CN100589465C - Automatic frequency control system in receiver - Google Patents

Abstract

This invention puts forward a new automatic frequency control (AFC) system used in the Rake receiver, which utilizes all locked time synchronous pilot I/Q symbol sequences given by finger to compute all the complex phase errors contained in said sequences then to be filtered to get the filtered phase errors. Said AFC controller regulates the control voltage of the PDM of the VCTCXO in terms of thestep length and utilizes the output of the VCTCXO PDM to regulate the output frequency of the VCTCXO.

Description

Automatic frequency controller in the receiver

Technical field:

The present invention proposes automatic frequency control (AFC) system of a kind of Rake of being used for receiver.This AFC system is used to control the frequency of the output of temperature-compensating VCXO, with the carrier frequency of accurate tracking base stations emission.The patented technology that the present invention proposes belongs to moving communicating field.

Background technology:

In mobile communication, travelling carriage needs to produce a local carrier frequency consistent with the centre frequency of base station carrier wave in order to finish the demodulation to the radiofrequency signal that receives.

VCTCXO (temperature-compensating VCXO) is used for producing and base station carrier frequency f at travelling carriage _cThe local carrier frequency that is complementary.Because various factors, there is the frequency accuracy error in VCTCXO.As a rule, the carrier frequency of generation is f _c+ Δ f.Wherein, Doppler effect is to cause the main cause that produces Δ f, and this effect is relevant with car speed.

If the precision of the local carrier frequency that VCTCXO produces is not enough and untimely correction, the input signal demodulation performance may severe exacerbation.

The frequency drift that automatic frequency control is used for following the tracks of VCTCXO.It can be in a preset range, the effective signal of locking on some Frequency point.

In brief, automatic frequency control is used for correction of frequency drift and uncertain, and Δ f reaches frequency stability and remains on the level that can guarantee that demodulation is carried out smoothly.It is pointed out that in mobile communication there is decline in radio wave in communication process, thereby 3-G (Generation Three mobile communication system) adopts the Rake receiver more.Rake receiver can make up the signal from a plurality of Finger.

Therefore, automatic frequency control circuit/logic is used with Finger.Each finger has its independently automatic frequency control discriminator (frequency discriminator), and it is irrelevant to carry out frequency error detection process and other finger.Automatic frequency control loop merges the input from the finger of all lockings, adjusts the error signal of output frequency as the actuation temperature compensated voltage controlled oscillator.

Obtain at original frequency after (being downlink pilot search) finish, it is 1/2 of sample frequency that automatic frequency is controlled repairable maximum frequency error.If frequency error exceeds sample frequency 1/2, can cause phase ambiguity problem (having a plurality of possible phase places).

The minimum frequency resolution that individual mobile terminal is required is 150Hz, and therefore, promptly minimum measurement unit also is to be step-length with 150Hz.Actual conditions require to have better resolution.

The speed of frequency change depends on Doppler effect.Under the condition of 2GHz, the maximum doppler frequency of expectation is 300Hz, and nominal rated speed is 80Hz, corresponding to about 43km/hr.Do individual the comparison, walking speed generally is less than 5km/hr, and generally about 50km/hr, running on expressway speed is generally 100km/hr to Vehicle Speed in the urban district.The frequency error of 100Hz and cause and be approximately 6 * 10 ^-3The mis-behave of dB.

Accompanying drawing one illustrates the system level block diagram of a conceptual automatic frequency control.It is pointed out that in a travelling carriage each finger adopts an automatic frequency control discriminator (frequency discriminator).To the correction of VCTCXO, be based on that the error signal weighted sum of frequency discriminator output of the finger of each locking finishes, that is, be used to produce the error measuring value E (t) of voltage-controlled voltage, be the frequency error signal e that the finger of all lockings provides _i(t) weighted sum:

$E\left(t\right)=\underset{i=0}{\overset{3}{\mathrm{\Σ}}}{w}_i\·e_i\left(t\right)$

I=0 wherein, 1,2,3, be the numbering of the finger of locking.w _iBe e _i(t) weight coefficient.

Automatic frequency control loop is a kind of feedback control loop.Because therefore changes in channel conditions needs by the control loop bandwidth relevant with signal quality.Simultaneously and since proofread and correct with tracking require different, need be according to system mode adjustment loop bandwidth.

Accompanying drawing two is linear automatic frequency control analysis block diagrams.For this block diagram, the closed loop transfer function, that can derive automatic frequency control is:

H(s)＝A ₀KF(s)/(1+A ₀KF(s))

In the formula

A ₀The signal amplitude that the control of=process automatic gain obtains

K=closed loop gain constant

And for F (s), we adopt firstorder filter, thereby have

F(s)＝1/s

With

H(s)＝A ₀K/(s+A ₀K)

Studies show that the frequency error under the stable state levels off to zero.The K constant has merged discriminator gain (no Board Lot) and VCTCXO sensitivity (radian per second/volt).F (s)=1/s filter block meets the Laplace transform characteristic of VCTCXO.Phi ₁And Phi ₂Be input signal phase place and fixed phase.The first-order loop design parameter only with A ₀K is relevant.Thereby the error signal bandwidth B L of automatic frequency control loop is defined as follows:

BL＝A ₀K/4

Summary of the invention:

Purpose of design: the present invention proposes automatic frequency control (AFC) system of a kind of Rake of being used for receiver.The time synchronized pilot tone I/Q symbol sebolic addressing that this system utilizes the finger of all lockings to provide calculates the plural phase error that comprises in the time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides by complex multiplier.And then, obtain filtered plural phase error by this plural number phase error is carried out filtering.This automatic frequency controller is pressed the control voltage that the pulse density modulator (PDM) of VCTCXO is adjusted in the step-length adjustment according to the value of filtered plural phase error, utilizes the output of VCTCXO PDM further to adjust the frequency that VCTCXO exports.

Design:

The simplified block diagram of the structure of the automatic frequency control loop that the present invention proposes is shown in accompanying drawing three.

According to the present invention, automatic frequency control loop is by constituting with lower member:

(1) the direct frequency converter of RF;

(2) high-speed a/d (modulus) transducer;

(3) Finger demodulating unit (a plurality of);

(4) automatic frequency controller (AFC);

(5) VCTCXO (temperature-compensating VCXO);

(6) frequency synthesizer.

In this automatic frequency control loop, the direct frequency converter of RF is finished the down-converted to the radio signal that receives (RF signal), extracts modulated-analog signal.The A/D change-over circuit converts modulated-analog signal to original I/Q chip sequence.After original I/Q chip sequence was sent into Finger demodulating unit (a plurality of), the demodulation from original I/Q chip sequence of Finger demodulating unit recovered time synchronized pilot tone I/Q symbol sebolic addressing.

The time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides is admitted in the automatic frequency controller.Automatic frequency controller at first synchronously calculate each Finger output time synchronized pilot tone I/Q symbol sebolic addressing delay the conjugate complex number multiplied result.Then, merge each Finger synchronously by the time and delay the real part and the imaginary part of conjugate complex number product sequence, thereby obtain a new sequence of complex numbers (sequence of real numbers of perhaps regarding two quadratures as).According to this new sequence of complex numbers, utilize each real part and imaginary part constantly of this sequence then, just can calculate each phase error of (time synchronized pilot tone) constantly.

Utilize each phase error of (time synchronized pilot tone) constantly, just can obtain to adjust the control voltage of VCTCXO output frequency.Method is that the phase place in each moment (time synchronized pilot tone) is sent into filtering in the first order pole window index iir filter, result according to filtering output, adjustment is used to control the control voltage of the pulse density modulator (PDM) of VCTCXO output frequency, thereby realizes adjusting with the output of VCTCXO PDM the frequency of VCTCXO output.

Above-mentioned is the basic principle and the course of work of automatic frequency control loop.Automatic frequency controller is parts complicated, the most most crucial in the automatic frequency control loop.The block diagram of automatic frequency controller is shown in accompanying drawing four.Automatic frequency controller is by constituting with lower member:

(1) complex multiplier;

(2) postpone conjugator;

(3) adder;

(4) divider;

(5) first order pole window index iir filter;

(6) pulse density modulator.

The time synchronized pilot tone I/Q symbol sebolic addressing that this automatic frequency controller utilizes the finger of all lockings to provide calculates the plural phase error that comprises in the time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides by complex multiplier.And then, obtain filtered plural phase error by this plural number phase error is carried out filtering.This automatic frequency controller is pressed the control voltage that the pulse density modulator (PDM) of VCTCXO is adjusted in the step-length adjustment according to the value of filtered plural phase error, utilizes the output of VCTCXO PDM further to adjust the frequency that VCTCXO exports.

The time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides is admitted in the automatic frequency controller.Automatic frequency controller at first synchronously calculate each Finger output time synchronized pilot tone I/Q symbol sebolic addressing delay the conjugate complex number multiplied result.Then, merge each Finger synchronously by the time and delay the real part and the imaginary part of conjugate complex number product sequence, thereby obtain a new sequence of complex numbers (sequence of real numbers of perhaps regarding two quadratures as).According to this new sequence of complex numbers, utilize each real part and imaginary part constantly of this sequence then, just can calculate each plural phase error of (time synchronized pilot tone) constantly.

Utilize each plural phase place phase place of (time synchronized pilot tone) constantly, just can obtain to adjust the control voltage of VCTCXO output frequency.Method is that the plural phase error in each moment (time synchronized pilot tone) is sent into filtering in the first order pole window index iir filter, result according to filtering output, adjustment is used to control the control voltage of the pulse density modulator (PDM) of VCTCXO output frequency, thereby realizes the frequency with output control and the adjustment VCTCXO output of VCTCXO PDM.

The above is the basic principle and the course of work of the automatic frequency controller that proposes of the present invention, below sets forth the relevant algorithm that this automatic frequency controller uses.

1. the time synchronized pilot tone I/Q symbol sebolic addressing of the Finger output of N locking is remembered respectively do I (N, k) and Q (N, k)

(1) I (N, k) k symbol in the time synchronized pilot tone I symbol sebolic addressing of the Finger output of N locking of expression;

(2) Q (N, k) k symbol in the time synchronized pilot tone Q symbol sebolic addressing of the Finger output of N locking of expression;

In the formula, N is the numbering of the Finger of locking, and k represents k symbol in the I/Q symbol sebolic addressing.

2. to the time synchronized pilot tone I/Q symbol sebolic addressing of the Finger of all lockings output, computing relay conjugate complex number product respectively.The real part of this conjugate complex number product (homophase symbol) C _i(N, k) and imaginary part (quadrature phase symbol) C _q(N k) is calculated as follows respectively:

(1)C _i(N，k)＝I(N，k)·I(N，k-1)+Q(N，k)·Q(N，k-1)；

(2)C _q(N，k)＝Q(N，k)·I(N，k-1)-I(N，k)·Q(N，k-1)。

3. to the Finger of all lockings, calculate C respectively by following formula _i(N, k) and C _q(N, stack k):

$\left(1\right)C_i\left(k\right)=\underset{N=0}{\overset{3}{\mathrm{\Σ}}}{C}_i(N,k);$

$\left(2\right)C_q\left(k\right)=\underset{N=0}{\overset{3}{\mathrm{\Σ}}}{C}_q(N,k).$

4. be calculated as follows by C _i(k) and C _q(k) the plural phase error of Gou Chenging:

$\mathrm{\α}\left(k\right)=\frac{C_q\left(k\right)}{C_i\left(k\right)};$

5. α (k) sends into first order pole window index iir filter, carries out filtering by following formula, calculates filtered plural phase error:

θ(k)＝(1-β)·θ(k-1)+β·α(k)

In the formula, θ (k) is filtered plural phase error, and k is the subscript of α (k) and θ (k) sequence.β is the forgetting factor of first order pole iir filter.

6. according to the value of θ (k), adjust pulse density modulator (PDM) the control voltage of VCTCXO, to adjust the frequency of VCTCXO output:

(1) if θ (k) 〉=0 then increases a voltage with the control voltage of the pulse density modulator (PDM) of VCTCXO and adjusts step delta c;

(2) if θ (k)＜0 then reduces by a voltage with the control voltage of the pulse density modulator (PDM) of VCTCXO and adjusts step delta c.

The automatic frequency controller that the present invention proposes, the time synchronized pilot tone I/Q symbol sebolic addressing that utilizes the finger of all lockings to provide calculates the plural phase error that comprises in the time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides by complex multiplier.And then, obtain filtered plural phase error by this plural number phase error is carried out filtering.This automatic frequency controller is pressed the control voltage that the pulse density modulator (PDM) of VCTCXO is adjusted in the step-length adjustment according to the value of filtered plural phase error, utilizes the output of VCTCXO PDM further to adjust the frequency that VCTCXO exports.In this automatic frequency controller, adopt the window index iir filter that the plural phase error that time synchronized pilot tone I/Q symbol sebolic addressing comprises is carried out filtering, improved the accuracy and the reliability of automatic frequency control.The output of window index iir filter is used to adjust the control voltage of the pulse density modulator of VCTCXO, thereby utilizes the output of the pulse density modulator of VCTCXO to adjust exactly, control the output frequency of VCTCXO.This automatic frequency controller is simple in structure, complexity is little, computing cost is low, the reliability height of automatic frequency control, be easy to realize, practical, be very suitable for being embedded in the communication chip and realize, directly commercialization with hardware mode.

Description of drawings:

Fig. 1 is the automatic frequency control loop block diagram.

Fig. 2 is the linear analysis block diagram of automatic frequency control loop.

Fig. 3 is the simplified block diagram of the structure of automatic frequency control loop.

Fig. 4 is the block diagram of automatic frequency controller.

Embodiment:

Embodiment 1: with reference to accompanying drawing 1.Shown in the accompanying drawing 1 is the system level block diagram of a conceptual automatic frequency control, comprises a frequency error detector, a loop filter and a reference carrier generator.The output of frequency error detector is through loop filtering, as the input of reference carrier generator.

Embodiment 2: with reference to accompanying drawing 2.Shown in the accompanying drawing 2 is the linear analysis block diagram of automatic frequency control loop, comprises a closed loop gain amplifier A ₀K, a first-order loop filter F (s) and an adder.The input of adder is respectively the anti-phase of the phase place of tracking signal and automatic frequency control loop feedback phase.

Embodiment 3: with reference to accompanying drawing 3 and accompanying drawing 4.Shown in the accompanying drawing 3 is the structured flowchart of automatic frequency control loop, and shown in the accompanying drawing 4 is the block diagram of automatic frequency controller.

In accompanying drawing 3 and the accompanying drawing 4, the direct frequency converter of RF is finished the down-converted to the radio signal that receives (RF signal), extracts modulated-analog signal.The A/D change-over circuit converts modulated-analog signal to original I/Q chip sequence.After original I/Q chip sequence was sent into Finger demodulating unit (a plurality of), the demodulation from original I/Q chip sequence of Finger demodulating unit recovered time synchronized pilot tone I/Q symbol sebolic addressing.

The time synchronized pilot tone I/Q symbol sebolic addressing that the finger of all lockings provides is admitted in the automatic frequency controller.Automatic frequency controller at first synchronously calculate each Finger output time synchronized pilot tone I/Q symbol sebolic addressing delay the conjugate complex number multiplied result.Then, merge each Finger synchronously by the time and delay the real part and the imaginary part of conjugate complex number product sequence, thereby obtain a new sequence of complex numbers (sequence of real numbers of perhaps regarding two quadratures as).According to this new sequence of complex numbers, utilize each real part and imaginary part constantly of this sequence then, just can calculate each phase error of (time synchronized pilot tone) constantly.

Utilize each phase error of (time synchronized pilot tone) constantly, just can obtain to adjust the control voltage of VCTCXO output frequency.Method is that the phase error in each moment (time synchronized pilot tone) is sent into filtering in the first order pole window index iir filter, result according to filtering output, adjustment is used to control the control voltage of the pulse density modulator (PDM) of VCTCXO output frequency, thereby realizes adjusting with the output of VCTCXO PDM the frequency of VCTCXO output.

Claims (1)

1, a kind of automatic frequency controller that is used for the Rake receiver, the direct frequency converter of this automatic frequency controller and RF, high-speed a/d converter, Finger demodulating unit, temperature-compensating VCXO, frequency synthesizer constitute automatic frequency control loop; The feature of this automatic frequency controller is:

(1) in this automatic frequency control loop, there is interface between direct frequency converter of RF and the A/D converter, the direct frequency converter of RF is finished the down-converted to the RF signal that receives, and extracts modulated-analog signal, outputs to A/D converter;

(2) in this automatic frequency control loop, there is interface between A/D converter and the Finger demodulating unit, after the direct frequency converter of RF outputed to A/D converter with modulated-analog signal, the modulated-analog signal of the direct frequency converter of RF being exported by A/D converter converted original I/Q chip sequence to; Then, A/D converter outputs to the Finger demodulating unit with original I/Q chip sequence;

(3) in this automatic frequency control loop, there is interface between Finger demodulating unit and the automatic frequency controller, after A/D converter outputs to the Finger demodulating unit with original I/Q chip sequence, the Finger demodulating unit demodulates time synchronized pilot tone I/Q symbol sebolic addressing from original I/Q chip sequence, output to automatic frequency controller;

(4) in this automatic frequency control loop, there is interface between automatic frequency controller and the temperature-compensating VCXO, after the time synchronized pilot tone I/Q symbol sebolic addressing of Finger demodulating unit output is admitted in the automatic frequency controller, automatic frequency controller comprises complex multiplier, postpones conjugator, adder, divider, first order pole window index iir filter, pulse density modulator, is used to finish following calculating:

1) automatic frequency controller at first calculates the real part C that delays the conjugate complex number product of the time synchronized pilot tone I/Q symbol sebolic addressing of each Finger demodulating unit output respectively by following formula _i(N is k) with imaginary part C _q(N, k):

C _i(N，k)＝I(N，k)·I(N，k-1)+Q(N，k)·Q(N，k-1)

C _q(N，k)＝Q(N，k)·I(N，k-1)-I(N，k)·Q(N，k-1)

In the formula, I (N, k) k symbol in the time synchronized pilot tone I symbol sebolic addressing of N Finger demodulating unit output of expression, Q (N, k) k symbol in the time synchronized pilot tone Q symbol sebolic addressing of N Finger demodulating unit output of expression; C _i(N, the k) real part of delaying the conjugate complex number product that calculates of expression, C _q(N, k) imaginary part of delaying the conjugate complex number product that calculates of expression; N is the numbering of Finger demodulating unit, k symbol in the k express time synchronizing pilot I/Q symbol sebolic addressing;

2) afterwards, automatic frequency controller calculates the C of all Finger demodulating units that obtained by above-mentioned formula more respectively by following formula _i(N, stack result C k) _i(k) and C _q(N, stack result C k) _q(k):

$C_i\left(k\right)=\underset{N=0}{\overset{3}{\mathrm{\Σ}}}{C}_i(N,k)$

$C_q\left(k\right)=\underset{N=0}{\overset{3}{\mathrm{\Σ}}}{C}_q(N,k)$

3) then, automatic frequency controller is calculated as follows the C that is obtained by following formula again _i(k) and C _q(k) the plural phase error (k) of Gou Chenging:

$\mathrm{\α}\left(k\right)=\frac{C_q\left(k\right)}{C_i\left(k\right)}$

4) automatic frequency controller calculates above-mentioned plural phase error (k) afterwards, and α (k) is sent into first order pole window index iir filter, by following formula α (k) is done first order pole window index IIR filtering, calculates filtered plural phase error theta (k):

θ(k)＝(1-β)·θ(k-1)+β·α(k)

In the formula, θ (k) is filtered plural phase error, and k is the subscript of α (k) and θ (k) sequence, simultaneously k and above-mentioned steps 1) in the definition of k identical, also k symbol in the express time synchronizing pilot I/Q symbol sebolic addressing, β is the forgetting factor of first order pole window index iir filter;

5) last, automatic frequency controller is used the control voltage of θ as a result (k) the adjustment pulse density modulator of first order pole window index IIR filtering output by following condition judgment method,

If a) θ (k) 〉=0, then the control voltage with pulse density modulator increases a voltage adjustment step delta c;

B) if θ (k)＜0 then adjusts step delta c with voltage of control voltage minimizing of pulse density modulator;

6) in this automatic frequency control loop, between pulse density modulator in the above-mentioned automatic frequency controller and the temperature-compensating VCXO interface is arranged, output by the pulse density modulator of automatic frequency controller adjustment is fed to the temperature-compensating VCXO, is used to control the output frequency of temperature-compensating VCXO.

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