Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> 1. Introduction to Optical Phase Conjugation.- 2. Physics of Stimulated Scattering.- 3. Properties of Speckle-Inhomogeneous Fields.- 4. OPC by Backward Stimulated Scattering.- 5. Specific Features of OPC-SS.- 6. OPC in Four-Wave Mixing.- 7. Nonlinear Mechanisms for FWM.- 8. Other Methods of OPC.- References. <s> BIB001 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB002 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> Accurate focusing in an inhomogeneous medium is difficult to implement. In order to focus on a reflective target we have extended the concept of optical phase‐conjugate mirrors, valid for monochromatic signals, to broadband pulses such as those used in ultrasound echography. The transducer’s linear response to the acoustic pressure allows one to replace 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 time, and then reemitted. The main advantage of this process is that waves distorted by the propagation through an 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. We present theoretical results on this principle, nu... <s> BIB003 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB004 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> The current state of the acoustic wave phase conjugation (PC) problem is reviewed. The generation of phase conjugate ultrasonic waves is discussed with emphasis on the parametric method using electromagnetic pumping in solids. The giant-amplification supercritical parametric PC mode is considered in detail. Ultrasonic PC with a gain in excess of 80 dB is demonstrated for a soft magnetic ceramics-based amplifier. The high quality of supercritical parametric PC sound is confirmed by acoustic-optical visualization. Acoustic PC effects, such as anomalous sound reflection at the PC mirror and the autofocusing and self-targeting of phase conjugate sound beams incident on a scatterer in a liquid, are shown. Recent experimental results demonstrating the potential of PC for applications are presented. <s> BIB005 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB006 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB007 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB008 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB009 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> Time reversal (TR) is a technique that can achieve remarkable temporal focusing (delay spread reduction) and spatial focusing (low spatial interference) in the context of wideband multiple/single input-single output systems. In this paper, we investigate a simplified form of time-reversal, referred to as one-bit time reversal. The efficiency of this technique has already been demonstrated for ultrasound applications. We apply it to radio waves and demonstrate that it can preserve the spatial focusing properties of TR, while requiring much simpler transmitters. However, it does not have as good temporal focusing properties as pure TR <s> BIB010 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> Time reversal (TR) communications marks a paradigm shift in ultrawideband (UWB) communications. The system complexity can be shifted from the receiver to the transmitter, which is ideal to some applications. UWB multiple input-single output (MISO) is enabled by the use of the TR scheme. Two basic problems are investigated experimentally using short UWB radio pulses (nanosecond duration). Temporal focusing and increase in collected-energy with the number of antennas are verified. Also, the reciprocity of realistic channels, the cornerstone of TR, is demonstrated perhaps for the first time in electromagnetics <s> BIB011 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> Time reversal (TR) communications marks a paradigm shift in UWB communications. The system complexity can be shifted from the receiver to the transmitter, which is ideal to UWB sensors. UWB multiple input single output (MISO) is enabled by the use of the TR scheme. Two basic problems are investigated experimentally using short UWB radio pulses (nanosecond duration). Temporal focusing and SNR increase with the number of antennas are verified. Also, reciprocity of realistic channels, the foundation of TR, is demonstrated for the first time in electromagnetics. <s> BIB012 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> This paper presents a reduced-complexity time reversal technique for ultra-wideband (UWB) communications. Time reversal takes advantage of rich scattering environments to achieve signal focusing via transmitter-side processing, which enables the use of simple receivers. The goal of this paper is to demonstrate a UWB time reversal system architecture based on experimental results and practical pulse waveform, taking into account some practical constraints, and to show feasibility of UWB time reversal. Pre-decorrelating in addition to time reversal processing is considered for a downlink multiuser configuration. Multiple transmit antennas are employed to improve the performance. <s> BIB013 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> We present an approach for subwavelength focusing of microwaves using both a time-reversal mirror placed in the far field and a random distribution of scatterers placed in the near field of the focusing point. The far-field time-reversal mirror is used to build the time-reversed wave field, which interacts with the random medium to regenerate not only the propagating waves but also the evanescent waves required to refocus below the diffraction limit. Focal spots as small as one-thirtieth of a wavelength are described. We present one example of an application to telecommunications, which shows enhancement of the information transmission rate by a factor of 3. <s> BIB014 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles 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> BIB015 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> This paper proposes a new pre-coding technique called Frequency Division Duplex Time Reversal (FDD TR) for FDD Multiple Input Multiple Output (MIMO) wireless communications which relies on unquantized uncoded channel feedback. This technique is compared in terms of performance and overhead to Time Reversal (TR), 1bit Time Reversal (1bit TR). The performance assessment of a 16x2 MIMO system, is based on post receiver Signal To Noise Ratio (SINR) formulas and simulated indoor propagation channels. Results show that FDD TR outperforms 1bit TR in terms of overhead and throughput. <s> BIB016 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> The multi-path effect makes high speed broadband communications a very challenging task due to the severe inter-symbol interference (ISI). By concentrating energy in both the spatial and temporal domains, time-reversal (TR) transmission technique provides a great potential of low-complexity energy-efficient communications. In this paper, a novel concept of time-reversal division multiple access (TRDMA) is proposed as a wireless channel access method based on its high-resolution spatial focusing effect. It is proposed to use TR structure in multi-user downlink systems over multi-path channels, where signals of different users are separated solely by TRDMA. Both the single-transmit-antenna scheme and its enhanced version with multiple transmit antennas are developed and evaluated in this paper. The system performance is investigated in terms of its effective signal-to-interference-plus-noise ratio (SINR), the achievable sum rate and the achievable rates with outage. And some further discussions regarding its advantage over conventional rake receivers and the impact of spatial correlations between users are given at the end of this paper. It is shown in both analytical and simulation results that desirable properties and satisfying performances can be achieved in the proposed TRDMA multi-user downlink system, which makes TRDMA a promising candidate for future energy-efficient low-complexity broadband wireless communications. <s> BIB017 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> Utilizing channel reciprocity, the traditional time- reversal technique boosts the signal-to-noise ratio at the receiver with very low transmitter complexity. However, the large delay spread gives rise to severe inter-symbol interference (ISI) when the data rate is high, and the performance is further degraded in the multiuser downlink due to the inter-user interference (IUI). In this work, we study the weighted sum rate optimization problem in the time-reversal multiuser downlink using waveform design. By exploiting the relation between the allocated power and the SINR targets, an iterative algorithm is proposed to optimize the waveform design through a new power allocation algorithm. Simulation results are shown to demonstrate the superior performance of the proposed algorithm in comparison with other traditional methods. <s> BIB018 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> With the advance of semiconductor technologies, the performance of analog-to-digital converter (ADC) has been improved a lot during the past decade in terms of both sampling rate and resolution. The cost has gone down dramatically that makes the wideband communication much affordable. Under such circumstances, a natural question to ask is: Is there a low complexity high energy-efficient solution to high throughput wideband communications? In this letter, we explore this question by studying time-reversal communications. We find that compared with that of OFDM system, the computational complexity of a time-reversal system at the transmitter side is lower since it requires similar additions but no multiplications. Furthermore, the computational complexity of a time-reversal system at the receiver side is negligible since only onetap detection is performed, which means that the overall computational complexity of time-reversal system is much lower than that of OFDM system. Moreover, we find that when the bandwidth is large enough, time-reversal system can achieve much higher achievable rate than OFDM system. Therefore, time-reversal technique is a desired solution to low complexity high throughput wideband communications when more bandwidth is available. <s> BIB019 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> A. The Basic Principles of TR <s> The concept of Time-Reversal Division Multiple Access (TRDMA) has recently been proposed as a promising medium access technology for wireless broadband communications. Previous work on TRDMA mainly focuses on the multi-user downlink system. In this paper, we first introduce a TRDMA-based multi-user uplink architecture and then propose a 2D parallel interference cancellation scheme to enhance system performance. The TRDMA uplink architecture keeps the cost of end-users at a minimum level, and reuses the processing power at the base station that has already been made available for the downlink. The proposed 2D parallel interference cancellation scheme utilizes tentative decisions of detected symbols to effectively cancel both inter-symbol interference and inter-user interference, which significantly improve the bit-error-rate (BER) performance in high SNR regime. To further improve BER performance, a multi-stage processing can be implemented by cascading multiple stages of the proposed 2D interference cancellation, with a total delay that increases linearly with the number of stages, but independent of the number of users. The BER performance of the single-stage cancellation is analyzed, and the approximated theoretical result well matches the simulation results. Simulations are provided for up-to 3 stages of interference cancellation and compared with the basic TRDMA system without interference cancellation. <s> BIB020
The TR signal processing is a technology to focus the power of signal waves in both time and space domains. The research of TR can date back to early 1970s, when phase conjugation was first observed and studied by Zeldovich et al. BIB001 . Unlike the phase conjugation that uses an holographic or parametric pumping BIB005 , the TR uses transducers to record the signal waves and enables signal processing on the recorded waveforms. The TR signal processing was applied by Fink et al. in 1989 [30] , followed by a series of theoretical and experimental works in acoustic communications BIB003 -  . As found in acoustic physics BIB002 - BIB004 and then further validated in practical underwater propagation environments BIB006 - BIB007 , the energy of the TR acoustic waves from transmitters could be refocused only at the intended location with very high spatial resolution. Since TR can make full use of multi-path propagation and also requires no complicated channel processing and equalization, it was later verified and tested in wireless radio communication systems. Experimental validations of TR technique with EM waves have been conducted in BIB015 , BIB008 -  , including the demonstration of spatial and temporal focusing properties BIB015 , BIB008 -  and channel reciprocity BIB015 , BIB011 . The feasibility of applying TR technique into ultra-wideband (UWB) communications has been studied in BIB009 - BIB013 with the focus on the bit error rate (BER) performance through simulation. A system-level theoretical investigation and comprehensive performance analysis of a TR-based multiuser communication system was conducted in BIB017 , where the concept of TRDMA was proposed. To improve the performance of the TRDMA systems, interference suppression through waveform design BIB018 ,  and interference cancelation BIB020 are proposed. The implementation complexity issue is studied in BIB019 , BIB010 , and BIB016 . Moreover, as shown in BIB014 ,  , with random scatterers, TR can achieve focusing that is far beyond the diffraction limit, i.e., half wavelength. The principle of TR transmission is very simple, as demonstrated in Fig. 2 . In this figure, when transceiver A wants to transmit information to transceiver B, transceiver B first has to send an impulse-like pilot signal that propagates through a scattering and multi-path environment and the resulting waveforms are received and recorded by transceiver A. This is called channel probing phase. After that, transceiver A simply timereverses (and conjugated, if the signal is complex valued) the received waveform and then transmits it back through the same channel to transceiver B. This is called TR-transmission phase. There are two basic assumptions for the TR communication system to work. 1) Channel reciprocity: the impulse responses of the forward link channel and the backward link channel are assumed to be identical. 2) Channel stationarity: the channel impulse responses (CIRs) are assumed to be stationary for at least one probing-and-transmitting cycle. These two assumptions generally hold in reality, especially for indoor environment, as validated through real experiments in BIB012 and BIB015 . In BIB012 , Qiu et al. conducted experiments in a campus lab area and showed that the correlation between the impulse response of the forward link channel and that of backward link channel is as high as about 0.98, which means that the channel is highly reciprocal. In BIB015 , Wang et al. showed with experimental results that the multi-path channel of an office environment is actually not changing a lot. In their experiment, Want et al. measured the channel information every one minute, and a total of 40 channel snapshots were taken and stored, where the first 20 snapshots correspond to a static environment, snapshots 21 to 30 correspond to a moderately varying environment, and snapshots 31 to 40 correspond to a varying environment. The experimental results are shown in Fig. 3 , where we can see that most the correlation coefficients between different snapshots are higher than 0.8 and those between static snapshots are above 0.95, which means that the channel is highly stationary. By utilizing channel reciprocity, the re-emitted TR waves can retrace the incoming paths, ending up with a constructive sum of signals of all the paths at the intended location and a "spiky" signal-power distribution over the space, as commonly referred to as spatial focusing effect. Also from the signal processing point of view, in the point-to-point communications, TR essentially leverages the multi-path channel as a matched filter and focuses the wave in the time domain as well, as commonly referred as temporal focusing effect. By treating the environment as a facilitating matched filter computing machine, the complexity of TR systems is significantly reduced, which is ideal for IoT applications as we will discuss later.