⚡ electrical engineering

Toward Safe and Energy-Efficient 5G NR V2X Communications in Rural Environments

This paper proposes an adaptive strategy for configuring subcarrier spacing, modulation and coding schemes, and transmit power in rural 5G NR V2X systems to optimize the trade-off between communication safety (measured by packet receive ratio) and energy efficiency under varying traffic densities.

Original authors: Zhanle Zhao, Son Dinh-Van, Yuen Kwan Mo, Siddartha Khastgir, Matthew D. Higgins

Published 2026-03-03

📖 5 min read🧠 Deep dive

CC0 1.0

Original authors: Zhanle Zhao, Son Dinh-Van, Yuen Kwan Mo, Siddartha Khastgir, Matthew D. Higgins

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a rural highway as a long, quiet stretch of country road. In the future, the cars driving on this road will be "smart" (autonomous), and they will talk to each other to avoid accidents. This paper is about making sure these cars can talk to each other safely without wasting too much battery power, especially when the road is far away from cities and power sources are scarce.

Here is the story of the paper, broken down into simple concepts and analogies.

1. The Problem: The "Silent" Country Road

In big cities, there are lots of cell towers and power lines. But in the countryside, these are rare. If a smart car needs to send a "BRAKE NOW!" message to the car behind it, it has to do it using its own limited battery and a weak connection.

If the connection is bad, the message might get lost. If the message is lost, the car behind doesn't brake in time, and crash.

The Goal: We need the message to get through 100% of the time (Safety), but we also don't want to drain the car's battery or the roadside unit's power (Energy Efficiency).

2. The Three "Knobs" on the Radio

The researchers looked at three settings (knobs) on the car's radio that control how it talks. They wanted to find the perfect combination of these knobs for different traffic situations.

Knob A: The Subcarrier Spacing (SCS) – The "Pacing" of the Message
- Analogy: Imagine sending a message by tapping on a wall. You can tap slowly (15 taps per second) or very quickly (30 taps per second).
- The Finding: In rural areas, cars move fast and the signal bounces off hills and trees. Tapping faster (30 kHz) is better. It's like speaking quickly and clearly so the wind doesn't mess up your words. It helps the message arrive faster and clearer.
Knob B: The Modulation and Coding Scheme (MCS) – The "Complexity" of the Message
- Analogy: Imagine writing a letter.
  - High MCS: You write in a fancy, complex code with tiny handwriting. It fits a lot of info, but if the paper gets wet (bad signal), no one can read it.
  - Low MCS: You write in big, bold, simple letters. It takes up more space, but even if the paper is a bit crumpled, it's easy to read.
- The Finding: The researchers found that simple is better. Using the "big, bold letters" (Low MCS) is much more reliable. Even though it's not as "fancy," it ensures the "BRAKE" message gets through without needing to be sent again and again.
Knob C: The Transmit Power (Pt) – The "Volume" of the Voice
- Analogy: How loudly you shout.
  - Low Volume: Saves energy, but if the road is busy or far away, no one hears you.
  - High Volume: Everyone hears you, but it burns a lot of energy.
- The Finding: This depends on how busy the road is.
  - Empty Road (Light Traffic): You can whisper (Low Power). The message travels far because there's no noise.
  - Crowded Road (Heavy Traffic): You need to shout (High Power) to be heard over the noise of other cars.

3. The "Critical Distance" (The Safety Zone)

The paper introduces a concept called $D_{comm}$ .

The Metaphor: Imagine you are driving a car at 60 mph. It takes a split second for your brain to realize you need to brake, and another split second for the brakes to engage. In that tiny window, your car travels a certain distance.
The Rule: The "BRAKE" message must arrive before your car travels that distance. If the message is delayed, you travel too far, and you might hit the car in front.
The Goal: The researchers wanted to make sure the message arrives within this "Safety Zone" no matter what.

4. The Big Discovery: It's All About Balance

The researchers ran thousands of simulations (like a video game) to see what happens when they mix these knobs. Here is what they learned:

Scenario 1: The Quiet Country Lane (Light Traffic)
- The Situation: Only a few cars, no noise, clear air.
- The Strategy: Turn the volume down (Low Power) and keep the message simple (Low MCS).
- Why: You don't need to shout to be heard. This saves a massive amount of battery while still keeping everyone safe.
Scenario 2: The Rush Hour Jam (Heavy Traffic)
- The Situation: Lots of cars, lots of interference, signals getting mixed up.
- The Strategy: Turn the volume up (High Power) and keep the message simple (Low MCS).
- Why: You must shout to cut through the noise. If you don't, the message gets lost, and the "Safety Zone" ( $D_{comm}$ ) gets too long, making crashes more likely.
- The Catch: Shouting uses a lot of energy. The study found that in heavy traffic, you might need double the energy to keep everyone safe compared to a quiet road.

5. The Conclusion: Be Smart, Not Just Loud

The paper concludes that we cannot use the same settings for every situation.

If we always shout (High Power), we waste battery and run out of power in rural areas where charging is hard.
If we always whisper (Low Power), we might miss a crash in a busy jam.

The Solution: We need an Adaptive System.
Think of it like a smart thermostat.

When the road is empty, the system automatically turns down the volume to save energy.
When the road gets crowded, it automatically turns up the volume to ensure safety.

In a nutshell: To make self-driving cars safe in the countryside without killing their batteries, we need to be smart about how and how loudly they talk. Sometimes a whisper is enough; sometimes you need a shout, but you should only shout when you absolutely have to.

1. Problem Statement

The deployment of Connected and Autonomous Vehicles (CAVs) in rural areas faces unique challenges compared to urban settings. While urban CAVs benefit from dense infrastructure, rural environments suffer from sparse Road Side Units (RSUs) and limited power resources.

Safety vs. Energy Trade-off: In rural scenarios, safety-critical applications like "connected braking" (where vehicles receive alerts to brake early) require reliable, low-latency communication. However, the infrastructure is often power-constrained.
The Gap: Existing research often focuses on urban environments or evaluates communication parameters (Subcarrier Spacing, Modulation and Coding Scheme, Transmit Power) in isolation. There is a lack of holistic analysis on how to jointly configure these factors to balance safety reliability (ensuring packets arrive in time to prevent collisions) and energy efficiency (minimizing RF consumption) specifically in rural traffic conditions (light vs. heavy).

2. Methodology

The authors propose a comprehensive framework combining analytical modeling and extensive simulations using the WiLabV2XSim simulator.

A. System Model

Scenario: A rural highway (2 lanes, 2000m length) with an RSU, an "Ego" vehicle, and a "Target" vehicle.
Mechanism: The RSU detects a hazard on the Target vehicle and sends a braking signal to the Ego vehicle via 5G NR Sidelink V2X.
Traffic Conditions: Simulated under Light Traffic (30–50 vehicles/km) and Heavy Traffic (80–100 vehicles/km) with vehicle speeds ranging from 50 to 110 km/h.
Key Metric ( $D_{comm}$ ): The paper introduces $D_{comm}$ as a critical safety metric. It represents the distance the Ego vehicle travels from the moment a hazard occurs until it successfully receives a valid braking packet. A smaller $D_{comm}$ indicates higher safety.
Control Factors Investigated:
- Subcarrier Spacing (SCS): 15 kHz vs. 30 kHz.
- Modulation and Coding Scheme (MCS): Ranging from 8 (QPSK) to 18 (16-QAM).
- Transmit Power ( $P_t$ ): 23 dBm to 26 dBm.

B. Analytical Formulations

The authors derived mathematical models to quantify performance:

Packet Reception Ratio (PRR): The probability of successfully receiving a packet.
Critical Distance ( $D_{comm}$ ): Calculated based on vehicle speed, packet generation rate, and PRR. It accounts for delays caused by retransmissions due to packet errors.
Energy Consumption:
- RF Energy: Calculated based on transmit power, data rate, and the expected number of retransmissions (using a truncated HARQ model).
- Total Communication Energy: Includes RF transmission energy plus circuit power (idle and active).

3. Key Contributions

Joint Optimization Framework: The paper provides a holistic evaluation of three key 5G NR control factors ( $P_t$ , SCS, MCS) specifically tailored for rural V2X scenarios, addressing the gap in existing literature which often ignores rural constraints or treats factors separately.
Safety-Energy Metric ( $D_{comm}$ ): The development of an analytical formulation for $D_{comm}$ that maps communication performance (latency/reliability) directly into a spatial safety margin interpretable by vehicle dynamics.
Adaptive Strategy Guidelines: The study identifies specific configuration strategies that optimize the trade-off between safety and energy consumption based on traffic density (light vs. heavy).

4. Key Results

The simulation results yielded the following insights:

Subcarrier Spacing (SCS):
- 30 kHz is superior to 15 kHz for rural high-mobility scenarios.
- 30 kHz offers better resilience to Doppler shifts and phase noise, resulting in higher PRR, especially at the critical distance $D_{comm}$ .
Modulation and Coding Scheme (MCS):
- Lower MCS (e.g., MCS 8) is preferred over higher MCS (e.g., 12–18).
- While higher MCS offers higher spectral efficiency, it is highly sensitive to interference and channel impairments in rural environments, leading to frequent retransmissions and higher energy costs without significant PRR gains.
- MCS 8 (QPSK) provides the most robust balance of reliability and energy efficiency.
Transmit Power ( $P_t$ ) & Traffic Density:
- Light Traffic (30–50 veh/km): A lower transmit power (23 dBm) is sufficient to maintain high PRR and acceptable $D_{comm}$ . Increasing power here yields diminishing returns and wastes energy.
- Heavy Traffic (80–100 veh/km): A higher transmit power (26 dBm) is necessary to overcome interference and maintain safety.
- Energy Cost of Safety: In heavy traffic, increasing $P_t$ from 23 dBm to 26 dBm improves safety (reduces $D_{comm}$ and increases PRR) but results in a >100% increase in total RF energy consumption.

5. Significance and Conclusion

This paper demonstrates that a "one-size-fits-all" configuration is inefficient for rural V2X. The findings advocate for an adaptive, energy-aware strategy:

In Light Traffic: Use SCS = 30 kHz, MCS = 8, and low $P_t$ (23 dBm) to maximize energy efficiency while maintaining safety.
In Heavy Traffic: Maintain SCS = 30 kHz and MCS = 8, but increase $P_t$ (up to 26 dBm) to ensure safety, accepting the higher energy cost as a necessary trade-off for collision avoidance.

Impact: The study provides a foundational baseline for designing future adaptive control algorithms in rural CAVs. By explicitly incorporating $D_{comm}$ as a design metric, it bridges the gap between communication engineering and vehicle safety dynamics, ensuring that rural V2X systems remain both safe and sustainable in terms of energy usage.