Vehicle-to-Vehicle Communication using Free Space Optics

Overview

Radio frequency (RF) bands utilized for vehicular communications face increasing spectrum congestion and susceptibility to electromagnetic interference. Free-Space Optics (FSO) offers an alternative, highly directional line-of-sight communication medium with high data integrity.

In this project, developed for ECE4005 - Optical Communication Networks at Vellore Institute of Technology, I designed and prototyped an end-to-end Free-Space Optical Vehicle-to-Vehicle (V2V) telemetry system from scratch, complete with custom transceivers, a physical-layer framing protocol, and error-recovery mechanisms.

System Architecture

Figure 1: Transmitter and receiver hardware architecture for optical vehicle telemetry.

Hardware Implementation

1. Transmitter Circuit

Input telemetry (braking status, lane-change indicators, speed) is collected from vehicle control interfaces, framed by an embedded microcontroller, and modulated onto a collimated laser diode transmitter.

Figure 2: Laser transmitter circuit schematic and driver stage.

2. Receiver Circuit

A high-sensitivity photodetector array receives the modulated optical signal, applies thresholding to resolve binary pulses, and passes the bitstream to the receiver microcontroller for frame synchronization and telemetry decoding.

Figure 3: Optical receiver circuit schematic with signal conditioning.

Protocol Design & Synchronization

Frame Structure

A custom 17-bit frame was designed to balance throughput with error recovery over optical channels:

• Header: 4 bits (1011)
• Data Payload: 9 bits:
  • Lane change indicator: 1 bit (Left / Right)
  • Braking alert: 1 bit (Active / Inactive)
  • Vehicle speed: 7 bits (0 – 127 km/h range)
• Trailer: 4 bits (1010)

Figure 4: 17-bit optical telemetry packet structure.

Error Recovery & Bit Stuffing

• Bit Stuffing: Implemented bit-stuffing rules to ensure that reserved header and trailer flag sequences do not appear within the data payload, preventing false synchronization.
• Stop-and-Wait ARQ: Sender transmits a frame and waits for an optical acknowledgment (ACK). If no ACK is received within a 1.5-second timeout window, the frame is retransmitted. The receiver dynamically resynchronizes to the next incoming frame using header flag detection if intermediate bits are dropped.

Figure 5: Oscilloscope traces showing transmitted and received optical pulse streams under synchronization.