OFDM transmitter for blocking distortion of OFDM signal generated according to insertion of synchronous symbol
Technical field
The present invention relates to Orthodoxy Frequency Division Multiplex (OFDM) transmitter, be specifically related to insert the OFDM transmitter of carrying out Domain Synchronous (TDS) transmission in the ofdm signal by pseudo noise (PN) sequence that will be the synchronizing information of ofdm signal.
Background technology
High definition TV (HDTV) broadcast system generally can roughly be divided into image encoding unit and modulating unit.The image encoding unit will be compressed to the data of 15-18Mbps from the input digital data of about 1Gbps in high-definition image source.Modulating unit sends the numerical data of this tens mbit/(Mbps) to the recipient by the finite bandwidth channel of about 6-8MHz.The digital HDTV broadcasted application is used the ground while broadcast system that divides the very high frequency(VHF)/hyperfrequency (VHF/UHF) that is used in television broadcasting.
In Europe, Orthodoxy Frequency Division Multiplex (OFDM) system is adopted as HDTV terrestrial broadcast system of future generation, and wherein, ofdm system is a kind of digital modulation system of the double effects that can obtain to improve every bandwidth for transmission speed and prevent to disturb.
This OFDM technology is a kind of in predetermined module unit, and delegation's symbol transformations that serial is imported is a parallel data, and this paralleled code element is multiplexed with the interior technology of different subcarrier frequencies.Such OFDM technology is used multicarrier, and is very different with the prior art of using single carrier wave.Has orthogonality between these multicarriers each other." orthogonality " means a kind of feature, and promptly two carrier multiplication will be null value (0), and this is to use the necessary condition of these multicarriers.This OFDM technology realizes by fast Fourier transform (FFT) and inverse fast fourier transform (IFFT), and the definition by orthogonality between subcarrier and IFFT obtains simply.
Use the advantage of OFDM technology as follows:
Depend on the TV ground transmission system of the characteristic of channel of the reflected wave that produces in the signals transmission and cochannel and adjacent-channel interference as having transmission quality, the design condition of transmission system is very complicated.Yet OFDM is powerful in multi-path environment.In other words, when using a plurality of carrier wave, the code element transmission time can be lengthened out.Therefore, therefore the OFDM technology has the less distorted signals that echo-signal took place owing to length comparatively speaking for insensitive by the caused interference of signal that multipath sends by OFDM.And ofdm system also is powerful for existing signal, and therefore, the influence that disturbed by cochannel is less.Because these characteristics can be set up Single Frequency Network (SFN)." SFN " mean single broadcast by single frequency to home broadcasting.If the use Single Frequency Network can use limited frequency resource effectively.
Simultaneously, ofdm signal is made up of multicarrier, and each carrier wave has very narrow bandwidth.Because entire spectrum has near foursquare shape, multicarrier has frequency efficiency relatively preferably than single carrier wave.Another advantage of OFDM technology is, because the waveform of ofdm signal is identical with white Gaussian noise (WGN), ofdm signal is compared as the broadcasting service of line-by-line inversion (PAL), SEQUAM system (SECAM) etc. with use, has less interference.Therefore, because the OFDM technology is used different modulator approaches for each carrier wave, can realize hierarchical transmission.
Use the OFDM transmitter of Domain Synchronous (TDS), provide distribution service for predetermined frequency band and the ofdm signal that on frequency axis, forms along time axis conversion usually.The OFDM transmitter inserted protection (GI) at interval before forming ofdm signal along time shaft, be used to suppress inter-signal interference, and the insertion synchronizing information sends then before GI.
Fig. 1 is a block diagram, shows the general purpose O FDM transmitter that sends TDS.This OFDM transmitter has forward error correction (FEC) coding unit 10, inverse discrete Fourier transform (IDFT) unit 20, protection, and (GI) inserts unit 30 at interval, synchronizing information is inserted unit 40, pulse shaping filter 50 and RF (radio frequency) up-conversion unit 60.
FEC coding unit 10 forward error correction coding source datas are to proofread and correct in the transmission course mistake that business information was produced about each ofdm signal that will be sent out.
IDFT unit 20 is a signal in the time domain with the coded data inverse discrete Fourier transform in the frequency domain.
GI inserts unit 30 and inserts protection at interval, is used for being suppressed at ofdm signal and the adjacent OFDM interference between signals that IDFT is carried out in IDFT unit 20.
Synchronizing information is inserted unit 40 pseudo noise (PN) sequence is inserted in the ofdm signal of insertion GI in the GI insertion unit 30.This synchronizing information is inserted the impulse wave that unit 40 produces corresponding to the ofdm signal that inserts the PN sequence.Should occupy wideer bandwidth than inserting PN sequence ofdm signal before for frequency axis from the impulse wave corresponding to the ofdm signal that inserts the PN sequence of synchronizing information insertion unit 40 outputs.
This impulse wave of pulse shaping filter 50 filtering is to form corresponding to the impulse wave that inserts the ofdm signal of unit 40 outputs from synchronizing information, to be suitable for being assigned the occupied bandwidth of ofdm signal of this impulse wave.RF up-conversion unit 60 is for carried out the high frequency up conversion by the ofdm signal of pulse shaping filtering in pulse shaping filter 50.In RF up-conversion unit 60, sent to receiver from antenna 70 radiation and by transmitting channel by the ofdm signal after the high frequency up conversion.
Fig. 2 is the figure that illustrates by the frame structure of Domain Synchronous (TDS) ofdm signal that type sent.The frame of this ofdm signal comprises PN sequence, GI and OFDM data.These OFDM data are to be used in the information of receiver with the true reappearance that is performed.GI is allocated for the interference between the ofdm signal that suppresses to send.This PN sequence comprise be used for ofdm signal that the estimating OFDM receiver receives synchronously and the synchronizing information of channel.
Fig. 3 is the figure that the frequency spectrum that each square produced among Fig. 1 is shown.This frequency spectrum of pointing out is based on frequency axis (F) and the figure of formation.In the figure, the OFDM frequency spectrum of process IDFT unit 20 can be shown in figure " A ", and the frequency spectrum of PN sequence is shown in figure " B ", and the frequency characteristic of pulse shaping filter 50 is shown in figure " C ".Therefore, if insert the pulse shaping filter 50 of ofdm signal through having " C " frequency characteristic of the PN sequence of the frequency spectrum with " B ", output has the ofdm signal of " D " frequency spectrum.
Yet, through having " C " frequency characteristic pulse shaping filter 50 ofdm signal with from the IDFT unit 20 output " A " Frequency spectrum ratio, have the partial distortion signal, as " D ".This problem is: if ofdm signal distortion in frequency domain, when the OFDM transmitter used frequency domain to send ofdm signal, the ofdm signal that is received in the receiver can not be reproduced exactly.
Summary of the invention
The OFDM transmitter of the distortion that the present invention that proposition is used to address the above problem is the restriction pulse shaping filtering of inserting the occupied frequency band of ofdm signal of PN sequence and carrying out so that a kind of foundation to be provided, can prevent in ofdm signal is produced.
According to the present invention, above-mentioned purpose can realize that this transmitter comprises by Domain Synchronous (TDS) OFDM transmitter: forward error correction (FEC) coding unit is used for coded input data to proofread and correct the mistake at the receiver place that transmission course was taken place; Inverse discrete Fourier transform (IDFT) unit is used to modulate described coded data, and the coded data of this modulation is defined as the OFDM code element; The unit is inserted in protection (GI) at interval, is used for that GI is inserted into the OFDM code element and is used to suppress ISI (disturbing in the code element); The pulse shaping filter unit is used to limit the frequency bandwidth of PN (pseudo noise) sequence that the PN generator produced; PN inserts the unit, is used for described filtered PN sequence is inserted the OFDM code element; With RF up-conversion unit, the OFDM code element that is used for having filtered PN sequence up-converts to the RF signal.
PN sequence by pulse shaping filter filtering preferably has infinitely-great value for frequency axis.When the ofdm signal that is received in the OFDM receiver was reproduced, it is synchronous and balanced that this PN sequence can be used to symbol timing.The type of pulse shaping filter unit comprises raised cosine (RC) filter and square root raised cosine (SRRC) filter.
Simultaneously, according to the present invention, above-mentioned purpose can be realized that this method may further comprise the steps by the signal processing method in a kind of Domain Synchronous (TDS) OFDM transmitter:
(1) coded input data is to proofread and correct the mistake at the receiver place that is taken place in the transmission course;
(2) described coded data is carried out inverse discrete Fourier transform (IDFT), with modulation encoded data, the coded data after this modulation is defined as the OFDM code element;
(3) GI is inserted into is used to suppress ISI (code element in disturb) in the OFDM code element;
(4) PN (pseudo noise) sequence that produced of filtering PN generator is to limit the frequency bandwidth of this PN sequence;
(5) described filtered PN sequence is inserted in the OFDM code element; And
(6) the OFDM code element that will have a filtered PN sequence up-converts to the RF signal.
PN sequence by the pulse shaping filtering unit filters preferably has infinitely-great value for frequency axis.When the ofdm signal that is received in the OFDM receiver was reproduced, it is synchronous and balanced that this PN sequence can be used to symbol timing.
The type of pulse shaping filter unit preferably includes RC filter and SRRC filter.
According to the present invention, by before having inserted the ofdm signal of GI with PN sequence insertion ofdm signal with its insertion, carry out pulse shaping filtering for the PN sequence, can prevent by the expansion that occupies frequency band of the caused ofdm signal of insertion of PN sequence and the distortion of GI being inserted ofdm signal.Therefore, the OFDM receiver that receives the ofdm signal of the PN sequence have pulse shaping filtering can stably receive and reproduce ofdm signal.
Description of drawings
The description of the preferred embodiment by with reference to the accompanying drawings, above-mentioned purpose of the present invention and characteristic will be more obvious, wherein:
Fig. 1 shows the block diagram of the general purpose O FDM transmitter that sends TDS;
Fig. 2 shows the frame structure of the ofdm signal that is sent by the TDS type;
Fig. 3 shows the frequency spectrum that each square frame produced of Fig. 1;
Fig. 4 shows the block diagram according to the preferred embodiment of OFDM receiver of the present invention;
Fig. 5 shows the signal processing method in Domain Synchronous (TDS) the OFDM transmitter according to embodiments of the invention.
Embodiment
Hereinafter with preferred embodiments of the present invention will be described in detail with reference to the annexed drawings.
Fig. 4 shows the block diagram according to the preferred embodiment of OFDM receiver of the present invention.
As shown in Figure 4, the OFDM transmitter has forward error correction (FEC) coding unit 100, inverse discrete Fourier transform (IDFT) unit 200, protection (GI) insertion unit 300, pulse shaping filter 400, PN insertion unit 500 and RF (radio frequency) up-conversion unit 600 at interval.
FEC coding unit 100, forward error correction coding input data are to proofread and correct in the transmission course mistake for the receiver place that business information was taken place of each ofdm signal that will be sent out.
IDFT unit 200 is carried out IDFT and is modulated described coded data, and the coded data of this modulation is defined as the OFDM code element.Be that to be used to the ofdm signal inverse discrete Fourier transform in the frequency domain be signal in the time-based time domain in IDFT unit 200.
GI inserts unit 300 and inserts protection at interval, is used for being suppressed at ofdm signal and the adjacent OFDM interference between signals that IDFT is carried out in IDFT unit 200.In other words, this protection is used to suppress the ISI (inter symbol interference) of ofdm signal at interval.
Pulse shaping filter unit 400, the frequency bandwidth of PN (pseudo noise) sequence that restriction PN generator is produced.Be that 400 filtering of pulse shaping filter unit will be inserted in the frequency band that GI inserts the PN sequence in the ofdm signal that 300 places, unit are inserted into GI.This PN sequence comprise be used for estimating the ofdm signal that received at the OFDM receiver synchronously and the synchronizing information of channel.The PN sequence forms the impulse wave for time shaft, and has infinitely-great value for frequency axis.In addition, when reproducing the ofdm signal that the OFDM receiver received, it is synchronous and balanced that this PN sequence can be used to symbol timing.
The type of pulse shaping filter unit 400 comprises raised cosine (RC) filter and square root raised cosine (SRRC) filter.
PN inserts unit 500, will be in pulse shaping filter unit 400 filtered PN sequence be inserted into to insert in the unit 300 and inserted in the ofdm signal of GI at GI.This PN inserts the impulse wave that unit 500 produces corresponding to the ofdm signal that inserts the filtered PN sequence of pulse shaping.Insert in the unit 500 by the PN sequence of pulse shaping filtering for the expansion of the frequency band that in ofdm signal, distributes without any influence at PN.In other words, because ofdm signal is not pulse shaping filtering, the frequency spectrum of ofdm signal keeps with original the same good.
Therefore, by the PN sequence is inserted ofdm signal and its is inserted inserted the ofdm signal of GI before, carry out pulse shaping filtering for the PN sequence, can prevent to be occupied the expansion of frequency band and the distortion of GI being inserted ofdm signal by the insertion of PN sequence is caused.Therefore, the OFDM receiver can more stably receive and reproduce ofdm signal.
RF up-conversion unit 600, the OFDM code element that will have filtered PN sequence up-converts to the RF signal.That is, high frequency conversion is carried out for being inserted in the pulse shaping filter unit 400 by the ofdm signal of the PN sequence of pulse shaping filtering in RF up-conversion unit 600.In RF up-conversion unit 600, sent to receiver from antenna 700 radiation and through transmitting channel by the ofdm signal after the high frequency conversion.
Hereinafter, will describe in detail according to the signal processing method in Domain Synchronous (TDS) the OFDM transmitter of embodiments of the invention with reference to figure 5.
Fig. 5 shows the signal processing method in Domain Synchronous (TDS) the OFDM transmitter according to embodiments of the invention.In step S1, FEC coding unit 100 coding from the input data in source to proofread and correct the mistake at the receiver place that is taken place in the transmission course; In step S2, the 200 pairs of described coded datas in IDFT unit are carried out inverse discrete Fourier transform (IDFT), and with modulation encoded data, the coded data after this modulation is defined as the OFDM code element; In step S3, GI inserts unit 300 and will protect at interval (GI) to be inserted into to be used to suppress ISI (interference in the code element) in the OFDM code element; In step S4, PN (pseudo noise) sequence that pulse shaping filter unit 400 filtering PN generators are produced is to limit the frequency bandwidth of this PN sequence; In step S5, PN inserts unit 500 described filtered PN sequence is inserted in the OFDM code element; And in step S6, the OFDM code element that RF up-conversion unit 600 will have filtered PN sequence up-converts to the RF signal also through the OFDM code element after antenna 700 these up-conversions of transmission.
In the best PN sequence that produces of step (4) by the institute filtering of RC filter and SRRC filter.
According to the present invention, by before having inserted the ofdm signal of GI with PN sequence insertion ofdm signal with its insertion, carry out pulse shaping filtering for the PN sequence, can prevent to be occupied the expansion of frequency band and the distortion of GI being inserted ofdm signal by the insertion of PN sequence is caused.Therefore, the OFDM receiver that receives the ofdm signal of the PN sequence have pulse shaping filtering can stably receive and reproduce ofdm signal.
Although described the preferred embodiments of the present invention, it should be appreciated by those skilled in the art that to the invention is not restricted to above preferred embodiment.Under the situation of the defined the spirit and scope of the present invention of appended claims, can make various changes and modification.