Objectives

• • To become familiar with the concept of smart lighting
• • To become familiar with the development of smart lighting
• • To become familiar with the applications of smart lighting

24.2 Background

The development of smart lighting technique consists of two stages [7]. The first stage is that the conventional bulbs are replaced by the new technology, LEDs. In addition, LEDs are used as dominant light source for public illumination. In this stage, the researches are focused on improving the performance of LEDs. Higher efficiency, lower cost, and longer lifetime are the characteristics of LEDs that we pursue. The second stage of development of smart lighting is the lighting control algorithm. Adaptive lighting control solutions are the aims for this stage to make smart lighting system have higher performance efficiency, satisfy users' illumination requirements, and provide wireless network connections.

For the first stage, the semiconductor materials, structure, and fabrication can affect the performance of LEDs. Some researchers develop a high-efficiency InGaN quantum-well LEDs with microspheres [8]. With the help of this special structure and material, the efficiency enhancement of 1.7 times is achieved. In order to improve the quantum efficiency of LEDs, another research group proposed a method to combine high flexibility of polymer films with nitride nanowires [9]. The output light power has an enhancement of to the conventional GaN-based LEDs by using a hybrid structure of straight nanorods [10].

In the second stage, the smart lighting systems are illumination systems that use feedback measurements from a network of various sensors. Considering the quality of output light and power consumption, some researchers propose an optimization framework for control of non-square smart lighting systems with saturation constraints [11]. In [11], two solutions are proposed: one is based on the interior point algorithm, and the other one is based on Newton–Raphson method with projection. With the help of wireless sensor network, smart lighting systems can be controlled efficiently [12]. After experiments, the total power consumption is reduced by more than . Results of the successive 6 days are shown in Table 24.1.

GROUP	DAY 1	DAY 2	DAY 3	DAY 4	DAY 5	DAY 6
Box office 1
Box office 2
Meeting room
Showroom
Total average power saving

Source: Magno et al. (2015) [12].

24.3 Smart Lighting Applications

Smart lighting technique is one of the most significant branches in smart cities, which can be used in many aspects such as dimmable outdoor environment lighting, vehicle safety, indoor illumination and communications, smart lighting positioning, and so on.

Outdoor smart lighting systems consist of public lighting, wireless sensor network, Internet, cloud computing, and other techniques. The diagram of a typical outdoor smart lighting system is shown in Figure 24.1. With the help of sensor networks, the outdoor public lighting can be dimmed. To save energy, the system maintains lighting at a low level until vehicle or pedestrian motion is detected [13]. When the area is occupied, the street light can be dimmed to a higher level to satisfy the illumination requirements and keep safety. To save energy and prolong lamps lifetime, some researchers propose an optimization algorithm to reduce the energy consumption and increase lamp life [14]. From the results in [14], the total energy consumption is decreased by , and the average lamp life is increased by in 12 working hours. Similar to the smart lighting system for the street illumination, parking lot can also use the smart lighting control technique to save energy [15]. The dimming and brightening depend on whether people or vehicles are detected in the monitored area.

The smart lighting can also be applied in vehicles for vehicle safety. The smart automotive lighting can provide illumination and signaling, reliable communications, and so on [16]. An example of smart lighting for vehicles in a highway configuration is shown in Figure 24.2. This vehicle smart lighting system can detect potential risks in advance and provide early warnings to the driver. Therefore, the probability of traffic accidents can be reduced [16].

Indoor and outdoor positioning is another important application for smart lighting technique. Since the lighting system might be installed everywhere, the smart lighting positioning must be a trend of development. An example of indoor positioning using smart lighting is shown in Figure 24.3. For outdoor smart lighting positioning system, bad weather conditions such as rain and fog can easily affect the performance of positioning systems. However, with the help of image sensor, multiple photodetectors, and some algorithms, the outdoor smart lighting positioning is still a promising technique [18]. Indoor positioning systems can be installed on the base of the existing lighting systems. The average positioning error in the smaller environment is 3.78 cm [19].

The most popular application for smart lighting is smart lighting communications. This system uses visible light as communication media and specially designed LEDs work as data transmitters. In the following section, smart lighting communication is introduced in detail. In most cases, the smart lighting communication is known by visible light communication (VLC).

24.4 Visible Light Communication (Smart Lighting Communication) System

With the rapid development of the technology, high data rate transmission will be demanded in our daily lives. Considering the spectrum of radio frequency (RF) communications is so congested, and the data transmission rate of RF communications cannot satisfy the huge demand of the big data transmission, VLC has emerged as a new possible technology for the next generation communications [21]. In VLC systems, white LEDs are used as transmitters for communications and can become the dominant indoor communication method due to its many advantages compared with RF communications. The VLC systems are built as a dual system (illumination and data transmission) and have higher privacy than RF communication systems. LEDs are efficient light sources, and they have long life expectancy [22]. Because of the advantages over RF communications, VLC can become the dominant indoor communication method [23–25].

VLC can also be applied not only in indoor communications but also in outdoor short-range communications, such as vehicle-to-vehicle communications. As the number of vehicles increases every year, an urgent action is needed to prevent and reduce the traffic accidents as well as improve road safety [26]. Since the vehicles' headlights and taillights are usually composed of LEDs, the vehicles can communicate with each other using VLC. Then, an intelligent transportation system (ITS) can be built to improve the road safety and traffic flow based on VLC network.

In addition, the VLC technique can be used for many applications, such as smart lighting, mobile connectivity, healthcare, underwater communications, location-based services, and so on. Therefore, the applications in VLC have a great potential to increase in the next decades. These applications using VLC technique can change the pattern of people's life. According to the latest market research report [27], the VLC market is expected to grow from USD 327.8 million in 2015 to USD 8502.1 million by 2020, at a compound annual growth rate (CAGR) of between 2015 and 2020.

24.4.1 System Description

24.4.1.1 Transmitter and Receiver Model

The quality of signals received at the users depends on the transmitters, the receiver, and the channel models. In this section, we describe the transmitter and receiver models.

Multiple-LED lamp model in Figure 24.4 is proved to have a better coverage of illumination and better communication performance [25]. For this model, there are multiple LEDs with different inclination angles for each lamp, and each LED can be controlled separately.

The receivers' model we used is proposed in [28]. For this model, there are photodetectors with different inclination angles for each receiver; the structure of the multi-detector model is shown in [29]. On this basis, the received signal for each user is a combination of the signals from each detector. To get the optimal signal-to-interference-plus-noise ratio (SINR), we propose a combined optical power allocation and received signal linear combination scheme (Figure 24.5).

24.4.1.2 Indoor VLC Channel Model

In this section, we analyze the indoor VLC channel model and show the derivation of the channel model with the 1-LED and 25-LED lamp models.

For indoor VLC systems, white LEDs work as transmitters and photodetectors work as receivers. Since the visible light is incoherent, intensity modulation and direct detection (IM–DD) are employed in VLC systems (Figure 24.6). After the receiver, the received signal can be represented as

24.1

where represents the responsivity, is convolution, is the transmitted optical intensity, and is the additive noise. is the indoor channel impulse response, which, using ray tracing, can be modeled as

24.2

where and represent the path gain and transmission time delay, respectively. is the number of multipath components. The channel gains and transmission time delays in (24.2 ) depend on the light paths to the receiver.

Because of the principles of optics, the light rays from the transmitter can be classified into two types. They are the line of sight (LOS) rays and diffused rays, as shown in Figure 24.7. These two types cause the multipath effect in indoor VLC system. Thus, the indoor VLC channel transfer function can be approximated by Ghassemlooy and Popoola [30]

24.3

where is the contribution due to the LOS, which is basically independent of the modulation frequency, and it depends on the distance between transmitter and receiver and on their orientation with respect to the LOS. is the diffused part, the intensity of which is less than the LOS part. The impulse response of the indoor channel can also be represented as

24.4

where is the number of reflections.

The intensity of the LOS rays and diffused rays follows the Lambertian law. The Lambertian radiant intensity model can be defined as [31]

24.5

where is the Lambertian mode of the light source and is the radiation angle for transmitters as shown in Figure 24.8. The maximum radiated power is reached when . The Lambertian mode is related to LED's semiangle by

24.6

The detector effective area can be modeled as a function of the incident angle, , as [31]

24.7

where is the area of the detector as shown in Figure 24.8. We assume that the detector cannot be active beyond the field of view (FOV) angle . Therefore, the LOS link can be described as

24.8

Thus, the impulse response of LOS part can be described as

24.9

The diffused part is

24.10

where represent the link attenuations, is the reflection coefficient, and is the area of the detector:

24.11

where is the distance of the LOS link, represents the distance of the th bounce link, is the radiation angle, represents the incident angle, and and represent distance between transmitter and receiver and light speed, respectively [32].

The impulse response of the indoor channel can be obtained by ray tracing algorithm, and an example of the impulse response is shown in Figure 24.9. In this figure, we can find the impulse response composed of the LOS and diffused components. The very long tail represents the diffused components. This long tail can introduce intersymbol interference (ISI) when symbol rate is high. The ISI would affect the communication performance.

24.4.2 VLC MIMO Technology

In an RF multiple input and multiple output (MIMO) system, the signal can be sent by multiple antennas from transmitters and received by multiple antennas at receivers. Similarly, for VLC systems, the MIMO technique can also be used. Figure 24.10 shows a configuration diagram of a typical indoor VLC MIMO system. In this figure four LED lamps are used for room lighting as well as for transmitting independent data streams simultaneously [33].

In a VLC MIMO system, if the number of transmitters is and the number of receivers is , this MIMO system channel can be represented as a matrix. In this section, we assume that there is no ISI; thus the MIMO channel matrix can be described as

24.12

where represents the channel attenuation from the th transmitter to the th receiver. For the indoor VLC system, has been described in [25]:

24.13

Using the MIMO technique, the received signal is given by

24.14

where , represents the signal received by user , , is the transmitted signal from transmitter , , and is the additive noise at receiver . The noise we model here is the thermal noise and background noise (shot noise is assumed to be small).

24.4.2.1 Modulation Schemes in VLC Systems

The conventional modulation schemes adopted in RF communications cannot be readily applied in VLC directly, because the visible light is incoherent. IM–DD are used, so nonnegative signals are transmitted.

24.4.3 On–Off Keying (OOK)

OOK is the simplest technique that can be used in VLC systems. In OOK, the intensity of an optical source is directly modulated by the information sequences, which is usually binary. For a sequence, a bit “one” can be represented by an optical pulse, we call it “on.” For the “on,” the entire bit duration is occupied. On the contrary, a bit “zero,” we call it “off,” can be represented as a blank duration.

For OOK, both the non-return-to-zero (NRZ) and return-to-zero (RZ) schemes can be applied. In the NRZ, the whole bit duration is occupied by the pulse when transmitting “1.” But in the RZ scheme, only partial duration of bit can be occupied.

24.4.3.1 -ary Pulse Amplitude Modulation (-PAM)

M-ary Pulse Amplitude Modulation (M-PAM) can offer higher bandwidth efficiency than OOK, since more bits can be transmitted using one pulse in M-PAM. In M-PAM, a pulse is sent in each symbol duration, where the pulse amplitude takes on one of the possible levels. The bandwidth efficiency is defined as

24.15

where is bit rate and represents the bandwidth. The block diagram of -PAM scheme is shown in Figure 24.11. In this block diagram, represents the noise, and is the received signal at the receiver. The channel impulse response is described in Section 24.4.1. After the matched filter and sampling, we can get the signal ready for demodulation and decision.

An example for 4-PAM modulation is given in Figure 24.12 to help us understand the principle of the -PAM scheme. The data stream ready for transmission is “000111100111” in Figure 24.12. For 4-PAM, we divide the stream into groups containing 2 bits. Here, the stream can be divided into “00”, “01”, “11”, “10”, “01”, and “11”. Converting these binary numbers into 4-ary ones, we get 0, 1, 3, 2, 1, and 3 for each group. For 4-PAM, the transmitters just need to send the corresponding power levels.

24.4.3.2 Pulse Position Modulation (PPM)

PPM is an orthogonal modulation technique that has a higher power efficiency than OOK. However, to achieve a same bit rate with OOK, more bandwidth is required when using PPM. Thus, for the band-limited indoor channel, the PPM scheme may introduce some ISI. The comparison of waveforms between OOK and PPM is shown in Figure 24.13.

24.4.4 Multiuser VLC Systems

In this section, some conventional multiple access schemes for VLC systems are introduced. Studying from RF communication systems, a cellular structure based on MIMO technique is introduced.

24.4.4.1 Multiple Access Schemes

Time Division Multiple Access (TDMA)

Time division multiple access (TDMA) is a conventional multiple access scheme, which is used in the digital 2G cellular system such as global system for mobile (GSM) communications in RF. The users in TDMA transmit signals in rapid succession, one after the other, each using its own time slot as shown in Figure 24.14. For VLC systems, the TDMA can be used directly. The data can be modulated using OOK, PAM, or PPM. The advantages for TDMA are very obvious. The structures of transmitter and receiver can be designed simply. There is almost no multiple access interference (MAI) if synchronization between different users can be guaranteed.

Space Division Multiple Access (SDMA)

Space division multiple access (SDMA) is a multiple access scheme that uses the space information of users to separate them. In RF communications, this scheme is usually used in satellite communication systems. In SDMA, an antenna array is used as the transmitter. From the antenna array, multiple narrow beams are generated according to the users' positions. In this way, the multiple users using SDMA can share the same time slot. In VLC system, SDMA technique can also be applied [34]. In this paper, LED arrays with narrow beamwidth were used as transmitters as shown in Figure 24.15. By turning on different transmitter elements, the angle diversity transmitter can generate narrow light beams of different directions.

Optical Code Division Multiple Access (OCDMA)

In our indoor VLC system, a code division multiple access (CDMA) technique is employed to provide multiple access for simultaneous users. Because of the intensity modulation used in VLC, the CDMA code is different in optical communication than in RF communication. To implement the CDMA technique in optical communications, choosing the proper optical CDMA (OCDMA) code is a significant step.

Nowadays, OCDMA is receiving increasing attention due to its enhanced information security. In an OCDMA system, different users share a common communication medium; thus multiple access is achieved by assigning OCDMA codes to different users [35]. Therefore, the transmitted signal for user can be described as , where is the intended data for user and represents the length- OCDMA code for user .

An important type of OCDMA code is the optical orthogonal code (OOC) that was proposed for IM–DD OCDMA systems [36]. OCDMA codes must satisfy the following conditions [37]:

1. 1. The peak autocorrelation function of the code should be maximized.
2. 2. The cross-correlation between any codes should be minimized.
3. 3. The side lobes of the autocorrelation function of the code should be minimized.

Conditions (1) and (2) ensure that the MAI is minimized, and condition (3) ensures the synchronization process at the receiver. In this chapter, since we consider only the downlink, the codes for all users are transmitted synchronously.

An OOC is usually represented by , where is the code length, is the code weight, is the upper bound on the autocorrelation value for a nonzero shift, and is an upper bound on cross-correlation values. The conditions for OOC are [35]

24.16

where is the th code word and

24.17

There is a special case of that the OOC is represented by [35]. Table 24.2 gives us examples for code word indexes (the positions of the “ones” in the code word). For example, the index with length 7 represents the code word in Figure 24.16.

	Sequence index, when ,
7
13
19
25
31
37
43

Source: Ghafouri-Shiraz and Karbassian (2012) [37].

Orthogonal Frequency-Division Multiple Access (OFDMA)

Orthogonal frequency-division multiple access (OFDMA) is a novel multiple access scheme based on OFDM technique, which is used in 4G LTE. Multiple access in OFDMA is achieved by assigning subsets of subcarriers to individual users, which is a multiuser version of OFDM. The basic principle of OFDMA is shown in Figure 24.17.

Conventional OFDM generates complex-valued bipolar signals, which need to be modified in order to become suitable for VLC. This operation effectively maps the information symbols onto subcarriers in different frequency bands. A real OFDM signal can be obtained but reduces the system bandwidth by half. This approach has been widely accepted in the literature for the generation of a real OFDM signal. The resulting waveform, however, is still bipolar in nature (it has a positive and negative part). A number of approaches have been proposed for the creation of a unipolar signal. For example, a DC bias can be added to the original bipolar signal. This scheme is known as DC-biased optical OFDM (DCO-OFDM) [38]. Another approach is asymmetrically clipped optical OFDM (ACO-OFDM) [39]. In this scheme, only the odd-indexed subcarriers in the OFDM frame are modulated with information.

24.4.4.2 Cellular Structure for VLC Systems

In VLC systems, a tiny cellular network, attocell network, can provide wireless access for users in an indoor environment. Different from the hexagonal cell in RF communication network, we propose a circular cell as shown in Figure 24.18. The radius of the cell is artificially defined for users in the indoor environment, such that only if the user is located in the access area can it be served by this lamp. To make sure that all area in an indoor environment can be covered, there may be some overlap area between cells as shown in Figure 24.19. If there are some users located in the overlapped area, they can be served by both lamps.

Centralized Power Allocation Algorithm for Multicell Indoor VLC Systems

A centralized power allocation joint optimization (PAJO) algorithm is introduced in this section [25]. According to this algorithm, we maximize the minimum SINR of all the users and design an MMSE receiver for each user. A block diagram of the PAJO algorithm is shown in Figure 24.20. For PAJO, the transmitted signal from each LED depends on to as shown in Figure 24.20. is the intended data for user . The power allocation for each LED is different, which depends on the channel feedback from the users. Since the location of each user is different, the channels of the different users are different. Therefore, we need to allocate the transmitted power to compensate the channel loss for different users.

Distributed Power Allocation Algorithm for Multicell Indoor VLC Systems

A distributed power allocation scheme, weighted distributed power allocation joint optimization (WD-PAJO) method is introduced in this section [40].

For WD-PAJO, the indoor area can be classified into a single-covered area, double-covered area, triple-covered area, and so on, as shown in Figure 24.21a. In this algorithm each LED lamp works independently and just serves the users that are located in its access area. Thus, there is only one lamp per optimization thread. To eliminate the MAI, each lamp does power allocation optimization by maximizing the minimum weighted SINR for all the users in its own access area. The SINR is weighted by the number of lamps transmitting to the user so that users served by many lamps are not unduly advantaged. For lamp , the algorithm computes

24.18

where is the power allocation matrix for the th lamp, is the access area of lamp , and is the number of lamps serving user .

Figure 24.21b shows an example of how WD-PAJO works. In this example, lamp 1 serves users a, b, and c. Lamps 2 and 3 serve users b and c and users c and d, respectively. Unlike the PD-PAJO, all the three lamps must work independently. Since the three lamps do not communicate with each other, they can do the power allocation optimization in parallel. For this example the objective functions for the three lamps can be represented as

24.19

In general, the WD-PAJO algorithm can be accomplished via the same steps as the PD-PAJO. The only difference is that for WD-PAJO there is only one lamp in each thread, so no communication between lamps is needed.

24.4.5 Practical Considerations

In this section, some practical considerations such as ISI, nonlinearity of LEDs, illumination requirements, and dimming control in indoor VLC systems are analyzed and discussed.

24.4.5.1 Intersymbol Interference (ISI)

In a practical indoor VLC system, because of reflections of walls and furniture, the indoor channel is dispersive. In addition, the bandwidth of LEDs is bandlimited (usually 3 dB bandwidth is 100 MHz); therefore, when the transmitting symbol rate is higher than the overall bandwidth of the channel (LED and indoor channel), the signal must be distorted and the ISI is introduced.

There are two approaches to diminish the ISI. First approach is to reduce the symbol rate. To make sure a relatively high bit rate is achieved, OFDM technique and multilevel modulation schemes such as -PAM can be used. However, either OFDM or multilevel modulation scheme may introduce nonlinear distortion because of nonlinearity of LEDs. Therefore, the second approach, equalization, becomes more and more popular. Some researchers have proposed methods to design an equalization circuit in [41, 42] to improve the transmitted data rate, which can achieve up to 340 Mb/s using OOK modulation scheme with BER at [43]. There are also some related works to VLC system equalization concentrating on the ISI elimination, which is caused by the dispersive indoor channel. The zero forcing algorithm is generally applied to mitigate the effects of ISI in infrared wireless systems in [44]. Artificial neural network (ANN)-based equalizer is a feasible way to build an equalizer at the users' port [45]. In this paper, with the help of the ANN equalizer, the bit rate can achieve 170 Mb/s using OOK. Some researchers proposed frequency domain equalization algorithms in VLC, which can reduce the ISI and improve the data rate [46] by designing an equalizer in frequency domain. Linear equalizer (LE) and decision feedback equalizer (DFE) can also be applied in VLC systems. For LE and DFE, training sequences are needed. The coefficients of the LE and DFE can be trained. From the results in [47], both the LE and DFE can reduce the ISI.

24.4.5.2 Nonlinearity of LEDs

LEDs are commonly used light sources for lighting systems, and they also serve as transmitters in indoor VLC systems. The optical power from LEDs is driven by an input electrical signal that carries information. Due to the structure of LEDs and the principles of generating light, the relation between the output optical power and the input current can be modeled as a nonlinear function as shown in Figure 24.22. This nonlinearity of LEDs introduces a nonlinear distortion on transmitted optical signals.

24.4.5.3 Illumination Requirements and Dimming Control

Considering the total radiation power limit and the illumination requirements of the users, the constraints can be represented as

24.20

where is the required received power for illumination of user and is the illumination tolerance. This constraint is to make sure that the illumination level at these users will not be too dark or too bright. represents the CDMA code weight to CDMA code length ratio, which decides the illumination level.

To illuminate the room uniformly in space, we define virtual users that must have a fixed illumination but no data transmission. These users are distributed uniformly in the room. For these virtual users that require just illumination, we use the constraint

24.21

where is the channel between the th transmitter to the th virtual user, is the required received power illumination for user , and is the number of virtual users that only need illumination.

Figure 24.23a shows a contour plot of the illumination distribution for 4 users with both data transmission and illumination requirements, plus 16 virtual users with illumination requirements only. Figure 24.23b shows the illumination distribution without data transmission requirements. Comparing these two figures, the illumination distribution in (a) is still smooth and flat. That is to say, setting illumination constraints prevents the lighting system from creating too dark and too bright spots in the room, and the illumination requirements at all the user locations are satisfied.

24.5 Conclusion and Outlook

This chapter introduces the smart lighting system and its applications. The smart lighting system is an energy-efficient system that can be widely used in indoor and outdoor public illumination. Using LEDs as light sources and data transmitters, VLC system can support high rate wireless connections. In this chapter, we analyze the brief principles of VLC system. For multiuser VLC system, we propose centralized and distributed power allocation algorithms using MIMO technique. From the theoretical analysis and numerical results, we conclude that the smart lighting system can support high rate wireless access and specific illumination requirements with low energy consumption and high power efficiency.

Although some practical problems need to be considered, smart light VLC can become a practical augmentation technology with high security. Increasing interest from academia and industry shows that smart light VLC system can be successfully commercialized in the coming years [48].

Final Thoughts

In this chapter, we discussed the concept of smart lighting, its development, and its applications. The smart lighting technique is one of the most significant branches in smart cities. Smart lighting can be applied in outdoor public lighting systems, vehicle safety lighting, positioning systems, and smart lighting communication systems. In this chapter, we emphatically introduced indoor smart lighting communication principles, power allocation algorithms, and practice considerations. After analysis, we can define smart lighting system as a combination system, which includes illumination, safety alarm, positioning, and communication. From the experiment results, the smart lighting system is an environmental friendly system that can save energy.

Questions

1. 1 What is the definition of smart lighting?

2. 2 Name and describe four common applications for smart lighting technique.

3. 3 Name four multiple access modulation schemes and describe the advantages for each scheme.

4. 4 What are the limitations of indoor smart lighting communication systems?

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