privateline.com logo: Welcome to my site!


Privateline.com: Digital Wireless Basics

Google
The Web Privateline.com


 
SITE MENU
HOME PAGE
Old Home Page
Advertise here
Cell Phone Plans
Cell Phone Basics
Clip Art/Images
Contact Me!
Daily Notes
Digital Basics
Telecom History
Links
Miscellany
Telecom News
Website Docs
Wired Telecom
Wireless Pages
Writers

Sub-Menu
Digital Wireless Basics:
Introduction
Wireless History
Standards
Radio Principles
Cellular defined
Frequency reuse
Cell splitting
Cellular frequencies
Transmitting digital
Wireless systems
Network elements
Wireless categories
Digital principles
Modulation
Speech into digital
Frames, slots & channels
IS-54: D-AMPS
IS-136: TDMA cellular
Call processing
Appendix
Wireless systems
Frequency chart

Reserved

Reserved


 
Basic Wireless Principles: Frequencies

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

V. Cellular, PCS, GSM, and Japanese Digital Cellular Frequencies

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.

 

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 -->

CellTowerInfo.com Good folksText link to CellTowerInfo.com

privateline.com logo http://www.privateline.com: West Sacramento, California, USA. A Tom Farley production

 

 

 
Sponsor

Sponsor

Sponsor

Reserved