CN105610747A - Pilot-aided time-frequency interpolation weighted channel estimation method for OFDM (Orthogonal Frequency Division Multiplexing) system - Google Patents

Abstract

The invention discloses a pilot-aided time-frequency interpolation weighted channel estimation method for an OFDM (Orthogonal Frequency Division Multiplexing) system, and mainly solves the problem that the interpolation precision of the traditional cascade one-dimensional interpolation algorithm declines under the condition of Doppler shift and multiple effects. The method comprises the following steps: separately calculating a time-domain directional channel frequency response value and a frequency-domain directional channel frequency response value at a virtual pilot by using the calculated cannel frequency response value at a scattered pilot; calculating a weight factor by simultaneously using multi-path delay and Doppler shift of a channel parameter channel; performing weighted average on the time-domain directional channel frequency response value and the frequency-domain directional channel frequency response value by using the weight factor to obtain an extended pilot channel frequency response value; and finally, performing frequency directional interpolation by using the extended pilot channel frequency response value to obtain all received data channel frequency response values. The method improves the precision of channel estimated values, and can be used for a communication system based on scattered pilots.

Description

The time-frequency domain interpolation weighting channel evaluating method that ofdm system pilot tone is auxiliary

Technical field

The invention belongs to wireless communication field, relate to the auxiliary time-frequency domain interpolation weighting channel estimating side of ofdm system pilot toneMethod, can be used for the communication system based on scatter pilot, as second generation digital ground video broadcasting DVB-T2 system.

Background technology

Wireless communication system is growing to the demand of high data rate, impels researcher and production firm to adoptState-of-the-art technology meets these demands. Ofdm system is a kind of technology maturation and perfect wireless communication transmissions system,Channel is divided into some orthogonal subchannels by ofdm system, changes transmission data higher data rate into biography that speed is lowerTransmission of data. Ofdm system relies on high spectrum utilization, it is excellent effectively to overcome frequency selective fading that multipath effect causes etc.Gesture, has been widely recognized and applies.

As the important component part in ofdm communication system, channel estimation and equalization technique guarantee the property of communication systemCan, be the hot research problem in communication system always. The transmission such as multipath effect and the Doppler frequency shift spy that wireless channel is intrinsicProperty makes communication system easily produce intersymbol interference, can effectively resist intersymbol interference and design good channel estimation method, therebyReduce the communication system bit error rate. Channel estimation method is divided into pilot tone assisted channel estimation algorithm, Blind channel estimation algorithm and half-blindnessChannel estimation method, wherein the auxiliary channel estimation method of pilot tone is because of its effectively compromise quilt between estimated accuracy and pilot tone loadExtensive use. The auxiliary channel estimation method implementation procedure of pilot tone is divided into two steps: the channel estimating of pilot tone point and the letter of data pointRoad interpolation, the channel interpolation algorithm accuracy of data point determines the accuracy of channel estimation value, is that guarantee balanced device is effectively eliminatedThe key of channel effect, therefore the channel interpolation algorithm accuracy of data point is particularly important.

Within 2008, ETSI has promulgated second generation Europe digital video broadcast-terrestrial transmission standard DVB-T2, than the first generationStandard, DVB-T2 standard has adopted the modulating mode of high-order more, pilot-frequency expense and more pilot frequency mode still less, is obtaining moreWhen high data rate and the availability of frequency spectrum, also the precision of channel estimating is proposed to higher requirement. For this problem,Article " the Adaptive that GuanghuiLiu in 2014 delivers on IEEETransactionsonBroadcastingInterpolationforPilot-AidedChannelEstimatorinOFDMSystem " the middle cascade one dimension that adoptsInterpolation algorithm is realized ofdm system channel estimating, and this algorithm is owing to not considering time domain directional interpolation and frequency domain direction interpolationThe Degree of Accord Relation of interpolation scope and channel fading, therefore the frequency that the time variation causing at Doppler frequency shift and multipath effect cause choosingIn selecting property channel, its interpolation accuracy is lost, and causes the performance of channel estimating to reduce.

Summary of the invention

The object of the invention is to the deficiency for above-mentioned prior art, propose the auxiliary time-frequency of a kind of ofdm system pilot toneTerritory interpolation weighting channel evaluating method, to improve the interpolation accuracy in pilot tone assisted channel estimation.

For achieving the above object, technical scheme of the present invention comprises as follows:

(1) in ofdm system receiver, calculate scattered pilot place channel frequency domain response valueWherein, [l, k]The position number of k subcarrier in l OFDM symbol in expression ofdm system;

(2) by the channel frequency domain response value at scattered pilot placeCalculate respectively the time domain direction at virtual pilot frequency placeChannel frequency domain response valueWith frequency domain direction channel frequency domain response value

(3) utilize channel parameter channel maximum multipath time delay τ_maxWith Doppler frequency shift f_dCalculate weight:

(3a) calculate normalization channel the most mostly through time delay: τ=τ_max/τ′_max, wherein, τ '_maxFor scattered pilot can be supportedChannel maximum multipath time delay;

(3b) calculate normalization Doppler frequency shift: f '_d＝f_d/f′_dmax, wherein, f '_dmaxFor scattered pilot can be supportedLarge Doppler frequency shift;

(3c) utilize normalization channel the most mostly through time delay τ and normalization Doppler frequency shift f '_dCalculate weight λ:

λ＝(τ+1-f′_d)/2；

(4) with weight λ, (2) are drawnWithBe weighted on average, obtain virtual pilot frequency placeTime-frequency domain weighting channel frequency domain response value

$\stackrel{\‾}{H}\[l,k\]=\mathrm{\λ}{\hat{H}}_1\[l,k\]+(1-\mathrm{\λ}){\hat{H}}_2\[l,k\],k\∈{\mathrm{\ψ}}_{vp}\[l\],$

Wherein, ψ_vp[l] is the virtual pilot frequency collection of l OFDM symbol,ψ_expFor expansion pilot set, ψ_exp＝{k:k＝N_min+tD_x,t≥0andk≤N_max, t is more than or equal to 0 integer, N_minWithN_maxRepresent respectively minimum of a value and the maximum of OFDM symbol sub-carriers sequence number, ψ_sp[l] represents the discrete of l OFDM symbolPilot set, ψ_sp[l]＝{k:k＝N_min+D_x(lmodD_y)+mD′_x,m≥0andk≤N_max}，D_xIt is the subcarrier of scattered pilotInterval, D_yThe OFDM mark space of scattered pilot, D '_x＝D_x·D_y, m is more than or equal to 0 integer;

(5) by the time-frequency domain weighting channel frequency domain response value at virtual pilot frequency placeAnd step (1) calculate discretePilot tone place channel frequency domain response valueCalculate spread pilot channel frequency domain response value

(6) to spread pilot channel characteristic valueCarry out frequency domain direction interpolation, show that all reception data channels frequentlyDomain response value

Tool of the present invention has the following advantages:

1) the present invention utilizes Doppler frequency shift and channel maximum multipath time delay to calculate weight, while utilizing weights factor pairTerritory direction channel frequency domain response value and frequency domain direction channel frequency domain response value are weighted on average, by adjusting both direction channelThe weight of frequency domain response value, has obviously improved the precision of interpolation method, has further improved the channel frequency domain response of data point and has estimatedThe accuracy of evaluation.

2) the present invention, compared with DFT interpolation algorithm, has avoided DFT map function in implementation procedure, has reduced and has calculated againAssorted degree.

Brief description of the drawings

Fig. 1 is of the present invention at realization flow figure;

Fig. 2 is pilot configuration figure of the present invention;

Fig. 3 is that the present invention is the bit error rate performance analogous diagram under 80Hz condition at channel Doppler frequency shift;

Fig. 4 is that the present invention is the bit error rate performance analogous diagram under 50Hz condition at channel Doppler frequency shift;

Fig. 5 is that the present invention is the least mean-square error curve of the channel estimation value under 80Hz condition at channel Doppler frequency shiftFigure;

Fig. 6 is that the present invention is the least mean-square error curve of the channel estimation value under 50Hz condition at channel Doppler frequency shiftFigure.

Detailed description of the invention

Below by drawings and Examples, technical method of the present invention is further described.

See figures.1.and.2, specific implementation step of the present invention is as follows:

Step 1: the channel frequency domain characteristic value of calculating scattered pilot point.

The present embodiment, taking DVB-T2 system as example, is established k pilot sub-carrier place in l OFDM symbol of DVB-T2 systemThe pilot data sending is X[l, k], in l OFDM symbol, the pilot data of k pilot sub-carrier place reception is Y[l,k]；

Utilize the pilot data X[l sending, k] with the pilot data Y[l receiving, k], calculate scattered pilot place channel frequentlyDomain response value

Calculate scattered pilot place channel frequency domain response valueAlgorithm has: least-squares algorithm, least mean-square error are calculatedMethod and linear minimum mean-squared error algorithm etc., the present embodiment adopts least-squares algorithm to calculate scattered pilot place channel frequency domain responseValue, computing formula is as follows:

$\tilde{H}\[l,k\]=\left\{\begin{array}{cc}\frac{Y\[l,k\]}{X\[l,k\]},& k\∈{\mathrm{\ψ}}_{sp}\[l\]\\ 0,& k\∉{\mathrm{\ψ}}_{sp}\[l\]\end{array}\right.,$

Wherein, [l, k] is the position number of k subcarrier in l OFDM symbol in ofdm system, ψ_sp[l] representsThe scattered pilot collection of l OFDM symbol, ψ_sp[l]＝{k:k＝N_min+D_x(lmodD_y)+mD′_x,m≥0andk≤N_max}，N_minWith N_maxRepresent respectively minimum of a value and the maximum of an OFDM symbol sub-carriers sequence number, D_xIt is the subcarrier of scattered pilotInterval, D '_x＝D_x·D_y，D_yBe the OFDM mark space of scattered pilot, m is more than or equal to 0 integer.

Step 2: calculate time domain direction channel frequency domain response value and frequency domain direction channel frequency domain response at virtual pilot frequency placeValue.

2a) the channel frequency domain characteristic to scattered pilot placeCarry out time domain directional interpolation, draw virtual pilot frequency placeTime domain direction channel frequency domain response valueTime domain directional interpolation method can adopt classical one dimension interpolation method, as:Polynomial interopolation algorithm and transform domain interpolation algorithm etc., the present embodiment adopts polynomial interopolation algorithm, and computing formula is as follows:

${\hat{H}}_1\[l,k\]=\left\{\begin{array}{cc}\underset{i=-M_1}{\overset{M_2}{\mathrm{\Σ}}}{\mathrm{\α}}_p\left(i\right)\tilde{H}\[l-p-{\mathrm{iD}}_y,k\],& k\∈{\mathrm{\ψ}}_{exp}\\ 0,& k\∉{\mathrm{\ψ}}_{exp}\end{array}\right.,$

Wherein, M₁、M₂Be the integer that is greater than 0, α_p(i) be that length is M₁+M₂+ 1 time domain directional interpolation coefficient, differentThe interpolation coefficient α that time domain directional interpolation method adopts_p(i) difference, p span is [0, D_y), the value of p need ensure [l-p,K] be scattered pilot sequence number, ψ_expFor expansion pilot set, ψ_exp＝{k:k＝N_min+tD_x,t≥0andk≤N_max, t is for being greater thanOr equal 0 integer;

2b) the channel frequency domain characteristic to scattered pilot placeCarry out frequency domain direction interpolation, show that virtual pilot frequency place frequentlyTerritory direction channel frequency domain response valueFrequency domain direction interpolation and time domain directional interpolation are similar, and frequency domain direction interpolation method canTo adopt classical one dimension interpolation method, as: polynomial interopolation algorithm and transform domain interpolation algorithm etc.; The present embodiment adopts multinomialFormula interpolation algorithm, computing formula is as follows:

${\hat{H}}_2\[l,k\]=\left\{\begin{array}{cc}\underset{j=-N_1}{\overset{N_2}{\mathrm{\Σ}}}{\mathrm{\β}}_q\left(j\right)\tilde{H}\[l,k-q-{\mathrm{jD}}_x\],& k\∈{\mathrm{\ψ}}_{vp}\\ 0,& k\∉{\mathrm{\ψ}}_{vp}\end{array}\right.,$

Wherein, N₁、N₂Be the integer that is greater than 0, β_q(j) be that length is N₁+N₂+ 1 frequency domain direction interpolation coefficient, differentThe interpolation coefficient β that frequency domain direction interpolation method adopts_q(j) difference, q span is [0, D_x) integer, the value of q need protectCard [l, k-q] is scattered pilot sequence number, ψ_vp[l] represents the virtual pilot frequency collection of l OFDM symbol, ${\mathrm{\ψ}}_{vp}\[l\]=\{k:k\∈{\mathrm{\ψ}}_{\exp }andk\∉{\mathrm{\ψ}}_{sp}\[l\]\}.$

Step 3: utilize channel parameter channel maximum multipath time delay τ_maxWith Doppler frequency shift f_dCalculate weight.

(3a) calculate normalization channel the most mostly through time delay τ, computing formula is:

τ＝τ_max/τ′_max，

Wherein, τ '_maxThe channel maximum multipath time delay that can support for scattered pilot;

(3b) calculate normalization Doppler frequency shift f '_d, computing formula is:

f′_d＝f_d/f′_dmax，

Wherein, f '_dmaxThe maximum doppler frequency that can support for scattered pilot;

(3c) utilize normalization channel the most mostly through time delay τ and normalization Doppler frequency shift f '_d, calculate weight λ:

λ＝(τ+1-f′_d)/2。

Step 4: the time-frequency domain weighting channel frequency domain response value of calculating virtual pilot frequency place.

With weight λ to the time domain direction channel frequency domain response value drawing in step 2With frequency domain direction channelFrequency domain response valueBe weighted on average, calculate the time-frequency domain weighting channel frequency domain response value at virtual pilot frequency placeComputing formula is as follows:

$\stackrel{\‾}{H}\[l,k\]=\mathrm{\λ}{\hat{H}}_1\[l,k\]+(1-\mathrm{\λ}){\hat{H}}_2\[l,k\],k\∈{\mathrm{\ψ}}_{vp}\[l\].$

Step 5: calculate spread pilot channel frequency domain response value.

Utilize the time-frequency domain weighting channel frequency domain response value at the virtual pilot frequency place that step 4 calculatesCount with step 1The scattered pilot place channel frequency domain response value calculatingCalculate spread pilot channel frequency domain response valueMeterCalculation formula is as follows:

Step 6: calculate all reception data channel frequency domain response values.

The spread pilot channel characteristic value that step 5 is calculatedCarry out frequency domain direction interpolation, calculate all connecingReceive data channel frequency domain response valueThis step frequency domain directional interpolation method can adopt classical one dimension interpolation sideMethod, as: polynomial interopolation algorithm and transform domain interpolation algorithm etc.; The present embodiment adopts polynomial interopolation algorithm, computing formula asUnder:

Wherein, in different frequency domain direction interpolation method frequency domain directional interpolation factor beta_q(j) value difference;

All reception data channel frequency domain response values that this step calculates are institute in DVB-T2 system channel estimatorThere is the estimated value of the characteristic of channel of data subcarrier.

Effect of the present invention can further illustrate by following simulation result:

1. simulated conditions

Analogue system is DVB-T2 system, and channel is 21 footpath Rice channel models of DVB-T2 standard definition, adopts PP1 to leadFrequently pattern, coded system adopts 16200, and code check is 2/3, and protection is spaced apart 1/4, FFT and counts as 2K, and emulation Doppler frequency shift isTwo kinds of situations of 80Hz and 50Hz.

2. emulation content

The inventive method has been done to two kinds of emulation:

Emulation one: under the Rice channel model that is 80Hz at Doppler frequency shift, to cascade one dimension interpolation algorithm and DFT algorithmAnd these three kinds of methods of the inventive method carry out emulation, simulate the bit error rate with the performance curve of signal to noise ratio and channel estimation valueLittle mean square error curve, as shown in Figure 3 and Figure 4, wherein Fig. 3 is the performance change curve of the bit error rate with signal to noise ratio, Fig. 4 is channelEstimated value least mean-square error is with the change curve of signal to noise ratio.

Emulation two: under the Rice channel model that is 50Hz at Doppler frequency shift, to cascade one dimension interpolation algorithm and DFT algorithmAnd these three kinds of methods of the inventive method carry out emulation, simulate the bit error rate all minimum with performance and the channel estimation value of signal to noise ratioSide's error, as shown in Figure 5 and Figure 6, wherein, Fig. 5 is the performance change curve of the bit error rate with signal to noise ratio, Fig. 6 is channel estimation valueLeast mean-square error is with the change curve of signal to noise ratio.

3. analysis of simulation result:

Can find out from simulation result Fig. 3 and simulation result Fig. 5, although the performance curve of the present invention and ideal communication channel estimated valueStill have the gap of 2dB left and right, but compare traditional cascade one dimension interpolation 2 × 1D algorithm, at BER=10^-4Time, of the present inventionBER performance can be improved 1dB;

Can find out from simulation result Fig. 4 and simulation result Fig. 6, compare traditional cascade one dimension interpolation 2 × 1D algorithm, thisThe bright channel estimation value least mean-square error performance 1dB that moved to left. Result shows, by taking into full account time variation and frequency selectionProperty fading severity, adjust the weight of the characteristic of channel at the interpolation place of both direction, obviously improve the precision of interpolation method, furtherImprove the precision of channel estimation value.

Claims (4)

1. The time-frequency domain interpolation weighting channel evaluating method that 1.OFDM system pilot is auxiliary, comprising:

   (1) in ofdm system receiver, calculate scattered pilot place channel frequency domain response valueWherein, [l, k] representsThe position number of k subcarrier in l OFDM symbol in ofdm system;

   (2) by the channel frequency domain response value at scattered pilot placeCalculate respectively the time domain direction channel at virtual pilot frequency placeFrequency domain response valueWith frequency domain direction channel frequency domain response value

   (3) utilize channel parameter channel maximum multipath time delay τ_maxWith Doppler frequency shift f_dCalculate weight:

   (3a) calculate normalization channel the most mostly through time delay: τ=τ_max/τ′_max, wherein, τ '_maxThe letter that can support for scattered pilotRoad maximum multipath time delay;

   (3b) calculate normalization Doppler frequency shift: f '_d＝f_d/f′_dmax, wherein, f '_dmaxFor scattered pilot can support the most generalStrangle frequency displacement;

   (3c) utilize normalization channel the most mostly through time delay τ and normalization Doppler frequency shift f '_dCalculate weight λ:

   λ＝(τ+1-f′_d)/2；

   (4) with weight λ, (2) are drawnWithBe weighted on average, obtain the time-frequency at virtual pilot frequency placeTerritory weighting channel frequency domain response value

   $\stackrel{\‾}{H}\[l,k\]=\mathrm{\λ}{\hat{H}}_1\[l,k\]+(1-\mathrm{\λ}){\hat{H}}_2\[l,k\],k\∈{\mathrm{\ψ}}_{vp}\[l\],$

   Wherein, ψ_vp[l] is the virtual pilot frequency collection of l OFDM symbol,ψ_expFor expansion pilot set, ψ_exp＝{k:k＝N_min+tD_x,t≥0andk≤N_max, t is more than or equal to 0 integer, N_minWithN_maxRepresent respectively minimum of a value and the maximum of OFDM symbol sub-carriers sequence number, ψ_sp[l] represents the discrete of l OFDM symbolPilot set, ψ_sp[l]＝{k:k＝N_min+D_x(lmodD_y)+mD′_x,m≥0andk≤N_max}，D_xThe son that is scattered pilot carriesWave spacing, D '_x＝D_x·D_y，D_yBe the OFDM mark space of scattered pilot, m is more than or equal to 0 integer;

   (5) by the time-frequency domain weighting channel frequency domain response value at virtual pilot frequency placeAnd the scattered pilot that calculates of step (1)Place's channel frequency domain response valueCalculate spread pilot channel frequency domain response value

   (6) to spread pilot channel characteristic valueCarry out frequency domain direction interpolation, show that all reception data channel frequency domains ringShould be worth
2. 2. the auxiliary time-frequency domain interpolation weighting channel evaluating method of ofdm system pilot tone according to claim 1, Qi ZhongbuSuddenly in (1) to scattered pilot place channel frequency domain response valueBe calculated as follows:

   $\tilde{H}\[l,k\]=\left\{\begin{array}{cc}\frac{Y\[l,k\]}{X\[l,k\]},& k\∈{\mathrm{\ψ}}_{sp}\[l\]\\ 0,& k\∉{\mathrm{\ψ}}_{sp}\[l\]\end{array}\right.$

   Wherein, the position number of k subcarrier in l OFDM symbol in [l, k] expression ofdm system, X[l, k] and Y[l,K] represent that respectively ofdm system sends pilot data and receives pilot data, ψ_sp[l] represents the scattered pilot of l OFDM symbolCollection, ψ_sp[l]＝{k:k＝N_min+D_x(lmodD_y)+mD′_x,m≥0andk≤N_max}，N_minWith N_maxRepresent respectively oneMinimum of a value and the maximum of OFDM symbol sub-carriers sequence number, D_xThe subcarrier spacing of scattered pilot, D '_x＝D_x·D_y，D_yBeThe OFDM mark space of scattered pilot, m is more than or equal to 0 integer.
3. 3. the auxiliary time-frequency domain interpolation weighting channel evaluating method of ofdm system pilot tone according to claim 1, Qi ZhongbuSuddenly in (2) in the time domain direction channel frequency domain response value at virtual pilot frequency placeWith frequency domain direction channel frequency domain response valueBe calculated as follows:

   ${\hat{H}}_1\[l,k\]=\left\{\begin{array}{cc}\underset{j=-M_1}{\overset{M_2}{\Σ}}{\mathrm{\α}}_q\left(j\right)\tilde{H}\[l-p-{\mathrm{iD}}_y,k\],& k\∈{\mathrm{\ψ}}_{\exp }\\ 0,& k\∉{\mathrm{\ψ}}_{\exp }\end{array}\right.$

   ${\hat{H}}_2\[l,k\]=\left\{\begin{array}{cc}\underset{j=-N_1}{\overset{N_2}{\Σ}}{\mathrm{\β}}_q\left(j\right)\tilde{H}\[l,k-q-{\mathrm{jD}}_x\],& k\∈{\mathrm{\ψ}}_{vp}\\ 0,& k\∉{\mathrm{\ψ}}_{vp}\end{array}\right.$

   Wherein, M₁、M₂、N₁、N₂Be the integer that is greater than 0, D_xThe subcarrier spacing of scattered pilot, D_yThe OFDM of scattered pilotMark space, α_p(i) be that length is M₁+M₂+ 1 time domain directional interpolation coefficient, p span is [0, D_y), the value of p needsEnsure that [l-p, k] is scattered pilot sequence number, ψ_expFor expansion pilot set, ψ_exp＝{k:k＝N_min+tD_x,t≥0andk≤N_max, t is more than or equal to 0 integer, N_minWith N_maxThe minimum of a value that represents respectively an OFDM symbol sub-carriers sequence number withMaximum, β_q(j) be that length is N₁+N₂+ 1 frequency domain direction interpolation coefficient, q span is [0, D_x) integer, the value of qNeed to ensure that [l, k-q] is scattered pilot sequence number, ψ_vpFor virtual pilot frequency collection,ψ_sp[l] represents the scattered pilot collection of l OFDM symbol, ψ_sp[l]＝{k:k＝N_min+D_x(lmodD_y)+mD′_x,m≥0andk≤N_max}，D′_x＝D_x·D_y, m is more than or equal to 0 integer.
4. 4. the auxiliary time-frequency domain interpolation weighting channel evaluating method of ofdm system pilot tone according to claim 1, Qi ZhongbuSuddenly in (6), receive data channel frequency domain response valueBe calculated as follows:

   Wherein, β_q(j) be that length is N₁+N₂+ 1 frequency domain direction interpolation coefficient, q span is [0, D_x) integer, the value of qNeed to ensure that [l, k-q] is scattered pilot sequence number, D_xIt is the subcarrier spacing of scattered pilot.