arXiv subjects

Hideki Ochiai

Publications and source records attributed to Hideki Ochiai.

24 records · Page 2Linked to original sources

arXiv2022

On Rate-Splitting With Non-unique Decoding In Multi-cell Massive MIMO Systems

We consider the downlink of a multi-cell massive MIMO system suffering from asymptotic rate saturation due to pilot contamination. As opposed to treating pilot contamination interference as noise (TIN), we study the performance of decoding the pilot contamination interference. We model pilot-sharing users as an interference channel (IC) and study the performance of schemes that decode this interference partially based on rate-splitting (RS), and compare the performance to schemes that decode the interference in its entirety based on simultaneous unique decoding (SD) or non-unique decoding (SND). For RS, we non-uniquely decode each layer of the pilot contamination interference and use one common power splitting coefficient per IC. Additionally, we establish an achievable region for this RS scheme. Solving a maximum symmetric rate allocation problem based on linear programming (LP), we show that for zero-forcing (ZF) with spatially correlated/uncorrelated channels and with a practical number of BS antennas, RS achieves significantly higher spectral efficiencies than TIN, SD and SND. Furthermore, we numerically examine the impact of increasing the correlation of the channel across antennas, the number of users as well as the degree of shadow fading. In all cases, we show that RS maintains significant gain over TIN, SD and SND.

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arXiv2022

A New Polar Code Design Based on Reciprocal Channel Approximation

This paper revisits polar code design for a binary-input additive white Gaussian noise (BI-AWGN) channel when successive cancellation (SC) decoding is applied at the receiver. We focus on the reciprocal channel approximation (RCA), which is often adopted in the design of low-density parity-check (LDPC) codes. In order to apply RCA to polar code design for various codeword lengths, we derive rigorous closed-form approximations that are valid over a wide range of SNR over an AWGN channel, for both the mutual information of BPSK signaling and the corresponding reciprocal channel mapping. As a result, the computational complexity required for evaluating channel polarization is thus equivalent to that based on the popular Gaussian approximation (GA) approach. Simulation results show that the proposed polar code design based on RCA outperforms those based on GA as well as the so-called improved GA (IGA) approach, especially as the codeword length is increased. Furthermore, the RCA-based design yields a better block error rate (BLER) estimate compared to GA-based approaches.

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arXiv2021

On Downlink Interference Decoding In Multi-Cell Massive MIMO Systems

In this paper, the downlink of a multi-cell massive MIMO system is considered where the channel state information (CSI) is estimated via pilot symbols that are orthogonal in a cell but re-used in other cells. Re-using the pilots, however, contaminates the CSI estimate at each base station (BS) by the channel of the users sharing the same pilot in other cells. The resulting inter-cell interference does not vanish even when the number of BS antennas $M$ is large, i.e., $M\rightarrow\infty$, and thus the rates achieved by treating interference as noise (TIN) saturate even if $M\rightarrow\infty$. In this paper, interference aware decoding schemes based on simultaneous unique decoding (SD) and simultaneous non-unique decoding (SND) of the full interference or a part of the interference (PD) are studied with two different linear precoding techniques: maximum ratio transmission (MRT) and zero forcing (ZF). The resulting rates are shown to grow unbounded as $M\rightarrow\infty$. In addition, the rates achievable via SD/SND/PD for finite $M$ are derived using a worst-case uncorrelated noise technique, which are shown to scale as $\mathcal{O}(\log M)$. To compare the performance of different schemes, the maximum symmetric rate problem is studied, where it is confirmed that with large, yet practical, values of $M$, SND strictly outperforms TIN, and also that PD strictly outperforms SND.

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arXiv2020

Capacity-Approaching Polar Codes with Long Codewords and Successive Cancellation Decoding Based on Improved Gaussian Approximation

This paper focuses on an improved Gaussian approximation (GA) based construction of polar codes with successive cancellation (SC) decoding over an additive white Gaussian noise (AWGN) channel. Arikan has proven that polar codes with low-complexity SC decoder can approach the channel capacity of an arbitrary symmetric binary-input discrete memoryless channel, provided that the code length is chosen large enough. Nevertheless, how to construct such codes over an AWGN channel with low computational effort has been an open problem. Compared to density evolution, the GA is known as a low complexity yet powerful technique that traces the evolution of the mean log likelihood ratio (LLR) value by iterating a nonlinear function. Therefore, its high-precision numerical evaluation is critical as the code length increases. In this work, by analyzing the asymptotic behavior of this nonlinear function, we propose an improved GA approach that makes an accurate trace of mean LLR evolution feasible. With this improved GA, through numerical analysis and simulations with code lengths up to $N=2^{18}$, we explicitly demonstrate that various code-rate polar codes with long codeword and capacity approaching behavior can be easily designed.

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arXiv2011

A Generalized MMSE Detection with Reduced Complexity for Spatially Multiplexed MIMO Signals

In multiple-input multiple-output (MIMO) spatially multiplexing (SM) systems, achievable error rate performance is determined by signal detection strategy. The optimal maximum-likelihood detection (MLD) that exhaustively examines all symbol candidates has exponential complexity and may not be applicable in many practical systems. In this paper, we consider a generalized minimum mean square error (MMSE) detection derived from conditional mean estimation, which in principle behaves equivalently to MLD but also includes a linear MMSE detection as a special case. Motivated by this fact, we propose a low-complexity detection which significantly reduces the number of examined symbol candidates without significant error rate performance degradation from MLD. Our approach is to approximate the probability density function (pdf) of modulated symbols that appears in the exact conditional mean expression such that the decision metric can be cast into a partially closed form. It is found that uniform ring approximation in combination with phase shift keying (PSK) and amplitude phase shift keying (APSK) is promising, as it can achieve a performance even comparable to MLD, while its complexity is linear when the number of transmit antennas is two.

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arXiv2005

Collaborative Beamforming for Distributed Wireless Ad Hoc Sensor Networks

The performance of collaborative beamforming is analyzed using the theory of random arrays. The statistical average and distribution of the beampattern of randomly generated phased arrays is derived in the framework of wireless ad hoc sensor networks. Each sensor node is assumed to have a single isotropic antenna and nodes in the cluster collaboratively transmit the signal such that the signal in the target direction is coherently added in the far- eld region. It is shown that with N sensor nodes uniformly distributed over a disk, the directivity can approach N, provided that the nodes are located sparsely enough. The distribution of the maximum sidelobe peak is also studied. With the application to ad hoc networks in mind, two scenarios, closed-loop and open-loop, are considered. Associated with these scenarios, the effects of phase jitter and location estimation errors on the average beampattern are also analyzed.

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