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Keywords: LMDS system interface business
With the rapid popularization of the Internet and the rapid development of media technology, users have higher and higher requirements for access bandwidth, and traditional copper wire and cable access can no longer meet the transmission requirements. All-fiber access is a relatively perfect solution, but the requirements on the network are too high and the cost is relatively high, even if developed countries can not achieve it in the near future. As an effective supplement to optical fiber access, the new technology of broadband wireless access, LMDS, has gained more and more attention, especially for those emerging telecommunications operators that are relatively lacking in telephone lines and cables. The platform from the terminal to the core network has low investment and fast recovery, and is a very flexible networking solution.
1 LMDS system composition LMDS is the abbreviation of Local MulTIpoint DistribuTIon Service (local multipoint distribution service), which is a microwave broadband service. The working frequency band is generally 20-40 GHz, and the bandwidth can reach 1 GHz. LMDS adopts the cell structure, and the cell radius can reach 2 ～ 5km (theoretical value is 5km).
LMDS system is usually composed of multiple cells, each cell is composed of a base station and many user stations, and each base station is connected by a high-speed fiber loop with self-healing function.
The base station system is composed of a network interface unit and a radio frequency unit. The network interface unit provides the interface of the backbone network, including PSTN, Internet, Frame Relay, ATM, ISDN, etc. It completes the signal encoding / decoding, compression / decompression, error correction, complex / tap, modulation / demodulation, combining / division And other functions; the radio frequency unit up-converts the intermediate frequency signal from the network interface unit to the corresponding radio frequency signal, which is transmitted by the antenna, and at the same time down-converts the radio frequency signal sent by the user received by the antenna to the intermediate frequency signal and sends it to the network interface unit deal with.
The user station is also composed of a network interface unit and a radio frequency unit. Its network interface unit provides interfaces for various user services, including interfaces such as telephones, switches, images, frame relays, and Ethernet. The available interface rates are: ISDN, N × 64K, 2M, N × 2M, etc. Basically, common services can be directly accessed.
2 LMDS transmission characteristics Due to the use of advanced error correction algorithms, the bit error rate of the LMDS system is reduced to 10-10, and the transmission quality is close to the fiber level.
Because the LMDS uses the 20GHz to 40GHz frequency band, the strong rain makes the signal attenuation very strong. To this end, the LMDS system uses dynamic adaptive transmission power control technology, in the case of large signal attenuation, automatically increase the signal transmission power in order to provide sufficient power reserve for the signal. Therefore, LMDS solves the rain attenuation problem well, and the system work is not affected by the weather.
Because LMDS uses the 20GHz ～ 40GHz frequency band, it has very strict requirements for the “view through†of each communication point. When designing the LMDS system, technologies such as cell overlap, adding relay stations and reflectors can be used to enable more cell users to receive The signal from the central station increases the number of users that the LMDS system can cover.
3 Advantages of LMDS The advantages of LMDS include:
(1) It has huge bandwidth resources and can provide multiple services. Common wireless access systems are narrow-band systems, and are only suitable for low-speed voice and data services. The available spectrum bandwidth of LMDS can reach more than 1GHz, and can carry almost any type of business, including ordinary voice, data, and images.
(2) The initial investment is small, the recovery is fast, and the capacity expansion is strong. At the initial stage of the LMDS system's network construction, only a small part of the investment is required to build a base station with a simple configuration that covers several users to operate. With the increase in the number of users, the configuration of base station equipment can be improved, user-side equipment can be added, capacity can be gradually expanded, and additional investment can be gradually added, resulting in better investment economy. According to relevant information, LMDS operators can generally recover all their investments within 5 years.
(3) Low network operation and maintenance costs LMDS does not need to lay and maintain lines and cables to users, and the network operation and maintenance costs are low.
(4) Good compatibility LMDS integrates the advantages of the existing network. Its protocol is neutral and has excellent compatibility. It supports almost all current transmission standards, including ATM, TCP / IP, MPEG-2, etc. LMDS network can be easily interconnected with various existing networks (Internet, LAN, CATV network, etc.).
4 Services provided by LMDS LMDS can provide the following services:
(1) Voice service The LMDS system can provide high-quality voice services without delay. The interface between the user and the system is the RJ-11 telephone standard, which is compatible with all commonly used telephone interfaces.
(2) Data services LMDS data services include low speed, medium speed, and high speed. The low-speed data service has a rate of 1.2 kbps to 9.6 kbps and can handle open protocol data. The network allows local access points to connect to value-added service networks and provides low-speed data on standard voice circuits. The medium-speed data service rate is 9.6kbps to 2Mbps, such data interface is usually a local access point of value-added network. The high-speed data service rate is 2Mbps to 155Mbps. To provide such data service, there must be a 100Mbps Fast Ethernet and optical fiber distribution data interface.
Specifically, due to the sufficient bandwidth, LMDS can not only provide high-speed Internet access, as well as the interconnection of campus network, local area network, metropolitan area network, and wide area network, but also can be used as an ATM, SONET medium.
(3) Image service · Cable TV access LMDS system can provide cable TV access, which provides much more TV channels than traditional cable TV, it can also provide pay-per-view time service and interactive VOD (video On demand) business.
Other image services With the development of the business, LMDS can also provide services such as high-speed conference TV, remote medicine, remote teaching, remote image monitoring, online shopping, and online interactive games.
5 Prospects of LMDS As an emerging broadband wireless access method, the above characteristics and advantages of LMDS will provide telecom operators with an economical and simple solution for broadband access and interactive multimedia applications "in the last mile", Has a very broad market prospect.
2 LMDS—Use microwave to solve the access problem. http: //. com. cn / tech / getpage. php3? TItle ï¼ lmds
3 Che Lijun. Access network development trends and existing problems. Shtt p 21st.com/hottech/subject/wireless-access/a16. HTM
4 Qu Hua et al. Access network and V5 interface. Beijing: People's Posts and Telecommunications Press, October 1999
New technology of broadband wireless access-LMDS
ã€Abstract】 Introduce the system composition of LMDS and the interface between base station and user station; explain the transmission characteristics of LMDS system; explain the superiority, practical application ability and broad development prospect of LMDS system.
references
1 Xi Ning. Broadband wireless access LMDS technology. p-com company. 2000