Data communication and network-mobile phone

A mobile phone is designed to provide communication between two mobile units called a mobile base station (MS), or between a mobile unit and a fixed unit. The service provider must be able to locate and track the caller and assign a channel to the call. When the caller moves beyond the permitted range, the communication should be transferred from one base station to another.

To make this tracing possible, each mobile phone service area is divided into small areas called cells ). Each cell contains an antenna. It is also controlled by a network site called a base station (BS). Each base station is controlled by an exchange board called a mobile exchange center (MSC. MSC coordinates communication between all base stations and telephone centers. It is a computing center that estimates call connections, records call information and billing.

The cell size is not fixed. The typical radius is 1 ~ 12 miles. Once the cell size is determined, it should be the best size to prevent interference between neighboring cell signals. The transmission energy of each cell is kept at a low level to prevent interference by other cells.

Principle of frequency reuse

Normally, adjacent cells cannot communicate with each other at the same frequency, because interference occurs when the user is located at the cell boundary, but the available frequency sets are limited, so the frequency must be reused, the Frequency Multiplexing mode is the configuration of N cells, and N is the multiplexing factor.

In, each label uses the desired frequency, but because it is not adjacent, it will not produce interference.

1st Generation Mobile Communication

AMPS (Advanced Mobile Phone System) is a leading analog mobile phone system that uses FDMA in links (see http://blog.csdn.net/todd911/article/details/9303685 for details) to separate channels.

Amps works in the ism m band. The system uses two separate analog channels, one for forwarding (from base station to mobile phone), and the other for receiving (from mobile phone to base station ), 824mhz ~ The 849mhz frequency band is used to receive communications ~ The MHz band is used for forwarding communication.

Each frequency band is divided into 832 channels. However, if two providers share one region, this means that each provider has only 416 channels, 21 of which are used for control, there are 395 remaining, and the Frequency Multiplexing factor of APMs is 7, which means that only 395 of the 1/7 channels in a cell can be used.

2nd Generation Mobile Communication

GSM (Global System for Mobile Communication) is a European standard developed to provide universal second-generation mobile communication for all European countries. It aims to replace many incompatible 1st-generation mobile communication technologies.

GSM uses two frequency bands to implement duplex communication. The width of each frequency band is 25 MHz, and the frequency ranges from 905 to MHz ~ The uplink frequency is 950 to MHz ~ 124 MHz is the downlink frequency. Each frequency band is divided into channels of kHz, which are separated by the Protection frequency band.

A gsm system is displayed. Each voice channel is digitalized and compressed into a digital signal of 13 kbps, each carrying 156.25 bits. 8 time slots are combined to form a TDM frame, and 26 frames are combined to form a multi-frame (TDMA ). We can calculate the bit rate of each channel as follows:

(1/120 ms) * 26*8*156.25 = 270 kbps

Each 270kbps digital channel is modulated into a carrier signal using GMSK. The result is a 124 kHz analog signal. Finally, the kHz analog channels are combined using FDMA and the result is a 25 MHz band.