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In this paper, we present a simple analytic method that can be used to predict potential energy depletion in off-grid wireless backbone network nodes serving mobile users. The instantaneous energy depletion of the batteries of the network nodes is determined by random energy arrivals and departures and modeled as a G/G/1 queue. To evaluate the online energy depletion probability (EDP), an integral-free asymptotic approach is typically used by assuming that the prediction horizon approaches infinity. However, in many practical cases, the time required by user connections can be rather short. This indicates the need for proactive resource management decisions over finite horizons so that the transmission opportunities with limited energy and time horizons are not wasted. Using Hölder's inequality, we obtain a novel finite-horizon upper bound for the EDP, and the result is compared with the infinite-horizon method. The accuracy of the proposed bound, which is addressed both analytically and numerically, proves to be better for shorter prediction horizons. The finite- and infinite-horizon methods are then applied for an energy provisioning admission control (EP-AC) framework using off-grid backbone network nodes. The key observation of this paper is that the proposed finite-horizon prediction approach admits significantly more users to the network when the connection times are relatively short, while retaining an integral-free closed-form structure suitable for the online evaluation of the EDP. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the linear precoder design for three dimensional (3D) massive multi-input multi-output (MIMO) downlink with uniform planar array (UPA) and imperfect channel state information (CSI). We introduce a beam based statistical channel model (BSCM) by using sampled steering vectors, and then an a posteriori channel model which includes the channel aging is established. On the basis of the a posteriori channel model, we consider the robust precoder design by maximizing an upper bound of the expected weighted sum-rate under a total power constraint. We derive two concave minorizing functions of the objective function. With these minorizing functions and the minorize-maximization (MM) methodology, we derive two iterative algorithms that converge to stationary points of the optimization problem. Simulation results show that the proposed precoders can achieve a significant performance gain than the widely used regularized zero forcing (RZF) precoder and the signal to leakage noise ratio (SLNR) precoder in median to high mobility scenarios. [ABSTRACT FROM AUTHOR]

Relays have been deployed to provide connectivity to heavily blocked and isolated radio environments, but suffer from severe data rate losses due to the half-duplex nature of relays. This paper exploits non-orthogonal multiple access (NOMA) to enhance the sum-rate of two heavily blocked users to and from a base station (BS) via two-way relays (TWRs). By carefully arranging the NOMA transmissions, only two time slots are needed to complete a round of two-way transmissions between the BS and the two users. Unlike existing works where users associated with the same relay form a NOMA pair, the two users are connected to two different relays in the proposed scheme, due to the blockage. We analyze the sum-rate of the proposed NOMA-based TWR scheme with two popular relaying modes, namely, amplify-and-forward (AF) and decode-and-forward (DF) relaying, and derive closed-form expressions for the sum-rate upper bounds of the scheme. We also analyze the asymptotic sum-rate of the NOMA-based TWR schemes in high signal-to-noise ratio (SNR) regions. Simulations verify the analysis, and show that the proposed schemes outperform existing TDMA-based alternatives and a NOMA-based benchmark scheme. It is shown that the sum-rate of the schemes is independent of the transmit power of the BS in the high SNR regions, given the transmit powers of the relays and users. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA), as a codebook-based non-orthogonal multiple access (NOMA) technique, has received research attention in recent years. The codebook design problem for SCMA has also been studied to some extent since codebook choices are highly related to the system’s error rate performance. In this paper, we approach the SCMA codebook design problem by formulating an optimization problem to maximize the minimum Euclidean distance (MED) of superimposed codewords under power constraints. While SCMA codebooks with a larger MED are expected to obtain a better BER performance, no optimal SCMA codebook in terms of MED maximization, to the authors’ best knowledge, has been reported in the SCMA literature yet. In this paper, a new iterative algorithm based on alternating maximization with exact penalty is proposed for the MED maximization problem. The proposed algorithm, when supplied with appropriate initial points and parameters, achieves a set of codebooks of all users whose MED is larger than any previously reported results. A Lagrange dual problem is derived which provides an upper bound of MED of any set of codebooks. Even though there is still a nonzero gap between the achieved MED and the upper bound given by the dual problem, simulation results demonstrate clear advantages in error rate performances of the proposed set of codebooks over all existing ones not only in AWGN channels but also in some downlink scenarios that fit in 5G/NR applications, making it a good codebook candidate thereof. The proposed set of SCMA codebooks, however, are not shown to outperform existing ones in uplink channels or in the case where non-consecutive OFDMA subcarriers are used. The correctness and accuracy of error curves in the simulation results are further confirmed by the coincidences with the theoretical upper bounds of error rates derived for any given set of codebooks. [ABSTRACT FROM AUTHOR]

Precoding-aided spatial modulation (PSM) is a multi-antenna communication scheme, which jointly modulates information bits into both the spatial index and the signaling constellation. Specifically, the spatial bits are conveyed by the index of a receive antenna, which is steered at by precoding at the transmitter. In this paper, the indices of the receive antennas are called a spatial constellation in contrast to the signaling constellation. We investigate the symbol error rate (SER) of the PSM scheme to strike a fine tradeoff between the signaling and the spatial constellation, i.e., the signaling-spatial constellation tradeoff, which minimizes the system's SER. First, we formulate the SER of the PSM scheme. Then, an explicit upper bound on the SER is determined based on an approximation using a weighted sum of exponentials. By minimizing the SER upper bound, we show that for phase-shift keying modulation, it achieves a sufficiently good tradeoff when the size of the signaling constellation is 4, i.e., the quadrature phase-shift keying. The numerical results show that the signaling-spatial constellation tradeoff determined by the derived SER upper bound significantly outperforms other PSM schemes with equal spectral efficiency. [ABSTRACT FROM AUTHOR]

Multiple-input-multiple-output (MIMO) systems significantly increase the network throughput and coverage probability for short-packet communications. In this paper, a tight lower bound on the average achievable rate of MIMO short-packet communication systems is theoretically derived, considering flat uncorrelated Rayleigh fading MIMO channels. The derivations are based on the eigenvalue analysis of a Wishart matrix and Jensen's inequality. Given the actual transmission rate, a tight approximation on the average block error rate (BLER) of MIMO short-packet communication systems is also provided, which is based on the moment generating function and inverse Laplace transform. Numerical results demonstrate that both our theoretically derived analytical average achievable rate lower bound and approximate average BLER are almost the same as the simulation results. [ABSTRACT FROM AUTHOR]

This paper presents an optimization approach for a system consisting of multiple bidirectional links over a two-way amplify-and-forward relay. Improvement to the fairness of the system is desired. All user pairs exchange information over one relay station (RS) with multiple antennas. Due to the joint transmission to all users, the users are subject to mutual interference. A mitigation of the interference can be achieved by max–min fair precoding optimization where the relay is subject to a sum power constraint. The resulting optimization problem is nonconvex. This paper proposes a novel iterative low-complexity approach based on a modified Levenberg–Marquardt (LM) method to find near optimal solutions. The presented method finds solutions close to the standard convex-solver-based relaxation approach. [ABSTRACT FROM PUBLISHER]

In this paper, the source estimation problem is investigated in a wireless sensor network (WSN) with energy-harvesting (EH) nodes. We first consider a deterministic EH model where the amount of harvested energy at each node is known prior to transmissions. For such a model, we investigate the average mean-square-error (MSE) minimization problem over a finite horizon of multiple estimation periods and derive the optimal solution. Moreover, a more realistic stochastic EH model is considered, where only the current amount of harvested energy is known. The corresponding MSE minimization problem over an infinite horizon is formulated. The Lyapunov optimization technique is adopted to obtain a solution for the stochastic EH problem. Simulation results are conducted to validate the effectiveness of the proposed solutions for both the deterministic and stochastic EH models. [ABSTRACT FROM AUTHOR]

In this paper, we consider an intelligent reflecting surface (IRS) enabled mobile network, where a fixed access point (AP) communicates with a mobile user (MU) via the aid of an IRS. We assume that the MU moves from one elementary square to another following a Markov random walk within a grid, and propose a maximum a posteriori (MAP) criterion to track the movement of the MU by leveraging the line-of-sight component of the IRS-MU link. Since it is infeasible to derive an explicit expression for the average probability of estimation error (APEE) for the proposed MAP criterion, we derive a closed-form upper bound for the APEE, which is used as the cost function to optimize the phase shifts of the IRS units. Considering the unit modulus constraints incurred by the IRS units, a manifold optimization (MO) method is firstly employed to gain a favorable solution to the formulated optimization problem, followed by a low-complexity codebook based solution to circumvent the high computational cost of the MO method. Our numerical results demonstrate the superior performance of the proposed IRS phase shift designs over the benchmark method. [ABSTRACT FROM AUTHOR]

In this paper, a cooperative spatial modulation (SM) based non-orthogonal multiple access (NOMA) scheme with simultaneous wireless information and power transfer is proposed. In particular, NOMA is applied at the base station (BS) and the near user (U1) acts as an energy harvesting full-duplex relay to assist the far user (U2). The data bits of U1 and U2 are mapped into SM symbols and amplitude/phase modulation symbols, respectively. Furthermore, the rate of U1 is maximized while satisfying the target rate of U2 by jointly designing the power allocation coefficient, power-splitting ratio, and transmit power of U1. To solve the non-convex problem, a successive convex approximation-based iterative algorithm is developed. Finally, numerical results reveal that the proposed scheme can achieve the superior performance compared to existing schemes. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of an intelligent omni-surface (IOS) assisted downlink non-orthogonal multiple access (NOMA) network with phase quantization errors and channel estimation errors, where the channels related to the IOS are spatially correlated. First, upper bounds on the average achievable rates of the two users are derived. Then, channel hardening is shown to occur in the proposed system, based on which we derive approximations of the average achievable rates of the two users. The analytical results illustrate that the proposed upper bound and approximation on the average achievable rates are asymptotically equivalent in the number of elements. Furthermore, it is proved that the asymptotic equivalence also holds for the average achievable rates with correlated and uncorrelated channels. Additionally, we extend the analysis by evaluating the average achievable rates for IOS assisted orthogonal multiple access (OMA) and IOS assisted multi-user NOMA scenarios. Simulation results corroborate the theoretical analysis and demonstrate that: i) low-precision elements with only two-bit phase adjustment can achieve the performance close to the ideal continuous phase shifting scheme; ii) The average achievable rates with correlated channels and uncorrelated channels are asymptotically equivalent in the number of elements; iii) IOS-assisted NOMA does not always perform better than OMA due to the reconfigurability of IOS in different time slots. [ABSTRACT FROM AUTHOR]

In this paper, we derive an upper bound on load-threshold of coded slotted ALOHA (CSA) for slot erasure channel (SEC) and that for packet erasure channel (PEC), respectively. In SEC, by exploiting the analogy between Gaussian elimination of solving linear equations and the iterative successive interference-cancellation (SIC) decoding procedure of CSA, we derive the upper performance bound for SEC. Subsequently, in PEC, utilizing density evolution to track the average probability of packet-decoded failure within the SIC decoding process, we obtain the upper bound for PEC. Moreover, we propose a numerical search algorithm of these bounds under a given calculating precision. The numerical results indicate that the asymptotic upper limit on load-threshold of CSA over SEC is always less than that over PEC with any given erasure probability and average transmission rate. [ABSTRACT FROM AUTHOR]

As the first class of constructive channel codes achieving the symmetric capacity of the binary-input discrete memoryless channels, polar codes have been extended to the fading channel. But most constructions of polar codes in fading channel are only suitable for the Rayleigh fading and involve complex iterative-calculation. In this paper, we establish a systematic framework in term of the polar spectrum to analyze and construct polar codes in various fast fading channels, such as Rician, Rayleigh and Nakagami channels. In these three fading channels, by using the polar spectrum, we derive the upper bound of the error probability of the polarized channel and that of block error rate of polar codes. The analysis based on the polar spectrum explicitly reveals the relationship between the diversity order and the codeword weight. Furthermore, we propose two construction metrics, named logarithmic upper-bound weight (LUW) and minimum-weight LUW (MLUW) respectively, to design the polar codes in fast fading channels. These two constructions are simple with a linear complexity and explicit for the practical implementation. In addition, for various fading channels, they can construct polar codes with similar or better performance over those based on traditional methods. [ABSTRACT FROM AUTHOR]

This paper investigates the capacity results of dimmable visible light communications (VLCs) channel under practical range-limited constraints. Upper and lower bounds for both single-input single-output (SISO) and multiple-input single-output (MISO) scenarios which are valid in all signal-to-noise (SNR) region are derived. We reveal that the SISO capacity is a symmetrical function of the dimming target, and is strictly increasing at its left half. Tight MISO upper bounds are obtained by solving non-convex problems, and alternating optimization method is employed with constraint transformations. [ABSTRACT FROM AUTHOR]

Modern vehicle-based distributed storage networks need to provide efficient data recovery services due to the high mobility nature of storage vehicles. Compared with the simple replication scheme, erasure codes have been increasingly deployed in practical systems to achieve better storage performance. Specifically, fractional repetition (FR) codes are a class of repair efficient erasure codes characterized by the features of uncoded exact repair and minimum repair bandwidth. In this paper, we consider the capacity of FR codes, which is the maximum amount of information that can be obtained by any user contacting $k$ out of $n$ storage nodes in the code based system. We present several families of FR codes that achieve a tight upper bound on the FR capacity, which are derived from combinatorial designs. The proposed constructions extend the parameter values of capacity-achieving FR codes to larger sets, and the resulting codes are of practical interest for vehicle-based distributed storage systems. [ABSTRACT FROM AUTHOR]

The channel feedback overhead for massive multiple-input multiple-output systems with a large number of base station (BS) antennas is very high since the number of feedback bits of traditional codebooks scales linearly with the number of BS antennas. To reduce the feedback overhead, an effective codebook based on channel statistics has been designed, where the required number of feedback bits only scales linearly with the rank of the channel correlation matrix. However, this attractive conclusion was only proved under a particular channel assumption in the literature. To provide a rigorous theoretical proof under a general channel assumption, in this paper, we quantitatively analyze the performance of the channel-statistics-based codebook. Specifically, we first introduce the rate gap between the ideal case of perfect channel state information at the transmitter and the practical case of limited channel feedback, where we find that the rate gap depends on the quantization error of the codebook. Then, we derive an upper bound of the quantization error, based on which we prove that the required number of feedback bits to ensure a constant rate gap only scales linearly with the rank of the channel correlation matrix. Finally, numerical results are provided to verify this conclusion. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the capacity and delay in mobile ad hoc networks (MANETs), considering the correlation of node mobility (correlated mobility). Previous studies on correlated mobility investigated the maximum capacity with the corresponding delay in several subcases; the problem of optimal capacity under various delay constraints (the optimal capacity–delay tradeoff) still remains open. To this end, we deeply explore the characteristics of correlated mobility and figure out the fundamental relationships between the network performance and the scheduling parameters. Based on that, we establish the overall upper bound of the capacity–delay tradeoff in all the subcases of correlated mobility. Then, we try to obtain the achievable lower bound by identifying the optimal scheduling parameters on certain constraints. Results demonstrate the whole picture of how the correlation of node mobility impacts the capacity, the delay, and the corresponding tradeoff between them. [ABSTRACT FROM PUBLISHER]

In this paper, an emerging wireless communication concept, which is termed as spatial modulation (SM) for large-scale multiple-input–multiple-output (MIMO), is considered. The results show that from the information-theoretic perspective, SM achieves capacity comparable to the open-loop MIMO capacity, although a subset of transmit antennas is activated in every channel use because both the channel coefficients and the input symbols carry information in SM. As a result, SM compensates the loss of information capacity due to a subset of antennas being activated by modulating information in the antenna index; therefore, the total information rate remains high. In particular, an upper bound for the capacity of SM is derived in closed form, and it is shown analytically that this upper bound is almost certainly achievable in the massive MIMO regime. Moreover, it is shown that the upper bound is achievable with no channel state information at the transmitter (CSIT) but with channel distribution information (CDI) at the transmitter (CDIT). The optimum transmission strategy should adapt the channel input distribution to fading using CDI such as the $K$ factor in Rician fading or the shape parameter $m$ in Nakagami- $m$ fading. [ABSTRACT FROM PUBLISHER]

In this paper, we consider secure communications in a multi-cell downlink multi-antenna system, where each multi-antenna base station (BS) sends confidential messages to one intended legitimate user (LU) with a passive multi-antenna eavesdropper trying to intercept the messages in this cell. To enhance the secure communications of the cell-edge users, the multiple BSs employ coordinated beamforming with the aid of artificial-noise (AN) beamforming. We conduct a mathematically rigorous secrecy performance analysis of the considered system and obtain new accurate closed-form expressions of a lower bound on the ergodic rate of each LU and an upper bound of the ergodic rate of each eavesdropper without assuming asymptotes for any system parameter (except for the “worst-case” scenario with zero noise power). Then, a lower bound on the ergodic secrecy rate (ESR) of each LU follows. Based on the derived analytical results, we propose a low-complexity algorithm to optimize the channel sate information (CSI) feedback bits allocation for the channels of target signal link and inter-cell interference links. Furthermore, we develop explicit sufficient conditions on the set of system parameters under which there are at least two BSs to coordinated secure transmission. Through both theoretical analysis and numerical results, we can verify that the obtained analytical lower bound on ESR of each LU as a function of the set of power allocation coefficients is generally neither convex nor concave. Thus, we propose to employ some numerical method to find a sub-optimal power allocation solution. Finally, numerical results are also provided to validate our theoretical results and the proposed feedback bits and power allocation methods. [ABSTRACT FROM AUTHOR]

This paper proposes a new location-based multicasting technique, for dual phase amplify-and-forward (AF) large networks, aiming to improve the security in the presence of non-colluding passive eavesdroppers. These eavesdroppers could also be part of this cooperative network as relays. In order to reduce the impact of these eavesdroppers on the network security, we propose a new transmission strategy where, for the first hop of each transmission time, while the destination is jamming, the source randomly chooses a different subset $K$ of the total $T$ relays, to transmit its message toward the destination. For practical implementation, sectoral transmission can be achieved with analog beamforming at the source's side. In the second hop, using the distributed beamforming technique, the $K$ AF relays retransmit the received signal to the destination. We analytically demonstrated that the proposed technique decreases the probability of choosing the same sector that has certain eavesdroppers again, for each transmission time, to $K/T$. Moreover, we also show that the secrecy capacity scaling of our technique is still the same as for broadcasting. Hereafter, the lower and upper bounds of the secrecy outage probability are calculated, and it is shown that the security performance is remarkably enhanced, compared to conventional multicasting technique. [ABSTRACT FROM AUTHOR]

Batched sparse (BATS) codes are a promising technique for reliable data transmission in multihop wireless networks. A BATS code consists of an outer code and an inner code, the design of which can be separated. In this paper, we investigate the design of the inner code and focus on maximizing the expected batch transfer matrix rank normalized by the total number of packets transmitted by both the source and intermediate nodes. By allowing different recoding decisions at different nodes, the problem is formulated as a nonlinear integer programming (NLIP) problem, which is non-deterministic polynomial (NP)-hard in general. We derive a noniterative formula to simplify the evaluation of the expected batch transfer matrix rank. By considering large finite fields and relaxing the integer constraints, we obtain an upper bound on the optimal value of the original maximization NLIP problem. We also derive a centralized and a decentralized algorithm to find suboptimal solutions of the NLIP problem, with different tradeoffs between complexity and performance. Numerical results show that our proposed inner codes outperform the standard BATS, and achieve very close to the upper bound, which verifies the near optimality of our algorithms. [ABSTRACT FROM AUTHOR]

In this paper, we propose a generalized channel decoding scheme for nonbinary physical-layer network coding (CD-NC) in two-way relay channels (TWRCs), where two source nodes A and B exchange their nonbinary symbols via a relay. The two sources use the same nonbinary low-density parity-check (LDPC) channel code over the integer ring $\mathbb {Z}_M$ and $M$ -pulse-amplitude modulation, respectively. The existing channel decoding schemes for nonbinary network coding suffer severe rate loss compared with the cut-set bound of TWRC, especially in the low-to-medium signal-to-noise ratio regime. The proposed CD-NC can decrease the rate loss. Our contributions are as follows: 1) We develop a generalized nonbinary sum product algorithm (G-SPA) for CD-NC according to the principle of virtual encoding of the superimposed symbols. Simulation results show that our CD-NC can achieve significant performance gains over the conventional nonbinary network coding for both additive white Gaussian noise and fading channels; and 2) We exploit two-dimensional fast-Fourier-transform-based belief propagation (2-D-FFT-BP) and extended min-sum (EMS) decoding algorithms to reduce the decoding complexity of G-SPA. Simulation results show that the 2-D-FFT-BP has the same performance as G-SPA, while EMS can greatly reduce the decoding complexity of G-SPA at the cost of slight performance degradation. [ABSTRACT FROM AUTHOR]

In this paper, multidimensional constellation designs of spatial modulated sparse code multiple access (SM-SCMA) systems with arbitrary number of transmit antennas are investigated to improve the system performance. Firstly, the symbol error rate (SER) performance of single-user and multi-user systems under flat independent Rayleigh fading channels is analyzed. It is shown that except for the product distance of the sub-constellation of each antenna, the product of the norm of symbols on each-dimension can affect single-user performance. Then, based on the performance analysis, a symmetric multidimensional constellation (SMC) and a golden-angle-modulation-based constellation (GAMC) are proposed for the general case where the number of transmit antennas is an arbitrary integer, and a low-complexity SMC (LSMC) is proposed for the special case where the number of transmit antennas is a power of two. Finally, the three proposed design schemes are extended to transmit correlated scenarios. Simulation results show that for uplink systems, compared with existing schemes, SMC provides the best SER performance and has no limitation on the number of transmit antennas. When the transmitter has no spatial correlation and the number of transmit antennas is a power of two, LSMC can achieve comparable performance to SMC with reduced computational complexity. For downlink systems, GAMC can obtain a considerable performance gain (up to 3 dB) over other solutions under transmit correlated channels. [ABSTRACT FROM AUTHOR]

In this paper, we establish an analytical framework to derive a closed form for a close approximation of the optimal angle of transform in fractional-Fourier-transform orthogonal-frequency-division-multiplexing (FrFT-OFDM) systems. In this framework, we derive a tight upper bound for the intercarrier interference (ICI) power and use this bound instead of the exact ICI power to obtain the near-optimal angle of transform. We also analyze the sensitivity of the FrFT-OFDM systems to the angle of transform. In other words, we derive the increase in the ICI power when we exploit a nonoptimal angle of transform due to channel estimation errors. We show that once the error in the channel state information (CSI) is reasonable, the ICI power increase is tolerable. For instance, for a two-path channel, if the error in estimating the slope of the channel spread function in the delay–Doppler plane is less than 20%, the ICI power increase is smaller than 0.1 dB. [ABSTRACT FROM AUTHOR]

This paper proposes a relay selection (RS) scheme that aims to improve the end-to-end symbol error rate (SER) performance of a two-way relay network (TWRN). The TWRN consists of two single-antenna sources and multiple relays employing a decode-and-forward (DF) protocol. It is shown that the SER performance is determined by the minimum decision distance (DD) observed in the TWRN. However, the minimum DD is likely to be made arbitrarily small by channel fading. To tackle this problem, a phase rotation (PR)-aided RS scheme is proposed to enlarge the minimum DD, which, in turn, improves the SER performance. The proposed PR-based scheme rotates the phases of the transmitted symbols of one source and of the selected relay according to the channel state information (CSI), aiming for increasing all DDs to be above a desired bound. The lower bound is further optimized by using a MaxMin-RS criterion associated with the channel gains. It is demonstrated that the PR-aided MaxMin-RS approach achieves full diversity gain and improved array gain. Furthermore, compared with the existing DF-based schemes, the proposed scheme allows more flexible relay antenna configurations. [ABSTRACT FROM AUTHOR]

This paper considers a two-cell uplink cochannel multiple-input–multiple-output (MIMO) network with users sequentially arriving at the network. We study the problem of sequential base-station (BS) selection for the users, with the selection criterion based on the degrees of freedom (DoFs) available for the new arriving user. We find that different sequential BS selections affect individual and network performance in terms of the individual and network sum DoF, as well as the number of admissible users in the network. We propose a method to build the tree structure for sequential BS selection, which carries trellis information for individual and system-wide selections. The properties of the tree are analytically studied. It turns out that by adopting an interference coordination strategy based on the concept of interference alignment (IA), better individual and network performance can be achieved. Simulation compares the proposed DoF-based BS selection and traditional BS selection schemes and highlights the advantages of the proposed scheme. [ABSTRACT FROM PUBLISHER]

Using a spatial multiplexing transmission scheme can improve the data rate in multiple-input–multiple-output (MIMO) relaying systems, while making signal detection more difficult at receivers. Aiming at lowering the computational complexity of the receiver, this paper proposes a maximum likelihood combining (MLC) algorithm for spatial multiplexing MIMO amplify-and-forward (AF) relaying systems in a Rayleigh flat-fading environment, which is implemented before maximum likelihood (ML) detection. The combining signal and equivalent channel are opportunely designed based on the ML rule in the MLC algorithm. We also formulate the diversity gain of the systems that employ the MLC algorithm mathematically, induced by the Chernoff bound of pairwise error probability (PEP). An upper bound on the symbol error probability (SEP) for the MLC algorithm with multiple modulations is also given, based on the derived bound of PEP. Moreover, the complexities of ML receivers adopting the MLC algorithm and the conventional vector combining (VC) algorithm are analyzed. Numerical simulations indicate that systems with the MLC algorithm achieve the same performance while consuming lower computational complexity compared to that with the VC algorithm. [ABSTRACT FROM PUBLISHER]

This paper presents a robust predictive control scheme with a graphic-based delay boundary analysis to mitigate the electric vehicle (EV) drivetrain oscillating issues, subject to the multi-channel compounding-construction loop delays. The application of Controller Area Network (CAN) in autonomous electric vehicles (AEVs) inevitably induces multi-channel compounding-construction loop delays into the control loop. The in-deep analyzing and understanding of the network-induced loop delays is critical for the electrified powertrain and its motion control. This study aims to guarantee, explicitly, the motion stability of AEV drivetrains as safe-critical and hard real-time applications. Firstly, a graphic-based constructional representation approach is presented for modeling of the compounding-construction loop delays. To resolve the upper bound of the compounding-construction loop delays further, a mathematic expression of delay boundary-envelopment analysis is derived. Secondly, based on the reasonable upper bound, Taylor series expansion is applied to make the system model with nonlinear uncertainties caused by the network-induced loop delays represent in the form of the convex polytope. Then, with the convex polytope of the drivetrain system model, a robust model predictive control (RMPC) approach is developed to enhance the system robustness against the unexpected network-induced delays. To attenuate the online calculation burden, a scheme combining off-line design and on-line synthesis is provided. Finally, the satisfactory motion control performance in both the co-simulations (Matlab&Carsim) and bench experimental tests can strongly verify the effectiveness of the proposed approaches. [ABSTRACT FROM AUTHOR]

In this paper, the adaptive fault compensation problem is investigated for high-speed trains in the presence of time-varying system parameters, disturbances, and actuator failures. To deal with the time-varying system parameters, a new time-varying indicator function instead of commonly used 0–1 function, is proposed to model the train dynamics as a piecewise model with unparameterizable time-varying disturbances, which can cover more time variations and help parametrization for adaptation. A backstepping adaptive controller is designed for the healthy system with unknown piecewise model parameters and known piecewise bounds on disturbances. For both the parameterizable and unparameterizable failures, the backstepping adaptive failure compensation with the adaptive laws are derived to achieve the position tracking under the known bound disturbances. The adaptive failure compensation for unknown bounds on disturbances is also discussed under the parameterizable failure. Through introducing the nonlinear damping in the proposed controller, the failure compensation controller is proposed for the model with unparameterizable system parameters to achieve an arbitrary degree of position tracking accuracy. The stability of the corresponding closed-loop system and asymptotic state tracking are proved via Lyapunov direct method, and validated using a high-speed train model. [ABSTRACT FROM AUTHOR]

In this paper, we address the received-signal-strength (RSS) based source localization problem in mixed line-of-sight/non-line-of-sight (LOS/NLOS) environments, where the model parameters are unknown. To accommodate the additional path losses incurred by NLOS signal propagations, we introduce a random variable to the outdoor RSS measurement model to represent the additional path loss. Based on this modified model, we propose a robust localization method for the case of unknown model parameters. Specifically, we introduce a balancing parameter and express the additional path loss term as the sum of the balancing parameter and an error term, and then formulate a robust weighted least squares (RWLS) problem to jointly estimate the source location, the unknown model parameters, and the balancing parameter. The RWLS problem is solved in an iterative manner, where the S-Lemma and the semidefinite relaxation technique are used. Both simulated and real experimental data verify that the proposed method works well in both dense and sparse NLOS environments. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) and millimeter-wave (mmWave) communication are two promising techniques to increase the system capacity in the fifth-generation (5G) mobile network. The former can achieve high spectral efficiency by modulating the information in power domain and the latter can provide extremely large spectrum resources. Fluctuating two-ray (FTR) channel model has already been proved to accurately agree with the small-scale fading effects in mmWave communications in experiments. In this paper, the performance of NOMA-based communications over FTR channels in mmWave communication systems is investigated in terms of outage probability (OP) and ergodic capacity (EC). Specifically, we consider the scenario that one base station (BS) transmits signals to two users simultaneously under NOMA scheme. The BS and users are all equipped with a single antenna. Two power allocation strategies are considered: the first one is a general (fixed) power allocation scheme under which we derive the OP and EC of NOMA users in closed form; the other one is an optimal power allocation scheme that can achieve the maximum sum rate for the whole system. Under the second scheme, not only the closed-form OP and EC but also the upper and lower bounds of EC are derived. Furthermore, we also derive the asymptotic expression for the OP in high average SNR region to investigate the diversity order under these two schemes. Finally, we show the correctness and accuracy of our derived expressions by Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

While capacities of discrete memoryless channels are well studied, it is still not possible to obtain a closed-form expression for the capacity of an arbitrary discrete memoryless channel (DMC). In this paper, we study a class of DMCs whose channel matrix is an invertible positive matrix. This class of channel matrices can be used to model many real-world settings. Next, an elementary technique based on Karush-Kuhn-Tucker (KKT) conditions is used to obtain (1) a good upper bound of a discrete memoryless channel having an invertible positive channel matrix and (2) a closed-form expression for the capacity if the channel matrix satisfies certain conditions related to its singular value and its Gershgorin's disk. [ABSTRACT FROM AUTHOR]

Recently, a new mutual information preserving quantize-and-forward (MIP-QF) is proposed for the half-duplex relay channel, where the relay node does not need to decode the source signal correctly. In this paper, a new MIP-QF upper bound on the achievable rate of the half-duplex relay channel is proposed, which is shown to be lying between the compress-and-forward (CF) bound and the cut-set bound. For the half-duplex additive white Gaussian noise (AWGN) relay channel, the specific MIP-QF bound and the optimal duplex ratios of the MIP-QF upper bound and the MMSE-CF bound are also derived. In addition, we also show that the decode-and-forward (DF) bound can exceed the MIP-QF bound in some special cases that the relay is close to the source and far away from the destination. The validity and accuracy of the proposed theoretical results are verified by numerical results under the half-duplex AWGN relay channel. [ABSTRACT FROM AUTHOR]

In this paper, analytical expressions for the channel capacity, channel cutoff rate, and achievable bandwidth efficiency associated with $M$-ary signaling and Joint Fading Two-path Shadowing (JFTS) fading/shadowing channel model that characterize mobility-constrained indoor propagation scenarios are derived. Expressions for the bandwidth efficiency of a JFTS fading channel involve infinite-series summation since the probability density function of the JFTS signal-to-noise ratio (SNR) contains infinite series. We derive lower and upper bounds on the errors resulting from truncating the infinite series in the bandwidth efficiency expression. The cutoff rate and bandwidth efficiency analysis can be used to demonstrate how much throughput can be practically achieved when $M$-ary signaling techniques are implemented over indoor composite fading/shadowing channel conditions. Simulation results show that in order to achieve 3 bits/symbol, the average SNR for a 16-quadrature-amplitude-modulation (16-QAM) system requires almost 10 dB more for a JFTS channel model compared with a traditional Rayleigh channel model, thus reinforcing the necessity of using proper channel models for indoor communication systems. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the secure relay beamforming problem for simultaneous wireless information and power transfer (SWIPT) in an amplify-and-forward (AF) two-way relay network. We consider scenarios that the eavesdropper's channel state information (CSI) is and is not available. When the eavesdropper's CSI is available, our objective is to maximize the achievable secrecy sum rate under transmit power constraint and energy harvesting constraint. Since the optimization problem is nonconvex, we derive its performance upper bound, which requires 2-D search, where a semidefinite programming is solved in each step. We also propose an upper bound-based rank-one solution by employing the Gaussian randomization method. To reduce computational complexity, we transform the optimization problem into a difference-of-convex programming and propose a sequential parametric convex approximation (SPCA)-based iterative algorithm to find a locally optimal solution. Furthermore, we also propose a zero-forcing (ZF)-based suboptimal solution. Simulation results demonstrate that the upper bound-based rank-one solution archives the performance almost the same as the upper bound that has high computational complexity. The low-complexity SPCA-based locally optimal solution performs close to the upper bound. The ZF-based suboptimal solution has the lowest computational complexity among the proposed solutions. When the eavesdropper's CSI is not available, we propose an artificial noise-aided secure relay beamforming scheme. [ABSTRACT FROM AUTHOR]

In this paper, the problem of joint data-rate allocation and mobile terminal (MT) assignment is investigated in a heterogeneous wireless network (HetNet) environment that consists of wireless local area network (WLAN) access points (APs) and cellular base stations (BSs) for a best-effort service. MTs are equipped with multiple radio interfaces and have multihoming capabilities. As a result, MTs can connect simultaneously to more than one wireless network (e.g., cellular network BS and WLAN AP) and aggregate the offered bandwidths from these networks to support applications with high required data rates. Unlike the existing research, to account for the MT's limited number of radio interfaces and the abundant wireless network options, the joint MT assignment and data-rate allocation problem is formulated to select the optimal subset of networks for each MT and allocate the optimal data rate share from this subset to maximize the HetNet total utility. The problem is formulated as a mixed-integer nonlinear program (MINLP), and due to its intractability and computational complexity, we transform the problem into a convex optimization problem via a binary variable relaxation approach. Based on the mathematical analysis of the problem, we present a heuristic algorithm for joint MT assignment and data-rate allocation. Numerical results demonstrate that the proposed solution achieves a near-optimal MT assignment and data-rate allocation at reduced computational complexity. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate power-allocation and relay-selection algorithms for amplify-and-forward (AF) cooperative relay systems under imperfect channel estimation. Based on maximizing a tight capacity lower bound, a closed-form formula for optimal power allocation under a given relay is first derived and then used to establish an optimal relay-selection algorithm. By exploring the asymptotic behavior of the capacity lower bound, a suboptimal but reduced-complexity relay-selection algorithm is further presented. Simulation results demonstrate that significant capacity gains can be achieved by using the proposed optimal and suboptimal algorithms. [ABSTRACT FROM PUBLISHER]

Place recognition and user positioning methods are becoming trending topics among the research community thanks to the need of providing cutting-edge solutions to enable innovative and efficient Location-Based Services (LBS) and applications. However, the mobile framework suffers from typical constraints on energy and computational capabilities due to the inherent characteristics of wireless mobile devices. For this reason, in order to be suitable for deploying in such framework, a place recognition algorithm should be thrifty in terms of resource consumption. In this paper we consider six different WiFi-based outdoor positioning approaches and we analytically derive an upper-bound for their computational burden in terms of number of FLoating-point OPerations (FLOPs). [ABSTRACT FROM AUTHOR]

This paper investigates pilot design and allocation in multi-cell massive multiple-input multiple-output systems with two-tier receiving. As the length of pilot is limited, the pilots cannot be orthogonal and cause interference. The sum rate as well as the signal-to-interference-plus-noise ratio (SINR) of the system are firstly analyzed. The sum rate upper bound and the asymptotic SINR are derived. The analysis shows that the pilot correlation and the spatial correlation jointly affect the sum rate. Based on this observation, the discrete Fourier transform vectors are employed in designing pilots with various correlation values. Then, with the channel correlation information and the designed pilots, the pilots are allocated to the cells in a greedy way with the object of maximizing the sum rate upper bound. Moreover, the sum rate analysis, the pilot design, and the pilot allocation are extended to the scenario with grouped correlations. Simulation results show that the proposed pilot design and allocation performs better than other pilots and the derived upper bound is effective. [ABSTRACT FROM AUTHOR]

This article establishes an analytical framework for the upper bound on the average Age-of-Information (AoI) in first-come-first-served (FCFS) wireless communications where a certain level of outage probability is unavoidable. To begin with, we analyze the average AoI and derive a general upper bound for G/G/1 systems with a certain outage probability. Subsequently, for an M/M/1 system with the FCFS scheme, we obtain a concise closed-form expression of the upper bound, and further refine the upper bound after analyzing the relative error. Interestingly, it is found by the analysis that the relative error is independent of the service rate, and the upper bound becomes tighter as the outage probability increases. Based on the refined upper bound, we minimize the average AoI for the communications suffering from block Rayleigh fading. We derive a closed-form expression of the outage probability over a fading channel, and then prove that the refined upper bound is a convex function with respect to the average update generating rate. Consequently, we optimize the AoI performance by solving a convex optimization problem formulated utilizing the refined upper bound expression. The numerical results indicate that the minimum average AoI can be reduced by either increasing the service rate or the transmission power. [ABSTRACT FROM AUTHOR]

In this paper, we study completely irrepressible (CI) sequences. For a slot-asynchronous communication system supporting K users with such sequences, a key feature is that each user is guaranteed to be able to send out at least one contention-free packet in one common sequence period. This is a desirable property since it provides a bounded delay guarantee for medium access control (MAC) layer contention, in contrast to random access schemes. Generalizing previous studies on CI sequences, we investigate systems endowed with multiple-packet reception (MPR) capability \gamma, \mbox2 \leq \gamma < K. That is, a packet transmission is successful if and only if the total number of transmissions in the channel at any point in time during its transmission is less than or equal to \gamma. We investigate the minimum period L of CI sequences for MPR as L is a fundamental factor that affects the worst-case delay. The main result is that L is asymptotically upper bounded by when \gamma \geq \mbox{3}. For \gamma = \mbox{2}, the corresponding bound is \mbox2K^2. In contrast, the bound for the single-packet reception system (\gamma=\mbox1) is \mbox4K^2. Simulation results verify our analysis and present comparative studies between CI sequences and random access in an application of group-based detection in a wireless sensor network. [ABSTRACT FROM AUTHOR]

Interference alignment (IA) is a promising technique that can eliminate interference in wireless networks effectively and has been applied to spectrum sharing in cognitive radio (CR) networks. However, most existing IA schemes neglect the quality of the desired signal, which may lead to poor performance, particularly at poor channel status. In this paper, we analyze the problem of the decrease in the signal-to-interference-plus-noise ratio (SINR) of the desired signal and propose a novel IA scheme based on antenna selection (AS) to improve the received SINR of each user in IA-based CR networks. In the proposed scheme, multiple antennas are equipped at each secondary receiver, and some of them are chosen to achieve optimal performance. Furthermore, the condition of imperfect channel state information (CSI) is also considered, which can impact the performance of IA-AS. To face this problem, a scheme called CSI filtering is proposed to weaken the influence of the imperfect CSI. Moreover, considering the considerable computational complexity brought by the selection among mass of antenna combinations, an efficient IA-AS algorithm based on discrete stochastic optimization (DSO) is thus proposed, which can converge quickly to the optimum with low computational complexity. To further improve the tracking performance of the algorithm under a time-varying channel environment, we propose an adaptive DSO scheme with window CSI filtering for IA-AS to give the algorithm a good tracking capability. Simulation results are presented to show that the proposed schemes can significantly improve the performance of IA-based CR networks. [ABSTRACT FROM AUTHOR]

Selection combining (SC) with full channel state information (CSI) achieves full diversity order in a decode-and-forward (DF) relaying system. However, the estimation of CSI of all the links is a cumbersome process. As an alternative, switched-diversity techniques have been proposed in the literature, in which CSI of all the links are not simultaneously required. In this paper, we compare the variants of the switched-diversity and full-CSI SC schemes through a paired error approach for a multi-DF-relaying-based cooperative diversity system. We derive the exact symbol error probability (SEP) expressions for the full-CSI SC, switch-and-examine combining (SEC), and SEC with post-selection (SECps) schemes, considering $M$-ary phase-shift keying (MPSK) signaling over slow, flat, independent, and nonidentical Nakagami- $m$ fading channels. It is to be noted that the derived SEP expressions are valid for integer values of fading severity parameter $m$. In addition, we also present the closed-form asymptotic SEP expressions for MPSK signaling to evaluate the diversity order. Concisely, from the numerical results, it is observed that the SECps scheme is able to offer the same performance as that of the full-CSI SC scheme. Furthermore, the SECps scheme outperforms the SEC scheme with more CSI. [ABSTRACT FROM AUTHOR]

Large intelligent surface (LIS)-assisted wireless communications have drawn attention worldwide. With the use of low-cost LIS on building walls, signals can be reflected by the LIS and sent out along desired directions by controlling its phases, thereby providing supplementary links for wireless communication systems. In this paper, we evaluate the performance of an LIS-assisted large-scale antenna system by formulating a tight upper bound of the ergodic spectral efficiency and investigate the effect of the phase shifts on the ergodic spectral efficiency in different propagation scenarios. In particular, we propose an optimal phase shift design based on the upper bound of the ergodic spectral efficiency and statistical channel state information. Furthermore, we derive the requirement on the quantization bits of the LIS to promise an acceptable spectral efficiency degradation. Numerical results show that using the proposed phase shift design can achieve the maximum ergodic spectral efficiency, and a 2-bit quantizer is sufficient to ensure spectral efficiency degradation of no more than 1 bit/s/Hz. [ABSTRACT FROM AUTHOR]

In this paper, the time-of-arrival-based localization problem under mixed line-of-sight/non-line-of-sight (LOS/NLOS) conditions is addressed. Previous studies show that existing robust methods perform well in dense NLOS environments, but generally perform badly in sparse NLOS environments. To alleviate this problem, we introduce a “balancing parameter” related to the NLOS errors and formulate a new robust weighted least squares (RWLS) problem with the source position and the NLOS balancing parameter as the estimation variables. The proposed method does not require the statistics of NLOS errors and the path status. By leveraging the S-Lemma, the RWLS problem is transformed into a non-convex optimization problem, which is then relaxed into a convex semidefinite program. Simulation results show that the proposed method works well for both the sparse and dense NLOS environments. [ABSTRACT FROM AUTHOR]

In this paper, we propose the proactive and the reactive cumulative distribution function (cdf)-based relay selection schemes with a control parameter (CP) in a dual-hop wireless network to easily control the fairness among relays such as the probability of selecting each relay and achieve full diversity. We analyze the probability of selecting a relay and the average fairness among relays for the proposed schemes. The upper bound of outage probability is derived in closed form for the proactive cdf-based relay selection (CRS) scheme. Exact outage probability is derived in closed form for the reactive CRS scheme. Moreover, we derive the asymptotic outage probabilities at a high-signal-to-noise-ratio region. Numerical results show that the analytical results of the proposed CRS schemes match the simulation results well. It is shown that both the proactive and reactive CRS schemes achieve the same diversity as the channel-gain-based relay selection (CGRS) scheme, which is outage optimal. It is shown that when all CPs are equally set for all relays, the proposed CRS schemes guarantee strict fairness among relays. It is shown that when CPs are set based on the channel condition, the outage probability of the proposed CRS schemes is close to that of the CGRS scheme. [ABSTRACT FROM PUBLISHER]

In this paper, the problem of reducing the peak-to-average power ratio (PAPR) of a discrete Fourier transform spread orthogonal frequency-division multiplexing signal is considered for the transmission of BPSK symbols. In particular, a joint optimization of the constellation-rotation angle and the pulse-shaping vector is conducted with or without the allocation of additional sub-carriers. To avoid the exponential complexity required to evaluate the PAPR, a computationally efficient upper bound on the worst-case PAPR is derived and adopted as the objective function. Unlike conventional upper bounds, the proposed upper bound exploits the structure in the relative phase among symbol waveforms and incorporates the constellation-rotation effect. The signal-to-interference-plus-noise ratio (SINR) at the output of a constellation-derotating matched filter is also derived along with a generalized Nyquist criterion and adopted as the inequality constraint function. Then, joint optimizations are conducted through numerical search for various signal parameters and target SINRs. Surprisingly, the optimal rotation angle is neither 0 nor $\pi /2$. It is shown that the proposed combinations significantly outperform the existing combinations of the rotation angle and the shaping vector when the signal is amplified by a nonlinear power amplifier. [ABSTRACT FROM AUTHOR]

For an uplink sparse code multiple access (SCMA) system, a codebook-domain superposition transmission can be applied to increase the multiplexing gain by efficient utilization of the codebooks as multiple access resources. However, the destructive collision of superposed codewords can restrict the performance improvement of the system because of the increasing interlayer interferences. In this paper, we analyze the effects of the interlayer interferences on the bit error rates, and optimize the SCMA codebooks that can mitigate the interlayer interferences. Simulation results show that the optimized codebooks can provide a noticeable gain to improve the performance of uplink SCMA systems applied with superposition transmission. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA) is a codebook-based nonorthogonal multiple access that exploits the coding gain of designed codebooks and the near-optimum performance of belief-propagation (BP) detection. In this paper, we study an uplink SCMA system equipped with multiple receive antennas utilized to get a diversity gain. Assuming that the maximum likelihood detection is applied to the system, we derive the upper bound on the average bit error rate (ABER) and analyze the diversity order which is shown to be the product of the number of receive antennas and the spreading gain. Exploiting the multiple received signals, we also propose a simplified detection method called Gaussian approximation based BP (GA-BP) detection with linear prefiltering. As the first step of the proposed detection, a conventional linear filter is applied to mitigate the multiuser interferences. Then, the GA-BP iteratively removes the residual interferences which may be approximated to a Gaussian random variable for the massive number of user equipments. Numerical results show that for moderate and high signal to noise ratio region the analyzed upper bounds on ABER of SCMA system tightly coincide with the simulation results. Also, for the moderate numbers of antennas, the ABER of GA-BP with matched filter closely approaches to that of the conventional BP detection with much lower computational costs. [ABSTRACT FROM AUTHOR]

Dense wireless local area networks (WLANs) have emerged as a promising design paradigm recently. While partially overlapped channels (POCs) have proved to be able to improve network capacity significantly in the dense network, they are usually considered the fixed number of access points (APs) in traditional research works. The network capacity can be scaled by providing additional APs in a given area since the expected distance from users to the associated APs becomes shorter. On the other hand, the additional increment of network capacity by means of deploying more APs is limited. It can be accredited to the substantial interference among the high number of deployed APs assigned with POCs. Furthermore, the impact of the POC model makes the design more complex. To cope with these challenges in this work, we study the problem of interaction between density of APs and POC assignment with parameter tuning and propose the approach called DAPA, in which we derive that the optimal density of APs lies in the feasible region consisting of the lower bound and upper bound density. Thus, the solution can be obtained by searching for the feasible region by means of the proposed POC assignment. Through analysis and numerical results in DAPA, we provide recommendations on the optimal dimension of high density and POC assignment, with consideration of the network configuration. [ABSTRACT FROM AUTHOR]