Fixed wireless access (FWA) refers to an access mode in which part or all of the service node to the user terminal uses wireless transmission. Fixed wireless access is mainly for fixed terminals, and only provides limited terminal mobility, such as users walking. The purpose of fixed wireless access is to connect users to the wired telephone network through wireless channels, mainly to provide ordinary telephone services. The service quality of fixed wireless access is based on the quality of wired telephones. The blocking probability and delay characteristics of the wired system are similar.
1. System composition of fixed wireless access
A typical fixed wireless access system consists of the following elements.
1.1 Subscriber unit: Receives the wireless signal from the base station, restores it to an analog audio signal after demodulation and decoding processing, and then sends it to the user terminal (telephone, fax machine, data modem, etc.), which is carried by the user or fixed at a certain location The wireless transceivers are divided into three types: fixed, mobile and portable. They provide users with standard interfaces for terminals such as telephones, faxes, and data modems, and can also be integrated with terminals. It is connected with the base station through a wireless interface, and transparently transmits the services and functions that the switch can provide to end users. It can be divided into single-user units and multi-user units.
1.2 Base station: The base station is controlled by the controller and serves as a wireless transceiver device for a cell. It provides a wireless channel with the user unit through the wireless interface; the wireless base station is usually a multi-channel wireless transceiver located in the center, and its transmission The range is called a "cell" (omnidirectional antenna) or a "sector" (directional antenna), and the coverage range can vary from a few hundred meters to tens of kilometers.
1.3 Base station controller: also known as central office or wireless port controller, its function is to provide the interface with the base station, network and operation and maintenance center, provide functions such as wireless channel control and base station monitoring, and complete the transfer with the switch. It determines the circuit allocation of each user and monitors the performance of the system. The base station and the controller can choose to form star, tree, ring and other transmission networks according to factors such as user distribution, geographical environment, and loop reliability, and use twisted pair, optical fiber, or digital microwave transmission systems.
1.4 Network management center: manages the entire fixed wireless access system. Its functions include configuration management, fault management, performance management, security management and billing management.
2. Application of fixed wireless access
According to the application position of wireless transmission technology in the access network, FWA mainly has the following three application methods.
2.1 All-wireless local loop: From the local exchange to the user end, all use wireless transmission, that is, use wireless instead of feeder lines, wiring lines, and lead-in lines.
2.2 Wireless distribution leads/lead-in local loop: From the local exchange to the flexible point or distribution point, use wired transmission, and then use wireless to connect to the user, that is, use wireless instead of wiring and lead-in or lead-in.
2.2 Wireless feeder/feeder distribution local loop: From the local exchange to the flexible point or distribution point, use wireless transmission, and then use wired methods such as optical cables or copper cables to connect to users, that is, replace the feeder or feeder and wiring with wireless.
3. Classification of fixed wireless access
3.1 According to the transmission rate provided to users, fixed wireless access technology can be divided into narrowband wireless access (≤64kbit/s), medium broadband wireless access (64~2048kbit/s) and broadband wireless access (≥2048kbit/s) /s).
3.1.1 Narrowband Fixed Wireless Access Technology
Narrowband fixed wireless access is characterized by low-speed circuit switching services, providing services such as voice, low-speed data, Modem, and ISDN, and its data transmission rate is generally less than or equal to 64kbit/s. Cellular technology and fixed cordless technology etc.
3.1.2 Medium Broadband Fixed Wireless Access Technology
The medium-broadband fixed wireless access system can provide users with a wireless access rate of 64~2048kbit/s, and open access services such as ISDN. The system structure is similar to the narrowband system, consisting of base station controllers, base stations and subscriber units. The interface between the base station controller and the switch is generally a V5 interface, and the controller and the base station are usually connected by optical fiber or wireless. Users of this type of system mostly adopt TDMA access mode and work in the frequency band of 3.5GHz or 10GHz.
3.1.3 Broadband Fixed Wireless Access Technology
The above-mentioned narrowband and medium broadband wireless accesses are based on circuit switching, and their system structures are similar. However, the broadband fixed wireless access system is based on packet switching and mainly provides video services. At present, it has developed from the initial one-way broadcast service to provide two-way video services, such as video on demand (VOD). The technologies it adopts mainly include direct broadcast satellite (DBS) system, multi-channel multipoint distribution service (MMDS) and local multipoint distribution service (LMDS).
3.2 Fixed wireless access can be divided into three categories according to the target audience and coverage area:
3.2.1 Macro area: Also known as large area, the coverage area ranges from 5 to 55km in radius, and is mainly used in rural areas, mountainous areas, coastal areas, and desert areas.
3.2.2 Micro-area: The radius of the coverage area is between 0.5 and 5km. This type of product is mainly used in cities, suburbs and rural areas.
3.2.3 Pico area: The radius of the coverage area is between 50 and 500m. This type of product is mainly used in densely populated areas of large cities.
Because in non-adjacent coverage areas, the same channel can be used by different users at the same time, so the same bandwidth is occupied, and the number of users that the system can accommodate is related to the size of the coverage area.
4. Operating frequency bands for fixed wireless access
Fixed wireless access is a fixed application of wireless technology, and its working frequency bands can be 450MHz, 800/900MHz, 1.5GHz, 1.8/1.9GHz, 3GHz or so.
For the 1.8~1.9GHz frequency band, it should strictly follow the corresponding provisions of the "Notice on Some Ground Radio Service Planning and Related Issues in the 2000MHz Frequency Band" issued by the National Radio Commission:
The working frequency bands of wireless access (FDD mode) are 1880~1900MHz and 1960~1980MHz, and the above frequency bands are used by the public network.
The working frequency band of wireless access (TDD mode) is 1900~1920MHz, and the above frequency bands are used by public network and private network.
5. Advantages of fixed wireless access
Compared with mobile communication wireless access systems, fixed wireless access has incomparable advantages in terms of installation and maintenance costs, service speed, and flexible and variable access network coverage.
5.1 Economy: The installation and maintenance costs of the wireless access network are much lower than that of the wired user loop, and the cost of the access network loop has nothing to do with the user distance, which is especially economical in underdeveloped rural areas. The capacity of the wireless access network is dynamic. Under the same investment situation, at the cost of sacrificing the call loss rate, the capacity can be doubled or even more. Therefore, at the initial stage of investment, the base station with the least channels can cover the largest area strategy.
5.2 Can quickly provide services: The wireless access network is easy to install and has a short construction period, while the wired system not only has a long period, but also occupies land resources, construction and connection are difficult, equipment is easy to be stolen and destroyed by man, and it is also affected by natural disasters. The advantage that the wireless access network can still ensure the smooth communication of users in natural disasters is incomparable to the wired system.
5.3 Flexible: The radio access network is flexible and variable, and does not need to predict the location of users. The capacity can be large or small, easy to expand. In some places with special needs, public wireless telephones can also be installed to meet the needs of users.
5.4 Large coverage: the longest distance of a wired user circuit is generally about 8 kilometers, while the wireless access network can cover an area of more than 30 kilometers under the condition of a reasonable location of the base station. Therefore, some rural telephone exchanges and The setting of the module bureau optimizes the network structure and reduces network investment.
6. Disadvantages of fixed wireless access
There are many difficulties in the development of fixed wireless access. Although fixed wireless access has great advantages, there are still many difficulties in the actual development. For example, the technical development is not perfect, the frequency band allocation is not uniform, and the cost of fixed base station equipment is relatively high. High, user recognition is not enough and so on.
6.1 The technology is not perfect: Taking the United States as an example, Teligent, one of the largest fixed wireless operators in the United States, obtained an operating license in the 24GHz frequency band in the early 1990s. At that time, it agreed with some equipment manufacturers on the development of LMDS. It is possible to transform LMDS military technology, and it is possible in terms of cost, but the reality is that the development of LMDS technology is much slower than expected.
6.2 Inconsistent allocation of frequency bands: At present, many countries are trying to vigorously promote the development of fixed wireless services, but it is difficult to form a consistent standard when allocating frequency bands. In Europe, some countries have chosen 35GHz and 26GHz, while others have chosen 105GHz and 40GHz. In the United States, 24GHz, 28GHz, 38GHz, 39GHz and other frequency bands are selected to develop LMDS. In the 26GHz frequency band, the wireless signal is transmitted in a direct way, which is very suitable for densely populated metropolises, but in the suburbs, the use effect is not good, especially when the network traffic is overloaded. And if Spain chooses the 3GHz frequency band, in the densely populated Madrid, it feels that the capacity is insufficient. Therefore, the non-uniform selection of frequency bands not only leads to the lack of standards for the development of fixed wireless services, but also further aggravates the difficulty of further improvement of related technologies.
6.3 Cost is still an obstacle: Some analysts pointed out that the high cost of fixed wireless base station equipment may make many small and medium-sized telecom companies stay away and dare not pay attention. At present, the cost of base station equipment for each low-frequency band such as 35GHz is US$1,000, and the cost will increase to US$5,000 for base station equipment in the 26GHz frequency band. In this situation, if the cost cannot be reduced, it will take at least 7-8 years before considering realizing economies of scale in operating broadband wireless services.
6.4 Insufficient recognition by users directly leads to insufficient market demand: From the analysis of market status and development, the demand for fixed wireless is not as good as that of DSL. The main reason is that end users lack understanding and awareness of fixed wireless access technology. "Users don't understand it, and of course they won't buy it," said European broadband operator Formus. In this way, if the market demand continues to be insufficient, it will be impossible for manufacturers to produce related equipment on a large scale, and the cost of related equipment will remain high, and a virtuous circle cannot be formed.
