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Abstract
Fog has a strong attenuation effect on the optical channel, which will greatly degrade the performance of visible light communication (VLC). Studying the effect of the fog on communication performance is crucial to realize outdoor VLC for next generation networks, but there is little research on this topic. In this work, the transmission characteristics of visible light band in the foggy channel were measured and a high-speed VLC system based on a 450 nm blue laser diode (LD) and 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM-OFDM) in the artificial fog environment was demonstrated experimentally. Through a foggy channel of 60 cm, a maximum data rate of up to 4 Gbps was achieved at the pass loss of 13.06 dB with a bit error rate (BER) of 3.5 × 10−3 below the forward error correction (FEC) limit (3.8 × 10−3), which was the highest data rate ever reported for VLC in the foggy channel. Even at a higher pass loss of 17.32 dB, the proposed system still could achieve a data rate of 2.84 Gbps with a BER of 2.8 × 10−3. Further extending the distance to 16.9 m for a more practical application, a data rate of 2.0 Gbps was also demonstrated successfully.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The traditional radio frequency (RF) communication is limited by the scarce frequency band resources and is not sufficient to meet the requirements of high-speed wireless communication in the future. Visible light communication (VLC) has the advantages of high data rate and rich spectrum resources, which is one of the advanced research fields and has gained numerous researches in recent years [1–3]. Besides, Outdoor VLC or optical wireless communication (OWC) is considered to be an important part of the next generation networks and has great application potential in vehicle-to-vehicle (V2V) communication, intelligent transportation, marine communication and other fields for its merits of high data rate, low latency and easy integration with the infrastructure [1,3,4]. To break through the limitations of RF, lots of work on VLC in different channels including atmosphere and underwater has been carried out based on different light sources [5,6]. However, the atmospheric channel is unstable due to the changeable meteorological conditions such as rain, snow, dust, fog and haze, which will severely degrade the performance of outdoor VLC for the absorption and scattering mechanism [7]. Researches about the impacts of atmospheric channel on outdoor VLC are crucial and inevitable.
Among different meteorological phenomena, fog will greatly attenuate light propagation compared to others, which significantly limits the transmission distance and signal-to-noise ratio (SNR) of VLC [3,8]. In recent years, there have been many studies on the VLC system in the foggy channels. Because natural fog is of uncertainty and variability, experiments in natural fog environments are unreliable and unrepeatable. A chamber with artificial fog inside could be used to simulate the natural fog environment [9–11]. Kim et al. studied the propagation characteristics of laser beams at different wavelengths and found that the attenuation of laser beam was independent of wavelength in the dense fog channel (visibility < 500 m) [12]. Then, the spectral attenuation in visible and near-infrared bands for free space optical (FSO) communications was theoretically and experimentally investigated in artificial fog and haze environment [8]. V2V communication is an important application and requires high data rate and low latency for safety driving, which are exactly the advantages of outdoor VLC, but it is susceptible to fog, haze and other bad weathers. Lots of studies have been done to investigate the feasibility of light emitting diode (LED)-based OWC systems for V2V communication [4,13–15]. Compared with LED, laser diodes (LDs) have the advantages of high efficiency, high optical power and high modulation bandwidth, which is an outstanding front-end transmitter for VLC and able to be integrated with solid-state lighting (SSL) [16–18]. Most of reports about FSO communications in the foggy channel aimed at the theoretical transmission characteristics and were based on near-infrared or infrared light, which only could achieve low data rates. And there are few reports about high-speed VLC in dense fog. In reality, however, the high-speed short-distance communication in dense or thick fog environments is necessary in the field of outdoor VLC.
In this paper, a high-speed short-range VLC system employing a modulation scheme of 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM-OFDM) under the artificial fog environment was established. First, we measured the transmission characteristics of laser beams at wavelengths of 405, 450, 520 and 660 nm in the visible light range. Then, a 450 nm blue gallium nitride-based LD with high modulation bandwidth and high light output power was used to transmit the optical signal aiming at achieving high data rates. A glass fog chamber with a length of 60 cm and a fog generator were used to simulate the natural dense fog environment in laboratory. By controlling the fog mass concentration, the curves of BER versus pass loss at different OFDM bandwidths were obtained. Below the forward error correction (FEC) limit (3.8 × 10−3), a maximum data rate of 4.0 Gbps was achieved at the pass loss of 13.06 dB. When the pass loss increased to 17.32 dB, the proposed VLC system still could maintain a data rate of 2.84 Gbps. In order to meet the needs of a more practical application, furthermore, we extended the link to 16.9 m using high-reflectivity mirrors and obtained a data rate of 2.0 Gbps with pre-equalization. This work demonstrates the feasibility of LD-based high-speed outdoor VLC, which could be improved to higher data rate and longer distance in the future.
2. Attenuation effect of fog
Fog has a strong attenuation effect on light which will limit the transmission distance. Lots of work has been carried out to study the relationship between attenuation and wavelength. In general, light with long wavelength has weaker attenuation than that with short wavelength under fog environment. Absorption and scattering are two main factors that contribute to the attenuation. Similar to that in the underwater channel in our previous work [19], light propagation in the foggy channel can be also described by the Beer-Lambert’s law [20]:
where Pr and Pt represent the received and transmitted optical power, respectively. γ(λ) is the attenuation coefficient related to wavelength λ and L is the transmission distance. γ(λ) is related to the absorption and scattering by molecules and fog droplets: where αm and αd are the absorption coefficients caused by molecules and fog droplets, respectively, βm and βd are the scattering coefficients caused by molecules and fog droplets, respectively. In dense fog environment, the scattering caused by fog droplets plays a dominant role according to Mie scattering theory [12]. There have been many theories and empirical formulas trying to describe the light transmission characteristics in the foggy channel, but it is still controversial and hard to find a unified model applicable to all cases. It has been proposed that the attenuation in dense fog (visibility < 500 m) is independent of wavelength [12]. This property has been experimentally confirmed in the infrared wavelength band [21,22], but has not been proven in the visible wavelength band. In this study, the fog mass concentration ranged from 15 to 400 mg/m3 whose visibility was lower than 200 m and the visibility could be defined as the distance at which the optical power was reduced to 2% [23]. As shown in Fig. 1, we first measured the transmissive characteristics of laser beams at different visible wavelengths using a glass fog chamber with a length of 60 cm. Due to the instability and non-uniform distribution of the fog, however, it was hard to obtain precise measurements. So, we used the method of averaging multiple measurements to reduce the deviation. It was found that the theory in [12] could not be applicable in the visible wavelength band. The result shows that the path loss is clearly dependent of wavelength. In general, short-wavelength laser is susceptible to the larger pass loss and cannot transmit over a long distance in the foggy channel and the long-wavelength laser has smaller attenuation.
Fig. 1. Path loss as a function of fog mass concentration measured at wavelengths of 405, 450, 520 and 660 nm in visible light range based on LDs.
As we mentioned above, the V2V communication is an important application and could be integrated with LD-based lighting. Some automakers are already using LD-based white light sources and it has great potential to realize smart vehicle lighting and smart transportation in the future [18]. At present, there are mainly two technical routes to achieve LD-based white light illumination, both of which include the blue LD. One is blue-emitting LD covered with yellow phosphor, and the other is mixing the three primary-colors LDs [18,24]. Blue LD is a good choice for outdoor VLC and V2V communication. And blue LD has a high modulation bandwidth and high output optical power which will benefit the outdoor VLC performance. So, we chose the 450 nm blue LD as the transmitter for an expectation to achieve high-speed outdoor VLC under dense fog environment in this work.
3. Experiment setup
The schematic diagram of the experiment setup for the proposed VLC system in the artificial foggy channel is presented in Fig. 2. First, the 16-QAM-OFDM digital signal was derived from the pseudorandom binary sequences and offline generated by a self-compiled program in MATLAB. The specific description about OFDM will be discussed later. The digital signal was uploaded into an arbitrary waveform generator (AWG, Tektronix AWG710B, 4.2 Gsa/s), which converted the digital signal to an analog one. Through a bias-tee (Mini-circuit ZFBT-6GW+, 0.1-6000 MHz), the OFDM analog signal was biased by a direct current (DC) signal from a DC source (Yokogawa GS610) to generate the positive signal, which was used to drive a 450 nm blue LD (Osram PL 450B). The blue light emission was collimated by a lens at the receiver side and transmitted through a glass tank with artificial fog inside. Afterwards, the light was collected and focused onto a high sensitivity 1 GHz Hamamatsu C5658 avalanche photodiode (APD) with an active area diameter of 0.5 mm by another lens. The artificial fog was generated by a commercial fog generator to simulate the natural fog and had a droplet diameter of 3–10 µm. The distances from the glass tank side walls to the LD and the APD were 5.2 and 8.0 cm, respectively. And the length of the fog chamber was 60 cm. The APD converted the light signal into the electrical signal, which was captured by a high-speed digital signal analyzer (DSA, Agilent DSA90604A Infiniium, 20 Gsa/s). Finally, the received data was offline demodulated and analyzed with a MATLAB program for calculating the BER, SNR, constellation diagram and other parameters. The photos of the transmitter, fog chamber and receiver are shown from left to right in the Fig. 3, respectively.
Fig. 2. Schematic diagram of the experiment setup for the proposed VLC system based on a 450 nm blue LD in the foggy channel.
Fig. 3. Captured photos of (a) a 450 nm blue LD at the transmitter side, (b) the overall transceiver and fog chamber with a length of 60 cm and (c) a high sensitivity 1 GHz APD at the receiver side of the proposed VLC system in laboratory.
Considering the strong multipath effect in a lossy channel and the SNR at the receiver side, the modulation scheme adopted in this work was chosen as 16-QAM-OFDM that could suppress the inter-symbol interference (ISI) by adding the cyclic prefix (CP). The diagram of the 16-QAM-OFDM is illustrated in Fig. 4. First, the serial binary bit stream was transformed to parallel blocks and then mapped to 16-QAM symbols. The total subcarrier number NSC was 512 and pilot sequences with a length of 1/16 FFT size were then uniformly inserted into the blocks at an even interval for channel estimation. For a 60 cm link, the upsampling rate and FFT size were set to 2 and 1024, respectively, and for a 16.9 m link, they were set to 4 and 2048, respectively. Before the inverse fast Fourier transform (IFFT), another half of FFT size in the OFDM symbols was filled with the conjugate complex values of the mapped 16-QAM symbols for obtaining a real-value OFDM signal, which was known as the Hermitian symmetry. After, the 16-QAM-OFDM frequency-domain signal was converted to temporal signal using IFFT algorithm. The CP with a length of 1/16 FFT size was added to the front of the OFDM signal which was loaded into the AWG subsequently. In this work, the AWG sampling rates were adjusted to obtain varied OFDM bandwidths. The subcarrier number Nsc could be calculated by following formula [25]:
where fB, NFFT and fAWG represent the OFDM signal bandwidth, FFT size and the sampling rate of AWG, respectively. The data rate is related to fB and fAWG, could be calculated by [26]:Fig. 4. The diagram of the 16-QAM-OFDM. The inner image in the left shows the impact of noise and distortion on the 16-QAM constellation. The right one presents the structure of the generated OFDM symbols.
The received digital signal from the DSA was converted to the frequency-domain signal by the FFT algorithm after removing CP. Then, the channel estimation with the simple least square (LS) algorithm was employed to alleviate the distortion of the frequency selective fading in the foggy channel. Consequently, the frequency-domain signal was remapped to 16-QAM symbols and converted to serial binary data. The received data was processed and analyzed offline to obtain the constellation diagram, SNR and EVM of each subcarrier. Comparing the input and output binary data could yield the BER. All the digital data was processed using the offline MATLAB program. SNRs of different subcarriers showed how the signals in different frequency bands impacted the performance and the SNR of the nth subcarrier was obtained indirectly by calculating the EVM [27]:
It is worth noting that the inner surface of the glass fog chamber was sprayed with antifogging agent to prevent the fog droplet from condensing. The average mass concentration with a unit of mg/m3 was used to evaluate the density of fog. In addition, the fog mass concentration in fog chamber was controlled and measured by the fog generator and dust meter (DUSTTRAK II 8530EP, 0.001–400 mg/m3), respectively. An optical power meter (Thorlabs PM100D) was used to measure the optical powers at the transmitter and receiver sides under the condition of varying fog mass concentrations. This test was repeated several times to reduce the measurement errors caused by the instability and uneven spatial distribution of the fog.
4. Results and discussion
The voltage versus current (V-I) and output optical power versus current (P-I) characteristics of the 450 nm blue LD are shown in Fig. 5(a). Figure 5(b) presents the spectrum of the blue LD at 37 mA measured by a spectrometer (Ocean Optics USB4000). The inset is a captured photo of the packaged blue LD. The peak wavelength is at ∼446.7 nm and the full width at half maximum (FWHM) is ∼2.63 nm. The narrow peak of LD shows a good monochromaticity. The drive current and modulation depth are two vital parameters that influence the performance of the OWC in fog environment, because they determine the utilization of the active linear range of LD. The tested BERs under different currents and peak-to-peak voltages (Vpp) of the OFDM temporal signal were obtained and presented in Fig. 6. Vpp is also known as the modulation depth. Both curves in Fig. 6 demonstrate a shape of valley. The lowest valley points in Fig. 6(a) and Fig. 6(b) were chosen as the drive current and peak-to-peak voltage of OFDM signal for the subsequent VLC test, which were 37 mA and 1.8 V, respectively. And the spectrum was also measured at 37 mA corresponding the biased voltage of 4.52 V in Fig. 5(b).
Fig. 5. (a) The V-I and P-I characteristics of the 450 nm LD. (b) The measured spectrum of the 450 nm LD biased at 37 mA. Inset: the photo of packaged 450 nm blue LD mounted on a Peltier thermo-electric cooler and a collimating lens.
Fig. 6. BER of 1 GHz 16-QAM-OFDM signal at (a) different drive current of LD and (b) peak-to-peak voltage.
The communication performance of the proposed VLC system was tested under the conditions of optimum drive current and modulation depth. At the transmitter side, the optical powers close to outer and inner front glass surfaces were 17.22 and 15.89 mW, respectively. At the receiver side, the optical powers close to outer and inner back glass surfaces were 14.51 and 13.57 mW, respectively. Hence the transmittances of front and back glasses were roughly equal to 92.3% and 93.5%, respectively. First, the BER performances at fixed bandwidth of 16-QAM-OFDM under different pass losses were obtained as shown in Fig. 7(a). It can be seen that the BER increases monotonically with the increasing of pass loss. Under the same pass loss, the BER of a high-bandwidth OFDM signal is larger than that of a low-bandwidth OFDM signal. The BER of 1.0 GHz 16-QAM-OFDM at 13.06 dB is 3.5 × 10−3 below the FEC threshold, which corresponds a raw data rate of 4.0 Gbps. After removing the CP and pilots, the effective data rate is 3.41 Gbps. It is noticed that the 0.71 GHz OFDM data still meets the FEC criterion at high pass loss. The BER of 0.71 GHz OFDM data at 17.3 dB is 2.8 × 10−3 below the FEC limit, corresponding a data rate of 2.84 Gbps. The pass loss (dB) of light beam in the foggy channel is calculated by:
where both Pt and Pr have a unit of mW. It is worth noting that the transmittances of front and back glasses were taken into account when calculating the pass loss using formula (6). In addition, the transmissive characteristics of 450 nm laser in foggy environment were tested. The average pass loss versus fog mass concentration at wavelength of 450 nm was measured as also shown in Fig. 1. It is clear that the path loss rises rapidly with the increase of fog mass concentration due to the strong Mie scattering effect in dense fog.Fig. 7. (a) BER versus path loss under different bandwidths of 16-QAM-OFDM signal. The dash line represents the FEC limit. (b) The SNRs and BERs of the different subcarriers that carrying effective data of 1 GHz 16-QAM-OFDM signal at 13.06 dB.
As is shown in Fig. 7(b), the performances of subcarriers that carry effective data were also obtained for further analysis. It can be seen that, with the increase of frequency, the SNR curve first rises in a narrow low-frequency band and then basically decreases in the high-frequency band, while the BER curve presents an increasing trend by and large. It is worth noting that the BERs of the subcarriers in the low frequency band are almost zero, but the BERs of the subcarriers in the high frequency band rise rapidly. Therefore, the overall BER can be considered to be mainly caused by the high BERs of high-frequency subcarriers. This is mainly attributed to the fact that the VLC system has a relatively low response to high-frequency signal, which is can be proved in the frequency spectra above. To further improve the communication performance, the pre-equalization can be used to compensate the attenuations of high-frequency subcarriers, or the method of power and bit loading can be used to increase the data rate.
The electrical power spectra of 0.71 and 1.0 GHz 16-QAM-OFDM signals were captured by the DSA under the same pass loss as shown in Fig. 8(a) and 8(d), respectively. It can be clearly seen that the power spectra present a decaying trend owing to the fading frequency response of LD. The cut-off frequency is corresponded to the bandwidth of OFDM signal. In addition, the constellation diagrams of 0.71 GHz signal at 11.3, 17.32 dB and 1.0 GHz at 11.3, 13.06 dB are presented in Fig. 8(b), 8(c), 8(e) and 8(f), respectively. When the fog mass concentration rises, the increase of pass loss induces the declined SNR. The decrease of SNR is reflected in the constellation diagram, showing that the distribution of points is more scattered.
Fig. 8. The electrical spectra are captured by the oscilloscope. The constellation diagrams are drawn by the MATLAB. 0.71 GHz 16-QAM-OFDM: (a) the frequency spectrum at 11.3 dB, (b) constellation diagram at 11.3 dB and (c) constellation diagram at 17.32 dB. 1 GHz 16-QAM-OFDM: (d) the frequency spectrum at 11.3 dB, (e) constellation diagram at 11.3 dB and (f) constellation diagram at 13.06 dB.
Furthermore, in order to meet the needs of a more practical outdoor VLC, we used another fog chamber with a larger length of 3.38 m and several mirrors with high reflectivity to extend the VLC link to 16.9 m. Such distance is sufficient for some short-range outdoor VLC application scenarios. The schematic diagram and photos of the extended link are presented in Fig. 9(a) and 9(b), respectively. The setup of 16.9 m link was similar to that in Fig. 1, but the lens at the receiver side was replaced by a Fresnel lens with a diameter of 10 cm to collect larger light spots. To obtain a better communication performance, the optimum drive current was changed as 50 mA but the modulation depth remained the same. And the upsampling rate was set as 4 to suppress the spectral sidelobes. The optical power at the transmitter side was 20.2 mW close to the outer glass surface. The optical powers at the receiver side were 0.2 and 7.1 mW with and without fog inside, respectively. The mass concentration for the 16.9 m link was not specified, because the instability and uneven spatial distribution of fog would make it hard to obtain a precise measurement in such a long chamber. However, the range of fog mass concentration was obtained, which was 15–50 mg/m3.
Fig. 9. Extending the transmission distance to 16.9 m using four mirrors. (a) Schematic diagram of the extended VLC link in fog environment. (b) The photos of the 16.9 m VLC link.
Due to the high modulation bandwidth and high optical power of blue LD, several Gbps data rates can still be achieved over 16.9 m link. The BER versus data rate before pre-equalization under the 16.9 m foggy channel is shown in Fig. 10(a). A maximum data rate of 1.4 Gbps was demonstrated with a BER of 3.2 × 10−3 below the FEC limit. The constellation plots corresponding to 0.8, 1.2 and 1.4 Gbps were also obtained as shown in Fig. 10(a), which present a more scattered trend with the increase of BER. The frequency spectrum corresponding to 1.4 Gbps is shown in Fig. 10(b) with a cut-off frequency of 350 MHz. It is also clear to observe a decay in Fig. 10(b) which will limit the available OFDM bandwidth. To expand the signal bandwidth and obtain higher transmission speed, a simple zero-forcing pre-equalization with one-tap was adopted to mitigate the decay at high frequency before IFFT. Meeting the FEC criterion, a maximum data rate of 2.0 Gbps was obtained with a BER of 2.0 × 10−3 as shown in Fig. 10(c). And the frequency spectrum corresponding to 2.0 Gbps is shown in Fig. 10(d), which is flatter than that in Fig. 10(b) for the use of pre-equalization.
Fig. 10. Under the 16.9 m foggy VLC channel, (a) the tested BER versus data rate and (b) the frequency spectrum corresponding to 1.4 Gbps before pre-equalization; (c) the tested BER versus data rate and (d) the frequency spectrum corresponding to 2.0 Gbps after pre-equalization.
5. Conclusion
Aiming at short-range high-speed wireless communication in the dense fog environment, this paper proposes a VLC system based on a 450 nm blue LD and 16-QAM-OFDM. To achieve a high data rate and easy integration with LD-based white light illumination, a 450 nm blue LD with high optical power and high modulation bandwidth was selected as the transmitter. Under the conditions of optimum drive current and modulation depth, the proposed system achieved a high data rate of up to 4.0 Gbps in 60 cm free space, with a pass loss of 13.06 dB and a corresponding BER of 3.5 × 10−3 below the FEC limit. At higher pass loss of 17.3 dB, moreover, the data rate could still be up to 2.84 Gbps with a BER of 2.8 × 10−3. Then, we extended the VLC link to 16.9 m to demonstrate a more practical VLC link and obtained a data rate of 2.0 Gbps with a BER of 2.0 × 10−3. This proposed system could maintain high data rates in severe weather conditions or some special scenarios and has great application potential in the field of outdoor VLC. And it could be integrated with LD-based white light illumination to realize the smart lighting and smart transportation. The data rate can be improved further by employing the bit loading algorithm and optimizing the optics design in the future.
Funding
National Natural Science Foundation of China (NSFC) (61974031); Fudan University-CIOMP Joint Fund (FC2020-001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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