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frequency-hopping spread spectrum (FHSS)

By

Katie Terrell Hanna

John Burke,
Nemertes Research

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Frequency-hopping spread spectrum (FHSS) transmission is the repeated switching of the carrier frequency during radio transmission to reduce interference and avoid interception.
FHSS is useful to counter eavesdropping, as well as to obstruct the frequency jamming of telecommunications and to enable code-division multiple access communications. It can also minimize the effects of unintentional interference.
The idea behind FHSS was discovered and rediscovered several times in the 20th century. German physicist and electrical engineer Jonathan Zenneck initially mentioned the concept in print in 1908.
Credit for FHSS belongs to actress Hedy Lamarr , however, who worked with composer George Antheil to bring the technology into existence during World War II.
Another more practical application of frequency hopping was created by Ray Zinn, co-founder of Micrel Inc. It enabled radio communication systems to function without synchronizing the receiver with a transmitter in low data rate wireless applications.
Examples include equipment monitoring and utility metering in environments such as the internet of things .
In FHSS, the transmitter hops between available narrowband frequencies within a specified broad channel in a pseudo-random sequence known to both sender and receiver.
A short burst of data is transmitted on the current narrowband channel, and then the transmitter and receiver tune to the next frequency in the sequence for the next burst of data. In most systems, the transmitter will hop to a new frequency more than twice per second.
Because no channel is used for long and the odds of any other transmitter being on the same channel at the same time are low, FHSS is often used as a method to allow multiple transmitter and receiver pairs to operate in the same space on the same broad channel at the same time.
Because a frequency band can be divided into subbands, user devices can rapidly change their carrier frequencies, or frequency hop, with minimal interference.
This wideband or broadband strategy provides a few benefits over fixed analog transmissions:
For those reasons, military radios commonly use spread-spectrum signals for spreading coded language because they need to be stealthy and resilient against jamming techniques. To disrupt an FHSS signal, an opponent's equipment would need to have insight into the frequency-hopping pattern.
Governments regulate the broadcast spectrum and often dictate some aspects of spread-spectrum communications.
For example, in North America, the industrial, scientific and medical waveband is divided into 75 hopping channels, and the devices using them may not transmit with more than 1 watt of power on any one channel.
These restrictions ensure a single device does not consume too much bandwidth or linger too long on a single frequency.
In the 2000s, the U.S. Federal Communications Commission ( FCC ) allowed FHSS systems to operate in the unregulated 2.4 gigahertz band in order to support the use of FHSS in 802.11b, 802.11g and 802.11n Wi-Fi deployments.
FCC Code of Federal Regulations 47 part 15.247 provides guidance for frequency hopping for several megahertz frequency bands as well.
Additional spread spectrum techniques and variations of FHSS are commonly used today. A few examples follow.
DSSS is a related technique to FHSS. It also spreads a transmitted signal across a wide channel, but it does so all at once instead of in discrete bursts separated by hops.
It can achieve higher throughput, but DSSS systems are more susceptible to interference and less effective as a spectrum-sharing method.
AFH is used in Bluetooth wireless technology to improve resistance to radio frequency interference by circumventing high-traffic frequencies in the hopping sequence. This type of adaptive transmission is simpler to implement with FHSS than with DSSS.
Chirp modulation is another form of frequency hopping that simply scans all available frequencies in consecutive order.
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Brooklyn, NY
http://witestlab.poly.edu/~ffund/

This is an experimental demonstration of frequency hopping spread spectrum , a wireless technology that spreads a signal over rapidly changing frequencies. You will learn how a frequency hopping transmitter works and observe a FHSS signal transmitted over the air using software defined radio devices.
It should take about 60-120 minutes to run this experiment, but you will need to have reserved that time in advance. This experiment uses wireless resources (specifically, any one of the sb3 or sb7 sandbox at ORBIT ), and you can only use wireless resources on GENI during a reservation.
To reproduce this experiment on GENI, you will need an account on the GENI Portal , and you will need to have joined a project . You should have already uploaded your SSH keys to the portal . The project lead of the project you belong to must have enabled wireless for the project . Finally, you must have reserved time on a sandbox at ORBIT and you must run this experiment during your reserved time. (You may use sb3 or sb7 on ORBIT.)
In a frequency-hopping spread spectrum (FHSS) system, the transmitted signal is spread across multiple channels, as shown in Figure 1 below. In the example of Figure 1, the full bandwidth is divided into 8 channels, centered at f 1 through f 8 . The signal "hops" between them in the following sequence: f 5 , f 8 , f 3 , f 6 , f 1 , f 7 , f 4 , f 2 .

Figure 1: Frequency hopping example, from William Stallings "Data and Computer Communications".
Figure 2 shows the block diagram of a typical FHSS transmitter. First, digital data is modulated using some digital-to-analog scheme. This baseband signal is then modulated onto a carrier c(t) .

Figure 1: Block diagram of FHSS transmitter, from William Stallings "Data and Computer Communications".
The frequency of the carrier c(t) , i.e. the sequence of channels, depends on the spreading code , which is generated by a pseudonoise (PN) source. Every T C seconds, the PN source produces a new k-bit value. This value is then used to look up a channel in the channel table, and that determines the frequency of c(t) for that time interval.
For example, consider a system where k=4, and for the first time interval, the PN source generates the value 1101 2 . For that time interval, the signal will be transmitted on channel 13. (The channel table will have 2 k -1=2 4-1 =15 entries, indexed from 1 to 15. The PN sequence will repeat itself with a period of 2 k -1.)
One popular way to generate a PN sequence is with a linear feedback shift register (LFSR). Consider the LFSR described by the sequence 10011, which can also be described by the following block diagram:

Figure 3: The LFSR corresponding to the generator 10011.
In the FHSS implementation used in this experiment, the spreading code is determined from the state of the registers as shown in Figure 4. After each time instance, the values in each register "shift" as follows (for the LFSR generated by "10011"):
(where the addition in the last item is an XOR operation). Then, the "value" used for that time instance is the decimal value corresponding to the binary digits
e.g. if the values are r 3 =1, r 2 =1, r 1 =0, r 0 =1, then the channel at f 13 will be used (1101 2 =13).

Figure 4: The first four values for the LFSR corresponding to the generator 10011, with initial register values 1111, would be: 15, 7, 14, 5.
For the example above, the complete frequency hopping sequence would be:
f 15 , f 7 , f 14 , f 5 , f 10 , f 13 , f 3 , f 6 , f 12 , f 1 , f 2 , f 4 , f 8 , f 9 , f 11
The following video shows the FHSS transmission due to a PN generator with generator (1,0,0,1,1) and initial values (1,1,1,1):
Here's a screenshot of that hopping pattern, annotated with channel numbers:
First, you will have to reserve time on an SDR testbed: either sb3 or sb7 on ORBIT. Log on with your GENI account at http://geni.orbit-lab.org , then click "Control Panel" to access the schedule page.
At your reserved time, open a terminal and log in to the console of the testbed that you have reserved. For example, if you have reserved sandbox 3 on ORBIT,
where GENI-WIRELESS-USERNAME is your wireless username assigned by GENI. This is usually your regular GENI username with a geni- prefix, e.g. geni-ffund . Also specify the path to the key you have uploaded to the GENI Portal as the /PATH/TO/KEY .
If you are using sandbox 7, log in to sb7.orbit-lab.org.
Then, you must load a disk image onto the testbed nodes. From the testbed console, run:
This disk image has the GNU Radio software suite, and the ShinySDR spectrum analyzer , both of which we'll use for this experiment, pre-installed.
This process can take 5-10 minutes. Don't interrupt it in middle - you'll just have to start again, and it will only take longer.
If it's been successful, then once the process finishes running completely you should see output similar to:
Sometimes, transient errors can cause the process to fail - if you haven't successfully imaged 2 nodes, wait a few minutes and try again.
When you've finished loading the disk image, turn on your nodes with
Wait a few minutes for your testbed nodes to turn on, then continue with the experiment.
Open a new terminal window, and run
again, using the correct GENI-WIRELESS-USERNAME , /PATH/TO/KEY , and substituting for sb3.orbit-lab.org the hostname of the console of the testbed that you have reserved.
Then, in that terminal window (which should now be logged in to your testbed console), run
This last command should start the Shiny server, which, when it is running successfully, will say something like:
In a Google Chrome browser window, open the URL that is shown in the Shiny server output. ( http://localhost:8100/ShinySDR/ ).
Configure your Shiny window as follows:
Because FHSS is highly sensitive to frequency, you will also have to calibrate your receiver against the transmitter to adjust for any frequency offset that might exist between them. (See Why is my signal located at an offset from the expected center frequency? for more explanation of the frequency offset phenomenon.)
Open a new terminal, and log in to the testbed console again. From the testbed console, log in to the transmitter node:
On the transmitter node, generate a reference signal at 2480 MHz using the following command:
While this is running, in the ShinySDR interface, adjust the "Freq.corr. (PPM)" option until you can see the 2480 MHz signal in the center of the display, at the tick mark labeled 2480 MHz:
(If the signal already appears at 2480 MHz, there is no frequency offset and you don't have to adjust the frequency correction.)
When you are satisfied, use Ctrl+C to stop the transmitter on node1-1.
On node1-1, download the FHSS transmitter from GitHub with
To run the FHSS transmitter for the generator "10011", on node1-1 run
(All registers will be initialized with value of 1.)
You should observe the hopping pattern f 15 , f 7 , f 14 , f 5 , f 10 , f 13 , f 3 , f 6 , f 12 , f 1 , f 2 , f 4 , f 8 , f 9 , f 11 :
(Note that for some channels, especially those far from the center frequency, a lower-power "mirror image" of the signal may appear at the same time at its "opposite" channel (relative to the center frequency). This is not a cause for concern; the "mirror" signal can be ignored.)
The offset of each channel relative to the center frequency will be printed in the terminal output, e.g.:
Run the frequency hopping spread spectrum transmitter with the PN code generator "11001". Take a screenshot of the ShinySDR receiver showing the complete hopping pattern (show at least 20 hops). Annotate your screenshot - add the channel numbers at the bottom, similar to this image (although the specific hopping pattern will be different since the generator is different!).
List the hopping sequence for the first 20 hops observed in your screenshot.
Also, create a figure similar to this one , showing how the FHSS transmitter determines the first four channels in the sequence for the generator "11001".
Did you reproduce this experiment? Have useful information to share with other intrepid researchers?
Post it here! Comments are posted following moderation.
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These packets are then transmitted sequentially in a pseudo-random manner over the various frequency channels within the spread-spectrum band being used.
In other words, the frequency of the carrier signal keeps hopping around. Synchronization between the master transmitter and slave devices is achieved by modulating the center or carrier frequency of the communication band according to a preset algorithm.
Both the mobile and the base station know the modulation algorithm, which enables them to keep in communication with each other.
Adaptive frequency-hopping spread spectrum (AFH) as used in Bluetooth improves resistance to radio frequency interference by avoiding crowded frequencies in the hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with DSSS.
The key idea behind AFH is to use only the “good” frequencies, by avoiding the “bad” frequency channels – perhaps those “bad” frequency channels are experiencing frequency selective fading, or perhaps some third party is trying to communicate on those bands, or perhaps those bands are being actively jammed.
AFH should be complemented by a mechanism for detecting good and bad frequency channels.
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