Zujun Liu

Impact and compensation of I/Q imbalance on channel reciprocity of time-division-duplexing multiple-input multiple-output systems

Direct-conversion-based wireless communication systems suffer from I/Q imbalances in the front-end analog processing. In this study, the authors investigate the impact of frequency-independent (FI) I/Q imbalance on the channel reciprocity of time-division-duplexing (TDD) multiple-input multiple-output (MIMO) systems over frequency flat-fading channels. Moreover, in order to compensate the channel non-reciprocity caused by FI I/Q imbalance, the authors propose two pre-compensation algorithms with and without I/Q imbalance parameters. The performance of the TDD MIMO system under study is evaluated in terms of downlink capacity and bit-error rate. Simulation results show that the proposed algorithms can effectively mitigate the negative effect of FI I/Q imbalance on the channel reciprocity.

Relay-Assisted Opposite Directional Interference Alignment: Feasibility Condition and Achievable Degrees of Freedom

This letter proposes an opposite directional interference alignment (ODIA) algorithm for the general K-user Gaussian interference channel with a half-duplex MIMO relay, where the numbers of transmit and receive antennas are arbitrary. By exploiting the additional space dimensions introduced by the MIMO relay, the algorithm requires only global channel state information (CSI) available at the relay to align the interference from the relay link along the opposite direction of the interference from the direct link. The ODIA feasibility involves the solvability of a linear system. We then derive a sufficient and necessary feasibility condition by using the properties of Kruskal-rank of Khatri-Rao product, and give a closed-form solution. Furthermore, the exact expression of the sum degrees of freedom (DoF) achieved through ODIA is provided and is also verified by simulation results.

Channel Prediction Using IAA-Based Spectral Estimation in Precoded TDD-MIMO Systems

This letter proposes a channel prediction scheme for precoded time division duplex multiple-input multiple-output (TDD-MIMO) systems. Based on the sum-of-sinusoidal (SoS) channel model, the proposed scheme first uses iterative adaptive approach (IAA)-based spectral estimation algorithm for sinusoidal components estimation according to the channel state information (CSI) detected in the discontinuous uplink periods. Then the minimum description length (MDL) criterion is used to choose the proper number of sinusoidal components for the channel model. Finally, time extrapolation is performed to the estimated channel model to predict the future downlink CSI that is needed for transmit precoding. The scheme is verified by simulations.

Reducing specific inter-symbol interference of constant envelope OFDM system through optimizing zero-padding pattern

The application of phase modulation in Constant-Envelope Orthogonal Frequency Division Multiplexing (CE-OFDM) system transforms OFDM signal to a signal with a unity PAPR and exploits frequency diversity, but also induces one specific kind of inter-symbol interference (ISI). In this paper, the expression of such kind of ISI is defined, and indicates that the ISI is dependent on the zero-padding pattern. Hence a method is proposed to reduce such kind of ISI through optimizing zero-padding pattern. Simulation results show that the proposed optimization method can bring obvious BER performance improvement.

Unified SER Performance Analysis and Improvement for Multiuser MIMO Downlink with Correlation Rotation Linear Precoding

In multiuser multiple-input multiple-output (MIMO) downlink communications, system performance suffers from multiuser interference (MUI). Contrary to common practice where MUI is suppressed, correlation rotation (CR) linear precoding makes MUI constructive via rotating the angle of correlation between transmitted symbols. With the derived closed-form instantaneous SINR expression, we analyze the unified SER performance for CR precoding, and identify the channel condition under which the MUI exploitation via CR precoding will fail and consequently lead to the unified SER performance degradation. To solve the problem, two power allocation schemes that are subject to power constraint are proposed: one is iterative scheme that directly minimizes the unified SER, and the other has a closed-form solution that maximizes the minimum user SINR. The asymptotic behavior and computational complexity of the proposed schemes are analyzed. Numerical and simulation results show that both schemes can avoid the degradation of unified SER performance and furthermore offer performance gains. Moreover, the closed-form solution with much lower complexity can provide a comparable performance to that of the iterative scheme.

A Robust NDA Carrier Tracking Method for LDPC-Coded Systems With Symbol Rate Sampling

In this letter, a robust nondata-aided (NDA) carrier tracking method is proposed for low-density parity-check (LDPC) coded systems with symbol rate sampling. The proposed method has a novel and hybrid carrier tracking loop combined with the frequency-locked loop (FLL), increasing/decreasing sampling rate blocks, phase-locked loop (PLL), and modified symbol decision method, which is capable of improving estimation accuracy of the frequency offsets and phase offsets at low signal-to-noise ratios (SNRs). When applied to the

Joint Interference Suppression for Multi-User TDD MIMO Downlink with Other-Cell Interference

(4096,3072)

Space-time cooperative transmission scheme for asynchronous environment

LDPC coded system, numerical simulations and comparisons are provided to validate the effectiveness of the proposed carrier tracking method in the case of relatively low SNRs.

Ultra wideband stratum communication system

In multiuser TDD MIMO downlink communications, system performance suffers from multiuser interference(MUI) and other-cell interference(OCI). This paper proposes designing a precoder to jointly suppress MUI and OCI. Firstly, a precoding matrix is designed for multiuser interference suppression, with the help of the QR decomposition of generalized MMSE channel inversion. And then, using a signal leakage criterion, the second precoding matrix is designed to suppress OCI and residual MUI. The proposed algorithm can avoid user feedback in multiuser TDD MIMO systems. Simulation results demonstrate the effectiveness of the proposed scheme.

Time-variant TDD-MIMO communication channel reciprocity compensation method based on channel prediction

A space-time cooperative transmission scheme based on orthogonal space-time block codes (OSTBC) and single-carrier block transmission (SCBT) is proposed. The scheme can be applied to asynchronous cooperative transmission scenarios with any number of relay nodes. By simple linear combination and minimum-mean-square-error frequency-domain equalization (MMSE-FDE), the scheme can obtain full diversity gains.