A Rate Splitting Strategy for Massive MIMO with Imperfect CSIT

Abstract: In a multiuser MIMO broadcast channel, the rate performance is affected by the multiuser interference when the Channel State Information at the Transmitter (CSIT) is imperfect. To tackle the detrimental effect of the multiuser interference, a Rate-Splitting (RS) approach has been proposed recently, which splits one selected user's message into a common and a private part, and superimposes the common message on top of the private messages. The common message is drawn from a public codebook and should be decoded by all users. In this paper, we generalize the idea of RS into the large-scale array regime with imperfect CSIT. By further exploiting the channel second-order statistics, we propose a novel and general framework Hierarchical-Rate-Splitting (HRS) that is particularly suited to massive MIMO systems. HRS simultaneously transmits private messages intended to each user and two kinds of common messages that can be decoded by all users and by a subset of users, respectively. We analyse the asymptotic sum rate of RS and HRS and optimize the precoders of the common messages. A closed-form power allocation is derived which provides insights into the effects of system parameters. Finally, simulation results validate the significant sum rate gain of RS and HRS over various baselines.

• The paper introduces a Rate Splitting (RS) strategy that divides user messages into common and private parts to alleviate multiuser interference in massive MIMO systems.
• It further proposes Hierarchical-Rate-Splitting (HRS) to manage both inter-group and intra-group interference using closed-form power allocation analysis.
• Simulations demonstrate that RS and HRS outperform conventional methods, showing robustness against CSIT errors and significant sum rate gains.

Evaluation of a Rate Splitting Strategy for Massive MIMO Systems with Imperfect CSIT

This paper addresses the challenge of multiuser interference in Massive MIMO systems when Channel State Information at the Transmitter (CSIT) is imperfect. The authors put forward a Rate Splitting (RS) strategy which is expanded upon under a new framework called Hierarchical-Rate-Splitting (HRS). This proposal is particularly designed for massive MIMO systems where imperfect CSIT is a critical concern.

Key Contributions

The paper discusses several significant contributions:

1. Rate Splitting Strategy: The paper extends the concept of Rate-Splitting (RS) to a massive MIMO environment with imperfect CSIT. By splitting a selected user's message into a common part and a private part, RS introduces a mechanism to alleviate multiuser interference. The common part can be decoded by all users, while the private parts are targeted specifically to individual users.
2. Hierarchical-Rate-Splitting (HRS): Building on RS, the authors propose HRS. This framework introduces an additional layer of message splitting, categorizing messages into outer and inner common streams. This approach caters to both inter-group and intra-group interference in large-scale antenna systems by utilizing a two-tier precoder handling.
3. Performance Analysis: The paper provides an asymptotic performance analysis of RS and HRS using random matrix theory. It delivers closed-form equations for power allocation that show how system parameters like CSIT quality and spatial correlation affect performance. The analysis is complemented with simulation results, illustrating sum rate gains over existing baselines.

Strong Numerical Results

The paper demonstrates that RS and HRS outperform conventional Zero-Forcing (ZF) and broadcast strategies in simulations by achieving significant sum rate gains. RS, in particular, exhibits robustness against CSIT errors by increasing its sum rate without saturation at high SNRs, contrasting sharply with the traditional multiuser broadcast methods that plateau due to interference limitations. HRS, designed for scenarios with spatial correlation, achieves superior performance by effectively managing both inter-group and intra-group interferences.

Implications and Future Scope

This research presents important practical and theoretical implications. The introduction of RS and HRS offers a viable approach to mitigating interference in massive MIMO systems, potentially enhancing spectral and energy efficiency in future wireless networks. The methodology and insights could be extended to multi-cell and highly distributed networks, where managing differential quality of CSIT becomes increasingly challenging.

For future research, several avenues are suggested. One critical area is refining power allocation strategies in RS and HRS frameworks as network densities increase. Another opportunity is exploring the application of these strategies in millimeter-wave frequencies, where beamforming intricacies and CSIT imperfections are inherent. Furthermore, investigating the potential integration with advanced scheduling algorithms could further leverage the divide between the common and private message handling.

In summary, the paper signifies a valuable contribution to the ongoing efforts to enhance how massive MIMO networks operate under imperfect CSI conditions, and sets the path for further investigations into interference management and transmission strategies.