With the rapid development of communications and the increasing popularity of telephones, the installation of telephones in densely populated cities or remote mountainous areas faces a series of problems when laying the last section of subscriber lines. Problems that are difficult to solve: the length of copper wires and twisted-pair wires is 4-5 kilometers, and high loop resistance problems occur, and the communication quality is difficult to guarantee: in mountainous areas, islands, and cities with high user density and tight pipelines, it is difficult to set up user lines. Lead to time-consuming, laborious and high cost. In order to solve the so-called "last mile" problem and achieve the goal of rapid installation and low price, wireless access technology, an important part of the access network technology, came into being. Wireless access refers to the use of wireless means in part or all from the switching node to the user terminal. A typical wireless access system is mainly composed of a controller, an operation and maintenance center, a base station, a fixed subscriber unit, and a mobile terminal. The functions completed by each part are as follows.
The controller is connected to these functional entities through the interfaces it provides with switches, base stations, and operation and maintenance centers. The main function of the controller is to handle user calls (including call establishment, disconnection, etc.), to manage the base station, to perform wireless channel control, base station monitoring, and to monitor and manage fixed subscriber units and mobile terminals through the base station.
Operation and Maintenance Center
Operation and Maintenance Center is responsible for the operation and maintenance of the entire wireless access system. Its main function is Operation of various configuration data: monitor and collect data for each part of the system during the operation of the system; record and alarm the faults that occur in the operation of the system. In addition, the performance of the system can also be tested.
The base station provides wireless channels with fixed termination equipment and mobile terminals through wireless transceivers, and completes voice calls and data transmission through wireless channels. The controller manages the wireless channel through the base station. The wireless interface between the base station and the fixed terminal equipment and the mobile terminal can use different technologies and determine the characteristics of the entire system, including the used wireless frequency and its certain scope of application.
Fixed terminal equipment
Fixed terminal equipment provides users with the standard interface-Z interface for user terminals such as telephone, fax, and data modem. It is connected with the base station through a wireless interface. And transparently transmit the services and functions that the switch can provide to end users. The fixed terminal equipment can use a directional antenna or a non-directional antenna. Using a directional antenna to point directly in the direction of the base station can improve the signal transmission quality in the wireless interface and increase the coverage of the base station. According to the number of user terminals that can be connected; fixed termination equipment can be divided into single-user units and multi-user units. A single user unit (SSU) can only be connected to one user terminal; it is suitable for situations where user density is low and the distance between users is long; a multi-user unit can support multiple user terminals, generally 4 or 8 are supported , 16 and 32 user multi-user units, multi-user units are more economical when the distance between users is very close (such as a user upstairs).
Functionally, a mobile terminal can be regarded as a physical entity that combines a fixed termination device and a user terminal. Because it has a certain degree of mobility, the wireless access system supporting mobile terminals should not only have the functions of a fixed wireless access system, but also have certain mobility management and other functions of a cellular mobile communication system. If there is a breakthrough in price, mobile terminals will be more popular with users and operators.
Classification of wireless access technology
GSM access technology
GSM is a mobile communication technology standard that originated in Europe and is the second generation of mobile communication technology . This technology is a common technical representative of personal communication at present. It uses narrow-band TDMA, which allows 8 groups of calls to be carried out at the same time on a radio frequency, or "cellular". GSM was put into use in 1991. By the end of 1997, it had been operating in more than 100 countries and became the de facto standard in Europe and Asia. The GSM digital network has strong confidentiality and anti-interference, clear sound quality, stable calls, and has the advantages of large capacity, high frequency resource utilization, open interfaces, and powerful functions. my country introduced this technical standard in the early 1990s. Prior to this, it has always used cellular analog mobile technology, the first generation of GSM technology (the analog mobile network was closed on December 31, 2001). At present, China Mobile and China Unicom each have a GSM network. The total number of GSM mobile phone users exceeds 140 million, making it the world's largest mobile communication network.
CDMA access technology
CDMA is the abbreviation of code-pision multiple access, translated as "code division multiple access packet data transmission technology", known as the 2.5th generation mobile communication technology. CDMA mobile phones have the characteristics of clear voice, not easy to drop calls, low transmit power and strong confidentiality, and are called "green mobile phones". More importantly, CDMA based on broadband technology makes video applications in mobile communications possible. CDMA, like GSM, is also a relatively mature wireless communication technology. Unlike GSM, which uses Time-DivisionMultiplexing technology, CDMA does not assign a certain frequency to each caller, but allows each channel to use all the spectrum that can be provided. Therefore, the CDMA digital network has the following advantages: efficient frequency band utilization and larger network capacity, simplified network planning, high call quality, confidentiality and signal coverage, and it is not easy to drop calls. In addition, the CDMA system adopts encoding technology, and its encoding has 440 million digital permutations. The encoding of each mobile phone is also changing at any time, which makes the theft of codes only a theoretical possibility.
GPRS access technology
Compared with the original GSM dial-up circuit-switched data transmission method, GPRS is a packet-switched technology. Due to the use of "grouping" technology, users can avoid the pain of disconnection when surfing the Internet. The situation is probably the same as using the download software NetAnts. In addition, the method of using GPRS to surf the Internet is different from that of WAP. Using WAP to surf the Internet is like surfing the Internet at home. First, "dial-up connection", and then you cannot use the phone line at the same time. Simultaneously. Technically speaking, if you are simply making a voice call, you might as well continue to use GSM, but if you have data transmission needs, it is best to use GPRS, which raises the application of mobile phones to a higher level. At the same time, the development of GPRS technology is also very "economical" because it only needs to upgrade the existing GSM network. GPRS has a wide range of uses, including sending and receiving e-mails via mobile phones, and browsing on the Internet. The biggest advantage of GPRS is that its data transmission speed is not comparable to that of WAP. The data transmission speed of the current GSM mobile communication network is 9.6K bytes per second, while GPRS has reached 115kbps. This speed is twice the ideal rate of the commonly used 56Kmodem. In addition to the speed advantage, GPRS also has the "always-on" feature, that is, users keep in touch with the network at any time.
CDPD access technology
The biggest feature of CDPD access technology is its fast transmission speed, and the highest communication speed can reach 19.2kbps. In addition, in terms of data security, due to the use of RC4 encryption technology, the security is relatively high; the forward and reverse channel keys are asymmetrical, and the key is controlled by the switching center. The mobile terminal logs in once, and the switching center automatically checks the old secret. The key is replaced with a new key once for dynamic management. In addition, since the CDPD system is an open system based on TCP/IP, we can easily access the Internet. All TCP/IP-based application software can be used directly without modification; application software development is simple; mobile terminal communication numbers are direct Use IP address. The CDPD system also supports user handover and entire network roaming, broadcast and group calling, and can be interconnected with public wired data networks.
Fixed broadband wireless access (MMDS/LMDS) technology
The broadband wireless access system can be divided into MMDS (Multi-channel Multi-point Distribution Service) and LMDS (Local Multi -pointDistributionService) two series, the Chinese meaning is called local multi-point distribution service. This is a kind of microwave broadband technology, which is also referred to as "wireless optical fiber" technology. It can realize two-way transmission of broadband services such as voice, data image, video, conference TV, etc. in a relatively short distance, and supports standards such as ATM, TCP/IP and MPEG2. A cell-like service area structure is adopted to divide an area that needs to provide services into several service areas. Each service area is equipped with a base station, and the base station equipment communicates with users in the service area via a point-to-multipoint wireless link. Each service area covers a range of several kilometers to more than ten kilometers, and can overlap each other. Because NMDS/LMDS has the characteristics of higher bandwidth and two-way data transmission, it can provide a variety of broadband interactive data and multimedia services, overcome the bottleneck of the traditional local loop, and meet the increasing needs of users for high-speed data and image communication. It is a powerful tool to solve the problem of communication network access.
DBS satellite access technology
DBS technology is also called digital direct broadcast satellite access technology, which uses communication satellites located in geosynchronous orbits to send high-speed broadcast data to the user’s receiving antenna , So it is generally called high-orbit satellite communication. Its characteristic is that the communication distance is long, the cost has nothing to do with the distance, the coverage area is large and is not restricted by geographical conditions, the frequency bandwidth, the capacity is large, and it is suitable for multi-service transmission. It can provide global users with large-span, large-scale and long-distance roaming and Flexible mobile communication services, etc. In the DBS system, a large amount of data is broadcast by the high-speed uplink channel and satellite transponder of the satellite master station after modulation by frequency division or time division. The user receives the data through the satellite antenna and the satellite receiving Modem. The diameter of the receiving antenna is generally 0.45m or 0.53m. Due to the high reliability of the digital satellite system, unlike the analog phone using twisted pair in the PSTN network, which requires more signal error correction, the download rate can reach 400kbps, while the actual DBS broadcast rate can reach up to 12Mbps. At present, the United States can already provide DBS services, mainly for Internet access. The largest DBS network is DirectPC from Hughes Networks Systems. DirectPC's data transmission is also asymmetrical. When accessing the Internet, the download rate is 400kbps and the uplink rate is 33.6kbps. Although this rate is much higher than that of ordinary dial-up modems, it is still incomparable with DSL and CableModem technologies.
The English name of Bluetooth is "Bluetooth". In fact, it is a protocol that realizes wireless connection between multiple devices. Through this agreement, information can be exchanged among many devices including cellular phones, palmtop computers, notebook computers, related peripherals, and home Hubs, including home RF. Bluetooth is used in the connection of mobile phones and computers, which can save mobile phone costs and realize data sharing, Internet access, wireless hands-free, synchronized data, image transmission, etc. Although Bluetooth has great advantages in multi-directional transmission, if there are many devices, the identification method and speed will also be problematic; Bluetooth has one-to-multipoint data exchange capability, so it needs a security system to prevent unauthorized The basic communication speed of Bluetooth is 750Kbps, but products with 4Mbps IR port are now very common, and the 16Mbps extension has also been approved recently.
HomeRF is mainly designed for home networks to reduce the cost of voice data. In order to achieve efficient transmission of data packets, HomeRF adopts the CSMA/CA mode in the IEEE802.11 standard, which is similar to CSMA/CD. It obtains control of the channel in a competitive manner. There can only be An access point transmits data in the network. Unlike other protocols, HomeRF provides true support for streaming services (StreamMedia). As a high-level priority is prescribed for streaming services and a retransmission mechanism with priority is adopted, the bandwidth required for real-time streaming services, low interference, and low error codes are ensured in this way. HomeRF works in the 2.4GHz frequency band. It uses digital frequency hopping spread spectrum technology, with a rate of 50 hops/s, and a total of 75 frequency hopping channels with a bandwidth of 1MHz. The modulation method is constant envelope FSK modulation, which is divided into 2FSK and 4FSK. FM modulation can effectively suppress interference and fading in the wireless environment. In 2FSK mode, the maximum data transfer rate is 1Mbps; in 4FSK mode, the rate can reach 2Mbps. In the latest version of HomeRF2.x, WBFHwidebandfrequencyhopping technology is used to increase the frequency hopping bandwidth, from the original 1MHz to 3MHz, 5MHz, the frequency hopping rate is also increased to 75 hops/s, and its data peak value is as high as 10Mbps, which is close to IEEE802.11b. The standard 11Mbps can basically meet the future home broadband communications.
WCDMA access technology
WCDMA technology can bring users a data transmission rate of up to 2Mbps. Under such conditions, any media currently used in computers can pass wirelessly Easy delivery through the network. The advantage of WCDMA lies in its high chip rate, effective use of frequency selective diversity and spatial reception and transmission diversity, which can solve multipath and fading problems. It uses Turbo channel coding and decoding to provide higher data transmission rates. FDD The standard can provide full coverage of a wide area, and the downlink base station uses a unique cell search method to distinguish between the base stations without strict synchronization between the base stations. Using continuous pilot technology, it can support high-speed mobile terminals. Compared with the second-generation mobile communication technology, WCDMA has larger system capacity, better voice quality, higher spectrum efficiency, faster data rate, stronger anti-fading ability, and better anti-multipath performance. , Can be applied to the technical advantages of mobile terminals up to 500km/h, and can be used from G