Liyu Cai

A Novel Timing Synchronization Method for Distributed MIMO-OFDM System

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. Its capability of turning a frequency-selective channel into a parallel collection of frequency flat subchannels has attracted huge interest from the research community as well as the industry. Multiple input and multiple output (MIMO) configuration is also a hot topic for the merit of an increased diversity gain and/or a higher system capacity. In this contribution, a novel timing synchronization method is proposed for the distributed MIMO-OFDM configuration. The performance of this proposed scheme is evaluated by simulations. The simulation results show that the proposed method offers an accurate estimation on the different time delay caused by the distributed transmitters of the MIMO-OFDM system

EESM Based Link Error Prediction for Adaptive MIMO-OFDM System

Exponential effective SNR mapping (EESM) based link error prediction is widely used for adaptive MIMO-OFDM in system evaluations to simplify simulation complexity. In MIMO system, EESM prediction method is more sophisticated due to specific MIMO detection techniques and spatial selectivity of multiple antennas. Especially for maximum likelihood (ML) detection it is hard to determine post-detected SNR due to non-linearity of ML detection, so penalty method based on MMSE detection has been proposed for EESM mapping. Simulations show EESM based effective SNR for ML receiver has obtained high fitting precision compared with AWGN performance in QPSK or 16QAM modulation, convolution coding with rate 1/2 in 2 times 2 MIMO-OFDM system. This method can be naturally generalized into other modulation and coding rates, higher level of MIMO modes, and other fading channels with frequency selectivity, time selectivity or spatial selectivity.

Multi-user subcarrier allocation with minimum rate requests for downlink OFDM packet transmission

In multi-carrier systems, dynamic resource allocation (DRA) has been developed to improve system throughput by taking the benefit of channel variance among users. For practical use, it is important for DRA algorithms to be both fair and efficient. Resource allocation algorithms are measured in terms of fairness and efficiency; then, a new scheduling algorithm (MRR algorithm) considering a users QoS provision is proposed. The MRR (minimum rate request) algorithm is designed to meet individual users minimum required rate while maximizing the fairness and efficiency of the whole system. Simulation results have proved that the new algorithm can reach its target with fairly good fairness and efficiency.

Improved HARQ scheme using channel quality feedback for OFDM systems

An improved Chase combining scheme is proposed to take advantage of the channel quality feedback information, which is mandatory for adaptive modulation and coding (AMC) schemes, to enhance the throughput of OFDM based mobile communication systems. By comparing the signal-to-noise ratio (SNR) measured on sub-carriers with a predetermined threshold, the data to be retransmitted are selected and mapped onto the sub-carriers that have better channel quality to achieve higher diversity gain in the next retransmission. Simulation results prove that such an operation can reduce the amount of retransmitted data and the resource usage in retransmission, which leads to higher throughput efficiency than conventional Chase combining schemes.

Enhanced DFT Interpolation-based Channel Estimation for OFDM Systems with Virtual Subcarriers

In practical OFDM systems, some virtual subcarriers are reserved for easing the requirements on the filter. In this case, the channel impulse response (CIR) estimated by DFT approaches appears a spectral leakage, which results in an error floor for the MSE performance. This paper proposes two enhanced DFT interpolation-based channel estimation methods, termed as EDFTI-CE-A and EDFTI-CE-B, based on the robust Wiener filtering, which can eliminate most of the leakage effect owning to the absence of pilot symbols in the virtual subcarriers and further suppress the effect of additive noise. The analysis and simulation results show that the proposed EDFTI-CE-A can almost eliminate the MSE floor of conventional DFTI-CE with small complexity increase and the proposed EDFTI-CE-B can achieve the same performance as linear MMSE channel estimator but with much lower computational complexity

A low-complexity equalization technique for OFDM system in time-variant multipath channels

In this paper, an equalization technique with low computational complexity is proposed for time-variant multipath channel in OFDM system. Under the assumption that channel impulse response (CIR) varies linearly within one OFDM symbol, this technique utilizes the property of Toeplitz matrix and fast Fourier transform (FFT) matrix to obtain the interchannel interference (ICI) terms in frequency domain with very low complexity, and Jacobi stationary iterative algorithm is proposed to compensate these ICI terms in manner of matrix.

Joint Multiuser Precoding and Scheduling with Imperfect Channel State Information at the Transmitter

This paper addresses the joint design of multiuser precoding and scheduling for downlink multiuser (MU-) multiple-input-multiple-output (MIMO) systems with imperfect channel state information at the transmitter (CSIT). The major issue is to control the increased co-channel-interference (CCI) caused by CSIT error among users sharing the same time-frequency resource. We propose an advanced joint multiuser precoding and scheduling technique, which uses the statistical information of the CSIT error to predict the relationship among CCI, precoding matrixes and scheduling results, and utilizes such information to improve the precoding and scheduling algorithms for better CCI control. Simulation results have shown that considerable capacity improvement can be achieved by the proposed technique in the case of imperfect CSIT.

New iterative detection algorithm for V-BLAST

The vertical Bell Laboratories layered space-time (V-BLAST) scheme achieves very high spectral efficiency while maintaining low implementation complexity. However, the error propagation resulting from the low-diversity substreams constrains the overall system performance greatly. To solve this problem, this paper proposes a new iterative V-BLAST detection algorithm, which greatly improves the performance of the traditional detection algorithm with a reasonable increase of computation complexity. The new iterative detection algorithm also has better performance and lower complexity than another iterative approach in (Cong Shen et al, Elect. Lett. vol.39, no.13, 2003).

Single-symbol preamble design and channel estimation for filter bank multi-carrier modulations

In this paper, the channel estimation related issues in FBMC systems are discussed. A single-symbol preamble structure is proposed and the corresponding channel estimation methods are provided for filter bank multicarrier (FBMC) systems. The simulation results show that compared with the existing methods, the proposed scheme achieves impressive performance gains (up to 4.0dB) with much smaller preamble resource overhead. This is obtained through effective channel filtering for the intrinsic imaginary interference and additive noise exploiting the sparse property of the channel response in delay domain. With the proposed solution, the FBMC system can even save half resources for pilots compared with OFDM systems.

Field trials of downlink multi-cell MIMO

Multi-cell MIMO technique theoretically promises large improvement in spectral efficiency and fairness in cellular networks and is regarded as the key enabling technique for solving the troublesome issue of inter-cell interference. To evaluate its practical performance in the cellular network, we have implemented the real-time downlink multi-cell MIMO trial system operating over bandwidth of 20MHz. The initial field trial indeed shows a significant spectral efficiency gain of downlink multi-cell MIMO over the traditional single-cell transmissions in an indoor deployment of two operating base stations and two terminals. Particularly, several principal operation modes of multi-cell MIMO are evaluated and compared in the field trial measurements, including coherent joint transmission (JT), non-coherent JT and interference nulling. The field trial results agree quite well with expectations from theory.