An automotive optical fiber communication system architecture and reliability verification example using WDM wavelength division multiplexing

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We designed an in-vehicle communication system using wavelength division multiplexing and studied its characteristics in various environments. Since a single optical fiber can transmit multiple wavelengths of light, it is assumed that this will lead to a reduction in the number and weight of cables required for communication compared to conventional coaxial cables. As a result, vehicle maintenance and fuel efficiency are improved. Using coaxial cable data as a reference, the transmission characteristics of single-mode fiber (SMF), wavelength division multiplexing (WDM) systems, and optical fibers were measured and compared in an in-vehicle environment.

Introduction

In recent years, due to the rapid development of autonomous driving technology and the increasing requirements for vehicle safety, the number and weight of wires used in each vehicle have increased [1]. This has reduced the maintainability and fuel efficiency of the vehicle, which has become a long-standing problem. To solve these problems, the Controller Area Network (CAN) communication protocol was invented in 1985, which connects each electronic control unit (ECU) with a single wire. The CAN protocol used in vehicles is a bus-type protocol called CAN bus communication. CAN bus, CAN star, and CAN ring protocols are currently widely used [2]. This CAN bus communication has reduced vehicle wiring to a certain extent. However, even with the adoption of the CAN protocol, the number of vehicle wires and the overall weight of the vehicle continue to increase.

Therefore, we propose an in-vehicle communication method using CAN communication and optical division multiplexing [3]. This will solve the problem and improve the maintainability and fuel efficiency of the vehicle. In the conventional method, each coaxial cable transmits one electrical signal, but in the proposed method, since light of different wavelengths can pass through the optical fiber, multiple signals can be sent and received through a single optical fiber. This will eliminate the increase in wiring and weight caused by conventional coaxial cables, thereby improving vehicle maintainability and fuel efficiency.

On the other hand, the disadvantages of this method include

1. Signal attenuation when converting electrical signals to optical signals (optical to electrical signals)

2. Physical attenuation of fiber coupling due to vibration.

However, regarding signal attenuation, attenuation can be suppressed by adding an amplifier, and this shortcoming can be solved relatively easily. In this study, we will measure the transmission characteristics of optical fibers under various environments and the use of wavelength division multiplexing (WDM).

Proposed Methodology and Conventional Techniques

In the conventional method, each coaxial cable sends and receives one electrical signal to communicate between each ECU. In the proposed method, multiple wavelengths of light are passed through one optical fiber to reduce the number of cables.

A. Optical fiber

Optical fiber is a method used for communication systems that started to be implemented in the 1970s. Compared to the coaxial cables used before the 1970s, optical fiber has various advantages such as light weight, low loss, delay and noise.

Optical fibers can be broadly classified into two categories: single-mode fiber (SMF) and multimode fiber (MMF) [4].

SMF is an optical fiber in which light passes only through the center of the fiber. The core system is as small as 9μm, so the transmission loss is low[5]. Therefore, a large amount of data can be transmitted at a time. In addition, since only one wavelength passes through the optical fiber, it has the advantage of not being affected by light of different wavelengths. One disadvantage is that the core system is as small as 9μm, making it easy to bend and not suitable for installation in places where wiring is difficult. In addition, in order to achieve the transmission of large amounts of data, high-purity glass materials are used as core materials, making the price of SMF uneconomical.

MMF is an optical fiber with multiple light transmission paths. The core system is 50μm or 62.5μm larger than SMF[6], and can transmit a variety of data. Unlike SMF, since multiple wavelengths of light pass through the inside of the optical fiber, it cannot send and receive large amounts of data. However, its large core system and bending resistance allow it to be installed in locations where wiring is difficult. The disadvantage is that light in MMF repeatedly undergoes total reflection as it passes through the optical fiber, so in the case of long-distance communication, the transmission speed may vary depending on each wavelength.

B. Wavelength Division Multiplexing

B. Wavelength Division Multiplexing

WDM is one of the optical communication technologies used to transmit large-capacity signals. On the transmission side of WDM, multiple semiconductor lasers that emit light of different wavelengths are prepared, and each laser is modulated to generate signal light. These signal beams are then transmitted in a single optical fiber by using a combining device. At the receiving end, the signal light is separated into lights of different wavelengths using a demodulator and then received by a photodetector. The more wavelengths used, the more signals can be transmitted. This study uses this feature to reduce the number and weight of wires. WDM can be divided into two categories: Corough WDM (CWDM) and Dense WDM (DWDM) [8], [9].

CWDM is a type of WDM that uses 18 wavelengths divided into 20nm intervals from 1271nm to 1611nm. However, in practice, it is rare to use all 18 wavelengths, and usually 8 wavelengths from 1471nm to 1611nm or 4 wavelengths from 1531nm to 1611nm are used. It is suitable for medium and short distance transmission. DWDM is a type of WDM that uses 160 wavelengths divided into 1.6nm intervals from 1531nm to 1611nm, which produces 9 times the wavelength of CWDM and is able to transmit a wider range of wavelengths. The transmission distance is long, and data transmission can reach 1000 kilometers.

With conventional multiplexing methods such as time division multiplexing, if another data is transmitted after that, the line cannot be used until the data transmission is completed, but with this wavelength division multiplexing method, data can be transmitted faster. In addition, since multiple optical fibers can be integrated into one optical fiber, this system has significant advantages in terms of ease of maintenance and cost.

Conventional methods use electrical signals for in-vehicle communications. Therefore, each signal requires a wiring line. The proposed method applies the two conventional technologies described in Section 2 to in-vehicle communications. By using optical fiber and WDM, multiple signals can be transmitted using a single fiber optic cable, reducing the wiring weight and the number of wires.

SECTION III.

Experimental Equipment and Environment

In this experiment, we will measure the bit error rate (BER) of one signal using SMF, the bit error rate of a combination of two signals using WDM, and the BER of a coaxial cable signal based on the traditional method. The pulse signal is generated using MATLAB and sent to the Lime SDR (Software Defined Radio) through GNURadio. The electrical signal is then converted to an optical signal through RoF-Link, and the converted optical signal is converted back to an electrical signal before reception, thereby measuring the bit error rate in the optical signal.

A. Transmission signal

generation

MATLAB is used to create the transmission signal. The transmission signal is a 10,000-bit pulse signal of random 0s and 1s. The generated pulse signal is output as a wave file (.wav), and the generated signal is transmitted by reading the wave file in GNU_Radio.

B. Radio Over Fiber (RoF)

-Link

RoF-Link is a device that converts electrical signals into optical signals[10].

Two types of RoF-Links are used in this experiment, with wavelengths of 1310nm and 1530nm. Using these RoF-Links, the transmission characteristics of the two signals in SMF using WDM will be compared with the coaxial cable used in the conventional method.

When the optical input level of the RoF-Link decreases by 1 dB, the gain of the RoF-Link decreases by 2 dB. Therefore, in order to stabilize the output, an amplifier is inserted inside the system to stabilize the gain within a certain range when the optical reception level varies within the range of +6 dB to -1.5 dB.

C. LimeSDR and GNU_Radio C. LimeSDR and GNU_Radio

LimeSDR sends and receives pulse signals generated in MATLAB. In this experiment, the device is used at each wavelength (1310nm and 1550nm). GNU_Radio is the software that controls the above LimeSDR and will be used to adjust the transmission power (gain) of LimeSDR and check the input and output signals. GNU_Radio achieves communication by connecting blocks written in C++ and Python. Each signal processing module written in C++ has an input and output interface, and by combining these modules, the sending and receiving of signals can be achieved. In the experiment, the transmission power (gain) is adjusted by manipulating the module screen.

D. WDM Fiber D. Wavelength Division Multiplexing Fiber

Unlike ordinary optical fibers, WDM fibers combine two wavelengths of light into one fiber. By combining the two optical fibers in this experiment, the two wavelengths are combined into one wavelength, and then the signal combined into one wavelength is restored to two wavelengths. In addition, due to the use of RoF-Link equipment, this experiment will use WDM fibers with adaptive wavelengths of 1310nm and 1550nm.

Each wavelength bandwidth is ±15.0nm, the maximum transmission power is 1W, the polarization loss dependence is ≥0.2dB, the directivity is ≥60dB, and the optical fiber is SMF before wavelength combination.