CN102420794B - 4M transmission method of OFDM structure of satellite CMMB system - Google Patents
4M transmission method of OFDM structure of satellite CMMB system Download PDFInfo
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Abstract
The invention discloses a 4M transmission method of an orthogonal frequency division multiplexing (OFDM) structure of a satellite China mobile multimediabroadcasting (CMMB) system. According to the method, at a transmitting terminal, pilot frequency insertion, OFDM moulding and cyclic prefix insertion are carried out on a data subcarrier that has been coded and mapped and then the processed data subcarrier is sent to a receiving terminal; at the receiving terminal, cyclic prefix deletion, OFDM deframing, and pilot frequency deletion are correspondedly carried out on a received signal and then sent data are recovered by demapping and channel decoding processes. According to the invention, a balanced and eclectic optimized scheme is provided for a CMMB standard in aspects of a system data transmission rate and a frequency spectrum utilization rate, so that a technical problem of unbalanced functions in the aspects of a transmission rate and a frequency utilization rate in a traditional system is solved. Besides, a pilot frequency pattern arrangement mode is designed reasonably and optimized, so that the system has good channel estimation precision and a system transmission performance of a 4M mode in a satellite mobile multipath environment is also improved; moreover, a transmission mode of the OFDM structure of the CMMB system is expanded.
Description
Technical field
The invention belongs to mobile communication technology field, relate to a kind of 4M transmission method of satellite CMMB system OFDM system.
Background technology
In October, 2006, China Mobile multimedia broadcasting (China Mobile MultimediaBroadcasting, CMMB) technical research working group has issued < < Mobile Multimedia Broadcasting part 1: broadcast channel frame format, this industry standard of Channel Coding and Modulation > >.(hereinafter to be referred as " this standard ").
This standard definition in S frequency range by the broadcast system of the multi-media signals such as ground (or satellite) TV transmission, broadcast, data message, wherein physical layer adopts Orthodoxy Frequency Division Multiplex transmission (OrthogonalFrequency Division Multiplexing, OFDM) technology, signal bandwidth has 8M and two kinds of transmission modes of 2M are available.These two kinds of patterns can provide one or more independently broadcast channel, support respectively Multi-encoding and modulation system in order to meet different business, the different requirements of different transmission environment to signal quality.Wherein, 8M pattern has 4096 OFDM subcarriers, is applicable to the demand of middle and high bit rate business, and 2M pattern has 1024 subcarriers, in being applicable to, the demand of low bit rate rate business.Physical layer is also supported multiple services mixed mode, reaches the coupling of traffic performance and transmission mode, thereby realizes flexibility and the diversity of service operation.
But there is following three point problem in above-mentioned standard:
1, the system spectrum utilization ratio of existing 8M pattern is lower, under the demand of the mobile multi-media broadcasting service speed of identical middle and high data transfer rate (4Mbps～8Mbps), on average lower than 1 (excursion is about 0.51～1.02);
2, the data transmission service speed of existing 2M pattern is lower, (constellation mapping: BPSK/QPSK/16QAM under identical constellation mapping+chnnel coding condition, chnnel coding: LDPC1/2, LDPC3/4 etc.), be only on average 1.26Mbps (excursion is about 0.55Mbps～2.17Mbps);
3, existing 8M pattern ofdm signal bandwidth is 8MHz, relatively large, and 2M pattern ofdm signal bandwidth is 2MHz, relatively little again, not yet on the resource-constrained satellite of transmission bandwidth, band efficiency, applies at present, and transformation parameter need further optimization.
Growing along with DMB communication requirement, existing transmission mode cannot meet the requirement that transmission rate variation is selected, and system spectrum utilization ratio is not high yet.The well designed of existing transmission mode and optimal design are just become to the new problem of filed of mobile multimedia broadcasting.
Summary of the invention
Technology of the present invention is dealt with problems and is: for the deficiencies in the prior art, provide a kind of 4M transmission method of satellite CMMB system OFDM system.The present invention aspect system data transfer rate and the availability of frequency spectrum two for CMMB standard provides a kind of compromise preferred version of weighing, solved legacy system unbalanced technical problem of function aspect transmission rate and the availability of frequency spectrum two.
Technical solution of the present invention is:
A 4M transmission method for satellite CMMB system OFDM system, at transmitting terminal, carries out sending to receiving terminal after pilots insertion, OFDM moulding and insertion Cyclic Prefix step to the data subcarrier completing after coding and mapping; At receiving terminal, after deleting accordingly to the received signal Cyclic Prefix, OFDM solution frame and deleting pilot tone, by demapping and channel decoding, recover the data of transmission.
In described pilots insertion step: every 8 subcarriers are divided into one group, comprising 7 data subcarriers and 1 scattered pilot subcarrier; In frequency direction and time slot direction, at interval of 7 data subcarriers, insert 1 scattered pilot subcarrier respectively; Thereby on frequency domain direction, form an OFDM frequency domain symbol, each OFDM frequency domain symbol comprises 1540 effective subcarriers;
In described OFDM forming step: supplement 507 virtual subnet carrier waves and 1 center frequency point subcarrier in each OFDM frequency domain symbol; By two groups of 1540 effective subcarrier average marks, every group 770, be arranged in symmetrically the both sides of described center frequency point subcarrier, then 507 virtual subnet carrier waves be filled in to the both sides of 1540 effective subcarriers, form altogether 2048 total subcarriers, form the spectrum structure of nominal bandwidth 4MHz; 2048 total subcarriers are carried out to IFFT computing, form an OFDM time domain moulding symbol;
In described insertion Cyclic Prefix step: first, using the beginning that in each OFDM time domain moulding symbol, 256 last subsymbols copy to OFDM time domain moulding symbol as Cyclic Prefix, make duration of each OFDM time domain moulding symbol equal FFT integration period and cyclic prefix period and; Then, before the beginning of each OFDM time domain moulding symbol and after ending place, add protection interval.
In OFDM frequency domain symbol described in each, have 192 scattered pilot subcarriers, in time slot direction or frequency direction, the ratio at pilot sub-carrier interval is 1: 1, and stagger mutually in the position of scattered pilot subcarrier in different OFDM frequency domain symbols, form star-like pilot frequency design spread pattern.
In described star-like pilot frequency design spread pattern, for the Position Number m of scattered pilot subcarrier in n OFDM frequency domain symbol in each time slot, meet following rule:
Wherein, MOD (n, 8) is for getting remainder operation; N span is that 0≤n < 53,53 is the sum of OFDM frequency domain symbol in a time slot; Integer p represents the sequence number of scattered pilot subcarrier in OFDM frequency domain symbol, and p span is 0≤n≤191.
The length of described Cyclic Prefix is 1/8 of FFT integration period.
The present invention compared with prior art tool has the following advantages:
(1) on the one hand in order to expand and to improve system data transfer rate, under certain constellation mapping+chnnel coding condition restriction, effective method is raising system transmission bandwidth, and this need to expand the bandwidth of existing 2M pattern, to improve system data transfer rate.There is certain constraints in choosing of nominal bandwidth, need to meet on the one hand the rule of IFFT/FFT computing, needs on the other hand to consider the implementation complexity of satellite system and the balance of operational efficiency, and the present invention is designed to 4MHz by nominal bandwidth.Adopt this kind of design to there is higher data transmission rate than existing 2M pattern, under identical constellation mapping+chnnel coding condition, select the average data transfer rate of 2M pattern to be about 1.26Mbps, and select the average data transfer rate of 4M pattern to be about 3.14Mbps.
(2) on the other hand in order to optimize and improve system spectrum utilance, under the restriction of the mobile multi-media broadcasting service rate requirement of certain middle and high data transfer rate, effective method is to reduce system transmission bandwidth consumption, this need to reduce the bandwidth of existing 8M pattern, to improve system spectrum utilance.According to the bandwidth selection principle described in advantage (1), and take into account and consider the operational efficiency of satellite system and the balance of implementation complexity and IFFT/FFT algorithm.Design of the present invention is compared with existing 8M pattern, under the demand of the mobile multi-media broadcasting service speed of identical middle and high data transfer rate (4Mbps～8Mbps), there is the higher availability of frequency spectrum, selecting the system spectrum utilance of 8M pattern is approximately only on average 0.8, and selects the availability of frequency spectrum of 4M pattern to be on average about 1.58.
(3) in order to improve and elevator system transmission performance, at satellite, move under Multipath Transmission environmental limitations, the present invention is by the position relationship of pilot sub-carrier in reasonable Arrangement pilot frequency design and data subcarrier, in time slot direction and frequency direction, make pilot sub-carrier interval equate, and the pilot initial position of different OFDM symbols is staggered mutually, make each subcarrier in each OFDM symbol can obtain good precision of channel estimation, thereby the accuracy that elevator system overall channel is estimated, and then reduction system transmission error rates, final elevator system transmission performance.Compare with Europe 4k transmission mode of the same type, under identical simulated conditions, pilot frequency design distributes more even, it is more reasonable to design, make system there is better channel estimating performance and more excellent transmission error performance, effectively promoted adaptability and the integrality of ofdm system under multiple mobile multipath reception environment, and the diversity of transport service selection.
Accompanying drawing explanation
Fig. 1 is 4M pattern OFDM Equivalent Base-Band transmission schematic diagram of the present invention;
Fig. 2 is that 4M pattern physical layer frame structure time slot of the present invention is divided schematic diagram;
Fig. 3 is 4M mode spectrum structure of the present invention and subcarrier allocation order figure;
Fig. 4 is 4M pattern pilot frequency design structural design schematic diagram of the present invention;
Fig. 5 is that 4M pattern of the present invention is inserted Cyclic Prefix and protection spacer structure figure;
Fig. 6,7 is 4M mode channel transmission performance comparison diagram of the present invention.
Embodiment
The present invention proposes and the 4M transmission mode of design adopts and the compatible design in physical layer transmission standard of existing 8M/2M pattern.The 4M transmission mode proposing can provide one or more independently broadcast channel, can support respectively Multi-encoding and modulation system in order to meet different business, the requirement of different transmission environment to signal quality.Physical layer is also supported multiple services mixed mode, reaches the coupling of traffic performance and transmission mode, thereby realizes flexibility and the diversity of service operation.And in order to keep the self-consistentency of three kinds of transmission modes, 4M pattern has the basic framework identical with 8M/2M transmission mode, physical layer Equivalent Base-Band mode is basic identical, and difference is the design of system important technological parameters index.Fig. 1 has provided the Equivalent Base-Band mode of 4M pattern, and the basic framework of this model is compatible up and down with existing 8M/2M transmission mode, and design parameter preparation need to design separately for 4M pattern.
The performing step of the inventive method is as follows:
1, important technological parameters index
The 4M transmission mode that the present invention is designed, physical layer bandwidth is 4MHz (nominal value), number of subcarriers is 2048 subcarriers, wherein effectively subcarrier (not containing virtual subnet carrier wave) quantity is 1540 subcarriers, comprise 1304 data subcarriers, 192 scattered pilot subcarriers and 44 continuous pilot subcarriers, actual effective bandwidth is 3.76MHz, and the minimum symbol duration of time domain is 0.2 μ s, the total symbolic number of every time slot is 125,000 data symbols; Other parameters such as sub-carrier frequencies interval, FFT integration period, every frame duration is identical with existing 8M/2M transmission mode, has realized and existing 8M/2M pattern compatible this principle in physical layer transmission standard.System parameters and three kinds of mode parameter comparisons are referring to table 1.
Table 1
2, physical layer frame structure and time slot are divided design
It is unit definition that the physical layer frame structure of 4M transmission mode be take a frame equally, it within every 1 second, is 1 frame, every 1 frame is divided into 40 time slots, wherein first time slot (time-gap number 0) is distributed to fixing control logic channel use, and the service logical channels that all the other 39 time slots (time-gap number 1～39) are distributed to by system configuration is used.The duration of each time slot is 25ms, and in each time slot, all includes 1 beacon symbols and 53 OFDM symbols.Fig. 2 provides 4M pattern physical layer frame structure time slot and divides schematic diagram.
3, pilot configuration design
In ofdm system, the channel estimating of receiving terminal is very important, this completes by insert pilot sub-carrier in transmitting terminal data subcarrier, conventionally by data subcarrier and pilot sub-carrier weave in, the OFDM frequency domain symbol of complete, effectively carries out the operations such as channel estimating and compensating equalization thereby realize receiving terminal.The present invention is the reference diagram that 4M transmission mode has designed a kind of pilot sub-carrier and the star-like distribution patterns of data subcarrier, as shown in Figure 4.
In Fig. 4 on frequency domain direction, by every 7 data subcarriers and 1 scattered pilot subcarrier totally 8 subcarriers be divided into one group, every 7 data subcarriers, insert 1 scattered pilot subcarrier; In time slot direction, be that every 8 subcarriers are one group equally, every 7 data subcarriers, insert 1 scattered pilot subcarrier; Thereby form a kind of star-like pilot tone arrangement mode, and time slot direction pilot interval becomes 1: 1 ratio setting with the value of sub-carrier frequencies direction pilot interval, pilot frequency design is distributed more even.In order to make there is pilot frequency information on the sub-carrier frequencies point in each OFDM frequency domain symbol as far as possible, here the pilot initial position of different OFDM frequency domain symbols is staggered mutually, make each sub-carrier positions in each OFDM frequency domain symbol have an opportunity to obtain good precision of channel estimation, thereby elevator system overall channel estimated performance and data symbol isostatic correction effect, also make system have time-frequency equilibrium and the consistency of good channel estimating.
In 4M transmission mode, each OFDM frequency domain symbol comprises N
vindividual effective subcarrier, considers N in conjunction with the design of 8M/2M pattern
vshould be taken as 75% left and right of the total number of subcarriers of present mode FFT, consider the folding neutral principle of 4M mode parameter design, in 4M pattern, the value of the total number of subcarriers of FFT should be between 2M pattern and 8M pattern, and is the minimum base 2 power integers that meet the demands, therefore should be taken as 2048, N
vshould be taken as 1540.
In these 1540 effective subcarriers of each OFDM frequency domain symbol, contain 192 scattered pilot subcarriers, 44 continuous pilot subcarriers, and 1304 data subcarriers.Continuous pilot subcarrier is used for transmitting identical transmission indication (binary bit stream, 1 or 0) information in each OFDM frequency domain symbol; Scattered pilot subcarrier has been used for channel estimating, and according to the arrangement scheme of Fig. 4, in each time slot, in n OFDM frequency domain symbol, the Position Number m (0 is initial) of scattered pilot subcarrier meets following rule:
In above formula, MOD (n, 8) is for getting remainder operation; N span is that 0≤n < 53,53 is the sum of OFDM frequency domain symbol in a time slot; Integer p represents the sequence number that adds up to the scattered pilot subcarrier of 192 in OFDM frequency domain symbol, and p span is 0≤n≤191.
4, OFDM moulding
The modulation and demodulation process of ofdm system is unique, it is, by IFFT/FFT algorithm, the subcarrier of all OFDM is carried out to modulation and demodulation conversion, can guarantee orthogonality relation strict between subcarrier like this, so the modulating/demodulating of ofdm system is also known as " quadrature modulation/demodulation ".
Consider the compromise of 4M mode parameter design, in 4M transmission mode, the total sub-carrier number value of FFT should be between 2M pattern and 8M pattern, the minimum base 2 power integers that simultaneously will meet the demands, therefore total number of subcarriers of 4M transmission mode is 2048, wherein 1540 subcarriers are evenly distributed in center frequency point around, and 507 virtual subnet carrier waves are distributed in the both sides of bandwidth of operation symmetrically, form the spectrum structure of actual bandwidth 3.76MHz.According to FFT Transformation Properties, subcarrier number 0～1023 is distributed between positive spectrum region, and subcarrier number 1024～2047 is distributed between negative spectrum region.The OFDM baseband frequency spectrum structure of 4M transmission mode and subcarrier distribute schematic diagram as shown in Figure 3.
5, design is inserted at Cyclic Prefix and protection interval
Insert Cyclic Prefix (Cyclic Prefix, CP) object is to eliminate to greatest extent intersymbol interference (Inter-Symbol Interference, ISI), concrete grammar is the beginning to symbol duration by the signal replication in a last time slot of symbol duration, the duration of such OFDM symbol just become FFT integration period and cyclic prefix period and.
In 4M pattern, select the length of CP to account for 1/8 of FFT integration period, this is approximately last 256 the word symbols of IFFT integration period, as shown in Figure 5.
After inserting cyclic prefix CP; also to locate to add respectively protection interval (Guard Interval in the beginning (or ending) of each symbol; GI); its object is to avoid producing between adjacent two symbols ISI equally; one section of useful signal can be copied in this protection interval as CP; also can be null symbol (Null Symbol); but it is conventionally short a lot of than CP; the length of general selection GI only accounts for 0.5%～1% left and right of IFFT integration period; in 4M pattern of the present invention, this is approximately last 12 the word symbols of I FFT integration period.
6, other designs
Physical layer is carried out independent coding and modulation to each physical layer logic channel, wherein control logic channel adopts fixing Channel Coding and Modulation pattern, the coding of service logical channels and modulating mode can be adjusted flexibly according to system requirements, configuration mode is broadcasted to terminal by system control information, according to the difference of coding and modulating mode, physical layer can provide different transmission net load service rates.4M pattern physical layer system net load speed is referring to table 2.
Table 2
In table 3, selecting under identical constellation mapping+chnnel coding condition, for example, selecting " BPSK+1/2 " mode, the data transmission rate of 2M pattern is only 0.553Mbps, and 4M pattern has higher data transmission rate, is about 1.382Mbps; " QPSK+1/2 " mode of selection, the data transmission rate of 2M pattern is 1.106Mbps, 4M pattern is 2.763Mbps; Select alternate manner, also have identical Changing Pattern.
Table 3
In table 4, under identical middle and high data transfer rate (4Mbps～8Mbps) data traffic transmission rate requirement, for example select 4Mbps speed access way, the system spectrum utilance of 8M pattern is approximately only 0.51,4M pattern has the higher availability of frequency spectrum, is about 1.02; Select 8Mbps speed access way, the system spectrum utilance of 8M pattern is that 1.02,4M pattern is 2.03; Select alternate manner, also have identical Changing Pattern.
Table 4
The present invention is also to proposed this novel 4M transmission mode, carried out the comparative analysis of transmission performance with the 4k transmission mode of the same type that European DVB-H released in recent years, it is benchmark that two kinds of transmission modes be take identical FFT integration period (409.6 μ s), and to take the subcarrier spacing of identical magnitude be analogy.Research and analyse and show, 4M pattern has been simplified the quantity of system subcarrier, makes spectrum structure compacter, thereby realizes the optimization of system frequency/power resource.Notice that the average pilot that DVB-H arranges 4k pattern is spaced apart 12, the average pilot of 4M pattern is spaced apart 8, although this than 4k mode losses approximately 10% sub-carrier resources, yet but brought the lifting of precision of channel estimation, moreover the subcarrier spacing of 4M pattern has expanded nearly 10% than 4k pattern, this makes 4M pattern compare the severe interference that 4k pattern more can adapt to larger Doppler frequency deviation under multiple mobile multipath fading reception environment, and finally make system bit error rate be significantly reduced, in Fig. 6 and 7, choosing identical satellite moves multi-path fading channel and carries out simulation analysis, title and the implication of model are listed in table 5.
Table 5
Simulation result shows, the system average error rate level of 4M transmission mode is all better than 4k pattern under multiple mobile multipath fading environments, especially when signal to noise ratio (SNR) is 20dB, on average error rate of system (BER) has been reduced to approximately 75%, the pilot tone and the data subcarrier distribution that show 4M pattern are more reasonable, and this novel pilot configuration can also be promoted the use of in 2M and 8M pattern.Finally, compare with 4k pattern, the transmitting base station average headway of 4M pattern has expanded approximately 50%, can effectively alleviate to a certain extent difficulty and the density at base station construction and cloth station, as shown in table 6.
Table 6
The unspecified part of the present invention belongs to general knowledge as well known to those skilled in the art.
Claims (3)
1. a 4M transmission method for satellite CMMB system OFDM system, at transmitting terminal, carries out sending to receiving terminal after pilots insertion, OFDM moulding and insertion Cyclic Prefix step to the data subcarrier completing after coding and mapping; At receiving terminal, after deleting accordingly to the received signal Cyclic Prefix, OFDM solution frame and deleting pilot tone, by demapping and channel decoding, recover the data of transmission, it is characterized in that:
In described pilots insertion step: every 8 subcarriers are divided into one group, comprising 7 data subcarriers and 1 scattered pilot subcarrier; In frequency direction and time slot direction, at interval of 7 data subcarriers, insert 1 scattered pilot subcarrier respectively; Thereby on frequency domain direction, form an OFDM frequency domain symbol, each OFDM frequency domain symbol comprises 1540 effective subcarriers;
In described OFDM forming step: supplement 507 virtual subnet carrier waves and 1 center frequency point subcarrier in each OFDM frequency domain symbol; By two groups of 1540 effective subcarrier average marks, every group 770, be arranged in symmetrically the both sides of described center frequency point subcarrier, then 507 virtual subnet carrier waves be filled in to the both sides of 1540 effective subcarriers, form altogether 2048 total subcarriers, form the spectrum structure of nominal bandwidth 4MHz; 2048 total subcarriers are carried out to IFFT computing, form an OFDM time domain moulding symbol;
In described insertion Cyclic Prefix step: first, using the beginning that in each OFDM time domain moulding symbol, 256 last subsymbols copy to OFDM time domain moulding symbol as Cyclic Prefix, make duration of each OFDM time domain moulding symbol equal FFT integration period and cyclic prefix period and; Then, before the beginning of each OFDM time domain moulding symbol and after ending place, add protection interval;
The length of described Cyclic Prefix is 1/8 of FFT integration period.
2. the 4M transmission method of a kind of satellite CMMB system OFDM system as claimed in claim 1, it is characterized in that: in the OFDM frequency domain symbol described in each, there are 192 scattered pilot subcarriers, in time slot direction or frequency direction, the ratio at pilot sub-carrier interval is 1:1, and stagger mutually in the position of scattered pilot subcarrier in different OFDM frequency domain symbols, form star-like pilot frequency design spread pattern.
3. the 4M transmission method of a kind of satellite CMMB system OFDM system as claimed in claim 2, it is characterized in that: in described star-like pilot frequency design spread pattern, for the Position Number m of scattered pilot subcarrier in n OFDM frequency domain symbol in each time slot, meet following rule:
Wherein, mod (n, 8) is for getting remainder operation; N span is 0≤n<53, and 53 is the sum of OFDM frequency domain symbol in a time slot; Integer p represents the sequence number of scattered pilot subcarrier in OFDM frequency domain symbol, and p span is 0≤n≤191.
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