In an indoor bidirectional visible light communications (VLC), a line-of-sight (LOS) transmission plays a major role in obtaining adequate performance of a VLC system. Signals are often obstructed in the LOS transmission path, causing an effect called optical shadowing. In the absence of LOS, the performance of the VLC system degrades significantly and, in particular, at uplink transmission this degradation becomes severe due to design constraints and limited power at uplink devices. In this paper, a novel concept and design of an optical bidirectional beacon (OBB) is presented as an efficient model to counter the performance degradation in a non-line-of-sight (NLOS) VLC system. OBB is an independent operating bidirectional transceiver unit installed on walls, composed of red, green, and blue (RGB) light emitting diodes (LEDs), photodetectors (PDs) and color filters. OBB improves the coverage area in the form of providing additional or alternate paths for transmission and enhances the performance of the VLC system in terms of bit error rate (BER). To verify the effectiveness of the proposed system, simulations were carried out under optical shadowing conditions at various locations in an indoor environment. The simulation results and analysis show that the implementation of OBB improves the performance of the VLC system significantly, especially when the LOS bidirectional transmission paths are completely or partially obstructed.
© 2015 Optical Society of America
With various advantages such as license-free spectrum, electromagnetic interference-free transmission and high security, visible light is now able to provide a data rate of gigabits per second (Gbps) . As a result, it is believed that visible light communications (VLC) with abundant bandwidth (in terahertz) will emerge as a compelling technology that has potential to change the face of future wireless communications .
A VLC system using light emitting diodes (LEDs) as transmitters and photodetectors (PDs) as receivers relies predominantly on intensity modulation and direct detection (IM/DD) technique for communication. VLC used to offer the two unique functionalities: illumination and communication. Interestingly, another unique functionality of VLC based motion detection has recently been reported in the literature . Together with other known advantages, such as cost-effectiveness and no harm to humans and electronic devices, the deployment of VLC systems is envisioned to grow rapidly for years to come.
In recent advancements, a data rate of 3 Gbps was achieved using a single µLED and adaptive data loading technique , while a very high data rate of 4.22 Gbps was achieved with a red, green, and blue (RGB) LED using hybrid time-frequency adaptive equalization algorithm at a distance of 1 cm . Up until now, the highest throughput achieved in a mobile indoor VLC system is 5 Gbps, which utilizes a laser diode (LD) with an imaging receiver and delay adaptation technique . Extensive works have been reported by extending the point-to-point communication link mentioned in the IEEE standard . In this respect, authors proposed resource allocation  and user allocation scheme  to address multiuser bidirectional transmission scenarios in a VLC environment, while all optical bidirectional links over a more realistic transmission span of 2.3 m has been reported in . In the area of smart home applications, VLC has also been considered to efficiently support various smart devices [3,10].
In designing a high-speed VLC system, there are several challenges to resolve. Among these, a low modulation bandwidth of an LED and intersymbol interference (ISI) are most prominent. Undoubtedly, significant efforts were directed on addressing these issues. Most notable solutions are the use of a blue optical filter , artificial neural network based equalizer , pre- and post-equalization with orthogonal frequency division multiplexing (OFDM)  and parallel transmission in the structure of color clustered multiple-input multiple-output . To reduce the effect of ISI, an adaptive recursive least square (RLS) decision feedback equalizer (DFE) with reduced training sequences was employed in , while the authors in  used zero forcing equalization with an LED transmitter arrangement to reduce ISI.
The performance of a VLC system depends largely on both the optical power received from the line-of-sight (LOS) transmission path and the distance between the LED transmitter and the receiver. However, the transmission path is prone to obstruction due to various movements, objects or man-made structures in an indoor VLC environment. As a consequence, the quality of service of a VLC system is vulnerable to the optical shadowing in the optical wireless transmission. A few studies were reported in the literature to counter this optical shadowing effect. In , Burton et al. presented an angle diversity receiver that employs selection combining to achieve mobility and counteract the optical shadowing, while several LEDs were utilized at downlink based on time division multiple access to address the effect of shadowing on a VLC link . Focusing on the impacts of shadowing and blocking on mobility, the characterization of the channel was carried out in , considering random movement of people within the room. Another group of researchers proposed an imaging LD-VLC system with delay adaptation approach . In , Xiang et al. analyzed the effect of human shadowing on the indoor VLC channel; however, an effective solution to improving the performance for a bidirectional VLC link under this shadowed channel was not presented. Therefore, the reported schemes lack an efficient solution to overcoming the optical shadowing effect in a bidirectional VLC link.
In this paper, a conceptual design and analysis of an optical bidirectional beacon (OBB) is proposed to improve the performance of a bidirectional VLC system. OBB is an independent operating bidirectional transceiver unit consisting of RGB LEDs, PDs and color filters. The installation of OBB is critical in hostile environments where attenuation and signal loss are significant and are required to be compensated in order to provide adequate performance, regardless of any obstructive objects present along the path in the underlying indoor environment. Therefore, the present system aims to provide performance and coverage improvements with additional paths created from the OBBs so that the proposed VLC can be a future compelling indoor wireless transmission system.
In the proposed OBB based bidirectional VLC system, on-off keying (OOK) is employed, which is a PHY I modulation scheme for optical wireless communications in the IEEE standard 802.15.7 . The OBB unit consists of RGB LEDs where the red color is used for downlink transmission, while the blue color is used for uplink transmission. The BER performance is investigated at different locations in an indoor VLC environment under optical shadowing conditions. The proposed OBB units installed on each wall decode and forward the received signal to enhance the performance of the VLC system in terms of bit error rate (BER), received power and illumination, especially when the LOS path is optically shadowed or occluded.
Section 2 presents the description of the indoor VLC with OBB. Theoretical analysis is presented in Section 3 and results are analyzed in Section 4, demonstrating the effectiveness of OBB. Conclusions are drawn in Section 5.
2. Proposed optical bidirectional beacon model
Figure 1(a) shows a probable scenario for the optical shadowing in an indoor VLC environment. In order to counter this shadowing effect, a simple solution can be considered by installing a larger number of LEDs and PDs in the underlying indoor environment. However, this solution suffers from obvious drawbacks. As a large number of LEDs and PDs are installed, the operational complexity associated with these LEDs and PDs becomes significant, let alone a complicated circuitry or algorithm for the operation. As an alternative to this and other previous solutions, OBB is considered in an indoor VLC environment. This OBB based model is a simple and viable solution to mitigating the optical shadowing. Figure 1(b) shows a conceptual design of an OBB based VLC system.
OBB is a transceiver unit composed of N RGB LEDs, a PD with a red optical color filter (BP635) for downlink signal and another PD with a blue optical filter (BP470) for uplink signal. The number of LEDs deployed in OBB may vary in accordance with practical applications and the dimension of an indoor environment. In the present study, 100 RGB LEDs are utilized in OBB. Figure 2 shows the design of an OBB unit.
An OBB can be placed at any location within the indoor environment in accordance with the physical dimensions of the room. For the current study, an indoor environment having a dimension of 5m×5m×3m (length×width×height) is considered. For the ease of demonstration, OBBs are considered to be installed at the center of each wall at a height of 1.5 m above the ground. Figure 3 shows an actual OBB based VLC transmission environment.
As the considered VLC system operates in a bidirectional environment, the red color of RGB LEDs is used for downlink transmission, whereas the blue color of a phosphor based white LED is employed for uplink transmission to minimize the interference at bidirectional transmission . A single PD installed at the ceiling is used as an uplink receiver for the bidirectional communication. For performance analysis, we designate three locations marked as Device 1, Device 2 and Device 3 in Fig. 3.
To demonstrate the effectiveness of the proposed OBB based VLC, we employ on-off keying (OOK) modulation scheme, which is the simplest modulation mentioned for optical wireless communications in PHY I of IEEE 802.15.7 standards . It should be noted, however, that the identical or similar performance can be obtained with other modulation formats.
At uplink transmission, the modulated OOK signal is transmitted using the blue color of a phosphor based white LED and is received at the OBB unit installed on the wall. At the OBB, the blue color filter filters out for the optical signal transmitted using the blue light and the PD converts this optical signal to electrical signal. The electrical signal is decoded and retransmitted using the blue color of the RGB LEDs toward the uplink receiver. It is worth noting that during the transmission from the OBB, the average power in other two color components is changed according to the power variation in the blue color with a view to maintaining flicker free white color . This transmitted signal from the OBB is then received by the receiver installed at the ceiling that consists of a PD and the blue color filter. It is important to note that the PD installed at the ceiling is designed to detect the signal that is considered to be the largest over the bit time span, when the signals from both the device and the OBB are received at the PD. The block diagram of the OBB operations is shown in Fig. 4.
Similarly, the downlink transmission is also performed using the red color of RGB LEDs (transmission) and the red color filter (reception). Clearly, it is impossible to predict human movement in an indoor environment and therefore the OBBs are designed to transmit and receive the data continuously. In the current study, perfect synchronization is assumed among the three components, i.e. the downlink transmitter, the uplink transmitter and OBB.
In the design of the OBB based VLC, it should be noted that additional time dispersion is intentionally created from the OBB unit. It is known that all the rays from LOS and non-line-of-sight (NLOS) paths arrive at the destination in less than 20 ns and also this time dispersion depends heavily on both room geometry and wall materials . The OBB induced additional time dispersion may be excessive so that compensatory measures such as advanced equalizers need to be employed to avoid significant ISI and subsequent adverse effect on performance and data rate [14, 15]. In the OOK based OBB VLC, ISI occurs when the time dispersion caused by the sources and the destination via OBB is greater than the bit duration. Since the focus in the present study is placed on the investigation of the OBB design to combat the optical shadowing, the bit duration is set to 100 ns. That is, the bit duration is sufficient to avoid the adverse effect of the ISI. Although this increased bit duration would inevitably lessen the maximum achievable data speed to 10 Mbps, this data speed can usually be enhanced with higher modulation schemes such as direct current biased optical OFDM (DCO-OFDM), M-quadrature amplitude modulation-OFDM (M-QAM-OFDM), etc. [1,4,8].
To verify the proposed OBB scheme comprehensively, we introduce two conventional optical shadowing models. The first model is that there is a single downlink transmitter and an uplink receiver at the center of the ceiling. This model is referred to as Reference 1 (R1) model and is shown in Fig. 5(a). It is this model that most VLC studies consider in terms of LED arrangement . The second model is that there are 4 transmitters and 5 PDs. This model is usually employed to exploit transmitter and receiver diversity from multiple LEDs and PDs and can be considered a widely employed LED arrangement as well as a possible effective solution to the optical shadowing problem. This model is referred to as Reference 2 (R2) model and shown in Fig. 5(b). These reference models are directly compared against the proposed OBB based VLC in terms of link quality.
3. Theoretical analysis
A theoretical illumination performance of the proposed VLC system is discussed. The brightness of an illuminated surface is expressed in luminance. It is assumed that the light intensity emitted from the transmitter has a cosine dependence at the angle of emission with respect to the normal surface . For the proposed OBB model, the luminous intensity is given by the total luminance from the transmitter installed at the ceiling and the OBB unit as22]
The light intensity emitted from OBB, I(ϕb), at an angle ϕb with the center luminance intensity, Ib(0), is given by
A horizontal illuminance Eh at a point (x,y) is given by
3.2. Received power
For the OBB based indoor environment, the power received at the receiving plane, Pbeacon(t), is the sum of optical power from the transmitter installed at the ceiling, Pr(t), and the OBB power, Pb(t) and is given by22] 22]
As a measure of the optical shadowing, we define an optical shadowing indicator (OSI) in percentage terms. Given an Nshadow as an OSI, the received power, Pshadow, can be obtained as
3.3. Signal-to-noise ratio and bit error rate
A SNR value of the detected electrical signal is given by22].
In the present analysis, the modulation format employed for data transmission is OOK. Therefore, the BER of the OOK modulation scheme is expressed as 
4. Results and analysis
An indoor VLC environment must fulfill the need of sufficient illumination. For practical applications, however, the number of LEDs should be determined by the room dimension for adequate illumination distribution. For the current study, 1600 RGB LEDs were implemented in the considered indoor environment. Without loss of generality, the total number of LEDs in both R1 and R2 is set to 1600. That is, each transmitter in R2 has 400 RGB LEDs. As noted earlier, the OBB unit consists of 100 RGB LEDs and 2 PDs with respective color filters (red for downlink and blue for uplink). Over the transmitter on the ceiling, 1200 LEDs were implemented for downlink transmission. That is, the total number of LEDs in the proposed OBB model is 1600, i.e. 1200 LEDs + 4 OBBs with each OBB having 100 LEDs. Therefore, R1, R2 and the OBB model have the identical number of LEDs throughout for the purpose of conducting a fair comparison.
The transmitted optical power of a single LED is 60 mW and the center luminous intensity of 750 mcd with 60° semi-angle at half power. The reception was made at 0.85 m above the floor by a PD with a red optical filter (BP635) having optical filter gain, TR(ψ), equal to 1. The PDs in the downlink receiver, the uplink receiver and the OBB have a physical area of 1.0 cm2 and have responsivity, R, equal to 1 and the field of view, FOV, equal to 60°. The blue optical filter (BP470) for the uplink receiver also has optical filter gain, TB(ψ), equal to 1.
Figure 6 shows the illumination distribution for R1, R2 and OBB models. It can be observed from Fig. 6 that the current LED setup provides sufficient illuminance of 500 to 1200 1x, defined by ISO , over the entire room for all 3 models. Hence, it can be said that the installation of the OBBs does not cause any impact on illumination.
We further analyzed the performance of a single user bidirectional transmission using the OOK modulation at various locations, i.e. L1 (0.2, 0.2, 0.85), L2 (0.6, 1.2, 0.85) and L3 (1.2, 2.5, 0.85) at different OSI values. Figures 7(a) and 7(b) show the BER variation for R1 and R2 models. It is observed that the BER performance degrades severely when the OSI values are higher than 50%, which makes communication nearly impossible, especially for L1. This is due to the fact that the receiver is located in the far corner region and suffers most from the optical shadowing.
Similarly, the impact of the optical shadowing for uplink transmission was analyzed. The results are shown in Figs. 8(a) and 8(b). For an uplink transmission, the performance degradation becomes even more severe, due largely to the design constraints and limited power at uplink devices. It is evident that the BER performance of R1 is severely degraded even at relatively low OSI values, especially for L1 and L2. For the R2 model, however, the performance is less affected by the optical shadowing than R1, due to the receiver diversity. Nevertheless, it degrades severely at the OSI values higher than 50%.
The proposed OBB assisted VLC model is considered to address the optical shadowing induced poor link quality in a room or smart homes environment. The OBBs are placed on each wall as shown in Fig. 3 and the BER performance was analyzed for both downlink and uplink transmission.
Figure 9 shows the BER performance comparison at downlink transmission for L1. The proposed OBB exhibits an improved performance at the OSI value of 50%. Figures 10 and 11 show the BER performance comparison for L2 and L3, respectively. The OBB model still gives better performance than the reference models at both L2 and L3 at the OSI values of 30% and 50% at downlink transmission.
Similarly, the performance improvement from the OBB based VLC at uplink transmission can be observed in the indoor environment for all 3 locations. Figures 12, 13 and 14 show the simulation results. In Fig. 12, the OBB model gives better performance than R1 and R2 at L1 when the OSI is 50%. In respect of L2, the OBB model offers more improved performance than the other reference models even at the OSI values of 30% and 50%. When compared at L3, the OBB model outperforms the two models at all OSI values. Hence, it can be said that the proposed OBB model provides the best performance at uplink transmission over two reference models and two locations (L2 and L3) at the OSI value of 30% or higher.
It is evident from the results that the performance of the bidirectional communication link in a VLC system is significantly enhanced with the installation of the OBBs in terms of BER performance while maintaining the illumination level. The advantage of the OBB implementation is more distinct for uplink transmission as anticipated. Hence, the OBB unit considerably improves the performance especially at the OSI values of 50% or higher at downlink and uplink alike. Moreover, the installation of the OBB units offers this performance improvement with relatively less complicated circuitry and with no additional LEDs required, thus saving power consumption. It should be noted that OBB is designed to be an independent operating unit and can be battery operated or connected to main power supply.
To evolve VLC into an indoor high speed last-mile wireless communication system, an efficient bidirectional transmission in an indoor environment such as smart homes, offices, convention halls, exhibition centers, etc. is a prerequisite for its successful deployment even under severe optical shadowing conditions. Performance degradation has been observed under the optical shadowing conditions, thereby VLC being unable to deliver an efficient link performance. We proposed an efficient OBB based VLC system to address this NLOS transmission environment. It was demonstrated that while maintaining the illumination level, the proposed OBB system outperforms the conventional VLCs over the OSI values greater than 50% at both uplink and downlink transmission. Therefore, it can be said that the installation of OBB significantly enhances the performance and proves to be a viable solution to the deployment of an effective indoor bidirectional VLC system, when the channel is optical shadowed or completely occluded.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education ( 2015R1D1A3A01017713).
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