Cell_ phone_technology.wpd 8-22-05 clips from www

In AMPS and all existing TDMA systems (including GSM) the bandwidth allocated per user is constant and inherent in the system design. (In IS-136 it's 10 kbps per call, and in GSM it's 25 kbps. Some of that is used up by overhead; it's effectively somewhat less.) When a cell is deployed in a given area, then based on the technology and the amount of spectrum assigned to it there is a hard upper limit on the number of simultaneous calls which can be carried, and nothing can change that. GSM is allocated in 200 khz channels each of which can carry 8 calls. If a cell has 5 of them, it's limited to 40 calls simultaneously.

In CDMA systems (older IS-95, new CDMA2K and the upcoming GSM UMTS system) the bandwidth used by any given call is always been dynamic; it uses as much as it needs, though there were upper limits. CDMA systems use what are called variable rate codecs which means that the actual amount of data sent for any frame (20 milliseconds of sound transmitted as a digital packet) depends on how complex the sound was. It can be "full rate", "half rate", "quarter rate" or "eighth rate", using respectively the full specified bandwidth of the codec, half, a quarter or an eighth of it, and this choice is made each 20 milliseconds based on the sound being sent. And because of the way CDMA works, there are no allocations per phone at the level of bandwidth. They're assigned "channels", but all that means is that they're assigned a Walsh code no one else will be using in that sector. It's not like in GSM where they're assigned one of the 8 slots on a given carrier frequency. Whatever a given CDMA phone isn't using at any given instant is available for the other phones in the sector.

Which is one of the big reasons why CDMA systems are referred to as having "soft capacity". The overall bandwidth available isn't really exact for a number of complicated technical reasons, and in any case there's no way of knowing how much bandwidth any given call will be using at any given instant. CDMA was designed from the beginning to let each active call dynamically change how much data it sent and received, with the ability to change every 20 milliseconds. That was done so that the system as a whole could take advantage of the unneeded bandwidth during the majority of the frames where a phone's codec doesn't need to send a full-rate packet. But they later took advantage of it to support different codec rates, and then to permit digital calls with higher allocations.

What these new features do is to permit a given phone to use more of the available bandwidth, but they don't actually change the overall ceiling for the system. The capacity of a system a function of the spectrum allocated, the number of cells, the geography, and the work of Papa Shannon, and beyond that you cannot go. CDMA2K and UMTS come with reasonable striking distance of the theoretical limit; they're already designed to take advantage of spectrum bandwidth about as effectively as the state of the art permits. Which is not surprising because bandwidth is a serious bottleneck, and there's no good reason to waste it. CDMA as a transmission technology is usually between 3 and 5 times better at this than TDMA depending on circumstances, and most of that gain comes from the fact that TDMA is enormously wasteful, while CDMA is not. (GSM switched to a CDMA air interface with the UMTS system because it really had no choice if it wanted to remain competitive.)

CDMA systems aren't wasting bandwidth now; there isn't any reserve to make available during emergencies.

In an emergency the cell system could temporarily ban all high-bandwidth calls, which would prevent some small number of users from hogging the carrier. And in IS-95 and CDMA2K, the cell system could order all phones to use low-bandwidth vocoders, but they do that anyway. Originally IS-95 had an 8 kilobit-per-second vocoder but it really sounded terrible. They also had a 13 kbps codec which was a lot better, but it uses more bandwidth per call. About five years ago they rolled out a third codec called EVRC (Enhance Variable Rate Codec) which used 8 kbps but sounded nearly as good as the 13K codec, and most systems have already switched to using EVRC as their standard codec. There isn't anything lower.

Text messaging in IS-95 doesn't compete with voice for bandwidth; it's all handled through the paging channel. If, for instance, the system banned all voice calls, it turns out that this would relieve the paging channel of carrying call setup traffic and that would allow somewhat more messaging traffic. But not all that much more as a practical matter; it isn't really worthwhile.

So in AMPS and TDMA systems there isn't any way at all to increase system capacity in emergencies. In CDMA systems you get rid of premium services, but the gain still wouldn't be all that great because most voice traffic is already operating at the lowest level it can be.

The only real way to significantly increase the number of calls during emergencies would be to limit their duration by automatically terminating them after some fixed short interval (e.g. 3 minutes).

Update: Folks, forget about easy answers. There are no easy answers.

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V. Cellular, PCS, GSM, and Japanese Digital Cellular Frequencies http://www.privateline.com/PCS/Frequencies.htm

The following table lists a few frequency allocations for common cellular and PCS services around the world. It's immediately clear why you can't easily use your cell phone while traveling: different countries use different frequencies. Today's mobiles can't tune themselves automatically to the frequencies they find, they need the right hardware, not just software, to use different frequencies. That's why you need a so called dual or triple band phone to use a mobile overseas; these units have additional circuitry built in to use the different frequencies in the countries you might travel to.

Dual mode phones, by comparison, are those few that operate in, say, a digital CDMA operating system, but use a non digital system like AMPS when no PCS signal is found. Sprint and others make these phones. The future promises more operating systems than today and far more different frequency allocations. A single wireless standard based on a common frequency and operating system will be nearly impossible to achieve. It makes sense then to build radios which accommodate different frequencies and protocols. "Smart" radios and "smart" antennas. But I am getting ahead of myself. Back to frequencies.

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Article written by John Scourias, with comments in maroon by Tom Farley

http://www.privateline.com/PCS/GSM05.htmlhttp://www.privateline.com/PCS/GSM05.html

4.1 Multiple access and channel structure

Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.

This is the correct, complete view of GSM. It's not enough to say, as I have too many times, that GSM and conventional cellular (IS-136) are TDMA based. While that it is true, it is more true to say such systems are TDMA and FDM based. First, we have a number of radio frequencies, each separated by 200khz. This is the frequency division multiplexing part. (Or the FDMA part, a minor semantic difference.) Secondly, we have the transmission technology, TDMA, by which we put several calls on a single frequency. These calls are broken into many pieces, each piece of each call sent one after another. Each call separated by slight differences in time. GSM is a TDMA/FDMA system.

Weick calls a burst "a sequence of signals counted as a unit in accordance with some specific criterion or measure." Bits are single pulses of electrical energy. Much like the single dash of a Morse Code key. With Morse code we use long and short pulses of energy to stand for letters. Although of uniform length, the pulses we use in digital radio do the same thing. Bits grouped in patterns represent voice and data. We also use bits, as shown in the diagram below, for signaling. In the channel depicted a burst of bits is a marker, an indicator, a signal within a signal. It's what the mobile first looks for in the digital stream flowing from the base station. More on this on the next page.

Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.

Terminology alert! Cellular radio uses the word channel in many ways. It is a pair of radio frequencies. And channels are part of the digital stream that flows back and forth from the mobile to the base station. Channels, therefore, can be carried on a channel. Confusing, isn't it? The discussion below focuses on data channels, not radio channels.

4.1.1. Traffic channels

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused (see Figure 2). TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics.

1. General frequency table

American Cellular

AMPS, N-AMPS, D-AMPS (IS-136) CDMA

824-849 MHz

869-894 MHz

Mobile to base

Base to mobile

American PCS

Narrowband	901-941 MHz

Broadband

1850-1910MHz

1930-1990 MHz

Mobile to base

Base to mobile

E-TACS

872-905 MHz

917-950 MHz

Mobile to base

Base to mobile

GSM

930-960MHz

890-915MHz

JDC

810-826 MHz

940-956 MHz

1429-1441 MHz

1477-1489 MHz

Mobile to base

Base to mobile

Base to mobile

Mobile to base

2. Wireless frequencies and the microwave band

United States cellular and PCS frequencies lie in the ITU (external link) recognized UHF or ultra high frequency band. That band runs between 300 MHz and 3000 MHz (3GHz). T.V. channels 14 to 70 also occupy this large band, ranging from 470 to 806 MHz. More specifically, cellular frequencies start at 824 MHz and end at 894 MHz. PCS broadband freqs go from1850 MHz to1990 MHz. The radio spectrum cellular and PCS occupy also places them in the arbitarily termed microwave band, encompassing frequencies between 1 GHz (1000 MHz) and 100 GHz. This means many things.

At these wavelengths radio frequencies behave like light. For the mobile, low powered light waves since the FCC lets mobile use just a few watts and, in actual practice, more often milliwatts. (The base station, by comparison, uses much more power. "PCS base stations put out more than 200 watts. A Motorola 800 MHz CDMA system is putting out more than twice that. In analog, we often used 100 watts per channel in rural areas." [Van Der Hoek] ) I digressed. I was trying to compare microwaves to lightwaves and the problems that causes.

To use Cannon and Luecke's analogy, microwaves act like narrowly focused flashlights: they travel short distances, are directional, work best in a straight line, and get reflected or absorbed by obstacles. Tall buildings, billboards, and even large trucks cause havoc. What Lee calls 'local scatterers.' Unless a system is properly engineered, especially one using 1900 MHz frequencies within a large city, dropped calls may frequently result. Omnipoint, for example, initially employed only 160 base stations for New York City, an inadequate number for the conditions. They now have over 500, with base stations nearly every ten blocks and some cells covering particular streets. [The New York Times]

3. Frequencies and bandwidth

Cellular and PCS occupy 50 megahertz and 140 MHz worth of radio frequency spectrum respectively. By comparison, the entire AM broadcast band takes up only 1.17 megahertz. That band, however, provides only 107 broadcast frequencies. Cellular provides thousands of frequencies to carry conversations and data. The many frequencies and their large channel width account for the large amount of spectrum used. Advanced Mobile Phone Service or AMPS uses 832 channels that are 30 kHz wide. Digital systems like IS-95 (CDMA) and the TDMA based IS-54B (now folded into IS-136), provide more channels in the same space. Let's back up a little.

I mentioned that a typical cell channel is 30 kilohertz wide compared to the ten kHz allowed an AM radio station How is it possible, you might ask, that a one to three watt cellular phone call takes up a path three times wider than a 50,000 watt broadcast signal? Power does not necessarily relate to bandwidth. A high powered signal might take up lots of room or a high powered signal might be narrowly focused. A wider channel helps with audio quality, that's what's important. An FM stereo station, for example, uses a 150 kHz channel to provide the best quality sound. A 30 kHz cellular channel gives you good sound almost automatically, nearly on par with the normal telephone network. We'll see later how TDMA puts three calls within a 30KHz channel, and describe the technological struggle to keep up sound quality.

4. Offsets: Transmit and Receive Frequencies

In AMPS, IS-54B, IS-36, and PCS 1900, 45 MHz speparates transmit and recieve frequencies. That keeps them from interfering with each other and allows simultaneous talking. For example, in the conventional cellular band, mobiles use frequencies 824.04 MHz to 848.97MHz and the base stations operate on 869.04 MHz to 893.97 MHz.

To see how this works, let's look at eight frequencies in a single cell of a single carrier. Assume for the moment that this is one of 21 cells in either an AMPS or or IS-136 system. For IS-136 at 1900 MHz and PCS the channel width (30KHz) remains the same but the offset is greater: 80 Mhz.

Cell#1 of 21 in Band A (The nonwireline carrier)

Channel 1 (333) Tx 879.990 Rx 834.990(The control channel in AMPS)

Channel 2 (312) Tx 879.360 Rx 834.360

Channel 3 (291) Tx 878.730 Rx 833.730

Channel 4 (270) Tx 878.100 Rx 833.100

Channel 5 (249) Tx 877.470 Rx 832.470

Channel 6 (228) Tx 876.840 Rx 831.840

Channel 7 (207) Tx 876.210 Rx 831.210

Channel 8 (186) Tx 875.580 Rx 830.580

etc., etc., etc.,

(Each cell has at least 15 frequencies or channels)

Get the idea of offsets? Check out the animated gif below, modified only slightly from Marshall Brain's award winning, very cool site. Note what we call these frequencies: the reverse channel and the forward channel. They're what makes talking at the same time possible. In the case of analog and TDMA systems the cellular carrier assigns each transmit and receive frequency for each cell in advance. The MTSO or base station controller then chooses from those frequencies for your call.

Animated gif

Frequency offsets and forward and reverse channels depicted. The base station transmits

on the forward channel and the mobile transmits on the reverse channel.

PCS frequencies as mentioned above are offset as well. One more thing. A transmit and receive frequency are often called paired frequencies. That seems logical enough since it takes two frequencies to pass information. Unfortunately, the forward and reverse channels refer to just a single frequency, making a channel definition muddy. For now, think of a channel as a communication path, no matter what form or frequencies make it up. Still following me? Good. Since we've been talking about frequencies, for the most detailed diagram of cellular and PCS frequencies on the web, click here or on the chart below. It's from the Webproforum.

(back to Cell Basics article)

Click on the chart below!

Frequency chart

American cell phone frequencies start at 824 MHz and end at 894 MHz. The band isn't continuous, though, it runs from 824 to 849MHz, and then from 869 to 894. Airphone, Nextel, SMR, and public safety services use the bandwidth between the two cellular blocks. Cellular takes up 50 megahertz total. Quite a chunk. By comparison, the AM broadcast band takes up only 1.17 megahertz of space. That band, however, provides only 107 frequencies to broadcast on. Cellular provides thousands of frequencies to carry conversations and data. T

5. Frequency blocks and licenses

a. Cellular - 800 MHz

Now things get really dry. Hold on. As we'll see in detail later, North American cellular development got going in earnest after the Bell System breakup in 1984. To foster competition in a limited radio spectrum, the United States licensed two carriers in every large metropolitan area. One license went automatically to the local telephone company, the local exchange carriers or LECs. Or as telco talk puts it, the wireline carriers. Companies like Ameritech or Pacific Bell. The other went to an individual, a company or a group of investors who met a long list of requirements and who properly petitioned the FCC. The non-wireline carriers. Groups like Cellular One.

Each company in each area took half the spectrum available. What's called the "A Band" and the "B Band." The nonwireline carriers usually got the A Band and the wireline carriers got the B band. There's no real advantage to having either one. It's important to remember, though, that depending on the technology used, one carrier might provide three times the connections a competitor does with the same amount of spectrum. Now that we've gotten through the cellular band, let's move up the spectrum.

b. PCS-1900 MHz

From 1995 to 1997 the FCC licensed the so called PCS or Personal Communication Service spectrum, the area around 1900 MHz and some additional radio space around 900 Mhz. It's here where most TDMA based GSM systems are, as well as the CDMA based IS-95 system.

The FCC calls the two PCS spectrum blocks broadband and narrowband frequencies. To make things confusing, PCS licenses differ in bandwidth size from cellular licenses. PCS operators can have two different sized licenses: 30 MHz and 10 MHz, of which they are allowed to put together. Six PCS licenses exist for each market. It's said that "the real advantage for PCS is that the 30 MHz and 10 MHz licenses are contiguous, which cuts down on the cost of infrastructure and subscriber equipment. So, the advantages for PCS are more capacity, lower infrastructure cost, and lower subscriber costs." Speaking of the Personal Communications Service, the FCC divided it into two sections, which we should look at now.

6.The PCS band

a. Narrowband

Lower in the spectrum than wideband PCS, Narrowband PCS uses narrower frequency blocks. Less room means N-PCS is better suited for advanced paging services. Narrowband's spectrum falls into these frequency ranges: 901-902MHz, 930-931 MHz, and 940-941 MHz

50 kHz wide paired and unpaired channels make up narrowband's frequency ranges. 12.5 kHz response channels for existing paging licenses also exist. Besides paging services, something this spectrum isn't limited to by regulation, N-PCS can be used for telemetry, such as remotely monitoring gas and electric meters. Even keeping track of copier usage or vending machines. I won't discuss PCS narrowband very much because, quite honestly, I'm not that interested. I like voice communications, not data comms. In addition, each technology can differ widely from another. So little would be gained in understanding PCS in general by exploring paging system nuances. But feel free to go further by exploring these company websites: all external links: SkyTel, Paging Network, AT&T Wireless Services.

B. Broadband

Broadband PCS belongs in the microwave band near 2GHz., utilizing 30 MHz wide frequency blocks. This room allows voice, data, and video. Of the 140 MHz allotted, 20MHz is reserved for "unlicensed applications that could include both data and voice services." [FCC external link] Broadband's spectrum falls into the frequency range of 1850MHz to 1990.

Within each range are scattered frequency blocks. The A, B, and C blocks are 30 MHz wide while the D, E, and F blocks are 10 MHz wide. Check out this illustration from the Cellular Development Group (external link.)

frrequency chart

MS means Mobile Station and BS means base station. Don't worry about remembering exact frequency allocations; it's enough to know that most voice based PCS telephony operates around 2GHz. To remember it by, GSM 1900 refers to 1900 MHz. And IS-136 is often called D-AMPS 1900. For a much more detailed look at the cellular and PCS spectrum, click here to go to Webforum's most excellent PCS primer:

http://www.iec.org/online/tutorials/ (external link)

Sources

Don L. Cannon and Gerald Luecke. Understanding Communications Systems, (Indianapolis: Howard W. Sams & Co. 1984) p. 94

47 CFR24 Title 47--Telecommunication Chapter 1, FCC, Part 24 Personal Communications Services

Mark Van Der Hoek, personal correspondence. "Consider the differences in the antenna. The mobile has, typically, a unity gain (0 dB gain) antenna. The base station (PCS) will have 18 to 20 dBi gain. So the big signal put out by the base station is received by the puny little mobile's ears, while the puny little signal put out by the mobile is heard by the base station big ears."

Landler, Mark. "Yipes! Invasion of the 9-inch antennas! A new form of wireless phone service is in the works for New York City." (Omnipoint Communications to offer wireless personal communications services) (Company Business and Marketing) NewYork Times v145 (August 19, 1996):C1(N), D1(L).

Meyers, Jason. "To the point" Telephony, Aug 18, 1997,30-32. Intertec Publishing Corp 1997 Company Profile with some interesting operating details.

<-- Last topic: Wireless Principles Next topic: Multiplexing -->

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4.2 Speech coding

Speech coding means turning voice into digital. I've written much on this subject so be sure to click on the links below if there are points you don't understand . . .

GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal, although simple to implement, contains much redundancy. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop.

Conventional cellular uses an equally intimidating algorithm named Vector Sum Excited Linear Predictive speech compression. Ugh. Click here to learn about it.

Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.

This is the subject of digital signal processing. Read about it here.

This is the so-called Full-Rate speech coding. Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate.

Nokia said in January, 1997 that they would start shipping Enhanced Full Rate voice codecs by March 1997: http://press.nokia.com/PR/199701/775480_5.html; I must assume their use is now wide spread.

4.3 Channel coding and modulation

Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Radio waves are a rough medium to transmit fragile data over; we need a way to protect that information. We do so with error checking, mathematical routines that check and then double-check the integrity of our data. These methods contribute greatly to the overhead in a digital stream, adding a tremendous amount of bits, and thus dramatically cutting down on data speed. It's one reason data transfer rates are only 9.6kbs. This is a complex subject, one I haven't written much on.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:

* Class Ia 50 bits - most sensitive to bit errors

* Class Ib 132 bits - moderately sensitive to bit errors

* Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co-existence of GSM and the older analog systems (at least for the time being).

For much, much more on GMSK, read Professor Levine's comments by clicking here. This discussion is quite advanced.

You can read my writing on modulation by clicking here.

4.4 Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

Here are two old Western Union images. The top graphic shows transmission without a delay equalizer. The image below it shows the same transmission corrected by a delay equalizer.

Equalizer illustration

Above. No equalizer.

Above. Delay equalizer introduced. Pretty dramatic difference, eh?

4.5 Frequency hopping

The mobile station already has to be frequency agile, meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame, which normally are on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping, where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is dependent on carrier frequency, slow frequency hopping helps alleviate the problem. In addition, co-channel interference is in effect randomized.

Here's a huge difference between conventional cellular (IS-136) and GSM: frequency hopping. When enabled, slots within frames can leapfrog from one frequency to another. In IS-136, by comparison, once assigned a channel your call stays on that pair of radio frequencies until the call is over or you have moved to another cell.

Three frogs a leapin . . .

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