Cells distribution for visible light communication can enhance the capacity of the data transmission by the reuse of optical spectrum. In this paper, we adopt three modulation formats as OOK, PPM and PWM for neighboring cells A, B and C respectively. The prototype experiment results demonstrate the error free transmission of 1.0Mbit/s and 6.25Mbit/s visible light communication system with our scheme. With the available LED, we can expect that the data rate of a visible light communication system with seamless connectivity can be up to 71.4Mbit/s.
© 2012 OSA
Nowadays, light emitting diode (LED) as the new generation of solid-state light sources has been intensively studied in various fields of applications, such as lighting, automobiles, transportation and communication [1–4]. Owing to the advantage of fast response in switching on and off, on-off-keying (OOK) modulation can be easily introduced into the LED based visible light communication (VLC) . The works of Dominic O’Brien et al. show that the typical lighting levels can provide a communications channel with the signal to noise ratio (SNR) in excess of 40 dB for indoor visible light communication . IEEE 802.15.7 standard for short-range wireless optical communication using visible light has been published in last year , which includes the MAC and PHY specifications, but does not mention the cells distribution as the cellular communication. Hyun-Seung Kim et al. proposed the way of utilizing carrier allocation to mitigate the inter-cell interference in visible light communication system . The information is modulated into the radio frequency (RF) at first, and then the RF signal as a carrier is used to modulate the LED. In this case, the bandwidth limitation of the LED is the obstacle for the application with high data rate. Handover in VLC systems, realized by cooperating mobile devices, has been investigated in Ref . To avoid connectivity interruptions, the buffer is used to store data. However, the buffer is possibly exhausted before a new link is established.
Pulse position modulation (PPM) is widely used in optical wireless communication system due to its high energy efficiency , and the pulse-width modulation (PWM) is a very popular way for dimming control . In this paper, we adopt OOK, PPM and PWM as the modulation schemes for three adjacent regions respectively in the cells distribution. With our modulation schemes, we verified that the cells distribution could provide the seamless coverage and avoid connectivity interruption during switching the data streaming from the different cells. The proposed schemes can provide a way for position estimation inherently, and the simulated results support this point.
2. Cells distribution with the modulation schemes
Simplified cells distribution for seamless coverage of data transmission is illustrated in Fig. 1 . Three modulation formats such as OOK, PPM and PWM are used in the adjacent cells A, B and C respectively. The centers of the cells A, B, and C construct an equilateral triangle. The LED lamps are mounted at the three vertices of the equilateral triangle. To set the original point at the center of the equilateral triangle as shown in Fig. 1, we can establish the Cartesian coordinate system in Fig. 2 .
The Lambertian radiation pattern is applied to model the LED radiant irradiance. The power of the LED lamps is 1 W and the other parameters are the same as the model in Ref . The receiver plane with the detectors is located 2.15 meter below the coordinate system. With the distance between the lamps and the original point of 1.5 meter, we obtain the intensity of optical distribution as illustrated in Fig. 3 .
The rise and fall time of the available RC-LED-650-02 (produced by ROITHNER LASERTECHNIK) is 3ns, so the minimum pulse-width should be no less than 6ns. The proposed modulation schemes are illustrated in Fig. 4 . The bit duration of the three kinds of modulation is the same as 14ns. For PWM modulation, pulse-width of 6ns represents bit ‘0’ and 8ns represents bit ‘1’. For PPM modulation, the pulse-width is set as 7ns. In this way, the brightness of each modulation can be 50% on average and the date rate of 71.4Mbit/s can be achieved for the seamless coverage.
In order to provide the distinction of different cells and modulation formats at the receiver plane, we employ 11 foregoing bits, like ‘100-1111000’ for A, ‘010-00110011’ for B, and ‘001-01010101’ for C before the subscribed data in a frame. In the first three-bit duration, LED at A, B and C lights on in turn, which is used to indicate the location of the receiver. The positioning is made according to the maximum of the intensity integration in one-bit duration. At the receiver, the following eight symbols are used as the maximum likelihood reference to decode the information carried in the OOK, PPM and PWM format, including eight possible combinations of the three modulation formats as 100, 101, 110, 111, 000, 001, 010 and 011. The receiver first determines which cell it locates, then stores the signal of the following eight-bit duration, defined as . When the signal of the subscribed data is received, defined as the error function is computed. The minimum determines that the ith bit is the same as the jth bit. It should be noted that when the receiver is at the border of neighboring cells, two pulses are detected at the first three-bit duration. In this case, a signal waveform can be decoded to the bits corresponding to two neighboring cells respectively, and handover between two cells occurs.
3. Position estimation
The first three bits are introduced to help the receiver estimate the location by calculating the received signal intensity. The distribution of optical power for the cells A, B and C are depicted in Fig. 5 respectively. The parameters for the simulation are the same as those in section 2. The received signal intensity is fluctuated because the field of view (FOV) of receiver is deviated from the vertical direction , therefore we consider the vertical FOV of receiver to estimate position.
For the proposed cell distribution, the receiver is located in only one region with triangle pattern such as illustrated in Fig. 1. We calculate the ratio of power distribution from LED A over that from LED B and LED C respectively as shown in Fig. 6 . The curves at the top-right corner are the ratios of LED A over LED C, and the curves at the bottom-right corner are the ratios of LED A over LED B. To consider the area where the signal intensity from LED A is higher than that from LED B and LED C, the position of cell A is reduced from the triangle ABC to the quadrangle OEAD, the area of which is one third of the triangle ABC. The closer the location is to the point A, the more accurate the positioning estimation is. On the other hand, the closer the location is to the point O, the more fuzzy the positioning estimation is.
4. Experiment results
The prototype experiment setup is shown in Fig. 7 . At the transmitter, the FPGA (EP2C8Q208C8) is used to drive LED. The blue LED is used to avoid the blue filtering for the phosphorescent white LED . The power of the LED is about 20mW. Three pieces of LED are mounted in an equilateral triangle with 1cm far away from each other. The photon-detector (PD) with the active area of 0.8mm2, provided by THORLABS (PDA10A), is placed 2cm far away from the LED. The oscilloscope (OSC) produced by Tektronix Corp. measures the received waveform from the PD.
In order to measure the response of the LED used in the experiment, we programmed the FPGA and generated the pulses with pulse-width of 20, 40, 60, 80, 100, and 120ns to drive the LED. Figure 8(a) illustrated the temporal response of the LED. From Fig. 8(b), we can see that the rise and fall time is 40 and 20ns respectively, so the minimum pulse-width should be no less than 60ns. In this case, the proposed modulation schemes for OOK, PPM and PWM are illustrated in Fig. 8(c). The bit duration of three modulation formats is the same as 160ns. For PWM modulation, pulse-width of 60ns represents bit ‘0’ and 100ns represents bit ‘1’. For PPM modulation, the pulse-width is set as 80ns. In this way, the brightness of each modulation can be 50% on average and the date rate of 6.25Mbit/s can be achieved for the seamless coverage.
To turn off the LEDs in the neighboring cells, the waveforms received in cell A, B and C without interference from adjacent cells are shown in Fig. 9 . The subscribed data for cell A, B and C are ‘0011-0001-0010-0000-100…’, ‘1011-0111-1100-0010-001…’ and ‘1110-0110-0101-1100-000…’ respectively. In the first three-bit duration, three LEDs light on in turn as the location indication, with the pulse width of 100ns. Figure 9(a) shows the signal received at the center of cell A with OOK modulation. The first four of the following eight reference symbols are ‘1111’ with pulse width of 160ns and the rest four are ‘0000’. The subscribed data can be correctly decoded with the rule of maximum likelihood described in section 2. To take the ‘0’ symbols in the subscribed data as example, no matter which of the last four reference symbols is decoded as the output result by the rule of maximum likelihood, the decoded symbol is ‘0’. The decoding of the ‘1’ symbols is in a similar way. In Fig. 9(b), the 3rd, 4th, 7th and 8th of the eight reference symbols are ‘1’, occupying the front 80ns of a symbol duration, and the 1st, 2nd, 5th and 6th are ‘0’, occupying the hinder 80ns of a symbol duration. In Fig. 9(c), the 2nd, 4th, 6th and 8th of the eight reference symbols are ‘1’ with the pulse-width of 100ns, and the1st, 3rd, 5th and7th are ‘0’ with pulse-width of 60ns. Similarly, the subscribed data in Fig. 9(b) and 9(c) can also be correctly decoded by the rule of maximum likelihood.
Then, the waveforms received in cell A, B and C with interference from adjacent cells are measured as shown in Fig. 10 . Figure 10(c) shows that the interference from cell B is obvious. we calculated the error function εfor the first four waveforms of the subscribed data as shown in Fig. 11 . According to the maximum likelihood rule, the first four waveforms of the subscribed data should be decoded as the 8th, 6th, 4th and 3rd reference symbols, i.e., ‘1110’, the same as the transmitted bits.
Figure 12 are the waveforms received at the boundary of the neighboring cells. Two pulses with the same signal intensity in the first three-bit duration indicate that the receiver is at the borderline of two cells as shown in Fig. 12(a), 12(b) and 12(c). In this case, the data transmission switching from one cell to the other cell occurred, and the data received without error for both cells is required. Three pulses with the same signal intensity in Fig. 12(d) indicates that the receiver is at the center of the cells A, B and C. we calculated the error function for the first four waveforms of the subscribed data as shown in Fig. 13 . According to the maximum likelihood rule, the first four waveforms of the subscribed data should be decoded as the 8th, 6th, 4th and 3rd reference symbols, which represent ‘011’, ‘001’, ‘111’ and ‘110’ respectively. It means that the bits for cell A, B and C are ‘0011’, ‘1011’ and ‘1110’, which is the same as the transmitted bits. The correct decoding in the worst situation verifies that the seamless connectivity without interruption can be realized with our proposed scheme.
5. FPGA based receiver
In this part, the signal processing with FPGA at the receiver is carried and the 1Mbit/s prototype experiment setup is depicted in Fig. 14 . At the receiver, the analog signal from the photo-detector is amplified with the gain of 17dB, sampled by AD with the sampling frequency 60MHz and finally processed by the FPGA board. For this system, the modulation schemes for OOK, PPM and PWM are illustrated in Fig. 15 . The bit duration of three modulation formats is the same as 1μs. For PWM modulation, pulse-width of 400ns represents bit ‘0’ and 600ns represents bit ‘1’. For PPM modulation, the pulse-width is set as 500ns.
The output of FPGA is illustrated in Fig. 16(a) and 16(b) when PD is located 10cm and 20cm away from three LEDs respectively. Three pulses appearing in the first three-bit duration indicates that the interference of the neighboring cells is significant. The signal intensity in Fig. 16(b) is about one fourth of that in Fig. 16(a). By further processing the results in Fig. 16, the error function of the subscribed data is calculated. Figure 17 is the error function corresponding to the first four waveforms of the subscribed data. By the maximum likelihood rule, the decoded data are the same as the transmitted ones in cells A, B and C.
The results of the prototype experiment demonstrate that our scheme can provide the error free data transmission for seamless coverage. Considering that the optical power of available visible LED can be 20dB higher than the LED we used and the collector lens can be mounted in PD, we can expect the indoor communication systems with the seamless coverage of several meters.
In this paper, we proposed a modulation based cell distribution scheme, OOK, PPM and PWM for neighboring cells A, B and C, to realize the seamless connectivity of visible light communication. By introducing the positioning pulses in the first three-bit duration, the location of the receiver can be estimated. With the following eight reference symbols, the maximum likelihood rule is employed to decode the subscribed data. The prototype experiment results demonstrate the error free transmission of 1.0Mbit/s and 6.25Mbit/s visible light communication system with our scheme. With the available LED, we can expect that the data rate of a visible light communication system with seamless connectivity can be up to 71.4Mbit/s.
The project was supported by Open Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), China. The work was financially supported by the National Natural Science Foundation of China under Grant No. 60978007, 61027007 and 61177067.
References and links
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