Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> III. ASYMMETRIC TRDMA ARCHITECTURE FOR IOT <s> Abstract We present a method to transmit digital information through a highly scattering medium in a MIMO-MU (multiple input multiple output multiple users) context. It is based on iterations of a time-reversal process, and permits us to focus short pulses, both spatially and temporally, from a base antenna to different users. This iterative technique is shown to be more efficient (lower inter-symbol interference and lower error rate) than classical time-reversal communication, while being computationally light and stable. Experiments are presented: digital information is conveyed from 15 transmitters to 15 receivers by ultrasonic waves propagating through a highly scattering slab. From a theoretical point of view, the iterative technique achieves the inverse filter of propagation in the subspace of non-null singular values of the time-reversal operator. We also investigate the influence of external additive noise, and show that the number of iterations can be optimized to give the lowest error rate. (Som... <s> BIB001 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> III. ASYMMETRIC TRDMA ARCHITECTURE FOR IOT <s> We show that all the spatiotemporal degrees of freedom available in a complex medium can be harnessed and converted into spatial ones. This is demonstrated experimentally through an instantaneous spatial inversion, using broadband ultrasonic waves in a multiple scattering sample. We show theoretically that the inversion convergence is governed by the total number of degrees of freedom available in the medium for a fixed bandwidth and demonstrate experimentally its use for complex media investigation. We believe our approach has potential in sensing, imagery, focusing, and telecommunication. <s> BIB002 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> III. ASYMMETRIC TRDMA ARCHITECTURE FOR IOT <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> BIB003 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> III. ASYMMETRIC TRDMA ARCHITECTURE FOR IOT <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> BIB004 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> III. ASYMMETRIC TRDMA ARCHITECTURE FOR IOT <s> The time reversal (TR) technique consists of transmitting in the channel a signal filtered by the time-reversed channel impulse response. To assess the performance of a TR system using impulse radio ultrawideband signals, one interesting parameter is the focusing gain defined as the ratio of the maxima in the time waveforms of the received signal with and without the time-reversal operation. We propose a heuristic approach to get an approximate analytical expression for the distribution function of this gain, as a function of the channel delay spread normalized to the pulse width and assuming a dense multipath environment. Predicted results are compared to experimental values obtained in reverberating chambers. <s> BIB005
Based on TR technique, we recently introduced in BIB004 a novel multi-user media access scheme, TRDMA, for wideband communication. Leveraging the unique temporal and spatial focusing effects of the TR technique BIB003 , BIB005 , the TRDMA exploits the spatial degrees of freedom of the environment and uses the multipath channel profile associated with each user's location as a location-specific signature for the user. Moreover, such channel profiles may be further improved by mixing spatial degrees of freedom and temporal degrees of freedom as shown in BIB001 and BIB002 . With the concept of TRDMA, in this section, we propose an asymmetric TRDMA architecture for IoT, where most of the computational complexities are concentrated at the more powerful base station (BS), resulting in a minimal complexity and cost at the terminal devices in both uplink and downlink. As shown in Fig. 1 , the proposed IoT system consists of multiple TR BSs and each BS serves multiple heterogeneous terminal devices, which ranges from laptop and TV to light and clothes. In the following, we will first focus on the single BS scenario and then discuss the multiple-BS scenario in Section III-E.