{"id":168,"date":"2014-02-21T21:02:38","date_gmt":"2014-02-21T21:02:38","guid":{"rendered":"http:\/\/blogs.stthom.edu\/cameron\/?p=168"},"modified":"2017-04-13T20:54:51","modified_gmt":"2017-04-13T20:54:51","slug":"spread-spectrum","status":"publish","type":"post","link":"https:\/\/blogs.stthom.edu\/cameron\/spread-spectrum\/","title":{"rendered":"Spread Spectrum"},"content":{"rendered":"

Spread spectrum is probably today\u2019s most important wireless networking protocol.\u00a0 As unlikely as it might seem, a 26-year-old Hollywood movie star, a screen siren named Miss Hedy Lamarr<\/a>, invented spread spectrum.\u00a0 In 1942 during World War II, she and a partner (who was a music composer and producer) received a US patent (#2,292,387) for inventing spread spectrum.\u00a0 The essential insight is to spread a signal across many channels over a much wider bandwidth than would normally be used and therefore to make jamming or intercepting that signal much more difficult.\u00a0 This idea has military significance because assuring reliable communications during a battle has high priority.\u00a0 Almost immediately, the US government realizing the import of her ideas snapped up the patent and classified it as \u2018top secret.\u2019\u00a0 After her film career when the scientific community began to discover her contributions, she received several prestigious awards as a premiere inventor.<\/p>\n

Every telecommunications protocol standard has its own language and terminology, and this is no exception.\u00a0 The concept of spread spectrum is based upon a sequence of digits known as a \u2018spreading code.\u2019\u00a0 At the source, this code is used to spread a single stream of signals across a range of frequencies; the same code is used at reception to \u2018de-spread\u2019 the signals back into the original form.\u00a0 Spreading the signal in this manner uses a lot more bandwidth for each transmission.\u00a0 Therefore, on the surface, this approach would appear to waste a critical resource.\u00a0 But there are very significant advantages here.\u00a0 Spread spectrum does prevent military opponents from jamming battlefield communications or torpedo guidance systems. \u00a0And these same characteristics provide commercial networks with effective immunity from various forms of interference, noise, and distortion.\u00a0 Spread spectrum protocols can also be used to help secure signals, because a receiver cannot decode the incoming signal without the original spreading code.\u00a0 Most importantly, several users can use overlapping ranges of high bandwidth frequencies at the same time with very little effective operational interference.\u00a0 These properties make spread spectrum especially desirable for cellular telephony applications.<\/p>\n

User Code<\/b><\/p>\n

There are three forms of spread spectrum, frequency hopping spread spectrum, direct sequence spread spectrum, and code division multiple access<\/a>.\u00a0 The last of these is the newest, and most important, and is used in modern cellular telephony.\u00a0 Essentially, CDMA systems mix a long binary spreading code called a \u2018user code\u2019 with a small amount of communications data to produce a combined signal that is then spread over a very wide range of frequencies.\u00a0 The same user code is also used at the destination to reconstruct the original digital signal.\u00a0 In this approach, every device that connects to the system (such as a digital mobile cell phone) is dynamically assigned a unique user code when a connection is established.\u00a0 This code is typically more than a hundred digits in length.\u00a0 The number of digits used in the code is called the \u2018spreading factor.\u2019\u00a0 The code uses binary digits with each digit being interpreted as plus or minus one.\u00a0 So, each active device in the system has associated with it a unique code made up of a long sequence of intermixed plus ones and minus ones.<\/p>\n

When a digital cellular device communicates with a cellular tower, the user code is transmitted across multiple channels to indicate a ONE, and its complement is transmitted to indicate a ZERO.\u00a0 (The complement of the user code is the same code with all the bits flipped; that is, with all the pluses and minuses reversed.) \u00a0Both the device and the tower use the user code and its complement to communicate with one another in this fashion.\u00a0 The number of channels used for transmission is the same as the spreading factor, which is the number of digits in the user code, so the entire pattern of plus and minus ones arrives at the destination for each transmission, in unison each on a different frequency.\u00a0 The receiver then decodes the incoming signal to get back the original bit.\u00a0 A stream of these transmissions effectively sends a bit stream of information between the cellular device and the tower, as required.<\/p>\n

However, the cellular airways can be jammed full of traffic.\u00a0 Using spread spectrum means that channels are shared and allocated bandwidths overlap.\u00a0 So, how does a receiver figure out if a message it hears in the air is meant for it, and also what is being sent, a zero or a one? <\/i>\u00a0This is the ingenious part of CDMA.\u00a0 All it requires is a bit of mathematics.\u00a0 The computer in the cellular tower knows the user codes for all active devices in its area because it assigned those codes originally.\u00a0 The codes are just long sequences of plus or minus ones.\u00a0 And they can be treated as vectors, and manipulated using vector algebra.\u00a0 When a transmission is received, the computer can quickly calculate a dot-product between the received code pattern and each of the active user codes for all the devices in its area to get answers to these questions.[1]<\/a><\/p>\n

Historically, one of the greatest problems with cellular telephony has been finding a way to share bandwidth efficiently.\u00a0 Failure to share dramatically limits the capacity of a cellular system and severely restricts the number of devices that an individual cell tower can support at any one time.\u00a0 Spread spectrum protocols change all of this.\u00a0 And these limitations are lifting.\u00a0 The future of cellular telephony lies with advanced forms of spread spectrum.\u00a0 And to think it all began with Miss Hedy Lamarr!<\/p>\n

\n
\n
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[1]<\/a> A dot-product of two vectors is a scalar.\u00a0 It is the summation of position by position multiplications.\u00a0 For example, to multiply these two four-dimensional vectors, (1,-1,-1,1) times (1,1,-1,1), do the following:\u00a0 (1×1) + (-1×1) + (-1x-1) + (1×1) = 2.\u00a0 So, the scalar \u20182\u2019 equals the dot-product of these two vectors.<\/p>\n

 <\/p>\n

Charles K. Davis, Ph.D.<\/a>
\nProfessor; Cameron Endowed Chair of Management & Marketing<\/strong><\/p>\n

See more posts by this author<\/a><\/p>\n

\u00a9 Copyright by Charles K. Davis, 2014<\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Spread spectrum is probably today\u2019s most important wireless networking protocol.\u00a0 As unlikely as it might seem, a 26-year-old Hollywood movie star, a screen siren named Miss Hedy Lamarr, invented spread spectrum.\u00a0 In 1942 during World War II, she and a partner (who was a music composer and producer) received a US patent (#2,292,387) for inventing…<\/p>\n","protected":false},"author":5,"featured_media":170,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[44,33,37,51],"tags":[15,63,54,64],"class_list":["post-168","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-critical-thinking","category-commentary","category-csb-faculty","category-managment-information-systems","tag-charles-k-davis","tag-hedy-lamarr","tag-management-information-systems","tag-spread-spectrum"],"yoast_head":"\nSpread Spectrum - 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