Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> C. TR Communication System <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> BIB001 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> C. TR Communication System <s> The combination of the Internet and emerging technologies such as nearfield communications, real-time localization, and embedded sensors lets us transform everyday objects into smart objects that can understand and react to their environment. Such objects are building blocks for the Internet of Things and enable novel computing applications. As a step toward design and architectural principles for smart objects, the authors introduce a hierarchy of architectures with increasing levels of real-world awareness and interactivity. In particular, they describe activity-, policy-, and process-aware smart objects and demonstrate how the respective architectural abstractions support increasingly complex application. <s> BIB002 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> C. TR Communication System <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> C. TR Communication System <s> 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 spatial and temporal domains, time-reversal (TR) transmission technique provides a great potential of low-complexity energy-efficient communications. In this paper, we develop a new concept of time-reversal division multiple access (TRDMA) as a wireless channel access method based on its high-resolution spatial focusing effect. We propose to use TR structure in a multi-user downlink system over large delay spread channels, where the signals of different users are separated solely by TRDMA. Both the single-transmit-antenna scheme and its enhanced version with multiple-transmit-antenna are developed and evaluated in this paper. We investigate the system performance in terms of the effective SINR and the achievable sum rate of the multi-user system. It is shown in both analytical and simulation results that satisfying performances can be achieved in the proposed TRDMA multi-user downlink system. <s> BIB004 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> C. TR Communication System <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> BIB005
A very simple TR-based communication system is shown in Fig. 6 . The CIR between the two transceivers is modeled as where is the complex channel gain of the th path of the CIR, and is the corresponding path delay, and the is the total number of the underlying multi-paths (assuming infinite system bandwidth and time resolution). Without loss of generality, we assume that in the following discussion, i.e., the first path arrives at time , and as a result, the delay spread of the multipath channel is given by . Constrained by the limited bandwidth of practical communication system, pulse shaping filters are typically used to limit the effective bandwidth of the transmission. Generally, the duration of the pulse is limited by the available bandwidth through the simple relation . 1) Channel Probing Phase: Prior to transceiver A's TRtransmission, transceiver B first sends out a pulse of duration (other than an ideal impulse which demands infinite bandwidth) which propagates to transceiver A through the multi-path channel , where transceiver A keeps a record of the received waveform , which is the convolution of and , represented as follows: ⩽ ⩽ where can be treated as an equivalent channel response for the system with a limited bandwidth . From (2), one can see that for those paths whose time differences are less than the pulse duration , they are mixed together due to the limited system bandwidth . Also for > , the received values and are determined by completely different sets of paths. Therefore, given a limited bandwidth , the corresponding pulse duration determines the time-domain resolution to resolve two adjacent paths. In other words, from the system's perspective, those paths whose time differences are within the duration are treated like one path in the equivalent channel response . 2) Data Transmission Phase: Upon receiving the waveform, transceiver A time-reverses (and conjugate, when complexvalued) the received waveform , and uses the normalized TR waveform as a basic signature waveform , i.e., Defining ≜ and ≜ , in (3) can be represented as At transceiver A, there is a sequence of information symbols to be transmitted to transceiver B. Typically, the symbol rate can be much lower than the system chip rate. BIB002 Therefore, a rate backoff factor is introduced to match the symbol rate with the chip rate by inserting zeros between two symbols BIB003 , BIB005 , BIB001 , BIB004 . Applying the pulse shaping filter , Z and the transmitted signal 2 can be expressed as