1. Introduction

2. System Architecture

2.3. Hybrid OCC Scheme

As is shown in Figure 3, a hybrid OCC system is proposed herein that can communicate with both high and low data-rate streams using the same light source. Two OCC signals are transmitted at the same time through the hybrid waveform. This proposed system reduces cost while providing various services to users via a low-complexity light source for the communication network. Energy consumed is reduced as the required number of lamps for the communication system are reduced.

In the hybrid OCC scheme, OFDM packets of the high-frequency OCC signals are transmitted during the high period of the FSK signal and low period of the FSK signal, as is shown in Figure 7. In [15], the authors proposed a hybrid OCC system that allowed transmission of VLC signals during the high period of the OCC signal. In the proposed scheme, the high-frequency OCC signal is transmitted during both the high and low periods of the FSK signals. Therefore, the data rate increases by two times as compared to that of the conventional system. On the receiver side, two cameras detect the low-frequency and high-frequency signals, and this controls the exposure times for both the high-frequency and low-frequency waveforms. The image sensor functions as a lowpass filter, meaning that extending the exposure time results in attenuation of the high-frequency signals. As the exposure time increases, the communication bandwidth diminishes, leading to a reduction in the overall noise power distributed across the bandwidth.

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Pilot

For estimating and equalizing the optical channel, the pilots should insert the OFDM signals before the OFDM waveforms are transmitted. Minimal pilot density and pilot position are important for the OFDM system for optimal performance. In [19], the pilot spacing used in each OFDM symbol was investigated and evaluated.

The maximum pilot spacing value of the OFDM symbol is $\Delta p$, as shown in Equation (4).

$$\Delta p\le \frac{N\Delta f}{2\tau /T_s}$$

(4)

where N stands for the OFDM symbol, $\Delta f$ is the frequency spacing between the subcarriers, $N^{*}\Delta f$ is the OFDM bandwidth, $\tau$ is the time delay, and $T_s$ is the spatial sampling period.

The pilot spacing needs to be short for suitable interpolation performance. However, the estimation performance is not relative to the number of pilots. If the pilots are too close together, the system performance is reduced because they do not carry the desirable information. Figure 4 shows an example of the pilot positions.

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Equalizer

Channel equalization is a procedure for reducing the amplitude and phase alterations. During channel equalization, the channel effect is decreased to increase system performance. The equalization technique is then applied to balance the tradeoff between the efficiency and complexity of the processes [20]. For example, this model has two adjacent pilot points: H₀, H₁. Based on linear interpolation, the H(x) point between H₀ and $H_1$ is depicted as in Equation (5):

$$H(x)=H_0+\left(x-x_0\right)\cdot \frac{H_1-H_0}{x_1-x_0} \mathrm{with} 0\le x\le 1$$

(5)

$$Y_{\mathit{equalied}}=\frac{Y_{\mathit{non}\_\mathit{equalized}}}{H}d$$

(6)

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Rolling-Shutter OFDM Packet

The frame-rate variation is an important parameter in the OCC system. In most cases, it is supposed that a camera’s frame rate is fixed (e.g., 30 fps or 1,000 fps). In fact, all cameras have their own frame rates that differ based on the technical parameters. These parameters are unpredictable, making it even more difficult to synchronize the transmitter and receiver. To resolve this issue, a sequence number part is inserted into each OFDM packet, as shown in Figure 5. This assists the receiver side with assembling the packets in order and detecting any missing packets. The serial number of a packet is represented by SN. In reality, we can categorize situations based on the transmitter’s packet rate and camera’s frame rate into two scenarios. Case 1 pertains to undersampling, where the camera’s frame rate is lower than the transmitter’s (LED) packet rate. Case 2 corresponds to oversampling, where the camera’s frame rate significantly exceeds the packet rate of the transmitter. Our suggested data frame arrangement comprises numerous data packet frames, with each data subpacket (DS) containing payload data and a sequence number (SN). The DS components consist of multiple units. The SN serves as sequence information for a data packet, aiding a receiver in determining the arrival status of a new payload in situations with variable oversampling and detecting any missed payloads during undersampling conditions.

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M-FSK Packet

The M-FSK modulation data structure is as shown in Figure 6 and includes two parts: the asynchronous bit (representing the clock information of the data packet) for synchronization and payload for communication data. This was proposed by us [3] as the CM-FSK scheme. The Ab bit is generated by a specific frequency of the optical light source. The data packet length should consider the image sensor frame rate and application scenario. The preamble frequency is generated by a specific frequency of the optical light source to synchronize the head and tail of the data packet, as represented by $f_0$ and $f_5$ in Table 1.

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OFDM Symbol Synchronization

Before performing further activities required by the OFDM system, such as frequency synchronization and channel estimation, the symbol’s correct starting point must be determined. The goal of symbol synchronization is to achieve the start point of the OFDM symbol. In this work, we use two waveforms to transmit data based on a single LED, so that the confusion between two waveforms is reduced. The M-FSK waveform received by a smartphone can be easily detected and decoded by counting the row pixels between the “ON” and “OFF” statuses of M-FSK, as proposed in [5]. However, in the high-speed stream, we receive hybrid (O-OFDM and M-FSK) waveforms. To decode the OFDM signal in the hybrid waveform, we split the OFDM symbol from the hybrid waveform; then, the start of OFDM symbol detection is important before decoding the data. In this work, we introduced the Van de Beek [21] method for real-time detection of the O-OFDM symbol’s frame start, which relies on correlation with the CP part.

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Hybrid Waveforms

As mentioned in [14], RS-OFDM scheme was designed based on intensity modulation/direct detection. It is applied to generate a multicarrier waveform, which is then converted to a voltage signage for driving the LED light source. With M-FSK, the data is transmitted based on different frequencies of the on/off statuses of the LED lamp. On the receiver side, based on the values and ranges of the on/off strips in the images, it is simple to decode the data. There are two options available to combine the FSK and OFDM waveforms. Case 1: As shown in Figure 7a, only the OFDM symbols will be placed in the high period of the FSK signal as the FSK signal is based on the LED’s on and off statuses. Since the value is 0 V in the FSK signal’s low period, no OFDM signals can be placed there. Case 2: The conventional M-FSK scheme will be updated. Two intensities are used to describe the on/off statuses on the transmitter side. For example, 5 V and 10 V are used to describe the on/off statuses instead of 0 V and 10 V in the conventional M-FSK scheme. Therefore, the OFDM symbols can be placed in both the high and low FSK signal periods. In case 2, the rate of the high data stream is increased by two times as compared to that of case 1. The hybrid waveform is shown in Figure 7b.

As mentioned previously, the hybrid waveform is created from two waveforms: M-FSK and RS-OFDM. Therefore, the M-FSK frequencies must carefully consider the clock rate of the RS-OFDM and OFDM symbol length (shown in Table 1). Table 1 illustrates the relationship between the clock rate and FSK frequencies. As n OFDM symbols (n = 1, 2, 3, 4, ...) are required in each period of the FSK signal, the $f_0$ illustrate the preamble frequency of the M-FSK signal. This signal is calculated based on the clock rate and length of the OFDM symbol as follows:

$$f_0=\frac{f_{clock-rate}}{N_{OFDM\_frame}}$$

(7)

$$f_n=\frac{f_0}{n}$$

(8)

The clock rate of the hybrid scheme is $f_{clock-rate}$, length of the OFDM frame is $N_{OFDM\_frame}$, and n is the number of OFDM symbols in each period (low or high) of the FSK signal (n = 1, 2, 3, ...). The cycle of a hybrid signal has two cases (mentioned above). These are shown in Equations (9) and (10).

Figure 7. Hybrid waveform: (a) OFDM symbols embedded only in the high period of the FSK signal; (b) OFDM symbols embedded in the high and low periods of the FSK signal.

Figure 7. Hybrid waveform: (a) OFDM symbols embedded only in the high period of the FSK signal; (b) OFDM symbols embedded in the high and low periods of the FSK signal.

Table 1. 4-FSK encoding table.

Table 1. 4-FSK encoding table.

A packet of bits input	Frequency output
Preamble 1	f0
00	$f_1=\frac{f_0}{2}$
01	$f_2=\frac{f_0}{3}$
10	$f_3=\frac{f_0}{4}$
11	$f_4=\frac{f_0}{5}$
Preamble 2 (Ab bit frequency)	$f_5=\frac{f_0}{6}$

Here, $x_{OFDM,l}(t)$ is the l^th OFDM symbol, and n is the number of OFDM symbols in each FSK signal period; T is the cycle of FSK waveforms. The two direct current voltage bias values are $A_1,A_2$ ($A_1>A_2$) for the OFDM symbols. As is shown in Table 1, s_n(t_k) is the f_n−1 waveform. As noted in Table 1, the six frequencies correspond to n = 1 to 6. The cycle of the OFDM symbol is $T_{OFDM}$. The relationship between the cycle of OFDM symbol and cycle of M-FSK waveform, T, is as in Equation (11).

$$T_{OFDM}=T\cdot \frac{1}{2n}$$

(11)

To guarantee a flicker-free condition, the full hybrid waveform (multiple waveform cycles) is expressed as follows:

$$s_n(t)={\displaystyle \sum _{k=0}^M{s}_n}(t_k+kT)$$

(12)

As shown in Equations (9) and (10), it is assumed that the OFDM symbol’s cycle is constant, so the cycle of the FSK signal is based on the number of OFDM symbols in each high period of the FSK signal. Accordingly, the frequencies of the FSK signal are shown in Table 1. The relationships between the optical clock rate (carrier frequency) and FSK scheme frequencies are depicted in Table 1.

3. Implementation Results

Table 2. Hybrid OCC system parameters.

4. Conclusions

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