White Paper on UAV Interference Technology Based on VCO DDS and SDR Technology (1)

Overview

In recent years, the drone industry has developed rapidly, and the application of drones has become increasingly widespread, with the number of drones also showing an increasing trend year by year. However, at the same time, drones also pose serious security threats to various places in society. In recent years, incidents of drones impacting and attacking critical infrastructure have occurred frequently, and there is an urgent need for effective drone countermeasures technology. There are several common drone countermeasures techniques, including:

• Wireless signal interference technology: By transmitting interference radio frequency signals, it interferes with wireless signals such as remote control, image transmission, navigation, etc. of unmanned aerial vehicles to achieve the purpose of driving away, interfering, or forcing landing of drone.
• Wireless signal deception technology: By transmitting deceptive wireless signals to drones, the drones can obtain incorrect information, thereby achievingthe purpose of hijacking drones. There are two main types of wireless signal deception techniques: location signal deception and remote control signal hijacking.
• Directed energy destruction technology: By emitting high-energy laser or electromagnetic signals, it physically destroys unmanned aerial vehicles, mainly through two technical routes: laser destruction and high-energy microwave destruction.
• Physical damage/capture technology: By launching bullets, cruise missiles, or colliding with drones, invading drones can be physically damaged, orshoot capture nets to capture invading drones.

This article mainly discusses wireless signal interference technology.

UAV Communication Protocol

Drones generally use the following four types of radio signals:

Figure 1 Typical wireless signal schematic of unmanned aerial vehicle

1. RC：Remote Control：Transmitting instructions from the operator to the drone through remote control signals, allowing the drone to perform corresponding flight actions;
2. Videotransmission：The video signal captured by the drone camera is transmitted back to the remote control, and the operator operates the drone based on the transmitted image signal to select the appropriate flight path and avoid hitting obstacles;

Navigation: Drones receive positioning signals from navigation satellites for their own positioning

The navigation signals seen include GPS, Beidou, GLONASS, etc., and the working frequency bands are mainly distributed around 1.2GHz and 1.6GHz.

3. Telemetry：Used to distribute telemetry information such as the location of drones, which will be received by the remote control and nearby monitoring stations.

Among them, remote control signals and image feedback signals are the main targets of radio frequency interference countermeasures, and in some cases, they can also interfere with navigation and positioning signals. When interfering with remote control signals, the drone cannot receive instructions from the operator and will perform hovering or returning actions; When interfering with the image feedback signal, the remote control cannot display the image seen by the drone, which may cause the drone to lose control; When interfering with both remote control signals and navigation positioning signals, the drone cannot obtain accurate positioning information and lands directly, relying on ultrasonic sensors to avoid touching the ground and hovering at a certain height above the ground.

The following table lists some common drone remote control and image transmission protocols. Strong manufacturers such as DJI and AUTEL have developed dedicated remote control image transmission protocols, among which DJI's OcuSync and LightBridge are the most common and perform the best. For manufacturers who do not have self-developed remote control image transmission protocols, Wi Fi protocol is generally chosen. For DIY FPVs, the ELRS protocol and TBSCrossFire have become the actual standards.

 OFDM technology introduction

LightBridge、OcuSync、SkyLink protocol, and Wi-Fi, the encoding technology of its physical layer adopts OFDM technology. This section will briefly introduce OFDM technology.

OFDM technology is a multi carrier modulation multiplexing technique that uses multiple subcarriers to transmit data simultaneously, with equal frequency intervals between each subcarrier. Although there is some spectral overlap between adjacent subcarriers, they are orthogonal to each other, so the signals transmitted by each subcarrier do not affect each other. This allows data information to be transmitted simultaneously on many subcarriers.

OFDM technology is usually based on digital signal processing technology, and the specific implementation process is as follows: the data source to be modulated is allocated to N subcarriers, each subcarrier is IQ modulated, and then the IQ modulated data of N subcarriers is subjected to IFFT inverse Fourier transform to obtain the time-domain IQ data of an OFDM symbol.

Figure 2 Overview of OFDM Modulation Technology Principle

A complete OFDM frame usually contains several OFDM symbols, and the duration of the OFDM symbols is the reciprocal of the subcarrier spacing. For example, when the subcarrier spacing is 15KHz, the length of the OFDM symbol is 66.67us. At the beginning of each OFDM symbol, a shorter cyclic prefix (CP) is extended and inserted. The content of the CP is a copy of the content at the end of the OFDM symbol. The purpose of extending the CP is to resist inter symbol interference caused by dispersion.

Figure 3 OFDM symbols and subcarriers

OFDM spectrum utilization efficiency of multiplexing technology is very high. In the frequency domain, OFDM signals consist of many subcarriers, and the energy allocation of each subcarrier is relatively even, so the spectrum of OFDM signals is close to a flat straight line. In the time domain, OFDM signals consist of several symbols, each with a fixed length.

Figure 4 OFDM symbols and subcarriers

 Dji LIGHTBRIDGE/OCUSYNC Protocol

Dji LightBridge and OcuSync protocols are the technical benchmarks for civilian image transmission remote control protocols, with LightBridge protocol developed earlier and applied to models such as Phantom 3 and Inspire; The OcuSync protocol was developed relatively late and is applied to models such as Phantom 4, Mavic series, Air series, etc. The OcuSync protocol has been iteratively updated, and its latest version is OcuSync 4.0. The OcuSync 4.0 protocol has strong transmission performance and anti-interference ability.

Figure 5 Time frequency diagram of DJI OcuSync protocol

LightBridge &OcuSync physical layer of the protocol is based on OFDM encoding technology, but the various parameters of OFDM encoding are different. The LightBridge protocol uses a physical layer similar to WiMAX, with a subcarrier spacing of 10.9375KHz. The downlink uses 864 subcarriers, occupying a bandwidth of approximately 9.46MHz; The OcuSync protocol uses a physical layer similar to LTE, with a subcarrier spacing of 15KHz. The 10M bandwidth downlink uses 600 subcarriers, occupying a bandwidth of approximately 9.02MHz, while the 20M bandwidth downlink uses 1200 subcarriers, occupying a bandwidth of approximately 18.02MHz.

 SKYLINK Protocol

Skylink protocol is also a common image transmission remote control protocol. The Skylink protocol is widely used in the Dao Tong EVO series of drones.

The physical layer of the Skylink protocol is also based on OFDM technology, occupying a bandwidth of approximately 10MHz and a subcarrier spacing of 15KHz.

Figure 6 Time frequency diagram of SkyLink protocol

 The Skylink protocol adopts a physical layer similar to LTE, with a subcarrier spacing of 15KHz.

 The downlink (image transmission signal) uses 600 subcarriers, occupying a bandwidth of about 9.02MHz, and the uplink (remote control signal) uses 72 subcarriers, occupying a bandwidth of about 1.1MHz.

 WI-FI protocol

Wi-Fi communication technology is very popular in consumer electronics, and many civilian unmanned aerial vehicles use Wi-Fi protocol to transmit remote control signals and image feedback signals. The Wi-Fi communication protocol has undergone many years of technological iteration. In addition to the early Wi-Fi 1 using DSSS spread spectrum, subsequent Wi Fi uses OFDM technology with different technical parameters such as bandwidth.

Wi-Fi standard	Wi-Fi version	Standard release	Work frequency	Physical layer reuse technology	Number of spatial flows	Wide-band channel	Data rate
802.11	Wi-Fi1	1997	2.4GHz	DSSS	1	20MHz	2 Mbps
802.11b	Wi-Fi1	1999	2.4GHz	DSSS	1	20MHz	11Mbps
802.11a	Wi-Fi2	1999	5GHz	OFDM	1	20MHz	54Mbps
802.11g	Wi-Fi3	2003	2.4GHz	OFDM	1	20MHz	54Mbps
802.11n	Wi-Fi4	2009	2.4GHz,5 GHz	MIMO-OFDM	Upto4	20/40MHz	Upto 600 Mbps
802.11ac	Wi-Fi5	2013	5GHz	MIMO-OFDM	Upto8	20/40/80/160MHz	Upto 3.47Gbps
802.11ax	Wi-Fi6	2019	2.4GHz,5 GHz	OFDMA,MU-MIMO	Upto8	20/40/80/160MHz	Upto 9.6Gbps
802.11be	Wi-Fi7	2024	2.4GHz,5GHz,6 GHz	OFDMA,MU-MIMO	8	20/40/80/160/320MHz	Upto 23Gbps

The Wi-Fi used in the field of drones is usually 802.11n or 802.11ac, as the Wi-Fi chips for these two standards are very mature. Taking 802.11n as an example, there are usually two bandwidth modes to

standards are very mature. Taking 802.11n as an example, there are usually two bandwidth modes to choose from, 20M and 40M, with subcarrier spacing of 312.5KHz. In 20M mode, there are 56 subcarriers, and the actual occupied bandwidth is about 17.8MHz. In 40M mode, there are 114 subcarriers, and the actual occupied bandwidth is about 35.9MHz.

Figure 7 Time frequency diagram of Wi-Fi protocol

 FPV Protocol ELRS/TBS

The remote control protocol and image transmission protocol of FPV are separate. The remote control protocol usually uses ELRS or TBS Crossfire, while the image transmission protocol is usually simulated to achieve lower latency.

ELRS, also known as ExpressLRS, is an open-source remote control protocol that provides ultra-low latency and longer remote control distances. The physical layer of ELRS adopts the LoRA protocol and is implemented based on SEMTECH's SX127x/SX1280 chips. ELRS adopts frequency hopping and spread spectrum technology, which can achieve strong anti-interference ability. The spread spectrum of ELRS is based on chirp (linear frequency modulation) spread spectrum technology. The larger the spreading factor, the higher the spreading gain, sensitivity, and transmission rate. The spreading bandwidth of ELRS is 500KHz, and the spreading factor is generally selected from SF6 to SF9. The physical layer encoding of TBS Crossfire is similar to ELRS, both using chirp (linear frequency modulation) spread spectrum technology, but the spread spectrum bandwidth is only 250KHz.

spreading factor	Spread spectrum code length	Spreading gain（dB）
SF6	64	5
SF7	128	7.5
SF8	256	10
SF9	512	12.5
SF10	1024	15
SF11	2048	17.5
SF12	4096	20

Figure 8 Time frequency diagram of ELRS protocol