Search

In this paper, we design receivers for filter bank multicarrier-based (FBMC-based) massive MIMO considering practical aspects such as channel estimation and equalization. In particular, we propose a spectrally efficient pilot structure and a channel estimation technique in the uplink to jointly estimate all the users’ channel impulse responses. We mathematically analyze our proposed channel estimator and find the statistics of the channel estimation errors. These statistics are incorporated into our proposed equalizers to deal with the imperfect channel state information (CSI) effect. We revisit the channel equalization problem for FBMC-based massive MIMO, address the shortcomings of the existing equalizers in the literature, and make them more applicable to practical scenarios. The proposed receiver in this paper consists of two stages. In the first stage, a linear combining of the received signals at the base station (BS) antennas provides a coarse channel equalization and removes any multiuser interference. In the second stage, a per subcarrier fractionally spaced equalizer (FSE) takes care of any residual distortion of the channel for the user of interest. We propose an FSE design based on the equivalent channel at the linear combiner output. This enables the applicability of our proposed technique to small and/or distributed antenna setups such as cell-free massive MIMO. Finally, the efficacy of the proposed techniques is corroborated through numerical analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the capacity of radio channels when iterative channel estimation, data detection, and decoding are employed. Knowing the capacity gain from iterative detection versus purely pilot-based channel estimation helps a designer compare the performance of an iterative receiver against a noniterative receiver and select the best balance between performance and cost. A bound is put on the linear minimum mean square error (LMMSE) channel estimation error, based on which a bound on the capacity is obtained. The attainable capacity is related to the channel estimation error of the receiver. The bounds take into account the uncertainty in symbol detection on channel estimation and incorporate the effect of channel estimation error on channel capacity. The interaction between the symbol detector and the decoder is analytically characterized and depicted in an extrinsic information transfer (EXIT) chart, where a bound on the detector curve is found. With optimal LMMSE pilot-based channel estimation, the results of this paper demonstrate that iterative channel estimation provides insignificant capacity advantage at fading rates below 1% of the symbol rate, although a computational-cost gain is still available. Iterative channel estimation provides a capacity benefit if suboptimal pilot signaling is used to provide initial channel estimates. [ABSTRACT FROM PUBLISHER]

Faster-than-Nyquist (FTN) signaling is regarded as a potential modulation format of 5G communication system for its high spectral efficiency. To further improve the spectral efficiency, researchers have extended the FTN signaling to the frequency dimension. However, the two-dimensional FTN signaling called multicarrier FTN (MFTN) signaling induces intersymbol interference and intercarrier interference (ICI), which challenges the receiver. Even the subcarriers are orthogonal, the MFTN signaling still induces the ICI. In this paper, we propose a new MFTN signaling transceiver with an FTN mapper and a linear minimum mean square error (LMMSE) equalizer. The FTN mapper is deployed on every subcarriers independently, which means that the MFTN is ICI-free. The proposed transceiver is compatible with the existing orthogonal frequency division multiplexing (OFDM) system and can detect the MFTN signal efficiently. The numerical results show that the bit error rate performance of the transceiver is close to the convolution code line. At the same time, the MFTN system with high-order modulation has power gains than the traditional OFDM system. [ABSTRACT FROM AUTHOR]

A plethora of new applications will be enabled by next generation wireless networks. These demand sophisticated medium access control and physical layer protocols in order to fulfill the requirements in terms of spectral efficiency, bit error rate (BER), energy efficiency, reliability, and security, among others. This paper develops an advanced space-time multiple-input, multiple-output (MIMO) solution that builds on the concept of space-time shift keying (STSK) for single-carrier frequency domain equalization (SC-FDE). STSK-aided SC-FDE, or SSF, ensures space-time and frequency diversity in dispersive channels, but can only support suboptimal linear receiver schemes. We therefore introduce a turbo equalizer for SSF in frequency-selective multipath channels. It enables inter-symbol-interference cancellation by means of iterative space-time processing. We coin it the iterative space-time block equalizer (SSF-ISTBE). We provide the mathematical formulation for the proposed SSF-ISTBE framework in non-linear channel conditions. We demonstrate that the SSF-ISTBE achieves a substantial 4 dB gain in terms of BER over the linear minimum mean square error equalizer while significantly reducing the computational complexity of the optimal maximum likelihood SSF. When considering practical transceivers, in particular non-linear power amplifiers, the gain of the SSF-ISTBE becomes more prominent over state-of-the-art approaches, demonstrating its robustness against non-linear distortion. [ABSTRACT FROM AUTHOR]

Chained turbo equalization (CHATUE) has been recently recognized as a low-complexity frequency-domain turbo equalization technique that eliminates the necessity of transmitting the cyclic prefix (CP), and hence allows for spectrally efficient signaling in wireless communications. However, two issues arise from the original version of CHATUE (referred to as CHATUE1) as a consequence of eliminating the CP, which are the noise enhancement and the latency due to the time-concatenated structure. This paper proposes a new version of CHATUE (referred to as CHATUE2) to solve the noise enhancement problem. CHATUE2 retrieves the circulant structure of the channel matrix, originally inherent within the CP transmission, by utilizing composite replica signals that combine the received signals and the soft reference signals replicated from the log-likelihood ratio (LLR) fed back from the decoder. For this purpose, this paper determines the optimal combining ratio based on the minimum-mean-square-error (MMSE) criterion. CHATUE2 is hence able to achieve an improvement in bit-error-rate (BER) performance over CHATUE1. In addition, this paper provides a solution to solving the latency problem by making a practical assumption on the training sequence (TR) transmission, which is required to perform channel estimation generally, in practical systems. Furthermore, this paper proposes a new channel estimation technique, i.e., chained turbo estimation (CHATES), which improves the spectrum efficiency and asymptotically achieves the Cramér–Rao bound (CRB). CHATES assumes that the TR length is exactly equal to the channel impulse response (CIR) length, although the conventional technique requires twice as long or even longer TR lengths. Numerical results show that CHATUE2 with CHATES achieves a gain of 1 dB over conventional turbo equalization with a CP at a BER of \10^-5 in realistic propagation scenarios represented by channel sounding measurement data and in model-based frequency-selective fading channels. [ABSTRACT FROM PUBLISHER]

Battery equalizers are widely used in multibattery systems to maintain balanced charge among individual battery cells. While the research on the hardware realization of battery equalizers has received significant attention, rigorous analysis of the battery equalization process at the system level remains largely unexplored. In this paper, we study three types of battery equalization system structures: series-based, layer-based, and module-based. Specifically, we derive mathematical models that describe the system-level behavior of the battery equalization processes under these equalization structures. Then, based on the mathematical models, analytical methods are developed to evaluate the performance of the equalization processes. In addition, we carry out statistical analysis to compare the performance of the three equalization structures considered. [ABSTRACT FROM PUBLISHER]

This paper introduces a new adaptive nonlinear equalizer relying on a radial basis function (RBF) model, which is designed based on the minimum bit error rate (MBER) criterion, in the system setting of the intersymbol interference channel plus cochannel interference (CCI). Our proposed algorithm is referred to as the online mixture of Gaussian-estimator-aided MBER (OMG-MBER) equalizer. Specifically, a mixture of Gaussian-based probability density function (pdf) estimator is used to model the pdf of the decision variable, for which a novel online pdf update algorithm is derived to track the incoming data. With the aid of this novel online mixture of Gaussian-based sample-by-sample updated pdf estimator, our adaptive nonlinear equalizer is capable of updating its equalizer's parameters sample by sample to aim directly at minimizing the RBF nonlinear equalizer's achievable bit error rate (BER). The proposed OMG-MBER equalizer significantly outperforms the existing online nonlinear MBER equalizer, known as the least bit error rate equalizer, in terms of both the convergence speed and the achievable BER, as is confirmed in our simulation study. [ABSTRACT FROM AUTHOR]

This paper analyzes the achievable rate of the single-carrier system with a frequency-domain equalizer (SC-FDE) under the receiver in-phase/quadrature phase (I/Q) imbalance in the radio frequency front-end. We consider linear schemes based on the minimum mean square error (MMSE), zero-forcing equalization, and a matched filter. We derive the achievable rate of the SC-FDE system hit by the I/Q imbalance for the mentioned scenarios. We show that depending on the value of I/Q imbalance parameters, the achievable rate of the system approaches a saturation level. To examine the validity of our analysis, we compare it with the simulation results and exhibit their consistency. We also consider nonlinear schemes such as an iterative block decision feedback equalizer (IB-DFE), which is an iterative scheme. We obtain the mutual information achieved by the IB-DFE in the high-signal-to-noise-ratio regime and after enough number of iterations. [ABSTRACT FROM AUTHOR]

In this paper, a pair of novel frequency domain equalizers for broadband single-carrier (SC) spatial modulation (SM) systems over dispersive channels are proposed by integrating with the concept of generalized approximate message passing (GAMP). Specifically, we first propose a GAMP-aided frequency domain turbo equalizer (FDTE-GAMP) operating on a vector-by-vector basis, which exploits the linear mixing channel structure composed of channel matrix and fast Fourier transforms. In the design of this novel detector, the non-Gaussian property of the transmitted SM symbol is fully investigated, in order to provide a more reliable belief of the channel symbols, leading to an accurate extrinsic information. For the sake of simplification, we propose a low-complexity version of FDTE-GAMP by forcing the variance components of transmitted symbols to be the same scalar, and thus avoiding the matrix inversion operation and maintaining a sequence of scalar operations. Simulation results show that the proposed equalizers are capable of providing a better tradeoff between the attainable bit error rate and the imposed computational complexity than the conventional FDTEs conceived for SC-SM systems. [ABSTRACT FROM PUBLISHER]

In this paper, we propose an improved hybrid turbo equalizer (TEQ) for single-carrier transmission over multipath channels with symmetric alpha-stable noise. First, a nonlinear myriad filter is adopted for combating the effect of impulsive noise. The noise output of this nonlinear filter is analyzed by an asymptotic statistic method and shown to be asymptotically Gaussian. Then, a matched minimum mean square error based linear TEQ is proposed for further mitigating the effects of the residual impulsive noise and the intersymbol interference caused by multipath fading. In our proposed TEQ design procedure, the instantaneous estimation of the noise variance is exploited to obtain robust TEQ coefficients. Simulation results show that the proposed hybrid TEQ provides geometric signal-to-noise ratio improvements (of up to 4.5 dB) compared to conventional algorithms designed for Gaussian channels. [ABSTRACT FROM PUBLISHER]

In this correspondence paper, an adaptive turbo equalizer for differential OFDM systems in underwater acoustic communications is proposed. This scheme first reconstructs the demodulation symbols using the soft information fed back from the channel decoder, and then eliminates the residual intercarrier interference (ICI) induced by estimated symbols using an adaptive turbo equalizer. Moreover, a pilot-based differential encoding method is presented to mitigate the impact of the feedback error, thereby improving the convergence of turbo equalization. Theoretical analysis and simulation results demonstrate that, the proposed equalizer outperforms the linear minimum mean-squared error (MMSE) estimation-based turbo equalizer for coherent systems in medium-to-high signal-to-noise ratio (SNR) region, while enjoys significantly reduced computational complexity, and its performance is more robust against the change of Doppler frequency. [ABSTRACT FROM AUTHOR]

In this paper, a new time-domain soft-decision feedback (TDSDF) aided frequency domain turbo equalizer (FDTE) is proposed for broadband single-carrier (SC) spatial modulation (SM) systems. Specifically, the proposed equalizer operates on a frame-by-frame basis and is designed based on the minimum block-averaged mean-square error (MBMSE) criterion. To tackle the error propagation problem, a metric in consideration of the reliability of the feedback information is considered. The optimal coefficients of the forward and feedback filters involved in the TDSDF-FDTE design are derived analytically. Simulation results show that the proposed nonlinear TDSDF-FDTE detector is capable of offering a better tradeoff between the bit-error-rate (BER) performance and the imposed computational complexity than the conventional linear frequency domain turbo demapper (FDTD) conceived for SC-SM systems. Furthermore, a 4 dB improvement is observed for the unbalanced channel. [ABSTRACT FROM AUTHOR]

On account of complementary control, reduced size, and energy saving, the switched-capacitor (SC) based equalizer becomes promising for the energy management of energy storage system. Traditionally, the number of the bypass capacitor in the SC based equalizer equals to the number of the battery module in series or parallel connections. The amount of bypass capacitors is enormous, thus costly and bulky for electric vehicles (EVs). In this paper, the integrated energy storage is proposed to reduce cost and save space, meanwhile, the equalizations between the batteries and the ultracapacitors (UCs) are achieved by the reutilization of SC network. UCs not only perform as energy storage units, but also work as energy exchange units. Thus, the number of balanced elements can be reduced significantly. The operational principles of the storage system and the analyses of the balancing process are examined in detail. The system simulation is performed for functionality evaluations. Meanwhile, an experimental prototype is also implemented for six cells to demonstrate the balancing process. The accordance of experiment and simulation can further testify the feasibility and effectiveness. [ABSTRACT FROM AUTHOR]

This paper presents the analysis and design procedure of a voltage equalizer, which consists of a boost full-bridge input cell and a symmetrical voltage multiplier (SVM). The batteries under different initial voltage conditions can be charged to a desired uniform voltage level by the proposed voltage equalizer. The fundamental operation principle of the voltage equalizer is described, and a dc equivalent resistance model is derived to analyze the charging and equalization behaviors. The design procedure based on analytical models is presented to give the optimal parameter configuration for the voltage equalizer to reduce voltage imbalance among the batteries and improve the power conversion efficiency. The model of the equivalent resistance is verified by simulation study of an equivalent circuit. A 10-W prototype for 3 series-connected energy storage modules was built and tested with voltage imbalance. Experimental results confirm the validity of the design method, and demonstrate that the proposed voltage equalizer can achieve a good voltage equalization with a standard deviation reduced to a few millivolts. [ABSTRACT FROM AUTHOR]

Obtaining channel state information in massive multiple-input multiple-output (MIMO) systems is challenging because of significant multiple-access interference and very limited resources available for channel estimation. Most existing equalization schemes for massive MIMO systems assume perfect channel knowledge at the receivers. When channel estimation error is present, however, significant performance degradation will be experienced, which has been demonstrated in the literature. In this paper, we model the maximum-likelihood channel estimation as the sum of real channel and random channel estimation error, and develop a minimum mean-square error (MMSE) based turbo equalization method conditioned on the channel estimate. We demonstrate that our newly developed iterative processing approach is a general expression, whereas existing methods can be viewed as special cases. We further derive an MMSE-sorted QR decomposition (MMSE-SQRD) based turbo equalization to further improve the performance for massive MIMO systems under large user numbers and severe channel estimation error conditions. Numerical results and performance comparison show that the proposed MMSE-based turbo equalization scheme greatly outperforms the existing schemes. With a large number of users being simultaneously served, our proposed MMSE-SQRD-based scheme can further enhance system performance over the MMSE-based turbo equalization scheme. [ABSTRACT FROM AUTHOR]

In mobile orthogonal frequency-division multiplexing (OFDM) systems, a frequency-domain channel matrix represents the same-carrier channel frequency response in the diagonal and intercarrier interference (ICI) between the subcarriers in the off-diagonals, respectively. A variety of the banded equalizers manipulated the banded approximation of the channel matrix to be exploited by the low-complexity equalizations. In this paper, we derive a simple and tight lower bound on the variance of the individual coefficients in the channel matrix for insights into the banded approximations. We obtain the errors introduced with the banded approximation and the ICI-mitigation gains of the banded equalizers in simple closed forms. The derivations of the banded approximation errors (BAEs) are beneficially applicable to the equalizers that perform the minimum mean square error (MMSE) estimation with the banded channel matrix. Simulations show that both the block MMSE banded equalizers and the block turbo MMSE banded equalizers significantly reduce the error floors by considering the BAEs. [ABSTRACT FROM PUBLISHER]

In this paper, a new class of soft-decision feedback aided time-domain turbo equalizer (TDTE) based on the minimum mean-square error criterion is proposed for broadband single-carrier (SC) generalized spatial modulation (GSM) systems. First, a time-domain soft-decision feedback (TDSDF) aided TDTE is proposed, which operates on a vector-by-vector basis. In order to provide a flexible tradeoff between complexity and performance, time-invariant filter coefficients are derived to provide a low-complexity approximate solution to the proposed TDSDF-TDTE. Furthermore, a soft-feedback interference cancelation (SFIC) aided TDTE is proposed for SC-GSM systems with a lower complexity, where the computation of the second-order moment of matrices is completely avoided. Finally, bi-directional soft-decision feedback aided TDTEs are proposed by using a time-reversed TDSDF-TDTE/SFIC-TDTE in conjunction with a ordinary TDSDF-TDTE/SFIC-TDTE so as to harvest time-reverse diversity and to mitigate the error propagation. An extrinsic information transfer chart based analysis and bit error rate simulation results are presented to demonstrate the superior performance of the proposed soft-decision feedback aided TDTEs as opposed to the original TDTEs, while achieving a lower complexity. [ABSTRACT FROM AUTHOR]

Cell voltage equalization is inevitable to ensure years of safe operation of series-connected energy storage cells, such as lithium-ion batteries and supercapacitors (SCs). Although various kinds of cell voltage equalizers have been proposed, most equalizer topologies require multiple switches and/or a multiwinding transformer, resulting in complex circuitry and poor modularity. In this paper, a single-switch single-transformer equalizer using a forward–flyback resonant inverter (FFRI) with a voltage multiplier is proposed. The required switch count of the proposed equalizer is the minimum without impairing modularity due to the single-switch circuitry with no need for a multiwinding transformer. An experimental equalization test performed for eight SCs connected in series successfully demonstrated the equalization performance of the proposed equalizer. The FFRI can be extended as a “resonant input cell,” and by combining one of resonant input cells and a voltage multiplier, a single-switch resonant equalization charger that is basically a charger with an equalization function is also derived. An experimental charging test using the resonant equalization charger was also performed, and its equalization-charging performance was demonstrated. [ABSTRACT FROM AUTHOR]

This paper proposes an interleaved equalization architecture for series-connected lithium-ion battery strings, which can deliver energy from a battery module to the cells in the next adjacent battery module, resulting in an enhancement of the equalization current and efficiency. Particularly, the global equalization is achieved without the need of additional stage circuits to balance among modules, leading to a small size and low cost. A two-module equalizer based on resonant LC converter and buck converter with soft switching is implemented to verify the validity of the proposed interleaved architecture. Furthermore, a self-learning fuzzy logic control (SLFLC) algorithm is employed to online regulate the equalization period based on the voltage difference among cells and the cell voltage, not only greatly abbreviating the balancing time but also effectively preventing overequalization. The SLFLC has the outstanding advantages of high balancing precision, easy implementation, and strong robustness. Experimental results demonstrate that the proposed equalizer has good balancing performances with soft switching and high efficiency, and achieves zero-voltage gap among cells. Moreover, the proposed SLFLC algorithm abridges the total equalization time by about 27%, and reduces the equalization cycles by about 59% compared with the traditional control algorithm. [ABSTRACT FROM AUTHOR]

This paper proposes a modularized global architecture using multi-winding transformers for battery cell balancing. The global balancing for a series-connected battery string is achieved based on forward conversion in each battery module and based on flyback conversion among modules. The demagnetization of the multiwinding transformers is also simultaneously achieved by the flyback conversion among modules without the need of additional demagnetizing circuits. Moreover, all MOSFET switches are driven by two complementary pulse width modulation signals without the requirement of cell voltage sensors, and energy can be automatically and simultaneously delivered from any high voltage cells to any low voltage cells. Compared with existing equalizers requiring additional balancing circuits for battery modules, the proposed modularized equalizer shares one circuit for the balancing among cells and modules. The balancing performance of the proposed equalizer is perfectly verified through experimental results, and the maximum balancing efficiency is up to 91.3%. In summary, the proposed modularized equalizer has the advantages of easier modularization, simpler control, higher efficiency, smaller size, and lower cost, ensuring the battery system higher reliability and easier implementation. [ABSTRACT FROM PUBLISHER]

The demand for high-data-rate radio systems that can robustly operate in multipath environments presents challenging technical issues. In this paper, we propose a passive phase conjugation (PPC)-based orthogonal frequency-division multiplexing (OFDM) system. Compared with the conventional OFDM system, the proposed system achieves lower computational complexity since the need for conventional channel estimation and equalization can be eliminated. The performance gain of the proposed technique is shown to be particularly advantageous in systems with multiple receiver array elements. Numerical results are provided to illustrate the performance of the system when operating through a multipath radio channel. We also derive a closed-form formula that can be used to predict the performance of the proposed scheme over multipath channels with a uniform power delay profile. Our analysis validates the simulation results and provides insights into the behavior of wireless centric OFDM-PPC systems. [ABSTRACT FROM PUBLISHER]

Channel estimation is an important issue for wireless communication systems. A channel estimation scheme using a Takagi-Sugeno (T-S) fuzzy-based Kalman filter under the time-varying velocity of the mobile station in a multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) system is proposed in this paper. The fuzzy technique is used to interpolate several linear models to approximate the nonlinear estimation system. A MIMO system with the orthogonal space-time block coding (OSTBC) scheme is considered, where the radio channel is modeled as an autoregressive (AR) random process. The parameters of the AR process and the channel gain are simultaneously estimated by the proposed method. One-step-ahead prediction can be obtained during this estimation procedure. This is useful for the decision-directed channel-tracking design, particularly in the fast-fading channel. Furthermore, the robust minimum mean-square error (MMSE) equalization design can be achieved by considering the channel prediction error to improve the performance of symbol detection. To validate the performance of our proposed method, several simulation results are given and compared with those of other methods. When considering the time-varying velocity of the mobile station communication in the MIMO-OFDM system, the enhanced equalizer based on the T-S fuzzy-based Kalman filter performs better than those based on the conventional channel estimators in terms of symbol error rate. [ABSTRACT FROM PUBLISHER]

Many communication systems today encounter the problem of data transmission over a channel with intersymbol interference (ISI). The purpose of this paper is to develop a low-complexity iterative soft-decision feedback equalization (SDFE) receiver for severe frequency-selective ISI channels in multiple-input–multiple-output (MIMO) communication systems. The proposed SDFE algorithm offers a novel approach to combat error propagation. In addition, its computational complexity only linearly grows with the number of equalizer coefficients, compared with the quadratic complexity of minimum mean square error (MMSE)-based linear turbo equalizer with time-varying coefficients. The performance of the proposed detection scheme is verified through simulations using different signal constellations. In addition, convergence properties of the proposed SDFE algorithm are also analyzed using an extrinsic information transfer (EXIT) chart and verified by simulations in a severe ISI channel. Simulation results show that our proposed algorithm has significant improvement over the Approximate MMSE linear turbo equalizer proposed by Tüchler et al. Moreover, we show that the performance of the proposed equalization scheme significantly improves when higher order constellations are used for digital modulation. [ABSTRACT FROM PUBLISHER]

Various types of cell voltage equalizers that exchange charges among cells have been proposed for series-connected lithium-ion cells. During equalization, cells are possibly cycled not only over a conventional period that ranges from seconds to hours but at a high frequency that is equal to the operating frequency of the equalizers as well. This paper investigates how the high-frequency cycling occurs and the factor that affects the life performance of the cells during high-frequency cycling. Experiments on charge–discharge cycling at frequencies of 1–100 kHz were performed for lithium-ion cells. The capacity of cells cycled at frequencies below 10 Hz significantly deteriorated, whereas the capacity decay of cells cycled at higher frequencies were identical to the corresponding calendar degradation. Simulation analyses revealed that the cycling current at high frequencies served only to charge and discharge the double-layer capacitance C \rm dl, whereas the current that flows through the charge-transfer resistance Rct, indicating the charge-transfer process, dominated at low frequencies. A correlation between the degradation trend and current distribution to Cdl and Rct was observed, implying that the calendar degradation and the degradation due to the charge-transfer process were the major causes of capacity decay in the frequency range below 10 Hz. [ABSTRACT FROM PUBLISHER]

A spatial multiplexing single-input–multiple-output (SM-SIMO) ultrawideband (UWB) communication system using the time-reversal (TR) technique is proposed in this paper. The system with only one transmit antenna, using a spatial multiplexing scheme, can transmit several independent data streams to achieve a very high data rate. To cope with the long delay spread of the UWB channel, the TR technique is adopted. TR can mitigate not only the intersymbol interference (ISI) but the multistream interference (MSI) caused by multiplexing several data streams simultaneously as well. Preequalization using the channel state information (CSI), which is already available at the transmitter in TR systems, is proposed to further eliminate the ISI and MSI. Simulation results show that the preequalization based on shortened equivalent channels can considerably improve the bit error rate (BER) performance of the system. [ABSTRACT FROM PUBLISHER]

Single-carrier (SC) block transmission with frequencydomain equalization (FDE) is an efficient scheme that avoids complex time-domain equalization in multipath channels. There are also several systems that are proposed that apply SC-FDE in code-division multiple access (CDMA). However, the present CDMA systems with FDE may not be suitable for asynchronous users for lack of a signal model that can suitably express the despread signal in the frequency domain. In this paper, we first propose circularly sliding (CS) despreading that can attain preliminary separation of interference among asynchronous users. Then, we derive a signal model that represents spreading, channel effect, and despreading as a series of circular convolutions. This signal model helps to derive a new FD minimum mean-square error (MMSE) equalization algorithm for SC block transmission asynchronous CDMA. We further extend the signal model by including equalization as another circular convolution. The extended signal model indicates several equivalent architectures of the receiver. For these architectures, despreading and equalization can be implemented in either the time or frequency domain, and the order is exchangeable. Based on the proposed signal model, we also derive a RAKE combiner that can be implemented in the frequency domain. Our architectures and signal model can be applied for further development of multiuser-detection algorithms. [ABSTRACT FROM AUTHOR]

In this paper, a two-stage receiver structure for interference cancellation in multiuser spatially multiplexed multiple-antenna systems is presented. A space-time equalizer is used in the first stage for joint coantenna/cochannel-interference suppression and shortening of the effective channel for each transmit stream of the desired user. The channel shortening, combined with independent detection, helps reduce the complexity of the second stage Viterbi equalizer, which is used for separate intersymbol-interference equalization for each of the streams. Three objective functions are proposed for determining the coefficients of the space-time equalizer using a direct training data-based approach, which does not require estimation of the interferer's channel. Simulation results show good symbol error performance as compared to existing algorithms with asynchronous multiple-input multiple-output interferers. [ABSTRACT FROM AUTHOR]

Multiple-input multiple-output (MIMO) relays can improve cell coverage and data throughput for wireless networks. The key challenge for the success of MIMO relay networks is effectively managing the intersymbol interference (ISI) and multiantenna interference (MAI) in multipath channels. In this paper, equalize-and-forward (EF) relaying strategies are employed to mitigate the interference by jointly optimizing equalizer weights and power allocation for dual-hop MIMO relay networks. Two scenarios with different channel state information (CSI) knowledge are investigated: 1) full CSI at the relays and 2) only backward CSI at the relays and the destination. By considering CSI availability and using the minimum-mean-square-error (MMSE) criterion, iterative algorithms are proposed for the joint design of equalizer weights and power allocation to resolve the interference problem. We then extend the design to a more general case, in which the direct link between the source and the destination is taken into account. Furthermore, two relay selection algorithms based on allocated power and MSE performance are investigated for the two scenarios, which attain a performance that is comparable to that of cases with brute-force search or without relay selection. The design framework can capture the impact of the available CSI at the relays and the destination on the performance of MIMO multirelay networks with multipath receptions. [ABSTRACT FROM AUTHOR]

We study the robust linear equalizer for the uplink of massive multiple-input–multiple-output (MIMO) systems with multicell pilot reuse. We use the worst-case approach for robust design to combat pilot contamination. When the number of base station (BS) antennas is large, we build a novel model to indicate the relationship between the instantaneous channel matrix and the imperfect channel estimation in the presence of channel model uncertainty. Based on this model, we formulate the robust equalizer design problem into a min–max problem. Furthermore, we transform the min–max problem into an unconstrained one, and the optimality conditions are derived. With the resulting optimality conditions, two iterative algorithms and a simple approximation method are proposed to compute the optimal robust equalizers. Simulations are adopted to evaluate the performance of the proposed algorithms and the approximation method. Compared with the conventional equalizers, the proposed robust equalizers achieve a better bit error rate (BER) performance, particularly in the regime of high signal-to-noise ratio (SNR), where pilot contamination is significant. [ABSTRACT FROM AUTHOR]

In this paper, we study the performance of single carrier (SC) modulation with frequency-domain equalization (FDE) over underwater acoustic (UWA) channels. The underlying channels are time-varying intersymbol interference (ISI) channels, where time variation arises from the relative motion between the transceivers, which induces one or more Doppler scaling factors. We study two scenarios: a point-to-point (P2P) system, where each cluster of paths has its own distinct Doppler scaling factor, and a multiple access (MAC) system, where $M$ users communicate with a multiple-antenna common receiver in which each user has its own Doppler scaling factor. First, the maximum likelihood (ML) receiver is derived for both scenarios, and it is shown that a preprocessing stage is necessary to reduce the time variation; this is referred to as multiple resampling (MR). The proposed receiver consists of multiple branches where each branch corresponds to a cluster/user and performs frequency shifting and resampling followed by an integrator. Since the output of the MR stage is contaminated by ISI for P2P systems and ISI and interuser interference for MAC systems, an additional equalization stage is necessary, which is ideally the ML sequence detector (MLSD). Since the complexity of MLSD exponentially grows with the number of symbols per block, the alphabet size, and the number of users, a linear minimum-mean-square-error FDE equalizer is used instead. To further reduce the complexity, instead of resampling the received signal multiple times by different scaling factors, it is resampled only by one scaling factor, which is a function of all Doppler scaling factors. The resulting suboptimal preprocessing scheme is called single resampling (SR). Simulation results for uncoded systems show that MR outperforms its SR counterpart at the expense of some additional hardware complexity. Moreover, it is shown that SC-FDE is more resilient than orthogonal frequency-division multiplexing (OFDM) to the Doppler scaling effect in UWA channels at lower overall complexity for the MR case, whereas both have the same overall complexity for the SR case. [ABSTRACT FROM AUTHOR]

We propose a unified approach to transceiver optimization for nonregenerative multiple-input–multiple-output (MIMO) relay networks. This approach leads to new transceiver designs and reduces algorithmic complexity with adaptive implementations. First, we formulate a generic system model that accommodates various network topologies by imposing structural constraints on the source precoder, the relaying matrix, and the destination equalizer. Based on the minimum-mean-square-error (MMSE) criterion, we derive the optimal relaying matrix as a function of the other two matrices, thereby freeing the optimization problem from this matrix and its associated power constraint with no loss of optimality. Subsequently, we study how to optimize either the precoder or the equalizer under different structural constraints and propose an alternating algorithm for the joint design. When optimizing the equalizer, the optimum from the previous transmission block is chosen as the initial search point to speed up convergence and, hence, to reduce computational complexity. The proposed framework is further explained and numerically validated within the context of different system configurations. [ABSTRACT FROM PUBLISHER]

Interference alignment (IA) has been shown to be a promising technique for achieving the optimal capacity scaling of a multiuser interference channel at asymptotically high-signal-to-noise ratio (SNR). However, in practical communication systems, mitigating interference from all interferers via IA is not necessary since some users' interference have negligible effect due to large path-loss. Moreover, the feasibility constraint and the heavy signaling overhead hinder applying IA on interference from all interferers. Clustered IA puts users in disjoint clusters where IA is applied to users within each cluster. It provides a mechanism for mitigating the signaling overhead and maximizing the achievable rate. However, how to properly form IA clusters has not been well studied. We consider the application of clustered IA in a multiuser interference network with asymmetric channel attenuation at finite SNR. We model the interference network as a connected graph, transforming the clustering problem into a graph partitioning problem. By exploiting the variation on the interference levels from multiple interferers, efficient clustering algorithms are proposed such that clusters formed can capture strong interference as intracluster interference, leaving relatively weak interference as intercluster interference. Then, the intercluster interference can be coarsely modeled as noise. We also consider the precoder/equalizer design in a clustered system and show the importance of incorporating the aggregated intercluster interference in the design. Simulation results show that proper clustering combined with generalized IA precoder/equalizer design leads to significant gains on the achievable sum rate. [ABSTRACT FROM PUBLISHER]

Cell voltage equalizers are necessary to ensure years of operation for series-connected supercapacitors (SCs) by eliminating cell voltage imbalance. Conventional equalizers require numerous switches and/or magnetic components; hence, the circuit complexity and size tend to increase. In this paper, a double-switch single-transformer equalizer, which is based on the combination of a half-bridge inverter and a voltage multiplier, is proposed. The operation of the proposed equalizer is analyzed in detail, and a dc equivalent circuit is mathematically derived. Experimental equalization tests were performed for eight SCs connected in series under a voltage-imbalanced condition. The energy of the series-connected SCs was preferentially redistributed to SCs with low voltages, and eventually, the voltage imbalance was eliminated, demonstrating the equalization performance. Simulation-based equalizations using the derived dc equivalent circuit were also performed under the same condition as the experiments. The experimental and simulation results were in good agreement, indicating that the derived dc equivalent circuit can adequately express real equalization profiles. [ABSTRACT FROM PUBLISHER]

The cell voltage imbalance of series-connected energy storage cells, such as supercapacitors (SCs) and lithium-ion cells, causes premature deterioration and a decrease in the available energies of the cells. Various equalization techniques have been developed for diminishing such imbalances. However, since the number of switches, sensors, and/or multiwinding transformers present in conventional equalizers is directly proportional to the number of series connections of the cells, the circuit complexity and cost of the equalizers are prone to increase with the number of series connections. In this paper, single-switch cell voltage equalizers using multistacked buck-boost converters, such as the single-ended primary inductor converter (SEPIC), Zeta, and Ćuk converters, are proposed. These equalizers consist of passive components and a single switch, significantly reducing the complexity of the circuit when compared with that of conventional equalizers. In addition, when the proposed equalizers operate in discontinuous conduction mode, feedback control is not required to limit currents flowing through cells and circuit components. The proposed equalizers are compared with conventional topologies in terms of the number of active and passive components required. Operating analyses were conducted under both cell-voltage-balanced and -imbalanced conditions. Experimental equalization tests were performed for four series-connected SCs using the SEPIC-based single-switch equalizer. The energies of the series-connected SCs were preferentially redistributed by the equalizer to the cell(s) having the lowest voltage, resulting in the elimination of the cell voltage imbalance and subsequent uniformity of the cell voltages. [ABSTRACT FROM PUBLISHER]

This paper is concerned with the challenging and timely problem of data detection for coded orthogonal frequency-division multiplexing (OFDM) systems in the presence of frequency-selective and very rapidly time varying channels. New low-complexity maximum a posteriori probability (MAP) data detection algorithms are proposed based on sequential detection with optimal ordering (SDOO) and sequential detection with successive cancellation (SDSC). The received signal vector is optimally decomposed into reduced dimensional subobservations by exploiting the banded structure of the frequency-domain channel matrix whose bandwidth is a parameter to be adjusted according to the speed of the mobile terminal. The data symbols are then detected by the proposed algorithms in a computationally efficient way by means of the Markov chain Monte Carlo (MCMC) technique with Gibbs sampling. The impact of the imperfect channel state information (CSI) on the bit error rate (BER) performance of these algorithms is investigated analytically and by computer simulations. A detailed computational complexity investigation and simulation results indicate that, particularly, the algorithm based on SDSC has significant performance and complexity advantages and is very robust against channel estimation errors compared with existing suboptimal detection and equalization algorithms proposed earlier in the literature. [ABSTRACT FROM PUBLISHER]

In this paper, the problem of channel equalization in filter bank multicarrier (FBMC) transmission based on the offset quadrature-amplitude modulation (OQAM) subcarrier modulation is addressed. Finite impulse response (FIR) per-subchannel equalizers are derived based on the frequency sampling (FS) approach, both for the single-input multiple-output (SIMO) receive diversity and the multiple-input multiple-output (MIMO) spatially multiplexed FBMC/OQAM systems. The FS design consists of computing the equalizer in the frequency domain at a number of frequency points within a subchannel bandwidth, and based on this, the coefficients of subcarrier-wise equalizers are derived. We evaluate the error rate performance and computational complexity of the proposed scheme for both antenna configurations and compare them with the SIMO/MIMO OFDM equalizers. The results obtained confirm the effectiveness of the proposed technique with channels that exhibit significant frequency selectivity at the subchannel level and show a performance comparable with the optimum minimum mean-square-error equalizer, despite a significantly lower computational complexity. The possibility of tolerating significant subchannel frequency selectivity gives more freedom in the multicarrier system parameterization. For example, it is possible to use significantly wider subcarrier spacing than what is feasible in OFDM, thus relieving various critical design constraints. [ABSTRACT FROM PUBLISHER]

This paper analyzes the outage performance of soft-cancellation frequency-domain minimum-mean-square error (SC FD-MMSE) turbo equalization in a serially concatenated coded modulation scheme over frequency-selective Rayleigh fading channels with exponential delay-power profile. The convergence behavior of this iterative three-stage system is evaluated with the aid of correlation charts. Based on the convergence characteristic of the equalizer and the inner and outer channel decoders, we derive a simple closed-form approximation on the outage probability of the turbo receiver. In particular, using a union-bounding technique and a specific central limit theorem, it is shown that the outage probability can be well approximated by a sum of complementary Gaussian error functions. A design criterion based on the resultant analytical expression is then proposed to construct outer encoders that minimize the outage probability of three-stage turbo systems employing irregular recursive convolutional (IRC) codes. We provide examples of optimized IRC-based outer encoders and investigate by simulation the performance improvement obtained by optimized IRC-coded turbo equalization over turbo equalization using regular recursive convolutional codes. Numerical results of outage performance simulations in channels with different delay-power profiles are presented to verify the analysis. [ABSTRACT FROM PUBLISHER]

This paper presents a multiple-input-multiple- output (MIMO) receiver design with integrated channel estimation and tracking for a time-varying frequency-selective Rician or Rayleigh fading environment. It first extends a polynomial- predictor-based channel estimation and tracking approach to a MIMO system. The structure and complexity of the estimator are similar to that of an optimum estimator using a Kalman filter, but it does not require a priori knowledge of the channel statistics. It employs a fixed-state transition matrix using precomputed polynomial coefficients and can be used in a Rician fading environment without reconfiguration. It is integrated with a MIMO minimum-mean-squared-error decision feedback equalizer, and simulation results show that the system performance using the estimator can be made comparable to that employing a Kalman estimator under a broad range of channel conditions. [ABSTRACT FROM AUTHOR]

A low-complexity minimum mean square error (MMSE) equalizer with successive interference cancellation (SIC) for orthogonal frequency-division multiplexing (OFDM) over time-varying multipath channels is presented. The equalization algorithm exploits the property of matrix LDL^H factorization to avoid direct or recursive computation of matrix inversion, thereby reducing the computational complexity. This algorithm also allows us to exploit the band approximation of the frequency-domain channel matrix to further reduce the complexity with minor performance degradation. Theoretical analysis and simulations show that the proposed algorithms reduce the computational complexity of the MMSE-SIC scheme while preserving a good performance. [ABSTRACT FROM PUBLISHER]

Ultraviolet (UV) communication has emerged as a promising solution for providing non-line-of-sight (NLOS) wireless connectivity due to strong molecular and aerosol scattering at UV wavelength. However, performance of UV based communication systems is severely impaired due to nonlinear transfer-characteristics of light emitting diode (LED), which degrades the overall symbol error rate (SER) performance. In addition, UV based communication systems are also impaired by multiplicative distortion due to turbulence, that causes detrimental instantaneous outages. Hence, in this work, first an expression for the outage probability is derived for a nonlinear UV communication system via analytical characterization of the statistics of the additive distortion. Further, utilizing the proposed analytical model for additive distortion, the error rate of reproducing kernel Hilbert space (RKHS) based detectors is quantified for the nonlinear outdoor NLOS UV channel. Additionally, using the derived expression for error-rate, an RKHS based minimum symbol error rate (MSER) equalizer is formulated to mitigate the distortion due to LED nonlinearity, and to enhance the error-rate performance of the considered nonlinear NLOS UV link. Convergence of the proposed MSER equalizer is analyzed, and improvements in error-rate promised by the proposed equalizer are validated by computer simulations over typical NLOS UV channels. [ABSTRACT FROM AUTHOR]

A series of integrated equalizers based on joint buck-boost (BB) and switched-capacitor (SC) converters are proposed for balancing the voltages of series-connected battery packs. All these equalizers realize the any-cells-to-any-cells (AC2AC) equalization mode without increasing the count of MOSFETs and drivers. Corresponding operational principles are analyzed and the expressions of balancing currents are derived by analytical methods and verified by experimental waveforms. According to the comparative balancing experiments for four and six series-connected Li-ion cells, one proposed CBB-PCSC equalizer, which achieves the dual AC2AC balancing modes through the integration of both coupled buck-boost (CBB) and parallel-connected switched-capacitor (PCSC) converters, leads to the highest balancing speed and efficiency. Moreover, compared with several conventional equalizers, this CBB-PCSC topology also has the compact size and low cost, making it become a well-performing integrated topology for automotive battery voltages equalization. [ABSTRACT FROM AUTHOR]

The large delay spread of ultra-wideband (UWB) channels causes intersymbol interference and therefore increases the complexity of a UWB Rake receiver. To resolve these problems, we present a new method for the design of channel shortening equalizers (CSE) in time-hopping UWB systems. Our proposed system is based on an eigenfilter method using a new objective function. In contrast to existing methods based essentially on the optimization of the CSE following one single criterion, our design allows using complementary criteria. The proposed system aims at minimizing most of the energy of the shortened channel impulse response outside the desired delay-spread window and the spectral distortion related to the filtering function. Furthermore, it simultaneously aims at reducing the multiple access interference and noise power. Indeed, the proposed CSE reduces the number of effective channel taps before the correlation receiver, which renders its architecture less complex. [ABSTRACT FROM AUTHOR]

Based on the zero prefix (ZP) structure, we propose an efficient block transmission algorithm using distributed space-time codes in the asynchronous cooperative communication systems. To accomplish the new frame, ZP is inserted in front of the data block to combat the relative delay between the asynchronous relays, while zero suffix (ZS) is appended at the end of the block to remove the interblock interference caused by the time dispersing channel; so, we employ this ZP–ZS precoding form to make the coded frames synchronous. It should be noted that single carrier transmission and orthogonal frequency division multiplexing (OFDM) are both compatible with the frame. For single carrier transmission, conventional space-time equalizers can be investigated at the destination node to cancel the multipath interference. To further improve the performance, an improved QR decomposition-based recursive interference cancellation (QRD-RIC) detector is employed, where joint QRD is adopted on the combination of channel matrices from different relays, and then spatial-and-multipath diversity combinations are introduced and improved into the proposed system, where the spatial and multipath diversities are acquired. For OFDM, the fractionally spaced frequency-domain MMSE diversity receiver is employed, since the guard interval in our framing scheme implies oversampling in the frequency domain. Numerical simulations show that the proposed algorithms are effective for the asynchronous transmissions under frequency selective fading channels. [ABSTRACT FROM AUTHOR]

In view of the nonuniform voltage of series-connected supercapacitors (SCs), a new voltage equalizer was designed. This voltage equalizer is composed of a switching network circuit and an isolated dc/dc converter. A switching network greatly simplifies the circuit over a traditional switch driver by using electrical potential difference between series-connected SCs to drive switches. It provides pathways for all SCs that are series connected. Each SC can be connected to an isolated dc/dc converter individually by the use of a switching network. Energy will be transmitted between any two cells directly; thus, it is also called point-to-point energy transmission. Detailed operation analyses and an equalizing strategy were presented for the voltage equalizer. A prototype for 12 SCs that are series connected was built based on the new voltage equalizer, and an experiment was performed for series-connected SCs with a nonuniform initial voltage. The experimental results showed that the voltage equalizer has the ability to eliminate nonuniform voltage of the series-connected SCs effectively. Furthermore, this voltage equalizer not only can be used for SCs but can be used for traditional secondary batteries as well. [ABSTRACT FROM PUBLISHER]

An adaptive subband chip-level equalizer is investigated for downlink multiple-input–multiple-output (MIMO) direct-sequence (DS) ultrawideband (UWB) systems to cope with intersymbol interference (ISI) and multiple-access interference (MAI) in multipath fading channels. The subband decomposition technique with multiple analysis and synthesis filter banks (FBs) is employed to substantially improve the downlink reception of mobile-terminal (MT) receivers. This chip equalizer utilizes subband adaptive interference suppression techniques based on the minimum mean square error (MMSE) criterion. A reduced-complexity chip-level MMSE equalization scheme is proposed by making full use of the reduced-rank (RR) multistage matrix Wiener filter (MMWF) and the matrix conjugate gradient (MCG) architectures in subbands. Simulations are conducted to provide a comparative evaluation of the proposed subband chip equalizer with other existing full-band and subband chip equalizers. [ABSTRACT FROM PUBLISHER]

The mitigation of 2-D intersymbol interference (ISI) has recently emerged as an important problem in the field of storage technology and wireless communications. Several solutions based on message passing have been proposed to perform equalization on 2-D channels with ISI. We present an algorithm based on Gaussian belief propagation (GaBP). We show that the proposed algorithm has interesting features compared with the corresponding message passing solutions available in the literature. First, the complexity of the proposed equalizer is independent of the size of the constellation used for modulation. Second, since the complexity is not exponential in the size of the state space, the computational burden is reduced for channels with long memory. As an application, we consider decentralized intercell interference mitigation in uplink cellular networks, where base stations communicate with their neighbors to recover their own users' signals. [ABSTRACT FROM PUBLISHER]

A sequence-based multiple-input-multiple-output receiver incorporating channel estimation and tracking for a frequency selective fading environment is developed. It employs near-maximum likelihood sequence estimation using the partitioned Viterbi algorithm and offers linearly increasing complexity with the number of transmit antennas. The receiver implements channel estimation and tracking using a vector-polynomial-based generalized recursive least squares algorithm. The resulting integrated receiver can operate in a continuously time-varying Rayleigh or Rician fading environment. Simulation results show that it offers a good tradeoff between complexity and performance. Further complexity reduction can be achieved using tentative decisions as reference signals to the estimator and a reduced complexity form of the estimator. [ABSTRACT FROM AUTHOR]

The authors present a chip-level blind frequency domain equalizer (FEQ) for the forward-link channel of a cyclic-prefix code division multiple access system. The FEQ coefficients are obtained without the need of training symbols or knowledge of channel state information. The coefficients are instead acquired by solving a constraint energy minimization problem involving the subspace spanned by the active and passive spreading codewords. They also prove that the random scrambling code sequences is required for the operation of the proposed equalization algorithm. Results from computer simulations are provided to verify the performance of the proposed FEQ. [ABSTRACT FROM AUTHOR]