To enable the low-cost ubiquitous Internet of Things, passive backscatter communication is envisioned as one of the most prominent and promising techniques; however, the underlying security issues associated with practical finite-alphabet signaling from the perspective of physical-layer security (PLS) have not been well studied. Despite several preliminary efforts on improving the eavesdropper's decoding error probability through PLS approaches, this article comprehensively investigates the secrecy rate performance of a secure multiantenna radio-frequency identification (RFID) system with a finite-alphabet input at the RFID tag. Unlike conventional noise-injection schemes, a randomized continuous wave (CW) signal is exploited at the RFID reader for security enhancement, and an analytical framework is proposed to evaluate the impact of exploiting either full or only statistical knowledge of the randomized CW signal at the reader and the eavesdropper, respectively. The secrecy rate is maximized by designing the transmitted randomized CW signal to tackle the stability-variance tradeoff between balancing legitimate signal reception and eavesdropper mitigation. In particular, we show that the proposed scheme also poses a tradeoff between the received additive and multiplicative noise at the eavesdropper for the special case of a single-antenna eavesdropper. Moreover, the more practical case where the eavesdropper's instantaneous channel state information is unavailable is studied under different fading conditions. The numerical results verify the accuracy of the proposed approximations and show that introducing a small variance into the CW signal can greatly improve the system secrecy.

The problem of power allocation with uncoordinated cooperative jamming is addressed in this paper to enhance the physical layer security of the single-input-single-output wiretap channel with single-antenna jammers. Differing from the existing works concerning the secrecy rate maximization, this paper studies the problem of minimizing the secrecy outage probability with fixed secrecy rate, where the channel state information concerning the eavesdropper is only known statistically. This problem is formulated as an optimization problem of power allocation for the jamming signals. We prove that the optimization problem is convex in essence, and the optimal solution is thus conveniently obtained by using the methods of golden section search and bi-section search. Simulation results show that the proposed approach is very efficient and can significantly enhance the physical layer security by reducing the secrecy outage probability.

In this paper, we investigate physical-layer transmission security problem of a two-way relay wireless network consisting of two legitimate terminal nodes who wants to communicate with each other in presence of one eavesdropper, with help of N intermediate nodes. We propose a cooperative relaying and jamming strategy, which selecting one intermediate node to create intentional interference upon the eavesdropper, while other K = N -1 intermediate nodes operating in relay mode and assisting the source nodes to deliver data to their corresponding destinations using an amplify-and-forward (AF) protocol. All N intermediate nodes in this system are under a per-node power constraint. When K ≥ 3, we propose two different system design criteria based on null-space beamforming, to maximize secrecy sum rate by rate-splitting method with a bisection numerical algorithm iteratively getting the maximal secrecy sum rate, or to maximize the minimum rate between the two terminals. And we also propose the modified security schemes when the number of intermediate nodes is not enough for the previously mentioned schemes (K <; 3).

Intravascular optical coherence tomography is the state-of-the-art imaging modality in percutaneous coronary intervention planning and evaluation, in which side branch ostium and main vascular measurements play critical roles. However, manual measurement is time consuming and labor intensive. In this paper, we propose a fully automatic method for side branch ostium detection and main vascular segmentation to make up manual deficiency. In our method, side branch ostium points are first detected and subsequently used to divide the lumen contour into side branch and main vascular regions. Based on the division, main vascular contour is then smoothly fitted for segmentation. In side branch ostium detection, our algorithm creatively converts the definition of curvature into the calculation of the signed included angles in global view, and originally applies a differential filter to highlight the feature of side branch ostium points. A total of 4618 images from 22 pullback runs were used to evaluate the performance of the presented method. The validation results of side branch detection were TPR = 82.8 $\%$, TNR = 98.7$\%$ , PPV = 86.8$\%$, NPV = 98.7$\%$. The average ostial distance error (ODE) was 0.22 mm, and the DSC of main vascular segmentation was 0.96. In conclusion, the qualitative and quantitative evaluation indicated that the presented method is effective and accurate.

Mm-wave offers a sensible solution to the capacity crunch faced by 5G wireless communications. This paper comprehensively studies physical layer security in a multi-input single-output mm-wave system, where multiple single-antenna eavesdroppers are randomly located. Concerning the specific propagation characteristics of mm-wave, we investigate two secure transmission schemes, namely maximum ratio transmitting beamforming and artificial noise (AN) beamforming. Specifically, we first derive closed-form expressions of the connection probability for both schemes. We then analyze the secrecy outage probability in both non-colluding eavesdroppers and colluding eavesdroppers scenarios. Also, we maximize the secrecy throughput under a secrecy outage probability constraint, and obtain optimal transmission parameters, especially the power allocation between AN and the information signal for AN beamforming. Numerical results are provided to verify our theoretical analysis. We observe that the density of eavesdroppers, the spatially resolvable paths of the destination and eavesdroppers all contribute to the secrecy performance and the parameter design of mm-wave systems.

The artificial noise (AN) aided beamforming design in the MISOME (multiple-input, single-output, multiple-eavesdropper) wiretap channel is studied from the perspective of secrecy outage. Unlike many existing works which directly adopted the traditional null-space AN scheme, we start with a general assumption on the structure of the transmit signal, and seek to find the optimal structure by solving a secrecy outage probability (SOP) constrained secrecy rate maximization problem. By generalizing several existing conclusions regarding a single-antenna eavesdropper, we prove that the null-space AN scheme is indeed optimal for an arbitrary number of the eavesdropper's antennas from the perspective of secrecy outage. The power allocation problem is also studied and the closed-form optimal solution is obtained. It is found that the increase in the eavesdropper's antenna number is equivalent to requiring a smaller SOP.

Bioresorbable Vascular Scaffolds (BVS) are the most promising type of stent in percutaneous coronary intervention. For accurate BVS struts apposition assessment, intravascular optical coherence tomography (IVOCT) is the state-of-the-art imaging modality. However, manual analysis for IVOCT frames is time consuming and labor intensive. In this paper, we propose an automatic method for BVS struts center and region detection based on Adaboost algorithm and Haar-like features. Then, dynamic programming algorithm is applied to segment the contour of BVS struts. Based on the segmentation results, the apposed or malapposed struts can be automatically distinguished. By comparing the manual and automatic detection and segmentation results, our method correctly detected and segmented 87.7% of 4029 BVS struts with 18.6% false positives. The average Dice's coefficient for the correctly detected struts was 0.78. In conclusion, the evaluation suggested that this method is accurate and robust for BVS struts detection and segmentation.

We investigate multi‐user (MU) multiple‐input multiple‐output (MIMO) scheduling under the practical constraints in long‐term evolution advanced (LTE‐A) downlink cellular networks. The authors first derive the received signal model in MU‐MIMO systems when there exist both inter‐stream interference (ISI) and inter‐user interference (IUI) at each user device. Based on this, they formulate the optimisation problem as joint user pairing, precoding matrix indicator (PMI) selection, and resource block (RB) allocation to maximise the total system throughput. The authors then develop a codebook grouping technique for user pairing and PMI selection in MU‐MIMO. With the help of codebook grouping, the authors propose some suboptimal and low‐complexity scheduling algorithms to improve system throughput. From system‐level simulation, the proposed algorithms can improve the network throughput significantly when exploiting only limited feedback designed for single‐user (SU) MIMO in the LTE‐A specification.

Exploiting millimeter wave is an effective way to meet the data traffic demand in the 5G wireless communication system. In this paper, we study secure transmissions under slow fading channels with multipath propagation in millimeter wave systems. Concerning the new propagation characteristics of millimeter wave, we investigate three transmission schemes, namely, maximum ratio transmitting (MRT) beamforming, artificial noise (AN) beamforming, and partial MRT (PMRT) beamforming. We evaluate the secrecy performance by analyzing both the secrecy outage probability (SOP) and the secrecy throughput for each scheme. Particularly, for the AN scheme, we derive a closed-form expression for the optimal power allocation ratio of the information signal power to the total transmit power that minimizes the SOP, as well as obtain an explicit solution on the optimal transmission parameters that maximize the secrecy throughput. By comparing the secrecy performances achieved by different strategies, we demonstrate that the secrecy performance of the millimeter wave system is significantly influenced by the relationship between the legitimate user's and the eavesdropper's spatially resolvable paths, which is different from the wireless systems with statistically independent channel models. In the absence of the common path between the legitimate user and the eavesdropper, MRT beamforming is the best scheme. In the presence of common paths, AN beamforming and PMRT beamforming show their respective superiorities depending on the transmit power and the number of common paths. Numerical results are provided to verify our theoretical analysis.

In this paper, we study the secrecy throughput in millimeter wave systems under slow fading channels considering multipath propagation. For the specific propagation characteristics of millimeter wave, we provide transmission scheme designs and a comprehensive secrecy performance analysis. Specifically, we maximize the secrecy throughput under a secrecy outage probability (SOP) constraint through a dynamic parameter transmission scheme, and provide the optimal solution to transmission parameters, including the codeword rate and the power allocation ratio of the information signal power to the total transmit power. We find that the secrecy performance of the millimeter wave system under investigation is significantly influenced by the relationship between spatially resolvable paths of the legitimate user and those of the eavesdropper, which differs from those wireless systems with statistically independent channel. Numerical results are provided to verify our theoretical analysis.

In this paper, we investigate three-dimensional (3D) multi-user (MU) multiple-input multiple-output (MIMO) transmission for long term evolution advanced (LTE-A) downlink systems. We investigate some key techniques for 3D MU-MIMO to improve the performance of LTE-A systems, including rank and precoding matrix (PM) determination and user pairing. To reduce the complexity caused by a large number of co-scheduled users in 3D MU-MIMO, we develop a simplified and high efficient 3D MU-MIMO scheduling algorithm. The performance improvement of the proposed algorithm is demonstrated by system level simulation.

The use of millimeter wave is an effective way to meet the data traffic demand of the 5G wireless communication system. For the new propagation characteristics of millimeter wave, in this paper, we study the secure transmission with artificial noise under slow fading channels considering multipath propagation in millimeter wave systems. Firstly, when partial eavesdropper's channel state information is known at the transmitter, an artificial noise transmission strategy which depends on directions of the destination's and the eavesdropper's propagation paths is proposed. Then we analyze the secrecy outage probability (SOP) through an on-off transmission scheme. Furthermore, we minimize the SOP subject to a secrecy rate constraint and derive a closed-form optimal power allocation between the information bearing signal and artificial noise. Numerical results are provided to show the superiority of the proposed scheme.

In this paper, we investigate secure communications uplink transmissions, where there are a base station (BS) with M receive antennas, K mobile users each with a single antenna, and an eavesdropper with N receive antennas. The closed-form expressions of the achievable ergodic secrecy sum rates (ESSRs) for a random k users selection scheme in the high and low signal-to-noise ratio regimes are presented. It is shown that the scaling behavior of ESSR with respect to the number served users k can be quite different under different system configurations, determined by the numbers of the BS antennas and that of the eavesdropper antennas. In order to achieve multiuser gain, two low-complexity user selection schemes are proposed under different assumptions on the eavesdropper's channel state information. The closed-form expressions of the achievable ESSRs and the multiuser secrecy gains of the two schemes are also presented in both low and high SNR regimes. observe that, as k increases, the multiuser secrecy gain increases, while the ESSR may decrease. Therefore, when N much larger than M, serving one user with the strongest channel (Time Division Multiple Address-like) is a favorable secrecy scheme, where the ESSR scales with √2 log K.

In this paper, we propose a new computationally efficient blind carrier frequency offset (CFO) estimator for multi-input multi-output orthogonal frequency division multiplexing systems. A cost function is carefully designed and can be exactly expressed as the superposition of very few harmonically related cosine waves even with the effect of the noise. Using this property, the minimization of the designed cost function can be solved in a quite computationally efficient manner without any exhaustive grid search procedure. It is seen that the proposed estimator can achieve comparable estimation performance as existing competitors with substantially reduced computational burden. Moreover, in order to improve the estimation performance, we further develop an enhanced CFO estimator by introducing an additional one-step adjustment. We find that the enhanced estimator can attain almost the same estimation performance as the existing maximum likelihood estimator but with a lower order of computational burden. We provide both numerical results and theoretical performance analysis to corroborate the proposed studies.

In this paper, we propose a new efficient blind carrier frequency offset (CFO) estimator for multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. A cost function is carefully designed which can be exactly expressed as a superposition of very few harmonically related cosine waves even with the effect of noise. By using this property, the minimization of the designed cost function can be solved in a quite computationally efficient manner without any exhausted grid searching procedure. We provide numerical results to corroborate the proposed studies. It is seen that the proposed estimator can achieve comparable estimation performance with existing competitors with substantially reduced computational burden.

In this paper, we investigate three-dimensional (3D) multiple-input multiple-output (MIMO) techniques with dynamic rank selection for long term evolution advanced (LTE-A) downlink cellular networks. To facilitate users to transmit different numbers of data streams, we develop a new structure for designing 3D precoding matrix (PM). Then based on the designed PM, 3D MIMO transmission with rank adaptation is proposed. The performance improvement of the proposed algorithms is demonstrated by system level simulation.

The secrecy outage of a two-user slow fading broadcast channel is studied in this paper. We consider the scenario where independent and confidential messages are transmitted to each user using superposition coding in the presence of an external eavesdropper. We assume the instantaneous channel state information (CSI) of the eavesdropper's channel is not available and the secrecy outage can be claimed for each user. The secrecy outage probability of each user conditioned on the instantaneous CSI of the legitimate channel is studied and used to derive the achievable secrecy rate region under the conditional secrecy outage probability constraints. The average achievable secrecy rate, i.e. the secrecy throughput, is studied as the measure of performance under the slow fading. The optimal secrecy rate allocation strategy that achieves the boundary of the achievable secrecy throughput region is characterized for the general case and for the maximum sum secrecy throughput. Numerical results of the secrecy throughput regions are shown, and the parameters that influence the secrecy throughput are discussed.

Distance estimation using multifrequency phases measurement is a common practice in many areas of engineering. This method brings in the issue of phase ambiguity. Some Chinese remainder theorem (CRT) based phase unwrapping algorithms have been suggested to solve the problem, however, these algorithms either have high complexity or lack precision. In this paper, we propose an efficient algorithm to reconstruct the unknown distance from the contaminated wrapped phases. The proposed method can be separated into two stages. The first stage is to obtain the optimal estimate of the common remainder which is significant to the estimation. In the second stage, the indefinite distance is estimated by using the extended CRT. Simulations test the validity of the proposed algorithm.

This paper investigates the secret key generation from multiple cooperative sources. We assume that the public communication between the legitimate users has unlimited capacity. In this case, the secret key rate is the upper bound of a practical system with limited public communication. The random signals from the multiple helpers can help to create correlated Gaussian sources for the legitimate users to generate secret key, and only local channel state information (CSI) is needed by the helpers to cooperate with others. The power allocation between the helpers is studied with a total transmit power constraint, which results in a non-convex optimization problem. A high efficiency sub-optimal solution is proposed. And even when the legitimate users have no idea about the eavesdropper, the scheme still can help the system to achieve fairly good performance.

In this paper, we propose a new blind carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) with multi-antenna receiver. The proposed method is one-shot and works with only a single OFDM block, which differs from the existing works for multi-antenna receiver. We show that, the proposed method not only supports fully loaded transmission, but also outperforms the existing maximum likelihood estimator for multi-antenna receiver. The numeral results are provided to corroborate the proposed studies.

In this paper, we investigate multi-user (MU) multiple-input multiple-output (MIMO) scheduling for long term evolution advanced (LTE-A) downlink cellular networks, where only limited feedback designed for single-user (SU) MIMO is available. To help the base station (BS) allocate resource blocks (RBs), we develop codebook grouping and MU-SINR estimation techniques. Two adaptive SU/MU-MIMO scheduling methods are proposed to improve system throughput. Although the proposed adaptive SU/MU- MIMO scheduling algorithms exploit the only limited feedback in the LTE-A specification, they can improve the network throughput by 13% from system level simulation.

In this study, for interleaved orthogonal frequency division multiple access uplink with multi‐antenna receiver, we propose two generalised carrier frequency offset estimators which, respectively, exploit the subspace theory and maximum‐likelihood (ML) criterion. We find that, as long as the numbers of multipaths from two of the users are smaller than the number of antennas at the receiver, both the proposed estimators support fully loaded transmissions. We also derive the theoretical performance lower bound for the proposed ML estimator. The numerical results are then provided, which corroborate the proposed studies.

In this paper, we address the problem of distributed angle estimation in wireless sensor localization system. Given that the practical limitations on size and cost, i.e., anchor node cannot equip a complicated antenna or antenna array, we devise an alternative measure based on a fixed-spacing two-antenna equipment. Through the multiple frequencies tuning, a virtual non-uniform linear array, called multiple frequencies synthetic array (MFSA), can be rebuilt independently at each sensor node. We first provide in theory the angle unambiguity conditions for such virtual array and then propose two different algorithms to achieve the distributed angle of departure (AOD) estimation. The first algorithm is based on the spectral searching technique, which is an improved version of the conventional rank reduction estimator (RARE) and has the same performance but less computational complexity. To further alleviate the computational burden at sensor node end, we convert the above angle estimation into a congruence problem and provide another closed-form solution based on Chinese remainder theorem (CRT). We also derive the Cramer-Rao bound for the MFSA and demonstrate theoretically that the latter algorithm from the perspective of cumulative circular distance is suboptimal when compared with the former one. Numerical examples are provided to show the validity of the proposed algorithms and the corresponding conclusions.

In this paper linear precoders for non-cooperative transmitters and iterative multiuser detectors (MUD) for the receiver are jointly designed to achieve the individual quality-of-service (QoS). An evolutional analysis framework is first developed for predicting and tracking the performance evolution for each user along iterations. This semi-analytical tool then leads to a joint QoS and quality-of-convergence (QoC) constrained precoder design problem formulation that is complicated and non-convex. Due to the competitive nature of a multiaccess system, a game-theoretic reformulation is adopted to reveal the underlying partial convexity structure of the problem, which further facilitates the development of an efficient iterative solver for the problem. Extensive simulation results are given to support the mathematical development of the problem.

Cooperative beamforming and jamming are two efficient schemes to improve the physical-layer security of a wireless transmission in the presence of a passive eavesdropper. However, in most works they are discussed separately. In this paper, we propose a joint cooperative beamforming and jamming scheme to enhance the security of a cooperative relay network, where some intermediate nodes adopt distributed beamforming while the others jam the eavesdropper, simultaneously. Subjected to the more practical individual power constraint of each node, we propose a null-space beamforming based secrecy strategy. The beamformer design can be optimized by a bisection method and second-order convex cone programming (SOCP). Simulations show the joint scheme greatly improves the security.

In this paper, we propose a novel angle of departure (AOD) aided sensor localization algorithm. Through the multiple frequency tuning technique at a fixed-spaced two-antenna anchor node, a non-uniform linear array can be rebuilt at each ground sensor node. We convert the angle estimation into a real number reconstruction by Chinese remainder theorem (CRT) and then provide a new algorithm to solve the congruence problem. Compared with the conventional spectral searching algorithms, the proposed one is computationally efficient and also performs much better than other CRT reconstruction algorithms. The validity and the advantages of the proposed algorithm is proved by numerical simulations.

In this paper, we consider a random wireless sensor network where the sensor nodes are distributed randomly and form clusters to cooperatively transmit data packets using cooperative multi-input-multi-output (CMIMO) technique. We focus on analyzing the overall system performance in terms of packet error rate (PER). And then propose a new node sleep strategy to minimize the overall energy consumption. We take the random distribution of sensor nodes into consideration and get a closed form of overall PER expression that simplifies the overall energy consumption optimization. We analyze the tradeoff between the overall energy consumption and sensor nodes sleep rate. The simulation results corroborate our analysis and show that the proposed sensor node sleep strategy has significant energy saving compared with other transmission schemes.

A distributed localization method has been proposed recently based on a novel concept of angle of departure (AOD) to estimate the angle information and achieve localization. Based on the AOD estimation, we introduce a synthetic uniform circular array (SUCA) synthesized by a rotary antenna to get the distributed localization of the sensors in multi-path environments. The rotated operation of antenna can create the SUCA with a larger aperture to deal with AOD of multi-path components (MPCs) and relaxes the condition that array aperture should be greater than the number of MPCs. This method only relies on radio transceivers without requiring extra measuring equipments, and the sensor nodes conduct localization in a completely distributed manner instead of centralized processing.

In this paper, we propose an Alamouti coded orthogonal frequency-division multiplexing (OFDM) scheme for amplify-and-forward (AF) relay networks that contain multiple distributed relay nodes. The frequency asynchronous nature of the distributed system is considered in our design. By implementing simple operations, e.g., time reversal, conjugation and amplification, the proposed scheme eliminates the needs of knowledge of both channel states and the frequency offsets at the relay nodes. We prove that when the frequency offsets among the relay nodes are smaller than a certain threshold, full spatial diversity can be achieved at the destination node even with the linear receivers, e.g., zero-forcing (ZF) receiver. Numerical results are provided to corroborate the proposed studies.

Exploiting the idea of cooperative communications is an efficient way to improve the physical-layer security of a wireless transmission in the presence of passive eavesdroppers. In this paper, we propose a hybrid cooperative relaying and jamming scheme to enhance the security of a two-way relay network, where some intermediate nodes help to relay the signal to the legitimate terminals via distributed beamforming and the others jam the eavesdropper simultaneously. In such a way, both two cooperative phases of the data transmission are protected. Subjected to the more practical per-node power constraint of each node and without the channel state information (CSI) of the eavesdropper, we propose a scheme to enhance the secrecy of the two terminals. It is shown that the problem can be transformed into a semidefinite programming (SDP) problem with an additional rank-1 constraint. We then develop a penalty function method and an iterative algorithm to solve such a problem efficiently, instead of the popular semi-definite relaxation (SDR) and randomization techniques proposed in the previous literatures. Simulations show that the proposed hybrid scheme greatly improves the security of the two-way relay networks.

In this study, the authors develop a distributed space–time coding scheme for a two‐way relay network that contains multiple distributed relay nodes. Both the time and frequency asynchronous nature of the distributed system are considered in the design. Distributed convolutional coding is employed to handle multiple timing errors in the networks. The authors prove that under perfect frequency synchronisation, the proposed scheme can achieve both spatial and multipath diversity by linear receivers, such as linear zero‐forcing or minimum mean‐square‐error receiver, thus providing a low decoding complexity. The authors further find that frequency asynchronism has little effect on the designed scheme and show that the diversity can still be achieved almost surely (in the measured theoretic sense) under both time and frequency asynchronous scenarios. The authors also provide numerical results to corroborate the proposed studies.

Cooperative beamforming and jamming are two efficient schemes to improve the physical-layer security of a wireless relay system in the presence of passive eavesdroppers. However, in most works these two techniques are adopted separately. In this letter, we propose a joint cooperative beamforming and jamming scheme to enhance the security of a cooperative relay network, where a part of intermediate nodes adopt distributed beamforming while others jam the eavesdropper, simultaneously. Since the instantaneous channel state information (CSI) of the eavesdropper may not be known, we propose a cooperative artificial noise transmission based secrecy strategy, subjected to the individual power constraint of each node. The beamformer weights and power allocation can be obtained by solving a second-order convex cone programming (SOCP) together with a linear programming problem. Simulations show the joint scheme greatly improves the security.

This letter investigates a novel two-stage cooperative scheme to secure the communication system with one source, one destination and multiple helpers. Herein, the source mixes the confidential messages with the interferences transmitted by the helpers and destination. Since the helpers and destination serve to suppress interferences, the destination can acquire an interference-free signal, however the eavesdropper is still confused. This scheme can achieve positive secrecy rate even when the eavesdropper's channel is much better than the legitimate link, and only requires local channel state information (CSI) and a few parameters of the main link. An efficient sub-optimal algorithm for the majorization of system parameters is proposed to avoid global search and the practical case without eavesdropper's CSI is also considered. Simulation results demonstrate the effectiveness of the proposed scheme and algorithm.

In this paper, we propose a hybrid cooperative beamforming and jamming scheme to enhance the physical-layer security of a single-antenna-equipped two-way relay network in the presence of an eavesdropper. The basic idea is that in both cooperative transmission phases, some intermediate nodes help to relay signals to the legitimate destination adopting distributed beamforming, while the remaining nodes jam the eavesdropper, simultaneously, which takes the data transmissions in both phases under protection. Two different schemes are proposed, with and without the instantaneous channel state information of the eavesdropper, respectively, and both are subjected to the more practical individual power constraint of each cooperative node. Under the general channel model, it is shown that both problems can be transformed into a semi-definite programming (SDP) problem with an additional rank-1 constraint. A current state of the art technique for handling such a problem is the semi-definite relaxation (SDR) and randomization techniques. In this paper, however, we propose a penalty function method incorporating the rank-1 constraint into the objective function. Although the so-obtained problem is not convex, we develop an efficient iterative algorithm to solve it. Each iteration is a convex SDP problem, thus it can be efficiently solved using the interior point method. When the channels are reciprocal such as in TDD mode, we show that the problems become second-order convex cone programming ones. Numerical evaluation results are provided and analyzed to show the properties and efficiency of the proposed hybrid security scheme, and also demonstrate that our optimization algorithms outperform the SDR technique.

This paper considers random wireless sensor networks where nodes are distributed randomly and form clusters to transmit the packets to relay clusters using cooperative multi-input-multi-output (CMIMO) technique. First, we study CMIMO energy consumption and divide it into intra-cluster energy consumption and inter-cluster energy consumption. Then we focus on analyzing the system performance in terms of overall packet error rate (PER) and energy consumption. Based on system performance metrics, we propose a new node sleep strategy, where overall energy consumption is minimized by coordinating inter-cluster transmitting energy consumption and the number of sensor nodes in activation. Compared to existing works, our contributions are (1) we consider random distribution of nodes and get a closed-form expression of overall PER that simplifies the energy minimization problem; (2) we analyze the tradeoff between overall energy consumption and nodes active rate. In simulation, we investigate the relationships among nodes active rate, inter-cluster transmitting energy consumption and the overall energy consumption. Furthermore, we discuss how these parameters are affected by the nodes density, the inter-cluster transmitting distance, the cluster radius and the intra-cluster energy consumption. Finally, we validate that the proposed nodes sleep strategy has significant energy savings compared with non-sleep cooperative transmission and direct transmission.

In this paper, we design a new distributed angle estimation method for localization in wireless sensor networks (WSNs) under multipath propagation environment. We employ a two-antenna anchor that can emit two linear chirp waves simultaneously, and propose to estimate the angle of departure (AOD) of the emitted waves at each receiving node via frequency measurement of the local received signal strength indication (RSSI) signal. An improved estimation method is further proposed where multiple parallel arrays are adopted to provide the space diversity. The proposed methods rely only on radio transceivers and do not require frequency synchronization or precise time synchronization between the transceivers. More importantly, the angle is estimated at each sensor in a completely distributed manner. The performance analysis is derived and simulations are presented to corroborate the proposed studies.

In this paper, we investigate multiuser pairing and resource allocation with interference avoidance in LTE uplink cellular networks. We first derive the received signal-to-interference-plus-noise ratio (SINR) in spatial multiuser SC-FDMA systems with frequency-domain minimum mean-square error (MMSE) equalization. Based on it, we formulate an optimization multiuser pairing and resource allocation problem to maximize system weighted throughput. With the help of the exchange of high interference indicators among multiple base stations, we develop a distributed joint optimal algorithm and a distributed low-complexity algorithm. Simulation results show that the proposed algorithm with interference avoidance significantly outperforms the algorithm without interference avoidance in [12].

In this paper linear precoder at the transmitter and iterative frequency domain equalizer (FDE) at the receiver are jointly designed to achieve the target quality-of-service (QoS) with minimum transmission power consumption. We first devise a semi-analytical tool that precisely tracks the performance evolution of the iterative process, and then provide a set of quality-of-convergence (QoC) constraints such that the target QoS can be achieved after iterations. We thus formulate the design problem as a joint QoS/QoC constrained power allocation problem, which is shown to be convex. Despite the widely adopted suboptimal solvers, we develop a specific IPM solver by revealing the underlying sparsity in the mathematical structure of the problem, which significantly reduces the computational cost, and keeps the performance advantages as well. Extensive simulation results are given to support the mathematical development of the problem.

In this paper, we address the physical-layer security of a two-way wireless transmission with the help of multiple inter-mediate nodes in the presence of an eavesdropper. We propose a hybrid cooperative relaying and jamming mechanism to enhance the security of the data exchange, where some intermediate nodes adopt distributed beamforming to improve the channel quality to legitimate terminals and others jam the eavesdropper, simultaneously. Two different schemes are proposed with and without the instantaneous channel state information (CSI) of the eavesdropper, respectively. The solutions of both schemes can be obtained by convex optimization techniques. Comparisons show that the proposed hybrid schemes greatly improve the security of the two-way relay networks.

A new angle of departure (AOD) aided distributed localization approach for wireless sensor node is proposed in this paper. An L-shaped antenna array is arranged on anchor node end and the ground single antenna sensor node conduct its own line of sight AOD estimation for self-localization. Considering the limited degree of freedom provided by the anchor node can not guarantee a successful recognition to the LOS signal component in multipath environment, we adopt the minimum redundancy configuration. Via the intrinsic character of wireless channel, the maximum degree of freedom through performing two-dimension joint difference co-array operation can be obtained at sensor node end. Therefore, the sensor node can identify the LOS signal component in a very high probability with a rebuild degree of freedom increased virtual array. The validity of proposed methods are verified by numerical examples.

In this paper, we propose an energy-efficient cooperative geographic routing (ECGR) to reach energy-efficient routing in wireless sensor networks(WSN). ECGR fully takes advantage of cooperative diversity and geographic routing. At physical layer, the sensor nodes cooperatively transmit the same packet to achieve farther transmission distance of each hop. While at the network layer, ECGR adaptively selects the appropriate cooperative nodes to form forwarding clusters to forward the packet when we consider energy consumption and geographic information of the sensor nodes. In order to fully assess the energy consumption, we take circuit energy consumption of the sensor nodes into consideration. According to the description above, we increase the transmitting distance of each hop but decrease the hop number. Meanwhile, the overall network energy consumption is reduced and balanced among nodes to extend the life of the network.

In this paper, we develop a new carrier frequency offset (CFO) estimation scheme for interleaved orthogonal frequency division multiple access (OFDMA) transmission. We employ multi-antenna at the receiver and exploit the rank reduction approach to blindly estimate the CFOs of multiple users. The proposed scheme supports full load transmission that allows all sub-carriers being allocated to users, which is a significant advantage over the existing schemes. Both performance analysis and numerical results are provided to corroborate the proposed studies.

A novel bind algorithm is proposed for ranges and direction-of-arrivals (DOAs) estimation of the hybrid narrow-band sources, which contain both the near field sources and the far field ones. To separate them, a two-step algorithm is exploited. In the first step, the far-field sources are identified by exploiting the MUSIC method. After eliminating the far field source components in the auto correlation matrix and adding the conjugate formulation, the dimensions of covariance matrix for near field sources are increased. Therefore, compared with the conventional methods, more independent near filed sources, near 4p/3 ones can be identified with a uniformly linear array (ULA) of 2p + 1 elements. Furthermore, in the near-field source estimation of our method, the peak search and pairing operations, needed in most algorithms for near field sources, can be omitted completely. Simulation results show that our ESPRIT-based near-field method provides the improved performance over conventional ones because of the dimensions increment of the covariance matrix.

To distributed sources, the usage of conventional point-source model will cause inaccuracy in the direction-of-arrival (DOA) estimation. Conventional methods for distributed sources usually estimate the DOAs and the distribution parameters with multi-dimensional peak search operations, which will lead to much higher computational complexity than that in the point source model. In this paper, a blind parameter estimation method is proposed for distributed sources, where the DOAs are estimated in a closed form. As a result, the computational burden is decreased a lot. Furthermore, the proposed method can work in the scenario with mixture sources of many kinds of distributions. The selection method and its spatial-time domain version are also discussed in this paper. Simulation results show the effectiveness of our method.

In this paper, we investigate joint user pairing and resource allocation under the practical constraints in single-carrier frequency-division multiple access (SC-FDMA) LTE uplink systems. We first introduce a joint optimal algorithm based on branch-and-bound search as a benchmark. To reduce complexity, we divide the joint optimization problem into two subproblems: user pairing and resource allocation. For both these subproblems, we develop several low-complexity algorithms by exploiting the properties of the application of the optimization problem itself. It is shown by the simulation results that the proposed algorithms have better throughput and fairness than the conventional algorithms in and in for LTE uplink no matter whether power control is perfect or not.

In this paper, we investigate joint user pairing and resource allocation under the practical constraints in single carrier frequency-division multiple access (SC-FDMA) LTE uplink systems. We first introduce a joint optimal algorithm based on branch-and-bound search as a benchmark. To reduce complexity, we divide the joint optimization problem into two subproblems: user pairing and resource allocation. For these subproblems, we develop several suboptimal but low-complexity algorithms. The simulation results show that the proposed algorithms outperform the conventional one.

In multiple-input multiple-output (MIMO) systems, transmit antenna selection is a very promising technique for reducing implementation cost and computational complexity. In this paper, we propose two novel fast near-optimal antenna selection algorithms based on channel capacity maximization, whose core idea is to use an effective iterative process to reduce computational complexity and retain near-optimal performance. Compared with the conventional algorithm, the proposed algorithms have lower computational complexity and achieve almost the same capacity and bit-error rate (BER) performance as the optimal one. In particular, the BER performance gain of the proposed algorithms is much larger than the conventional algorithms in correlated fading channels. Finally, computer simulations are presented to validate and demonstrate performance of our algorithms.

Single carrier cyclic prefix system with frequency domain equalization (SC-CP-FDE) is an attractive technology for broadband wireless communications. In this paper, we investigate the diversity order of a SC-CP-FDE system without channel coding in frequency-selective channels. Existing analytic proofs in [6], [7] indicate that the achievable diversity order is only 1, which contradicts the conclusion in [11] and simulation results in [1]-[3]. We explain why the proofs in [6], [7] can not support the claim that SC-CP system is unable to provide any frequency diversity gain. We then re-investigate the diversity order of a SC-CP system with frequency domain decision feedback equalizer (FD-DFE) based on MMSE criteria at the receiver end. Adopting outage analysis instead of pairwise error probability (PEP) analysis, we show that with MMSE-FD-DFE, the asymptotic achievable diversity order is Lc+1 when the block length N is sufficiently large, where Lc+1 is the number of independent channel paths, i.e., full multipath diversity is asymptotically achieved.

In this paper, we address the physical-layer security of two-way (bi-directional) transmissions of two terminals with the help of multiple relay nodes in the presence of an eavesdropper, where each node in the network is only equipped with single antenna. The secrecy sum rate is used as the metric of security, which is the rate difference between the legitimate information exchange between the terminals and the information leakage to the eavesdropper. We propose two-phase analog network coding (ANC) and power allocation scheme for the relay nodes and terminals to enhance the security of the data exchange. Three different schemes are proposed. The first scheme is the optimal ANC to achieve the maximum secrecy sum rate, i.e. secrecy capacity, which is found to be mathematically difficult to solve. Two suboptimal schemes are proposed with and without the instantaneous channel state information (CSI) of the eavesdropper, respectively. The solutions of both schemes are shown to be unique and globally optimum, which are solved by convex optimization techniques. Numerical evaluation results of the obtained secrecy sum rate and transmit power are provided and analyzed to show the properties and efficiency of the proposed ANCs.

In this paper, we develop a new subspace based multiuser carrier frequency offset (CFO) estimation scheme for multi-input multi- output (MIMO) orthogonal frequency division multiple access (OFDMA) uplink transmission. We exploit the rank reduction approach by equipping multiple antennas at the receiver, in which the CFO of each user is derived blindly using one dimension (1-D) search individually. The proposed scheme supports the generalized subcarrier assignment scheme and full loaded transmission with all subcarriers being allocated to users. Numerical results are provided to corroborate the proposed studies.

In this paper, we develop a distributed space-time coding scheme to cope with both time and frequency asynchronism in two-way relay network (TWRN). The convolutional coding is employed to handle multiple timing errors in the networks. We prove that both the cooperative and the multipath diversities can be achieved by linear receivers, e.g., linear zero-forcing (ZF) or minimum mean square error (MMSE) receivers. Moreover, the diversity can be guaranteed almost surely under frequency asynchronism. Numerical results are then provided to corroborate the proposed studies.

In MB-OFDM-based UWB systems, the different CFOs for different bands can be obtained by only estimating the OFO. To achieve OFO estimators with wide estimation ranges, we benefit from the certain relationship between the different CFOs. The OFO is attained by estimating the CFO difference and the correlations between different bands are used. The estimator I for the TFC 1 to 4 derives the BLUE estimator with the phases of the correlations. The estimator II dedicated for the TFC 1 and 2 can achieve an even wider estimation range by exploiting the equally spaced preamble structures.

When the arrival timing difference exists in the uplink MUMIMO systems, the conventional MUD techniques, without timing control, failed under this circumstance. In this paper, we proposed a joint distortion precancelation and iterative FDE architecture, without timing control, for the asynchronous system. The proposed method derives the iterative FDE filters, via the MMSE criterion, by treating the precanceled distortion components as colored noise. The asymptomatic MSE performance is shown to approach MFB, and the achievable SER performance is evaluated in various system settings, which confirms its effectiveness.

Single-carrier block transmission systems with frequency domain equalization (FDE) suffers from power and bandwidth efficiency loss due to the cyclic extension. Conventional schemes to reduce the cyclic extension needed rely heavily on Turbo architecture which has much higher computational complexity. To make good compromise between performance and complexity, we proposed a precoding aided iterative FDE technique with reduced-length cyclic extensions. We emphasized that the proposed scheme effectively redistributes the dominant part of residual distortion and ensures reliable symbol estimate, which is superior to the conventional non-Turbo schemes. Low complexity algorithms are presented for computing key parameters and corresponding FDE filters. Simulation results confirmed the effectiveness of the proposed scheme for both uncoded and coded systems, which achieves a bandwidth efficiency increase of 29.4% and a power efficiency gain of 1.12dB, in the Hiperlan2/E system settings.

In this paper, a novel cooperative transmission scheme based on orthogonal signaling is proposed for two-user cooperative systems. Compared to the traditional cooperation alternatives, in our scheme, each user utilizes the in-phase and quadrature components of the QPSK signal constellation to forward its own information as well as the partner's. The approximate closed form of its frame-error-rate (FER) is derived and the principle of the optimal power allocation is also provided. Simulation results demonstrate that our proposed scheme outperforms traditional alternatives and can still achieve satisfiable FER performance even in low inter-user channel quality due to the power adjustment.

Time asynchronism is a practical issue needs to be addressed for a general distributed two-way relay network, where two terminal nodes exchange information through multiple spatial-separated relay nodes. In this paper, we propose an analog network coding (ANC) scheme for a time asynchronous two-way relay network. In the proposed scheme, each relay node linearly transforms the received mixed asynchronous signals in the first time-slot by a Toeplitz matrix, and then broadcasts them back to the terminals in the second time-slot. A sufficient condition is derived for the proposed ANC to achieve full cooperative diversity using only linear receivers at the terminal nodes, such as zero-forcing (ZF), or minimum mean square error (MMSE) receivers, with any delay profiles of the timing errors. The design of the coefficients in the Toeplitz matrix to satisfy the sufficient condition is also proposed, which is shown coincides with the design we proposed in the previous work. Simulation results verify our analysis of the diversity order.

Distributed beamforming is a form of collaborative communication, in which wireless nodes adjust the phases of their transmitted signals so that the signals are combined coherently at the destination. This enhances the received signal power and extends the communication range. In this paper, we propose a transmit beamforming method that does not require any explicit feedback information from the receiver for the phase adjustment. Instead, sensors use phase-locked loops (PLL) to synchronize their carriers to the received reference signal broadcast by the destination / receiver. Based on the fact that the change of phase over some distance depends not only on the distance but also on the frequency, the destination indirectly adjusts the sensors'phase by changing the frequency of the reference signal. The destination scans all the frequencies within a feasible range to find the optimal frequency at which the received power gets its maximum. The advantage of this method is that it does not require any direct interaction among the sensors and feedback between the sensors and the destination. Every sensor adjusts its own phase independently and simultaneously. Therefore, the proposed method has good scalability and is suitable for the distributed system. We also derive some performance analysis of the method and present simulation results that validate the analysis.

Cooperative relay networks are inherently time- and frequency- asynchronous due to their distributed nature. The existence of multiple time and frequency offsets results in a time-varying and frequency-selective equivalent channel between relay nodes and destination node. In this paper, we propose a transceiver scheme to combat both time and frequency offsets for cooperative relay networks. At the relay nodes, the distributed linear convolutive space-time coding is adopted to achieve the time-asynchronous full cooperative diversity. This transmission scheme leads to an equivalent channel with special structure at the destination node. By taking full advantage of this special structure, fast Kalman equalizations based on linear minimum mean square error (LMMSE) and MMSE decision feedback equalizers (MMSE-DFE) are proposed for the receiver, where the estimation of the state vector (information symbols) can be operated recursively and computational efficiently. The proposed scheme can achieve considerable diversity gain with both time and frequency offsets, and also applies to frequency-selective fading channels.

In this paper, we propose a distributed angle estimation method for wireless sensor network localization with multipath fading. The multiple antenna system is equipped at the anchor and the angle of departure (AOD) can be estimated individually at each sensor node with single antenna system. Further, we exploit the space diversity to improve the estimation performance by deploying multiple parallel arrays at the anchor. Analysis and simulations demonstrate the effectiveness of our method.

In this paper, we deal with channel estimation and intercarrier interference mitigation for very high mobility OFDM systems, the communication is between a BS and a train traveling at a speed of 500 km/hr. The scheme proposed here uses piece-wise polynomial expansion to approximate time-variations of multipath channels. Two adjacent symbols are utilized to estimate the first and second-order parameters. In order to improve the estimation accuracy and mitigate the intercarrier interference caused by pilot-aided channel estimation, the multipath channel parameters were re-estimated in time-domain employing the tentative decision of current OFDM symbol. Simulation results show that this method would improve estimation accuracy and system performance in a high-mobility environment.

In this paper, we investigate adaptive resource allocation in downlink transmission of orthogonal frequency division multiple access (OFDMA) networks. A block-level resource allocation scheme is developed to maximize the overall throughput of the networks. We focus on application in long term evolution (LTE) systems where all resource blocks (RB's) allocated to the same user must use the same modulation and coding scheme (MCS). The complexity of such optimization problem is usually high. In order to reduce the complexity, we divide the original optimization problem into two suboptimal ones. We first allocate appropriate RB's to users with the best channel quality and then perform power allocation. Finally, a more effective MCS is selected, which not only ensures block-error rate (BLER) performance of RB with the poor channel condition but also makes full use of the RB's with good channel condition. Simulation results show that the proposed resource allocation scheme can improve the overall throughput of the network by 20% compared with the existing scheme when the number of users is 4 and SINR = 10 dB.

In this paper, we investigate resource block (RB) assignment and modulation-and-coding scheme (MCS) selection to maximize downlink throughput of long-term evolution (LTE) systems, where all RB's assigned to the same user in any given transmission time interval (TTI) must use the same MCS. We develop several effective MCS selection schemes by using the effective packet-level SINR based on exponential effective SINR mapping (EESM), arithmetic mean, geometric mean, and harmonic mean. From both analysis and simulation results, we show that the system throughput of all the proposed schemes are better than that of the scheme in. Furthermore, the MCS selection scheme using harmonic mean based effective packet-level SINR almost reaches the optimal performance and significantly outperforms the other proposed schemes.

In this paper, an energy-efficient cooperative geographic routing (ECGR) is proposed for Wireless Sensor Networks(WSN). ECGR fully takes advantage of cooperative diversity and "multi-sensor diversity" to reach energy-efficient routing. At physical layer, thanks to the high redundancy of the nodes in WSN, the nodes that have correctly decoded the packet are selected to realize "multi-sensor diversity" gain, and these nodes form a new forwarding cluster in a distributed way to obtain transmit diversity. At the network layer, based on the geographic routing, ECGR makes the packet always rout toward the destination and avoids diverging routes, simultaneously. In this way, we increase the transmitting distance of each hop, meanwhile, the overall network energy consumption is reduced and are balanced among the sensor nodes to extend the life of the network. We also discuss an appropriate packet error rate (PER) to optimize average transmitting distance.

Self-localization system of wireless sensor networks based on angle of departure (AOD) is studied in this paper. In AOD model, anchor node with multi-antenna transmits orthogonal pilot signals to sensor node with single antenna, which entitles the sensor node to function as equipping multi-antenna. Given that the limited number of antenna at anchor nodes will result in degree of freedom (DOF) deficiency at sensor node when dealing with more multipath components (MPC), we adopt a novel method without requiring extra antennas. The proposed algorithm simultaneously includes the oscillator mismatch between anchor node and sensor node, i.e., carrier frequency offset (CFO). With the aid of anchor's movement and CFO, the equivalent antenna array at sensor node is expended by synthetic aperture procedure to a much larger one, which subsequently improve the estimation ability to MPCs. In addition, the close-form solutions of CFO and AOD are also derived. The effectiveness and performance of proposed algorithm are demonstrated by numerical simulations.

In this paper, we propose a family of distributed space-frequency codes (SFCs) for frequency asynchronous cooperative relay networks with multiple carrier frequency offsets (CFOs) and flat fading channels. The codes have the special structures that the columns of each code matrix can be divided into several orthogonal groups and each group is stacked by several sub-blocks with Toeplitz structure. These codes possess two interesting properties: a) Even under the frequency-asynchronous scenario, the codes can , namely, if R relay nodes participate the cooperation adopting the proposed SFCs, then at the destination node, the achievable diversity order is R; b) Only , such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, are required to collect the full cooperative diversity, instead of the maximum likelihood (ML) detector as utilized in most existing STC (SFC) schemes. The codes in this family have different symbol rates, orthogonality and performances for different numbers of relay nodes, which can be adjusted by choosing different design parameters. It is shown that frequency-reversal SFC (FR-SFC), and frequency-shift SFC (FS-SFC) proposed in previous papers are only two special members of the code family, and new SFCs outperforming both of them are proposed.

In this paper, we investigate adaptive resource allocation in downlink transmission of orthogonal frequency division multiple access (OFDMA) networks. A block-level resource allocation scheme is developed to maximize the overall throughput of the networks by appropriately allocating resource blocks (RB's) and power to different users. We focus on application in long term evolution (LTE) systems where all RB's allocated to the same user must use the same modulation and coding scheme (MCS). Unfortunately, the complexity of solving such an optimization problem is prohibitively high in general. In order to reduce the complexity, we divide the original joint optimization problem into two subproblems. We first allocate appropriate RB's to users with the best channel condition and then perform power allocation. Then, a more effective MCS is selected in our scheme, which not only ensures the block-error rate (BLER) of the RB with the worst channel condition but also makes full use of the RB's with better channel conditions. Simulation results show that the proposed resource allocation scheme can improve the overall throughput of the network by 20% compared with the existing scheme when the number of users is 4 and the signal-to-interference-plus-noise ratio (SINR) is 10 dB.

Opportunistic relay selection is considered as an efficient approach to implement cooperative communication in multi-relay cooperative networks owing to its low implementation complexity. However, due to channel fluctuations, the channel state information (CSI) employed in relay selection process may differ from the exact CSI in data forwarding, in other words, the CSI is outdated. Addressing this issue, this paper focuses on the design of relay selection strategies in decode-and-forward (DF) cooperative networks with outdated CSIs being available in relay judgement, as well as their performance analysis. We resort to two main cases, 1) only outdated CSIs are available in relay selection process; 2) both outdated CSIs and statistical channel information are available in selection procedure, and propose three different relay selection schemes. The expressions of outage probability are derived, and asymptotic analysis in high SNR region is also carried out. Simulation results are finally provided, which not only validate our theoretical analysis, but also shed light on the way of designing relay selection strategies in various scenarios.

We consider a two-way relay network consisting of multiple pairs of single-antenna users and multiple distributed single-antenna relays. The two communication peers in each pair of users transmit simultaneously to the relays in the first time slot, and the relays rebroadcast the received sum signal weighted by a complex gain, in the second time slot. For multi-user systems, the signal arriving at the users contains not only self interference from the back-propagation of user signals, but also inter-pair interferences from other pairs of users. In this paper, we use zero-forcing (ZF) to cancel the inter-user interference, assuming that channel-state information for all relay-peer connections are known at every relay, but no data exchange occurs between relays. We also derive two closed-form expressions for zero-forcing beamforming weights, corresponding to two different relay power constraints, which can be implemented in a distributed manner. The first approach uses standard ZF to null out every inter-pair interference and the second approach sets the total inter-pair interference to zero. We also derive a closed-form upper bound of the achievable sum-rate and show that both methods achieve the same multiplexing gain when the number of relays N is sufficient for perfect zero-forcing, namely 2K2+ K, where K is the number of user pairs. For the case of insufficient number of relays, we also propose two solutions for beamforming weights, i.e., based on diagonal loading and use of the pseudo-inverse, and compare their advantages and weaknesses.

The opportunistic cooperation schemes, where only the “best” relay is selected to forward the message, are well performed in outage probability but with poor fairness among relays. In this paper, we introduce the concept “outage priority based fairness” (OPF), aiming at improving the fairness appropriately without outage performance deterioration. A novel cooperation scheme is proposed to meet the concept and theoretical analysis is provided further. Then, based on OPF, the achievable upper bound of the fairness is derived and an optimal cross-layer designed scheme is also provided to achieve the bound. Numerical simulations are carried out finally, which validate the correctness of our analysis and show that in the proposed schemes, the fairness among all relays is greatly improved especially in high SNR regime, without any loss of outage performance.

A novel distributed angle estimation method for wireless sensor network localization is proposed in this paper. The anchor emits two linear frequency modulation waves simultaneously using its equipped two antenna elements, while the angle of departure of the emitted waves can be estimated independently at each sensor. The method only relies on radio transceivers and requires no time or carrier frequency synchronization between the transceivers. It is also shown that the method can be improved to applied to mobile networks. Besides, the performance analysis and simulations are both presented to evaluate the proposed approach.

Adaptive noise cancellation (ANC) is used widely to reduce noise from a noisy speech sound. However the least-mean-square (LMS) algorithm and its variants, such as the normalized (N)LMS, the modified (M)-LMS and the constrained stability (CS)-LMS algorithms do not perform well in ANC since the desired speech signal has a bad effect on the convergence rate and steady state misadjustment of these algorithms. Thus, we propose a new adaptive algorithm that further relaxes the constraint in the CS-LMS algorithm. The new algorithm attempts to minimize the estimation error of the a posteriori error and the estimation is obtained using the concept of Taylor's expansion. The analysis and simulation results show that the proposed new algorithm outperforms the NLMS and CS-LMS algorithms.

The Radio Interferometric Positioning System (RIPS) provides a localization method with high accuracy and simple hardware configuration for sensor networks by means of measuring the phase difference of the interference signal. However, due to the periodicity of phase, it is hard to determine the actual distance difference only from a single phase measurement. To solve the problem, RIPS makes multiple measurements at different frequencies so as to convert phase difference to distance difference, which is a computationally intensive searching process and not suitable for the energy-constrained wireless sensor nodes. In this paper, we propose an efficient ranging method based on Chinese Remainder Theorem (CRT). Meanwhile, we utilize some properties of the coefficients in the CRT algorithm to avoid the over-sensitivity of the traditional CRT, which increases the robustness of the algorithm. We apply this robust CRT algorithm to the ranging process which calculates the distance difference directly in a closed form equation and therefore reduces the response time and energy consumption of the ranging procedure.

A novel distributed TDoA (Time-Difference-of-Arrival) estimation method for wireless sensor networks (WSN) is proposed in this letter. Linear frequency modulation (LFM) waves are emitted from two anchors simultaneously to create an interference field. Through the frequency measurement of local RSSI (Received Signal Strength Indication) signal, the TDoA is estimated independently at each sensor to detect range difference from the sensor to the two anchors. Furthermore, an extended method applied to mobile sensors is also presented. The proposed method only relies on radio transceivers and requires no time synchronization for sensors. Simulations results demonstrate the effectiveness of our method.

In cooperative communication systems, due to their distributed nature, multiple carrier frequency offsets (CFOs) may occur and make the channel time-varying, thus most existing space-time codes (STC) to collect diversity gain for the co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we propose a family of distributed space-frequency codes (SFCs) for cooperative transmissions with multiple CFOs and flat fading channels. These codes can achieve full cooperative (spatial) diversity with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, instead of the computationally exhausted maximum likelihood (ML) detector. The codes in this family have different symbol rates, orthogonality and performances for different number of relay nodes, which can be adjusted by choosing different design parameters. Actually, frequency-reversal SFC (FR-SFC), and frequency-shift SFC (FS-SFC) proposed in our previous paper are special cases of the new codes. Furthermore, new SFCs outperform both FD-SFC and FS-SFC can be obtained, from the systematic design.

Power saving in wireless sensor networks (WSNs) has been the focus of much recent research due to the fact that the nodes are usually battery-powered. Wireless communication accounts for a strikingly high proportion of the total power consumption, so we want to reduce the power consumption by adopting a low-power transceiver in the physical layer. IEEE 802.15.4 is a popular communication standard for WSNs which has many good features that can be used for power-saving. In this paper, we propose a simple transceiver for IEEE 802.15.4. It utilizes the constant-envelope feature of the offset quadrature phase shift keying (O-QPSK) signal with half-sine shaping defined in the standard to simplify the structure of the transceiver. Compared with conventional transceivers, the proposed transceiver adopts a hard-limiter rather than a digital-to-analog converter (DAC) and an analog-to-digital converter (ADC), which results in a simpler structure and lower power consumption, furthermore, the correlation operation could be efficiently implemented just by a reversible counter. The performance of the proposed transceiver is effectively demonstrated by both computer simulation and hardware test.

The paper proposes a new iterative method to estimate the carrier frequency offset (CFO) with scattered pilot symbols inserted for channel estimation in orthogonal frequency division multiplexing (OFDM) systems. Assuming that the channel remains constant during a few consecutive OFDM blocks, the new method uses the correlations of the pilot symbols in these consecutive OFDM blocks. And an iterative concept is also employed to reduce the inter-carrier interference (ICI) that deteriorates the CFO estimation performance. After a few iterations, the proposed method can achieve the accuracy with no ICI. The mean square error (MSE) of the new method is analyzed and simulation results show the new method outperforms existing methods.

A novel distributed TDoA (Time-Difference-of-Arrival) estimation method for wireless sensor networks (WSN) in multipath environment is proposed in this paper. For the conventional narrow band system usually adopted in the wireless sensors, the frequency-hopping technique can be utilized to perform the estimation of the wide band frequency-domain channel response, by which the super-resolution TDoA estimation is conducted independently at each sensor to detect range difference from the sensor to the two anchors. The proposed method only relies on the conventional narrow band radio transceiver and each sensor conducts range estimation independently without any information exchange or time synchronization requirement, which is a distributed range estimation method more suitable for WSN.

In this paper, we propose an oblique projection-based robust linear receiver for spatial multiplexing systems in multi-cell environments. In our receiver design, we first obtain an oblique projection operator by using the received data, and then use such operator to extract the desired signal subspace and cancel the interference-plus-noise subspace from the received signals. After suppressing the unknown cochannel interference (CCI) included in the interference-plus-noise subspace, we further use zero-forcing (ZF) receiver to suppress the coantenna interference (CAI) and detect the desired transmitted symbols in the desired signal subspace. Finally, simulation results show that the proposed linear receiver obtains significant performance improvement over ZF receiver without considering the unknown CCI in the practical environments.

A novel distributed localization method for wireless sensor networks (WSN) is proposed in this paper. Linear frequency modulation (LFM) waves are emitted from two anchors simultaneously to create an interference field. Through the frequency measurement of local RSSI (Received Signal Strength Indication) signal, the TDoA (Time-Difference-of-Arrival) is estimated independently at each sensor to detect range difference from the sensor to the two anchors, by which the position can be determined. Furthermore, an extended method applied to mobile sensors is also presented. The proposed method only relies on radio transceivers and requires no time synchronization for sensors. Simulations results demonstrate the effectiveness of our method.

Time asynchronism is a practical issue need to be addressed for a general distributed two-way relay network, where two terminal nodes exchange information through multiple spatial-separated relay nodes. In this letter, we propose an analog network coding (ANC) scheme for a time asynchronous two-way relay network. In the proposed scheme, each relay node linearly transforms the received mixed asynchronous signals in the first time-slot by a Toeplitz matrix, and then broadcasts them back to the terminals in the second time-slot. A sufficient condition is derived for the proposed ANC to achieve full cooperative diversity using only linear receivers at the terminal nodes, such as zero-forcing (ZF), or minimum mean square error (MMSE) receivers, with any delay profiles of the timing errors. The decoding of the proposed ANC scheme is computationally efficient and the symbol rate can approach 1, when the coding block length is sufficiently large compared to the number of relay nodes R and the timing errors.

In this paper, we propose a family of space-time block codes (STBC) that achieve full diversity when linear receivers, such as zero-forcing (ZF) or minimum mean square error (MMSE) receivers, are used. Our proposed STBC family is a combination/overlay between orthogonal STBC and Toeplitz codes, which can be viewed as a generalization of overlapped Alamouti codes (OAC) and Toeplitz codes recently proposed in the literature. Simulation results show that the newly proposed STBC may outperform the existing codes when linear receivers are used.

Two novel algorithms are proposed for ranges, frequencies and direction-of-arrivals (DOAs) estimation of the narrow-band near field sources. By exploiting the time-domain and space-domain correlations of impinging signals jointly, we can identify 2p independent sources and estimate 3 groups of parameters with a uniformly linear array (ULA) of 2p + 1 elements, being double of the conventional methods. In addition, the peak search and pairing operations, needed in most algorithms for near field sources, can be omitted completely in our methods. Simulation results show that our two ESPRIT-based methods provide the improved performance over conventional ones based on the second-order statistics.

Multiple frequency offsets (CFO) in cooperative communications are difficult, if not impossible, to compensate completely at the destination node. The multiple CFO make the channel time-varying, and thus the conventional space-time codes to collect diversity gain for co-located multi-input multi-output (MIMO) systems may not be applied directly. In a previous paper, we proposed a distributed space-frequency code (SFC), called frequency-reversal SFC (FR-SFC), for cooperative transmissions with multiple CFOs and flat fading channels. We have shown that this code can achieve the full cooperative diversity with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers. However, the code is not bandwidth efficient when the relay nodes are more than 3. To have a higher bandwidth efficiency, in this paper we propose a new distributed SFC called frequency-domain linear convolutive SFC (FLC-SFC). The symbol rates of the new codes approach 1 when code block length is sufficiently larger than the number of relay nodes. Furthermore. It achieves the full cooperative diversity with linear receivers.

Two novel algorithms which exploit the fourth-order statistics are proposed for DOA and range estimation of the near-field narrow-band sources. Compared with most proposed methods which can identify only p sources with a (2p + 1)-element ULA (uniformly linear array), the proposed methods can identify 2p or 2p − 1 sources with the same one. Moreover, our methods calculate much fewer fourth-order statistics than conventional ones. Simulation results show that our two ESPRIT-based methods also provide improved performance over conventional ones because of more efficient use of received signals.

Based on the finite alphabet property of the communication signals and the subspace theory, a blind source separation algorithm for co-channel signals is presented. A comparison between the proposed method and MUSIC algorithm is made. Although their application backgrounds are different, their principles are similar. On the viewpoint of MUSIC algorithm, a new explanation for the proposed approach is given. Based on the subspace perturbation theory, the bit error rate (BER) performance analysis is also presented. Some key parameters, such as channel vector correlation coefficient, signal vector correlation coefficient, number of antennas and length of code vectors, are discussed. Finally, simulation results confirm our performance analysis.

Cyclic delay diversity (CDD) is a low-complexity transmit diversity technique for coded orthogonal frequency division multiplexing (OFDM), which transforms a mutiple-input channel into an equivalent single-input channel with increased frequency selectivity and provides additional frequency diversity. However, it makes channel estimation more difficulty due to the increased frequency selectivity. In this paper, we focus on the direct estimation of the equivalent single-input channel for CDD-OFDM systems. We first propose a cyclic cross-correlation channel estimation scheme based on the equispaced constant-modulus pilot structure. Using the statistical parameters of the channel and noise derived by the result of the cyclic cross-correlation channel estimation, we propose an improved channel estimation scheme based on the Wiener filter to reduce the estimation error further. Computation simulations are given to demonstrate the effectiveness of two proposed channel estimation schemes for CDD-OFDM systems.

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.

In cooperative communications, time and frequency synchronization is an important issue needed to be addressed in practice. Due to the nature of cooperative communications, multiple frequency offsets may occur and the traditional frequency offset compensations may not apply. For this problem, equalization for the time-varying channel has been used in the literature, where the equalization matrix inverse needs to be retaken every symbol. In this paper, we propose computationally efficient minimum mean square error (MMSE) and MMSE decision feedback equalizers (MMSE-DFE) when multiple frequency offsets are present, where the equalization matrix inverses do not need to be retaken every symbol. Our proposed equalization methods apply to linear convolutively coded cooperative systems, where linear convolutive space-time coding is used to achieve the full cooperative diversity when there are timing errors from the cooperative users or relay nodes, i.e., asynchronous cooperative communication systems.

In cooperative communications, due to the distributed nature, multiple different carrier frequency offsets (CFOs) may occur and make the channel time-varying. It is hard for the receiver to compensate multiple CFOs from multiple relay nodes simultaneously. Thus, the conventional space-time codes to collect cooperative diversity for co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we consider the cooperative transmission with multiple CFOs when the channels from relay nodes to destination node are flat (frequency-non-selective) fading. We approximate a flat fading channel with CFO as a block time-invariant intersymbol-interference (ISI) channel in the frequency domain. We then propose two distributed space-frequency codes (SFC) for such ISI channels in the frequency domain to achieve the cooperative full spatial diversity, where the space-frequency coding concept is different from the one in the literature and also has a different role. One is called frequency-reversal SFC and the other is called frequency-domain linear convolutive SFC. Furthermore, we show that, with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, our codes achieve the full cooperative diversity.

The frequency synchronization issue of multiuser in the uplink of an orthogonal frequency-division multiple-access (OFDMA) system is investigated in this letter. We develop a subspace based blind carrier frequency offsets (CFOs) estimation algorithm for the base station (BS) equipped with uniform linear array (ULA). Due to the adoption of ULA at the BS and the narrowband signal assumption, CFOs of different users can be separated by their spatial information. Thus, unlike existing blind alternatives, which are subject to some specific carrier assignment schemes (CAS) or rely on null subcarriers, our proposed method can support generalized carrier assignment scheme (GCAS) and fully loaded systems, i.e., all subcarriers are available to users. Consequently, the dynamic channel assignment is available and the bandwidth efficiency is higher. Moreover, the closed-form directions of arrival (DOAs) of all active users are also obtained, which can be used in downlink beamforming in frequency division duplex (FDD) systems. Performance of the proposed scheme is evaluated by computer Monte Carlo simulations.

In cooperative communications, due to the distributed nature, multiple different carrier frequency offsets (CFOs) may be present and make the channel time-varying. It is hard for the receiver to compensate multiple CFOs from multiple relay nodes simultaneously. Thus, the conventional space-time codes to collect cooperative diversity for co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we consider the cooperative transmission with multiple CFOs when the channels from relay nodes to destination node are flat fading. We approximate a flat fading channel with CFO as a block time-invariant intersymbol-interference (ISI) channel in the frequency domain. We then propose a distributed space- frequency code (SFC), called frequency-reversal SFC, for such ISI channels in the frequency domain to achieve the cooperative spatial diversity, where the space-frequency coding concept is different from the one in the literature and also has a different role. Furthermore, we show that, with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, our code achieves the full cooperative diversity.

In this paper, we develop a blind carrier frequency offsets (CFOs) estimation algorithm for multiusers in the uplink of an orthogonal frequency-division multiple-access (OFDMA) system, where the base station equipped with uniform linear array (ULA). Due to the ULA at base station, different users could be separated by their spatial information. So unlike existing blind alternatives, which are subject to some specific carrier assignment schemes or rely on null carriers, our proposed method can support generalized carrier assignment scheme (GCAS) and full loaded systems. Thus, our method is more flexible and bandwidth efficient. Besides, the closed-form directions of arrival (DOAs) of all active users are also obtained, which can be used in downlink beamforming in frequency division duplex (FDD) systems. Performance of the proposed scheme is evaluated by computer Monte Carlo simulations.

We present a smart antenna assisted gradient broadcasting (SA-GRAB) data delivery protocol for large-scale WSN, where only the sink is equipped with a low-cost smart antenna offering a relative small coverage. SA-GRAB partitions the smart antenna's coverage into N inner sectors directly and the rest part of WSN into N outer sectors during the cost field building by means of a "dyeing" procedure. Once an event occurs, data are forwarded jointly by sector aware gradient broadcasting and direct communication with smart antenna. Simulations show that SA-GRAB achieves better energy-efficiency and scalability for large-scale WSN applications than prior work.

The optimal maximum-likelihood (ML) detector for a single-carrier (SC) multiple-input multiple-output (MIMO) system is often prohibitive due to its enormous computational complexity. In this paper, we propose a low complexity layered space-frequency equalization with time domain noise prediction (LSFE-NP) structure, where at each stage of the detector, a given data stream is detected by a multiple-input single-output (MISO) frequency domain equalization with time domain noise prediction (FDE-NP). It is shown that the proposed structure is optimal in the minimum mean square error (MMSE) sense, and the coefficients of the feedback filter are independent of the feedforward equalizer. Therefore, an alterable feedback taps LSFE-NP scheme is outlined for the coded SC MIMO system, which can feed as many reliable decisions as possible back to the equalizer to achieve better performance. Simulation results show that our proposed scheme can outperform the conventional LSFE and the MIMO FDE-NP significantly.

A novel closed-form subspace-based blind CFO estimation approach for MIMO-OFDM systems is proposed. Our key observation is that the CFO information is redundant in spectrum structure. By allocating null carriers equispaced at the transmitter and downsampling and collecting the received blocks from all antennas at the receiver, the dimension of parameter space is greatly reduced compared with previous proposed subspace- based methods. Consequently, a lower computational complexity approach is obtained. Furthermore, estimation performance is also improved significantly. Analysis and simulation results demonstrate the effectiveness of our proposed method.

In cooperative communications, due to distributed nature, cooperative transmission may not be either time or frequency synchronized. In this paper, we propose computationally efficient minimum mean square error (MMSE) and MMSE decision feedback equalizer (MMSE-DFE) equalizers for cooperative communication systems when multiple time offsets and frequency offsets present, where the equalization matrix inverses do not need to be retaken every symbol. Our proposed equalization methods apply to linear convolutively coded cooperative systems, where the linear convolutive space-time coding is used to achieve the full cooperative diversity when there are timing errors from the cooperative users or relay nodes, i.e., asynchronous cooperative communication systems.

This paper addresses the application of particle filtering to DOA estimation and tracking, adaptive uplink beam- forming and multiple user separation in SDMA digital com- munication systems. According to the aposteriori probabilities provided by the particle filter, uplink receivers are designed for both AWGN channel and time-selective channel. In AWGN channel, transmitted symbols can be directly detected at the output end of the particle filter. In time-selective channel, channel coding is introduced to combat fading, and correspondingly two different receiver structures without channel estimation are proposed: one is to concatenate a Viterbi / MAP decoder to the output of the particle filter; the other is to employ the particle filter as the first stage demodulator and the MAP channel decoder as the second stage demodulator to construct a turbo receiver. Finally, the performance of the proposed particle filter receivers is demonstrated through computer simulations.

We propose a new DOA-Matrix based blind CFO estimation method for a MIMO-OFDM system. This method allows arbitrary use of the subcarriers, with no need of virtual carriers. Since the Cyclic Prefix (CP) is often longer than the channel length, the last part (at least the last symbol) in CP is contaminated by IBI to a little extent, we can use some of the symbols in CP to span the signal space for DOA-Matrix, eliminating the need of virtual carriers. In contrast to the blind methods with virtual carriers, the proposed method is bandwidth efficiency, and has lower computational complexity. We also present numerical simulation results to illustrate the validity of the method for different transmit and receive antenna configurations.

A novel subspace-based blind carrier frequency offset (CFO) estimation approach for orthogonal frequency-division multiplexing (OFDM) systems is proposed. The key observation is that the CFO information is redundant in spectrum structure. By locating null carriers equispaced and remodeling the received blocks through downsampling, we compact parameter space and obtain a low complexity estimator with the performance of high-resolution. Analysis and simulations show the estimator is robust in both AWGN channel and frequency-selective channel.

Synchronization is an important issue in orthogonal frequency-division multiplexing (OFDM) systems including symbol timing and carrier frequency offset (CFO) estimation. In this paper, we derive a new subspace based CFO estimation method exploiting the cyclic prefix (CP) samples which are not disturbed by the preceding OFDM symbol. As a special and more practicable case, we provide an adaptive joint symbol timing and CFO estimation scheme. It is robust in both AWGN channel and frequency selective channel.

Two novel algorithms are proposed for ranges and direction-of-arrivals (DOAs) estimation of the narrow-band near field sources. By exploiting the time-domain and space-domain correlations of impinging signals jointly, we can estimate 2p independent sources with a uniformly linear array (ULA) of 2p + 1 elements, being double of the conventional methods. In our methods, the peak search and pairing operations, needed in most algorithms for near field sources, can be omitted completely. Simulation results show that our two ESPRIT-based methods provide the improved performance over conventional ones based on the second-order statistics.

The combined use of the multicarrier (MC) technique with the vertical Bell Labs layered space-time (V-BLAST) architecture, i.e. the MC V-BLAST system is well suited to the frequency-selective fading environment. A modified rotation invariance coding scheme and corresponding decoding algorithm without knowing channel state information (CSI) are proposed under this environment. Utilizing the time redundancy, this method has the advantage that the number of receive antennas is less than that of the transmit ones. Simulation results demonstrate the validity of the coding scheme and the performance of the direct decoding algorithm.

A new algorithm is proposed for bearing estimation of the narrow-band near field sources in the spherical coordinates (azimuth, elevation and range), which can distinguish 2p independent sources with a cross array of 2p + 3 elements by exploiting the time-domain and space-domain correlations of impinging signals jointly. The number of distinguishable sources in our algorithm is more than double of that in conventional algorithms when the number of array elements is the same. In addition, our algorithm exploits the eigen values and vectors simultaneously, and then the calculated parameters only need simple pairing and the multidimensional searching can be omitted completely. Simulation results demonstrate the novelty, validity and effectiveness of the proposed algorithm.

A new practical computationally efficient power and bit allocation algorithm for wireless OFDM communication systems is proposed aiming at minimization of transmit power under the constraints of data rate and bit error rate. The proposed algorithm exploits the relationship between the water-filling level and the system data rate, and the differences between the bit allocations of two consecutive water-filling iterations in order to derive an efficient adaptive water-filling level searching method without need of preset parameters and a simplified greedy search method to allocate final bits, respectively. By combining them effectively, the proposed algorithm converges faster to the optimal solution than existing techniques. Computer simulation demonstrates the computational efficiency of the proposed algorithm in frequency selective fading environments.

In this paper, a blind adaptive signature vectors estimation algorithm for Multicarrier (MC) CDMA system in rapidly time-varying multipath fading channel is proposed. This algorithm adopts PROTEUS algorithm to estimate signal subspace and is an adaptive low dimension implementation of subspace projection method in virtue of the special update form of PROTEUS algorithm. In the case that multiple antennas are deployed to increase the capacity of CDMA systems, the proposed algorithm can estimate time-varying spatial-temporal signature vectors. This method has low computational complexity and good real-time tracking capability for rapidly time-varying channel. Monte-Carlo simulations results demonstrate the estimation and convergence performance of the proposed algorithm.

A novel blind symbol detection algorithm is proposed for time-selective fading multiple-input-multiple-output (MIMO) systems, whose every channel is described as a second-order autoregressive (AR) model. This algorithm employs the particle filter as the demodulator stage in turbo receivers because the particle filter has the natural characteristics of soft-input soft-output. Moreover, the particle filter has the advantage of parallel computation. Computer simulation results demonstrate the performance of the proposed algorithm.

The uplink of a space-time block coded multicarrier code division multiple access (MC-CDMA) system endowed with a uniform linear array (ULA) at the base station of macrocell is studied. Transmission takes place over frequency-selective fading channels and the transmitted symbol of a user is encoded by Tarokh's rate 1 space-time block encoder, (V. Tarokh and H. Jafarkhani, July 1999) from real orthogonal design, with three transmit antennas. An algorithm is proposed to blindly estimate the data symbol sequence and the direction of arrival (DOA) of each user without determining the space-time vector channels. The algorithm employs an ESPRIT- like method (Y. Zeng et al.) to separate multiple co-channel users with different impinging DOAs, then exploits the structure of space- time block coding (STBC) in combination with the subspace method to estimate the original symbol sequences for multiple users. In particular, the theoretical analysis shows that the signal subspace defined by the received data covariance matrix can be separated by users' DOA, so the joint diagonalization step of many matrices due to the method presented by A. Swindlehurst (April 2002) is avoided, which saves large computations. Computer simulations illustrate the overall performances of the proposed algorithm.

The paper integrates space-time coding (STC) techniques and the multiple-input multiple-output (MIMO) antenna architecture with multicarrier code-division multiple access (MC-CDMA) systems. For brevity, we call such integration space-time coded MIMO MC-CDMA systems. A numerical model for the space-time coded MIMO MC-CDMA system is presented in detail. Successively, by exploiting subspace decomposition techniques, a novel blind identification method is derived for downlink wireless channels in the space-time coded MIMO MC-CDMA system. As there is no need of pilot symbols to identify wireless channels between multiple-transmit and multiple-receive antennas, our proposed algorithm can finely keep the spectrum efficiency of the space-time coded MIMO MC-CDMA system. Computer simulation results demonstrate the validity and the performance of our proposed algorithm.

Space-time coding and modulation exploit the presence of multiple transmit antennas to improve performance on multipath radio channels. Thus far, most work on space-time coding has assumed that perfect channel estimates are available at the receiver. In certain situations, however, it may be difficult or costly to estimate the channel accurately. We propose a new decoding algorithm for full rate space-time block coding in frequency selective channels, which accomplishes the decoding directly from the over-sampled system outputs without the knowledge of the channel. Combined with the unique feature of space-time block coding, the subspace of the transmitted signal is exploited to carry out the decoding. Monte Carlo simulations give the performance comparison of this algorithm against that with available channel information.

In order to hold fine spectral efficiency, it is meaningful to investigate multicarrier code-division multiple access (MC-CDMA) systems without cyclic prefix (CP), which are unavoidably corrupted by the hostile intersymbol interference (ISI) so that we can call them as ISI-corrupted MC-CDMA systems. Moreover, combining multiple antennas with ISI-corrupted MC-CDMA systems is advantageous in suppressing cochannel interference. Hence, this paper applies multiple antennas to the base station of ISI-corrupted MC-CDMA systems over frequency-selective fading channels. A joint spatio-temporal estimation method of uplink vector channel is then derived based on the singular value decomposition (SVD) technique. Computer simulations illustrate that our algorithm is more robust against noise and can well mitigate multiple access interference (MAI) in multiuser scenarios.

In multicarrier code-division multiple access (MC-CDMA) systems over the finite impulse response (FIR) channel, a usual approach for combating the resultant hostile intersymbol interference (ISI) is via addition of a cyclic prefix (CP) to each transmitted data block. However, ISI is eliminated at the cost of reduction in spectral efficiency. In order to recover spectral efficiency, it is meaningful to investigate MC-CDMA systems without CP. This paper explores the multiuser detection for the MC-CDMA system without CP. We interpret the MC-CDMA system without CP as an equivalently time-domain ISI-corrupted direct sequence (DS-) CDMA system with specific spreading codes. Successively, based on this representation, a time-domain linear minimum mean square error (MMSE) multiuser detection scheme is derived for the MC-CDMA system without CP. Computer simulations illustrate both the validity and the overall performance of the proposed multiuser detection scheme.

The constant modulus property of communication signals, such as FM and phase-modulated signals, are often used in blind separation of multiple users. Because there is no training sequence needed, the system transmission efficiency is increased. However the CMA suffers from the complex iteration computation and local minimums. The analytical constant modulus algorithm (ACMA) replaces the problem of finding the weight vector blindly with the equivalent problem of blind source separation in signal subspace. Via joint diagonalization of matrices, each user's signal can be obtained analytically. Expanding the ACMA of space domain beamforming to space-time domain can improve the performance of blind signal separation of the system. The computation complexity of ST-ACMA is only related to the amount of constant modulus sources. Its performance is similar to ST-CMA algorithm, but it has less computation than ST-CMA and the faster convergence rate. Finally, numerical simulations can illustrate the effectiveness of this algorithm.

The multicarrier code division multiple access (MC-CDMA) is a flexible and adaptive radio interface to provide dynamical bit rate transmission and a variety of services on radio channels. It can sufficiently utilize the subcarrier diversity and meet the demand of diverse multimedia services; therefore it is broadly considered as an effective approach for future wireless multimedia communications. However, it is very sensitive to carrier drifts, while the orthogonal property between the subcarriers is lost with frequency offset. An accurate channel estimate allows the use of multiuser detection. After reviewing some blind channel estimation algorithms with the signal time domain structure and statistic characteristic, we propose a constant modulus based adaptive blind channel estimation at the base station for MC-CDMA systems in a frequency-selective fading channel environment with the consideration of carrier offset compensation. Compared with the SVD-based method and finite alphabet-based method, it illustrates the effectiveness with numerical simulations.

We investigate the transmit diversity for orthogonal frequency division multiplexing code division multiple access (OFDM-CDMA). Diversity transmission at the base station is an effective technique to combat channel fading. Space-time (ST) coding has been proposed as coding technique to improve the system performance with additional gains. By combing the spatial transmit diversity with the space-time codes for OFDM systems it shows high performance improvement. But it is important to remark that ST decoding requires multichannel state information at the receiver. Thus the achievable diversity gain comes at the price of proportional increase in the amount of training or computation complexity, which incurs efficiency loss especially in a rapidly varying environment. In an OFDM system, there exist pilots used for frame detection, carrier frequency offset estimation and channel estimation. We proposed a receive scheme, which use the pilot structure to estimate the multichannel state information and use the CMA-based channel tracking in the space-time coded transmit diversity OFDM-CDMA systems. Numerical simulations illustrate the effectiveness of this algorithm.

In this paper, we propose a subspace tracking method for blind signature waveform estimation. In practice, the signature waveform can not change suddenly. So, we can take use of the past information of the signal subspace to update the signal subspace gradually for blind signature waveform estimation. After the algorithm proposed in this paper, the computation complexity is reduced substantially.

Based on the ESPRIT-like method, a closed-form CDMA signal and DOA estimation approach without the knowledge of space-time channel and training sequence is presented in this paper. An auxiliary matrix is constructed by exploiting the special structure of the antenna array. The symbol vector and DOA are obtained from an eigenvalue-eigenvector pair of the auxiliary matrix even under imperfect carrier recovery. The carrier offset can be acquired easily by exploiting the finite alphabet property of the transmitted symbols. In comparison with conventional space-time linear multiuser detector, in which the knowledge of space-time channel is assumed known, the proposed method has the advantage of lower computation at the expense of little performance degradation. Finally, numeral simulations are presented.

Multicarrier modulation code division multiple access (MCM-CDMA) technique or orthogonal frequency division multiplexing code division multiple access (OFDM-CDMA) scheme can sufficiently utilize the available spectrum and meet the demand of diverse multimedia services, therefore it is broadly considered as an effective approach for future wireless multimedia communications. In this paper, based on software-defined radio (SDR), by exploiting the discrete Fourier transform (DFT) and inverse discrete Fourier transform (IDFT), a general baseband digital model for different OFDM-CDMA schemes is developed. Moreover, the corresponding matrix representation is addressed.

The constant modulus property of communication signals, such as FM and phase-modulated signals, are often used in blind separation of multiple users. Because there are no training sequences needed, the system transmission efficiency is increased. When extended to the space-time domain, the new schemes have potentially greater performance than conventional ones. In this paper a feedback adaptive space-time constant modulus algorithm (FAST-CMA) is proposed for DS-CDMA multiuser detection. First the uplink channel model is built with receiving K synchronous communication signals that are transmitted through a MIMO-FIR channel. Based on this channel model, we compare and analyze the FAST-CMA with cascade-type constant modulus algorithm (CT-CMA) and space-time type constant modulus algorithm (ST-CMA). The FAST-CMA can combat both ISI and MAI without stringent power control and training sequence, and it is preferable to CT-CMA and approximately has the same performance as the ST-CMA. It has faster convergence rate, and less computational complexity than ST-CMA. Numerical simulations can illustrate the effectiveness of this algorithm.

The direction-of-arrival (DOA) is one of important parameters, in addition to time-of-arrival (TOA) and attenuation, to describe most radio propagation channel models. Time-frequency analysis is a powerful technique to deal with time-variant or non-stationary signals. Although M.G. Amin (see Gersham, A.B. and Amin, M.G., IEEE Sig. Proc. Letters, vol.7, no.6, p.152-5, 2000; Belouchrani, A. and Amin, M.G., IEEE Sig. Proc. Letters, vol.6, no.5, p.109-10, 1999) and L. Jin et al. (see IEEE Int. Symp. on Circuits and Systems, p.375-8, 2000) combined it with a coherent signal subspace (CSS) approach, it is not easy to extend from the narrowband to the wideband case. We first overview the fast chirplet-based signal decomposition algorithm, then build the general time-frequency covariance matrix to overlap the narrowband and wideband of CSS. After discussing the difference between the two cases, we propose a wideband DOA estimation algorithm with fast chirplet-based adaptive signal decomposition. The algorithm has no restriction to the array manifold. We give the comparison of our proposed algorithm with the optimal solution of maximum likelihood (ML). We can see that the proposed algorithm has very low complexity. Its robust performance is demonstrated with numerical simulations.

Based on the subspace method, a closed-form CDMA signal and DOA estimation approach without the knowledge of space-time channel and training sequence is presented. An auxiliary matrix is constructed by exploiting the special structure of the antenna array. Then the symbols instead of their signal subspace are achieved by eigendecomposition of the auxiliary matrix. Therefore, the puzzle of separating the different users' signal from their signal subspace in conventional blind symbol estimation method is avoided. In comparison with the conventional space-time linear multiuser detector, in which the knowledge of space-time channel is assumed to be known, the proposed method has the advantage of lower computation without degrading the near-far resistance performance. Finally, numeral simulations are presented.

The multicarrier code division multiple access (MC-CDMA) technique is known to be appropriate for high-data-rate wireless communications due to its robustness to multipath fading and its capability of handling high-data-rate transmissions with a simple one-tap equalizer. This paper investigates a uniform linear antenna array at the base station for macrocell MC-CDMA systems in a frequency-selective fading channel environment, and develops a blind space-frequency channel estimator for an MC-CDMA system with antenna array. As a by-product, the direction-of-arrivals (DOAs) can also be estimated. By constructing a specific auxiliary matrix including both the space-frequency channel and DOA information, and performing eigen decomposition on it, we can obtain the closed-form solution of space-frequency channels and DOAs for all active users within the macrocell. The overall performance of this novel scheme is demonstrated by extensive computer simulations.

Many blind channel estimation methods have been proposed for direct sequence (DS) CDMA systems, so we can certainly use them to estimate the finite impulse response (FIR) channel for the multicarrier code division multiple access (MC-CDMA) system. In this paper, we interpret the MC-CDMA system as an equivalently DS-CDMA system with specific spreading codes. Then, an equivalently blind time-domain channel estimator is derived for the MC-CDMA uplink, which is based on second-order statistics of the received data. Because there is no need of a large amount of sampling data, this method is greatly suitable for the fast time varying wireless channels. By exploiting singular value decomposition (SVD) and the finite alphabet property of transmitted symbols, the time-domain channel impulse response (CIR) for the MC-CDMA can be accurately identified. Computer simulations illustrate both the validity and the overall performance of the proposed estimator.

Joint spatial-temporal multiuser detection (MUD) schemes for DS-CDMA systems with antenna arrays have drawn much attention, which has continued unabated. We derive the equivalent spatial-temporal MUD scheme for MC-CDMA systems with antenna arrays. A uniform linear array (ULA) is applied to the base station of the MC-CDMA system. We first describe the equivalent time-domain DS-CDMA interpretation for the MC-CDMA system. Based on this interpretation, by utilizing the finite alphabet property of the transmitted symbols, an equivalent spatial-temporal multiuser detector is derived for the MC-CDMA system with the ULA over frequency-selective fading channels. Computer simulations illustrate that our algorithm is more robust against noise and can well mitigate multiple access interference (MAI) in multiuser scenarios. In particular, our novel scheme has the potential of providing the anticipated MUD performance gains with a complexity that would be manageable for MC-CDMA systems.

Joint space-time multiuser detection provides a significant performance improvement over cascade space-time multiuser detection. But its computation load makes it impossible to be applied in practical situations. We propose a wavelet-based multiresolution-beamspace approach. Using the multiresolution of wavelets, it reduces the computation load of joint space-time multiuser detector in DS-CDMA. It is a tradeoff between the cascade and joint space-time processing. In applying wavelets to an array problem, we discuss the multiresolution in an array signal. Simulations show that there is a reduction of the computational load and compared with other beamspace methods, it improves the performance.

In many wireless communication systems, two-dimensional Directions-Of-Arrival (DOA) of wideband signals needs to be found for spatial selective transmission. In this paper, based on the generalized spectral correlation signal subspace fitting (GSC-SSF), we propose a new method to estimate 2-D elevation/azimuth angle of wideband signals by exploiting the temporal feature of signals, i.e., cyclostationarity. The theoretical analysis shows that, without need of 2-D search or 2-D parameter match, this method is bandwidth-independent and suitable for multi-rate multimedia communications. Simulation results are given to illustrate the performance of the new algorithm.

A novel DOA estimation method called Generalized Spectral Correlation Signal Subspace Fitting (GSC-SSF) is proposed in this paper, which is applicable in both wide band and narrow band scenarios. Especially, in narrow band cases, the method is capable of enlarging the effective aperture of the sensor array considerably. It is proven herein that several present DOA estimation methods based on cyclostationarity, including SC-SSF, Cyclic MUSIC and Cyclic ESPRIT, are just special cases or approximations of this method. Its effectiveness and generality are also illustrated by simulation results.

A beamforming algorithm suitable for space-time Rake receiver in CDMA system is addressed. The algorithm is the optimal solution in the sense of minimum variance when interference number approaches to infinite. Simulation results demonstrate that the proposed algorithm approximates to the Capon's MVDR (minimum variance distortionless response) beamformer when the interference number is much more than the number of array elements. In comparison with Capon's, the algorithm has the advantages of lower computation and no coherent problem. Based on the method described above, a space-time searcher and a spatial frequency detector are presented, by which a beam tracking algorithm is proposed. Simulation results demonstrate that the algorithm can efficiently track the direction of desired signals and has the advantages of lower static error and better dynamic tracking performance.

A closed-form array response estimation (CARE) technique for blind source separation in wireless communication is developed. By exploiting the data structure of second-order statistics of the array output in the presence of multipath, we construct a signature matrix in such a way that its eigenvectors corresponding to non-zero eigenvalues are just the array response vectors. Thus a closed-form solution of array response can be obtained by eigen-decomposition. The theoretical analysis and the simulations show that the proposed method achieves array response estimation with little constraint on signal property and propagation environment such as scatters or angular spread. Moreover, the array considered here can be of arbitrary geometry and even uncalibrated.

This paper presents a new cyclic direction-of-arrival (DOA) estimation algorithm. By exploiting a new minimum-redundancy linear array (MRLA) configuration and cyclostationarity, the proposed algorithm constructs a matrix pencil from the pseudo-data matrix of 2N-1 virtual sensors, which are the results of temporal and spatial processing with designed M-sensor Sum-MRLA. Theoretical analysis and computer simulations show that, for each cyclic frequency, the algorithm can estimate 2N-2 DOAs with M-sensor Sum-MRLA (N>M).

In smart antenna systems (SAS), the most compelling research work is the space-division multiple-access (SDMA) which includes uplink source separation and downlink selective transmission. We propose a subspace-based spatial signature estimation algorithm of cochannel multiuser signals by jointly exploiting cumulant and cyclostationarity, and apply it to multiuser signal copy in uplink SDMA. Our method constructs a new matrix called the spatial signature matrix (SS matrix), and estimates the multiuser spatial signature through the eigen-decomposition of the SS matrix. Based on estimated spatial signature, a spatial filter bank is designed for cochannel multiuser waveform estimation. Computer simulations show that the spatial response of the filter bank can be used to find the direction-of-arrival (DOA) of the correlated multiuser signals.

The chirp function is one of the most fundamental functions in nature. Many natural events can be roughly approximated by a group of chirp functions. In this paper, we present a practical adaptive chirplet based signal approximation algorithm. Unlike the other chirplet decompositions known so far, the elementary chirplet functions employed in this algorithm are adaptive. Therefore, the resulting approximation could better match the underlying signal and uses fewer coefficients. The effectiveness of the algorithm is demonstrated by numerical simulations.

In this paper, we propose a neural method to solve the orthogonality search problem arising in direction-of-arrival (DOA) estimation. The most important feature of this method hinges upon the fact that it can offer the potential of real-time solutions to the above problem by utilizing the fast relaxation properties of the Hopfield's linear programming neural network. Theoretical analysis and simulation results show that the performance of neural method is exactly equivalent to that of the standard MUSIC method or the Real Domain DOA estimation method (RD method). That is to say, the method proposed in this paper is a neural implementation of the MUSIC method and RD method.>

In this paper, we give a simple analysis of the DOA estimation method based on Hopfield's optimization neural network. In this case, the problem is mapped onto the quadratic energy function of the network as an optimization problem. Although the method can avoid the eigendecomposition of the data autocorrelation matrix and the orthogonality search of parameter space, theoretical analysis and computer simulation results show that the construction of the DOA cost function is incorrect on the condition that there is no constraint on the outputs of the network.>

A novel approach for high resolution estimates of two-dimensional (2-D) angles of arrival of multiple narrowband sources for two parallel linear arrays is presented. By fully using the properties of the autocovariance matrices, the authors construct a direction-of-arrival (DOA) matrix. The theoretical analysis shows that the eigenvalues and corresponding eigenvectors of the DOA matrix are related to the 2-D angles of arrival of sources. Thus, they can be used to estimate 2-D angles of arrival separately, and the large computations required for 2-D searches can be avoided. Simulation results are given, showing that the method yields excellent estimates with low variance.>

The authors present a novel eigenstructure-based method for spatial-temporal estimating of coherent narrowband sources impinging on a uniform passive sensor array. Based on a decomposition of 2-D covariance matrix and by using a signal-eigenvector-smoothing technique, the directions of arrival and the center frequencies of multiple sources can be separately estimated in each dimension regardless of coherence. Thus, a great amount of computational work required in searching peaks from 2-D polynomial is saved, and the aperture can be used more effectively. Simulation results are given.>