Cellular mobile communication system

Author：admin  Clicks：9  Release time：2026-01-19

Cellular mobile communication system
A cellular mobile communication system provides wireless connectivity to the Public Switched Telephone Network (PSTN) for users within its radio wave coverage area. By utilizing limited spectrum resources, cellular systems can accommodate a large number of users distributed over a large geographical area and provide high-quality service comparable to that of wired telephones. In a cellular mobile communication system, the coverage area of each base station transmitter is limited to a small geographical area called a "cell". Cellular mobile communication systems use a sophisticated switching technology called "handoff" that allows users to move from one cell to another without interrupting their calls.

Figure 3-8 shows a basic cellular system consisting of a Mobile Station (MS), a Base Station (BS), and a Mobile Switching Center (MSC). (In the figure, the tower represents the base station that provides wireless access between mobile users and the MSC.) The Mobile Switching Center is sometimes referred to as a Mobile Telephone Switching Office (MTSO) because it is responsible for connecting all mobile telephones to the PSTN in the cellular system. Each mobile station communicates with the base station in its cell via radio waves. During a call, when it moves into another cell, it can hand off to a new base station to continue the call. The mobile station includes a radio transceiver, an antenna, and control circuitry. It can be installed in a car or be a portable mobile phone. The base station consists of several sets of radio transceivers for duplex communication. It connects mobile phones to the MSC in parallel via telephone lines or microwave links, acting as a bridge. The MSC coordinates the work of all base stations and connects the entire cellular system to the PSTN. A typical MSC can manage 100,000 cellular subscribers, handle 5,000 simultaneous calls, and perform all billing and system maintenance functions. In large cities, a cellular mobile communication system typically includes several MSCs.

The Common Air Interface (CAI) standard specifies the communication interface standard between the base station and the mobile phone, and it defines four different channels. The voice transmission channel from the base station to the mobile phone is called the Forward (Downlink) Voice Channel (FVC), and the voice transmission channel from the mobile station to the base station is called the Reverse (Uplink) Voice Channel (RVC). The two channels responsible for establishing and controlling the call process are the Forward (Downlink) Control Channel (FCC) and the Reverse (Uplink) Control Channel (RCC). The control channels are only responsible for establishing a call and transferring it to an unused voice channel. The control channels themselves are not used for voice communication but only send and receive data messages related to call establishment and teardown and service requests.

Based on the concept of Frequency Reuse, cellular systems require that the Forward (Downlink) Frequency Control Channels (FCC) in adjacent cell areas be different. By specifying a small number of FCCs as part of the common air interface, cellular mobile stations can be manufactured by multiple companies. In this way, the cellular mobile station can quickly search all possible Forward (Downlink) Control Channels at any time and finally determine the channel with the strongest signal, thus determining the cell in which it is located. Once the strongest signal is found, the mobile station locks onto that particular FCC. The MSC broadcasts the corresponding system data on all FCCs, which can send signals to all users of the cellular system. When the MSC receives a call to a mobile subscriber from the PSTN, it can cause the called mobile subscriber to receive a signal.

When a mobile station (such as a mobile phone) is turned on, it cannot immediately make or receive calls. The mobile phone first searches the Forward (Downlink) Control Channel group to determine the channel with the strongest signal and then monitors that control channel. Once the signal level is too low, it searches the control channel again to find the strongest base station signal. For each cellular system, its control channels are generally defined and standardized over the entire geometric area covered. Typically, about 5% of the total number of available channels in the system is used as control channels (the other approximately 95% of the channels are used for voice and data transmission). Control channel standardization ensures that different coverage areas within a country or region have the same control channel groups, so every mobile phone searches the same control channel groups. When there is a call to a mobile subscriber, the MSC sends a command to all base stations in the relevant area of the cellular system, and then broadcasts the Mobile User Identification Number (MIN) and the mobile user's telephone number as a paging message on all Forward (Downlink) Control Channels within the relevant base stations. After the mobile station receives the information sent by the base station it is monitoring, it sends a confirmation message back through the Reverse (Uplink) Control Channel. The base station forwards the confirmation message sent by the mobile station, notifying the MSC that a link has been established. Then, the MSC instructs the base station to transfer the call to an unused idle voice channel within the cell. In this way, the base station notifies the mobile station to change the frequency to a pair of unused Forward and Reverse (Downlink and Uplink) Voice Channels, and also sends some additional information on the Forward Voice Channel to cause the mobile phone to ring, reminding the user to answer the phone. All these steps are completed in a few seconds, and the user will not notice it at all.

When a mobile station initiates a call, the mobile station sends its Mobile User Identification Number (MSIN), its own telephone number, and the called party's telephone number on the Reverse (Uplink) Control Channel. The base station receives this data and transmits it to the MSC, which confirms the request, connects the called party through the PSTN, and then instructs the base station and the mobile user to transfer to a pair of unused idle Forward and Reverse (Downlink and Uplink) Voice Channels to begin the call.

When a mobile user gradually enters or leaves the coverage area of a base station during a call, the MSC automatically adjusts the mobile station's transmit power to ensure call quality. If this still cannot meet the requirements of call quality, the MSC will, through a certain procedure, allow another base station closer to the mobile station to take over the original base station and continue to communicate with the mobile station, and correspondingly change the channel between the mobile station and the base station. This is the so-called "handoff". By inserting special control signaling in the voice channel, the base station and the MSC can control the mobile station during the call. All cellular systems provide a service called "Roaming". This service allows users to continue to receive the same communication services in other service areas outside the paid service area. When a mobile user leaves their registered area and enters another city or geographic area, they register as a roamer in the new service area. If a roamer has paid and has roaming rights, the MSC registers them as a legitimate roamer. Once registered, the roaming mobile user can make two-way calls in the area, just as they would in their original registered area.