Channel Hardening-Exploiting Message Passing (CHEMP) Receiver in Large-Scale MIMO Systems (1310.3062v2)

Abstract: In this paper, we propose a MIMO receiver algorithm that exploits {\em channel hardening} that occurs in large MIMO channels. Channel hardening refers to the phenomenon where the off-diagonal terms of the ${\bf H}^H{\bf H}$ matrix become increasingly weaker compared to the diagonal terms as the size of the channel gain matrix ${\bf H}$ increases. Specifically, we propose a message passing detection (MPD) algorithm which works with the real-valued matched filtered received vector (whose signal term becomes ${\bf H}^T{\bf H}{\bf x}$, where ${\bf x}$ is the transmitted vector), and uses a Gaussian approximation on the off-diagonal terms of the ${\bf H}^T{\bf H}$ matrix. We also propose a simple estimation scheme which directly obtains an estimate of ${\bf H}^T{\bf H}$ (instead of an estimate of ${\bf H}$), which is used as an effective channel estimate in the MPD algorithm. We refer to this receiver as the {\em channel hardening-exploiting message passing (CHEMP)} receiver. The proposed CHEMP receiver achieves very good performance in large-scale MIMO systems (e.g., in systems with 16 to 128 uplink users and 128 base station antennas). For the considered large MIMO settings, the complexity of the proposed MPD algorithm is almost the same as or less than that of the minimum mean square error (MMSE) detection. This is because the MPD algorithm does not need a matrix inversion. It also achieves a significantly better performance compared to MMSE and other message passing detection algorithms using MMSE estimate of ${\bf H}$. We also present a convergence analysis of the proposed MPD algorithm. Further, we design optimized irregular low density parity check (LDPC) codes specific to the considered large MIMO channel and the CHEMP receiver through EXIT chart matching. The LDPC codes thus obtained achieve improved coded bit error rate performance compared to off-the-shelf irregular LDPC codes.

Channel Hardening-Exploiting Message Passing (CHEMP) in Large-Scale MIMO Systems

This paper introduces a novel approach to MIMO systems utilizing a Channel Hardening-Exploiting Message Passing (CHEMP) receiver, specifically tailored for large-scale MIMO environments. The CHEMP architecture fundamentally alters conventional channel estimation and detection schemes by directly leveraging the channel hardening effect inherent in large MIMO systems, resulting in reduced computational complexity without substantially compromising on performance.

Core Concepts and Methodology

In the field of large-scale MIMO systems, channel hardening manifests when off-diagonal elements of the channel matrix become negligible, simplifying interference management. This phenomenon makes large MIMO channels much more stable, thus allowing for the simplification of signal processing techniques.

The CHEMP approach builds upon this concept by utilizing a message passing detection (MPD) algorithm. This algorithm operates on real-valued matched filtered signals, approximating off-diagonal elements of the channel matrix with Gaussian distributions. The algorithm circumvents the need for matrix inversion, a computationally intensive requirement in traditional MMSE-based detectors, by directly estimating ${\bf H}^T{\bf H}$ instead of ${\bf H}$. This shifts complexity away from channel inversion operations.

Performance and Complexity

The proposed CHEMP receiver exhibits complexity nearly comparable to MMSE detectors but affords marked performance improvements. In systems ranging from 16 to 128 users with hundreds of antennas, CHEMP showed substantial BER performance gains over MMSE, notably so in high-loading scenarios where spatial interference is prominent.

Empirical results indicate that CHEMP outperforms both MMSE and other message passing detectors that rely on MMSE channel estimates. The complexity of CHEMP remains advantageous, as its operations primarily entail matrix multiplications rather than inversions, a crucial consideration in scalable implementations.

Implications and Future Directions

The CHEMP's ability to operate effectively without direct channel gain estimation paves the way for low-complexity implementations of MIMO systems. Its robustness to channel estimation errors suggests applicability in environments with constrained computational resources. Moreover, the successful integration with irregular LDPC codes, optimized via EXIT chart matching, presents opportunities for improved coding strategies that are tailored to these conditions.

For future research, expanding the CHEMP framework to frequency-selective channels could deliver further enhancements. Additionally, refining convergence conditions beyond the current sufficiency could unlock greater system robustness, ensuring the algorithm remains effective across diverse operational environments.

In summary, the CHEMP receiver represents a significant stride in efficient signaling within large-scale MIMO systems by capitalizing on channel hardening properties. Its innovative design principles could inform further advancements in message passing paradigms and set the stage for next-generation wireless communications.