In this paper, a unique and novel visible light communication based motion detection is presented. The proposed motion detection is performed based on white light LEDs and an array of photodetectors from existing visible light communication (VLC) links, thus providing VLC with three functionalities of illumination, communication and motion detection. The motion is detected by observing the pattern created by intentional obstruction of the VLC link. Experimental and simulation results demonstrate the validity of the proposed VLC based motion detection technique. The VLC based motion detection can benefit smart devices control in VLC based smart home environments.
© 2015 Optical Society of America
Radio frequency (RF) based wireless communication networks are in constant search for high speed, high efficiency and wide frequency spectrum to cope with an ever-increasing volume of data traffic. As an alternative to this RF based wireless communications, visible light communication (VLC) has been emerged over the last decade [1–5]. Up until now, VLC has been conceived a transmission technology providing illumination plus communication in an indoor environment.
We propose motion detection as another paradigm of functionality in VLC that can efficiently be used to control smart devices in conjunction with emerging VLC based smart home environments . Motion detection techniques have been studied in terms of optical motion detection or motion gesture control (MGC) using infrared (IR) [6, 7] and optical cameras . The drawback of the IR based motion detection techniques is that the techniques need extra hardware and circuitry to transmit the detected motion information. Moreover, it is certain that it cannot be used for illumination. On the other hand, the optical camera based motion detection can cause an intrusion of privacy and requires more complex processing circuitry and high processing power. As an additional functionality to the existing VLC links, the VLC based motion detection is considered. This technique offers low-cost and efficient motion detection and thus will exhibit great potential in indoor VLC smart devices environments.
In the proposed technique, we present a unique and novel motion detection using VLC; therefore, three independent functionalities are provided from the existing VLC links, i.e. illumination, communication and motion detection. Based on an array of photodetectors (PDs), which is used for communication and motion detection, the motion is detected by observing the pattern created by intentional obstruction of the VLC link. We also propose an algorithm to identify the motion created by the user. Experimental and simulation results demonstrate the validity of the proposed VLC based motion detection technique.
Section 2 provides details of the proposed motion detection technique. Experimental setup is explained in Section 3 and results are discussed and analyzed in Section 4. Conclusions are drawn in Section 5.
2. Proposed motion detection technique
2.1. Basic principle
The basic principle behind the proposed technique is to utilize an array of PDs for the detection of the pattern created by intentional obstruction in the established VLC link. This array also contributes to enhanced VLC performance via receiver diversity obtainable from multiple PDs. Without loss of generality, we assume a total of 9 PDs employed in the present motion detection. Figure 1 shows the principle of the proposed motion detection technique. For the data detection, we define two threshold levels, Th1 and Th0. That is, the intensity detected above Th1 is considered “1” and the intensity detected between Th1 and Th0 is deemed “0”. If the intensity goes below Th0, it is “No data” (ND) condition.
This ND condition occurs when the obstruction is intentionally created in the VLC link. In other words, the ND detection for a period of time in a predefined fashion provides the exact detection of the pattern created by the motion. As an example, the pattern for “ON” command can be created by intentionally making a straight line over any of the three sets of PDs. Likewise, the pattern for “OFF” command can be created by making a circle. Figures 1(c) and 1(d) show these patterns. The detailed algorithm for the detection of these command signals is explained in the subsequent section. Note that with the present PD array, we can create not only the basic motion control signals, i.e. “ON” and “OFF”, but also advanced control signals such as “Increase” and “Decrease”. Figure 2(b) shows these motion control signals.
2.2. Motion detection technique
The proposed technique is based on the user’s intentional obstruction of the VLC link followed by the detection of the pattern created by that motion, while the VLC link operates for data transmission. We use an array of light emitting diodes (LEDs) at the transmitter that fulfils the need of illumination as well as data communication to transmit the modulated data [1–3]. In the present study, the modulation format employed for the data transmission is non-return-to-zero (NRZ) on-off keying (OOK) that is the simplest modulation scheme mentioned in PHY I of IEEE standards . Prior to the modulation, encoding schemes can be applied to remove a long trail of 0 and 1. We use Manchester code as a run length limited (RLL) line code mentioned in .
As noted earlier, the array of PDs acts as the VLC receiver as well as the motion sensor. Figure 3 shows the structure of the proposed VLC based motion detection technique where there are two paths, i.e. the VLC data transmission and the motion detection. For the data transmission, we have the three groups formed from all PDs with each group having three PDs. This grouping will facilitate efficient decoding of the transmitted data from the PDs, even when the motion detection technique is in operation. The received signals from the three groups of the PDs are first fed into the threshold detector and demodulator (TDD). The TDD block estimates the received intensity, detects the transmitted symbols, and converts the symbol into a bit stream. Since we employ the OOK modulation, symbols are interchangeable with bits. The binary data from each PD through the TDD block is then passed to the selection combining (SC) blocks where the most probable bit is detected . The detected bits from the three different blocks are provided to the decision circuit to accurately decide whether the bits are either “0” or “1”. Finally, the line decoding is performed to recover the transmitted bits. Note that when the obstruction occurs, the detected bits from one or two particular SC blocks may not be accurate. Therefore, the decision circuit is employed for further reliability of the data detection.
On the other hand, for the motion detection, the signals from all PDs are fed into the motion detection circuit so as to detect the pattern created by the user as shown in Fig. 3. The motion detection circuit detects the ND condition as mentioned in Section 2.1 and subsequently identifies the pattern created by the user. Then, this detected motion will eventually initiate the intended control of the devices.
2.3. Algorithm for motion detection technique
The proposed detection technique is solely based on the obstruction created by the user and initiates a command to control smart devices. Therefore, the detection of the ND condition for a specific period of time, Δt, at a PD is important. Apparently, the value of Δt needs to be determined on the basis of the speed of hand movement of the user. An empirical value obtained from the experiments for Δt is 100μs with a tolerance of ±10μs. If the motion is faster or slower than Δt, it is not considered as any signal. Likewise, if the movement is irregular, the pattern is not recognized. It should be noted, however, that the algorithm is designed to allow an extra time of 10μs as an additional margin for irregular motion. Nevertheless, this interval can readily be calibrated for a particular user prior to practical applications.
The algorithm for the motion detection is illustrated in Fig. 4. It is important to note that the user is assumed to create the predefined patterns and the algorithm detects the patterns over the VLC link. The proposed algorithm is designed to provide a degree of flexibility. For example, the pattern for the “ON” signal can be drawn, starting from either PD 1 or PD 2 or PD 3, not in the reverse order. Similarly, for the pattern of the “OFF” signal, the user can start from any of PD 2, 4, 6 and 8.
Figure 5(a) shows the pattern detection starting from any of the three PDs, i.e. PD 1, 2 or 3 for the “ON” condition. It can be observed that the obstruction occurs at the designated PDs after an interval of Δt. Likewise, Fig. 5(b) shows the circular pattern for the “OFF” condition with the assumption that the pattern begins at PD 2. It also shows flexible pattern detection capability for the “OFF” signal when the user does not complete a full circle or the user starts from PD 6. For the “ON” signal, the detection algorithm is also flexible that it can be designed to detect from PD 7 or 8 or 9. It was found, however, that the “ON” signal requires at least 3 PDs for accuracy and reliability of the motion detection.
It should be noted that the present study focuses on the validity of the proposed motion detection technique while providing an efficient VLC link. For this reason, we employ the simplest modulation scheme of OOK to demonstrate the effectiveness of the proposed technique. Nevertheless, the proposed technique can readily be extended simply by defining the minimum threshold (Th0) for the ND condition, to accommodate other modulation schemes such as orthogonal frequency division multiplexing (OFDM) , pulse position modulation (PPM) , pulse width modulation (PWM)  etc.
3. Experimental setup
In order to verify its effectiveness, we performed experiments with an array of LEDs comprised of 20 RGB LEDs having a modulation bandwidth of 120 MHz with an optical output power of 60mW each. This optical output power is considered adequate in fulfilling the need of illumination. We used the NRZ OOK modulation scheme as described previously. In addition, prior to the modulation, a line encoding scheme mentioned in  was employed to remove a long trail of 1 and 0 from the data. The data transmission was performed at a data rate of 10 kbps, based on Arduino ATMEGA 2560.
The experimental setup for the demonstration of the proposed motion detection is shown in Fig. 6(a). Figure 6(b) shows an array of 9 PDs in a more detail at the receiver side with a field of view of (FOV) of 60°, a physical area of 1.0 cm2 and responsitivity equal to 1. Along with the PD array, Arduino ATMEGA 2560 board was also utilized at the receiver end.
4. Result and analysis
Experimental results are shown in Fig. 7. Figure 7(a) shows the detection of the “ON” signal based on PD 1, 4 and 7. It can be observed that the sequential ND occurrence is detected for a period of Δt from PD 1, 4 and 7. Therefore, this pattern is identified as the “ON” signal according to the proposed algorithm. For the “OFF” condition, Fig. 7(b) shows the received signal. The sequential ND measurement is also observed from PD 2, 6, 8 and 4, which is interpreted as the “OFF” control signal.
In the experiments, it was found that over a distance of up to 60cm as shown in Fig. 6(a), no bit errors were observed from 21,000 bits transmitted, while maintaining a data rate of 10 kbps. Moreover, a high level of accuracy for the motion detection of the “ON” and “OFF” conditions was obtained from the proposed technique. With respect to the threshold values, they are determined by performing the transmission of a known trail of 1s and 0s prior to actual transmission of the data. The two threshold levels are so obtained and fixed. The values of Th0 and Th1 are experimentally found to be 25 lx and 100 lx, respectively.
In addition to the detection accuracy analysis, we conducted performance evaluation of the VLC link using MATLAB. It was ensured that the simulation parameters are identical to the parameters of the experiment, except for the data rate, distance and number of transmitted bits. We used a data rate of 96Mbps and transmitted 108 bits. The distance of the data transmission ranges 40cm to 100cm. The simulation results are shown in Fig. 8. It reveals that the conventional OOK with SC is superior to the conventional OOK without SC [1, 4]. This performance gain is due to the receiver diversity in the form of SC, obtained from the PD array. A further analysis was conducted for the effect of the motion detection on the VLC link performance. It is observed that the effect of the proposed motion detection on the existing VLC link is negligible in terms of the BER performance, regardless of whether it is “ON” or “OFF” condition. In other words, it can be said that the communication quality remains unchanged due to SC. Therefore, the proposed motion detection constitutes an additional functionality of the existing VLC link. It is also worth noting that the impact of Δt is negligible on the communication link, because the ND condition of either “ON” or “OFF” signal does not affect the link performance.
For more advanced motion gestures in addition to the present gestures considered, the proposed detection technique can further be extended by defining the patterns and subsequently the detection algorithm with a denser PD array having a large number of PDs or the use of imaging receivers. This denser PD array or imaging receivers can also enhance detection accuracy for the advanced motion gestures at the expense of more complex detection algorithm.
A unique and novel technique for the VLC based motion detection is presented. The proposed technique is found to provide accurate detection of the motion while satisfying illumination requirement. The detection algorithm is also presented on the basis of the obstruction duration (Δt) created by the user. The experiments and simulation results show that the proposed technique is able to accurately detect the motion using an array of PDs and also does not affect existing VLC link performance. The proposed VLC based motion detection can be considered an additional functionality of emerging VLC systems and is envisioned to perform a relatively basic control of present or future smart devices in environments such as VLC based smart homes where the device control via motion detection can aptly be facilitated.
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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