Performance Analysis of IEEE 802.11p Preamble Insertion in C-V2X Sidelink Signals for Co-Channel Coexistence

This paper proposes and evaluates a mitigation strategy for co-channel interference between IEEE 802.11p and LTE-V2X Mode 4 by inserting an 802.11p preamble into LTE-V2X transmissions, demonstrating through analysis and simulations that this approach effectively reduces collisions and improves performance across various traffic densities, particularly when combined with congestion control mechanisms.

The Gist

Imagine a busy highway where two different types of drivers are trying to talk to each other to avoid accidents and keep traffic flowing smoothly.

• Driver A (IEEE 802.11p): This driver is like a cautious old-school motorist. Before they honk or shout a warning, they listen carefully to the road. If they hear anyone else talking, they wait their turn. They are very polite but can be slow to react if the road gets too crowded.
• Driver B (LTE-V2X): This driver is like a high-tech, autonomous robot. They have a pre-planned schedule. They don't really "listen" before they speak; they just start talking at their exact scheduled time, regardless of what's happening around them. They are fast and efficient, but they can accidentally shout over Driver A.

The Problem: The "Shouting Match"

The paper tackles a growing problem: Spectrum Scarcity. Both drivers are trying to use the same radio frequency (the same "road") to talk.

Because Driver B (the robot) doesn't listen before speaking, they often start talking right when Driver A (the cautious motorist) is trying to send a critical safety message. Driver A hears the noise, thinks the road is busy, and stays silent. But Driver B keeps shouting, drowning out the important safety warnings. This creates a "co-channel coexistence" nightmare where safety messages get lost in the noise.

The Proposed Solution: The "Pre-Warning Bell"

The authors propose a clever, low-cost fix: Insert a "Pre-Warning Bell" into Driver B's speech.

Here is how it works:

1. The Old Way: Driver B starts talking immediately. Driver A, who is listening, might miss the very first split-second of the noise because it's too short or looks like background static. Driver A thinks, "Oh, the road is clear!" and starts talking, causing a crash (collision).
2. The New Way: Before Driver B starts their main message, they ring a specific, recognizable "bell" (an IEEE 802.11p preamble) for a tiny fraction of a second.
   • This bell is a fixed, known sound that Driver A is trained to recognize instantly.
   • As soon as Driver A hears this bell, they know, "Oh! Driver B is about to talk for a full second. I better wait."
   • Driver A waits, and Driver B speaks clearly without interruption.

Why is this cool?

• It's a one-way street: Driver B (the robot) doesn't need to learn how to speak Driver A's language. They just need to play a pre-recorded sound file at the start of their message. They don't need to be "smart" enough to listen; they just need to be "loud" enough to ring the bell.
• It's compatible: It works with the old cars (Driver A) without changing their software, and it only requires a tiny tweak to the new cars (Driver B).

The Results: What the Simulations Showed

The researchers ran computer simulations to see how this "bell" worked in different traffic conditions:

1. Light Traffic (The Open Highway):

   • Result: The bell works perfectly. Driver A almost never gets interrupted. The "collision rate" drops significantly. It's like having a polite traffic cop at a quiet intersection.

2. Heavy Traffic (The Rush Hour Jam):

   • Result: The bell helps, but it's not a magic wand. When the road is packed with thousands of cars, even with the bell, Driver B still tends to hog the road because they have a rigid schedule. Driver A gets squeezed out again.
   • The Fix: The paper suggests that in heavy traffic, we need to combine the "bell" with Traffic Control. We need to tell Driver B (the robot) to slow down their speaking rate or take breaks when the road is too crowded.
   • The Winner: The best solution was The Bell + Smart Traffic Control. By telling the robots to be a bit more flexible when the road is jammed, both drivers can share the road effectively, even in a massive traffic jam.

The Big Picture

This paper is about making sure our future self-driving cars and current smart cars can talk to each other without shouting over one another.

• The Analogy: Imagine a party where one group of people (Driver A) waits for silence to speak, and another group (Driver B) just starts talking on a timer. The "Bell" is like the timer group ringing a gong before they speak, so the waiting group knows to stay quiet.
• The Takeaway: By adding this simple "gong" (preamble) to the new technology, we can prevent the old technology from being silenced. It's a small change that could save lives by ensuring safety messages get through, whether the road is empty or packed.

Technical Summary

Here is a detailed technical summary of the paper "Performance Analysis of IEEE 802.11p Preamble Insertion in C-V2X Sidelink Signals for Co-Channel Coexistence."

1. Problem Statement

The paper addresses the critical challenge of spectrum scarcity and co-channel interference in Vehicle-to-Everything (V2X) communications. Specifically, it focuses on the coexistence of two competing access technologies operating in the same 5.9 GHz Intelligent Transport Systems (ITS) band:

• IEEE 802.11p (ITS-G5): Uses CSMA/CA (Listen-Before-Talk), where stations sense the channel before transmitting.
• LTE-V2X (Mode 4): Uses Sensing-Based Semi-Persistent Scheduling (SB-SPS). Stations autonomously select resources based on past sensing but transmit without re-verifying channel availability immediately before transmission.

The Core Issue:

• Asymmetric Interference: LTE-V2X transmissions are "blind" regarding immediate channel state. If an LTE-V2X station transmits while an IEEE 802.11p station is sensing, the 802.11p station may fail to detect the LTE signal in time (due to the LTE signal structure and the 802.11p Clear Channel Assessment (CCA) threshold).
• Hidden Node Effect: The LTE-V2X signal often appears as noise to the 802.11p receiver until it is too late, causing the 802.11p station to transmit, resulting in a collision.
• Congestion: In high-density scenarios, LTE-V2X tends to dominate channel resources, forcing 802.11p to defer excessively or reduce its transmission rate, leading to unfairness and safety risks.

2. Proposed Methodology

The authors propose a mitigation solution involving the insertion of a fixed IEEE 802.11p preamble at the very beginning of the LTE-V2X sidelink transmission.

Technical Mechanism:

• Preamble Structure: The first 40 µs of the LTE-V2X signal (which normally contains an Automatic Gain Control (AGC) symbol and redundancy) is replaced by a standard IEEE 802.11p preamble (Short Training Field, Long Training Field, and Signal Field).
• Signaling: This preamble indicates a channel occupancy duration of 1 ms (the length of one LTE-V2X Transmission Time Interval, TTI).
• Implementation:
  • No changes to 802.11p: Existing 802.11p receivers can decode this standard preamble without modification.
  • Minor changes to LTE-V2X: LTE-V2X transmitters need to store the preamble sequence and insert it. Since the preamble is fixed and pre-defined, no complex standard implementation is required.
• Benefits:
  1. Extended Detection Range: The preamble allows 802.11p receivers to detect the channel as "busy" at a much lower power level (approx. -100 dBm) compared to the standard CCA threshold (-65 dBm) used for undecodable signals.
  2. Gap Elimination: It eliminates the idle gap at the end of the LTE-V2X TTI that previously allowed 802.11p to mistakenly sense the channel as free.

3. Key Contributions

The paper provides a comprehensive evaluation of this proposal through three distinct phases:

1. Analytical Modeling (Free-Flow Scenarios):

   • Developed a mathematical model based on a 1-D Poisson Point Process (PPP) to analyze packet reception probability (PRP).
   • Derived a closed-form expression for PRP considering the "protected area" created by the extended detection range of the preamble.
   • Finding: The model demonstrates a significant reduction in inter-technology collisions in low-to-medium traffic densities.

2. Simulation in Dense Scenarios:

   • Used the open-source simulator WiLabV2Xsim to evaluate performance in highway scenarios with varying vehicle densities (up to 150 vehicles/km per technology).
   • Evaluated metrics including Packet Reception Ratio (PRR) and Data Age (DA).
   • Finding: The preamble insertion significantly improves PRR for 802.11p and maintains LTE-V2X performance, even in dense traffic, provided the traffic load is not extreme.

3. Congestion Management Strategies:

   • Investigated that in highly congested scenarios (e.g., 150+150 v/km), preamble insertion alone is insufficient because LTE-V2X still consumes too many resources.
   • Compared three additional countermeasures combined with preamble insertion:
     • Disabling HARQ (Blind Retransmissions): Reduces LTE traffic but hurts LTE reliability.
     • Half Pool: Restricting LTE to 50% of subframes. This improves 802.11p slightly but degrades LTE PRR significantly.
     • Modified LTE-V2X Congestion Control (CC): Halving the thresholds for channel occupation limits.
   • Finding: The Modified Congestion Control approach is the most effective. It balances the load, allowing both technologies to coexist with high reliability in congested conditions.

4. Key Results

• Detection Range: The preamble insertion extends the "protected range" (where 802.11p senses the channel as busy) from approximately 55 meters (legacy) to 390 meters.
• Packet Reception Ratio (PRR):
  • In free-flow, preamble insertion drastically reduces collision probability for 802.11p.
  • In dense scenarios (100 v/km), preamble insertion restores 802.11p PRR to near-single-technology levels.
• Data Age (DA): The solution reduces the latency (Data Age) for 802.11p messages in coexistence scenarios.
• Congestion: In high-density scenarios (150 v/km), combining preamble insertion with Modified LTE Congestion Control yields the best balance, preventing 802.11p from being starved of resources while maintaining acceptable LTE performance.

5. Significance and Implications

• Standardization Relevance: The proposal is compatible with current deployments in Europe (where 802.11p is standard) and requires no changes to the 802.11p protocol stack. It is also forward-compatible with IEEE 802.11bd and 5G-NR V2X.
• Practical Deployment: It offers a "low-hanging fruit" solution for immediate coexistence issues without requiring centralized coordination or complex spectrum sharing protocols.
• Safety Impact: By mitigating the "unfairness" where LTE-V2X dominates the spectrum, this solution ensures that safety-critical 802.11p messages (like emergency braking alerts) are not lost due to interference, directly contributing to road safety.
• Future Work: The paper highlights that while preamble insertion solves the detection problem, resource management (Congestion Control) remains essential for high-density traffic, suggesting a hybrid approach for future V2X standards.