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For multiple-input multiple-output (MIMO) Rayleigh channels, it has been shown that transmitter correlations always degrade the performance of general space–time codes (STCs) in high signal-to-noise ratio (SNR) regimes. In this correspondence, however, we show that when MIMO channels experience single-keyhole conditions, the effect of spatial correlations between transmission antennas is more sophisticated for general STCs: When M>N (i.e., the number of transmission antennas is greater than the number of receiving antennas), depending on how the correlation matrix {\bf P} beamforms the codeword difference matrix {\mmb \Delta}$, the pairwise error probability performance of general STCs can be either degraded or improved in high-SNR regimes. We provide a new measure, which is based on the eigenvalues of {\mmb \Delta} and the numbers of transmission and receiving antennas, to examine if there exist certain correlation matrices that can improve the performance of general STCs in high-SNR regimes. Previous studies on the effect of spatial correlations over single-keyhole channels only concentrated on orthogonal STCs, whereas our study here is for general STCs and can also be used to explain previous findings for orthogonal STCs. Simulations also show that the results are applicable with imperfect channel knowledge. [ABSTRACT FROM PUBLISHER]

This paper proposes a joint timing synchronization and channel estimation scheme for communications systems with multiple transmit antennas based on a well-designed training sequence arrangement. In addition, a generalized maximum-likelihood (ML) channel estimation scheme is presented, and this one-shot scheme is applied to obtain all channel impulse responses (CIR) from different transmit antennas. The proposed approach consists of three stages at each receive antenna. First, coarse timing and frequency offset estimates are obtained. Then, an advanced timing, relative timing indices, and the corresponding CIR estimates at the second stage are obtained using the generalized ML estimation based on a sliding observation vector. Finally, the fine time adjustment based on the minimum mean squared error criterion is performed. From the simulation results, the proposed approach has excellent performance in timing synchronization under several channel models at signal-to-noise ratio smaller than 1dB. [ABSTRACT FROM AUTHOR]

Space-time coding (STC) has been shown to play a key role in the design of MIMO radars with widely spaced antennas: In particular, rank-one coding amounts to using the multiple transmit antennas as power multiplexers, while full-rank coding maximizes the transmit diversity, compromises between the two being possible through rank-deficient coding. In detecting a target at known distance and Doppler frequency, no uniformly optimum transmit policy exists, and diversity maximization turns out to be the way to go only in a (still unspecified) large signal-to-noise ratio region. The aim of this paper is to shed some light on the optimum transmit policy as the radar is to detect a target at an unknown location: To this end, at first the Cramér–Rao bounds as a function of the STC matrix are computed, and then waveform design is stated as a constrained optimization problem, where now the constraint concerns also the accuracy in target ranging, encapsulated in the Fisher Information on the range estimate. Results indicate that such accuracy constraints may visibly modify the required transmit policy and lead to rank-deficient STC also in regions where pure detection would require pursuing full transmit diversity. [ABSTRACT FROM AUTHOR]

In this paper, we propose high-rate space–time coding for cooperative wireless networks to reduce the overall delay incurred in relaying signals to multiple receivers. The relay structure is optimized in order to achieve maximum SNR at the receiver nodes. The proposed scheme provides a significant reduction in the delay required for the relaying and transmission of the signals to the multiple receivers with a minute loss in performance. We have also shown by simulation that this loss in the performance could be recovered by selecting more number of relays. We propose two relaying strategies for high-rate space–time codes, which are very useful in providing high data-rate in wireless cooperative networks. [ABSTRACT FROM AUTHOR]

The goal of combining beamforming and space-time coding is to obtain full-diversity order and to provide additional received power (array gain) compared to conventional space-time codes. In this work, a class of code constellations is proposed, called generalized partly orthogonal designs (PODs) and both high-rate and low-rate feedback information is incorporated with possible feedback errors. A binary symmetric channel (BSC) model characterizes feedback errors. Two cases are studied: first, when the BSC bit error probability is known a priori to the transmission ends, and second, when it is not known exactly. Based on a minimum pairwise error probability (PEP) design criterion, we design a channel optimized vector quantizer (COVQ) for feedback information and a precoder matrix codebook to adjust the transmission codewords. The attractive property of our combining scheme is that it converges to conventional space-time coding with low-rate and erroneous feedback and to directional beamforming with high-rate and error-free feedback. This scheme also shows desirable robustness against feedback channel modeling mismatch. [ABSTRACT FROM AUTHOR]

This paper presents serially concatenated trellis coded modulations (SCTCMs) that perform consistently close to the available mutual information for periodic erasure channel (PEC), periodic fading channel (PFC) and the 2 x 2 compound matrix channel. We use both the maximum-likelihood decoding criteria and iterative decoding criteria to design universal SCTCMs for the PEC and the PFC. For the space-time channel, by demultiplexing the symbols across the antennas, the proposed universal SCTCMs for the period-2 PFC deliver consistent performance over the eigenvalue skew of the matrix channel. Within the family of channels having the same eigenvalue skew, a time-varying linear transformation (TVLT) is used to mitigate the performance variation over different eigenvectors. The proposed space-time SCTCMs of 1.0, 2.0 and 3.0 bits per transmission require excess mutual information in the ranges 0.11-0.15, 0.23- 0.26 and 0.35-0.53 bits per antenna, respectively. Because of their consistent performance over all channels, the proposed codes will have good frame-error-rate (FER) performance over any quasi-static fading distribution. In particular, the codes provide competitive FER performance in quasi-static Rayleigh fading. [ABSTRACT FROM AUTHOR]

We analyze decoupling structures of MIMO DS-CDMA channels with general multiuser detector front ends, using the replica method, in order to compare the space-time spreading (STS) and time spreading (TS) schemes. In the many-user limit, a MIMO DS-CDMA channel with the STS scheme is decoupled into a bank of single-user SIMO channels. On the other hand, a MIMO DS-CDMA channel with the TS scheme is decoupled into a bank of single-user MIMO channels. In view of performance,, the STS scheme outperforms the TS scheme in the fast fading situation if transmit spatial correlations exist. In terms of complexity, the STS scheme does not require any space-time coding. On the other hand, the TS scheme does require space-time coding in order to achieve comparable performance to the STS scheme. The STS scheme improves the performance of communications and reduces the complexity of transmitter and receiver architectures. [ABSTRACT FROM AUTHOR]

It is a general consensus that an orthogonal space-time block code can achieve full diversity, and due to its orthogonal nature, the multiple-input--multiple-output (MIMO) maximum-likelihood (ML) decoding metrics can be decoupled into single-input-single-output (SISO) ML metrics based on linear processing at the receiver, thus greatly reducing the decoding complexity. In fact, nonorthogonal codes also currently exist that can achieve better symbol-error-rate performance without rate reduction and complexity increase for correlated fading channels. In this paper, we show by detailed derivations that nonorthogonal linear space-time block codes can also be decoded by ML decoupling through receiver linear processing. Our derived expressions for the decoupled ML metrics automatically contain the design information for the receiver linear processors. [ABSTRACT FROM AUTHOR]

A multiple input, multiple output (MIMO) diffuse optical communications link is implemented and evaluated using a 2 × 2 Alamouti-type space-time coding scheme to increase link performance beyond that of single input, single output (SISO) and multiple input, single output (MISO) systems. We present a representative profile of received power versus distance, and comparative bit-error probability performance. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1108–1110, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21558 [ABSTRACT FROM AUTHOR]