Talk:Code-division multiple access/Archive 1

Archive 1

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."

CDMA 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.

CDMA as a cellular technology didn't come out until the 1990s. GPS, designed in the 1970s, is not a communication system but it also uses CDMA and direct sequence spread spectrum to permit the receivers to distinguish among satellites even though they all transmit simultaneously on the same channel. Karn (talk) 08:40, 16 September 2008 (UTC)

The concept of spread spectrum as it is understood today, originally invented by an American, Heady LaMarr (spelling), was never used in WW II.

As far as I know this is correct but I'd leave it up to a WW II technology historian. Karn (talk) 08:40, 16 September 2008 (UTC)

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)

The CDMA was analysed in the late 1970 as technology for even Arpanet.

No, it was for the experimental PRNet -- Packet Radio network. It was also funded by DARPA but had nothing to do with the ARPANET, a packet switched network using fixed computers and point-to-point links. An early motivation for what became TCP/IP was to interconnect the ARPANET with the PRNet. Karn (talk) 08:40, 16 September 2008 (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.

Yes, Qualcomm has supresses a slight error in their math equation. As long as there are few users, CDMA is a good technology - but once the air gets crowded, the O-residual that they claim is "insignificant" just shows when they fell asleep during Calulus in high school. —The preceding unsigned comment was added by Khflottorp (talk • contribs) 12:43, 9 July 2006.

The above two paragraphs are utter nonsense. Every multiple access system has a capacity limit. That capacity is very well defined with FDMA and TDMA; assign all the frequency or time slots and there aren't any more. The CDMA capacity limit is less well defined but, I emphasize, considerably higher than that of either FDMA or TDMA when used in a cellular telephone network with frequency reuse. Part of this comes from voice activity detection, transmitting only when you have something to say, reducing average interference levels when it would be impractical to reallocate time or frequency slots so rapidly. Most comes from the inherent interference resistance that allows full reuse of frequency channels by all cells instead of allowing only a subset of channels to be used at each cell site to protect its neighbors. The full derivation of the system capacity of CDMA is in . Disclaimer: I work for Qualcomm. I dare say we've been pretty successful with CDMA despite having fallen asleep in high school calculus. But don't believe me. Check it all out for yourself. Karn (talk) 08:40, 16 September 2008 (UTC)

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? — Preceding unsigned comment added by 64.114.27.59 (talk • contribs) 12:23, 16 January 2007 (UTC)

TDMA/GSM are DIGITAL cellular/PCS technologies – very poor CDMA Comparisons

It'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)

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)

Dot Product Problems

It's been a while since I've done it, but I'm fairly confident the dot products on this page are incorrect. — Preceding unsigned comment added by 70.144.11.71 (talk • contribs) 13:47, 7 December 2004 (UTC)

Why are there dot product and orthogonality explanations on this page? Just link to the other wikipedia pages that already do a better job of explaining them.

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.

• • The dot products are correct in that they are normalized dot products (i.e. divide the result of the dot product by the number of terms in the respective vectors). I edited the description to reflect this, as it confused me a lot when I was reading it as well.

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.

Two meaning of CDMA

The article does a good deal of explaining the two meaning of the initials. See this post for more info [1]

Vector components

Agh. Yes, it was a mistake, and the statement was wrong anyway. I've fixed it. I don't know what I was thinking. Dysprosia 14:04, 10 July 2005 (UTC)

Power control and more

The 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?

With respect to the first problem, this is dependent on whether the receiver is detecting for sign or for value. For sign, it isn't a problem, but for value, it is. In implementation, I don't know what is more common/used. If you do know, please be bold, go ahead and rectify the article directly. It is a wiki after all.

The article says that the amount of CDMA codes are infinite in that, for example, if there are two senders, one can use chip codes of "length" 2, if there are up to four senders, one can use chip codes of "length" 4, and so on, so theoretically there is no hard protocol-imposed limit to how many senders can use the system, but practically there may need to be caps placed on the number of senders.

I'll try and add a clarification here and there in regards to these matters, and the article would probably be served by a few more passes over it -- but again, if you see problems, go ahead and edit. Dysprosia 06:53, 11 July 2005 (UTC)

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!

Complete your PhD - and then get cracking! But beware the readers of Wikipedia are many-fold: Some require high-level abstracts that easy to read, others need toi reference parts to allow them to understand the text books they read. Just beware:"interference generated by each user" is not a uniform distribution - best a poission - probably an expotential since they all use the same frequency - making your assumptions a dangerous one. —The preceding unsigned comment was added by Khflottorp (talk • contribs) 12:43, 2006 July 9.

comment

Comment 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.

I came to this article looking for a reasonably concise and understandable explanation of CDMA after reading an article in the Wall Street Journal. I was impressed. While I understand the concerns of the editor (above), the article satisfied my needs and I commend those who have worked on it and who have tried to improve it. Walter Siegmund (talk) 20:02, 27 February 2006 (UTC)

Comparision of CDM and CDMA is not satisfactory

CDM/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.

Re: comment

Again, 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.

You have also, however, removed rather pertinent details that need not have been removed. Dysprosia 01:08, 4 May 2006 (UTC)

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.

If by near-far you mean that the statement that differing signal strengths can disrupt orthogonality, I don't believe you are correct. Furthermore, I don't understand what you mean by "correlate". However, let me try and explain what is meant by using the example in the article. If we model signal fading by multiplication by a constant less than 1, suppose that v = (1,-1,-1,1,1,-1,1,-1) is transmitted by one sender V with chip code (1,-1), and w = (-1,-1,-1,-1,1,1,1,1) is sent by the other sender W with chip code (1,1). Suppose that V transmits less strongly than W, so we end up with 0.3v and w "adding up in the air" to get (-0.7, -1.3, -1.3, -0.7, 1.3, 0.7, 1.3, 0.7). Take the dot product of (-0.7, -1.3) and (1,-1) and we get 0.6 -- not 2 as expected. So something has gone wrong. If we take the dot product of (-0.7, -1.3) and (1,1), however, we get -2 as expected, so W is swamping V.

This is all according according to the mathematical model. Dysprosia 07:02, 4 May 2006 (UTC)

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.

Perspective from someone who just wants the basics, not the thesis

None 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 agree. I was lost after 2 paragrgaphs, although im only 17 doing senior high school physics. It would be nice if you could please explain some of the jargon (if not all) that is used. Also providing a summary in simple language to give an overview for the people that dont want the specifics would be nice.

Its ok to have the detailed version aswell, but have this simple stuff in there as well.

I came here looking for "the frequency of CDMA" as pertaining in my assessment questions but could not find the answer in the article. --202.139.23.75 07:51, 26 July 2006 (UTC) Nick

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)

I rewrote the lead a bit to make it less confusing. Let me know if this helps, and what other parts you'd like to see work on. Dicklyon 23:02, 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

A little bit of historical perspective

While 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.

See "Re: comment" above

If by near-far you mean that the statement that differing signal strengths can disrupt orthogonality, I don't believe you are correct. Furthermore, I don't understand what you mean by "correlate". However, let me try and explain what is meant by using the example in the article. If we model signal fading by multiplication by a constant less than 1, suppose that v = (1,-1,-1,1,1,-1,1,-1) is transmitted by one sender V with chip code (1,-1), and w = (-1,-1,-1,-1,1,1,1,1) is sent by the other sender W with chip code (1,1). Suppose that V transmits less strongly than W, so we end up with 0.3v and w "adding up in the air" to get (-0.7, -1.3, -1.3, -0.7, 1.3, 0.7, 1.3, 0.7). Take the dot product of (-0.7, -1.3) and (1,-1) and we get 0.6 -- not 2 as expected. So something has gone wrong. If we take the dot product of (-0.7, -1.3) and (1,1), however, we get -2 as expected, so W is swamping V. This is all according according to the mathematical model. Dysprosia 07:02, 4 May 2006 (UTC)

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.

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.

Copy editing to improve clarity

After 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)

Use of talk pages

The 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)

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.

Same here. Simplify it a bit, for crying out loud! Leave all the tech stuff where it is, but introduce something for us lamers. :P 91.150.117.216 (talk) 19:11, 9 October 2009 (UTC)

(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)

QUALCOMM IN ALL CAPS

The word "QUALCOMM" should always be in all 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)

It is conventional to just say Qualcomm when referring to the company. There's no need for us to try to copy their trademark in referring to them. Do a Google search and you'll that most writers outside of Qualcomm don't try to copy their trademark when referring to the company, as with most companies. Dicklyon 23:04, 19 February 2007 (UTC)

by say do you mean write?

Their legal page has "Qualcomm" as the trademark and "Qualcomm Incorporated" as the company name, and the usual name used on site navigation is "Qualcomm" (even though the bottom of the page says it's © QUALCOMM Incorporated). There's no reason why we should change the page to fit Qualcomm's branding du jour. ⇌Elektron 14:41, 5 December 2007 (UTC)

CDMA trivia section

Who 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)

I've removed it again. Wikipedia has a very high standard when it comes to including personal details about living people. The facts must be verifiable and significant enough to be included in an article. I don't believe revealing license plate details qualifies. See WP:OR and WP:BLP. JonHarder ^talk 12:48, 20 December 2006 (UTC)

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)

It may also be called a "chipping code". That's all I'll have to say on the matter, I hope that helps. Dysprosia 00:35, 25 February 2007 (UTC)

That does appear to be more common. I'll change chip code to chipping code and/or just code. Dicklyon 02:59, 25 February 2007 (UTC)

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?

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

I skipped over the math, and just read the plain English. It's perfectly clear how CDMA works (the example of many people talking using different languages). HOWEVER I'm glad the math is there, in case I would want or need to look it up. (I am an EE.) IMHO it's better that the reader be presented ALL of the information, so he can self-select which sections he wants to review, rather than have the article be too simplistic & therefore not useful to people who might need the Math.

I don't need the math right now, but maybe tomorrow I will need it. I would be quite annoyed to see it's gone. - Theaveng 19:36, 24 September 2007 (UTC)

I wish i could understand it, but I can't. Maybe this is just how complicated CDMA is. TDMA is relatively easy to understand, but all this technical stuff should be kept, maybe move it towards the end Towel401 (talk) —Preceding comment was added at 23:00, 8 December 2007 (UTC)

-Why bother even writing the "math language" definition if nobody can understand it.

Somebody will understand it, and a lot more people will want to try to understand it. Over time, many will even add/improve the technical treatment of this article.

-WP is not a textbook.

Yes, Wikipedia is an ENCYCLOPEDIA, which should explain in more depth than just a textbook.

-This article gave me absolutely nothing. I came to get a rundown on basic CDMA, the speeds it offers and ...

CDMA does not formally offer any "speeds", as you can build a real slow system, or a super high-speed system, both using the CDMA technique. CDMA actually is a METHOD (or technique) to enable multiple transmitters/receivers (transceivers) to be able to communicate simultaneously (Multiple Access) using something called Code Division to be able to properly distinguish between the different transceivers via different "codes". TDMA or FDMA uses similar methods, but instead their "division" technique is using Time, and Frequency, rather than the Code, to distinguish between different transceivers. 121.120.184.46 (talk) 05:28, 14 March 2009 (UTC)daniel.kho

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)

Just understand what you can, and leave what you can't understand to another day, or just don't try to understand it if you're not interested or in the field. Just as we EEs (I am an EE too) aren't really that interested in microbiology for example, we'll just read an interesting article about it, and don't really bother about the details. We tend to respect microbiologists more, and appreciate that they're doing a good job for the community. Similarly, if you can't understand the article (I don't follow the whole thing either), just appreciate that CDMA is a good technology for the benefit of everyone, and appreciate what EEs do. 121.120.184.46 (talk) 05:28, 14 March 2009 (UTC)daniel.kho

Possible reorganization of this article

Here'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)

I disagree. When I was reading the article I skipped over the math, and just read the plain English. It's perfectly clear how CDMA works (the example of many people talking using different languages). HOWEVER I'm glad the math is there, in case I would want or need to look it up. (I am an EE.) IMHO it's better that the reader be presented ALL of the information, so he can self-select which sections he wants to review (or skip), rather than have the article be too simplistic & therefore not useful to people who might need the Math.

I don't need the math right now, but maybe tomorrow I will need it. I would be quite annoyed to see it's gone. - Theaveng 19:39, 24 September 2007 (UTC)

I agree with all 4 points here. The article is a total mess, not really differentiating between CDMA as a multiple access method and CDMA as a phone technology. Luckly for me I knew what CDMA was and how it worked before reading this article, I don't think i would of been any wiser if I did not have the previous knowlage. I think the article needs any references to individual cellular technologies removing, it should be compared to other multiple access methods. A disambiguation page may be necessary as users will search for CDMA looking for the is-95/2000 articles. A section on technologies using CDMA could be used to link to the cellular systems etc. that use CDMA.

For the maths I think that the plain english description should come first. Orthogonality is an easy concept to understand using bit patterns, e.g. 11111111, 11110000, 11001100, and 10101010 are all orthogonal to each other. The maths does not demonstrate this simple concept easily. The maths should be there, possibly after the description or in a section all of its own. Simon alfie 16:14, 26 October 2007 (UTC)

Most readers aren't EEs. This article should be fully comprehensible to non-specialists. An encyclopedia does not fill the same function as a professional textbook. I strongly agree that accessible narrative that describes this topic to the informed layperson is the correct level of writing. Heavy math should be moved toward the end, into an appendix or maybe even a link to an external source. NuclearWinner (talk) 17:29, 5 March 2008 (UTC)

This article confuses the CDMA vs. GSM discussion and the CDMA vs. TDMA vs. FDMA discussion

Wikipedia 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:

• devide the channels by frequency (FDMA)
• devide them by time (TDMA)
• devide them by code (CDMA)

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)

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)

As mentioned earlier, CDMA does not provide formally for any bitrate (or speed). As far as codecs are concerned, you can think of CDMA being another type of codec, where the transmitter would multiplex (or encode) the outgoing serial bitstream, and the receiver would demultiplex (or decode) the incoming serial bitstream. 121.120.249.70 (talk) 17:53, 14 March 2009 (UTC)daniel.kho

CDMA and IS-95

As 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)

Also, "CDMA"/IS-95 appears to be officially "cdmaOne" now. It really shouldn't be mentioned in this article. —Preceding unsigned comment added by Elektron (talk • contribs) 14:42, 5 December 2007 (UTC)

This article should cover only the multiplexing scheme, and nothing about mobile phone operators. Towel401 (talk) 22:58, 8 December 2007 (UTC)

Yup. It seems to do so now, although it's appropriate at the top to say "if you're looking for the "CDMA as opposed to GSM", try these pages" (as it does) and to mention cdmaONE, CDMA2000, and W-CDMA, along with GPS, etc., in the "Uses" section. Guy Harris (talk) 07:11, 6 October 2011 (UTC)

Excellent 2nd paragraph

Thanks for the 2nd paragraph which explains the concepts very clearly. Todd (talk) 01:19, 16 December 2007 (UTC)

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)

Hi. Well, although CDMA seems to be a popular name for IS-95/cdmaONE, it's actually a channel access method, and the merger is not a good idea as you say above. On the other hand, just to clear some things out, GSM and cdmaONE DON'T rely on a single channel acces method; GSM combines FDMA, TDMA and frequency hopping to further insure logical channel separation, and cdmaONE probably does the same with FDMA and CDMA (not so sure about cdmaONE anyway; I'm European, you know!). Thank you for your attention. 62.210.194.58 (talk) 19:30, 3 June 2008 (UTC)

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)

Cross-correlation wrong

A resultant signal is erferred to in the article as cross-correlation. It is wrong. Poppafuze (talk) 15:30, 3 June 2008 (UTC)

Eh, well, actually it's the product of the received signal with the user code, or so I understood. Could you please help us correct this error? I'm translating this article into es:wiki and would love to tell my readers nothing but the truth ;). And by the way, thanks for pointing out the error, but probably next time it's better to, eh, correct it yourself? Thanks for your cooperation anyway, 62.210.194.58 (talk) 19:32, 3 June 2008 (UTC)

It may not be particularly well written, but it's correct. The cross-correlation of the user's code with another code results in a low, noise-like signal. The cross-correlation of the user's code with itself (auto-correlation) results in a low, noise-like signal, except when they are time-aligned. Oli Filth^(talk) 19:37, 3 June 2008 (UTC)

yet another meaning of CDMA

Currently 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)

Unfortunately, that app note doesn't make it at all clear how they see CDMA and FHSS interacting. However, perhaps they're referring to MC-CDMA. Oli Filth^{(talk|contribs)} 19:59, 28 July 2008 (UTC)

What the appnote basically means in lay terms is that it uses FHSS to divide down the "code" for a CDMA system. Simply put, any CDMA system specifies a unique "code" for each unique user, and this type of code division can be done using spread-spectrum methods like DSSS and FHSS too. For FHSS, there will be a state machine with a pseudorandom number (PN) sequence generator (a.k.a. pseudorandom bit sequence - PRBS) that "decides" which frequency the transmitter should switch (or hop) to next. The receiver would have a similar state machine for synchronisation with the transmitter. The number generated by the PN sequence generator in any point in time (or any hop interval), can be used to determine the unique "code" allocated to any particular user.

For the case of this appnote, it is a little more complicated in the sense that not only one number is used for any one user, but a set of numbers (or a set of individual narrowband frequencies) is used to specify a unique combination for a particular user. A user will have a set of allowable frequencies it can hop about, which will never interfere with another user's set of allowable frequencies. This means that a set of frequencies of a particular user will be mutually-exclusive to every other user's sets of frequencies, in other words (sorry if I'm being a little technical here), the cross-correlation of each set of frequencies with one another will be zero. This simply means that no two sets of frequencies will be able to "correlate" with each other, or to put it even more simply, no two sets of frequencies will be able to find any similarity whatsoever with each other.

This does not mean allocating a frequency range to a user, but rather, allocating a set of allowable narrowband frequencies to a user. Note that a user's set of frequencies may overlap another user's set of frequencies within the same frequency band, however, each individual allowable narrowband frequency components within a user's set of frequencies do not overlap with another user's individual frequency components. Also note that every user in a CDMA system shares the same bandwidth with everybody else in the same network. Hence, there should only be one frequency band in a CDMA system, which has to be wideband in this case, to be able to support so many narrowband frequency components from each user. For example, a user can be allocated a set of hoppable frequency components (11MHz, 33MHz, 55MHz, and 77MHz), while another user may be allocated the set (13MHz, 37MHz, 57MHz, and 79MHz). Note that each individual elements in the set are actually narrowband frequency components of probably 1-MHz bandwidth or narrower, but both the users are within the same band of frequencies (10MHz to 80MHz). For a real FHSS-CDMA system, the bandwidth would be much larger than this, and a user would probably have more narrowband frequencies to hop about. This can better be explained graphically, so I'm hoping someone to be able to draw it and post it here. :)

Please note that this is just one way to implement CDMA by using FHSS; there can be numerous other "ways" to define a protocol that is also CDMA technology. Note that CDMA is a scheme, not a standard, which means it is a method to transmit and receive information, but it does not define any particular protocol, i.e. you can use other suitable spread-spectrum (DS, FH, FFH, SFH, etc.) or digital modulation techniques to implement a CDMA system. This means you would have to define your own protocol if you were to implement a CDMA system. Qualcomm's IS-95 is just one particular implementation of CDMA (I'd like to call it Qualcomm's standard, or IS-95 standard, rather than CDMA). If you're smart enough, you could even come up with your own. :) —Preceding unsigned comment added by Daniel.kho (talk • contribs) 05:41, 5 August 2008 (UTC)

If the appnote does indeed imply that each user has a dedicated set of carriers from which to choose, this would be incredibly spectrally inefficient, and would render the "code-division" aspect pointless. The only benefit of the hopping would be the mitigation of frequency-selective fading.

What would be more likely is that at any one time, each set of users has a dedicated set of carriers between them, and they all hop within that set. However, this would still render "code-division" pointless, unless there was a chance that during any one hop period, more than one user could transmit on the same carrier frequency. Oli Filth^{(talk|contribs)} 12:14, 5 August 2008 (UTC)

The appnote didn't imply that each user has a dedicated set of carriers, in fact, there is no mention of any specific carrier frequency dedicated for each user. This is why CDMA is wideband. The frequency range for all users can be very large. Being wideband can mean having many (and I mean real many) individual narrowband frequency components combined together to form the whole range of frequencies for the CDMA system.

As mentioned in the example earlier, for a frequency-hopping spread-spectrum (FHSS) system, one user's frequency band will be spread across the whole range of the CDMA system, as with all other users in the system. However, one user's individual hoppable frequency components will differ from any other user's individual hoppable frequency components, even though all of the users' frequencies will be spread evenly across the whole CDMA wideband spectrum. If I had the time, I would draw a diagram and post it here. But for now, please bear my rough sketch.

   1    2     9     1     9      2    ...   1           9           2
 1MHz  3MHz  5MHz  7MHz  9MHz  12MHz  ...  30.01GHz   30.03GHz   30.05GHz  

Note that the frequencies (3MHz, 5MHz, ... , 30.05GHz) are all narrowband individual frequency components combined together to form a wideband CDMA system.

There are 2 lines in the sketch. The first line, is the user's identifier (1 means User1, 2 means User2, etc.). These numbers are pseudorandomly chosen by a PRBS generator in both the transmitter and the receiver. I'm sorry I just wrote those numbers by hand, without calculating the autocorrelation of a user with itself, and the cross-correlation between different users.

For a real system, to make sure that you select the users fairly and to provide security, the autocorrelation of the output of the PRBS generator has to be verified to provide the required level of randomness based on the number of users connected at any one time. This means that all the users will be uncorrelated with one another. For the example above, I have 3 users connected, and the probability that one of the 3 will be able to transmit and receive information at any particular point in time, is the same as the other 2 users.

Anyway Oli, you brought up a good point. It is possible also that we can have more than one user transmit on the same narrowband carrier with all other users for a CDMA system, but only one user can be allowed to transmit on (or hop to) that frequency at any one time. I could have drawn the sketch above with all users allowed to hop to any narrowband frequency, but only one user is allowed to hop to any particular frequency at any one time.

This again can be decided by a PRBS generator + state machine, where it would decide whether or not a user is allowed to hop to a frequency at any particular time. Daniel.kho (talk) 07:42, 9 August 2008 (UTC)

Whilst I see we agree that it wouldn't make sense for each user to have a dedicated set of carriers (by that, I meant "frequency components"), I'm not sure the explanation you've written above makes complete sense either! If each user is broadcasting on a different set of components at any given point in time, then the cross-correlation of their signal with other users will be zero (or nearly zero, depending on the relationship between sub-carrier spacing and symbol rate). It doesn't matter what the hopping sequence is; indeed even with a static allocation, there will be zero cross-correlation, it's simply FDM.

Personally, I really wouldn't call this "CDMA" at all, it's just frequency-hopping spread spectrum (otherwise, GSM would be classed as CDMA!). "Code-division" implies that the codes themselves are the basis functions providing orthogonality. Of course, that's not the case, it's really the frequency-division that is providing the multiple access. Oli Filth^{(talk|contribs)} 11:39, 9 August 2008 (UTC)

Verizon rip

I am speaking as a Verizon employee - the "people in a room" analogy came from Verizon material. Just saying.

Heh. 'Tis a nice analogy though. Is it somehow trademarked/copyrighted? Andipi (talk) 10:27, 4 June 2009 (UTC)

Advantages of Synchronous CDMA

Why is there a section on asynch advantages but not for the synchronous case? I'm afraid I don't understand what the advantage of using CDMA (as opposed to, say FDMA) are - it wouldn't appear to be a more efficient use of bandwidth, since (as in the example) it would appear that for a system with four users, four times as much data would need to be transmitted (data rate multiplied by length of code). Can anyone clear this up? Andipi (talk) 10:27, 4 June 2009 (UTC)

Broken reference

I think fourth reference ""CDMA Spectrum". Retrieved 2008-04-29." has moved. I didn't find it on site linked. Alfaisanomega (talk) 13:28, 6 January 2010 (UTC)

pabitra:cdma"code devision maltiple access" —Preceding unsigned comment added by 117.197.234.148 (talk) 10:54, 4 February 2010 (UTC)

Broken external link

The external link at the end of the article "3G Radio Network Planning - Managing Cell Breathing" (to macltd.com) is broken. Anyone have the correct intended link? Youblend2 (talk) 16:12, 16 August 2010 (UTC)

With respect to the article, if the length of the chip sequence of the CDMA is doubled , what impact will it have on the bandwidth of the transmitted signal ?

Because, the paragraph on "Steps in CDMA Modulation" does not say anything about the variation in chip sequences and is misleading ...

Thanks — Preceding unsigned comment added by Upendwar (talk • contribs) 06:10, 12 February 2011 (UTC)

With respect to the article, if the length of the chip sequence of the CDMA is doubled , what impact will it have on the bandwidth of the transmitted signal ?

Because, the paragraph on "Steps in CDMA Modulation" does not say anything about the variation in chip sequences and is misleading ...

Thanks

the length of the chip sequence

With respect to the article, if the length of the chip sequence of the CDMA is doubled , what impact will it have on the bandwidth of the transmitted signal ?

Because, the paragraph on "Steps in CDMA Modulation" does not say anything about the variation in chip sequences and is misleading ...

Thanks — Preceding unsigned comment added by Upendwar (talk • contribs) 06:19, 12 February 2011 (UTC)

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