CN1264228A - Automatic frequency control method and device in broadband CDMA system - Google Patents
Automatic frequency control method and device in broadband CDMA system Download PDFInfo
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Abstract
An automatic frequency control method for the wide-band CDMA system in frequency-divided duplex (FDD) mode of the third generation mobile communication includes such steps as small-area search to obtain time slot synchronization, field synchronization and master scramble code numbers, escrambling and band-compressing the common pilot channel (CPICH) to obtain common pilot sign, fast Fourier transform to transform the signals into frequency domain, and energy analysis to exract frequency deflection information. An automatic frequency control device is also disclosed.
Description
The present invention relates to broadband CDMA system (WCDMA), relate more specifically to automatic frequency control (AFC) method and apparatus of broadband CDMA system receiving terminal under Frequency Division Duplexing (FDD) (FDD) pattern.
In general wireless communication system, receiver all will comprise the AFC module.This is the local frequency of using because of signal mixing in the local frequency of the modulation of signal in transmitter usefulness and the receiver inconsistent (we are referred to as fixedly frequency deviation), and baseband signal can residually have the fixedly frequency component of frequency deviation after the mixing; Simultaneously, also can be retained in the baseband signal owing to moving the Doppler frequency deviation that causes by mixing and filtering.We are collectively referred to as frequency deviation with above-mentioned two kinds of frequencies.It has the process of a quantitative change to qualitative change to the influence of base band signal process: if frequency deviation is very little, it is also very little to the influence of signal processing results; If frequency deviation is big more, its influence is also big more; When frequency deviation surpassed a certain value, the phase place aliasing can appear in signal, and directly cause can't the correct decision data content.Therefore, in communication system, generally all there is the AFC device to proofread and correct frequency deviation, makes transmitter and receiver under different situations, can both keep the synchronous of certain precision.
In U.S. Pat 5,828,710, in " AFC FREQUENCY SYNCHRONIZATION NETWORK ", the AFC bearing calibration of a kind of Eureka-147 of being applicable to digital audio broadcast system (DAB) has been proposed, it is earlier with signal digitalized, convert the signal into frequency domain with fast Fourier transform (FFT) then, decide the direction (plus or minus) of frequency shift (FS) according to the statistics variations trend of frequency energy, and according to direction stepping adjustment voltage controlled oscillator (VCO), the stepping of Tiao Zhenging each time is 1/2 of the last time.Every adjustment once all will be estimated a frequency deviation direction according to the statistics variations of frequency energy again.So repeatedly, less than a certain set point, think that frequency deviation compensates substantially this moment up to stepping.As can be seen, this method requires: every adjustment once, difference should surpass Doppler's decline and the noise synergy influence to frequency energy before and after the frequency energy that obtains, otherwise just can not decide the frequency deviation direction according to the variation of frequency energy.
But in the WCDMA system, this method is just inapplicable.Because above-mentioned digital audio broadcasting DAB system is a frequency modulation system, its carrier wave is the order of magnitude of 100MHz, and the carrier wave of WCDMA system is 2GHz, therefore under the situation of identical ground environment and receiving terminal translational speed, the signal fadeout of WCDMA system is more much bigger and faster than the signal fadeout of DAB system.Therefore, in the WCDMA system, the frequency energy change direction of statistics can not be represented the direction of frequency deviation.
Purpose of the present invention is exactly AFC method and the device that proposes a kind of WCDMA of being applicable to system and mobile terminal so that carry out effective automatic frequency control, guarantee transmitter and receiver under certain precision synchronously.
The invention provides the auto frequency control method in a kind of broadband CDMA system, comprise the steps:
1) Cell searching is to obtain slot synchronization, frame synchronization and main scrambler number;
2) descrambling and de-spreading utilizes by the information that step 1) obtained, and (CPICH) carries out descrambling to Common Pilot Channel
Despreading obtains common pilot symbols;
3) convert the signal into frequency domain by fast fourier transform, extract frequency deviation information through energy spectrometer;
Realize the automatic frequency control apparatus of above-mentioned auto frequency control method, comprising:
Cell search module obtains slot synchronization, frame synchronization and the main scrambler number of signal after the primary synchronization channel in being used for to received signal, auxiliary synchronization channel and Primary Common Control Physical Channel are handled;
It is characterized in that it also comprises:
PN code generator module utilizes main scrambler that cell search module generates number to produce corresponding PN sign indicating number, under the control of slot synchronization signal and frame synchronizing signal, to the CPICH channel descrambling and de-spreading of being correlated with;
Correlator, the symbol of output CPICH, each symbol all contains frequency deviation information;
The FFT module with the discrete CPICH symbol fast fourier transform of correlator output, obtains the frequency-region signal of CPICH;
The frequency energy comparison module compares the frequency-region signal segmentation of gained, obtains the crest frequency in each section, and obtains frequency deviation information by the relative position of analyzing crest frequency.
Compare with the method for other frequency offset estimating, the frequency deviation estimating method that the present invention proposes has the following advantages:
1) realizes simply that this method does not propose any extra requirement to system on hardware is realized, promptly utilize the original hardware resource of system just can realize.
2) precision is adjustable.If three information that Cell searching provides are accurate, then the estimated accuracy of this method depends on the transform length of FFT.Transform length is long more, and then precision is high more.But because transform length is long more, operation time is also long more.Therefore, during realization, the scope that precision is adjustable is in the operation time that allows.
3) be particularly suitable for the WCDMA system.Because in the WCDMA system, before the Cell searching, the signal of all channels all is a spread-spectrum signal, its content all is the combination at random of "+1 " and " 1 ", though through pulse shaping and mobile channel, but the still similar noise spectrum of the frequency spectrum of signal, the frequency of frequency offset signal is almost flooded by signal spectrum, is difficult to extract.And general frequency deviation estimating method is directly to estimate to received signal, is impracticable with general frequency deviation estimating method therefore.After Cell searching, behind CPICH despreading descrambling, the signal content of CPICH is resumed helps " 1 ", and this moment, signal spectrum was 0 substantially, so just signal spectrum and frequency deviation frequency separation had been opened, and just be easy to extract frequency deviation information with FFT this moment again.Therefore, we can say that the frequency deviation estimating method that the present invention proposes aims at the WCDMA system design.
The present invention is described in further detail below in conjunction with accompanying drawing.
Fig. 1 is general wireless telecommunication system receiver block diagram.
Fig. 2 is that the typical case of the AFC device of the WCDMA system that is applicable to that proposes of the present invention uses block diagram.
Fig. 3 is the committed step in the auto frequency control method that proposes of the present invention---to the signal schematic representation of the process of CPICH descrambling and de-spreading.
Hereinafter will do further detailed description to the present invention.
Received signal can be expressed as after being down-converted to base band and sampling:
K=0 wherein, 1,2..., I (k), Q (k) initial data for receiving, n (t) is an additive white Gaussian noise, and α is a channel fading, and is the random phase that is evenly distributed in the mobile channel, N ω
0Be the sample frequency of chip (chip), ω
0Be spreading rate 3.84MHz, N is the sampling number of a chip, and N=1 is all got in following discussion, and Δ ω is a frequency deviation, and it comprises the fixedly frequency deviation Δ ω that transmits and receives between two local oscillators
0With the mobile Doppler frequency deviation Δ ω that causes
d
Suppose that Cell searching well finished slot synchronization and frame synchronization and scrambler identification having under the situation of initial frequency deviation, then we can utilize Common Pilot Channel to estimate.Before the estimation, earlier to Common Pilot Channel despreading descrambling, obtain the data symbol (SYMBOL) of Common Pilot Channel, carry out fast Fourier transform (FFT) after these data are done simple filtering, obtain the frequency spectrum of Common Pilot Channel symbol, through relatively, get the frequency deviation Δ ω of the highest frequency of energy, as shown in Figure 2 as gained.If
, (1) formula is splitted into real part and imaginary part gets:
I
c(k)＝α[I(k)cosθ(k)-Q(k)sinθ(k)]+n
1(k)
Q
c(k)＝α[I(k)sinθ(k)+Q(k)cosθ(k)]+n
2(k) (2)
Ic (k) and Qc (k) to CPICH carry out the despreading descrambling, and under synchronous fully situation, the symbol of CPICH becomes " 1 " entirely, and the data of other channels are because channel code or scrambler difference separately, be pseudo random number still, be similar to white noise, be combined into new white noise n with former noise
1(k) and n
2(k).Therefore the relevant back result that adds up is (accumulation length is SF, the SF=256 of CPICH):
M=0 wherein, 1,2..., because white noise is in sufficiently long integrating range, its average is " 0 ", therefore,
With
All be approximately " 0 ", then:
Wherein
When frequency deviation Δ ω got certain value, Is (m) and Qs (m) were
Function, 256m is meant 256m point in the sampled point (speed 3.84MHz) of chip here, it is equivalent to m point in the sampled point (speed 15kHz) of symbol, therefore, symbol function Is (m) and Qs (m) are actually
Function, ω ' wherein
0Be symbol speed, it satisfies
, like this,, can obtain frequency deviation information Δ ω as long as the frequency spectrum of Is (m) and Qs (m) is handled.Because FFT does under the speed of symbol, so the frequency shift (FS) of the ceiling capacity of gained is with respect to symbol speed, as: be the FFT of 256 points, the frequency of gained ceiling capacity is located at the 38th, and then the actual frequency of this point is:
(38/256)
*15kHz＝2.23kHz
Present problem is: under the effect of initial frequency deviation, whether Cell searching can well finish slot synchronization, frame synchronization and scrambler identification.
Because be the primary synchronization channel sign indicating number in the synchronizing channel to be done relevant in the slot synchronization process, its relevant output is the same with the correlated results of foregoing CPICH, therefore relevant after the quadratic sum of I, Q two-way be
Because frequency deviation Δ ω is with respect to spreading rate ω
0Very little, therefore
Be approximately 0, (6) formula can be approximately equal to
I
s 2(t)+Q
s 2(t)＝(256α)
2 (7)
It and frequency deviation are irrelevant, so slot synchronization hour is not influenced by frequency deviation in frequency deviation.
The identification of frame synchronization and scrambler group number is to utilize the judgement of number tabling look-up of auxiliary synchronization channel sign indicating number to obtain, and sign indicating number number to be peak according to the quadratic sum of the relevant output of FHT get, based on same reason, it is not influenced by frequency deviation substantially.
Equally, in frequency deviation hour, the process that produces main scrambler biasing is influenced by frequency deviation also hardly, and therefore, this moment, the generation of main scrambler can think irrelevant with frequency deviation.Through emulation, under the situation of initial frequency deviation Δ f＜3000Hz, Cell searching still can be finished slot synchronization, frame synchronization and the identification of main scrambler, and this illustrates that the method for foregoing correcting frequency offset is feasible.
Common wireless telecommunication system receiver as shown in Figure 1.After signal is accepted into from antenna, at first carry out the frequency band gating through 1 pair of signal of band pass filter, enter high frequency power amplifier 2, enter down-conversion 3 then and (be down-converted to intermediate-frequency bandwidth from 900MHz or 1800MHz such as GSM, IF-FRE is regulation not, generally be tens MHz), by becoming baseband signal behind the if demodulator 4, then through low pass filter 5 and A/D conversion 6, become digital signal, through digital baseband signal processor 7, extract the frequency offset correction signal by the AFC8 loop and deliver to phase-locked loop pll 9 and go correct frequency error.
Auto frequency control method provided by the invention is described in detail as follows:
1, Cell searching
Cell searching divides three steps to finish:
1) utilizes a kind of certain relevant device relevant, obtain slot synchronization primary synchronization channel (P-SCH) sign indicating number;
2) on the basis of slot synchronization, auxiliary synchronization channel (S-SCH) sign indicating number is carried out a series of processing, obtain the auxiliary synchronization channel sign indicating number number of each time slot, again these yards number are handled and obtained the block number of frame synchronization and main scrambler;
3) under the situation that slot synchronization and frame synchronization are all finished, it is relevant to utilize known block number that Primary Common Control Physical Channel (P-CCPCH) is done, and obtains the biasing of main scrambler in this group after relatively, thereby obtains the main scrambler number of this sub-district.
After three steps are finished, obtain slot synchronization respectively, frame synchronization and main scrambler number.These three bases that information is the whole system operate as normal.
The Chinese patent application 99117207.8 that detailed small region search method was submitted on November 12nd, 1999 referring to the applicant, " the slot synchronization device in the WCDMA Cell searching "; And 99117209.4, " method of discrimination of scrambler group number and frame-synchronizing device in the WCDMA Cell searching ".
2, descrambling and de-spreading
On the basis of slot synchronization and frame synchronization, utilize known main scrambler number Common Pilot Channel (CPICH) to be carried out descrambling and de-spreading (spreading channel code of CPICH is known as " 1 " entirely), obtain common pilot symbols.Common pilot symbols was complete " 1 " originally, but through behind the mobile channel, frequency pilot sign is modulated by frequency deviation, so the common pilot symbols of gained contains frequency deviation information.Select for use the CPICH channel to do frequency offset estimating, be because the content of CPICH channel is known, be complete " 1 ", and channel code (determining) and scrambler (promptly being discerned the main scrambler of this sub-district that obtains by Cell searching) they also are as can be known, like this, and the pseudo noise code of CPICH (PN sign indicating number, comprise channel code and scrambler, be used for final adding and expand scrambling or despreading descrambling) just determined, therefore can after Cell searching is finished, directly utilize the PN sign indicating number that CPICH is carried out descrambling and de-spreading, and need not other any information; And its transmitting power is bigger, helps differentiating.
The process of despreading can be described with Fig. 3.Signal 110 is the data-signal a (t) that add before expanding, Here it is above-mentioned symbol, and symbol intervals is Ts; Signal 111 is spreading code b (t), also is channel code, and its width is Tc, and it satisfies Tc=Ts/SF, and SF is exactly a spreading factor; Signal 112 is data-signal c (t) behind the spread spectrum, and it is that signal 111 multiply by signal 110 and gets.Because data symbol and spreading code all include only two kinds of level :+1 (high level) and-1 (low level), therefore take advantage of the result as shown in Figure 3, the signal frequency bandwidth is before the spread spectrum SF a times behind the visible spread spectrum, so this frequency expansion process is " spread spectrum "; Foregoing is the process that adds expansion, and the process of despreading is the inverse process that adds expansion, and both are complementary.Signal 113 is despreading spreading code d (t), and it is exactly to add spreading code used when expanding, i.e. d (t)=b (t) in fact.During despreading, remove to take advantage of signal 112 with signal 113, so just can recover signal 110, Here it is recovers back data-signal 114, when correct e (t)=a (t) should be arranged.Be expressed as follows with expression formula:
e(t)＝c(t)
*d(t)＝[a(t)
*b(t))]
*d(t)＝a(t)
*[b(t)
*d(t)]＝a(t)
*b
2(t)＝a(t) (1)
Attention: in adding expansion or despreading process, the necessary sequential of two signals that multiply each other is synchronous, and b (t)=d (t) could set up, b (t) in (1) formula
*D (t)=b
2(t)=1 also just set up, could correctly recover former data-signal like this.
The process and the despreading of descrambling are just the same.
3, extract frequency deviation information
Convert the signal into frequency domain by FFT, comparison can extract frequency deviation information through energy.It must be noted that, all there is multipath effect in any wireless channel, for each bar footpath, fixedly frequency deviation is the same, Doppler frequency deviation is because every footpath is different and different with the angle of the moving direction of portable terminal, and Cell searching is finished just in the multipath most powerful path synchronously, so the frequency deviation that this method obtains also is the frequency deviation of most powerful path, Doppler frequency deviation wherein can only be represented most powerful path.But because when multipath signal merges, generally adopt the high specific merging, i.e. the strong more footpath of energy, added weights are big more during merging.Therefore, the most powerful path frequency deviation that estimates of this method is used for AFC and proofreaies and correct still very resultful.
Fig. 2 shows the AFC device of WCDMA provided by the invention system and realizes schematic diagram.
In the AFC device that the present invention proposes, mainly contain cell search module, correlator, PN code generator, FFT computing module and five parts of energy comparison module.Correlator and the PN code generator that use correlator herein and PN code generator and broadband CDMA system many places are duplicate, and available field programmable logic array (FPGA) or digital signal processor (DSP) are realized; FFT computing and comparator can be realized with DSP.
As shown in Figure 2, empty frame is wherein partly represented the automatic frequency control apparatus that the present invention proposes.Analog-to-digital conversion module 101 is transformed into digital signal with analog receiving signal and delivers to slot synchronization, frame synchronization and the main scrambler number that obtains signal after primary synchronization channel, auxiliary synchronization channel and the Primary Common Control Physical Channel of cell search module 102,102 modules in to received signal handled; PN code generator module 106 utilizes main scrambler that 102 modules generate number to produce corresponding PN sign indicating number, under the control of slot synchronization signal and frame synchronizing signal, to the CPICH channel descrambling and de-spreading of being correlated with, the symbol of correlator 103 output CPICH, each symbol all contains frequency deviation information; Discrete CPICH symbol is sent into FFT module 104 be FFT, obtain the frequency-region signal of CPICH; The frequency-region signal segmentation of 105 pairs of gained of frequency energy comparison module relatively obtains the crest frequency in each section; By analyzing the relative position of crest frequency, can obtain frequency deviation information; The frequency deviation information of gained is sent into loop filter module 107, and in fact this module is exactly a low pass filter, and it is frequency offset signal smoothly; Frequency offset signal after level and smooth is sent to voltage-control oscillator module (VOC) 108, it is according to the output signal frequency of the signal voltage size adjustment oscillator of input, and this output signal is to participate in the intermediate frequency mixing, like this, just reach the purpose that changes frequency deviation, realized automatic frequency correction.
Claims (2)
1, the auto frequency control method in a kind of broadband CDMA system comprises the steps:
1) Cell searching is to obtain slot synchronization, frame synchronization and main scrambler number;
2) descrambling and de-spreading utilizes by the information that step 1) obtained, and Common Pilot Channel is carried out descrambling and de-spreading, obtains common pilot symbols;
3) convert the signal into frequency domain by fast fourier transform, extract frequency deviation information through energy spectrometer.
2, realize the device of auto frequency control method as claimed in claim 1, comprising:
Cell search module (102) obtains slot synchronization, frame synchronization and the main scrambler number of signal after the primary synchronization channel in being used for to received signal, auxiliary synchronization channel and Primary Common Control Physical Channel are handled;
It is characterized in that it also comprises:
PN code generator module (106) utilizes main scrambler that cell search module (102) generates number to produce corresponding PN sign indicating number, under the control of slot synchronization signal and frame synchronizing signal, to the CPICH channel descrambling and de-spreading of being correlated with;
Correlator (103), the symbol of output CPICH, each symbol all contains frequency deviation information;
FFT module (104) with the discrete CPICH symbol fast fourier transform of correlator (103) output, obtains the frequency-region signal of CPICH;
Frequency energy comparison module (105) compares the frequency-region signal segmentation of gained, obtains the crest frequency in each section, and obtains frequency deviation information by the relative position of analyzing crest frequency.
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