Time-reversal Space-time Block Coding Research Articles

Joint Time-Reversal Space-Time Block Coding and Adaptive Equalization for Filtered Multitone Underwater Acoustic Communications.

Underwater acoustic (UWA) sensor networks demand high-rate communications with high reliability between sensor nodes for massive data transmission. Filtered multitone (FMT) is an attractive multicarrier technique used in high-rate UWA communications, and can obviously shorten the span of intersymbol interference (ISI) with high spectral efficiency and low frequency offset sensitivity by dividing the communication band into several separated wide sub-bands without guard bands. The joint receive diversity and adaptive equalization scheme is often used as a general ISI suppression technique in FMT-UWA communications, but large receive array for high diversity gain has an adverse effect on the miniaturization of UWA sensor nodes. A time-reversal space-time block coding (TR-STBC) technique specially designed for frequency-selective fading channels can replace receive diversity with transmit diversity for high diversity gain, and therefore is helpful for ISI suppression with simple receive configuration. Moreover, the spatio-temporal matched filtering (MF) in TR-STBC decoding can mitigate ISI obviously, and therefore is of benefit to lessen the complexion of adaptive equalization for post-processing. In this paper, joint TR-STBC and adaptive equalization FMT-UWA communication method is proposed based on the merit of TR-STBC. The proposed method is analyzed in theory, and its performance is assessed using simulation analysis and real experimental data collected from an indoor pool communication trial. The validity of the proposed method is proved through comparing the proposed method with the joint single-input–single-output (SISO) and adaptive equalization method and the joint single-input–multiple-output (SIMO) and adaptive equalization method. The results show that the proposed method can achieve better communication performance than the joint SISO and adaptive equalization method, and can achieve similar performance with more simpler receive configuration as the joint SIMO and adaptive equalization method.

Equalization in aeronautical telemetry using multiple transmit antennas

This paper introduces a generalized time-reversed space-time block code (GTR-STBC) for single-carrier systems employing multiple transmit antennas and operating over frequency-selective multipath fading channels. The generalization involves unequal power allocation parameterized by ρ. The criteria for selecting the optimum ρ is minimizing the postequalizer mean-squared error for a minimum mean-squared error (MMSE) equalizer. GTR-STBC is applied to measured channel impulse responses and a simple statistical channel model. The results show that 1) the optimum value of ρ gives the best trade-off between signal-to-noise ratio and intersymbol interference (ISI); 2) equal power allocation may not be the optimum power allocation when channel state information is available; and 3) the optimum profile of ρ over measured channels is significantly different than from that in statistical channel models.

Cooperative diversity routing and transmission for wireless sensor networks

In this study, the authors present a novel design framework aimed at developing `cooperative diversity' in 802.11-based wireless sensor networks. The proposed scheme is a combination of a time-reversed space-time block code scheme at the physical layer and a cooperative routing protocol at the network layer. The core feature of this architecture is that the multiple routes are capable of assisting the transmission of each other, hence the reliability of `all' the wireless links are enhanced simultaneously by cooperative diversity. This will involve the design of physical layer transmission schemes, medium access protocols and routing strategies. For the latter in particular, the authors present a cooperative routing protocol that is capable of exploiting full transmit diversity in wireless sensor networks. The authors restrict ourselves by imposing as few modifications to existing schemes as possible, so that integration to the existing infrastructure will be straightforward. Comprehensive simulations have been carried out to demonstrate the end-to-end performance of the proposed scheme. It is shown that a substantial gain can be achieved by cooperative diversity using a virtual multiple input multiple output system architecture.

Two time slots distributed time‐reversal space‐time block coding for single‐carrier block transmissions

Distributed space-time block coding (STBC) is a promising technique for future broadband wireless communication system, because of its substantially improving the reliability of wireless channel by exploiting cooperative spatial diversity. In this study, the authors propose a novel two time slots distributed time-reversal STBC scheme for amplify-and-forward relay-assisted single-carrier (SC) block transmissions over frequency-selective fading channel. They first exploit the discrete Fourier transform extended properties to construct a linear precoding matrix. They then employ a low-complexity suboptimal frequency domain decision feedback equalisation (FD-DFE) to collect potential multipath diversity at high signal-to-noise ratio. Simulation results demonstrate that the proposed scheme provides better performance than the conventional distributed SC-STBC scheme with minimum-mean-square error FD linear equalisation.

A Low-Complexity Solution to Decode Diversity-Oriented Block Codes in MIMO Systems with Inter-Symbol Interference

In this paper we first propose a block-code based general model to combat the Inter-Symbol Interference (ISI) caused by frequency selective channels in a Multi-Input Multi-Output (MIMO) system and/or by asynchronous cooperative transmissions. The general model is not only exemplified by the Time-Reversed Space-Time Block Code (TR-STBC) scheme, but also by the Asynchronous Cooperative Liner Dispersion Codes (ACLDC) scheme. In these schemes a guard interval has to be inserted between adjacent transmission blocks to mitigate the effect of ISI. Consequently, this could degrade the effective symbol rate for a small block size. A larger block size would enhance the effective symbol rate and also substantially increase the decoding complexity. In the general model proposed in this paper, we further present a novel low-complexity breadth-adjustable tree-search algorithm and compare it with Sphere-Decoding (SD) based algorithms. With simulation results we will illustrate that our algorithm is able to achieve the optimal performance in terms of Bit Error Rate (BER) with a complexity much lower than the SD-based algorithms, whether the ACLDC or TR-STBC scheme is employed. Through simulations we further demonstrate that when the block size of the ACLDC is equivalent to 20, the complexity of the proposed algorithm is only a fraction of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> that of the Maximum Likelihood (ML) algorithm. This would allow us to practically enhance the effective symbol rate without any performance degradation.

A Simplified MMSE Equalizer for Distributed TR-STBC Systems with Multiple CFOs

In distributed wireless systems, the traditional carrier frequency offset (CFO) compensation methods may not be applied due to the existence of multiple CFOs. In this paper, we address the equalization issue for distributed time-reversal space-time block coded (TR-STBC) systems when multiple CFOs are presented. A simplified minimum mean-square error (MMSE) equalizer is proposed, which exploits the nearly-banded structure of the channel matrices and utilizes the {LDL}^H factorization to reduce the computational complexity. Simulation results show that the proposed equalizer can achieve the similar performance as the traditional MMSE equalizer while possessing much less complexity.

Frequency domain decision feedback equalizer for time-reversal space-time block coding

In this paper, a frequency domain decision feedback equalizer is proposed for single carrier transmission with time-reversal space-time block coding (TR-STBC). It is shown that the diagonal decision feedback equalizer matrix can be calculated from the frequency domain channel response. Under the perfect feedback assumption, the proposed equalizer can approach matched filter bound (MFB). Compared with the existing time domain decision feedback equalizer, the proposed equalizer exhibits better performance with the same equalization complexity.

Channel and delay estimation for base-station–based cooperative communications in frequency-selective fading channels

A channel and delay estimation algorithm for both positive and negative delay, based on the distributed Alamouti scheme, has been recently discussed for base-station-based asynchronous cooperative systems in frequency-flat fading channels. This paper extends the algorithm, the maximum likelihood estimator, to work in frequency-selective fading channels. The minimum mean square error (MMSE) performance of channel estimation for both packet schemes and normal schemes is discussed in this paper. The symbol error rate (SER) performance of equalisation and detection for both time-reversal space-time block code (STBC) and single-carrier STBC is also discussed in this paper. The MMSE simulation results demonstrated the superior performance of the packet scheme over the normal scheme with an improvement in performance of up to 6 dB when feedback was used in the frequency-selective channel at a MSE of 3 x 10-2. The SER simulation results showed that, although both the normal and packet schemes achieved similar diversity orders, the packet scheme demonstrated a 1 dB coding gain over the normal scheme at a SER of 10-5. Finally, the SER simulations showed that the frequency-selective fading system outperformed the frequency-flat fading system.

Cyclic-delay time-reversal space-time block codes for single-carrier transmission with frequency-domain decision-feedback equalization

Transmit diversity is an important multiple-input multiple-output (MIMO) approach to improve the transmission reliability; however, it is usually designed with maximum-likelihood detection. In this paper, we investigate a simple transceiver structure for single-carrier MIMO systems, in which the cyclic-delay time-reversal space-time block code (CDTR-STBC) is employed at the transmitter, and the frequency-domain decision-feedback equalization (FD-DFE) is used at the receiver. We separate the transmit antennas into two groups, and then, encode the groups by STBC and cyclically delay the data streams within each group. Such processing can mitigate the impact of error-propagation and reduce the signal-to-noise ratio (SNR) loss when FD-DFE is employed, in which SNR is defined as the ratio of the received signal power over the noise power at each receive antenna. To support our proposal, this paper also provides the performance analysis of FD-DFE. It is shown that the proposed scheme can achieve full diversity and full transmission rate for a MIMO system employing arbitrary antennas. Therefore, it has considerable performance improvement over the existing schemes.

Equalization of rotationally variant signals

Several equalization algorithms utilizing the rotationally variant nature of the received signals are presented in this paper to combat the detrimental effect of intersymbol interference (ISI) introduced by frequency selective channels. Their adaptive implementations and application to a time-reversal space-time block coded (TR-STBC) system are also considered. In addition, a turbo equalization algorithm is derived for systems employing the error correction code. The proposed equalizers and turbo equalizer are evaluated over broadband fixed wireless access channels, and are shown to yield superior performance compared to the conventional equalization schemes.

Low Complex Decision-Feedback Equalization for Time-Reversal Quasi-Orthogonal Space-Time Block Codes

In this paper, we design a practical time-reversal quasi-orthogonal space-time block code (TR-QO-STBC) system for broadband multi-input multi-output (MIMO) communications. We first modify the TR-QO-STBC encoding structure so that the interference between the transmitted blocks can be completely removed by linear processing. Two low complex decision-feedback equalization (DFE) schemes are then proposed. One is built from the frequency-domain decision-feedback equalization (FD-DFE). The derived bi-directive FD-DFE (BiD-FD-DFE) cancels the interference among the successive symbols along the time axis. The other one is the enhanced V-BLAST, which cancels the interference between the real and imaginary parts of the spectral components. They have distinct performance characteristics due to the different interference-cancellation strategies. The underlying orthogonal and symmetric characters of TR-QO-STBC are exploited to reduce the computational complexity. Computer simulations confirm that the proposed equalizers can achieve better performance than the existing schemes.

A family of space-time block codes achieving full diversity with linear receivers

In this paper, we propose a design scheme for a family of space-time block codes (STBCs) that achieve full diversity when linear receivers, such as zero-forcing (ZF) or minimum mean square error (MMSE) receivers, are used. New STBCs obtained based on this design scheme are characterized by their special structure that combines/overlays orthogonal STBCs (OSTBCs) and Toeplitz codes together, which are called Group Orthogonal-Toeplitz Codes (GOTCs). By choosing different parameters, codes with different symbol rates and orthogonality can be obtained. It is indicated that the existing STBCs achieving full diversity with linear receivers, such as the Toeplitz codes and overlapped Alamouti codes (OACs) recently proposed in the literature and the OSTBCs are three members of the GOTCs with different parameters. It is also shown that the time-reversal space-time block codes (TR-STBCs) and delay diversity codes (DDCs) for frequency selective channels are equivalent to OACs and Toeplitz codes, respectively, and thus they both can achieve full space-frequency diversity with only linear receivers. Furthermore, simulations show that with linear receivers, GOTCs outperforming the above mentioned three codes can be obtained from the proposed design scheme.

Comments on "Successive Interference Cancellation Schemes for Time-Reversal Space-Time Block Codes

This correspondence points out some mathematical typos in a previous paper by Song and corrects them; however, the algorithms proposed in that paper do work in time-reversal space-time block-coded (STBC) block transmission systems in spite of the typos.

Adaptive Joint Decoding and Equalization for Space-Time Block-Coded Amplify-and-Forward Relaying Systems

We develop practical time- and frequency-domain adaptive equalizers for distributed time-reversal space-time block-coded (TR-STBC) systems. The adaptive equalizers eliminate the need for explicit channel impulse response estimation at the receiver. The length of the time-domain adaptive equalizer is also independent of the data block length, making it particularly suitable for frequency selective fading channels with short delay spreads. We derive the block minimum mean square error solutions for both time- and frequency-domain distributed TR-STBC systems and from these we develop recursive least squares adaptive algorithms for the block structures. We use computer simulations to compare both the time- and frequency-domain adaptive equalizers for amplify-and-forward relay networks. We show that the adaptive algorithms work well for Protocols I and III proposed by Nabar The time-domain adaptive algorithms perform better than the frequency-domain algorithms, and overall the Protocol I receivers outperform the Protocol III receivers. We also show that, if only the Protocol III receiver is used, it can be susceptible to noise amplification due to a weaker source-to-relay link compared to the relay-to-destination link. This problem can be mitigated by using the Protocol I receivers with some extra complexity but much superior diversity performance.

Robust transmit processing for frequency-selective fading channels with imperfect channel feedback

Reliable channel state information at the transmitter (CSIT) can improve the throughput of wireless networks significantly. In a realistic scenario, there is a mismatch between the CSIT and the true channel. The CSIT may be outdated due to channel variations or could be erroneous due to quantization effects. In this paper, robust transmit beamforming (rBF) and the combination of time-reversal space-time block codes (TR-STBCs) with rBF, both optimized for transmission over frequency-selective fading channels with decision-feedback equalization (DFE), are considered. The reliability of the CSIT is taken into account for the design of channel state dependent transmit signal processing algorithms to achieve robustness against imperfect CSIT. Noisy (quantized) CSIT, outdated CSIT, and the combination of both cases are considered. We show that the performance of the rBF scheme is upper and lower bounded by that of transmit BF with perfect CSIT and optimized delay diversity (ODD) without instantaneous CSIT, respectively. The combination of a TR-STBC with rBF yields significant BF gains in addition to the diversity gains of the TR-STBC for reliable CSIT, whereas for unreliable CSIT, the performance is identical to that of the TR-STBC.

Distributed Space–Time Cooperative Schemes for Underwater Acoustic Communications

In resource limited, large scale underwater sensor networks, cooperative communication over multiple hops offers opportunities to save power. Intermediate nodes between source and destination act as cooperative relays. Herein, protocols coupled with space-time block code (STBC) strategies are proposed and analyzed for distributed cooperative communication. Amplify-and-forward-type protocols are considered, in which intermediate relays do not attempt to decode the information. The Alamouti-based cooperative scheme proposed by Hua (2003) for flat-fading channels is generalized to work in the presence of multipath, thus addressing a main characteristic of underwater acoustic channels. A time-reversal distributed space-time block code (TR-DSTBC) is proposed, which extends the dual-antenna TR-STBC (time-reversal space-time block code) approach from Lindskog and Paulraj (2000) to a cooperative communication scenario for signaling in multipath. It is first shown that, just as in the dual-antenna STBC case, TR along with the orthogonality of the DSTBC essentially allows for decoupling of the vector intersymbol interference (ISI) detection problem into separate scalar problems, and thus yields strong performance (compared with single-hop communication) and with substantially reduced complexity over nonorthogonal schemes. Furthermore, a performance analysis of the proposed scheme is carried out to provide insight on the performance gains, which are further confirmed via numerical results based on computer simulations and field data experiments.

A Novel Single Carrier Space-Time Block-Coded CDMA System with Iterative Receiver

In this paper, we employ time-reversal space-time block coding (TR-STBC) in single-carrier direct sequence code-division multiple access (DS-CDMA) block transmission in the presence of multiple access interference (MAI) as well as intersymbol interference (ISI), which is subject to fairly long delay spread. We introduce the transmission rate improvement by capitalizing on the assignment of additional spreading codes to each user so as to expand the cardinality of space-time code matrix with no sacrifice of diversity order. Given perfect channel state information at the receiver, a simple linear frequency-domain interference suppression scheme on a basis of symbol-by-symbol processing is developed under certain circumstances. A “turbo principle” receiver is facilitated by exploiting the serially concatenated structure at the transmitter to further enhance system performance. Simulation results justify the efficacy of our proposed system and also present performance comparisons with some existing systems in terms of bit error rate (BER).

Transmission and Reception Concepts for WLAN IEEE 802.11b

State-of-the-art wireless local area network (WLAN) IEEE 802.11b terminals employ complementary code keying (CCK) as modulation format. In this paper, receiver concepts tailored for CCK transmission over frequency-selective fading channels are presented in a unified and systematic framework. First, optimum maximum-likelihood (ML) detection for CCK signaling is considered. Second, for complexity reduction, minimum mean-squared error block decision-feedback equalization (MMSE-DFE) is investigated and reduced-state sequence estimation (RSSE) is considered on the basis of an Ungerbock-like set partitioning of the multidimensional CCK code wordset. In order to improve the reliability of CCK transmission over fading channels, time-reversal space-time block codes (TR-STBCs) combined with receive diversity are applied. Simulation results of the considered suboptimum receivers are compared with a performance approximation for optimum detection. Our results demonstrate the excellent performance of the advocated equalization schemes and the significant gains that can be achieved with TR-STBCs and receive diversity in typical WLAN environments

Single-Carrier Frequency-Domain Equalization With Decision-Feedback Processing for Time-Reversal Space–Time Block-Coded Systems

In this letter, we propose to introduce frequency-domain equalization with decision-feedback processing (FD-DFE) for time-reversal space-time block-coded (TR-STBC) systems with multiple transmit antennas to effectively achieve both spatial diversity and multipath diversity over frequency-selective channels. This letter will also present a theoretical performance analysis result that relates the minimum mean-squared error (MMSE) to a new tight upper bound for the probability of error. It is shown that the new bound is not only valid for the TR-STBC systems, but also valid for single-input single-output, single-input multiple-output, and multiple-input multiple-output systems with MMSE equalization.

Time-reversal space-time block coded WCDMA receiver in urban and suburban environments

Time-reversal space-time block coding (TR-STBC) allows spatial diversity gains to be achieved over time-dispersive channels. The technique can be applied to the WCDMA downlink in order to exploit both spatial and multipath diversity. In the paper, a low-complexity frequency-domain implementation of TR-STBC for a WCDMA chip equalisation receiver with complexity approximately one-third that of the time-domain equivalent, enabling implementation on existing DSP devices is proposed the performance of the proposed receiver is evaluated in a macro-cellular train (suburban) scenario and a micro-cellular street (urban) scenario. The effects of base-station antenna spacing are investigated. It is shown that the proposed TR-STBC receiver can provide significant performance benefits over SISO chip equalisers and TR-STBC RAKE receivers in a rich-scattering urban environment, even for small base-station antenna spacings.