Hsuan-Fu Wang

Performances of OFDM-CDMA receivers with MIMO communications

This paper considers the joint design of OFDM and code division multiple access CDMA with MIMO communications. The receivers for both Bell Labs Layered Space Time system and space-time block code are designed in this paper. The performances of the MIMO-OFDM-CDMA receivers are investigated in several typical fading channels. The maximum likelihood, zero forcing, and minimum mean square error receivers for the MIMO-OFDM-CDMA systems are derived, and the performances in the typical fading channels are verified by the simulations. Copyright

Iterative multiuser detection for LDPC MIMO SC-FDMA communication systems

In order to provide high quality services, current communication systems require high-data transmission rate and good error performance. Because single-carrier frequency division multiple access SC-FDMA is with the desirable characteristics of orthogonal frequency division multiple access and the low peak-to-average power ratio of single-carrier transmission schemes, it has been adopted as the uplink transmission scheme in 3GPP long-term evolution to approach the demand for data transmission rates and error performance. By combining multiple-input multiple-output MIMO techniques with the SC-FDMA modulation scheme, MIMO SC-FDMA systems can achieve high data rates over broadband wireless channels. In this paper, we propose the iterative multiuser detection scheme for the low-density parity-check-coded MIMO SC-FDMA communication systems. On the basis of the property of space-frequency block coding, we develop the frequency domain minimum mean-squared error and zero-forcing equalisers with multiuser interference cancellation technique on each subcarrier to recover the transmitted symbols. Some simulation examples for uplink scenario are given to demonstrate the effectiveness of the proposed schemes. Copyright

Multiuser MIMO receivers for space frequency block coded SC-FDMA systems

Single carrier-frequency division multiple access SC-FDMA has been adopted as the uplink transmission standard in fourth generation cellular network to enable the power efficiency transmission in mobile station. Because multiuser MIMO MU-MIMO is a promising technology to fully exploit the channel capacity in mobile radio network, this paper investigates the uplink transmission of SC-FDMA systems with orthogonal space frequency block codes SFBC. Two linear MU-MIMO receivers, orthogonal SFBC OSFBC and minimum mean square error MMSE, are derived for the scenarios with limited number of users or adequate receive antennas at base station. In order to effectively eliminate the multiple access interference MAI and fully exploit the capacity of MU-MIMO channel, we propose a turbo MU-MIMO receiver, which iteratively utilizes the soft information from maximum a posteriori decoder to cancel the MAI. By the simulation results in several typical MIMO channels, we find that the proposed MMSE MU-MIMO receiver outperforms the OSFBC receiver over 1dB at the cost of higher complexity. However, the proposed turbo MU-MIMO receivers can effectively cancel the MAI under overloaded channel conditions and really achieve the capacity of MU-MIMO channel. Copyright

Zero Forcing and Minimum Mean Square Error Equalization for OFDM-CDMA Multiuser Detection in Multipath Fading Channels

This paper introduces four block joint detection (JD) receivers which combining the orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) techniques, i.e., OFDM-CDMA receiver. The performance of the OFDM-CDMA receiver with orthogonal and non-orthogonal spreading sequences is investigated in mobile radio channels. The sequences considered as spreading codes are orthogonal Walsh-Hadamard, Orthogonal Gold, Gold and maximim length sequences or m - sequences. The JD algorithms for the OFDM- CDMA receiver are Zero Forcing Block Linear Equalizer (ZF- BLE), Minimum Mean Square Error Block Linear Equalizer (MMSE-BLE), with and without decision feedback (DF). Time- domain equalization does not require DFT or FFT operations and can be used to compensate the channel fading effect. Hence for hardware implementation, the time domain interpretation can lead to a more efficient structure when compared with the frequency domain approach. The bit error rate (BER) performance comparisons of different spreading codes and equalizers are shown in the simulations.

Turbo OFDM-CDMA receiver with varying stepsize in multipath fading channels

The receiver performance can be improved dramatically by employing equalizer and turbo code. This paper investigates adaptive receivers with turbo decoder for orthogonal frequency division multiplexing-code division multiple access (OFDM-CDMA) systems in multipath channels. The LMS algorithm is used as the adaptive algorithm for the de-spreading equalizers which jointly perform equalization and single user detection in downlink multiuser environment. The varying stepsize equalizer jointed with turbo decoder is presented. Simulation results show that the system performances can substantially be improved if the adaptive equalizer can properly utilize the extrinsic information from the turbo decoder.

Iterative Equalizer for OFDM-CDMA Multiple Access Communication Systems

In this paper, the receivers with adaptive equalizer and turbo decoder are investigated for orthogonal frequency division multiplexing-code division multiple access (OFDM-CDMA) systems in multipath channels. The de-spreading equalizer which combines de-spreading and equalization is used to mitigate the MAI and ISI. The stochastic gradient based LMS algorithm is used as the adaptive algorithm for the equalizer. Three receiver architectures are investigated: The first one is the traditional receiver called de-spreading equalizer disjointed with turbo decoder. In this scheme, the equalizer and turbo decoder operate independently and the equalizer relies on slicer output to adapt its filter coefficients. However, the extrinsic information from turbo decoder can provide more reliable information of the transmitted data. Hence the second receiver architecture is called as de-spreading equalizer jointed with turbo decoder, where turbo decoder gives the extrinsic information as feedback to the adaptive equalizer as the desired information. Finally, the equalizer jointed with turbo decoder is extended to differential coded signaling in OFDM-CDMA system and its performance is presented and discussed.

Optimal step-size mechanism for stochastic gradient algorithm applied to CDMA receiver in multipath fading channels

This paper investigates the optimal step-size mechanism for a stochastic gradient (SG) based direct sequence (DS-) code division multiple access (CDMA) receiver under the influence of the multiple access interference (MAI) and intersymbol interference (ISI) in multi-path environments by utilizing a simple stochastic model. The convergence of the derived optimal step-size sequence is verified analytically. The learning curve is applied to analyze and compare the speed of convergence and residual mean-squared error (MSE) for the fixed and time-varying step-size sequence methodologies. Simulations show that the receiver utilizes the derived optimal step-size sequence not only has faster convergent speed and smaller MSE, but also is robustly to different multi-path channels.

Convergence analysis of adaptive DS-CDMA receivers in multi-path channels

This paper objectively investigates the convergence and transient behaviors of symbol-rate adaptive receivers for direct sequence-code division multiple access (DS-CDMA) communication system in dispersive multi-path environment. The input signal vector is modeled as a simple stochastic model to simplify the analyses of the least mean square (LMS) and normalized least mean square (NLMS) adaptive receivers. The mean square error (MSE) learning curve, which is based on ensemble averaging of the squared estimation error, is applied to analyze the convergences of the adaptive receivers for DS-CDMA systems interfered with multiple access interference (MAI) and inter-symbol interference (ISI). The analytical results are verified with computer simulations. Computer simulations show that the analytical results are consistent with simulations.

Convergence behaviors of adaptive DS-CDMA receivers

This paper aims to investigate the convergence and transient behaviors of the symbol-based adaptive receivers for direct sequence-code division multiple access (DS-CDMA) communication system in multipath channels. The input signal vector is characterized as a simple stochastic model to simplify the analysis of the least mean square (LMS) and the normalized least mean square (NLMS) adaptive receivers. The learning curve, which is the ensemble average of the squared estimation error is applied to analyze the convergence of adaptive receivers for DS-CDMA system in multipath channels. Computer simulations show that the analytical results of the learning curve obtained in this work are consistent with the simulation results.

BER Performance Comparison of Low Processing Gain TDD-CDMA System with Various Spreading Sequences in Mobile Fading Channels

The bit-error-rate (BER) performance of low processing gain (PG) time division duplex CDMA (TDD-CDMA) system with joint detection (JD) multi-user detection (MUD) algorithms and different spreading sequences in dissimilar mobile radio propagation environments is compared in this paper. The sequences considered as spreading codes are maximum-length, gold, orthogonal-gold, random and Walsh-Hadamard sequences. The zero forcing-block linear equalizer (ZF-BLE), minimum mean square error-block linear equalizer (MMSE-BLE), zero forcing block decision feedback equalizer (ZF-BDFE) and minimum mean square error block decision feedback equalizer (MMSE-BDFE) JD algorithms are adopted in simulations. The mobile radio channels are rural area (RA), typical urban (TU) and bad urban (BU) channels. The result shows that under different propagation environments, the BER performance of each spreading sequence are different. The results obtained in this paper are very useful in the selection of sequence sets for TDD-CDMA system applications