CDMA FAQ: Technical and Operational Questions.

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CDMA FAQ


Is there any danger associated with using a CDMA phone?
Short answer: Nothing extraordinary, if you use common sense.
Long answer: It is not recommended that you try to swallow it. (Sorry, small joke.)
Don't try to take apart either NiMH or LiIon batteries, because the contents are volatile and poisonous. Definitely don't try to charge either kind of battery by directly connecting them to a "dumb" current source (i.e. a simple power supply) without the phone being involved. This is particularly a problem with NiMH batteries, because they can overheat and become a fire or explosion hazard. Unlike NiCad or Lead Acid batteries, charging of LiIon and NiMH batteries is a science and is more complicated than simply back-flowing current.
Using a phone while you are driving can distract you from the road, which can increase your chance of a traffic accident. Particularly, dialing a phone or looking up a phone number in the phone book can be a problem, since that usually requires your eyes. Answering a call and talking is usually less of a problem since many phones permit "any key answer". But a car with a manual transmission requires both hands to drive safely, so even using a phone in such a car can be a hazard. A hands-free carkit will largely eliminate this problem.

Is it dangerous to leave my phone connected to a charger?
Short answer: No.
Long answer: The external power source simply supplies current to the phone, and the microprocessor inside the phone supervises the charging process. When the battery is fully charged, the phone will stop charging its battery even if it continues to be connected to the charger. (While it remains connected to the charger, it won't draw power from the battery, however. So it can operate indefinitely when connected to the charger.)
I've read about cases which decrease the radiation generated by the phone. Is this a good idea?
Short answer: They're a complete waste of money.
Long answer: Suppose, for a moment, that such a case was perfectly efficient and prevented any RF (Radio Frequency) from being broadcast by the phone. That would prevent the phone from working. The RF is an essential part of the operation of the phone.
So if you're using one of these cases, and if the phone still works properly, then you can be sure that the case is not eliminating RF emissions.
Actually, those cases don't cover the phone's antenna, which is where the majority of the RF is emitted.
But suppose that the case could attenuate the signal generated by the phone by 50%. What would that do?
It turns out that in CDMA it would have little effect. It would increase power usage and thus decrease battery life, and it would increase the chance that the phone would drop the call. It would not decrease the amount of RF to which your head was exposed most of the time, surprisingly.
CDMA operates best when everything in the system uses the lowest transmit power it can. That not only makes all the mobiles get the best life out of their battery, but more to the point it maximizes the capacity of the system.
The cell constantly monitors the integrity of the signal it receives from the phone, and 800 times per second it sends power control bits to the phone telling the phone to raise or lower its transmit power by very small increments. The cell does this so as to make the power level received by the cell remain constant.
The signal path from the phone to the cell can vary a great deal, especially if the phone is moving while in a call. The efficiency of transmission can improve or degrade rather rapidly, and this feedback mechanism is intended to compensate for these kinds of changes. The cell makes sure that the phone uses enough power to get its signal through, but no more than that.
Since this mechanism is measuring the power received by the cell, it takes into account every source of attenuation -- including the case.
Taking again that hypothetical 50% attenuation by the case, the result would simply be that the cell would command the phone to raise its transmitter power. It would operate at twice the power level that it would need to without the case, so as to generate the same RF level at the cell as it would have without the case. This means that outside the case the phone will still broadcast the same amount of RF.
But to do so it has to operate its transmitter at a higher power level, which means that it drains the battery faster. So your "talk time" goes down considerably even though your exposure to RF remains the same.
Also, one of the ways that a call can be dropped is that the signal conditions degrade so far that the transmitter is told to boost, and can't go any higher. At that point, there will be enough errors in the data stream that the cell will give up and drop the call.
CDMA phones top out at 200 milliwatts (a very small amount of power, by the way). Once the phone reaches this ceiling, if the cell asks for more, there is no more to give, and the call will probably drop.
With this hypothetical 50% attenuation, when the phone reaches 200 milliwatts it would actually be broadcasting 100 milliwatts. This means that it's more likely to have its call drop, since it runs out of ceiling sooner.
But that discussion is academic, because the cases can't attenuate the power that much since they don't cover or affect the phone's antenna.
Does my CDMA phone drain the battery faster when it says it is "searching"?
Short answer: Yes, quite a lot.
Long answer: In order to make the battery last as long as possible, all CDMA phones turn on and off portions of themselves constantly.
When the phone is normally idle, the receiver is only on perhaps 2% of the time, if that. But when the phone says it is searching, that means it is attempting to get in contact with the cellular system. To do that, it has to have the receiver on most of the time.
The receiver consumes quite a lot of power, and therefore the phone will drain the battery much faster if it spends long periods searching. If you're going into an area where you can be pretty certain the phone can never contact a cell (such as riding a subway), you might want to turn the phone off in order to preserve the battery. (Note that it does not harm the phone or the battery to search; it just runs the battery down.)

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I have a phone from CDMA service provider XYZ. CDMA service provider ABC sells exactly the same phone (or operates a network which should be compatible with it). Can I use the one I already have?
Short answer: Probably not.
Long answer: The reason isn't technical, it's economic.
Most phones are sold by the service providers for less than they actually cost. The cellular phone companies aren't in the business of selling phones, they're in the business of selling air time. But since customers can't use air time without a phone, the service providers want to get as many phones out into circulation as possible. To that end, they subsidize them.
It's like Polaroid cameras. When you buy one of the low cost cameras, you're actually paying less than it costs Polaroid to make and sell it. That's because if you own a camera, you'll be buying film for it. Only Polaroid makes film for those cameras, and that's where they make their money off of you. By taking a one-time loss on the camera, they hope to make a continuing profit off the film.
Now of course, that's not necessarily going to happen for every single camera they sell. But statistically speaking, the more cameras there are out there, the more film they'll sell.
The service providers do the same thing. But while only Polaroid sells film for their cameras, all the CDMA phones are compatible with all the CDMA phone systems in the same frequency band (generally speaking).
If provider XYZ sells you a phone at a loss, they do so in expectation that statistically speaking you'll pay as much or more in air-time fees as they lost on the phone. For a one-time loss on the phone they expect to make continuing profit off of your airtime use.
But that's just what won't happen if you take that phone to some other service provider. In that case, XYZ is out the subsidy and gets nothing in return. Needless to say, they take a dim view of this prospect.
So the phone manufacturers have implemented what are known as subsidy locks. What that means is that the phones are inhibited so that a phone sold by XYZ won't work with carrier ABC, even though ABC sells exactly the same model. Usually this is done by locking the phone's NAM, so that it can't be reprogrammed to use a different service provider as the home system. (This does not prevent you from roaming to the other system.)
If the phone has a subsidy lock on it, and most do, then only the provider which originally sold the phone can lift the lock. You can ask, but they probably won't do so for the simple reason that there is absolutely no incentive for them to. It doesn't make for customer good-will, since you're trying to be someone else's customer anyway. It merely makes it easier and more attractive for you to leave, which clearly they don't want. After all, if you have to buy a new phone to switch service providers, you're less likely to do so

Can I use two phones on the same phone number?
Short answer: Maybe. Talk to your service provider.
Yes, but there are restrictions. Each individual phone must have a unique Electronic Serial Number (ESN). The ESN is a unique number programmed into each cellular telephone at the time it is manufactured and is the means by which a cellular carrier identifies a telephone to determine whether the user of that phone is entitled to obtain service and to insure that the proper accounting is made of all activity. Most cellular phone emulators or extension services simply "clone" cellular phones, duplicating not only the telephone number but also the ESN. This activity is in violation of current Commission rules.

The Code of Federal Regulations Title 47, Section 22.915, entitled Cellular System Compatibility Specifications, generally sets forth the standards of cellular operation as reflected in the Cellular System Mobile Station-Land Station Compatibility Specification (April 1981 ed.), Appendix D to the Report and Order in CC Docket No. 79-318, 86 FCC 2d 469, 567 (1981). It is a violation of Section 22.915 of the Commission's rules for an individual or company to alter or copy the ESN of a cellular telephone so that the telephone emulates the ESN of any other cellular telephone. Moreover, it is a violation of the Commission's rules to operate a cellular telephone that contains an altered or copied ESN.

Part 22 of the Commission's rules was recently revised to add a new rule Section 22.919, to further clarify the issue of ESNs. Pursuant to subpart (c) of the referenced section, it is a violation to remove, tamper with, or change the ESN chip, its logic system, or firmware originally programmed by the manufacturer.

It currently is possible to obtain two cellular phones with the same telephone number if the cellular carrier in the market has the software in place to handle the billing and its fraud detection system has been notified not to be triggered by the use of two phones with the same phone number in suspicious circumstances.

However, there are technical reasons why it may be impossible to do this with CDMA specifically..

The phone identifies itself to the network in a registration message by including the NAM [effectively, its phone number]. From that, the network looks up its ESN. The phone knows its own ESN. The ESN never passes over the RF link in CDMA.

The long code used on the reverse link is modified by using the ESN of the phone. Since both the network and the phone know the ESN, they both modify it in the same way. As mentioned above, only one phone in existence can have any given ESN.

If a second phone tries to identify itself using the same NAM, the table lookup at the network will turn up the first phone's ESN rather than the second phone's ESN -- and the reverse link will be severely degraded (by a high "chip error rate"), probably to the point of uselessness, due to the network and phone using different ESNs to modify the long code.

Can I use two phones on the same phone number?
Short answer: Maybe. Talk to your service provider.

Long answer: Here's the FCC's answer to this question:

Why doesn't an incoming call automatically switch to voicemail if the cell I'm in is overloaded?
Short answer: Because the cell system doesn't actually know.

Long answer: There are two reasons why the cell system can't do this, and both come down to the fact that it doesn't know that the cell sector you're in is at capacity.

First off, it's not easy to say exactly when a cell sector is "at capacity". CDMA doesn't work that way. Every simultaneous call uses the entire spectrum, with separate encoding. Each call looks like noise to all the others, and every time you add a call to the sector, the noise floor rises for all the others. This decreases the signal-to-noise ratio for all the other calls.

But the amount of noise that each call adds to the noise floor is not constant. It's a function of the location of the phone and thus how much power it needs to use to get its signal through to the cell, and it's also a function of the actual voice traffic it is carrying, which varies from moment to moment.

The theoretical maximum number of calls a sector can carry is 61, because there are 64 Walsh codes (which is how "channels" are differentiated from each other under CDMA, since each channel uses a different Walsh code) and three of them are reserved. This leaves 61 for traffic channels. In practice, however, no cell ever can support 61 calls simultaneously.

But exactly how many they do get is very much dependent on circumstances, and it changes from second to second. Where are all the phones with existing calls? What's the terrain like? Is it raining? How are all the neighboring cells behaving and how busy are they? Which codec is each phone using and what is its bandwidth? Many factors feed in. (I've heard that about 50 simultaneous calls is pretty much all anyone ever gets, but I don't know how true that is.)

So when the cell system tries to create a new call, it doesn't actually know whether the cell can sustain just one more.

But there's another factor which is even more important: the cell system doesn't actually know which cell sector your phone is in. What it knows is which zone you are in, but a zone usually consists of more than one cell, usually having more than one sector each. Any time your phone moves from one zone to another it has to register, which means that it turns its transmitter on and sends a brief message to the cell system announcing some important information about itself. The cell system knows where it received that message, and thus at that moment knows which sector of which cell your phone is locate within. However, thereafter you can wander at will within the whole zone and the cell system won't have the faintest idea where within it you are.

This is actually a good thing. If your phone had to transmit every time you moved from one sector to another, your standby time would suffer badly, because ordinarily phones can perform idle handoffs quite commonly even if the phone is standing still.

Whenever there is an incoming call for your phone, the cell system broadcasts a page on the paging channel of every sector of every cell in the last zone within which it received a registration from you (if it thinks your phone is still turned on). But within that zone, some sectors might be extremely busy and others might have very little traffic. If your phone is in one of the busy sectors, it might miss the page entirely or it might receive it and try to set up a traffic channel – but fail. On the other hand, if it is within one of the less busy sectors, it probably receives the call normally.

So the cell system doesn't know which cell sector you are in, and even if it did it doesn't know whether that sector is at capacity. Therefore it has to attempt to page your phone before it routes the call to voicemail.

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Phone 1 shows 4 bars and Phone 2 shows 2 bars in the same location. Phone 2 is worse, right?
Short answer: Not necessarily.
Long answer: The "bars" display shows a single number, quite coarsely. But to actually understand your reception conditions, there are two numbers you'd really like to know.
One is the absolute signal strength. That's what most phones display on their bars display. But there's no industry standard indicating what "one bar" means, or "two bars" means, so comparing the number of bars between phones of different models, even from the same manufacturer, doesn't tell you anything at all. One phone showing four bars and another showing two bars may actually be displaying the same value for the signal strength.

The other number is called EC/I0 (pronounced eee-see over eye-not) and it refers to the amount of the signal which is usable by the phone. In CDMA, every phone which is currently in a call using a given carrier frequency looks like noise to all the other phones using that carrier frequency. This is the major contributor to what is known as the noise floor. The difference between the total signal strength and the noise floor is usable signal, known as EC/I0.

The problem is that you really need to know both numbers to perform a scientific comparison of phones, and you need to know them to much more accuracy than five or six steps, which is all the "bars" display gives you.

The EC/I0 value can never exceed the absolute signal strength, but it can be far less as the cell becomes busier with other calls and the noise floor rises.

The problem with displaying the absolute signal strength is that it doesn't tell you enough. Even with "four bars", if the cell is overloaded then the EC/I0 value would be too low to support a call. On the other hand, with zero bars, if the cell is not busy then there may still be sufficient EC/I0 to support a call.

The problem with displaying the EC/I0 value instead is that it fluctuates very rapidly, since it is load dependent. It becomes very difficult to explain to a non-technical user why their phone display changed from four bars to zero bars and back to four bars again within a 30 second period, while they're sitting still, which can easily happen. It might convince them that their phone is malfunctioning, even though it is actually acting properly.

In any case, even knowing the absolute EC/I0 value to a sufficient degree of precision doesn't tell you everything you need to know. The state of the art is improving with each generation, and the receivers are getting more sensitive. One phone may be able to carry a call with a certain low EC/I0 value while another older phone, or one from a different manufacturer using different silicon, might drop a call even though its EC/I0 value is larger.

The joker in the deck is that this all tells only half the story, because all of it is concerned with the forward link. But a successful call also requires a reverse link from the phone to the cell, and that's entirely separate. Even if both the signal strength and EC/I0 values are sufficiently high to permit a phone to carry a call on the forward link, the environmental conditions might be such that the reverse link won't work. And there's no measurement the phone is capable of making which will tell you that, because the phone itself doesn't know!

Don't take the "number of bars" display on the phone too seriously. It's much too coarse and it contains far too little information. You can't use it to perform scientific comparisons of phones.

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I just got my service switched to a new phone. Can I keep using my old phone
Short answer: Except for 911 calls, no.

Long answer: You can use it to make 911 calls, and you can use it to contact the service provider. That's it.

It's important to understand that 911 is very special and should not be abused. "I'm trapped on a lonely road and I need a tow truck" is not a valid 911 call, for instance. 911 is for summoning police, fire trucks or ambulances, and nothing else. This is not a joke. If too many people use it for trivial things, then it may not be available for the person who really needs it. (Do not call it just to see if it works, for instance.)

The phone can't be used for anything else, because of a technical aspect of how CDMA works, which was actually put in there precisely to defeat this sort of thing. (What it actually defeats is phone cloning: it keeps someone else from using your phone number to make calls with their phone, thus making you pay the bill for their hour-long calls to Mozambique or Paraguay.)

A CDMA phone uses something called the long code to spread the chip sequence that it sends on the RF link. This works because the cell and phone both use exactly the same long code, precisely synchronized. On the reverse link the long code is modified using the phone ESN. The ESN is never transmitted by the phone to the cell, so it can't be intercepted by cloners snooping on the radio link. Rather, when the phone registers with the cell, it sends its NAM. The cell system then looks this up in its database and retrieves the ESN from there. The phone itself also knows the ESN because it is stored locally.

Thus both the cell and the phone modify the long code the same way because they're using the same ESN, and the signal gets through.

A cloner could conceivably intercept your NAM, but if he changed his NAM to match yours, he would not meet with the same success. His phone would register using your NAM, but his phone would use his ESN on the reverse link. The cell system, on the other hand, would use your ESN on the reverse link, and they wouldn't match. Without going into too much technical detail, the effect of this is to substantially reduce the amount of signal the cell can derive from the RF with its rake receiver. Usually there's too little to reconstruct the bit sequence, and after missing a certain number of packets in a row, the cell will give up and drop the call. At best the phone won't work reliably, at worst it won't work at all.

Well, with your old phone that's exactly the situation. The old phone still identifies itself to the cell system using the same NAM as was originally programmed into it, but the cell system has updated its records for that NAM to indicate the ESN from your new phone rather than the one from your old phone. Thus when you try to make a normal call with your old phone, the ESN doesn't match and the call won't work.

This does not apply to 911 calls because 911 calls are special. The reverse link is not modified using the phone's ESN on a 911 call, so the call will work normally. Equally, a call to the service provider using a *-code is not modified using the phone's ESN, so that too will work properly.

But nothing else will work.

The only way the old phone could continue to work was if its ESN could be changed to match the new phone. But that's both illegal and extremely difficult to do. In fact, the phone manufacturers make it as difficult as they possibly can, because if you could do it then a cloner could do it, and could steal service and stick you with the bill.
Note: If a phone is dual-band or dual-mode, and thus supports AMPS, it is sometimes possible to force such an old phone into AMPS mode and to make a credit-card call. However, you can't rely on this, which is why it's not a good idea to store a deactivated phone for use as an emergency phone in a car. Even if AMPS coverage is available, the carrier may not permit you to make a call in that way.
(Another reason this is not a good idea is that LiIon batteries do not have a really long shelf-life; left alone they discharge themselves in a couple of months. When you need the phone, you might find that it doesn't work.)
My new phone shows the battery at half charge. What should I do?
Short answer: Fully charge it once, then full discharge it once.
Long answer: It's necessary to condition a LiIon battery in order for it to perform optimally. They ship with half a charge, and if you proceed to use it like that and drain it, you will damage the battery and it will never hold as much charge as it should.
When a LiIon battery is first new, you should completely charge it once before you use it for any long period. (Turning the phone on for a few minutes won't matter; it's not that critical.) Also, within the first week or so you should let it fully discharge once by leaving the phone on until the phone turns itself off due to the battery being used up.
Once you've done that, you can largely ignore the battery; use it when you feel like, charge it when you can.
You may encounter people who will tell you that, when the battery is new, you have to "charge it for a full 24 hours". That's not really necessary.
The confusion there comes from the fact that the charging process involves two stages. In the first stage, the phone backflows current continuously into the battery. This is fairly fast and gets the battery up to perhaps 80% of its full charge. After that, there's a process called "topping off".
During the topping off segment, the phone moves current into the phone for a short duration, then pauses for a somewhat longer duration. The reason for this is that when the battery gets near full charge, it begins to heat up while current is backflowing. The pause is to permit the battery to cool again. (If this is not done, the battery can melt or explode. Kids, don't try this at home! That's why you should never try to charge a LiIon battery yourself; the process requires more than just a current source.)
This topping-off process takes an additional hour or three, depending on the size of the battery. Where the confusion comes in is when exactly the phone announces that the battery is "fully charged".
On some phones, "fully charged" means that the topping off process has completed. However, on other phones, "fully charged" means that the main charging process is finished, but topping off may still be going on even though that's been displayed.
Another source of confusion is an assumption people make that charging continues as long as the phone is connected to the charger. That's not correct; in fact, it would be dangerous to do that. On the contrary, once the battery is fully charged, the phone will cease charging it whether it remains connected to the external power source or not.

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Is there a master security code which will permit me to make changes to my phone?
Short answer: No.

Long answer: When this question gets asked, usually what the asker is looking for is a single universal number which will work with any phone of a given model, so as to lift the subsidy lock on their phone without going to the carrier which sold it.

Sorry, there's no such capability. Each phone has a unique code, and only the cellular provider which sold you the phone knows what it is.

Can I use a normal modem with my CDMA cell phone?
Short answer: No.

Long answer: Sorry, but the long answer is really long.

This isn't a plot by the cell phone manufacturers to force you to discard your perfectly good modem. To understand why, you have to understand how normal modems work and how they relate to the regular land-line phone system.

Originally the phone system was a circuit switch, and modems were analog. This is in the days of the cross-bar switch, and after a circuit had been established a single wire carried traffic both ways (possibly with an analog amplifier in between). Modems were devices which moved digital information from one place to another using what was known as "frequency shift keying".

That meant that the modem alternated between two frequencies when it transmitted, with one frequency meaning "1" and the other "0".

Time marches on, and the modems got faster. The new approach was called "phase shift keying", and it worked by encoding four bits into a single wave.

There things stalled. Meanwhile, the phone companies had been switching to digital systems which were not cross-bars. Instead, they worked by digitizing the analog waveform being fed to them, packetizing the results, and interleaving it at very high speed into a TDMA bit stream inside the switch. So to connect two lines to each other, instead of a physical switch making an electrical connection between them, the switch tells each which TDMA time slot to use. This massively simplifies the switch and improves reliability by removing all the mechanical moving parts which were associated with the older cross-bar approach.

But it also meant that the connection from one end of the phone line to the other was no longer electrically isolated. Rather, it was being digitized and thus converted to a step function. Any approach which depended on pure analog waveforms was doomed to fail. But it opened the way for a crafty approach to modem design.

The fastest modems in existence today for standard phone lines (and the fastest which will ever exist, because they use the full bandwidth which is available) are designed specifically for how the phone system A/D converters work. They happen to be non-linear, but the real point is that the threshold for each A/D step is published. When two of these modems establish contact with each other, they "negotiate" and test the line to see just how high an amplitude the line will permit. (They also test various steps in between to make sure they understand how the line is being amplified.) If they can get up to step 237, for instance, then thereafter they will communicate with each other in modulo 237. On each A/D digitization time (which is also published) the transmitting modem sends to the phone system a flat voltage representing one of the 237 voltages which the line permits, and thus this is what will pop out at the receiving modem. Converting binary streams into arbitrary modulus transmissions and back out again is left as an exercise to the student.

So the modems are designed precisely around the exact characteristics of the land-line, in published standards.

Unfortunately, CDMA cell phones work entirely differently. For one thing, while the transmissions in the land-line system are lossless (as long as you stick to the amplitude and step-function levels which the line will support) that is not the case for CDMA.

CDMA begins with a far more restricted bandwidth per phone call than a landline does. When you speak into your phone, it is digitized, but then it is passed through a lossy compression device called a codec. The codec algorithm is specifically designed to take advantage of the fact that human spoken language is enormously redundant and that the human ear can compensate for certain kinds of distortion. In fact, humans are extremely good at this, as you'll realize if you've ever had a conversation at a busy party, or next to high surf, or by a revving motorcycle. Needless to say, CDMA's codecs don't induce distortion like that.

The CDMA codec deliberately discards useless detail, and by doing so is capable of achieving a tenfold reduction in the data stream -- or even more in some cases. Now the emphasis here is on the word useless. Human ears will barely notice that anything has changed, but test equipment (and modems) can pinpoint the differences very clearly.

This works beautifully for a human voice, and most people find that CDMA with a 13K codec actually sounds as good as or even better than a landline does. (Landlines suffer from the fact that the voice traffic covers several miles from the last stage switch to the home, in analog, on copper wires with little shielding. Distortion and noise are inevitable.) But what the CDMA codec is doing is completely wrong for how a standard modem wants to use the link.

For one thing, the traffic that a standard modem tries to feed to the phone looks nothing like a human voice, and the codec is lost at sea. If a standard modem were connected to such a phone, what would come out at the far end would bear only a passing resemblance to what went in. The negotiation between the two modems would fail completely and no connection would take place.

The highest transmission rate available as this is written (May 1999) in most CDMA systems is 14.4 kilobits. There simply is no way to cram 56 kilobits through such a channel; Claude Shannon's Information Theory doesn't permit it. And even at lower rates, what the modem is feeding the phone is not what the phone is designed to carry. [Soon the cell systems will deploy a new form of data services which will support much higher data rates. SCDB 2/2000]

The right solution is for digital communications through CDMA to take advantage of the characteristics of the medium, just as it does through a landline. In this case, it takes advantage of the fact that the actual link between the cell system and the phone is digital, unlike landlines. When the phone carries digital data, it bypasses the codec entirely and transmits the digital information given it by some external digital device (typically a notebook computer or PIM). At the cell system the resulting digital stream either is passed through a standard modem before interfacing into the standard landline phone system, or increasingly it will be gatewayed directly onto the Internet. By this means, the full digital bandwidth of the phone channel is made available to the user.

What are the differences between NiMH and LiIon batteries?
Short answer: Size and weight.

Long answer: NiMH means "Nickel Metal Hydride". LiIon means "Lithium Ion". Nickel is a metal related to Iron, well up on the periodic chart, and it's quite dense. Lithium is in the same family as Sodium and Potassium, and it is the lightest metal there is.

For the same energy storage capacity, a NiMH battery will be smaller than a LiIon battery, but the LiIon battery will weigh less. They cost about the same.

Generally speaking, for most users weight is more important than size, and because the weight difference is really quite dramatic (a factor of two or more) generally LiIon batteries are better for most users.

Also, NiMH batteries do suffer somewhat from is known as "memory effect". (NiCad batteries were much more subject to this.) This means that it's necessary to fully discharge the battery before charging it again. If you routinely recharge a NiMH battery after only partially discharging it, the battery will "learn" that point and not have as much energy storage capacity. LiIon batteries are almost wholly immune to this kind of thing, and with a LiIon battery you can recharge any time you want without harming the battery. [However, when a LiIon battery is first new, you should always fully charge it before using it the first time, and it's a good idea to let it fully discharge once or twice. This conditions the battery and gives it maximum energy storage capacity and maximum life. SCDB 2/2000]

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Is it possible to jam CDMA?
Short answer: It's possible, but it's impractical.

Long answer: People who ask this question tend to divide into two groups. The first group are those who are concerned about people who are stupid enough to use their cell phones in environments where the radio frequency emissions (RF) could cause problems, like near operating jets, or in hospitals where people are wired to pacemakers. The second group are those who are fed up with listening to phones ringing in theaters and restaurants.

RF jamming divides into active and passive. Passive jamming means shielding; the ultimate form of this is known as a "Faraday cage" and it means you are surrounded by conductive metal or fine screen on all sides, including top and bottom. Active jamming means to broadcast meaningless RF at the frequencies in question at sufficient power level to disrupt the behavior of the device -- in this case, the CDMA cell phone.

Active jamming is a lost cause. Not only would it be a violation of FCC regulations (or those of comparable authorities in other countries) but CDMA uses spread spectrum. Spread spectrum was developed during World War 2 precisely because it is exceedingly difficult to jam with active jamming. And in many cases (the hospital heart ward) the cure would be worse than the disease because the transmit power levels required would cause more harm than the phones could.

Passive jamming would require that metallic shielding be built into the walls of the structure as it was being constructed. While this might work for a theater, it's impossible for any structure which has windows unless you put grounded screens over every single one of them. (Which might actually be possible for the heart ward, but is probably impractical for a restaurant or an airport.) And it certainly wouldn't be cheap in any case.


What is a PRL?
Short answer: It stands for "Preferred Roaming List", and it is used by the phone to locate different cell systems.

Long answer: The PRL is a list of bands and channels in order of preference which the phone uses when it attempts to locate and connect to a cell system, such as when you first turn the phone on.

Among other things, when your cell provider makes a deal with some other cellular provider to give you a roaming discount, the PRL will be updated to include that second provider's systems. Your phone can find a system which is not on the PRL, but it will try to find a system on the PRL before it uses one which isn't on it.

But the PRL is more than that. It's also used to find your own provider's systems. That's because your provider may not be using the same band in every market.

In 800 MHz cellular, each market has two bands called A and B. When the 1900 MHz PCS bands were opened, there were six named A, B, C, D, E and F. These have been auctioned off in groups by the FCC, and even providers trying to build nationwide systems have ended up with different bands in different areas. When you travel from one city covered by your provider to a different one, the PRL tells your phone how to locate your provider there.

If you have an 800 MHz dual-mode phone or a 1900 MHz dual-band phone, or a tri-mode phone, then you have the ability to use AMPS if your phone can't find CDMA coverage. The PRL tells your phone how to locate AMPS coverage.

And even when you stay at home, the PRL helps. CDMA uses two spread-spectrum carriers which utilize 1.25 MHz each, but each of the licensed bands is actually much larger than that. Initially, most CDMA providers have deployed to use a single carrier system-wide in any given coverage area. But as traffic levels rise, they can and do deploy a second or third carrier frequency within the band, in order to increase capacity. The PRL helps your phone to find these.

The PRL is stored in your phone, but it can be updated. This can be done at a phone store with proper equipment, but increasingly it is becoming more common to do it over the air. To make that happen, you would dial a certain phone number. The process takes a couple of minutes. Not all providers support this capability. For more information about this, consult your provider.

Typically, if you are not having trouble with coverage, you don't need to concern yourself with PRL updates. But it doesn't hurt anything to ask about it every six months or so.

What is the phone doing when it's idle?
Short answer: It's actually very busy!

Long answer: It's constantly turning parts of itself on and off; on to perform vital functions and off again to save power so that the battery lasts longer.

By far the most important thing it does is to wake periodically and turn on its receiver briefly to see if it has been paged, which means to find out if there is an incoming call. This happens on what is known as a slot cycle, and the period of the slot cycle is controlled by the cell (for all intents and purposes). Slot cycle indices are numbers from 0 to 7, and for any index the period is 1.28 seconds multiplied by 2^index. In North America, by far the most common slot cycle indices are 1 and 2, which indicates a period of 2.56 seconds or 5.12 seconds respectively. I haven't heard of anyone using anything longer than this, though the specification supports slot cycles of 163.84 seconds.

The receiver consumes quite a lot of power. relatively speaking, and the purpose of the slot cycle is to permit the phone to keep the receiver turned off most of the time. This is vital to extend battery life. When the phone first registers with a cell, the cell and phone determine which paging channel the phone will use (if there is more than one) and what phase of the slot cycle that phone will use. Thereafter, the phone wakes periodically, turns its receiver on briefly to see if it has an incoming call or if there is other traffic from the cell it must respond to, and if there is nothing then it shuts the receiver down again and waits until the next slot time.

When an incoming call arrives at the cell for a given phone, the phone system generates the sound of a phone ringing as a comfort tone back to the caller, and the cell waits until the slot time for the phone. When it comes around, the cell sends a message to the phone telling it that there is an incoming call. This causes the phone to waken and set up the call, and to begin to ring.

If the phone doesn't respond to the page, the cell may try again on the next slot.

The advantage of a longer slot cycle is that the phone spends a lower percentage of the time with its receiver on and thus the battery will last longer. It also means there is more capacity on the paging channel. The advantage of a shorter slot cycle is that the phone gets more chances to receive the page, and will receive the page sooner.

When the cell system needs to send out that page, it obviously needs to know where to broadcast it. The cell system as a whole will be divided into zones, and when a phone is paged, every sector of every cell in the zone it's in will carry the page. This means that no matter where the phone is located in that zone, it will receive the page. When the phone moves from one zone to another, it registers again, which permits the cell to know where it is located. The size and layout of the zones is another tradeoff: if the zones are large, the traffic channels will carry a great deal of redundant paging information and can become overloaded, but the phone doesn't have to perform zone-based registration very often as it moves around, which means its battery will last longer. On the other hand, if the zones are small then the paging channels are used more efficiently but the phone will need to register more often and thus will use more battery power.

You may have noticed that when you turn your phone off it takes several seconds for it to actually shut down. That's because it is sending a message to the cell to tell the cell that the phone is going offline. However, the phone can go down unexpectedly without having the chance to send this to the cell (for instance, the battery could be popped from the phone unexpectedly while the phone is operating, which is generally not recommended), and in that case the phone would be offline but the cell wouldn't know it. That would then mean that the cell would try to handle an incoming call for that phone by paging it even though the phone was off, and it generally means that the cell's database would be loaded with entries for phones which aren't available. As a long term recovery for that, the phone is required in most systems to do timer-based registration, which means that every ten or twenty minutes it turns its transmitter on to let the cell know that it's still there. If the cell misses a couple of these registrations in a row, it decides that the phone has gone offline and removes it from the database of "phones which are currently turned on".

Under some circumstances, the cell system can directly challenge the phone for a registration. This happens on the paging channel at the slot, and when the phone receives this message, it turns its transmitter on and sends a registration immediately.

If there is pending voice mail for the phone, the phone will be told on a slot to alert its user of this fact.

All of these registration messages sent by the phone are nearly identical, and they simply identify the phone and contain a few other important pieces of information about it. Despite how it sounds, they (deliberately) don't happen very often and (deliberately) represent a negligible impact on standby time. But they are necessary for the phone system as a whole to work properly.

Of course, the phone is also updating its display to show the current date and time and signal strength and amount of energy remaining in the battery, and perhaps other things depending on the phone model.

That "idle" phone is plenty busy!

[reemusk]

How easy is it to eavesdrop on CDMA cellular?
Short answer: Harder than a landline phone.

Long answer: Eavesdropping on the radio link is prohibitively difficult. Any law enforcement agency which wanted to listen to your calls wouldn't bother with that.

The cellular and PCS carriers are required to cooperate with law enforcement agencies armed with proper warrants for line taps. If they wish to listen to calls, they tap in at the service provider's central office. It's approximately comparable to what they would do to tap a landline phone.

It's possible to illicitly tap a landline by having someone climb a phone pole (or go down into a hole) and tap the wires near your home. The equivalent of this for AMPS was a simple FM radio scanner that cost a few hundred dollars. But whoever decided to try something like that for CDMA would be stumped. Even if he had all the information necessary (like your phone's ESN, which is required to be able to intercept the reverse link) the equipment needed would cost tens of thousands of dollars, well beyond the means of any private detective or creepy voyeur.

When you speak into your CDMA phone, your voice is digitized and compressed into 50 digital packets per second. These are then spread, interleaved, passed through a Viterbi forward-error-correction encoder, scrambled using the Walsh code for the channel you've been assigned, scrambled again with the short code, possibly encrypted, scrambled yet again with a modified version of the long code and then transmitted in quadrature with spread spectrum. The creepy voyeur with his FM scanner can't even pick up spread spectrum, and if he had the right receiver it would just sound like a very high frequency hiss (well beyond the range of human hearing) bearing no resemblance whatever to your voice.

The modification of the long code includes knowledge of the ESN (the unique serial number of your phone) which the phone keeps in its memory and the cell system knows. The ESN is not transmitted, and thus can't be intercepted. Rather, your phone sends its phone number to the cell system, which looks the ESN up in its database. (If you're roaming, it gets it from your home system.) Both your phone and the cell system know the ESN and modify the long code the same way. Without it, the resulting chip sequence is gibberish.

It would not only take a lot of very expensive and customized hardware to do all this, it would also take espionage. It's been truly said that if you have someone after you who can intercept your CDMA radio link and is inclined to do so, you've got a lot worse problems than just this.


Short answer: There's an ideal length for the antenna.

Long answer: The ideal length of an antenna is half a wavelength of whatever the frequency is that it's designed to operate with.

800 MHz cellular has a wavelength of approximately 37 centimeters, about 15 inches. So an ideal antenna would be half that, about seven and a half inches. This refers to the dipole, the distance from the tip of the antenna to the opposite end of the antenna buried inside the phone somewhere (usually near the bottom). 1900 MHz PCS has a wavelength of approximately 16 centimeters, about six inches. So the ideal antenna dipole is about 3 inches.

The ideal antenna performs best if it is exactly perpendicular to the impinging waveform. In practice the orientation of the phone is somewhat random; the antenna will be pointed approximately upward, but probably at a slant. So cell phone manufacturers generally try to make the antenna 5/8's of a waveform, because if the antenna is at a slant, its cross-section relative to the impinging waveform will be near to the ideal half a wavelength. For a dual-band phone, one which operates at both 1900 and at 800 MHz, it's obvious that determining the antenna length is a bit of a problem. (But not insoluble; it's just a compromise. Since digital is usually more resilient than AMPS, usually the length is optimized for 800 MHz.)

Making the antenna shorter will both decrease the amount of incoming signal the phone receives, and will make the phone's transmitter less efficient. But CDMA operates over a very wide range of effective powers, and it can usually compensate. That's why the phone will usually work with the antenna down. And because it's digital, if it is working it will sound exactly the same. This has lead some people to conclude that the antenna is not actually doing anything for them, which is not quite correct. While the phone can operate with the antenna down, it's easier on the phone if you raise the antenna; it has more signal ceiling to work with and will be less likely to drop the call. Also, it will use somewhat less transmit power, and your battery will last somewhat longer.

Making it longer with some sort of extension is worse than useless; it actually degrades the signal. If the antenna is exactly one wavelength long and is exactly perpendicular to the impinging waveform, it will pick up essentially no signal at all.

When it reaches one and a half wavelengths, signal strength is again maximized, but for physical reasons it's a bit lower than the strength with a half-wavelength antenna. (The physical reason is that the antenna is not an ideal conductor.)

Short answer: The network tells it.

Long answer: CDMA requires that every component of the system, including all the handsets, have a very precise knowledge of exactly what absolute time it is. This is necessary in order to synchronize the long code, one of the modulating chip-patterns used to make spread spectrum work. The long code cycles only once every six weeks (41.4 days) and if the phone's long code is out of sync, it won't work with the network.

What follows is a bit esoteric, since it gets into the guts of how CDMA works.

The system acquisition process involves three steps. In the first step it has to find the pilot. The pilot is channel 0 (whose Walsh Code is all zeros) and it broadcasts a signal of constant zeros, which is not modulated with the long code. In essence, that means that what it is transmitting is the cell's short-code at whatever phase offset the cell is using. (Phase offset of the short code is how cells are differentiated from each other, since they all use the same frequencies.)

Once the phone has found that, it can synchronize its short code. Step 2 is to find the sync channel and to process a sync channel message. The sync channel message contains many interesting things, but one of the things it contains is "At the tone, the time will be...". Actually, the "tone" is the next PNROLL(0), which is known to cognoscenti as an "80" because they happen every 80 milliseconds. (It's the next time that the PNROLL, which happens every 26.666 milliseconds, coincides with a frame, which happens ever 20 milliseconds. There are three PNROLLs for every four frames.)

The sync channel message also tells the phone what timezone the cell is in (in increments of plus or minus half hour relative to Universal Time) and the number of leap seconds there have been since "the beginning of time" (which happens to be time 0 for GPS, sometimes called the epoch. It happens to have been midnight on January 6, 1980.) Note that the cell and phone won't necessarily be in the same time zone, which is why your phone may seem to be an hour off if you're right next to a time-zone line. That happens if it synchronized with a cell on the opposite side of the line.

This idea of the time is accurate to a few microseconds. The inaccuracy comes from the speed of light delay between the cell and the phone, and the fact that the phone doesn't know how far away the cell is. (The speed of light is about 980 feet per microsecond, almost exactly 300 meters. If you're a mile from a cell, then it takes about five microseconds for the signal to reach you.)

It would actually be useless to know that delay. The purpose of knowing the time is to permit initialization of the long code generator, and the long code being received from the cell is being delayed by the same amount of time as the sync channel message was. Therefore, it's good that the sync channel message is delayed by the transmission path length.

Once the phone initializes its long code generator, it moves to step 3, which is to listen to the paging channel. After that, if it doesn't decide that it can't use that cell, it will register, and then your phone is online.

This always happens when you first power up your phone. It always happens just after you finish a call. It happens at other times, too.

Whenever the phone processes a sync-channel message, it sets its internal representation of the time of day. On most phones, that's what's being displayed on the screen.

The IS-95 specification requires that all the cells be synchronized to within a few microseconds of each other. In actuality, they do it by having a fixed GPS receiver at each cell, from which the equipment gets the time very precisely.

Most CDMA phones don't let you manually set the time of day, mostly because doing so would be pretty useless. The phone would override your time each and every time it acquired a cell, anyway.

So why does the minute display on my phone click over several seconds late compared to WWV?

The phone is usually in a mode called slotted sleep during which as much of the phone as is possible is shut down to save power, including the CPU. There would be a significant power cost (manifesting as a significant hit on standby time) in order for the phone to update its display more often during slotted sleep.

You might note that the "elapsed time" display during a call ticks seconds very accurately. That's because the CPU is on anyway to handle the call, so there's no additional cost to speak of in maintaining the display accurately.

[reemusk]

How is CDMA superior to TDMA?
Short answer: It supports more calls in the same spectrum, and it dynamically allocates bandwidth more easily.

Long answer: Spectrum is extremely expensive; it has to be purchased from various governmental licensing authorities at auction, and sometimes these auctions have involved billions of dollars (or equivalent monetary value in other currencies). It represents a considerable investment by a carrier.

Generally speaking, CDMA will carry between two and three times as many calls simultaneously as TDMA in the same amount of bandwidth. This is due to something known as "frequency reuse" and is very well explained on this page.

The other major advantage of CDMA is dynamic allocation of bandwidth. To understand this, it's important to realize that in this context in CDMA, "bandwidth" refers to the ability of any phone to get data from one end to the other. It doesn't refer to the amount of spectrum used by the phone, because in CDMA every phone uses the entire spectrum of its carrier whenever it is transmitting or receiving.

TDMA works by taking a channel with a fixed bandwidth and dividing it into time slots. Any given phone is then given the ability to use one or more of the slots on an ongoing basis, if it is in a call. For instance, if the channel is 200 kHz wide with 8 slots, and the phone is allocated one of them, then the phone has effective bandwitdth of 200/8 = 25 kHz. This bandwidth is allocated to that phone while the call proceeds, whether the phone actually uses it or not. In other words, when you're in a call with TDMA and being silent because you're listening to the other person speak, your phone still uses that full bandwidth to transmit silence.

CDMA is more efficient about that kind of thing. In both TDMA and CDMA, the outgoing voice traffic is digitized and compressed. But the CDMA codec can realize when the particular packet is noticeably simpler (e.g. silence, or a sustained tone with little change in modulation) and will compress the packet far more. Thus the packet may involve fewer bits, and the phone will take less time to transmit it.

And that's where this odd idea of what "bandwidth" means in CDMA comes in. For in a very real sense, bandwidth in CDMA equates to received power at the cell. CDMA systems constantly adjust power to make sure as little is used as necessary, and compensate for this by using coding gain through the use of forward error correction and other approaches which are much too complicated to go into here. The chip rate is constant, and if more actual data is carried by the constant chip rate, then there will be less coding gain. Therefore, it's necessary to use more power instead.

Conceptually, a given cell sector can tolerate a certain amount of total received power before it becomes difficult to decipher all the channels being received. If one phone uses more of that power allocation, there is less available for the others.

But this is an advantage, not a disadvantage, for it can be stated a different way: if one phone uses less of that power allocation, there is more available for the others. This is the right way to look at it, because this is going on constantly.

In a TDMA system, suppose that the phone needed more or less than the 25 kHz slot. "Less" is a non-issue because there's no way to get smaller. "More" would require that an additional slot be allocated to the phone, which would require a protocol-level exchange: the phone says to the cell "I need more bandwidth", the cell finds some other phone on that same channel and tells it to move, clearing an additional slot, then sends a message back to the phone telling it "OK, you can use this slot in addition". This might take quite a while, and by the time it's complete the need may have passed.

But CDMA actually does this dynamically and on the fly. When the CDMA phone realizes that it doesn't need to transmit a full digital packet, it will use a "half rate" packet, or "quarter rate" or "eighth rate", and will transmit for less time. Packet transmissions happen fifty times per second in current CDMA systems, but a phone with a half-rate packet to send will pseudo-randomly send half the symbols during the 20 millisecond packet.

Received power at the cell is an instantaneously measured quantity. If two phones are transmitting at half rate but at different times, the cell is actually only receiving power from one phone at a time. Effective bandwidth in CDMA is thus actually being dynamically allocated at all times. And when you are listening and silent, the phone drops to eighth rate and uses virtually no bandwidth at all.

This is very nice for voice traffic and is an additional reason why CDMA is more efficient in use of spectrum, but where it will become particularly valuable is when data transmission becomes a significant use. That's because common data use is very bursty, even more than is voice traffic.

Consider how you use a browser, for instance: you click a link and in a short interval your computer downloads many kilobytes of data. You then sit and read what was downloaded, and there's virtually no data traffic going on.

In a CDMA system, it would be very easy to allocate a considerable proportion of the bandwidth of a sector to a single phone for that interval. Nothing special needs to be done except to allocate that phone a considerable proportion of the power, which it could do without requesting permission from the cell.

High spectrum efficiency and dynamic allocation of bandwidth are the principle reasons why the entire wireless telecommunications industry is moving to CDMA. The current generation of GSM is based on TDMA, but the next generation will use a CDMA air interface.


What is "Soft Handoff"?
Short answer: One of the advantages of CDMA over TDMA.

Long answer: In TDMA or AMPS, due to spectrum reuse, a given slot on a given frequency channel can't be used by neighboring cells. So when a phone which is in a call moves from one cell to another, at a certain point it has to switch between cells. In AMPS and TDMA it will be commanded by the system to change frequencies, all at once. This is called a hard handoff, so called because it's all or nothing: the transition is a hard one.

In CDMA, on the other hand, all the cells operate on the same frequency. The phone still has a single RF receiver which converts radio frequency down to baseband, but behind that it has a rake receiver with multiple fingers. Since all the cells operate on the same frequency, the single RF receiver picks up all of those which are within range. The phone then assigns fingers from the rake receiver to various signals, and these are added together to create the full signal the phone utilizes.

Sometimes these are multiple paths from the same cell. For instance, if there's a direct route from the cell to the phone, and in addition the signal travels to a large building and reflects off it before reaching the phone, then the CDMA phone can utilize both of these signals for additional clarity. This is called multipath. (Similar conditions degrade TDMA and AMPS performance.)

But even more useful is when the phone is about halfway between two cells. While in a call, the phone is not only handling its transport of data back and forth to the cell, but it's also actively looking for other cells. When it finds one whose signal strength is good (on the same frequency, remember) it will inform the cell system of this. The cell system might decide at that point to route the call through both cells simultaneously. The specification actually permits a phone to talk to six cells at once, though no phone currently in existence has this capability.

So when a CDMA phone in a call moves from one cell to another, the handoff process happens in multiple steps. First the phone notices the second cell, and the cell begins to carry the call on both cells. As the phone continues to move, eventually the signal strength from the one the phone is moving away from will drop to the point where it isn't useful any longer. Again, the phone will inform the cell system of this fact, and the system will drop the original cell. Thus it isn't an all-or-nothing transition, which is why it is called soft.

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Is it possible to update the firmware of a phone over-the-air?
Short answer: No.

Long answer: This sometimes comes up in the context of discussions of the "Preferred Roaming List", or PRL. Some cellular or PCS service providers have the ability to update the PRL over the air, usually by dialing a special phone number or by talking to a customer service representative.

But there is a massive difference between updating the PRL this way and updating the firmware itself. The PRL is only a couple of kilobytes long at worst. The actual operating firmware of the phone, on the other hand, can easily exceed a megabyte. Where the download time for a PRL is very short, even at 14 kilobaud (the data rate available on most CDMA networks now 5/1999), the download time for the operating firmware would be 15-20 minutes.

Worse, the phone would have to continue to operate while the new firmware was being downloaded. The air interface isn't trivial to maintain, so the phone would actually have to be running during this download. The only way this could be done is for the phone to contain enough flashROM (or whatever other medium contains the firmware) to hold two copies of the operating code instead of only one. This would significantly increase the component cost of the phone, because flashROM isn't cheap. It would also decrease the standby time of the phone, because it would increase the power draw on the battery.

Finally, it would massively increase the load on the phone system.