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Frequency diverse arrays (FDAs) provide range-angle time-invariant transmit–receive beamforming, although a fine focus of their radiation patterns is costly since high-resolution mixers and phase shifters are required. An innovative approach to FDAs is proposed in which mixers and phase shifters are replaced by the application of periodic linear-phase time-modulating waveforms to the antenna excitations implemented with 2-bit variable phase shifters controlled with periodic on–off instants of binary sequences, which leads to a simple and precise method for FDA beam focusing. [ABSTRACT FROM AUTHOR]

Frequency diverse arrays (FDAs) allow the synthesis of dot-shaped radiated patterns in the range-angle space by applying frequency offsets to the antenna elements. Conventional FDAs are usually implemented with costly variable phase shifters and mixers and require an independent feeding network per beam for multibeam operation. In this letter, we propose an innovative approach to FDA that makes use of time-modulated array (TMA) building blocks modeled with inexpensive single-pole dual-throw switches. Our approach is able to simultaneously and independently steer two dot-shaped beam patterns, over the two most significant TMA harmonics, with an almost 90% time modulation efficiency. [ABSTRACT FROM AUTHOR]

The application of frequency offsets to the elements of a conventional phased array with variable phase shifters (VPSs) leads to frequency diverse arrays (FDAs) with the ability to provide range-angle spatial focusing. In this letter, we propose an innovative approach to FDA using switched time-modulated arrays (TMAs) instead of VPSs. The time modulation efficiency of the combined FDA-TMA approach is over 94% when using Kaiser windows to determine the frequency offsets. Such windows have the advantage of flexibly handling the tradeoff between the beam collection efficiency and the half power beamwidth without the need of difficult optimizations. [ABSTRACT FROM AUTHOR]

In this letter, sinusoidal pulsewidth modulation (PWM) is applied to the design of time-modulated arrays (TMAs). The array excitations are modulated using PWM-synthesized sine waveforms. This provides increased signal bandwidth, excellent rejection of the undesired harmonics, and modulation efficiencies aligned with the best ones in the literature. The simplicity of the technique allows for an easily implementable antenna array feeding network. [ABSTRACT FROM AUTHOR]

Time-modulated arrays (TMAs) can effectively perform beamsteering over the first positive harmonic pattern by applying progressively delayed versions of stair-step approximations of a sine waveform to the antenna excitations. In this letter, we consider synthesizing such stair-step sine approximations by means of Haar wavelets. Haar functions constitute a complete orthonormal set of rectangular waveforms, which have the ability to represent a given function with a high degree of accuracy using few constituent terms. Hence, when they are applied to the TMA synthesis, employing single-pole double-throw switches, such a feature leads to an excellent rejection level of the undesired harmonics as well as a bandwidth greater than that supported by conventional TMAs with on-off switches. [ABSTRACT FROM AUTHOR]

Doubly-selective channels, such as those that occur when the transmitter and the receiver move relative to each other at high speeds, are a key scenario for fifth generation (5G) cellular systems, which are mostly based in the use of the orthogonal frequency-division multiplexing (OFDM) modulation. In this paper, we consider an OFDM system using quadrature amplitude modulation (QAM) symbols and we show that, when transmitting over deterministic doubly-selective channels, the inter-carrier-interference (ICI) affecting a symbol can be well approximated by a complex-valued normal distribution. Based on this, we derive a lower bound for the link capacity using the Shannon-Hartley theorem. Finally, we provide an approximation of the bit error probability (BEP) using the well-known BEP expressions for Gray-coded QAM constellations over additive white Gaussian noise (AWGN) channels, and show numerical results that confirm that the proposed BEP expression approximates accurately the bit error ratio (BER) of the OFDM system for standardized channel models. The proposed closed-form analytical expressions for the capacity and the BEP do not only allow for discarding the need of computationally-costly Monte-Carlo system simulations, but also provide a theoretical framework to optimize the system parameters directly impacting on the achievable performance. [ABSTRACT FROM AUTHOR]

Presents informationon on the 27th European Signal Processing Conference in A Coruna, Spain: Looking Into the Future of Signal Processing [Conference Highlights] [ABSTRACT FROM AUTHOR]

The design of zero-delay Joint Source-Channel Coding (JSCC) schemes for the transmission of correlated information over fading Multiple Access Channels (MACs) is an interesting problem for many communication scenarios like Wireless Sensor Networks (WSNs). Among the different JSCC schemes so far proposed for this scenario, Distributed Quantizer Linear Coding (DQLC) represents an appealing solution since it is able to outperform uncoded transmissions for any correlation level at high Signal-to-Noise Ratios (SNRs) with a low computational cost. In this paper, we extend the design of DQLC-based schemes for fading MACs considering sphere decoding to make the optimal Minimum Mean Squared Error (MMSE) estimation computationally affordable for an arbitrary number of transmit users. The use of sphere decoding also allows to formulate a practical algorithm for the optimization of DQLC-based systems. Finally, non-linear Kalman Filtering for the DQLC is considered to jointly exploit the temporal and spatial correlation of the source symbols. The results of computer experiments show that the proposed DQLC scheme with the Kalman Filter decoding approach clearly outperforms uncoded transmissions for medium and high SNRs. [ABSTRACT FROM AUTHOR]

Bipolar ($\pm$ 1) sequences with no zero state suit particularly well for safeguarding the switched feeding network efficiency when applied to time-modulated arrays. During the zero state of a conventional time-modulating sequence, if a given array element is switched off, a certain amount of energy of the transmitted/received signal is wasted. We propose a novel single sideband time-modulated phased array (TMPA) architecture governed by realistic bipolar squared sequences in which the rise/fall time of the switches is considered. By using single-pole dual-throw switches and nonreconfigurable passive devices, the TMPA exploits, exclusively, the first positive harmonic pattern while exhibiting an excellent performance in terms of efficiency and control level of the undesired harmonics without using synthesis optimization algorithms (software simplicity). [ABSTRACT FROM AUTHOR]

This paper jointly addresses user scheduling and precoder/combiner design in the downlink of a wideband millimeter-wave communications system. We consider the orthogonal frequency-division multiplexing modulation to overcome the channel frequency selectivity and obtain a number of equivalent narrowband channels. Hence, the main challenge is that the analog preprocessing network is frequency flat and common to all the users at the transmitter side. Moreover, the effect of the signal bandwidth over the uniform linear array steering vectors has to be taken into account to design the hybrid precoders and combiners. The proposed algorithmic solution is based on the linear successive allocation, which greedily allocates streams to different users and computes the corresponding precoders and combiners. By taking into account the rank limitations imposed by the hardware at transmission and reception, the performance loss in terms of achievable sum rate for the hybrid approach is negligible. The numerical experiments show that the proposed method exhibits excellent performance with reasonable computational complexity. [ABSTRACT FROM AUTHOR]

We present a single sideband time-modulated multibeam phased array governed by periodic Nyquist pulsed signals. A Nyquist pulse is a physically realizable approach to the ideal sinc function. Hence, its low-pass spectrum suits particularly well for time-modulated arrays (TMAs) to perform harmonic beam steering. Contrarily to switched TMAs and standard solutions based on variable phase shifters, the performance and complexity of the proposed time modulation scheme is rather robust when increasing the number of multibeams. [ABSTRACT FROM AUTHOR]

This paper focuses on distributed linear precoding when users transmit correlated information over a fading multiple-input and multiple-output multiple access channel. The precoders are optimized in order to minimize the sum-mean square error (MSE) between the source and the estimated symbols. When sources are correlated, minimizing the sum-MSE results in a non-convex optimization problem. The precoders for an arbitrary number of users and transmit and receive antennas are thus obtained via a projected steepest-descent algorithm and a low-complexity heuristic approach. For the more restrictive case of two single-antenna users, a closed-form expression for the minimum sum-MSE precoders is derived. Moreover, for the scenario with a single receive antenna and any number of users, a solution is obtained by means of a semi-definite relaxation. Finally, we also consider precoding schemes where the precoders are decomposed into complex scalars and unit norm vectors. Simulation results show a significant improvement when source correlation is exploited at precoding, especially for low signal-to-noise ratios and when the number of receive antennas is lower than the number of transmitting nodes. [ABSTRACT FROM AUTHOR]

Time-modulated arrays (TMAs) receiving beamforming provide a noteworthy hardware simplicity given the ability of this multi-antenna technique to transform spatial diversity into frequency diversity. However, the dual behavior at transmission seems to be as simple as limited: a given signal is simultaneously transmitted over all the different TMA harmonic patterns. We investigate the efficient and simultaneous transmission of two different signals over the same physical antenna using two independent harmonic beam-patterns of the TMA. For that purpose, we propose an innovative dual-signal TMA transmitter based on two complementary operations: the complex mixing of the baseband signals and the TMA processing with quadrature and time-delayed periodic pulses. [ABSTRACT FROM AUTHOR]