3. 2.    Operation of cellular systems

The operation of a cellular mobile system can be described as five major functionalities and four additional utilities. All the functions together make a complete mobile cellular system.

Mobile unit initialization

1. When mobile unit is turned on, it scans and selects the strongest setup control channel used for system.
2. Cells with different frequency bands repetitively broadcast on different setup channels.
3. The receiver selects the strongest setup channel and monitors that channel.
4. With this the mobile station has automatically selected the BS antenna of the cell within which it will operate.
5. Then handshake takes place b/w the mobile unit and MTSO controlling this cell through the BS in this cell.
6. Handshake is used to identify the user and register its location.
7. As long as the mobile station is on, scanning is repeated periodically to account for the motion of the unit.
8. If the unit enters a new cell, then a new BS is selected.

Mobile-originated call

9. A mobile unit originates a call by sending the number (Mobile Identification Number, MIN) of the called unit on the preselected setup channel.
10. The receiver of mobile unit checks if the forward channel (from BS) is idle.
11. If idle the mobile may transmit over the reverse channel( To base station)
12. BS sends request to the MTSO.

Paging

13. MTSO attempts to complete connection
14. MTSO sends a paging message to certain BSs depending on called mobile number.
15. BS sends paging signal on its own assigned setup channel.

Call accepted

16. Called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO.
17. MTSO stes up a circuit between calling and called BSs.
18. MTSO selects available traffic channel within each BS’s cell and notifies each BS, which in turn notifies its  mobile unit (a data msg called alert is transmitted over FVC to instruct the mobile to ring).
19. The two mobile units tune to their respective channels.

Ongoing call

20. While connection is maintained, two mobile stations exchange voice or data, through BSs and MTSO.

Handoff

21. If a mobile unit moves from range of one cell to another the traffic channel has to change.
22. System makes this change without either interrupting the call or alerting the user.

 

Call blocking: If all traffic channels are busy even after multiple attempts a busy tone is returned.

Call termination: When one of the users hangs up, MTSO is informed and the traffic channels are released

Call drop: during a connection if because of interference or weak signal spots, the BS can’t maintain the minimum required signal strength for a certain period of time the traffic channel is dropped and MTSO is informed.

Calls to/from fixed and remote mobile subscriber: MTSO connects to the public switched telephone network. Thus it can setup calls b/w mobile user in its area and fixed subscriber via telephone network, remote MTSO.