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Orthogonal frequency-division multiplexing access (OFDMA) based wireless visible light communication (VLC) system
Optics Communications 355 (2015) 261–268
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Orthogonal frequency-division multiplexing access (OFDMA) based wireless visible light communication (VLC) system Jiun-Yu Sung a, Chien-Hung Yeh b,n, Chi-Wai Chow a, Wan-Feng Lin a, Yang Liu c a
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Photonics, Feng Chia University, Taichung 40724, Taiwan c Philips Electronics Ltd., Hong Kong b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2015 Received in revised form 27 June 2015 Accepted 29 June 2015
An orthogonal frequency-division multiplexing access (OFDMA) based visible light communication (VLC) system is proposed in this paper. The architecture of the proposed system is divided into several VLC cells, which is defined in this paper. The deployment and upgrade of the system involve only simple combination of the VLC cells. Hence it is economically advantageous. To guarantee smooth communication, nearly equal data rate is provided at every location within the system with no concern on the system scale. The user location monitor strategy is also discussed to solve the region division issues. The characteristics of the proposed system are analyzed in detail in this paper. A one-dimensional experiment was demonstrated with 13.6 Mb/s data rate. & 2015 Elsevier B.V. All rights reserved.
Keywords: Visible light communication (VLC) Phosphor-LED Orthogonal frequency division multiplexing access (OFDMA) Discrete-multi tone (DMT)
1. Introduction Growing requirement of mobile application encourages the development of wireless communication. Wireless communication is typically achieved by radio frequency (RF) signal transmission. There are usually four protocols used for short-range wireless communication [1]. However, the RF spectra of the four protocols are licensed and may be congested. For additional applications and economical consideration, it may be preferable to develop new techniques for wireless communication. Prosperous development of Light-emitting diode (LED) technology has made LED a promising luminance source. Compared to traditional fluorescent lamp, LED is advantageous of being brighter and lower power consumed. These characteristics guarantee more efficient power usage. For environmentally friendly and green concern, this is especially a significant advantage. LED also has higher modulation bandwidth than fluorescent lamp. Application of simultaneous lighting and visible light communication (VLC) is thus widely investigated [2–9]. Related standards for VLC have also been built [8]. High speed VLC systems were also demonstrated [10–13]. Moreover, VLC is license-free; and for economical consideration, this may be a dominant reason of choosing VLC as a wireless choice. Compared to typical RF wireless communication, one main n
Corresponding author. E-mail address: [email protected] (C.-H. Yeh).
http://dx.doi.org/10.1016/j.optcom.2015.06.070 0030-4018/& 2015 Elsevier B.V. All rights reserved.
disadvantage of VLC is its low bandwidth-distance product. For short-range indoor wireless communication, vertical transmission distance as far as 10 m height may be unnecessary. The vertical transmission distance around several meters height using a set of red (R), green (G), and blue (B) LEDs was demonstrated [14]. For expanding transmission distance of a VLC system, VLC-over fiber technique may be used [14–16]. Unflatten spatial power distribution of a single LED source makes it difficult to achieve uniform power distribution over a specific region. In this paper, a LED source can mean a LED chip or a LED array (with higher power than a single chip) depending on the requirement of the system). Moreover, the practical field distribution of a LED source is typically limited. Hence, to provide sufficient illuminance within an indoor space, more than one LED source separately allocated apart is commonly involved. This way, uniform brightness within the indoor space may be achieved thanks to the non-linear relation between eye sensation and input power. However, power difference at different locations within the VLC system may induce signal-to-noise ratio (SNR) and maximum data rate difference. This is unfavorable for smooth communication under mobile application. Relating solutions have been proposed and analyzed in [4,17]. Real-time continuous communication is also important for practical application, such as voice and video. The smooth power distribution strategies proposed in [4,17] may eliminate SNR fluctuation inside the VLC system. However, time-division multiplexing (TDM) mechanism for sharing bandwidth among users
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around a specific LED sources or a specific set of LED sources may be unfavorable for real-time continuous applications. The bandwidth allocation among different users may involve complicated quality of service (QoS) algorithm [18]. In this paper, a novel VLC system was proposed. Each user at different locations may get nearly equal data rate in the proposed VLC system. Smooth mobile communication is thus satisfied. Realtime signal transmission can be achieved everywhere in the VLC system. Hence, real-time services, such as voice and video, can be used in the VLC system. Mb/s data rate using two LEDs was demonstrated. Related analyses were also provided.
2. Principle of the VLC system We identify that a desirable VLC system should have the following characteristics. First, any location inside the VLC system should have nearly identical capacity for smooth communication. Second, any possible communication delay should be minimized for providing high quality real-time continuous applications, such as voice and video. Third, the deployment of the VLC system should be simple and scalable for economical consideration. In the following analyses, to get a more practical emulation, the radial power distribution of a LED from Cree incorporation (Cree LED) is measured. The analyses of the proposed VLC system are based on this measured power distribution of the Cree LED. Fig. 1 shows the radial power distribution of the Cree LED. Our proposed VLC zone is constructed by stacks of unit bases, each named a VLC cell. Here, the VLC cell is defined for the convenience of deploying a group of LED sources. The concept of a VLC cell is slightly different to the conventional cell defined in a cellular communication system. Each VLC cell comprises 4 4 rectangular sub-regions and four distinguished LED sources placed at different locations of the indoor system. Fig. 2(a) shows the relative position of the 4 4 sub-regions and the power distribution inside the VLC system. Unique indices, from “1” to “9,” are marked on each sub-region. For simplicity, hereafter, we will directly denote sub-region-(x) as the sub-region indexed by number x. The region circled by the bold black line is an instance of a VLC cell. Each of the sub-region-(1) is placed with a LED source. Fig. 2 (b) shows the power distribution inside the VLC system while the distance between neighbor LED sources is 5 cm. It is seen that maximum power appears at the sub-regions between any two neighbor LED sources. This is a result of power superposition. Fig. 2(c) shows an instance of the proposed VLC system which is
700 Linear Power [a.u.]
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Fig. 2. Architecture of the VLC system. (a) The relative relation between the power distribution and the VLC cell. (b) The power distribution inside the VLC system while neighbor LEDs are separated at 5 cm. (c) An instance of the proposed VLC system composed by stacks of the VLC cells.
400 300 200 100 0 0
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2 3 4 5 Location [cm]
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Fig. 1. The measured radial power distribution of the Cree LED.
constructed with repetitive stacks of VLC cells. To maximize the capacity of each LED source with limited available bandwidth, discrete-multi tone (DMT) is used in the proposed VLC system. The subcarriers of each DMT signal emitted from a specific LED source are divided into 9 sets. Hereafter, each of the 9 subcarrier sets is denoted by DMT-(y), where y is the index number. Each subcarrier set comprises of different subcarriers of the totally used subcarriers of the DMT signal. Each sub-region communicates using orthogonal frequencydivision multiplexing access (OFDMA) based mechanism. For
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SNR for single LED SNR
Resulted SNIR SNIR
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SNIR (e)
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Fig. 3. Illustration of the working principle of the VLC zone using two LEDs.
avoiding misunderstanding, we will focus on the 3 3 sub-regions (sub-region-(1)- sub-region-(9)) of a VLC cell. (The remaining 7 (4 4–3 3) sub-regions of a VLC cell are not discussed since their behavior is theoretically identical to one of our considered 3 3 sub-regions according to the power-symmetrical relations). subregion-(1) is located at the center of the 3 3 sub-regions. DMT signals are emitted from each LED source at the center of subregion-(1). For a specific sub-region, the sub-region index x determines which of the nine DMT subcarrier sets is used for communication. For example, DMT-(1) is used for communication at sub-region-(1). Because the transmission frequency window does not have flat frequency response, different DMT subcarriers may have different maximum transmission capacities. Hence, different DMT sub-regions may use different number of DMT subcarriers. Refer to Fig. 2(c) for a more general VLC system comprises of multiple stacks of VLC cells. LED sources neighbor to each of subregion-(2)–(5) transmit the same signal at the same DMT subcarriers allocated to the specific region. A one dimension illustration is depicted in Fig. 3. The sub-region setup of Fig. 3 follows the dash arrow of Fig. 2(c). For simplicity, only the interference between two LED sources is considered in Fig. 3. Inset (a) of Fig. 3 shows the SNR which may be achieved if only a single LED source is used in the system. Higher subcarriers have worse SNR performance because of the limited frequency response of a LED source. The pink subcarriers may be used in sub-region-(2), which was surrounding by two nearest LED sources. Inset (f) of Fig. 3 shows the resulted signal-to-noise-plus-interference ratio (SNIR) while the interference of the two LED sources is considered. It is seen that the SNIR performance of the pink subcarriers is enhanced. The SNIR improvement results from the power superposition of the two LED sources, which transmitted the same data at the pink subcarriers. In inset (f) of Fig. 3, the SNIR performance of blue and green subcarriers degrades because of signal interference between the two LED sources, which transmit different data at blue and green subcarriers. The degradation is not important because only pink subcarriers are used for communication at sub-region-(2). Inset (d), (f), and (h) of Fig. 3 have similar SNIR improvement and
degradation as described above. Inset (e) and (g) both belong to sub-region-(1). Blue subcarriers of this region may slightly degrade because of interference of neighbor LED sources. If the distance between neighbor LED sources is optimally determined, the interference power will be nearly negligible at sub-region-(1). Hence, the degradation will not be significant. All the above mentioned mechanism will make the resultant SNIR performance equivalently look like inset (b) of Fig. 3. The blue subcarriers may slightly degrade in SNIR, but the SNIR of both the green and pink subcarriers will be enhanced. The resultant frequency response thus becomes mote flat. This may increase the capacity limited by lower response of higher DMT subcarriers. Fig. 4 shows the SNIR distribution inside the VLC system for LED distances respectively being 4–7 cm. Here, the LED separation distances of 4–7 cm are chosen according to the characteristics of the LEDs available in our lab. For practical cases, higher separation distance among neighbor LED sources can be used if the LED sources have higher power and wider field distribution. For 4 cm LED distance, the SNIR can be as low as 1.7 dB. The low SNIR phenomenon results from the interference between the nearby LED sources. This distance setup is not suitable for our VLC zone because the bit error rate (BER) at SNIR 1.7 dB is bad. While the distance between neighbor LEDs sweeps from 5 cm to 7 cm, it is seen that the maximum and minimum SNIR performances are similar. The best LED separation distance is either 5 cm or 6 cm. For 6 cm case, we can see the minimum SNIR is about 11.8 dB, which is higher than the minimum SNIR of the 4–7 cm cases. This provides higher data rate while a preliminary BER threshold is required. For 5 cm case, though smaller minimum SNIR is observed compared to the 6–7 cm cases, only negligible areas are suffered from this lower minimum SNIR. In average, the 5 cm case has more flat SNIR distribution. This may provide lower average BER for the 5 cm case. For the 7 cm cases, higher minimum SNIR compared to the 5 cm case is also observed. However, tremendous areas suffer from the low SNIR performance for the 7 cm case. This will result in worse average BER performance for the 7 cm case. For even longer LED separation distance, the minimum SNIR will
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Fig. 4. The SNIR distribution of the 3 3 sub-regions inside the VLC system. (a) The neighbor LEDs are separated by 4 cm. (a) The neighbor LEDs are separated by 5 cm. (a) The neighbor LEDs are separated by 6 cm. (a) The neighbor LEDs are separated by 7 cm. Unit for side color bar: [dB].
be even worse than that of the 7 cm separation condition. Hence, we will not discuss it. To sum up the above analyses, our VLC system is constructed by periodic stacks of VLC cells. DMT is used for communication. At sub-region-(1), a LED source is placed. Sub-regions with different indices communicate using different sets of DMT subcarriers. Each sub-region has its signal from all of its nearest light sources. The power superposition phenomenon enhances the SNR of the subregions. This induces equivalently equalizing mechanism for the VLC system. Hence, the capacity of the VLC system may increase. Because each sub-region communicates with independent DMT subcarrier set, signal delay aroused by switching can be neglected. This is especially beneficial for real-time continuous applications, such as video and voice. Finally, because subcarriers used for each sub-region is adjusted, each sub-region has nearly identical capacity for smooth communication.
3. The monitor mechanism For smooth mobile application, it is insufficient to provide merely equal capacity at every position inside the VLC system. Another important issue is how the user may know which place he is located at, so that he knows which DMT subcarrier set he should use for communication. It is inferior for TDM-based VLC system to achieve the mobile application. This will be further discussed in the following section. Here, only the VLC systems with LED separation distance 5 cm and 6 cm is discussed. Referring to Fig. 4, it is seen that there exists significantly sharp SNIR differences at the boundaries between any two neighbor sub-regions. If each user monitors the SNIR values of all the DMT subcarriers, the SNIR difference will help him know if he is facing a sub-region transition. Because of the sharp SNIR differences at the boundaries between neighbor sub-regions, the user can accurately know which sub-region he is located.
4. Comparison between the TDM mechanism and the OFDMAbased mechanism TDM-based VLC is easy to achieve. There are typical two
approaches to provide TDM-based VLC application. One approach is to have all the LED sources shared by the whole VLC system. This means all the positions within the VLC system share their bandwidth in TDM mechanism. Because all the LED sources in the VLC system transmit the same signal at the same time, interference can be neglected and SNR is the main parameter to determine the signal performance. By moderate design of the power distribution of the VLC system, this approach can also provide nearly flat and high SNR at every position within the whole VLC system. However, while the number of sub-regions for independent communication increases, the capacity for a single sub-region will significantly decrease. Moreover, it is uneconomical to have a LED source far away from a specific position transmit data for the specific position because the received power is low. This kind of TDM based communication mechanism will be called “TDM-as-a-whole” in this paper. Another TDM-based VLC system first divides the VLC system into several small groups of area (GA). The TDM-based bandwidth is shared only within each GA. LED sources belongs to different GA may transmit different data at the same time. This will more efficiently use the power of the LED sources because the LED sources are used for communication locally. Meanwhile this approach may arouse two problems. First, the signal interference between nearby GAs may degrade the signal performance. To eliminate the interference, it may require the power of the edge of each GA being low. If the power at the edge of each GA is low, another problem will arise. The users at the edge of the VLC system will suffer from bad communication performance and thus lower capacity. The unequal capacity at different positions within the VLC is disadvantageous for practical system. In this paper, this kind of TDM based mechanism will be called “TDM-division”. Moreover, it is difficult for a TDM user to know his actual position by merely monitoring his signal SNR because the SNR is nearly identical at several positions. Lack of knowledge of user's self-position may make it difficult for a user to determine when the broadcast signal is sending for his message. One crucial advantage for TDM-as-a-whole is that it no interference occurs while communication; hence higher SNIR can be achieved. Fig. 5 shows the SNR distribution inside the VLC system for TDM-as-a-whole mechanism. Comparing the results of Figs. 4 and 5, it can be seen that high and flat SNR distribution can
Fig. 5. The SNR distribution inside the VLC system for TDM-as-a-whole mechanism. (a) The neighbor LEDs are separated by 4 cm. (a) The neighbor LEDs are separated by 5 cm. (a) The neighbor LEDs are separated by 6 cm. (a) The neighbor LEDs are separated by 7 cm. Unit for side color bar: [dB].
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5. Simulation
Fig. 6. The data rate transition while different number of VLC cell areas is involved in the VLC system.
be achieved while TDM-as-a-whole mechanism is used. However, only a small scale VLC system can benefit from this advantage. Fig. 6 shows the differences in capacity of a single sub-region between the TDM-as-a-whole and OFDMA mechanisms at different scales of the VLC system. The background noise and other electronic induced statistical noises are assumed to be 20 dB relative to the center power of a single LED source as measured in Fig. 1. DMT is used for both mechanisms for optimal bit-loading. It can be seen that the data rate at each sub-region decreases inverse proportionally to the number of sub-regions. However, while the OFDMA mechanism is used, the data rate is not influenced with the change of number of sub-regions. Referring to the 7 cm condition, it is worth mentioning that there also exists conditions which OFDMA will be superior to TDM-as-a-whole mechanism in low scale VLC system. While the separation of LED becomes 7 cm, some sub-regions will have significant lower power and SNR. While operating in TDM-as-a-whole mechanism, the low SNR region will be used in 4/9 sweeping time within a 3 3 sub-regions. This makes the bandwidth un-efficiently used. However, for OFDMA case, even though the corner four sub-regions have low SNR, the bandwidth of the high SNR parts works eternally. This makes more efficient usage of the bandwidth. The analysis for TDM-division mechanism is more complicated because it also involves how the regions are divided. It will not be further discussed here. Besides, we can also observe that the deployment strategies between TDM-as-a-whole and OFDMA mechanisms are different. As described above, while the Cree LED is used in the VLC system, the optimal LED separation is around 5–6 cm for OFDMA mechanism. However, referring to the SNR distribution of Fig. 5, while TDM-as-a-whole is used, the optimal LED separation is even shorter than 5 cm. We may conclude that for filling a fixed area, more LED sources may be used while TDM-as-a-whole mechanism is used. This also symbols un-economical use of LEDs. The proposed OFDMA mechanism is also beneficial for system upgrade. It is clear that every VLC cell works independently. We can easily remove or add any VLC cells from the VLC system with no influence on the other VLC cells. The TDM-based mechanism is disadvantageous for upgrading because any change of the LED setup may significantly influence the power distribution over the nearby working areas. As described in the above sections, our proposed OFDMA based VLC system can effectively use the bandwidth of the available LED sources. Its superiority becomes significant while the division of sub-region of the VLC system becomes large.
For further analyses of the system performance, simulation is launched. Assume the intensity of the Cree LED at different distances only involve linear distribution scaling. Assume the power of the LED source is high enough, such that the noise level is nearly constant. Thus, we can approximate our simulation space into a two dimensional plane. In this simulation, the background noise and electronic induced statistical noises are set to be 20 dB of 640.88, which is the central power of a single LED source as shown in Fig. 1. Both 5 cm and 6 cm LED separation distance conditions are discussed. The fast Fourier transform (FFT) size of the DMT signal is 128 and the CP is 1/32. Fig. 7(a) and (b) shows the SNR performance at different DMT subcarriers respectively for 5 cm and 6 cm cases. The three SNR distributions for “P1,” “P2,” and “P3” correspond to the three power symmetry characteristics in a VLC cell. The locations of the three types of power symmetry characteristics are respectively shown in Fig. 2(a). As described above, the DMT subcarriers of each LED source are divided into nine subcarrier sets, each worked for different VLC sub-regions. Each sub-region has different SNIR performances; hence the strategy for assigning a specific DMT subcarrier set for a specific sub-region is important to efficiently use the LED bandwidth. The black curve shows the resulted effective SNIR performance over different subcarriers. For the 6 cm case, it is seen that the black curve shows around 3 dB SNIR improvement for the red curve (P3 region). This means that the bandwidth is more efficient used. For the 5 cm case, the improvement of bandwidth usage efficiency become trifling because the three power curves have more consistent behaviors This result also corresponds to Fig. 6. It is shown in Fig. 6 that, for 6 cm LED separation case, the average data rate between using OFDMA mechanism and TDM-asa-whole mechanism is close. As explained above the data rate degrade phenomenon for TDM-as-a-whole mechanism results from the inefficient usage of bandwidth. Fig. 7(c) and (d) shows the bit-loading and subcarrier assignment strategies of the VLC system. The total data rate of a DMT signal for both cases is 7.125 Mb/s. The data rate achieved at different subcarrier sets is shown in Table 1. It is worth mentioning that only one assignment strategy exists to achieve 7.125 Mb/s data rate for the 6 cm case in our simulation condition. However, for 5 cm case, more than one strategy can be used to achieve the same level of data rate. This results show that 5 cm separation is more optimal. Fig. 7(e) and (f) shows the simulated BER performance at different locations within a 3 3 sub-regions, which correspond respectively to sub-region-(1)-sub-region-(9). One LED source is placed at the center of the central sub-region. BER o3.8 10 3 can be achieved at every location within the simulated regions. It can be seen within each sub-region, the BER distributions pattern for both cases are close to the SNIR distribution pattern as shown in Fig. 4. This supports the statement of high correlation between the SNIR and the BER performance. For both cases, the sub-regions with the same power symmetry characteristic do not have identical BER performance because different subcarrier sets and bitloading strategies are used for them.
6. Experiment and results In our proposed VLC system, the enhancement of signal performance mainly results from the superposition of power among different LED sources. In this section, we will first prove that the signal performance can be improved by power superposition; then a one dimensional demonstration of out VLC system will be
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Fig. 7. The simulation results. For (a) and (b), the SNIR performance at different DMT subcarriers. (a) Neighbor LEDs separation is 5 cm. (b) Neighbor LEDs separation is 6 cm. For (c) and (d), the bit-loading scheme of the DMT signal Each color specified for a specific DMT subcarrier set. The marks at each DMT subcarrier sets correspond to the y value of DMT-y. (a) Neighbor LEDs separation is 5 cm. (b) Neighbor LEDs separation is 6 cm. For (e) and (f), the BER performance at different locations of the 3 3 sub-regions inside the VLC system. Values for side color bar: [log10(BER)]. Table 1 The data rate achieved at different subcarrier sets. Set #1 [Mb/ s]
Set #2 [Mb/ s]
Set #3 [Mb/ s]
Set #4 [Mb/ s]
Set #5 [Mb/ s]
Set #6 [Mb/ s]
Set #7 [Mb/ s]
Set #8 [Mb/ s]
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Total data rate [Mb/ s]
5 cm 0.75 6 cm 0.75
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1 0.875
0.875 0.75
0.75 0.875
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0.75 0.875
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SNR [dB]
performed. Fig. 8 shows our experimental setup. Two Cree LEDs separated by 4 cm is used in our experiment. Differ to 5 cm or 6 cm used in the simulation, 4 cm separation is chosen because the un-
symmetrical power distribution of the two distinguished LED sources. The 4 cm separation is chosen to have our photo-diode receive identical power from both LED sources. The arbitrary waveform generator (AWG) used is AWG 33220a from Agilent with 16 MS/s sample rate. Fig. 9 is the resulted SNR performance at different DMT subcarriers. In Fig. 9, “SNR_1” and “SNR_2” respectively denote the SNR performance of the two LEDs while each was turned on respectively. “SNR_Add” is the SNR performance while the respectively received two waveforms from the two LEDs are added numerically. “SNR_Both” is the SNR performance while both LEDs transmitted identical DMT signal simultaneously. All the four SNR curves are smoothed for clearly observing the curve behavior. It can be seen that, at the high frequency region, nearly 3 dB enhancement was achieved by viewing the “SNR_Both” curve. This proves the statement that the superposition of power of LED sources is proportional to the SNR performance; and the noise level is nearly constant. The “SNR_Add” curve does not have corresponding SNR enhancement as the “SNR_Both” curve. Numerical addition of the two signals makes the noise level also slightly increased, and thus, degrades the SNR
Fig. 8. Experimental setup.
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Fig. 9. The SNR performance at different subcarriers.
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7. Conclusion In this paper, a novel OFDMA based VLC system is proposed. The propose VLC system could efficiently use the limit bandwidth of the LED sources. Nearly equal capacity can be guaranteed at every location in the VLC system. Position monitor can be easily achieved by the sharp SNIR difference at the boundaries of different sub-regions. Hence, it is advantageous for mobile smooth communication. The OFDMA mechanism minimizes the possible switching delay; hence real-time continuous applications, such as voice and video, can be used. The architecture of the proposed system is simple and economical. Upgrade solution is also included in the system. Detail analyses and discussion of the proposed are performed in the context. Demonstration of the proposed system with data rate 13.6 Mb/s is performed.
Acknowledgment This work was supported by Ministry of Science and Technology, Taiwan (ROC), under Contract MOST 103-2218-E-035-011MY3 and MOST 103-2221-E-009-030-MY3, and the Institute for Information Industry (III), Taiwan.
References Fig. 10. The SNR performance at different DMT subcarriers at different sub-regions. Inset: Constellations for different conditions.
performance. At low frequency region, the SNR enhancement behavior is not significant. This may be limited by the sensitivity of the scope. Because we do not have enough equipment, a one dimension OFDMA-based VLC system was demonstrated. The one dimension demonstration is similar to the concept shown in Fig. 3. The experimental setup is identical to Fig. 8. The transmission distance is 50 cm. The relation between our experimental setup and the proposed VLC system is shown as the solid line in Fig.2(c). Two different DMT signals transmitted from different LEDs were received respectively. The received two signals were added numerically to emulate the power superposition mechanism. Because only two LEDs are used in the experimental demonstration, there are only two power symmetry sub-regions. Here, only two DMT subcarrier sets were demonstrated, each for sub-region(2) and sub-region-(1). The DMT subcarrier set at lower frequency has higher SNR performance; hence 8-phase shift keying (PSK) was loaded at each of the subcarriers of this set. QPSK was loaded for the subcarriers at high frequency. Fig. 10 shows the experimental result. It can be seen that the superposition of the two signals can provide around 1.5 dB gain at the QPSK region while numerical addition was performed. This corresponds to the results provides in the superposition principle verification section. The low frequency subcarriers suffer from serious interference; hence has poor SNR performance. Inset (a) of Fig. 10 shows the constellation of the “ADD_8PSK” signal. It can be seen that the constellation is massed up because of strong interference between the two LEDs. While the numerical addition is not performed, the low frequency at “Region1_8PSK” has good SNR performance and its corresponding constellation is shown in inset (b) of Fig. 10. Inset (c) of Fig. 10 shows the constellation of “Add_QPSK” signal. The experimental results shows that 13.6 Mb/s total data rate can be achieved. The data rate at the QPSK band is 6.4 Mb/s, and the data rate at the 8-PSK band is 7.2 Mb/s. It is seen that both sub-regions have nearly equal data rate.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Orthogonal frequency-division multiplexing access (OFDMA) based wireless visible light communication (VLC) system Jiun-Yu Sung a, Chien-Hung Yeh b,n, Chi-Wai Chow a, Wan-Feng Lin a, Yang Liu c a
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Photonics, Feng Chia University, Taichung 40724, Taiwan c Philips Electronics Ltd., Hong Kong b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2015 Received in revised form 27 June 2015 Accepted 29 June 2015
An orthogonal frequency-division multiplexing access (OFDMA) based visible light communication (VLC) system is proposed in this paper. The architecture of the proposed system is divided into several VLC cells, which is defined in this paper. The deployment and upgrade of the system involve only simple combination of the VLC cells. Hence it is economically advantageous. To guarantee smooth communication, nearly equal data rate is provided at every location within the system with no concern on the system scale. The user location monitor strategy is also discussed to solve the region division issues. The characteristics of the proposed system are analyzed in detail in this paper. A one-dimensional experiment was demonstrated with 13.6 Mb/s data rate. & 2015 Elsevier B.V. All rights reserved.
Keywords: Visible light communication (VLC) Phosphor-LED Orthogonal frequency division multiplexing access (OFDMA) Discrete-multi tone (DMT)
1. Introduction Growing requirement of mobile application encourages the development of wireless communication. Wireless communication is typically achieved by radio frequency (RF) signal transmission. There are usually four protocols used for short-range wireless communication [1]. However, the RF spectra of the four protocols are licensed and may be congested. For additional applications and economical consideration, it may be preferable to develop new techniques for wireless communication. Prosperous development of Light-emitting diode (LED) technology has made LED a promising luminance source. Compared to traditional fluorescent lamp, LED is advantageous of being brighter and lower power consumed. These characteristics guarantee more efficient power usage. For environmentally friendly and green concern, this is especially a significant advantage. LED also has higher modulation bandwidth than fluorescent lamp. Application of simultaneous lighting and visible light communication (VLC) is thus widely investigated [2–9]. Related standards for VLC have also been built [8]. High speed VLC systems were also demonstrated [10–13]. Moreover, VLC is license-free; and for economical consideration, this may be a dominant reason of choosing VLC as a wireless choice. Compared to typical RF wireless communication, one main n
Corresponding author. E-mail address: [email protected] (C.-H. Yeh).
http://dx.doi.org/10.1016/j.optcom.2015.06.070 0030-4018/& 2015 Elsevier B.V. All rights reserved.
disadvantage of VLC is its low bandwidth-distance product. For short-range indoor wireless communication, vertical transmission distance as far as 10 m height may be unnecessary. The vertical transmission distance around several meters height using a set of red (R), green (G), and blue (B) LEDs was demonstrated [14]. For expanding transmission distance of a VLC system, VLC-over fiber technique may be used [14–16]. Unflatten spatial power distribution of a single LED source makes it difficult to achieve uniform power distribution over a specific region. In this paper, a LED source can mean a LED chip or a LED array (with higher power than a single chip) depending on the requirement of the system). Moreover, the practical field distribution of a LED source is typically limited. Hence, to provide sufficient illuminance within an indoor space, more than one LED source separately allocated apart is commonly involved. This way, uniform brightness within the indoor space may be achieved thanks to the non-linear relation between eye sensation and input power. However, power difference at different locations within the VLC system may induce signal-to-noise ratio (SNR) and maximum data rate difference. This is unfavorable for smooth communication under mobile application. Relating solutions have been proposed and analyzed in [4,17]. Real-time continuous communication is also important for practical application, such as voice and video. The smooth power distribution strategies proposed in [4,17] may eliminate SNR fluctuation inside the VLC system. However, time-division multiplexing (TDM) mechanism for sharing bandwidth among users
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around a specific LED sources or a specific set of LED sources may be unfavorable for real-time continuous applications. The bandwidth allocation among different users may involve complicated quality of service (QoS) algorithm [18]. In this paper, a novel VLC system was proposed. Each user at different locations may get nearly equal data rate in the proposed VLC system. Smooth mobile communication is thus satisfied. Realtime signal transmission can be achieved everywhere in the VLC system. Hence, real-time services, such as voice and video, can be used in the VLC system. Mb/s data rate using two LEDs was demonstrated. Related analyses were also provided.
2. Principle of the VLC system We identify that a desirable VLC system should have the following characteristics. First, any location inside the VLC system should have nearly identical capacity for smooth communication. Second, any possible communication delay should be minimized for providing high quality real-time continuous applications, such as voice and video. Third, the deployment of the VLC system should be simple and scalable for economical consideration. In the following analyses, to get a more practical emulation, the radial power distribution of a LED from Cree incorporation (Cree LED) is measured. The analyses of the proposed VLC system are based on this measured power distribution of the Cree LED. Fig. 1 shows the radial power distribution of the Cree LED. Our proposed VLC zone is constructed by stacks of unit bases, each named a VLC cell. Here, the VLC cell is defined for the convenience of deploying a group of LED sources. The concept of a VLC cell is slightly different to the conventional cell defined in a cellular communication system. Each VLC cell comprises 4 4 rectangular sub-regions and four distinguished LED sources placed at different locations of the indoor system. Fig. 2(a) shows the relative position of the 4 4 sub-regions and the power distribution inside the VLC system. Unique indices, from “1” to “9,” are marked on each sub-region. For simplicity, hereafter, we will directly denote sub-region-(x) as the sub-region indexed by number x. The region circled by the bold black line is an instance of a VLC cell. Each of the sub-region-(1) is placed with a LED source. Fig. 2 (b) shows the power distribution inside the VLC system while the distance between neighbor LED sources is 5 cm. It is seen that maximum power appears at the sub-regions between any two neighbor LED sources. This is a result of power superposition. Fig. 2(c) shows an instance of the proposed VLC system which is
700 Linear Power [a.u.]
600 500
Fig. 2. Architecture of the VLC system. (a) The relative relation between the power distribution and the VLC cell. (b) The power distribution inside the VLC system while neighbor LEDs are separated at 5 cm. (c) An instance of the proposed VLC system composed by stacks of the VLC cells.
400 300 200 100 0 0
1
2 3 4 5 Location [cm]
6
Fig. 1. The measured radial power distribution of the Cree LED.
constructed with repetitive stacks of VLC cells. To maximize the capacity of each LED source with limited available bandwidth, discrete-multi tone (DMT) is used in the proposed VLC system. The subcarriers of each DMT signal emitted from a specific LED source are divided into 9 sets. Hereafter, each of the 9 subcarrier sets is denoted by DMT-(y), where y is the index number. Each subcarrier set comprises of different subcarriers of the totally used subcarriers of the DMT signal. Each sub-region communicates using orthogonal frequencydivision multiplexing access (OFDMA) based mechanism. For
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SNR for single LED SNR
Resulted SNIR SNIR
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SNIR (e)
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subcarrier
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Fig. 3. Illustration of the working principle of the VLC zone using two LEDs.
avoiding misunderstanding, we will focus on the 3 3 sub-regions (sub-region-(1)- sub-region-(9)) of a VLC cell. (The remaining 7 (4 4–3 3) sub-regions of a VLC cell are not discussed since their behavior is theoretically identical to one of our considered 3 3 sub-regions according to the power-symmetrical relations). subregion-(1) is located at the center of the 3 3 sub-regions. DMT signals are emitted from each LED source at the center of subregion-(1). For a specific sub-region, the sub-region index x determines which of the nine DMT subcarrier sets is used for communication. For example, DMT-(1) is used for communication at sub-region-(1). Because the transmission frequency window does not have flat frequency response, different DMT subcarriers may have different maximum transmission capacities. Hence, different DMT sub-regions may use different number of DMT subcarriers. Refer to Fig. 2(c) for a more general VLC system comprises of multiple stacks of VLC cells. LED sources neighbor to each of subregion-(2)–(5) transmit the same signal at the same DMT subcarriers allocated to the specific region. A one dimension illustration is depicted in Fig. 3. The sub-region setup of Fig. 3 follows the dash arrow of Fig. 2(c). For simplicity, only the interference between two LED sources is considered in Fig. 3. Inset (a) of Fig. 3 shows the SNR which may be achieved if only a single LED source is used in the system. Higher subcarriers have worse SNR performance because of the limited frequency response of a LED source. The pink subcarriers may be used in sub-region-(2), which was surrounding by two nearest LED sources. Inset (f) of Fig. 3 shows the resulted signal-to-noise-plus-interference ratio (SNIR) while the interference of the two LED sources is considered. It is seen that the SNIR performance of the pink subcarriers is enhanced. The SNIR improvement results from the power superposition of the two LED sources, which transmitted the same data at the pink subcarriers. In inset (f) of Fig. 3, the SNIR performance of blue and green subcarriers degrades because of signal interference between the two LED sources, which transmit different data at blue and green subcarriers. The degradation is not important because only pink subcarriers are used for communication at sub-region-(2). Inset (d), (f), and (h) of Fig. 3 have similar SNIR improvement and
degradation as described above. Inset (e) and (g) both belong to sub-region-(1). Blue subcarriers of this region may slightly degrade because of interference of neighbor LED sources. If the distance between neighbor LED sources is optimally determined, the interference power will be nearly negligible at sub-region-(1). Hence, the degradation will not be significant. All the above mentioned mechanism will make the resultant SNIR performance equivalently look like inset (b) of Fig. 3. The blue subcarriers may slightly degrade in SNIR, but the SNIR of both the green and pink subcarriers will be enhanced. The resultant frequency response thus becomes mote flat. This may increase the capacity limited by lower response of higher DMT subcarriers. Fig. 4 shows the SNIR distribution inside the VLC system for LED distances respectively being 4–7 cm. Here, the LED separation distances of 4–7 cm are chosen according to the characteristics of the LEDs available in our lab. For practical cases, higher separation distance among neighbor LED sources can be used if the LED sources have higher power and wider field distribution. For 4 cm LED distance, the SNIR can be as low as 1.7 dB. The low SNIR phenomenon results from the interference between the nearby LED sources. This distance setup is not suitable for our VLC zone because the bit error rate (BER) at SNIR 1.7 dB is bad. While the distance between neighbor LEDs sweeps from 5 cm to 7 cm, it is seen that the maximum and minimum SNIR performances are similar. The best LED separation distance is either 5 cm or 6 cm. For 6 cm case, we can see the minimum SNIR is about 11.8 dB, which is higher than the minimum SNIR of the 4–7 cm cases. This provides higher data rate while a preliminary BER threshold is required. For 5 cm case, though smaller minimum SNIR is observed compared to the 6–7 cm cases, only negligible areas are suffered from this lower minimum SNIR. In average, the 5 cm case has more flat SNIR distribution. This may provide lower average BER for the 5 cm case. For the 7 cm cases, higher minimum SNIR compared to the 5 cm case is also observed. However, tremendous areas suffer from the low SNIR performance for the 7 cm case. This will result in worse average BER performance for the 7 cm case. For even longer LED separation distance, the minimum SNIR will
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Fig. 4. The SNIR distribution of the 3 3 sub-regions inside the VLC system. (a) The neighbor LEDs are separated by 4 cm. (a) The neighbor LEDs are separated by 5 cm. (a) The neighbor LEDs are separated by 6 cm. (a) The neighbor LEDs are separated by 7 cm. Unit for side color bar: [dB].
be even worse than that of the 7 cm separation condition. Hence, we will not discuss it. To sum up the above analyses, our VLC system is constructed by periodic stacks of VLC cells. DMT is used for communication. At sub-region-(1), a LED source is placed. Sub-regions with different indices communicate using different sets of DMT subcarriers. Each sub-region has its signal from all of its nearest light sources. The power superposition phenomenon enhances the SNR of the subregions. This induces equivalently equalizing mechanism for the VLC system. Hence, the capacity of the VLC system may increase. Because each sub-region communicates with independent DMT subcarrier set, signal delay aroused by switching can be neglected. This is especially beneficial for real-time continuous applications, such as video and voice. Finally, because subcarriers used for each sub-region is adjusted, each sub-region has nearly identical capacity for smooth communication.
3. The monitor mechanism For smooth mobile application, it is insufficient to provide merely equal capacity at every position inside the VLC system. Another important issue is how the user may know which place he is located at, so that he knows which DMT subcarrier set he should use for communication. It is inferior for TDM-based VLC system to achieve the mobile application. This will be further discussed in the following section. Here, only the VLC systems with LED separation distance 5 cm and 6 cm is discussed. Referring to Fig. 4, it is seen that there exists significantly sharp SNIR differences at the boundaries between any two neighbor sub-regions. If each user monitors the SNIR values of all the DMT subcarriers, the SNIR difference will help him know if he is facing a sub-region transition. Because of the sharp SNIR differences at the boundaries between neighbor sub-regions, the user can accurately know which sub-region he is located.
4. Comparison between the TDM mechanism and the OFDMAbased mechanism TDM-based VLC is easy to achieve. There are typical two
approaches to provide TDM-based VLC application. One approach is to have all the LED sources shared by the whole VLC system. This means all the positions within the VLC system share their bandwidth in TDM mechanism. Because all the LED sources in the VLC system transmit the same signal at the same time, interference can be neglected and SNR is the main parameter to determine the signal performance. By moderate design of the power distribution of the VLC system, this approach can also provide nearly flat and high SNR at every position within the whole VLC system. However, while the number of sub-regions for independent communication increases, the capacity for a single sub-region will significantly decrease. Moreover, it is uneconomical to have a LED source far away from a specific position transmit data for the specific position because the received power is low. This kind of TDM based communication mechanism will be called “TDM-as-a-whole” in this paper. Another TDM-based VLC system first divides the VLC system into several small groups of area (GA). The TDM-based bandwidth is shared only within each GA. LED sources belongs to different GA may transmit different data at the same time. This will more efficiently use the power of the LED sources because the LED sources are used for communication locally. Meanwhile this approach may arouse two problems. First, the signal interference between nearby GAs may degrade the signal performance. To eliminate the interference, it may require the power of the edge of each GA being low. If the power at the edge of each GA is low, another problem will arise. The users at the edge of the VLC system will suffer from bad communication performance and thus lower capacity. The unequal capacity at different positions within the VLC is disadvantageous for practical system. In this paper, this kind of TDM based mechanism will be called “TDM-division”. Moreover, it is difficult for a TDM user to know his actual position by merely monitoring his signal SNR because the SNR is nearly identical at several positions. Lack of knowledge of user's self-position may make it difficult for a user to determine when the broadcast signal is sending for his message. One crucial advantage for TDM-as-a-whole is that it no interference occurs while communication; hence higher SNIR can be achieved. Fig. 5 shows the SNR distribution inside the VLC system for TDM-as-a-whole mechanism. Comparing the results of Figs. 4 and 5, it can be seen that high and flat SNR distribution can
Fig. 5. The SNR distribution inside the VLC system for TDM-as-a-whole mechanism. (a) The neighbor LEDs are separated by 4 cm. (a) The neighbor LEDs are separated by 5 cm. (a) The neighbor LEDs are separated by 6 cm. (a) The neighbor LEDs are separated by 7 cm. Unit for side color bar: [dB].
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Fig. 6. The data rate transition while different number of VLC cell areas is involved in the VLC system.
be achieved while TDM-as-a-whole mechanism is used. However, only a small scale VLC system can benefit from this advantage. Fig. 6 shows the differences in capacity of a single sub-region between the TDM-as-a-whole and OFDMA mechanisms at different scales of the VLC system. The background noise and other electronic induced statistical noises are assumed to be 20 dB relative to the center power of a single LED source as measured in Fig. 1. DMT is used for both mechanisms for optimal bit-loading. It can be seen that the data rate at each sub-region decreases inverse proportionally to the number of sub-regions. However, while the OFDMA mechanism is used, the data rate is not influenced with the change of number of sub-regions. Referring to the 7 cm condition, it is worth mentioning that there also exists conditions which OFDMA will be superior to TDM-as-a-whole mechanism in low scale VLC system. While the separation of LED becomes 7 cm, some sub-regions will have significant lower power and SNR. While operating in TDM-as-a-whole mechanism, the low SNR region will be used in 4/9 sweeping time within a 3 3 sub-regions. This makes the bandwidth un-efficiently used. However, for OFDMA case, even though the corner four sub-regions have low SNR, the bandwidth of the high SNR parts works eternally. This makes more efficient usage of the bandwidth. The analysis for TDM-division mechanism is more complicated because it also involves how the regions are divided. It will not be further discussed here. Besides, we can also observe that the deployment strategies between TDM-as-a-whole and OFDMA mechanisms are different. As described above, while the Cree LED is used in the VLC system, the optimal LED separation is around 5–6 cm for OFDMA mechanism. However, referring to the SNR distribution of Fig. 5, while TDM-as-a-whole is used, the optimal LED separation is even shorter than 5 cm. We may conclude that for filling a fixed area, more LED sources may be used while TDM-as-a-whole mechanism is used. This also symbols un-economical use of LEDs. The proposed OFDMA mechanism is also beneficial for system upgrade. It is clear that every VLC cell works independently. We can easily remove or add any VLC cells from the VLC system with no influence on the other VLC cells. The TDM-based mechanism is disadvantageous for upgrading because any change of the LED setup may significantly influence the power distribution over the nearby working areas. As described in the above sections, our proposed OFDMA based VLC system can effectively use the bandwidth of the available LED sources. Its superiority becomes significant while the division of sub-region of the VLC system becomes large.
For further analyses of the system performance, simulation is launched. Assume the intensity of the Cree LED at different distances only involve linear distribution scaling. Assume the power of the LED source is high enough, such that the noise level is nearly constant. Thus, we can approximate our simulation space into a two dimensional plane. In this simulation, the background noise and electronic induced statistical noises are set to be 20 dB of 640.88, which is the central power of a single LED source as shown in Fig. 1. Both 5 cm and 6 cm LED separation distance conditions are discussed. The fast Fourier transform (FFT) size of the DMT signal is 128 and the CP is 1/32. Fig. 7(a) and (b) shows the SNR performance at different DMT subcarriers respectively for 5 cm and 6 cm cases. The three SNR distributions for “P1,” “P2,” and “P3” correspond to the three power symmetry characteristics in a VLC cell. The locations of the three types of power symmetry characteristics are respectively shown in Fig. 2(a). As described above, the DMT subcarriers of each LED source are divided into nine subcarrier sets, each worked for different VLC sub-regions. Each sub-region has different SNIR performances; hence the strategy for assigning a specific DMT subcarrier set for a specific sub-region is important to efficiently use the LED bandwidth. The black curve shows the resulted effective SNIR performance over different subcarriers. For the 6 cm case, it is seen that the black curve shows around 3 dB SNIR improvement for the red curve (P3 region). This means that the bandwidth is more efficient used. For the 5 cm case, the improvement of bandwidth usage efficiency become trifling because the three power curves have more consistent behaviors This result also corresponds to Fig. 6. It is shown in Fig. 6 that, for 6 cm LED separation case, the average data rate between using OFDMA mechanism and TDM-asa-whole mechanism is close. As explained above the data rate degrade phenomenon for TDM-as-a-whole mechanism results from the inefficient usage of bandwidth. Fig. 7(c) and (d) shows the bit-loading and subcarrier assignment strategies of the VLC system. The total data rate of a DMT signal for both cases is 7.125 Mb/s. The data rate achieved at different subcarrier sets is shown in Table 1. It is worth mentioning that only one assignment strategy exists to achieve 7.125 Mb/s data rate for the 6 cm case in our simulation condition. However, for 5 cm case, more than one strategy can be used to achieve the same level of data rate. This results show that 5 cm separation is more optimal. Fig. 7(e) and (f) shows the simulated BER performance at different locations within a 3 3 sub-regions, which correspond respectively to sub-region-(1)-sub-region-(9). One LED source is placed at the center of the central sub-region. BER o3.8 10 3 can be achieved at every location within the simulated regions. It can be seen within each sub-region, the BER distributions pattern for both cases are close to the SNIR distribution pattern as shown in Fig. 4. This supports the statement of high correlation between the SNIR and the BER performance. For both cases, the sub-regions with the same power symmetry characteristic do not have identical BER performance because different subcarrier sets and bitloading strategies are used for them.
6. Experiment and results In our proposed VLC system, the enhancement of signal performance mainly results from the superposition of power among different LED sources. In this section, we will first prove that the signal performance can be improved by power superposition; then a one dimensional demonstration of out VLC system will be
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Fig. 7. The simulation results. For (a) and (b), the SNIR performance at different DMT subcarriers. (a) Neighbor LEDs separation is 5 cm. (b) Neighbor LEDs separation is 6 cm. For (c) and (d), the bit-loading scheme of the DMT signal Each color specified for a specific DMT subcarrier set. The marks at each DMT subcarrier sets correspond to the y value of DMT-y. (a) Neighbor LEDs separation is 5 cm. (b) Neighbor LEDs separation is 6 cm. For (e) and (f), the BER performance at different locations of the 3 3 sub-regions inside the VLC system. Values for side color bar: [log10(BER)]. Table 1 The data rate achieved at different subcarrier sets. Set #1 [Mb/ s]
Set #2 [Mb/ s]
Set #3 [Mb/ s]
Set #4 [Mb/ s]
Set #5 [Mb/ s]
Set #6 [Mb/ s]
Set #7 [Mb/ s]
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Set #9 [Mb/ s]
Total data rate [Mb/ s]
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1 0.875
0.875 0.75
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performed. Fig. 8 shows our experimental setup. Two Cree LEDs separated by 4 cm is used in our experiment. Differ to 5 cm or 6 cm used in the simulation, 4 cm separation is chosen because the un-
symmetrical power distribution of the two distinguished LED sources. The 4 cm separation is chosen to have our photo-diode receive identical power from both LED sources. The arbitrary waveform generator (AWG) used is AWG 33220a from Agilent with 16 MS/s sample rate. Fig. 9 is the resulted SNR performance at different DMT subcarriers. In Fig. 9, “SNR_1” and “SNR_2” respectively denote the SNR performance of the two LEDs while each was turned on respectively. “SNR_Add” is the SNR performance while the respectively received two waveforms from the two LEDs are added numerically. “SNR_Both” is the SNR performance while both LEDs transmitted identical DMT signal simultaneously. All the four SNR curves are smoothed for clearly observing the curve behavior. It can be seen that, at the high frequency region, nearly 3 dB enhancement was achieved by viewing the “SNR_Both” curve. This proves the statement that the superposition of power of LED sources is proportional to the SNR performance; and the noise level is nearly constant. The “SNR_Add” curve does not have corresponding SNR enhancement as the “SNR_Both” curve. Numerical addition of the two signals makes the noise level also slightly increased, and thus, degrades the SNR
Fig. 8. Experimental setup.
24 21 18 15 12 9 6 3 0
SNR_1 SNR_2 SNR_Add SNR_Both
0
1
2 3 4 5 6 Frequency [MHz]
Fig. 9. The SNR performance at different subcarriers.
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7. Conclusion In this paper, a novel OFDMA based VLC system is proposed. The propose VLC system could efficiently use the limit bandwidth of the LED sources. Nearly equal capacity can be guaranteed at every location in the VLC system. Position monitor can be easily achieved by the sharp SNIR difference at the boundaries of different sub-regions. Hence, it is advantageous for mobile smooth communication. The OFDMA mechanism minimizes the possible switching delay; hence real-time continuous applications, such as voice and video, can be used. The architecture of the proposed system is simple and economical. Upgrade solution is also included in the system. Detail analyses and discussion of the proposed are performed in the context. Demonstration of the proposed system with data rate 13.6 Mb/s is performed.
Acknowledgment This work was supported by Ministry of Science and Technology, Taiwan (ROC), under Contract MOST 103-2218-E-035-011MY3 and MOST 103-2221-E-009-030-MY3, and the Institute for Information Industry (III), Taiwan.
References Fig. 10. The SNR performance at different DMT subcarriers at different sub-regions. Inset: Constellations for different conditions.
performance. At low frequency region, the SNR enhancement behavior is not significant. This may be limited by the sensitivity of the scope. Because we do not have enough equipment, a one dimension OFDMA-based VLC system was demonstrated. The one dimension demonstration is similar to the concept shown in Fig. 3. The experimental setup is identical to Fig. 8. The transmission distance is 50 cm. The relation between our experimental setup and the proposed VLC system is shown as the solid line in Fig.2(c). Two different DMT signals transmitted from different LEDs were received respectively. The received two signals were added numerically to emulate the power superposition mechanism. Because only two LEDs are used in the experimental demonstration, there are only two power symmetry sub-regions. Here, only two DMT subcarrier sets were demonstrated, each for sub-region(2) and sub-region-(1). The DMT subcarrier set at lower frequency has higher SNR performance; hence 8-phase shift keying (PSK) was loaded at each of the subcarriers of this set. QPSK was loaded for the subcarriers at high frequency. Fig. 10 shows the experimental result. It can be seen that the superposition of the two signals can provide around 1.5 dB gain at the QPSK region while numerical addition was performed. This corresponds to the results provides in the superposition principle verification section. The low frequency subcarriers suffer from serious interference; hence has poor SNR performance. Inset (a) of Fig. 10 shows the constellation of the “ADD_8PSK” signal. It can be seen that the constellation is massed up because of strong interference between the two LEDs. While the numerical addition is not performed, the low frequency at “Region1_8PSK” has good SNR performance and its corresponding constellation is shown in inset (b) of Fig. 10. Inset (c) of Fig. 10 shows the constellation of “Add_QPSK” signal. The experimental results shows that 13.6 Mb/s total data rate can be achieved. The data rate at the QPSK band is 6.4 Mb/s, and the data rate at the 8-PSK band is 7.2 Mb/s. It is seen that both sub-regions have nearly equal data rate.
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