CN102215203A - Method and system for estimating integer multiples of frequency offsets - Google Patents

Abstract

The invention relates to a method and a system for estimating integer multiples of frequency offsets. The method comprises the following steps of: a, receiving all synchronous symbols of each frame, and obtaining a relational expression between each synchronous symbol and a transmitted frequency-domain synchronous symbol; b, performing Fourier demodulation on each received synchronous symbol respectively to obtain a frequency domain of the synchronous symbol; c, performing conjugate multiplication on the adjacent code elements of each received synchronous symbol respectively; and d, obtaining an estimation value of the integer multiples of frequency offsets according to a maximum correlation of a result of the step c and frequency-domain data of a transmitting end after cyclic shifts. According to the method and the system for estimating the integer multiples of frequency offsets, disclosed by the invention, the integer multiples of frequency offsets in a wireless communication, in particular a China mobile multimedia broadcasting (CMMB) system can be correctly estimated, the correctness and the stability of the system are improved, the hardware is easy to realize, and the arithmetic process is fast and convenient.

Description

Integer frequency offset estimation approach and system

Technical field

The present invention relates to integer frequency offset estimation approach and system, is integer frequency offset estimation approach and system in the CMMB system concretely.

Background technology

China Mobile multimedia broadcasting is called for short CMMB (China Mobile Multimedia Broadcasting), is the multimedia broadcasting industry standard that has independent intellectual property right that China national General Bureau of Radio, Film and Television proposed in 2006.This standard adopts OFDM (orthogonal frequency division multiplexi) as modulation-demodulation technique.The OFDM technology has following outstanding advantage:

1, high spectrum utilization;

2, have very strong anti-multipath interference capability, can guarantee the quality of reception under the high-speed moving state;

3, realization is simple relatively, and the OFDM modulation can pass through IFFT (Fourier inverse operation) and FFT (Fourier computing) realizes.

But ofdm system also has a shortcoming clearly: to the carrier frequency shift sensitivity.The carrier wave frequency deviation of OFDM receiver can cause subcarrier to lose orthogonality, and therefore introduces the performance of disturbing (ICI) and seriously damaging system between subcarrier.According to the relation of carrier wave frequency deviation and subcarrier spacing, carrier wave frequency deviation can be divided into integer frequency offset and fractional part of frequency offset.

If there is integer frequency offset in system, the sampling location will be the unit skew with the subcarrier spacing, though the sampling location can not produce ICI on OFDM subcarrier frequency, but can cause the complete mistake of whole demodulation result, and error rate of system is almost 0.5.

Traditional integer frequency bias estimation is according to the principle design of " the normalization integer frequency offset shows as the integral multiple displacement of frequency domain data ", described normalization is meant in one group of data, get a number as unit 1, other number is divided by this unit 1 resulting value.Its first-selection parses the pilot frequency information on the fixed position in 1 continuous OFDM symbol, then pilot frequency information in this symbol and the local pilot frequency information displacement that produces is multiplied each other, and detects correlation peak, thereby determines integer frequency offset.

In the CMMB system, the frame structure of physical layer is a kind of timeslot-based structure.It is divided into 40 time slots with each second, and each time slot comprises 1 beacon and 53 OFDM symbols.Beacon is made up of with two identical synchronizing signals 1 identification signal of transmitter.Each OFDM symbol is formed by sending data, scattered pilot and continuous pilot.Each OFDM symbol has comprised continuous pilot and two kinds of pilot frequency informations of scattered pilot.Because the information bit that transmits in the continuous pilot is unknown for receiving terminal, can't produce corresponding information at receiving terminal.Because the fixing data that send of the scattered pilot of transmitting terminal are 1 bits, because effectively subcarrier wants scrambling to handle before the OFDM modulation, receiving terminal is before descrambling, and in fact pilot frequency information is exactly the scrambler value.Scrambler is to be produced by pseudo random sequence (PN sequence), has 8 kinds of initial values, and is bigger according to the autocorrelation between the random sequence, and the less characteristic of cross correlation obtains peak value.

Multiply each other and estimate integer frequency offset because conventional method is synchronizing symbol and the local training sequence (the local frequency domain synchronizing symbol that produces) that will receive, and in the CMMB system, can't obtain local training sequence, so conventional method can't directly realize, thereby the method that synchronizing symbol is divided by before and after will adopting usually in conventional method reduces error, but this mode need be used division, and the hardware implementation complexity is higher.

Summary of the invention

At above-mentioned problem, the invention provides a kind of integer frequency offset estimation approach and system, can to wireless telecommunications particularly the integer frequency offset in the CMMB system correctly estimate, improve the correctness and the stability of system, and hardware is realized simple.

Integer frequency offset estimation approach of the present invention is that all synchronizing symbols (every frame has two synchronizing symbols) by each frame that will receive are relevant with the local synchronizing symbol that produces, and the peak-peak of being tried to achieve is integer frequency offset, and it comprises step:

A. receive all synchronizing symbols of each frame, obtain the relational expression between the frequency domain synchronizing symbol of each synchronizing symbol and transmission;

B. each synchronizing symbol that is received is carried out the frequency domain that the Fourier demodulation obtains this synchronizing symbol respectively;

C. each synchronizing symbol that is received is carried out adjacent code element respectively and do conjugate multiplication;

D. the maximum correlation that the frequency domain data after cyclic shift exists according to result of step c and transmitting terminal obtains the integer frequency offset estimated value.

In order to guarantee method of the present invention robustness in actual applications, in step a, comprised the processing of symbol timing offset and sampling frequency offset to the influence of the synchronizing symbol that received.

Concrete, the subcarrier number in each synchronizing symbol is 2048.

The present invention also provides a kind of integer frequency offset estimating system that is used for said method, in described system, comprise conjugate unit, memory cell and decision unit, the conjugate module and first that in conjugate unit, the includes receiving inputted signal module that multiplies each other, multiply each other at conjugate module and first and to be connected with Postponement module between the module, signal multiplies each other module to memory cell through described first, comprise memory module in the memory cell, signal through storage after second module that multiplies each other obtains peak value, output to accumulator module again and add up, after comparator in the decision unit and threshold judgement device output integer frequency offset.

Further scheme is that the memory module described in the memory cell comprises dual port RAM and twoport ROM, and described dual port RAM and twoport ROM are controlled by the control module in the memory cell.The RAM of twoport and twoport ROM are 2048 bits, and wherein RAM is used to store the calculating output valve of conjugate unit, ROM storage preset parameters value.

Further again, in decision unit, also include the data selector and the maximum related value memory that are connected, described data selector is controlled by described comparator, and signal outputs to described threshold judgement device through the maximum related value memory.

Integer frequency offset estimation approach of the present invention and system, can to wireless telecommunications particularly the integer frequency offset in the CMMB system estimate that correctly improved the correctness and the stability of system, and hardware realizes simply, calculating process is quick.

Below in conjunction with embodiment, foregoing of the present invention is described in further detail again by the accompanying drawing illustrated embodiment.But this should be interpreted as that the scope of the above-mentioned theme of the present invention only limits to following example.Do not breaking away under the above-mentioned technological thought situation of the present invention, various replacements or change according to ordinary skill knowledge and customary means are made all should comprise within the scope of the invention.

Description of drawings

Fig. 1 is the relevant analogous diagram of carrying out frequency domain synchronizing symbol conjugate multiplication by integer frequency offset estimation approach of the present invention.

Fig. 2 is the structural representation of the system of integer frequency offset estimation of the present invention.

Embodiment

Embodiment 1:

A. receive all synchronizing symbols (not having frame to have two synchronizing symbols) of each frame, obtain relational expression 1 between the frequency domain synchronizing symbol of each synchronizing symbol and transmission (with first synchronizing symbol r of each frame _Sync(1, n) be example, the relational expression of second synchronizing symbol is by that analogy):

X wherein _Sync(1, k) be the frequency domain synchronizing symbol, N _SyncBe the subcarrier number in first synchronizing symbol, H (k) is the channel function,

Be the normalization frequency deviation,

Carrier wave in the time of for modulation.

In order to guarantee robustness in actual applications, to adding symbol timing offset and sampling frequency offset in each the synchronizing symbol relational expression that is received, with first synchronizing symbol r of each frame _Sync(1, n) for the calculating formula 2 of example be:

Wherein the symbol timing offset is n _o, the normalization sampling frequency offset is η.

B. the synchronizing symbol to each frame carries out the frequency domain that the Fourier demodulation obtains this synchronizing symbol respectively, with first synchronizing symbol r of each frame _Sync(1, n) for the calculating formula 3 of example be:

$z_{\mathrm{sync}}(1,m)\&ap;H\left({<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)X_{\mathrm{sync}}(1,{<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}})$

$*e^{j2\mathrm{\&pi;}{n}_{o}m(1+\mathrm{\&eta;})/N_{\mathrm{sync}}}{e}^{j2\mathrm{\&pi;m\&eta;}(N_{\mathrm{sync}}-1)/N_{\mathrm{sync}}}+\mathrm{ICI}\left(q\right)+N_m$

Z wherein _Sync(1, m) be the frequency domain of synchronizing symbol;

It is channel function cyclic shift λ value afterwards; $X_{\mathrm{sync}}(1,{<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}})$ Frequency for synchronizing symbol displacement λ;

The expression frequency deviation.ICI in the following formula (q) is at first synchronizing symbol r _Sync(1, n) in all the other N _Sync-1 subcarrier is to the influence of m subcarrier, intersymbol interference just.Visicode timing offset n _oCause the receiving terminal data to have phase rotation coefficient with sampling frequency offset η

This twiddle factor is relevant with subcarrier number m.

C. the adjacent code element of each synchronizing symbol of being received is done conjugate multiplication, with first synchronizing symbol r of each frame _Sync(1, n) for the calculating formula 4 of example be:

$z_{\mathrm{sync}}(1,m+1)z^{*}(1,m)$

$\&ap;H\left({<m+1-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)X(1,{<m+1-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}})H^{*}\left({<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)X^{*}\left({1,<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)$

$*e^{j2\mathrm{\&pi;}{n}_o(1+\mathrm{\&eta;})/N_{\mathrm{sync}}}{e}^{j2\mathrm{\&pi;\&eta;}(N_{\mathrm{sync}}-1)/N_{\mathrm{sync}}}+{N^{,}}_m$

In following formula, ignore the influence of additive noise,

Be only relevant with sampling frequency offset with a symbol timing offset the amount of phase rotation, it remains unchanged in a synchronizing symbol.

D. the frequency domain data after cyclic shift according to result of step c and transmitting terminal

The maximum correlation that exists, in each synchronizing symbol, the frequency domain data of transmission is exactly the PN sequence, therefore obtains the calculating formula 5 of integer frequency offset estimated value λ:

$\widehat{\mathrm{\&lambda;}}=\arg \underset{\mathrm{\&lambda;}}{\max }\left(\right|\underset{m-0}{\overset{N_{\mathrm{sync}}-1}{\mathrm{\&Sigma;}}}{z}_{\mathrm{sync}}(1,m+1)z^{*}(1,m)X(1,{<m+1-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}})X^{*}(1,{<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}})\left|\right)$

$\arg \underset{\mathrm{\&lambda;}}{\max }\left(\right|\underset{m-0}{\overset{N_{\mathrm{sync}}-1}{\mathrm{\&Sigma;}}}H\left({<m+1-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)H^{*}\left({<m-\mathrm{\&lambda;}>}_{N_{\mathrm{sync}}}\right)\left|\right)$

Wherein λ is the shift amount of first synchronizing symbol frequency.

By calculating formula 5 as can be seen, in the end eliminated symbol timing offset and sampling frequency offset among Shu Chu the integer frequency offset estimated value λ, therefore method of the present invention is not subjected to the influence of symbol timing offset and sampling frequency offset.

Fig. 1 is in the channel of the white Gaussian noise of signal to noise ratio snr=0dB, and the symbol timing offset is 10 sampled points, normalization sampling frequency offset 5.0 * 10 ^-4The time, by the analogous diagram of correlation peak that method of the present invention is tried to achieve.Abscissa in Fig. 1 is near the point that a peak value maximum is arranged 10.According to the emulation principle of relevant peaks, when in the abscissa scope, the peak value that any is arranged shows that this point is a correlation peak, i.e. the maximum frequency deviation position during far away above other.Therefore as shown in Figure 1, it is inclined to one side to obtain the maximum integer frequency multiplication by method of the present invention, even under the influence of symbol timing offset and sampling frequency offset, still can capture integer frequency offset by method of the present invention.

Embodiment 2:

The integer frequency offset estimating system that is used for embodiment 1 of the present invention comprises conjugate unit, memory cell and decision unit in described system.Wherein conjugate unit is used for realizing the z of embodiment 1 calculating formula 5 _Sync(1, m+1) z ^*(1, m) part, memory cell are used for realizing embodiment 1 calculating formula 5

Part, the conjugate module and first that in conjugate unit, the includes receiving inputted signal module that multiplies each other, multiply each other at conjugate module and first and to be connected with Postponement module between the module, signal multiplies each other module to memory cell through described first, comprise memory module in the memory cell, memory module comprises the dual port RAM of 2048 bits and the twoport ROM of 2048 bits, and described dual port RAM and twoport ROM are controlled by the control module in the memory cell.Signal, outputs to accumulator module again and adds up after second module that multiplies each other obtains peak value through memory module storage, passes through comparator, data selector and maximum related value memory in the decision unit at last, by threshold judgement device output integer frequency offset Δ

, wherein data selector is controlled by comparator, when the value of " 1 " number port during greater than the value of " 0 " number port, is output as the value of " 1 " number port, on the contrary the value of output " 0 " number port.The maximum related value memory is common memory, the maximum related value that storage obtains behind data selector.

By system of the present invention processing based on frame is converted into processing mode based on data flow.The data bit of input is 13bit, 1bit sign bit wherein, 3bit integer-bit, 9bit decimal place.Method in 1 in conjunction with the embodiments, the course of work of native system is as follows:

Read 1536 effective subcarrier datas the synchronizing symbol from FIFO (first in first out storage organization), adjacent sub-carrier position conjugate multiplication writes dual port RAM.

The capacity of dual port RAM is 2048 * 13bit, and the capacity of twoport ROM is 2048 * 1bit, all is twoport output, and purpose is to search for frequency deviation value simultaneously in positive negative direction.What store among the twoport ROM is the sign bit of the data that multiply each other of local synchronizing symbol PN sign indicating number adjacent position, with the dual port RAM Data Matching, obtain peak value.

Control module writing of dual port RAM of control and reading of dual port RAM and twoport ROM, delivering to accumulator module after the data of reading from dual port RAM and twoport ROM multiply each other adds up, through obtaining the correlation of a frequency deviation point after inferior the adding up of 1535 (also can be the data of other suitable actual environment), value to correlation delivery and maximum related value memory the inside is compared, if＞value of maximum related value memory the inside then deposits this maximum related value memory in.Because the is-symbol position of twoport ROM output, therefore second module that multiplies each other is just carried out negate to the data of dual port RAM output.

Each frequency deviation point is all pressed the said process operation, carries out point by point search, up to the maximum search scope that searches setting.After having searched for, the value of maximum related value memory the inside storage is comparing in the threshold judgement device with preset threshold, if greater than threshold value, then the search point d of output maximum related value correspondence this moment is as the normalization integer frequency offset

Claims (6)

1. integer frequency offset estimation approach is characterized by and comprises step:

A. receive all synchronizing symbols of each frame, obtain the relational expression between the frequency domain synchronizing symbol of each synchronizing symbol and transmission;

B. each synchronizing symbol that is received is carried out the frequency domain that the Fourier demodulation obtains this synchronizing symbol respectively;

C. each synchronizing symbol that is received is carried out adjacent code element respectively and do conjugate multiplication;

D. the maximum correlation that the frequency domain data after cyclic shift exists according to result of step c and transmitting terminal obtains the integer frequency offset estimated value.

2. integer frequency offset estimation approach as claimed in claim 1 is characterized by comprised the processing to the influence of the synchronizing symbol that received of symbol timing offset and sampling frequency offset in step a.

3. integer frequency offset estimation approach as claimed in claim 1 or 2, the subcarrier number that it is characterized by in described each synchronizing symbol is 2048.

4. the system that estimates of integer frequency offset, it is characterized by and comprise conjugate unit, memory cell and decision unit, the conjugate module and first that in conjugate unit, the includes receiving inputted signal module that multiplies each other, multiply each other at conjugate module and first and to be connected with Postponement module between the module, signal multiplies each other module to memory cell through described first, comprise memory module in the memory cell, signal through storage after second module that multiplies each other obtains peak value, output to accumulator module again and add up, after comparator in the decision unit and threshold judgement device output integer frequency offset.

5. the system that integer frequency offset as claimed in claim 4 is estimated is characterized by the memory module described in the memory cell and comprises dual port RAM and twoport ROM, the control module control in described dual port RAM and the twoport ROM hand memory cell.

6. the system that estimates as claim 4 or 5 described integer frequency offsets, it is characterized by and in decision unit, also include data selector and the maximum related value memory that is connected, described data selector is controlled by described comparator, and signal outputs to described threshold judgement device through the maximum related value memory.

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