CN1677876A - Method for removing phase noise for time domain synchronous or thogonal frequency-division multiplex receiver and system and thereof - Google Patents
Method for removing phase noise for time domain synchronous or thogonal frequency-division multiplex receiver and system and thereof Download PDFInfo
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Abstract
The method includes procedures: separating received digital signal at receiving end; PN head in use for evaluating channel and TPS in signal frame body in use for evaluating phase noise; obtaining N pieces of TPS in M groups of TPS after carrying out discrete Fourier transform for signal frame body; selecting the TPS with largest amplitude, the selected TPS through extraction of band-pass filter, channel and phase noise interference being as received TPS; multiplying complex conjugate of TPS estimated value passed through channel transmission so as to obtain frequency domain baseband signal of phase noise. Finally, time domain evaluation of phase noise and phase compensation is obtained from discrete Fourier inverse transform. The method is capable of tracking phase transient of received signal at receiving end in time, removing phase noise, recovering system synchronization precisely.
Description
Technical field
The invention belongs to digital information transmission technical field, (TimeDomain Synchronous OFDM, TDS-OFDM) receiver is removed the method and system of phase noise to be specifically related to a kind of time-domain synchronization OFDM.
Background technology
In the digital communication or broadcast system of synchronous transmission, sent be with sequence x corresponding continuous time of waveform s (t, x), rather than sequence itself.The channel symbol sequence is finished by modulator to the distribution of channel waveform.Except that channel sequence x, waveform also depends on parameter set θ=(θ
T, θ
c).Subclass θ
TBe transmitter parameter, subclass θ
cBe channel parameter, these parameters are unknown for receiver.For recovering symbol sebolic addressing x, receiver must estimate these unknown parameters from received signal.Then, these estimated values will be used as actual value, must set up synchronously, comprise timing information, carrier frequency, sign synchronization etc.
The baseband signal of receiving terminal enters receiver synchronous circuit part, and as shown in Figure 1, synchronous circuit is estimated synchronous parameters such as frequency shift (FS), regularly recovery, phase places according to this baseband signal, obtained carrier wave and recover.The operation that carrier wave recovers mainly comprises following three parts: 1) carry out phase estimation, phase estimation is after matched filter, and it is the execution of carrying out with symbol rate (owing to regularly recovering before the phase bit recovery); 2) phase place rotation will be multiplied each other through the sampling of data and a plural exp (j θ (nT)) of regularly recovery and matched filtering; 3) Frequency Synchronization, under the situation that has an a certain amount of frequency shift (FS), wide ranges, the thick frequency adjustment of precision are necessary.
Exist to disturb in the Digital Transmission process and noise, wherein phase noise is because the influence of thermal noise and cause the change at random of oscillator phase by the noise that externally fed and control circuit are introduced in the oscillator of sending and receiving end.The phase fluctuation of oscillator and frequency fluctuation have caused the phase noise of carrier wave.Phase noise can both be introduced at transmitting terminal transmitter and receiving terminal tuner etc.
For quadrature amplitude modulation (QAM) modulation, it is circular fuzzy that the constellation point (sampling point) on the I/Q plane appears in these phase jitter meetings, causes intercarrier and intersymbol interference, directly increases bit error rate.
Usually eliminating the most direct method of phase noise is to adopt phase-locked loop (PLL) to come the filtering phase noise.
In receiver, one of method that the reinsertion of carrier is synchronous is to make a start to insert a special pilot signal at frequency domain, and receiver uses a PLL to obtain and follow the tracks of this weight of pilot frequency, and makes the carrier frequency and the Phase synchronization of local oscillator and received signal.
Generally PLL is designed to have narrow bandwidth, but the selection of PLL bandwidth need be compromised between the noise in response speed and the phase estimation value (estimation accuracy) in practice, on the one hand, when there is the phase jitter component in input signal, requirement can not suppress to reflect the component of input phase variation too much, therefore, wish to select the bandwidth of loop enough wide, so that become when following the tracks of receiving phase any in time; On the other hand, a broadband P LL allows more noise to enter loop, thereby worsens phase estimation, and require passband narrow this moment, will get well the noise component(s) inhibition.These two aspects are contradiction to the requirement of PLL bandwidth.Same, settling time and retention time also are contradiction to the requirement of PLL bandwidth synchronously.So the PLL bandwidth that need choose suitable compromise according to actual needs alleviates this contradiction, perhaps take other measure to avoid this contradiction.
Tell about below in ground digital television broadcast and specifically how to do.
Current ground digital TV transmission system in the world mainly contains two kinds of the DVB-T COFDM OFDM of the coding (modulation) in the ASTC 8-VSB (modulation of eight level vestigial side bands) of the U.S. and Europe.
U.S.'s ATSC standard has adopted eight level vestigial side bands (8-VSB) modulating system.ATSC 8VSB pattern can be transmitted the information code check of 19.28Mbps in the 6MHz bandwidth, the input bit rate that is input to transmission system from transfer system is 19.39Mbps, each packet 188Byte, the wherein synchronous and 187Byte information (187/188=19.28/19.39) of 1Byte.Input information at first carries out randomization, carries out the preceding paragraph error correction coding then, and after the additional 20Byte error correcting code, each packet becomes 208Byte, outputs to multiplexer through 2/3 lattice code again, with data segment sync and data fields synchronized mixes.Randomization and forward error correction are not added to the synchronous Byte in the former bag.Synchronous Byte in the bag changes into the segment data synchronizing signal when multiplexing.Two all synthetic at last Frames of data.
8 level symbols and binary data section vestigial sideband amplitudemodulation synchronous and field synchronization employing suppressed carrier is modulated.VSB allows a sideband all pass through, and another sideband only residual a part of train.The root mean square of respectively having arranged to be shaped as normalizing in the sideband both sides rises the transition region of surplus profound response stroke 310KHz, and the baseband signal of generation converts analog form (D/A converter) to, is modulated to intermediate frequency carrier then, and generates the intermediate-freuqncy signal of residual sideband.Normalized transmission frequency spectrum such as Fig. 2 in the 6MHz bandwidth.
Can see in Fig. 2 that at 310KHz place, low side border a little pilot signal is arranged, this pilot signal is used for carrier lock in vsb receiver, and pilot signal power makes gross power increase 0.3dB, helps to reduce the loss in the enforcement.And, NTSC is not produced co-channel interference because pilot signal is positioned at the residual sideband zone of co-channel NTSC signal.
What ATSC used is single-carrier system, and it is subjected to the influence of phase noise littler than multicarrier system.Its uses carrier pilot phase-locked loop to finish locking to frequency, next just uses a single order feedback loop based on the judgement data to follow the tracks of remaining phase noise behind the carrier lock.This feedback loop obtains the initial phase estimated value of making an uproar in when beginning according to frame synchronizing signal, follows the tracks of according to phase constellation rotation the carrying out phase noise of judgement data at data segment then.
The ATSC carrier wave recovers to finish on pilot tone shown in Figure 2 with the PLL circuit, and the loop bandwidth of its PLL is enough wide, can provide ± the wide frequency of 100kHz is introduced scope, and the phase noise of following the tracks of about 2kHz on the filtered signal.But being too narrow to again, the PLL bandwidth is enough to suppress effectively strong white noise and the co-channel interference signal of NTSC.
The ASTC section synchronously and symbol clock recovery also be employing PLL circuit.
When but ATSC existed near strong multipath to change (phase place), above-mentioned single little pilot signal can be had a strong impact on, and carrier wave recovers the difficulty that becomes.
Europe DVB-T system adopts the modulation technique COFDM (Coded Orthogonal Frequency Division Multiplexing (COFDM)) different with U.S. 8-VSB.OFDM belongs to multi-carrier modulation technology, and it is divided into transmitted bit on the low bit rate subcarrier of thousands of meters, and each subchannel is the arrowband flat channel.
The data frame structure of DVB-T is formed a Frame to 4 OFDM symbolic frames, and each OFDM symbolic frame comprises 68 OFDM symbols.The carrier number of an OFDM symbol is constant, and 1705 carrier waves are arranged under the 2K pattern, and subcarrier spacing is 4.46KHz, and the effective information carrier number is 1512 carrier waves; 6817 carrier waves are arranged under the 8K pattern, and subcarrier spacing is 1.11KHz, and the effective information carrier number is 6048 carrier waves.In an OFDM Frame, use QPSK, 16QAM or the 64QAM modulation of the same Gray code mapping on all carrier waves.Each V bit symbol of exporting from interleaver is mapped to a constellation point in the modulation constellation.
One of the implication of " coding " among the Coded Orthogonal Frequency Division Multiplexing (COFDM) COFDM is meant has inserted some " pilot tone " signals in the OFDM frequency spectrum, here so-called " pilot tone " is meant the carrier wave of some OFDM like this, they are by the known data-modulated of receiver, what they transmitted is not modulating data itself, because it is known that these data receivers are systems, the purpose that pilot tone is set is that system transmits the parameter of some transmitter or the characteristic of test channel by the data on the pilot tone.
The effect of pilot tone in COFDM is very important, and its use comprises that frame synchronization, Frequency Synchronization, time synchronized, channel transfer characteristic are estimated, transmission mode is discerned and follows the tracks of phase noise etc.The data of modulated pilots are the pseudo random sequences that generates from a pseudo-random sequence generator of stipulating in advance.Stipulated scattered pilot, continuous pilot and transmission parameter signaling (TPS) pilot tone among the DVB-T.
Scattered pilot is as the valuation of the characteristic of channel; Continuous pilot is synchronous as timing and carrier frequency.Their amplitude is than data subcarrier height+2.5dB.The position of continuous pilot in each COFDM symbol all fixed, and inserted 177 continuous pilot in the 8k pattern, inserted 45 continuous pilot in the 2k pattern.The position of scattering pilot tone is different in different COFDM symbols, but with four COFDM symbols is loop cycle, as shown in Figure 3, white circle representative data subcarrier wherein, gray circles is represented the scattered pilot subcarrier, black circles is represented the TPS pilot sub-carrier, and the oblique line circle is represented the continuous pilot subcarrier.
The TPS carrier wave of DVB-T is used for transmission system parameters, i.e. chnnel coding and modulation parameter.Each OFDM symbol contains 1 TPS bit, in the OFDM symbolic frame 68/17bit is arranged, and the fixed position is arranged, as shown in Figure 3 in planisphere.The TPS carrier wave adopts the DBPSK modulation.
Because subcarrier spacing is very little, therefore, OFDM multicarrier system is more responsive to phase noise than single-carrier system.The influence of phase noise can be modeled as two parts: 1) public part, it causes the phase place rotation of the data symbol that all receive in the present frame, cause the integral body rotation of signal constellation (in digital modulation), it is the monotonically decreasing function of sub-carrier number N, as N=1 when (being equivalent to single-carrier system), reach maximum, when N was tending towards infinite, this part also was tending towards 0; 2) disperse part, it is similar to white Gaussian noise, will cause defocusing of corresponding received signal constellation point, and this part is the dull increasing function of sub-carrier number N, as N=1 when (being equivalent to single-carrier system), reaches minimum.
First's phase noise is eliminated by the continuous pilot signal that PLL follows the tracks of among the DVB-T easily.But for the second portion phase noise, DVB-T compensates difficulty, and promptly DVB-T is bad to the elimination performance of random phase noise, can not in time reflect the variation of phase of received signal.
In recent years Tsing-Hua University also proposed " the ground digital multimedia TV broad cast host-host protocol (and Terrestrial DigitalMultimedia-Television Broadcasting; DMB-T) ", time-domain synchronization OFDM (TimeDomain Synchronous Orthogonal-Frequency-Division-Multiplex, TDS-OFDM) modulation technique have been adopted.(number of patent application: 00123597.4 " ground digital multimedia TV broad cast system " and number of patent application: 01124144.6 " protection fill method at interval in the orthogonal FDM modulation system ").
The structure of DMB-T system has the frame structure of layering; pseudo random sequence (PN sequence) is as synchronous head; variable guard intervals (filling PN sequence, Cyclic Prefix or null value); length is no more than 1/4 of IDFT block length; be the cyclical transmission scheme of consecutive days circulation timei, has unique frame address, and the support time is shared multiple access; support continuously and the bursty data mixed transport, and MPEG sign indicating number bag and second time are synchronous.
The physical channel frame structure of DMB-T host-host protocol as shown in Figure 4, it is classification.A basic frame is called signal frame.Signal frame is made up of two parts, i.e. frame synchronization and frame.The frame group is defined as a group signal frame, and its first frame definition is a frame group head (control frame).Superframe is defined as a framing group.The top layer of frame structure be called a day frame (Calendar Day Frame, CDF).Physical channel is the cycle, and and Absolute Time Synchronization.
Signal frame is the elementary cell of DMB-T system frame structure.A signal frame is formed (see figure 4) by frame synchronization and frame two parts.The baseband signalling rate of frame synchronization and frame is identical, is defined as 7.56MSps.
The PN sequence of base band frame synchronizing signal has 420 symbols.Unlike signal frame among the signal frame group has different frame synchronizing signals.So frame synchronization can be used for identification as the frame synchronization feature of signal specific frame.
The PN sequence definition is one 8 rank m sequence, and its proper polynomial is defined as x
8+ x
6+ x
5+ x+1, the initial condition template will be determined the phase place of the m sequence that generates.For a specific signal frame, the initial condition of its signal frame number decision PN sequence.After " 0 " was arrived "+1 " value and " 1 " and arrive the mapping that " 1 " be worth, the PN sequence transformation was the binary signal of non-return-to-zero.
The baseband signal of a frame is an OFDM (OFDM) piece.OFDM piece further is divided into protection leaf inverse transformation (IDFT) piece at interval and in discrete answering.For TDS-OFDM, the PN synchronizing sequence is both as frame synchronization, and again as the protection of OFDM at interval, and frame is as the DFT piece, as shown in Figure 4.
The DFT piece has 3780 symbols (subcarrier) and continues 500us, and adjacent sub-carrier is spaced apart 2KHz, and each sub-carrier adopts QPSK, 16QAM and even or non-homogeneous 64QAM mapping.
Because the OTDM of PN sequence and DFT piece, and the PN sequence is known array for receiving terminal, and therefore, PN sequence and DFT piece are can be separated at receiving terminal.The signal frame of receiving terminal can be regarded as the OFDM with zero padding protection interval after removing the PN sequence, is of equal value and have zero padding protection OFDM at interval in theory with the OFDM with Cyclic Prefix protection interval.
In a frame, 3780 symbols (carrier wave) are arranged, wherein 36 subcarriers are used to carry transmission parameter signaling (Transmission Parameter Signaling, TPS), remaining 3744 subcarriers are used for the payload user data transmission.TPS repeats in each signal frame in the frame group.The formation of frame as shown in Figure 5.Transmission mode only could change when a new frame group begins.
In DMB-T, the PN synchronizing sequence be used for frame synchronization, Frequency Synchronization, regularly synchronously, channel estimating and equilibrium, transmission frame identification and follow the tracks of phase noise etc.
Also there is phase noise in the receiving terminal of the DMB-T system that proposes in Tsing-Hua University, need take certain measure to eliminate phase noise, recover (number of patent application: 01124144.6 " utilizing transmission parameter signaling to suppress the method for phase noise ") so that obtain reliable carrier wave.After the inside and outside sign indicating number of the digital signal process error correction coding with input; carry out the symbol-modulated mapping; then N TPS is divided into the M group and is inserted in the information symbol sequence according to certain rule, handle through inverse discrete fourier transform (IDFT), protection is inserted IDFT time domain piece at interval and is formed frame.A phase reference TPS who is used for the phase noise reference is arranged in every group of TPS, other TPS signal during its amplitude is higher than on the same group, other TPS constitutes the frequency protection band of phase reference TPS.The distribution of TPS subcarrier in the TDS-OFDM frame as shown in Figure 6.36 TPS subcarriers are divided into 4 groups, equally spaced insert in the DFT piece.Every group comprises 9 subcarriers, and wherein the power of the 5th subcarrier (we are referred to as phase reference TPS) is higher than other 8 subcarriers, is used for the estimation and the compensation of random phase noise.The frequency protection band that 4 subcarrier spacings are respectively arranged on the both sides of phase reference TPS to extract main phase noise as far as possible, reduces data simultaneously and disturbs.The frame multiple connection of frame head and the above-mentioned TPS of carrying is become signal frame,, eliminate phase noise accordingly by receiving end and handle through giving Channel Transmission after the processing such as digital to analog conversion, shaping filter, radio frequency up conversion.
Summary of the invention
The present invention is according to the characteristic of OFDM multicarrier modulation system phase noise, utilize transmission parameter signaling (TPS) in the time-domain synchronization OFDM modulation (TDS-OFDM), a kind of implementation method and system of eliminating phase noise proposed, in time lock the transient change of phase of received signal, accurately recovery system is synchronous.
TDS-OFDM receiver of the present invention is removed the method for phase noise, it is characterized in that it realizes that in digital circuit it contains following steps successively:
(1) separator is the TDS-OFDM that receives that the time domain orthogonal frequency-division multiplex singal separates, and isolated PN head is carried out channel estimating, and isolated signal frame body is carried out phase noise estimation and compensation;
(2) signal frame body being carried out the phase reference command transmitting that phase reference that discrete Fourier transform (DFT) obtains receiving obtains receiving is TPS, and total N of TPS is divided into the M group;
(3) compare N TPS, and choose that group TPS that wherein has maximum phase reference TPS subcarrier, give following band pass filter it;
(4) band pass filter that uses a centre frequency to be positioned at phase reference TPS place extracts through the phase reference TPS after channel and the phase noise interference, and the bandwidth of described filter is 2 subcarrier bandwidth;
(5) according to the channel estimation results of step (1)
And the TPS of selected position in the step (3), obtain after multiplying each other through estimating the TPS value behind the channel, with ν (k) expression,
(6) the filtering result that step (4) is obtained multiply by the complex conjugate ν (k) of ν (k)
*-1, the frequency domain baseband signal of acquisition phase noise;
(7) the frequency domain baseband signal of the phase noise that step (6) is obtained is converted into time domain phase noise estimated vector through inverse discrete fourier transform;
(8) complex conjugate of the time domain phase noise estimated vector that step (7) is obtained multiply by the baseband signal of signal frame body data after time domain obtains phase compensation of not making discrete Fourier transform (DFT) accordingly.
The described N=36 of step (2), M=4.
The bandwidth of the described band pass filter of step (4) is
Wherein Δ f is the OFDM subcarrier spacing.
TDS-OFDM receiver of the present invention is removed the phase noise system, it is characterized in that it contains:
The discrete Fourier transform (DFT) circuit, i.e. DFT circuit, it has a signal frame body input;
Relatively and select circuit, the output of it and dff circuit links to each other;
Band pass filter, its input links to each other with the output that compares with selecting circuit, and its centre frequency is positioned at comparison and selects the phase reference TPS place of circuit output, and its bandwidth is
M is the group number of TPS, and M=4, N are the TPS number of M group, and N=36, Δ f are the OFDM subcarrier spacing, and bandwidth is got 2 Δ f;
First multiplier, its two inputs respectively with outside channel estimating and equalizing system in the channel estimation circuit output and the output of comparison and selection circuit link to each other;
The first complex conjugate computing circuit, its input links to each other with the output of first multiplier;
Second multiplier, its two inputs link to each other with the output of complex conjugate computing circuit output and band pass filter respectively;
The inverse discrete fourier transform circuit, i.e. IDFT circuit, its input links to each other with the output of second multiplier;
The second complex conjugate computing circuit, its input links to each other with the output of IDFT circuit;
Phase compensating circuit, it is the 3rd multiplier, its two inputs respectively with the second complex conjugate circuit output end with separate after do not make the signal frame body that DFT handles and link to each other, its output is the time domain baseband signal after the phase compensation.
The receiver system principle composition frame chart of a described method of employing present embodiment as shown in figure 12.The signal of having introduced phase noise is through behind the separator, and isolated PN head carries out channel estimating, and isolated signal frame body is carried out phase estimation and compensation, thereby eliminates and follow the tracks of phase noise.Carrying out after the phase compensation ofdm signal being carried out discrete Fourier transform (DFT), promptly is to the ofdm signal demodulation, carries out signal equalization then, makes receiver performance improve greatly.
The ground digital multimedia TV broad cast receiving system principle composition frame chart of a described method of employing present embodiment as shown in figure 13.Antenna or other signal receiver receive modulation signal, give after down conversion module carries out frequency translation, give analog-to-digital conversion and become digital signal, utilize the time domain PN synchronizing sequence reinsertion of carrier synchronously, sign synchronization, regularly synchronous etc., and adopt the present invention to carry out phase noise and estimate, eliminate and follow the tracks of phase noise, after process OFDM multicarrier is separated the processing of mediation error correction decode method then, recover MPEG TS code stream at last.
The Computer Simulation that the ground digital multimedia TV broad cast receiving system of above-mentioned employing the method for the invention is carried out is tested, on the basis of computer artificial result, be the function model machine that FPGA has realized adopting the ground digital multimedia TV broad cast receiving system of the method for the invention with field programmable gate array.
Description of drawings
Fig. 1 is a typical digital receiver synchronous circuit block diagram.
Fig. 2 is the position of pilot tone in channel spectrum among the U.S. ATSC.
Fig. 3 is the locus of pilot tone among the European DVB-T.
Fig. 4 is the hierarchical frame structure of DMB-T host-host protocol of the present invention.
The TDS-OFDM frame that Fig. 5 has TPS forms block diagram.
Fig. 6 is the distribution of TPS subcarrier in the TDS-OFDM frame.
Fig. 7 is the TPS signal through recovering behind the multidiameter fading channel.
The graph of a relation of Fig. 8 phase noise estimate variance and phase reference TPS amplitude A c.
Fig. 9 is the relation of normalization NMSE and the boundary belt and the filter bandwidht of the reinsertion of carrier.
The graph of a relation of phase noise estimate variance and filter BW and phase reference amplitude A c when Figure 10 SNR=25dB and 30dB.
Figure 11 is a specific implementation method block diagram of the present invention.
Figure 12 utilizes the receiver internal frame diagram of removal phase noise of the present invention.
Figure 13 receives theory diagram for the ground digital multimedia TV broad cast system that adopts the present invention to remove phase noise.
Embodiment
Below in conjunction with accompanying drawing specific embodiments of the invention are described in detail.
The present invention proposes a kind ofly eliminates the phase noise estimation approach to be used for the performing step of embodiment of TDS-OFDM modulating system receiving terminal as follows:
(1) separator is the TDS-OFDM that receives that time domain synchronous OFDM signal separates, and isolated PN head is carried out channel estimating, and isolated signal frame body is carried out phase noise estimation and compensation;
(2) signal frame body being carried out the phase reference command transmitting that discrete Fourier transform (DFT) obtains receiving is TPS;
(3) N TPS relatively, and choose and wherein have maximum phase and organize TPS with reference to that of TPS subcarrier, following band pass filter given it;
(4) band pass filter that uses a centre frequency to be positioned at phase reference TPS place extracts through the phase reference TPS after channel and the phase noise interference, and the bandwidth of described filter is 2 subcarrier bandwidth;
(5) according to the channel estimation results of step (1)
And the TPS of selected position in the step (3), obtain after multiplying each other through estimating the TPS value behind the channel, with ν (k) expression,
(6) ν (k) that step (5) is obtained carries out the complex conjugate computing and obtains ν (k)
*-1
(7) the filtering result that step (4) is obtained multiply by the complex conjugate ν (k) that step (6) obtains
*-1, the frequency domain baseband signal of acquisition phase noise;
(8) the frequency domain baseband signal of the phase noise that step (7) is obtained is converted into time domain phase noise estimated vector through inverse discrete fourier transform;
(9) complex conjugate of the time domain phase noise estimated vector that step (8) is obtained multiply by the baseband signal of signal frame body data after time domain obtains phase compensation of not making discrete Fourier transform (DFT) accordingly, thereby removes phase noise.
TDS-OFDM be time domain synchronous OFDM modulating system to belong to OFDM be orthogonal frequency division multiplexi, therefore, the phase noise of TDS-OFDM system also comprises public rotating part and external dispersion part.Public rotating part is followed the tracks of by the PN synchronizing sequence of TDS-OFDM signal frame; And for the random phase noise that external dispersion partly produces, the present invention proposes and utilize that transmission parameter signaling is that the TPS subcarrier carries out phase estimation among the TDS-OFDM, thereby remove phase noise, so that become when in time reflecting phase of received signal any instantaneous.
It is transmission parameter signaling that the implementation method of removing phase noise among the present invention has been utilized TPS, in the embodiment of the method for removing phase noise, influenced by multipath effect, the channel fading that 4 groups of TPS subcarriers are subjected to is different, receiving end signal is through behind the Fourier transform, each is organized the TPS amplitude and changes, as shown in Figure 7.The accuracy that phase noise is estimated is relevant with the amplitude of phase reference TPS.According to simulation result, the amplitude A of phase reference TPS as can be seen
cWith the relation of phase noise estimate variance, as shown in Figure 8.So when carrying out filtering, the higher phase reference TPS of selecting range at first.
Phase noise can be with a phase place rotation e
J (n)Be model, TDS-OFDM frame signal is through after the Channel Transmission, and the frame signal before receiving terminal enters the discrete Fourier transform (DFT) circuit can be expressed as:
r(n)＝e
j(n)x(n)*h(n)+n′(n) (1)
Wherein, r (n) is the TDS-OFDM frame signal of receiving terminal, (n) is illustrated in the phase noise of n sample point, x (n) is the TDS-OFDM frame signal after modulating, h (n) is a channel, n ' (n)=n (n) * h (n), n (n) is a white Gaussian noise, n ' is n (n) (n) through the noise behind the channel.According to the model of phase noise, phase noise variance is compared very for a short time with signal as can be known, always sets up so formula (2) is approximate:
e
j(n)≈1+j(n) (2)
Among the present invention, the signal that receiver receives is through channel h (n) transmission.By formula (1) and formula (2) as can be known, the base band frame signal r (n) that obtains at receiving terminal can be approximately:
r(n)≈x(n)*h(n)+j(n)·(x(n)*h
c(n))+n′(n) (3)
After the discrete Fourier transform (DFT) through implementation step (2), obtain formula (4):
y(k)＝DFT(r(n))
≈H(k)·X(k)+j·[φ(k)*(H(k)·X(k))]+N′(k)
(4)
Wherein y (k), H (k), X (k), φ (k), N ' (k) are respectively r (n), h (n), x (n), (n), n ' (n) through corresponding frequency domain representation after the discrete Fourier transform (DFT).
The TPS that obtains
iGroup can be expressed as formula (5):
y
TPS(i)(k)≈H(k)·TPS
i(k)+j·[φ(k)*(H(k)·TPS
i(k))]+N′(k)
＝ν(k)+j·φ(k)*ν(k)+N′(k)
(5)
Y wherein
TPS (i)(k) the i group TPS that receives, ν (k)=H (k) TPS
i(k).
At receiving terminal, can finish carrier wave with the band pass filter that a centre frequency is positioned at phase reference TPS place and recover, therefore, except phase reference TPS must have the boundary belt, the bandwidth BW of filter also is an important parameter.Theory analysis shows that the normalization mean square error (NMSE) of the reinsertion of carrier is relevant with the bandwidth that the frequency protection band bandwidth of phase reference TPS and carrier wave recover filter, and its schematic diagram as shown in Figure 9.The graph of a relation of phase noise estimate variance and filter BW and phase reference amplitude A c when Figure 10 has provided SNR=25dB and 30dB.Phase reference TPS subcarrier amplitude A
cBig more, estimate variance is more little, but estimated performance increases improvement not quite with Ac after Ac=2.And estimate variance and BW concern more complicated: under the lower situation of signal to noise ratio snr,, work as A along with BW increases
cEstimate variance had improvement slightly in＞2 o'clock, and A
cVariance worsened in≤2 o'clock.Under the signal to noise ratio snr condition with higher,, work as A along with BW increases
cEstimate variance progressively improved in＞1 o'clock, A
cVariance worsened in=1 o'clock.This is because along with BW increases, the phase noise component of extraction increases, but the white noise in the band also increases thereupon, has only in band white noise to compare with pilot tone hour, and estimate variance just has improvement.Simultaneously, the estimate variance during BW=4 is generally greater than BW=3.This is the protection limited bandwidth owing to us, and when BW was too big, it is bigger that the subcarrier of band edge is disturbed by data subcarrier.Take all factors into consideration above various situation, get BW=2 in the present embodiment, Ac=3.The band pass filter bandwidth of Shi Yonging is 2 subcarrier spacing Δ f, i.e. bandwidth 4kHz like this.Wherein Δ f is the OFDM subcarrier spacing.
At receiving terminal, the implementation method that the above-mentioned TPS of utilization carries out the phase noise estimation is: the signal of receiving at first compares 4 groups of phase reference TPS subcarriers, choose wherein maximum that phase reference TPS and carry out bandpass filtering treatment, obtain the frequency domain baseband signal of phase noise.Multiply by the complex conjugate of the phase noise vector of estimation then, obtain the baseband signal of phase compensation, as shown in figure 11.Carrying out channel frequency response based on the PN synchronizing sequence among the TDS-OFDM then estimates, the estimated channel response is used for the signal frame after the balanced phase compensation, control is to form baseband signal behind the AGC through automatic gain at last, gives the processing section of back, recovers the data of making a start and transmitting.
Described Fig. 3 can see from the previous technique background, a large amount of pilot signal (comprising the TPS signal) has been placed by Europe DVB-T system in the OFDM symbol, but the frequency protection band is not left on the both sides of DVB-T system pilot, the both sides of each pilot tone all are actual data-signals, from above-mentioned analysis as can be known, DVB-T will adopt method of the present invention, and the one, need filter bandwidht very narrow and precipitous, realize complexity, data also can exist as disturbing; The 2nd, even realized filtering, but there is not the frequency protection band, from above-mentioned Fig. 8, Fig. 9, Figure 10 as can be known, it is also bad that phase noise is eliminated performance.
Above-mentionedly in conjunction with the accompanying drawings specific embodiments of the invention are had been described in detail, but the present invention is not restricted to the foregoing description, under the spirit and scope situation of the claim that does not break away from the application, those skilled in the art can make various modifications or remodeling.
Claims (4)
1, time-domain synchronization OFDM receiver is removed the method and system of phase noise, it is characterized in that it realizes that in digital circuit it contains following steps successively:
(1) separator is the TDS-OFDM that receives that the time domain orthogonal frequency-division multiplex singal separates, and isolated PN head is carried out channel estimating, and isolated signal frame body is carried out phase noise estimation and compensation;
(2) signal frame body being carried out the phase reference command transmitting that phase reference that discrete Fourier transform (DFT) obtains receiving obtains receiving is TPS, and total N of TPS is divided into the M group;
(3) compare N TPS, and choose that group TPS that wherein has maximum phase reference TPS subcarrier, give following band pass filter it;
(4) band pass filter that uses a centre frequency to be positioned at phase reference TPS place extracts through the phase reference TPS after channel and the phase noise interference, and the bandwidth of described filter is 2 subcarrier bandwidth;
(5) according to the channel estimation results of step (1)
And the TPS of selected position in the step (3), obtain after multiplying each other through estimating the TPS value behind the channel, with v (k) expression,
(6) the filtering result that step (4) is obtained multiply by the complex conjugate v (k) of v (k)
*-1, the frequency domain baseband signal of acquisition phase noise;
(7) the frequency domain baseband signal of the phase noise that step (6) is obtained is converted into time domain phase noise estimated vector through inverse discrete fourier transform;
(8) complex conjugate of the time domain phase noise estimated vector that step (7) is obtained multiply by the baseband signal of signal frame body data after time domain obtains phase compensation of not making discrete Fourier transform (DFT) accordingly.
2, time-domain synchronization OFDM receiver according to claim 1 is removed the method for phase noise, it is characterized in that: the described N=36 of step (2), M=4.
3, time-domain synchronization OFDM receiver according to claim 1 is removed the method for phase noise, and it is characterized in that: the bandwidth of the described band pass filter of step (4) is
Wherein Δ f is the OFDM subcarrier spacing.
4, time-domain synchronization OFDM receiver is removed the system of phase noise, it is characterized in that it contains:
The discrete Fourier transform (DFT) circuit, i.e. DFT circuit, it has a signal frame body input;
Compare and the selection circuit, it links to each other with the output of DFT circuit;
Band pass filter, its input links to each other with the output that compares with selecting circuit, and its centre frequency is positioned at comparison and selects the phase reference TPS place of circuit output, and its bandwidth is
M is the group number of TPS, and M=4, N are the TPS number of M group, and N=36, Δ f are the OFDM subcarrier spacing, and bandwidth is got 2 Δ f;
First multiplier, its two inputs respectively with outside channel estimating and equalizing system in the channel estimation circuit output and the output of comparison and selection circuit link to each other;
The first complex conjugate computing circuit, its input links to each other with the output of first multiplier;
Second multiplier, its two inputs link to each other with the output of complex conjugate computing circuit output and band pass filter respectively;
The inverse discrete fourier transform circuit, i.e. IDFT circuit, its input links to each other with the output of second multiplier;
The second complex conjugate computing circuit, its input links to each other with the output of IDFT circuit;
Phase compensating circuit, it is the 3rd multiplier, its two inputs respectively with the second complex conjugate circuit output end with separate after do not make the signal frame body that DFT handles and link to each other, its output is the time domain baseband signal after the phase compensation.
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