Analysis of broadband wireless access technology in local multi-point distribution service

I. Overview

Scientific and technological progress has always been a powerful driving force for the development of transportation, especially since the application of steam power in the second half of the 18th century to promote the beginning of the British Industrial Revolution, scientific and technological progress has promoted the upgrading of traditional modes of transport and the emergence of new modes of transport, greatly promoting the progress and development of Today, the new technological revolution is for the development of the transportation industry has brought fresh vitality.

With the development of the society, the demand for information is more and more, the provision of high-speed and high-capacity information has become an important topic of communication technology, especially with the rapid development of the Internet, people's demand for broadband services is more and more high. Today's technology has solved the core network part of the high-speed broadband switching technology, for users, the provision of high-speed broadband services bottlenecks in the access network, that is, the so-called "last kilometer." Narrowband access mode has become the main bottleneck of high-speed Internet and broadband service development. At present, it is urgent to provide high-speed broadband access technology for user access section.

Compared with other access technologies, broadband wireless access technology has the unique advantages such as low initial investment, short network construction period, fast service, reusable resources, and wide application foreground. Broadband wireless access technology has become one of the fastest growing areas of communication network. There are three main broadband wireless access technologies, namely the MMDS (multi-point distribution service) and DBS (direct-seeding satellite system) and the emerging LMDS (local multicast), and LMDS is the hottest technology in this field.

LMDs is a kind of microwave broadband technology. Because the technology utilizes high capacity point to multi point microwave transmission, it can provide almost any kind of business, support bidirectional voice, data and video image business, can realize user access rate from 64kbit/s to 2mbit/s, even up to mbit/s. With high reliability, it is likened to "Wireless optical fiber" technology, it can realize bidirectional transmission of voice, data image, video, conference TV and other broadband services in the near distance, and support ATM, TCP/IP and MPEG-2.

Each word in the LMDS has its own meaning: local refers to the range that a single base station can cover, ldms the radius of a single base station in the urban environment is usually less than 5km due to the limitation of the propagation characteristics of the frequency wave. Multipoint (multiple points) is the signal from the base station to the client is the point to multicast broadcast mode, the signal from the user to the base station is transmitted in a point-to-point manner; distribution (allocation) refers to the signal that the base station will emit (possibly including voice, data and Internet , video service) are assigned to each user respectively; Service (business) is the relationship between the service of the system operator and the user, i.e. the business that the user can get from the LMDS network depends entirely on the choice of the business. (Computer science)

Second, LMDS system network structure

A complete LMDS system includes the network Operation Center (NOC), backbone network, base station system and the four parts of the remote station.

(1) Network Operation Center. The network Operation Center is based on the software platform, it is responsible for the management of the user network in many regions, and completes the basic functions including fault diagnosis and alarm, System configuration management, billing management, performance analysis management, security management, etc. The large LMDS system should have several network operation centers, which can be divided into central management and multiple local management.

(2) Backbone network. Backbone network can be composed of optical fiber transmission network, ATM Exchange or IP exchange frame of the core Exchange platform and the Internet, public telephone network and other interconnection modules. The signal of each base station is fed into backbone network, complete various business exchanges and so on.

(3) Base Station system. The base station is the interface between the backbone network and the remote station, which realizes the signal transmission and conversion between the backbone network and the remote station. The base station can be designed to contain local switching functions, or it can be designed to not include local switching functions, either as a omni-base station or as a multiple-sector base station.

(4) Remote station. The role of the remote station starting (base station) (fixed user). The base station system to the remote station is the downlink link, using Time Division Multiplexing (TDM), remote station to base station system is uplink, using time Division Multiple access (TDMA) mode or frequency division multiple access (FDMA) mode, TDMA is suitable for sudden or low speed data accessing, FDMA suitable for continuous data access.

Third, the technical characteristics of LMDS

(1) LMDs in the band of more than 10GHz work, this band of technology to achieve a high degree of difficulty, the past rarely used, the frequency band is more relaxed, the available frequency band at least 1GHz, more suitable for broadband data transmission; At present, most countries LMDS spectrum distribution generally concentrated in 24, 26,28,31 and 38 GHz and other frequencies, of which 27.5~29.5 GHz is the most concentrated, almost 80% of the country's spectral distribution of the state of the wood is within this band.

(2) In the service area generally divides the sector, the radio frequency carries on the reuse, thus increases the user the usable bandwidth, usually uses the orthogonal polarization the way to reduce the neighboring sector the interference. Within the service area, the remote station and the base station are in full duplex communication, in which the uplink of the remote station can be TDMA, FDMA or CDMA mode, most of the equipment used TDMA, FDMA, downlink is using TDM or CDM way. The choice of multiple access mode is related to the business status, business development strategy and target market of the system operators. For enterprise users, the traffic flow is not large, you can use FDMA mode, for sudden strong business flow, then choose TDMA mode;

(3) The modulation mode of LMDS system is divided into two kinds of orthogonal phase shift keying (QPSK) and orthogonal amplitude modulation (QAM), in which QAM mainly adopts 4-qam, 16-qam,64-qam and so on, and the modulation mode on the downlink can choose Qpsk,16qam or 64QAM according to the specific conditions. If using 16QAM, the same band can support the capacity of QPSK 2.3 times times, if the use of 64QAM is 3.5 times times. But the more complex the modulation technology, the smaller the coverage under the same conditions, and the cost will increase correspondingly.

(4) The air interface between the base station outdoor unit and the remote station generally adopts the frequency band above 10GHz, which determines that the LMDS base station and the remote station can only use Line-of-sight (LOS) transmission, and all kinds of environmental factors such as buildings have great influence on the LDMS. For developing cities, the emergence of new buildings may affect the wireless transmission of LDMS, which brings difficulties to operation and maintenance.

Iv. typical application fields of LMDS system

(1) Application of LMDS in data communication network

As the access part of the data communication network, the LMDS system plays the role of connecting the Service node interface and the user network interface. This application is the traditional telecom operators on the basis of existing networks, as a complementary means of optical fiber access, in the short term to meet user demand for broadband services effective way. Based on the Shanghai ATM Broadband platform, the Shanghai test network transmits data between the base station and the Subscriber station based on the ATM signal multiplexing continuous carrier. The test results show that the LMDS system can be used as a flexible access mode for broadband ATM network, which provides a good service for broadband users.

(2) Application of LMDS in cellular service

Many cellular operators want to be able to continuously expand their business scope, and urgently take advantage of new business opportunities. The LMD system can truly dynamically allocate functions within a sector, thus supporting the transmission trunk of mobile networks and providing new broadband services for enterprise customers within their sector areas.

(3) LMDS building Local Information loop

Based on the backbone of the LMDS system, we use reasonable networking technology to build a high-speed exchange platform, enrich the local information service through own construction, website mirroring or the introduction of information from outside, and rent the Telecom department's export line to realize the interconnection of local loop and Internet. Considering that the local information exchange platform needs to provide users with a variety of services, and the future of the network will be the main business from IP, so the use of IP core network, at the same time with the ATM Exchange layer, that is, the LMDS system to achieve the air ATM interface, and effectively support various types of business (such as DDN, FRN, etc.). The application can realize the advantages of less one-time investment, faster opening, flexible networking and convenient expansion, and the broadband and cheapness of the LMDS system also meet the demand of broadband service for the current users.

(4) Application of LMDS in radio and television network

Celluar Vision Company built a 28GHZ LMDS broadcast video system in New York. The system is currently the world's most mature lmds system, mainly for users to provide multi-channel television programs, the price is lower than their competitors 30%, by the end of 1997 has a user of 12,000. This system can provide 48Mbps downlink data rate, because of the early opening, the system itself is one-way, users need to achieve uplink through the PSTN connection. There are no technical problems in building a local interactive TV distribution network using LMDS system.

(5) LMDs Construction of integrated service access network

The construction of integrated service access network, which integrates voice, data and vision, conforms to the trend of "three nets in one" and satisfies the needs of users in multimedia business. It is generally considered that the realization of all-optical (FTTH) of access network is the best solution for constructing integrated service access network. However, there are still many difficulties in the use of all-optical access network, such as the construction and operation of optical fiber is expensive, the laying of optical fiber is related to municipal, transportation and environment and so on. The maximum access rate supported by the LMDS system is up to 155Mbps, to meet the user's increasing demand for communication bandwidth, and it has less start-up funds, opening speed and flexibility to expand the advantages of the operators do not have the ability to access optical fiber, is undoubtedly and short-term expansion of broadband services to meet market demand, a powerful means.

V. Development status and prospect of LMDS

LMDS technology was originally developed by the United States Cellular vision, a pilot license for the technology in 1991, and is now running a LMDS network in New York, mainly providing multi-channel television services. January 7, 1999, P-com and Winstar company jointly announced that the world's first commercial LMDS system for the official operation of the company's LMDS system, to provide users with high-speed Internet access, conference television, video-on-demand, LAN interconnection and voice services, Have achieved very good business results. At present, many communications equipment manufacturers have launched or are about to launch LMDS products, such as Alcatel, Ericsson and so on. And some LMDS technology or commercial test systems have been built around the world. At home, the telecom operators have also embarked on the construction of the network. July 1999, p-com Company established China Unicom's first set of LMDS experimental network. Since then, China Netcom, Chinese mobile and so on have said the LMDS experiment network work.

From this perspective, based on the development and application of millimeter wave communication, image processing and VLSI technology, LMDS Access system is becoming more and more mature in solving technical problems. There is no doubt that broadband wireless access system is the hotspot of network development in the future, and in this field, LMDS is the shortest way to realize broadband communication, It has received great attention in China and the world, and has a very broad market prospect.