Daoben Li

Effect of fading correlation on capacity of distributed MIMO

Distributed multiple-input multiple-output (MIMO) system combines the advantages of point-to-point MIMO systems and distributed antenna systems, and improves link quality and coverage. A distributed MIMO channel model which comprises relative path loss, correlated shadow fading and correlated small scale fading is proposed. The impact of two kinds of fading correlation on capacity of distributed MIMO channel is investigated and compared carefully by Monte Carlo simulations, and the characteristics of capacity space distribution of distributed MIMO channel with spatial shadowing correlation are also obtained.

Convolutional Multi-Code Multiplexing for OFDM Systems

The paper proposes a new multiplexing scheme for the OFDM-based systems with significant coding gain and diversity gain, which is called convolutional multi-code multiplexing (CMM). In this scheme, the data symbols are spread onto orthogonal subcarriers by convolutional spreaders, and multiple data streams spread by different codes can be transmitted on the same frequency band simultaneously. The free Euclidean distance and the free Hamming distance are found to dominate the performance of the OFDM-CMM system in AWGN channel and frequency selective fading channel, respectively. Some codes with good performance are found by computer search. Compared to the conventional uncoded OFDM system with the same spectrum efficiency, the OFDM-CMM system can achieve significant diversity gain in frequency selective fading channel, and notable coding gain both in AWGN channel and fading channel.

Analysis of Overlapped Code Division Multiple Access System in Gaussian Multiple Access Channel

This paper proposes an overlapped code division multiple access (OVCDMA) system where each user is assigned a unique convolutional spreading code and all the users access the channel simultaneously. Node error probability and symbol error probability upper bounds of OVCDMA in Gaussian multiple access channel are derived based on the random coding arguments. The former one ensures the existence of convolutional spreading code dedicated to M-ary (M>2) modulation and the later one is tight for moderate signal-to-noise ratio. In addition, the random coding simulation results show that the symbol error rate gets lower by increasing the convolutional spreading code length. Finally, several convolutional spreading codes dedicated to QPSK are found by random coding search and free Euclidean distance calculation. They are compared with conventional modulation scheme under the condition of same spectral efficiency. Simulations for these fixed convolutional spreading codes show that notable coding gain are achieved in Gaussian multiple access channel.

Performance Analysis Framework of ML MIMO Receiver over Correlated Rayleigh Fading Channel

In this paper, a novel performance analysis frame-work of Maximum Likelihood (ML) receiver for Vertical Bell Labs Layered Space-Time (V-BLAST) architecture over correlated Rayleigh fading channel is presented. Based on the statistical property of squared Euclidean distance, the exact expressions for Pairwise Error Probability (PEP) over transmit, receive, and joint correlated fading channel are derived. The approximate expression of the PEPs for high signal-to-noise ration (SNR) region is used to quantitatively analyze the loss of the diversity order and the penalty of SNR. Three interesting results of this analysis are: 1) the loss caused by transmit and receive correlation is independent. 2) the transmit correlation has no impact on the diversity order, and the loss of SNR caused by transmit correlation is determined by the transmit correlation matrix and modulation scheme. 3) the loss of diversity order due to receive correlation is equals to the difference of the numbers of receiver antenna and the rank of receive correlation matrix, under the assumption that the receive correlation matrix has full rank, the penalty of SNR is jointly determined by the determinant of the receive correlation matrix and the number of receive antenna. Simulation results are given to corroborate the theoretical analysis.

Channel estimation based on continuous pilot channel with regression in LAS-CDMA

For downlink transmission, known pilot symbols are usually used to evaluate the channel characters through the common pilot channel. Contrasting to traditional CDMA, LS spreading codes in LAS-CDMA (large area code division multiple access) have IFW (interference free window). Thus when pilot symbols are used to estimate the channel coefficients. AWGN (additive white Gaussian noise) is the main obstacle to the accuracy. To suppress the noise degradation, in this paper, a new channel estimation method for LAS-CDMA system is introduced. In the proposed method, the channel coefficients obtained through common pilot channel are first fed to a regression process and these updated coefficients are then for the corresponding data symbol recovery. Linear and nonlinear regression schemes are both considered according to vehicle speeds and regression process lengths over the ITU-VA channel. Simulation results prove that this method can improve the channel estimation accuracy and system BER (bit error rate) performance greatly.

Optimal pilot interval design for channel estimation

In the wireless communication system, interpolated pilot scheme is usually applied for the dedicated pilot channel. It means that the known pilot symbols are time multiplexed with data symbols at the regular intervals within the time duration of one frame. Whether the length of the pilot symbol interval will affect the performance of channel estimation and what is the optimal pilot interval? They are the main problems studied in this paper. Considering linear interpolation channel estimation, the pilot interval design will be investigated in LAS/CDMA system. Based on the channel coherence time and noise accumulation property, the optimal design rule for the pilot symbol interval is given. Simulations are done over the ITU-VA channel at different vehicle speeds, and the results prove that the proposed design rule is reasonable and the experiential value for the optimal pilot interval is also given from lots of the simulation results, which is significative for the improvement of the system performance.

A novel method used for the performance analyzing of channel estimation in time-varying fast fading channel - equivalent channel analysis method

In a time-varying fast-fading channel, due to the influence of amplitude fading and fast phase changing, the performance of a QAM system deteriorates seriously; there are also restraints on using high-dimension modulation in fast mobile communications. To solve this problem, many estimation methods have been presented. The process of channel estimation is practically that of compensating the channel characteristic. Thus, analyzing the performance of channel estimation methods also analyzes the equivalent channel after compensation. Characteristics of equivalent channel by compensation of some estimation is conducted in the course of equivalent channel estimation. Various channel estimations essentially estimate and compensate the channel with the channel correlation. The correlation of amplitude can be analyzed by amplitude ratio, while that of phase by phase difference. Hence, above all, we analyze the channel estimation performance by calculating the probability distribution of the amplitude ratio and phase difference at two random times. We name this method equivalent channel analysis method. By using this method, system performance is only the function of the Doppler shift and channel correlation coefficient that depend on the channel estimation method. We only need to calculate the correlation coefficient of two neighbouring symbols to obtain the relation between error probability and Doppler shift as used in channel estimation, thus simplifying the analysis of system performance.

A Novel Adaptive Maximum Likelihood Sequence Detection Receiver for MIMO-OFDM System

Recently many detection methods for MIMO-OFDM system have been proposed in order to achieve high data rate. In this paper, we propose an adaptive maximum likelihood sequence detection (MLSD) receiver that can combat intersymbol interference without guard interval in MIMO-OFDM system. We present an implementation of the MLSD architecture for Viterbi algorithm detection based on Per-survivor processing (PSP) and adaptive least mean square (LMS) channel estimation algorithms. Performance comparison between this novel MLSD receiver without guard interval and that of conventional MLSD receiver is made. The bit error rate (BER) curve under different MIMO-ISI channel length is gave out

Optimal pilot interval design for the dedicated pilot channel

Channel estimation plays a critical role in wireless communication systems where it is necessary to acquire channel state to compensate the received user data. For dedicated pilot channel, the known pilot symbols are usually time multiplexed with the user data symbols. When the carrier frequency is fixed, the channel coherence time is related with the vehicle speed. Thus in order to evaluate the channel state at the user symbol positions correctly, the time duration between two neighboring symbols must be less than the aforementioned channel coherence time. Is it right that the nearer of the two pilot symbols, the better of the system BER performance? Considering linear interpolation channel estimation, optimal pilot interval design is studied in this paper. Combined the noise accumulation property of interpolation method with the channel coherence time, the optimal design rule is presented. Simulations about the channel estimation performance for the different pilot intervals are done in LAS/CDMA system over ITU-VA channel. The results prove that the proposed design rule is reasonable and the experiential value for the optimal pilot interval is also given from lots of the simulation results, which is helpful for the dedicated pilot channel design.

Lattice reduction aided MMSE-SIC detection for non-orthogonal frequency division multiplexing signals

A lattice reduction aided MMSE detection combined with successive interference cancellation (SIC) scheme is proposed in this paper to suppress the noise amplification in the commonly used MMSE detection of non-orthogonal frequency division multiplexing (NFDM) signals. By adopting Lenstra-Lenstra-Lovasz (LLL) algorithm to reduce condition number of the nearly singular modulation matrix in NFDM, the new scheme improves bit error rate (BER) performance of MMSE detection as expected. Another advantage of the new scheme is that lattice reduction adds very little complexity to on line decoding of NFDM signals in AWGN channels because the reduction part of it could be done beforehand. Simulations show that complexity of the new scheme is much lower than the recently proposed decoding algorithm of randomized rounding semidefinite relaxation (SDR) in NFDM system, while more competitively, the BER performance of the new decoding scheme outruns direct randomized rounding SDR in high signal to noise ratio (SNR) when the frequency span is small or the number of sub-carriers is large.