Space-time RAKE Receiver Research Articles

Space-Time Two Dimensional RAKE Receiver in Cooperative Communications Systems with Interference Cancellation Technique

In this paper, space-time two dimensional RAKE receiver was introduced into the asynchronous cooperative communication systems, and a high-performance RAKE receiving algorithm was proposed. Firstly, the transmitted signal was estimated by traditional space-time RAKE receiver, and then, decision feedback technique was investigated into the new algorithm, where each multipath component in the received signal was composed using the estimated results of the transmitted signal and then cancelled successively from the received signal, so the inter symbol interference in the received signal which was generated by the multipath fading channel was eliminated, and the line of sight (LOS) component in the received signal was obtained. Finally, the LOS component was combined together by the space-time block code combining rule to obtain the spatial diversity gain. Based on that, the iterative decision feedback architecture was further employed, where one stage stands for the process from multipath components reconstitution to space time combination, and the performance of the new scheme can be improved effectively by the multiple decision feedback stages. Computer simulations show that the new algorithm can improve the performance of traditional space-time RAKE dramatically when SNR is higher than 5 dB in asynchronous cooperative communication systems.

An Interference Cancellation Space-Time RAKE Receiver in Asynchronous Cooperative Communications Systems

Space-time two dimensional RAKE receiver was introduced into asynchronous and cooperative communication system, and a high-performance RAKE receiving algorithm was proposed. In this algorithm, the transmitting signals were estimated coarsely by traditional space-time rake receiver, then each multipath signal was reconstructed using the estimated result as well as the channel impulse response, and then eliminated successively from reception signals, so it can eliminate Inter Symbol Interference of receiver signals which generate by multipath fading, and the line of sight (LOS) component in the received signal was obtained, and finally, the space-time combination was carried out to the LOS component, and the spatial diversity was achieved. This algorithm can effectively reduce the bit error rate of the asynchronous cooperative communication system. Simulation result shows that the new algorithm can improve the performance of traditional space-time RAKE when SNR is higher than 5dB dramatically.

Efficient Space-Time Two Dimensional RAKE Receiver in Asynchronous Cooperative Communications Systems with Full Diversity

A high-performance space-time two dimensional RAKE receiving algorithm was proposed in asynchronous cooperative communication system. the transmitting signals were estimated coarsely by traditional space-time rake receiver, then on the basis of that we introduce feedback technology, each multipath signal of transmitted signals was composed by the decision results, and then every multipath component in the received signal can be obtained by eliminating the composed multipath signals from the received signal. Finally, the multipath components were combined by space-time combination and multipath combination rules accordingly, so spatial as well as multipath diversities were achieved. Simulation results show that the new scheme can reduce the bit error rate dramatically compare with traditional space-time RAKE in asynchronous cooperative communication system.

An Adaptive Multiuser Detection Algorithm for CDMA Systems Using Antenna Diversity

Based on the minimum mean squared error (MMSE) between the data stream and the linear combiner output, a new multiuser detection (MUD) algorithm that combines space–time (ST) processing and antenna array on direct-sequence CDMA signals is proposed. The proposed ST-MUD algorithm is proved to be equivalent to two existing MMSE-based ST-MUD algorithms, and the theoretical BER performances for all the three algorithms are the same. The most attractive feature of the new ST-MUD algorithm is based on the fact that the new method does not require explicit estimation of channel and signaling information. This avoids any channel estimation error, and the method is thus more robust and more accurate than the other two ST-MUD algorithms in practical implementation. Adaptation of the proposed ST-MUD algorithm is implemented by using training sequences. Performance of this new multiuser detector is compared with that of two existing MMSE multiuser detectors and the conventional single-user space–time rake receiver through simulations. The proposed ST-MUD algorithm provides a performance better than existing algorithms and is especially suitable for practical CDMA systems.

Effect of Slow Power Control Error on the Reverse Link of OSTBC DS-CDMA in a Cellular System With Nakagami Frequency-Selective MIMO Fading

In this paper, the performance of a multicell orthogonal space-time block-coding direct-sequence code-division-multiple-access system with base station diversity is studied for the reverse link in terms of bit error rate, taking into account the effects of frequency-selective Nakagami-m fading with arbitrary parameters, correlated lognormal shadowing, power control imperfections, selection-based macroscopic diversity, and space-time rake receiver diversity. How the transmitter and receiver antenna configuration setups, the number of rake fingers, and the number of resolvable paths affect the reverse-link capacity of the system is discussed in detail. Analytical results are also given for systems with different processing gains and for propagation environments with different multipath intensity profile distributions

New pre-processing technique to enhance single-user-type DS-CDMA detectors in "blind" space-time rake receivers

Proposed herein is a pre-processing technique applicable to maximum-SINR beamformers for blind space-time Rake receivers in single-user-type DS-CDMA detection. This smart-antennas signal-processing technique enhances the constructive summation of multipaths and the rejection of co-channel interference, both of which decrease the bit-error-rate and lower the error-floor of the cellular uplink's near-far problem at high SNR. This technique is blind in that it needs neither prior knowledge nor explicit estimation of (1) the channel-faded multipaths' arrival angles, relative arrival delays or power profiles, (2) the receiving antenna array's nominal or actual gain-phase-polarization response, and (3) the co-channel multi-access users' signature-spreading codes. Monte Carlo simulations show that the proposed scheme can enhance the output-SINR by 1 - 10 dB. A mathematical analysis shows this proposed pre-processing scheme to always improve the asymptotic output-SINR. A formula is analytically derived to predict this asymptotic output-SINR. This formula lies within 1 dB of limited simulation results at an input-SIR above -7 dB and a chip-rate space time-sampling, but the formula is less accurate otherwise

Capacity losses in wireless CDMA networks using imperfect decorrelating space-time Rake receiver in fading multipath channel

In this paper we analyze the impact of system imperfections on the overall cellular code-division multiple access (CDMA) radio network performance. The theory is general and some examples of practical sets of channel and system parameters are used as illustration. A flexible signal model, based on the complex signal format, is used enabling us to model all wideband CDMA standards. For such a signal, we first derive a complex decorrelator structure. In the next step imperfections in the operation of a decorrelating space-time Rake combiner are modeled and analyzed. Relative capacity losses and the network sensitivity function are used as performance measures. Simulations are also performed to confirm some of the key assumptions made in the analysis. Numerical results show that the user capacity varies significantly depending on the multipath profile, diversity order, fading rate, and code crosscorrelations. It is shown that up to 97 % of the system capacity can be lost due to the system imperfections. More, advanced and robust parameter estimators and/or multiuser detectors are needed to alleviate these degradations at the cost of increased complexity

Space-time selective RAKE receiver with finger selection strategies for UWB overlay communications

This paper proposes a space-time selective RAKE (SRAKE) receiver with maximum signal-to-interference-plus-noise ratio (MSINR) for direct-sequence ultra-wideband (UWB) communications in the presence of narrowband interference (NBI) and multiple-access interference. For effectively extracting a fixed number of the UWB signal components (fingers) from numerous resolvable paths, four finger selection strategies (FSSs) are considered for the proposed space-time SRAKE receiver, including the optimum FSS (with MSINR), which is not very computationally feasible, and three feasible FSSs: an energy-based FSS (EB-FSS), a constrained energy-based FSS (CEB-FSS), and a hybrid energy-based FSS, which is also a combination of the EB-FSS and CEB-FSS. Through a performance analysis, we show that the performance of the proposed receiver in the presence of NBI not only depends on the power ratio, bandwidth ratio, and relative spectrum location of NBI with respect to the UWB signal, but also on the FSS used. Some simulation results are then presented to show that the proposed space-time MSINR-SRAKE receiver with the preceding FSSs used can provide a larger system capacity and better immunity to strong NBI than the existing time-only SRAKE receivers and space-time SRAKE receivers.

Joint Channel Parameter Estimation and Signal Detection for Downlink MIMO DS-CDMA Systems

SUMMARY This paper proposes two space-time joint channel parameter estimation and signal detection algorithms for downlink DS-CDMA systems with multiple-input-multiple-output (MIMO) wireless multipath fading channels. The proposed algorithms initially use the space-time MUSIC to estimate the DOA-delays of the multipath channel. Based on these estimated DOA-delays, a space-time channel decoupler is developed to decompose the multipath downlink channel into a set of independent parallel subchannels. The fading amplitudes of the multipath can then be estimated from the eigen space of the output of the space-time channel decoupler. With these estimated channel parameters, signal detection is carried out by a maximal ratio combiner on a pathwise basis. Computer simulations show that the proposed algorithms outperform the conventional space-time RAKE receiver while having the similar performance compared with the space-time minimum mean square error receiver.

A Blind Interference-Blocking RAKE Receiver for CDMA Communications Systems

A space-time RAKE (ST-RAKE) receiver with a blind interference-blocking (IB) pre-processor, termed as the IB-RAKE receiver, is proposed for spread spectrum communications systems. The design of the proposed architecture consists of three components. A blind IB transformer is first constructed based on the received data, and then applied on the undespread data for the suppression of strong interference. After despreading, optimal beamforming is then performed on the IB despread data to extract the signals of interest (SOIs) from the desired user. Finally, a RAKE receiver with a maximum ratio combining technique is employed to constructively collect all the SOI energies. Since strong interference has been removed in the first stage, the RAKE receiver combines only those SOIs plus negligible interference, leading to robustness against strong interference. Numerical results have shown that substantial improvement can be obtained from the proposed ST-RAKE receiver with the blind IB preprocessing scheme.

“Fractional Self-Decorrelation” to Enhance Single-User-Type DS-CDMA Detectors' Output SINR in Blind Space-Time RAKE Receivers

In this letter, a new "fractional self-decorrelation" technique is proposed to enhance the multipath-constructive-summation and interference-rejection capability of single-user-type DS-CDMA detectors at maximum-SINR blind space-time RAKE receivers, to tackle the near-far problem in wireless mobile communications. This technique can significantly decrease the error rate and can reduce the near-far problem's error floor at high SNR. This "blind" space-time processing receiver architecture needs no prior knowledge nor explicit estimation of: 1) the channel's multipath arrival angle or arrival delay or power profile-the fading channel's delay spread may be of arbitrary length, even as long as the symbol duration; 2) the receiver's nominal or actual antenna array manifold; and 3) the other DS-CDMA users' signature spreading codes.

Space-Time Chip Equalization for Maximum Diversity Space-Time Block Coded DS-CDMA Downlink Transmission

In the downlink of DS-CDMA, frequency-selectivity destroys the orthogonality of the user signals and introduces multiuser interference (MUI). Space-time chip equalization is an efficient tool to restore the orthogonality of the user signals and suppress the MUI. Furthermore, multiple-input multiple-output (MIMO) communication techniques can result in a significant increase in capacity. This paper focuses on space-time block coding (STBC) techniques, and aims at combining STBC techniques with the original single-antenna DS-CDMA downlink scheme. This results into the so-called space-time block coded DS-CDMA downlink schemes, many of which have been presented in the past. We focus on a new scheme that enables both the maximum multiantenna diversity and the maximum multipath diversity. Although this maximum diversity can only be collected by maximum likelihood (ML) detection, we pursue suboptimal detection by means of space-time chip equalization, which lowers the computational complexity significantly. To design the space-time chip equalizers, we also propose efficient pilot-based methods. Simulation results show improved performance over the space-time RAKE receiver for the space-time block coded DS-CDMA downlink schemes that have been proposed for the UMTS and IS-2000 W-CDMA standards.

Prototype experience for MIMO BLAST over third-generation wireless system

In this paper, a multiple-input-multiple-output (MIMO) extension for a third-generation (3G) wireless system is described. The integration of MIMO concepts within the existing UMTS standard and the associated space-time RAKE receiver are explained. An analysis is followed by a description of an actual experimental MIMO transmitter and receiver architecture, both realized on digital signal processors (DSPs) and FPGAs within a precommercial OneBTS base station. It uses four transmit and four receive antennas to achieve downlink data rates up to 1 Mb/s per user with a spreading factor of 32 and the UMTS chip rate of 3.84 MHz. Furthermore, different MIMO detectors are evaluated, comparing their performance and complexity. System performance is evaluated through simulations and indoor over-the-air measurements. Capacity and bit-error rate measurement results are presented.

Interference-blocking RAKE receiver for CDMA systems

A space-time RAKE receiver with enhanced interference suppression is proposed for code division multiple access systems. Simulation results demonstrate that the proposed interference-blocking RAKE receiver exhibits robustness against the strong interference and significant performance enhancement as compared with conventional schemes.

Differential Space-Time Block Code Modulation for DS-CDMA Systems

A differential space-time block code (DSTBC) modulation scheme is used to improve the performance of DS-CDMA systems in fast time-dispersive fading channels. The resulting scheme is referred to as the differential space-time block code modulation for DS-CDMA (DSTBC-CDMA) systems. The new modulation and demodulation schemes are especially studied for the down-link transmission of DS-CDMA systems. We present three demodulation schemes, referred to as the differential space-time block code Rake (D-Rake) receiver, differential space-time block code deterministic (D-Det) receiver, and differential space-time block code deterministic de-prefix (D-Det-DP) receiver, respectively. The D-Det receiver exploits the known information of the spreading sequences and their delayed paths deterministically besides the Rake type combination; consequently, it can outperform the D-Rake receiver, which employs the Rake type combination only. The D-Det-DP receiver avoids the effect of intersymbol interference and hence can offer better performance than the D-Det receiver.

Reduced-dimension blind space-time 2-D RAKE receivers for DS-CDMA communication systems

Blind space-time RAKE receivers for classical DS-CDMA that cancel strong multiuser access interference while optimally combining the desired user's multipath are presented. The delay spread is assumed to be fraction of a symbol interval as in the IS-95 CDMA standard. The post-correlation symbol interval is segmented into that which encompasses the RAKE fingers and that away from the RAKE fingers. The signal-plus-interference and interference alone space-time correlation matrices are estimated during the former and latter, respectively. Reduced-dimension space-time RAKE receivers are investigated for alleviation of the computational burden of computing the largest generalized eigenvector of the resulting matrix pencil. Theoretical analysis and supporting simulations reveal that if the space and time compressing transformations are designed judiciously, reduced-dimension two-dimensional (2-D) RAKE receivers offer faster convergence at the expense of only a slight loss in asymptotic signal-to-interference-plus-noise ratio (SINR) relative to the full-dimension space-time RAKE receiver. Applications to the IS-95 uplink are also presented along with supporting simulations.

A low-complexity space-time RAKE receiver for DS-CDMA communications

An algorithm is presented for estimating the quantities needed by a space-time RAKE receiver for DS-CDMA to achieve maximum SINR for the desired user. The proposed algorithm, which is applicable in the case of either periodic or aperiodic spreading codes, asymptotically provides the exact time of arrival of each dominant multipath within a bit period, and the optimum beamformer for extracting each multipath. This is achieved by using cyclostationarity to exploit the difference between the respective spectra of the desired user and each multiuser access interferer at the output of the correlator based on the desired user's code. Estimates of the relative time delay and optimal beamformer for each RAKE finger are used by the space-time RAKE receiver to optimally combine the desired user's multipath while simultaneously canceling strong multiuser access interference. Simulations of a near-far scenario are presented demonstrating the efficacy of the proposed algorithm.