mobility.

Proposed method

• Models the channel as a time-correlated Rayleigh fading process with an initial state and innovation component to capture channel aging.
• Proposes a channel estimator that leverages the correlation between channel states across time instants to improve estimation accuracy.
• Derives closed-form expressions for uplink SE using large-scale fading decoding (LSFD) and matched-filter (MF) receivers under time-varying channels.
• Incorporates pilot contamination effects by modeling the correlation between channel estimates of different users served by the same APs.
• Applies fractional power control (FPC) to reduce inter-user interference and evaluates its performance gain under varying channel aging conditions.
• Uses stochastic geometry and statistical channel state information to compute the expectation of achievable rates, accounting for time-varying channel statistics.

Experimental results

Research questions

• RQ1 How does channel aging affect the uplink spectral efficiency of cell-free massive MIMO systems with pilot contamination?
• RQ2 How does the performance of cell-free massive MIMO compare to that of a conventional small-cell system under both low- and high-mobility conditions?
• RQ3 What is the impact of fractional power control (FPC) on system performance when channel aging is present?
• RQ4 How does the robustness of cell-free massive MIMO to channel aging compare to that of small-cell systems?
• RQ5 What are the closed-form expressions for uplink SE in cell-free massive MIMO with LSFD and MF receivers under time-correlated fading?

Key findings

• Cell-free massive MIMO achieves higher 95%-likely uplink spectral efficiency than small-cell systems in both low- and high-mobility scenarios.
• Cell-free massive MIMO is more robust to channel aging than small-cell systems, maintaining better performance as mobility increases.
• Fractional power control (FPC) reduces inter-user interference, but its performance gain diminishes as channel aging becomes stronger.
• The derived closed-form expressions for uplink SE are accurate and account for time-correlated channel variations, pilot contamination, and receiver types (LSFD and MF).
• The LSFD receiver provides a two-fold gain in 95%-likely per-user SE over the MF receiver in the presence of channel aging.
• The performance gap between FPC and full power transmission narrows with increasing channel aging, indicating diminishing returns of power control under high mobility.