CN1367615A - Time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio - Google Patents
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Abstract
The present invention relating to a time domain synchronous quadrature frequency division multiplex modulation method with low peak value average power ratio belongs to digital information transmission technical field. The method includes follows, after forward error correction and modulation, to send inputted data code stream to PADR processing module formed by FFT conversion and frequency reshaping and then to obtain a OFDM letter of low peak value average power ratio after it having been converted by IFFI and to be sent out. At the same time receiving end can adopt different equalizing methods. The present invention has overcome the shortcomings of high time peak value average power ratio in multicarrier frequency modulation method and frequency domain or/and time domain channel equalizing method can be applied flexibly.
Description
Technical field
The invention belongs to digital information transmission technical field, relate to and a kind ofly have low peak average power than (Peak-to-Average Power Ratio, PAPR) multi-carrier modulation technology, relate more specifically to a kind of time-domain synchronization OFDM (the Time Domain Synchronous in the digital television broadcasting system (ground, wired), Orthogonal Frequency Division Multiplexing, TDS-OFDM) modulator approach.
Background technology
The formation of a typical digital communication system as shown in Figure 1, modulation system is a core technology of communication system.Digital modulation is that numerical chracter is converted to and the be complementary process of waveform of the characteristic of channel.From the frequency characteristic of transmission channel, the channel that has is the low pass type, as pair cable, coaxial cable etc.; The channel that has then is the logical type of band, as various wireless channels, fiber channel etc.Obviously, low pass type channel is suitable for the transmission of digital baseband signal, and the logical type channel of band must carry out could transmitting after the necessary modulation to digital baseband signal.Under the situation of baseband modulation, these waveforms are pulses.Bandpass modulation then is that data signal modulation (is generally sinusoidal wave) to the carrier wave of a certain fixed frequency.In fact, broadly the modulation be exactly from information source message to channel waveform or mapping the vector and frequency spectrum shift, by modulation system with digital information transmission to corresponding media.
Corresponding to different transmission mediums, the structure of emission system is identical, but the modulation system difference.Modulation system is distinguished according to quality, noise resisting ability and the complexity of the speed of the required business that provides, required circuit, exists multiple modulation system at present, and these modulation systems roughly belong to two classes: single-carrier modulated technology and multi-carrier modulation technology.
In single-carrier modulated, single channel takies all bandwidth.The single-carrier modulated technology has changed all three features of analog waveform: amplitude, frequency and phase place.Be defined in 2B1Q (1 group of 4 level sign indicating number of 2 bits) the modulated carrier amplitude of ANSI T1.601 in 1988; Quadrature amplitude modulation (qam) on a carrier wave simultaneously modulated amplitude and and phase place, be expressed as mQAM, QAM commonly used has 4QAM, 16QAM, 64QAM, 256QAM etc.; (Quadrature Phase Shift Keying QPSK) is special circumstances of QAM modulation to the quadrature phase keying, and it waits and is used for 4QAM; Another variation of amplitude and phase modulated be carrierfree amplitude modulation/phase modulation (Carrierless AmplitudeModulation/Phase Modulation, CAP).These single-carrier modulated modes are comparative maturities, are used widely, and for example use QPSK many years in the satellite communication.
Along with the development of Digital Signal Processing, multi-carrier modulation technology more and more is widely used now.Multi-transceiver technology is used the set of massive band width, and it is divided into subchannel, therefore, has produced a plurality of parallel, narrower subchannels.Each subchannel uses single-carrier modulated technology, for example QPSK or mQAM modulation system, and the code stream of each subchannel is combined in together at the receiver place.The important example of multi-transceiver technology be OFDM (Orthogonal Frequency Division Multiplexing, OFDM) and Discrete multi-tone (DiscreteMulti-tone, DMT) technology.
In OFDM, for improving band efficiency, the signal spectrum on each carrier wave is overlapped each other, on whole symbol period, be quadrature but the selection in carrier spacing makes these carrier waves, promptly the product of any two carrier waves on symbol period all is zero.Like this, even exist overlappingly between the signal spectrum on each carrier wave, also can restore undistortedly.
Because the number of subcarriers in the ofdm system often reaches hundreds of and even several thousand, so can not resemble in the practical application and use hundreds of and even several thousand oscillators or phase-locked loop traditional FDM, but adopt the IFFT conversion to obtain ofdm signal, after the D/A conversion, just obtained the ofdm signal waveform.This signal times just can be moved ofdm signal on the required channel with actual carrier.
When symbol was made up of the rectangle time pulse, the frequency spectrum of each carrier signal was the sinx/x shape, and its peak value is seen Fig. 2 corresponding to the zero point of all other carrier spectrums, and the border of frequency spectrum is very precipitous, and empty housing is the curve of the actual test of OFDM frequency spectrum.
Well-known problem of ofdm system is that it may export a great amplitude peak, be that great peak-to-average power is than (PAPR), for example United States of america digital television ATSC has adopted single-carrier modulated 8-VSB, its PAPR is 7.0dB, and European digital video broadcasting DVB-T has adopted multi-carrier modulation COFDM, its PAPR is 9.5dB, than the high 2.5dB of PAPR of the U.S..This is that each complex random variable can be regarded the QAM signal on the different carrier frequencies as because signal is the complex random variable sum of N independent (not necessarily with distributing).Under opposite extreme situations, a certain instantaneous, different carrier wave homophase superpositions get up, and therefore produce an amplitude peak, equal different carrier amplitude and.
Occur significantly at the reflector output that the situation of peak value is very serious, it makes signal be subjected to amplitude limit, and the noise that amplitude limit produced is additive white Gaussian noise or impulsive noise.In addition, the instantaneous signal that is limited exists the harmonic energy of base band frequency to leak into outside the band, thereby has reduced the energy of inband signaling.The band external leakage that amplitude limit produces has improved the performance requirement of filter, and causes adjacently disturbing frequently, needs to use suitable filter to eliminate, and it is higher to cause inband signaling to rise and fall behind the filtering harmonic wave, therefore causes the signal peak average power ratio that receives very high.。Envelope be limited (shearing) can not produce harmonic components, but the energy that leaks on other adjacent frequency is higher.
For in transmission not to the peak value amplitude limit, not only the D/A converter needs enough bits, and power amplifier will keep linearity in comprising between the amplification region of this peak value.This has caused expensive and high power loss.
For eliminating amplitude peak, Many researchers has proposed certain methods, normally eliminates the possible signal combination that causes big peak value appearance or is making a start to signal precorrection by introducing redundancy in the glossary of symbols of emission.Clearly, reduce big peak amplitude and will require to increase redundancy.This redundancy is added to improving some coding techniquess that performance adopts, and has caused the decline of the bit rate of actual transmissions.
In a word, one of problem of OFDM (OFDM) modulation is exactly that peak-to-average power is higher than single-carrier modulated than (PAPR).Therefore, the PAPR that how to reduce multi-carrier modulation OFDM just becomes an important need.
On the other hand, in signals transmission,, cause frequency selective fading, can between receiving symbol, produce intersymbol interference (ISI), make correct demodulation become very difficult because echo disturbs and the nonlinear distortion of channel.When this serious interference, depending merely on increases the signal to noise ratio that transmitter power improves when receiving and can not reduce the error rate.The technology of effectively eliminating ISI at present has two kinds: adaptive equalization technique and OFDM (OFDM) technology.
Adaptive equalization technique is the conventional method a kind of commonly used of antagonism intersymbol interference, and it has the branch of time domain equalization and frequency domain equalization.The response that time-domain equalizer utilizes it to produce goes to compensate the earth signal waveform that distorted, the intersymbol interference that engraves in the time of finally eliminating the sampling judgement effectively, and it is widely used in the every field of digital communication.
Time domain equalization generally is to insert a transversal filter (also claiming transversal filter) behind matched filter, and it is made of the delay line of a band tap, and tap is the equal symbol cycle at interval.The time delayed signal of each tap is delivered to an adder circuit output after weighting, its form is identical with finite impulse response filter (FIR), and the signal after the addition is sent to decision circuit through sampling.The weight coefficient of each tap is adjustable, can eliminate ISI by adjusting weight coefficient.Theory analysis shows, could eliminate intersymbol interference fully when having only the endless of use transversal filter, and normally used limited tap transversal filter can stay some remainder error at last.The portfolio effect of equalizer is mainly by the decision of tap number and equalization algorithm, and equalization algorithm is commonly used zero forcing algorithm, peak distortion algorithm and lowest mean square distortion algorithm (LMS) etc.Equalizer divides two kinds of presetting system and self-adapting types.Also have noise jamming in actual channel, it can exert an influence to the convergence of equalizer.In order further to improve performance, often adopt decision-feedback formula equalizer in the practical application, the tap coefficient of feedback equalizer is determined by the channel impulse response hangover that forward equalizer caused.
For modulation system and the balanced way in the different modulating system is described, be that example is illustrated below with the qam mode.
In single carrier modulation system, as shown in Figure 3, the signal after the QAM modulation has adopted the time domain equalization technology to proofread and correct the distortion that transmission produces through after the Channel Transmission, gives the QAM demodulation of back then.The Digital Television ATSC of the U.S. just belongs to single carrier modulation system shown in Figure 3, and its equalizer has just adopted time domain decision-feedback formula equalizer (DFE).
Time-domain equalizer technology comparative maturity is widely used in the various communications fields, but it is only relatively good to the short ISI effect of time delay, and is poor to the ISI effect that time delay is long, complex structure, and cost is higher.At this moment, adopt OFDM (OFDM) technology better.
In multicarrier modulation system as shown in Figure 4, making a start, after modulating through QAM, gives institute's transmission signals the IFFT inverse fourier transform of back, realize OFDM (OFDM) multi-carrier modulation, this moment, modulation signal changed to time domain by frequency domain.Through after the Channel Transmission, receiving terminal at first carries out the FFT Fourier transform, and signal changes to frequency domain from time domain, adopts equalizer to proofread and correct the distortion that transmission produces then, gives the QAM demodulation of back at last, finishes the demodulation of OFDM.Therefore, in multicarrier system shown in Figure 4, equalizer is finished at frequency domain.Adopt the OFDM COFDM transmission of coding in the DVB-T system in Europe, belong to multi-carrier modulation technology.DVB-T has inserted some " pilot tone " signals at random in the OFDM frequency spectrum, as transmission mode identification, Phase Tracking, synchronously, channel equalization etc.
From the analysis of front as can be known, multicarrier modulation system shown in Figure 4 is compared with single carrier modulation system, and one of its shortcoming is that the PAPR that makes a start is very high.Therefore, after the system that has now moves before Channel Transmission to the FFT conversion of making a start among Fig. 4 from Channel Transmission, become single-carrier system as shown in Figure 5, but and the difference of single-carrier system shown in Figure 3 is that its equalizer is to realize at frequency domain, is a single carrier frequency domain equalization (SC-FDE) system.Present IEEE 802.11b adopts this mode exactly.
The core of the digital television transmission system DMB-T of Tsing-Hua University has adopted time-domain synchronization OFDM (the Time Domain Synchronous Orthogonal-Frequency-Division-Multiplex of mQAM/QPSK, TDS-OFDM) modulation technique (is seen Tsing-Hua University's patent of application in the past: ground digital multimedia TV broad cast system, application number: 00123597.4), also belong to multi-carrier modulation technology, its system block diagram as shown in Figure 6.
The elementary cell of TDS-OFDM is a signal frame.A signal frame is made up of frame synchronization (time domain) and frame (frequency domain) two parts, and as shown in Figure 7, the baseband signalling rate of frame synchronization and frame is identical, is defined as 7.56MSps.
The base band frame synchronizing signal constitutes synchronously by behind a preamble, PN sequence and one.Preamble is defined as 0,24 and 25 symbol, after be defined as 1,25 and 104 symbol synchronously, the PN sequence has 255 symbols.Preamble and after be defined as the cyclic extensions of PN sequence synchronously.Frame synchronization adopt the BPSK modulation with obtain stable synchronously.The baseband signal of a frame is an OFDM (OFDM) piece.An OFDM piece further is divided into a protection interval and a DFT piece, as shown in Figure 7.Because select different protections at interval, the OFDM piece has different sample numbers in the time domain.The DFT piece has 3780 sample values in its time domain, they are contrary discrete fourier transforms of 3780 subcarriers in the frequency domain.DFT piece time-domain signal continues 500us, and it is spaced apart 2kHz corresponding to adjacent sub-carrier in the frequency domain.6 kinds of optionally protection gap size are arranged, promptly 0 of the DFT block size, 1/6,1/9,1/12,1/20,1/30.The signal in the protection at interval and the final stage sample value identical (cyclic extensions) of DFT piece time-domain signal.A signal frame will have different symbolic number (sample number), depend on selected frame synchronization and OFDM protection at interval.
Therefore, the technology of the time-domain synchronization OFDM TDS-OFDM that adopts in Tsing-Hua University's Digital Television is the scheme that a kind of time-domain and frequency-domain mixes.In TDS-OFDM, do not insert the pilot signal in the European DMB-T COFDM modulation, but utilized spread spectrum in the spread spectrum communication, on time domain, insert the PN spread-spectrum signal as Domain Synchronous, be used for frame synchronization, Frequency Synchronization, time synchronized, channel transfer characteristic estimation and equilibrium etc.It had both had the advantage of OFDM multi-carrier modulation like this, had avoided some shortcomings of DVT-T COFDM again.But TDS-OFDM also exists peak-to-average power total in the multi-carrier modulation than very high shortcoming.
Summary of the invention
The peak-to-average power that the objective of the invention is to exist in the digital communication system at above-mentioned multi-carrier modulation is than high and intersymbol interference problem, and provide a kind of time-domain synchronization OFDM (TDS-OFDM) modulator approach of low peak average power ratio, make its peak-to-average power that has reduced transmitter ratio, improved the intersymbol interference performance.The present invention also can be applicable in the digital television broadcastings such as ground and wired, satellite.
The time-domain synchronous orthogonal frequency division multiplex modulation method of a kind of low peak average power ratio provided by the invention is characterized in that, may further comprise the steps:
1) in making a start, the symbol that send obtains discrete values through discrete Fourier transform (DFT) (DFT);
2) this discrete values is through frequency shaping; Described discrete Fourier transform (DFT) of making a start (DFT) and frequency shaping constitute the PAPR pre-treatment;
3) signal carries out IDFT inverse discrete fourier transform (IDFT) after the frequency shaping, obtains the multicarrier modulated signal output of low peak average power ratio.
Receiving terminal can also can increase the PAPR post-processing step in receiving end according to traditional demodulation method restoring signal,
Specifically comprise:
1) receiving end signal obtains discrete signal through discrete Fourier transform (DFT) (DFT);
2) discrete signal after the DFT conversion carries out channel equalization;
3) signal after the equilibrium carries out discrete Fourier and sends out conversion (IDFT) and handle the symbol that is restored.
Can increase channel CSF filter before in described receiving end discrete Fourier transform (DFT) (DFT).
The modulation system of described symbol can be QPSK, 64QAM and 256QAM.
Described multi-carrier modulation can be OFDM (OFDM) and Discrete multi-tone (DMT).
Characteristics of the present invention:
The present invention is according to the characteristic of digital communication and transmission and digital television broadcasting, time-domain synchronization OFDM (TDS-OFDM) modulator approach of concrete a kind of low peak power ratio has been proposed, thereby the peak-to-average power shortcoming higher when having overcome multi-carrier modulation method, and adopt frequency domain flexibly or/and the time domain channel equalization methods than (PAPR).
Description of drawings
Fig. 1 is a typical digital communication system pie graph.
Fig. 2 is the frequency spectrum of OFDM modulation.
Fig. 3 is the formation block diagram of existing single carrier modulation system.
Fig. 4 is the pie graph of existing multicarrier modulation system.
Fig. 5 is the pie graph of the single carrier modulation system of existing employing frequency domain equalization.
Fig. 6 is the simplified block diagram of existing TDS-OFDM multicarrier modulation system.
Fig. 7 is existing DMB-T signal frame structure schematic diagram.
Fig. 8 constitutes block diagram for the TDS-OFDM modulating system embodiment 1 with low peak average power ratio of the present invention.
Fig. 9 is the isoboles with TDS-OFDM modulating system of low peak average power ratio of the present invention.
Figure 10 is that the TDS-OFDM modulating system embodiment 2 of the low peak average power ratio of band CSF of the present invention constitutes block diagram.
Figure 11 has only the TDS-OFDM modulating system embodiment 3 of the low PAPR of time domain equalization to constitute block diagram for of the present invention.
Figure 12,13 is for adopting the transmitting terminal and the receiving terminal block diagram of ground digital multimedia TV broad cast system of the present invention.
Embodiment
Below in conjunction with accompanying drawing specific embodiments of the invention are described in detail.
TDS-OFDM is a kind of multi-carrier modulation technology, compares with single carrier, exists peak-to-average power than very high shortcoming.Therefore, in order to overcome this problem, proposed the method that a kind of PAPR handles in the present invention, this method combines reducing PAPR and channel equalization, is commonly referred to as the PAPR processing module.That adopts the inventive method a kind ofly has low peak average power and constitutes as shown in Figure 8 than the TDS-OFDM system embodiment 1 of PAPR.PAPR pre-processing module in the present embodiment is made up of FFT conversion and frequency shaping, and the signal after the modulation at first passes through the FFT discrete Fourier transform (DFT), and then carries out frequency shaping, makes it to satisfy Nyquist criterion; The PAPR post-processing module is made up of equalizer and IFFT inverse discrete fourier transform.From its formation, system shown in Figure 8 still is an OFDM multicarrier modulation system, has just increased PAPR pre-process and post-process module.
Say that from another point of view low PAPR TDS-OFDM modulating system shown in Figure 8 can be regarded as a single carrier modulation system again, but not exclusively the same, now be analyzed as follows.
As can see from Figure 8, the receiving terminal block diagram is the same with the composition frame chart of frequency domain equalization multicarrier modulation system shown in Figure 5 after the Channel Transmission, just in the present invention equalizer and IFFT conversion is consisted of the PAPR post-processing module.
See the composition of making a start of Fig. 8 again, suppose that this system is a linear time invariant, then FFT conversion in the PAPR pre-processing module and frequency shaping part can switch, and the DFT conversion is the computing of completely reversibility in theory, and signal does not change after through FFT and two adjacent links of IFFT in theory.Therefore, shown in Figure 8 making a start can be equivalent for removing two modules of FFT and IFFT, so the equivalence of making a start of Fig. 8 is shown in Figure 9, finds out that therefrom the TDS-OFDM modulating system of low PAPR can be regarded as a single carrier modulation system.But both are not exclusively the same, symbol after the single-carrier modulated meets even distribution, after the FFT conversion, its distribution becomes Gaussian Profile, pass through the IFFT conversion again after, it distributes and to become even distribution again, but both are on all four at sampling point just, be that interpolation obtains between sampling point, under non-ideality, both have error and exist.
Know that from the analysis of front the peak-to-average power of single carrier modulation system will be far below multicarrier modulation system than PAPR.Therefore, the TDS-OFDM multicarrier modulation system that has increased PAPR pre-process and post-process module of the present invention has lower peak-to-average power than PAPR, has overcome the shortcoming of multicarrier modulation system.
Embodiments of the invention 2 are on the basis of system shown in Figure 8, increase short time domain channel equilibrium or filter (Channel Shorten Filter in the receiving terminal input, CSF), its structure as shown in figure 10, this embodiment further improves the performance of channel equalization.
The TDS-OFDM modulating system embodiment 3 that has only the low PAPR of time domain equalization of the present invention, its system configuration as shown in figure 11, compare with 2 with embodiment 1, difference is to have removed CSF filter and PAPR post-processing module, because in actual applications, may not need so complicated channel equalization, for example in digital cable TV broadcasting, receiving terminal can only adopt time-domain equalizer so.
In sum, as follows for adopting low peak average power of the present invention than the implementation method step of making a start of the TDS-OFDM modulating system embodiment of PAPR:
1. the digital signal of input is carried out the QAM modulation;
2. qam signal is formed the DFT data block, carry out the FFT conversion, obtain discrete sample value;
3. the digital samples behind the FFT is carried out frequency shaping, make it to meet Nyquist criterion;
4. the FFT data block after adopting IDFT with shaping is transformed to the discrete sample value of time domain;
5. will protect and insert DFT time domain piece formation frame at interval;
6. frame head and frame are combined as signal frame;
7. give Channel Transmission after last composite signal being formed processing such as filtering, baseband signal frame up conversion.
Low peak average power of the present invention is as follows than the receiving end implementation method step of the TDS-OFDM modulating system embodiment of PAPR:
1. the baseband signal that receives is carried out CSF filtering or time domain equalization (can select for use);
2. the baseband signal that receives is carried out the FFT discrete Fourier transform (DFT), obtain discrete values;
3. carry out channel equalization;
4. the signal after the equilibrium is carried out inverse discrete fourier transform, the symbol that obtains modulating;
5. carry out the QAM demodulation, the signal after the demodulation is given the processing module of back.
The ground digital multimedia TV broad cast emission system embodiment of an employing the method for the invention as shown in figure 12.The MPEG TS code stream of input can be multimedia messagess such as video, audio frequency, figure, data, in order to resist the error code that produces in the transmission course, TS stream at first passes through forward error correction coding (FEC), give modulator and become the digital QAM modulation signal, after passing through low PAPRTDS-OFDM modulation treatment of the present invention then, give D/A converter module again, be converted to suitable analog signal, the radio frequency module receives this analog signal, and the result after the processing gives transmitting antenna or other signal transmitter.
The ground digital multimedia TV broad cast receiving system embodiment of an employing the method for the invention 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.After this digital signal is handled through low PAPR TDS-OFDM method of the present invention, give the QAM demodulation of back, obtain the bit stream after the demodulation, give corresponding decoding FEC (FEC) then, recover MPEG TS code stream.
In conjunction with the accompanying drawings specific embodiments of the invention are had been described in detail above, but the present invention is not restricted to above-mentioned those embodiment, 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, for example system's receiving end only adopts time domain equalization, and remove FFT and IFFT conversion, perhaps wherein QAM is changed into QPSK modulation etc.
Claims (5)
1, a kind of time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio is characterized in that, may further comprise the steps:
1) in making a start, the symbol that send obtains discrete values through discrete Fourier transform (DFT);
2) this discrete values is through frequency shaping; Described make a start discrete Fourier transform (DFT) and frequency shaping constitute low peak average power and compare pre-treatment;
3) signal carries out inverse discrete fourier transform after the frequency shaping, obtains the multicarrier modulated signal output of low peak average power ratio.
2, the time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio as claimed in claim 1 is characterized in that, also being included in receiving end increases low peak average power than post-processing step:
1) receiving end signal obtains discrete signal through discrete Fourier transform (DFT);
2) discrete signal after the DFT conversion carries out channel equalization;
3) signal after the equilibrium carries out inverse discrete fourier transform processing, the symbol that is restored.
3, the time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio as claimed in claim 2 is characterized in that, increases channel CSF filter before described receiving end discrete Fourier transform (DFT).
4, the time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio as claimed in claim 1 is characterized in that, the modulation system of described symbol can be QPSK, 64QAM and 256QAM.
5, the time-domain synchronous orthogonal frequency division multiplex modulation method of low peak average power ratio as claimed in claim 1 is characterized in that, described multi-carrier modulation can be OFDM and Discrete multi-tone.
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