Overlapping PPM for band-limited visible light communication and dimming
© Gancarz et al. 2015
Received: 22 December 2014
Accepted: 5 May 2015
Published: 27 May 2015
The synthesis of visible light communication (VLC) and lighting state control necessitates data-light modulation that can accommodate intensity control. A number of techniques that enable both optical wireless data transmission and intensity control of light-emitting diodes (LEDs) have been proposed as a response to this need. Relevant schemes leverage amplitude modulation (AM)/continuous current reduction (CCR) and/or pulse-width modulation (PWM) for dimming capability. Two-level schemes related to PWM, such as on-off keying with compensation time (OOK + CT), variable pulse position modulation (VPPM), and multiple pulse position modulation (MPPM), are most commonly investigated. In this paper, we survey and compare OOK + CT, VPPM and MPPM. Moreover, we propose a novel approach towards dimming and data transmission through the variation of codeword weights in overlapping pulse-position modulation (OPPM). The proposed approach has comparatively high spectral efficiency. Using realistic constraints of a practical VLC system, analysis reveals that OPPM can increase data rates by more than 20Mbps over expected performance of related, two-level schemes, when using LEDs suitable for lighting that have relatively low modulation bandwidths.
Future lighting systems will be expected to be optimized to meet strict energy efficiency and light rendering quality goals. To be commercially competitive, they will increasingly have new functions providing adaptability, self-provisioning capabilities, and intelligence to react to human needs. These functions will be provided by embedded control and communications. Visible light communications (VLC) seeks to provide high-speed optical communications as an additional function, delivering additional capacity in indoor wireless networks. Although these functions leverage one another (embedded control, communications, and VLC), the common lighting task of intensity control (dimming) is in conflict with achieving optical data modulation. VLC systems, which stream data wirelessly by high frequency modulation of LED drive currents, must incorporate new modulation schemes for compatibility with lighting control . These usually result in increased complexity and decreases in performance as compared to systems designed for fixed lighting conditions. Dimming schemes inherently limit the transmitted optical power and place restrictions on the modulated waveform. Additionally, reducing light intensity can result in an undesired chromaticity shift of the emitted light . While a number of modulation schemes compatible with dimming requirements have been proposed to mitigate data rate and light quality losses, as described in [2–5], analysis of schemes from a comprehensive perspective is often lacking. Consequently, this paper seeks to incorporate realistic constraints, including the limited bandwidths of LEDs, in evaluating two-level baseband data modulation and dimming schemes.
We propose a novel operation through the variation of codeword weights of overlapping pulse position modulation (OPPM) to achieve dynamic dimming and data transmission at higher data rates compared to well-known common two-level modulation and dimming schemes. For noise-limited environments, other schemes may yield higher data rates than OPPM; yet, the subsequent analysis reveals that within a bandlimited regime, the higher spectral efficiency of OPPM results in significant data rate gains over comparable schemes.
In this paper, we review the VLC channel model, followed by an overview of common two-level modulation and dimming, a description of OPPM and its dimming functionality, and a presentation of the analysis revealing the advantages of OPPM. The final section concludes the findings.
VLC channel model
Dimming and two-level modulation techniques
AM dimming has a few significant drawbacks. AM may induce a noticeable chromaticity shift in the emitted light, i.e. the light color will change as the LED is dimmed, especially if dimming at low light levels. Additionally, AM dimming is nonlinear; a change in the forward current is not directly proportional to the luminance of the emitted light. Conversely, PWM generally does not induce a perceptible chromaticity shift and features a near-linear relationship between luminance and duty cycle. Dyble et al. demonstrated the superior color integrity properties of PWM versus AM in their evaluation of white LEDs and dimming .
OOK with compensation time
where the average power P avg is half of the peak power P pk . It is assumed that the bandwidth of OOK is approximately equal to R b .
OOK with compensation time (OOK + CT) consists of a data subframe followed or preceded by a CT subframe. CT is utilized to raise or lower average power of a frame to brighten or dim. The CT subframe is fixed at a high or low level, i.e. the LED is completely driven to its on state or off state, depending on whether one wishes to brighten above 50 % or dim below 50 %. The duration of the CT determines the precise proportion of peak power, p, that is emitted (p = P avg /P pk ).
This takes into account the fact that Manchester line-coded OOK requires twice the bandwidth of standard OOK. OOK + CT as presented is the elementary version of the first physical type of IEEE 802.15.7 Visible Light Communication Task Group standard .
Variable Pulse-Position Modulation (VPPM)
VPPM is a modified version of 2-PPM, in which each symbol is divided into two slots, or chips (see Fig. 2). In classic 2-PPM, a 0-bit is communicated by a pulse in the first slot and a 1-bit is communicated by a pulse in the second slot. VPPM differs in that, for a fixed symbol duration, the pulses can shrink or expand in width to achieve the desired average power corresponding to the dimming set point. Performance necessarily decreases when deviating from 2-PPM (when p = 0.5) due to a reduction in the Euclidean distance of the 0-bit and 1-bit symbols. VPPM has the same achievable R b and bandwidth constraint as OOK + CT in (6) and (7). The technique is also a basic version of the second physical type of the IEEE 802.15.7 standard .
Multiple Pulse-Position Modulation (MPPM)
MPPM as a modulation technique for simultaneous communication and dimming was presented in . MPPM is similar to classic L-PPM; however, instead of allowing for one pulse per symbol period in one of L chips, it permits multiple pulses in any of the L chips (see Fig. 2). Each unique combination of pulses within a symbol period is represented by a codeword. A possible codeword is c1 = [0,1,1,0], representing a symbol where there is no pulse in chips 1 and 4, a pulse in chip 2, and a pulse in chip 3. MPPM is characterized by two values, the number of chips per symbol, n, and the weight of accepted codewords, w. The weight, w, is equivalent to the sum of ones in a codeword. As in , we assume that the weight of codewords is fixed. Therefore, there are Q = (n,w) possible codewords where (n,w) is the binomial coefficient. Q is the alphabet size. L-PPM is a limiting case of MPPM where w = 1 and n = Q = L.
Lee and Park utilize MPPM to brighten or dim by varying the weight of the codewords . For example, (8, 2) MPPM will result in the average power of transmission being 1/4th of the peak power, p = 0.25. The ratio p is equivalent to w/n.
Overlapping Pulse-Position Modulation (OPPM)
Bo et al. discussed OPPM dimming in ; however, their approach utilizes AM dimming, i.e. the P pk is reduced to reduce light intensity. As explained in Section III, AM has certain disadvantages with regard to light quality. Nonetheless, reference  demonstrates that OPPM with AM dimming fares well considering a flicker severity index as compared to OOK and VPPM. Future research is necessary to conclude if a hybrid of OPPM dimming using AM and the varying of codeword weights may inherit benefits of both techniques.
Results and discussion
As evident from (6)-(11), the bit-rate can be noise-limited or band-limited at a particular perceived brightness. As one dims or brightens, changing p, it is possible to transition from a noise-limited regime to a band-limited regime (while keeping the same BER). The particular behavior is dictated by the noise and/or signal strength.
For the sake of comparison, a 5x5x3 m3 model room is studied having the same geometry and transmitter/receiver properties as scenario B of Grubor et al.’s work in reference . This configuration requires that 80 % of the illuminance at desktop level (0.85 m from floor) is above 400 lux, a typical requirement for office lighting. Four 1x1 m2 luminaires in the model room each contain 196 LED chips capable of emitting 12.3 W of radiant flux. A photodiode with responsivity 0.28 A/W is assumed as per conformity with .
In order to demonstrate that the bandwidth limitation is dominant even in dimly lit regions of the model room, the minimum recorded SNR, 47 dB, and received power, 0.1 mW, were adopted from the simulation of Grubor et al. The LEDs are at full-brightness for the SNR calculation, but certain regions of the room receive more light than others due the lighting configuration and reflections.
As shown in Fig. 4, OPPM (n = 32) achieves 40.9 Mbps at a perceived brightness of 70.7 %, which is 22.6Mbps greater than MPPM (n = 32) and 30.9 Mbps greater than OOK-CT/VPPM. If brightness is increased to 98.4 % (31/32 of peak power), OPPM (n = 32) drops to a bit rate of 19.4 Mbps, but still maintains superiority by achieving 16.3Mbps over MPPM (n = 32) and 18.8Mbps over OOK-CT/VPPM. Dimming to 17.7 % (1/32 of peak power), MPPM matches OPPM at a bit-rate of 3.1 Mbps, which is 2.5Mbps greater than the bit-rate achieved by OOK + CT/VPPM. MPPM and OPPM are essentially both 32-PPM at this percentage (since w = 1).
Dimming, or intensity control, is the most prevalent and popular form of lighting control. In addition to creating the desired ambience, dimming can also provide energy savings. Unsurprisingly however, dimming presents a number of immediate challenges to VLC as modulation schemes must enable both data transmission and light intensity adjustment.
In this paper, we proposed OPPM dimming, wherein the weight of codewords is modified according to the desired dimming percentage. The technique manifests its usefulness in a comprehensive analysis of a VLC environment, which takes into account LED bandwidth limitations. Analysis reveals gains of over 20 Mbps in using OPPM rather than other two-level schemes in band-limited environments. Noise levels must increase by over two orders of magnitude than the model case considered in the analysis in order for MPPM with the same number of chips to surpass the performance of OPPM.
This work is supported by the National Science Foundation under Grant No. EEC-0812056.
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