Transmit Diversity System Research Articles

Closed-loop transmit diversity systems with hybrid antenna selection

The paper investigates closed-loop transmit diversity (CLTD) systems with antenna selection technique. The expected received signal-noise-ratio (RxSNR) of the proposed systems is analyzed and compared with CLTD systems. An algorithm is proposed for determining the number of increased transmit antennas in terms of a reduced RF chains without performance degradation. Since a feedback channel is bandwidth-limited, we present a method of quantizing transmit-weight vectors. Simulation results demonstrate advantage of the proposed systems with full and quantized feedback information. Quantized feedback has less effect on the proposed systems than CLTD systems.

Approaching maximum-likelihood performance with reduced complexity for a double space–time transmit diversity system

A reduced-complexity detector approaching maximum-likelihood (ML) detection performance is presented for the double space–time transmit diversity system. The proposed scheme exploits both the special structure of equivalent channel matrix and decision-feedback detection. This accounts for accomplishing near-ML or ML performance with significantly relieved computational loads. Moreover, to moderate the average complexity, several distance metric selection criteria are proposed. We can control performance and computational savings according to different distance metric selection rules. Numerical results show that the proposed detector requires significantly fewer computations than that of the Schnorr–Euchner sphere-decoding algorithm in terms of both the worst-case and the average complexity.

Single Carrier Frequency-Domain Equalization with Transmit Diversity over Mobile Multipath Channels

This paper discusses a cyclic prefixed single carrier frequency-domain equalization (SC-FDE) scheme with two types of transmit diversity. Firstly, we propose a SC-FDE system with space-frequency block coding (SFBC). The transmit sequence of the proposed system is designed to have spatial and frequency diversities, which is equivalent to the SFBC. The corresponding combining receiver is derived under a minimum mean square error (MMSE) criterion. It is shown that the proposed system significantly outperforms the SC-FDE system with space-time block coding (STBC) over fast fading channels, while providing lower computational complexity than orthogonal frequency division multiplexing (OFDM) combined with SFBC. We verify the performance of two-branch transmit diversity systems including the proposed one through bit error rate (BER) analysis. Secondly, as a scheme that combines STBC and SFBC, a space-time-frequency block code (STFBC) SC-FDE system is presented. Computer simulation results show that the proposed STFBC SC-FDE system has better immunity to the distortion caused by both fast fading and severe frequency selective fading, compared to the SC-FDE system with the STBC or the SFBC scheme. Complexity analysis is also conducted to compare their computational loads of the transceiver. It is shown that the proposed STFBC SC-FDE system has lower computational complexity than the STFBC OFDM system.

A complementary codes pilot-based transmitter diversity technique for OFDM systems

This letter presents a complementary codes pilot-based space-time block code/orthogonal frequency division multiplex (STBC/OFDM) system. In this system, a pair of complementary codes transmitted in a pre-defined order with the OFDM data signals is used as the pilot signals in a two-antenna transmitter diversity system, and used to estimate the channels for optimal data detection at the receiver side. A complete receiver architecture has been designed and Monte Carlo simulations have been used to verify the performance of the system in mobile radio fading channels.

Performance of Feedback-Type Adaptive Array Antenna in FDD System with Rake Receiver

The performance of a feedback-type adaptive array antenna (AAA) system placed only at a base station (BS) in an FDD/DS-CDMA system remains insufficiently clear. We evaluate the performance of this system by considering the effect of a rake receiver, spacing distance between antennas, the maximum Doppler frequency (f d ), and control delay time (T d ) on BER performance. In this system, the mobile station (MS) determines optimum weights of antenna elements and sends them back to BS as feedback information. We assume that the optimum weights are not quantized. Thereby, we estimate the performance degradation of 3GPP transmit diversity system, where the feedback information is quantized using a few bits. Computer simulation results show that the rake receiver achieves better BER performance because of the time diversity effect with rake receiver. The AAA with a wide antenna spacing gives high diversity gain for the received signals. For a high value of f d T d , BER performance becomes worse because weighting factors cannot follow the changing speed of channel characteristics. The degradation in performance of a 3GPP system is clarified.

Cell search algorithm in beyond 3G systems

The downlink frame structure for beyond 3G mobile communication systems is presented. Beyond 3G systems utilize the OFDM technique. However, a problem encountered in OFDM is that channel dispersion destroys orthogonality between carriers, causing inter-symbol interference. It is also sensitive to high peak to mean power ratio (PAPR). Therefore it spends much time on obtaining frequency, time, and frame synchronization. This paper proposes to add a frame synchronization channel in the time domain to overcome the shortcoming of OFDM. As transmitter diversity improves the system performance, beyond 3G systems employ space-time block coded (STBC). Fast cell search algorithm including slot synchronization, frame synchronization and cell ID identification is then discussed, which is based on the frame synchronization channel in transmitter diversity systems. Detection and false alarm probabilities in AWGN and Rayleigh channels are analyzed, and the mean acquisition time is obtained. Computer simulations are conducted to evaluate the performance of the cell search algorithm under different channel conditions.

Power allocation in a transmit diversity system with mean channel gain information

We study the power allocation problem in a transmit diversity wireless system with mean channel gain information. In Rayleigh fading for a given set of mean channel gains and nodes, we seek to find the power allocation that minimizes the outage probability subject to a total power constraint. The optimal solution is shown to be computationally intensive when the number of channels is large. Instead, we derive a simple solution based on the upper bound to the outage probability which can be summarized as equal power allocation with channel selection. Numerical results show that the proposed solution is near-optimal over a wide range of parameter values. The problem addressed and the solution are relevant to a decode-and-forward cooperative relaying system with only partial channel information available to the relays.

Effect of angle-of-arrival on transmit diversity and beamforming systems under correlated fading

A comparative analysis of transmit diversity and beamforming systems, using outage capacity as performance measure is presented. The objective is to examine the effect of angle-of-arrival (AOA) distributions on two systems with angle spread. This is done by comparing the performance of transmit diversity with a system that uses beamforming to point a relatively narrow beam at the mobile station. Results show that the transmit diversity system using uniform linear arrays with truncated Laplacian AOA always performs better even at smaller angle spreads as compared to other energy distributions.

Exact Symbol Error Probability of a Cooperative Network in a Rayleigh-Fading Environment

In a distributed spatial diversity wireless system, not all antennas are located at one station as in classical transmit diversity systems, but are dispersed at different, possibly mobile, stations in the network. Transmit diversity is created when the selected stations assist a sender by relaying its information signal to the destination. In this letter, we present an exact average symbol error rate analysis for the distributed spatial diversity wireless system with K amplifying relays in a Rayleigh-fading environment. The average symbol error rate formula allows us to clearly illustrate the advantage that the distributed diversity system has in overcoming the severe penalty in signal-to-noise ratio caused by Rayleigh fading. Using simple bounds on the probability of error, we show that the cooperative network presented in this letter achieves full diversity order.

Iterative channel estimation approach for space‐time/frequency coded OFDM systems with transmitter diversity

AbstractFocusing on transmit diversity orthogonal frequency division multiplexing (OFDM) transmission through frequency selective channels, this paper pursues novel iterative channel estimation approaches for both space‐frequency OFDM (SF‐OFDM) and space‐time OFDM (ST‐OFDM) systems. Relying on the unifying signal model for SF‐OFDM and ST‐OFDM transmitter diversity systems, we develop computationally efficient, maximum a posteriori (MAP) channel estimation algorithms according to the MAP criterion. The algorithms require a convenient representation of the discrete multipath fading channel based on the Karhunen–Loeve (KL) orthogonal expansion and estimates the complex channel parameters of each subcarriers iteratively using the expectation‐maximisation (EM) method. In order to explore the performance, the closed‐form expression for the average symbol error rate (SER) probability is derived for the maximum ratio combiner (MRC). Furthermore, to benchmark performance of the MAP channel estimator, the modified Cramer‐Rao bound of channel estimates is also derived. Finally, we provide simulation results studying the influence of delay spread, propagation parameters and modelling mismatch on the performance of channel estimation techniques. Simulation results confirm our theoretical analysis and illustrate that the proposed algorithms are capable of tracking fast fading and improving overall performance. Copyright © 2004 AEI

Diversity Performance of Precoded Orthogonal Space-Time Block Codes Using Limited Feedback

Orthogonal space-time block codes (OSTBCs) are simple space-time codes that can be used for open-loop transmit diversity systems. OSTBCs, however, can only be designed for certain numbers of transmit antennas. Channel-dependent linear precoders have been proposed to overcome this deficiency, but it is not clear what conditions the precoder design must satisfy to guarantee full diversity order. In this letter, we show necessary and sufficient conditions for linear precoded OSTBCs to provide full diversity order. We show that limited feedback precoding can achieve full diversity order using fewer bits than limited feedback beamforming. We also present a simplified version of antenna subset selection for OSTBCs that can provide full diversity order with low complexity and only a small amount of feedback.

Transmit-Preprocessing Techniques With Simplified Receivers for the Downlink of MISO TDD-CDMA Systems

Time-division-duplex code-division multiple-access (TDD-CDMA) systems have recently gained attention with their inclusion in third-generation mobile systems. In this paper, we introduce two optimized transmit preprocessing techniques to reduce multiple access interference in the downlink of TDD-CDMA systems with multiple transmit antennas that are employed at the base station (BS). In these systems, signal preprocessing is performed at the BS, so that a simplified receiver structure that consists of a one-finger correlator can be utilized at the mobile station. Analytic solutions for both of the optimized transmit preprocessing techniques are derived by minimizing the transmit mean square error. Numerical results are also provided, which demonstrate significant performance improvement when compared to the conventional RAKE system and the pre-RAKE maximum ratio combining transmit diversity system. In particular, transmit antennas can be used to increase the system capacity.

Code acquisition in transmit diversity ds-cdma systems

In this paper, two code acquisition schemes are studied for use in conjunction with transmit diversity direct-sequence code-division multiple access (DS-CDMA). One is a training-based single-user maximum-likelihood (SUML) estimator, which can achieve code acquisition very fast at low computational complexity. The other is a blind estimator based on the multiple signal classification (MUSIC) algorithm. Two recently proposed transmit diversity schemes known as orthogonal transmit diversity simulcast (OTD-S) and space-time selective spreading transmit diversity (STSTD) are considered. While the advantages of transmit diversity from the detection standpoint are well known, less is known about how code acquisition performance is affected by the use of transmit diversity. Through the analysis in this paper, it is proven that the SUMI. estimator should give the same performance in both the OTD-S and STSTD schemes in a single-user environment. In a multiple-user environment, simulation results show that the STSTD system offers slightly better code acquisition performance. It is also seen that the SUML estimators provide significantly better code acquisition performance than the MUSIC estimators in either transmit diversity system. From the standpoint of robustness to carrier frequency offset, it Is found that the training-based SUML estimator is very sensitive to frequency offset, while the MUSIC estimator is quite robust. A simple frequency offset estimator to be used in conjunction with the SUML estimator is also proposed and is shown to make the timing estimator quite tolerant of substantial frequency offsets.

Estimation of nakagami-m fading channel parameters with application to optimized transmitter diversity systems

An optimum power loading algorithm for transmitter diversity systems over correlated and unbalanced Nakagami (1960) paths and its performance evaluation under perfect channel estimate conditions are derived. In addition, various online estimators of the required Nakagami channel parameters for optimized power loading and the comparison of their mean square error via Monte Carlo simulations are presented. Some of these estimators are used to obtain the performance of optimized transmitter diversity systems under imperfect channel estimation (ICE). These numerical results show that the diversity gain of these optimized systems compared with equipower systems increases as the severity of fading decreases and as the degree of branch imbalance increases even under ICE. On the other hand, in weakly correlated and (or) unbalanced branches, optimized transmitter diversity systems offer negligible gain or even losses compared with unoptimized systems because of the ICE.

Spatial Block Codes Based on Unitary Transformations Derived from Orthonormal Polynomial Sets

Recent work in the development of diversity transformations for wireless systems has produced a theoretical framework for space-time block codes. Such codes are beneficial in that they may be easily concatenated with interleaved trellis codes and yet still may be decoded separately. In this paper, a theoretical framework is provided for the generation of spatial block codes of arbitrary dimensionality through the use of orthonormal polynomial sets. While these codes cannot maximize theoretical diversity performance for given dimensionality, they still provide performance improvements over the single-antenna case. In particular, their application to closed-loop transmit diversity systems is proposed, as the bandwidth necessary for feedback using these types of codes is fixed regardless of the number of antennas used. Simulation data is provided demonstrating these types of codes′ performance under this implementation as compared not only to the single-antenna case but also to the two-antenna code derived from the Radon-Hurwitz construction.

Pilot-Symbol-Assisted Channel Estimation for Space-Time Coded OFDM Systems

Space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to provide an efficient means of achieving near optimal diversity gain in frequency-selective fading channels. For these systems, knowledge of the channel parameters is required at the receivers for diversity combining and decoding. In this paper, we propose a low complexity, bandwidth efficient, pilot-symbol-assisted (PSA) channel estimator for multiple transmitter OFDM systems. The pilot symbols are constructed to be nonoverlapping in frequency to allow simultaneous sounding of the multiple channels. The time-varying channel responses are tracked by interpolating a set of estimates obtained through periodically transmitted pilot symbols. Simulations are used to verify the effectiveness of the propsed estimator and to examine its limitations. It is also shown decision-directed minimum mean square error MMSE channel estimator for OFDM transmitter diversity systems.

MISO CDMA transmission with simplified receiver for wireless communication handsets

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors.

Transmit diversity system with linear prefilters exhibiting near-optimal performance

A transmit diversity system with liner prefilters exhibiting near-optimal performance is proposed. Without the need for additional antennas, a significant gain can be obtained by increasing the prefilter length and adopting optimal MAP equalisation. As the prefilter length increases, the system performance is improved and closely approaches optimal performance. At a BER of 10/sup -3/, the performance of the proposed system attains optimal performance within 0.5 dB.

Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels

Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.