Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <s> The use of RDF data published on the Web for applications is still a cumbersome and resource-intensive task due to the limited software support and the lack of standard programming paradigms to deal with everyday problems such as combination of RDF data from dierent sources, object identifier consolidation, ontology alignment and mediation, or plain querying and filtering tasks. In this paper we present a framework, Semantic Web Pipes, that supports fast implementation of Semantic data mash-ups while preserving desirable properties such as abstraction, encapsulation, component-orientation, code re-usability and maintainability which are common and well supported in other application areas. <s> BIB001 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <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> I. INTRODUCTION <s> The Internet of Things (IoT) is the logical further development of today’s Internet. Technological advancements lead to smart objects being capable of identifying, locating, sensing and connecting and thus leading to new forms of communication between people and things and things themselves. Ambient Assisted Living (AAL) encompasses technical systems to support elderly people in their daily routine to allow an independent and safe lifestyle as long as possible. Keep In Touch (KIT) uses smart objects and technologies (Near Field Communication and Radio Frequency Identification) to facilitate telemonitoring processes. Closed Loop Healthcare Services take use of KIT technology and are capable of processing relevant data and establishing communication channels between elderly people and their environment and different groups of care-givers (physicians, relatives, mobile care providers). The combination of KIT technology (smart objects) and Closed Loop Healthcare Services results in an applied IoT infrastructure for AAL scenarios. Already applied IoT and AAL applications in telemonitoring and medication intake compliance projects show that these applications are useful and accepted by the elderly and that the developed infrastructure enables a new form of communication between people and people, people-to-people (P2P) communication. The personal communication between elderly people, their environment and relevant groups of care givers is an important aspect in AAL. Through the combination of KIT and Closed Loop Healthcare, a central AAL paradigm can be realized through the IoT, where the elderly live in their homes with smart objects, thus smart homes, communicating to the outside world in an intelligent and goal-orientated manner. <s> BIB003 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <s> With the development of embedded technology and network technology, various networks become more and more interconnected and integrated. The gateway is one of very important device of networks combination, which realizes many functions such as protocol conversion and data buffer. Based on this demand, this paper designs an embedded gateway which can interconnect all kinds of heterogeneous networks. Firstly, its software framework is given, and then, the main data structures including internal data packet format, Internet client-side data format and information record tables are introduced, finally, the key point of main software modules of the gateway is discussed. This gateway has many characteristics of modularization, customizable and instantaneity. It can interconnect Internet, industrial field bus, wireless networks based on Zigbee or Bluetooth etc and can be applied to intelligent home network, remote measurement and control system and the applications of Internet of things. <s> BIB004 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <s> Internet of Things (IoT) applications are developed and implemented through enlightened communication networks. The phenomena is proving a fact that research in IoT architecture becomes important domain in realizing IoT framework. Therefore, in this paper a new IoT architecture is purposed combining two sophisticated communication networks: power line communication (PLC) and 3G networks. This combination is motivated by scalability advantages of (PLC) and 3G networks. Further, the 3G-PLC networks test bed is constructed and analyzed to show its feasibility in supporting IoT applications. <s> BIB005 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <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> BIB006 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <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> BIB007 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <s> With the design of data communications in mind, 3GPP LTE-advanced (LTE-A) is probably the most promising technology for Internet of Things (IoT). For IoT applications, continuous low-rate streaming data may be reported from devices over a long period of time, imposing stringent requirements on power saving. To manage power consumption, 3GPP LTE-A has defined the discontinuous reception/transmission (DRX/DTX) mechanism to allow devices to turn off their radio interfaces and go to sleep in various patterns. Existing literature has paid much attention to evaluate the performance of DRX/DTX; however, how to tune DRX/DTX parameters to optimize energy cost is still left open. This paper addresses the DRX/DTX optimization, by asking how to maximize the sleep periods of devices while guarantee their quality-of-service (QoS), especially on the aspects of traffic bit-rate, packet delay, and packet loss rate in IoT applications. Efficient schemes to optimize DRX/DTX parameters and schedule devices' packets with the base station are proposed. The key idea of our schemes is to balance the impacts between QoS parameters and DRX/DTX configurations. Simulation results show that our schemes can guarantee traffic bit-rate, packet delay, and packet loss rate while save energy of user equipments . <s> BIB008 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <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> BIB009 </s> Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview <s> I. INTRODUCTION <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> BIB010
D URING the past decade, the Internet of Things (IoT) has drawn great attention from both academia and industry, since it offers challenging notion of creating a world where all the things, known as smart objects BIB002 , around us are connected, typically in a wireless manner, to the Internet and communicate with each other with minimum human intervention  - BIB001 . An example of IoT system is shown in Fig. 1 . The ultimate goal of IoT is to create a better world where things around us know what we like, what we want, and what we need and act accordingly without explicit instructions BIB003 , and thus improve the quality of our lives and consistently reduce the ecological impact of mankind on the planet  . The term "IoT" was first proposed by Kevin Ashton in his presentation at Procter&Gamble (P&G) in 1999  . During the presentation, Ashton envisioned the potential of IoT by stating "The Internet of Things has the potential to change the world, just as the Internet did. Maybe even more so." Such a concept then became popular when the MIT Auto-ID center presented their vision of IoT in 2001  . In 2005, IoT was formally introduced by the International Telecommunication Union (ITU) through the ITU Internet report  . With a very broad vision, IoT has shown its great potential to improve the quality of our lives. However, the research into the IoT is still in its infancy and there are still a lot of challenges needed to be addressed before the realization of IoT. In the following, we summarize some key technical challenges from the perspective of wireless communication, which is the essential technology of IoT to allow people and things to connect to Internet anywhere at anytime  1) Better battery life: Typically the things in IoT are powered with small batteries, due to which the power consumption is low and thus requires low computational complexity of the wireless communication techniques. 2) Multiple active things: The IoT is expected to have many concurrent active things transmitting data, which leads to severe interference among things. Thus, low interference wireless technologies are desired. 3) Low-cost terminal devices: For widespread adoption of the IoT technology, the cost at the terminal devices, i.e., things, needs to be low. Therefore, simple processing at the terminal devices' side is preferred. 4) Heterogeneous terminal devices: Different from current wireless systems that have a collection of rather uniform devices, it is to be expected that the IoT will exhibit a much higher level of heterogeneity, as things that are totally different in terms of functionality, technology and application fields will belong to the same communication environment. Thus, the wireless solution to IoT should be able to support heterogeneous terminal devices with different quality-of-service (QoS) options such as from very low bit rate to very high. 5) Scalability: The density of the things in IoT may be very high or low, which requires the wireless technology to be highly scalable to provide satisfactory QoS for low to high density areas. 6) Privacy and security: Since every thing in IoT has a unique identification, there is a need to have a technically sound solution to guarantee privacy and the security of the customers in order to have a widespread adoption of IoT. To qualify as a good wireless communication solution to IoT, a technology should be able to handle the challenges raised above. Currently existing wireless technologies for IoT can be classified into two groups: 1) wireless technologies for low-data-rate and low-power applications such as remote control  ,  ; and 2) wireless technologies for high-data-rate applications such as video streaming  - BIB004 . Note that the technologies suitable for low-data-rate applications may not be able to meet the requirements of the high-data-rate applications. A typical wireless communication technology suitable for low power, low-data-rate applications is ZigBee  . Mainly based on IEEE 802.15.4, ZigBee can operate in the 868 MHz, 915 MHz, and 2.4 GHz bands with respective data rates of 20, 40, and 250 kb/s. A similar technology is Z-Wave  , whose main purpose is to enable short message transmission from a control node to multiple nodes. The maximum speed of Z-Wave is 200 kb/s working at 2.4-GHz band. The most significant advantage of ZigBee and Z-Wave is the low price  , [19] . For instance, there exist chips including RF module, the digital baseband module, and a programmable microcontroller. Both of these technologies were designed for low-power applications in battery-operated devices. Moreover, ZigBee even includes a sleep-mode mechanism to reduce power consumption. The complexity of hardware is quite low: 32-128 kbytes of memory is enough to implement the system including the higher layers. On the other hand, the most obvious disadvantage of ZigBee and Z-Wave is their low data rate. Moreover, the 2.4-GHz frequency band is already crowded with interfering devices, e.g., microwave ovens, WiFi equipment, and cordless phones. The sub-GHz electromagnetic (EM) waves propagate very far, so very high node density may not be achievable due to the high interference levels created by other similar devices. The most popular technologies for high-data-rate applications are Bluetooth  and WiFi  . Bluetooth, based on IEEE 802.15.1, is a wireless technology for exchanging data over short distance. Compared with ZigBee and Z-Wave, the data rate could be increased to megabit per second (Mbps). WiFi, based on IEEE 802.11, is a popular technology that allows an electronic device to exchange data or connect to the Internet wirelessly. The speed of WiFi can achieve up to several gigabit per second (Gbps) according to IEEE 802.11ac with the help of MIMO and very high-order modulation. The most important advantage of these two technologies is the high data rate. However, they require higher power consumption, higher complexity of hardware (MIMO in WiFi) and thus higher price  . Since both transmitter and receiver use the same architecture, i.e., symmetric architecture is used, the power consumption of terminal devices is high. In addition, a large number of WiFi access points (APs) deployed close to each other operating in the same or adjacent channels will severely interfere with each other. Thus, these technologies do not seem to offer robust performance in interference-limited scenarios even with costly terminal devices. Another possible technology is the 3G/4G mobile communications BIB005 - BIB008 . However, the poor indoor coverage of 3G/4G signals greatly limit its application to IoT, where communications mostly happen in indoor environment. From the above discussions, we can see that existing technologies can only address partial challenges while leaving the rest unaddressed, e.g., both the heterogeneity and scalability challenges cannot be handled by existing technologies. A natural question to ask is: is there a wireless communication technique that can address most, if not all, challenges? As pointed out in BIB006 , time-reversal (TR) signal transmission is an ideal paradigm for low-complexity, low energy consumption green wireless communication because of its inherent nature to fully harvest energy from the surrounding environment by exploiting the multi-path propagation to recollect all the signal energy that could be collected as the ideal RAKE receiver. The theoretic analysis in BIB006 shows that a typical TR system has a potential of over an order of magnitude of reduction in power consumption and interference alleviation, which means that TR system can provide better battery life and support multiple concurrent active users. Moreover, with our proposed asymmetric TR architecture, only one-tap detection is needed at the receiver side BIB007 , thus the computational complexity at the terminal devices is low, which means the cost of the terminal devices is also low. Note that the achievable rate can still be very high when the bandwidth is wide enough as shown in BIB009 . In addition, the TR system can easily support heterogeneous terminal devices by providing various QoS options through adjusting the waveform and backoff factor BIB007 , BIB010 . Finally, the unique location-specific signature in TR system can provide additional physical-layer security and thus can enhance the privacy and security of customers in IoT BIB006 . Overall, we will provide an overview to show that TR technique is an ideal paradigm for IoT. The rest of the paper is organized as follows. In Section II, we introduce some basic concepts of TR technique. Then, we propose in Section III an asymmetric TR division multiple access (TRDMA) architecture and discuss in details why TR is an ideal paradigm for IoT. In Section IV, we discuss other challenging issues and future directions including advanced waveform design, MAC layer issues, and low-cost high-speed analog-to-digital converter (ADC) and DAC. Finally, we draw conclusions in Section V.