Cell modem Glossary

Glossary

AMPS (Advanced Mobile Phone System) is a somewhat ironic name for the original cellular system authorized in the United States. It uses an analog FM radio link and it is very easy to eavesdrop on it. AMPS is particularly inefficient in use of spectrum compared to any of the digital standards. Generally AMPS still has the best coverage of any of the standards (solely due to its ten year head start on buildout), but that's about the only thing it has going for it. Sound quality is generally worse than any of the digital standards.

Bit within the context of CDMA is distinct from chip and refers to a payload binary digit. Each bit is represented by many chips. Bits contain information and are subject to the laws of Information Theory.

CDMA stands for Code Division Multiple Access and refers to a technology for the radio link which utilizes spread spectrum communication with very tightly controlled power levels by all participants. There is currently (2/2000) only one commercial system which uses CDMA, covered by the specifications IS-95 and J-STD-008, and thus the term CDMA is often used to refer to that system. In future, other systems will adopt a CDMA air interface. CDMA was designed by Qualcomm in the US.

Cellular is a word used to mean a lot of different things. According to the FCC, cellular refers to any portable phone system which operates in the 800 MHz band allocated for use by portable phone systems. This includes AMPS, IS-136 and IS-95. At one time, only AMPS existed, and in some contexts cellular is used as a synonym for AMPS. (In particular, a "cellular ready modem" usually refers to one which works in AMPS mode, and often only in AMPS mode.) In some contexts it is used generically to refer to any portable phone system which relies on a grid of service provider antennas, and thus the term is used sometimes to include PCS.

Chip in the context of CDMA is distinct from bit and refers to binary digits transmitted over the RF link. The chip rate in IS-95 is 1.2288 MHz (thus allowing adequate guard bands to permit the carriers to be spaced 1.25 MHz apart). Each bit is represented by many chips, and if a majority of the chips get through then the bit can be reconstructed properly. The number of chips representing each bit varies depending on the bit rate. When using an 8K Vocoder (such as EVRC) there are 128 chips for each bit. Chips as such don't contain data because both the sender and receiver know the spreading pattern used to create them from a bit, and as such are not directly subject to the laws of Information Theory. Though there are many phones simultaneously using a single frequency to transmit full chiprate, that means that the channel is not saturated unless the bitrate approaches the bandwidth of the carrier.

Codec (pronounced CO-deck, short for compressor/decompressor) refers to a device inside the phone (and at the cell system) which takes digitized voice and compresses it prior to transmission to the cell, and which takes compressed voice received from the cell and decompresses it prior to playing it out the speaker of the phone. Codec algorithms are extremely sophisticated and are designed specifically around the characteristics of human voices and human ears. There are three in common use in IS-95, called "8K", "13K" and EVRC. GSM and IS-136 have their own codec standards.

Coding gain in CDMA refers to the ability to use digital techniques and redundancy inherent in the chip sequence to reproduce the bit sequence without requiring much absolute power on the RF. Generally speaking, the more coding gain, the less absolute power is needed to get the signal through. CDMA uses very sophisticated error correction methods (such as the Viterbi FEC Encoder/Decoder) to increase the coding gain.

Dual band refers to a phone capable of operating in two different frequency bands, e.g. both at 800 MHz cellular and at 1900 MHz PCS. Not all PCS phones are dual-band. When a CDMA phone is dual-band, most commonly it means it supports CDMA at 1900 MHz and AMPS at 800 MHz. Some phones exist which do both of those and also support CDMA at 800 MHz; these are usually referred to as tri-mode.

Dual mode refers to a phone which operates in a single frequency band but which is capable of supporting two protocols in that band. All 800 MHz CDMA phones are dual-mode, because all of them are also capable of operating in AMPS mode.

EC/I0 (pronounced Eee-See over Eye-Not) in CDMA refers to the portion of the RF signal which is usable. It's the difference between the signal strength and the noise floor.

ESN (Electronic Serial Number) is a unique number assigned to the phone by the phone manufacturer. No two phones will ever have the same ESN. It is against the law to try to change the ESN in a phone.

EVRC (Enhanced Variable Rate Codec) is a new codec being rolled out as this is written (2/2000) for IS-95 and J-STD-008 systems. It uses 8Kbps bandwidth but sounds nearly as good as the standard 13K codec. Because of this, when the majority of phones can use EVRC the cell systems will have more capacity without having to deploy more equipment. This should yield better service.

Forward link refers to the radio link from the cell to the phone.

Frame is the name of a CDMA digital voice packet duration. Frames are 20 milliseconds long. IS-95 transmits 50 frames per second, with each frame containing sufficient information to reproduce 20 milliseconds of sound. It should be pointed out that it may not require the whole 20 milliseconds to transmit the frame. The IS-95 codecs can generate "half-rate", "quarter-rate" and "eighth-rate" packets if the sound in that 20 milliseconds is sufficiently simple to require fewer bits to represent. A half rate packet only requires 10 milliseconds to transmit. An eighth rate packet only requires 2.5 milliseconds to transmit.

GPS stands for Global Positioning System and it is a system where a receiver can capture signals from orbiting satellites which permit it to determine the time very precisely, and also its location very precisely. Originally deployed by and still maintained by the US Navy, it is now in very common use all over the world for a variety of civilian uses. CDMA cell systems use fixed GPS receivers to determine the time very precisely. This is needed to synchronize the long code and short code in the infrastructure.

IDEN is the name of a cellular standard designed by Motorola which isn't supported by any other manufacturer. It uses a TDMA air interface and has not been very successful.

IS-95 is a standard which describes a cell system which uses a CDMA link and operates at 800 MHz. Sometimes the term is also used to describe 1900 MHz CDMA, though that properly is covered by J-STD-008. The two standards are similar and as time has gone on they have been migrating towards each other and have become more similar.

J-STD-008 is a standard which describes a cell system which uses a CDMA link and operates at 1900 MHz. It is similar to, but not identical to, IS-95.

GSM (Global System for Mobile communications) is a sophisticated cellular system used worldwide which was designed in Europe, primarily by Ericsson and Nokia. It uses a TDMA air interface.

Idle handoff in CDMA is when the phone moves from one sector or cell to another while not in a call. If it moves from one zone to another it will register. If the new cell or sector is part of the same zone, it does not need to register.

LiIon (Lithium Ion) is a rechargeable battery technology which utilizes the metal Lithium. They are the preferred form of batteries for cell phones at this time (2/2000).

Long code in CDMA is a chip sequence which is 240 chips long, which repeats every 41.4 days. Its primary purpose is to assist in spreading the signal, to make spread spectrum work more efficiently. The Long code used on the reverse link is usually modified using the phone's ESN when in a call. See Long Code Mask.

Multipath refers to a common phenomenon in RF where the signal arrives multiple times at the receiver at slightly different times. If you've used a TV with an old-style rabbit-ear antenna, you've sometimes seen ghosting, where the video seems to have echos of itself extending to the right. This is due to multipath. Usually the strongest path is nearly direct from the transmitter to the receiver. However, the signal can reflect off of other objects (large buildings are particularly good at this) and that signal arrives somewhat later, since it follows a somewhat longer path. For most kinds of RF multipath is a form of interference and degrades the signal. CDMA is unique among cellular transmission standards in that it actually uses multipath to its advantage by using fingers in the rake receiver. As a result, CDMA performance is actually improved by multipath.

NAM is the data which describes the phone and its home system. The phone number is part of the NAM.

NiCad (Nickel Cadmium) is a rechargeable battery technology which utilizes the metals Nickel and Cadmium. Generally they don't have as much energy storage capacity as newer technologies like NiMH or LiIon and are not generally used for cell phones anymore.

NiMH (Nickel Metal Hydride) is a rechargeable battery technology which utilizes the metal Nickel. They tend to be heavier than LiIon and are not used much any more for cell phones. (In other contexts, it stands for National Institute of Mental Health.)

Noise floor in CDMA refers to the part of the incoming signal which is unusable. The primary component of the noise floor on the forward link is signals being sent by the cell to other phones in the same sector, and to a lesser extent other nearby cells and sectors transmitting to their phones. The primary component of the noise floor on the reverse link is other phones transmitting to this cell or to others nearby.

Orthogonal ("composed of right angles") is a technical term referring to a certain special characteristic of the long code, the short code and the Walsh codes. It refers to the fact that, for instance, if you take any two Walsh codes and XOR them together, the result will be 32 1's and 32 0's. But if you XOR a Walsh code with itself, the result is 64 0's. The short code and long code are orthogonal to themselves at different offsets. What this means is that if two short codes are synchronized, then the XOR of them is all 0's. If they are offset from each other, by any amount, then the XOR of them is about half 1's and about half 0's. The long code is also orthogonal to itself. This was done deliberately and without it CDMA wouldn't work. This fundamental characteristic of the long code, short code and Walsh codes is what makes it possible for the rake receiver to separate out the chip sequence intended for this phone from the ones being sent to all the other phones.

A Page is a message sent by the cell system on the paging channel to a particular phone which says that there's an incoming phone call. When the phone receives a page, it sends a message to the system requesting a traffic channel, and when it is granted one it then rings to tell its owner that a phone call is waiting.

Paging channel in CDMA is a channel used by the cell to send pages, which indicate incoming calls, to the phone. The Paging channel also carries other information, such as indications of voice mail, SMS indications, plus housekeeping information such as the PN Offsets of all nearby cells and sectors.

PCS stands for personal communication system and according to the FCC it refers to any portable phone system which operates in the 1900 MHz band allocated for such systems. Among others, this includes J-STD-008, GSM and IS-136. Some phone companies have used this term as a synonym for "portable phone", so they have sometimes referred to 800 MHz phones as being "PCS". This is a misuse of the term.

Pilot channel in CDMA is a special channel which the cell transmits constantly. It is not modulated using the long code and it uses Walsh code channel 0, which is all 0's, and it transmits a bit pattern of all 0's. That means that what it contains is the short code at the phase being used by the cell. System acquisition by the phone begins by locating the pilot channel, and this permits the phone to synchronize its short code with the cell. After this, the phone looks for the sync channel.

PN Offset See Short code

PN Roll See Short code

Power control bits in CDMA are chips which are altered in the forward link to permit the cell to adjust the transmit power of the mobile phone on the reverse link while in a call. They are transmitted 800 times per second and cause the phone to increase or decrease its transmit power by a small increment.

PRL stands for Preferred Roaming List. For more information, see this.

Rake receiver is the digital section of a CDMA receiver which permits the phone (or cell) to separate out the relevant signal from all the other signals. The relevant signal will be encoded with a known Walsh Code and a known phase of the Short code, and the rake receiver can do this because the Walsh codes are orthogonal and the Short code is orthogonal to itself at different offsets. The rake receiver is capable of receiving multiple signal sources and adding them together using multiple fingers, each of which has the ability to use a separate phase of the short code and long code and a separate Walsh code if necessary. Different fingers might track multiple signals from the same cell (arriving at slightly different times due to multipath) or might track separate cells due to soft handoff.

Registration in CDMA is a process where the phone turns its transmitter on briefly and sends a packet on the paging channel which identifies the phone to the cell system. The phone does this when it first acquires the system. On most systems, it does this periodically (at a time interval selected by the cell system, typically every ten or twenty minutes). The registration message contains part of the phone's NAM, which the phone system uses to look up the phone's ESN. (If you are roaming, the roaming system asks your home system to look up the ESN.) The phone also registers if it changes zones, and can be challenged by the system to register.

Reverse link in CDMA refers to the radio link from the phone to the cell.

RF stands for "Radio Frequency" and is a commonly used acronym to refer to a radio link, e.g. "goes over RF to the cell".

Searching in CDMA is a process where the phone scans the phase space of the short code looking for valid signals. Depending on when and how this is done, it may be looking for valid pilots, or it may be looking directly for valid paging channels. In a dual-band or dual-mode phone this may also involve attempt to find an AMPS system.

Sector refers to the fact that a typical cell divides its circular coverage into several slices, sort of like a pie. The number of sectors supported is variable, but it's common for there to be three. Each sector in CDMA will use a different PN Offset. From the point of view of the phone, there's no difference between moving between sectors and moving between cells.

Short code is a chip sequence which is 215 chips long which repeats every 26.666 milliseconds. Different cells and cell sectors all use the same short code, but use different phases of it, which is how the phone differentiates them from each other. The phase is known as the PN Offset. The moment when the Short code wraps around and begins again is called a PN Roll. (PN stands for Pseudo-Noise.) The chip sequence is designed to be orthogonal to itself at different phases.

Signal strength refers to the total amount of power of RF received by the receiver. This is divided into useful signal, referred to as EC/I0, and the noise floor which is useless.

Slot cycle is a setting which controls the length of a slot. A slot is (1.28 seconds) * (2 ^ slot cycle). So slot cycle 0 is 1.28 seconds, slot cycle 1 is 2.56 seconds, and slot cycle 2 is 5.12 seconds. The longest slot cycle is 7, which is 163.84 seconds. The slot cycle is controlled by the cell. The advantage of a short slot cycle is that it means that the phone gets more chances to receive a page before the call is routed to voice mail. However, this makes the phone use more power, so standby time is not as good. It also means that the paging channel has less capacity. A longer slot cycle provides more capacity on the paging channel and lengthens standby time, but also means that the phone has fewer opportunities to receive a page, so that it's more likely to miss it and have the call go to voice mail. You as a user cannot choose a slot cycle (even if your phone has a menu item for it).

Slotted sleep is a mode of phone operation where the phone shuts down nearly all of its electronics most of the time. (All digital phone standards include a form of this. AMPS does not, which is why AMPS battery life is so poor.) The phone wakes on every slot (see slot cycle) to see if it gets paged on the paging channel. Because most of the electronics is turned off most of the time, this uses very little power from the battery. More information about this can be found here.

Soft handoff refers to a feature of CDMA where a phone can communicate simultaneously to two or more cells, or in some cases with two sectors on the same cell. This often happens when the phone is about halfway between the cells or on the dividing line between sectors, and permits the call to continue even though the signal from any one cell would not be strong enough to keep the call up. For more on this, see here. No other phone standard has this ability.

SMS (Short Messaging Service) is a mechanism which allows brief text messages to be sent to the phone. Several of the major phone standards support it. In CDMA systems, this is controlled by TIA/EIA-637-A.

Subsidy lock refers to a feature of a phone put in there by the phone manufacturers because the service providers want it. It makes the phone only work with a single service provider, even if the phone is compatible with a system belonging to a different service provider. On any other system the phone will be roaming. This is a common feature in the industry with all phone standards. Yes, this is legal. No, there is no way to unlock the phone without the lock code. No, you cannot sue to get the lock code. No, there is no grounds for class action. No, this is not a violation of the antitrust laws. No, it is not a violation of consumer protection laws. For more on this, see here.

Sync channel in CDMA is a special channel which is always transmitted by the cell. It is not modulated by the long code. It repeatedly transmits a sync channel message which contains information about the cell and the phone system, and also contains information which permits the phone to determine the absolute wall clock time. The phone looks for the sync channel as the second step of system acquisition, and uses it to synchronize its long code generator. Once the sync channel message has been processed, the phone has sufficient information to begin to process the paging channel and to register.

TDMA (Time Division Multiple Access) refers to a digital RF link where multiple phones share a single carrier frequency by taking turns. Each phone gets the channel exclusively for a certain time slice, then gives it up while all the other phones take their turn. TDMA is also used sometimes to refer specifically to the standard covered by IS-136, which is a source of confusion because GSM also uses a TDMA air interface, as does IDEN, and neither of those systems are compatible with IS-136.

Traffic channel in CDMA is a channel which carries a phone call. When a phone wants to set up a call, it makes a request to the cell on the paging channel and the cell system sends back a message telling it which traffic channel to use (in other words, which Walsh Code to use).

Walsh code is one of 64 chip patterns which are 64 chips long. CDMA channels are differentiated by which Walsh code they use. They are carefully chosen to be orthogonal to each other.

Zone refers to one or more sectors of one or more cells. It is an administrative category in CDMA. Movement within a zone does not require the phone to reregister. The phone has to register if it crosses a zone boundary. Read this for more information.

 

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