Here is another question & answer (my last, yay!) from the ARRL Extra class 2002 question pool that irks me. This may be one of the 2 that I got wrong on the test. I vaguely remember seeing this question in the 16 practice tests I took and noting a confusion factor for me. (This is when you need to remember the literal answer for the literal question. Hey I don’t have a photographic memory.) Frequency-hopping (FH) and spread-spectrum together? It was confusing for me. I am interested in your expert comments.
“ E2C09 How does the spread-spectrum technique of frequency hopping (FH) 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.
D. FH spread-spectrum is a form of spreading in which the center frequency of a conventional carrier is altered many times per second in accordance with a list of frequency channels. The same frequency list is used by the receiving station which “hops” in sync with the transmitter. ”
In my mind, spread-spectrum (SS) and frequency-hopping (FH) have always been two different things. They both can be used to obscure the content of a radio transmission’s information. I think this is the only reason for FH.
Upon investigation, the problem with this Q&A is the wording of the question, which is incorrect in my opinion. What I thought of as spread-spectrum is specifically Direct Sequence Spread-Spectrum (DSSS). Frequency-hopping is actually Frequency-Hopping Spread-Spectrum (FHSS). Spread-Spectrum is the general term for the field of techniques apparently, I didn’t know this.
The Question E2C09 “How does the spread-spectrum technique of frequency hopping (FH) work?” should be worded “How does the frequency hopping (FH) technique of spread-spectrum work?” Does grammar allow both ways and the ambiguity?
Because of the way the question was worded, I chose C. which does describe DSSS. D. does describe FHSS. And so the study book explanation for the answer D. is correct. They did want you to think of frequency hopping and choose D.
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Some background on Spread-Spectrum:
The FH mode allows the use of narrow band receivers. I believe that many implementations may allow at least 10 milliseconds to be spent on a channel. Can be implemented with SSB transceivers, and is. “High” bandwidth transceivers and “high” bandwidth data can allow much higher hopping rates to be implemented, I presume.
DSSS has many other advantages including resistance to multi-path propagation and jamming. Many users can be on one r.f. channel, also known as CDMA (code division multiple access) for example, cell phone systems and reception of all of the GPS satellites in view are on the same channel. DSSS can also operate in a band with existing narrow band signals and vice-versa.
From ["Spread Spectrum Communications", Charles E. Cook et al (Ed.), IEEE Press, New York, 1983. ISBN 0-87942-170-3]. :
The most important practical modes of spread spectrum coding are Direct Sequence (DS) and Frequency Hopping (FH). In DS, a pseudo random sequence is phase-shift-keyed (PSK) onto the carrier. In FH, a frequency synthesizer is driven by a pseudo random sequence of numbers to generate output frequencies that "hop around" in the desired frequency range.
Spread Spectrum development began during World War II, with the earliest studies dating from the 1920s. Most papers remained classified until the 1980s.
Frequency hopping spread spectrum was invented by Hedy Lamarr ("the most beautiful girl in the world", Samson and Delilah etc.) and the composer George Antheil. They held a patent filed in 1942. Direct sequence spread spectrum was invented by Paul Kotowski and Kurt Dannehl at Telefunken.
The technique is used extensively in military communications today. Commercial applications include cellular telephony and mobile networking.
Hedy Lamarr,
http://www.ncafe.com/chris/pat2/.
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Any comments?
