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[edit] CDMA history"CDMA and is a military technology first used during World War II by English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal." [edit] gush ansCDMA is built on spread spectrum but is distinct from it. The forms of spread spectrum are frequency hopping, direct sequence and (rarely) time hopping. Spread spectrum is not always used to provide CDMA. For example, the military is more interested in jamming resistance and low probability of intercept. The famous Lamarr system was frequency hopped to resist jamming, not to provide CDMA. The implementation was completely impractical -- it used player piano rolls at both ends to hold the hopping sequence -- but the basic elements of frequency hopping were all there. CDMA is code division multiple access, the use of spread spectrum to provide multiple access. That's a means for multiple transmitters to communicate without (harmful) mutual interference. CDMA cellular uses direct sequence spread spectrum so it is arguably not a descendent of the Lamarr system. Karn (talk) 08:40, 16 September 2008 (UTC) ok, first of all, CDMA as such did not come out until the 90s.
The concept of spread spectrum as it is understood today, originally invented by an American, Heady LaMarr (spelling), was never used in WW II.
Finally, it is usually referred to as "Allied forces" or "American allies" not "English allies".71.116.106.31 00:50, 9 July 2006 (UTC)
It was proposed and analysed by the GSM as one alternative, and rejected as I say - because it does not have a finite time to deliver: Simple, when the number of users exceed a threshold, the time spend identifying and managing the packets is to high to be able to send everyone, so then you start to retransmit. Put it blunt: What do you consider will happen in a town where every time there was a read light request was issued for every car in the queue to fine a new car to line up for them? Yes - you would very soon run out of street and road space to hold the queue, and after that - run out of cars.
—The preceding unsigned comment was added by Khflottorp (talk • contribs) 12:43, 9 July 2006.
Maybe you have a point but it is lost in words. "What do you consider will happen in a town where every time there was a read light request was issued for every car in the queue to fine a new car to line up for them" - do you understand yourself what you saying? [edit] TDMA/GSM are DIGITAL cellular/PCS technologies – very poor CDMA ComparisonsIt's my suspicion that people employed from certain carriers contributed to this article and would like a reader to believe TDMA and GSM are analog multiple access technologies. In fact they are both completely DIGITAL, which then concerns me about many of the other explanations – to include many of the other comparisons, which are plagued by incorrect explanations and inconsistencies. It’s in technical articles like this one where Wiki will always fall short of factual documentation. Reader beware and scrutinize what you take out of here. OK. Yes, it was highly biased on the US understanding - afterall they invented the names "CDMA" and "TDMA". Outside the US it has several different names - mostly "Code Division Multiplexing" - replacing the "MA" with "mulitplexing" - which is the term applied in telecom for many logical streams sharing one physical - not only on the "Access Network" link. The Nordic countries developed the NMT system soon after SS7 had been finalised (1978). This served as prototype for GSM - as it was fully digital in the network and call management, while the voice codec was insufficient to support fully digital service (full service 1982?). The readers needs to be made aware that telecom was one of the first areas to be completely computerised - in Europe. Here the telecom companies had at the time protected national monopoly on the service, and was liable only to the government. Their last and final effort to withstand the expected competition was to digitise all the network - fibre in the ground, and digital service in the air - with a suite of services for computer networks based on IBM's then HDLC/LU6.2 - and a proposed network hierarchy/stack - refered to a s OSI - "Open System interconnect". We still refer to these "layers" - and honstly, they may come back and haunt the US. IP-level interfacing needs routing as a service, as found in SS7 connection set-up and routing. Here, the Europeans defined routing as a service. The net owner may set up own routing, but since the dominant capacity was available to lease to new operators, they had to allow "others" to define own routing - using foreign nets. This is the same as "Soft Handover" really - it is just that the routing table at the BSC will use not only one "next station" - but can also interroagate and find that you radio link quality is alsmost as good by another BST - but here there are better capacity, and move you to this to optimise the network. I would like the article to state that: "CDMA was made by Qualcomm on behalf of the US telephone companies following the HR decision, and later sanction by the US president as the law to "Liberate the frequencies". This basically bars GSM usage in the US, and protects the US companies to make something different (not better). The observation was that there was a severe danger at the time, that the US operators would be an easy prey for European takeovers." Qualcommwas most likely unaware that the GSM consortium had studied Code Division Mutliplexing" and rejected it. They hired some Indians to help them with the math, and the poor professor has published and article where he completes the calculation of the O-residual, but this is never refered to by them. Participants from Nokia/Finland argued strongly for CDMA indicating that they had reached very far in investigating this and exploring other properties. I cannot find the papers, but if I remember right, it is multi-channel code division, with the capability to cut-off to time-division when the network was congested. The terrain in the Nordic countries require extensive cell overlay, thus multiple channels had to be used (coverage is considered more important than a few as possible base stations - to allow the consumer later to be loyal to the first network and not move on to competitors that needed to provide the same coverage). The GSM network also has muliple frequncy bands to support different cell sizes. These systems are defined by engineers and not by company directors. The engineers knows the basic laws of physics and use them to acheive specific properties. By using the SS7, it is possible for GSM to use special modulation for data - know as EDGE, and also paves the way for easy upgrade to 3G - with new modulation technology already in place making the first specifications outdated. I know that a comittee is a beast with 1 head and 12 legs - but it is better then no head at all (where 2 ignorant corporate managers decide based on how they believe they can acheive a higher profit. I know give the people tin cans with a wire between - but that is not "wire less") I have tried to tidy up some. Most important is that Americans need to understand that regardless of numerous attempts to offer CDMA even for free - the rest of the world has choosen GSM. They also need to understand that the ITU is the holder of standards, and as long as the US decides to ignore important work going on in the ITU, they loose business opportunities - and the rest of the world goes on very well without their intereference. (See first wimper...:-) --KH Flottorp 12:23, 9 July 2006 (UTC) [edit] Umm...Whoever wrote this seems to be a European with some sort of axe to grind toward Americans in general and Qualcomm in particular. He also seems rather confused about the various technologies and how they relate to the layers of a network. Disclaimer: I work for Qualcomm, but I speak here only for myself. Since this is an article about the technology known as CDMA, not the politics of cellular telephony standards, I'll try to avoid the political revisionism and stick to technical and historical facts. I urge others to double check me and not just take what I say as gospel. Packet switching and digital radio result from decades of research and development all over the world, including Europe and North America. This article is specfically about one of the methods used to share wireless channels among multiple users. This is purely a physical layer concept; all these methods can be and are routinely used to support existing upper layer protocols such as those of the Internet. The "MA" in CDMA, FDMA and TDMA all mean Multiple Access. This refers to a very specific problem of allowing multiple transmitters to share the same radio band to the same receiving station without mutual interference. This is distinct from multiplexing, the transmission of independent streams of data over a single channel by the same transmitter. Even in mobile telephony only the reverse link -- mobile to base -- is actually multiple access. The forward link from base station to mobile phone is multiplexed (without the final "A") as there is only one transmitter, that at the base station. The forward link may have to fend off interference from other cell transmitters, but unlike the reverse link receiver at the base station a mobile isn't trying to listen to them all at once. CDMA, TDMA and FDMA are, to my knowledge, generic terms that have been in use by engineers all over the world for decades. CDMA and TDMA are usually both digital; FDMA can be either digital or analog. In the context of mobile telephony, "FDMA" usually refers to the older analog FM system. But because radio communications are still regulated by frequency band, every modern telecommunication system (with the possible exception of UWB) uses FDMA in addition to whatever other methods it has chosen. Here I will talk specifically about the use of CDMA in mobile cellular telephony even though CDMA (and TDMA and FDMA) are generic terms that find use in other communication and navigation systems. Ideally there should be separate Wikipeda articles for generic CDMA, TDMA and FDMA, and for their uses in mobile telephony (and other) systems. If they are split, what follows should be moved to the mobile telephony articles. CDMA and GSM are both digital mobile cellular systems. Both operate on various frequency bands around the world depending on local regulatory allocations. In the USA the traditional 800 MHz "cellular" band and the newer 1900 MHz "PCS" band are both used. Some carriers operate on both bands, and many if not most mobile phones are designed to switch between them automatically. Along with a now-obsolete US-specific variant (confusingly) called IS-54 TDMA, GSM was indeed progressing towards standardization and deployment when Qualcomm first proposed using code division multiple access on top of direct sequence spread spectrum for cellular mobile telephony. (Although CDMA is a generic term, in everyday usage the term began to apply to Qualcomm's specific use of it for mobile telephony.) The regulatory policy in the USA at the time was laissez-faire; the carriers and industry were free to develop whatever technologies they thought would best meet their needs. This was apparently different from the climate in Europe where governments and international standards bodies were more actively involved in digital cellular development. Qualcomm began to demonstrate CDMA in the late 1980s and moved toward development and deployment in the early 1990s. It was first deployed in several other countries, notably China, Russia and India and then in the USA. CDMA is now found worldwide with the notable exception of Europe, which remains predominantly GSM. Both CDMA and GSM were originally designed solely to carry voice. Both used digital means to provide better capacity and quality than the contemporary analog FM cellular system developed in the late 1970s by AT&T's Bell Laboratories and known as the Advanced Mobile Phone Service or AMPS. As they were deployed, both began to add features to carry user data. These began at voice bit rates and then offered higher rates by "bonding" multiple voice channels. Both systems then began to develop new physical layers specifically to carry packet data for Internet access, though to my knowledge all are based on variants of CDMA (e.g., WCDMA) and not TDMA because CDMA is inherently better suited to the unpredictable bursts of traffic associated with this use. Karn (talk) 09:48, 16 September 2008 (UTC) [edit] Dot Product ProblemsIt's been a while since I've done it, but I'm fairly confident the dot products on this page are incorrect.
[edit] Dot Product Problems and MORE<sigh> Those aren't the only problems on this page. Ah, the wonder of Wikipedia! Any amateur who knows just enough to be dangerous can spread his errors far and wide and claim to have authored an encyclopdia article on the subject! Corrections are futile because the amateur simply returns and re-errorizes the information.
How broadcasting +1.2/-1.2 or +0.8/-0.8 change the calculations or the orthogonality between the walsh codes? To me the dot products between the presented codes don't seem to suffer from power difference between the transmissions at all. Crosstalk between the codes comes from totally different phenomena. There is no infinite amount of CDMA codes; presented four chip long code has only four. [edit] Two meaning of CDMAThe article does a good deal of explaining the two meaning of the initials. See this post for more info [1] [edit] Vector components"Most generally these vectors are specially constructed for ease of decoding -- they are columns or rows from Walsh matrices that are constructed from Walsh functions, but strictly mathematically the only restriction on these vectors are that they have no nonzero components". That means they are null vectors doesn't it? Thats the way I read, so maybe its a mistake.
[edit] Power control and moreThe article has improved a lot but still the explanation why the orthogonality is lost is missing. Power difference would require better orthogonality but it doesn't remove the orthogonality as can be understood from the article now. All the shown equations apply to any power level. In real systems no 1's or -1's can be transmitted since they would require infinite bandwidth. On the other hand limited bandwidth chips would require infinite time. So waveforms somewhere in between must be used. They are unfortunately both non-orthogonal and sensitive to timing. If infinite amount of CDMA codes would be used one bit would be infinite chips long and bit rate would be zero bps. In practice the amount of CDMA codes in most applications is very limited. There is also other CDMA systems than the described direct sequence. Maybe someone familiar with these issues could rewrite this article one more time?
[edit] Where do I begin...I would completely re-write this article if I had time. It is wrong on so many levels. Perhaps after I finish my doctoral thesis. Until then, here are the important points that need to be made: CDM vs. CDMA: The biggest fundamental error with this article is there is no distinction drawn between CDM (Code Division Multiplexing) and CDMA (Code Division Multiple Access). This article discusses CDM, not CDMA! CDM is synchronous, and is used for all of the Base-to-Mobile links. These can be synchronized because the base-stations are completely stationary and the clocks can be synchronized with extremely fine precision. Orthogonality is only possible in this synchronous *multiplexed* scenario. CDMA vs. TDMA and FDMA: The entire point of using CDMA is so that the mobiles do not need to be synchronized to the base-station. If you could perfectly synchronize all of the users, you might as well use TDMA. To a first order, TDMA, FDMA and CDMA are all equivalent in the sense that they all provide a means of of orthogonally separating multiple users under ideal conditions. However, in practice they all have pros and cons. In FDMA, doppler spreading and imperfect filters creates a need for guard bands. In TDMA, the inability to perfectly synchronize the users creates a need for guard times. In CDMA, orthogonality is only mathematically possible if the users are perfectly synchronized. Thus in practice, TDMA and FDMA can never be made perfectly efficient, while the mobile-to-base links in CDMA can never be made orthogonal. Asynchronous CDMA: True CDMA (asynchronous mobile-to-base links) comes in two flavors, short code and long code CDMA. The long code CDMA is the most common type, typically a pseudo-random shift register sequence (a.k.a, an "m-sequence") that is longer than the code length (number of chips per symbol, a.k.a the "Processing Gain"). Solomon Golomb's book "Shift Register Sequences" is a classic reference for these sequences. Multiple Access Interference (MAI): In an asynchronous system, the users are *not*, and *cannot* be orthogonal, merely *uncorrelated*, i.e., the average cross-correlation is zero. The variance of the correlation, on the other hand, is inversely proportional to the code length, and directly proportional to the number of users. This is known as Multiple Access Interference (MAI), and is an asymptotically stationary zero-mean Gaussian noise process for a large number of users (via the central limit theorem). Sarwate and Pursely's 1977 paper is a classic reference for this result. Each user adds a small amount of additional interference, which is known as "soft degradation". Power-Control and Capacity This can be significant since each base station typically picks up transmissions from all of the users in the cell, plus a large number of interfering mobiles from adjacent cell sites. The near-far problem with power control is that the mobiles close to the base station must use significantly lower power than mobiles far from it. In effect, if a user transmits twice as much power, they generate twice as much interference as they are supposed to. The use of FEC (Forward Error Correction) is to mitigate the number of errors incurred from the total of all the sources of degradation (thermal noise, MAI, co-site interference, narrowband interference, etc.). The capacity is essentially constrained by the strength of the FEC and ability to control the power. The FEC can only tolerate interference down to a signal-to-interference ratio (SIR) where the bit error rate (BER) becomes unacceptably high. Thus, the capacity (number of users) is maximized by keeping the level of interference generated by each user the same. If the power control is imperfect, then this inefficiency leads to a degradation in capacity. There are so many concepts here that need to be properly explained, I could probably spend a month on this!
[edit] commentComment on “Where do I begin”*** WARNING, he grossly overstates the inconstancies found in this article. You can tell he is someone who has only studied the technology from the outside in and does not actually know or understand some of the specific features found in the equipment itself - beyond the EE textbook – different vendors include different specifications which makes it hard to technically define a multiple access technology like CDMA – different carriers deploy different manufacturers’ solutions and then use different management techniques, which can also vary from market to market.
[edit] Comparision of CDM and CDMA is not satisfactoryCDM/CDMA, code division CDM/CDMA is the orthogonal power distribution multiplex/demultiplex scheme. Different signals may coexist in the same frequency band at the same time. Existence of each signal means interference to other signals in channel. And this interference in determined by power distribution of all signals, phase difference, and correlation factors between random code of each signal. Hence we can conclude that CDM and CDMA is same and not different.—The preceding unsigned comment was added by 202.138.120.37 (talk • contribs) 10:38, 2006 May 6. [edit] Re: commentAgain, the "Technical Details" section is only applicable to CDM. The statement that orthgonality is the heart of CDMA is *completely* wrong and totally misleading. I have removed some of the blatantly false statements, such as the nonsense about how the near-far effect "destroys the orthogonality". While it is true that 64-ary Walsh sequences are used in a particular M-ary CDMA scheme (IS-95, I beleive) to transmit multiple bits per symbol (64-ary -> 6 bits/symbol), the multiple-access capability comes from using different pseudo-random sequences (or at least different shifts of the same pseudo-random sequence), which *are not* and *cannot* be orthogonal. In general, however, Walsh sequences have *nothing* to do with the general concept of CDMA. The entire point of this particular page is, in fact, to give a simple "textbook" overview of the *concept* of CDMA, eschewing the esoteric details of particular CDMA standards, each of which have their own pages. Therefore, I think the entire section about Walsh sequences and orthogonality should be done away with (or placed under an a proper explanation of either CDM or as a particular variant of M-ary CDMA). This page needs to bring home the point for the non-specialist audience that only a synchronous system, such as the base-to-mobile link, can have orthogonality (CDM, not CDMA), and that general asynchronous systems, such as the mobile-to-base link, (which is true CDMA) have Multiple Access Intereference (MAI) that is approximated by a Gaussian noise process, and requires power-control to reduce the near-far effect. I have a doctorate in communications theory, wrote my dissertation on M-ary CDMA techniques for optical communications, have published several technical papers on CDMA in peer-reviewed journals, and my advisor was one of the pioneers of CDMA technology. So I definitely know what I am talking about here, and am probably more qualified than any other contributor to this page to assess the technical accuracy of this article (or the lack thereof, unfortunately). What I currently lack is loads of spare time to properly fix it. Until then, readers must be cautioned that the "Technical Details" section is very misleading.
Again, near-far has no effect on orthogonality! It's the very essence of what orthogonal means. Case in point, take any one of the 4 sequences S_i shown in the figure by a constant A, and then chose another sequence S_j and mutiply it by B (i = 0..3, k=0..3, i != k). When you correlate A*S_i with S_i, you get A. When you correlate B*S_k with S_k, you get B. If you correlate A*S_i with B*S_k, you get two positive A*B terms and two negative A*B terms that sum to zero. Period. This is the very essence of an orthogonal basis of functions. For instance, the FFT is an orthogonal basis of complex exponentials, and the whole point of signal processing is that you can use a linear filter to modify the weight of each spectral component indepdendently from any of the others. What destroys the orthogonality of a basis is to introduce non-linearities (like squaring) or to shift the time intervals of the components differently. As an example of a nonlinearity, it is clear that squaring any of the Walsh sequences produces the all ones sequence, destroying the orthogonality. As an example of shifting the time interval, it is evident that by cyclically rotating the third sequence to the right (move every chip over the right and carry back the right-most chip to the first (left-most) position), you obtain the second sequence, also destroying the orthogonality. In other words, linearity and orthogonality are like two sides of the same coin. To be very precise, the near-far effect makes the statistics of the MAI (and therefore, the performance) very difficult to analyze. Suppose there are 100 interferers with unity amplitude, and one extremely dominant interferer with an amplitude of 100. The 100 equal users combine to produce MAI that is approximately stationary Gaussian with zero mean and a variance of 100. Note that the tails aren't infinite though, the extreme values are +100 and -100. When you add this to a PN sequence with an amplitude of 100, you get a zero-mean process with a variance of 100^2 + 100 = 10100, but it is *not* stationary Gaussian. When a chip from the dominant sequence is positive, the values range between zero and 200. For a negative chip, the values range between -200 and zero. So you get a bi-modal distribution with one mode at +100, and the other at -100. To characterize it only by the mean and variance (which only completely describe a true stationary Gaussian distribution) is very misleading; the performance will be significantly worse than if you had 10100 users with unity amplitude. Now, suppose that the processing gain is 100, so that the modes are at +1 and -1, with extreme values of -2 and +2. In this example, the error rate is 1/4 or 25% because the dominant MAI has equal power with the signal, destructively interfering half the time (to 0), and constructively interfering half the time (to -2 or +2). For a zero value, we'll get lucky half the time, and for +/-2, the remaining interference from the 100 users isn't strong enough to flip the sign (the extrema of this process are -1 and +1), so we'll always get it right. On the otherhand, for 10100 users we get Gaussian MAI with zero mean and a variance of 10100/100^2 = 1.01. The error probability in this case is erfc(sqrt(1.01)), which is 15.5%. So clearly, the near-far effect can have a far more drastic effect on the performance than simply decreasing the SIR, which is a reasonable assumption only when the power of the dominant interfer is close to the average power.
It dosen't matter what get's transmitted, it matters what the receiver is able to understand from the message. The receiver works by a process called correlation, which is the same as the dot-product for any orthogonal basis. Simply put, if user i has sequence S_i, and user k has sequence S_k, the receiver for user i(after synchronizing to know where the symbols start, which is not trivial) mutiplies each symbol by S_i, and takes the sum. If it is positive, decide 1. if negative, decide -1. So if you transmit (A S_i + B S_k), with A and B arbitrary constants, the receiver computes the correlation which is the dot-product (A S_i + B S_k) * S_i = A ||S_i||^2 + A*B (S_i * S_k). From the dot-product properties in the article, this gives you A + 0 since S_i and S_k are orthogonal. Similarly, the receiver for user k correlates with sequence S_k, producing the dot-product (A S_i + B S_k) * S_k = A*B (S_i * S_k) + B ||S_k||^2 = 0 + B. Let's modify your example to see what I mean: I'm not sure your sequence for V is orthogonal to W, so let's pick two that we know are (from the Walsh basis): S_1 = {++++++++}, S_2 = {++++----}. Let A = 1 and B = 0.3. We transmit T = A S_1 + BS_2 = {++++++++} + 0.3{++++----} = {1.3, 1.3, 1.3, 1.3, 0.7, 0.7, 0.7, 0.7}. The receiver for V correlates by multiplying by S_1, taking the sum and dividing by the length N (N = 8), which is the same thing as the normalized dot-product of T and S_1: T * S_1 = sum({1.3, 1.3, 1.3, 1.3, 0.7, 0.7, 0.7, 0.7})/8 = (2+2+2+2)/8 = 8/8 = 1 = A. The receiver for W correlates by multiplying by S_2, taking the sum and dividing by the length, i.e., the normalized dot-product of T and S_2: T * S_2 = sum({1.3, 1.3, 1.3, 1.3, -0.7, -0.7, -0.7, -0.7})/8 = (0.6+0.6+0.6+0.6)/8 = 0.6/2 = 0.3 = B. So the reciever for V correctly determines that T includes A S_1, and the receiver for W determines that T includes B S_2, and they have no bearing on eachother whatsoever. In practice, |A| and |B| are random variables due to fading, while sign(A) and sign(B) are random from the modulation. The receiver only needs knowledge of the signature sequence S_i (and synchronization!) to recover A. The larger |A| is, the more reliably it can determine sign(A) in the presence of noise. Same for knowlege of S_k to determine B. The value of B has no bearing whatsoever on the recieved value of A, and vice-versa, which is the very essence of the orthogonality of S_i and S_k. —The preceding unsigned comment was added by 71.136.55.139 (talk • contribs) 09:11, 2006 May 4. [edit] Perspective from someone who just wants the basics, not the thesisNone of this does the topic any good. Most of the information in the article is too detailed for the Wikipedia reader. Keep the super-detailed stuff to the textbooks and standards and take your political views about various cellular technologies and standards to another forum. —The preceding unsigned comment was added by 204.181.181.9 (talk • contribs) 21:03, 2006 June 28.
I also agree. the article on TDMA is easy to read and offers a good insight to the average reader. This article should try and mirror the TDMA article. --Tanner Waldo 2007 --142.59.116.235 22:39, 19 February 2007 (UTC)
–Perhaps more text introducing the basic elements of CDMA (a sentence or two explaining how the pseudo-random code is used). I'd like to see several more paragraphs on the basics, in lay terms, before getting into the details (and yes, it seems a great deal should be eliminated or reference to another location). I'm a 40 yr. old engineer (on satellites, I work indirectly with network & comm), I'm not afraid of technical jargon, but most of these terms need to be defined or further explained within the description. I currently don't follow the text either, so don't feel bad 17 yr old senior! 64.122.203.32 18:05, 2 May 2007 (UTC)LM [edit] A little bit of historical perspectiveWhile many people give actress Hedy Lamar credit for having "invented" CDMA during WWII, there was no digital transmission taking place, unless you count Morse code as a digital method. During WWII the Allied forces did use a code system which would transmit dashes on one frequency, and dots on another in an attempt to fool their enemies. My understanding is that the first military use of digital radio transmissions took place during the Cuban Missile crisis in 1962. Hedy's patent for a frequency agile transmission scheme, designed to avoid frequency jamming on torpedoes was certainly a breakthrough concept at the time, but as far as I know, it was never put into use during WWII. 03:45, 1 July 2006 (UTC) —The preceding unsigned comment was added by 70.20.208.13 (talk • contribs) 03:45, 2006 July 1. [edit] See "Re: comment" above
It dosen't matter what get's transmitted, it matters what the receiver is able to understand from the message. The receiver works by a process called correlation, which is the same as the dot-product for any orthogonal basis. Simply put, if user i has sequence S_i, and user k has sequence S_k, the receiver for user i(after synchronizing to know where the symbols start, which is not trivial) mutiplies each symbol by S_i, and takes the sum. If it is positive, decide 1. if negative, decide -1. So if you transmit (A S_i + B S_k), with A and B arbitrary constants, the receiver computes the correlation which is the dot-product (A S_i + B S_k) * S_i = A ||S_i||^2 + A*B (S_i * S_k). From the dot-product properties in the article, this gives you A + 0 since S_i and S_k are orthogonal. Similarly, the receiver for user k correlates with sequence S_k, producing the dot-product (A S_i + B S_k) * S_k = A*B (S_i * S_k) + B ||S_k||^2 = 0 + B. Let's modify your example to see what I mean: I'm not sure your sequence for V is orthogonal to W, so let's pick two that we know are (from the Walsh basis): S_1 = {++++++++}, S_2 = {++++----}. Let A = 1 and B = 0.3. We transmit T = A S_1 + BS_2 = {++++++++} + 0.3{++++----} = {1.3, 1.3, 1.3, 1.3, 0.7, 0.7, 0.7, 0.7}. The receiver for V correlates by multiplying by S_1, taking the sum and dividing by the length N (N = 8), which is the same thing as the normalized dot-product of T and S_1: T * S_1 = sum({1.3, 1.3, 1.3, 1.3, 0.7, 0.7, 0.7, 0.7})/8 = (2+2+2+2)/8 = 8/8 = 1 = A. The receiver for W correlates by multiplying by S_2, taking the sum and dividing by the length, i.e., the normalized dot-product of T and S_2: T * S_2 = sum({1.3, 1.3, 1.3, 1.3, -0.7, -0.7, -0.7, -0.7})/8 = (0.6+0.6+0.6+0.6)/8 = 0.6/2 = 0.3 = B. So the reciever for V correctly determines that T includes A S_1, and the receive" what the hell is this, i didn't come here to learn what "correlates", either simplifiy it or i will, manually. by cutting up this article. —The preceding unsigned comment was added by 71.160.54.34 (talk • contribs) 22:56, 2006 July 14. [edit] why no sims?Can someone with some knowledge on this subject explain why SIM cards arent used on CDMA phones? thanks 12.170.1.226 16:10, 10 August 2006 (UTC)LUID Please its kind of confusing. Can anyone please explain this line to me how did he get the transmitted vector if v=(1,-1), then the binary vector (1, 0, 1, 1) would correspond to (1,-1,-1,1,1,-1,1,-1) —The preceding unsigned comment was added by HassanHaider (talk • contribs) 17:31, 2006 August 19. [edit] Copy editing to improve clarityAfter reading the article I could see why it had the "needs more clarity" tag on it. The greatest source of confusion seemed to be that it said 'CDMA' in many places where it actually meant 'CDM' (that is, Synchronous code division using Walsh vectors). What seemed to be needed was a better distinction between the 'CDM' Synchronous technique using Walsh vectors, and the 'CDMA' Asynchronous technique using pseudo-noise sequences as the vectors. I also made some minor edits to header size and added a couple headers to mark the start of the Asynchronous discussion and the comparison of Asynchronous against the other techniques. Still not 'perfect' but hopefully better. Randy549 05:18, 17 September 2006 (UTC) [edit] Use of talk pagesThe discussion above is hard to follow since many editors have not signed their comments using 4 tildes, ~~~~, at the end of their contributions. Also, please separate your comments from previous ones by indenting using ":", not tab or space or ---, at the beginning of each paragraph. Please see WP:TALK for more on using talk pages. Walter Siegmund (talk) 21:17, 9 July 2006 (UTC) [edit] What?I came here to learn what CDMA is, I read the whole page, and I still have no idea. I'm a fairly smart person but not a rocket scientist.--> —The preceding unsigned comment was added by 4.228.180.173 (talk • contribs) 08:07, 4 December 2006 (UTC). Agreed. This article is really poorly explained. [edit] (See also the Market situation section of GSM.)The word Market doesn't appear in this article Global_System_for_Mobile_Communications. This article should list which cell phone operators use CDMA technology Mathiastck 23:08, 5 December 2006 (UTC) [edit] QUALCOMM IN ALL CAPSThe word "QUALCOMM" should always be in call caps. It is part of the trade mark or something so that the two double "M" will cause look like a radio wave or something. --Methgon (talk • contribs) 6:24, 20 December 2006 (UTC)
by say do you mean write?
[edit] CDMA trivia sectionWho removed the CDMA trivia section listing Kevin Kelley and Harvey White as both having vanity plates reading CDMA? It is a hilarious piece of trivia that needs not be removed. --Methgon (talk • contribs) 6:14, 20 December 2006 (UTC)
[edit] chip code?In this diff] called "rewrite" by User:Dysprosia, the terminology chip code, which previously had been used once by an anonymous editor, was used extensively all over the article. This is not a familiar term to me, and I've worked in this field, and I can't find any source for it. I recommend we clean it out and go back to using chip and code in more standard ways, as in a 4-chip code meaning a code that is 4 bits (chips) long. Dicklyon 16:39, 24 February 2007 (UTC)
[edit] W-EDGE?Cannot find reference to W-EDGE in WP... ? "W-EDGE has emerged as the de facto standard for new handsets operating across multiple regions by supporting W-CDMA, EDGE and legacy GSM and GPRS modes". Says a market research analyst so it sounds important enough to get mentioned here. Not easy to find with disam with 'wedge; etc let alone EDGE and 'edge'. Why can't they use unique abrevs like WCDMA etc? Cheers. 81.86.144.210 18:30, 15 March 2007 (UTC) 15 March 07 Why no sim on cdma phones? [edit] WE ARE NOT EEs (but some of us are)How come you need 4 years of colledge math to understand this? Why bother even writing the "math language" definition if nobody can understand it. WP is not a textbook. So how I think this article is a copy vio of some textbook. Why can we write this as a flow chart, or a series of steps, without any vectors, dot products, and voodoo math. A average wikipedian coming here for a answer, IS NOT A ELECTRICAL ENGINEER. As far as I understand this, the multiplexing is simply 2 radio talking ontop of each other, with a unique number added to each radio's output, so the receiver can simply subtract the unique number and get the original value. I dont know exactly how this number is picked, or how it is subtracted. Also the orthagonal makes no sense, how can you encode information in the OPPOSITE of a signal, something can only have 1 opposite right? Patcat88 05:46, 30 March 2007 (UTC) -- I totally agree. This article gave me absolutely nothing. I came to get a rundown on basic CDMA, the speeds it offers and if/how I can use Bold text
But how are we (non EEs) to know what's relevant and what's not? —Preceding unsigned comment added by 205.219.133.241 (talk) 03:06, 12 June 2008 (UTC) [edit] Possible reorganization of this articleHere's my views on how to restructure it. Might try it in a few weeks if there is any agreement: 1. CDMA is a basic technology for sharing spectrum among multiple users, like TDMA and FDMA. 2. CDMA is based on spread spectrum. 3. Practical CDMA is not perfectly orthogonal so there are near/far problems that have to be considered. Thus satellite uplinks, where near/far are minimal, were among the first practical applications. 4.CDMA technology is used in cellular telephone systems and thus "CDMA" sometimes refers to IS-95 and sometimes refers to the other cellular technologies in the 3G family. I don't think the current equations add much to the discussion. It confuses the math-phobic and doesn't enlighten many others. - Mjmarcus 20:38, 18 September 2007 (UTC)
[edit] This article confuses the CDMA vs. GSM discussion and the CDMA vs. TDMA vs. FDMA discussionWikipedia needs one class of articles that explain different xDMA principles. These are basic ideas how I share an ether between different communication channels. They do not only apply to air, but also to communication over wires. Here we have 3 possibilities:
And then we have the whole discussion about mobile standards. GSM uses code devision multiple access (called W-CDMA) by itself for its 3G standard. It would be helpful if that would be explained clearly in separate articles.--68.6.44.232 06:51, 22 September 2007 (UTC) [edit] What audio codec is used?Which codec and at what bitrate is CDMA sending audio? IMHO it would be worthwhile to add that to the article. - Theaveng 19:38, 24 September 2007 (UTC) [edit] CDMA and IS-95As far as I can tell, there are two distinct concepts here: CDMA and Qualcomm's IS-95 a.k.a. "CDMA" (I'm not sure if IS-95 and "CDMA" are effectively the same thing). The current way they're laid out makes the article very hard to read, and anything over a couple paragraphs about a proprietary trademark/"technology"/etc should be moved to its own article. —Preceding unsigned comment added by Elektron (talk • contribs) 14:34, 5 December 2007 (UTC)
[edit] Excellent 2nd paragraphThanks for the 2nd paragraph which explains the concepts very clearly. Todd (talk) 01:19, 16 December 2007 (UTC) [edit] CDMA is a "Channel Access Method", and is separate from cdmaOne (IS-95) which is a "Mobile Phone Standard"Hi guys! The heading pretty much sums my post up : ) I agree with "Towel401", in the post above. Therefore I think it would be unwise to merge the present article with cdmaOne. I have tried to clear up the present article to remove information relating specifically to cdmaOne, and hopefully clarify the distinction. InternetMeme (talk) 10:32, 9 January 2008 (UTC) Well, since nobody seems to disagree, I have removed the "merge" tag, using the following rationale: The TDMA channel access method is used by the GSM mobile phone standard, and yet those two articles haven't been merged. Therefore the CDMA channel access method shouldn't be merged with the cdmaOne mobile phone standard. InternetMeme (talk) 10:32, 9 January 2008 (UTC)
[edit] How do a receiver synchronize to a transmitter (or how does it know which code should it use)I'd wish to know how do the receiver and transmitter start the communication from the practical point of view. Supposing we have the base and several mobiles turned off and then we start to switch them on in the random order... 62.89.89.140 (talk) 08:58, 16 January 2008 (UTC) [edit] Cross-correlation wrongA resultant signal is erferred to in the article as cross-correlation. It is wrong. Poppafuze (talk) 15:30, 3 June 2008 (UTC)
[edit] yet another meaning of CDMACurrently this article describes CDMA as built using direct spread spectrum hardware. I've seen a few people claim that CDMA can be built using frequency-hopping spread spectrum hardware[2]. These people re-define CDMA as also including "CDMA (Code-division multiple access) operation, where several cooperating transmitters using different frequency hopping patterns can transmit in the same frequency range without disturbing each other". Is there a better name for this kind of cooperating FHSS system than "CDMA"? --68.0.124.33 (talk) 15:12, 25 July 2008 (UTC)
1 2 9 1 9 2 ... 1 9 2 1MHz 3MHz 5MHz 7MHz 9MHz 12MHz ... 30.01GHz 30.03GHz 30.05GHz
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