Towards Intelligent Reflecting Surface Empowered 6G Terahertz Communications:A Survey

Zhi Chen,Xinying Ma,*,Chong Han,Qiye Wen

1 University of Electronic Science and Technology of China(UESTC),Chengdu 611731,China

2 Shanghai Jiao Tong University,Shanghai 200240,China

Abstract:Terahertz(THz)communications have been widely envisioned as a promising enabler to provide adequate bandwidth and achieve ultra-high data rates for sixth generation(6G)wireless networks.In order to mitigate blockage vulnerability caused by serious propagation attenuation and poor diffraction of THz waves,an intelligent reflecting surface(IRS),which manipulates the propagation of incident electromagnetic waves in a programmable manner by adjusting the phase shifts of passive reflecting elements,is proposed to create smart radio environments,improve spectrum efficiency and enhance coverage capability.Firstly,some prospective application scenarios driven by the IRS empowered THz communications are introduced,including wireless mobile communications,secure communications,unmanned aerial vehicle(UAV)scenario,mobile edge computing(MEC)scenario and THz localization scenario.Then,we discuss the enabling technologies employed by the IRS empowered THz system,involving hardware design,channel estimation,capacity optimization,beam control,resource allocation and robustness design.Moreover,the arising challenges and open problems encountered in the future IRS empowered THz communications are also highlighted.Concretely,these emerging problems possibly originate from channel modeling,new material exploration,experimental IRS testbeds and intensive deployment.Ultimately,the combination of THz communications and IRS is capable of accelerating the development of 6G wireless networks.

Keywords:terahertz(THz)communications;intelligent reflecting surface(IRS);sixth generation(6G);application scenarios;enabling technologies;emerging challenges.

I.INTRODUCTION

With the ongoing deployment and forthcoming commercialization of fifth generation(5G),the sixth generation(6G)wireless communication networks have attracted more and more attention recently,which are anticipated to provide higher data rates,larger bandwidth,higher spectrum efficiency,higher connection density,lower latency,much wider coverage(near 100%),more intelligent communications and so on[1,2].These promising performance requirements of 6G networks push forward plenty of application scenarios in the near future,including but not limited to virtual reality(VR),holographic communications,intra-device communications,high speed wireless data centers,high speed kiosk downloads,high speed backhaul links[3].It is worth noting that the microwave and millimeter wave(mmWave)frequency bands are unable to meet the emerging applications in 6G wireless networks due to the limited bandwidth.In light of this reason,developing new spectrum resources becomes much more imperative.

Fortunately,Terahertz(THz)communication has been envisioned as one of the most promising technology candidates for 6G wireless networks,which possesses sufficient spectrum bandwidth from 0.1 THz to 10 THz and achieves ultra-high transmission rates from hundreds of gigabits per second(Gbps)to several terabits per second(Tbps)[4-6].Due to its distinguished features,THz frequency band is investigated popularly all over the world.For example,the world radio communication conference in 2019(WRC2019)has preserved the range of 0.275-0.475 THz frequency band for mobile and fixed communication services[7].However,except for the obvious benefits,THz communication still faces some shortcomings when it is applied to practical scenarios.Along with the increase of frequency,THz waves experience serious propagation attenuation,thereby the communication distance is restricted within a small area.In particular,the THz waves impinging upon the conventional surface(e.g.,building,wall,ceiling,concrete)results in extremely high reflecting attenuation,which severely weakens the signal coverage capability and brings about the communication performance deterioration[8].Furthermore,in contrast with microwave and mmWave frequency bands,the formed beam at THz frequency band is much narrower.This is hard for THz communications to realize the beam control and beam tracking[9].To tackle these challenges,conventional approaches hold that the wireless environment is fixed by nature and we can only optimize the transmitter or receiver design in wireless networks.Although improving the transmitting power and increasing the number of transmitters may be the possible alternatives,the system cost and the power consumption are unaffortable in turn.Therefore,exploring the feasibility of constructing a controllable and programable radio environment is urgently demanded and taking the modification of wireless propagation into consideration is an inevitable trend.

Motivated by these open problems discussed above,intelligent reflecting surface(IRS)has been treated as a significant paradigm shift to cooperate with the smart and reconfigurable wireless propagation environment for 6G THz communication systems[10-12].The definition of the IRS also has other equivalent notions,such as reconfigurable intelligent surface(RIS)[13-17],large intelligent surface(LIS)[18-20],reconfigurable metasurface[21,22],smart reflect-arrays[23],software-controlled metasurfaces(SCMs)[24].In essence,the concept of IRS belongs to the tunable metasurface that is composed of a large number of passive reflecting elements(also namely meta-atoms).Specifically,the IRS is usually controlled by a central processor,and each reflecting element has the ability to manipulate and change the reflecting amplitude and phase shift of the impinging THz waves.Once the IRS is installed in the communication environment,the wireless channel between the transmitter and receiver can be controllable and reconfigurable,which greatly contributes to enhancing the system capacity,guaranteeing the communication reliability and reducing the power consumption[25].More importantly,the IRS employed in the THz communication system is capable of combating the undesired propagation conditions and relieving the acute wireless channel fading impairment at high frequency band[26].By utilizing the dense deployment of the IRS,the abominable blockage existing in the LoS link can be well avoided,while the cunning corners of the propagation environment can be covered by the THz signals.Furthermore,differing from the conventional surfaces,the mentioned IRS is able to alter the propagation direction of THz waves with negligible reflection loss[27],and thus the transmission distance of THz communications can be enlarged correspondingly.Stated thus,with the assistance of IRS,the THz communications can adapt effortlessly to the practical 6G wireless networks.

Apart from the conceptually appealing superiorities,when it is implemented in practice,IRS still owns some special characteristics compared with other related technologies,including reflect-arrays[28,29],amplify-and-forward relaying[30,31],and backscatter communication[32-34].Firstly,each reflecting element of the IRS is passive and does not require any active devices(e.g.,radio frequency chains,amplifiers,analog-to-digital converters).The passive design ensures that an IRS possesses much lower deployment cost and lower power consumption in contrast with the active techniques.Secondly,an IRS is able to support the stable broadband response at THz band and can work well under the condition of full-duplex(FD)mode[25],which is crucial to improve the spectrum efficiency for THz communications.Thirdly,an IRS also prevents from producing any active circuit noise that is more likely to appear at high frequency band.Instead,the active relay in half-duplex(HD)undoubtedly endures the low spectral efficiency,while the active FD relay scheme demands the self-interference cancellation techniques.Moreover,the IRS is generally low cost,light weight,flexible installation,strong scalability and various shapes,and it can be easily deployed and dismantled from the coated objectives[10-12].Furthermore,the deployment of the IRS at the cell edge can be utilized to mitigate the multicell interference,which is well introduced in[35].Finally,the IRS provides excellent flexibility and compatibility with other wireless systems,and thus can be easily extended to diverse application scenarios,including remote sensing,precise localization and space communication.

This paper mainly presents a comprehensive survey on the combination of IRS and THz communications.Concretely,the driving applications,the key technologies as well as the emerging challenges brought by IRS empowered 6G THz communications will be investigated thoroughly in this paper.The reminder of this paper is organized as follows.In Section II,some promising application scenarios inspired by IRS aided THz communications are described in detail,such as coverage enhancement,secure communication,unmanned aerial vehicle(UAV),mobile edge computing(MEC),non-orthogonal multiple access(NOMA)and localization.In addition,some enabling technologies and faced challenges that may be encountered by IRS aided THz communications are put forward in Section III and Section IV,respectively.Finally,Section V concludes this paper.

II.DRIVING APPLICATION SCENARIOS

In this section,the application scenarios motivated by joint IRS and THz band are introduced in detail.By utilizing the unique features of IRS,THz communication scenarios become more likely to be carried out in practice.

2.1 Coverage Enhancement for THz Communications

Coverage capability is a pressing challenge that needs to be addressed urgently for THz communications.On the one hand,due to its ultra-high frequency,THz communications suffer from extremely high propagation attenuation and strong molecular absorption effect,and thus the THz signals only cover the shortrange area[36].On the other hand,THz waves at such a high frequency band possess poor diffraction,which makes THz communications be sensitive to the undesired blockages[8,37].In particular,when THz communication is applied to the typical indoor application scenario,the wireless LoS link can be easily blocked by the furniture,human bodies and other complex interior structure,leading to the high speed communication interruption.Therefore,solving the coverage hole appeared in the THz communication scenarios still remains as a tricky issue.

Figure 1.Illustration of an IRS assisted THz coverage enhancement scenario.

In order to tackle the coverage problem existing in THz communication scenarios,the IRS has been widely regarded as a promising solution to reconfigure the wireless propagation environment and improve the communication performance,as shown in Figure 1.There are some distinctive advantages once the IRS is applied to 6G THz communications.Firstly,the IRS can provide the virtual wireless LoS link when the practical LoS link between the transmitter and the receiver is blocked by the obstacles,where the virtual LoS link is composed of the transmitter-IRS link and the IRS-receiver link.Since the IRS is able to control the propagation direction of THz waves,the blockage vulnerability can be mitigated well.Secondly,one may note that the IRS can be manufactured into arbitrary shape and the thickness of the IRS is small(aboutλ/5～λ/10).These physical characteristics enable that the IRS is deployed almost everywhere to take advantage of the electromagnetic waves,which is conducive to enhance the THz signal coverage in space.In addition,the IRS is lack of the complex hardware circuit as well as the RF chains,so the weight of the IRS is very light.Based on this,the IRS can be conveniently installed in the wireless propagation environment,including the building facade,ceilings,furniture,clothes and other reflecting objects.Currently,research on the coverage problem of the THz communications is still in its infancy[38-41].For example,the work in[39]explores the effect of human blockage on performance of the THz system,and employs two transmitters to establish the LoS link and handle the human blockage.However,increasing the number of transmitters leads to extra system cost and power consumption.In[40],the application scenarios of wireless local area network at THz band is considered here.Besides,the semi-closed-form expressions for the coverage probability and the average achievable rate are derived by utilizing the stochastic geometry tool.It is worth noting that these research contents mentioned above do not consider the combination of IRS and THz communications.Interestingly,the work in[41]is the first attempt to solve the coverage problem emerged in indoor IRS-aided THz communication systems,and converts the coverage optimization problem into the discrete phase shift search problem.Then,the authors also develop an efficient phase shift search solution by selecting partially better phase shift combinations during the phase search process,which makes a better tradeoff between coverage performance and computational complexity.Although the coverage analysis at low frequency bands(e.g.,microwave band and mmWave band)has been extensively investigated[42-44],the coverage enhancement for IRSaided THz communications should be still addressed in the near future 6G wireless networks.

2.2 IRS Empowered Secure THz Communications

Figure 2.An IRS assisted secure THz communication scenario.

With the rapid development of 6G wireless communication networks,physical layer security has draw a substantial amount of attention,and thus the secure wireless communication technology has gradually become one of the most important research hotspots for security-sensitive applications,such as social surveillance,banking,military communication.Although THz communications possess much better security performance than microwave and mmWave communications to a certain extent,improving the security level of the THz communication system is still a pressing problem[45].Since THz communications provide ultra-high data transmission,so tremendous private data can be grabbed by the eavesdropper during a short time frame[46].Based on this,the secret data leakage problem needs to be investigated thoroughly for the communication scenarios with high data rates.Another reason to enhance the security performance is that,compared with wired optical fiber communication,THz communication is a wireless transmission mode and the security level is relatively lower.Moreover,in order to improve the coverage area of the THz communications,an effective way is to increase the number of the transmitter,which definitely aggravate the deployment cost and system consumption.Attractively,the IRS can be well used for the physical layer security to build the programmable propagation environment and to avoid information leakage[47].It is also worth noting that the IRS can be employed by convert communication systems and the anti-jamming communication systems[48].By converting the uncontrollable channel into the reconfigurable propagation environment,the IRS assisted THz communication system is able to enhance the signals of the legitimate users and suppress the signals of the eavesdroppers[49].Therefore,it is meaningful for THz communications to leverage the IRS to heighten the physical layer security level.

Considering the IRS assisted secure communication systems with legitimate users and eavesdroppers as shown in Figure 2,different optimization algorithms are proposed to maximize the secrecy rates[50-52].For the case that eavesdropping channel is stronger than the legitimate channel,a novel alternating optimization and semidefinite relaxation scheme is developed to solve such a problem in[53].Most of the existing literatures assume that the CSI of the eavesdropper is known to the transmitter,but this hypothesis is not reasonable in practice.For catering to the secure transmission without the CSI of the eavesdropper,a joint beamforming and jamming approach is considered to minimize a transmit power at legitimate transmitter so as to meet the quality of service at legitimate receiver[54].In addition,the robust and secure beamforming(RSBF)scheme is then proposed to overcome the imperfect CSI of cascaded wiretap channels[55],where a RSBF scheme based on alternating optimization and semidefinite relaxation(SDR)techniques is introduced in detail.

One may note that improving the security of THz communications is more important,so some related works have been made as follows.Under the hypothesis of rank-one THz channel,the transmit beamforming at the base station(BS)and the phase shift matrix at the IRS are jointly optimized with transmit power and discrete phase-shift constraints[56].Similarly,to enhance the security of the IRS assisted THz system,the work in[57]aims to maximize the secrecy rate with low complexity and high secure performance.In addition,the alternating optimization algorithm is also used to iteratively design the precoder and discrete phase-shifts matrix[58].In contrast with the multiple-input single-output(MISO)systems,the IRS assisted multiple-input multiple-output(MIMO)systems is more meaningful at high frequency bands[59-61],which employs a large number of array antennas to conquer the serious path loss at both transmitter and receiver.For instance,based on the deep reinforcement learning,the post decision state and prioritized experience replay are employed to improve the security performance for the MIMO communication system[62,63].Through the above discussion,the research on IRS assisted secure communications is still in infancy stage,especially for THz communications.For sake of promoting the coordination of IRS and secure THz communications,more comprehensive studies are required to study,such as channel estimation of eavesdropper,transmitting power minimization,secure capacity characterization.

2.3 IRS Empowered THz UAV Communications

Figure 3.Illustration of an IRS assisted THz UAV scenario.

In the field of UAV communication,compared with the traditional ground communication station,UAV can be flexibly and quickly deployed in the wireless networks with lower cost and higher networking efficiency.Besides,the coverage capability(e.g.,LoS transmission)can be enhanced by increasing the height of UAVs[64,65].Therefore,the UAV can be regarded as a mobile relay to expand the transmission distance of the 6G wireless networks,which makes up for the shortages of THz communication systems.Similarly,the IRS is usually placed in a stationary position on the ground,and can make the propagation signals keep away from the obstacles by smartly adjusting the phase shifts of the reflecting elements.However,deploying IRS in wireless networks only has obvious performance gain under the condition that receiver and the transmitter are installed on the same side of the IRS[25].To sum up,the UAV is more flexible than IRS,while the IRS possesses lower cost and lower power consumption.If the advantages of both UAVs and IRSs are integrated together as shown in Figure 3,the THz communications can realize the omni-bearing coverage[66].Therefore,on account of the flexibility of UAV and the economy of IRS,the combination of IRS and UAV for THz communications is to be a promising trend.

Early research has investigated the IRS assisted UAV communication system[67],in which the analysis results validate that the IRS can significantly enhance the signal strength at the receiver side.For example,the work in[68]investigates the downlink transmission scenario where the wireless links include the UAV-IRS link and the IRS-receiver link.Concretely,in order to maximize the average achievable rate of the RIS-assisted UAV communication system,the joint passive beamforming design and trajectory optimization problem is formulated.In framework[68],only the LoS path between UAV-IRS link and the LoS path between IRS-receiver link are considered in the optimization problem.In[69],the channel model with scattering effect is further taken into account,and this paper demonstrates that the deployment of IRS can effectively improve the coverage and reliability of the UAV system by deriving the analytical expressions of average bit error rate(BER),average capacity and outage probability.Since the current convex optimization algorithms depend on the time-consuming iterations to obtain the approximate solutions[70],a reinforcement learning algorithm based on deep Qnetwork(DQN)is proposed to optimize the UAV trajectory and the phase shift matrix of the IRS,so as to maximize the weighted data rate and geographical fairness of all terminal equipment[69].Differing from the narrow-band channel model in[71],another work applies orthogonal frequency division multiple access(OFDMA)technique to improve the sum-rate of the IRS-assisted UAV communication scenario[72].In particular,a parametric approximation approach is developed to establish an upper and a lower bound for the formulated non-convex optimization problem.

Due to the high propagation loss of THz signals,a large number of IRSs and UAVs is to be deployed densely.With the increasing number of the UAVs,the system security level is reduced accordingly.To ensure the security performance of IRS-assisted UAV communication system,an alternating optimization algorithm is developed to maximize the average worstcase secrecy rate[73].Except for the research directions mentioned above,there are still many tricky problems that need to be solved for the IRS assisted THz UAV communications.We know that more advanced power supply or charging technology should be developed to solve the endurance problem of UAV scenarios.Furthermore,since a large number of IRSs and UAVs are deployed for THz communications,space resources on the ground or in the air ought to be allocated suitably.More importantly,when we take advantage of auxiliary equipment(e.g.,IRS,UAV),maintaining a safe living environment for humans is still a primary thing.

2.4 IRS Empowered THz MEC Scenario

MEC is a network computing architecture derived from the evolution of centralized cloud computing to distributed local edge computing[74].By offloading the computing targets of mobile devices to the communication network edge with stronger computing power,the communication latency of 6G wireless networks can be greatly reduced.At the same time,since a large number of computing tasks is processed locally,the upper network congestion problem can be greatly alleviated,and the system cost and power consumption of the whole wireless network is cut down[75].Since THz communications provide immense data exchange and demand much more stringent requirements for communication latency,so developing MEC technology in THz communication systems undoubtedly obtain plenty of benefits,as shown in Figure 4.However,the THz signals are very sensitive to obstacles and the unstable offloading channel brings additional communication latency as well as power consumption,which definitely influence the combination of MEC scheme and THz communications.Therefore,for the sake of reducing the network latency and improving the system throughput concurrently,the IRS can be adopted to assist the THz MEC scenarios,which has the ability to change the propagation direction of impinging THz waves and is capable of helping THz signals to bypass the obstacles with negligible reflection loss.

Figure 4.Illustration of the IRS assisted THz MEC application scenario.

Before discussing the THz MEC systems,we give a simple review of the IRS aided MEC systems.With the assistance of the IRS,the work in[76]considers that a fraction of the computing tasks of a single antenna device can be offloaded to the edge computing node through the access point equipped with multiple antennas.In the case of elevating the edge computing capability,the latency minimization problem is formulated,and the single device and multiple device cases are considered respectively.This paper reveals that compared with the conventional MEC system without IRS,the latency of the IRS assisted MEC system is significantly reduced.In[77],the authors further consider the cost metric of mobile computing,and the mobile devices offload the computing requirements to the edge server at the access point.On the premise of offloading the transmission data of mobile devices,we need to guarantee that each device has a customized information rate.By adjusting phase shifts of the IRS,the edge server can maximize its computing performance.In addition,the authors also demonstrate that the IRS assisted MEC can significantly improve the data rate from the access point to mobile devices.Instead of the identical power distribution for mobile devices in[76],the work in[78]mainly studies the radio resource allocation for the IRS assisted MEC scenario.Given the predefined maximum latency requirement,the authors minimize the mobile multiuser power of the uplink MEC system through the joint design of individual device power,multiuser detection matrix and passive beamforming at the IRS side.The IRS can alleviate the congestion problem existing in the MEC systems with low cost and low latency,so as to ensure the real-time computation offloading for the computing requirements.Some works are devoted to minimizing the energy consumption of IRS assisted wireless MEC scenarios[79-81].

Although aforementioned researches have put forward diverse optimization schemes for the IRS assisted MEC system,taking THz communications into consideration is still absent.There is no doubt that the corresponding optimization problems in IRS assisted THz MEC system will become more complex,because such an application scenario requests more powerful computing capability,less system consumption and more stable transmission environment.Stated thus,these emerging challenges need to be tackled gradually.

2.5 IRS Empowered THz Localization and Sensing

Localization and sensing is a critical technology that is to be utilized in diverse wireless communication applications,and a typical localization scenario is shown in Figure 5.Sometimes we need to communicate with each other after achieving the accurate localization of terminal devices.With the deepening of research works,the localization problems in THz system appear to be more challenging,even though more precise localization and sensing can be realized owing to the short wavelength of THz waves and the high angular resolution[82].Firstly,the THz communication system uses narrow beams to achieve the ultra-high data rate,which requires the centimeter-level accuracy compared with low frequency bands.Secondly,the serious propagation loss of the THz waves makes the localization and sensing more difficult,because the transmission distance of the THz signals is limited within a small area.Finally,due to the strong directionality of THz waves,the propagation signals can hardly cover the blind area.Once the LoS link is blocked,the localization and sensing scheme of NLoS region becomes more laborious.Remarkably,with the help of IRS,high precision localization and sensing can be applied to practical application scenarios.In contrast with conventional reflection materials(e.g.,plastic,concrete,glass),an IRS is able to change the propagation direction of THz waves with negligible reflection loss.In other words,an IRS can convert the NLoS path to the virtual LoS path,and thus the multipath information of THz communications can be utilized to enhance the receiving power of terminal devices.In practice,an IRS provides location and orientation as prior information,thus we can greatly improve the localization and sensing accuracy.

In terms of IRS assisted localization and sensing scenarios,a large amount of successive works can be found in the literature[19,83-91].Current researches on IRS assisted localization and sensing mainly concentrate on microwave and mmWave frequency bands.The authors discuss the IRS assisted localization in centralized and distributed scenarios,and further validate the localization precision in distributed scenario is better than the centralized scenario[19,83].The work in[84]calculates the Cramer-Rao lower bounds(CBLBs)for positioning using an IRS with quantized phase and amplitude,and analyze the CRLB loss caused by the different quantization resolutions.In[85],an RIS-aided downlink positioning problem is considered from the Fisher Information perspective.Then this paper proposes a two-step optimization scheme that selects the best phase shift combination of the IRS so as to improve the wireless localization performance.Since the IRS has the ability to focus energy in the three-dimensional(3D)space,the authors extend the IRS model from the two-dimensional(2D)planar shapes from three-dimensional spherical shapes[86].Such a novel IRS model is more helpful to realize the 3D localization and sensing.Under the case of the MIMO mmWave system,the work evaluates the influence of the number of reflecting elements and the values of phase shifters on the position estimation accuracy[87].In addition,the authors[88]put forward an adaptive phase shifter design based on hierarchical codebooks and feedback from the receivers,the benefit of which boosts both accurate positioning and high data rate transmission.Except for the IRS enabled indoor and outdoor physical channel modeling,some valuable insights are provided to achieve the efficient positioning of the IRS assisted communication systems[89].The work in[90]focuses on optimizing the passive beamforming at the IRS to reduce the localization error.Furthermore,towards the localization performance limits for wireless communication scenarios,a general model is proposed for both near field and far field localization in 3D space[91].As mentioned before,THz communication requires centimeter-level or even millimeter-level localization and sensing,these research contents are based on traditional localization methods and have some certain limitations.In order to meet the localization requirements of THz communications[92],[93],the future challenges encountered by the IRS assisted THz localization and sensing system includes channel modeling,IRS deployment,ingenious algorithm and experimental platform of localization.

2.6 IRS Empowered THz NOMA Scenario

Figure 5.Illustration of an IRS assisted THz localization and sensing scenario:(a)Outdoor scenario;(b)Indoor scenario.

Figure 6.Illustration of the IRS assisted THz NOMA scenario.

NOMA has been considered as an indispensable technology to support massive connectivity and to improve spectrum efficiency for near future 6G wireless communication networks[94-96].Particularly,NOMA is able to serve plenty of users by using the same wireless resource,and outperforms the orthogonal multiple access(OMA)scheme(e.g.,OFDMA)in terms of enlarging the connectivity capability and balancing the user fairness.As shown in Figure 6,with the dramatically increasing number of connected communication devices in 6G wireless networks,taking advantage of NOMA technology also requires the adequate spectrum resources[97,98].Fortunately,THz frequency band is an appropriate choice to handle the bandwidth shortage problem for 6G NOMA systems.The cooperation between NOMA and THz communications is capable of realizing much lower latency,higher spectrum efficiency and more device connections.Owing to the above benefits,the THz NOMA scenario is a potential research focus in the near future.

Even though the THz NOMA system has some irresistible advantages,there are still some burning issues that should be further solved.We note that THz band can support much more connected devices,but the decoding complexity for the THz NOMA communication systems also increases sharply.Besides,the proposed NOMA system at THz band inherits serious propagation attenuation,which limits the communication distance between the transmitter and the receiver.Furthermore,if the connected devices are blocked by the obstacles,the system performance will be affected obviously.In other words,the THz NOMA scenario requires that the wireless network provides quite good coverage capability.Interestingly,the IRS is one of the most promising solutions to enhance the network coverage and improve the spectrum efficiency by smartly controlling the channel conditions[99].Hence,it is of great interest to investigate the combination of IRS,THz and NOMA to further promote the 6G wireless communications.

To our best knowledge,there is no existing research on IRS assisted THz NOMA systems,although some heuristic research hotspots can be obtained according to the related research status[100-102].In[103],the authors aim to jointly optimize the channel assignment,decoding order,power allocation,and phase shift optimization for the IRS assisted NOMA systems.Then,to solve the formulated throughput maximization problem,the alternating optimization algorithm is developed by optimizing the power allocation and reflection coefficients alternately.In[104],the IRS enhanced mmWave NOMA system is studied and the virtual LoS link can be created to improve the signal coverage.However,since the active beamforming vector at the transmitter,phase shift matrix at the IRS and power allocation coefficients are coupled together,the formulated multi-variable optimization problem is non-trivial to solve.Against this condition,the joint alternative optimization and successive convex approximation based iterative algorithm is proposed to strengthen sum-rate gains and system throughput.Unlike the above downlink NOMA systems,the authors show solicitude for the IRS assisted uplink NOMA system,and then optimize the sum rate maximization problem under individual power constraints.Specifically,the semidefinite relaxationbased method is leveraged to achieve the near optimal performance.In order to pander to the requirements of 6G networks,these research aspects mentioned above need to take THz communications into consideration.Subsequently,by further exploiting the peculiarities of IRS,THz and NOMA,the research gap is more likely to be filled for the 6G wireless networks.

III.ENABLING TECHNOLOGIES FOR IRS EMPOWERED THZ COMMUNICATIONS

In this section,we analyze fundamental technologies associated with the IRS empowered THz communications,including efficient hardware design of IRS,channel acquisition,capacity optimization,beam control,resource allocation and robustness design.It is worth mentioning that all of these presented technologies are very momentous for the IRS aided THz systems.

3.1 Hardware Design of IRS

The concept of IRS is derived from the metasurface,which is the 2D form of metamaterial,refering to a series of artificial structures with special physical properties not normally found in nature,such as negative refractive index,near-zero index and so on[105].To further improve the performance and reduce the volume,metamaterials with subwavelength thickness,i.e.metasurfaces,are widely applied to electromagnetic wave control,including but not limited anomalous reflection[106]and refraction[107],polarization conversion[108],flat focusing[109],holographic imaging[110].The IRS deployed in the THz communication systems is a kind of tunable metasurfaces which can manipulate the propagation direction of reflected waves and realize the reconfigurable wireless environment.In particular,the IRS usually consists of periodically arranged reflecting elements(also namely meta-atoms),and each IRS element can flexibly control the amplitude and phase of the impinging THz waves.Since the size of each reflecting element is at a subwavelength level(aroundλ/5～λ/10),it can be treated as a point source.In light of the Huygens principle,the propagation state of the reflected waves can be controlled through the arrangement of reflecting elements.In general,the hardware structure of each reflecting element is designed to achieve the amplitude and phase control of the impinging waves,and then these reflecting elements will be arranged properly to construct the reflective wavefront.However,at present,the hardware design of the IRS at the THz frequency band is immature and needs to be further investigated in the near future.

For the sake of accelerating the development of the IRS,there are some imperative challenges that should be addressed.First of all,due to the short wavelength of the THz waves,the size of each reflecting element becomes much smaller,and thus much more IRS elements can be integrated within a small area.For example,the size of IRS is 20mm×20mmwhile such an array contains 100×100 reflecting elements.As a result,it is difficult to combine the massive IRS elements and the semiconductor devices(e.g.,varactor[111],transistor[112])together to actively control the reflected waves.In contrast with the IRS at microwave and mmWave frequency bands,the novel design theory and working principle of IRS at THz band is urgently required.Secondly,designing an efficient hardware architecture as well as the reflection coefficients of each IRS element is another important aspect,where the reflection coefficients is composed of amplitude response and phase response.It is worth noting that all the current research contents consider the ideal reflection coefficients.But in practice,the reflecting amplitude and the phase shift of the IRS elements are critically restricted by the hardware structure[113]and are not perfect.In our recent work,the relationship between the phase response and the conductivity of graphene is obtained.Specifically,the phase shift of impinging THz waves can be altered by changing the conductivity of graphene through the applied voltages.Simulation results of the reflecting element using CST Microwave Studio is shown in Figure 7.For a normally incident wave at 1.75 THz,the amplitude response(also namely reflectivity)is almost maintained at a high level above 70%and the phase response reaches to 275 degrees with the chemical potential(namely Fermi level[114])of the graphene ranging from 0 to 0.8 eV.Except for a wide range of amplitude response and phase response,the IRS also requires the high speed,high density and low crosstalk array structure to realize the high-speed THz beam scanning technology.Moreover,in order to enhance the reflection efficiency,the reasonable feeding network and the phase coded modulation should be employed according to the hardware features.Therefore,to meet the practical applications,more efficient hardware design of the IRS is demanded for 6G THz wireless communications.

Figure 7.Simulation results of the amplitude and phase response changing with Fermi level when the incident wave is at 1.75 THz.

3.2 Channel Estimation

Channel estimation is a fundamental problem for IRS assisted THz communication system.Before the data transmission,the acquisition of channel state information(CSI)is a priority to guarantee the reliable wireless communications in 6G wireless networks[11,115,116].In addition,the joint optimization of the transceiver and the IRS also depends on the knowledge of CSI[117,118].Nevertheless,the utilization and deployment of IRS in wireless communications leads to some critical challenges for the channel estimation,especially for THz communications.First of all,since the IRS is composed of passive reflecting elements,it does not possess any active components and has no ability to implement the signal processing.Thus,it is hard for the IRS assisted wireless systems to estimate the transmitter-IRS channel and the IRSreceiver channel simultaneously without transmitting pilots at the IRS side.Secondly,in contrast to the conventional wireless systems at low frequency band,a larger number of array antennas as well as reflecting elements can be integrated together at THz band,so another challenge is that plenty of channel parameters need to be estimated and the pilot overhead is extremely high.Finally,the future application scenarios require the low latency,but the channel estimation for IRS-enabled THz system is more time-consuming in contrast with conventional system without IRS.

In terms of solving the channel estimation problem,current researches on the IRS aided communication system are still preliminary.In[119],a binaryreflection(full or no)controlled channel estimation scheme for downlink MISO systems,but the proposed method requires to estimate all the channel vectors one-by-one and results in high network latency.Intriguingly,the authors put forward a novel hardware structure that some active reflecting elements are randomly distributed in the IRS[120].With the assistance of the active elements,the training overhead can be greatly reduced.However,using active channel sensors definitely increases extra system cost and power consumption.In addition,a two-stage algorithm that includes a sparse matrix factorization stage and a matrix completion stage is developed in[121],and a novel message-passing based algorithm is proposed to solve the matrix-calibration based matrix factorization problem in[122].In practice,the IRS reflects the incident signals to a specific direction,leading that the location of the mobile receiver should be considered.Thus,we need to take the time-varying channel estimation into consideration.Unfortunately,the mobile user tracking problem is not analyzed by the aforementioned research work.To this end,a three-stage framework is presented to jointly settle time-varying channel estimation,beamforming and user tracking for the practical IRS assisted mmWave MIMO systems[123].However,the research work mentioned above concentrates on the CSI acquisition at low frequency band.

With regard to the channel estimation at THz band,the work in[113]proposes a compressed sensing(CS)based method by leveraging the sparse nature of the THz channel,and then optimizes the data rate of the IRS aided THz MIMO system with the estimated CSI.In the downlink IRS assisted THz MIMO communication system,the channel estimation problem can be formulated as the sparse recovery problem.To solve the sparse reconstruction problem,an efficient deep learning based channel estimation method is put forward,which utilizes a two-stage neural network to obtain the sensing matrix and to achieve the signal reconstruction simultaneously[27].According to the research contents discussed above,the IRS aided THz communications are more sensitive to the small changes brought by the channel coefficients,such as path gain,angle of arrival(AoA)and angle of departure(AoD),location information.Hence,it is significant to further probe into the efficient time-varying channel estimation schemes with low latency as well as the multiuser beam tracking schemes for the practical IRS enabled THz communication scenarios.

3.3 Capacity Optimization

The IRS can improve the capacity of the THz communication systems by smartly adjusting the phase shift of each reflecting element for a time-varying environment.By controlling the phase shifts of the IRS,3D passive beamforming can be produced to reconfigure the wireless transmission channels and to obtain a high beamforming gain,which is similar to the conventional massive MIMO system for improving the capacity performance[124].Numerically,the received power increases asymptotically in the order ofN2asNgoes to infinity[11],whereNrepresents the number of reflecting elements.Compared with the massive MIMO system without IRS,the IRS aided MIMO THz system is capable of achieving much better capacity performance with the help of good channel environment[10].With regard to a poor channel condition that the LoS link between the transmitter and the receiver is weak or blocked down by obstacles(e.g.,thick walls,indoor furniture),the capacity performance of the THz communication system without IRS sharply drops,while the IRS aided THz MIMO system is feasible to maintain a good capacity performance when the transmitter adjusts the beam direction toward the IRS so as to maximize the reflected signal strength of the related receiver.

Recent researches on the capacity optimization of IRS-aided communication system mainly focus on jointly optimizing the active beamforming at the transmitter and the passive beamforming at the IRS[118,125-130].Specifically,a locally optimal solution for the achievable rate maximization problem can be obtained by iteratively optimizing one matrix variable(i.e.the active beamforming matrix or the phase shift matrix of the IRS)with the other matrix variable that is fixed[126].The prior works in[118,125-128]all consider that the IRS uses the continuous phase shift.However,the reflecting elements with continuous phase shifts are practically difficult to implement due to the complex hardware design,and definitely result in much higher system cost.For ease of practical applications,the IRS elements with discrete phase shifts are considered[129,130].When the phase shifts are discrete,the capacity optimization problem of IRSaided THz system become a non-convex problem due to the constraints of discrete variables.To this end,the heuristic optimization algorithms can be well utilized to solve the optimization problem,such as exhaustive search,genetic algorithm and so on.One may note that,the research contents mentioned above suppose that the IRS has perfect amplitude and phase shift model.But in practice,the amplitude response of each reflecting element is generally non-uniform as well as its discrete phase shift.Remarkably,the authors propose a practical phase shift model[131],and simulation results demonstrate that capacity performance achieved by the proposed phase shift model is better than that of the ideal model.

The aforementioned literature only involves the communication systems at low frequency band,while the capacity analysis for IRS-aided THz systems is still missing.Based on the IRS-aided THz system,a multi-hop IRS-assisted communication network is developed in[132].It shows that the IRS-aided THz system obviously improve the capacity performance than the THz system without IRS under the condition of the same communication distance.On the other hand,the THz system with multi-hop IRS can achieve a better coverage capability than the THz system with a single IRS.Although the large-scale fading model of THz communications with narrowband is considered in[132],the wideband characteristics of THz communications are not studied.Apart from the propagation loss affected by the distance and frequency,the absorbing molecule effect leads to several peaks of channel attenuation.For this reason,the total bandwidth of THz band needs to be divided into several sub-bands,and the capacity of the THz system is calculated by adding up the data rates of all the sub-bands.Based on this,the capacity of the IRS-aided THz communication system can be maximized by optimizing the phase shifts of the IRS,the IRS location,and sub-band allocations of THz communications[133].In[134]we consider the IRS-aided point-to-point THz MIMO system and present the simulation results as shown in Figure 8.In particular,compared with conventional gradient descent(C-GD),our developed Taylor expansion aided gradient descent(TE-GD)scheme achieves obvious capacity improvement.In addition,more peculiarities that combine THz communications and the IRS need to be considered together in order to meet the requirements of the practical application scenarios.

Figure 8.Capacity comparisons of considered schemes versus different values of SNR[134].

3.4 Beam Control

To begin with,massive MIMO are required to compensate for the serious path loss and molecular absorbing loss during the THz wave propagation process[135].The large-scale array antennas are feasible for THz communication systems and are able to concentrate the electromagnetic energy into a certain direction via controlling the phase and amplitude of THz waves.In this way,the high gain and high directivity beams can be generated at the transmitter to prolong the transmission distance of THz communications[136].In terms of the THz MIMO system,the key point of beamforming technique is to design an optimal precoding matrix to maximize the beam gain of the array antennas[137].Since the beamforming design is significantly dependent on the CSI acquisition[138,139],it is really important to obtain the accurate AoDs at the transmitter and the AoAs at the receiver to further realize the beam control techniques.

With regard to the beam tracking scheme,the transmitter is required to generate a sequence of training beams for multiple spatial directions(namly beam switch).Then,the receiver is able to estimate the AoD by dealing with the received signal beams[140,141].Due to the narrow beam at THz band,the highresolution codebooks are demanded to maintain the accurate beam alignment of THz beams.However,the beam switch of THz communications suffers from extremely high complexity.In general,the threedimensional beamforming scheme based on the largescale phased arrays needs to be considered in the THz system.Unfortunately,the 3D beamforming is timeconsuming and inefficient during the beam switch process.To balance the tradeoff between the beam precision and the beam switch complexity,a fast beam switch method is studied in[142],whose key idea is to decompose the complex beam switch process into two stages.At the first stage,the coarse beams at low frequency(sub 6 GHz)are employed.And at the second stage,the beam switch scheme based on precise beams is operated at THz frequency band.To further decrease the complexity of the beam switch in THz communications,some literatures aim to use spatial information to assist the beam tracking procedure,such as trajectory prediction of the targets[143-145].In[143],a uniform linear motion model of the target for time-varying channels is studied,and the CSI at the next time slot is obtained by analyzing the CSI at the previous time slots.In[145],a realistic human movement model is analyzed for indoor THz communications and the direction of human movement at the next time slot can be predicted by the reinforcement learning techniques.

Recently,the IRS has been widely considered as a new technique to enhance the coverage capability of THz communications.By making the amplitude and phase shift adjustment,the IRS are able to refract,absorb,reflect,and focus the incoming waves to any desired directions.Based on these functionalities,the IRS is able to be a major solution to overcome the high propagation loss and molecular absorbing attenuation by controlling the THz beams.Specifically,the ability of the IRS fulfills the concepts of beamforming and beam tracking.With respect to the beam control,although the beamforming technique for the IRS assisted communication system is extensively studied in[118,125-130],the beam tracking technique,which requires the dynamic CSI acquisition between the IRS and the mobile station(MS),is still a major challenge for IRS assisted THz communications due to the passive reflecting elements at IRS side.As shown in Figure 9,a typical IRS-aided beam tracking scenario is presented,where a BS serves a MS via the IRS.In order to realize the beam tracking in this scenario,the BS-IRS channel and the IRS-MS channel should be estimated separately[123].Given the estimated CSI,a two-stage beam tracking method is proposed in[123].It is worth noting that the beam control exploration in IRS assisted THz communications is still in its infancy.Since THz channel is sensitive to the small changes,it is more challenging for the IRS assisted THz system to realize the real-time beam tracking.

Figure 9.Beam tracking in an IRS-aided THz communication scenario.

3.5 Resource Allocation

One of the conspicuous features of THz communication is that it can fully take advantage of adequate spectral resources to realize ultra-high data rates,thus providing decent communication services for massive terminal devices in 6G wireless networks.To achieve this goal,we need to note that the THz frequency band is still a non-renewable and valuable wireless spectrum resource for wireless communication systems.Thus,the efficient allocation schemes for spectral resources not only reduce the spectrum overhead,but also enhance the communication performance of the THz systems[146].With the working frequency increasing,although the system capacity can be enlarged,the propagation loss of high frequency signals sharply aggravates in turn.Specifically,THz signals undergo much more severe atmospheric attenuation and path attenuation than microwave and mmWave bands.To maintain good communication conditions,a large number of new front-end devices and antenna technologies is required to be installed in the wireless communication systems.In addition,the IRS is compelling as an energy efficient technology to overcome the transmission problems existing in THz systems[147],which enhances the coverage capability and increases communication distance of THz systems.Hence,how to optimize the power allocation and data rate performance for IRS assisted THz communications is also a major research direction.

Nowadays,the existing researches have not been widely carried out to optimize resource allocation in IRS aided THz communication systems.With regard to the resource types and communication scenarios,there are some promising research directions.For the case of multiple RIS enhanced multiuser communication scenario,when a mobile user is associated with IRSs,the maximum and minimum signal-tointerference-plus-noise ratio(SINR)problem can be transformed into a user-IRS association problem because the automatic interference cancellation feature is applicable to an infinite IRS,and the greedy search algorithm is efficient to solve such a nonconvex problem[123].In the multi-cell communication scenarios,by alternately optimizing the coordinated transmit beamforming at the BS and the reflected beamforming at the IRS,the weighted SINR of the user can be maximized,which enables the IRS to improve the communication performance of the mobile users at the edge of the cell[35,148].Since all subchannels in an IRS-enhanced network share a common IRS phase shift matrix,more efficient bandwidth allocation methods need to be developed in this new scenario by assigning different users to different subchannels.In[149],the passive beamforming optimization schemes with different IRS phase shifts can be obtained by dynamically allocating resource blocks to different user groups in different time slots.The optimization problem of joint subchannel assignment,power allocation and reflecting coefficient design is settled in downlink IRS enabled NOMA wireless networks[103].In the case of IRS assisted vehicle communications,the combined optimization schemes of power positioning,IRS phase shift matrix and spectrum allocation are proposed to maximize the total system capacity of the vehicle-to-infrastructure(V2I),and meanwhile guarantees the signal-to-noise ratio(SNR)of the vehicleto-vehicle(V2V)link[150,151].Some optimization algorithms are also available for promoting the resource allocation design for the IRS assisted cognitive radio systems[146,150,151],IRS assisted multiuser MISO secure communication systems[49]and IRS assisted THz simultaneous wireless information and power transfer(SWIPT)systems[152].

As for the THz band,an adaptive distance-aware bandwidth resource allocation scheme can be used to capture the distinctive channel characteristics,e.g.,the relationship between distance and bandwidth[153].In addition,to meet the quality of service(QoS)requirements of THz wireless network,a joint bandwidth allocation and concurrent scheduling algorithm is able to achieve more successful traffic scheduling and higher network throughput by utilizing QoS awareness and bandwidth allocation[154].As discussed in[154],the authors also put forward a multibeam on-demand power allocation media access control protocol for distributed MIMO THz wireless networks.In[155],for the sake of increasing the weighted sum of completed flows with QoS constraints appeared in THz wireless backhaul network,a heuristic algorithm that contains QoS-aware bandwidth allocation and concurrent scheduling(IHQB)is developed.The existing researches on power allocation at THz band have also been extended to NOMA[156]and THz backhaul networks[157].However,the resource allocation problem related to IRS assisted THz wireless network is much more complicated than that of the THz wireless network without IRS.We note that most of the resource allocation schemes for IRS empowered wireless communication system are based on alternating optimization algorithms to solve the nonconvex optimization problems.One of the drawbacks of the alternating optimization algorithm is that only a suboptimal solution can be obtained instead of an optimal solution.Furthermore,the alternating optimization algorithm is not convergent under some special conditions.Therefore,how to further develop the more advanced resource allocation strategies for IRS aided THz communications.

3.6 Robustness Design

With the increasing wireless communication requirements for QoS level and system reliability,robustness design has always been a pivotal technology to judge the performance of wireless communication systems.Considering the existence shortcomings of serious path loss and poor coverage capability THz communications are easy to be affected by small environmental changes,such as weather conditions,estimated channel parameters,mobile obstacles and so on.Hence,the robustness design for the THz system becomes a much more significant and challenging task in contrast with low frequency communication systems.To this end,the IRS has been recognized as an effective technology to enhance the THz signal strength and spectrum efficiency owing to the passive beamforming gain and the ability of environment reconfiguration[158].More importantly,the IRS is able to enable the cost-effective and energy-efficient THz communications.In terms of improving the robustness performance,the IRS can provide more virtual LoS propagation paths for THz communications even the unexpected mobile objects appear stochastically.On the other hand,the IRS is capable of collecting the THz signals together and launching in a specific direction,which greatly counters the signal attenuation and improves the THz system robustness[159].According to these motivations,it is worth further exploring the IRS enabled robustness design of THz systems.

In the scope of our knowledge,the current researches on IRS assisted robustness design mainly focus on the following three application scenarios:wireless mobile communications with beamforming design,secure communications and cognitive radio communications.To begin with,considering the CSI acquisition error that is defined to follow the complex Gaussian distribution in IRS enabled MISO system,an alternating optimization and majorizationminimization method are selected to realize the beamforming design and obtain the satisfied data rate performance[160].To expand the number of terminal devices,an iterative algorithm based on the convexconcave procedure and approximation transformations is proposed to meet the QoS requirements and obtain the robustness design for IRS aided multiuser MISO system[161].Furthermore,on account of the imperfect cascaded channel model(e.g.,bounded and statistical CSI error model)at the transmitter,the transmit power minimization problems are settled subject to the worst-case rate constraints and the rate outage probability constraints[162].

With regard to the IRS enabled THz communication system,the authors develop a joint robust beamforming and phase shift matrix design based on imperfect CSI to deal with the originally formulated non-convex optimization problem[163].In terms of the IRS assisted secure communications,we need to optimize the robustness design to seek much better secrecy rate by using the IRS when the channel propagation environment is poor.Taking the imperfect CSI of eavesdropping channel into consideration,the successive convex approximation,semidefinite relaxation and penaltybased approaches are used to enhance the secrecy rate and achieves the robustness design[45].On the basic of[45],the authors model the wireless channel as cascaded statistical channel,and select weighted sum of discrete samples to set up a convex hull,which uses semidefinite relaxation to achieve robustness design for IRS assisted communication system[54].Moreover,with the CSI error in cascaded statistical channel model,the work in[164]adopts Bernstein-type inequality to approximate outage rate probability constraints and use penalty-based semidefinite relaxation algorithm to minimize the transmission power under condition of the required secrecy rate.For the IRS aided cognitive radio communication scenarios,the function of robustness design is applied to improve the performance of cognitive radio.Specifically,under the case of bound and statistical CSI error,the authors firstly investigate the robust beamforming design for primary user-related channels,and then the optimization problem of jointly phase shift matrix at the IRS and transmit precoding matrix at the secondary user is proposed to minimize the transmit power for the IRS assisted cognitive radio system when the QoS requirement of secondary user is satisfied[165].In addition,considering the worst-case CSI error,the authors model this situation as a maximization problem in the ellipsoid region,which can be tackled by using alternating optimization and semidefinite relaxation[166-168].When it comes to the robustness design in the IRS assisted THz communications,there are few studies related to this field.In the near future,we intend to combine the hardware features of IRS and the peculiarities of THz communications to further increase the robustness performance.

IV.CHALLENGES AND OPEN PROBLEMS FOR IRS EMPOWERED THZ COMMUNICATIONS

In this section,we mainly discuss the ineluctable challenges and open problems faced by the IRS empowered THz communications.Firstly,the channel measurements and modeling are the foundation of wireless communications.Then,exploring the new materials of IRS is an effective way to realize a wide range of phase response.In addition,the experimental platform and IRS deployment are studied in detail.

4.1 Channel Measurements and Modeling

For the sake of optimizing the IRS aided THz wireless communication system,sufficiently realistic,accurate and tractable communication models,which reveal the propagation peculiarities of THz waves and the physics properties of the IRS are required in the future 6G wireless networks.In[169],the authors propose three free-space path loss models for IRS assisted wireless communications from the perspective of electromagnetic theory,which takes into account some important factors such as the physical dimension of IRSs,and the radiation pattern of the reflecting elements.These models capture three relevant scenarios including i)the far field beamforming case where both the transmitter and receiver are in the far field of the IRS and the signals reflected by all the IRS elements to the receiver can enhance the received signal power,ii)the near field beamforming case where the transmitter or the receiver is in the near field of the IRS and enables the IRS to focus the impinging signals to the receiver by properly adjusting the reflection coefficients,and iii)the near field broadcasting case where the IRS is employed for beamforming and the received power is maximized for one specific user.Furthermore,the three propagation models are assessed and validated by experimental results.In[170],the authors propose a path-loss model for the IRS aided wireless system with far-field and near-field transmission,which leverages the vector theory of scattering and is formulated in terms of a computable integral that depends on the transmission distances,the polarization of the radio waves,the size of the IRS,and the desired surface transformations.This study demonstrates that the path-loss highly relies on the size of the IRS and the transmission distances,especially in the near-field regime.In[131],the authors propose a practical phase shift model that captures the phase-dependent amplitude variation in the element-wise reflection design of the IRS.Specifically,the amplitude response of a typical passive reflecting element is generally non-uniform with respect to its phase shift.The amplitude typically exhibits its minimum value at the zero phaseshift,but monotonically increases and asymptotically approaches unity amplitude at the phase shift of the 180 degrees or-180 degrees.Based on the practical phase shift model,a beamforming optimization scheme is further proposed to achieve better substantial performance gains than the conventional ideal IRS model that the amplitude is independent with the phase shift.

The existing channel models are generated by the common modeling techniques for the IRS aided wireless systems which are available for both low frequency communications and high frequency communications.However,the peculiarities of THz waves are lack of being considered.In terms of the THz communications,the severe path loss and molecular absorbing attenuation is diverse at different THz bands.Based on this phenomenon,the IRS-aided THz wireless channel is considered in[133],which consists of a set of frequency and distance-dependent sub-bands,and the channel fading in each sub-band is affected by both spreading loss and absorbent molecules.To combat the serious propagation attenuation of THz waves,a multi-hop IRS assisted THz model is considered in[132],where multiple IRSs are used to overcome severe signal blockage between the BS and users,and to achieve better service coverage for THz communications.Compared with the single-hop IRS aide THz model,the formulation of the multihop IRS aided model is more complex due to the multiple cascaded IRSs.To validate these proposed IRS-assisted THz channels,channel measurements are needed for testing the propagation peculiarities of the THz waves and the electromagnetic properties of the IRS.At the THz band,graphene predominantly exhibits a Drude-like response due to its easily generated and controlled free carriers and therefore is considered as a suitable platform for dynamically tunable metasurface components[171].Based on this concept,an electrostatic gate-tunable perfect absorber based on a monolayer graphene over a grounded dielectric is demonstrated experimentally in the low THz frequency regime[172].An experimentally ultra-fast THz amplitude modulation of a graphene based ultrathin absorber induced by photoexcitation via an optical pump signal is demonstrated in[173].Unfortunately,most of the existing researches on IRS at THz band are still in the stage of theoretical analysis and software simulation,and there is no real intelligent programmable wireless communication environment.Therefore,the primary target is to investigate a new channel model,and meanwhile the channel modeling of IRS assisted THz communication system should be implemented by the practical channel measurements.

Figure 10.Illustration of a graphene based reflecting element,where W0=42 μm,W1=40 μm,and h=30 μm.The permittivity of SiO2(Quartz)is 3.75.

4.2 New Materials Exploration of IRS

The special electromagnetic characteristics of IRS has inspired a research upsurge for making a thorough inquiry about the controllable wireless environment,which is unable to be implemented by the conventional nature materials without the THz beam control function.The pattern of the IRS is formed on the subwavelength scale by utilizing the structure of materials,so as to indirectly change the physical properties of the materials at THz band.In fact,the mentioned IRS is a two-dimensional meta-material with negligible thickness,and the IRS can modulate the impinging electromagnetic wave by generating phase gradient on its surface.Then,the IRS can freely manipulate the propagation direction of THz beam by designing different arrangement of reflecting elements,including polarization state and wave front shape.As discussed before,the key problem of designing IRS is to optimize the dynamic sub-wavelength reflecting elements,and the popular control methods make full use of dynamic materials,such as are composed of VO2[174-176],liquid crystal[177-179],graphene[180]and so on.It is worth noting that the hardware structure of graphene-based IRS has been designed in[41],as shown in Figure 10.In addition,the VO2-based IRS changes its conductivity by using thermal pumping and laser pumping methods,and thus the phase response of each reflecting element can be altered accordingly.

Current research on hardware design of IRS is still immature when we consider that the IRS is applied to practical scenarios in the near future 6G wireless networks.Apart from exploring new materials of IRS,there are also other open problems that should be addressed,including theoretical design of IRS,practical integration of IRS,and engineering manufacture of IRS.For the IRS that intends to realize beam scanning,the active device design in each reflecting element is a hard point.There are commercially available semiconductor devices at microwave band[181-183],such as PIN diodes,varactor diodes(VCDs),or micro motors.Unfortunately,the existing commercial devices at the THz band are unable to be processed into sub-wavelength size.One may note that,some special materials can be employed to meet certain modulation requirements in communication scenarios,such as VO2and liquid crystal.However,both VO2and liquid crystal inherits the drawback of low modulation speed.To solve this problem,the idea of applying high electron mobility transistor(HEMT)to IRS has been proposed[184-186],whose nanoscale two-dimensional electron gas and high electron mobility can provide the possibility for active THz devices.Recently,the HEMT-based IRS has become a research hotspot,while the power consumption produced by this kind of IRS is unbearable.Therefore,it is a pressing mission to develop more useful material of IRS.

4.3 Experimental Testbed Establishment

Though the related techniques of IRS-aided system,such as passive beamforming and cascaded channel estimation,have been widely studied in recent years,there are still some challenges and problems for the IRS-assisted THz communication system to develop the realistic channel models.To meet these demands,it is necessary to realize and develop the experimental assessment and testbeds for the IRS-aided THz system in order to substantiate theoretical findings and mathematical assumptions.

Figure 11.Illustration of VO2-based experimental IRS testbed developed by UESTC.

For the experimental IRS assessment and testbeds at sub-THz frequency band,a few research works can be found in[23,187-191].In[187],NTT DOCOMO and Metawave Corporation have announced the demonstration of a 5G mobile communication system using 28 GHz-band and the world’s first meta-structure reflect-array technology.As a result of this demonstration,the communication system with meta-structure reflect-array is able to achieve 560 megabits per second(Mbps),while the conventional system without reflector only has the data rate of 60 Mbps.In[188]and[23],an experimental testbed using the reconfigurable reflect-array at 60 GHz is introduced for enhancing the performance of indoor connectivity and network capacity.In detail,the reflect-array consists of small rectangle-structure patches whose size is with 25mmfor the width and 25.5mmfor the length,respectively.For a reflect-array with 14×16=224 patches(reflector units),the dimension of the panel is 337mm×345mm×0.254mm(length×height×thickness).In addition,the distance between two adjacent patches is designed to be longer than one wavelength in order to prevent the patches from coupling.Based on the experimental testbed,the authors show that the smart reflect-array can be used for multiple simultaneous communications without any significant interference,and with a reduced link outage probability.Then,the work in[189]establishes a 2.4 GHz RFocus prototype with 3200 inexpensive antennas on a 6 square-meter surface.This configuration may be the largest published number of antennas ever used for a single communication link.Moreover,the experiments show that in a typical indoor office environment,RFocus can improve the signal strength by 9.5 times and enhance the channel capacity by 2 times.In[190],the authors develop a new type of high-gain and lowcost IRS that contains 256 reflecting elements.The proposed IRS combines the functions of phase shift and radiation together on an electromagnetic surface,where positive intrinsic-negative(PIN)diodes are used to realize 2-bit phase shifting beamforming.This radical design forms the basis for the world’s first wireless communication prototype using IRS having 256 reflecting elements with 2-bit quantification.By testing this IRS prototype,the experimental results show that the proposed IRS achieves a 21.7 dBi antenna gain at 2.3 GHz and attains a 19.1 dBi antenna gain at 28.5 GHz.In[191],the authors design an RF sensing system for posture recognition based on IRS.This proposed system can actively customize the environments to provide the desirable propagation properties and diverse transmission channels.Experimental results show that compared to the random configuration and non-configurable environment cases,the designed system can greatly improve the recognition accuracy.

However,the aforementioned experimental testbeds are based on the IRS at low frequency bands,whose design and manufacturing process is easier in contrast with high frequency bands.There are some experimental IRS testbeds at THz band that can be found in[192-195].In[192],the authors present a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1-bit coding metamaterials.Specific coding sequences result in various THz far-filed reflection and scattering patterns,ranging from a single beam to two,three,and numerous beams.Experimental results in[192]show that a reflectivity less than-10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz.In[193],the design and fabrication of a THz metasurface with ultralow reflections is presented.Meanwhile,the numerical results show that THz diffusions can be invoked at the meta-surface due to the randomly distributed phase gradient among the metamaterial structures.Moreover,the experimentally demonstrated one-bit coding metasurface with a dual-band is proposed that could manipulate THz waves at two distinct frequencies(0.78 THz,1.19 THz)independently[194].In[195],the authors present a new strategy to design the low-scattering metasurfaces with the desired bandwidth,based on the deconstructive interference from partial meta-atoms.The metasurface is fabricated on the substrate of polymide with an overall size of 9504um×9504um,and is composed of three kinds of sub-wavelength ring resonators with different phase responses.Besides,the proposed metasurface is tested by a commercial THz time-domain spectroscopic system(Zomega-Z3,USA),which can measure the specular reflectance at different angles of incidences.One may note that,our group at UESTC has also developed an experimental IRS platform by leveraging the phase transition characteristics of VO2as shown in Figure 11,and this IRS prototype works at 300 GHz.However,when the IRS is applied to practical THz communication scenarios,the hardware design of IRS with multiple functions remains as a promising research area.

Figure 12.Illustration of different IRS deployment strategies in THz communications:(a)Distributed deployment strategy;(b)Centralized deployment strategy.

4.4 Deployment and Networking

The IRS deployment and networking technology is an indispensable research field for THz wireless communication systems.As discussed before,THz communication is restricted by the transmission distance due to the huge propagation attenuation.In order to guarantee the workable communication distance,massive IRSs are essential to be deployed in the wireless environment[25,35].Thus,the optimization strategies of IRS deployment at different locations(e.g.,transmitter side,receiver side and distributed in the environment)are also worth exploring.On the one hand,the IRS is leveraged to create the virtue LoS link for THz communications,so the IRS deployment possesses a significant impact on the reflected wireless channels as well as the system performance[196].On the other hand,when the IRS is installed in practice,we need to consider some other influence factors,including deployment cost,practical wireless conditions,building distribution,the number of IRSs,maintenance difficulty,aesthetic degree and so on.These challenges and open problems mentioned above are crucial for IRS deployment in 6G THz communications,but require to be further solved eagerly.

Current research on the IRS intensively considers the capacity characterization by jointly optimizing the phase shift matrix at IRS and the precoding matrix at the transmitter for both single user and multiple user scenarios.Nevertheless,the IRS deployment and IRS position determination for THz communications are still treated as paucity studies.There are some reasons that make the IRS deployment differ from the maturely active relays[197-199].Firstly,due to the passive characteristics,the IRS just reflects the impinging THz signals,and does not have the ability of signal processing.Thus,it is better for THz communication system to combat the serious pass loss by placing IRSs in close vicinity to the transmitter or receiver.Secondly,owing to the advantages of simple structure,light weight and low power consumption,the proposed IRSs can be much more densely deployed in 6G wireless networks,which contributes to extending the communication distance and thus panders to the propagation features of THz waves.Furthermore,we know that active relays suffer from the mutual interference,which definitely aggravates the deployment management of IRSs.On the contrary,the IRSs are composed of passive reflecting elements,and the reflected THz signals that comes from different IRSs can be separated.Based on this,the deployment design of IRSs can be well simplified.In[200],the stochastic geometry tool is utilized to study the performance improvement of cellular networks brought by the massive deployment of IRSs.To solve the deployment and passive beamforming design problem for IRS assisted NOMA systems,the authors propose a twostep machine learning based approach by optimizing the energy efficiency with considering particular data constraints of users[201].In addition,the work in[202]investigates two different IRS deployment cases as shown in Figure 12,including distributed IRS deployment and centralized IRS deployment.More importantly,the capacity region is derived to seek the optimal deployment of IRS assisted systems in practical.Since THz communications may adopt a novel wireless network architecture,the IRS deployment and networking need to consider the compatibility with traditional cellular network architecture.Moreover,considering that the THz beam is narrow,the IRS deployment and media access control(MAC)protocol should be jointly designed to achieve the optimal network performance.

V.CONCLUSION

In this paper,we provide a comprehensive survey on the cooperation of IRS and THz communications,which can realize a reconfigurable and programmable wireless communication environment.Even though the research on IRS empowered THz communications is still in its infancy,it has been shown that the IRS is regarded as a promising paradigm shift to enhance the coverage capability and improve the data rate for THz communications.With the assistance of IRS,THz communications can be well applied to diverse application scenarios in the near future 6G networks.In addition,to guarantee the practical implementation of IRS,both physical-layer techniques and hardware design methods are thoroughly investigated.Despite the notable advantages and benefits brought by IRS,this paper also addresses some emerging challenges and significant problems that will be faced in the future wireless communications.To sum up,this paper hopefully serves as the meaningful attempt and insightful guidance to further give impetus to the IRS empowered THz communications as well as the 6G wireless networks.

ACKNOWLEDGEMENT

The work was supported by the National Key Research and Development Project of China under Grant 2018YFB1801500.Meanwhile,The work was supported in part by The National Natural Science Foundation of China under Grant 6162780166 and Grant 61831012.