EC312 Lesson 25 – : Electronic Protection Techniques

Objectives: a) Describe the purpose of spread spectrum, the challenges it overcomes, and the advantages it provides for EW and commercial applications of digital communication. b) Describe the difference between Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) in terms of data and frequencies. c) Analyze the engineering factors associated with an FHSS signal (e.g., dwell time, bandwidth, data rate). d) Given a data signal and associated pseudorandom sequence for a DSSS scheme, determine what the transmitted DSSS signal would be. (Also be able to go in the reverse order, given a received DSSS signal.)

Jamming and Interference Our last lecture discussed the use of jamming for electronic warfare purposes. In some senses, jamming can be thought of as intentional signal interference. Even if there is no jamming occurring, we are always concerned about the possibility of unintentional signal interference, especially as the types of wireless technologies and users continues to proliferate. Today we’ll consider the use of wideband modulation as a method to prevent interference.

Historical Note In the midst of World War II, many noted the vulnerability of guided torpedoes and other radio controlled weapons to jamming and interference. An unlikely candidate, Hollywood starlet Hedy Lamarr, is responsible for the solution. Hedwig Kiesler was born in 1914 to a Jewish family in Austria. She launched into stardom and notoriety by starring in Ecstasy, a Czech film that was pretty controversial for its time. Right before her 20th birthday, she married a Viennese arms merchant. With her husband, she hosted lavish parties attended by Hitler and Mussolini, where she learned about military technology despite her lack of formal education. From “Hedy Lamarr: The Incredible Mind Wary of the Nazi Party and unhappy in her marriage, she Behind Secure WiFi, GPS and Bluetooth,” disguised herself as her maid and fled to Paris in 1937. Soon Forbes Magazine, Feb. 28, 2018. https://www.forbes.com/sites/shivaunefield/ after that, she met Louis Mayer, co-founder of MGM studios 2018/02/28/hedy-lamarr-the-incredible- in London. She went on to star in dozens of films. Eventually, mind-behind-secure-wi-fi-gps- bluetooth/#704006ed41b7 she became one of the first female Hollywood producers. In her spare time, she enjoyed playing with inventions, including an improved traffic stoplight and a tablet for creating carbonated drinks. Recalling what she had learned about torpedo vulnerabilities at her first husband’s dinner parties, she worked with her musician friend George Antheil to develop and patent the technique of frequency-hopping spread spectrum. Lamarr and Antheil’s idea was simple: instead of a single carrier frequency, why not cycle the carrier frequency for a transmission through a pattern of frequencies where the pattern would be known only by the sender and receiver? An eavesdropper with an antenna would only perceive a seemingly random sequence of blips at all different frequencies, and therefore would not be able to intercept or jam the signal. To accomplish this, Lamarr and Antheil proposed using a player-piano mechanism. The patent, titled “Secret Communication System,” was filed in 1942, and first used in 1957. Wideband Modulation Lamarr’s invention effectively spreads the signal transmission over a wider bandwidth, which is what makes the signal difficult to detect or jam. This is called “wideband modulation.” We’re particularly interested in a form of wideband modulation known as spread spectrum (i.e. SS). The two most common forms of spread spectrum are Frequency Hopping Spread Spectrum (FHSS), and Direct Sequence Spread Spectrum (DSSS), also known as code-division multiple access (CDMA).

Benefits of Spread Spectrum Spread spectrum has several benefits over communications: • Security – to receive the signal you need a wide BW receiver and precise knowledge and timing of the pseudorandom sequence • Resistance to jamming and interference– jamming signals are usually restricted to one frequency • Band sharing – many signals can use the same frequency band; but… many spread spectrum signals raise the overall background noise level • Resistance to fading – fading is when a signal is attenuated by variables such as atmospheric phenomena and geographical position. Fading is a frequency-selective phenomenon, and a spread spectrum signal doesn’t reside at only one frequency

Frequency-Hopping Spread Spectrum (FHSS) The FHSS communication system that you are probably most familiar with is Bluetooth (the IEEE 802.15 standard). The block diagram for an FHSS transmitter is shown on the top right. The output of the pseudorandom code generator is shown in the figure below it. (In Lamarr’s patent, the piano player roll was used as the pseudorandom code generator.) The carrier frequency is cycled through a number of discrete frequency steps, called the “hop sequence.” The carrier frequency stays at each frequency for a set length of time called the “dwell time”. The inverse of the dwell time is called the “hopping rate.” In order to receive and properly demodulate the signal, the receiver needs to know the same pseudorandom code that the transmitter used. One of the challenges of frequency-hopping systems is needing to synchronize the transmitter and receiver (if you can’t use a player piano roll). One approach is to start the communication with the transmitter initiating a “handshake” by sending out a particular code using all the channels for a short time. The receiver can find the transmitter by picking any random channel and listening for valid data. Once communication is established, the transmitter and [Figures from Principles of Electronic Communication receiver both reference fixed tables of frequency- Systems by Louis E. Frenzel, Jr., McGraw-Hill, 2016.] hopping patterns, so that once synchronized they can maintain communication by following the table. The hopping rate varies. Bluetooth began with a hopping rate of 10 Hz but now uses 1600 hops per second. This is fast enough to thwart an eavesdropper who would need to first lock onto the signal before they could intercept or jam it, but slow enough to include thousands of periods of the carrier signal in each hop. Bluetooth, for example, operates in the range of 2.402 to 2.480 GHz divided up into 1 MHz channels. For an example of the resulting signal, see https://youtu.be/kAvQ7O6W9J8. In addition to providing greater security, another advantage of FHSS is that multiple communication links can use the same larger frequency band, simply by using a different hop sequence. If each link hops frequently and uses lots of possible channels (Bluetooth has 75 to 79 channels), a collision is unlikely. When a collision between links does occur, it is likely to be only for the duration of one dwell time, and therefore has little effect. The “symbol” utilized in FHSS consists of several cycles of the carrier signal, modulated in frequency, phase and/or amplitude as was discussed in the digital signals lesson. Although FHSS systems started with FSK encoding, modern FHSS systems typically utilize encoding schemes with multiple symbol options, like M-ary PSK and QAM, so each symbol conveys multiple bits. The symbol rate is typically much higher than the hopping rate. The data rates for Bluetooth are as high as 10Mbps.

Direct Sequence Spread Spectrum (DSSS) Another method of realizing spread spectrum is called direct sequence spread spectrum (DSSS). In DSSS, the serial binary data is XORed with a pseudo-random binary code (also called the “spreading code”) which has a faster than the binary data rate, and the result is used to phase- modulate a carrier. The bit rate of the [Figure from Principles of Electronic Communication Systems pseudorandom code is called the chipping rate. by Louis E. Frenzel, Jr., McGraw-Hill, 2016.] 1 1 0 1

data time of one data bit carrier modulated by the data

Pseudo Random Sequence

“chip”

data ⊕ PRS XOR ↑ carrier modulated by the data ⊕ PRS

UNMODULATED power Instead of using the original data to phase CARRIER modulate a carrier, we would now use the output of the XOR process. This will change SLOW SPEED PSK the phase of the carrier much more frequently than the original data would, so the signal HIGH SPEED PSK power would be spread over a wider bandwidth.

frequency

Sudden phase changes to a carrier signal (like those occurring in simple BPSK) cause the bandwidth of the resulting modulated signal to increase. Beyond that, if we increase the rate of the phase changes of the carrier the bandwidth also increases. If we can get the phase change rate up high enough (thanks to a high chipping rate), the resulting modulated signal will look more like noise than an actual signal. It would look like noise just from the rapid phase changes. However, it becomes even more noise-like since the phase changes are generated in a pseudorandom fashion.

In DSSS, a signal that would normally occupy a few kHz of bandwidth (say like your regular FM broadcast) is spread out 10 to 10,000 times its normal bandwidth. The energy that would normally be concentrated in that narrow bandwidth is still there in DSSS, but has been spread out, too. In fact the energy is so spread out, that it appears as noise in a conventional receiver.

The energy of a DSSS signal is spread over such a large bandwidth that the energy is indistinguishable from naturally occurring background noise. [Figure from Principles of Electronic Communication Systems by Louis E. Frenzel, Jr., McGraw-Hill, 2016.]

As was the case with frequency-hopping spread spectrum, a DSSS receiver must know the pseudorandom sequence of the transmitter and have a synchronizing circuit to get in step with this pseudorandom digital signal. The receiver using an identically programmed spreading code sequence can then “see” incoming matched signal clearly from the noise.

The measure of the spreading is called the processing gain, G, which is the ratio of the DSSS bandwidth, BW, divided by the data rate, Rb. The higher the processing gain, the greater the DSSS signal’s ability to fight interference.

Code Division Multiple Access (CDMA) is a form of DSS used by satellites and digital cell phones in which all the terminals on the network utilize orthogonal spreading codes, which allow multiple broadcasts to overlap each other without causing any interference.

Example. We wish to transmit the message 01101 using a DSSS scheme, and our assigned pseudo-random sequence is shown in the table below. Fill in the blanks. TRANSMIT SIDE Original binary message to transmit 0 1 1 0 1 Assigned pseudo-random sequence 1 0 0 0 1 0 1 1 0 1 1 1 0 1 0 0 1 1 1 1 Transmitted DSSS signal (XOR result) 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 0 0 0

RECEIVE SIDE Received DSSS signal 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 0 0 0 Same pseudo-random sequence 1 0 0 0 1 0 1 1 0 1 1 1 0 1 0 0 1 1 1 1 XOR Result 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 Recovered original message 0 1 1 0 1