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Color Shift Keying (CSK) is a new modulation scheme for visible light communication systems using RGB LEDs which has been standardized in the PHY III level of the IEEE 802.15.7. This paper proposes some modifications so as to include multiuser capabilities provided by a time-based multiplexing, with the modulation constellation symbols being adapted to encode data with the luminux powers of the red, green and blue color bands respectively. This is achieved by employing a simple and low-complexity time-based pulse signals structure to separate the users’ data symbols, while a three-dimensional signal constellation design is merged to improve data throughput. Numerical simulations are carried out to assess the performance of this novel architecture. The statistical properties of the transmitted RGB signals ensure dimming capabilities and that the illumination function is unaffected by flickering.
© 2014 Optical Society of America
1. Introduction
Visible light communication (VLC) is a wireless technology based on the use of Light Emitting Diodes (LEDs) for data communication [1]. Distinct to any other wireless communication system, VLC has the uniqueness of allowing the functions of illumination and communication simultaneously. This attractive feature and the rapid growth of the LED lighting industry are driving the interest into new applications of this technology [2–6]. Research on VLC systems has been actively supported by a number of universities and organizations around the world during the last years [7–9]. As a result, the first VLC standard for Wireless Personal Area Networks (WPAN) has been released by the IEEE 802.15.7 working group [10]. This standard supports transmissions from 11.67 kbit/s to 96 Mbit/s through a variety of modulation schemes which are categorized into low, moderate and high data rate links. The envisaged application scenarios for VLC are extensive but their attainable data rates are strongly dependent on the propagation conditions of the optical channel. Thus, line of sight conditions will clearly prompt the achievable capacity of the VLC application. It is also important to mention that the IEEE 802.15.7 standard concentrates on the point to point communication scenario and it does not contemplate multiuser transmissions yet.
There are two conventional ways to generate white-light with LED lamps: using a phosphor cover or by combining the output from red, green and blue (RGB) LEDs. Although the phosphor-based white-light LED has the advantage of low-cost, its limited modulation bandwidth poses a constraint to enable high-speed wireless VLC links. In that case, the aggregated color channels in RGB transmitters offer the opportunity of higher data rates but at the expense of increasing the implementation costs. Despite these inherent limitations, recent advances in VLC technology have demonstrated to achieve high data transmission connections. Unless VLC commercialization efforts are currently focused on low-cost applications and services, complex solutions can become cost-effective and practical in a near future. It is from this perspective that this paper aims to devise a VLC system with the flexibility to extend its operation to a multiuser scenario. To date, research in VLC systems has been primarily focused in single-user applications. Few works can be found in the literature that address the VLC multiuser environment, e.g. [11] shows one proposal based on a precoding scheme. Although such scheme eliminates the effect of multiple access interference, it requires complex signal processing at the transmitter where knowledge of the channel is required. Orthogonal frequency division multiplexing (OFDM) has also been extensively studied for single-user VLC communications where it has been suggested as an accessible and straightforward technology for multiple access schemes [12–15]. In line with the above challenge, in this paper we proposed a new CSK-based VLC communication scheme that supports multiuser communications. Different to the CSK modulation scheme introduced in the standard IEEE 802.15.7, a time-based approach is employed to separate the users’ signals which is combined with a three-dimensional (3D) signal constellation design to enhance the throughput on each user. The benefit of this novel scheme is that it extends the operation of VLC to scenarios with multiple users under the basis of the IEEE 802.15.7 standard. In more detail, time-based pulse signals are used at the transmitter to sent data to different users in such a way that CSK color coding transformation is not longer needed. It will be shown that the constellation symbols can be represented directly by the luminux powers of the red, green and blue color bands respectively. The RGB components can be balanced so as to maintain the color of the emitted lights [16]. At the receiver, its structure is simplified by applying a single user detector and removing the CSK color decoding block from the IEEE 802.15.7 standard and by the fact that the use of pulse-coded signals can easily recover clocks. The uncomplicated architecture of this novel VLC multiuser scheme will prove to be effective and appealing for low-cost implementations. Numerical simulations are carried out to evaluate the performance of this CSK-based multiuser VLC scheme.
The structure of this paper is organised as follows: Section II describes the multiuser CSK (MU-CSK) system model of interest. The detection problem for the multiuser CSK signals is also formulated in Section II where a maximum likelihood detector is suggested. Then, Section III presents the design criterion used for the proposed 3D modulation block. Section IV discusses error and throughput evaluation results for the proposed MU-CSK scheme. Finally, the conclusions and future work are drawn in Section V.
2. Multiuser CSK communication model
Figure 1 presents the proposed multiuser CSK scheme model for visible light communications. This structure considers a multi-color transmitter that sends a modulated optical signal to U different terminals in an indoor environment. A description of the main components of the system is given in the next sections.
2.1. RGB transmitter
The transmitter consists of a time-multiplexing scheme which is used to support multiuser communications, followed by a 3D modulation scheme suitable for a multi-color transmitter. At the input of the transmitter, multiple data bits streams are first processed by the time-multiplexing scheme which basically performs a parallel to serial (P/S) conversion in groups of k bits per every user. The output serial data stream is then passed to the modulation block where each k bits pattern is mapped to a constellation symbol into a three-dimensional signal space. Therefore, each constellation signal point of a particular user can be geometrically represented by the vector
where defines the power intensity assigned to the i-th colour band, for i = 1, 2, 3, m = 1,···, M, and j = 1,···, U, and 𝕏j as the constellation set for user j. Without loss of generality, it is assumed that all users employ the same constellation size M. For transmission, each element of vector sj,m is thus transformed to a waveform in the following way where characterises the signal transmitted over the i-th colour band of the RGB transmitter and ϕj(t) a rectangular pulse given as in which Ts = U · Tp establishes the relation between the non-zero pulse width (Tp) and the symbol time duration (Ts). The time property of the signals ϕj(t) ∀j ensures orthogonality in the multiuser scheme, i.e. with δi,l as the Kronecker delta for j, l = 1, 2, ···, U. In this way, the transmitted signal of the multiuser CSK system over the i-th colour band can be written as Figure 2 illustrates an example of the proposed MU-CSK scheme using an RGB transmitter. This example considers the case of 4 users multiplexed in time where the intensity levels in sj,m are indicated in an arbitrary fashion. In section III, we will introduce the 3D constellation design criterion to yield the values for these constellation symbols. We remark also that scrambling sequences can be applied to add security to the users data streams, however, this is not within the scope of this work and therefore this procedure has been neglected.
Fig. 2 Example of multiple data communication using the proposed MU-CSK scheme with RGB light sources.
2.2. Indoor VLC channel
For the system described in Fig. 1 and after the electrical to optical conversion, the multi-colour source sends a set of three intensity modulating signals in parallel on different wavelength over an indoor VLC communication channel. At the receiver side, optical filters are used to separate the propagated multi-wavelength signal. In order to demonstrate the achievable data rates of the proposed MU-CSK system, the channel assumes a line of sight path between the emitters and each of the U receivers in the system. Therefore, the received electrical signal for user j after passing through the i-th colour band channel can be represented as
where is used to describe the effect of the optical channel impulse response over band i and Ri the responsivity of the photodiode. The signal defines a white Gaussian noise process with N(0, σ2/3) in band i and * the convolution operator. A discrete-time model representation of Eq. (6) can be derived as follows where a Nyquist rate sampling of N/Ts is considered such that N samples are taken per symbol period. Assuming that for the LOS channels with δ(t) as the Dirac delta function, then matrix with I as an N × N identity matrix. The sampled representation for the transmitted information and noise signals are given by and nj ∈ ℝN ∀j, respectively. Finally, we state that the proposed VLC system model in Fig. 1 is also valid to diffuse multipath lighting environments but dispersion of the channel must be taken into account for defining the parameter Tp to avoid intersymbol interference.2.3. Detector
The effect of time synchronization is not addressed in this paper, therefore, for signal detection it is assumed that the process of synchronization has already been done between the emitter and the j-th receiver without error. Thus, the corresponding received discrete-time signal is then correlated with the sampled version of signal ϕj(t), denoted as zj ∈ ℝN, which is used to eliminate the unwanted information from other users in the system. Then by multiplying Eq. (7) with and using Eq. (2), it is straightforward to show that
Now, maximum likelihood (ML) detection is applied to estimate the transmitted 3D modulating symbol of user j. The ML detector can be formulated by the following optimization problem where the i-th component of yj ∈ ℝ3 is given by , ○ denotes the Hadamard product and ‖·‖2 as the Euclidean norm. Finally, the estimated symbol ŝj,m is demapped to recover the binary sequence of length k according to the 3D modulation mapping rule which is presented in next section.3. 3D constellation design
In this section, we describe the constellation design criterion used for the 3D modulation scheme given in Fig. 1. Different to the conventional CSK modulation scheme introduced by the IEEE 802.15.7 for RGB LEDs, the proposed 3D modulation block aims to adapt the signal constellation to the incoherent multi-colour emitter in order to improve the users’ throughput while maintaining the multiuser structure. From the signal space viewpoint of the RGB transmitter, it is suggested here to choose the constellation symbols from the 3D signal space created by the red, green and blue transmitted signals. Various sets of constellation points can be selected from this three-dimensional space. However, it is our interest to construct power-efficient constellations, therefore, a search for the best constellation sets under the power constraints of the system is required. Using the well-known minimum Euclidean distance parameter, the constrained optimization problem for user j can be formulated as follows
where with (ℝ+)3 as the set of positive real numbers defined in a three dimensional space, PT is the total transmit power of the RGB source for user j and # denotes the cardinality of set 𝕏j. Since the optimization problem in (10) is not convex, the analytical solution of it becomes a difficult challenge. Instead of this, we approach this problem by applying a numerical solution to obtain those constellations with the highest minimum Euclidean distance. Also, to avoid the problem of local minima, the numerical solution was executed repeatedly in order to find all possible local minima under the system’s constraints. Table 1 shows the obtained constellation sets with the highest minimum Euclidean distance found for the cases of M = 4, 8 and 16 given that PT = 1. These constellation signal points will improve further the power efficiency of the multiuser CSK scheme by means of the 3D modulation block.
Table 1. Optimized 3D Constellations for M = 4, 8 and 16.
4. Performance analysis
The analysis presented in this section evaluates the performance of the proposed MU-CSK system in terms of the averaged symbol error rate (SER) and system’s overall throughput. In order to validate the proposed scheme, we first evaluate the channel propagation conditions of the VLC system for the room configuration described in Table 2. Following a Monte Carlo ray-tracing method [17], Fig. 3 shows the corresponding channel impulse response of the single user CSK communication system. For this case, the channel response consists of a line of sight (LOS) component and a maximum of three reflections. From these simulations, it is found that about 90% of the received power concentrates on the LOS component. Similar results were obtained by changing the location of the receiver at least within a radio of 2 meters of the receiver position given in Table 2. Therefore, the contribution of the received powers coming from reflections is assumed to be small enough that it can be neglected when compared to the LOS component. This assumption is considered to be valid and practical since we are looking upon a system for illumination purposes. It is also assumed that the photodiode responsivity related to each wavelength channel is constant, i.e. Ri = 1 ∀ i. Finally, the received signal to noise ratio (SNR) quantity is defined as
where is the total received signal power for user j, N0 = 2σ2 the noise power and W the corresponding bandwidth.Table 2. Room configuration for simulations.
Figure 4 shows the SER results as a function of the SNR of MU-CSK with only one user and various constellation sizes. The simulations consider a VLC system transmitting over a multipath channel with a dominant LOS component, where the effect of the reflection components are neglected, using the same room configuration presented in Table 2. The single-user case is first addressed in order to examine the effect of the 3D modulation scheme. For these evaluations we use the optimized constellations obtained in Table 1 weighted by the signal power required to reach the desired SNR. It can be seen that SER degrades when the constellation size increases. There is an SNR penalty of about 5 dB at the SER of 1×10−3 when the constellation size increases from M = 4 to M = 8 and an extra 4 dB when it grows to M = 16. Clearly, these results are somehow expected since the minimum Euclidean distance of the constellation is reduced when M gets larger for a fixed transmitted power. Notice that the minimum Euclidean distance values in Table 1 validate the attained SER results.
Fig. 4 SER performance of the proposed MU-CSK scheme for a single-user case using different signal constellations (M=4, 8 and 16).
Next, in Fig. 5 we plot again the SER versus SNR but now allowing multiple users in the MU-CSK system. The room configuration used in Fig. 4 is also applied here but where the receiver of each user in the system is now placed randomly within an area with a radio of 2 meters around the emitter. Every user employs the same optimized constellation given in Table 1 for M = 4. As a reference point, the performance of MU-CSK is compared with that of a single-user VLC system using CSK modulation as outlined in the IEEE 802.15.7 standard. We will refer to this latter scheme as SU-CSK. We remark that SU-CSK does not support multiuser communications, however, it will be helpful in order to provide insights in the MU-CSK performance. It is also important to mention that we do not contemplate other multiuser VLC systems such as that given in [11] or other OFDM based scheme since their structure and complexity are very different to MU-CSK and then their comparison would be unfair. Thus, the evaluations of MU-CSK and SU-CSK provide a meaningful comparison of systems with similar complexity and hardware requirements. In particular, this comparison is yielded on the basis that both systems support RGB transmissions with compatible IM/DD channels and using the same symbol time duration. Furthermore, detection is performed by a ML based receiver in both systems. In this manner, the curves in Fig. 5 show the SER evaluations of MU-CSK for U = 1, 2, 4 and 8 using the same symbol time duration, Ts. For SU-CSK, we use the constellation set defined for M = 4 in the IEEE 802.15.7 standard [18]. The comparison reveals that for high SNR, MU-CSK manages to allocate approximately 4 users with a similar performance to SU-CSK system. Meanwhile there is a degradation close to 4 dB at a SER of 1 × 10−3 for MU-CSK with 8 users with respect of SU-CSK.
Fig. 5 SER performance of the proposed MU-VLC system for different number of users, the constellation size is fixed to M = 4 for all users.
We now present the throughput results for MU-CSK and compared them with those obtained by SU-CSK. The achievable throughput is determined by
Figure 6 illustrates these results in terms of the constellation size and clock speed. It is observed that throughput increases logarithmically with M as it is shown from Eq. (12). More interestingly, there are two important observations that can be deduced from these evaluations. First, since Ts = U · Tp then the overall throughput of MU-CSK does not depend on the number of active users. Second, assuming that all users employ the same constellation size M, the total throughputs for MU-CSK and SU-CSK are identical for a given clock rate. To explain this behaviour, we recall that the Nyquist sampling rate is defined as N/Ts, then with N = U MU-CSK achieves its maximum throughput given a clock rate. For a fair comparison, the SU-CSK throughput is evaluated with N = 1 in order to assume the same bandwidth requirements than MU-CSK. Thus, we can state that the throughput penalty in MU-CSK of using a time division multiplexing scheme is compensated with the cumulative throughput from U users using the 3D modulation scheme for a fixed symbol time duration.Finally, the effects of flickering and dimming control in the proposed MU-CSK transmission scheme are analysed. From (5), it can be observed that the overall light intensity on each colour band behaves as a random variable since each user transmits an unpredictable constellation symbol. To illustrate the flickering and dimming control attributes of MU-CSK, we characterize the quantity of light during the symbol time period. Figure 7 shows the distribution function, f (Xi), for the random variable, Xi, defined by
with M = 8 and U = 8. It is seen that the light fluctuation approximates to a Normal Gaussian distribution, regardless of the colour band. Thus, using a high clock speed ensures that the light fluctuations are sufficiently rapid, in this sense, the human eye will only perceive the time average of Xi which avoids the effect of flickering. The overall average transmitted power over the symbol period Ts can be expressed as where E[·] denotes the expectation operator. Substituting first Eq. (2) in Eq. (5) and then Eq. (5) in Eq. (13), we obtain that Thus, the estimated average transmitted power is given by with as the average transmitted power of user j over the three bands. Therefore, we can conclude that the illumination requirements are satisfied according to the sum of all users average transmitted power. Finally, light dimming can be integrated into the MU-CSK scheme by regulating PTs that depends on Tp and the variables P̄j ∀ j. Alternatively, a number of empty slots of duration Tp can be added to the transmission signals xi(t) on each band which can be interpreted as adding more users to the system with zero transmission power. We must also remark that due to the eye’s relatively long integration time, the regulation of the total average transmitted power over a longer period o time (> Ts) is feasible which may provide a better control of the illumination features. However, further studies need to be carried out to analyse these properties.5. Conclusions
In this paper, a novel VLC scheme has been proposed to enable multiple data communication in an indoor environment using multi-colour LEDs. The strength of the MU-CSK structure is that it combines the energy efficiency of pulse-coded signals and the multiple inputs feature of RGB channels to provide a simplified and robust multiuser VLC link that is potentially attractive for low-cost applications. The numerical results show that MU-CSK can enhance the SER performance as compared to the single-user CSK scheme outlined in the IEEE 802.15.7 even with several users allocated in the system. In addition, it is found that the MU-CSK overall throughput does not depend on the number of users but the clock rate. Thus, an important remark from this result is that more users can be allocated in the MU-CSK with faster clock rates and a bandwidth expansion. Although different power intensities are used on each light source, the statistical properties of the transmitted RGB signals ensure dimming capabilities and that the illumination function is unaffected by flickering.
Acknowledgments
This work has been supported by the Spanish MINECO (ARIES Project) and CONACYT, Mexico.
References and links
1. M. Nakagawa, “Visible light communications,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems TechnologiesOctober 2007.
2. A. Jovicic, J. Li, and T. Richardson, “Visible Light Communication: Opportunities, Challenges and the Path to Market,” IEEE Communications Magazine 51(12), 26–32 (2013). [CrossRef]
3. J. Gancarz, H. Elgala, and T. D.C. Little, “Impact of Lighting Requirements on VLC Systems,” IEEE Communications Magazine 51(12), 34–41 (2013). [CrossRef]
4. L. Grobe, A. Paraskevopoulos, J. Hilt, and D. Schulz, “High-Speed Visible Light Communication Systems,” IEEE Communications Magazine 51(12), 60–66 (2013). [CrossRef]
5. J. Armstrong, Y. Sekercioglu, and A. Neild, “Visible light positioning: a roadmap for international standardization,” IEEE Communications Magazine 51(12), 68–73, (2013). [CrossRef]
6. Y. Shun-Hsiang, O. Shih, T. Hsin-Mu, and R. Roberts, “Smart automotive lighting for vehicle safety,” IEEE Communications Magazine 51(12), 50–59 (2013). [CrossRef]
7. Visible Light Communication Consortium., VLCC: Home. [Online]. Available: http://www.vlcc.net(Accessed: 28 May 2014)
8. European Commission., Project hOME Gigabit Access. [Online]. Available: http://www.ict-omega.eu/(Accessed: 28 May 2014)
9. Byte Light., Indoor Location Based Software. [Online]. Available: http://www.bytelight.com/(Accessed: 28 May 2014)
10. S. Rajagopal, R.D. Roberts, and S. Lim, “IEEE 802.15.7 Visible Light Communication: Modulation Schemes and Dimming Support,” IEEE Communications Magazine 50(3), 72–82 (2012). [CrossRef]
11. Y. Hong, J. Chen, Z. Wang, and C. Yu, “Performance of a Precoding MIMO System for Decentralized Multiuser Indoor Visible Light Communications,” IEEE Photonics Journal 5(4), 7800211 (2013). [CrossRef]
12. O. Gonzalez, R. Perez-Jimenez, S. Rodriguez, J. Rabadan, and A. Ayala, “OFDM over Indoor Wireless Optical Channel,” IEE Proc. Optoelectronics 152(4), 199–204 (2005). [CrossRef]
13. Y. Wang, Y. Shao, H. Shang, X. Lu, Y. Wang, J. Yu, and N. Chi, “875-Mb/s Asynchronous Bi-Directional 64QAM-OFDM SCM-WDM Transmission over RGB-LED Based Visible Light Communication System,” 2013 OSA OFC/NFOEC Technical Digest paper OTh1G.3.
14. H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Communications Magazine 49(9), 56–62 (2011). [CrossRef]
15. O. Gonzalez, R. Perez-Jimenez, S. Rodriguez, J. Rabadan, and A. Ayala, “Multi User OFDM System for Communications over the Indoor Wireless Optical Channels,” IET Optoelectronics 1(2), 68–76 (2007). [CrossRef]
16. E. Monteiro and S. Hranilovic, “Constellation design for color-shift keying using interior point methods,” in IEEE Globecom Workshops1224–1228 (2012).
17. O. Gonzalez, S. Rodriguez, R. Perez-Jimenez, B. R. Mendoza, and A. Ayala, “Comparison of Monte Carlo ray-tracing and photon-tracing methods for calculation of the impulse response on indoor wireless optical channels,” Opt. Express 19(3), 1997–2005 (2011). [CrossRef] [PubMed]
18. “IEEE Standard for Local and Metropolitan Area Networks–Part 15.7: Short-Range Wireless Optical Communication Using Visible Light,” September 2011.
