Fubin Wang，Xuan Huang，F(xiàn)ang Yang，2，Hui Yang，2，Jun Wang，2，Jian Song，2，*

2 Key Laboratory of Digital TV System of Guangdong Province and Shenzhen City，Research Institute of Tsinghua University in Shenzhen，Shenzhen 518057，China

1 Department of Electronic Engineering，Tsinghua National Laboratory of Information Science and Technology，Tsinghua University，Beijing 100084，China

*The corresponding author，email: jsong@tsinghua.edu.cn

Abstract: The ubiquitous deployment and restricted consumption are the requirements restricting the development of Internet of Things.Thus，a promising technology named Internet of Lamps(IoL)is discussed in this paper to address these challenges.Compared with other communication networks，the remarkable advantage of IoL is that it can make full use of the existing lighting networks with sufficient power supply.The lamps can be connected to the Internet through wired power line communication and/or wireless communication，while the integration of integrated sensing，hybrid interconnection，and intelligent illumination is realized.In this paper，the IoL is discussed from three aspects including sensing layer，network layer，and application layer，realizing the comprehensive upgrade based on the conventional communication and illumination systems.Meanwhile，several novel technologies of IoL are discussed based on the requirements of sensing，communication，and control，which have put forward practical solutions to the issues faced by IoL.Moreover，the challenges and opportunities for IoL are highlighted from various parts of the system structure，so as to provide future insights and potential trends for researchers in this field.

Keywords: Internet of Lamps (IoL); integrated sens-

I.INTRODUCTION

Smart city is a vision of urban development，which aims to integrate a variety of information and communication technologies to improve the quality of life[1，2].The Internet of Things(IoT)is considered as a promising technology for smart cities[3]，but the conventional IoT still faces with the issues like high cost and power consumption，so that the concept of the Internet of Lamps(IoL)is proposed in this paper.

The IoL will use the existing infrastructure to deploy the IoT，so as to achieve intelligent lighting for multifarious applications[4].Taking advantage of the natural links between lamps and the power line，the IoL will solve the power supply problem based on the wired communication to assist the wireless communication [5].The IoL can be adopted as a platform for IoT applications to realize sensing，communication，positioning，data analysis and other services under the constraints of illumination requirements.

In this paper，the IoL is addressed from the perspective of the sensing layer，network layer，and application layer to achieve the ubiquitous deployment with low costs and sufficient power supply.In the IoL，the lamps can be connected to the Internet through the wired power line communication (PLC) and/or wireless communication，thus realizing the integra-tion of lighting network，power network，and information network [5].Then，several important technologies in the IoL are introduced.In the aspect of sensing，multiple low-cost sensors are arranged to realize the measurement by the multi-node joint sensing technique [6，7].For the network interconnection，a hybrid routing scheme based on wireless and wired technologies is proposed to realize the routing in the IoL，while the communication is achieved based on integrated PLC，visible light communication (VLC)，and radio frequency (RF) communications [5，8].Moreover，the technique of joint illumination and communication is introduced[9，10].Finally，the challenges，opportunities，and future trends for the IoL are proposed to provide some insight.

The concept and system of IoL are proposed in this paper，integrating the advantages of wired and wireless communications，as well as the RF and VLC technologies.The contributions and innovations of this paper are summarized as follows.

1.The IoL is addressed as the three-layer framework including sensing，network，and application layers，so that the communications and illumination systems are upgraded comprehensively.

2.The significant techniques of IoL are invesitgated and several practical solutions are proposed，in order to satisfy the requirements of sensing，communication，computing and control.

3.Combined with promising future technologies，the relationship and interaction of different layers in IoL are analysed，while several research trends are also prospected.

The rest of this paper is organized as follows.In Section II，the related works are investigated，and the IoL is introduced from the perspective of the sensing layer，network layer，and application layer in Section III.In Section IV，the novel technologies of IoL are discussed，while the challenges，opportunities，and future trends for the IoL are highlighted in Section V.Finally，the conclusions are drawn in Section VI.

II.RELATED WORKS

The IoT is an extended network based on the Internet [11，12]，realizing the interconnection of human，machines，and things at any time and place [13，14].However，the cost of deploying new infrastructure is very high to achieve ubiquitous IoT [15，16].In addition，with the increase of network devices，the power consumption is also greatly increased[17].The IoL deployed on lighting infrastructure will solve the power supply problem of IoT，and integrate with PLC，RF，and other communication technologies to improve robustness of communications.Moreover，the IoL will expand the sensing capability and control intelligence further to realize the integration of multiple functions.

The VLC is one of the methods to support wireless communication in IoL，where the lamps can be used as access points(APs)[18，19].Most of the light emitting diodes (LEDs) for lighting are excited for communication and illumination simultaneously in VLC [20].Once the lamp is off，the APs can support wireless communication with RF or infrared light[21].Meanwhile，the lamps are also adopted for integrated sensors，communication modules，and computing components in IoL，so as to realize the integration of positioning，communication，and computing[22，23].

The IoL concentrates on heterogeneous communication，integrated networking，and intelligent illumination based on existing lighting infrastructure，which integrates PLC，VLC，RF，and other technologies for communication，sensing，and control [24，25].Not only the lighting effects will be controlled intelligently based on human feelings and demands，but also a variety of communication methods and functions are combined in IoL to expand the IoT capability in multiple aspects，making it more diversified，more flexible，and more reliable.Moreover，the IoL can perceive human behaviors and environment situations through lamps [26]，and use cloud computing and big data analysis to realize intelligent control of smart home appliances，thus achieving intelligent and personalized services[27，28].

In this paper，the IoL is compared with the conventional lighting infrastructure，VLC，and IoT，as listed in Table 1.For sensing capability and intelligent control，the IoL integrates sensors with lamps to realize ubiquitous sensing，and there are flexible control strategies including field control，remote control，and smart control.For human-centric illumination，VLC may consider the illumination constraints for dimming control，but IoL will pay close attention to various lighting requirements in different scenarios.Furthermore，the IoL realizes the integration of multiple dimensions，including diverse communication approaches such as PLC，RF，VLC，and infrared communication，as well as multifarious functions like sensing，control，communication，and illumination.

Table 1. Comparisons of IoL and other state-of-the-art solutions.

Table 2. Simulation configurations.

III.IOL SOLUTIONS

The conventional lighting networks have neither Internet access nor communication function.They can only obey the manual control passively，but cannot realize automatic perception and change accordingly.Therefore，many intelligent applications are not supported.In this paper，the IoL combines the information technologies with lighting networks，so as to promote the process of ubiquitous communication and illumination in the future.The structural diagram of the IoL is shown in Figure 1，which can achieve the intelligent lighting for various applications.

Figure 1. An illustration of the IoL system.

The IoL has the ability of integrated sensing，hybrid interconnection，and intelligent illumination.Integrated sensing refers to the realization of environment sensing and human behavior sensing through the integration and miniaturization of sensors.Hybrid interconnection includes the inter-communication between smart terminals and lamps，among lamps，as well as the inter-connection between lamps and the Internet.Intelligent illumination refers to analyzing human behavioral preferences based on big data，and then controlling lighting and smart home appliances，thus achieving personalized service.In this section，new features for the sensing layer，the network layer，and the application layer of the IoL are introduced.

3.1 New Features for the Sensing Layer

The basic lighting network has almost no sensing function，while the conventional sensing systems with single functions require additional deployment，and there will be a sharp rise of cost with higher accuracy requirements.In the IoL，the function of the sensing layer is to obtain environmental information through the ubiquitous sensors，aiming at the deployment characteristics of dense，distributed，and sensor-integrated lamps.Its main objective is to achieve the integration and miniaturization of sensors，so as to realize the environmental sensing and human behavior sensing.

As shown in Figure 2，the sensing layer is the core of the IoL as the bottom layer[29].On the one hand，the related technologies about lighting sources should be investigated，in order to realize the integration of multi-functional and high-efficiency lamps in complex electromagnetic and thermal environments.The lighting efficiency can be improved by adopting highefficiency chips，non-lead packaging，and using micronano structure for antireflection.For the differentiated demand for illumination，a personalized database about the lighting quality should be constructed.From the perspective of lighting quality，the light source with high lighting efficiency should be fabricated.In addition，the integration of functional and lighting modules needs to be addressed.Specifically，focusing on the requirements of intelligent lamps，the integrated functional modules of sensing，communication，and control should be investigated[30].

Figure 2. The architecture of the IoL.

On the other hand，the sensing technology related to intelligent lighting should be addressed.The infrared radiation source and the array infrared detector module should be investigated.According to various requirements，novel light sensors are designed for different scenarios.For the optical sensing，the photo diode(PD)and the avalanche PD(APD)can be adopted for wireless optical communication，and the infrared sensors are appropriate for motion capture of human beings，while the image receiver can be integrated with lamps for environmental monitoring.For example，a new type of occupancy sensors should be designed by using the ultrasonic or far-infrared isomeric technique.And a centimeter-level positioning sensor with highprecision is realized by the visible light positioning technique.Aiming at the sparse characteristic of distributed sensing data，the compressed sensing theory could be adopted to reduce data sizes significantly.

3.2 Novel Characteristics for the Network Layer

Most the conventional communication systems work in single service modes，and the performance will be limited by the single system characteristics [31].For example，RF systems suffers from the shortage of spectrum resources and the co-channel interference，while VLC systems are influenced by the lack of lineof-sight (LoS) links and the poor mobility.In the network layer of IoL，the technologies of PLC，RF，VLC，and others are integrated，so that the complementary advantages can be achieved [18].VLC can realize higher data rates of IoL with the large bandwidth，while alleviating the overcrowding problem of RF spectra.In order to realize the reduction of switching overhead，RF is helpful with the ability of non-LoS(NLoS) communication and larger coverage.As for the backhaul，PLC can solve the problem of“information isolated island”，and the infrared communication will achieve the uplinks of optical communication.

As a link between the sensing layer and the application layer，the network layer is the middle layer of the IoL，as shown in Figure 2 [14].Specifically，the network layer in the IoL is aiming at the demand of intelligent lighting applications for mobile，selforganizing，and ultra-dense network.Its main objective includes the inter-working between smart terminals and lamps，among lamps，as well as the interconnection between lamps and the Internet.Therefore，the network layer needs a new type of the wired and wireless cooperative access protocol to construct heterogeneous convergence of multiple wireless communications and PLCs，so as to achieve robust multi-node interconnection，support low-cost and ubiquitous interconnection，and improve the transmission reliability as well.

Figure 3. Different controls in the network layer of IoL.

In the aspect of intelligent lighting，the IoL needs to realize three different types of controls，i.e.，field control，remote control，and smart control，as shown in Figure 3，while the bluetooth low energy(BLE)technology will be utilized to significantly improve the coverage of sensors.Field control refers to that use hand-held terminals or intelligent remote controllers to control the switch of lamps through wireless communication.It mainly includes the positioning technique and the on-off control technique.Remote control refers to that use remote terminals to access the cloud server through the Internet，and after the cloud server accesses the gateway，it sends control information and remotely controls the status of the lamps based on this information.Smart control refers to the overall control of the whole interconnection system of the lamp.It mainly includes cloud control，gateway control，device control，and multicast mode control.

3.3 Novel Features for the Application Layer

The requirement of lighting is more and more humancentric for indoor applications，which is difficult to be satisfied by the conventional lighting technology.The application layer is at the top layer the IoL，as shown in Figure 2.Its function is processing，i.e.，to process information through the cloud computing platform to provide services or operations required by users[27].Its main objective is to realize intelligent illumination，i.e.，to analyze human preferences based on big data，so as to achieve intelligent and personalized services.In order to support smart services，the data of users will be transmitted to the cloud with the permission of users.Then，the behavior of the user can be analyzed and distinguished through corresponding algorithms，so that the lighting can be controlled intelligently[28].

Figure 4. Hybrid routing and cross-media communication in IoL.

Figure 5. Joint illumination and communication in IoL.

The lighting control intentions and demands of the users need to be identified intelligently.On the one hand，the multi-channel signal processing technique and microphone array can be used to enhance the ability of voice recognition.In order to enhance the effect of far-field voice recognition and improve the antinoise ability，the multi-channel signal processing and back-end processing can be combined to obtain the overall optimization from voice source to recognition system.On the other hand，in order to understand implicit lighting requirements of users，continuous semantic judgment of the voice should be made.When the users comment the lighting in the room，the evaluation and demand for the lighting will be analyzed，and then control instructions for lighting system should be generated according to the evaluation and demand.

Moreover，it is necessary to model and evaluate the illumination comfortness，so that lighting can serve people actively，intelligently，and in real time.A comfortness evaluation model can be established by using the model identification and order reduction technologies.Based on multi-dimensional sensing data，environment situation and human behavior are perceived，and the comfortness level can be evaluated by the comfortness model.Therefore，user experience can be improved by adjusting controllable factors.

IV.NOVEL TECHNOLOGIES IN THE IOL

The existing lighting facilities can be used to achieve VLC，without the need for new infrastructure.In addition，the cost of LED transformation is much lower than the deployment of IoT equipments.Furthermore，the power consumption of LED is low，and it is more green and environmentally friendly.Besides，as long as the light is blocked，the information cannot be leaked to people outside the lighting area，so compared with the conventional IoT，the IoL has the higher confidentiality and security.Therefore，the IoL can be an attractive candidate for ubiquitous communications in the future due to its existed ubiquitous infrastructure，low costs，and sufficient power supply[4].

Specifically，the IoL has the following three characteristics compared with the conventional IoT [17].First，sensors can be integrated into lamps to achieve ubiquitous deployment，and the lighting network can be easily upgraded to the IoL.Secondly，PLC or even VLC can be used to realize communication besides the conventional wireless sensor network.Thirdly，the supply of power grid can be used to relieve the power limitation of the conventional IoT，so that many complex technologies can be introduced，such as the edge computing and artificial intelligence(AI).In this section，novel technologies in the IoL，such as sensing，routing，and communication，will be investigated.

4.1 Multi-Node Joint Sensing Technique

In the conventional IoT，some sensing data needs to be measured by expensive sensors with high precision.In the sensing layer of IoL，the lighting lamps are ubiquitous with intensive deployment，while the convenience of power supply eliminates the power consumption limit of frequent sensing.Therefore，the technique of joint sensing is possible，where the measurement can be realized by the multi-node joint sensing technique with arranging multiple low-cost sensors[6].

Based on the rational adoption of multiple sensors and their observation information，the complementary and redundant information of various sensors in space and time is combined according to some optimization criteria.Then，the accurate results are approximated by AI and other methods[7].The theory and method of multi-sensor based on big data are utilized to reduce the pressure of data transmission，improve the response speed，and achieve the accurate environment sensing.Furthermore，based on the abundant and convenient power supply in IoL，the communication and calculation of the edge terminals are no longer limited by the problem of energy consumption，so that the frequency of observations can be further increased to improve the sensing accuracy.

Specifically，sensors at the bottom layer are responsible for preprocessing and forwarding the original sensing data，so as to reduce the overhead of directly transmitting data.As the sensing range of the terminal is limited and the sensing information of the adjacent terminals is highly correlated，the terminals are clustered according to the geographical location，and an edge terminal is deployed for each cluster.In addition，all the edge terminals form the middle layer，and use the edge computing technique to filter，extract，and fuse the received original data，so as to remove redundant information and improve the sensing accuracy.

4.2 Hybrid Routing and Cross-media Communication

For the connection of IoL networks，not only the dynamic switching of wireless and wired routing，but the integration of PLC，VLC，and RF can be realized as well.In this way，the equipment interconnection and access control in the network layer of IoL are achieved based on the mutual cooperation of heterogeneous links and the complementary advantages of cross-media technologies.

If only wireless routing is used in the IoL，the instability of the network connection may occur due to the path loss，user mobility，user interference，and other problems [8].However，if only the power line routing is used in the IoL，noise interference，the on-off of the line，and other unstable factors will also cause the route to break from time to time.Thus，a hybrid routing scheme based on wireless (such as Zigbee)and wired PLC technologies is proposed to realize the routing in the IoL，as shown in Figure 4.The communication technology which integrates the Zigbee，WiFi，and other methods with PLC as the backbone is proposed based on the low-cost lighting facilities to ensure the self-healing of interruption in the Internet with intelligent control.The integration of RF and VLC also provides complementary advantages for the reliability and flexibility of wireless links in IoL.

Figure 6. Comparison of data rates under different dimming requirements.

Both optical and RF communication will face the problem of limited coverage，so that the cross-media communication is taken into consideration，which is the integration of PLC，RF，VLC，and infrared communication.Specifically，LEDs and power supply lines have a natural coexistence relationship，so that each LED can be connected to the lighting network with the help of PLC [5].As long as the PLC chip is embedded in the LED，the LED can receive and transmit the information conveyed on the power line.In order to obtain the required data from the gateway，each router is integrated with the modem for signal modulation from the wireless routing or wired PLC onto the visible light or the radio wave.Each wireless or wired router is equipped with the LED lamp and the RF AP for downlink coverage，while the uplink transmission is supported by infrared or RF communication[21].

4.3 Joint Illumination and Communication

The most significant feature of IoL is realizing illumination and communication at the same time，which are the most principal requirements and services in the application layer of IoL.In the process of channel coding，modulation，framing，and other signal processing，the illumination constraints should also be taken into consideration，such as uniformity constraints and illumination intensity as shown in Figure 5.

The better the illumination uniformity，the superior the communication mobility.In order to satisfy the mobility of IoT devices，it is necessary to put forward the illumination uniformity for better user experience and communication mobility improvement.The illuminance of light is standardized in IEEE 802.15.7，the minimum of which should be no less than 0.7 of the average[32].The illumination uniformity can be improved with the design of LED arrays and power allocation[33].In addition，the techniques of lens design and space-time coding can also be adopted.

In addition，the illumination intensity of VLC needs to meet the requirements of human vision in different scenarios.For example，the appropriate illumination for human eyes is 300 lux in reading and writing work，while it should be around 30 lux in computer work[25].Therefore，the amplitude and average value of visible light signals are limited，which ought to be controlled with relevant dimming technologies [9].Multiple dimensions of digital signals can be adopted for dimming control，including intensitydomain，frequency-domain，time-domain，etc.[10]，and they can also be controlled and optimized jointly.

Some numerical results are illustrated of dimming techniques in Figure 6，and the relevant parameters are listed in Table 2.The normal directions of LEDs and PDs are vertical，and the locations of them adopt the three-dimensional rectangular coordinate system，where a corner of the room ground is taken as the zero point(0 m，0 m，0 m).The locations of LED and PD in the simulations of Figure 6(a)are(2.50 m，2.50 m，3.00 m)and(0.50 m，1.00 m，0 m)，respectively.The number of subcarriers is 256，and there are 4 groups of subcarriers.As for the simulations in Figure 6(b)，the locations of LEDs are (1.00 m，1.00 m，3.00 m)，(4.00 m，1.00 m，3.00 m)，(4.00 m，4.00 m，3.00 m)，and (1.00 m，4.00 m，3.00 m)，while the locations of PDs are (2.45 m，2.45 m，0.70 m)，(2.55 m，2.45 m，0.70 m)，(2.55 m，2.55 m，0.70 m)，and (2.45 m，2.55 m，0.70 m).The number of subcarriers is 64，and the layer number of layered orthogonal frequency division multiplexing(LACO-OFDM)in[10]is 3.

Figure 7. Future applications of IoL with emerging technologies.

The joint dimming in [9] optimizes the peak-toaverage ratio by selecting the number of carriers in the frequency domain，while achieving a larger range of dimming by adjusting the proportion of symbols in the amplitude domain.Compared with the amplitudedomain dimming in [34] and the time-domain dimming in [35]，the joint frequency-amplitude-domain dimming can adapt to more diversified dimming requirements，as reflected in Figure 6(a).The data rates are larger under different dimming levels，which has more obvious advantages under high dimming demand.

Moreover，the joint design of spatial and amplitude domains can also improve the dimming performances.By deploying multiple LED transmitters and PD receivers，the preliminary dimming is realized based on selecting the index of LEDs in the spatial domain，and the precise dimming is achieved by adjusting the symbol proportions in the amplitude domain.As shown in Figure 6(b)，the joint spatial-amplitude-domain dimming proposed in [10] brings out superior performances over the spatial dimming and bias adjustment in [36] under each dimming demand.It can also realize larger data rates than the conventional dimming technique via spatial modulation in both higher and lower dimming requirements，while the implementation is simpler without adjusting the direct current(DC)bias.

V.CHALLENGES，OPPORTUNITIES，AND FUTURE RESEARCH TRENDS

At present，the researches and applications of IoL has made great breakthrough.However，for the technologies and services of IoL，there are still some challenges and possible opportunities worthy of discussion in the future research，as shown in Figure 7.For the sensing layer，the technique of simultaneous lightwave information and power transfer(SLIPT)can be adopted to achieve energy harvesting(EH)and power supply for sensors.For the network layer，the requirements from the sensing layer can be dealt with by AI technologies to realize human-centric illumination and communication.Meanwhile，a large amount of computing burden required by network controlling can be allocated to multiple edge devices based on the technique of edge computing.For the application layer，it is no longer to realize the single sensing，communication，or illumination，but concentrate on the comprehensive consideration of integrated services，determining the functions and behaviors of the whole IoL system.

5.1 Simultaneous Lightwave Information and Power Transfer

In the face of the explosive growth of network equipment in the sensing layer of IoL，the power supply problem of mobile terminals needs to be solved.SLIPT is a kind of EH technologies，which can retain the energy of visible light while achieving wireless communication.IoL provides opportunities for SLIPT due to the ubiquity of illumination networks.SLIPT can achieve power collection and supply at the level of mW [37]，and is more suitable for IoT scenarios.It makes full use of radiated light energy to improve energy efficiency without additional power supply.

Therefore，the design of VLC systems in IoL needs to consider not only the current communication performance，but the requirements of SLIPT and following communication as well.For example，the power allocation of DC and alternating current (AC) is adopted to realize the multi-hop cascade communication[38].The improvement of communication characteristics such as spectrum efficiency needs to be further explored，and SLIPT will also bring new constraints on time slots and power[39].

5.2 Artificial Intelligence for the IoL

With the development of humanistic lighting，only using big data and cloud computing cannot meet human intelligent requirements for lighting.AI can approximate functional relationships that are difficult to express explicitly with training of multi-layer networks，in order to discover the implicit interaction of between system parameters and optimization objectives.In the future，in order to realize the human-centric lighting，it is necessary to introduce AI for scenario modeling and system optimization in IoL[40].

AI can be adopted to create lighting models for different scenarios[41].Firstly，data acquisition and fusion technologies are used to collect the factors affect the satisfactory on lighting，including external light intensity，weather，season，temperature，mood，and so on.Then，based on these collected factors，through AI technologies，combined with big data analysis，feature mining，and clustering analysis，the personalized lighting model can be created，and then the personalized lighting parameters can be predicted.

5.3 Edge Computing Based on IoL

The number of connected devices in the network layer of IoL will grow to billions in a few years，which makes the conventional cloud computing not sufficiently efficient to deal with all these data[42].Thus，most of the data generated by the IoL will never be sent to the cloud，but will be consumed at the edge of the network.In addition，with the increasing amount of edge data，data transmission speed becomes the bottleneck of cloud computing paradigm[43].Thus，it is necessary to process the data at the edge of IoL to reduce the response time and improve the processing efficiency，while the acceptable computational capacity and sufficient storage space can be ensured.Furthermore，compared with the conventional IoT，the IoL has abundant power supply to support the edge computing.

In the systems of edge computing，connected devices in the IoL not only play as data consumers，but also as data producers，which can not only request services and contents from the cloud，but also perform the computing tasks.In addition，owing to the diverse computing burden of multifarious devices at the edge，the cloud server can also distribute requests and deliver services among different devices.However，edge nodes produce data all the time，thus，the amount of data tends to be infinite，while the storage capacity of edge nodes is limited.In addition，at the edge nodes，it is easy for the illegal intruders to obtain information.Thus，how to process and save these data and how to ensure the security of edge computing have become the challenges of future IoL.

5.4 Integration of Sensing，Computing，Communication，and Control

The poor communication environment will lead to the inaccessibility of the system，and the incomplete information will affect the control performance.If the sensing，computing，communication，and control algorithms are designed separately，the performance of the whole network will be restricted.Therefore，in the future，sensing，computing，communication，and control will be jointly designed.The performance of IoL is determined by the communication performance and the control strategy of the network.Simply optimizing any one of them cannot meet the requirements of the overall design of the network.

The integrated design of sensing，computing，communication，and control makes the sensing process provide information support for the control process.The communication process is responsible for realizing the real-time and reliable interaction of the sensing process，while the control process provides the control decision to ensure the stable and efficient operation of the network.Moreover，all the processing can be realized based on the collaboration of the centralized cloud computing and decentralized edge computing.

Furthermore，the integration also needs to be extended to multi-mode，multi-parameter，and multiobjective scenarios，including wireless and wired routing，cross-media links，joint illumination and communication，etc.Specifically，the implementation of dynamic routing needs to consider wireless/wired transmission capacity and routing algorithms.The integration of PLC and VLC realizes power supply and network connection at the same time without extra cables，and the joint of RF can compensate for the limited coverage and frequent switching of VLC.Moreover，communication is no longer the only pursuit in IoL，while the illumination effect is important，and dimming indicators based on human feelings need to be proposed.

VI.CONCLUSION

In this paper，the concept of the IoL is proposed，which has the ubiquitous deployment with low costs and sufficient power supply.The IoL was discussed from the aspects of sensing layer，network layer，and application layer，which were aimed at realizing integrated sensing，hybrid interconnection，and intelligent illumination，respectively.Then，the novel technologies of IoL were introduced，including the multi-node joint sensing，hybrid routing，and integrated communication technologies.In addition，the lamps can be connected to the Internet through PLC and/or wireless communication to achieve the integration of the lighting network，power network，and information network.We also highlighted the key challenges，opportunities，and future research trends of the IoL.It is expected that the IoL will play an important role in future ubiquitous communications.

ACKNOWLEDGEMENT

This work is based upon researches supported by Tsinghua University-China Mobile Research Institute Joint Innovation Center，in part by the Science，Technology and Innovation Commission of Shenzhen Municipality (No.JSGG20201103095805015)，in part by the National Natural Science Foundation of China(No.61871255)，in part by the Fok Ying-Tung Education Foundation，and in part by Beijing National Research Center for Information Science and Technology(No.BNR2022RC01017).