Wen Road, Luoyang flower just, every family households cover spring. Taoist drink at one hundred empty pot. Annual spend the next drunk, and seen repeated red.
This Shui and from where to go, term drift east and west. Although different voice with laughter. A clear Night Moon, where we may not meet.
---- Linjiangxian · Liu Shui
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Summary
\ (~~~~~~ \) Intelligent reflecting surface (IRS) is a revolutionary new technology, which can be integrated on a plane by a large number of low-cost passive reflective element, reconfiguring smart radio propagation environment, thereby significantly improving the performance of a wireless communication network. Specifically, IRS different elements can be independently controlled to reflect incident signal amplitude and / or phase, so as to cooperatively achieve a directional signal enhancement or zero trap formed of a fine three-dimensional (3D) passive beam. In this article, we outlined the IRS technology, including its main applications in wireless communications, compared with the prior art competitive advantage, new hardware architecture and the corresponding signal model. Compared with traditional network consisting only of active elements, we focus on the key challenges in wireless network design and implement new IRS auxiliary mixing (including active and passive components) is. Numerical results show that, in a typical application of IRS wireless network greatly improve the performance of the system.
1 Introduction
\ (~~~~~~ \) benefited from the support of key technologies such as ultra-dense network (UDN), a large-scale multiple-input multiple-output (MIMO), millimeter wave (mmWave) communications, upcoming the fifth-generation (5G) wireless network has achieved a 1000-fold increase in network capacity and generally at least 100 billion wireless devices connected to the target. However, the required high complexity and hardware costs, and increased energy consumption remains the key issue remains unresolved. For example, in the UDN densely deployed base stations (BSs) or access point (AP) will not only increase the cost of hardware and maintenance costs, but also exacerbate network interference problems. Further, the UDN is how to provide a reliable, scalable backhaul transmission is a challenging task, especially in the chamber is not completely covered by the optical deployment. In addition, large-scale MIMO extended from less than 6 Ghz frequency band to mmWave usually require more complex and more expensive signal processing hardware and higher energy consumption (e.g., radiofrequency (RF) chain). Therefore, how to research to find innovative wireless network outside of the future / 5G, spectrum and energy-saving and cost-effective solution remains imperative.
\ (~~~~~~ \) Furthermore, although the physical layer technology 5G generally able to adapt to space and time of the wireless environment, but signal propagation is random in nature, largely uncontrollable. For these reasons, a smart reflective surface (IRS) is considered a promising new technology, you can reconfigure the wireless propagation environment reflected by the control software [3] - [6]. Specifically, IRS is a plan of a large number of low cost of passive reflective elements, each element capable of independently inducing amplitude and / or phase variation of the incoming signal, thereby to achieve synergistic fine three-dimensional (3D) form a reflected beam. Conventional transmitter / receiver radio link adaptation techniques contrast, IRS is actively modified by the radio channel between them highly controllable and intelligent signal reflection. This provides a new degree of freedom in order to further improve the performance of wireless links for the realization of intelligent programmable wireless environment paved the way. By appropriately adjusting the 3D passive beamforming, the signal may be reflected IRS constructively summed with the signal from the other paths, in order to enhance the desired signal power at the receiver, or destructively eliminate co-channel interference, such as unwanted signal. Since the IRS eliminating the use of RF transmit chain, and operates only in a short distance, it can be densely deployed, scalable and low energy cost, without complex interference management between the passive IRS.
\ (~~~~~~ \)Figure 1 shows a typical application of several IRS auxiliary radio network. In Figure 1 (a), the user is in the dead zone, the dead zone, the direct link between the user and its serving BS is blocked serious obstacle. In this case, the deployment of a clear link with the base station and the user IRS help through the intelligent signal reflections around obstacles, thus creating a virtual line of sight (LoS) link between them. This is particularly useful for coverage mmWave communications vulnerable to the effects of indoor blocked extension. FIG 1 (b) shows the use of physical layer used by the IRS to improve security. When the link is less than the distance from the BS to qietingqi legitimate user (e.g., user 1) link distance, or qietingqi located legitimate user (e.g., user 2) in the same direction, secure communication speed achievable by the height limit (at the BS using transmission beam forming by even in the latter case). However, if deployed near qietingqi IRS, IRS is the reflected signal may be tuned to cancel out at the BS from qietingqi (non-reflective IRS) signal, thereby effectively reducing leakage of information. In FIG. 1 (c), for simultaneously subjected to a high signal attenuation from its serving BS and severe co-channel from a neighbor BS interfering cell edge users can be deployed IRS at the cell edge, by appropriately designing the reflected beam forming, As well as enhancing the desired signal power, but also helps to suppress interference, thereby forming a "hot spot signal" and "interference-free zone" in the vicinity thereof. FIG 1 (d) shows the use of communication used by the IRS to implement device (D2D) of large-scale equipment, which simultaneously acts as a signal reflection IRS hub to support interference mitigation by low power transmission. Finally, FIG. 1 (e) shows an application IRS achieve simultaneous wireless information and power transmission of a miscellaneous equipment (SWIPT) of things (the IoT) network [7], wherein the large aperture of the IRS is used to compensate by passive beam forming network to a remote device near was significant power loss, to improve efficiency of wireless power transmission thereto.
\ (~~~~~~ \)In addition to the application of the above promising, from an implementation perspective, IRS also have attractive advantages. First, IRSs usually low profile, lightweight and conformal geometry produced, which makes it easy to install / remove them on walls, ceilings, facades, advertising panel. Further, since the IRS is a complementary device in a wireless network, which was deployed in existing wireless systems (e.g., cellular or WiFi) standard and does not need to change its hardware, and only the communication protocol necessary to modify . Thus, the IRS can be integrated into the wireless network transparent to the user, thereby providing a high degree of flexibility and superior compatibility with existing wireless systems. Thus, IRS can be deployed at a lower cost and the actual integrated in a wireless network.
\ (~~~~~~ \) Next, we highlight compared to other existing technologies related to the IRS, the main difference between the IRS and the competitive advantage that active repeater, backscatter-based communication and have mass MIMO source surface. First, compared to the signal reproduced by and retransmitted to assist the active destination wireless relay communication source, IRS does not use any active transmission module (e.g., power amplifier), and only the reflected signal received by an array of passive . In addition, the active half-duplex relay in the normal operation mode, so that spectrum efficiency is lower than IRS operating in full-duplex mode. Although full-duplex relay is achievable, but it requires advanced powerful self-interference cancellation technology, and high implementation costs of these technologies. Secondly, IRS backscatter communication with a conventional (e.g. by identifying the reflected signal from the reader the reader sends a radio frequency identification (RFID) communications tag) different, IRS for facilitating existing communication link, rather than send any information itself. Thus, backscatter communication from the reader need to implement interference cancellation in which the receiver to decode the message tag [9]. In contrast, the IRS auxiliary communication, the direct path signal and the reflected signal paths are useful to carry the same information, can be added to improve the coherent strength of the decoded signal at the receiver. Third, since the IRS array architecture (passive and active) and operating mechanism (reflection and transmission) is different, so it is different from the mass-based MIMO active surface.
Although IRS auxiliary radio network has many advantages, but it also contains active (BS, AP, a user terminal) and passive (IRS) component, and therefore only the traditional networks include active components are very different. This causes the paper IRS signal model, hardware configuration, a passive beam forming design, channel acquisition, and other aspects are reviewed nodes are deployed. With particular emphasis on the design and implementation of wireless networks IRS auxiliary main challenges and potential solutions to inspire future research. Numerical results show the effectiveness of IRS in typical wireless applications.
2 architecture
\ (~~~~~~ \) the IRS hardware implementation based on the concept of "surface element", which is digitally controllable by the concept of two-dimensional (2D) material element [10]. Specifically, a sub-surface planar array of a large number of sub-elements or atoms of a so-called electrically thickness are arranged in order of sub-wavelength of the operating frequency of interest. By appropriate design elements, including geometric shapes (e.g., square or ring opening), the size / dimensions, arrangement, etc., you can modify their single signal response (reflection amplitude and phase shift). In wireless communication applications, the reflection coefficient of each unit should be adjustable to adapt to the dynamic mobility of a radio channel generated by the user, thereby requiring real-time reconfiguration. This can be achieved by using an electronic device such as a positive intrinsic negative (PIN) diode, a field effect transistor or microelectromechanical system (MEMS) switch. As shown in FIG. 2, IRS typically a three-layer architecture and intelligent controller. In the outer layer, a large number of metal sheets (elements) are printed on a dielectric substrate, and the direct interaction of the incident signal. In the back layer, a copper plate to prevent leakage of signal energy. Finally, the inner layer is a control circuit board, each of the reflective elements responsible for adjusting the amplitude / phase shift, triggered by an intelligent controller attached IRS. In practice, a field programmable gate array (FPGA) may be implemented as a controller, it also acts as a gateway to communicate and coordinate through a separate wireless link to other network components (e.g., BSs, AP, and user terminal), so as they low exchange rate information.
Figure 2 also shows an exemplary structure of a single element, wherein the PIN diode embedded in each element. A bias voltage controlled by a direct current (DC) feed line, PIN diodes can be shown in an equivalent circuit "on" and "off" switching between the states, thereby producing a phase shift difference. Thus, the intelligent controller by setting the appropriate bias voltage can be independently implemented in different phase shifts IRS element. On the other hand, in order to effectively control the reflection amplitudes, may be employed in a variable resistance load element design. For example, by changing the resistance value of each element in the different parts of the incident signal energy is dissipated, thereby achieving a controllable reflection amplitudes in [0,1]. In practice, it is desirable to have independent control of amplitude and phase shift of each element, for this purpose, the circuit needs to be effectively integrated.
\ (~~~~~~ \) launched Massive MIMO technology to promote the development of high-speed wireless communication system, however, the performance of the wireless system still depends on its channel. Specifically, a transmitter of electromagnetic radiation reaching the receiver before the channel undergoes reflection, refraction, diffraction, and path loss. In conventional communications channel it is considered uncontrollable, and therefore usually probabilistic model of the channel. In fact, many studies presented in most communication technologies (e.g., beam forming, diversity, channel coding) is to change without affecting the behavior of the cancellation channel or utilizing a channel designed. In contrast, IRS recently proposed concept based on electromagnetic wave propagation control to improve the performance of the communication channels of the communication system. Specifically, IRS is a surface formed by a plurality of element microfacet composition, which can control the phase microfacet incoming signal. Ideally, the phase is controlled to be continuous, the use of several discrete element surface subwavelength size "elements" to approximate that each "element" different phase shifts caused. Thus, when the signals are incident, so that the combined effect of all of the phase shift the radiation beam in a selected direction. This is similar to conventional phased array beam forming, but the main difference is that the conventional phased array phase shift is not generated in the array. 3 depicts different phase shifts between the modes membered atoms CAUSED incident signals in a different direction as the light beam reflected.
3. IRS applications
\ (~~~~~~ \) with no large-scale cell and cooperative relaying different MIMO system, cooperative relaying active components typically used to improve the propagation environment, IRS only requires a small operating power, and therefore suitable for implementing the limited energy system. For example, when the size of the micro-membered to 8 × 8mm, the energy consumption of only 125mW / m2, which is much lower than many existing wireless communication devices. In addition, IRS can be very thin material, allowing the deployment of the exterior walls, ceilings and other places. Therefore, IRS is suitable for airport terminals, stadiums and other places. And compatibility with existing conventional network good IRS, i.e., in an existing network, IRS can be flexibly deployed to enhance the performance of the current communication network.
\ (~~~~~~ \)In practice, IRS deployed in the conventional MIMO system facilitates two kinds of beam forming shown in FIG. 4. In FIG. 4 (a), a deployed system IRS communication between multiple antennas at the transmitter and to assist users. The information signal sent from the transmitter, there may be a direct communication path between the transmitter and the user, and at the transmitter beamforming to improve signal reception user. Also, because of the broadcast nature of the wireless channel, the received signal is also IRS, IRS case the reflected signal, by means of IRS controller may control the main direction of reflection. In particular, the introduction of all the element surfaces in appropriate phase shift to create constructive coherent combining their respective scattered signal, thereby creating the signal beam is focused on the user, the larger the surface, the narrower the beam, this strategy also referred to as focused energy. On the other hand, if the shadow due to severe obstruction or without the direct path, the transmitter should be performed with respect to the beamforming IRS. Then, IRS may act as a non-amplified relays the event signal reflected and focused to the user to assist in end to end communication. In FIG. 4 (b), consider a case where the presence of the user 2, user multi-antenna transmitters to a service scenario. We assumed that the two users with different levels of security clearance, wherein a user message at the user 2 should be decoded. In this case, the reflection may be performed in a destructive IRS by adjusting the phase of the scattered signal, so that the signal at the user 2 is zero. This strategy is known as zero energy. These two beamforming technique has wide application, such as a physical layer security, interference management, coverage expansion, the capacity improvement.
3．参考文献
[1] Qingqing, W., & Rui, Z. (2019). Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network. arXiv preprint arXiv:1905.00152.
[2] Zhang, J., Björnson, E., Matthaiou, M., Ng, D. W. K., Yang, H., & Love, D. J. (2019). Multiple antenna technologies for beyond 5G. arXiv preprint arXiv:1910.00092.
More exciting content please pay attention to micro-channel public number " Optimization and Algorithms "
