Vehicular visible light communications (V2LC) has recently gained popularity as a complementary technology to radio frequency (RF) based vehicular communication schemes due to its low-cost, secure and RF-interference free nature. In this paper, we propose outdoor vehicular visible light communications (V2LC) frequency domain channel sounding based channel model characterization under night, sunset and sun conditions with the usage of vector network analyzer (VNA) and commercial offthe-shelf (COTS) automotive light emitting diode (LED) light. We further bring forward a new practical system bandwidth criteria named as effective usable bandwidth (EUB) for an endto-end V2LC system with respect to real world measurements. We demonstrate outdoor static V2LC channel measurement results, taking into account vehicle light emitting diode (LED) response, road reflections from nearby vehicles and various day light conditions with respect to varying inter-vehicular distances. Measurement results indicate that, sun light decreases system effective usable bandwidth due to the limited dynamic range of avalanche photodiode (APD), nearby vehicles cause constructive interference whereas road reflections change time dispersion characteristics of the V2LC channel.
Future connected vehicles are expected to require fast and reliable exchange of road information to increase safety and enable cooperative driving. Currently, standardized vehicular communication technologies aim to enable basic safety message exchanges with limited bandwidth. Recently, alternative technologies, based on millimeter-wave (mmWave) and visible light spectrum are proposed as complementary vehicle-toeverything (V2X) communication schemes, provisioned to support future connected vehicles with high bandwidth and increased security. However, the understanding of channel propagation characteristics is the key to achieve reliability, due to higher path loss compared to 5.8 GHz band. In this work, we compare channel path loss characteristics of mmWave and vehicular visible light communication (VVLC) schemes to provide an overview regarding technology selection in an indoor parking garage. Path loss measurements are conducted with respect to various intervehicular distances, receiver angles, nearby vehicle existence, and lane occupation scenarios. Measurement results indicated path loss of 21.47 dB for VVLC from 3 m to 20 m distances. Moreover, path loss for mmWave 26.5 GHz and 38.5 GHz channels increased 12.5 dB, and 12.7 dB, respectively. Nearby vehicles are shown to decrease path loss of 26.5 GHz and 38.5 GHz signals up to 9.78 dB, and 9.56 dB, respectively, whereas VVLC channel path loss decreases 0.4 dB at the same scenario. Channel frequency response (CFR) measurements indicated frequency flat behavior of VVLC channels while mmWave channel exhibits frequency selectivity induced dispersion due to parking garage structure. Obstructed line-of-sight (OLoS) measurements further reveal that blocking vehicle interrupts VVLC signals while selecting a favorable antenna location leads up to 30 dB less path loss for mmWave signals.
Vehicular visible light communication (V2LC) is expected to complement radio frequency (RF) technologies for higher reliability in vehicular connectivity. Since high mobility makes the line-of-sight V2LC channel very dynamic, an adaptive physical layer (PHY) design is required for realizing a rateoptimal and reliable V2LC system. Existing studies on adaptive PHY designs have mostly considered indoor scenarios with low mobility and require a feedback channel for both reporting the received signal-to-noise ratio (SNR) to the transmitter and channel equalization (CE), which increases system complexity and introduces overhead. This paper presents a novel lowcomplexity adaptive PHY design that provides rate-optimal and reliable V2LC without a feedback channel. The proposed design utilizes a priori measurements of the BER with respect to SNR, which are static for V2LC on the road. SNR is predicted in real-time based on the relative locations of the transmitting (TX) and receiving (RX) vehicles using a path loss model based on a priori measurements of the SNR-distance relationship and the polar beam pattern for a given TX/RX pair, in a given setting. The proposed design is validated via night-time experiments with On-Off-Keying (OOK), 4-Pulse-Position Modulation (4-PPM) and Direct Current-Biased Optical OFDM (DCO-OFDM). The proposed location-aware adaptive PHY design can be expanded for general reliable rate-optimal V2LC use by updating the path loss model with additional measurements for different settings.
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