CN1142644C - Filling method of protecting gap in orthogonal frequency division multiplexing modulation system - Google Patents
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Abstract
The present invention relates to information transmission, particularly to information communication in the application of the Internet, digital televisions, data broadcasting, etc. The present invention relates to a method for realizing the antimultipath of a transmission signal and the timing estimation of the starting and the ending of a transmission signal section by using a PN sequence to replace a guard space (zero guard space) when orthogonal frequency division multiplexing modulation (OFDM) has time delay in a transmission channel. Each signal frame sequence is provided with a pseudo-random m sequence PN, a convolution signal is composed of a received signal and the PN sequence, and the starting and the ending of the PN sequence are recognized by subtracting the convolution signal from the received signal. The PN sequence is used for timing recovery, carrier recovery, channel estimation, frame synchronization and the replacement of the guard space in the OFDM.
Description
Technical field
The invention belongs to information modulation technique field, particularly protection filling technique at interval in the orthogonal FDM modulation system.
Background technology
The core of communication system is how to improve efficiency of transmission and reliability in limited bandwidth.Therefore, in Digital Television or computer system, the payload of each transmission frame or valid data part should account for the big as far as possible part of transmission frame, so that improve the efficiency of transmission of system.Simultaneously, system should be able to discern and compensate the characteristic variations of transmission channel, comprises the time delays of each transmission frame, so that realize reliable transmission.
At OFDM modulation (Orthogonal Frequency Division Multiplexing; OFDM) in the system; the ofdm signal structure is a block structure; each block is called the OFDM symbol; it is made up of two parts in time domain; one is data division, and another is the protection compartment.The data division of OFDM symbol defines in frequency domain.For anti-multipath disturbs, the protection of OFDM symbol must have at interval, and its protection gap length generally greater than the propagation delay of multipath signal in the transmission, disturbs so that eliminate multipath, improves the reliability of system transmissions.
Difference according to filling signal in the OFDM protection at interval, filling signal has two kinds of independently definition in the present OFDM symbol: first kind is that null value is filled the protection interval of (Zero-padding), promptly in OFDM protection at interval, fill " 0 ", because the receiving terminal Processing Algorithm is complicated a little, never is used widely; Second kind then be the Cyclic Prefix of being used widely at present fill (Cyclic-Prefix-padding) protection at interval; the transmission system of existing employing OFDM all is to adopt this protection at interval, international standards such as for example European digital video broadcast-terrestrial (DVB-T), digital audio broadcasting (DAB), Japanese ground digital television broadcast (ISDB-T), IEEE802.11a and IEEE802.16a.
Realize above-mentioned reliability, need system can be from the signal specific of Channel Transmission recovered clock, the reinsertion of carrier and the estimation characteristic of channel.
In European DVB-T OFDM and other ofdm system, system synchronization realizes by insert pilot tone in frequency domain 0FDM symbol data part, promptly adopts the frequency domain simultaneous techniques.The core technology of Europe DVB-T is called COFDM (i.e. Bian Ma OFDM), wherein one of implication of so-called " coding " (i.e. " C ") is to have inserted some " pilot tones " (Pilot) signal in the OFDM frequency spectrum at random, these pilot tones are divided into continuous pilot (promptly the position in the COFDM symbolic blocks is fixed) again and scatter pilot tone (promptly different in different COFDM symbolic blocks, but be loop cycle with 4 COFDM symbolic blocks).Continuous pilot is used to synchronously and is phase-locked etc., scatters pilot tone and then forms frequency domain sample to frequency selective fading channels, and then be used for channel estimating.Europe DVB-T is used for synchronously and the pilot frequency carrier wave quantity of channel estimating purpose accounts for 10% of total carrier wave; and DVB-T has kept another characteristics of traditional OFDM technology; promptly before the OFDM data block, insert one section protection of filling at interval by the Cyclic Prefix sign indicating number; obviously the setting of protection interval and pilot tone is a cost with the efficiency of transmission that has reduced system.
Summary of the invention
The objective of the invention is by pseudorandom or pseudo noise (Pseudo Noise are provided in each transmission frame, PN) sequence satisfies the needs of recovered clock, the reinsertion of carrier and estimation channel, and the PN sequence here satisfies orthogonality and closure for algebraic operation (for example convolution, Boolean add and section internal shift).The received signal (through Channel Transmission) and the PN sequence of one or more selections are carried out convolution, obtain convolution signal.Analyze this convolution signal, can discern beginning or concluding time, the identification channel time delay of PN sequence and recover timing.The carrier frequency of received signal can be recovered from the symbol (bit, nibble, byte etc.) that constitutes the PN sequence, and realizes that signal frame is synchronous, estimates transmission channel characteristic from time delay and corresponding phase shift.Above-mentioned PN sequence is positioned at signal frame, and as the protection interval of OFDM (OFDM) modulation scheme, the protection that is equivalent among the common OFDM has become zero at interval simultaneously.All these characteristics concentrate in together, and can realize the OFDM demodulation, and obtain high efficiency and high reliability.
The present invention proposes protection fill method at interval in a kind of orthogonal FDM modulation system, under the life period time-delay situation, replaces protection at interval with the PN sequence in the signal channel of ofdm system, may further comprise the steps:
1) provides pseudorandom m sequence PN (t; K) (k=1 ..., K; K 〉=1) collection, wherein the data of dissimilar symbols are etc. general; The boolean sum of any two m sequences, and be still the m sequence after the cyclic shift of any m sequence; The convolution signal of any two sequences satisfies PN (t; I) * PN (t+ Δ t; J)=and δ (Δ t) δ (i, j), δ (Δ τ) is the δ function here, and δ (i, j)=0, unless i=j;
2) the additional sequence PN (t that selects behind at least one signal frame that will transmit; K), constitute the signal frame of filling;
3) by the signal frame of at least one filling of Channel Transmission, the transmission signals that receives may have uncontrollable time delays Δ t;
4) one of the reception transmission signals receives version Rc (t), and through behind the sectional convolution, cascade constitutes composite signal:
Here Δ t is the incremental time of selecting, and k1 and k2 be the integer of selecting, and k2 〉=k1; Form residual signal Rc (t; Rem)=Rc (t)-Rc (t; Δ t; Comp); And from this multipath information, can discern each (time delay) DFT piece and corresponding PN sequence, thereby each signal frame in the identification signal frame sequence.
The present invention can realize the OFDM demodulation, and obtains high efficiency and high reliability.
Description of drawings
Figure 1A and 1B have described the optional mode of traditional OFDM frame structure.
Fig. 2 A, 2B and 2C have described the frame structure mode of utilizing the PN sequence proposed by the invention.
Fig. 3 has described the composition of frame synchronization PN sequence.
Fig. 4 has described a typical transmission signals.
Fig. 5 A and 5B have described the non-overlapped component of desirable signal frame sequence.
Fig. 6 A and 6B have described typical multipath and have disturbed (pecked line) influence to frame synchronization sequence and data division.
Fig. 7 has described the signal flow graph of implementation procedure of the present invention.
Fig. 8 and 9 has described and has been used for the system that transmits and receives of the present invention.
Embodiment
Usually first and second kinds of formations of OFDM frame format are shown in Figure 1A and 1B.In the form of Figure 1A, DFT (or FFT) piece 13B is positioned at after the Cyclic Prefix section 13A, and Cyclic Prefix is as the protection interval of DFT.The OFDM modulation needs to use protection interval or its equivalent, so that offset the multipath signal that may exist in the received signal, prevents intersymbol interference.In the form of Figure 1B, the zero padding section is followed in DFT piece back, and this zero padding section is as the protection interval of DFT piece.
Pseudorandom of the present invention or pseudo noise PN sequence (the m sequence of coding) are used for the protection interval of OFDM modulation.This m sequence is the symbol sebolic addressing of certain-length, is generally the set of " 0 " and " 1 ", and its satisfies three needs: 1) number of dissimilar symbols (for example data " 0 " and " 1 ") is etc. general; 2) boolean sum of any two m sequences, and be still the m sequence after the cyclic shift of any m sequence; 3) two m sequence MS (t; I) and MS (t; J) convolution satisfies orthogonality condition:
MS(t+Δt；i)*MS(t；j)＝δ(Δt)·δ(i，j) (1)
Here δ (Δ t) be the δ function revised (for | Δ t |＞Δ t
1, δ (Δ t)=0), (i j) is Kronecker δ function (=0, unless i=j) to δ.If the m sequence does not rely on call number i, perhaps call number is well-known satisfies i=j, and Kronecker δ function can be ignored so.Usually the m sequence length elects 2 as
J-1, J is a positive integer here, for example J=7,8 or 9.
Subclass to all m arrangement sets all is suitable for, the m sequence PN (t of as noted above coding; J).The set of (coding) PN sequence can be adopted walsh code or similarly Haar sign indicating number generation." data compression " (Data Data Compression that the method for generation walsh code or Haar sign indicating number is write referring to T.J.Lynch, Van Nostrand, NewYork, 1985, pp.79-85) and " in wireless/personal communication CDMA use " (Applications of CDMA in Wireless/personal Communications, Prentice Hall of writing of K.Smolik, J.E.Wilker, Upper Saddle River, New Jersey, 1997, pp.23-39.).Also can produce and use other set of PN sequence, as long as it satisfies equation (1).
The PN sequence is M rank m sequences, and is well-known, and (Linearfeedback shift register LFSR) realizes available Fibonacci molded lines feedback shift register.For LFSR and M=9, a suitable feature multinomial is:
P(x；4∶9)＝1+X
4+X
9 (2)
Initial mask has determined the initial condition of LFSR, thereby has determined the phase place of the PN sequence of generation.Here also can select other suitable feature multinomial, and corresponding exponent number M.
In first kind of embodiment of the present invention shown in Fig. 2 A, the PN sequence 23A of coding comprises or replaces the Cyclic Prefix section 13A shown in Figure 1A, and 21A links to each other with the DFT piece.In second kind of embodiment of the present invention shown in Fig. 2 B, the PN sequence 23B of coding comprises or replaces the zero padding section 11A shown in Figure 1B, and 21B links to each other with the DFT piece.
In the third embodiment of the present invention shown in Fig. 2 C, the PN sequence 23B of the coding in Fig. 2 B is included among the frame sync segment 26C, follows by DFT piece 21C.Frame sync segment comprises an optional preamble 24C among Fig. 3, closelys follow PN sequence 23C thereafter and closelys follow PN sequence optional back afterwards 25C synchronously.Preamble, the synchronous length of PN sequence and back are respectively F1, F2 and F3, and F1 and F3 are chosen as nonnegative integer here, and selection F2 is a positive integer.Frame sync segment length is F1+F2+F3 symbol.
In the form shown in Fig. 2 C, in order to realize synchronously that fast and reliably preamble and back sync section are not optional.The most preferably preamble also is the PN sequence with the frame sync segment (length is F1+F2+F3) that the selection of synchronizing symbol afterwards will make any order cluster of F2 continuous symbol close formation; More particularly, be arranged in frame sync segment F1+1 among Fig. 3 and form new PN sequence after through some or cyclic shift, synchronously as preamble and back to " original " PN sequence of F1+F2 position.Frame sync segment if exist in signal frame, can characterize with relevant PN sequence.
Here used information transmission unit is a signal frame, and it comprises shown in Fig. 2 A and the 2B frame 21A or 21B, perhaps frame sync segment 26C in the form and frame 21C shown in Fig. 2 C in the form.Frame 21A, 21B or 21C, comprise DFT piece and header, header has the signal code (Octets, bytes, Nibbles or bit) of fixed number, it provides frame source, frame destination, sequence number, frame length, data type (supervision, information, unnumbered), payload user data size, flow process control, message priority, Error detection and correction, other check information etc., the most preferably frame has fixed size, perhaps in the magnitude range of regulation.
Suppose that Tr (t) is the frame signal of representative transmission, as shown in Figure 4.Come the identification signal frame by separating with the relevant PN sequence of analysis, this need know the beginning and the end position of each PN sequence in the signal frame sequence exactly.
The signal frame sequence of desirable correction can be decomposed into two non-overlapping sequence: first sequence-PN sequence PN (t; I; Ideal) and second sequence-DFT piece DFT (t; I; Ideal), shown in Fig. 5 A and 5B.
In any case in practice, the signal frame of each transmission all postpones if having time, comprises the multipath signal distortion,, need quite regularly accurate for other data segment difference in PN sequence and the signal frame is come.Fig. 6 A and 6B have described the influence after increasing time delay on frame synchronization shown in Fig. 5 A, 5B and the DFT.Though desirable correction PN sequence PN (t; I; Ideal) when t=`` (i), sport 0, receiving sequence PN (t; I; Rc) in regular hour t`` (i)≤t≤t``` (i+1), reduce to 0 lentamente, as shown in Figure 6A.Similarly, though desirable DFT piece DFT (t; I; Ideal) when t=` (i+1), sport 0, the DFT (t of reception; I; Rc) in regular hour t` (i+1)≤t≤t```` (i+1)＜t`` (i+1), reduce to 0 lentamente, shown in Fig. 6 B.Therefore, receiving terminal is wanted t=t` (i positioning time; Rc) and t=t`` (i; Rc) very difficult, and these time correspondences beginning and the end position that does not have the PN sequence that receives under the multipath situation.
H (t) is pulse δ (t) (correction δ (t) function with the infinitely small width Delta t1) response of transmission channel TC.After signal Tr (t) transmitted by channel TC, received signal Rc (t) was expressed as the convolution of transmission signals Tr (t) and impulse response signal h (t):
Rc (t
2)=Tr (t
1) * h (t
2-t
1) (3) here * represent two signal Tr (t
1) and h (t
2-t
1) convolution and related operation.And Tr (t) signal is
Tr (t)=PN (t; I; Ideal)+mp (t) (t ' (i; Rc)≤t＜t ' (i+1; Rc) (4) here, mp (t) expression multipath interference components.Because the orthogonality of each PN sequence is then provable in the equation (1)
PN(t+Δt；i；ideal)*PN(t；j；ideal)＝δ(Δt)δ(i，j) (5)
At time interval t` (i; Rc)≤t≤t`` (i; Rc) in, if PN sequence PN is (t; I; Ideal) do not rely on index i, perhaps PN sequence (i) is known, and i=j, and then (i, j) (=0 or 1) can abandon the Kronecker δ index δ here.Then equation (3) and PN sequence convolution get: PN (t+ Δ t; I; Ideal) * Rc (t)=PN (t+ Δ t; I; Ideal) * { Tr (t
1) * h (t
2-t
1)
＝PN(t+Δt；i；ideal)*{[PN(t；i；ideal)+mp(t)]*h(t
2-t
1)}
＝PN(t+Δt；i；ideal)*PN(t；i；ideal)*h(t
2-t
1) (6)
+PN(t+Δt；i；ideal)*mp(t)*h(t
2-t
1)
=δ (Δ t) * h (t)+(because mp (t) and little residual)
Be used to the sequence PN (t that multiplies each other in the following formula; I; Ideal) be known, and the impulse response h (t) of system can calculate, therefore, can know, for channel TC, the convolution signal of statement can be used for minute point (t=t` (i in the equation (6); Rc) and t=t`` (i; Rc)), corresponding to " edge " of the desirable PN signal by channel TC transmission.
The segmentation definition of received signal is as follows:
Thereby can cascade constitute composite signal
For the d frame signal frame sequence of analyzing, here k1 and k2 (〉=k1) be the integer of selecting.From the signal Rc (t) that receives, cut composite signal Rc (t; Δ t; Comp), obtain residual signal
Rc(t；rem)＝Rc(t)-Rc(t；Δt；comp) (9)
Following formula has been explained multipath signal clearly at each time interval t`` (i; Rc)≤t≤t`` (i+1; Rc) to the received signal influence in.From this multipath information, can discern each (time delay) DFT piece and corresponding PN sequence, thus each signal frame in the identification signal frame sequence.
After time delays is determined or estimated, use that known symbol pattern comes the restoring signal carrier frequency in the signal frame PN sequence, and estimate frequency shift (FS) and/or frequency departure.
Time-delay service time and frequency shift (FS) and/or frequency departure can estimate one or more transmission channel characteristics among frame or the frame group.
In frequency domain, signal frame has certain bandwidth deltaf f
1In order fully to cover this bandwidth, do not produce spectral aliasing, the time domain sampling frequency of DFT piece and its corresponding PN sequence equals the Nyquist frequency at least.In some cases, sampling rate need be greater than the symbol rate of signal frame.
The δ function that convolution algorithm produces in the equation (1) has time width Δ t very little but non-zero
1, respective bandwidth is Δ f in its frequency domain
2The actual bandwidth Δ f that provides for signal frame
1Should equal needed bandwidth deltaf f at least
2
One of method of estimating one or more transmission channel characteristics is to analyze the Fourier transform FT (f of received signal Rc (t); Rc), its corresponding impulse response function is h (t).Desirable Fourier transform FT (f; Rc) be approximately a synchronous function:
Represent the reference frequency f of frequency domain bandwidth
0Be approximate the selection.Actual Fourier transform FT (f; Rc) and desirable Fourier transform FT (f; Rc; Ideal) deviation can be used for estimating one or more (time change) characteristic of frame of transmission channel or frame sequence.
In case the time delays that the PN sequence is relevant determines and has compensated that PN just can be used for the synchronous of signal frame.
In any form shown in Fig. 2 A, 2B and the 2C, when detection or the estimation time delays relevant with frame, the protection that the PN sequence can be used as the DFT piece at interval because the symbol sebolic addressing of (1) PN sequence is known; (2) beginning of PN sequence and/concluding time is known.Thereby the protection that does not need to provide other in frame at interval, was used to protect part at interval to can be used for the payload user data part of frame now in the past, and/or reduces the length of frame.Therefore, improved efficiency of transmission.Simultaneously, aforesaid information also is used for the demodulation of received signal.
The insertion of PN sequence and analyzed following effect in each transmission signal frame: (1) clock recovery and multipath signal aberration estimation frame by frame; (2) recovery of carrier frequency; (3) estimation of one or more transport channel parameters; (4) deletion only as protection protection at interval at interval; (5) each signal frame that receives synchronously.
According to the present invention, under the situation of life period time-delay, the timing estimation flow process of the beginning of at least one transmission signals section and end as shown in Figure 7 in the signal channel of ofdm system.In step 71, provide K (〉=1) rank pseudorandom m sequence PN (t; K), satisfy the orthogonality of the convolution signal in equation (1) or (5).In step 72, the sequence of selection appends at least one signal frame, forms the signal frame of filling.In step 73, the signal frame of at least one filling is by a signal time uncontrollable transmission channel of delaying time.In step 74, one that receives transmission signals receives version Rc (t), for k1≤k≤k2, forms convolution signal PN (t+ Δ t; K) * Rc (t; I) summation Rc (t; Δ t; Comp), as equation (8).In step 75, form residual signal Rc (t; Rem)=Rc (t)-Rc (t; Δ t; Comp), as equation (9).In step 76, analysis residual signal is so that determine at least one sequence PN (t among the received signal Rc (t); K) beginning and end.
Fig. 8 principle has been described an emission system 80 that adopts the inventive method.Receive the input data at DFT (or FFT) conversion block 81, and be converted to suitable number format.PN block 83 provides one or more PN sequences.Give multiplexer (MUX) 85 from the signal of DFT piece 81 and 83 outputs of PN sequence blocks, the MUX output signal comprises DFT piece and PN sequence alternately.RF module 87 receives the MUX output signal, and the result after the processing gives transmitting antenna or other signal transmitter 89.
Described to Fig. 9 principle a receiving system 90 that adopts the inventive method, antenna or other signal receiver 91 receive modulation signal, give down conversion module 93 and carry out frequency translation.Signal frame after the down-conversion comprises DFT piece and additional PN sequence, gives PN sequence processor 95, and it finishes clock recovery, carrier wave recovers (option), channel estimating (option) and frame synchronization channel condition informations such as (options).Signal frame after the down-conversion is also given the PN sequence and is removed module 97.After PN sequence processor 95 is finished its task, also give the PN sequence control signal that obtains and remove module 97, remove additional PN sequence from signal frame, the DFT piece that obtains is given DFT conversion module 99.The DFT of " original " signal frame after the conversion is partly as restituted signal (data).Obtain channel condition information from PN sequence processor 95, can analyze (optionally) separately so that determine channel condition information based on the signal frame that receives.
Claims (5)
1. protect fill method at interval in an orthogonal FDM modulation system, in the signal channel of ofdm system, under the life period time-delay situation, replace the protection interval, may further comprise the steps with the PN sequence:
1) provides pseudorandom m sequence PN (t; K) (k=1 ..., K; K 〉=1) collection, wherein the data of dissimilar symbols are etc. general; The boolean sum of any two m sequences, and be still the m sequence after the cyclic shift of any m sequence; The convolution signal of any two sequences satisfies PN (t; I) * PN (t+ Δ t; J)=and δ (Δ t) δ (i, j), δ (Δ τ) is the δ function here, and δ (i, j)=0, unless i=j;
2) the additional sequence PN (t that selects behind at least one signal frame that will transmit; K), constitute the signal frame of filling;
3) by the signal frame of at least one filling of Channel Transmission, the transmission signals that receives may have uncontrollable time delays Δ t;
4) one of the reception transmission signals receives version Rc (t), and through behind the sectional convolution, cascade constitutes composite signal:
Here Δ t is the incremental time of selecting, and k1 and k2 be the integer of selecting, and k2 〉=k1; Form residual signal Rc (t; Rem)=Rc (t)-Rc (t; Δ t; Comp); And from this multipath information, can discern each DFT piece and corresponding PN sequence, thereby each signal frame in the identification signal frame sequence.
2. according to the method for claim 1, it is characterized in that, comprise that further known symbol pattern comes the restoring signal carrier frequency in the use signal frame PN sequence.
3. according to the method for claim 1, it is characterized in that, further comprise at least one parameter of using described PN sequence to estimate described transmission channel.
4. according to the method for claim 1, it is characterized in that, further comprise with at least one described signal frame replacing at least one protection at interval with described PN sequence.
5. according to the method for claim 1, it is characterized in that, further comprise after the relevant time delays of PN sequence is determined and compensated, use the PN signal of described filling, for signal frame provides time synchronized.
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| CN101383795B (en) * | 2007-09-07 | 2010-12-08 | 北京大学 | Method for enhancing valid data transmission efficiency by OFDM transmitter and OFDM transmitter |
| CN101127750B (en) * | 2007-09-18 | 2011-06-15 | 清华大学 | A single carrier or multi-carrier block transmission system and filling method for protection interval |
| CN101136884B (en) * | 2007-09-28 | 2010-06-02 | 清华大学 | Channel estimation method for TDS-OFDM system |
| CN101540895B (en) * | 2008-03-18 | 2011-08-31 | 卓胜微电子（上海）有限公司 | Method for identifying channel of national standard digital television terrestrial broadcasting signal |
| CN101252363B (en) * | 2008-04-03 | 2012-03-21 | 北京创毅视讯科技有限公司 | Transmitting system, multiplexer and method for reducing power unevenness in sending machine band |
| CN101888359B (en) * | 2009-05-13 | 2012-12-26 | 清华大学 | Multi-address access system based on TDS-OFDM (Time-Domain Synchronous Orthogonal Frequency Division Multiplexing) as well as signal frame structure and receiving method thereof |
| CN103581096B (en) * | 2013-03-25 | 2017-05-10 | 上海数字电视国家工程研究中心有限公司 | OFDM modulating and demodulating method and digital signal emitting and receiving system |
| CN105812062B (en) * | 2016-03-04 | 2018-11-30 | 烽火通信科技股份有限公司 | Optical transfer network MLD method for interface adaptation and system |
-
2001
- 2001-08-17 CN CNB011241446A patent/CN1142644C/en not_active Expired - Lifetime
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1787507B (en) * | 2004-12-10 | 2010-04-28 | 清华大学 | Method for producing frame synchronizing in time domain synchronizing orthogonal frequency division multiplex system |
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