Xiaolin Hou

DMRS Design and Channel Estimation for LTE-Advanced MIMO Uplink

In 3GPP long term evolution (LTE) uplink, single transmit antenna is adopted due to its simplicity and acceptable performance. However, in order to satisfy the higher requirement of LTE-Advanced (LTE-A) on uplink spectrum efficiency, multi- ple transmit antennas must be supported in LTE-A uplink. At the same time, the backwards compatibility to LTE should be taken into considerations. Therefore, the demodulation reference signal (DMRS) as well as channel estimation have to be re-designed for LTE-A uplink to support multiple-input multiple-output (MIMO) transmission. In this paper, we propose the DMRS design for LTE-A MIMO uplink via frequency domain code division multiplexing (FD-CDM) with the maximum distance binding (MDB), which can minimize the interference among multiple transmit antennas. More importantly, it is backwards compatible to the DMRS in LTE uplink, i.e., few modifications to the current specifications are needed. Furthermore, the improved uplink channel estimation with dynamic channel impulse response reser- vation (DCIR 2 ) and frequency domain windowing/dewindowing

Experimental study of advanced MU-MIMO scheme with antenna calibration for the evolving LTE TDD system

As the 3GPP long term evolution (LTE) system actively evolves, advanced multi-user multiple-input multiple-output (MU-MIMO) schemes have attracted increasing interest in the engineering field. We have proposed a QR decomposition (QRD) based MU-MIMO scheme for the evolving LTE time division duplex (TDD) system to achieve high spectrum efficiency, assuming ideal channel reciprocity. In order to investigate the promising performance of TDD MU-MIMO in practice, we implement the proposed QRD-based TDD MU-MIMO scheme together with an actual over-the-air (OTA) antenna calibration scheme into our FPGA-based hardware testbed for the evolving LTE TDD system to examine its performance under the constraint of practical channel reciprocity. Measurement results show that channel reciprocity indeed could be achieved with the OTA antenna calibration and the QRD-based MU-MIMO scheme can effectively make use of this channel reciprocity to achieve high spectrum efficiency data transmission for both downlink and uplink. Moreover, the influence of antenna calibration on the TDD MU-MIMO performance is further investigated and several considerations related to antenna calibration are addressed.

Low-Complexity Enhanced DFT-based Channel Estimation for OFDM Systems with virtual Subcarriers

Traditional discrete Fourier transform (DFT) based channel estimation will suffer from significant performance degradation due to the edge effect in practical orthogonal frequency division multiplexing (OFDM) systems with virtual subcarriers. In this paper, a novel enhanced DFT-based channel estimation method is proposed, which can effectively mitigate the edge effect caused by virtual subcarriers via edge value repetition in the frequency domain and zero insertion in the time domain, thus to improve the overall channel estimation performance for the usable subcarriers. Moreover, it does not need a priori statistical characteristics of the wireless channel and the complexity of channel estimation is not increased. Therefore, it is especially suitable for hardware implementation.

Two-Cell Coordinated Transmission Scheme Based on Interference Alignment and MU-MIMO Beamforming

We investigate the problem of two-cell multi-user co-channel transmission, wherein both inter-cell interference and intra-cell inter-user interference should be well managed. Interference alignment (IA) is theoretically proved to be a promising technology for managing co-channel interference. It can be used to mitigate inter-cell interference. When multiple users are served by a single cell, advanced multi- user multiple-input multiple output (MU-MIMO) transmission can be used to manage the intra-cell inter-user interference. In this paper, we present an efficient two-cell coordinated linear beamforming method where IA is combined with MU-MIMO to achieve high performance multi-cell cooperative transmission. The effectiveness of the proposed method is verified by numerical evaluations.

Adaptive Multi-Tx Multi-Rx MIMO Transmission Scheme for LTE-Advanced Downlink

Multiple-input multiple output (MIMO) spatial multiplexing has been adopted by 3GPP long term evolution (LTE) to increase the peak spectrum efficiency. However, it will lose its effectiveness for cell edge users due to the inter-cell interference (ICI). Therefore, 3GPP LTE-Advanced (LTE-A) is considering to introduce some multi-cell cooperative transmission techniques to solve this problem. In this paper, we first propose the distributed multi-Tx multi-Rx (MTMR) MIMO spatial multiplexing for LTEA downlink, which only needs partial cooperation, so the heavy backhauling overhead and the high implementation complexity encountered in the traditional full cooperation scheme can be avoided. Meanwhile, because of the efficient utilization of multi-cell transmit power and spatial degree of freedom, a higher spectrum efficiency can be achieved by MTMR MIMO spatial multiplexing in good channel conditions compared with the other partial cooperation schemes. Then in order to ensure the highest possible cell edge spectrum efficiency to be achieved in different channel conditions, we further integrate multiple distributed cooperation modes into the same framework and introduce a mechanism of adaptive cooperation mode selection. Both qualitative analysis and quantitative simulations show the proposals effectiveness and indicate it a promising candidate for the cooperative transmission in LTE-A downlink.

Enhanced iterative max-sum-rate algorithm for linear MU-MIMO precoding

Multi-user multiple input multiple output (MU-MIMO) systems can achieve high spectral efficiency for wireless communication. The design of optimal transmit precoding for max-sum-rate (MSR) MU-MIMO downlink transmission is a non-trivial problem. Recent research approached this problem by exploiting the relationship between max-sum-rate and the minimum-mean-squared-error (MMSE) and proposed several iterative schemes. Based on these research results, we propose enhanced iterative MSR algorithms for linear MU-MIMO precoding which improve the sum rate performance and reduce the computational complexity. The effectiveness of the proposed method is verified by numerical simulations.

QRD-based MU-MIMO transmission scheme towards evolved LTE TDD system

With the active evolvement of 3GPP long term evolution (LTE) system, advanced multi-user multiple-input multiple-output (MU-MIMO) transmission schemes attract more and more interests in engineering field. However, in order to bring advanced MU-MIMO techniques from theory to practice, there are two major obstacles to overcome, i.e., how to obtain accurate channel state information at the transmitter side (CSIT) and how to achieve high spectrum efficiency with low computational complexity. In this paper, we propose QR decomposition (QRD) based MU-MIMO transmission scheme towards evolved LTE time division duplex (TDD) system, which can orthogonalize the multiple data streams to/from multiple users in downlink/uplink with TDD ping-pong transmission to realize high spectrum efficiency, thanks to the accurate downlink/uplink CSIT obtained via the channel reciprocity and the downlink/uplink reference signal (RS) in evolved LTE TDD system. Furthermore, it only needs low-complexity QRD operation, which can simplify the transceiver structure significantly to facilitate its implementation in practice. Computer simulation results demonstrate the effectiveness of our proposal and suggest it a promising candidate of advanced MU-MIMO transmission schemes towards evolved LTE TDD system.

Time-Domain MMSE Channel Estimation Based on Subspace Tracking for OFDM Systems

Channel state information (CSI) is critical for the performance of orthogonal frequency division multiplexing (OFDM) systems. In this paper, we propose time-domain minimum mean square error (MMSE) channel estimation based on subspace tracking for OFDM systems, which can increase the channel estimation accuracy as well as lower the computational complexity. Furthermore, it requires little prior channel statistical information and can track the time-varying wireless channel effectively. With proper training sequences design on transmitter antennas, it can be easily applied to multiple inputs multiple outputs (MIMO) OFDM systems. Computer simulations demonstrate its good performance

Interference alignment based dynamic TDD for small cells

Dynamic time-division-duplexing (TDD) uplink (UL) and downlink (DL) reconfiguration is a promising solution to the problem of traffic adaptation in future small cell deployment, wherein the traffic demand may vary dynamically in volume and in transmission direction. Applying dynamic TDD UL-DL reconfiguration in a multi-cell scenario may lead to new challenges caused by inter-cell interference. In this paper, we propose an interference alignment based multiple-input-multiple-output (MIMO) transmission scheme that effectively addresses the interference problem in dynamic TDD systems and improves the system capacity. The proposed scheme may be relevant to 3GPP LTE standardization of enhanced interference mitigation and traffic adaptation (eIMTA), small cell enhancement and coordinated multi-point (CoMP) topics.

Doubly-Selective Channel Estimation for Packet OFDM Systems with Virtual Subcarriers

Channel estimation based on two-dimensional discrete- time Fourier transform interpolation (2D-DFTI) is suitable for orthogonal frequency division multiplexing (OFDM) systems because of its robustness to doubly-selective fading channels and high computational efficiency. However, its performance will degrade significantly for packet OFDM systems with virtual subcarriers due to the Gibbs phenomenon in both frequency- domain and time-domain. We have already proposed the frequency- domain enhanced DFT interpolation (FD-EDFTI) to effectively suppress the Gibbs phenomenon in frequency-domain, while in this paper the time-domain enhanced DFT interpolation (TD-EDFTI) is further proposed to mitigate the Gibbs phenomenon in time-domain. Therefore, doubly-selective channel estimation based on 2D-EDFTI can be easily implemented for packet OFDM systems with virtual subcarriers by concatenating FD-EDFTI with TD-EDFTI to efficiently suppress the Gibbs phenomenon in both frequency- domain and time-domain, while keeping good robustness and high computational efficiency.