Article Category: Question of the Week

(E8D03) Spread Spectrum

E8D03: How does the spread spectrum technique of frequency hopping work?

A. If interference is detected by the receiver it will signal the transmitter to change frequencies
B. If interference is detected by the receiver it will signal the transmitter to wait until the frequency is clear
C. A pseudo-random binary bit stream is used to shift the phase of an RF carrier very rapidly in a particular sequence
D. The frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station

You may have heard the term spread spectrum (try saying it three times really fast), or frequency hopping, but perhaps you do not have a solid grasp of what’s going on with these emission types. Let’s take a closer look while addressing some of the various question item concepts in the Extra Class pool about this mode.

The general concept of spread spectrum is to spread the RF signal across a broad range of the frequency domain. As an example comparison, a typical SSB signal is “spread” across a relatively narrow 3 kHz of bandwidth, while an amateur mesh network using spread spectrum in the 2.4 GHz band may spread its signal across more than 80 MHz of bandwidth. (A 27,000 to 1 ratio!) Generally with spread spectrum, the carrier frequency of the transmitted signal will continuously change over time, dragging the signal bandwidth with it all across the breadth of the amateur band from moment to moment.

Why bother? What’s the advantage of spread spectrum?

Because of the unique methods used to spread the signal across broad bandwidth, such as frequency hopping that will be described below, spread spectrum is resistant to interference, both intentional or unintentional. Signals not using the spread spectrum algorithm for carrier frequency changes across the broad bandwidth are suppressed in the receiver. A spread spectrum signal can effectively overlay in a band with other signals, expanding the utility of the band. Because many spread spectrum techniques employ code division multiplexing, some privacy of communications can be had through pseudo-random sequencing, but the transmissions must not be used to obscure the meaning of communications. Further, due to the wide bandwidth used by spread spectrum signals, transmissions are permitted only on amateur frequencies above 222 MHz, and the transmitting station must be in an FCC regulated area or other country that permits spread spectrum emissions.

spread spectrum frequency hopping

Frequency hopping spread spectrum technique moves the transmitted carrier frequency from one frequency channel to another in rapid sequence, usually dwelling on any one frequency on the order of 1/10 second. A pseudo-random sequence of frequency hops is generated with a code shared by both transmitting and receiving stations that keeps them synchronized through the frequency hops.

Frequency Hopping: There are multiple methods of spreading signals, but our question relates to just the method of frequency hopping. The carrier frequency of a frequency hopping spread spectrum system changes rapidly (hops) from one frequency value to another over time. The specific frequency change sequences are determined by a pseudo-random code used by both the transmitting and receiving stations in coordination with one another. That is, each station uses the same code sequence to shift their tuning among the carrier frequency hops in perfect union. You may think of this coded sequence as identifying numerous individual channels across the breadth of an amateur band, and the carrier frequency randomly bouncing from channel to channel over time in quick hops. The dwell time on any single channel is usually on the order of 100 ms, or about one-tenth second. While the instantaneous power of the spread spectrum signal may be relatively high, the average power over time on any singular frequency in the amateur band will be very low.

spread spectrum and narrow band signals

A narrow band signal, such as SSB or a digital packet signal will mask only a small fraction of the spread spectrum signal, and the narrow band signal is suppressed in reception. The average power of the spread spectrum signal across the band over time is low compared to narrow band signals that have a static carrier frequency.

Other methods of spread spectrum sweep the carrier frequency up or down the amateur band with specified timing and shift the phase of the carrier, and yet others use a repeating sequence rather than a continuing pseudo-randomized algorithm. A pulsed RF carrier can also be used to affect time multiplexed signals, often combined with frequency hopping in a hybrid technique of spread spectrum. You may wish to research Direct Sequencing, Time Hopping, and Pulsed FM Chirp spread spectrum systems.

Wifi Protocols: Many Part 15 computer devices utilize frequency hopping spread spectrum. Bluetooth devices utilize frequency hopping, and many computer network wifi standards implement some type of spread spectrum technique. For instance, the 802.11b wifi protocol takes advantage of spread spectrum, although later versions such as 802.11n use modern multiplexing schemes rather than true spread spectrum techniques.

The answer to Extra Class question E8D03, “How does the spread spectrum technique of frequency hopping work?” is “D. The frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station”

Related Questions: E1F01, E1F09, E1F10, E2C04, E8D01, E8D02