How Cell Phones Works
1. What is a Cell Phone?
Millions of people around the world use cellular phones. With a cell phone, you can talk to anyone from just about anywhere on the planet!
These days, cell phones provide an incredible array of functions. Depending on the cell-phone model, you can:
2. Cell-Phone Frequencies
Before cell phones, people who needed mobile-communications installed radio-telephones in their cars. In the radio-telephone, there were one central antenna tower per city, and perhaps 25 channels available on that tower. This central a powerful transmitter--big enough to transmit about 70km. It also mean meant that not many people could use radio telephones--there just were not enough channels.
The genius of the cellular system is the division of a city into small cells. this allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.
A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.
Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use one frequency for talking and a second, separate frequency for listening.
Channels - A walkie-talkie typically has one typical has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more!
Range - A walkie-talkie can transmit about 1.6km. A CB radio can transmit about 8km. Cell phones operate within cells, and they can switch ells as they move around. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.
In a typical analog cell-phone, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 26 square kilometers.
Each cell has a base station that consists of a tower and a small building containing the radio equipment.
3. Cell-Phone Channels
In any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA - based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available.
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts. The base station is also transmitting at low power.
The cellular approach requires a large number of base stations in a city of any size.
4. Digital Cell Phone
Digital phones convert your voice into binary information (1s and 0s) and then compress it. This compression allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.
Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the cell tower and the phone. Clever modulation and encoding schemes are required to convert the analog information to digital, compress it and convert it back again while maintaining an acceptable level of voice quality. All of this means that digital cell phones have to contain a lot of processing power.
Prepare by: Ung Yean
Reference
Millions of people around the world use cellular phones. With a cell phone, you can talk to anyone from just about anywhere on the planet!
These days, cell phones provide an incredible array of functions. Depending on the cell-phone model, you can:
- Store contact information
- Make task or to-do lists
- Keep track of appointments and set remiders
- Use the built-in calculator for simple math
- Send or receive SMS and e-mail
- Get information (news, entertainment, stock quotes) from the Internet
- Play games
- Watch TV
- Integrate other devices such as PDAs, MP3 Players and GPS receives
2. Cell-Phone Frequencies
Before cell phones, people who needed mobile-communications installed radio-telephones in their cars. In the radio-telephone, there were one central antenna tower per city, and perhaps 25 channels available on that tower. This central a powerful transmitter--big enough to transmit about 70km. It also mean meant that not many people could use radio telephones--there just were not enough channels.
The genius of the cellular system is the division of a city into small cells. this allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.
A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.
Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use one frequency for talking and a second, separate frequency for listening.
Channels - A walkie-talkie typically has one typical has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more!
Range - A walkie-talkie can transmit about 1.6km. A CB radio can transmit about 8km. Cell phones operate within cells, and they can switch ells as they move around. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.
In a typical analog cell-phone, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 26 square kilometers.
Each cell has a base station that consists of a tower and a small building containing the radio equipment.
3. Cell-Phone Channels
In any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA - based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available.
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts. The base station is also transmitting at low power.
The cellular approach requires a large number of base stations in a city of any size.
4. Digital Cell Phone
Digital phones convert your voice into binary information (1s and 0s) and then compress it. This compression allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.
Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the cell tower and the phone. Clever modulation and encoding schemes are required to convert the analog information to digital, compress it and convert it back again while maintaining an acceptable level of voice quality. All of this means that digital cell phones have to contain a lot of processing power.
Prepare by: Ung Yean
Reference
- NU Newsletter, Updating Edition, September 2011, p. 168
