Massive MIMO Transmission for LEO Satellite Communications (2002.08148v1)

Abstract: Low earth orbit (LEO) satellite communications are expected to be incorporated in future wireless networks, in particular 5G and beyond networks, to provide global wireless access with enhanced data rates. Massive MIMO techniques, though widely used in terrestrial communication systems, have not been applied to LEO satellite communication systems. In this paper, we propose a massive MIMO transmission scheme with full frequency reuse (FFR) for LEO satellite communication systems and exploit statistical channel state information (sCSI) to address the difficulty of obtaining instantaneous CSI (iCSI) at the transmitter. We first establish the massive MIMO channel model for LEO satellite communications and simplify the transmission designs via performing Doppler and delay compensations at user terminals (UTs). Then, we develop the low-complexity sCSI based downlink (DL) precoder and uplink (UL) receiver in closed-form, aiming to maximize the average signal-to-leakage-plus-noise ratio (ASLNR) and the average signal-to-interference-plus-noise ratio (ASINR), respectively. It is shown that the DL ASLNRs and UL ASINRs of all UTs reach their upper bounds under some channel condition. Motivated by this, we propose a space angle based user grouping (SAUG) algorithm to schedule the served UTs into different groups, where each group of UTs use the same time and frequency resource. The proposed algorithm is asymptotically optimal in the sense that the lower and upper bounds of the achievable rate coincide when the number of satellite antennas or UT groups is sufficiently large. Numerical results demonstrate that the proposed massive MIMO transmission scheme with FFR significantly enhances the data rate of LEO satellite communication systems. Notably, the proposed sCSI based precoder and receiver achieve the similar performance with the iCSI based ones that are often infeasible in practice.

• The paper introduces a sCSI-based transmission strategy that achieves near-optimal performance by maximizing ASLNR and ASINR in LEO satellite systems.
• It develops a comprehensive channel model with Doppler and delay compensation to enhance both uplink and downlink transmission efficiency.
• A novel space angle-based user grouping algorithm minimizes intra-beam interference, paving the way for advanced 5G and beyond networks.

Massive MIMO Transmission for LEO Satellite Communications

The paper proposes a novel approach to integrate massive Multiple-Input Multiple-Output (MIMO) systems into Low Earth Orbit (LEO) satellite communications. The research explores the theoretical underpinnings and practical implications of applying massive MIMO with full frequency reuse (FFR), presenting a significant step forward in satellite communication technology. The central focus is on addressing the difficulty of obtaining instantaneous channel state information (iCSI) at the transmitter, which becomes challenging due to the inherent dynamics and the long propagation delay between the satellites and user terminals (UTs). Instead, the authors advocate for the use of statistical channel state information (sCSI) as a more feasible alternative.

Key Contributions and Methodologies

1. Channel Modeling and Delay Compensation: The authors first establish a comprehensive channel model for LEO satellite communications that accounts for signal propagation characteristics such as Doppler shifts and delays. They implement Doppler and delay compensation mechanisms at the user terminals to streamline the transmission process at both uplink and downlink paths.
2. sCSI-Based Transmission Strategy: The paper presents a low-complexity transmission strategy utilizing sCSI for both downlink (DL) precoders and uplink (UL) receivers. By formulating the problem in closed-form solutions, the proposed scheme aims to maximize the average signal-to-leakage-plus-noise ratio (ASLNR) and the average signal-to-interference-plus-noise ratio (ASINR). This sCSI-based approach theoretically achieves near-optimal performance comparable to iCSI-based strategies, which are often too costly and impractical due to system constraints.
3. User Grouping via Space Angle: A novel user grouping algorithm based on space angle information is introduced. The space angle based user grouping (SAUG) aims to minimize intra-beam interference by ensuring the orthogonality of user channel direction vectors. This grouping is asymptotically optimal when the number of satellite antennas or user groups is sufficiently large.
4. Practical and Theoretical Implications: The simulation results demonstrate the efficacy of the proposed massive MIMO transmission technique, indicating a marked improvement in data rates over conventional methods. This work paves the way for efficient utilization of frequency resources in LEO satellite systems, aligning closely with emerging requirements in 5G and beyond networks.

Implications and Future Research Directions

The integration of massive MIMO into LEO satellite communications represents a significant advancement, with potential implications across various facets of wireless communication technology. The ability to utilize sCSI provides a realistic path for implementing massive MIMO in LEO satellite systems. This approach mitigates the challenges posed by long propagation delays and high mobility, which are predominant in non-terrestrial environments.

Moreover, this research underscores the importance of efficient user grouping strategies and the use of space angle information, which can be crucial for reducing inter-user interference and optimizing resource allocation.

Potential future research directions might include the exploration of low-complexity sCSI estimation techniques, adaptations for multi-antenna or directive antenna user terminals, as well as extension to multi-LEO satellite systems. Furthermore, practical implementations of such systems would benefit from further investigations into hardware constraints and energy efficiency, which remain critical challenges in the deployment of satellite-based communication networks.

In conclusion, the paper provides a comprehensive exploration of massive MIMO application in LEO satellite communications, addressing critical challenges with feasible solutions and opening avenues for further advancements in satellite communication research.