Shang-Chun Tsai

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.

Adaptive Detectors for MIMO DS/CDMA Communication Systems

In multiple-input-multiple-output (MIMO) communication systems, multiple antennas are employed at the transmitter and the receiver. Compared with conventional systems, it has been recognized that MIMO is an important technology in obtaining higher data rate and increasing the system capacity. This paper proposes two new linear multiuser detectors and their adaptive implementations for the synchronous multiuser direct- sequence code division multiple-access systems in a MIMO multipath fading environment. First, we employ the total least squares (TLS) method and the estimation of signal parameters via rotational invariance techniques (ESPRIT) to construct the multiuser detector structures. Second, we use the least mean square algorithm in the TLS-based and ESPRIT-based multiuser detectors to develop four adaptive receivers. We also analyze the probability of bit error of the proposed receivers. Simulation results show the performance comparisons of the proposed receivers.

Adaptive MDPSK DS/CDMA receivers with performance analysis

For direct-sequence code-division multiple-access (DS/CDMA) systems, the multiuser and multipath interferences are always present. When the demodulation is coherent, a coherent carrier reference must be generated prior to demodulation. The generation of coherent reference at low signal-to-interference-and-noise ratio (SINR) is difficult. In the downlink CDMA communications, numbers of active users transmit information simultaneously by different signature waveform. However, the multipath channel and multiple-access interference (MAI) limit the system capacity. Adaptive single-user receivers do not require knowledge of MAI parameters and need fewer computations. In this paper, we propose adaptive receivers with decision-feedback differential detection (DF-DD) and compare variations of the receiver structure proposed in Schrobers paper for noncoherent channels. In the absence of knowledge of the carrier phase, we derive the minimum mean square error (MMSE) of the proposed receivers to show that the performances of the proposed receivers with M-ary differential phase-shift keying (MDPSK) signals approach that of the coherent receiver with MPSK signal. Some simulation examples are given to demonstrate the effectiveness and compare the performances of the proposed adaptive receivers. Copyright

Smart Antennas for Multi-Rate MC-CDMA Communication Systems

This paper proposes and compares three multi- rate receiver structures with post-fast Fourier transform (FFT) type and the pre-FFT type for orthogonal frequency division multiplexing code-division multiple access (OFDM-CDMA) communication systems. In multipath fading channel, the transmitted signal suffers from the effects of inter-chip interference (ICI) and multiple access interference (MAI). In order to solve these problems and support multimedia traffic, we propose three multi- rate methodologies- variable processing gain (VPG), variable chip rate (VCR) and multi-code systems to investigate the performance of the OFDM-CDMA communications in multipath fading environments. Simulations confirm the good performance of the proposed receiver structures.

Adaptive MC-CDMA and MC-DS-CDMA receivers for multiuser and multipath fading communications

OFDM is a promising scheme for multicarrier (MC) transmission, since it has the advantage of high spectral efficiency, robustness to frequency selective fading, and feasibility of low-cost transceiver implementation. The combination of OFDM and CDMA, referred to as OFDM-CDMA, recently attracted most research interests in wireless communications. In this paper, adaptive receivers for OFDM-CDMA communication systems are developed based on the least mean square (LMS) algorithm. We evaluate the down-link performance for two forms of OFDM-CDMA schemes which are MC-CDMA and MC-DS-CDMA in multipath fading channels. A drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. Thus we propose adaptive receiver structure that exploits both frequency-domain and time-domain equalization to improve the performances of MC-DS-CDMA systems with shorter or no CP. Simulation results show performance comparisons between MC-CDMA and MC-DS-CDMA systems, which also verify the performance improvement of our proposed receiver for MC-DS-CDMA systems exploiting shorter or no CP.

Multirate CDMA Receivers with Adaptive Antennas in Multipath Fading Channels

Smart antennas, with spatial processing, used in multirate code division multiple access (CDMA) systems can enhance range, reliability and capacity. In this paper, we propose three multirate methodologies, including variable processing gain, variable chip rate and multicode systems in adaptive antenna receiver structures for direct-sequence (DS) CDMA multiuser communications in multipath fading channels. Least mean square (LMS) algorithm is employed in narrowband and wideband receiver structures to adjust the spreading code coefficients. Simulation examples are given to compare the performances of the three proposed multirate receiver structures

CorrectionS to “Blind Adaptive DS/CDMA Receivers for Multipath and Multiuser Environments”

Manuscript received March 2, 2006. F.-B. Ueng, J.-D. Chen, and S.-C. Tsai are with the Department of Electrical Engineering, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C. (e-mail: fbueng@ee.nchu.edu.tw; d9064102@mail.nchu.edu.tw; d9264104@ mail.nchu.edu.tw). P.-Y. Chen was with the Department of Electrical Engineering, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C. He is now with RDC Semiconductor Company, Ltd., Hsinchu 30078, Taiwan, R.O.C. Digital Object Identifier 10.1109/TCSI.2006.873917

Adaptive Noncoherent DS/CDMA Receivers with MDPSK Signals

The capacity of a direct-sequence code-division multiple-access (DS/CDMA) system is limited by multiple-access interference (MAI) and the near-far problem. Multiuser receivers provide solutions to these problems, but they require knowledge of parameters of the MAI and are computationally complex. Adaptive single-user receivers do not require knowledge of MAI parameters and need fewer computations. In this paper, we propose noncoherent receivers for M-ary differential phase-shift keying (MDPSK) signals. The performance of the proposed noncoherent receiver approaches that of a coherent MPSK receiver. The noncoherent receiver consists of a linear equalizer and a decision-feedback differential detector (DF-DD). The bit error rate (BER) performance comparisons of different proposed receivers are shown by the simulations.

Multiuser detectors for MIMO DS/CDMA systems

In this paper, the authors consider a DS-CDMA communication system with K active users. Each user is assigned a unique spreading sequence with finite support. The paper also considers a MIMO channel with N transmitted and M received antennas. The detector has a knowledge of the carrier frequency and phase shift, and cancels out in the receiver front end. The fading is slow such that the channel remains constant during one bit time slot of J chip intervals.

An Iterative Method for Blind Equalization of Multiple FIR Channels

In this paper, the performance of the multiple-shift correlation (MSC) based blind channel equalizer (BCE) under channel order mismatch due to small head and tail channel coefficients is investigated. The analysis results show that the output signal-to-interference plus noise ratio (SINR) of the MSC-based BCE is degraded due to the small channel coefficients. The performance degradation is function of the optimal output SINR, the optimal output power, and the control vector. In such channel order mismatch environment, this paper proposes a simple but effective iterative method to improve the performance. Analysis of the iterative method is also performed. Simulation examples are demonstrated to show the effectiveness of the proposed method and the analyses