Julio C. Bregáins

On the Feasibility of Time-Modulated Arrays for Digital Linear Modulations: A Theoretical Analysis

Wireless communications are widely based on linear digital modulation methods, which introduce the digital baseband information in the amplitude and/or in the phase of a carrier sinusoid. Time-modulated arrays, on the other hand, constitute an interesting technique capable of adapting the power radiation pattern of an antenna array. In this work, we study from a theoretical point of view the impact of time-modulated arrays on the transmission of linearly modulated digital communication signals. The work focuses on the requirements to safeguard the integrity of the signal and on the quantification of the useful power radiated within the desired band.

Performance Analysis of Time-Modulated Arrays for the Angle Diversity Reception of Digital Linear Modulated Signals

Diversity occurs whenever several copies of the same transmitted signal arrive at a receiver. Such a situation allows for improving the performance of a wireless communication system transmitting over a radio channel without increasing the transmit power. Previous works have shown that time-modulated arrays are capable of faithfully acquiring digital signals while exploiting angular diversity through the adaptive beamforming of their harmonic patterns. In this work, we take a step further and consider a wireless communication system employing digital linear modulated signals and an innovative receiver that includes a time-modulated array and a maximum ratio combining subsystem. The maximum ratio combiner is adapted to optimally exploit the multipath channel angular diversity. The performance of the system is analyzed in terms of two metrics: the signal-to-noise ratio and the symbol error rate. The results are compared to those achieved with other receivers that include conventional antenna arrays, exhibiting the time-modulated array solution a good tradeoff between performance and hardware complexity.

Enhanced Time-Modulated Arrays for Harmonic Beamforming

One of the primary features of time-modulated arrays (TMAs) is their ability to adapt their harmonic patterns to exploit the angular diversity of multipath wireless channels. In such a case, the TMA excitations must provide an adequate level of harmonic windowing in order to safeguard both the antenna efficiency and the signal-to-noise ratio at reception. In this work, we propose to time-modulate the array excitations with periodic sum-of-weighted-cosine (SWC) pulses rather than with the periodic rectangular pulses corresponding to the on–off switches of conventional TMAs. We show that how the larger flexibility of the SWC pulses allows for a more versatile design of the TMA harmonic radiation patterns without penalizing the array gain for each exploited harmonic.

Time-modulated arrays for Digital Communications in multipath scenarios

We study the performance of a Digital Communication System (DCS) when transmitting over a multipath wireless channel and using a Time-Modulated Array (TMA) at the receiver. We show that the TMA Sideband Radiation can be usefully exploited to extract the angle diversity from the wireless channel. Such angle diversity is used to combine the signals at the TMA output to improve the performance of the DCS in terms of Symbol Error Rate.

Time Modulated Arrays: From their Origin to Their Utilization in Wireless Communication Systems

Time-modulated arrays (TMAs) are electromagnetic systems whose radiated power pattern is controlled by the application of variable-width periodical pulses to the individual elements. The nonlinear nature of the array operation causes the appearance of radiation patterns at the harmonic frequencies of such periodic pulses. The technique can be used for improving the side-lobe level (SLL) topology of the radiation pattern at the central frequency and/or to profitably exploit the harmonic patterns in order to supply smart antenna capabilities. Among the latter features, the TMA harmonic beamforming takes on special importance due to its attractive trade-off performance-hardware complexity. From this perspective, TMAs are sensors capable of transforming the spatial diversity of a communication channel into frequency diversity, thus improving the performance of a wireless communication. In addition to a walk through the origins of the concept, and a brief analysis of the mathematical fundamentals, this paper organizes the prolific state of the art of TMAs in two major thematic blocks: (1) TMA design from an antenna perspective; and (2) TMA design from a signal processing perspective.

Time-modulated arrays with Sum of Weighted Cosine pulses

Time-Modulated Arrays (TMAs) perform angular diversity through the adaptive beamforming of their harmonic patterns with the advantage of employing a single Radio Frequency (RF) front-end. Rectangular pulses typically used in TMAs are not the best ones to efficiently distribute the spectral energy among the harmonics exploited. We propose a family of pulses, the so-called Sum of Weighted Cosines (SWC), which despite its simple form, allows for a flexible windowing of the harmonics involved while opening the door to a multimode TMA.

Impact of Time-Modulated Arrays on the BER of Linear Digital Modulations

Time modulation provides a simple way to control the power radiation pattern of an antenna array. By appropriately selecting the parameters of a Time-Modulated Array (TMA) it is possible to obtain a reconfigurable pencil beam adequate for wireless communications. This work focuses on the impact of a TMA on the Bit Error Ratio (BER) performance of a wireless communication receiver with a linear digital modulation. We show how the BER of such a receiver is affected by the TMA synthesis variables.

Time-modulated arrays for digital communications

Time-Modulated Arrays (TMAs) are an example of adaptive arrays that support radiation power pattern reconfigurability attractive for transmitting both analog and digital communication signals. While the study of the parameters specifications of TMAs for transmitting analog signals has been addressed in the literature, the detailed study of their use in digital communication systems has not been considered yet. In this work we present a mathematical analysis that establishes both the conditions that TMAs have to satisfy in order to effectively transmit linearly modulated digital signals, and the implications on the power radiated by such an array.

Frequency-Domain synthesis of time-modulated arrays

We address the design of Time-Modulated Arrays (TMAs) excitations in the frequency domain. The design is based on the particular features of the spectrum of the periodic pulses which modulate the excitations: a discrete spectrum with impulses at multiples of the time-modulation frequency, and whose corresponding areas are 2π times the associated exponential Fourier series coefficients. In this view, a simplified design of ad-hoc pulses for beamforming purposes is proposed by applying the Fourier series coefficients properties to preprocess conventional rectangular pulses before they are applied to the antenna elements.