Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> Focusing on a reflective target in an inhomogenous medium is difficult. In order to obtain good focusing properties, the concept of optical phase-conjugate mirrors, valid for monochromatic signals, is extended to large-bandwidth pulses such as those used in ultrasound echography. The transducer's linear response to the acoustic pressure allows replacement of the phase conjugation by a time-reversal operation on the pulse echo signals. The time-reversal mirror is an array of transmit-receive transducers. A first incident wave is reflected by the target. The received signals are stored in shift registers, reversed in times and then reemitted. The major advantage of this process is that the waves distorted by the aberrating medium are corrected by the mirror operation and the back propagation through the medium. When the medium contains several reflectors, this time-reversal process can be iterated in order to focus on the most-reflective one. > <s> BIB001 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> We report the first experiments showing the reversibility of transient acoustic waves through high-order multiple scattering by means of an acoustic time-reversal mirror. A point source generates a pulse which scatters through 2000 steel rods immersed in water. The time-reversed waves are found to converge to their source and recover their original wave form, despite the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. Surprisingly, the observed resolution was one-sixth of the theoretical limit for the mirror's aperture. <s> BIB002 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental resu... <s> BIB003 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments. <s> BIB004 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> We demonstrate experimentally that phase-conjugate techniques can be used to achieve super-resolution focusing of electromagnetic waves in a multipath indoor environment at 2.45 GHz. The focusing phenomena was used to direct independent signals to two locations separated by approximately one-half wavelength, thereby creating two simultaneous channels at the same frequency. An increase in channel capacity is shown to be achievable by an experimental transmission of a 1 Mbps signal over two channels created using a four element phase-conjugate array. <s> BIB005 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> We report the first experimental demonstration of time-reversal focusing with electromagnetic waves. An antenna transmits a 1-micros electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time-reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity. <s> BIB006 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> We study the possibility to transmit data over channels with large delay spreads under the constraint of a very simple receiver which has only one tap. Such a scheme is of interest when a cost-efficient way to transmit potentially high data rates are sought after. We investigate the performance of the optimal prefilter for this scheme and compare it to a simplified, so-called time-reversal (TR) prefilter which has very low complexity. At low SNRs, the TR prefilter and the optimal prefilter are equivalent. At high SNRs, the TR prefilter achieves a performance that is independent from the delay spread of the channel and hence its performance is the same for any bandwidth. In applications where bandwidth is abundant, such as ultra-wide band (UWB) communications, any required performance can be obtained by TR prefilters with a rate back-off transmission (i.e. transmission rate lower than the allowable bandwidth). Similar performance can also be obtained with full-rate transmission using several transmit antennas. This performance is guaranteed, since the high diversity of the large delay spread channel effectively eliminates any fading. <s> BIB007 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> This paper studies the binary hypothesis test of detecting the presence or absence of a target in a highly cluttered environment by using time reversal. In time reversal, the backscatter of a signal transmitted into a scattering environment is recorded, delayed, energy normalized, and retransmitted through the medium. We consider two versions of the test-target channel frequency response assumed known or unknown-and, for each version, contrast two approaches: conventional detection (where no time reversal occurs) and time reversal detection. This leads to four alternative formulations for which we derive the optimal detector and the generalized likelihood ratio test, when the target channel frequency response is known or unknown, respectively. We derive analytical expressions for the error probabilities and the threshold for all detectors, with the exception of the time reversal generalized likelihood ratio test. Experiments with real-world electromagnetic data for two channels (free space with a target immersed in 20 scatterers and a duct channel) confirm the analytical results and show that time reversal detection provides significant gains over conventional detection. This gain is explained by the empirical distribution or type of the target channel frequency response-richer scattering channels induce types with heavier tails and larger time reversal detection gains <s> BIB008 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> We develop the time reversal adaptive interference canceler (TRAIC) time reversal beamformer (TRBF), a new algorithm to detect and locate targets in rich scattering environments. It utilizes time reversal in two stages: (1) Anti-focusing: TRAIC time reverses and then reshapes the clutter backscatter to mitigate the clutter response; (2) Focusing: TRBF time reverses the residual backscatter to focus the radar image on the target. Laboratory experiments with electromagnetic radar data in a highly cluttered environment confirm the superiority of TRAIC-TRBF over conventional direct subtraction (DS) beamform imaging. <s> BIB009 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> The paper studies detection of a target buried in a rich scattering medium by time reversal. We use a multi-static configuration with receive and transmit arrays of antennas. In time reversal, the backscattered field is recorded, time reversed, and retransmitted (mathematically or physically) into the same scattering medium. We derive two array detectors: the time-reversal channel matched filter when the target channel response is known; and the time-reversal generalized-likelihood ratio test (TR-GLRT) when the target channel response is unknown. The noise added in the initial probing step to the time-reversal signal makes the analysis of the TR-GLRT detector non trivial. The paper derives closed form expressions for the signal-to-noise ratio gain provided by this detector over the corresponding conventional clutter subtraction energy detector in the two extreme conditions of weak and strong (electronic additive) noise and shows that time reversal provides, under weak noise, the optimal waveform shape to probe the environment. We analyze the impact of the array configuration on the detection performance. Finally, experiments with electromagnetic data collected in a multipath scattering laboratory environment confirm our analytical results. Under the realistic conditions tested, time reversal provides detection gains over conventional detection that range from 2 to 4.7 dB. <s> BIB010 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> B. Temporal Focusing and Spatial Focusing of TR <s> Green wireless communications have received considerable attention recently in hope of finding novel solutions to improve energy efficiency for the ubiquity of wireless applications. In this paper, we argue and show that the time-reversal (TR) signal transmission is an ideal paradigm for green wireless communications because of its inherent nature to fully harvest energy from the surrounding environment by exploiting the multi-path propagation to re-collect all the signal energy that would have otherwise been lost in most existing communication paradigms. A green wireless technology must ensure low energy consumption and low radio pollution to others than the intended user. In this paper, we show through theoretical analysis, numerical simulations and experiment measurements that the TR wireless communications, compared to the conventional direct transmission using a Rake receiver, reveals significant transmission power reduction, achieves high interference alleviation ratio, and exhibits large multi-path diversity gain. As such it is an ideal paradigm for the development of green wireless systems. The theoretical analysis and numerical simulations show an order of magnitude improvement in terms of transmit power reduction and interference alleviation. Experimental measurements in a typical indoor environment also demonstrate that the transmit power with TR based transmission can be as low as 20% of that without TR, and the average radio interference (thus radio pollution) even in a nearby area can be up to 6dB lower. A strong time correlation is found to be maintained in the multi-path channel even when the environment is varying, which indicates high bandwidth efficiency can be achieved in TR radio communications. <s> BIB011
In principle, the mechanisms of reflection, diffraction, and scattering in wireless medium give rise to the uniqueness and independence of the CIR of each multi-path communication link  . Obtained from real indoor experiments BIB011 , Figs. 4 and 5 show that, when the re-emitted TR waves from transceiver A propagate in the wireless medium, the location of transceiver B is the only location that is associated with the reciprocal CIR. That is to say that given the re-emitted TR waveform from transceiver A that is specific to the CIR between transceivers A and B, the environment will serve as a natural matched filter only for the intended transceiver B. As a result, the temporal focusing effect of the specific re-emitted TR waveform can be observed only at the location of transceiver B. It means that at the time instance of time focusing, the signal power not only exhibits a strong peak in the time domain at transceiver B as shown in Fig. 4 , but also concentrates spatially only at the location of transceiver B in the rich multi-path environments as shown in Fig. 5 . A physical meaning is that at this moment, it creates a resonating effect due to the TR transmission. Experimental results in both acoustic/ultrasound domain and radio frequency (RF) domain further verified the temporal focusing and spatial focusing effects of the TR transmission, as predicted by theory. Fink et al. BIB001 - BIB002 found that acoustic energy can be refocused on the source with very high resolution (wavelength level). In BIB003 - BIB004 , acoustics experiments in the ocean were conducted to validate the focusing effects of TR in real underwater propagation environments. In the RF domain, experiments in BIB005 , BIB006 , and BIB007 demonstrated the spatial and temporal focusing properties of EM signal transmission with TR by taking measurements in RF communications. Furthermore, a TR-based interference canceler to mitigate the effect of clutter was presented in BIB009 , and target detection in a highly cluttered environment using TR was investigated in BIB008 and BIB010 . In BIB011 , real-life RF experiment results were obtained in typical indoor environments, which shows the great potential of TR as a new paradigm of the green wireless communications. In the context of communication systems, the temporal focusing effect concentrates a large portion of the useful signal energy of each symbol within a short time interval, which effectively suppresses the inter-symbol interference (ISI) for high-speed broadband communications. The spatial focusing effect allows the signal energy to be harvested at the intended location and reduces leakage to other locations, leading to a reduced required transmit power consumption and lower co-channel interference to other locations. The benefits and unique advantages of TRbased communication systems due to the temporal and spatial focusing effects promise a great potential for the applications of IoT, as will be discussed in the remaining parts of this paper.