The full name of GSM is the Global System for Mobile Communications, commonly known as Global GSM Communication system.
It is a digital mobile phone network standard developed by Europe. Its development purpose is to let all parts of the world use a mobile phone network standard so that users can use a mobile phone to travel everywhere globally.
The GSM communication system includes several frequency bands such as GSM 900: 900MHz, GSM1800: 1800MHz, and GSM-1900 and 1900MHz.
History of GSM communication system
The Global System for Mobile Communications (GSM) is by far the most successful global mobile communications system. Its development began in 1982.
The European Telecommunications Management Conference (CEPT), the predecessor of the European Telecommunications Standards Institute (ETSI), established the Groupe Speciale Mobile, which has been authorized to improve many of the recommendations related to the pan-European digital mobile communication system. The two goals I tried to accomplish were:
First, better and more effective technical solutions for wireless communication at that time, digital systems were superior to analog systems that were still popular at the time in terms of user capacity, ease of use, and the number of possible additional services. The system is already obvious.
Second, to achieve a unified standard across Europe to support roaming across national borders. This was impossible in the past because all countries use incompatible analog systems.
In the early 1990s, people realized that GSM should have some features that were not included in the original standard. Therefore, the so-called phase-2 specifications including these functions were not developed until 1995. However, further functional enhancements including the more efficient modulation schemes adopted by packet radio (GPRS) and EDGE were gradually introduced later.
Based on these expansions, GSM is often referred to as a 2.5-generation system, because its functions are more powerful than those of the second-generation system, but it does not have all the functions of the third-generation system.
There are three versions of GSM, each of which uses a different carrier frequency. The original GSM communication system used a carrier frequency of around 900 MHz. Later, GSM-1800, or DCS-1800, was added to support the ever-increasing number of users. The carrier frequency it uses is around 1800MHz, the total available bandwidth is about three times the available bandwidth of around 900MHz, and it reduces the maximum transmit power of the mobile station.
GSM-1800 is exactly the same as the original GSM. Therefore, signal processing, switching technology, etc. can be used in the same way without any changes.
A higher carrier frequency means greater path loss, and at the same time, a reduction in transmit power will result in a significant reduction in cell size. This practical effect, together with the wider available bandwidth, allows the network capacity to be considerably expanded.
The third system is called GSM1900 or PCS-1900 (Personal Communication System), which works on the 1900MHz carrier frequency and is mainly used in the United States.
GSM is an open standard. As long as the interface is specified, the specific implementation form is not restricted. Let us consider the modulation method adopted by GSM, namely GMSK. The GSM standard specifies the upper limit of out-of-band emission, phase jitter, intermodulation products, and other content to achieve the required linearity.
Technical Features of GSM Communication System
GSM has the following technical characteristics:
(1) The GSM communication system is composed of several subsystems that can be interconnected with various public communication networks. Standardized interface specifications are clear and in detail defined between each sub-system or between each sub-system and various public communication networks to ensure GSM communication systems or subsystems provided by any manufacturer can be interconnected.
(2) The GSM communication system has a flexible and convenient network structure, and the frequency repetition rate is high.
(3) In addition to providing voice services, the GSM communication system can also provide various digital services.
(4) GSM communication system can provide international automatic roaming function.
(5) A multiple access method combining time division multiple access (TDMA) and frequency division multiple access (FDMA) is adopted. Frequency division multiple access is used to share frequency bands between different cells, and time division multiple access is used to distinguish the same frequency point. Different users use different time slots.
(6) Adopting digital voice and digital modulation technology, the modulation method is Gaussian Minimum Frequency Shift Keying (GMSK), which has good performance in the case of co-channel interference.
The network structure and functions of the GSM communication system
The GSM communication system is composed of four major parts: mobile station (MS), base station subsystem (BSS), network subsystem (NSS) and operation support system (OSS). The network structure of the GSM communication system is shown in the figure, and the main functions of each part are summarized as follows.
(1) Mobile Station (MS) Mobile stations are user equipment, such as vehicle-mounted, portable, and handheld types. The mobile station (MS) is composed of a mobile terminal and a customer identification card (SIM card).
The mobile terminal completes functions such as voice coding, channel coding, information encryption, modulation and demodulation, and information sending/receiving; the SIM card stores information and part of the management information required for customer identity authentication.
(2) The base station subsystem (BSS) is composed of a base station controller (BSC) and a base station receiving/transmitting station (BTS).
A base station controller (BSC) controls one or more BTSs and is responsible for functions such as wireless network resource management, cell configuration, data management, power control, positioning, and handover. Each base station receiving/transmitting station (BTS) serves a cell and is responsible for providing an air interface U for mobile terminals.
BTS includes various hardware and software required for wireless transmissions, such as transmitters, receivers, antennas, interface circuits, and detection and control devices.
(3) The network subsystem (NSS) mainly plays the functions of switching, mobility management, and security management.
The mobile switching center (MSC) is the core of the network. It manages one or more BSCs. It is a functional entity that controls the mobile terminals in the covered area and completes the voice signal exchange.
It is also the mobile communication network and other public communication networks (ISDN). , PSTN) interface (GMSC is the gateway mobile switching center), so that various services can be used by users.
The Home Location Register (HLR) is a database that stores the information of mobile users registered in the area under the jurisdiction of the MSC, which mainly includes two types of information, the user’s subscription information, and the current location.
The visitor location register (VLR) is also a database that stores all the information of mobile users entering its coverage area, enabling the mobile switching center to establish incoming and outgoing calls.
(4) The operation support system (OSS) is composed of an authentication center, a mobile device identification register, and an operation and maintenance center.
The Authentication Center (AUC) stores necessary information such as authentication parameters to ensure that mobile users’ communications are not violated.
The mobile equipment identification register (EIR) realizes the functions of identifying, monitoring, and locking mobile equipment, and prohibits the use of some illegal mobile stations.
The Operation and Maintenance Center (O&M/OMC) is used to monitor and maintain all network units.
The GSM communication system mode and principle
Early analog mobile communication systems adopted a large-area networking method, that is, a base station was used to cover the entire service area, but the number of channels that a base station can provide is limited, and the coverage area is limited.
In order to break through the limitation of user capacity, the current cell system networking method is adopted, that is, the entire service area is divided into several cells, and the radius of each cell is about 1-30km depending on the user density.
The cells generally use hexagons, because the overall layout of multiple regular hexagonal cells resembles a honeycomb, and each cell resembles a honeycomb, so it is called a cellular mobile communication system.
In each cell, a low-power base station that can only effectively cover the cell is set up, responsible for wireless communication with mobile stations in the cell, and an MSC is set up to control several base stations.
The base station is connected to the MSC through microwave, terrestrial cable, or optical fiber transmission, and the MSC is connected to public networks such as PSTN, ISDN, etc., so as to realize the transfer of mobile user communication between the cells, and the communication link between mobile users and fixed users.
Cellular telephone networks can solve the problem of lack of frequency resources through frequency reuse. Frequency reuse refers to the use of the same frequency group for coverage in different cells separated by a certain distance. These cells must maintain sufficient spacing to suppress co-frequency interference.
In GSM, 3, 4, or 7 cells form a cluster (cluster), and the same frequency channels are not used in the cluster. The frequency group of each cell contains multiple frequency channels.
The typical allocation scheme is shown in the figure below, and the number of frequency reuse factors is N=7. Obviously, the distance between two base stations using the same frequency group will increase with the increase of v, thereby reducing co-channel interference;
On the other hand, under the same frequency resources, increasing N will reduce the frequency channels allocated for each cell, which will reduce the frequency resource utilization efficiency of the cellular system.
Pictured: Frequency reuse of cellular system when N=7
All base stations serving a cell phone cluster are permanently connected to one MSC. If the mobile station moves to another cell, it will send a signal to the MSC through the BSC so that the HLR, the new VLR, and the old VLR will all be notified to update the corresponding information.
When a phone calls a certain mobile station, the call is first sent to the gateway mobile switching center GMSC. After querying the HLR to obtain the MSC in the area where the mobile station is currently located, it sends the first address information to the MSC, and the MSC queries the VLR The location of the mobile station and the base station in the jurisdiction to page this mobile station, and then the called mobile station will send a paging response to the base station to establish a call.
GSM communication system interface and business support
Network interface
The network interface of GSM mainly includes Um interface, Sm interface, A interface, MAP interface, MSC and PSTN interface, and so on. At present, these interfaces have achieved relatively complete standardization, achieving the purpose of interconnecting products of different manufacturers.
Wireless interface
The interface between the mobile terminal and the network is a wireless interface, which can be divided into three layers from bottom to top according to the protocol, the physical layer, the data link layer, and the network layer. The third layer can be divided into three sub-layers:
The wireless resource (I flood) management layer is responsible for completing the establishment, operation, and release of the dedicated wireless channel connection. It is carried out between the mobile station and the base station subsystem.
The mobility management (MM) layer is responsible for the completion of location updates, authentication, and temporary mobile subscriber number assignment.
The connection management (CM) layer is responsible for completing circuit-switched call establishment, maintenance, and termination, and supports supplementary services and short message services. MM, layer, and CM layer is the communication between the mobile station and the MSC directly.
GSM business support
GSM can provide two major types of services, namely basic services and supplementary services. Among them, the basic services include telecommunications services and bearer services; supplementary services are supplements to the basic services and cannot be provided separately.
Telecommunications business refers to end-to-end business, which includes the OSI seven-layer protocol, and has services such as telephone, short message, and three types of fax.
The short message service mentioned here can be divided into two types, one is the point-to-point short message service, which can be sent directly from the mobile station;
If your mobile phone is connected to the PC, you can input it through the PC, send it to the Short Message Center (SC) from the GSM network, and then forward it to the other party.
The second is the cell broadcast short message service, which can download data to the BSC through the SC, and the BSC controls sending some public information (such as traffic, stock market, etc.) to a certain cell or certain cells.
If you want to implement data services, you can connect the data card (ie, terminal adapter card) that matches the mobile phone to a PC, fax machine, and other equipment to perform data communication.
The bearer service is a service provided at two terminal/network interfaces (access point R/S), and it only includes the lower three layers of OSI.
The development of the GSM communication system
At present, the development trend of GSM is to evolve to the third-generation mobile communication-wcDMA (Wideband Code Division Multiple Access) technologies through GPRS technology.
GPRS stands for General Radio Packet Service and is a data service enhancement technology based on the GSM communication system.
It superimposes a new network on the basis of GSM, and at the same time adds some hardware devices and software upgrades to the network, forming a new network logical entity, providing end-to-end, wide-area wireless IP connection, the introduction of packet switching technology into the existing GSM communication system and the realization of high-speed data transmission on the existing network will greatly meet the growing demand for mobile data services.
Through the dynamic allocation of GSM original time slots, each user can occupy multiple wireless channels at the same time, and the same wireless channel can be shared by multiple users, which enhances the data communication capability of the GSM communication system.
The emergence and practical use of GPRS will enable mobile communications to enter a new stage of development and continue to evolve into the third generation of mobile communications.
The frame structure of the third-generation mobile communication system is to combine satellite networks with terrestrial mobile communication networks to form a seamless global three-dimensional communication network to meet the communication requirements of users with different densities in cities and remote areas.
It will use a common frequency band and a unified global standard;
Capable of supporting voice, data, and multimedia services, especially supporting internet services;
Able to achieve high spectrum efficiency, high service quality, high confidentiality, and low cost;
At the same time, it is necessary to extend the services of the integrated broadband network to the mobile environment as much as possible and be able to transmit high-quality images up to 2Mbit/s, so as to truly realize the personal communication desire of any human being to facilitate communication with anyone at any place and at any time.
GSM mobile phone
The GSM frequency bands commonly used by Chinese mobile phones mainly include the CDMA1X and 800MHz frequency bands occupied by CDMA mobile phones; the 900/1800MHz frequency bands occupied by GSM mobile phones; the 900/1800MHz frequency bands occupied by GSM1X dual-mode in the past two years; and the 1900/2000/2100MHz frequency bands occupied by 3G.
Working frequency: GSM Tx: 890～915MHz
Rx: 935～960MHz
DCS Tx: 1 710～1 785MHz
Rx: 1 805～1 880MHz
PCS Tx: 1 850.2～1 909.8MHz
Rx: 1 930.2～1 989.8MHz
Frequency error: ±0.1×f0×10
Working temperature: -10～+55°C
Reference frequency source: VCTCXO 13MHz
GSM dual frequency
Using a GSM900/GSM1800 dual-band mobile phone, users can freely switch between GSM900 and GSM1800, which can effectively avoid the problems of dropped calls, difficult calls, and poor sound quality in the past, which is more convenient than the previous calls using only the GSM900 network.
GSM tri-band
Tri-frequency means to include 3 working frequencies. These three working frequencies are GSM900Mhz, DCS1800Mhz, and PCS1900Mhz.
A tri-band mobile phone means that the mobile phone can receive the signals of the three frequency bands of GSM900M, DCS1800Mhz, and PCS1900Mhz at the same time, and make a choice from them.
If the signal of that frequency band is strong, choose the signal of the base station. If one of them cannot be connected, you can freely switch to a signal in another frequency band. It actually increases the connection rate of the mobile phone.
In some areas where mobile phone users are concentrated, it is especially suitable to use tri-band mobile phones, because tri-band mobile phones can flexibly switch between GSM900, DCS1800, and PCS1900 in order to always maintain continuous calls and call quality.
PCS1900 is a network segment commonly used in the field of communication networks in North America (the United States, Canada) and European countries.
Because tri-band mobile phones can work in three different frequency network segments at the same time, tri-band mobile phones undoubtedly have the characteristics of these three networks.
From a technical point of view, because of the high-frequency band, GSM1800 has strong signal penetration ability, so it can bring good call quality and communication coverage in a complex environment with tall buildings;
The PCS1900 channel has good communication capabilities in North America (the United States, Canada) and Europe, which undoubtedly provides the services they need for those who frequently travel between continents.
For operators, the construction of a three-band network has completely alleviated the frequency band and capacity problems of GSM900 and further optimized the network. The peak traffic in hotspots has been effectively alleviated and the connection rate is higher. Greatly increase the volume of business.
Mobile phone standard
The GSM digital mobile communication system was designed by a standardization committee composed of major European telecommunication operators and manufacturers, and it was developed on the basis of a cellular system.
Due to the rapid development of GSM, after its 900MHZ frequency band was full, the GSM1800 frequency band was opened up, and the mobile phone worked in the 900MHZ and 1.8GHZ frequency bands, as well as several frequency bands such as GSM1900MHz.
GPRS is a high-speed data processing technology, the method is to transmit data to users in the form of packets. Although GPRS is a transitional technology for the transition from the existing GSM network to the third generation of mobile communications, it has significant advantages in many aspects.
TDMA is the abbreviation of Time Division Multiple Access, which is a technology that uses Time-Division Multiplexing (Time Division Multiple Access) to provide wireless digital services. It represents a digital signal transmission technology for mobile phone systems.
TDMA divides a radio frequency into multiple time slots and then allocates these time slots to multiple groups of calls. In this way, one radio can support multiple data channels at the same time, and this technology has become the basis of today’s D-AMPS and SM systems.
The advantages of GSM mobile phones are:
Efficient frequency band utilization and larger network capacity
Simplify network planning
Improve call quality
Increase confidentiality
Improve coverage characteristics
Extend user talk time
Soft volume and “soft” switching
Faster internet speed
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