1. Introduction
With the rapid development of Internet and the rapid development of multimedia technology, users have higher and higher requirements. At present, the core network construction of countries is initially ubiquitous, and the main communication network of the optical cable is basically able to meet the current needs of current communications. The prominent contradiction is reflected in the access network, namely the connection part of the user and the core network. In theory, the full-fiber access network will be a relatively perfect solution. However, all-fiber access achievement is too high, even in developed countries, it is far from being implemented. Currently, only fiber optics (FTTC) or fiber optic buildings (FTTF), and FTTF is only a small part. According to recent statistics, FTTF is only 1% in the 46 million commercial building in the United States. In fact, more than 95% of commercial and enterprises is less than 20 employees, small businesses, providing fiber access to such small and medium-sized users, and economically not costly. At present, it is mainly used to use the optical fiber coaxial mixing mode (HFC) access. However, due to factors such as prices and regulations, it is difficult to achieve difficult heaviness, and its bandwidth is still limited. Therefore, wired access is no matter from the bandwidth or technology. This creates a so-called "last-mail" problem.
2. "Last km" problem
2.1 What is the "last km" problem?
"Last-Mile" issues are due to the development of fiber optic networks. The "last kilometer" is the earliest part of the telecommunications service provider in the construction of the public analog telephone communication network, the real "last kilometer" refers to no more than 2 kilometers from the optical node to each user. the distance. Since the fiber can provide extremely high bandwidth (currently only 1 Ghz (GHz) is only a very small part of its bandwidth), and the traditional telephone line bandwidth is only a few tens of kilohertz (kHz), so from the optical node to the user The narrow information channel between the terminals forms the "bottleneck" of the access network, resulting in slow information transmission speed. This is the "last-mail" problem that has been plaguing considerable part of the user and the telecom operator, and the main bottleneck developed to broadband, intelligence, and personalization.
2.2 Existing access technology is resolving the limitations of the "last km" issue
Existing network access has a variety of implementation, the most common thing is to access telephone line, and can also be accessed through a wired TV network, satellite communication network. The reason why telephone line access has the most widely used application, the key is to popularize the telephone line of the telecommunications system. But it has several great problems:
1) Since the traditional telephone line only supports the transfer rate of 64kb / s, in addition to many users, the line is often crowded, and the speed is slow, and it is unbearable.
2) The telephone line is an analog line and can only transmit analog signals. To provide a data service, A / D, D / A conversion must be made, so that it will become unstable and often disconnect.
Therefore, the telephone line cannot achieve broadband access. The broadband access scheme currently adopted in the world has the following: FTTH (Direct home), ADSL (asymmetric digital user line), Cable Modem (cable modem), local area network access. In China, due to the high-performance factors such as optical exchange equipment, the cost of the fiber is extremely high and the labor injury; the ADSL access program is relatively high, and it is not convenient for future technical upgrade, and has never been able to break through the asymmetrical The Cable Modem Access is a broadband access plan in the United States. Because the US population is scarce, Cable Modem can complete broadband access issues within 30-40 kilometers, so Cable Modem The access scheme is a technology that is more suitable for US national conditions. However, the situation in my country is the urban population density, and there are many residents in a relatively small range. At this time, broadband access is to solve the "last kilometer" problem, not 30-40 km. Therefore, the wired access is stretched when solving the "last kilometer" problem. With the rapid development of wireless communication technology, an emerging wireless access technology that provides communication capacity is close to the fiber, known as "wireless fiber", is a local multi-point distribution business - LMDS.
3. LMDS technology
Local multi-point distribution business is also known as local multi-channel distribution services (Local MultiPoint Distribution Service, abbreviated as LMDS) is a representative of broadband wireless access systems. The words in "local multi-point allocation business" have their own meaning.
"Local" is actually the range that can be covered by a single base station. LMDS is limited by the radius covered by a single base station in a urban environment because of the limitations of operating frequency and radio wave propagation characteristics.
"Multipoint" means that the signal is transmitted by the base station to the client side in a point-to-multipoint manner, and the signal is transmitted by the user to the base station;
"Distribution" means that the base station will be sent to the respective users, respectively, including voice, data, and Internet, image services.
"Service" refers to the service providing and use relationship between system operators and users, that is, the business that users can get from the LMDS network depends entirely on operators' choice of business.
LMDS provides high-rate, large capacity, point-to-multi-point high-reliability full-duplex wideband wireless access, using wireless channels to solve the data backbone network, local switches to users with wireless channels. The inter-access problem is forwarded by the ground forwarding station instead of satellite. It provides services based on millimeter wave technology, working in a frequency band near 28GHz (around 80%, defines 27.5 GHz ~ 29.5 GHz as the frequency band of LMDS). The available bandwidth reaches 1 GHz, which can provide any kind of business, and support ATM, TCP / IP and MPEG-2 and other standards, high reliability. Due to the characteristics of LMDS with higher bandwidth and bidirectional data transmission, a variety of broadband interactive data and multimedia services can be provided to overcome traditional local loop bottlenecks, which meet the growing demand for high-speed data and image communications, is known as The ATM and IP extension in the air are therefore a worker that solves the communication network access problem.
3.1 Working principle of LMDS
Intermediate frequency signal
Intermediate frequency
Carrier source
Intermediate frequency
Modulator
power
Medium
Microwave
Upconversion
Microwave power amplifier
Microwave
filter
antenna
Vibrator
Code transformation
Microwave
Terminal device signal
LMDS indoor unit
LMDS outdoor unit
Figure 1 LMDS device sending principle block diagram
LMDS
With a service area similar to a honeycomb, it is divided into a number of service areas that need to be provided in areas. There is a base station in each serving area, and the base station device is connected to a multi-point wireless link to communicate with the remote station (client) in the service area. LMDS
The principle of device sending information is shown in Figure 1.
The downlink multiplexing mode between the base station and the remote station is a time division multiplex (TDM) or frequency division multiplexing (FDM), and the upper behavior time division multiple access (TDMA) or frequency division multiple access (FDMA) can also The code division multiplex (CDMA) is used. The RF modulation mode is orthogonal phase shift keying (QPSK) or orthogonal amplitude modulation (QAM). The base station and the remote station are also divided into two parts of the indoor unit and outdoor units, respectively. The indoor unit provides a part of the business (such as the accommodation and aggregation of the business), and the signal from the terminal device is modulated in the medium-frequency modulator to obtain the intermediate frequency modulation signal; the outdoor unit is provided The radio frequency transmission function between the base station and the distal station includes two parts: RF transceiver and radio frequency antenna. The radio frequency transceiver performs an intermediate frequency signal from the indoor unit to the radio frequency band, and outputs a radio frequency signal to the radio frequency antenna by the microwave filter. Since there is a multi-radius effect and random interference in the wireless channel, the transmission performance of the wireless channel is relatively harsh, so the transmitting end generally uses the RS encoded and dendered weaving, improve the system's error correction performance.
3.2 LMDS system structure
Figure 2 LMDS system composition
normally,
A perfect
LMDS
The system is made of telecom backbone network, base station, remote station and network management system
(NOC)
)
Four parts composition.
LMDS
The system composition is shown in Figure 2.
1)
Basic backbone network is also known as the core network. because
LMDS
The system can provide a variety of integrated business, this backbone network can be transmitted from the fiber optic transmission network.
ATM
switch,
IP
Switch or
IP ATM
The core switching platform made in the architecture and
Internet
Public telephone network (
PSTN
Interconnect module, etc.
each
The base station's data is sent to the backbone network, complete the voice exchange,
ATM
Exchange and
IP
Exchange, etc., and join the international export.
2) The base station device consists of a plurality of sector devices. Each fan is divided into two parts: ODU and the indoor unit (IDU). The outdoor unit includes an antenna, a microwave transceiver module; the indoor unit includes a modem module, a network interface module. The network interface module provides interfaces with telecom backbone, such as ATM, IP, Frame Relay, PSTN, ISDN, etc., and implements the signal between the backbone network and the wireless transmission.
The base station of the LMDS system is covered with a multi-zone, and the user-end device is covered with a speaker antenna focused within a certain angle range. The LMDS system can be divided into 15 degrees, 22.5 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degree sectors in accordance with the antenna, which is up to 24 sectors. The base station indoor unit will aggregate multiplexed services from different users of each sector, submit to different service nodes, and then complete the access service of the user service by the network interface module.
3) The remote station (also known as the user terminal) is located in the user station, which can be a cell or home user. Also consist of the outdoor unit (ODU) and the indoor unit (IDU). The outdoor unit includes a directional antenna, a microwave transceiver module; the indoor unit includes a modem module, a network interface module (NIU). The directional antenna of the outdoor unit is directed to accept signals from the base station antenna, and the indoor unit will receive the service adaptation, aggregate from the user's resident network, and then transmitted to the outdoor unit through the intermediate frequency cable, and transmitted to the base station through the wireless link; In contrast, the indoor unit extracts this business from the downstream service and distributes it to the user. The network interface module provides interfaces for various user services and completes multiplexing / demultiplexing. 4) The network management system is the monitoring center of the entire LMDS system, which is responsible for completing the alarm and fault diagnosis, network configuration, system counting, performance analysis, and security management.
3.3 Limds provided by LMDS
The broadband characteristics of LMDS determine that it can provide any kind of business, including voice, data, and images. It is also possible to provide carrier services such as transmission between cellular systems or PCS / PCN base stations.
1) The voice service LMDS system is a high-capacity point to multi-point microwave transmission technology, providing high quality voice services, and there is no time delay. The system can provide standard interfaces such as RJ-11, 10Base-T, etc.
2) Data service LMDS system data services include low, medium and high speeds of 3 files. Low speed data service, 1.2 ~ 9.6kb / s, can handle the data of the open protocol, the network allows the network to receive the value-added service network; the medium speed data service, 9.6kb / s ~ 2Mb / s, such data interfaces are usually It is a value-added network local contact; high-speed data service, 2 ~ 155MB / s, BER below 10-9. This data service must have an Ethernet and fiber distribution data interfaces.
3) Image service LMDS supports simulation and digital image services, such as telemedicine, high-speed conference TV, distance education, remote business and user TV, video on demand (VOD), etc. The available image channels include 150 remote programs, 10 local programs. The signal of the system can be from satellite, or it can be localized; it can be encrypted or unencrypted.
3.4 Advantages of the LMDS system
LMDS is essentially a fixed broadband wireless access technology, so it has the advantages of broadband and wireless access, which is reflected in:
1) Width of the operating bandwidth can provide broadband access. At present, the LMDS operating bandwidth allocated by countries can reach 1GHz, which is much higher than the bandwidth of the cellular communication system (50MHz), the PCS system (140MHz), the MMDS system (140MHz), and the supportable user access data rate is as high as 155MB / s, can meet the needs of users to grow in communication bandwidth.
2) High frequency complexity and large system capacity. The multi-sector base station can make more effectively utilize the spectrum to further expand system capacity. If the polarization is used, the system capacity can be further added. The capacity of the LMDS base station is likely to exceed the total amount of user service within its coverage area, so the LMDS system is likely to be a "range" limited system instead of "capacity" restricted system. Therefore, the LMDS system is particularly suitable for use in high-density users.
3) The system expansion is flexible. When the user increases, the sector can be greatly improved by increasing the number of carriers per sector or a sector antenna with a smaller angle, can also be greatly improved by combining two means.
4) Advantages with millimeter wave communications. The millimeter wave communication is good, and the performance is stable, which is smaller than the external interference. Due to the work in the millimeter wave section, the antenna diameter and the radio frequency front end device can be made small, the complexity of the equipment is reduced, which is especially important for the user. 5) It is generally advantageous with wireless communication systems. Such as low construction costs, small launching funds, short construction cycle, fast investment recovery, low network operation and maintenance costs.
3.5 Limitations of LMDS systems
LMDS works in a millimeter wave band, making it some limitations inherent in millimeter wave communication:
1) The LMDS service area coverage is small and is not suitable for remote users. Mmmm Mime wave is very propagated in free space. In the actual transmission, it is also subject to factors such as reflection, refraction, scattering, and absorption, making the propagation losses greater. Therefore, the typical coverage radius of LMDS is 3 to 5 km.
2) Need a transmission of a line. The millimeter wave is shorter and is only suitable for the transmission. In order to avoid new high-rise buildings, it may be necessary to adjust the LMDS transceiver equipment placed in the roof or tower, which will increase the cost of network maintenance.
3) Due to the high operating frequency, the quality of communication is raining, the weather is more affected, and the communication is caused when it is severe.
4) The base station apparatus of the high-frequency section is relatively complicated, the price is more expensive, so when the user is small, the average cost of each user is high.
5) Technology is still not perfect. Factory with LMDS network devices is currently available, and there is also a lack of equipment performance. And there is no unified LMDS air interface standard, so that there are many problems with different manufacturers' equipment interconnection.
4. Conclude
LMDS is a brand new broadband wireless access technology, although there are still some relevant technical issues that have not been well resolved, and there is no unified standards and norms in the international. But because it has many advantages such as large capacity, multi-service, easy construction, efficiency, etc., has attracted the widespread concern of many telecom operators at home and abroad. LMDS is undoubtedly a better solution for solving the "last km" problem in the case where the fiber is entered.