1 Technical advantages of fixed wireless access The fixed wireless access system is generally composed of three parts: the central station (CS), the terminal station (TS) and the network management system. Among them, the central station and terminal station usually have their own indoor and outdoor units. The indoor unit (IDU) handles the adaptation and convergence of services, connecting different service networks. The outdoor unit (ODU) provides the radio frequency transmission function between the central station and the terminal station, and is generally installed on the roof of the building. Under special circumstances, a relay station (RS) can be used to relay between the central station and the terminal station. Compared with the metropolitan area network composed of fixed lines, wireless access has the following advantages:
1.1 Wireless access does not require the laying of pipeline lines, which provides the possibility of business access for areas where optical cables or cables cannot be laid, shortens the time of engineering projects, and saves investment in pipeline lines.
1.2 With the development of access technology, wireless access equipment can simultaneously solve the access of multiple services such as data and voice.
1.3 Adjust the bandwidth flexibly according to the increase and decrease of regional traffic.
1.4 It is very convenient for business migration and expansion. The advantage is more obvious in the application of temporary business establishment.
2 Features of fixed wireless access technology The characteristics of fixed wireless access technology are mainly reflected in multiple access mode, modulation mode, duplex mode, support for circuit switching and packet switching, dynamic bandwidth allocation, air wireless protocol, OFDM technology, etc.
2.1 Multiple access methods There are currently three main multiple access methods in fixed wireless access-FDMA, TDMA and CDMA. It is rare to use FDMA as the multiple access method. The current practical systems mostly use TDMA or FDMA + TDMA.
2.2 Modulation methods At present, fixed wireless access mainly chooses the following modulation methods: QPSK, 16QAM and 64QAM, which are adapted to different bandwidth and coverage requirements.
2.3 Duplex mode The fixed wireless access system is a two-way transmission system. There are two types of duplex modes, TDD and FDD, according to the different principles of equipment composition.
2.4 Support for circuit switching and packet switching For a long period of time in the future, the main task of telecom operators is to support both circuit switching and packet switching networks, especially at the access network level. There is still a certain demand for the access equipment in the switching mode, and the fixed wireless access system's support for circuit switching is very important.
2.5 Dynamic allocation of bandwidth Fixed wireless access systems must support dynamic allocation of bandwidth, and bandwidth is allocated only after connection requests. The FDD method can only dynamically allocate each user in the total bandwidth in one direction of uplink or downlink, while the TDD method can realize dynamic bandwidth allocation between uplink and downlink channels.
2.6 Over-the-Air Wireless Protocol There are currently three over-the-air protocols: DOCSIS, ATM and TDM. In fact, these three air interfaces belong to the TDMA mode at the physical layer, but the method of filling the business data is different, and the protocol for handling multiple access in the air is different.
2.7 OFDM technology The application of OFDM (Orthogonal Frequency Division Multiplexing) technology in the field of wireless access is gradually becoming a development trend. OFDM has good frequency selection fading and anti-multipath interference capabilities, which makes the wireless access system's requirements for line-of-sight transmission reduced, and is particularly suitable for the increasingly complex urban propagation environment.
3 Main technologies of fixed wireless access The main technologies of fixed wireless access can be divided into LMDS and MMDS.
3.1 LMDS Fixed wireless local multipoint distribution service (LMDS) provides a low-cost and effective solution for densely populated urban communications. Using high-capacity wireless local loops, it can quickly provide data and voice services for a large number of user areas , Suitable for fast access to small and medium-sized enterprises, small offices and home offices in commercial buildings.
3.2 MMDS The MMDS system is currently widely used in the 3.5GHz and 5.8GHz band point-to-multipoint systems. The main features are good transmission performance, wide coverage, mature technology, good rain attenuation resistance, strong scalability, and networking Flexible and low cost pressure, it is an ideal wireless access method. The system has a radio frequency bandwidth of 30MHz and is suitable for 1E1 + 10Base-T users. Due to the long transmission distance, it is suitable for large-area coverage and quickly provides services to users.
4 Strategies for fixed wireless access in metropolitan area network construction 4.1 Fixed wireless access application strategies The lower the frequency band of fixed wireless access, the lower the information rate that can be transmitted, but the better the non-line-of-sight performance and the better the wireless performance it is good. The 3.5GHz frequency is allocated by the state through unified bidding. After winning the bid, you can enjoy a certain frequency resource. The frequency band has good transmission performance, wide coverage, relatively simple and mature technology, good rain attenuation resistance, strong scalability, and networking. Flexible and cost competitive, it is an ideal wireless access method. The 26GHz frequency band is allocated by the country to the four major operators as a commercial test. 5.8GHz is a shared frequency band and uses a reporting coordination mechanism. If all three frequency bands can be used in a city, 3.5GHz can be used as the access method based on voice services. 5.8GHz has a large bandwidth and is a TDD system. It is suitable for IP services, and 26GHz is suitable for large service coverage. And the area where the business is concentrated. In the 26GHz band, due to heavy rain attenuation, for cities with heavy rainfall, the application of the 26GHz band will greatly reduce the quality of wireless access services.
In general, the cost of a 5.8GHz wireless access system based on the IP protocol is relatively low. The price of equipment based on a circuit-type 3.5GHz system is medium, while the price of equipment of a 26GHz system is high. Therefore, from the perspective of investment income, the construction of metropolitan area networks requires the selection of different frequency bands and systems for different occasions. 4.2 Frequency planning Frequency planning is the most important link in wireless network design planning, and it has an important impact on the performance of the network. influences. Effective frequency planning allows each cell to use more frequencies, construct a network that can use fewer cells, reduce the interference of uplink and downlink wireless links, and improve the quality of service provided by the network to users. The basic principles of frequency planning are as follows:
In the initial design, comprehensively consider the future cell size and number of sectors, and build in phases based on actual capacity to save investment; during the initial design, determine a reasonable sector polarization direction. When the network is upgraded, the sector polarization direction is not Change, the previous terminal station does not need to change; regardless of the polarization mode, adjacent sectors use different frequencies, and adjacent sectors use polarization isolation to achieve isolation requirements; in the case of a given bandwidth, in order to obtain Larger system capacity, using channel grouping, grouping radio frequency frequencies to meet the system's anti-interference requirements; carrier bandwidth and modulation have a great impact on system performance, therefore, choosing the appropriate carrier bandwidth and modulation technology is critical ; Calculate the signal-to-noise ratio according to the distance, azimuth of the interference source and the antenna pattern, etc., so as to configure the appropriate frequency point, polarization mode and multiplexing times; as the user's access bandwidth requirements increase, the base station sector can be used Split mode, increase the capacity of a single base station, and improve investment efficiency.
Integrate various favorable factors, such as a reasonable frequency reuse scheme, a reasonable division of users between adjacent sectors, and a reasonable reuse of services within a single carrier frequency, etc., so that the network can achieve the maximum degree of use of frequency resources in the most economical situation User needs are met.
4.3 Base station selection The selection of fixed wireless access system site needs to consider a variety of factors, a reasonable layout is conducive to reducing the construction cost of the entire network. The selection of the site should mainly pay attention to the following aspects: ①user needs; ② line-of-sight propagation; ③ scalability; ④ electromagnetic environment; ⑤ cooperation with the property department.
4.4 Avoidance of interference According to the national standard, microwave systems need to avoid three aspects of interference, namely internal system interference, interference between adjacent systems and interference from outside the system.
5 Conclusion The effective construction of the metropolitan area network can not only solve the access "bottleneck" problem currently faced by telecom operators, but also the focus and key to the future development of national information infrastructure. Solving the problem of access can win the initiative in the future market competition and be in an advantage in the competition of the information superhighway.
The application of fixed wireless access system should be adapted to local conditions, give full play to its advantages, rely on the existing network, adopt the principle of combining wired and wireless; use fixed wireless access as an important and effective supplementary means of optical fiber access to better Serve telecommunications users.
1.1 Wireless access does not require the laying of pipeline lines, which provides the possibility of business access for areas where optical cables or cables cannot be laid, shortens the time of engineering projects, and saves investment in pipeline lines.
1.2 With the development of access technology, wireless access equipment can simultaneously solve the access of multiple services such as data and voice.
1.3 Adjust the bandwidth flexibly according to the increase and decrease of regional traffic.
1.4 It is very convenient for business migration and expansion. The advantage is more obvious in the application of temporary business establishment.
2 Features of fixed wireless access technology The characteristics of fixed wireless access technology are mainly reflected in multiple access mode, modulation mode, duplex mode, support for circuit switching and packet switching, dynamic bandwidth allocation, air wireless protocol, OFDM technology, etc.
2.1 Multiple access methods There are currently three main multiple access methods in fixed wireless access-FDMA, TDMA and CDMA. It is rare to use FDMA as the multiple access method. The current practical systems mostly use TDMA or FDMA + TDMA.
2.2 Modulation methods At present, fixed wireless access mainly chooses the following modulation methods: QPSK, 16QAM and 64QAM, which are adapted to different bandwidth and coverage requirements.
2.3 Duplex mode The fixed wireless access system is a two-way transmission system. There are two types of duplex modes, TDD and FDD, according to the different principles of equipment composition.
2.4 Support for circuit switching and packet switching For a long period of time in the future, the main task of telecom operators is to support both circuit switching and packet switching networks, especially at the access network level. There is still a certain demand for the access equipment in the switching mode, and the fixed wireless access system's support for circuit switching is very important.
2.5 Dynamic allocation of bandwidth Fixed wireless access systems must support dynamic allocation of bandwidth, and bandwidth is allocated only after connection requests. The FDD method can only dynamically allocate each user in the total bandwidth in one direction of uplink or downlink, while the TDD method can realize dynamic bandwidth allocation between uplink and downlink channels.
2.6 Over-the-Air Wireless Protocol There are currently three over-the-air protocols: DOCSIS, ATM and TDM. In fact, these three air interfaces belong to the TDMA mode at the physical layer, but the method of filling the business data is different, and the protocol for handling multiple access in the air is different.
2.7 OFDM technology The application of OFDM (Orthogonal Frequency Division Multiplexing) technology in the field of wireless access is gradually becoming a development trend. OFDM has good frequency selection fading and anti-multipath interference capabilities, which makes the wireless access system's requirements for line-of-sight transmission reduced, and is particularly suitable for the increasingly complex urban propagation environment.
3 Main technologies of fixed wireless access The main technologies of fixed wireless access can be divided into LMDS and MMDS.
3.1 LMDS Fixed wireless local multipoint distribution service (LMDS) provides a low-cost and effective solution for densely populated urban communications. Using high-capacity wireless local loops, it can quickly provide data and voice services for a large number of user areas , Suitable for fast access to small and medium-sized enterprises, small offices and home offices in commercial buildings.
3.2 MMDS The MMDS system is currently widely used in the 3.5GHz and 5.8GHz band point-to-multipoint systems. The main features are good transmission performance, wide coverage, mature technology, good rain attenuation resistance, strong scalability, and networking Flexible and low cost pressure, it is an ideal wireless access method. The system has a radio frequency bandwidth of 30MHz and is suitable for 1E1 + 10Base-T users. Due to the long transmission distance, it is suitable for large-area coverage and quickly provides services to users.
4 Strategies for fixed wireless access in metropolitan area network construction 4.1 Fixed wireless access application strategies The lower the frequency band of fixed wireless access, the lower the information rate that can be transmitted, but the better the non-line-of-sight performance and the better the wireless performance it is good. The 3.5GHz frequency is allocated by the state through unified bidding. After winning the bid, you can enjoy a certain frequency resource. The frequency band has good transmission performance, wide coverage, relatively simple and mature technology, good rain attenuation resistance, strong scalability, and networking. Flexible and cost competitive, it is an ideal wireless access method. The 26GHz frequency band is allocated by the country to the four major operators as a commercial test. 5.8GHz is a shared frequency band and uses a reporting coordination mechanism. If all three frequency bands can be used in a city, 3.5GHz can be used as the access method based on voice services. 5.8GHz has a large bandwidth and is a TDD system. It is suitable for IP services, and 26GHz is suitable for large service coverage. And the area where the business is concentrated. In the 26GHz band, due to heavy rain attenuation, for cities with heavy rainfall, the application of the 26GHz band will greatly reduce the quality of wireless access services.
In general, the cost of a 5.8GHz wireless access system based on the IP protocol is relatively low. The price of equipment based on a circuit-type 3.5GHz system is medium, while the price of equipment of a 26GHz system is high. Therefore, from the perspective of investment income, the construction of metropolitan area networks requires the selection of different frequency bands and systems for different occasions. 4.2 Frequency planning Frequency planning is the most important link in wireless network design planning, and it has an important impact on the performance of the network. influences. Effective frequency planning allows each cell to use more frequencies, construct a network that can use fewer cells, reduce the interference of uplink and downlink wireless links, and improve the quality of service provided by the network to users. The basic principles of frequency planning are as follows:
In the initial design, comprehensively consider the future cell size and number of sectors, and build in phases based on actual capacity to save investment; during the initial design, determine a reasonable sector polarization direction. When the network is upgraded, the sector polarization direction is not Change, the previous terminal station does not need to change; regardless of the polarization mode, adjacent sectors use different frequencies, and adjacent sectors use polarization isolation to achieve isolation requirements; in the case of a given bandwidth, in order to obtain Larger system capacity, using channel grouping, grouping radio frequency frequencies to meet the system's anti-interference requirements; carrier bandwidth and modulation have a great impact on system performance, therefore, choosing the appropriate carrier bandwidth and modulation technology is critical ; Calculate the signal-to-noise ratio according to the distance, azimuth of the interference source and the antenna pattern, etc., so as to configure the appropriate frequency point, polarization mode and multiplexing times; as the user's access bandwidth requirements increase, the base station sector can be used Split mode, increase the capacity of a single base station, and improve investment efficiency.
Integrate various favorable factors, such as a reasonable frequency reuse scheme, a reasonable division of users between adjacent sectors, and a reasonable reuse of services within a single carrier frequency, etc., so that the network can achieve the maximum degree of use of frequency resources in the most economical situation User needs are met.
4.3 Base station selection The selection of fixed wireless access system site needs to consider a variety of factors, a reasonable layout is conducive to reducing the construction cost of the entire network. The selection of the site should mainly pay attention to the following aspects: ①user needs; ② line-of-sight propagation; ③ scalability; ④ electromagnetic environment; ⑤ cooperation with the property department.
4.4 Avoidance of interference According to the national standard, microwave systems need to avoid three aspects of interference, namely internal system interference, interference between adjacent systems and interference from outside the system.
5 Conclusion The effective construction of the metropolitan area network can not only solve the access "bottleneck" problem currently faced by telecom operators, but also the focus and key to the future development of national information infrastructure. Solving the problem of access can win the initiative in the future market competition and be in an advantage in the competition of the information superhighway.
The application of fixed wireless access system should be adapted to local conditions, give full play to its advantages, rely on the existing network, adopt the principle of combining wired and wireless; use fixed wireless access as an important and effective supplementary means of optical fiber access to better Serve telecommunications users.
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