Re: cell phone communication frequencies

Hello, Jeremy,

There are currently several different types of cellular phones - what we refer to as 1G, the first generation, is still in use - Analog Cellular, which uses frequency modulation of carrier waves:

We also use digital cellular, which is referred to as 2G, the second generation of cellular phones. There are three standards in use for 2G cellular phones in the U.S. - TDMA, CDMA, and GSM. These standards transmit series of 1's and 0's , so to transmit your voice, the cellular phone must first convert your voice to digital - a series of 1's and 0's - and use that series of 1's and 0's to modulate the carrier wave. Cellular phones use either

frequency (FSK)

or

phase modulation (PSK) to modulate the carrier wave with the series of 1's and 0's.

Analog cellular phones, 1G, use the frequency range of 824 MHz to 849 MHz to transmit to the cellular base station , and the cellular base station transmits to the analog cellular phone using the frequency range of 869 to 894 MHz.

The FCC (Federal Communications Commission) chooses what each frequency range is used for, and who has the right to use that frequency range. Some cordless phones operate on a frequency range of 902 to 928 MHz, for example. However, the equipment must pass rigorous tests to receive FCC approval, and so there is virtually no overlap of the frequencies used even when devices operate on nearby frequencies.

However, one of the problems with analog cellular phones is exactly what you refer to -- the security issue. People can listen in on analog cellular phone calls with the right equipment. That is one of the selling points of digital cellular, or 2G - it is very difficult for anyone to listen in or "tap into" digital cellular conversations. GSM and TDMA are somewhat similar in that the series of 1's and 0's for someone conversation are assigned a frequency channel within the frequency range, and a time slot for that frequency channel. In the United States, GSM, TDMA, and CDMA are assigned two frequency ranges that include the frequency ranges assigned to analog cellular, 824 MHz to 849 MHz and 869 MHz to 894 MHz, and also the frequency ranges of 1850 to 1910 MHz and 1930 MHz to 1990 MHz. In order to use any of these ranges for digital cellular, a carrier in an area is allowed to use some portion of the frequency ranges for analog cellular or GSM or TDMA or CDMA. For example, Verizon Wireless in Knoxville could use frequency channels from 824 MHz to 830 MHz for analog cellular and combine the frequency channels from 835 to 849 MHz for CDMA.

Digital cellular is very difficult for others to listen in on. With TDMA and GSM, someone trying to listen in on a conversation would have to determine which time slot and frequency channel is assigned to the person under surveillance --- however, the time slot and frequency channel will change - for example, if the person is in a car, he or she will likely have to be handed off from one cellular base station to another as he or she moves out of range of the first base station. Also, GSM has a "slow frequency hopping" approach that changes the frequency and timeslot assignment even if someone does not move out of range.

With CDMA, it is even harder to listen in. In fact, CDMA is based on Spread Spectrum technology that was originally developed for military communication applications specifically to prevent enemies from listening in on our military's secure communications. As part of the Spread Spectrum approach used in CDMA, the series of 1's and 0's are combined (the logical combination is called an "exclusive OR") with pseudorandom numbers - numbers that don't repeat for about 2^42-1 bits - even at a million bits per second continually transmitting on the CDMA phone, the pattern wouldn't repeat for over a month!

Anyway, Jeremy, Wireless Communication is a very interesting but also very complex field. You might want to read some books on wireless systems first, like "Wireless Communication" by Theodore Rappaport, and "Applications of CDMA in Wireless/Personal Communications"  by Vijay Kumar Garg,  Ken Smolik  and Joseph Wilkes.

You can also do searches on the web for sites that explain wireless communication. Here are a few:

http://www.privateline.com/Cellbasics/Cellbasics.html - Cellular Basics

http://www.ami.bolton.ac.uk/courseware/comms/ch1/comms01notes.html Introduction to Communication Circuits and Radio Architectures

http://sss-mag.com/ss.html - Tutorial on Spread Spectrum

http://www.electronics-tutorials.com/receivers/receivers.htm - Radio Receiver design