Application point of view: Brief Introduction to LMDS Broadband Access Network Technology

Broadband access network technology is worth learning a lot. Here we mainly introduce LMDS broadband access network technology. From the perspective of the entire telecommunication network, the whole network can be divided into two major parts: public network and user resident network CPN. CPN belongs to the user. Therefore, the general meaning of the telecommunication network refers to the public telecommunication network.

Public telecommunication networks can be divided into long-distance networks, relay networks, and access networks. The combination of long distance network and relay network is called the core network. Compared with the core network, the access network is between a local switch and a user. It is mainly used to connect the user to the core network. The access network consists of a series of transmission devices between the business node interface SNI) and the user network interface UNI. In recent years, the new technological revolution represented by the Internet is profoundly changing the traditional telecom concept and architecture. With the gradual opening of access network markets in various countries, the telecom control policy has relaxed, with the increasing competition and the rapid emergence of new business demands, and the development of wired and wireless technologies, access networks have become the focus of attention. Driven by the huge market potential, various broadband access network technologies have emerged. The bandwidth of the backbone network is relatively adequate due to the large adoption of optical fiber, and the main bottle diameter that limits bandwidth needs is in the access segment. Optical access network is a long-term solution for the development of broadband access network technology. However, this method still has the disadvantages of High Engineering Cost and slow construction speed. For some network operators, local network resources are not available. In this case, to enter and occupy the access market, using the broadband access network technology is a more appropriate entry point.

With the rapid development of wireless communication technology, a new wireless communication technology that provides an approximate bandwidth for optical fiber communication has opened up a new world for the access network, that is, the local multi-point distribution service-LMDS. Technically, the access networks can be divided into Wired Access Networks and Wireless Access Networks Based on the used media. Wireless Access is divided into mobile wireless access and fixed wireless access. LMDS is a fixed wireless access method.

I. Concept of LMDS

LMDS has gradually developed over the past few years. It is a 10 GHz or above band and broadband wireless point-to-point multi-point broadband access network technology. In some countries, such as Canada and South Korea, it is also called the local multi-point communication system (LMCS ). The so-called "local" refers to the range that can be covered by a single base station. LMDS is limited by the radio frequency transmission characteristics, and the coverage radius of a single base station in the urban environment is generally less than 5 kilometers; "Multi-point" means that the signal is transmitted from the base station to the client through point-to-point broadcast, while the signal is transmitted through point-to-point transmission from the client to the base station; "distribution" means that the base station allocates the signals sent to various users, including voice, data, Internet, and video services; "Business" refers to the relationship between the system operator and the user, that is, the business that the user can obtain from the LMDS network depends on the operator's choice of business. In different countries or regions, the specific frequency band and frequency band width assigned to LMDS vary, with about 80% of the countries going to be 27.5GHz ~ 29.5 GHz is used as the LMDS band.

LMDS works in 24 ~ The 38 GHz band is generally near the millimeter wave band, and the available spectrum is usually above 1 GHz. Because the technology uses high-capacity Point-to-Point Microwave Transmission and transmission through Millimeter waves, it can provide almost any type of business and support bidirectional voice, data, and video and image services, it can achieve user access rates from 64 KB/s to 2 mb/s, or even up to 155 mb/s. It has high reliability and is called a "Wireless Optical Fiber" technology. At present, the LMDS standardization work is carried out by organizations such as ATM forums, DAVIC, ETSI, and ITU. Most standardization organizations use ATM cells as the basic wireless transmission mechanism.

Ii. Components of the LMDS system

A complete LMDS network consists of four parts: basic backbone network, base station, client device, and network management system.

1. Basic Backbone Network

A basic backbone network is also called a core network. To enable the LMDS system to provide diversified integrated services, the core network can be composed of a core exchange platform consisting of an optical fiber transmission network, an ATM exchange, an IP exchange, or an IP + ATM architecture, as well as interconnection modules with the Internet and Public Telephone Network (PSTN.

2. Base Station

The base station directly enters the telecom backbone network or core network. Because LMDS directly supports the ATM protocol, the link efficiency can be improved by using the wireless ATM protocol. The base station is responsible for covering the user end and providing backbone network interfaces, including PSTN, Internet, ATM, frame relay, and ISDN. The Base Station converts signals between the basic backbone network and wireless transmission. The base station equipment includes the interface module connected to the basic backbone network, the modulation and demodulation module, and the microwave receiving and receiving module usually placed on the roof or top of the tower.

The Base Station of the LMDS system uses multi-sector coverage and uses a horn antenna that is focused within a certain angle to cover the client devices. The Base Station capacity depends on the following technical factors: available spectrum bandwidth, number of sectors, Frequency Multiplexing, modulation, multiple access, and System Reliability Indicators, the number of users supported by the system depends on the system capacity and the business required by each user. The coverage radius of the base station is closely related to many factors, such as System Reliability Indicators, microwave transceiver performance, signal modulation methods, radio wave transmission paths, and local rainfall.

3. client devices

The configurations of client devices vary greatly, and different device vendors have different options. Generally, this module includes an outdoor unit, a targeted antenna, and a microwave transceiver device. The Indoor Unit includes a modulation and demodulation module and a network interface module connected to the user's indoor equipment. Most LMDS wireless transceiver duplex modes are FDD ). Downlink devices from the base station to the user end are generally reused in TDM mode. For uplink devices, multiple user end devices can use TDMA) and FDMA. FDMA is suitable for a large number of continuous non-burstable broadband access networks, while TDMA is suitable for supporting access to multiple burstable or low-rate data users. The LMDS operator should select an appropriate multi-access mode based on the characteristics and distribution of the user's business. The LMDS system can adopt modulation modes such as phase shift keying PSK including BPSK, DQPSK, and PSK) and orthogonal Amplitude Modulation QAM including 4QAM ). Currently, modulation technologies such as 6QAM and 16QAM can be provided.

4. Network Management System

The network management system is responsible for alarm and fault diagnosis, system configuration, billing, system performance analysis, and security management. Unlike traditional microwave technologies, LMDS can also form a cellular network to provide services in specific regions. When multiple base stations provide regional coverage, frequency multiplexing and polarization planning, wireless link computing, coverage and interference simulation and optimization are required.