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hide top 1 telecommunications 2 invention of frequency hopping 3 clock signal generation toggle clock signal generation subsection 3 1 background 3 2 method 3 3 effect 4 see also 5 notes 6 sources 7 external links toggle the table of contents spread spectrum 26 languages العربية български català čeština dansk deutsch ελληνικά español eesti فارسی suomi français bahasa indonesia italiano 日本語 한국어 nederlands norsk nynorsk norsk bokmål polski português русский simple english sunda українська 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia spreading the frequency domain of a signal passband modulation analog modulation am sm ssb angle modulation fm pm qam digital modulation ask apsk cpm fsk mfsk msk ook ppm psk qam sc fde tcm tc pam wdm hierarchical modulation qam wdm spread spectrum css dsss fhss thss see also capacity approaching codes demodulation line coding modem anm pom pam pcm pdm pwm δσm ofdm fdm multiplexing v t e multiplexing analog modulation am sm ssb angle modulation fm pm qam circuit mode constant bandwidth tdm fdm wdm sdma polarization spatial oam statistical multiplexing variable bandwidth packet switching dynamic tdma fhss dsss ofdma sc fdm mc ss related topics channel access methods medium access control v t e in telecommunications especially radio communication spread spectrum are techniques by which a signal e g an electrical electromagnetic or acoustic generated with a particular bandwidth is deliberately spread in the frequency domain over a wider frequency band spread spectrum techniques are used for the establishment of secure communications increasing resistance to natural interference noise and jamming to prevent detection to limit power flux density e g in satellite downlinks and to enable multiple access communications telecommunications edit spread spectrum generally makes use of a sequential noise like signal structure to spread the normally narrowband information signal over a relatively wideband radio band of frequencies the receiver correlates the received signals to retrieve the original information signal originally there were two motivations either to resist enemy efforts to jam the communications anti jam or aj or to hide the fact that communication was even taking place sometimes called low probability of intercept lpi 1 frequency hopping spread spectrum fhss direct sequence spread spectrum dsss time hopping spread spectrum thss chirp spread spectrum css and combinations of these techniques are forms of spread spectrum the first two of these techniques employ pseudorandom number sequences created using pseudorandom number generators to determine and control the spreading pattern of the signal across the allocated bandwidth wireless standard ieee 802 11 uses either fhss or dsss in its radio interface techniques known since the 1940s and used in military communication systems since the 1950s spread a radio signal over a wide frequency range several magnitudes higher than minimum requirement the core principle of spread spectrum is the use of noise like carrier waves and as the name implies bandwidths much wider than that required for simple point to point communication at the same data rate resistance to jamming interference direct sequence ds is good at resisting continuous time narrowband jamming while frequency hopping fh is better at resisting pulse jamming in ds systems narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth where it will often not be much stronger than background noise by contrast in narrowband systems where the signal bandwidth is low the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth resistance to eavesdropping the spreading sequence in ds systems or the frequency hopping pattern in fh systems is often unknown by anyone for whom the signal is unintended in which case it obscures the signal and reduces the chance of an adversary making sense of it moreover for a given noise power spectral density psd spread spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same but since the signal power is spread over a large bandwidth the signal psd is much lower often significantly lower than the noise psd so that the adversary may be unable to determine whether the signal exists at all however for mission critical applications particularly those employing commercially available radios spread spectrum radios do not provide adequate security unless at a minimum long nonlinear spreading sequences are used and the messages are encrypted resistance to fading the high bandwidth occupied by spread spectrum signals offer some frequency diversity i e it is unlikely that the signal will encounter severe multipath fading over its whole bandwidth in direct sequence systems the signal can be detected by using a rake receiver multiple access capability known as code division multiple access cdma or code division multiplexing cdm multiple users can transmit simultaneously in the same frequency band as long as they use different spreading sequences invention of frequency hopping edit further information frequency hopping spread spectrum the idea of trying to protect and avoid interference in radio transmissions dates back to the beginning of radio wave signaling in 1899 guglielmo marconi experimented with frequency selective reception in an attempt to minimize interference 2 the concept of frequency hopping was adopted by the german radio company telefunken and also described in part of a 1903 us patent by nikola tesla 3 4 radio pioneer jonathan zenneck s 1908 german book wireless telegraphy describes the process and notes that telefunken was using it previously 2 it saw limited use by the german military in world war i 5 was put forward by polish engineer leonard danilewicz in 1929 6 showed up in a patent in the 1930s by willem broertjes u s patent 1 869 659 issued aug 2 1932 and in the secret us army signal corps world war ii communications system named sigsaly during world war ii golden age of hollywood actress hedy lamarr and avant garde composer george antheil developed an intended jamming resistant radio guidance system for use in allied torpedoes patenting the device under u s patent 2 292 387 secret communications system on august 11 1942 their approach was unique in that frequency coordination was done with paper player piano rolls a novel approach which was never put into practice 7 clock signal generation edit this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed january 2020 learn how and when to remove this message spread spectrum of a modern switching power supply heating up period incl waterfall diagram over a few minutes recorded with a nf 5030 emc analyzer spread spectrum clock generation sscg is used in some synchronous digital systems especially those containing microprocessors to reduce the spectral density of the electromagnetic interference emi that these systems generate a synchronous digital system is one that is driven by a clock signal and because of its periodic nature has an unavoidably narrow frequency spectrum in fact a perfect clock signal would have all its energy concentrated at a single frequency the desired clock frequency and its harmonics background edit practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics resulting in a frequency spectrum that at certain frequencies can exceed the regulatory limits for electromagnetic interference e g those of the fcc in the united states jeita in japan and the iec in europe spread spectrum clocking avoids this problem by reducing the peak radiated energy and therefore its electromagnetic emissions and so comply with electromagnetic compatibility emc regulations it has become a popular technique to gain regulatory approval because it requires only simple equipment modification it is even more popular in portable electronics devices because of faster clock speeds and increasing integration of high resolution lcd displays into ever smaller devices as these devices are designed to be lightweight and inexpensive traditional passive electronic measures to reduce emi such as capacitors or metal shielding are not viable active emi reduction techniques such as spread spectrum clocking are needed in these cases method edit in pcie usb 3 0 and sata systems the most common technique is downspreading via frequency modulation with a lower frequency source 8 spread spectrum clocking like other kinds of dynamic frequency change can also create challenges for designers principal among these is clock data misalignment or clock skew a phase locked loop on the receiving side needs a high enough bandwidth to correctly track a spread spectrum clock 9 even though ssc compatibility is mandatory on sata receivers 10 it is not uncommon to find expander chips having problems dealing with such a clock consequently an ability to disable spread spectrum clocking in computer systems is considered useful 11 12 13 effect edit note that this method does not reduce total radiated energy and therefore systems are not necessarily less likely to cause interference spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths typical measuring receivers used by emc testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 khz wide 14 if the system under test were to radiate all its energy in a narrow bandwidth it would register a large peak distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits the usefulness of this method as a means to reduce real life interference problems is often debated 9 as it is perceived that spread spectrum clocking hides rather than resolves higher radiated energy issues by simple exploitation of loopholes in emc legislation or certification procedures this situation results in electronic equipment sensitive to narrow bandwidth s experiencing much less interference while those with broadband sensitivity or even operated at other higher frequencies such as a radio receiver tuned to a different station will experience more interference fcc certification testing is often completed with the spread spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits however the spread spectrum functionality may be disabled by the user in some cases as an example in the area of personal computers some bios writers include the ability to disable spread spectrum clock generation as a user setting thereby defeating the object of the emi regulations this might be considered a loophole but is generally overlooked as long as spread spectrum is enabled by default see also edit direct sequence spread spectrum electromagnetic compatibility emc electromagnetic interference emi frequency allocation frequency hopping spread spectrum george antheil have quick military frequency hopping uhf radio voice communication system hedy lamarr open spectrum orthogonal variable spreading factor ovsf spread spectrum time domain reflectometry time hopping spread spectrum ultra wideband notes edit torrieri don 2018 principles of spread spectrum communication systems 4th ed 1 2 kahn david january 17 2014 how i discovered world war ii s greatest spy and other stories of intelligence and code crc press isbn 9781466561991 retrieved november 9 2022 via google books tony rothman random paths to frequency hopping american scientist january february 2019 volume 107 number 1 page 46 americanscientist org jonathan adolf wilhelm zenneck wireless telegraphy mcgraw hill book company incorporated 1915 page 331 denis winter haig s command a reassessment danilewicz later recalled in 1929 we proposed to the general staff a device of my design for secret radio telegraphy which fortunately did not win acceptance as it was a truly barbaric idea consisting in constant changes of transmitter frequency the commission did however see fit to grant me 5 000 zlotys for executing a model and as encouragement to further work cited in władysław kozaczuk enigma how the german machine cipher was broken and how it was read by the allies in world war ii 1984 p 27 ari ben menahem historical encyclopedia of natural and mathematical sciences volume 1 springer science business media 2009 pages 4527 4530 spread spectrum clocking microsemi 1 2 item media 19 march 2013 spread spectrum clock generation theory and debate interference technology catc satracer trainer application note spread spectrum clocking pdf catc july 2 2003 retrieved 20 may 2023 western digital raid edition iii hdds werden vom raid controller nicht erkannt thomas krenn wiki intel speichersystem ss4000 e festplatten wie beispielsweise die western digital wd2500js sata werden nicht erkannt woran liegt das intel reseller center ssc toggle utility barracuda 7200 9 at the wayback machine archived 2010 04 29 seagate knowledge base american national standard for electromagnetic noise and field strength instrumentation 10 hz to 40 ghz specifications ansi c63 2 1996 section 8 2 overall bandwidth sources edit this article incorporates public domain material from federal standard 1037c general services administration archived from the original on 2022 01 22 in support of mil std 188 ntia manual of regulations and procedures for federal radio frequency management national information systems security glossary history on spread spectrum as given in smart mobs the next social revolution howard rheingold isbn 0 7382 0608 3 władysław kozaczuk enigma how the german machine cipher was broken and how it was read by the allies in world war two edited and translated by christopher kasparek frederick md university publications of america 1984 isbn 0 89093 547 5 andrew s tanenbaum and david j wetherall computer networks fifth edition external links edit a short history of spread spectrum cdma and spread spectrum archived 2009 04 16 at the wayback machine spread spectrum scene newsletter v t e telecommunications history beacon broadcasting cable protection system cable tv communications satellite computer network data compression audio dct image video digital media internet video online vid...