CN1716788A - Method and device for correcting frequency deviation - Google Patents

Abstract

The frequency deviation correcting method and device is suitable for EDGE system. The method includes turning the received signal, delay conjugate multiplying the turned and synchronized training sequence signals, reading out the weighted and averaged coefficient from the memory and weighting and averaging the result of delay conjugate multiplying for several times, calculating the angle of the weighted and averaged value and maintaining the frequency deviation estimating angle value of the time slot, and performing frequency deviation correction based on the frequency deviation estimating angle value. The device includes delay conjugate multiplying module, multiple weighting and averaging module, angle calculating and averaging treatment module, several time slot averaging module and frequency deviation correcting module. The present invention can generate reliable frequency correcting signal and improve the automatic frequency tracking performance and demodulation system stability.

Description

A kind of method and apparatus of frequency offset correction

Technical field

The invention belongs to field of mobile communication, relate in particular to the method and apparatus of the frequency offset correction of a kind of EDGE of being applicable to system.

Background technology

GSM has obtained using widely in worldwide as second generation mobile cellular communication system.But along with the development and the professional variation of mobile communication technology, people constantly increase the demand of data business.In order to satisfy people's demand, based on the gsm system of support voice business the model of two kinds of high-speed data services has been proposed in its PHASE2 and PHASE2+ standard, promptly based on high-speed data bit rate and Circuit-switched HSCSD (high speed circuit switched data) with based on the GPRS (GPRS) of packet switched data.Though HSCSD and GPRS have adopted the operator scheme of multi-slot, improved message transmission rate to a certain extent, what but it still adopted is the modulation system of GMSK (GMSK (Guassian Minimum Shift Keying)), cover with the wide area of the 384kbit/s data rate of 3-G (Generation Three mobile communication system) and approximately the local of 2Mbit/s data transfer rate cover and also greatly differ from each other, therefore be necessary to adopt more advanced communication and signal processing technology, with the capacity of further expansion gsm system.ETSI (ETSI) has determined to develop the GSM evolution scheme-EDGE (Enhanced Data rates for GSM Evolution) of enhancing data rate as the GSM evolution direction in future.So EDGE arises at the historic moment thus.In order to provide higher data communication rates in existing cellular system, EDGE has introduced many level digitals modulation system---the 8PSK modulation.Because the 8PSK modulation is a kind of linear modulation, 3 successive bits are mapped to a symbol of I/Q coordinate, thereby higher bit rate and spectrum efficiency can be provided.

In mobile communication, the channel fading that is caused by frequency change has two kinds: a kind of is because the caused decline of inherent frequency error between the frequency of this locality concussion of base station and travelling carriage, this be by physical device for a long time or short-term float frequently and the frequency departure that causes.In general, the crystal oscillator that is used for receiver can only be accurate to several ppm (1,000,000/), and for receiver, the frequency deviation that crystal oscillator causes can reach hundreds of even a few KHz, and obviously this can not ignore; Another is that travelling carriage moves the channel fading that Doppler frequency shift caused that causes.In system, need compensate correction to two kinds of frequency departures, to guarantee the correct reception of signal.

For the EDGE system, the frequency departure of supposing the signal that base band receives is the hundreds of hertz, therefore within each symbol period, and the signal 2 π Δ fT=2 π * 300/271/1000=0.002 π that will overturn.Consider that from the angle of a time slot each time slot has 156 symbols, so at the afterbody of each time slot, the signal 0.002 π * 156=0.345 π that will overturn.Because the EDGE system has adopted the modulation of 8PSK, angle phase difference of pi between the adjacent two symbols/4＜0.345 π on the planisphere.In other words, frequency departure will make the symbol on the constellation be turned on the another one character position, and the result that comes out of demodulation is wrong certainly like this.Therefore be necessary the frequency departure in the EDGE system is estimated to proofread and correct, to improve the stability and the reliability of demodulating system.

Be to have proposed a kind of automatic frequency tracking method that can in the GSM/EDGE system, use in 6,628,926 the United States Patent (USP) " Method for Automatic FrequencyControl " in the patent No..The frequency offset estimating algorithm of this patent was divided into for two steps: the first step utilizes known training sequence to come frequency departure in the "ball-park" estimate signal, and second step was to utilize data after the demodulation with the training sequence frequency deviation of coming together to estimate accurately.This invention is owing to having utilized the signal in the whole time slot to come estimating frequency offset, so the ratio of precision of estimating is higher.Shortcoming be computation complexity than higher, be not suitable in real system, adopting.In the thick frequency offset estimating of the first step, this invention at first will estimate channel parameter in addition, and then estimates frequency deviation according to minimum variance (LS-LeastSquare) or linear least mean-square poor (LMMSE-Linear Minimum Mean-Squared Error).Accuracy of frequency offset estimation will be subjected to the influence of channel parameter estimation value certainly like this.

Patent 02111067.0 has proposed a kind of method that realizes automatic frequency tracking of mobile terminal system.At first the digital quadrature signal to receiving terminal goes rotation processing; Calculate the phase place of the baseband signal after the rotation processing; Phase unwrapping is become a similar linear function; Utilize curve fit to ask the exemplary frequency deviation values of burst, the frequency deviation value of estimating is carried out peak error detect, carry out smoothing processing at last.This patent adopts the method for curve fit to come the estimated frequency error, and detects and the frequency deviation smoothing processing by peak error, makes frequency offset calculation precision height, and can reduce The noise.But the method only is directed to portable terminal, frequency correction channel and the synchronizing channel (FCCH and SCH) that need utilize base station down to send.For base station uplink, do not have similar channel and can utilize, so this patent is not suitable in the base station system.

Another common method of proofreading and correct frequency deviation is discrete Fourier transform (DFT) algorithm, and it is the energy by calculating each sampled point in the DFT transformation range, and finds the peak value of energy in frequency domain, and the pairing frequency values of this peak value is exactly the frequency offset estimating value.The computational speed of this algorithm is than very fast, but the precision of frequency offset estimating is not high, because in general, DFT realizes under character rate, so the estimated value of frequency deviation is also with respect to character rate.In the GSM/EDGE system, the character rate of signal is 270KHz, supposes that DFT adopts 256 points to estimate frequency deviation, and the estimated accuracy of DFT is 270/256=1.05KHz so, does not reach needed precision far away.If increase the sampled point number, though precision increases, the complexity of calculating also can increase thereupon.

In sum, algorithm in the above patent and DFT algorithm all can not satisfy the needs of EDGE system medium frequency offset correction.

Summary of the invention

The objective of the invention is to propose the method and apparatus of the frequency offset correction of a kind of EDGE of being applicable to system, not high to overcome in the prior art precision, can't satisfy mobile communication system, especially the shortcoming of EDGE system requirements.The present invention can produce reliable frequency correction signal, can improve the performance of automatic frequency tracking and the stability of demodulating system.

For achieving the above object, the invention discloses the method for the frequency offset correction of a kind of EDGE of being applicable to system, comprise the steps:

(1) processing of overturning to the received signal;

(2) with the upset on the training sequence position and the conjugate multiplication of delaying time of the signal synchronously;

(3) from memory, read average weighted coefficient, carry out the result of time-delay conjugate multiplication multi-time weighted average;

(4) calculate the angle of weighted average, preserve the frequency offset estimating angle value of this time slot;

(5), carry out frequency offset correction according to the frequency offset estimating angle value.

Described step (1) also comprises, adopts e ^{-j3 π k/8}Overturn, read training sequence then from memory, the estimation and the time delay of carrying out channel parameter are synchronous.

Described step (4) comprises that also the frequency deviation value that will estimate in former a plurality of time slots averages, as the final frequency offset estimating value of current time slots.

Described step (4) comprises does following processing to signal:

$\mathrm{\Δ}\hat{f}=\frac{1}{2\mathrm{\πTM}}\underset{m=0}{\overset{M-1}{\mathrm{\Σ}}}\arg \{\underset{k=0}{\overset{24}{\mathrm{\Σ}}}{a}_m\left(k\right)\·r\left(k\right)\}$

Wherein, T is a symbol period, a _m(k) be average weighted coefficient.

Described step (4) also comprises: the frequency offset estimating angle value is made normalized, $\mathrm{\Ω}=2\mathrm{\π}\·\mathrm{\Δ}\hat{f}\·T.$

Frequency offset correction in the described step (6) comprises the signal on each character position is rotated processing.

The invention also discloses the device of the frequency offset correction of a kind of EDGE of being applicable to system, comprise time-delay conjugate multiplication module, multi-time weighted average module, angle calculation and average treatment module, average module of a plurality of time slots and frequency offset correction module, wherein,

Time-delay conjugate multiplication module is used for the upset on the training sequence position and the conjugate multiplication of delaying time of the signal synchronously;

Multi-time weighted average module is used for carrying out the result of time-delay conjugate multiplication multi-time weighted average;

Angle calculation and average treatment module are used for according to formula

$\arg \left\{x\right\}=\arctan \left\{\frac{\mathrm{Im}\left(x\right)}{\mathrm{Re}\left(x\right)}\right\}$ Calculate the frequency offset estimating angle value, and according to formula

$\mathrm{\Δ}\hat{f}=\frac{1}{2\mathrm{\πTM}}\underset{m=0}{\overset{M-1}{\mathrm{\Σ}}}\arg \{\underset{m=0}{\overset{24}{\mathrm{\Σ}}}{a}_m\left(k\right)\·r\left(k\right)\}$ Average processing, wherein, T is a symbol period, a _m(k) be average weighted coefficient;

The average module of a plurality of time slots is used for the frequency deviation value that former a plurality of time slots estimate is averaged, as the final frequency offset estimating value of current time slots.

The frequency offset correction module is used for according to described frequency offset estimating value signal being rotated processing.

The method of the invention adopts time-delay conjugate multiplication and weighted-average method, according to the signal that overturns on the training sequence position after reaching synchronously frequency departure is proofreaied and correct, and adopts the average method of a plurality of time slot frequency offset estimating values to guarantee precision simultaneously.The present invention does not utilize the estimated value of channel parameter to proofread and correct frequency deviation, has eliminated the influence of channel parameter estimation error to the frequency offset estimating value.This invention computation complexity is low, and the estimated accuracy of frequency shift (FS) can satisfy the requirement of baseband system to frequency-tracking than higher, can improve the performance of base band demodulating system simultaneously.The present invention can adopt devices commonly used such as DSP to realize in addition, and is convenient and easy.

Description of drawings

Fig. 1 is the structure chart that the band receiver of base demodulation realizes in the EDGE system;

Fig. 2 is the planisphere of the 8PSK in the EDGE agreement;

Fig. 3 is the data format of conventional burst in the EDGE system;

Fig. 4 is the implementation structure figure of frequency deviation correcting method proposed by the invention.

Embodiment

Below in conjunction with accompanying drawing, the concrete enforcement of the method for the invention is described in further detail.For those skilled in the art that, from the description to the inventive method, above-mentioned and other purposes of the present invention, feature and advantage will be apparent.

The method of the invention has been taken all factors into consideration performance, stability and the arithmetic speed of 8PSK demodulating system, the algorithm of the lower frequency offset correction of a kind of computational complexity has been proposed, solved in the EDGE system, because the unsettled influence of demodulation performance that frequency departure brought.This invention computation complexity is low, and the estimated accuracy of frequency shift (FS) can improve the performance of base band demodulating system than higher.

Fig. 1 is the structure chart that the receiver demodulation realizes in the EDGE system.This device comprises: signal flip module 101, channel estimation module 103, time delay synchronization module 102, frequency offset correction module 104, matched filtering module 105 and equalizing demodulation module 106.At receiver end, digital I, Q signal through over-sampling obtains at first will pass through signal flip module 101.For the 8PSK signal, adopt e ^{-j3 π k/8}Overturn.Signal after the upset carries out the estimation of channel parameter, generally according to the training sequence of being imported with overturn after signal carry out relevant the acquisition.It is synchronous simultaneously to carry out time delay according to the estimated channel parameter value, the lead parameter of acquisition time.In frequency offset correction module 104, according to upset on the training sequence position and the signal estimating frequency offset synchronously and proofreading and correct.The channel parameter that estimates in channel estimation module 103 is as the input of matched filtering module 105, carries out matched filtering with signal after the upset.Its result enters equalizing demodulation module 106 and does the demodulation equilibrium, the result of output hard decision.Will carry out symbol transition at last, the value of symbol that equalizing demodulation is obtained converts corresponding with it bit value to.

Fig. 2 is the planisphere of the 8PSK that stipulates in the EDGE agreement.As can be seen, the frequency departure of hundreds of hertz will make the symbol on the planisphere be turned on the position adjacent, will inevitably bring EDGE demodulating system performance decrease like this.Therefore, frequency offset correction is a very important step in demodulating system.

In order to further specify frequency deviation correcting method of the present invention, see also the data format of the conventional burst that Fig. 3 provides.The present invention need utilize the training sequence in the conventional burst to carry out frequency offset correction.As can be seen from the figure, training sequence is positioned at the middle part of whole burst, and the duration is 26 values of symbol.

Fig. 4 has provided the implementation structure figure of frequency deviation correcting method proposed by the invention.This device is divided into 5 modules, is respectively: time-delay conjugate multiplication module 41, multi-time weighted average module 42, angle calculation and average treatment module 43, average module 44 of a plurality of time slots and frequency offset correction module 45.26 values of symbol after at first reaching the upset of the signal on the training sequence position synchronously take out the symbol conjugate multiplication of delaying time.From memory, read subsequently and respectively organize the weighted average coefficient, do weighted average and handle.Calculate the angle value of result of weighted average again.Average processing after repeatedly calculating.Adopt the average method of a plurality of time slots to guarantee precision then.Carry out frequency offset correction at last, to the signal processing of overturning.

The concrete implementation method of the present invention comprises following step:

1. the I to receiving, the Q signal processing of overturning.Agreement regulation according to EDGE adopts e ^{-j3 π k/8}Overturn, read training sequence then from memory, the estimation and the time delay of carrying out channel parameter are synchronous.

2. with the upset on the training sequence position and the conjugate multiplication of delaying time of the signal synchronously;

3. from memory, read average weighted coefficient, carry out the result of time-delay conjugate multiplication multi-time weighted average.And calculate the angle of weighted average.

4. preserve the frequency offset estimating angle value of this time slot.The frequency deviation value that estimated in former a plurality of time slots is averaged, as the final frequency offset estimating value of current time slots.

5. carry out frequency offset correction, promptly signal is done upset and handle.

Further specify the method for the invention below.

In the base band demodulating system, the signal after the process upset reaches synchronously can be expressed as

$y\left(k\right)=\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}d(k-i)\·h\left(i\right)+n\left(k\right)---\left(1\right)$

The data that sent of d (i) expression wherein.The data of training sequence part are known.Can be used for doing frequency offset estimating and channel estimating.H (i) is the composite channel parameter.L is the disperse length of channel.In the agreement regulation of EDGE, L=6.N (k) is a white Gaussian noise.

In training sequence part, at first the counting period is the time-delay conjugate multiplication of the signal of 1 symbol, that is:

R (k)=y (k) y* (k-1) (2) wherein, the span of k is 0-25.Then r (k) is weighted on average, and obtains its angle value.Formula is as follows:

$\mathrm{\Δ}\hat{f}=\frac{1}{2\mathrm{\πTM}}\underset{m=0}{\overset{M-1}{\mathrm{\Σ}}}\arg \{\underset{k=0}{\overset{24}{\mathrm{\Σ}}}{a}_m\left(k\right)\·r\left(k\right)\}---\left(3\right)$

Wherein T is a symbol period, a _m(k) be average weighted coefficient.Arg{} represents angle calculation, that is:

$\arg \left\{x\right\}=\arctan \left\{\frac{\mathrm{Im}\left(x\right)}{\mathrm{Re}\left(x\right)}\right\}---\left(4\right)$

It should be noted that average weighted coefficient is known, in system, it can be kept in the memory, during calculating it is read and get final product.In the present invention, the frequency offset estimating value is made normalized, so that the processing of frequency offset correction module.

$\mathrm{\Ω}=2\mathrm{\π}\·\mathrm{\Δ}\hat{f}\·T$

In the frequency offset correction processing module, need be rotated processing to the signal of whole time slot.For the signal on each character position, the rotation that only need carry out angle Ω is just passable.The rotation formula is as follows:

y′(k)＝y(k)·exp(-jkΩ)

＝y(k)·[cos(kΩ)-jsin(kΩ)] (5)

When calculating cos and sin value, adopt the method for tabling look-up to obtain.

The front provides the description to preferred embodiment, so that any technical staff in this area can use or utilize the present invention.Various modifications to these embodiment are conspicuous to those skilled in the art, can be applied to total principle described here each other embodiment and do not lose creativeness.Thereby, the present invention will be not limited to here shown in embodiment, and the wide region of principle that should disclose and new feature according to meeting here.

Claims (7)

1. a method that is applicable to the frequency offset correction of EDGE system is characterized in that, comprises the steps:

(1) processing of overturning to the received signal;

(2) with the upset on the training sequence position and the conjugate multiplication of delaying time of the signal synchronously;

(3) from memory, read average weighted coefficient, carry out the result of time-delay conjugate multiplication multi-time weighted average;

(4) calculate the angle of weighted average, preserve the frequency offset estimating angle value of this time slot;

(5), carry out frequency offset correction according to the frequency offset estimating angle value.

2. the method that is applicable to the frequency offset correction of EDGE system as claimed in claim 1 is characterized in that, described step (1) also comprises, adopts e ^{-j3 π k/8}Overturn, read training sequence then from memory, the estimation and the time delay of carrying out channel parameter are synchronous.

3. the method that is applicable to the frequency offset correction of EDGE system as claimed in claim 1 is characterized in that, described step (4) comprises that also the frequency deviation value that will estimate in former a plurality of time slots averages, as the final frequency offset estimating value of current time slots.

4. the method that is applicable to the frequency offset correction of EDGE system as claimed in claim 1 is characterized in that, described step (4) comprises does following processing to signal:

$\mathrm{\Δ}\hat{f}=\frac{1}{2\mathrm{\πTM}}\underset{m=0}{\overset{M-1}{\mathrm{\Σ}}}\arg \{\underset{k=0}{\overset{24}{\mathrm{\Σ}}}{a}_m\left(k\right)\·r\left(k\right)\}$

Wherein, T is a symbol period, a _m(k) be average weighted coefficient.

5. the method that is applicable to the frequency offset correction of EDGE system as claimed in claim 4 is characterized in that, described step (4) also comprises: the frequency offset estimating angle value is made normalized, $\mathrm{\Ω}=2\mathrm{\π}\·\mathrm{\Δ}\hat{f}\·T.$

6. as claim 1,2,3, the 4 or 5 described methods that are applicable to the frequency offset correction of EDGE system, it is characterized in that the frequency offset correction in the described step (6) comprises the signal on each character position is rotated processing.

7. a device that is applicable to the frequency offset correction of EDGE system is characterized in that, comprises time-delay conjugate multiplication module, multi-time weighted average module, and angle calculation and average treatment module, average module of a plurality of time slots and frequency offset correction module, wherein,

Time-delay conjugate multiplication module is used for the upset on the training sequence position and the conjugate multiplication of delaying time of the signal synchronously;

Multi-time weighted average module is used for carrying out the result of time-delay conjugate multiplication multi-time weighted average;

Angle calculation and average treatment module are used to calculate the frequency offset estimating angle value, and carry out average treatment;

The average module of a plurality of time slots is used for the frequency deviation value that former a plurality of time slots estimate is averaged, as the final frequency offset estimating value of current time slots;

The frequency offset correction module is used for according to described frequency offset estimating value signal being rotated processing.

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