TDD OFDM Bidirectional Link for UAV Data Transmission

How TDD OFDM Bidirectional Links Solve High-Rate Telemetry and Sensor Data Challenges in Drone Communication

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Introduction

Modern UAVs (Unmanned Aerial Vehicles) increasingly demand simultaneous transmission of multiple data types—telemetry, high-rate sensor or video data, and command/control feedback. Achieving all of these over a reliable and low-latency wireless link is not a trivial task, especially when dealing with real-time applications such as surveillance, mapping, or autonomous navigation.

A customer recently presented the following requirements for a drone communication system:

Customer’s Requirement Summary:

1. Sending telemetry data @40ms (40KB data) over a 100 Mbps Ethernet line. Data is sent as UDP packets.
2. Sending raw data of size 1.7 MB @40ms over a 1 Gbps Ethernet line using UDP protocol.
3. Command/response over RS422, to interact with the drone’s onboard computer to get IMU data (location and attitude) @40ms, and also interact with GCS over RF.

This set of requirements highlights the complexity of modern UAV data transmission needs: high throughput, low latency, and full-duplex interaction between air and ground.

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Challenges in the Original Setup

Let’s analyze the requirements in technical terms:

Data Type	Interval	Data Size	Equivalent Data Rate	Communication Type
Telemetry	40 ms	40 KB	~8 Mbps	UDP (100 Mbps Ethernet)
Raw Data	40 ms	1.7 MB	~340 Mbps	UDP (1 Gbps Ethernet)
Command / IMU	40 ms	<100 KB	Negligible	RS422 serial link

Key challenges:

• Throughput: 340 Mbps of raw data exceeds what traditional COFDM or narrowband systems can handle.
• Directionality: Telemetry and IMU data must flow in both directions between the drone and GCS.
• Synchronization: Data, video, and control links need to be synchronized at 25 Hz (40 ms period) without packet loss or high latency.

This makes it evident that a TDD (Time Division Duplex) OFDM-based bidirectional link is the ideal solution.

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Why TDD OFDM is the Right Solution

1. Bidirectional Transmission in a Single Frequency Channel

Unlike traditional unidirectional COFDM transmitters, a TDD OFDM link uses the same frequency band for both uplink and downlink, alternating transmission time slots. This allows:

• Full bidirectional communication (command, telemetry, and data)
• Reduced RF complexity and antenna requirements
• Perfect synchronization between drone and ground station

2. Efficient Spectrum Use

In narrowband environments or frequency-constrained UAV systems (e.g., 300 MHz–6 GHz), TDD OFDM allows adaptive allocation of uplink/downlink ratios:

• For example, 80% downlink for sensor/video data and 20% uplink for telemetry/commands.
• Dynamic switching based on link condition ensures optimal throughput and minimal delay.

3. UDP Compatibility

Since UDP is lightweight and real-time friendly, the TDD OFDM system can encapsulate UDP packets directly in its Ethernet bridge mode, ensuring transparent data transmission for both telemetry and sensor data.

4. Integration with RS422 / Serial Control

TDD OFDM modules often support transparent serial data (RS232/422/485) channels in parallel with the Ethernet stream. This allows:

• Real-time command/control exchange with the onboard computer
• IMU and GPS data synchronization with ground control software
• Unified data stream combining Ethernet and serial interfaces

5. Low Latency and Stability

With optimized framing, TDD OFDM systems can achieve end-to-end latency under 40–50 ms, which meets the customer’s requirement of 25 Hz telemetry update rate.
Additionally, the robust FEC (Forward Error Correction) and adaptive modulation maintain stable transmission even under dynamic flight conditions.

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System Implementation Example

[Drone Onboard Unit]
 ├── Sensors / Camera (Raw Data 1.7 MB @40ms)
 ├── Flight Controller (Telemetry + IMU via RS422)
 ├── TDD OFDM Transceiver Module (Ethernet + Serial Input)
 └── Power & Antenna System

       ⇅  (Bidirectional RF Link, e.g., 840 MHz / 1.4 GHz / 2.4 GHz)

[Ground Control Station]
 ├── TDD OFDM Receiver (Ethernet Output + RS422)
 ├── GCS Software (UDP-based Telemetry & Control)
 └── Data Storage / Video Processing System

This configuration enables a full-duplex data bridge between the drone and the ground—video, telemetry, and serial control signals all flow seamlessly through the same TDD OFDM radio link.

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Advantages of TDD OFDM Bidirectional Links

Feature	Benefit
Single Frequency for TX/RX	Simplifies antenna setup and spectrum licensing
Adjustable Time Slot Ratio	Customizable for uplink/downlink bandwidth balance
Transparent UDP / Ethernet Support	Directly compatible with existing IP-based systems
Integrated Serial Port	Ideal for command/control and telemetry exchange
Low Latency	<50 ms end-to-end delay
Strong Anti-Interference	OFDM modulation and FEC ensure stable performance

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Conclusion

The customer’s demanding setup—combining telemetry, raw data, and RS422 control—illustrates why modern UAV systems require adaptive, full-duplex data solutions.
A TDD OFDM bidirectional link provides:

• Real-time, two-way Ethernet and serial communication
• Scalable throughput for both compressed and raw data
• Reliable synchronization at 40 ms update intervals

By deploying a TDD OFDM system, UAV designers can achieve the perfect balance of throughput, latency, and reliability, making it the backbone of next-generation drone communication networks.

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FAQ