Modeling and analysis of variable PPM for visible light communications
© Yoo and Jung; licensee Springer. 2013
Received: 26 March 2012
Accepted: 8 May 2013
Published: 24 May 2013
Variable pulse position modulation (VPPM) is a new modulation scheme that supports simultaneously illumination with dimming control and communication. In this paper, the transmitter, optical wireless channel, and receiver structure of VPPM were modeled mathematically, and their error performance for examining the communication performance according to dimming level and data rate was analyzed. The results highlight the need for careful consideration of the tradeoff between the dimming flexibility and communication performance according to the channel condition in VPPM.
Recently, light-emitting diode (LED) research has focused on the emerging lighting infrastructure due to Green IT technical innovations. LED lighting is superior to existing incandescent and fluorescent lighting in terms of the long life expectancy, high tolerance to humidity, minimal heat generation, and low power consumption. Another important benefit of LEDs is that it is a controllable digital device. Therefore, many attempts have been made to merge LEDs with information technology (IT). Among them, visible light communication (VLC), which uses LED as a communicating device, has emerged as a new Green IT convergence technology [1–3]. Generally, VLC uses intensity modulation with a direct detection (IM/DD) scheme, which utilizes the amplitude (or intensity) of light to transmit data. Human eyes recognize only the mean intensity when light changes faster than the maximum flickering time period, which is defined as 5 ms. Therefore, both lighting and communication can be implemented simultaneously. In IEEE, the corresponding VLC standardization was recently published by the IEEE Standards Association .
This paper proposes the transmit-receive (TX-RX) modem structure, which is a mathematical model of VPPM and an optical wireless channel. The error performance of the model was then analyzed with regard to the dimming level for the illumination and channel condition. Based on the analytical and simulated results, this paper provides a guideline for the VPPM operation to satisfy both lighting and communication abilities for a given environmental condition.
2. System description
where R is the PD conversion responsivity (A/W), H(0) means the path loss gain of the signal, h(t) denotes the electrical impulse response of the optical wireless channel, and n(t) is the electrical additive white Gaussian noise.
where C is the number of channel cluster, G i is the channel gain of the i th cluster, τ i is the time constant of the i th cluster, and t d,i is the time delay of the i th cluster.
where q i (t) = φ i (t) ∗ h(t) (i = 0, 1) denotes the template pulse dispersed by the channel.
3. Performance analysis
where γ = R · H(0) means the scaling coefficient that contains the effect of the channel path loss gain and PD responsivity.
Accordingly, it affects the performance of VPPM. Therefore, it is important to consider this correlation factor carefully when operating VPPM.
4. Simulation result
The VPPM scheme was described, and its error performance was analyzed. This paper presents the analytical and simulated results to test the validity of the analysis and the relationship between the dimming and communication performance in the line-of-sight (LOS) and diffuse channel cases.
Channel parameters for simulations
h(t) = δ(t)
Number of clusters
Channel gain (G i )
Time delay t d,i
2.3 × 108
The error performance of VPPM was analyzed, and the results were confirmed by a simulation under LOS and diffuse channel conditions. An examination of the relationships among the BER curve, dimming level, and optical rate according to the reference SNRrx revealed tradeoffs between the dimming flexibility and communication performance according to the channel condition if the VPPM scheme for VLC is considered. Care should be taken when designing LED illumination infrastructure with a VLC support based on VPPM because the SNRrx in a VLC is related to the LED luminance distribution.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (No. 2010-0021118).
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