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Vehicular ad hoc networks (VANETs) are able to facilitate data exchange among vehicles and provide diverse data services. The benefits of cooperative communications are such as to make it a proper idea to improve the achievable rate and diversity in VANETs. We investigate a dual-hop relay vehicular network with different transmit powers in vehicles on non-uniform shadowed double Nakagami-m fading channels in the presence of non-uniform co-channel interferers. In this paper, we investigate the diversity-multiplexing tradeoff (DMT) in such a network for three different schemes, namely, dynamic decode and forward (DDF), dynamic quantize map and forward and static quantize map and forward and prove that the DDF strategy has the optimal DMT. Also, the optimal listening and transmitting time of vehicles in various strategies as a function of the channel conditions in vehicles are determined. Finally, we extend our results to a general multi-hop relay vehicular network model with multiple half-duplex relay terminals and different average SNRs over links.

In this study, the authors address the issue of clutter mitigation in monostatic pulse-Doppler radars, using sub-Nyquist sampling and compressed sensing (CS) recovery algorithms. Generally, clutter is modelled as a coloured random process and a whitening filter is employed to maximise the signal to clutter ratio. Here, the authors apply this approach to the case in which the received signal sampling rate is much smaller than the Nyquist rate. They also generalise the recently proposed sub-Nyquist method based on difference set (DS) codes, which uses very few samples to detect targets in the presence of only white Gaussian noise. By using the frequency-coded modulation waveform with the frequencies selected in accordance to the DS, the detection performance of the proposed method can be significantly improved compared with the previously proposed sub-Nyquist methods in the presence of clutter. Furthermore, the authors show that the proposed method can increase the target recovery resolution due to the features of the DS codes and the employed CS model.

Decentralized hierarchical coded caching is studied with two layers of caches which users receive their demands through intermediate helpers from a main server. Coded caching in hierarchical model reduces rates in both layers and this scheme has been shown to be order-optimal when all contents are uniformly popular and there is no tension between the rates in each of two layers. We consider two-layer coded caching over heterogeneous wireless networks which can reduce the rates more effectively in both layers. The contents in our work are multi-level, based on the different levels of popularity. We use two different approaches in our hierarchical model: when the coded caching is provided within each layer and when it is provided across multiple layers. We consider combination of these two approaches and then optimize the proportion of using each approach to minimize the rates of the first layer and the second layer. We show that the rates in our scheme are much lower than the traditional hierarchical model. Moreover, common-memory method is introduced in two-layer network and is compared with the other schemes. It can be observed that the rates of common-memory method are lower than the previous hierarchical model but are higher than our proposed multi-level scheme.

Dirty paper coding (DPC) is a non-linear transmit scheme in presence of known interference, which is capacity achieving. Unfortunately, the implementation complexity of DPC is usually prohibitive. A sub-optimal yet practical method to implement DPC in inter-symbol interference channels is Tomlinson-Harashima precoding (THP). However, THP is not applicable to the constant-envelope phase shift keying (PSK) modulation. In this study, a modified Tomlinson-Harashima precoder is proposed to implement DPC for PSK modulation. The technique involves design of decision region at the receiver side and an algorithm to select the phase of transmitted symbol optimally at the transmitter side accordingly. Various patterns, namely, striped, checked, hexagonal, and radial are proposed and examined. Simulation results show that by using the proposed equaliser, the error floor is eliminated and significantly smaller bit-error rate is achieved. Numerical and simulation results show that the striped pattern outperforms other decision region designs. Moreover, its implementation and computational complexities is almost the same as the Tomlinson-Harashima precoder.

In this paper, we consider compressive sensing (CS)-based recovery of delays and Doppler frequencies of targets in high resolution radars. We propose a novel sub-Nyquist sampling method in the Fourier domain based on difference sets (DS), called DS-sampling, to create dictionaries with highly incoherent atoms. The coherence of the dictionary reaches the Welch minimum bound if the DS-sampling is employed. This property let us to implement sub-Nyquist high resolution radars with minimum number of samples.1 Two low-complexity recovery methods are developed and sufficient condition of target recovery with specific resolution obtained theoretically for noisy and noiseless conditions. We also propose a new waveform, called DS-frequency coded modulated waveform, to boost the recovery performance of the sub-Nyquist radar in noisy environments. The proposed method solves some of the common problems in many CS-based radars and overcome disadvantages of the conventional Nyquist processing (i.e., matched filtering) in high resolution radar systems. The proposed method allows us to design sub-Nyquist radars, which require less than $\text{2}{\%}$ of Nyquist samples and recover targets without resolution degradation in comparison to the conventional Nyquist processing.

Frequency estimation is vital for monitoring, control, and protection of power systems. Multiple signal classification (MUSIC) method has been used for frequency estimation in communication systems. This method requires wide range of frequency scanning and heavy computations, which is inappropriate for power system applications. This paper proposes an adaptive accelerated MUSIC algorithm for frequency estimation in power systems. The algorithm accelerates the frequency scanning and adapts itself to transient conditions of power systems, to keep the accuracy of the frequency estimation with minimum computations. Experimental signals besides several static and dynamic test signals are used to evaluate and compare the performance of the proposed algorithm with recent methods. These comparisons show that the proposed algorithm provides faster convergence speed, lower computation burden, and more robustness to noise and grid disturbances. These significant advantages make the proposed algorithm appropriate for power system protection and control applications.

This paper presents a new filter-based algorithm to estimate the fundamental phasor of fault currents, using the proposed complex frequency filter. The presence of decaying dc component (ddc) in fault currents causes large error in discrete Fourier transform (DFT)-based phasor estimation methods. The proposed filter discriminates the ddc from dc and sinusoidal signals. In this algorithm, first, the ddc is filtered out by the proposed filter. Then, the fundamental phasor can be precisely extracted by applying DFT. Second and fourth order of the complex frequency filter and their design method are presented. To evaluate the performance of the proposed algorithm, several test signals are used. The algorithm is also compared with recent DFT-based and mimic-DFT methods, using four performance indices. The simulation results demonstrate the robustness of the proposed algorithm against harmonics, noise, multiple ddc components, and off-nominal frequency. It also offers faster convergence speed. The proposed complex frequency filter can also be used for the phasor estimation of harmonic components in the presence of ddc with no need to readjust its parameters.

An anti-deception jamming technique is proposed for moving target indication in a pulse-Doppler (PD) radar. The deceive targets are produced by digital radio frequency memory, which tries to pull off the range and velocity gates of real targets. Similar to orthogonal frequency division multiplexing, we use different sets of orthogonal sub-carriers in consecutive coherent pulse intervals (CPIs). By changing sub-carriers in different CPIs, we show that the deceive targets appear as interference in receiving signals. The generalized likelihood ratio test is used for detection and discrimination of real targets. The performance of the proposed method is achieved analytically and by simulations. Furthermore, we implement a hardware block using a TMS6416-DSK DSP for a PD radar prototype exploiting the proposed algorithm to deception discrimination. The presented results demonstrate the good accordance with theoretical predictions.

Cyclic frequency detection of cyclostationary signals and its application in modulation parameters estimation is the aim of this paper. A simple frequency shifted (FRESH) model is structured. The normalized least mean square (NLMS) algorithm is engaged to estimate the unknown weights of the considered model. The norm of the parameter estimate is related to the model accuracy. A high (low) value indicates that the FRESH structure is based on an accurate (inaccurate) cyclic frequency and the weight estimate of the NLMS algorithm converges to a nonzero (zero) vector. Accordingly for two given values which only one of them is the true cyclic frequency of the received signal, two FRESH structures are considered and simultaneously two NLMS algorithms are executed. If the norm of the weight estimate of the first algorithm exceeds the resulted norm of the second one, according to the proposed method, the first given value is the true cyclic frequency and vice versa. The method is used to estimate the modulation parameters of three different problems: chip rate estimation of a spread spectrum signal, carrier frequency and bit rate estimation of BPSK and QPSK signals, and symbol rate estimation of OFDM signals.

The diversity-multiplexing trade-off (DMT) of half-duplex single-relay networks is computed analytically, where all the channels in the networks are assumed to be quasi-static flat-fading channels with independent Rayleigh distributions. We have computed DMT of dynamic quantize-map-and-forward (DQMF) strategy analytically, which has the optimal DMT for half-duplex single-relay networks with local channel state information. As a result, the optimal listening time of the relay in the DQMF strategy as a function of the source-relay channel realization can also be determined using our solution.

In this paper, we derive an analytical expression for the bit error probability of a multi-threshold (MTh) detector in the downlink three-user non-orthogonal multiple access (NOMA) based vehicular communication system with unknown noise. The MTh detector directly detects the signal of vehicle user with the highest channel gain without detecting the signals of weaker users and removing their effects. It is shown that the bit error probability performance of the MTh detector is superior to that of the successive interference cancellation (SIC) detector in a special case. Then, we propose a blind empirical characteristic function (ECF) based method to estimate the signal levels of vehicle users, required to implement the MTh detector, in unknown noise. Next, we derive an analytical expression for the variance of the proposed ECF-based estimator which is validated via computer simulations. It is shown that the ECF-based estimator is asymptotically unbiased and consistent. Furthermore, we obtain the normalized Cramer-Rao lower bound (NCRLB) for the estimators of signal levels which shows that the ECF-based estimator is almost efficient in small generalized signal-to-noise ratio (GSNR) values. Numerical results show that the ECF-based MTh detector outperforms the absolute median-based SIC detector in the mixture of Gaussian and α-stable noise.

Two-hop parallel networks with half-duplex relays over quasi-static Rayleigh fading channels are considered. Assuming relays have only local channel state information, optimal communication schemes for these networks in terms of the diversity multiplexing trade-off (DMT) are obtained. It is shown that the dynamic quantize-map-and-forward (DQMF) and the static quantize-map-and-forward (SQMF) communication strategies achieve the optimal DMT at different ranges of the multiplexing gain. In the DQMF scheme, a relay sets its transmitting/listening times according to the instantaneous source-relay channel condition. However, in the SQMF scheme, a relay sets its transmitting/listening times to be a constant independent of the instantaneous source-relay channel gain. Essentially, we calculate the DMT of the network under the general setting for arbitrary average signal-to-noise ratios over links and it is shown that the DMT of the proposed schemes matches the upper bounds. Moreover, the optimal transmitting/listening times of the half-duplex relays as functions of the instantaneous channel realizations are derived for different communication strategies in the general setting. Finally, various simulation scenarios are considered and the DMT of the proposed schemes and the dynamic decode and forward communication strategy are compared.

Diversity-multiplexing tradeoff (DMT) of a full-duplex diamond network where the inter-relay interference (IRI) is suppressed, global channel state information (CSI) is available in all nodes, and perfect self-interference cancellation (SIC) technique is used, which we call it the perfect full-duplex diamond network, is an upper bound for the DMT of the equivalent half-duplex diamond network, i.e., the perfect full-duplex diamond network outperforms the half-duplex case. Nevertheless, the implementation of the perfect full-duplex diamond network is a high-cost operation and requires significant hardware changes in current systems. The main question addressed in this paper is: under which condition (or perhaps conditions) does a prevalent half-duplex diamond network provide the same performance as the perfect full-duplex one? In this regard, first, we calculate the DMT of a perfect full-duplex multiple-relay and multiple-antenna (MRMA) symmetric diamond network. Then, we demonstrate that for the same network but with half-duplex relays, when the number of antennas in relays satisfies a specific condition, there exists a fixed listen-transmit schedule combined with quantize-map-and-forward (QMF) relaying to achieve the optimal DMT. Also, we attain a lower bound for the total number of antennas of the MRMA symmetric diamond network to achieve the optimal DMT.

In this letter, we propose a novel estimator for pilot-aided orthogonal frequency division multiplexing (OFDM) channels in an additive Gaussian and impulsive perturbation environment. Due to sensor failure which might happen because of man-made noise, a number of measurements in high rate communication systems is often corrupted by impulsive noise. High power impulsive noise is generally an obstacle for OFDM systems as valuable information will be completely lost. To overcome this concern, an objective function based on a penalized atomic norm minimization (PANM) is provided in order to promote the sparsity of time dispersive channels and impulsive noise. The corresponding dual problem of the PANM is then converted to tractable semidefinite programming. It has shown that one can simultaneously estimate the time dispersive channels in a continuous dictionary and the location of impulsive noise using the dual problem. Several numerical experiments are carried out to evaluate the performance of the proposed estimator.

We consider load balancing problem in a cache network consisting of storage-enabled servers forming a distributed content delivery scenario. Previously proposed load balancing solutions cannot perfectly balance out requests among servers, which is a critical issue in practical networks. Therefore, in this paper, we investigate a coded cache content placement where coded chunks of original files are stored in servers based on the files popularity distribution. In our scheme, upon each request arrival at the delivery phase, by dispatching enough coded chunks to the request origin from the nearest servers, the requested file can be decoded. Here, we show that if $n$ requests arrive randomly at $n$ servers, the proposed scheme results in the maximum load of $O(1)$ in the network. This result is shown to be valid under various assumptions for the underlying network topology. Our results should be compared to the maximum load of two baseline schemes, namely, nearest replica and power of two choices strategies, which are $\Theta(\log n)$ and $\Theta(\log \log n)$, respectively. This finding shows that using coding, results in a considerable load balancing performance improvement, without compromising communications cost performance. This is confirmed by performing extensive simulation results, in non-asymptotic regimes as well., Comment: The paper is 12 pages and contains 8 figures

In this paper, we address the problem of recovering point sources from two-dimensional low-pass measurements, which is known as the super-resolution problem. This is the fundamental concern of many applications such as electronic imaging, optics, microscopy, and line spectral estimations. We assume that the point sources are located in the square $[0,1]^2$ with unknown locations and complex amplitudes. The only available information is low-pass Fourier measurements band limited to the integer square $[-f_c,f_c]^2$ . The sources are estimated by minimizing the total variation norm, which leads to a convex optimization problem. We observe in numerical results that if the sources are closer than $1.4/f_{c}$ , they might not be recovered by the proposed optimization. We theoretically show that there exists a dual certificate which guarantees exact recovery when the sources are separated by at least $1.68/f_c$ , reducing the gap between the available theoretical guarantee for the source separations and the observed results in the simulations.

This paper studies the diversity-multiplexing trade-off (DMT) of half-duplex diamond relay channels. Diversity and multiplexing gains usually evaluate the performance of any relaying strategy, in wireless relay networks with fading channels. DMT captures the fundamental trade-off between these two gains, for communications at high signal to noise ratios (SNR). In this paper, we propose an efficient algorithm that computes the DMT of static-QMF communication strategy in half-duplex diamond relay channels. To find DMT of static QMF, one needs to solve a cumbersome optimization problem. The proposed algorithm, breaks the given optimization to finite1 number of sub-optimization problems, each one of which is a quasi-concave optimization problem. Then, using a binary search method, each sub-optimization problem is solved efficiently and the results of sub-optimization problems are combined to find DMT of the static QMF communication in the network.

We study the diversity-multiplexing trade-off (DMT) of diamond relay channels. A diamond network contains a transmitter, two relays and a receiver. DMT of a network represents a fundamental trade-off between the reliability and rate of transmission with the diversity and multiplexing gains for wireless networks with fading channels. In this paper, we compute the DMT of full-duplex diamond relay channels and find the DMT of QMF communication strategy for half-duplex diamond relay channels. Also, a new genetic algorithm is introduced to solve the DMT of static QMF strategy for half-duplex diamond relay channels. By simulation, it is shown than the running time of the genetic algorithm is much better than the running time of finding the DMT of SQMF using grid based optimization techniques. The novelty of the proposed genetic algorithm is in using creative crossover and mutation operators in the algorithm which improved the results.