Le Ding

Multiuser Carrier Frequency Offset Estimation for OFDMA Uplink with Generalized Carrier Assignment Scheme

In this paper, we develop a blind carrier frequency offsets (CFOs) estimation algorithm for multiusers in the uplink of an orthogonal frequency-division multiple-access (OFDMA) system, where the base station equipped with uniform linear array (ULA). Due to the ULA at base station, different users could be separated by their spatial information. So unlike existing blind alternatives, which are subject to some specific carrier assignment schemes or rely on null carriers, our proposed method can support generalized carrier assignment scheme (GCAS) and full loaded systems. Thus, our method is more flexible and bandwidth efficient. Besides, the closed-form directions of arrival (DOAs) of all active users are also obtained, which can be used in downlink beamforming in frequency division duplex (FDD) systems. Performance of the proposed scheme is evaluated by computer Monte Carlo simulations.

Blind channel estimator for V-BLAST coded DS-CDMA system in frequency-selective fading environment

Most proposed space-time codec schemes focus on the single-user scenario, which cannot be applied directly in mobile communication systems. Hence, the paper focuses on a multiuser system with multi-transmit-multi-receive antennas in the frequency-selective fading environment. A vertical Bell Labs layered space-time (V-BLAST) coded direct-sequence code-division multiple-access (DS-CDMA) scheme and its blind channel identification algorithm are proposed. By exploiting an ESPRIT-like method and singular value decomposition, the channels between every transmit antenna of every user and every receive antenna of the base station can be estimated with a closed-form solution. The channel identification algorithm includes two steps and reduces the computational complexity dramatically.

A Practical Computationally Efficient Power and Bit Allocation Algorithm for Wireless OFDM Systems

A new practical computationally efficient power and bit allocation algorithm for wireless OFDM communication systems is proposed aiming at minimization of transmit power under the constraints of data rate and bit error rate. The proposed algorithm exploits the relationship between the water-filling level and the system data rate, and the differences between the bit allocations of two consecutive water-filling iterations in order to derive an efficient adaptive water-filling level searching method without need of preset parameters and a simplified greedy search method to allocate final bits, respectively. By combining them effectively, the proposed algorithm converges faster to the optimal solution than existing techniques. Computer simulation demonstrates the computational efficiency of the proposed algorithm in frequency selective fading environments.

Layered Space-Frequency Equalization with Time Domain Noise Prediction for a Single-Carrier Multiple-Input Multiple-Output System

The optimal maximum-likelihood (ML) detector for a single-carrier (SC) multiple-input multiple-output (MIMO) system is often prohibitive due to its enormous computational complexity. In this paper, we propose a low complexity layered space-frequency equalization with time domain noise prediction (LSFE-NP) structure, where at each stage of the detector, a given data stream is detected by a multiple-input single-output (MISO) frequency domain equalization with time domain noise prediction (FDE-NP). It is shown that the proposed structure is optimal in the minimum mean square error (MMSE) sense, and the coefficients of the feedback filter are independent of the feedforward equalizer. Therefore, an alterable feedback taps LSFE-NP scheme is outlined for the coded SC MIMO system, which can feed as many reliable decisions as possible back to the equalizer to achieve better performance. Simulation results show that our proposed scheme can outperform the conventional LSFE and the MIMO FDE-NP significantly.

Low Complexity Blind Carrier Frequency Offset Estimation for MIMO-OFDM Systems

A novel closed-form subspace-based blind CFO estimation approach for MIMO-OFDM systems is proposed. Our key observation is that the CFO information is redundant in spectrum structure. By allocating null carriers equispaced at the transmitter and downsampling and collecting the received blocks from all antennas at the receiver, the dimension of parameter space is greatly reduced compared with previous proposed subspace- based methods. Consequently, a lower computational complexity approach is obtained. Furthermore, estimation performance is also improved significantly. Analysis and simulation results demonstrate the effectiveness of our proposed method.

On the condition for STBC cooperative system outperforms direct system

This letter investigates a practical space time block coded (STBC) cooperative system to determine under what conditions a cooperative system outperforms a direct one, and then proposes two criteria for selecting proper relay based on outage probability and capacity, respectively. Performance analysis and numerical results indicate that the STBC cooperative system is superior in practical scenarios.

AN Improved Scheme on a Linear Pre-Coding Method of Multi-User MIMO Downlink Systems

An improved signal-to-jamming and noise ratio (SJNR) method is proposed for the linear precoding of multiuser MIMO downlink systems. To well balance suppression of co-channel interference and suppression of noise, the proposed method modifies the noise variance term to argument of a piecewise linear function, and then enhances the suppression of noise. A dynamic power allocation scheme is also introduced to further decrease the overall average bit error rate of the system. Simulation results prove the effectiveness of the proposed scheme.

A Practical Power Allocation Algorithm for Wireless OFDM Systems

A sub-optimal practical computationally efficient transmit power allocation algorithm is proposed under the constraints of data rate and max bit error rate (BER) in OFDM systems. By exploiting the relation between the water-filling level and the data rate, the proposed algorithm derives an efficient adaptive coarse searching method for water-filling level without need of preset step and other initial parameters, then determines the required water-filling level by the dichotomy method. When the channel state changes, the proposed algorithm works out a small domain of required water-filling level based on the latest allocation and then tracks the channel quickly. Simulation results show that the proposed algorithm converges quickly and is suitable for time-varying channels.

Time-varying Signature Vectors Estimation for Multicarrier CDMA Systems

In this paper, a blind adaptive signature vectors estimation algorithm for Multicarrier (MC) CDMA system in rapidly time-varying multipath fading channel is proposed. This algorithm adopts PROTEUS algorithm to estimate signal subspace and is an adaptive low dimension implementation of subspace projection method in virtue of the special update form of PROTEUS algorithm. In the case that multiple antennas are deployed to increase the capacity of CDMA systems, the proposed algorithm can estimate time-varying spatial-temporal signature vectors. This method has low computational complexity and good real-time tracking capability for rapidly time-varying channel. Monte-Carlo simulations results demonstrate the estimation and convergence performance of the proposed algorithm.

Blind low rate multiuser detection for multirate multicarrier CDMA systems using antenna array

A space-time low rate blind multiuser detector for synchronous uplink multirate multicarrier CDMA systems with antenna arrays at the base station is proposed to mitigate multiple access interference and introduce spatial diversity. After high rate physical users are modelled as several corresponding low rate virtual users, the space-time signature vectors of the low rate virtual users are derived. Then, these space-time signature vectors are estimated blindly based on a subspace projection algorithm to realize multirate multiuser detection without computing the ambiguous complex factor between the true and the estimated signature vectors. The proposed scheme can increase system performance and capacity by introducing spatial domain processing. Simulation results demonstrate that the performance of the proposed scheme is close to that of a minimum mean-square error multiuser detector with perfect channel information and that it can increase the system capacity about two times to employ a two-element antenna array.