Fundamentals of direction finding and radiolocation

Technology fundamentals

Fundamentals of hopping signals

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What are frequency-agile short-time signals?

Frequency hopping is a process used by many transceivers to make interception more difficult and reduce the impact of frequency-specific interference. Typically, the carrier frequency changes after each burst/frame. This means the content of the baseband signal is spread across a bandwidth which is defined by the lowest and highest frequencies used.

Frequency hopping also known as frequency hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching the carrier between different frequency channels. Switching occurs based on a pseudo-random sequence known to the transmitter and the receiver. FHSS is a wireless technology that spreads the signal over rapidly changing frequencies. Each frequency band is divided into sub-frequencies and the signal changes ("hops") rapidly between these in a pre-determined order. Hopping between different frequencies should reduce the interference of the signal. Interference will only affect the signal during short intervals.

Spread spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics. Military radios use cryptographic techniques to generate the channel sequence by implementing a secret transmission security key that the sender and receiver share in advance. By itself, frequency hopping provides only limited protection against eavesdropping and jamming. Most modern military frequency hopping radios also employ separate encryption devices.

Frequency hopping spread spectrum systems are very often used in the unregulated 2.4 GHz band in which many consumer devices have employed various spread spectrum modes. Some walkie-talkies that employ frequency hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Some of these radios are marketed under the name eXtreme Radio Service (eXRS). Despite the name's similarity to the FRS allocation, the system is a proprietary design. Different manufacturers have deployed a business-band, license-free digital radio that uses FHSS technology.

Signal analysis methods

Frequency hopping webcast

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What is frequency hopping?

Frequency hopping also known as frequency hopping spread spectrum (FHSS) is a state-of-the-art method for transmitting radio signals where carriers rapidly switch among many different frequency channels. The switching involves a pseudo random sequence known to the transmitter and the receiver.

Join the Rohde & Schwarz webinar and learn about:

The significant technical parameters of frequency-agile signals and why analyzing frequency-agile signals is difficult
How an online signal analysis solution can analyze signal scenarios for frequency-agile hopper communications
How to automatically recognize known frequency-agile signal types
How to dehop (recombine) frequency-agile signals for further signal processing and analysis with demodulation and channel code analysis

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All you need to know about hopper signals

Detection of hopper signals

Interception of short-time or FHSS signals is not as easy as intercepting a conventional signal. Due to the spreading of the energy to a wider spectrum, the transmission power can hide behind the noise level.

The signal is modulated with a seemingly random series of radio frequencies, which hops from frequency to frequency at fixed intervals. Due to this hopping, the transmitter and receiver must be synchronized to recombine the spread signal.

To enable detection, the following information is necessary:

Hop duration
Hop length
Used bandwidth
Center frequency
Number of used channels
Frequencies of used channels

Short-time or low-power signals

FHSS systems with one or more hop dwells per data bit are hard to intercept.

Due to the nature of fast frequency hop spread spectrum systems, the hop dwells have a very short duration and are therefore hard to detect by automatic emission detection.

Another reason why FHSS signals are hard to detect is the power of the emissions, which is spread over a wide spectrum.

Hop density waterfall with three hopper signals

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Hop density waterfall with three hopper signals

Multiple hoppers

When two (or more) transmitters hop across the same frequencies, there is chance that bursts from different transmitters will overlap.

Bursts with a frequency overlap will appear to be a single burst with a wide bandwidth.

Bursts with a time overlap will appear to be a single burst with a long duration.

Depending on how the detector is setup, this could result in:

False detections
Missed detections

Such effects will distort the analysis of the hopper scenario.

The varying density of the bursts is the first indicator that more than one hopper is present. A closer look reveals that some bursts are wider (i.e. they appear to have a larger bandwidth) than others. This is a second indicator. Zooming in reveals that some bursts have a longer duration. This is a third indicator.

Further (more reliable) indicators can only be obtained by analysis of the scenario.

Two hopper signals

Hop length and cycle time

Short-time or low-power signals

Technical parameters

When frequency agile signals have to be analyzed, it is expected that subsequent bursts are transmitted on different frequencies. The different used frequencies are within a certain range and the hopping pattern is (pseudo-)random. The percentage of usage of the different frequencies should be equal.

Under good radio conditions, we expect the RX power to be relatively constant. Effects such as fading, interference and multipath propagation mean that the RX power level is not a reliable basis for making analytical conclusions.

The following burst characteristics should be equal:

Duration
Bandwidth
Shift
Symbol rate
Modulation

To analyze a short-time signal, it is important to measure a wideband spectrum to ensure that all hops can be received and analyzed.

During analysis, the following measurements are performed on the measured signals:

Hop (frequency) range Frequency range between hop with lowest frequency and highest frequency
Channel spacing Frequency spacing between two neighboring hops
Number of channels Number of different frequencies used for hopping
Hop length Duration of a single hop
Hop bandwidth Bandwidth occupied by a single hop
Cycle time Time between the start times of consecutive hops
Guard period Time between the stop and start of consecutive hops (cycle time - hop length)
Hop rate Number of hops per second