CN1678044A - Automatic gain controlling method and system of time-domain synchronous orthogonal frequency-division duplex receiver - Google Patents

Abstract

Based on feature of TDS - OFDM signal frame, AGC control includes following steps: (1) completing frame synchronization at front end of receiver, determining separation of location between PN sequence and IDFT frame; (2) adopting time domain AGC circuit for PN sequence completes quick tracing for channel AGC; (3) DFT calculation converts IDFT frame into frequency domain, eliminating single frequency or narrow band interference through channel equalization to carry out AGC control in frequency domain; (4) weighted addition of time domain evaluation AGCPN and AGCIDFT obtains united AGC voltage, which is fed back to front end to carry put AGC amplitude control. The invention is realized at FPGA of Tsinghua DMB - T, ASIC version receiver. Favorable effect is obtained through actual try out broadcasting and testing, and performance is better than other system evidently.

Description

The auto gain control method of time-domain synchronization OFDM receiver and system

Technical field

The invention belongs to digital information transmission technical field, particularly a kind of time-domain synchronization OFDM (Time DomainSynchronous OFDM, TDS-OFDM) auto gain control method of digital television receiver and system.

Background technology

In communication control processor, the level output signal of receiver depends on the gain of incoming signal level and receiver.Because a variety of causes, the input range of receiver is often very big, differs between peak signal and the weak signal even can reach tens decibels.Can make receiver keep this excursion of operate as normal to be called the dynamic range of receiver.

The factor that influences the receiver input signal is a lot.For example, the size of transmitting station power, receiver is from the distance of transmitting station distance, the variation of signal propagation conditions in communication process is (as ionosphere and tropospheric disturbance, the variation of weather), the variation of receiver environment (as mobile receiver), and the artificial noise that produces is to the influence of receiver etc.

In order to overcome of the influence of extraneous various factors, need to use automatic gain control (Automatic Gain Control, AGC) technology to the receiver input signal.Automatic gain control circuit is a communication equipment, particularly one of important circuit of communications receiving equipment.Concerning digital receiver, it mainly acts on is that the signal level that makes receiver be used for digital processing remains certain numerical value, therefore, also can be described as automatic electric-level control (Automatic Level Control, ALC) circuit.It can guarantee when receiving weak signal, the gain height of receiver, and it is low then to gain when receiving strong signal.Make the digital processing signal keep suitable level, being unlikely to because input signal is too little can't operate as normal, also is unlikely to make receiver take place saturated or obstruction because input signal is too big.

Auto-gain circuit control block diagram is seen Fig. 1.Among the figure, the multiplication factor A of controllable gain amplifier _vControlled voltage V _cControl, system is to A after the closed loop _vAutomatically control.Detecting circuit detects the mean value of reflected signal level in the loop, behind low pass filter, compares with reference level E in comparator, produces control signal V _cRemove to control A _vIf input signal S _InAmplitude increases or circuit parameter variations makes gain become big and cause output signal S _OutWhen amplitude increased, loop produced a control signal and makes A _vReduce; Otherwise, cause S at various factors _OutWhen amplitude reduced, loop also can produce control voltage V _c, make A _vIncrease.Promptly by the loop control action, input signal S no matter _InChanges in amplitude or system parameter variations, output signal S _OutLevel all will remain on the level by reference level E decision almost constant.Among Fig. 1, the effect of low pass filter is the reaction speed of decision feedback branch, and therefore, the low pass filter time constant is the important parameter of whole automatic gain control loop.Time constant is little, and passband is wide, and reaction speed is fast, and promptly when the input end signal fluctuating frequency was higher, the feedback branch of AGC (automatic gain control) system also can react in time, and the signal of output is remained unchanged substantially.General agc circuit all has low-pass characteristic, and promptly loop changes reactionlessly to the signal amplitude that is higher than a certain frequency, and the signal amplitude that is lower than a certain frequency slowly changed control action is arranged.As in mobile communication system because multipath fading, cause the changes in amplitude of signal, this compensates because of the slow changes in amplitude that channel causes to received signal with regard to needing automatic gain.

Automatic gain can be divided into the control of inertia automatic gain, Instantaneous automatic gain control and near area gain control according to the control method difference; Can be divided into closed-loop system and open cycle system according to the circuit form difference; Can be divided into time domain AGC and frequency domain AGC again by the action scope difference.No matter take which kind of pattern, its basic principle all as mentioned above.

Ground TV broadcast is continuous data flow, so agc circuit also is continuous operation usually, to obtain stable output level.In three big international standards, the ATSC standard of the U.S. adopts the 8-VSB signal transmitting digital TV program of single carrier, and its signal carries information with 8 kinds of modulation amplitudes.Simultaneously because video signal information is bigger, show as signal amplitude change very fast, so the relative long-term average that its time domain AGC system that adopts usually should the receiver control output signal so that can carry the information distortion of changes in amplitude faster.The ISDB-T standard of the DVB-T in Europe and Japan all adopts the ofdm signal transmitting digital TV signal of multicarrier.The characteristics of ofdm signal are: signal amplitude is approximated to Gaussian Profile on time domain, and the ratio of peak value and mean value is very high; Frequency spectrum is comparatively smooth on frequency domain.If adopt time domain AGC, then filter time constant should be bigger, is unfavorable for following the tracks of fast-changing channel like this; Therefore can adopt frequency domain AGC, promptly the receiver input signal be transformed to frequency domain through DFT earlier, carry out AGC control again.

Ground digital multimedia TV broad cast (the Digital MultimediaTV Broadcasting-Terrestrial that Tsing-Hua University proposes; DMB-T) purpose of scheme provides a kind of digital information transmission method; adopted time-domain synchronization OFDM (Time Domain Synchronous OFDM; TDS-OFDM) modulation technique; about DMB-T; it is 00123597.4 " ground digital multimedia TV broad cast system " by name that the correlation circumstance of TDS-OFDM sees grant number for details; grant number is 01115520.5 " time-domain synchronous orthogonal frequency division multiplex modulation method " by name, and grant number is the Chinese invention patent of 01124144.6 Tsing-Hua University's applications such as " protection fill methods at interval in the orthogonal FDM modulation system " by name.

The TDS-OFDM of Tsing-Hua University also is a multi-carrier OFDM systems.With Europe and Japan ofdm system different be, the signal frame of DMB-T has particular structure, utilizes this structure, the present invention adopts time domain and frequency domain AGC simultaneously in the DMB-T signal, can obtain extraordinary control effect.

The structure of DMB-T system has the frame structure of layering, and its physical channel frame structure as shown in Figure 2.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 (CalendarDayFrame, CDF).Physical channel is the cycle, and and Absolute Time Synchronization.Signal frame is the elementary cell of DMB-T system frame structure, and a signal frame is formed (see figure 2) by frame synchronization and frame two parts, and the baseband signalling rate of frame synchronization and frame is identical, is defined as 7.56MSps.Frame synchronization is generated by the PN sequence cyclic-extension, and the conduct of PN sequence is synchronous, variable guard intervals (filling PN sequence, Cyclic Prefix or null value), and length is 1/4 or 1/9 of IDFT block length with the different application environment of signal.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 one 3780 OFDM (OFDM) piece.

As seen from Figure 2, the basic frame structure of DMB-T can be divided into PN sequence and two parts of IDFT frame.In DMB-T, the PN sequence is used for frame synchronization both as flag of frame, symbol and carrier synchronization and channel estimating, again at interval as the protection of OFDM.Therefore the OFDM among the DMB-T is called as the OFDM (TimeTDS-OFDM) of Domain Synchronous.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.In this basic frame structure, the PN sequence has constant amplitude on time domain, is fit to adopt time domain AGC, and the time constant of its AGC loop can be very little, so that follow the tracks of the quick variation of channel, this is most important for mobile receiver; And IDFT frame and common ofdm signal are just the same, can adopt frequency domain AGC control to it.

Summary of the invention

A kind of AGC control method and system of the low complex degree based on TDS-OFDM have been proposed.This method is handled the basic frame separated into two parts of TDS-OFDM respectively.The PN sequence is partly adopted time domain AGC, the IDFT frame is adopted frequency domain AGC.For whole signal flow, two kinds of agc modes are not continuous operations.This control method not only can guarantee the quick track channel change of time domain agc circuit, obtain processing signals output stably, make the performance of mobile receiver more superior, and the frequency domain agc circuit is easy to remove single-frequency and narrow band interference in the channel, improve the quality of reception of signal, be fit to very much be applied to digital TDS-OFDM receiver.

The present invention proposes a kind of AGC control method based on the TDS-OFDM signal frame structure, it is characterized in that, it is realized in digital circuit, contains following steps successively:

1) separator comes PN sequence and IDFT frame data separating according to the position of determined PN sequence of frame synchronization and IDFT frame:

2) to resulting PN sequence, carrying out the control of time domain automatic gain earlier is the estimation of AGC: the method for taking the expressed time-domain signal of following formula to be averaged obtains the control voltage of a time domain amplitude, records and narrates to be AGC _PN: the PN sequence that receives is added up by PN sequence length K obtains earlier: $x\left(j\right)=\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}{r}_j\left(k\right),$

r _j(k)＝c _j(k+ε)·exp(j(kΩT+θ))+n _j(k)，

Wherein, ε is normalized timing error, and j is signal frame sequence number O≤j≤i, and Ω is a carrier wave frequency deviation, and θ is the initial skew of carrier wave, n _i(k) additive white Gaussian noise of expression channel, c _j(k+ ε) is the receiving terminal PN sequence after the timing error normalization; Obtain the control voltage of a time domain amplitude after adding up again through a low pass filter, i.e. AGC _PN:

${\mathrm{AGC}}_{\mathrm{PN}}(i+1)-{\mathrm{AGC}}_{\mathrm{PN}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}x\left(j\right)\⊗f\left(j\right),i\≥0,$

Wherein, f (j) is the impulse response of low pass filter, and it carries out the low pass computing according to the cycle of signal frame; AGC _PN(i) be AGC _PNEstimated value when i signal frame, AGC when initial _PN(0)=0;

3) after receiver is finished synchronously, comprise that regularly synchronous and carrier wave recovers, just to N frame sampled point { r _n, 0≤n≤N-1} finishes frequency domain AGC at frequency domain according to the following steps and estimates:

(3.1) earlier processing transforms to frequency domain through DFT the IDFT frame, and receiver is output as in that DFT is later:

$Y_k=\sqrt{\frac{1}{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{r}_n{e}^{-j2\mathrm{\πk}\frac{n}{N}},$

R wherein _nThe TDS-OFDM signal of receiving for receiver;

(3.2) carry out frequency domain equalization, remove single-frequency or narrow band interference, obtained considering the frequency domain equalization data Z (k) after the noise jamming with smoothing circuit;

$Z\left(k\right)=Y\left(k\right)/\hat{H}\left(k\right),$

Y _k＝H _kX _k+N′(k)，

Wherein N ' is the DFT conversion of white Gaussian noise n (n) (k), For the channel frequency domain is estimated H _kExpression channel response h (l) is at the frequency response complex values at frequency k/T place, X _kBe transmitting of k subcarrier in the frame;

Single-frequency or the narrow band interference that frequency k place is existed with smoothing circuit removed again, i.e. Z (k)=0; (3.3) accumulator in the frequency domain agc circuit adds up according to DFT block length N, and the z that obtains after adding up (j) obtains the control voltage of a frequency domain amplitude again through a low pass filter, use AGC _IDFTExpression:

$z\left(j\right)=\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}Z\left(k\right)$

${\mathrm{AGC}}_{\mathrm{IDFT}}(i+1)-{\mathrm{AGC}}_{\mathrm{IDFT}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}z\left(j\right)\⊗y\left(j\right),i\≥0,$

Wherein, y (j) is the impulse response of low pass filter, and it carries out low pass computing, AGC according to the cycle of signal frame _IDFT(i) be AGC _IDFTEstimated value when i signal frame, wherein i represents the label of signal frame.AGC _IDFTInitial value AGC _IDFT(0)=0;

4) the i frame AGC (automatic gain control) voltage of receiver represents that with AGC (i) it can estimate AGC by time domain _PNEstimate AGC with frequency domain _IDFTUnite by the following formula weighting summation and to obtain:

AGC(i)＝α·AGC _IDFT(i)+β·AGC _PN(i)

Wherein, weight coefficient α, β obtains by emulation experiment, and α, β get bigger value under initial synchronisation or mobile channel, and get less value behind synchronism stability.

The present invention also proposes the AGC system of a kind of TDS-OFDM, it is characterized in that it contains:

The time domain automatic gain control circuit, it is composed in series successively by accumulator and low pass filter, and the input of accumulator links to each other with the PN sequence output of separator;

The frequency domain automatic gain control circuit, it is followed in series to form by DFT, smoothing circuit, accumulator and low pass filter, and the input of DFT links to each other with the frame data output end of separator;

Adder, its two inputs link to each other with the output of two low pass filters in two automatic gain control circuits time domain, frequency domain respectively.

TDS-OFDM belongs to ofdm system, handles but its unique frame structure makes it conveniently carry out time domain again.The AGC controlling schemes that the present invention proposes is utilized the characteristics of TDS-OFDM signal frame, with its two OTDM parts regarding time domain and frequency domain as, carries out AGC respectively and handles.For whole signal flow, two kinds of agc modes are not continuous operations.Not only can guarantee the quick track channel change of time domain agc circuit, obtain processing signals output stably, make the performance of mobile receiver more superior, and the frequency domain agc circuit is easy to remove single-frequency and narrow band interference in the channel, be not subjected to simultaneously the influence of synchronous error, improve the quality of reception of signal.Computer Simulation and real system show, this AGC controlling schemes can solve the automatic gain control problem in the ground TV broadcast transmission, be highly suitable for the TDS-OFDM system, piloting in laboratory tests and place proves that all its performance obviously is better than existing other system.

On base of digital simulation, the present invention is all accomplished in the on-site programmable gate array FPGA of DMB-T system of Tsing-Hua University and User Defined integrated circuit ASIC version receiver, actual pilot and test has obtained good effect.

Description of drawings

Fig. 1 is the AGC theory diagram.

Fig. 2 is the hierarchical frame structure of the DMB-T of Tsing-Hua University host-host protocol.

The TDS-OFDM receiver AGC control system theory diagram that Fig. 3 proposes for the present invention.

Embodiment

Specific embodiments of the invention as shown in Figure 3.

The signal frame of DMB-T transmission system uses the OFDM modulation (TDS-OFDM) of Domain Synchronous, and perhaps being called with the PN sequence is protection OFDM modulation at interval.A signal frame is made up of frame synchronization and frame two parts, and they have identical baseband signalling rate 7.56MS/s (1/T).The OTDM of PN sequence and IDFT piece, because the PN sequence is a known array for receiving terminal, PN sequence and IDFT piece are can be separated at receiving terminal.

The signal of receiver front end at first carries out a yard acquisition procedure, just received signal and local PN sequence is carried out related operation.Because the PN sequence has good autocorrelation, therefore the sign indicating number acquisition procedure can be determined the spaced-apart locations of PN sequence and IDFT frame at an easy rate.

Under this instructed, separator was divided into the signal frame that receives the part of PN sequence and two time-division quadratures of frame data.Time domain AGC estimates that it uses the PN sequence that obtains, and the method that adopts time-domain signal to be averaged realizes.This time domain agc circuit adds up according to PN sequence length K, obtains the control voltage AGC of a time domain amplitude after adding up through a low-pass filtering _PNBecause the PN sequence adopts the BPSK modulation, it is constant that its amplitude keeps in the whole sequence time-continuing process, can obtain the initial estimate of a more stable AGC after therefore adding up.

Suppose that transmission and receiving filter satisfy Nyquist (Nyquist) law, regularly the recovered data signal is with symbol period T sampling, and at awgn channel, when having frequency shift (FS) Ω, when j signal frame, the PN sequence that receives can be expressed as:

r _j(k)＝c _j(k+ε)·exp(j(kΩT+θ))+n _j(k)

c _j(k+ ε) is the receiving terminal PN sequence after the timing error normalization, and ε is normalized timing error, and Ω is a carrier wave frequency deviation, and θ is the initial skew of carrier wave, n _j(k) additive white Gaussian noise of expression channel, k is a PN sign indicating number sequence number.The time domain AGC control voltage AGC that obtains _PNCan be expressed as:

$x\left(j\right)=\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}{r}_j\left(k\right)$

${\mathrm{AGC}}_{\mathrm{PN}}(i+1)-{\mathrm{AGC}}_{\mathrm{PN}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}x\left(j\right)\⊗f\left(j\right),i\≥0$

K is the PN sequence length, and f (j) is the impulse response of low pass filter, and it carries out the low pass computing according to the cycle of signal frame.AGC _PN(i) be AGC _PNEstimated value when i signal frame, wherein i represents the label of signal frame.AGC _PNInitial value AGC _PN(0) can be set at 0, i.e. AGC _PN(0)=O.In actual applications, because the PN sequence can adopt two kinds of different lengths according to different applied environment (as single frequency network or multiple frequency network, MFN), be 1/4 or 1/9 of signal frame body length, so the time domain agc circuit should adopt different adding up the time according to different PN sequence length K.

Receiver does not have timing information when initial, can only use time domain AGC control this moment.After receiver is finished synchronously, comprise that regularly synchronous and carrier wave recovers.In each signal frame, obtain N frame sampled point { r _n, 0≤n≤N-1} will be input to and carry out demodulation among the DFT., receiver is output as in that DFT is later:

$Y_k=\sqrt{\frac{1}{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{r}_n{e}^{-j2\mathrm{\πk}\frac{n}{N}}$

By $r_n=\sqrt{N}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}{H}_k{X}_k{e}^{j2\mathrm{\πn}\frac{k}{N}}+n\left(n\right)$

X _kBe transmitting of k subcarrier in the frame, H _kExpression channel response h (l) is in the frequency response complex values at frequency k/T place, and n (n) is a Gaussian noise.

$H_k=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}h\left(l\right)e^{-j2\mathrm{\πk}\frac{l}{N}}$

Therefore have

Y _k＝H _kX _k+N′(k)

$N^{\′}\left(k\right)=\frac{1}{\sqrt{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}n\left(n\right)e^{-j2\mathrm{\πk}\frac{n}{N}}$

N ' is the DFT conversion of white Gaussian noise n (n) (k).Obtaining the channel frequency domain by channel estimation system estimates

Obtain balanced back data Z (k) by division:

$Z\left(k\right)=Y\left(k\right)/\hat{H}\left(k\right)$

If have single-frequency or narrow band interference in the channel, then the Z at corresponding frequencies k place (k) can exceed very large amplitude than adjacent data, because single-frequency is disturbed at frequency domain and is shown as " projection " on the smooth frequency band, this moment, the data of this frequency k were disturbed by single-frequency, can't recover, so these Z (k) can be arranged to zero,, obtain the frequency domain automatic gaining controling signal again by adding up to remove the influence of single-frequency interference to AGC.When j signal frame, can obtain the control voltage AGC of a frequency domain amplitude through a low-pass filtering _IDFT:

$z\left(j\right)=\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}Z\left(k\right)$

${\mathrm{AGC}}_{\mathrm{IDFT}}(i+1)-{\mathrm{AGC}}_{\mathrm{IDFT}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}z\left(j\right)\⊗y\left(j\right),i\≥0$

Y (j) is the impulse response of low pass filter, and it carries out the low pass computing according to the cycle of signal frame.AGC _IDFT(i) be AGC _IDFTEstimated value when i signal frame, wherein i represents the label of signal frame.AGC _IDFTInitial value AGC _IDFT(0)=0.

Time domain AGC does not obtain under the synchronous situation fully at receiver, and estimated value can be subjected to factor affecting such as timing error.And frequency domain AGC obtains under synchronous situation, so the accuracy height.Single-frequency or narrow band interference show as " projection " on the smooth frequency band, can be easy to eliminate, thereby remove the influence of these interference to the AGC loop.

Obtain above-mentioned a kind of TDS-OFDM receiver AGC control method, its AGC control voltage is estimated AGC by time domain _PNEstimate AGC with frequency domain _IDFTUnite by weighting summation and to obtain:

AGC(i)＝α·AGC _IDFT(i)+β·AGC _PN(i)

α wherein, β are respectively the weight coefficient of time domain and frequency domain AGC control voltage, and its value is relevant with the loop-locking time and the loop bandwidth of selection, can be according to system's needs selection.As obtaining by emulation: the mean-square value of AGC tracking error and α, β is inversely proportional to.Just entered and selected bigger α when following the tracks of, β can improve tracking velocity, but has caused bigger AGC shake; Select less α and follow the tracks of stable back, β can reduce the mean-square value of tracking error, suppresses shake better, locks the AGC level exactly.We when realizing AGC looped phase locking ring more thus adopts two groups of parameters: get higher value α under initial synchronisation or the mobile channel situation, and β, i.e. α=β=0.5, and behind synchronism stability, use little value α, β, i.e. α=β=0.05.

Claims (2)

1, the auto gain control method of time-domain synchronization OFDM receiver is characterized in that, it is realized in digital circuit, contains following steps successively:

1) separator comes PN sequence and IDFT frame data separating according to the position of determined PN sequence of frame synchronization and IDFT frame;

2) to resulting PN sequence, carrying out the control of time domain automatic gain earlier is the estimation of AGC: the method for taking the expressed time-domain signal of following formula to be averaged obtains the control voltage of a time domain amplitude, records and narrates to be AGC _PN:

Earlier the PN sequence that receives is added up by PN sequence length K and obtains: $x\left(j\right)=\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}{r}_j\left(k\right),$

r _j(k)＝c _j(k+ε)·exp(j(kΩT+θ))+n _j(k)，

Wherein, ε is normalized timing error, and j is signal frame sequence number 0≤j≤i, and Ω is a carrier wave frequency deviation, and θ is the initial skew of carrier wave, n _j(k) additivity Gauss's self noise of expression channel, c _j(k+ ε) is the receiving terminal PN sequence after the timing error normalization; Obtain the control voltage of a time domain amplitude after adding up again through a low pass filter, i.e. AGC _PN:

$\mathrm{AG}{C}_{\mathrm{PN}}(i+1)-\mathrm{AG}{C}_{\mathrm{PN}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}x\left(j\right)\⊗f\left(j\right)--i\≥0,$

Wherein, f (j) is the impulse response of low pass filter, and it carries out the low pass computing according to the cycle of signal frame; AGC _PN(i) be AGC _PNEstimated value when i signal frame, AGC when initial _PN(0)=0;

3) after receiver is finished synchronously, comprise that regularly synchronous and carrier wave recovers, just to N frame sampled point { r _n, 0≤n≤N-1} finishes frequency domain AGC at frequency domain according to the following steps and estimates:

(3.1) earlier processing transforms to frequency domain through DFT the IDFT frame, and receiver is output as in that DFT is later:

$Y_k=\sqrt{\frac{1}{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{r}_n{e}^{-j2\mathrm{\πk}\frac{n}{N}},$

R wherein _nThe TDS-OFDM signal of receiving for receiver;

(3.2) carry out frequency domain equalization, remove single-frequency or narrow band interference, obtained considering the frequency domain equalization data Z (k) after the noise jamming with smoothing circuit;

$Z\left(k\right)=Y\left(k\right)/\hat{H}\left(k\right),$

Y _k=H _kX _k+N′(K)

Wherein N ' is the DFT conversion of white Gaussian noise n (n) (k),

For the channel frequency domain is estimated H _kExpression channel response h (l) is at the frequency response complex values at frequency k/T place, X _kBe transmitting of k subcarrier in the frame; Single-frequency or the narrow band interference that frequency k place is existed with smoothing circuit removed again, i.e. Z (k)=0; (3.3) accumulator in the frequency domain agc circuit adds up according to DFT block length N, and the z that obtains after adding up (j) obtains the control voltage of a frequency domain amplitude again through a low pass filter, use AGC _IDFTExpression:

$z\left(j\right)=\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}Z\left(k\right)$

$\mathrm{AG}{C}_{\mathrm{IDFT}}(i+1)-\mathrm{AG}{C}_{\mathrm{IDFT}}\left(0\right)=\underset{j=0}{\overset{i}{\mathrm{\Σ}}}z\left(j\right)\⊗y\left(j\right),i\≥0,$

Wherein, y (j) is the impulse response of low pass filter, and it carries out low pass computing, AGC according to the cycle of signal frame _IDFT(i) be AGC _IDFTEstimated value when i signal frame, wherein i represents the label of signal frame; AGC _IDFTInitial value AGC _IDFT(0)=0;

4) the i frame AGC (automatic gain control) voltage of receiver represents that with AGC (i) it can estimate AGC by time domain _PNEstimate AGC with frequency domain _IDFTUnite by the following formula weighting summation and to obtain:

AGC(i)＝α·AGC _IDFT(i)+β·AGC _PN(i)

Wherein, weight coefficient α, β obtains by emulation experiment, and α, β get bigger value under initial synchronisation or mobile channel, and get less value behind synchronism stability.

2, the AGC (automatic gain control) system of time-domain synchronization OFDM receiver is characterized in that, it contains:

The time domain automatic gain control circuit, it is composed in series successively by accumulator and low pass filter, and the input of accumulator links to each other with the PN sequence output of separator;

The frequency domain automatic gain control circuit, it is followed in series to form by DFT, smoothing circuit, accumulator and low pass filter, and the input of DFT links to each other with the frame data output end of separator;

Adder, its two inputs link to each other with the output of two low pass filters in two automatic gain control circuits time domain, frequency domain respectively.

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