Multicoded Variable PPM for High Data Rate Visible Light Communications
 Author: Moon HyunDong, Jung SungYoon
 Organization: Moon HyunDong; Jung SungYoon
 Publish: Current Optics and Photonics Volume 16, Issue2, p107~114, 25 June 2012

ABSTRACT
In this paper, we propose a new modulation scheme called multicoded variable pulse position modulation (MCVPPM) for visible light communication systems. Two groups of signals (Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM) groups) are multicoded by orthogonal codes for transmitting data simultaneously. Then, each multilevel value of the multicoded signal is converted to pulse width and position which results in not only an improved data rate, but also a processing gain in reception. In addition, we introduce average duty ratio and cyclic shift concepts in PWM through which dimming control for light illumination can be supported without any degradation in communication performance. Through simulation, we confirm that the proposed MCVPPM shows a comparable BER curve and much greater achievable data rate than the conventional VPPM scheme using a visible light optical channel environment.

KEYWORD
Visible Light Communication (VLC) , Multicodes , Variable PPM , PPM , PWM

I. INTRODUCTION
Nextgeneration LED lighting is more advantageous than existing fluorescent and incandescent lighting in terms of long life expectancy, high tolerance to humidity, low power consumption, and minimal heat generation [1]. In addition, LEDs are used not only for illumination but also for many products such as monitors, cell phones, cars, and others. Recently, there have been many attempts to converge LED with IT technology [2,3]. Among them, Visible Light Communication (VLC), which is the convergence of illumination and communication, has emerged [46]. A corresponding VLC standardization was recently published by the IEEE Standards Association [7,8]. Generally, VLC uses the Intensity Modulation with a Direct Detection (IM/DD) scheme, which uses the amplitude (or intensity) of light to transmit data. Human eyes perceive only the average intensity when light changes faster than the Maximum Flickering Time Period (MFTP), which is defined as 5 ms [8]. Therefore, both lighting and communication can be simultaneously implemented. By considering both terms together, many modulation methods have been proposed, such as inverted pulse position modulation (IPPM), subcarrier inverted pulse position modulation (SCIPPM) [9], pulse width modulation (PWM) [10], and variable PPM (VPPM) [8]. Among them, VPPM is the modulation scheme proposed by the IEEE 802.15 standard group. To support illumination with dimming control and communication simultaneously, it uses binary PPM for communication and PWM for dimming control. However, the main drawback of the VPPM scheme is that the data rate is limited to the bandwidth of an LED since it uses only binary PPM modulation.
In this paper, we propose a new VLC modulation scheme called multicoded variable pulse position modulation (MCVPPM). Two groups of signals (PWM and PPM group) are multicoded by orthogonal codes for transmitting data simultaneously. Then, each multilevel value of the multicoded signal is converted to pulse width and position, which results in not only an improved data rate, but improved processing gain in reception as well. In addition, we introduce average duty ratio and cyclic shift concepts in PWM. To this effect, it is possible to support dimming control for light illumination without any degradation in communication performance. Through simulations, we compare BER and the achievable data rate of MCVPPM and conventional VPPM schemes in a visible light optical channel environment. When comparing BER to VPPM, much greater achievable data rate is shown in BER. The rest of the paper is organized as follows. Section II describes the proposed MCVPPM scheme. Simulation results and conclusions are given in Sections III and IV, respectively.
II. MULTICODED VARIABLE PPM SCHEME
Fig. 1 shows the block diagram of the proposed multicoded VPPM scheme for VLC. As shown in the Figure, the transmitting data block is divided into two separate parts. b
_{pwm} = [b _{1}, _{pwm}…b _{L1},_{pwm} ]^{T} is the PWM data block that contains theL_{1} antipodal signaled data. b_{ppm} = [b _{1}, _{ppm}…b _{L2},_{ppm} ]^{T} is the PPM data block that contains theL _{2} antipodal signaled data. Then, PWM and PPM data blocks are each encoded by usingL _{1} andL _{2} binary orthogonal multicodes of lengthN _{s}. Then two multicoded signal vectors of lengthN _{s} are obtained as follows:where
,C _{pwm} areC _{ppm}N_{s} ×L _{1} andN_{s} ×L _{2} binary orthogonalcode matrices.
[C _{l,pwm}c_{1,l,pwm} …c_{N}_{s},_{l,pwm} ]^{T} whereC _{n,l,pwm} ∈ { + 1, ？1 } denotes thel thN_{s} + 1 orthogonal codes of the PWM and PPM data group, respectively. Then, each element of andd _{pwm} haved _{ppm}L _{1} + 1 andL _{2} + 1 symmetric possible multilevel values, i.e.,d_{n,pwm} ∈ {？L _{1}, ？L _{1} + 2,… ,L _{1}？2,L _{1}},d_{n,ppm} ∈ {？L _{2}, ？L _{2} + 2,… ,L _{2}？2,L _{2}}In this paper, we propose a new concept called average duty ratio of pulse
X (%), which determines the average intensity of the light source. Dimming control can be supported by varying the average duty ratio. In the proposed scheme the following values are chosen as initial system parameters: the average duty ratio of pulseX (%) for dimming support; the pulse width variation intervalK (%) for PWM; the pulse width marginM (%) for limiting maximum and minimum pulse width; and the number of pulse positions N for PPM. Based on these basic system parameters, the possible amount of PWM and PPM data (L _{1},L _{2}) and the orthogonal codes of lengthN_{s} are determined as follows:2.1. Signaling Format
To transmit the signal, each element of a multicoded signal vector for PWM is converted to a pulse width vector as shown below:
where
and the corresponding pulse width
W_{n} is given asHere,
T_{f} is the duration of a frame, whereT_{f} = (L _{2}+1)δ, and denotes the pulse spacing for PPM. Table 1 shows the PWM mapping rule according to eq. (7) and (8).In the case of PPM, each element of a multicoded signal vector for PPM is converted to a pulse position vector as follows:
where
Table 2 shows the PPM mapping rule according to eq. (10).
Based on the pulse width and pulse position vector, the transmitted multicoded VPPM signal has the form
where
P _{mw},_{wn} represents the transmitting pulse of widthW_{n} , positionm_{n} , and energyThe transmitted signal that contains
L _{1} +L _{2} data occupies a total ofN_{s} frames of durationT_{f} , and requires a time duration ofT _{s} =N_{s}T_{f} . Because we set the average duty ratio of a pulse atX (%) for dimming support, the average pulse energysatisfies the following constraint:
The signaling example of the proposed multicoded VPPM is shown in Fig. 2.
In order to produce the corresponding signals
b _{pwm} = [b _{1,pwm}…b _{L1pwm}]^{T}= [1,1,  1 ]^{T} andb _{ppm} = [b _{1,ppm}…b _{L2ppm}]^{T}= [1,1,  1 ]^{T}; and the orthogonal code matrices;C _{pwm} andC _{ppm} are generated based on WalshHardamard (WH) orthogonal codes of length 4 as follows:From the Figure, we can see that there are pulses that exceed their own frame durations (
T_{f} ) and introduce Inter Frame Interferences (IFI). This is because we use PWM and PPM mapping simultaneously. Because IFI makes it difficult to detect and demodulate the transmitted MCVPPM signal on the receiver side, we introduce a Cyclic Shift scheme, which can remove IFI. By considering the energy constraints in Eq. (12) and (13) and providing Cyclic Shift, the shape of the transmitting pulse can be designed as follows:where
The graphical representation of the Cyclic Shift scheme by using the example given in Fig. 2 is shown as below:
Finally, an LED is driven by the current signal controlled by the MCVPPM signal
s (t ). The LED emits the light signalX (t ) ≥ 0, which has the average optical powerP_{t} given as follows:2.2. Receiver Design
After passing through the VLC optical channel
h (t ),X (t ) is received by a photodiode (PD). The received signalr (t ) is given as [4,5,11]:Here,
R is the detector responsibility [A/W],n (t ) is Additive White Gaussian Noise (AWGN), and * indicates the convolution operator.The average received optical power becomes
P_{r} =H (0)P_{t} , whereH (0)represents the channel DC gain. If we consider a LOS case with no reflections, the channel DC gain is:
where
m is the order related toΦ _{1/2}, the transmitter semiangle (at half power), given byFor example,
Φ _{1/2} = 60° (Lambertian transmitter) corresponds tom = 1.A is the physical detection area of the PD,d is the distance between the LED and the PD,？ is the angle of irradiance, andψ is the angle of incidence.T_{s} (ψ ) is the signal transmittance of the optical filter,g (ψ ) is the concentrator gain, and ψc is the concentrator field of view (FOV).We assume that the Gaussian noise
n (t ) has a total varianceN that consists of shot noise, thermal noise, and inter frame interferenceP_{rIFI} by an optical path difference.If the duration of the signal is long enough, the ISI is negligible. Therefore, the main noise sources become shot and thermal noises. A shot noise variance is given by [4]
where
q is the electronic charge,P_{rSingnal} is the received signal power,B is the equivalent noise bandwidth, andI_{bg} is the background current. We define the noise bandwidth factorI_{bg} =0.562. The thermal noise variance is also given as [4]:where the first two terms represent feedbackresistor noise and RET channel noise, respectively.
K is Boltzmann’s constant,T_{k} is absolute temperature,G is the openloop voltage gain,η is the fixed capacitance of a photo detector per unit area, ？ is the FET channel noise factor,g_{m} is the FET transconductance, andI _{3}=0.0868.Here, we assume that the detector responsibility is ideal (
R =1) and the receiver is exactly synchronized with the transmitter. Upon reception ofr (t ) , the receiver performs demodulation at the ith signal interval and pulse width, and produces the correlation metricr of length (L _{1}+1) ？ (L _{2}+ 1) ？N _{s}as follows:where
Here,
q_{a,b} (t ) =P_{a,b } (t )*h (t ) denotes the template pulse, which is the transmitted pulse dispersed by the VLC optical channel.Based on the maximumlikelihood (ML) decision rule, we detect the position and pulse width
of the transmitted pulses and regenerate the multicoded signal vector as shown below:
where
Then, the two multicoded signal vectors received,
are given as follows:
where
Finally,
L _{1} andL _{2} data contained inare decoded by orthogonal codes used at the transmitter with hard decision as follows:
III. SIMULATION RESULTS
Until now, we have discussed the overall description of the proposed MCVPPM scheme. Now, we present the simulated result to validate the proposed scheme. The initial system parameters of MCVPPM for simulation are shown in Table 3.
We used the VLC channel environment [12] and parameters for determining noise variances in [4] and [12]. For performance comparison, we include the conventional VPPM scheme [8], which is coded with an orthogonal code of length 4 to obtain the same processing gain as the proposed MCVPPM scheme. Note that we are focusing on evaluating the BER performance of the proposed MCVPPM modulation scheme. Therefore, we consider an uncoded system.
Fig. 4 shows the comparison of BERs for the proposed MCVPPM and VPPM scheme according to the received
E_{b} /N_{0} (E_{b} /N_{0} )_{rx}) by changing the dimming levels. BER plots were widely used to express the data transmission performance of the proposed scheme in regard to communication, including optical communication. We perform a MonteCarlo simulation that repeats data transmission 10^{9} times for obtaining a 10^{8} BER curve. In order to investigate the performance in the operating (E_{b} /N_{0} )_{rx} range of interest, we change 1) the dimming ratio and 2) the distance between the LED and PD. Fig. 5 illustrates the relationship between MCVPPM’s (E_{b} /N_{0} )_{rx} performance according to the dimming ratio under the VLC scenario in [12] and the distance between LED and PD. Because VLC performance based on MCVPPM affects the design of LED illumination infrastructure, it will be meaningful to express communication performance based on distance and dimming ratio under the given VLC scenario.
From the above result, it is observed that MCVPPM shows better BER performance as the dimming level decreases. This is because the interference caused by increased dimming levels in determining correlation metrics results in performance degradation, even though more energy is allocated to the transmitted signal thanks to higher dimming level increases in (
E_{b} /N_{0} )_{rx}. Compared with the conventional VPPM scheme, the BER of MCVPPM in 30% dimming levels always shows better BER performance than VPPM. In regard to BERs of the proposed MCVPPM with 50% and 70% dimming, the proposed MCVPPM still shows better performance in the low (E_{b} /N_{0} )_{rx} region even though the BERs of the proposed scheme with 50% and 70% dimming in higher (E_{b} /N_{0} )_{rx} regions are a little bit less than that of VPPM. However, as one can see in Fig. 6, the achievable data rate of the proposed MCVPPM scheme is much higher than that of the VPPM scheme (by a factor of 2 in our simulation). This means that it is possible to compensate for the degraded BER performance of the proposed MCVPPM scheme in high (E_{b} /N_{0} )_{rx} conditions by utilizing more power channel coding schemes with the benefit of a higher data rate. For example, MCVPPM can use 1/2 channel coding schemes under the same data rate condition while VPPM cannot use any kind of channel coding scheme. Therefore, the proposed MCVPPM scheme shows reasonably robust BER performance compared with the conventional VPPM scheme.As mentioned before, we compared the achievable data rate (ADR) between the proposed MCVPPM and VPPM schemes. ADR represents how much data can be transmitted without error per unit time. Therefore, ADR can be expressed as a function of the maximum data rate and the bit error rate (BER):
where
P_{b} andR _{max} denote BER and the maximum transmission rate per unit time (bits/sec), respectively. Here, the maximum transmission rateR _{max} is calculated as shown below:Finally, we can obtain the achievable data rate (ADR) of the proposed MCVPPM given as
Fig. 6 shows the comparison of achievable data rate between two schemes when the optical rate of the LED is 400kHz. As expected, the achievable data rate of MCVPPM is much greater than that of the VPPM scheme.
IV. CONCLUSION
We have proposed a new VLC scheme based on multicoded variable PPM (MCVPPM) for VLC. By converting multilevel values of the multicoded signals to pulse positions and pulse width, the proposed MCVPPM gives both improved data rate and robust BER. For light illumination, dimming can be also supported without any degradation in communication performance by introducing average duty ratio and cyclic shift concepts in to the PWM. Through simulations based on VLC scenarios, we confirmed the robust BER curves and muchgreater achievable data rates of the proposed MCVPPM in comparison with conventional VPPM schemes.

7.

[FIG. 1.] Block diagram of the proposed multicoded VPPM scheme.

[TABLE 1.] PWM mapping rule

[TABLE 2.] PPM mapping rule

[FIG. 2.] Signaling example of the proposed multicoded VPPM scheme. (X=70%, K=10%, M=10%, N=4, N1=3, N2=3, Ns=4)

[FIG. 3.] Cyclic Shifting example of the proposed multicoded VPPM scheme.

[TABLE 3.] The initial system parameters of MCVPPM

[FIG. 4.] The comparison of BERs between the proposed MCVPPM and VPPM.

[FIG. 5.] The (Eb/N0)rx performance of MCVPPM according to the dimming ratio under the VLC scenario in [12] and the distance between the LED and PD. ( LED power is 2 uW, optical rate : 400 kHz )

[FIG. 6.] A comparison of achievable data rate between the proposed MCVPPM and VPPM schemes. (optical rate of LED : 400 kHz)