Wang JinChun belong to mao only brilliant Southwestern Communications University
[Summary ]: Concept of wireless access technology LMDS of broadband that more exhaustive introduction to this text is new developing, the system forms and technological characteristic, the business that LMDS offers.
As demand for information of the social development being greater and greater, offer high-speed high-capacity information to already become the important subject in communication, especially the high-speed development of Internet, the demands for the broadband business of people are higher and higher. The technology of now has already solved the high-speed broadband swapper of the key network part, as to user, the bottleneck offering the high-speed broadband business exists in the part of the access network, the so-called one ” The last kilometer ” . The access way of the narrowband, has already become the main bottleneck of surfing the Net at a high speed and broadband business development. Need to offer the high-speed broadband access technology for the fact that users insert partly urgently at present, the representative broadband access technology has xDSL ‘ Cycle of digital user ,HFC mix the optic fibre co-axle ,Cable Modem the modem of cable ,FTTH
optic fibre get home With the wireless access technology of broadband.
As to other access technologies, the wireless access technology of broadband, with little investment in initial stage of having, unless short in network construction period, business will offer fast,as resource can re-utilizing advantages and extensive application prospect unique of respects. Broadband wireless access technology become current the intersection of communication network and with the fastest developing speed field one of already. There are three kinds of main wireless access technologies of broadband, namely MMDS many some many ways distribute business that has already come into operation With DBS broadcast directly the satellite system And LMDS rises ‘ Much some here distributes business ,And LMDS is the hottest technology in this field.
LMDS technology is put forward by the father Bernard Bossard of LMDS of U.S.A. at first, it is a one point to many point broadband wireless communication system, the work is in one millimetre of wave wave bands, 20~40GHz takes frequently, can reach 1G and can be compared with the optic fibre while transmitting the capacity, known as ” The wireless optic fibre ” . Can offer the two-way pronunciation, services such as the data, Internet, the video,etc..
The meaning of every word in LMDS:
Local here : Mean the range that the single base station can cover, because LMDS is propagated the limiting of the characteristic by the electric wave of operating frequency, the radius covered of the single base station with in the urban environment is usually smaller than 5 kilometers;
There is a bit more Multipoint : Mean signal to convey by piece to many the intersection of broadcast and way a bit from base station to the intersection of user and end, but the signal is conveyed in a point-to-point way to the base station by the user end;
Distribution assign : Mean the signal that the base station will send out ‘ May include speech, data, Internet, video business at the same time Assign to each user separately;
Service business : Mean the business between system operator and user is offered and used the relation, namely the business which users can get from LMDS network totally depends on the choice to the business of the operator.
2nd, form systematically
LMDS system may be regarded users and the connection between the networks as ‘ Network insertion Application. A typical LMDS application system is composed of four parts: Key network of optic fibre, Network Operation Centre NOC , the end apparatus of user in base station system and service area in the service area, as shown in Fig. 1. The data of each base station are sent into the key network, finish the speech exchanging, ATM exchanging, IP to exchange etc., and enter the international export in succession. In order to manage apparatus and user of the service area, the system often sets up Network Operation Centre NOC .
Fig. 1 LMDS systematic structure
The network operation center refers to the network administrative system N M S The apparatus, it responsible for managing the networks of the users of a plurality of areases, finishes functions such as system assigning, systematic perfomance analysis, safety management, charging,etc., a plurality of NOC can pass the optic fibre network interconnectedly. The optic fibre backbone network means the key floor of the network, it not merely offers more than one network platforms of business, it is the interconnected physical passway between each base station at the same time. One layer of network platforms of core can be formed by technology such as ATM, IP, ATM IP, SONET/SDH/WDM, the interconnection that took charge of with various existing networks, for instance with PSTN, FR, DDN, CATV, Internet,etc.
Wait, thus make LMDS base station offer the business that nearly all existing networks can offer.
The base station system is responsible for realizing the optic fibre backbone network to change with the wireless signal while transmitting. The base station apparatus includes the network interface of the optic fibre with interconnected backbone network, modulates and demodulates the module of function and place on the microwave in the rooftop and receive and dispatch the apparatus. In the base station design, whether the base station includes local exchange, it is an important problem that must be considered. If including exchanging here in the base station design, then the base station is covered in the area the information exchange between different users can be finished in the inside of this base station, do not need to exchange the optic fibre backbone network; If excluding exchanging here in the base station design, it covers in the district all users’ information exchange will not be finished unless get in the optic fibre backbone network.
The base station adopts many sectors to cover technology, namely cover user’s end apparatus within the range of certain angle with loudspeaker aerial focused on. Many sector base stations can utilize frequency spectrum effectively, expand systematic capacity. Can be divided into 15 degrees, 22.5, 30 degrees of, 45, 60 degrees of, 90 degrees of sectors according to adopting different LMDS of the aerial, 4 sectors at least, 24 sectors at most.
Modulating the way can adopt BPSK, DQPSK, QPSK and 4QAM, 16QAM, 64QAM. And adopt 16QAM and 64QAM and improve the utilization ratio of the frequency greatly, the base station can also offer higher sector capacity.
User’s end apparatus CPE Including outdoor unit ODU With the indoor unit IDU Two parts. The outdoor unit includes beam antenna and microwave and receives and dispatches the apparatus; The indoor unit includes and modulates and demodulates the network interface NIU that the module links with user’s inner apparatus of the room Module. The outdoor beam antennas and single / many mould optic fibres or copper cables that users generally adopt the requirements to be very little are connected to the indoor network interface unit, it offer multiple operation interface,it and finish of different business signals to reply with /if you can’t solve, if you can’t reply, use by function.
Indoor network interface unit, usually offer abundant business interface such as E1, POTS, 10Base-T, FR, ATM, ISDN, N * 64kb/s, therefore can support many kinds of application.
3, technological characteristics
3.1 The working frequency band distributes and frequency use
LMDS system work generally mainly has 10GHz, 24GHz, 26GHz, 31GHz and 38GHz on 10GHz- 40GHz frequency band in one millimetre of wave wave bands. A lot of countries in the world have planned the application frequency band of LMDS at present. American federal communication committee divides the frequency band of LMDS into two sections: 27.5- 28.35 GHz, 29.1-29.25 GHz and 31.075-31.225 GHz is the section A, 1150MHz altogether; 31.225- 31.3 GHz is the section B, 150MHz altogether. Ministry of Information Industry of our country is in 2001 too Issued in annual August <>,Determine to distribute the business LMDS as many some FDD ways some frequency of 24.45-27GHz frequency band Frequency of utilization try . The centeral station launches the frequency band: 24.507 -25.515 GHz; Carry the station and launch the frequency band far: 25.757 -26.765 GHz; It is 1250 MHz to receive and dispatch the interval of frequency. Though LMDS system can use frequency band in 1GHz above, reach each operator to use it specifically but speech, its spectral bandwidth that has is still limited. LMDS system mainly from two respects realizes the re-utilization of the frequency. Between different sectors of the same centeral station and the adjacent sectors of different centeral stations at first, polarise isolation technology and isolates the channel and dispose the frequency to reply and fully utilize frequency resources with the perpendicularity of a carrier frequency with technology through using. In the cover district of a centeral station, the more the sector divided, the higher the same frequency utilization ratio is, the utilization ratio that for example 24 sectors divide is 6 times that 4 sectors divided. Secondly, LMDS system is used in the same sector and adopted the capacity of improving the network greatly that many signal carrier technologies will come frequently in the neighbor.
3.2 Wireless transmission means
LMDS may be in FDM in system ‘ Divide and reply using frequently Or TDM reply and use time Work under the duplexing communication mode. The down wireless chain from base station to user terminal is one to way to click more, adopt the replying and using the way of FDM or TDM. The wireless up chain between user terminal and base station adopts some to finish to the way to click, insert many location ways and adopt FDMA divide much location frequently Or TDMA much location of time .
It is still TDMA way to adopt FDMA way, will choose according to the needs of customer service. FDMA divides designated frequency spectrum into several equi-spaced channels according to certain requirement, each user each accounts for a channel among them. Its characteristic is that there are few expenses, high in efficiency, the capacity is distributed fixedly, is suitable for big portfolio, steady, the invariable bit of business that lacks happens suddenly; TDMA cuts continuous time apart into several passages, called the time slot, each user takes up the time slot stipulated to receive and dispatch signals in the frequency section that assign. Carrier frequency share a plurality of user TDMA the same, be needed moving ahead simultaneously with timing, there is low efficiency than ,Need to allocate the time slot in advance, the bandwidth that support to distribute according to need, is suitable for Gao’s sudden and low speed data customer service.
3.3 Modulate the way
LMDS modulation way of system have PSK ‘ Move the key to accuse of With QAM orthogonal range is modulated Two kinds, mainly BPSK, DQPSK, QPSK, 8PSK, 4QAM, 16QAM, 64QAM,etc. modulate the way. Different modulation these choose, to FDMA and TDMA basically can realize.
FDMA adoptable modulation method in LMDS system, and estimate that is used in 2Mbps CBR invariable bit of rate The necessary bandwidth while connecting, it has reflected the relation between bandwidth and transfer rate.
At present, in LMDS system, TDMA can’t adopt the 64QAM modulation method yet while visiting the method, in addition, TDMA and FDMA can adopt the same modulation method. In the same modulation way, the data transmission speed of TDMA is lower than FDMA under 80% of the situations. Moreover, can’t adopt the 64QAM modulation, so the supreme speed that can obtain in FDMA system in TDMA system, can not reach in TDMA system. But realize the 64QAM modulation and demand to have higher signal level, and with conveying the increase of the distance in LMDS system, signal level will decay, so the adoptable 64QAM modulation way that be able to reach the speed of supreme data transmission of FDMA system can only be used for conveying at short range in LMDS system. Therefore the more advanced modulation ways adopted are, the smaller the cover radius of the base station is. Under the electromagnetic environment equally, after QPSK modulation, transmitting the distance can reach 10km, it is 5km after 16QAM modulation, and the distance of transmission after the 64QAM modulation is only 2.5km.
3.4 Wireless ATM
The transmission of LMDS system was realized by ATM and combination of the wireless technology mainly, adopt ATM letter yuan and transmit the mechanism as basic wireless. This way makes LMDS network have larger communication capacity, can carry on flexible, comprehensive treatment to the multiple operation. It replies wireless statistics by statistics of function and ATM and replies with the function and combines, have obviously improved the utilization ratio of the frequency band. This kind has the following main characteristic to realize the way: Can meet the access demands of different users, offer user dynamic bandwidth to distribute the function; Offer for different communication service, based on standard QoS ‘ Service quality Controlling mechanism; Can bear the weight of the wireless platforms of multimedia of multiple operation such as the speech, data, video; Offer the service of carrying the end to wired and wireless user; Interconnected with the extant broadband network.
As bearing the weight of the system of broadband LAN and wide-area network, ATM is approved by everybody more and more. In the wireless network of the honeycomb, the centeral station can be regarded as the node of ATM, carry on the seamless link with the wired network, offer the wireless broadband business. Most the intersection of LMDS and apparatus offer commercial city adopt the intersection of ATM and the intersection of letter and yuans of way to shed, assign and work LMDS business at present. Compared with standard ATM letter yuan, wireless the intersection of ATM and the intersection of letter and yuan increase the intersection of letter and yuans of sequence number, come and judge letter continuity and repetitiveness of yuan, and joined CRC at ending of yuan of the letter ‘ Circulate redundant yard ,Used in correcting error and check-up of the wireless chain, as shown in Fig. 2. Unless on ATMs wireless, getting more abominable relatively in terms in because channel,let’s adopt by FEC before in codes error-correcting With ARQ send again automatically asks The improvement that can be effective believes the losing rate of yuan.
The intersection of 2 Fig. and typical wireless the intersection of ATM and the intersection of letter and yuans of structure
In LMDS system, because of adopting wireless ATM technology, makes the centeral station offer the abundant network interface, such as ATM, TDM, FR, IP and ADSL,etc.. Pass the distribution of resources at the same time, the wireless ATM network technique can offer different QoS grades service. Combine wireless the intersection of ATM and technology and point to some wireless technology more, LMDS network can meet different the intersection of bandwidth and solution of demand with economic flexible various offering.
3.5 Wireless to propagate the characteristic
, in one millimetre of wave wave bands of 10GHz- 40GHz, it is extremely apt to receive rain, fog, snow and deciduous influence because of LMDS system work, cause decline and deterioration of the signal. So LMDS system is can only getting getting more shorter regard communication base station is usually covered by 2-5km with the radius in the distance as ,Namely there is LOS ‘ Look at the general character . To communication of low frequency it is multipath decline, and the frequency band above 10GHz of LMDS system work that mainly exist, the impact on it of the multipath decline is very small.
The signal level that the rainfall has decayed and received, make LMDS base station cover the radius to diminish, the reason is: It is relatively strong that the microwave frequency band that the rainwater works to LMDS absorbs and scatter function, make the signal decay relatively largely; The rainfall makes polarised direction of the signal deflect, the signal making the aerial receive is diminished. The deflection that the signal polarises will cause and isolate degree dropping to the adjacent sector interference too. In order to reduce the influence that the rain declines, LMDS systems generally adopt ATPC dynamic self-adaptation posts a letter the power is controlled With FEC to the error-correcting code ago Compensate the decline brought because of rainfall,etc..
Because the working frequency band of LMDS system is extremely wide, can offer the multiple operation, mainly face commercial user and group user, area when suitable for portfolio concentrating on, user’s trooping. The main business lines type that can be offered at present includes several kinds.
LMDS system can offer the high-quality voice service, the system offers the standard interface, can dispose the module of interface, realize it with traditional POTS Plain Old Telephone Service The business links up, offer PSTN speech business.
2Business of the data
The data business of LMDS system can be divided into 3 shelves of low, middle and high speed according to their transfer rate of data: The business of the low-speed data: 1.2~9.6kbit/s, can deal with the data of the open agreement, the network allows to receive the value-added service network from the local access point;
The data business of the moderate speed: 9.6 Kbit/s 2Mbit/s, such data interface is usually the local joint of value-added network;
The business of the high-speed data: 2~155MBit/s, BER one bit of mistake rate Lower than 1* 10-9, there must be Ethernet and optic fibre and is distributed in interface of the data to offer such data business.
LMDS can offer simulation and digital video business, such as tele-medicine, teleeducation, high-speed meeting TV, E-business and VOD,etc..
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